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Updated by: 3661 INFORMATIONAL
Errata Exist
Network Working Group                                       F. Andreasen
Request for Comments: 3435                                     B. Foster
Obsoletes: 2705                                            Cisco Systems
Category: Informational                                     January 2003


                 Media Gateway Control Protocol (MGCP)
                              Version 1.0

Status of this Memo

   This memo provides information for the Internet community.  It does
   not specify an Internet standard of any kind.  Distribution of this
   memo is unlimited.

Copyright Notice

   Copyright (C) The Internet Society (2003).  All Rights Reserved.

IESG Note

   This document is being published for the information of the
   community.  It describes a protocol that is currently being deployed
   in a number of products.  Implementers should be aware of RFC 3015,
   which was developed in the IETF Megaco Working Group and the ITU-T
   SG16 and which is considered by the IETF and ITU-T to be the
   standards-based (including reviewed security considerations) way to
   meet the needs that MGCP was designed to address.

Abstract

   This document describes an application programming interface and a
   corresponding protocol (MGCP) which is used between elements of a
   decomposed multimedia gateway.  The decomposed multimedia gateway
   consists of a Call Agent, which contains the call control
   "intelligence", and a media gateway which contains the media
   functions, e.g., conversion from TDM voice to Voice over IP.

   Media gateways contain endpoints on which the Call Agent can create,
   modify and delete connections in order to establish and control media
   sessions with other multimedia endpoints.  Also, the Call Agent can
   instruct the endpoints to detect certain events and generate signals.
   The endpoints automatically communicate changes in service state to
   the Call Agent.  Furthermore, the Call Agent can audit endpoints as
   well as the connections on endpoints.






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   The basic and general MGCP protocol is defined in this document,
   however most media gateways will need to implement one or more MGCP
   packages, which define extensions to the protocol suitable for use
   with specific types of media gateways.  Such packages are defined in
   separate documents.

Table of Contents

   1.     Introduction.................................................5
   1.1    Relation with the H.323 Standards............................7
   1.2    Relation with the IETF Standards.............................8
   1.3    Definitions..................................................9
   1.4    Conventions used in this Document............................9
   2.     Media Gateway Control Interface.............................10
   2.1    Model and Naming Conventions................................10
   2.1.1  Types of Endpoints..........................................10
   2.1.2  Endpoint Identifiers........................................14
   2.1.3  Calls and Connections.......................................16
   2.1.4  Names of Call Agents and Other Entities.....................22
   2.1.5  Digit Maps..................................................23
   2.1.6  Packages....................................................26
   2.1.7  Events and Signals..........................................28
   2.2    Usage of SDP................................................33
   2.3    Gateway Control Commands....................................33
   2.3.1  Overview of Commands........................................33
   2.3.2  EndpointConfiguration.......................................36
   2.3.3  NotificationRequest.........................................37
   2.3.4  Notify......................................................44
   2.3.5  CreateConnection............................................46
   2.3.6  ModifyConnection............................................52
   2.3.7  DeleteConnection (from the Call Agent)......................54
   2.3.8  DeleteConnection (from the gateway).........................58
   2.3.9  DeleteConnection (multiple connections from the Call Agent) 59
   2.3.10 AuditEndpoint...............................................60
   2.3.11 AuditConnection.............................................65
   2.3.12 RestartInProgress...........................................66
   2.4    Return Codes and Error Codes................................69
   2.5    Reason Codes................................................74
   2.6    Use of Local Connection Options and Connection Descriptors..75
   2.7    Resource Reservations.......................................77
   3.     Media Gateway Control Protocol..............................77
   3.1    General Description.........................................78
   3.2    Command Header..............................................79
   3.2.1  Command Line................................................79
   3.2.2  Parameter Lines.............................................82
   3.3    Format of response headers.................................101
   3.3.1  CreateConnection Response..................................104
   3.3.2  ModifyConnection Response..................................105



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   3.3.3  DeleteConnection Response..................................106
   3.3.4  NotificationRequest Response...............................106
   3.3.5  Notify Response............................................106
   3.3.6  AuditEndpoint Response.....................................106
   3.3.7  AuditConnection Response...................................107
   3.3.8  RestartInProgress Response.................................108
   3.4    Encoding of the Session Description (SDP)..................108
   3.4.1  Usage of SDP for an Audio Service..........................110
   3.4.2  Usage of SDP for LOCAL Connections.........................110
   3.5    Transmission over UDP......................................111
   3.5.1  Providing the At-Most-Once Functionality...................112
   3.5.2  Transaction Identifiers and Three Ways Handshake...........113
   3.5.3  Computing Retransmission Timers............................114
   3.5.4  Maximum Datagram Size, Fragmentation and Reassembly........115
   3.5.5  Piggybacking...............................................116
   3.5.6  Provisional Responses......................................117
   4.     States, Failover and Race Conditions.......................119
   4.1    Failover Assumptions and Highlights........................119
   4.2    Communicating with Gateways................................121
   4.3    Retransmission, and Detection of Lost Associations:........122
   4.4    Race Conditions............................................126
   4.4.1  Quarantine List............................................127
   4.4.2  Explicit Detection.........................................133
   4.4.3  Transactional Semantics....................................134
   4.4.4  Ordering of Commands, and Treatment of Misorder............135
   4.4.5  Endpoint Service States....................................137
   4.4.6  Fighting the Restart Avalanche.............................140
   4.4.7  Disconnected Endpoints.....................................143
   4.4.8  Load Control in General....................................146
   5.     Security Requirements......................................147
   5.1    Protection of Media Connections............................148
   6.     Packages...................................................148
   6.1    Actions....................................................150
   6.2    BearerInformation..........................................150
   6.3    ConnectionModes............................................151
   6.4    ConnectionParameters.......................................151
   6.5    DigitMapLetters............................................151
   6.6    Events and Signals.........................................152
   6.6.1  Default and Reserved Events................................155
   6.7    ExtensionParameters........................................156
   6.8    LocalConnectionOptions.....................................157
   6.9    Reason Codes...............................................157
   6.10   RestartMethods.............................................158
   6.11   Return Codes...............................................158
   7.     Versions and Compatibility.................................158
   7.1    Changes from RFC 2705......................................158
   8.     Security Considerations....................................164
   9.     Acknowledgments............................................164



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   10.    References.................................................164
   Appendix A: Formal Syntax Description of the Protocol.............167
   Appendix B: Base Package..........................................175
   B.1    Events.....................................................175
   B.2    Extension Parameters.......................................176
   B.2.1  PersistentEvents...........................................176
   B.2.2  NotificationState..........................................177
   B.3    Verbs......................................................177
   Appendix C: IANA Considerations...................................179
   C.1    New MGCP Package Sub-Registry..............................179
   C.2    New MGCP Package...........................................179
   C.3    New MGCP LocalConnectionOptions Sub-Registry...............179
   Appendix D: Mode Interactions.....................................180
   Appendix E: Endpoint Naming Conventions...........................182
   E.1    Analog Access Line Endpoints...............................182
   E.2    Digital Trunks.............................................182
   E.3    Virtual Endpoints..........................................183
   E.4    Media Gateway..............................................184
   E.5    Range Wildcards............................................184
   Appendix F: Example Command Encodings.............................185
   F.1    NotificationRequest........................................185
   F.2    Notify.....................................................186
   F.3    CreateConnection...........................................186
   F.4    ModifyConnection...........................................189
   F.5    DeleteConnection (from the Call Agent).....................189
   F.6    DeleteConnection (from the gateway)........................190
   F.7    DeleteConnection (multiple connections
          from the Call Agent).......................................190
   F.8    AuditEndpoint..............................................191
   F.9    AuditConnection............................................192
   F.10   RestartInProgress..........................................193
   Appendix G: Example Call Flows....................................194
   G.1    Restart....................................................195
   G.1.1  Residential Gateway Restart................................195
   G.1.2  Call Agent Restart.........................................198
   G.2    Connection Creation........................................200
   G.2.1  Residential Gateway to Residential Gateway.................200
   G.3    Connection Deletion........................................206
   G.3.1  Residential Gateway to Residential Gateway.................206
   Authors' Addresses................................................209
   Full Copyright Statement..........................................210










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1. Introduction

   This document describes an abstract application programming interface
   (MGCI) and a corresponding protocol (MGCP) for controlling media
   gateways from external call control elements called media gateway
   controllers or Call Agents.  A media gateway is typically a network
   element that provides conversion between the audio signals carried on
   telephone circuits and data packets carried over the Internet or over
   other packet networks.  Examples of media gateways are:

   * Trunking gateways, that interface between the telephone network and
     a Voice over IP network.  Such gateways typically manage a large
     number of digital circuits.

   * Voice over ATM gateways, which operate much the same way as voice
     over IP trunking gateways, except that they interface to an ATM
     network.

   * Residential gateways, that provide a traditional analog (RJ11)
     interface to a Voice over IP network.  Examples of residential
     gateways include cable modem/cable set-top boxes, xDSL devices, and
     broad-band wireless devices.

   * Access gateways, that provide a traditional analog (RJ11) or
     digital PBX interface to a Voice over IP network.  Examples of
     access gateways include small-scale voice over IP gateways.

   * Business gateways, that provide a traditional digital PBX interface
     or an integrated "soft PBX" interface to a Voice over IP network.

   * Network Access Servers, that can attach a "modem" to a telephone
     circuit and provide data access to the Internet.  We expect that in
     the future, the same gateways will combine Voice over IP services
     and Network Access services.

   * Circuit switches, or packet switches, which can offer a control
     interface to an external call control element.

   MGCP assumes a call control architecture where the call control
   "intelligence" is outside the gateways and handled by external call
   control elements known as Call Agents.  The MGCP assumes that these
   call control elements, or Call Agents, will synchronize with each
   other to send coherent commands and responses to the gateways under
   their control.  If this assumption is violated, inconsistent behavior
   should be expected.  MGCP does not define a mechanism for
   synchronizing Call Agents.  MGCP is, in essence, a master/slave
   protocol, where the gateways are expected to execute commands sent by
   the Call Agents.  In consequence, this document specifies in great



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   detail the expected behavior of the gateways, but only specifies
   those parts of a Call Agent implementation, such as timer management,
   that are mandated for proper operation of the protocol.

   MGCP assumes a connection model where the basic constructs are
   endpoints and connections.  Endpoints are sources and/or sinks of
   data and can be physical or virtual.  Examples of physical endpoints
   are:

   * An interface on a gateway that terminates a trunk connected to a
     PSTN switch (e.g., Class 5, Class 4, etc.).  A gateway that
     terminates trunks is called a trunking gateway.

   * An interface on a gateway that terminates an analog POTS connection
     to a phone, key system, PBX, etc.  A gateway that terminates
     residential POTS lines (to phones) is called a residential gateway.

   An example of a virtual endpoint is an audio source in an audio-
   content server.  Creation of physical endpoints requires hardware
   installation, while creation of virtual endpoints can be done by
   software.

   Connections may be either point to point or multipoint.  A point to
   point connection is an association between two endpoints with the
   purpose of transmitting data between these endpoints.  Once this
   association is established for both endpoints, data transfer between
   these endpoints can take place.  A multipoint connection is
   established by connecting the endpoint to a multipoint session.

   Connections can be established over several types of bearer networks,
   for example:

   * Transmission of audio packets using RTP and UDP over an IP network.

   * Transmission of audio packets using AAL2, or another adaptation
     layer, over an ATM network.

   * Transmission of packets over an internal connection, for example
     the TDM backplane or the interconnection bus of a gateway.  This is
     used, in particular, for "hairpin" connections, connections that
     terminate in a gateway but are immediately rerouted over the
     telephone network.

   For point-to-point connections the endpoints of a connection could be
   in separate gateways or in the same gateway.






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1.1 Relation with the H.323 Standards

   MGCP is designed as an internal protocol within a distributed system
   that appears to the outside as a single VoIP gateway.  This system is
   composed of a Call Agent, that may or may not be distributed over
   several computer platforms, and of a set of gateways, including at
   least one "media gateway" that perform the conversion of media
   signals between circuits and packets, and at least one "signaling
   gateway" when connecting to an SS7 controlled network.  In a typical
   configuration, this distributed gateway system will interface on one
   side with one or more telephony (i.e., circuit) switches, and on the
   other side with H.323 conformant systems, as indicated in the
   following table:

    ------------------------------------------------------------------
   | Functional|  Phone     |  Terminating    |  H.323 conformant     |
   | Plane     |  switch    |  Entity         |  systems              |
   |-----------|------------|-----------------|-----------------------|
   | Signaling |  Signaling |  Call agent     |  Signaling exchanges  |
   | Plane     |  exchanges |                 |  with the Call Agent  |
   |           |  through   |                 |  through H.225/RAS and|
   |           |  SS7/ISUP  |                 |  H.225/Q.931.         |
   |-----------|------------|-----------------|-----------------------|
   |           |            |                 |  Possible negotiation |
   |           |            |                 |  of logical channels  |
   |           |            |                 |  and transmission     |
   |           |            |                 |  parameters through   |
   |           |            |                 |  H.245 with the call  |
   |           |            |                 |  agent.               |
   |-----------|------------|-----------------|-----------------------|
   |           |            |  Internal       |                       |
   |           |            |  synchronization|                       |
   |           |            |  through MGCP   |                       |
   |-----------|------------|-----------------|-----------------------|
   | Bearer    |  Connection|  Telephony      |  Transmission of VoIP |
   | Data      |  through   |  gateways       |  data using RTP       |
   | Transport |  high speed|                 |  directly between the |
   | Plane     |  trunk     |                 |  H.323 station and the|
   |           |  groups    |                 |  gateway.             |
    ------------------------------------------------------------------

   In the MGCP model, the gateways focus on the audio signal translation
   function, while the Call Agent handles the call signaling and call
   processing functions.  As a consequence, the Call Agent implements
   the "signaling" layers of the H.323 standard, and presents itself as
   an "H.323 Gatekeeper" or as one or more "H.323 Endpoints" to the
   H.323 systems.




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1.2  Relation with the IETF Standards

   While H.323 is the recognized standard for VoIP terminals, the IETF
   has also produced specifications for other types of multi-media
   applications.  These other specifications include:

   * the Session Description Protocol (SDP), RFC 2327

   * the Session Announcement Protocol (SAP), RFC 2974

   * the Session Initiation Protocol (SIP), RFC 3261

   * the Real Time Streaming Protocol (RTSP), RFC 2326.

   The latter three specifications are in fact alternative signaling
   standards that allow for the transmission of a session description to
   an interested party.  SAP is used by multicast session managers to
   distribute a multicast session description to a large group of
   recipients, SIP is used to invite an individual user to take part in
   a point-to-point or unicast session, RTSP is used to interface a
   server that provides real time data.  In all three cases, the session
   description is described according to SDP; when audio is transmitted,
   it is transmitted through the Real-time Transport Protocol, RTP.




























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   The distributed gateway systems and MGCP will enable PSTN telephony
   users to access sessions set up using SAP, SIP or RTSP.  The Call
   Agent provides for signaling conversion, according to the following
   table:

    ------------------------------------------------------------------
   | Functional|  Phone     |  Terminating  |  IETF conforming systems|
   | Plane     |  switch    |  Entity       |                         |
   |-----------|------------|---------------|-------------------------|
   | Signaling |  Signaling |  Call agent   |  Signaling exchanges    |
   | Plane     |  exchanges |               |  with the Call Agent    |
   |           |  through   |               |  through SAP, SIP or    |
   |           |  SS7/ISUP  |               |  RTSP.                  |
   |-----------|------------|---------------|-------------------------|
   |           |            |               |  Negotiation of session |
   |           |            |               |  description parameters |
   |           |            |               |  through SDP (telephony |
   |           |            |               |  gateway terminated but |
   |           |            |               |  passed via the call    |
   |           |            |               |  agent to and from the  |
   |           |            |               |  IETF conforming system)|
   |-----------|------------|---------------|-------------------------|
   |           |            | Internal syn- |                         |
   |           |            | chronization  |                         |
   |           |            | through MGCP  |                         |
   |-----------|------------|---------------|-------------------------|
   | Bearer    |  Connection|  Telephony    |  Transmission of VoIP   |
   | Data      |  through   |  gateways     |  data using RTP,        |
   | Transport |  high speed|               |  directly between the   |
   | Plane     |  trunk     |               |  remote IP end system   |
   |           |  groups    |               |  and the gateway.       |
    ------------------------------------------------------------------

   The SDP standard has a pivotal status in this architecture.  We will
   see in the following description that we also use it to carry session
   descriptions in MGCP.

1.3 Definitions

   Trunk:  A communication channel between two switching systems, e.g.,
   a DS0 on a T1 or E1 line.

1.4 Conventions used in this Document

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED, "MAY", and
   "OPTIONAL" in this document are to be interpreted as described in BCP
   14, RFC 2119 [2].



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2. Media Gateway Control Interface

   The interface functions provide for connection control and endpoint
   control.  Both use the same system model and the same naming
   conventions.

2.1 Model and Naming Conventions

   The MGCP assumes a connection model where the basic constructs are
   endpoints and connections.  Connections are grouped in calls.  One or
   more connections can belong to one call.  Connections and calls are
   set up at the initiative of one or more Call Agents.

2.1.1 Types of Endpoints

   In the introduction, we presented several classes of gateways.  Such
   classifications, however, can be misleading.  Manufacturers can
   arbitrarily decide to provide several types of services in a single
   package.  A single product could well, for example, provide some
   trunk connections to telephony switches, some primary rate
   connections and some analog line interfaces, thus sharing the
   characteristics of what we described in the introduction as
   "trunking", "access" and "residential" gateways.  MGCP does not make
   assumptions about such groupings.  We simply assume that media
   gateways support collections of endpoints.  The type of the endpoint
   determines its functionality.  Our analysis, so far, has led us to
   isolate the following basic endpoint types:

   * Digital channel (DS0),

   * Analog line,

   * Announcement server access point,

   * Interactive Voice Response access point,

   * Conference bridge access point,

   * Packet relay,

   * ATM "trunk side" interface.

   In this section, we will describe the expected behavior of such
   endpoints.







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   This list is not final.  There may be other types of endpoints
   defined in the future, for example test endpoints that could be used
   to check network quality, or frame-relay endpoints that could be used
   to manage audio channels multiplexed over a frame-relay virtual
   circuit.

2.1.1.1 Digital Channel (DS0)

   Digital channels provide a 64 Kbps service.  Such channels are found
   in trunk and ISDN interfaces.  They are typically part of digital
   multiplexes, such as T1, E1, T3 or E3 interfaces.  Media gateways
   that support such channels are capable of translating the digital
   signals received on the channel, which may be encoded according to
   A-law or mu-law, using either the complete set of 8 bits per sample
   or only 7 of these bits, into audio packets.  When the media gateway
   also supports a Network Access Server (NAS) service, the gateway
   shall be capable of receiving either audio-encoded data (modem
   connection) or binary data (ISDN connection) and convert them into
   data packets.

                                         +-------
                           +------------+|
              (channel) ===|DS0 endpoint| -------- Connections
                           +------------+|
                                         +-------

   Media gateways should be able to establish several connections
   between the endpoint and the packet networks, or between the endpoint
   and other endpoints in the same gateway.  The signals originating
   from these connections shall be mixed according to the connection
   "mode", as specified later in this document.  The precise number of
   connections that an endpoint supports is a characteristic of the
   gateway, and may in fact vary according to the allocation of
   resources within the gateway.

   In some cases, digital channels are used to carry signaling.  This is
   the case for example for SS7 "F" links, or ISDN "D" channels.  Media
   gateways that support these signaling functions shall be able to send
   and receive the signaling packets to and from a Call Agent, using the
   "backhaul" procedures defined by the SIGTRAN working group of the
   IETF.  Digital channels are sometimes used in conjunction with
   channel associated signaling, such as "MF R2".  Media gateways that
   support these signaling functions shall be able to detect and produce
   the corresponding signals, such as for example "wink" or "A",
   according to the event signaling and reporting procedures defined in
   MGCP.





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2.1.1.2 Analog Line

   Analog lines can be used either as a "client" interface, providing
   service to a classic telephone unit, or as a "service" interface,
   allowing the gateway to send and receive analog calls.  When the
   media gateway also supports a NAS service, the gateway shall be
   capable of receiving audio-encoded data (modem connection) and
   convert them into data packets.

                                         +-------
                        +---------------+|
              (line) ===|analog endpoint| -------- Connections
                        +---------------+|
                                         +-------

   Media gateways should be able to establish several connections
   between the endpoint and the packet networks, or between the endpoint
   and other endpoints in the same gateway.  The audio signals
   originating from these connections shall be mixed according to the
   connection "mode", as specified later in this document.  The precise
   number of connections that an endpoint supports is a characteristic
   of the gateway, and may in fact vary according to the allocation of
   resources within the gateway.  A typical gateway should however be
   able to support two or three connections per endpoint, in order to
   support services such as "call waiting" or "three way calling".

2.1.1.3 Announcement Server Access Point

   An announcement server endpoint provides access to an announcement
   service.  Under requests from the Call Agent, the announcement server
   will "play" a specified announcement.  The requests from the Call
   Agent will follow the event signaling and reporting procedures
   defined in MGCP.

                  +----------------------+
                  | Announcement endpoint| -------- Connection
                  +----------------------+

   A given announcement endpoint is not expected to support more than
   one connection at a time.  If several connections were established to
   the same endpoint, then the same announcements would be played
   simultaneously over all the connections.

   Connections to an announcement server are typically one way, or "half
   duplex" -- the announcement server is not expected to listen to the
   audio signals from the connection.





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2.1.1.4 Interactive Voice Response Access Point

   An Interactive Voice Response (IVR) endpoint provides access to an
   IVR service.  Under requests from the Call Agent, the IVR server will
   "play" announcements and tones, and will "listen" to responses, such
   as DTMF input or voice messages, from the user.  The requests from
   the Call Agent will follow the event signaling and reporting
   procedures defined in MGCP.

                      +-------------+
                      | IVR endpoint| -------- Connection
                      +-------------+

   A given IVR endpoint is not expected to support more than one
   connection at a time.  If several connections were established to the
   same endpoint, then the same tones and announcements would be played
   simultaneously over all the connections.

2.1.1.5 Conference Bridge Access Point

   A conference bridge endpoint is used to provide access to a specific
   conference.

                                           +-------
               +--------------------------+|
               |Conference bridge endpoint| -------- Connections
               +--------------------------+|
                                           +-------

   Media gateways should be able to establish several connections
   between the endpoint and the packet networks, or between the endpoint
   and other endpoints in the same gateway.  The signals originating
   from these connections shall be mixed according to the connection
   "mode", as specified later in this document.  The precise number of
   connections that an endpoint supports is a characteristic of the
   gateway, and may in fact vary according to the allocation of
   resources within the gateway.

2.1.1.6 Packet Relay

   A packet relay endpoint is a specific form of conference bridge, that
   typically only supports two connections.  Packets relays can be found
   in firewalls between a protected and an open network, or in
   transcoding servers used to provide interoperation between
   incompatible gateways, for example gateways that do not support
   compatible compression algorithms, or gateways that operate over
   different transmission networks such as IP and ATM.




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                                           +-------
                   +---------------------+ |
                   |Packet relay endpoint|  2 connections
                   +---------------------+ |
                                           +-------

2.1.1.7 ATM "trunk side" Interface

   ATM "trunk side" endpoints are typically found when one or several
   ATM permanent virtual circuits are used as a replacement for the
   classic "TDM" trunks linking switches.  When ATM/AAL2 is used,
   several trunks or channels are multiplexed on a single virtual
   circuit; each of these trunks correspond to a single endpoint.

                                          +-------
                      +------------------+|
          (channel) = |ATM trunk endpoint| -------- Connections
                      +------------------+|
                                          +-------

   Media gateways should be able to establish several connections
   between the endpoint and the packet networks, or between the endpoint
   and other endpoints in the same gateway.  The signals originating
   from these connections shall be mixed according to the connection
   "mode", as specified later in this document.  The precise number of
   connections that an endpoint supports is a characteristic of the
   gateway, and may in fact vary according to the allocation of
   resources within the gateway.

2.1.2 Endpoint Identifiers

   Endpoint identifiers have two components that both are case-
   insensitive:

   * the domain name of the gateway that is managing the endpoint

   * a local name within that gateway

   Endpoint names are of the form:

      local-endpoint-name@domain-name

   where domain-name is an absolute domain-name as defined in RFC 1034
   and includes a host portion, thus an example domain-name could be:

      mygateway.whatever.net





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   Also, domain-name may be an IP-address of the form defined for domain
   name in RFC 821, thus another example could be (see RFC 821 for
   details):

      [192.168.1.2]

   Both IPv4 and IPv6 addresses can be specified, however use of IP
   addresses as endpoint identifiers is generally discouraged.

   Note that since the domain name portion is part of the endpoint
   identifier, different forms or different values referring to the same
   entity are not freely interchangeable.  The most recently supplied
   form and value MUST always be used.

   The local endpoint name is case-insensitive.  The syntax of the local
   endpoint name is hierarchical, where the least specific component of
   the name is the leftmost term, and the most specific component is the
   rightmost term.  The precise syntax depends on the type of endpoint
   being named and MAY start with a term that identifies the endpoint
   type.  In any case, the local endpoint name MUST adhere to the
   following naming rules:

   1) The individual terms of the naming path MUST be separated by a
      single slash ("/", ASCII 2F hex).

   2) The individual terms are character strings composed of letters,
      digits or other printable characters, with the exception of
      characters used as delimiters ("/", "@"), characters used for
      wildcarding ("*", "$") and white spaces.

   3) Wild-carding is represented either by an asterisk ("*") or a
      dollar sign ("$") for the terms of the naming path which are to be
      wild-carded.  Thus, if the full local endpoint name is of the
      form:

          term1/term2/term3

      then the entity name field looks like this depending on which
      terms are wild-carded:

          */term2/term3 if term1 is wild-carded
          term1/*/term3 if term2 is wild-carded
          term1/term2/* if term3 is wild-carded
          term1/*/*     if term2 and term3 are wild-carded, etc.

      In each of these examples a dollar sign could have appeared
      instead of an asterisk.




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   4) A term represented by an asterisk ("*") is to be interpreted as:
      "use ALL values of this term known within the scope of the Media
      Gateway".  Unless specified otherwise, this refers to all
      endpoints configured for service, regardless of their actual
      service state, i.e., in-service or out-of-service.

   5) A term represented by a dollar sign ("$") is to be interpreted as:
      "use ANY ONE value of this term known within the scope of the
      Media Gateway".  Unless specified otherwise, this only refers to
      endpoints that are in-service.

   Furthermore, it is RECOMMENDED that Call Agents adhere to the
   following:

   * Wild-carding should only be done from the right, thus if a term is
     wild-carded, then all terms to the right of that term should be
     wild-carded as well.

   * In cases where mixed dollar sign and asterisk wild-cards are used,
     dollar-signs should only be used from the right, thus if a term had
     a dollar sign wild-card, all terms to the right of that term should
     also contain dollar sign wild-cards.

   The description of a specific command may add further criteria for
   selection within the general rules given above.

   Note, that wild-cards may be applied to more than one term in which
   case they shall be evaluated from left to right.  For example, if we
   have the endpoint names "a/1", "a/2", "b/1", and "b/2", then "$/*"
   (which is not recommended) will evaluate to either "a/1, a/2", or
   "b/1, b/2".  However, "*/$" may evaluate to "a/1, b/1", "a/1, b/2",
   "a/2, b/1", or "a/2, b/2".  The use of mixed wild-cards in a command
   is considered error prone and is consequently discouraged.

   A local name that is composed of only a wildcard character refers to
   either all (*) or any ($) endpoints within the media gateway.

2.1.3 Calls and Connections

   Connections are created on the Call Agent on each endpoint that will
   be involved in the "call".  In the classic example of a connection
   between two "DS0" endpoints (EP1 and EP2), the Call Agents
   controlling the endpoints will establish two connections (C1 and C2):

                  +---+                            +---+
    (channel1) ===|EP1|--(C1)--...        ...(C2)--|EP2|===(channel2)
                  +---+                            +---+




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   Each connection will be designated locally by an endpoint unique
   connection identifier, and will be characterized by connection
   attributes.

   When the two endpoints are located on gateways that are managed by
   the same Call Agent, the creation is done via the three following
   steps:

   1) The Call Agent asks the first gateway to "create a connection" on
      the first endpoint.  The gateway allocates resources to that
      connection, and responds to the command by providing a "session
      description".  The session description contains the information
      necessary for a third party to send packets towards the newly
      created connection, such as for example IP address, UDP port, and
      codec parameters.

   2) The Call Agent then asks the second gateway to "create a
      connection" on the second endpoint.  The command carries the
      "session description" provided by the first gateway.  The gateway
      allocates resources to that connection, and responds to the
      command by providing its own "session description".

   3) The Call Agent then uses a "modify connection" command to provide
      this second "session description" to the first endpoint.  Once
      this is done, communication can proceed in both directions.

   When the two endpoints are located on gateways that are managed by
   two different Call Agents, the Call Agents exchange information
   through a Call-Agent to Call-Agent signaling protocol, e.g., SIP [7],
   in order to synchronize the creation of the connection on the two
   endpoints.

   Once a connection has been established, the connection parameters can
   be modified at any time by a "modify connection" command.  The Call
   Agent may for example instruct the gateway to change the codec used
   on a connection, or to modify the IP address and UDP port to which
   data should be sent, if a connection is "redirected".

   The Call Agent removes a connection by sending a "delete connection"
   command to the gateway.  The gateway may also, under some
   circumstances, inform a gateway that a connection could not be
   sustained.

   The following diagram provides a view of the states of a connection,
   as seen from the gateway:






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           Create connection
              received
                  |
                  V
         +-------------------+
         |resource allocation|-(failed)-+
         +-------------------+          |
                  |           (connection refused)
            (successful)
                  |
                  v
     +----------->+
     |            |
     |   +-------------------+
     |   |  remote session   |
     |   |   description     |----------(yes)--------+
     |   |    available ?    |                       |
     |   +-------------------+                       |
     |            |                                  |
     |          (no)                                 |
     |            |                                  |
     |      +-----------+                         +------+
     | +--->| half open |------> Delete   <-------| open |<----------+
     | |    |  (wait)   |      Connection         |(wait)|           |
     | |    +-----------+       received          +------+           |
     | |          |                 |                |               |
     | |   Modify Connection        |         Modify Connection      |
     | |      received              |            received            |
     | |          |                 |                |               |
     | | +--------------------+     |       +--------------------+   |
     | | |assess modification |     |       |assess modification |   |
     | | +--------------------+     |       +--------------------+   |
     | |    |             |         |          |             |       |
     | |(failed)     (successful)   |      (failed)     (successful) |
     | |    |             |         |          |             |       |
     | +<---+             |         |          +-------------+-------+
     |                    |         |
     +<-------------------+         |
                                    |
                           +-----------------+
                           | Free connection |
                           | resources.      |
                           | Report.         |
                           +-----------------+
                                    |
                                    V





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2.1.3.1 Names of Calls

   One of the attributes of each connection is the "call identifier",
   which as far as the MGCP protocol is concerned has little semantic
   meaning, and is mainly retained for backwards compatibility.

   Calls are identified by unique identifiers, independent of the
   underlying platforms or agents.  Call identifiers are hexadecimal
   strings, which are created by the Call Agent.  The maximum length of
   call identifiers is 32 characters.

   Call identifiers are expected to be unique within the system, or at a
   minimum, unique within the collection of Call Agents that control the
   same gateways.  From the gateway's perspective, the Call identifier
   is thus unique.  When a Call Agent builds several connections that
   pertain to the same call, either on the same gateway or in different
   gateways, these connections that belong to the same call should share
   the same call-id.  This identifier can then be used by accounting or
   management procedures, which are outside the scope of MGCP.

2.1.3.2 Names of Connections

   Connection identifiers are created by the gateway when it is
   requested to create a connection.  They identify the connection
   within the context of an endpoint.  Connection identifiers are
   treated in MGCP as hexadecimal strings.  The gateway MUST make sure
   that a proper waiting period, at least 3 minutes, elapses between the
   end of a connection that used this identifier and its use in a new
   connection for the same endpoint (gateways MAY decide to use
   identifiers that are unique within the context of the gateway).  The
   maximum length of a connection identifier is 32 characters.

2.1.3.3 Management of Resources, Attributes of Connections

   Many types of resources will be associated to a connection, such as
   specific signal processing functions or packetization functions.
   Generally, these resources fall in two categories:

   1) Externally visible resources, that affect the format of "the bits
      on the network" and must be communicated to the second endpoint
      involved in the connection.

   2) Internal resources, that determine which signal is being sent over
      the connection and how the received signals are processed by the
      endpoint.






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   The resources allocated to a connection, and more generally the
   handling of the connection, are chosen by the gateway under
   instructions from the Call Agent.  The Call Agent will provide these
   instructions by sending two sets of parameters to the gateway:

   1) The local directives instruct the gateway on the choice of
      resources that should be used for a connection,

   2) When available, the "session description" provided by the other
      end of the connection (referred to as the remote session
      description).

   The local directives specify such parameters as the mode of the
   connection (e.g., send-only, or send-receive), preferred coding or
   packetization methods, usage of echo cancellation or silence
   suppression.  (A detailed list can be found in the specification of
   the LocalConnectionOptions parameter of the CreateConnection
   command.)  Depending on the parameter, the Call Agent MAY either
   specify a value, a range of values, or no value at all.  This allows
   various implementations to implement various levels of control, from
   a very tight control where the Call Agent specifies minute details of
   the connection handling to a very loose control where the Call Agent
   only specifies broad guidelines, such as the maximum bandwidth, and
   lets the gateway choose the detailed values subject to the
   guidelines.

   Based on the value of the local directives, the gateway will
   determine the resources to allocate to the connection.  When this is
   possible, the gateway will choose values that are in line with the
   remote session description - but there is no absolute requirement
   that the parameters be exactly the same.

   Once the resources have been allocated, the gateway will compose a
   "session description" that describes the way it intends to send and
   receive packets.  Note that the session description may in some cases
   present a range of values.  For example, if the gateway is ready to
   accept one of several compression algorithms, it can provide a list
   of these accepted algorithms.













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                 Local Directives
                (from Call Agent 1)
                        |
                        V
                 +-------------+
                 | resource    |
                 | allocation  |
                 | (gateway 1) |
                 +-------------+
                   |         |
                   V         |
                 Local       |
              Parameters     V
                   |      Session
                   |    Description               Local Directives
                   |         |                   (from Call Agent 2)
                   |         +---> Transmission----+      |
                   |                (CA to CA)     |      |
                   |                               V      V
                   |                           +-------------+
                   |                           | resource    |
                   |                           | allocation  |
                   |                           | (gateway 2) |
                   |                           +-------------+
                   |                               |      |
                   |                               |      V
                   |                               |    Local
                   |                               |  Parameters
                   |                            Session
                   |                          Description
                   |         +---- Transmission<---+
                   |         |      (CA to CA)
                   V         V
                 +-------------+
                 | modification|
                 | (gateway 1) |
                 +-------------+
                   |
                   V
                 Local
              Parameters

      -- Information flow: local directives & session descriptions --








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2.1.3.4 Special Case of Local Connections

   Large gateways include a large number of endpoints which are often of
   different types.  In some networks, we may often have to set-up
   connections between endpoints that are located within the same
   gateway.  Examples of such connections may be:

   * Connecting a call to an Interactive Voice-Response unit,

   * Connecting a call to a Conferencing unit,

   * Routing a call from one endpoint to another, something often
     described as a "hairpin" connection.

   Local connections are much simpler to establish than network
   connections.  In most cases, the connection will be established
   through some local interconnecting device, such as for example a TDM
   bus.

   When two endpoints are managed by the same gateway, it is possible to
   specify the connection in a single command that conveys the names of
   the two endpoints that will be connected.  The command is essentially
   a "Create Connection" command which includes the name of the second
   endpoint in lieu of the "remote session description".

2.1.4 Names of Call Agents and Other Entities

   The media gateway control protocol has been designed to allow the
   implementation of redundant Call Agents, for enhanced network
   reliability.  This means that there is no fixed binding between
   entities and hardware platforms or network interfaces.

   Call Agent names consist of two parts, similar to endpoint names.
   Semantically, the local portion of the name does not exhibit any
   internal structure.  An example Call Agent name is:

      ca1@ca.whatever.net

   Note that both the local part and the domain name have to be
   supplied. Nevertheless, implementations are encouraged to accept call
   agent names consisting of only the domain name.

   Reliability can be improved by using the following procedures:

   * Entities such as endpoints or Call Agents are identified by their
     domain name, not their network addresses.  Several addresses can be





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     associated with a domain name.  If a command or a response cannot
     be forwarded to one of the network addresses, implementations MUST
     retry the transmission using another address.

   * Entities MAY move to another platform.  The association between a
     logical name (domain name) and the actual platform is kept in the
     domain name service.  Call Agents and Gateways MUST keep track of
     the time-to-live of the record they read from the DNS.  They MUST
     query the DNS to refresh the information if the time to live has
     expired.

   In addition to the indirection provided by the use of domain names
   and the DNS, the concept of "notified entity" is central to
   reliability and fail-over in MGCP.  The "notified entity" for an
   endpoint is the Call Agent currently controlling that endpoint.  At
   any point in time, an endpoint has one, and only one, "notified
   entity" associated with it.  The "notified entity" determines where
   the endpoint will send commands to; when the endpoint needs to send a
   command to the Call Agent, it MUST send the command to its current
   "notified entity".  The "notified entity" however does not determine
   where commands can be received from; any Call Agent can send commands
   to the endpoint.  Please refer to Section 5 for the relevant security
   considerations.

   Upon startup, the "notified entity" MUST be set to a provisioned
   value.  Most commands sent by the Call Agent include the ability to
   explicitly name the "notified entity" through the use of a
   "NotifiedEntity" parameter.  The "notified entity" will stay the same
   until either a new "NotifiedEntity" parameter is received or the
   endpoint does a warm or cold (power-cycle) restart.

   If a "NotifiedEntity" parameter is sent with an "empty" value, the
   "notified entity" for the endpoint will be set to empty.  If the
   "notified entity" for an endpoint is empty or has not been set
   explicitly (neither by a command nor by provisioning), the "notified
   entity" will then default to the source address (i.e., IP address and
   UDP port number) of the last successful non-audit command received
   for the endpoint.  Auditing will thus not change the "notified
   entity".  Use of an empty "NotifiedEntity" parameter value is
   strongly discouraged as it is error prone and eliminates the DNS-
   based fail-over and reliability mechanisms.

2.1.5 Digit Maps

   The Call Agent can ask the gateway to collect digits dialed by the
   user.  This facility is intended to be used with residential gateways
   to collect the numbers that a user dials; it can also be used with




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   trunking gateways and access gateways alike, to collect access codes,
   credit card numbers and other numbers requested by call control
   services.

   One procedure is for the gateway to notify the Call Agent of each
   individual dialed digit, as soon as they are dialed.  However, such a
   procedure generates a large number of interactions.  It is preferable
   to accumulate the dialed numbers in a buffer, and to transmit them in
   a single message.

   The problem with this accumulation approach, however, is that it is
   hard for the gateway to predict how many numbers it needs to
   accumulate before transmission.  For example, using the phone on our
   desk, we can dial the following numbers:

        ------------------------------------------------------
       |  0                     |  Local operator             |
       |  00                    |  Long distance operator     |
       |  xxxx                  |  Local extension number     |
       |  8xxxxxxx              |  Local number               |
       |  #xxxxxxx              |  Shortcut to local number at|
       |                        |  other corporate sites      |
       |  *xx                   |  Star services              |
       |  91xxxxxxxxxx          |  Long distance number       |
       |  9011 + up to 15 digits|  International number       |
        ------------------------------------------------------

   The solution to this problem is to have the Call Agent load the
   gateway with a digit map that may correspond to the dial plan.  This
   digit map is expressed using a syntax derived from the Unix system
   command, egrep.  For example, the dial plan described above results
   in the following digit map:

      (0T|00T|[1-7]xxx|8xxxxxxx|#xxxxxxx|*xx|91xxxxxxxxxx|9011x.T)

   The formal syntax of the digit map is described by the DigitMap rule
   in the formal syntax description of the protocol (see Appendix A) -
   support for basic digit map letters is REQUIRED while support for
   extension digit map letters is OPTIONAL.  A gateway receiving a digit
   map with an extension digit map letter not supported SHOULD return
   error code 537 (unknown digit map extension).

   A digit map, according to this syntax, is defined either by a (case
   insensitive) "string" or by a list of strings.  Each string in the
   list is an alternative numbering scheme, specified either as a set of
   digits or timers, or as an expression over which the gateway will
   attempt to find a shortest possible match.  The following constructs
   can be used in each numbering scheme:



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   * Digit:    A digit from "0" to "9".
   * Timer:    The symbol "T" matching a timer expiry.
   * DTMF:     A digit, a timer, or one of the symbols "A", "B", "C",
               "D", "#", or "*".  Extensions may be defined.
   * Wildcard: The symbol "x" which matches any digit ("0" to "9").
   * Range:    One or more DTMF symbols enclosed between square brackets
               ("[" and "]").
   * Subrange: Two digits separated by hyphen ("-") which matches any
               digit between and including the two.  The subrange
               construct can only be used inside a range construct,
               i.e., between "[" and "]".
   * Position: A period (".") which matches an arbitrary number,
               including zero, of occurrences of the preceding
               construct.

   A gateway that detects events to be matched against a digit map MUST
   do the following:

   1) Add the event code as a token to the end of an internal state
      variable for the endpoint called the "current dial string".

   2) Apply the current dial string to the digit map table, attempting a
      match to each expression in the digit map.

   3) If the result is under-qualified (partially matches at least one
      entry in the digit map and doesn't completely match another
      entry), do nothing further.

   If the result matches an entry, or is over-qualified (i.e., no
   further digits could possibly produce a match), send the list of
   accumulated events to the Call Agent.  A match, in this
   specification, can be either a "perfect match," exactly matching one
   of the specified alternatives, or an impossible match, which occurs
   when the dial string does not match any of the alternatives.
   Unexpected timers, for example, can cause "impossible matches".  Both
   perfect matches and impossible matches trigger notification of the
   accumulated digits (which may include other events - see Section
   2.3.3).

   The following example illustrates the above.  Assume we have the
   digit map:

      (xxxxxxx|x11)

   and a current dial string of "41".  Given the input "1" the current
   dial string becomes "411".  We have a partial match with "xxxxxxx",
   but a complete match with "x11", and hence we send "411" to the Call
   Agent.



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   The following digit map example is more subtle:

     (0[12].|00|1[12].1|2x.#)

   Given the input "0", a match will occur immediately since position
   (".") allows for zero occurrences of the preceding construct.  The
   input "00" can thus never be produced in this digit map.

   Given the input "1", only a partial match exists.  The input "12" is
   also only a partial match, however both "11" and "121" are a match.

   Given the input "2", a partial match exists.  A partial match also
   exists for the input "23", "234", "2345", etc.  A full match does not
   occur here until a "#" is generated, e.g., "2345#".  The input "2#"
   would also have been a match.

   Note that digit maps simply define a way of matching sequences of
   event codes against a grammar.  Although digit maps as defined here
   are for DTMF input, extension packages can also be defined so that
   digit maps can be used for other types of input represented by event
   codes that adhere to the digit map syntax already defined for these
   event codes (e.g., "1" or "T").  Where such usage is envisioned, the
   definition of the particular event(s) SHOULD explicitly state that in
   the package definition.

   Since digit maps are not bounded in size, it is RECOMMENDED that
   gateways support digit maps up to at least 2048 bytes per endpoint.

2.1.6 Packages

   MGCP is a modular and extensible protocol, however with extensibility
   comes the need to manage, identify, and name the individual
   extensions.  This is achieved by the concept of packages, which are
   simply well-defined groupings of extensions.  For example, one
   package may support a certain group of events and signals, e.g.,
   off-hook and ringing, for analog access lines.  Another package may
   support another group of events and signals for analog access lines
   or for another type of endpoint such as video.  One or more packages
   may be supported by a given endpoint.

   MGCP allows the following types of extensions to be defined in a
   package:

   * BearerInformation

   * LocalConnectionOptions

   * ExtensionParameters



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   * ConnectionModes

   * Events

   * Signals

   * Actions

   * DigitMapLetters

   * ConnectionParameters

   * RestartMethods

   * ReasonCodes

   * Return codes

   each of which will be explained in more detail below.  The rules for
   defining each of these extensions in a package are described in
   Section 6, and the encoding and syntax are defined in Section 3 and
   Appendix A.

   With the exception of DigitMapLetters, a package defines a separate
   name space for each type of extension by adding the package name as a
   prefix to the extension, i.e.:

      package-name/extension

   Thus the package-name is followed by a slash ("/") and the name of
   the extension.

   An endpoint supporting one or more packages may define one of those
   packages as the default package for the endpoint.  Use of the package
   name for events and signals in the default package for an endpoint is
   OPTIONAL, however it is RECOMMENDED to always include the package
   name.  All other extensions, except DigitMapLetter, defined in the
   package MUST include the package-name when referring to the
   extension.

   Package names are case insensitive strings of letters, hyphens and
   digits, with the restriction that hyphens shall never be the first or
   last character in a name.  Examples of package names are "D", "T",
   and "XYZ".  Package names are not case sensitive - names such as
   "XYZ", "xyz", and "xYz" are equal.






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   Package definitions will be provided in other documents and with
   package names and extensions names registered with IANA.  For more
   details, refer to section 6.

   Implementers can gain experience by using experimental packages.  The
   name of an experimental package MUST start with the two characters
   "x-"; the IANA SHALL NOT register package names that start with these
   characters, or the characters "x+", which are reserved.  A gateway
   that receives a command referring to an unsupported package MUST
   return an error (error code 518 - unsupported package, is
   RECOMMENDED).

2.1.7 Events and Signals

   The concept of events and signals is central to MGCP.  A Call Agent
   may ask to be notified about certain events occurring in an endpoint
   (e.g., off-hook events) by including the name of the event in a
   RequestedEvents parameter (in a NotificationRequest command - see
   Section 2.3.3).

   A Call Agent may also request certain signals to be applied to an
   endpoint (e.g., dial-tone) by supplying the name of the event in a
   SignalRequests parameter.

   Events and signals are grouped in packages, within which they share
   the same name space which we will refer to as event names in the
   following.  Event names are case insensitive strings of letters,
   hyphens and digits, with the restriction that hyphens SHALL NOT be
   the first or last character in a name.  Some event codes may need to
   be parameterized with additional data, which is accomplished by
   adding the parameters between a set of parentheses.  Event names are
   not case sensitive - values such as "hu", "Hu", "HU" or "hU" are
   equal.

   Examples of event names can be "hu" (off hook or "hang-up"
   transition), "hf" (hook-flash) or "0" (the digit zero).

   The package name is OPTIONAL for events in the default package for an
   endpoint, however it is RECOMMENDED to always include the package
   name.  If the package name is excluded from the event name, the
   default package name for that endpoint MUST be assumed.  For example,
   for an analog access line which has the line package ("L") as a
   default with dial-tone ("dl") as one of the events in that package,
   the following two event names are equal:







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      L/dl

   and

      dl

   For any other non-default packages that are associated with that
   endpoint, (such as the generic package for an analog access
   endpoint-type for example), the package name MUST be included with
   the event name.  Again, unconditional inclusion of the package name
   is RECOMMENDED.

   Digits, or letters, are supported in some packages, notably "DTMF".
   Digits and letters are defined by the rules "Digit" and "Letter" in
   the definition of digit maps.  This definition refers to the digits
   (0 to 9), to the asterisk or star ("*") and orthotrope, number or
   pound sign ("#"), and to the letters "A", "B", "C" and "D", as well
   as the timer indication "T".  These letters can be combined in "digit
   string" that represents the keys that a user punched on a dial.  In
   addition, the letter "X" can be used to represent all digits (0 to
   9).  Also, extensions MAY define use of other letters.  The need to
   easily express the digit strings in earlier versions of the protocol
   has a consequence on the form of event names:

   An event name that does not denote a digit MUST always contain at
   least one character that is neither a digit, nor one of the letters
   A, B, C, D, T or X (such names also MUST NOT just contain the special
   signs "*", or "#").  Event names consisting of more than one
   character however may use any of the above.

   A Call Agent may often have to ask a gateway to detect a group of
   events.  Two conventions can be used to denote such groups:

   * The "*" and "all" wildcard conventions (see below) can be used to
     detect any event belonging to a package, or a given event in many
     packages, or any event in any package supported by the gateway.

   * The regular expression Range notation can be used to detect a range
     of digits.

   The star sign (*) can be used as a wildcard instead of a package
   name, and the keyword "all" can be used as a wildcard instead of an
   event name:

   * A name such as "foo/all" denotes all events in package "foo".

   * A name such as "*/bar" denotes the event "bar" in any package
     supported by the gateway.



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   * The name "*/all" denotes all events supported by the endpoint.

   This specification purposely does not define any additional detail
   for the "all packages" and "all events" wildcards.  They provide
   limited benefits, but introduce significant complexity along with the
   potential for errors.  Their use is consequently strongly
   discouraged.

   The Call Agent can ask a gateway to detect a set of digits or letters
   either by individually describing those letters, or by using the
   "range" notation defined in the syntax of digit strings.  For
   example, the Call Agent can:

   * Use the letter "x" to denote" digits from 0 to 9.
   * Use the notation "[0-9#]" to denote the digits 0 to 9 and the pound
     sign.

   The individual event codes are still defined in a package though
   (e.g., the "DTMF" package).

   Events can by default only be generated and detected on endpoints,
   however events can be also be defined so they can be generated or
   detected on connections rather than on the endpoint itself (see
   Section 6.6).  For example, gateways may be asked to provide a
   ringback tone on a connection.  When an event is to be applied on a
   connection, the name of the connection MUST be added to the name of
   the event, using an "at" sign (@) as a delimiter, as in:

      G/rt@0A3F58

   where "G" is the name of the package and "rt" is the name of the
   event.  Should the connection be deleted while an event or signal is
   being detected or applied on it, that particular event detection or
   signal generation simply stops.  Depending on the signal, this may
   generate a failure (see below).

   The wildcard character "*" (star) can be used to denote "all
   connections".  When this convention is used, the gateway will
   generate or detect the event on all the connections that are
   connected to the endpoint.  This applies to existing as well as
   future connections created on the endpoint.  An example of this
   convention could be:

      R/qa@*

   where "R" is the name of the package and "qa" is the name of the
   event.




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   When processing a command using the "all connections" wildcard, the
   "*" wildcard character applies to all current and future connections
   on the endpoint, however it will not be expanded.  If a subsequent
   command either explicitly (e.g., by auditing) or implicitly (e.g., by
   persistence) refers to such an event, the "*" value will be used.
   However, when the event is actually observed, that particular
   occurrence of the event will include the name of the specific
   connection it occurred on.

   The wildcard character "$" can be used to denote "the current
   connection".  It can only be used by the Call Agent, when the event
   notification request is "encapsulated" within a connection creation
   or modification command.  When this convention is used, the gateway
   will generate or detect the event on the connection that is currently
   being created or modified.  An example of this convention is:

      G/rt@$

   When processing a command using the "current connection" wildcard,
   the "$" wildcard character will be expanded to the value of the
   current connection.  If a subsequent command either explicitly (e.g.,
   by auditing) or implicitly (e.g., by persistence) refers to such an
   event, the expanded value will be used.  In other words, the "current
   connection" wildcard is expanded once, which is at the initial
   processing of the command in which it was explicitly included.

   The connection id, or a wildcard replacement, can be used in
   conjunction with the "all packages" and "all events" conventions. For
   example, the notation:

      */all@*

   can be used to designate all events on all current and future
   connections on the endpoint.  However, as mentioned before, the use
   of the "all packages" and "all events" wildcards are strongly
   discouraged.

   Signals are divided into different types depending on their behavior:

   * On/off (OO):  Once applied, these signals last until they are
     turned off.  This can only happen as the result of a reboot/restart
     or a new SignalRequests where the signal is explicitly turned off
     (see later).  Signals of type OO are defined to be idempotent, thus
     multiple requests to turn a given OO signal on (or off) are







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     perfectly valid and MUST NOT result in any errors.  An On/Off
     signal could be a visual message-waiting indicator (VMWI).  Once
     turned on, it MUST NOT be turned off until explicitly instructed to
     by the Call Agent, or as a result of an endpoint restart, i.e.,
     these signals will not turn off as a result of the detection of a
     requested event.

   * Time-out (TO):  Once applied, these signals last until they are
     either cancelled (by the occurrence of an event or by not being
     included in a subsequent (possibly empty) list of signals), or a
     signal-specific period of time has elapsed.  A TO signal that times
     out will generate an "operation complete" event.  A TO signal could
     be "ringback" timing out after 180 seconds.  If an event occurs
     prior to the 180 seconds, the signal will, by default, be stopped
     (the "Keep signals active" action - see Section 2.3.3 - will
     override this behavior).  If the signal is not stopped, the signal
     will time out, stop and generate an "operation complete" event,
     about which the Call Agent may or may not have requested to be
     notified.  If the Call Agent has asked for the "operation complete"
     event to be notified, the "operation complete" event sent to the
     Call Agent SHALL include the name(s) of the signal(s) that timed
     out (note that if parameters were passed to the signal, the
     parameters will not be reported).  If the signal was generated on a
     connection, the name of the connection SHALL be included as
     described above.  Time-out signals have a default time-out value
     defined for them, which MAY be altered by the provisioning process.
     Also, the time-out period may be provided as a parameter to the
     signal (see Section 3.2.2.4).  A value of zero indicates that the
     time-out period is infinite.  A TO signal that fails after being
     started, but before having generated an "operation complete" event
     will generate an "operation failure" event which will include the
     name of the signal that failed.  Deletion of a connection with an
     active TO signal will result in such a failure.

   * Brief (BR):  The duration of these signals is normally so short
     that they stop on their own.  If a signal stopping event occurs, or
     a new SignalRequests is applied, a currently active BR signal will
     not stop.  However, any pending BR signals not yet applied MUST be
     cancelled (a BR signal becomes pending if a NotificationRequest
     includes a BR signal, and there is already an active BR signal). As
     an example, a brief tone could be a DTMF digit. If the DTMF digit
     "1" is currently being played, and a signal stopping event occurs,
     the "1" would play to completion.  If a request to play DTMF digit
     "2" arrives before DTMF digit "1" finishes playing, DTMF digit "2"
     would become pending.

   Signal(s) generated on a connection MUST include the name of that
   connection.



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2.2 Usage of SDP

   The Call Agent uses the MGCP to provide the endpoint with the
   description of connection parameters such as IP addresses, UDP port
   and RTP profiles.  These descriptions will follow the conventions
   delineated in the Session Description Protocol which is now an IETF
   proposed standard, documented in RFC 2327.

2.3 Gateway Control Commands

2.3.1 Overview of Commands

   This section describes the commands of the MGCP.  The service
   consists of connection handling and endpoint handling commands.
   There are currently nine commands in the protocol:

   * The Call Agent can issue an EndpointConfiguration command to a
     gateway, instructing the gateway about the coding characteristics
     expected by the "line-side" of the endpoint.

   * The Call Agent can issue a NotificationRequest command to a
     gateway, instructing the gateway to watch for specific events such
     as hook actions or DTMF tones on a specified endpoint.

   * The gateway will then use the Notify command to inform the Call
     Agent when the requested events occur.

   * The Call Agent can use the CreateConnection command to create a
     connection that terminates in an "endpoint" inside the gateway.

   * The Call Agent can use the ModifyConnection command to change the
     parameters associated with a previously established connection.

   * The Call Agent can use the DeleteConnection command to delete an
     existing connection.  The DeleteConnection command may also be used
     by a gateway to indicate that a connection can no longer be
     sustained.

   * The Call Agent can use the AuditEndpoint and AuditConnection
     commands to audit the status of an "endpoint" and any connections
     associated with it.  Network management beyond the capabilities
     provided by these commands is generally desirable.  Such
     capabilities are expected to be supported by the use of the Simple
     Network Management Protocol (SNMP) and definition of a MIB which is
     outside the scope of this specification.






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   * The Gateway can use the RestartInProgress command to notify the
     Call Agent that a group of endpoints managed by the gateway is
     being taken out-of-service or is being placed back in-service.

   These services allow a controller (normally, the Call Agent) to
   instruct a gateway on the creation of connections that terminate in
   an "endpoint" attached to the gateway, and to be informed about
   events occurring at the endpoint.  An endpoint may be for example:

   * A specific trunk circuit, within a trunk group terminating in a
     gateway,

   * A specific announcement handled by an announcement server.

   Connections are logically grouped into "calls" (the concept of a
   "call" has however little semantic meaning in MGCP itself).  Several
   connections, that may or may not belong to the same call, can
   terminate in the same endpoint.  Each connection is qualified by a
   "mode" parameter, which can be set to "send only" (sendonly),
   "receive only" (recvonly), "send/receive" (sendrecv), "conference"
   (confrnce), "inactive" (inactive), "loopback", "continuity test"
   (conttest), "network loop back" (netwloop) or "network continuity
   test" (netwtest).

   Media generated by the endpoint is sent on connections whose mode is
   either "send only", "send/receive", or "conference", unless the
   endpoint has a connection in "loopback" or "continuity test" mode.
   However, media generated by applying a signal to a connection is
   always sent on the connection, regardless of the mode.

   The handling of the media streams received on connections is
   determined by the mode parameters:

   * Media streams received through connections in "receive",
     "conference" or "send/receive" mode are mixed and sent to the
     endpoint, unless the endpoint has another connection in "loopback"
     or "continuity test" mode.

   * Media streams originating from the endpoint are transmitted over
     all the connections whose mode is "send", "conference" or
     "send/receive", unless the endpoint has another connection in
     "loopback" or "continuity test" mode.

   * In addition to being sent to the endpoint, a media stream received
     through a connection in "conference" mode is forwarded to all the
     other connections whose mode is "conference".  This also applies





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     when the endpoint has a connection in "loopback" or "continuity
     test" mode.  The details of this forwarding, e.g., RTP translator
     or mixer, is outside the scope of this document.

   Note that in order to detect events on a connection, the connection
   must by default be in one of the modes "receive", "conference",
   "send/receive", "network loopback" or "network continuity test".  The
   event detection only applies to the incoming media.  Connections in
   "sendonly", "inactive", "loopback", or "continuity test" mode will
   thus normally not detect any events, although requesting to do so is
   not considered an error.

   The "loopback" and "continuity test" modes are used during
   maintenance and continuity test operations.  An endpoint may have
   more than one connection in either "loopback" or "continuity test"
   mode.  As long as there is one connection in that particular mode,
   and no other connection on the endpoint is placed in a different
   maintenance or test mode, the maintenance or test operation shall
   continue undisturbed.  There are two flavors of continuity test, one
   specified by ITU and one used in the US.  In the first case, the test
   is a loopback test.  The originating switch will send a tone (the go
   tone) on the bearer circuit and expects the terminating switch to
   loopback the tone.  If the originating switch sees the same tone
   returned (the return tone), the COT has passed.  If not, the COT has
   failed.  In the second case, the go and return tones are different.
   The originating switch sends a certain go tone.  The terminating
   switch detects the go tone, it asserts a different return tone in the
   backwards direction.  When the originating switch detects the return
   tone, the COT is passed.  If the originating switch never detects the
   return tone, the COT has failed.

   If the mode is set to "loopback", the gateway is expected to return
   the incoming signal from the endpoint back into that same endpoint.
   This procedure will be used, typically, for testing the continuity of
   trunk circuits according to the ITU specifications.  If the mode is
   set to "continuity test", the gateway is informed that the other end
   of the circuit has initiated a continuity test procedure according to
   the GR specification (see [22]).  The gateway will place the circuit
   in the transponder mode required for dual-tone continuity tests.

   If the mode is set to "network loopback", the audio signals received
   from the connection will be echoed back on the same connection.  The
   media is not forwarded to the endpoint.

   If the mode is set to "network continuity test", the gateway will
   process the packets received from the connection according to the
   transponder mode required for dual-tone continuity test, and send the
   processed signal back on the connection.  The media is not forwarded



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   to the endpoint.  The "network continuity test" mode is included for
   backwards compatibility only and use of it is discouraged.

2.3.2 EndpointConfiguration

   The EndpointConfiguration command can be used to specify the encoding
   of the signals that will be received by the endpoint.  For example,
   in certain international telephony configurations, some calls will
   carry mu-law encoded audio signals, while others will use A-law.  The
   Call Agent can use the EndpointConfiguration command to pass this
   information to the gateway.  The configuration may vary on a call by
   call basis, but can also be used in the absence of any connection.

         ReturnCode,
         [PackageList]
         <-- EndpointConfiguration(EndpointId,
                                   [BearerInformation])

   EndpointId is the name of the endpoint(s) in the gateway where
   EndpointConfiguration executes.  The "any of" wildcard convention
   MUST NOT be used.  If the "all of" wildcard convention is used, the
   command applies to all the endpoints whose name matches the wildcard.

   BearerInformation is a parameter defining the coding of the data sent
   to and received from the line side.  The information is encoded as a
   list of sub-parameters.  The only sub-parameter defined in this
   version of the specification is the bearer encoding, whose value can
   be set to "A-law" or "mu-law".  The set of sub-parameters may be
   extended.

   In order to allow for extensibility, while remaining backwards
   compatible, the BearerInformation parameter is conditionally optional
   based on the following conditions:

   * if Extension Parameters (vendor, package or other) are not used,
     the BearerInformation parameter is REQUIRED,

   * otherwise, the BearerInformation parameter is OPTIONAL.

   When omitted, BearerInformation MUST retain its current value.

   ReturnCode is a parameter returned by the gateway.  It indicates the
   outcome of the command and consists of an integer number optionally
   followed by commentary.

   PackageList is a list of supported packages that MAY be included with
   error code 518 (unsupported package).




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2.3.3 NotificationRequest

   The NotificationRequest command is used to request the gateway to
   send notifications upon the occurrence of specified events in an
   endpoint.  For example, a notification may be requested for when a
   gateway detects that an endpoint is receiving tones associated with
   fax communication.  The entity receiving this notification may then
   decide to specify use of a different type of encoding method in the
   connections bound to this endpoint and instruct the gateway
   accordingly with a ModifyConnection Command.

         ReturnCode,
         [PackageList]
         <-- NotificationRequest(EndpointId,
                                 [NotifiedEntity,]
                                 [RequestedEvents,]
                                 RequestIdentifier,
                                 [DigitMap,]
                                 [SignalRequests,]
                                 [QuarantineHandling,]
                                 [DetectEvents,]
                                 [encapsulated EndpointConfiguration])

   EndpointId is the identifier for the endpoint(s) in the the gateway
   where the NotificationRequest executes.  The "any of" wildcard MUST
   NOT be used.

   NotifiedEntity is an optional parameter that specifies a new
   "notified entity" for the endpoint.

   RequestIdentifier is used to correlate this request with the
   notifications that it triggers.  It will be repeated in the
   corresponding Notify command.

   RequestedEvents is a list of events, possibly qualified by event
   parameters (see Section 3.2.2.4), that the gateway is requested to
   detect and report.  Such events may include, for example, fax tones,
   continuity tones, or on-hook transition.  Unless otherwise specified,
   events are detected on the endpoint, however some events can be
   detected on a connection.  A given event MUST NOT appear more than
   once in a RequestedEvents.  If the parameter is omitted, it defaults
   to empty.

   To each event is associated one or more actions, which can be:

   * Notify the event immediately, together with the accumulated list of
     observed events,




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   * Swap audio,

   * Accumulate the event in an event buffer, but don't notify yet,

   * Accumulate according to Digit Map,

   * Keep Signal(s) active,

   * Process the Embedded Notification Request,

   * Ignore the event.

   Support for Notify, Accumulate, Keep Signal(s) Active, Embedded
   Notification Request, and Ignore is REQUIRED.  Support for Accumulate
   according to Digit Map is REQUIRED on any endpoint capable of
   detecting DTMF.  Support for any other action is OPTIONAL.  The set
   of actions can be extended.

   A given action can by default be specified for any event, although
   some actions will not make sense for all events.  For example, an
   off-hook event with the Accumulate according to Digit Map action is
   valid, but will of course immediately trigger a digit map mismatch
   when the off-hook event occurs.  Needless to say, such practice is
   discouraged.

   Some actions can be combined as shown in the table below, where "Y"
   means the two actions can be combined, and "N" means they cannot:

       --------------------------------------------------------------
      |       | Notif | Swap | Accum | AccDi | KeSiA | EmbNo | Ignor |
      |--------------------------------------------------------------|
      | Notif |   N   |   Y  |   N   |   N   |   Y   |   Y*  |   N   |
      | Swap  |   -   |   N  |   Y   |   N   |   N   |   N   |   Y   |
      | Accum |   -   |   -  |   N   |   N   |   Y   |   Y   |   N   |
      | AccDi |   -   |   -  |   -   |   N   |   Y   |   N   |   N   |
      | KeSiA |   -   |   -  |   -   |   -   |   N   |   Y   |   Y   |
      | EmbNo |   -   |   -  |   -   |   -   |   -   |   N   |   N   |
      | Ignor |   -   |   -  |   -   |   -   |   -   |   -   |   N   |
       --------------------------------------------------------------

      Note (*):  The "Embedded Notification Request" can only be
      combined with "Notify", if the gateway is allowed to issue more
      than one Notify command per Notification request (see below and
      Section 4.4.1).

   If no action is specified, the Notify action will be applied.  If one
   or more actions are specified, only those actions apply.  When two or
   more actions are specified, each action MUST be combinable with all



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   the other actions as defined by the table above - the individual
   actions are assumed to occur simultaneously.

   If a client receives a request with an invalid or unsupported action
   or an illegal combination of actions, it MUST return an error to the
   Call Agent (error code 523 - unknown or illegal combination of
   actions, is RECOMMENDED).

   In addition to the RequestedEvents parameter specified in the
   command, some MGCP packages may contain "persistent events" (this is
   generally discouraged though - see Appendix B for an alternative).
   Persistent events in a given package are always detected on an
   endpoint that implements that package.  If a persistent event is not
   included in the list of RequestedEvents, and the event occurs, the
   event will be detected anyway and processed like all other events, as
   if the persistent event had been requested with a Notify action.  A
   NotificationRequest MUST still be in place for a persistent event to
   trigger a Notify though. Thus, informally, persistent events can be
   viewed as always being implicitly included in the list of
   RequestedEvents with an action to Notify, although no glare
   detection, etc., will be performed.

   Non-persistent events are those events that need to be explicitly
   included in the RequestedEvents list. The (possibly empty) list of
   requested events completely replaces the previous list of requested
   events.  In addition to the persistent events, only the events
   specified in the requested events list will be detected by the
   endpoint.  If a persistent event is included in the RequestedEvents
   list, the action specified will replace the default action associated
   with the event for the life of the RequestedEvents list, after which
   the default action is restored.  For example, if "off-hook"was a
   persistent event, the "Ignore off-hook" action was specified, and a
   new request without any off-hook instructions were received, the
   default "Notify off-hook" operation would be restored.

   The gateway will detect the union of the persistent events and the
   requested events.  If an event is not included in either list, it
   will be ignored.

   The Call Agent can send a NotificationRequest with an empty (or
   omitted) RequestedEvents list to the gateway.  The Call Agent can do
   so, for example, to a gateway when it does not want to collect any
   more DTMF digits.  However, persistent events will still be detected
   and notified.

   The Swap Audio action can be used when a gateway handles more than
   one connection on an endpoint.  This will be the case for call
   waiting, and possibly other feature scenarios.  In order to avoid the



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   round-trip to the Call Agent when just changing which connection is
   attached to the audio functions of the endpoint, the
   NotificationRequest can map an event (usually hook flash, but could
   be some other event) to a local swap audio function, which selects
   the "next" connection in a round robin fashion.  If there is only one
   connection, this action is effectively a no-op.  If there are more
   than two connections, the order is undefined.  If the endpoint has
   exactly two connections, one of which is "inactive", the other of
   which is in "send/receive" mode, then swap audio will attempt to make
   the "send/receive" connection "inactive", and vice versa.  This
   specification intentionally does not provide any additional detail on
   the swap audio action.

   If signal(s) are desired to start when an event being looked for
   occurs, the "Embedded NotificationRequest" action can be used.  The
   embedded NotificationRequest may include a new list of
   RequestedEvents, SignalRequests and a new digit map as well.  The
   semantics of the embedded NotificationRequest is as if a new
   NotificationRequest was just received with the same NotifiedEntity,
   RequestIdentifier, QuarantineHandling and DetectEvents.  When the
   "Embedded NotificationRequest" is activated, the "current dial
   string" will be cleared; however the list of observed events and the
   quarantine buffer will be unaffected (if combined with a Notify, the
   Notify will clear the list of observed events though - see Section
   4.4.1).  Note, that the Embedded NotificationRequest action does not
   accumulate the triggering event, however it can be combined with the
   Accumulate action to achieve that.  If the Embedded
   NotificationRequest fails, an Embedded NotificationRequest failure
   event SHOULD be generated (see Appendix B).

   MGCP implementations SHALL be able to support at least one level of
   embedding.  An embedded NotificationRequest that respects this
   limitation MUST NOT contain another Embedded NotificationRequest.

   DigitMap is an optional parameter that allows the Call Agent to
   provision the endpoint with a digit map according to which digits
   will be accumulated.  If this optional parameter is absent, the
   previously defined value is retained.  This parameter MUST be
   defined, either explicitly or through a previous command, if the
   RequestedEvents parameter contains a request to "accumulate according
   to the digit map".  The collection of these digits will result in a
   digit string.  The digit string is initialized to a null string upon
   reception of the NotificationRequest, so that a subsequent
   notification only returns the digits that were collected after this
   request.  Digits that were accumulated according to the digit map are
   reported as any other accumulated event, in the order in which they
   occur.  It is therefore possible that other events accumulated are




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   found in between the list of digits.  If the gateway is requested to
   "accumulate according to digit map" and the gateway currently does
   not have a digit map for the endpoint in question, the gateway MUST
   return an error (error code 519 - endpoint does not have a digit map,
   is RECOMMENDED).

   SignalRequests is an optional parameter that contains the set of
   signals that the gateway is asked to apply.  When omitted, it
   defaults to empty.  When multiple signals are specified, the signals
   MUST be applied in parallel.  Unless otherwise specified, signals are
   applied to the endpoint.  However some signals can be applied to a
   connection.  Signals are identified by their name, which is an event
   name, and may be qualified by signal parameters (see Section
   3.2.2.4).  The following are examples of signals:

   * Ringing,

   * Busy tone,

   * Call waiting tone,

   * Off hook warning tone,

   * Ringback tones on a connection.

   Names and descriptions of signals are defined in the appropriate
   package.

   Signals are, by default, applied to endpoints.  If a signal applied
   to an endpoint results in the generation of a media stream (audio,
   video, etc.), then by default the media stream MUST NOT be forwarded
   on any connection associated with that endpoint, regardless of the
   mode of the connection.  For example, if a call-waiting tone is
   applied to an endpoint involved in an active call, only the party
   using the endpoint in question will hear the call-waiting tone.
   However, individual signals may define a different behavior.

   When a signal is applied to a connection that has received a
   RemoteConnectionDescriptor, the media stream generated by that signal
   will be forwarded on the connection regardless of the current mode of
   the connection (including loopback and continuity test).  If a
   RemoteConnectionDescriptor has not been received, the gateway MUST
   return an error (error code 527 - missing RemoteConnectionDescriptor,
   is RECOMMENDED).  Note that this restriction does not apply to
   detecting events on a connection.






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   When a (possibly empty) list of signal(s) is supplied, this list
   completely replaces the current list of active time-out signals.
   Currently active time-out signals that are not provided in the new
   list MUST be stopped and the new signal(s) provided will now become
   active.  Currently active time-out signals that are provided in the
   new list of signals MUST remain active without interruption, thus the
   timer for such time-out signals will not be affected.  Consequently,
   there is currently no way to restart the timer for a currently active
   time-out signal without turning the signal off first.  If the time-
   out signal is parameterized, the original set of parameters MUST
   remain in effect, regardless of what values are provided
   subsequently.  A given signal MUST NOT appear more than once in a
   SignalRequests.  Note that applying a signal S to an endpoint,
   connection C1 and connection C2, constitutes three different and
   independent signals.

   The action triggered by the SignalRequests is synchronized with the
   collection of events specified in the RequestedEvents parameter.  For
   example, if the NotificationRequest mandates "ringing" and the
   RequestedEvents asks to look for an "off-hook" event, the ringing
   SHALL stop as soon as the gateway detects an off-hook event.  The
   formal definition is that the generation of all "Time Out" signals
   SHALL stop as soon as one of the requested events is detected, unless
   the "Keep signals active" action is associated to the detected event.
   The RequestedEvents and SignalRequests may refer to the same event
   definitions.  In one case, the gateway is asked to detect the
   occurrence of the event, and in the other case it is asked to
   generate it.  The specific events and signals that a given endpoint
   can detect or perform are determined by the list of packages that are
   supported by that endpoint.  Each package specifies a list of events
   and signals that can be detected or performed.  A gateway that is
   requested to detect or perform an event belonging to a package that
   is not supported by the specified endpoint MUST return an error
   (error code 518 - unsupported or unknown package, is RECOMMENDED).
   When the event name is not qualified by a package name, the default
   package name for the endpoint is assumed.  If the event name is not
   registered in this default package, the gateway MUST return an error
   (error code 522 - no such event or signal, is RECOMMENDED).

   The Call Agent can send a NotificationRequest whose requested signal
   list is empty.  It will do so for example when a time-out signal(s)
   should stop.

   If signal(s) are desired to start as soon as a "looked-for" event
   occurs, the "Embedded NotificationRequest" action can be used.  The
   embedded NotificationRequest may include a new list of
   RequestedEvents, SignalRequests and a new Digit Map as well.  The
   embedded NotificationRequest action allows the Call Agent to set up a



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   "mini-script" to be processed by the gateway immediately following
   the detection of the associated event.  Any SignalRequests specified
   in the embedded NotificationRequest will start immediately.
   Considerable care must be taken to prevent discrepancies between the
   Call Agent and the gateway.  However, long-term discrepancies should
   not occur as a new SignalRequests completely replaces the old list of
   active time-out signals, and BR-type signals always stop on their
   own.  Limiting the number of On/Off-type signals is encouraged.  It
   is considered good practice for a Call Agent to occasionally turn on
   all On/Off signals that should be on, and turn off all On/Off signals
   that should be off.

   The Ignore action can be used to ignore an event, e.g., to prevent a
   persistent event from being notified.  However, the synchronization
   between the event and an active time-out signal will still occur by
   default (e.g., a time-out dial-tone signal will stop when an off-hook
   occurs even if off-hook was a requested event with action "Ignore").
   To prevent this synchronization from happening, the "Keep Signal(s)
   Active" action will have to be specified as well.

   The optional QuarantineHandling parameter specifies the handling of
   "quarantine" events, i.e., events that have been detected by the
   gateway before the arrival of this NotificationRequest command, but
   have not yet been notified to the Call Agent.  The parameter provides
   a set of handling options (see Section 4.4.1 for details):

   * whether the quarantined events should be processed or discarded
     (the default is to process them).

   * whether the gateway is expected to generate at most one
     notification (step by step), or multiple notifications (loop), in
     response to this request (the default is at most one).

   When the parameter is absent, the default value is assumed.

   We should note that the quarantine-handling parameter also governs
   the handling of events that were detected and processed but not yet
   notified when the command is received.

   DetectEvents is an optional parameter, possibly qualified by event
   parameters, that specifies a list of events that the gateway is
   requested to detect during the quarantine period.  When this
   parameter is absent, the events to be detected in the quarantine
   period are those listed in the last received DetectEvents list.  In
   addition, the gateway will also detect persistent events and the
   events specified in the RequestedEvents list, including those for
   which the "ignore" action is specified.




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   Some events and signals, such as the in-line ringback or the quality
   alert, are performed or detected on connections terminating in the
   endpoint rather than on the endpoint itself.  The structure of the
   event names (see Section 2.1.7) allows the Call Agent to specify the
   connection(s) on which the events should be performed or detected.

   The NotificationRequest command may carry an encapsulated
   EndpointConfiguration command, that will apply to the same
   endpoint(s).  When this command is present, the parameters of the
   EndpointConfiguration command are included with the normal parameters
   of the NotificationRequest, with the exception of the EndpointId,
   which is not replicated.

   The encapsulated EndpointConfiguration command shares the fate of the
   NotificationRequest command.  If the NotificationRequest is rejected,
   the EndpointConfiguration is not executed.

   ReturnCode is a parameter returned by the gateway.  It indicates the
   outcome of the command and consists of an integer number optionally
   followed by commentary.

   PackageList is a list of supported packages that MAY be included with
   error code 518 (unsupported package).

2.3.4 Notify

   Notifications with the observed events are sent by the gateway via
   the Notify command when a triggering event occurs.

         ReturnCode,
         [PackageList]
         <-- Notify(EndpointId,
                    [NotifiedEntity,]
                    RequestIdentifier,
                    ObservedEvents)

   EndpointId is the name for the endpoint in the gateway which is
   issuing the Notify command.  The identifier MUST be a fully qualified
   endpoint identifier, including the domain name of the gateway.  The
   local part of the name MUST NOT use any of the wildcard conventions.

   NotifiedEntity is a parameter that identifies the entity which
   requested the notification.  This parameter is equal to the
   NotifiedEntity parameter of the NotificationRequest that triggered
   this notification.  The parameter is absent if there was no such
   parameter in the triggering request.  Regardless of the value of the
   NotifiedEntity parameter, the notification MUST be sent to the
   current "notified entity" for the endpoint.



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   RequestIdentifier is a parameter that repeats the RequestIdentifier
   parameter of the NotificationRequest that triggered this
   notification.  It is used to correlate this notification with the
   request that triggered it.  Persistent events will be viewed here as
   if they had been included in the last NotificationRequest.  An
   implicit NotificationRequest MAY be in place right after restart -
   the RequestIdentifier used for it will be zero ("0") - see Section
   4.4.1 for details.

   ObservedEvents is a list of events that the gateway detected and
   accumulated.  A single notification may report a list of events that
   will be reported in the order in which they were detected (FIFO).

   The list will only contain the identification of events that were
   requested in the RequestedEvents parameter of the triggering
   NotificationRequest.  It will contain the events that were either
   accumulated (but not notified) or treated according to digit map (but
   no match yet), and the final event that triggered the notification or
   provided a final match in the digit map.  It should be noted that
   digits MUST be added to the list of observed events as they are
   accumulated, irrespective of whether they are accumulated according
   to the digit map or not.  For example, if a user enters the digits
   "1234" and some event E is accumulated between the digits "3" and "4"
   being entered, the list of observed events would be "1, 2, 3, E, 4".
   Events that were detected on a connection SHALL include the name of
   that connection as in "R/qa@0A3F58" (see Section 2.1.7).

   If the list of ObservedEvents reaches the capacity of the endpoint,
   an ObservedEvents Full event (see Appendix B) SHOULD be generated
   (the endpoint shall ensure it has capacity to include this event in
   the list of ObservedEvents).  If the ObservedEvents Full event is not
   used to trigger a Notify, event processing continues as before
   (including digit map matching); however, the subsequent events will
   not be included in the list of ObservedEvents.

   ReturnCode is a parameter returned by the Call Agent.  It indicates
   the outcome of the command and consists of an integer number
   optionally followed by commentary.

   PackageList is a list of supported packages that MAY be included with
   error code 518 (unsupported package).










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2.3.5 CreateConnection

   This command is used to create a connection between two endpoints.

         ReturnCode,
         [ConnectionId,]
         [SpecificEndPointId,]
         [LocalConnectionDescriptor,]
         [SecondEndPointId,]
         [SecondConnectionId,]
         [PackageList]
         <-- CreateConnection(CallId,
                              EndpointId,
                              [NotifiedEntity,]
                              [LocalConnectionOptions,]
                              Mode,
                              [{RemoteConnectionDescriptor |
                              SecondEndpointId}, ]
                              [Encapsulated NotificationRequest,]
                              [Encapsulated EndpointConfiguration])

   A connection is defined by its endpoints.  The input parameters in
   CreateConnection provide the data necessary to build a gateway's
   "view" of a connection.

   CallId is a parameter that identifies the call (or session) to which
   this connection belongs.  This parameter SHOULD, at a minimum, be
   unique within the collection of Call Agents that control the same
   gateways.  Connections that belong to the same call SHOULD share the
   same call-id.  The call-id has little semantic meaning in the
   protocol; however it can be used to identify calls for reporting and
   accounting purposes.  It does not affect the handling of connections
   by the gateway.

   EndpointId is the identifier for the connection endpoint in the
   gateway where CreateConnection executes.  The EndpointId can be
   fully-specified by assigning a value to the parameter EndpointId in
   the function call or it may be under-specified by using the "any of"
   wildcard convention.  If the endpoint is underspecified, the endpoint
   identifier SHALL be assigned by the gateway and its complete value
   returned in the SpecificEndPointId parameter of the response.  When
   the "any of" wildcard is used, the endpoint assigned MUST be in-
   service and MUST NOT already have any connections on it.  If no such
   endpoint is available, error code 410 (no endpoint available) SHOULD
   be returned.  The "all of" wildcard MUST NOT be used.

   The NotifiedEntity is an optional parameter that specifies a new
   "notified entity" for the endpoint.



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   LocalConnectionOptions is an optional structure used by the Call
   Agent to direct the handling of the connection by the gateway.  The
   fields contained in a LocalConnectionOptions structure may include
   one or more of the following (each field MUST NOT be supplied more
   than once):

   * Codec compression algorithm:  One or more codecs, listed in order
     of preference.  For interoperability, it is RECOMMENDED to support
     G.711 mu-law encoding ("PCMU").  See Section 2.6 for details on the
     codec selection process.

   * Packetization period:  A single millisecond value or a range may be
     specified.  The packetization period SHOULD NOT contradict the
     specification of the codec compression algorithm.  If a codec is
     specified that has a frame size which is inconsistent with the
     packetization period, and that codec is selected, the gateway is
     authorized to use a packetization period that is consistent with
     the frame size even if it is different from that specified.  In so
     doing, the gateway SHOULD choose a non-zero packetization period as
     close to that specified as possible.  If a packetization period is
     not specified, the endpoint SHOULD use the default packetization
     period(s) for the codec(s) selected.

   * Bandwidth:  The allowable bandwidth, i.e., payload plus any header
     overhead from the transport layer and up, e.g., IP, UDP, and RTP.
     The bandwidth specification SHOULD NOT contradict the specification
     of codec compression algorithm or packetization period.  If a codec
     is specified, then the gateway is authorized to use it, even if it
     results in the usage of a larger bandwidth than specified.  Any
     discrepancy between the bandwidth and codec specification will not
     be reported as an error.

   * Type of Service:  This indicates the class of service to be used
     for this connection.  When the Type of Service is not specified,
     the gateway SHALL use a default value of zero unless provisioned
     otherwise.

   * Usage of echo cancellation:  By default, the telephony gateways
     always perform echo cancellation on the endpoint.  However, it may
     be necessary, for some calls, to turn off these operations.  The
     echo cancellation parameter can have two values, "on" (when the
     echo cancellation is requested) and "off" (when it is turned off).
     The parameter is optional.  If the parameter is omitted when
     creating a connection and there are no other connections on the
     endpoint, the endpoint SHALL apply echo cancellation initially.  If
     the parameter is omitted when creating a connection and there are
     existing connections on the endpoint, echo cancellation is
     unchanged.  The endpoint SHOULD subsequently enable or disable echo



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     cancellation when voiceband data is detected - see e.g., ITU-T
     recommendation V.8, V.25, and G.168.  Following termination of
     voiceband data, the handling of echo cancellation SHALL then revert
     to the current value of the echo cancellation parameter.  It is
     RECOMMENDED that echo cancellation handling is left to the gateway
     rather than having this parameter specified by the Call Agent.

   * Silence Suppression:  The telephony gateways may perform voice
     activity detection, and avoid sending packets during periods of
     silence.  However, it is necessary, for example for modem calls, to
     turn off this detection.  The silence suppression parameter can
     have two values, "on" (when the detection is requested) and "off"
     (when it is not requested).  The default is "off" (unless
     provisioned otherwise).  Upon detecting voiceband data, the
     endpoint SHOULD disable silence suppression.  Following termination
     of voiceband data, the handling of silence suppression SHALL then
     revert to the current value of the silence suppression parameter.

   * Gain Control:  The telephony gateways may perform gain control on
     the endpoint, in order to adapt the level of the signal.  However,
     it is necessary, for example for some modem calls, to turn off this
     function.  The gain control parameter may either be specified as
     "automatic", or as an explicit number of decibels of gain.  The
     gain specified will be added to media sent out over the endpoint
     (as opposed to the connection) and subtracted from media received
     on the endpoint.  The parameter is optional.  When there are no
     other connections on the endpoint, and the parameter is omitted,
     the default is to not perform gain control (unless provisioned
     otherwise), which is equivalent to specifying a gain of 0 decibels.
     If there are other connections on the endpoint, and the parameter
     is omitted, gain control is unchanged.  Upon detecting voiceband
     data, the endpoint SHOULD disable gain control if needed.
     Following termination of voiceband data, the handling of gain
     control SHALL then revert to the current value of the gain control
     parameter.  It should be noted, that handling of gain control is
     normally best left to the gateway and hence use of this parameter
     is NOT RECOMMENDED.

   * RTP security:  The Call agent can request the gateway to enable
     encryption of the audio Packets.  It does so by providing a key
     specification, as specified in RFC 2327.  By default, encryption is
     not performed.

   * Network Type:  The Call Agent may instruct the gateway to prepare
     the connection on a specified type of network.  If absent, the
     value is based on the network type of the gateway being used.





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   * Resource reservation:  The Call Agent may instruct the gateway to
     use network resource reservation for the connection.  See Section
     2.7 for details.

   The Call Agent specifies the relevant fields it cares about in the
   command and leaves the rest to the discretion of the gateway.  For
   those of the above parameters that were not explicitly included, the
   gateway SHOULD use the default values if possible.  For a detailed
   list of local connection options included with this specification
   refer to section 3.2.2.10.  The set of local connection options can
   be extended.

   The Mode indicates the mode of operation for this side of the
   connection.  The basic modes are "send", "receive", "send/receive",
   "conference", "inactive", "loopback", "continuity test", "network
   loop back" and "network continuity test".  The expected handling of
   these modes is specified in the introduction of the "Gateway Control
   Commands", Section 2.3.  Note that signals applied to a connection do
   not follow the connection mode.  Some endpoints may not be capable of
   supporting all modes.  If the command specifies a mode that the
   endpoint does not support, an error SHALL be returned (error 517 -
   unsupported mode, is RECOMMENDED).  Also, if a connection has not yet
   received a RemoteConnectionDescriptor, an error MUST be returned if
   the connection is attempted to be placed in any of the modes "send
   only", "send/receive", "conference", "network loopback", "network
   continuity test", or if a signal (as opposed to detecting an event)
   is to be applied to the connection (error code 527 - missing
   RemoteConnectionDescriptor, is RECOMMENDED).  The set of modes can be
   extended.

   The gateway returns a ConnectionId, that uniquely identifies the
   connection within the endpoint, and a LocalConnectionDescriptor,
   which is a session description that contains information about the
   connection, e.g., IP address and port for the media, as defined in
   SDP.

   The SpecificEndPointId is an optional parameter that identifies the
   responding endpoint.  It is returned when the EndpointId argument
   referred to an "any of" wildcard name and the command succeeded.
   When a SpecificEndPointId is returned, the Call Agent SHALL use it as
   the EndpointId value in successive commands referring to this
   connection.

   The SecondEndpointId can be used instead of the
   RemoteConnectionDescriptor to establish a connection between two
   endpoints located on the same gateway.  The connection is by
   definition a local connection.  The SecondEndpointId can be fully-
   specified by assigning a value to the parameter SecondEndpointId in



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   the function call or it may be under-specified by using the "any of"
   wildcard convention.  If the SecondEndpointId is underspecified, the
   second endpoint identifier will be assigned by the gateway and its
   complete value returned in the SecondEndPointId parameter of the
   response.

   When a SecondEndpointId is specified, the command really creates two
   connections that can be manipulated separately through
   ModifyConnection and DeleteConnection commands.  In addition to the
   ConnectionId and LocalConnectionDescriptor for the first connection,
   the response to the creation provides a SecondConnectionId parameter
   that identifies the second connection.  The second connection is
   established in "send/receive" mode.

   After receiving a "CreateConnection" request that did not include a
   RemoteConnectionDescriptor parameter, a gateway is in an ambiguous
   situation.  Because it has exported a LocalConnectionDescriptor
   parameter, it can potentially receive packets.  Because it has not
   yet received the RemoteConnectionDescriptor parameter of the other
   gateway, it does not know whether the packets that it receives have
   been authorized by the Call Agent.  It must thus navigate between two
   risks, i.e., clipping some important announcements or listening to
   insane data.  The behavior of the gateway is determined by the value
   of the Mode parameter:

   * If the mode was set to ReceiveOnly, the gateway MUST accept the
     media and transmit them through the endpoint.

   * If the mode was set to Inactive, Loopback, or Continuity Test, the
     gateway MUST NOT transmit the media through to the endpoint.

   Note that the mode values SendReceive, Conference, SendOnly, Network
   Loopback and Network Continuity Test do not make sense in this
   situation.  They MUST be treated as errors, and the command MUST be
   rejected (error code 527 - missing RemoteConnectionDescriptor, is
   RECOMMENDED).

   The command may optionally contain an encapsulated Notification
   Request command, which applies to the EndpointId, in which case a
   RequestIdentifier parameter MUST be present, as well as, optionally,
   other parameters of the NotificationRequest with the exception of the
   EndpointId, which is not replicated.  The encapsulated
   NotificationRequest is executed simultaneously with the creation of
   the connection.  For example, when the Call Agent wants to initiate a
   call to a residential gateway, it could:






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   * ask the residential gateway to prepare a connection, in order to be
     sure that the user can start speaking as soon as the phone goes off
     hook,

   * ask the residential gateway to start ringing,

   * ask the residential gateway to notify the Call Agent when the phone
     goes off-hook.

   This can be accomplished in a single CreateConnection command, by
   also transmitting the RequestedEvents parameters for the off-hook
   event, and the SignalRequests parameter for the ringing signal.

   When these parameters are present, the creation and the
   NotificationRequest MUST be synchronized, which means that both MUST
   be accepted, or both MUST be refused.  In our example, the
   CreateConnection may be refused if the gateway does not have
   sufficient resources, or cannot get adequate resources from the local
   network access, and the off-hook NotificationRequest can be refused
   in the glare condition, if the user is already off-hook.  In this
   example, the phone must not ring if the connection cannot be
   established, and the connection must not be established if the user
   is already off-hook.

   The NotifiedEntity parameter, if present, defines the new "notified
   entity" for the endpoint.

   The command may carry an encapsulated EndpointConfiguration command,
   which applies to the EndpointId.  When this command is present, the
   parameters of the EndpointConfiguration command are included with the
   normal parameters of the CreateConnection with the exception of the
   EndpointId, which is not replicated.  The EndpointConfiguration
   command may be encapsulated together with an encapsulated
   NotificationRequest command.  Note that both of these apply to the
   EndpointId only.

   The encapsulated EndpointConfiguration command shares the fate of the
   CreateConnection command.  If the CreateConnection is rejected, the
   EndpointConfiguration is not executed.

   ReturnCode is a parameter returned by the gateway.  It indicates the
   outcome of the command and consists of an integer number optionally
   followed by commentary.

   PackageList is a list of supported packages that MAY be included with
   error code 518 (unsupported package).





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2.3.6 ModifyConnection

   This command is used to modify the characteristics of a gateway's
   "view" of a connection.  This "view" of the call includes both the
   local connection descriptor as well as the remote connection
   descriptor.

         ReturnCode,
         [LocalConnectionDescriptor,]
         [PackageList]
         <-- ModifyConnection(CallId,
                              EndpointId,
                              ConnectionId,
                              [NotifiedEntity,]
                              [LocalConnectionOptions,]
                              [Mode,]
                              [RemoteConnectionDescriptor,]
                              [Encapsulated NotificationRequest,]
                              [Encapsulated EndpointConfiguration])

   The parameters used are the same as in the CreateConnection command,
   with the addition of a ConnectionId that identifies the connection
   within the endpoint.  This parameter was returned by the
   CreateConnection command, in addition to the local connection
   descriptor.  It uniquely identifies the connection within the context
   of the endpoint.  The CallId used when the connection was created
   MUST be included as well.

   The EndpointId MUST be a fully qualified endpoint identifier.  The
   local name MUST NOT use the wildcard conventions.

   The ModifyConnection command can be used to affect parameters of a
   connection in the following ways:

   * Provide information about the other end of the connection, through
     the RemoteConnectionDescriptor.  If the parameter is omitted, it
     retains its current value.

   * Activate or deactivate the connection, by changing the value of the
     Mode parameter.  This can occur at any time during the connection,
     with arbitrary parameter values.  If the parameter is omitted, it
     retains its current value.

   * Change the parameters of the connection through the
     LocalConnectionOptions, for example by switching to a different
     coding scheme, changing the packetization period, or modifying the
     handling of echo cancellation.  If one or more
     LocalConnectionOptions parameters are omitted, then the gateway



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     SHOULD refrain from changing that parameter from its current value,
     unless another parameter necessitating such a change is explicitly
     provided.  For example, a codec change might require a change in
     silence suppression.  Note that if a RemoteConnectionDescriptor is
     supplied, then only the LocalConnectionOptions actually supplied
     with the ModifyConnection command will affect the codec negotiation
     (as described in Section 2.6).

   Connections can only be fully activated if the
   RemoteConnectionDescriptor has been provided to the gateway.  The
   receive-only mode, however, can be activated without the provision of
   this descriptor.

   The command will only return a LocalConnectionDescriptor if the local
   connection parameters, such as RTP ports, were modified.  Thus, if,
   for example, only the mode of the connection is changed, a
   LocalConnectionDescriptor will not be returned.  Note however, that
   inclusion of LocalConnectionOptions in the command is not a
   prerequisite for local connection parameter changes to occur.  If a
   connection parameter is omitted, e.g., silence suppression, the old
   value of that parameter will be retained if possible.  If a parameter
   change necessitates a change in one or more unspecified parameters,
   the gateway is free to choose suitable values for the unspecified
   parameters that must change.  This can for instance happen if the
   packetization period was not specified.  If the new codec supported
   the old packetization period, the value of this parameter would not
   change, as a change would not be necessary.  However, if it did not
   support the old packetization period, it would choose a suitable
   value.

   The command may optionally contain an encapsulated Notification
   Request command, in which case a RequestIdentifier parameter MUST be
   present, as well as, optionally, other parameters of the
   NotificationRequest with the exception of the EndpointId, which is
   not replicated.  The encapsulated NotificationRequest is executed
   simultaneously with the modification of the connection.  For example,
   when a connection is accepted, the calling gateway should be
   instructed to place the circuit in send-receive mode and to stop
   providing ringing tones.  This can be accomplished in a single
   ModifyConnection command, by also transmitting the RequestedEvents
   parameters, for the on-hook event, and an empty SignalRequests
   parameter, to stop the provision of ringing tones.

   When these parameters are present, the modification and the
   NotificationRequest MUST be synchronized, which means that both MUST
   be accepted, or both MUST be refused.





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   The NotifiedEntity parameter, if present, defines the new "notified
   entity" for the endpoint.

   The command may carry an encapsulated EndpointConfiguration command,
   that will apply to the same endpoint.  When this command is present,
   the parameters of the EndpointConfiguration command are included with
   the normal parameters of the ModifyConnection with the exception of
   the EndpointId, which is not replicated.  The EndpointConfiguration
   command may be encapsulated together with an encapsulated
   NotificationRequest command.

   The encapsulated EndpointConfiguration command shares the fate of the
   ModifyConnection command.  If the ModifyConnection is rejected, the
   EndpointConfiguration is not executed.

   ReturnCode is a parameter returned by the gateway.  It indicates the
   outcome of the command and consists of an integer number optionally
   followed by commentary.

   PackageList is a list of supported packages that MAY be included with
   error code 518 (unsupported package).

2.3.7 DeleteConnection (from the Call Agent)

   This command is used to terminate a connection.  As a side effect, it
   collects statistics on the execution of the connection.

         ReturnCode,
         ConnectionParameters,
         [PackageList]
         <-- DeleteConnection(CallId,
                              EndpointId,
                              ConnectionId,
                              [NotifiedEntity,]
                              [Encapsulated NotificationRequest,]
                              [Encapsulated EndpointConfiguration])

   The endpoint identifier, in this form of the DeleteConnection
   command, SHALL be fully qualified.  Wildcard conventions SHALL NOT be
   used.

   The ConnectionId identifies the connection to be deleted.  The CallId
   used when the connection was created is included as well.

   The NotifiedEntity parameter, if present, defines the new "notified
   entity" for the endpoint.





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   In the case of IP multicast, connections can be deleted individually
   and independently.  However, in the unicast case where a connection
   has two ends, a DeleteConnection command has to be sent to both
   gateways involved in the connection.  After the connection has been
   deleted, media streams previously supported by the connection are no
   longer available.  Any media packets received for the old connection
   are simply discarded and no new media packets for the stream are
   sent.

   After the connection has been deleted, any loopback that has been
   requested for the connection must be cancelled (unless the endpoint
   has another connection requesting loopback).

   In response to the DeleteConnection command, the gateway returns a
   list of connection parameters that describe statistics for the
   connection.

   When the connection was for an Internet media stream, these
   parameters are:

   Number of packets sent:

      The total number of media packets transmitted by the sender since
      starting transmission on this connection.  In the case of RTP, the
      count is not reset if the sender changes its synchronization
      source identifier (SSRC, as defined in RTP), for example as a
      result of a ModifyConnection command.  The value is zero if the
      connection was always set in "receive only" mode and no signals
      were applied to the connection.

   Number of octets sent:

      The total number of payload octets (i.e., not including header or
      padding) transmitted in media packets by the sender since starting
      transmission on this connection.  In the case of RTP, the count is
      not reset if the sender changes its SSRC identifier, for example
      as a result of a ModifyConnection command.  The value is zero if
      the connection was always set in "receive only" mode and no
      signals were applied to the connection.

   Number of packets received:

      The total number of media packets received by the sender since
      starting reception on this connection.  In the case of RTP, the
      count includes packets received from different SSRC, if the sender
      used several values.  The value is zero if the connection was
      always set in "send only" mode.




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   Number of octets received:

      The total number of payload octets (i.e., not including header,
      e.g., RTP, or padding) transmitted in media packets by the sender
      since starting transmission on this connection.  In the case of
      RTP, the count includes packets received from different SSRC, if
      the sender used several values.  The value is zero if the
      connection was always set in "send only" mode.

   Number of packets lost:

      The total number of media packets that have been lost since the
      beginning of reception.  This number is defined to be the number
      of packets expected less the number of packets actually received,
      where the number of packets received includes any which are late
      or duplicates.  For RTP, the count includes packets received from
      different SSRC, if the sender used several values.  Thus packets
      that arrive late are not counted as lost, and the loss may be
      negative if there are duplicates.  The count includes packets
      received from different SSRC, if the sender used several values.
      The number of packets expected is defined to be the extended last
      sequence number received, as defined next, less the initial
      sequence number received.  The count includes packets received
      from different SSRC, if the sender used several values.  The value
      is zero if the connection was always set in "send only" mode.

   Interarrival jitter:

      An estimate of the statistical variance of the media packet
      interarrival time measured in milliseconds and expressed as an
      unsigned integer.  For RTP, the interarrival jitter J is defined
      to be the mean deviation (smoothed absolute value) of the
      difference D in packet spacing at the receiver compared to the
      sender for a pair of packets.  Detailed computation algorithms are
      found in RFC 1889.  The count includes packets received from
      different SSRC, if the sender used several values.  The value is
      zero if the connection was always set in "send only" mode.

   Average transmission delay:

      An estimate of the network latency, expressed in milliseconds. For
      RTP, this is the average value of the difference between the NTP
      timestamp indicated by the senders of the RTCP messages and the
      NTP timestamp of the receivers, measured when the messages are
      received.  The average is obtained by summing all the estimates,






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      then dividing by the number of RTCP messages that have been
      received.  When the gateway's clock is not synchronized by NTP,
      the latency value can be computed as one half of the round trip
      delay, as measured through RTCP.  When the gateway cannot compute
      the one way delay or the round trip delay, the parameter conveys a
      null value.

   For a detailed definition of these variables, refer to RFC 1889.

   When the connection was set up over a LOCAL interconnect, the meaning
   of these parameters is defined as follows:

   Number of packets sent:
      Not significant - MAY be omitted.

   Number of octets sent:
      The total number of payload octets transmitted over the local
      connection.

   Number of packets received:
      Not significant - MAY be omitted.

   Number of octets received:
      The total number of payload octets received over the connection.

   Number of packets lost:
      Not significant - MAY be omitted.  A value of zero is assumed.

   Interarrival jitter:
      Not significant - MAY be omitted.  A value of zero is assumed.

   Average transmission delay:
      Not significant - MAY be omitted.  A value of zero is assumed.

   The set of connection parameters can be extended.  Also, the meaning
   may be further defined by other types of networks which MAY
   furthermore elect to not return all, or even any, of the above
   specified parameters.

   The command may optionally contain an encapsulated Notification
   Request command, in which case a RequestIdentifier parameter MUST be
   present, as well as, optionally, other parameters of the
   NotificationRequest with the exception of the EndpointId, which is
   not replicated.  The encapsulated NotificationRequest is executed
   simultaneously with the deletion of the connection.  For example,
   when a user hang-up is notified, the gateway should be instructed to
   delete the connection and to start looking for an off-hook event.




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   This can be accomplished in a single DeleteConnection command, by
   also transmitting the RequestedEvents parameters, for the off-hook
   event, and an empty SignalRequests parameter.

   When these parameters are present, the DeleteConnection and the
   NotificationRequest must be synchronized, which means that both MUST
   be accepted, or both MUST be refused.

   The command may carry an encapsulated EndpointConfiguration command,
   that will apply to the same endpoint.  When this command is present,
   the parameters of the EndpointConfiguration command are included with
   the normal parameters of the DeleteConnection with the exception of
   the EndpointId, which is not replicated.  The EndpointConfiguration
   command may be encapsulated together with an encapsulated
   NotificationRequest command.

   The encapsulated EndpointConfiguration command shares the fate of the
   DeleteConnection command.  If the DeleteConnection is rejected, the
   EndpointConfiguration is not executed.

   ReturnCode is a parameter returned by the gateway.  It indicates the
   outcome of the command and consists of an integer number optionally
   followed by commentary.

   PackageList is a list of supported packages that MAY be included with
   error code 518 (unsupported package).

2.3.8 DeleteConnection (from the gateway)

   In some rare circumstances, a gateway may have to clear a connection,
   for example because it has lost the resource associated with the
   connection, or because it has detected that the endpoint no longer is
   capable or willing to send or receive media.  The gateway may then
   terminate the connection by using a variant of the DeleteConnection
   command:

         ReturnCode,
         [PackageList]
         <-- DeleteConnection(CallId,
                              EndpointId,
                              ConnectionId,
                              ReasonCode,
                              Connection-parameters)

   The EndpointId, in this form of the DeleteConnection command, MUST be
   fully qualified.  Wildcard conventions MUST NOT be used.





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   The ReasonCode is a text string starting with a numeric reason code
   and optionally followed by a descriptive text string.  The reason
   code indicates the cause of the DeleteConnection.  A list of reason
   codes can be found in Section 2.5.

   In addition to the call, endpoint and connection identifiers, the
   gateway will also send the connection parameters that would have been
   returned to the Call Agent in response to a DeleteConnection command.

   ReturnCode is a parameter returned by the Call Agent.  It indicates
   the outcome of the command and consists of an integer number
   optionally followed by commentary.

   PackageList is a list of supported packages that MAY be included with
   error code 518 (unsupported package).

   Note that use of this command is generally discouraged and should
   only be done as a last resort.  If a connection can be sustained,
   deletion of it should be left to the discretion of the Call Agent
   which is in a far better position to make intelligent decisions in
   this area.

2.3.9 DeleteConnection (multiple connections from the Call Agent)

   A variation of the DeleteConnection function can be used by the Call
   Agent to delete multiple connections at the same time.  Note that
   encapsulating other commands with this variation of the
   DeleteConnection command is not permitted.  The command can be used
   to delete all connections that relate to a Call for an endpoint:

         ReturnCode,
         [PackageList]
         <-- DeleteConnection(CallId,
                              EndpointId)

   The EndpointId, in this form of the DeleteConnection command, MUST
   NOT use the "any of" wildcard.  All connections for the endpoint(s)
   with the CallId specified will be deleted.  Note that the command
   will still succeed if there were no connections with the CallId
   specified, as long as the EndpointId was valid.  However, if the
   EndpointId is invalid, the command will fail.  The command does not
   return any individual statistics or call parameters.









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   It can also be used to delete all connections that terminate in a
   given endpoint:

         ReturnCode,
         [PackageList]
         <-- DeleteConnection(EndpointId)

   The EndpointId, in this form of the DeleteConnection command, MUST
   NOT use the "any of" wildcard.  Again, the command succeeds even if
   there were no connections on the endpoint(s).

   Finally, Call Agents can take advantage of the hierarchical structure
   of endpoint names to delete all the connections that belong to a
   group of endpoints.  In this case, the "local name" component of the
   EndpointId will be specified using the "all of" wildcarding
   convention.  The "any of" convention SHALL NOT be used.  For example,
   if endpoint names are structured as the combination of a physical
   interface name and a circuit number, as in "X35V3+A4/13", the Call
   Agent may replace the circuit number by the "all of" wild card
   character "*", as in "X35V3+A4/*".  This "wildcard" command instructs
   the gateway to delete all the connections that were attached to
   circuits connected to the physical interface "X35V3+A4".

   After all the connections have been deleted, any loopback that has
   been requested for the connections MUST be cancelled by the gateway.

   This command does not return any individual statistics or call
   parameters.

   ReturnCode is a parameter returned by the gateway.  It indicates the
   outcome of the command and consists of an integer number optionally
   followed by commentary.

   PackageList is a list of supported packages that MAY be included with
   error code 518 (unsupported package).

2.3.10 AuditEndpoint

   The AuditEndPoint command can be used by the Call Agent to find out
   the status of a given endpoint.











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         ReturnCode,
         EndPointIdList,|{
         [RequestedEvents,]
         [QuarantineHandling,]
         [DigitMap,]
         [SignalRequests,]
         [RequestIdentifier,]
         [NotifiedEntity,]
         [ConnectionIdentifiers,]
         [DetectEvents,]
         [ObservedEvents,]
         [EventStates,]
         [BearerInformation,]
         [RestartMethod,]
         [RestartDelay,]
         [ReasonCode,]
         [MaxMGCPDatagram,]
         [Capabilities]}
         [PackageList]
         <-- AuditEndPoint(EndpointId,
                           [RequestedInfo])

   The EndpointId identifies the endpoint(s) being audited.  The "any
   of" wildcard convention MUST NOT be used.

   The EndpointId identifies the endpoint(s) being audited.  The "all
   of" wildcard convention can be used to start auditing of a group of
   endpoints (regardless of their service-state).  If this convention is
   used, the gateway SHALL return the list of endpoint identifiers that
   match the wildcard in the EndPointIdList parameter, which is simply
   one or more SpecificEndpointIds (each supplied separately).  In the
   case where the "all of" wildcard is used, RequestedInfo SHOULD NOT be
   included (if it is included, it MUST be ignored).  Note that the use
   of the "all of" wildcard can potentially generate a large
   EndPointIdList.  If the resulting EndPointIdList is considered too
   large, the gateway returns an error (error code 533 - response too
   large, is RECOMMENDED).

   When a non-wildcard EndpointId is specified, the (possibly empty)
   RequestedInfo parameter describes the information that is requested
   for the EndpointId specified.  The following endpoint info can be
   audited with this command:

      RequestedEvents, DigitMap, SignalRequests, RequestIdentifier,
      QuarantineHandling, NotifiedEntity, ConnectionIdentifiers,
      DetectEvents, ObservedEvents, EventStates, BearerInformation,
      RestartMethod, RestartDelay, ReasonCode, PackageList,
      MaxMGCPDatagram, and Capabilities.



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   The list may be extended by extension parameters.  The response will
   in turn include information about each of the items for which
   auditing info was requested.  Supported parameters with empty values
   MUST always be returned.  However, if an endpoint is queried about a
   parameter it does not understand, the endpoint MUST NOT generate an
   error; instead the parameter MUST be omitted from the response:

   * RequestedEvents: The current value of RequestedEvents the endpoint
     is using including the action(s) and event parameters associated
     with each event - if no actions are included, the default action is
     assumed. Persistent events are included in the list. If an embedded
     NotificationRequest is active, the RequestedEvents will reflect the
     events requested in the embedded NotificationRequest, not any
     surrounding RequestedEvents (whether embedded or not).

   * DigitMap:  The digit map the endpoint is currently using.  The
     parameter will be empty if the endpoint does not have a digit map.

   * SignalRequests:  A list of the; Time-Out signals that are currently
     active, On/Off signals that are currently "on" for the endpoint
     (with or without parameter), and any pending Brief signals.  Time-
     Out signals that have timed-out, and currently playing Brief
     signals are not included.  Any signal parameters included in the
     original SignalRequests will be included.

   * RequestIdentifier:  The RequestIdentifier for the last
     NotificationRequest received by this endpoint (includes
     NotificationRequests encapsulated in other commands).  If no
     NotificationRequest has been received since reboot/restart, the
     value zero will be returned.

   * QuarantineHandling:  The QuarantineHandling for the last
     NotificationRequest received by this endpoint.  If
     QuarantineHandling was not included, or no notification request has
     been received, the default values will be returned.

   * DetectEvents:  The value of the most recently received DetectEvents
     parameter plus any persistent events implemented by the endpoint.
     If no DetectEvents parameter has been received, the (possibly
     empty) list only includes persistent events.

   * NotifiedEntity:  The current "notified entity" for the endpoint.

   * ConnectionIdentifiers:  The list of ConnectionIdentifiers for all
     connections that currently exist for the specified endpoint.

   * ObservedEvents:  The current list of observed events for the
     endpoint.



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   * EventStates:  For events that have auditable states associated with
     them, the event corresponding to the state the endpoint is in,
     e.g., off-hook if the endpoint is off-hook.  Note that the
     definition of the individual events will state if the event in
     question has an auditable state associated with it.

   * BearerInformation:  The value of the last received
     BearerInformation parameter for this endpoint (this includes the
     case where BearerInformation was provisioned).  The parameter will
     be empty if the endpoint has not received a BearerInformation
     parameter and a value was also not provisioned.

   * RestartMethod:  "restart" if the endpoint is in-service and
     operation is normal, or if the endpoint is in the process of
     becoming in-service (a non-zero RestartDelay will indicate the
     latter).  Otherwise, the value of the restart method parameter in
     the last RestartInProgress command issued (or should have been
     issued) by the endpoint.  Note that a "disconnected" endpoint will
     thus only report "disconnected" as long as it actually is
     disconnected, and "restart" will be reported once it is no longer
     disconnected.  Similarly, "cancel-graceful" will not be reported,
     but "graceful" might (see Section 4.4.5 for further details).

   * RestartDelay:  The value of the restart delay parameter if a
     RestartInProgress command was to be issued by the endpoint at the
     time of this response, or zero if the command would not include
     this parameter.

   * ReasonCode:  The value of the ReasonCode parameter in the last
     RestartInProgress or DeleteConnection command issued by the gateway
     for the endpoint, or the special value 000 if the endpoint's state
     is normal.

   * PackageList:  The packages supported by the endpoint including
     package version numbers.  For backwards compatibility, support for
     the parameter is OPTIONAL although implementations with package
     versions higher than zero SHOULD support it.

   * MaxMGCPDatagram:  The maximum size of an MGCP datagram in bytes
     that can be received by the endpoint (see Section 3.5.4).  The
     value excludes any lower layer overhead.  For backwards
     compatibility, support for this parameter is OPTIONAL.  The default
     maximum MGCP datagram size SHOULD be assumed if a value is not
     returned.







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   * Capabilities:  The capabilities for the endpoint similar to the
     LocalConnectionOptions parameter and including packages and
     connection modes.  Extensions MAY be included as well.  If any
     unknown capabilities are reported, they MUST simply be ignored.  If
     there is a need to specify that some parameters, such as e.g.,
     silence suppression, are only compatible with some codecs, then the
     gateway MUST return several capability sets, each of which may
     include:

     - Compression Algorithm:  A list of supported codecs.  The rest of
       the parameters in the capability set will apply to all codecs
       specified in this list.

     - Packetization Period:  A single value or a range may be
       specified.

     - Bandwidth:  A single value or a range corresponding to the range
       for packetization periods may be specified (assuming no silence
       suppression).

     - Echo Cancellation:  Whether echo cancellation is supported or not
       for the endpoint.

     - Silence Suppression:  Whether silence suppression is supported or
       not.

     - Gain Control:  Whether gain control is supported or not.

     - Type of Service:  Whether type of service is supported or not.

     - Resource Reservation:  Whether resource reservation is supported
       or not.

     - Security:  Whether media encryption is supported or not.

     - Type of network:  The type(s) of network supported.

     - Packages:  A list of packages supported.  The first package in
       the list will be the default package.

     - Modes:  A list of supported connection modes.

   The Call Agent may then decide to use the AuditConnection command to
   obtain further information about the connections.

   If no info was requested and the EndpointId refers to a valid
   endpoint (in-service or not), the gateway simply returns a positive
   acknowledgement.



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   ReturnCode is a parameter returned by the gateway.  It indicates the
   outcome of the command and consists of an integer number optionally
   followed by commentary.

   Note that PackageList MAY also be included with error code 518
   (unsupported package).

2.3.11 AuditConnection

   The AuditConnection command can be used by the Call Agent to retrieve
   the parameters attached to a connection.

         ReturnCode,
         [CallId,]
         [NotifiedEntity,]
         [LocalConnectionOptions,]
         [Mode,]
         [RemoteConnectionDescriptor,]
         [LocalConnectionDescriptor,]
         [ConnectionParameters,]
         [PackageList]
         <-- AuditConnection(EndpointId,
                             ConnectionId,
                             RequestedInfo)

   The EndpointId parameter specifies the endpoint that handles the
   connection.  The wildcard conventions SHALL NOT be used.

   The ConnectionId parameter is the identifier of the audited
   connection, within the context of the specified endpoint.

   The (possibly empty) RequestedInfo describes the information that is
   requested for the ConnectionId within the EndpointId specified.  The
   following connection info can be audited with this command:

      CallId, NotifiedEntity, LocalConnectionOptions, Mode,
      RemoteConnectionDescriptor, LocalConnectionDescriptor,
      ConnectionParameters

   The AuditConnection response will in turn include information about
   each of the items auditing info was requested for:

   * CallId, the CallId for the call the connection belongs to.

   * NotifiedEntity, the current "notified entity" for the Connection.
     Note this is the same as the "notified entity" for the endpoint
     (included here for backwards compatibility).




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   * LocalConnectionOptions, the most recent LocalConnectionOptions
     parameters that was actually supplied for the connection (omitting
     LocalConnectionOptions from a command thus does not change this
     value).  Note that default parameters omitted from the most recent
     LocalConnectionOptions will not be included.
     LocalConnectionOptions that retain their value across
     ModifyConnection commands and which have been included in a
     previous command for the connection are also included, regardless
     of whether they were supplied in the most recent
     LocalConnectionOptions or not.

   * Mode, the current mode of the connection.

   * RemoteConnectionDescriptor, the RemoteConnectionDescriptor that was
     supplied to the gateway for the connection.

   * LocalConnectionDescriptor, the LocalConnectionDescriptor the
     gateway supplied for the connection.

   * ConnectionParameters, the current values of the connection
     parameters for the connection.

   If no info was requested and the EndpointId is valid, the gateway
   simply checks that the connection exists, and if so returns a
   positive acknowledgement.  Note, that by definition, the endpoint
   must be in-service for this to happen, as out-of-service endpoints do
   not have any connections.

   ReturnCode is a parameter returned by the gateway.  It indicates the
   outcome of the command and consists of an integer number optionally
   followed by commentary.

   PackageList is a list of supported packages that MAY be included with
   error code 518 (unsupported package).

2.3.12 RestartInProgress

   The RestartInProgress command is used by the gateway to signal that
   an endpoint, or a group of endpoints, is put in-service or out-of-
   service.

         ReturnCode,
         [NotifiedEntity,]
         [PackageList]
         <-- RestartInProgress(EndPointId,
                               RestartMethod,
                               [RestartDelay,]
                               [ReasonCode])



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   The EndPointId identifies the endpoint(s) that are put in-service or
   out-of-service.  The "all of" wildcard convention may be used to
   apply the command to a group of endpoints managed by the same Call
   Agent, such as for example all endpoints that are attached to a
   specified interface, or even all endpoints that are attached to a
   given gateway.  The "any of" wildcard convention SHALL NOT be used.

   The RestartMethod parameter specifies the type of restart.  The
   following values have been defined:

   * A "graceful" restart method indicates that the specified endpoints
     will be taken out-of-service after the specified delay.  The
     established connections are not yet affected, but the Call Agent
     SHOULD refrain from establishing new connections, and SHOULD try to
     gracefully tear down the existing connections.

   * A "forced" restart method indicates that the specified endpoints
     are taken abruptly out-of-service.  The established connections, if
     any, are lost.

   * A "restart" method indicates that service will be restored on the
     endpoints after the specified "restart delay", i.e., the endpoints
     will be in-service.  The endpoints are in their clean default state
     and there are no connections that are currently established on the
     endpoints.

   * A "disconnected" method indicates that the endpoint has become
     disconnected and is now trying to establish connectivity (see
     Section 4.4.7).  The "restart delay" specifies the number of
     seconds the endpoint has been disconnected.  Established
     connections are not affected.

   * A "cancel-graceful" method indicates that a gateway is canceling a
     previously issued "graceful" restart command.  The endpoints are
     still in-service.

   The list of restart methods may be extended.

   The optional "restart delay" parameter is expressed as a number of
   seconds.  If the number is absent, the delay value MUST be considered
   null (i.e., zero).  In the case of the "graceful" method, a null
   delay indicates that the Call Agent SHOULD simply wait for the
   natural termination of the existing connections, without establishing
   new connections.  The restart delay is always considered null in the
   case of the "forced" and "cancel-graceful" methods, and hence the
   "restart delay" parameter MUST NOT be used with these restart
   methods.  When the gateway sends a "restart" or "graceful"




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   RestartInProgress message with a non-zero restart delay, the gateway
   SHOULD send an updated RestartInProgress message after the "restart
   delay" has passed.

   A restart delay of null for the "restart" method indicates that
   service has already been restored.  This typically will occur after
   gateway startup/reboot.  To mitigate the effects of a gateway IP
   address change as a result of a re-boot, the Call Agent MAY wish to
   either flush its DNS cache for the gateway's domain name or resolve
   the gateway's domain name by querying the DNS regardless of the TTL
   of a current DNS resource record for the restarted gateway.

   The optional reason code parameter indicates the cause of the
   restart.

   Gateways SHOULD send a "graceful" or "forced" RestartInProgress
   message (for the relevant endpoints) as a courtesy to the Call Agent
   when they are taken out-of-service, e.g., by being shutdown, or taken
   out-of-service by a network management system, however the Call Agent
   cannot rely on always receiving such a message.  Gateways MUST send a
   "restart" RestartInProgress message (for the relevant endpoints) with
   a null delay to their Call Agent when they are back in-service
   according to the restart procedure specified in Section 4.4.6 - Call
   Agents can rely on receiving this message.  Also, gateways MUST send
   a "disconnected" RestartInProgress message (for the relevant
   endpoints) to their current "notified entity" according to the
   "disconnected" procedure specified in Section 4.4.7.

   The RestartInProgress message will be sent to the current "notified
   entity" for the EndpointId in question.  It is expected that a
   default Call Agent, i.e., "notified entity", has been provisioned so
   that after a reboot/restart, the default Call Agent will always be
   the "notified entity" for the endpoint.  Gateways SHOULD take full
   advantage of wild-carding to minimize the number of RestartInProgress
   messages generated when multiple endpoints in a gateway restart and
   the endpoints are managed by the same Call Agent.

   ReturnCode is a parameter returned by the Call Agent.  It indicates
   the outcome of the command and consists of an integer number
   optionally followed by commentary.

   A NotifiedEntity may additionally be returned with the response to
   the RestartInProgress from the Call Agent - this SHOULD normally only
   be done in response to "restart" or "disconnected" (see also Section
   4.4.6 and 4.4.7):






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   * If the response indicated success (return code 200 - transaction
     executed), the restart in question completed successfully, and the
     NotifiedEntity returned is the new "notified entity" for the
     endpoint(s).

   * If the response from the Call Agent indicated an error, the restart
     in question did not complete successfully.  If a NotifiedEntity
     parameter was included in the response returned, it specifies a new
     "notified entity" for the endpoint(s), which MUST be used when
     retrying the restart in question (as a new transaction).  This
     SHOULD only be done with error code 521 (endpoint redirected).

   Note that the above behavior for returning a NotifiedEntity in the
   response is only defined for RestartInProgress responses and SHOULD
   NOT be done for responses to other commands.  Any other behavior is
   undefined.

   PackageList is a list of supported packages that MAY be included with
   error code 518 (unsupported package).

2.4 Return Codes and Error Codes

   All MGCP commands are acknowledged.  The acknowledgment carries a
   return code, which indicates the status of the command.  The return
   code is an integer number, for which the following ranges of values
   have been defined:

   * values between 000 and 099 indicate a response acknowledgement

   * values between 100 and 199 indicate a provisional response

   * values between 200 and 299 indicate a successful completion

   * values between 400 and 499 indicate a transient error

   * values between 500 and 599 indicate a permanent error

   * values between 800 and 899 are package specific response codes.

   A broad description of transient errors (4XX error codes) versus
   permanent errors (5XX error codes) is as follows:










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   * If a Call Agent receives a transient error, there is the
     expectation of the possibility that a future similar request will
     be honored by the endpoint.  In some cases, this may require some
     state change in the environment of the endpoint (e.g., hook state
     as in the case of error codes 401 or 402; resource availability as
     in the case of error code 403, or bandwidth availability as in the
     case of error code 404).

   * Permanent errors (error codes 500 to 599) indicate one or more
     permanent conditions either due to protocol error or
     incompatibility between the endpoint and the Call Agent, or because
     of some error condition over which the Call Agent has no control.
     Examples are protocol errors, requests for endpoint capabilities
     that do not exist, errors on interfaces associated with the
     endpoint, missing or incorrect information in the request or any
     number of other conditions which will simply not disappear with
     time.

   The values that have been already defined are the following:

   000 Response Acknowledgement.

   100 The transaction is currently being executed.  An actual
       completion message will follow later.

   101 The transaction has been queued for execution.  An actual
       completion message will follow later.

   200 The requested transaction was executed normally.  This return
       code can be used for a successful response to any command.

   250 The connection was deleted.  This return code can only be used
       for a successful response to a DeleteConnection command.

   400 The transaction could not be executed, due to some unspecified
       transient error.

   401 The phone is already off hook.

   402 The phone is already on hook.

   403 The transaction could not be executed, because the endpoint does
       not have sufficient resources at this time.

   404 Insufficient bandwidth at this time.

   405 The transaction could not be executed, because the endpoint is
       "restarting".



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   406 Transaction time-out.  The transaction did not complete in a
       reasonable period of time and has been aborted.

   407 Transaction aborted.  The transaction was aborted by some
       external action, e.g., a ModifyConnection command aborted by a
       DeleteConnection command.

   409 The transaction could not be executed because of internal
       overload.

   410 No endpoint available.  A valid "any of" wildcard was used,
       however there was no endpoint available to satisfy the request.

   500 The transaction could not be executed, because the endpoint is
       unknown.

   501 The transaction could not be executed, because the endpoint is
       not ready.  This includes the case where the endpoint is out-of-
       service.

   502 The transaction could not be executed, because the endpoint does
       not have sufficient resources (permanent condition).

   503 "All of" wildcard too complicated.

   504 Unknown or unsupported command.

   505 Unsupported RemoteConnectionDescriptor.  This SHOULD be used when
       one or more mandatory parameters or values in the
       RemoteConnectionDescriptor is not supported.

   506 Unable to satisfy both LocalConnectionOptions and
       RemoteConnectionDescriptor.  This SHOULD be used when the
       LocalConnectionOptions and RemoteConnectionDescriptor contain one
       or more mandatory parameters or values that conflict with each
       other and/or cannot be supported at the same time (except for
       codec negotiation failure - see error code 534).

   507 Unsupported functionality. Some unspecified functionality
       required to carry out the command is not supported. Note that
       several other error codes have been defined for specific areas of
       unsupported functionality (e.g. 508, 511, etc.), and this error
       code SHOULD only be used if there is no other more specific error
       code for the unsupported functionality.

   508 Unknown or unsupported quarantine handling.





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   509 Error in RemoteConnectionDescriptor.  This SHOULD be used when
       there is a syntax or semantic error in the
       RemoteConnectionDescriptor.

   510 The transaction could not be executed, because some unspecified
       protocol error was detected.  Automatic recovery from such an
       error will be very difficult, and hence this code SHOULD only be
       used as a last resort.

   511 The transaction could not be executed, because the command
       contained an unrecognized extension.  This code SHOULD be used
       for unsupported critical parameter extensions ("X+").

   512 The transaction could not be executed, because the gateway is not
       equipped to detect one of the requested events.

   513 The transaction could not be executed, because the gateway is not
       equipped to generate one of the requested signals.

   514 The transaction could not be executed, because the gateway cannot
       send the specified announcement.

   515 The transaction refers to an incorrect connection-id (may have
       been already deleted).

   516 The transaction refers to an unknown call-id, or the call-id
       supplied is incorrect (e.g., connection-id not associated with
       this call-id).

   517 Unsupported or invalid mode.

   518 Unsupported or unknown package.  It is RECOMMENDED to include a
       PackageList parameter with the list of supported packages in the
       response, especially if the response is generated by the Call
       Agent.

   519 Endpoint does not have a digit map.

   520 The transaction could not be executed, because the endpoint is
       "restarting".  In most cases this would be a transient error, in
       which case, error code 405 SHOULD be used instead.  The error
       code is only included here for backwards compatibility.

   521 Endpoint redirected to another Call Agent.  The associated
       redirection behavior is only well-defined when this response is
       issued for a RestartInProgress command.





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   522 No such event or signal.  The request referred to an event or
       signal that is not defined in the relevant package (which could
       be the default package).

   523 Unknown action or illegal combination of actions.

   524 Internal inconsistency in LocalConnectionOptions.

   525 Unknown extension in LocalConnectionOptions.  This code SHOULD be
       used for unsupported mandatory vendor extensions ("x+").

   526 Insufficient bandwidth.  In cases where this is a transient
       error, error code 404 SHOULD be used instead.

   527 Missing RemoteConnectionDescriptor.

   528 Incompatible protocol version.

   529 Internal hardware failure.

   530 CAS signaling protocol error.

   531 Failure of a grouping of trunks (e.g., facility failure).

   532 Unsupported value(s) in LocalConnectionOptions.

   533 Response too large.

   534 Codec negotiation failure.

   535 Packetization period not supported.

   536 Unknown or unsupported RestartMethod.

   537 Unknown or unsupported digit map extension.

   538 Event/signal parameter error (e.g., missing, erroneous,
       unsupported, unknown, etc.).

   539 Invalid or unsupported command parameter. This code SHOULD only
       be used when the parameter is neither a package or vendor
       extension parameter.

   540 Per endpoint connection limit exceeded.

   541 Invalid or unsupported LocalConnectionOptions. This code SHOULD
       only be used when the LocalConnectionOptions is neither a package
       nor a vendor extension LocalConnectionOptions.



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   The set of return codes may be extended in a future version of the
   protocol.  Implementations that receive an unknown or unsupported
   return code SHOULD treat the return code as follows:

   * Unknown 0xx code treated as 000.

   * Unknown 1xx code treated as 100.

   * Unknown 2xx code treated as 200.

   * Unknown 3xx code treated as 521.

   * Unknown 4xx code treated as 400.

   * Unknown 5xx-9xx code treated as 510.

2.5 Reason Codes

   Reason codes are used by the gateway when deleting a connection to
   inform the Call Agent about the reason for deleting the connection.
   They may also be used in a RestartInProgress command to inform the
   Call Agent of the reason for the RestartInProgress.

   The reason code is an integer number, and the following values have
   been defined:

   000 Endpoint state is normal (this code is only used in response to
       audit requests).

   900 Endpoint malfunctioning.

   901 Endpoint taken out-of-service.

   902 Loss of lower layer connectivity (e.g., downstream sync).

   903 QoS resource reservation was lost.

   904 Manual intervention.

   905 Facility failure (e.g., DS-0 failure).

   The set of reason codes can be extended.









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2.6 Use of Local Connection Options and Connection Descriptors

   As indicated previously, the normal sequence in setting up a bi-
   directional connection involves at least 3 steps:

   1) The Call Agent asks the first gateway to "create a connection" on
      an endpoint.  The gateway allocates resources to that connection,
      and responds to the command by providing a "session description"
      (referred to as its LocalConnectionDescriptor).  The session
      description contains the information necessary for another party
      to send packets towards the newly created connection.

   2) The Call Agent then asks the second gateway to "create a
      connection" on an endpoint.  The command carries the "session
      description" provided by the first gateway (now referred to as the
      RemoteConnectionDescriptor).  The gateway allocates resources to
      that connection, and responds to the command by providing its own
      "session description" (LocalConnectionDescriptor).

   3) The Call Agent uses a "modify connection" command to provide this
      second "session description" (now referred to as the
      RemoteConnectionDescriptor ) to the first endpoint.  Once this is
      done, communication can proceed in both directions.

   When the Call Agent issues a Create or Modify Connection command,
   there are thus three parameters that determine the media supported by
   that connection:

   * LocalConnectionOptions:  Supplied by the Call Agent to control the
     media parameters used by the gateway for the connection. When
     supplied, the gateway MUST conform to these media parameters until
     either the connection is deleted, or a ModifyConnection command
     with new media parameters (LocalConnectionOptions or
     RemoteConnectionDescriptor) is received.

   * RemoteConnectionDescriptor:  Supplied by the Call Agent to convey
     the media parameters supported by the other side of the connection.
     When supplied, the gateway MUST conform to these media parameters
     until either the connection is deleted, or a ModifyConnection
     command with new media parameters (LocalConnectionOptions or
     RemoteConnectionDescriptor) is received.

   * LocalConnectionDescriptor:  Supplied by the gateway to the Call
     Agent to convey the media parameters it supports for the
     connection. When supplied, the gateway MUST honor the media
     parameters until either the connection is deleted, or the gateway
     issues a new LocalConnectionDescriptor for that connection.




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   In determining which codec(s) to provide in the
   LocalConnectionDescriptor, there are three lists of codecs that a
   gateway needs to consider:

   * A list of codecs allowed by the LocalConnectionOptions in the
     current command (either explicitly by encoding method or implicitly
     by bandwidth and/or packetization period).

   * A list of codecs in the RemoteConnectionDescriptor in the current
     command.

   * An internal list of codecs that the gateway can support for the
     connection. A gateway MAY support one or more codecs for a given
     connection.

   Codec selection (including all relevant media parameters) can then be
   described by the following steps:

   1. An approved list of codecs is formed by taking the intersection of
      the internal list of codecs and codecs allowed by the
      LocalConnectionOptions. If LocalConnectionOptions were not
      provided in the current command, the approved list of codecs thus
      contains the internal list of codecs.

   2. If the approved list of codecs is empty, a codec negotiation
      failure has occurred and an error response is generated (error
      code 534 - codec negotiation failure, is RECOMMENDED).

   3. Otherwise, a negotiated list of codecs is formed by taking the
      intersection of the approved list of codecs and codecs allowed by
      the RemoteConnectionDescriptor. If a RemoteConnectionDescriptor
      was not provided in the current command, the negotiated list of
      codecs thus contains the approved list of codecs.

   4. If the negotiated list of codecs is empty, a codec negotiation
      failure has occurred and an error response is generated (error
      code 534 - codec negotiation failure, is RECOMMENDED).

   5. Otherwise, codec negotiation has succeeded, and the negotiated
      list of codecs is returned in the LocalConnectionDescriptor.

   Note that both LocalConnectionOptions and the
   RemoteConnectionDescriptor can contain a list of codecs ordered by
   preference. When both are supplied in the current command, the
   gateway MUST adhere to the preferences provided in the
   LocalConnectionOptions.





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2.7 Resource Reservations

   The gateways can be instructed to perform a reservation, for example
   using RSVP, on a given connection. When a reservation is needed, the
   call agent will specify the reservation profile to be used, which is
   either "controlled load" or "guaranteed service". The absence of
   reservation can be indicated by asking for the "best effort" service,
   which is the default value of this parameter in a CreateConnection
   command. For a ModifyConnection command, the default is simply to
   retain the current value. When reservation has been asked on a
   connection, the gateway will:

   * start emitting RSVP "PATH" messages if the connection is in "send-
     only", "send-receive", "conference", "network loop back" or
     "network continuity test" mode (if a suitable remote connection
     descriptor has been received,).

   * start emitting RSVP "RESV" messages as soon as it receives "PATH"
     messages if the connection is in "receive-only", "send-receive",
     "conference", "network loop back" or "network continuity test"
     mode.

   The RSVP filters will be deduced from the characteristics of the
   connection. The RSVP resource profiles will be deduced from the
   connection's codecs, bandwidth and packetization period.

3. Media Gateway Control Protocol

   The Media Gateway Control Protocol (MGCP) implements the media
   gateway control interface as a set of transactions. The transactions
   are composed of a command and a mandatory response. There are nine
   commands:

   * EndpointConfiguration

   * CreateConnection

   * ModifyConnection

   * DeleteConnection

   * NotificationRequest

   * Notify

   * AuditEndpoint

   * AuditConnection



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   * RestartInProgress

   The first five commands are sent by the Call Agent to a gateway. The
   Notify command is sent by the gateway to the Call Agent. The gateway
   may also send a DeleteConnection as defined in Section 2.3.8.  The
   Call Agent may send either of the Audit commands to the gateway, and
   the gateway may send a RestartInProgress command to the Call Agent.

3.1 General Description

   All commands are composed of a Command header, optionally followed by
   a session description.

   All responses are composed of a Response header, optionally followed
   by session description information.

   Headers and session descriptions are encoded as a set of text lines,
   separated by a carriage return and line feed character (or,
   optionally, a single line-feed character). The session descriptions
   are preceded by an empty line.

   MGCP uses a transaction identifier to correlate commands and
   responses. The transaction identifier is encoded as a component of
   the command header and repeated as a component of the response header
   (see sections 3.2.1.2 and 3.3).

   Note that an ABNF grammar for MGCP is provided in Appendix A.
   Commands and responses SHALL be encoded in accordance with the
   grammar, which, per RFC 2234, is case-insensitive except for the SDP
   part.  Similarly, implementations SHALL be capable of decoding
   commands and responses that follow the grammar.  Additionally, it is
   RECOMMENDED that implementations tolerate additional linear white
   space.

   Some productions allow for use of quoted strings, which can be
   necessary to avoid syntax problems.  Where the quoted string form is
   used, the contents will be UTF-8 encoded [20], and the actual value
   provided is the unquoted string (UTF-8 encoded).  Where both a quoted
   and unquoted string form is allowed, either form can be used provided
   it does not otherwise violate the grammar.

   In the following, we provide additional detail on the format of MGCP
   commands and responses.








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3.2 Command Header

   The command header is composed of:

   *  A command line, identifying the requested action or verb, the
      transaction identifier, the endpoint towards which the action is
      requested, and the MGCP protocol version,

   *  A set of zero or more parameter lines, composed of a parameter
      name followed by a parameter value.

   Unless otherwise noted or dictated by other referenced standards
   (e.g., SDP), each component in the command header is case
   insensitive.  This goes for verbs as well as parameters and values,
   and hence all comparisons MUST treat upper and lower case as well as
   combinations of these as being equal.

3.2.1 Command Line

   The command line is composed of:

   * The name of the requested verb,

   * The identification of the transaction,

   * The name of the endpoint(s) that are to execute the command (in
     notifications or restarts, the name of the endpoint(s) that is
     issuing the command),

   * The protocol version.

     These four items are encoded as strings of printable ASCII
     characters, separated by white spaces, i.e., the ASCII space (0x20)
     or tabulation (0x09) characters.  It is RECOMMENDED to use exactly
     one ASCII space separator.  However, MGCP entities MUST be able to
     parse messages with additional white space characters.















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3.2.1.1 Coding of the Requested Verb

   The verbs that can be requested are encoded as four letter upper or
   lower case ASCII codes (comparisons SHALL be case insensitive) as
   defined in the following table:

                  -----------------------------
                 |       Verb           | Code |
                 |----------------------|------|
                 | EndpointConfiguration| EPCF |
                 | CreateConnection     | CRCX |
                 | ModifyConnection     | MDCX |
                 | DeleteConnection     | DLCX |
                 | NotificationRequest  | RQNT |
                 | Notify               | NTFY |
                 | AuditEndpoint        | AUEP |
                 | AuditConnection      | AUCX |
                 | RestartInProgress    | RSIP |
                  -----------------------------

   The transaction identifier is encoded as a string of up to 9 decimal
   digits.  In the command line, it immediately follows the coding of
   the verb.

   New verbs may be defined in further versions of the protocol.  It may
   be necessary, for experimentation purposes, to use new verbs before
   they are sanctioned in a published version of this protocol.
   Experimental verbs MUST be identified by a four letter code starting
   with the letter X, such as for example XPER.

3.2.1.2 Transaction Identifiers

   MGCP uses a transaction identifier to correlate commands and
   responses.  A gateway supports two separate transaction identifier
   name spaces:

   * a transaction identifier name space for sending transactions, and

   * a transaction identifier name space for receiving transactions.

   At a minimum, transaction identifiers for commands sent to a given
   gateway MUST be unique for the maximum lifetime of the transactions
   within the collection of Call Agents that control that gateway.
   Thus, regardless of the sending Call Agent, gateways can always
   detect duplicate transactions by simply examining the transaction
   identifier.  The coordination of these transaction identifiers
   between Call Agents is outside the scope of this specification
   though.



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   Transaction identifiers for all commands sent from a given gateway
   MUST be unique for the maximum lifetime of the transactions
   regardless of which Call Agent the command is sent to.  Thus, a Call
   Agent can always detect a duplicate transaction from a gateway by the
   combination of the domain-name of the endpoint and the transaction
   identifier.

   The transaction identifier is encoded as a string of up to nine
   decimal digits.  In the command lines, it immediately follows the
   coding of the verb.

   Transaction identifiers have values between 1 and 999,999,999 (both
   included).  Transaction identifiers SHOULD NOT use any leading
   zeroes, although equality is based on numerical value, i.e., leading
   zeroes are ignored.  An MGCP entity MUST NOT reuse a transaction
   identifier more quickly than three minutes after completion of the
   previous command in which the identifier was used.

3.2.1.3 Coding of the Endpoint Identifiers and Entity Names

   The endpoint identifiers and entity names are encoded as case
   insensitive e-mail addresses, as defined in RFC 821, although with
   some syntactic restrictions on the local part of the name.
   Furthermore, both the local endpoint name part and the domain name
   part can each be up to 255 characters.  In these addresses, the
   domain name identifies the system where the endpoint is attached,
   while the left side identifies a specific endpoint or entity on that
   system.

   Examples of such addresses are:

    ------------------------------------------------------------------
   | hrd4/56@gw23.example.net     |  Circuit number 56 in             |
   |                              |  interface "hrd4" of the Gateway  |
   |                              |  23 of the "Example" network      |
   | Call-agent@ca.example.net    |  Call Agent for the               |
   |                              |  "example" network                |
   | Busy-signal@ann12.example.net|  The "busy signal" virtual        |
   |                              |  endpoint in the announcement     |
   |                              |  server number 12.                |
    ------------------------------------------------------------------

   The name of a notified entity is expressed with the same syntax, with
   the possible addition of a port number as in:

      Call-agent@ca.example.net:5234





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   In case the port number is omitted from the notified entity, the
   default MGCP Call Agent port (2727) MUST be used.

3.2.1.4 Coding of the Protocol Version

   The protocol version is coded as the keyword MGCP followed by a white
   space and the version number, and optionally followed by a profile
   name.  The version number is composed of a major version, coded by a
   decimal number, a dot, and a minor version number, coded as a decimal
   number.  The version described in this document is version 1.0.

   The profile name, if present, is represented by white-space separated
   strings of visible (printable) characters extending to the end of the
   line.  Profile names may be defined for user communities who want to
   apply restrictions or other profiling to MGCP.

   In the initial messages, the version will be coded as:

      MGCP 1.0

   An entity that receives a command with a protocol version it does not
   support, MUST respond with an error (error code 528 - incompatible
   protocol version, is RECOMMENDED).  Note that this applies to
   unsupported profiles as well.

3.2.2 Parameter Lines

   Parameter lines are composed of a parameter name, which in most cases
   is composed of one or two characters, followed by a colon, optional
   white space(s) and the parameter value.  The parameters that can be
   present in commands are defined in the following table:




















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    ------------------------------------------------------------------
   |Parameter name        | Code |  Parameter value                   |
   |----------------------|------|------------------------------------|
   |BearerInformation     |   B  |  See description (3.2.2.1).        |
   |CallId                |   C  |  See description (3.2.2.2).        |
   |Capabilities          |   A  |  See description (3.2.2.3).        |
   |ConnectionId          |   I  |  See description (3.2.2.5).        |
   |ConnectionMode        |   M  |  See description (3.2.2.6).        |
   |ConnectionParameters  |   P  |  See description (3.2.2.7).        |
   |DetectEvents          |   T  |  See description (3.2.2.8).        |
   |DigitMap              |   D  |  A text encoding of a digit map.   |
   |EventStates           |   ES |  See description (3.2.2.9).        |
   |LocalConnectionOptions|   L  |  See description (3.2.2.10).       |
   |MaxMGCPDatagram       |   MD |  See description (3.2.2.11).       |
   |NotifiedEntity        |   N  |  An identifier, in RFC 821 format, |
   |                      |      |  composed of an arbitrary string   |
   |                      |      |  and of the domain name of the     |
   |                      |      |  requesting entity, possibly com-  |
   |                      |      |  pleted by a port number, as in:   |
   |                      |      |    Call-agent@ca.example.net:5234  |
   |                      |      |  See also Section 3.2.1.3.         |
   |ObservedEvents        |   O  |  See description (3.2.2.12).       |
   |PackageList           |   PL |  See description (3.2.2.13).       |
   |QuarantineHandling    |   Q  |  See description (3.2.2.14).       |
   |ReasonCode            |   E  |  A string with a 3 digit integer   |
   |                      |      |  optionally followed by a set of   |
   |                      |      |  arbitrary characters (3.2.2.15).  |
   |RequestedEvents       |   R  |  See description (3.2.2.16).       |
   |RequestedInfo         |   F  |  See description (3.2.2.17).       |
   |RequestIdentifier     |   X  |  See description (3.2.2.18).       |
   |ResponseAck           |   K  |  See description (3.2.2.19).       |
   |RestartDelay          |   RD |  A number of seconds, encoded as   |
   |                      |      |  a decimal number.                 |
   |RestartMethod         |   RM |  See description (3.2.2.20).       |
   |SecondConnectionId    |   I2 |  Connection Id.                    |
   |SecondEndpointId      |   Z2 |  Endpoint Id.                      |
   |SignalRequests        |   S  |  See description (3.2.2.21).       |
   |SpecificEndPointId    |   Z  |  An identifier, in RFC 821 format, |
   |                      |      |  composed of an arbitrary string,  |
   |                      |      |  followed by an "@" followed by    |
   |                      |      |  the domain name of the gateway to |
   |                      |      |  which this endpoint is attached.  |
   |                      |      |  See also Section 3.2.1.3.         |
   |----------------------|------|------------------------------------|







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   |RemoteConnection-     |   RC |  Session Description.              |
   |         Descriptor   |      |                                    |
   |LocalConnection-      |   LC |  Session Description.              |
   |         Descriptor   |      |                                    |
    ------------------------------------------------------------------

   The parameters are not necessarily present in all commands.  The
   following table provides the association between parameters and
   commands.  The letter M stands for mandatory, O for optional and F
   for forbidden.  Unless otherwise specified, a parameter MUST NOT be
   present more than once.








































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    ------------------------------------------------------------------
   | Parameter name      | EP | CR | MD | DL | RQ | NT | AU | AU | RS |
   |                     | CF | CX | CX | CX | NT | FY | EP | CX | IP |
   |---------------------|----|----|----|----|----|----|----|----|----|
   | BearerInformation   |  O*|  O |  O |  O |  O |  F |  F |  F |  F |
   | CallId              |  F |  M |  M |  O |  F |  F |  F |  F |  F |
   | Capabilities        |  F |  F |  F |  F |  F |  F |  F |  F |  F |
   | ConnectionId        |  F |  F |  M |  O |  F |  F |  F |  M |  F |
   | ConnectionMode      |  F |  M |  O |  F |  F |  F |  F |  F |  F |
   | Connection-         |  F |  F |  F |  O*|  F |  F |  F |  F |  F |
   |   Parameters        |    |    |    |    |    |    |    |    |    |
   | DetectEvents        |  F |  O |  O |  O |  O |  F |  F |  F |  F |
   | DigitMap            |  F |  O |  O |  O |  O |  F |  F |  F |  F |
   | EventStates         |  F |  F |  F |  F |  F |  F |  F |  F |  F |
   | LocalConnection-    |  F |  O |  O |  F |  F |  F |  F |  F |  F |
   |            Options  |    |    |    |    |    |    |    |    |    |
   | MaxMGCPDatagram     |  F |  F |  F |  F |  F |  F |  F |  F |  F |
   | NotifiedEntity      |  F |  O |  O |  O |  O |  O |  F |  F |  F |
   | ObservedEvents      |  F |  F |  F |  F |  F |  M |  F |  F |  F |
   | PackageList         |  F |  F |  F |  F |  F |  F |  F |  F |  F |
   | QuarantineHandling  |  F |  O |  O |  O |  O |  F |  F |  F |  F |
   | ReasonCode          |  F |  F |  F |  O |  F |  F |  F |  F |  O |
   | RequestedEvents     |  F |  O |  O |  O |  O*|  F |  F |  F |  F |
   | RequestIdentifier   |  F |  O*|  O*|  O*|  M |  M |  F |  F |  F |
   | RequestedInfo       |  F |  F |  F |  F |  F |  F |  O |  M |  F |
   | ResponseAck         |  O |  O |  O |  O |  O |  O |  O |  O |  O |
   | RestartDelay        |  F |  F |  F |  F |  F |  F |  F |  F |  O |
   | RestartMethod       |  F |  F |  F |  F |  F |  F |  F |  F |  M |
   | SecondConnectionId  |  F |  F |  F |  F |  F |  F |  F |  F |  F |
   | SecondEndpointId    |  F |  O |  F |  F |  F |  F |  F |  F |  F |
   | SignalRequests      |  F |  O |  O |  O |  O*|  F |  F |  F |  F |
   | SpecificEndpointId  |  F |  F |  F |  F |  F |  F |  F |  F |  F |
   |---------------------|----|----|----|----|----|----|----|----|----|
   | RemoteConnection-   |  F |  O |  O |  F |  F |  F |  F |  F |  F |
   |          Descriptor |    |    |    |    |    |    |    |    |    |
   | LocalConnection-    |  F |  F |  F |  F |  F |  F |  F |  F |  F |
   |          Descriptor |    |    |    |    |    |    |    |    |    |
    ------------------------------------------------------------------

   Notes (*):

   * The BearerInformation parameter is only conditionally optional as
     explained in Section 2.3.2.

   * The RequestIdentifier parameter is optional in connection creation,
     modification and deletion commands, however it becomes REQUIRED if
     the command contains an encapsulated notification request.




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   * The RequestedEvents and SignalRequests parameters are optional in
     the NotificationRequest.  If these parameters are omitted the
     corresponding lists will be considered empty.

   * The ConnectionParameters parameter is only valid in a
     DeleteConnection request sent by the gateway.

   The set of parameters can be extended in two different ways:

   * Package Extension Parameters (preferred)

   * Vendor Extension Parameters

   Package Extension Parameters are defined in packages which provides
   the following benefits:

   * a registration mechanism (IANA) for the package name.

   * a separate name space for the parameters.

   * a convenient grouping of the extensions.

   * a simple way to determine support for them through auditing.

   The package extension mechanism is the preferred extension method.

   Vendor extension parameters can be used if implementers need to
   experiment with new parameters, for example when developing a new
   application of MGCP.  Vendor extension parameters MUST be identified
   by names that start with the string "X-" or "X+", such as for
   example:

      X-Flower: Daisy

   Parameter names that start with "X+" are critical parameter
   extensions.  An MGCP entity that receives a critical parameter
   extension that it cannot understand MUST refuse to execute the
   command.  It SHOULD respond with error code 511 (unrecognized
   extension).

   Parameter names that start with "X-" are non-critical parameter
   extensions.  An MGCP entity that receives a non-critical parameter
   extension that it cannot understand MUST simply ignore that
   parameter.







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   Note that vendor extension parameters use an unmanaged name space,
   which implies a potential for name clashing.  Vendors are
   consequently encouraged to include some vendor specific string, e.g.,
   vendor name, in their vendor extensions.

3.2.2.1 BearerInformation

   The values of the bearer information are encoded as a comma separated
   list of attributes, which are represented by an attribute name, and
   possibly followed by a colon and an attribute value.

   The only attribute that is defined is the "encoding" (code "e")
   attribute, which MUST have one of the values "A" (A-law) or "mu"
   (mu-law).

   An example of bearer information encoding is:

      B: e:mu

   The set of bearer information attributes may be extended through
   packages.

3.2.2.2 CallId

   The Call Identifier is encoded as a hexadecimal string, at most 32
   characters in length.  Call Identifiers are compared as strings
   rather than numerical values.

3.2.2.3 Capabilities

   Capabilities inform the Call Agent about endpoints' capabilities when
   audited.  The encoding of capabilities is based on the Local
   Connection Options encoding for the parameters that are common to
   both, although a different parameter line code is used ("A").  In
   addition, capabilities can also contain a list of supported packages,
   and a list of supported modes.

   The parameters used are:

   A list of supported codecs.
      The following parameters will apply to all codecs specified in
      this list.  If there is a need to specify that some parameters,
      such as e.g., silence suppression, are only compatible with some
      codecs, then the gateway will return several Capability
      parameters; one for each set of codecs.

   Packetization Period:
      A range may be specified.



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   Bandwidth:
      A range corresponding to the range for packetization periods may
      be specified (assuming no silence suppression).  If absent, the
      values will be deduced from the codec type.

   Echo Cancellation:
      "on" if echo cancellation is supported, "off" otherwise.  The
      default is support.

   Silence Suppression:
      "on" if silence suppression is supported for this codec, "off"
      otherwise.  The default is support.

   Gain Control:
      "0" if gain control is not supported, all other values indicate
      support for gain control.  The default is support.

   Type of Service:
      The value "0" indicates no support for type of service, all other
      values indicate support for type of service.  The default is
      support.

   Resource Reservation Service:
      The parameter indicates the reservation services that are
      supported, in addition to best effort.  The value "g" is encoded
      when the gateway supports both the guaranteed and the controlled
      load service, "cl" when only the controlled load service is
      supported.  The default is "best effort".

   Encryption Key:
      Encoding any value indicates support for encryption.  Default is
      no support which is implied by omitting the parameter.

   Type of network:
      The keyword "nt", followed by a colon and a semicolon separated
      list of supported network types.  This parameter is optional.

   Packages:
      The packages supported by the endpoint encoded as the keyword "v",
      followed by a colon and a character string.  If a list of values
      is specified, these values will be separated by a semicolon.  The
      first value specified will be the default package for the
      endpoint.

   Modes:
      The modes supported by this endpoint encoded as the keyword "m",
      followed by a colon and a semicolon-separated list of supported
      connection modes for this endpoint.



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   Lack of support for a capability can also be indicated by excluding
   the parameter from the capability set.

   An example capability is:

     A: a:PCMU;G728, p:10-100, e:on, s:off, t:1, v:L,
                              m:sendonly;recvonly;sendrecv;inactive

   The carriage return above is included for formatting reasons only and
   is not permissible in a real implementation.

   If multiple capabilities are to be returned, each will be returned as
   a separate capability line.

   Since Local Connection Options can be extended, the list of
   capability parameters can also be extended.  Individual extensions
   may define how they are reported as capabilities.  If no such
   definition is provided, the following defaults apply:

   * Package Extension attributes:  The individual attributes are not
     reported.  Instead, the name of the package is simply reported in
     the list of supported packages.

   * Vendor Extension attributes:  The name of the attribute is reported
     without any value.

   * Other Extension attributes:  The name of the attribute is reported
     without any value.

3.2.2.4 Coding of Event Names

   Event names are composed of an optional package name, separated by a
   slash (/) from the name of the actual event (see Section 2.1.7).  The
   wildcard character star ("*") can be use to refer to all packages.
   The event name can optionally be followed by an at sign (@) and the
   identifier of a connection (possibly using a wildcard) on which the
   event should be observed.  Event names are used in the
   RequestedEvents, SignalRequests, ObservedEvents, DetectEvents, and
   EventStates parameters.

   Events and signals may be qualified by parameters defined for the
   event/signal.  Such parameters may be enclosed in double-quotes (in
   fact, some parameters MUST be enclosed in double-quotes due to
   syntactic restrictions) in which case they are UTF-8 encoded [20].

   The parameter name "!" (exclamation point) is reserved for future use
   for both events and signals.




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   Each signal has one of the following signal-types associated with it:
   On/Off (OO), Time-out (TO), or Brief (BR).  (These signal types are
   specified in the package definitions, and are not present in the
   messages.) On/Off signals can be parameterized with a "+" to turn the
   signal on, or a "-" to turn the signal off.  If an on/off signal is
   not parameterized, the signal is turned on.  Both of the following
   will turn the vmwi signal (from the line package "L") on:

      L/vmwi(+)
      L/vmwi

   In addition to "!", "+" and "-", the signal parameter "to" is
   reserved as well.  It can be used with Time-Out signals to override
   the default time-out value for the current request.  A decimal value
   in milliseconds will be supplied.  The individual signal and/or
   package definition SHOULD indicate if this parameter is supported for
   one or more TO signals in the package.  If not indicated, TO signals
   in package version zero are assumed to not support it, whereas TO
   signals in package versions one or higher are assumed to support it.
   By default, a supplied time-out value MAY be rounded to the nearest
   non-zero value divisible by 1000, i.e., whole second.  The individual
   signal and/or package definition may define other rounding rules. All
   new package and TO signal definitions are strongly encouraged to
   support the "to" signal parameter.

   The following example illustrates how the "to" parameter can be used
   to apply a signal for 6 seconds:

      L/rg(to=6000)
      L/rg(to(6000))

   The following are examples of event names:

      -----------------------------------------------------------
     | L/hu        |   on-hook transition, in the line package   |
     | F/0         |   digit 0 in the MF package                 |
     | hf          |   Hook-flash, assuming that the line package|
     |             |   is the default package for the endpoint.  |
     | G/rt@0A3F58 |   Ring back signal on connection "0A3F58"   |
      -----------------------------------------------------------

   In addition, the range and wildcard notation of events can be used,
   instead of individual names, in the RequestedEvents and DetectEvents
   parameters.  The event code "all" is reserved and refers to all
   events or signals in a package.  The star sign ("*") can be used to
   denote "all connections", and the dollar sign ("$") can be used to
   denote the "current" connection (see Section 2.1.7 for details).




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   The following are examples of such notations:

      ---------------------------------------------------------
     | M/[0-9]   |   Digits 0 to 9 in the MF package.          |
     | hf        |   Hook-flash, assuming that the line package|
     |           |   is a default package for the endpoint.    |
     | [0-9*#A-D]|   All digits and letters in the DTMF        |
     |           |   packages (default for endpoint).          |
     | T/all     |   All events in the trunk package.          |
     | R/qa@*    |   The quality alert event on all            |
     |           |   connections.                              |
     | G/rt@$    |   Ringback on current connection.           |
      ---------------------------------------------------------

3.2.2.5 ConnectionId

   The Connection Identifier is encoded as a hexadecimal string, at most
   32 characters in length.  Connection Identifiers are compared as
   strings rather than numerical values.

3.2.2.6 ConnectionMode

   The connection mode describes the mode of operation of the
   connection.  The possible values are:

      --------------------------------------------------------
     |    Mode     |               Meaning                    |
     |-------------|------------------------------------------|
     | M: sendonly |  The gateway should only send packets    |
     | M: recvonly |  The gateway should only receive packets |
     | M: sendrecv |  The gateway should send                 |
     |             |  and receive packets                     |
     | M: confrnce |  The gateway should place                |
     |             |  the connection in conference mode       |
     | M: inactive |  The gateway should neither              |
     |             |  send nor receive packets                |
     | M: loopback |  The gateway should place                |
     |             |  the circuit in loopback mode.           |
     | M: conttest |  The gateway should place                |
     |             |  the circuit in test mode.               |
     | M: netwloop |  The gateway should place                |
     |             |  the connection in network loopback mode.|
     | M: netwtest |  The gateway should place the connection |
     |             |  in network continuity test mode.        |
      --------------------------------------------------------






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   Note that irrespective of the connection mode, signals applied to the
   connection will still result in packets being sent (see Section
   2.3.1).

   The set of connection modes can be extended through packages.

3.2.2.7 ConnectionParameters

   Connection parameters are encoded as a string of type and value
   pairs, where the type is either a two-letter identifier of the
   parameter or an extension type, and the value a decimal integer.
   Types are separated from value by an '=' sign.  Parameters are
   separated from each other by a comma.  Connection parameter values
   can contain up to nine digits.  If the maximum value is reached, the
   counter is no longer updated, i.e., it doesn't wrap or overflow.

   The connection parameter types are specified in the following table:

    -----------------------------------------------------------------
   | Connection parameter| Code |  Connection parameter              |
   | name                |      |  value                             |
   |---------------------|------|------------------------------------|
   | Packets sent        |  PS  |  The number of packets that        |
   |                     |      |  were sent on the connection.      |
   | Octets sent         |  OS  |  The number of octets that         |
   |                     |      |  were sent on the connection.      |
   | Packets received    |  PR  |  The number of packets that        |
   |                     |      |  were received on the connection.  |
   | Octets received     |  OR  |  The number of octets that         |
   |                     |      |  were received on the connection.  |
   | Packets lost        |  PL  |  The number of packets that        |
   |                     |      |  were lost on the connection       |
   |                     |      |  as deduced from gaps in the       |
   |                     |      |  RTP sequence number.              |
   | Jitter              |  JI  |  The average inter-packet arrival  |
   |                     |      |  jitter, in milliseconds,          |
   |                     |      |  expressed as an integer number.   |
   | Latency             |  LA  |  Average latency, in milliseconds, |
   |                     |      |  expressed as an integer number.   |
    -----------------------------------------------------------------

   The set of connection parameters can be extended in two different
   ways:

   * Package Extension Parameters (preferred)

   * Vendor Extension Parameters




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   Package Extension Connection Parameters are defined in packages which
   provides the following benefits:

   * A registration mechanism (IANA) for the package name.

   * A separate name space for the parameters.

   * A convenient grouping of the extensions.

   * A simple way to determine support for them through auditing.

   The package extension mechanism is the preferred extension method.

   Vendor extension parameters names are composed of the string "X-"
   followed by a two or more letters extension parameter name.

   Call agents that receive unrecognized package or vendor connection
   parameter extensions SHALL silently ignore these parameters.

   An example of connection parameter encoding is:

      P: PS=1245, OS=62345, PR=0, OR=0, PL=0, JI=0, LA=48

3.2.2.8 DetectEvents

   The DetectEvents parameter is encoded as a comma separated list of
   events (see Section 3.2.2.4), such as for example:

      T: L/hu,L/hd,L/hf,D/[0-9#*]

   It should be noted, that no actions can be associated with the
   events, however event parameters may be provided.

3.2.2.9 EventStates

   The EventStates parameter is encoded as a comma separated list of
   events (see Section 3.2.2.4), such as for example:

      ES: L/hu

   It should be noted, that no actions can be associated with the
   events, however event parameters may be provided.

3.2.2.10 LocalConnectionOptions

   The local connection options describe the operational parameters that
   the Call Agent provides to the gateway in connection handling
   commands.  These include:



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   * The allowed codec(s), encoded as the keyword "a", followed by a
     colon and a character string.  If the Call Agent specifies a list
     of values, these values will be separated by a semicolon.  For RTP,
     audio codecs SHALL be specified by using encoding names defined in
     the RTP AV Profile [4] or its replacement, or by encoding names
     registered with the IANA.  Non-audio media registered as a MIME
     type MUST use the "<MIME type>/<MIME subtype>" form, as in
     "image/t38".

   * The packetization period in milliseconds, encoded as the keyword
     "p", followed by a colon and a decimal number.  If the Call Agent
     specifies a range of values, the range will be specified as two
     decimal numbers separated by a hyphen (as specified for the "ptime"
     parameter for SDP).

   * The bandwidth in kilobits per second (1000 bits per second),
     encoded as the keyword "b", followed by a colon and a decimal
     number.  If the Call Agent specifies a range of values, the range
     will be specified as two decimal numbers separated by a hyphen.

   * The type of service parameter, encoded as the keyword "t", followed
     by a colon and the value encoded as two hexadecimal digits.  When
     the connection is transmitted over an IP network, the parameters
     encode the 8-bit type of service value parameter of the IP header
     (a.k.a. DiffServ field).  The left-most "bit" in the parameter
     corresponds to the least significant bit in the IP header.

   * The echo cancellation parameter, encoded as the keyword "e",
     followed by a colon and the value "on" or "off".

   * The gain control parameter, encoded as the keyword "gc", followed
     by a colon and a value which can be either the keyword "auto" or a
     decimal number (positive or negative) representing the number of
     decibels of gain.

   * The silence suppression parameter, encoded as the keyword "s",
     followed by a colon and the value "on" or "off".

   * The resource reservation parameter, encoded as the keyword "r",
     followed by a colon and the value "g" (guaranteed service), "cl"
     (controlled load) or "be" (best effort).

   * The encryption key, encoded as the keyword "k" followed by a colon
     and a key specification, as defined for the parameter "K" in SDP
     (RFC 2327).






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   * The type of network, encoded as the keyword "nt" followed by a
     colon and the type of network encoded as the keyword "IN"
     (internet), "ATM", "LOCAL" (for a local connection), or possibly
     another type of network registered with the IANA as per SDP (RFC
     2327).

   * The resource reservation parameter, encoded as the keyword "r",
     followed by a colon and the value "g" (guaranteed service), "cl"
     (controlled load) or "be" (best effort).

   The encoding of the first three attributes, when they are present,
   will be compatible with the SDP and RTP profiles.  Note that each of
   the attributes is optional.  When several attributes are present,
   they are separated by a comma.

   Examples of local connection options are:

      L: p:10, a:PCMU
      L: p:10, a:G726-32
      L: p:10-20, b:64
      L: b:32-64, e:off

   The set of Local Connection Options attributes can be extended in
   three different ways:

   * Package Extension attributes (preferred)

   * Vendor Extension attributes

   * Other Extension attributes

   Package Extension Local Connection Options attributes are defined in
   packages which provides the following benefits:

   * A registration mechanism (IANA) for the package name.

   * A separate name space for the attributes.

   * A convenient grouping of the extensions.

   * A simple way to determine support for them through auditing.

   The package extension mechanism is the preferred extension method.








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   Vendor extension attributes are composed of an attribute name, and
   possibly followed by a colon and an attribute value.  The attribute
   name MUST start with the two characters "x+", for a mandatory
   extension, or "x-", for a non-mandatory extension.  If a gateway
   receives a mandatory extension attribute that it does not recognize,
   it MUST reject the command (error code 525 - unknown extension in
   LocalConnectionOptions, is RECOMMENDED).

   Note that vendor extension attributes use an unmanaged name space,
   which implies a potential for name clashing.  Vendors are
   consequently encouraged to include some vendor specific string, e.g.,
   vendor name, in their vendor extensions.

   Finally, for backwards compatibility with some existing
   implementations, MGCP allows for other extension attributes as well
   (see grammar in Appendix A).  Note however, that these attribute
   extensions do not provide the package extension attribute benefits.
   Use of this mechanism for new extensions is discouraged.

3.2.2.11 MaxMGCPDatagram

   The MaxMGCPDatagram can only be used for auditing, i.e., it is a
   valid RequestedInfo code and can be provided as a response parameter.

   In responses, the MaxMGCPDatagram value is encoded as a string of up
   to nine decimal digits -- leading zeroes are not permitted.  The
   following example illustrates the use of this parameter:

      MD: 8100

3.2.2.12 ObservedEvents

   The observed events parameter provides the list of events that have
   been observed.  The event codes are the same as those used in the
   NotificationRequest.  Events that have been accumulated according to
   the digit map may be grouped in a single string, however such
   practice is discouraged; they SHOULD be reported as lists of isolated
   events if other events were detected during the digit accumulation.
   Examples of observed events are:

      O: L/hu
      O: D/8295555T
      O: D/8,D/2,D/9,D/5,D/5,L/hf,D/5,D/5,D/T
      O: L/hf, L/hf, L/hu







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3.2.2.13 PackageList

   The Package List can only be used for auditing, i.e., it is a valid
   RequestedInfo code and can be provided as a response parameter.

   The response parameter will consist of a comma separated list of
   packages supported.  The first package returned in the list is the
   default package.  Each package in the list consists of the package
   name followed by a colon, and the highest version number of the
   package supported.

   An example of a package list is:

     PL: L:1,G:1,D:0,FOO:2,T:1

   Note that for backwards compatibility, support for this parameter is
   OPTIONAL.

3.2.2.14 QuarantineHandling

   The quarantine handling parameter contains a list of comma separated
   keywords:

   * The keyword "process" or "discard" to indicate the treatment of
     quarantined and observed events.  If neither "process" or "discard"
     is present, "process" is assumed.

   * The keyword "step" or "loop" to indicate whether at most one
     notification per NotificationRequest is allowed, or whether
     multiple notifications per NotificationRequest are allowed.  If
     neither "step" nor "loop" is present, "step" is assumed.

   The following values are valid examples:

      Q: loop
      Q: process
      Q: loop,discard

3.2.2.15 ReasonCode

   Reason codes are three-digit numeric values.  The reason code is
   optionally followed by a white space and commentary, e.g.:

      E: 900 Endpoint malfunctioning

   A list of reason codes can be found in Section 2.5.

   The set of reason codes can be extended through packages.



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3.2.2.16 RequestedEvents

   The RequestedEvents parameter provides the list of events that are
   requested.  The event codes are described in Section 3.2.2.4.

   Each event can be qualified by a requested action, or by a list of
   actions.  The actions, when specified, are encoded as a list of
   keywords, enclosed in parenthesis and separated by commas.  The codes
   for the various actions are:

                -------------------------------------
               |          Action              | Code |
               |------------------------------|------|
               | Notify immediately           |  N   |
               | Accumulate                   |  A   |
               | Treat according to digit map |  D   |
               | Swap                         |  S   |
               | Ignore                       |  I   |
               | Keep Signal(s) active        |  K   |
               | Embedded Notification Request|  E   |
                -------------------------------------

   When no action is specified, the default action is to notify the
   event.  This means that, for example, ft and ft(N) are equivalent.
   Events that are not listed are ignored (unless they are persistent).

   The digit-map action SHOULD only be specified for the digits, letters
   and interdigit timers in packages that define the encoding of digits,
   letters, and timers (including extension digit map letters).

   The requested events list is encoded on a single line, with
   event/action groups separated by commas.  Examples of RequestedEvents
   encodings are:

      R: L/hu(N), L/hf(S,N)
      R: L/hu(N), D/[0-9#T](D)

   In the case of the "Embedded Notification Request" action, the
   embedded notification request parameters are encoded as a list of up
   to three parameter groups separated by commas.  Each group starts by
   a one letter identifier, followed by a list of parameters enclosed
   between parentheses. The first optional parameter group, identified
   by the letter "R", is the value of the embedded RequestedEvents
   parameter.  The second optional group, identified by the letter "S",
   is the embedded value of the SignalRequests parameter.  The third






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   optional group, identified by the letter "D", is the embedded value
   of the DigitMap.  (Note that some existing implementations and
   profiles may encode these three components in a different order.
   Implementers are encouraged to accept such encodings, but they SHOULD
   NOT generate them.)

   If the RequestedEvents parameter is not present, the parameter will
   be set to a null value.  If the SignalRequests parameter is not
   present, the parameter will be set to a null value.  If the DigitMap
   is absent, the current value MUST be used.  The following are valid
   examples of embedded requests:

      R: L/hd(E(R(D/[0-9#T](D),L/hu(N)),S(L/dl),D([0-9].[#T])))
      R: L/hd(E(R(D/[0-9#T](D),L/hu(N)),S(L/dl)))

   Some events can be qualified by additional event parameters.  Such
   event parameters will be separated by commas and enclosed within
   parentheses.  Event parameters may be enclosed in double-quotes (in
   fact, some event parameters MUST be enclosed in double-quotes due to
   syntactic restrictions), in which case the quoted string itself is
   UTF-8 encoded.  Please refer to Section 3.2.2.4 for additional detail
   on event parameters.

   The following example shows the foobar event with an event parameter
   "epar":

      R: X/foobar(N)(epar=2)

   Notice that the Action was included even though it is the default
   Notify action - this is required by the grammar.

3.2.2.17 RequestedInfo

   The RequestedInfo parameter contains a comma separated list of
   parameter codes, as defined in Section 3.2.2.  For example, if one
   wants to audit the value of the NotifiedEntity, RequestIdentifier,
   RequestedEvents, SignalRequests, DigitMap, QuarantineHandling and
   DetectEvents parameters, the value of the RequestedInfo parameter
   will be:

      F: N,X,R,S,D,Q,T

   Note that extension parameters in general can be audited as well.
   The individual extension will define the auditing operation.







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   The capabilities request, in the AuditEndPoint command, is encoded by
   the parameter code "A", as in:

      F: A

3.2.2.18 RequestIdentifier

   The request identifier correlates a Notify command with the
   NotificationRequest that triggered it.  A RequestIdentifier is a
   hexadecimal string, at most 32 characters in length.
   RequestIdentifiers are compared as strings rather than numerical
   value.  The string "0" is reserved for reporting of persistent events
   in the case where a NotificationRequest has not yet been received
   after restart.

3.2.2.19 ResponseAck

   The response acknowledgement parameter is used to manage the "at-
   most-once" facility described in Section 3.5.  It contains a comma
   separated list of "confirmed transaction-id ranges".

   Each "confirmed transaction-id range" is composed of either one
   decimal number, when the range includes exactly one transaction, or
   two decimal numbers separated by a single hyphen, describing the
   lower and higher transaction identifiers included in the range.

   An example of a response acknowledgement is:

      K: 6234-6255, 6257, 19030-19044

3.2.2.20 RestartMethod

   The RestartMethod parameter is encoded as one of the keywords
   "graceful", "forced", "restart", "disconnected" or "cancel-graceful"
   as for example:

      RM: restart

   The set of restart methods can be extended through packages.

3.2.2.21 SignalRequests

   The SignalRequests parameter provides the name of the signal(s) that
   have been requested.  Each signal is identified by a name, as
   described in Section 3.2.2.4.

   Some signals, such as for example announcement or ADSI display, can
   be qualified by additional parameters, e.g.:



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   * the name and parameters of the announcement,

   * the string that should be displayed.

   Such parameters will be separated by commas and enclosed within
   parenthesis, as in:

      S: L/adsi("123456 Francois Gerard")
      S: A/ann(http://ann.example.net/no-such-number.au, 1234567)

   When a quoted-string is provided, the string itself is UTF-8 encoded
   [20].

   When several signals are requested, their codes are separated by a
   comma, as in:

      S: L/adsi("123456 Your friend"), L/rg

   Please refer to Section 3.2.2.4 for additional detail on signal
   parameters.

3.3 Format of response headers

   The response header is composed of a response line, optionally
   followed by headers that encode the response parameters.

   An example of a response header could be:

      200 1203 OK

   The response line starts with the response code, which is a three
   digit numeric value.  The code is followed by a white space, and the
   transaction identifier.  Response codes defined in packages (8xx) are
   followed by white space, a slash ("/") and the package name.  All
   response codes may furthermore be followed by optional commentary
   preceded by a white space.

   The following table describes the parameters whose presence is
   mandatory or optional in a response header, as a function of the
   command that triggered the response.  The letter M stands for
   mandatory, O for optional and F for forbidden.  Unless otherwise
   specified, a parameter MUST NOT be present more than once.  Note that
   the table only reflects the default for responses that have not
   defined any other behavior.  If a response is received with a
   parameter that is either not understood or marked as forbidden, the
   offending parameter(s) MUST simply be ignored.





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    ------------------------------------------------------------------
   | Parameter name      | EP | CR | MD | DL | RQ | NT | AU | AU | RS |
   |                     | CF | CX | CX | CX | NT | FY | EP | CX | IP |
   |---------------------|----|----|----|----|----|----|----|----|----|
   | BearerInformation   |  F |  F |  F |  F |  F |  F |  O |  F |  F |
   | CallId              |  F |  F |  F |  F |  F |  F |  F |  O |  F |
   | Capabilities        |  F |  F |  F |  F |  F |  F |  O*|  F |  F |
   | ConnectionId        |  F |  O*|  F |  F |  F |  F |  O*|  F |  F |
   | ConnectionMode      |  F |  F |  F |  F |  F |  F |  F |  O |  F |
   | Connection-         |  F |  F |  F |  O*|  F |  F |  F |  O |  F |
   |   Parameters        |    |    |    |    |    |    |    |    |    |
   | DetectEvents        |  F |  F |  F |  F |  F |  F |  O |  F |  F |
   | DigitMap            |  F |  F |  F |  F |  F |  F |  O |  F |  F |
   | EventStates         |  F |  F |  F |  F |  F |  F |  O |  F |  F |
   | LocalConnection-    |  F |  F |  F |  F |  F |  F |  F |  O |  F |
   |            Options  |    |    |    |    |    |    |    |    |    |
   | MaxMGCPDatagram     |  F |  F |  F |  F |  F |  F |  O |  F |  F |
   | NotifiedEntity      |  F |  F |  F |  F |  F |  F |  O |  O |  O |
   | ObservedEvents      |  F |  F |  F |  F |  F |  F |  O |  F |  F |
   | QuarantineHandling  |  F |  F |  F |  F |  F |  F |  O |  F |  F |
   | PackageList         |  O*|  O*|  O*|  O*|  O*|  O*|  O |  O*|  O*|
   | ReasonCode          |  F |  F |  F |  F |  F |  F |  O |  F |  F |
   | RequestIdentifier   |  F |  F |  F |  F |  F |  F |  O |  F |  F |
   | ResponseAck         |  O*|  O*|  O*|  O*|  O*|  O*|  O*|  O*|  O*|
   | RestartDelay        |  F |  F |  F |  F |  F |  F |  O |  F |  F |
   | RestartMethod       |  F |  F |  F |  F |  F |  F |  O |  F |  F |
   | RequestedEvents     |  F |  F |  F |  F |  F |  F |  O |  F |  F |
   | RequestedInfo       |  F |  F |  F |  F |  F |  F |  F |  F |  F |
   | SecondConnectionId  |  F |  O |  F |  F |  F |  F |  F |  F |  F |
   | SecondEndpointId    |  F |  O |  F |  F |  F |  F |  F |  F |  F |
   | SignalRequests      |  F |  F |  F |  F |  F |  F |  O |  F |  F |
   | SpecificEndpointId  |  F |  O |  F |  F |  F |  F |  O*|  F |  F |
   |---------------------|----|----|----|----|----|----|----|----|----|
   | LocalConnection-    |  F |  O*|  O |  F |  F |  F |  F |  O*|  F |
   |         Descriptor  |    |    |    |    |    |    |    |    |    |
   | RemoteConnection-   |  F |  F |  F |  F |  F |  F |  F |  O*|  F |
   |         Descriptor  |    |    |    |    |    |    |    |    |    |
    ------------------------------------------------------------------

   Notes (*):

   * The PackageList parameter is only allowed with return code 518
     (unsupported package), except for AuditEndpoint, where it may also
     be returned if audited.







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   * The ResponseAck parameter MUST NOT be used with any other responses
     than a final response issued after a provisional response for the
     transaction in question.  In that case, the presence of the
     ResponseAck parameter SHOULD trigger a Response Acknowledgement -
     any ResponseAck values provided will be ignored.

   * In the case of a CreateConnection message, the response line is
     followed by a Connection-Id parameter and a
     LocalConnectionDescriptor.  It may also be followed a Specific-
     Endpoint-Id parameter, if the creation request was sent to a
     wildcarded Endpoint-Id.  The connection-Id and
     LocalConnectionDescriptor parameter are marked as optional in the
     Table.  In fact, they are mandatory with all positive responses,
     when a connection was created, and forbidden when the response is
     negative, and no connection was created.

   * A LocalConnectionDescriptor MUST be transmitted with a positive
     response (code 200) to a CreateConnection.  It MUST also be
     transmitted in response to a ModifyConnection command, if the
     modification resulted in a modification of the session parameters.
     The LocalConnectionDescriptor is encoded as a "session
     description", as defined in section 3.4.  It is separated from the
     response header by an empty line.

   * Connection-Parameters are only valid in a response to a non-
     wildcarded DeleteConnection command sent by the Call Agent.

   * Multiple ConnectionId, SpecificEndpointId, and Capabilities
     parameters may be present in the response to an AuditEndpoint
     command.

   * When several session descriptors are encoded in the same response,
     they are encoded one after each other, separated by an empty line.
     This is the case for example when the response to an audit
     connection request carries both a local session description and a
     remote session description, as in:















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          200 1203 OK
          C: A3C47F21456789F0
          N: [128.96.41.12]
          L: p:10, a:PCMU;G726-32
          M: sendrecv
          P: PS=1245, OS=62345, PR=780, OR=45123, PL=10, JI=27,LA=48

          v=0
          o=- 25678 753849 IN IP4 128.96.41.1
          s=-
          c=IN IP4 128.96.41.1
          t=0 0
          m=audio 1296 RTP/AVP 0

          v=0
          o=- 33343 346463 IN IP4 128.96.63.25
          s=-
          c=IN IP4 128.96.63.25
          t=0 0
          m=audio 1296 RTP/AVP 0 96
          a=rtpmap:96 G726-32/8000

     In this example, according to the SDP syntax, each description
     starts with a "version" line, (v=...).  The local description is
     always transmitted before the remote description.  If a connection
     descriptor is requested, but it does not exist for the connection
     audited, that connection descriptor will appear with the SDP
     protocol version field only.

   The response parameters are described for each of the commands in the
   following.

3.3.1 CreateConnection Response

   In the case of a CreateConnection message, the response line is
   followed by a Connection-Id parameter with a successful response
   (code 200).  A LocalConnectionDescriptor is furthermore transmitted
   with a positive response.  The LocalConnectionDescriptor is encoded
   as a "session description", as defined by SDP (RFC 2327).  It is
   separated from the response header by an empty line, e.g.:











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      200 1204 OK
      I: FDE234C8

      v=0
      o=- 25678 753849 IN IP4 128.96.41.1
      s=-
      c=IN IP4 128.96.41.1
      t=0 0
      m=audio 3456 RTP/AVP 96
      a=rtpmap:96 G726-32/8000

   When a provisional response has been issued previously, the final
   response SHOULD furthermore contain the Response Acknowledgement
   parameter (final responses issued by entities adhering to this
   specification will include the parameter, but older RFC 2705
   implementations MAY not):

      200 1204 OK
      K:
      I: FDE234C8

      v=0
      o=- 25678 753849 IN IP4 128.96.41.1
      s=-
      c=IN IP4 128.96.41.1
      t=0 0
      m=audio 3456 RTP/AVP 96
      a=rtpmap:96 G726-32/8000

   The final response SHOULD then be acknowledged by a Response
   Acknowledgement:

      000 1204

3.3.2 ModifyConnection Response

   In the case of a successful ModifyConnection message, the response
   line is followed by a LocalConnectionDescriptor, if the modification
   resulted in a modification of the session parameters (e.g., changing
   only the mode of a connection does not alter the session parameters).
   The LocalConnectionDescriptor is encoded as a "session description",
   as defined by SDP.  It is separated from the response header by an
   empty line.








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      200 1207 OK

      v=0
      o=- 25678 753849 IN IP4 128.96.41.1
      s=-
      c=IN IP4 128.96.41.1
      t=0 0
      m=audio 3456 RTP/AVP 0

   When a provisional response has been issued previously, the final
   response SHOULD furthermore contain the Response Acknowledgement
   parameter as in:

      200 1207 OK
      K:

   The final response SHOULD then be acknowledged by a Response
   Acknowledgement:

      000 1207 OK

3.3.3 DeleteConnection Response

   Depending on the variant of the DeleteConnection message, the
   response line may be followed by a Connection Parameters parameter
   line, as defined in Section 3.2.2.7.

      250 1210 OK
      P: PS=1245, OS=62345, PR=780, OR=45123, PL=10, JI=27, LA=48

3.3.4 NotificationRequest Response

   A successful NotificationRequest response does not include any
   additional response parameters.

3.3.5 Notify Response

   A successful Notify response does not include any additional response
   parameters.

3.3.6 AuditEndpoint Response

   In the case of a successful AuditEndPoint the response line may be
   followed by information for each of the parameters requested - each
   parameter will appear on a separate line.  Parameters for which no






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   value currently exists, e.g., digit map, will still be provided but
   with an empty value.  Each local endpoint name "expanded" by a
   wildcard character will appear on a separate line using the
   "SpecificEndPointId" parameter code, e.g.:

      200 1200 OK
      Z: aaln/1@rgw.whatever.net
      Z: aaln/2@rgw.whatever.net

   When connection identifiers are audited and multiple connections
   exist on the endpoint, a comma-separated list of connection
   identifiers SHOULD be returned as in:

      200 1200 OK
      I: FDE234C8, DFE233D1

   Alternatively, multiple connection id parameter lines may be returned
   - the two forms should not be mixed although doing so does not
   constitute an error.

   When capabilities are audited, the response may include multiple
   capabilities parameter lines as in:

      200 1200 OK
      A: a:PCMU;G728, p:10-100, e:on, s:off, t:1, v:L,
          m:sendonly;recvonly;sendrecv;inactive
      A: a:G729, p:30-90, e:on, s:on, t:1, v:L,
          m:sendonly;recvonly;sendrecv;inactive;confrnce

   Note:  The carriage return for Capabilities shown above is present
   for formatting reasons only.  It is not permissible in a real command
   encoding.

3.3.7 AuditConnection Response

   In the case of a successful AuditConnection, the response may be
   followed by information for each of the parameters requested.
   Parameters for which no value currently exists will still be
   provided.  Connection descriptors will always appear last and each
   will be preceded by an empty line, as for example:











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      200 1203 OK
      C: A3C47F21456789F0
      N: [128.96.41.12]
      L: p:10, a:PCMU;G728
      M: sendrecv
      P: PS=622, OS=31172, PR=390, OR=22561, PL=5, JI=29, LA=50

      v=0
      o=- 4723891 7428910 IN IP4 128.96.63.25
      s=-
      c=IN IP4 128.96.63.25
      t=0 0
      m=audio 1296 RTP/AVP 96
      a=rtpmap:96 G726-32/8000

   If both a local and a remote connection descriptor are provided, the
   local connection descriptor will be the first of the two.  If a
   connection descriptor is requested, but it does not exist for the
   connection audited, that connection descriptor will appear with the
   SDP protocol version field only ("v=0"), as for example:

      200 1203 OK

      v=0

3.3.8 RestartInProgress Response

   A successful RestartInProgress response may include a NotifiedEntity
   parameter, but otherwise does not include any additional response
   parameters.

   Also, a 521 response to a RestartInProgress MUST include a
   NotifiedEntity parameter with the name of another Call Agent to
   contact when the first Call Agent redirects the endpoint to another
   Call Agent as in:

      521 1204 Redirect
      N: CA-1@whatever.net

3.4 Encoding of the Session Description (SDP)

   The session description (SDP) is encoded in conformance with the
   session description protocol, SDP.  MGCP implementations are REQUIRED
   to be fully capable of parsing any conformant SDP message, and MUST
   send session descriptions that strictly conform to the SDP standard.






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   The general description and explanation of SDP parameters can be
   found in RFC 2327 (or its successor).  In particular, it should be
   noted that the

   * Origin ("o="),

   * Session Name ("s="), and

   * Time active ("t=")

   are all mandatory in RFC 2327.  While they are of little use to MGCP,
   they MUST be provided in conformance with RFC 2327 nevertheless.  The
   following suggests values to be used for each of the fields, however
   the reader is encouraged to consult RFC 2327 (or its successor) for
   details:

   Origin
   o = <username> <session id> <version> <network type> <address type>
       <address>

   * The username SHOULD be set to hyphen ("-").

   * The session id is RECOMMENDED to be an NTP timestamp as suggested
     in RFC 2327.

   * The version is a version number that MUST increment with each
     change to the SDP.  A counter initialized to zero or an NTP
     timestamp as suggested in RFC 2327 is RECOMMENDED.

   * The network type defines the type of network.  For RTP sessions the
     network type SHOULD be "IN".

   * The address type defines the type of address.  For RTP sessions the
     address type SHOULD be "IP4" (or "IP6").

   * The address SHOULD be the same address as provided in the
     connection information ("c=") field.

   Session Name
   s = <session name>

   The session name should be hyphen ("-").

   Time active
   t = <start time> <stop time>






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   * The start time may be set to zero.

   * The stop time should be set to zero.

   Each of the three fields can be ignored upon reception.

   To further accommodate the extensibility principles of MGCP,
   implementations are ENCOURAGED to support the PINT "a=require"
   attribute - please refer to RFC 2848 for further details.

   The usage of SDP actually depends on the type of session that is
   being established.  Below we describe usage of SDP for an audio
   service using the RTP/AVP profile [4], or the LOCAL interconnect
   defined in this document.  In case of any conflicts between what is
   described below and SDP (RFC 2327 or its successor), the SDP
   specification takes precedence.

3.4.1 Usage of SDP for an Audio Service

   In a telephony gateway, we only have to describe sessions that use
   exactly one media, audio.  The usage of SDP for this is
   straightforward and described in detail in RFC 2327.

   The following is an example of an RFC 2327 conformant session
   description for an audio connection:

      v=0
      o=- A7453949499 0 IN IP4 128.96.41.1
      s=-
      c=IN IP4 128.96.41.1
      t=0 0
      m=audio 3456 RTP/AVP 0 96
      a=rtpmap:96 G726-32/8000

3.4.2 Usage of SDP for LOCAL Connections

   When MGCP is used to set up internal connections within a single
   gateway, the SDP format is used to encode the parameters of that
   connection.  The connection and media parameters will be used as
   follows:

   * The connection parameter (c=) will specify that the connection is
     local, using the keyword "LOCAL" as network type, the keyword "EPN"
     (endpoint name) as address type, and the local name of the endpoint
     as the connection-address.






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   * The "m=audio" parameter will specify a port number, which will
     always be set to 0, the type of protocol, always set to the keyword
     LOCAL, and the type of encoding, using the same conventions used
     for the RTP AVP profile (RTP payload numbers).  The type of
     encoding should normally be set to 0 (PCMU).

   A session-level attribute identifying the connection MAY furthermore
   be present.  This enables endpoints to support multiple LOCAL
   connections.  Use of this attribute is OPTIONAL and indeed
   unnecessary for endpoints that only support a single LOCAL
   connection.  The attribute is defined as follows:

   a=MGCPlocalcx:<ConnectionID>
      The MGCP Local Connection attribute is a session level only case-
      insensitive attribute that identifies the MGCP LOCAL connection,
      on the endpoint identified in the connection information, to which
      the SDP applies.  The ConnectionId is a hexadecimal string
      containing at most 32 characters.  The ConnectionId itself is
      case-insensitive.  The MGCP Local Connection attribute is not
      subject to the charset attribute.

   An example of a LOCAL session description could be:

      v=0
      o=- A7453949499 0 LOCAL EPN X35V3+A4/13
      s=-
      c=LOCAL EPN X35V3+A4/13
      t=0 0
      a=MGCPlocalcx:FDE234C8
      m=audio 0 LOCAL 0

   Note that the MGCP Local Connection attribute is specified at the
   session level and that it could have been omitted in case only a
   single LOCAL connection per endpoint is supported.

3.5 Transmission over UDP

   MGCP messages are transmitted over UDP.  Commands are sent to one of
   the IP addresses defined in the DNS for the specified endpoint.  The
   responses are sent back to the source address (i.e., IP address and
   UDP port number) of the commands - the response may or may not arrive
   from the same address as the command was sent to.









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   When no port is specified for the endpoint, the commands MUST by
   default be sent:

   * by the Call Agents, to the default MGCP port for gateways, 2427.

   * by the Gateways, to the default MGCP port for Call Agents, 2727.

3.5.1 Providing the At-Most-Once Functionality

   MGCP messages, being carried over UDP, may be subject to losses.  In
   the absence of a timely response, commands are retransmitted.  Most
   MGCP commands are not idempotent.  The state of the gateway would
   become unpredictable if, for example, CreateConnection commands were
   executed several times.  The transmission procedures MUST thus
   provide an "at-most-once" functionality.

   MGCP entities are expected to keep in memory a list of the responses
   that they sent to recent transactions, and a list of the transactions
   that are currently being executed.  The numerical value of
   transaction identifiers of incoming commands are compared to the
   transaction identifiers of the recent responses.  If a match is
   found, the MGCP entity does not execute the transaction again, but
   simply resends the response.  The remaining commands will be compared
   to the list of current transactions, i.e., transactions received
   previously which have not yet finished executing.  If a match is
   found, the MGCP entity does not execute the transaction again, but a
   provisional response (Section 3.5.5) SHOULD be issued to acknowledge
   receipt of the command.

   The procedure uses a long timer value, noted T-HIST in the following.
   The timer MUST be set larger than the maximum duration of a
   transaction, which MUST take into account the maximum number of
   repetitions, the maximum value of the repetition timer and the
   maximum propagation delay of a packet in the network.  A suggested
   value is 30 seconds.

   The copy of the responses MAY be destroyed either T-HIST seconds
   after the response is issued, or when the gateway (or the Call Agent)
   receives a confirmation that the response has been received, through
   the "Response Acknowledgement".  For transactions that are
   acknowledged through this attribute, the gateway SHALL keep a copy of
   the transaction-id (as opposed to the entire transaction response)
   for T-HIST seconds after the response is issued, in order to detect
   and ignore duplicate copies of the transaction request that could be
   produced by the network.






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3.5.2 Transaction Identifiers and Three Ways Handshake

   Transaction identifiers are integer numbers in the range from 1 to
   999,999,999 (both included).  Call-agents may decide to use a
   specific number space for each of the gateways that they manage, or
   to use the same number space for all gateways that belong to some
   arbitrary group.  Call agents may decide to share the load of
   managing a large gateway between several independent processes.
   These processes MUST then share the transaction number space.  There
   are multiple possible implementations of this sharing, such as having
   a centralized allocation of transaction identifiers, or pre-
   allocating non-overlapping ranges of identifiers to different
   processes.  The  implementations MUST guarantee that unique
   transaction identifiers are allocated to all transactions that
   originate from a logical call agent, as defined in Section 4.
   Gateways can simply detect duplicate transactions by looking at the
   transaction identifier only.

   The Response Acknowledgement Attribute can be found in any command.
   It carries a set of "confirmed transaction-id ranges" for final
   responses received - provisional responses MUST NOT be confirmed.  A
   given response SHOULD NOT be confirmed in two separate messages.

   MGCP entities MAY choose to delete the copies of the responses (but
   not the transaction-id) to transactions whose id is included in
   "confirmed transaction-id ranges" received in the Response
   Confirmation messages (command or response).  They SHOULD then
   silently discard further commands from that entity when the
   transaction-id falls within these ranges, and the response was issued
   less than T-HIST seconds ago.

   Entities MUST exercise due caution when acknowledging responses.  In
   particular, a response SHOULD only be acknowledged if the response
   acknowledgement is sent to the same entity as the corresponding
   command (i.e., the command whose response is being acknowledged) was
   sent to.

   Likewise, entities SHOULD NOT blindly accept a response
   acknowledgement for a given response.  However it is considered safe
   to accept a response acknowledgement for a given response, when that
   response acknowledgement is sent by the same entity as the command
   that generated that response.

   It should be noted, that use of response acknowledgments in commands
   (as opposed to the Response Acknowledgement response following a
   provisional response) is OPTIONAL.  The benefit of using it is that
   it reduces overall memory consumption.  However, in order to avoid
   large messages, implementations SHOULD NOT generate large response



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   acknowledgement lists.  One strategy is to manage responses to
   commands on a per endpoint basis.  A command for an endpoint can
   confirm a response to an older command for that same endpoint.
   Responses to commands with wildcarded endpoint names can be confirmed
   selectively with due consideration to message sizes, or alternatively
   simply not be acknowledged (unless the response explicitly required a
   Response Acknowledgement).  Care must be taken to not confirm the
   same response twice or a response that is more than T-HIST seconds
   old.

   The "confirmed transaction-id ranges" values SHALL NOT be used if
   more than T-HIST seconds have elapsed since the entity issued its
   last response to the other entity, or when an entity resumes
   operation.  In this situation, commands MUST be accepted and
   processed, without any test on the transaction-id.

   Commands that carry the "Response Acknowledgement attribute" may be
   transmitted in disorder.  The union of the "confirmed transaction-id
   ranges" received in recent messages SHALL be retained.

3.5.3 Computing Retransmission Timers

   It is the responsibility of the requesting entity to provide suitable
   time outs for all outstanding commands, and to retry commands when
   time outs have been exceeded.  Furthermore, when repeated commands
   fail to be acknowledged, it is the responsibility of the requesting
   entity to seek redundant services and/or clear existing or pending
   associations.

   The specification purposely avoids specifying any value for the
   retransmission timers.  These values are typically network dependent.
   The retransmission timers SHOULD normally estimate the timer by
   measuring the time spent between the sending of a command and the
   return of the first response to the command.  At a minimum, a
   retransmission strategy involving exponential backoff MUST be
   implemented.  One possibility is to use the algorithm implemented in
   TCP/IP, which uses two variables:

   * the average acknowledgement delay, AAD, estimated through an
     exponentially smoothed average of the observed delays,

   * the average deviation, ADEV, estimated through an exponentially
     smoothed average of the absolute value of the difference between
     the observed delay and the current average.







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   The retransmission timer, RTO, in TCP, is set to the sum of the
   average delay plus N times the average deviation, where N is a
   constant.  In MGCP, the maximum value of the timer SHOULD however be
   bounded, in order to guarantee that no repeated packet will be
   received by the gateways after T-HIST seconds.  A suggested maximum
   value for RTO (RTO-MAX) is 4 seconds.  Implementers SHOULD consider
   bounding the minimum value of this timer as well [19].

   After any retransmission, the MGCP entity SHOULD do the following:

   * It should double the estimated value of the acknowledgement delay
     for this transaction, T-DELAY.

   * It should compute a random value, uniformly distributed between 0.5
     T-DELAY and T-DELAY.

   * It should set the retransmission timer (RTO) to the minimum of:
     - the sum of that random value and N times the average deviation,
     - RTO-MAX.

   This procedure has two effects.  Because it includes an exponentially
   increasing component, it will automatically slow down the stream of
   messages in case of congestion.  Because it includes a random
   component, it will break the potential synchronization between
   notifications triggered by the same external event.

   Note that the estimators AAD and ADEV SHOULD NOT be updated for
   transactions that involve retransmissions.  Also, the first new
   transmission following a successful retransmission SHOULD use the RTO
   for that last retransmission.  If this transmission succeeds without
   any retransmissions, the AAD and ADEV estimators are updated and RTO
   is determined as usual again.  See, e.g., [18] for further details.

3.5.4 Maximum Datagram Size, Fragmentation and Reassembly

   MGCP messages being transmitted over UDP rely on IP for fragmentation
   and reassembly of large datagrams.  The maximum theoretical size of
   an IP datagram is 65535 bytes.  With a 20-byte IP header and an 8-
   byte UDP header, this leaves us with a maximum theoretical MGCP
   message size of 65507 bytes when using UDP.

   However, IP does not require a host to receive IP datagrams larger
   than 576 bytes [21], which would provide an unacceptably small MGCP
   message size.  Consequently, MGCP mandates that implementations MUST
   support MGCP datagrams up to at least 4000 bytes, which requires the






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   corresponding IP fragmentation and reassembly to be supported.  Note,
   that the 4000 byte limit applies to the MGCP level.  Lower layer
   overhead will require support for IP datagrams that are larger than
   this:  UDP and IP overhead will be at least 28 bytes, and, e.g., use
   of IPSec will add additional overhead.

   It should be noted, that the above applies to both Call Agents and
   endpoints.  Call Agents can audit endpoints to determine if they
   support larger MGCP datagrams than specified above.  Endpoints do
   currently not have a similar capability to determine if a Call Agent
   supports larger MGCP datagram sizes.

3.5.5 Piggybacking

   There are cases when a Call Agent will want to send several messages
   at the same time to the same gateways, and vice versa.  When several
   MGCP messages have to be sent in the same datagram, they MUST be
   separated by a line of text that contains a single dot, as in for
   example:

      200 2005 OK
      .
      DLCX 1244 card23/21@tgw-7.example.net MGCP 1.0
      C: A3C47F21456789F0
      I: FDE234C8

   The piggybacked messages MUST be processed exactly as if they had
   been received one at a time in several separate datagrams.  Each
   message in the datagram MUST be processed to completion and in order
   starting with the first message, and each command MUST be responded
   to.  Errors encountered in a message that was piggybacked MUST NOT
   affect any of the other messages received in that datagram - each
   message is processed on its own.

   Piggybacking can be used to achieve two things:

   * Guaranteed in-order delivery and processing of messages.

   * Fate sharing of message delivery.

   When piggybacking is used to guarantee in-order delivery of messages,
   entities MUST ensure that this in-order delivery property is retained
   on retransmissions of the individual messages.  An example of this is
   when multiple Notify's are sent using piggybacking (as described in
   Section 4.4.1).






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   Fate sharing of message delivery ensures that either all the messages
   are delivered, or none of them are delivered.  When piggybacking is
   used to guarantee this fate-sharing, entities MUST also ensure that
   this property is retained upon retransmission.  For example, upon
   receiving a Notify from an endpoint operating in lockstep mode, the
   Call Agent may wish to send the response and a new
   NotificationRequest command in a single datagram to ensure message
   delivery fate-sharing of the two.

3.5.6 Provisional Responses

   Executing some transactions may require a long time.  Long execution
   times may interact with the timer based retransmission procedure.

   This may result either in an inordinate number of retransmissions, or
   in timer values that become too long to be efficient.

   Gateways (and Call Agents) that can predict that a transaction will
   require a long execution time SHOULD send a provisional response with
   response code 100.  As a guideline, a transaction that requires
   external communication to complete, e.g., network resource
   reservation, SHOULD issue a provisional response.  Furthermore
   entities SHOULD send a provisional response if they receive a
   repetition of a transaction that has not yet finished executing.

   Gateways (or Call Agents) that start building up queues of
   transactions to be executed may send a provisional response with
   response code 101 to indicate this (see Section 4.4.8 for further
   details).

   Pure transactional semantics would imply, that provisional responses
   SHOULD NOT return any other information than the fact that the
   transaction is currently executing, however an optimistic approach
   allowing some information to be returned enables a reduction in the
   delay that would otherwise be incurred in the system.

   In order to reduce the delay in the system, it is RECOMMENDED to
   include a connection identifier and session description in a 100
   provisional response to the CreateConnection command.  If a session
   description would be returned by the ModifyConnection command, the
   session description SHOULD be included in the provisional response
   here as well.  If the transaction completes successfully, the
   information returned in the provisional response MUST be repeated in
   the final response.  It is considered a protocol error not to repeat
   this information or to change any of the previously supplied
   information in a successful response.  If the transaction fails, an
   error code is returned - the information returned previously is no
   longer valid.



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   A currently executing CreateConnection or ModifyConnection
   transaction MUST be cancelled if a DeleteConnection command for the
   endpoint is received.  In that case, a final response for the
   cancelled transaction SHOULD still be returned automatically (error
   code 407 - transaction aborted, is RECOMMENDED), and a final response
   for the cancelled transaction MUST be returned if a retransmission of
   the cancelled transaction is detected (see also Section 4.4.4).

   MGCP entities that receive a provisional response SHALL switch to a
   longer repetition timer (LONGTRAN-TIMER) for that transaction.  The
   purpose of this timer is primarily to detect processing failures.
   The default value of LONGTRAN-TIMER is 5 seconds, however the
   provisioning process may alter this.  Note, that retransmissions MUST
   still satisfy the timing requirements specified in Section 3.5.1 and
   3.5.3.  Consequently LONGTRAN-TIMER MUST be smaller than T-HIST (it
   should in fact be considerably smaller).  Also, entities MUST NOT let
   a transaction run forever.  A transaction that is timed out by the
   entity SHOULD return error code 406 (transaction time-out).  Per the
   definition of T-HIST (Section 3.5.1), the maximum transaction
   execution time is smaller than T-HIST (in a network with low delay,
   it can reasonably safely be approximated as T-HIST minus T-MAX), and
   a final response should be received no more than T-HIST seconds after
   the command was sent initially.  Nevertheless, entities SHOULD wait
   for 2*T-HIST seconds before giving up on receiving a final response.
   Retransmission of the command MUST still cease after T-MAX seconds
   though.  If a response is not received, the outcome of the
   transaction is not known.  If the entity sending the command was a
   gateway, it now becomes "disconnected" and SHALL initiate the
   "disconnected" procedure (see Section 4.4.7).

   When the transaction finishes execution, the final response is sent
   and the by now obsolete provisional response is deleted.  In order to
   ensure rapid detection of a lost final response, final responses
   issued after provisional responses for a transaction SHOULD be
   acknowledged (unfortunately older RFC 2705 implementations may not do
   this, which is the only reason it is not an absolute requirement).

   The endpoint SHOULD therefore include an empty "ResponseAck"
   parameter in those, and only those, final responses.  The presence of
   the "ResponseAck" parameter in the final response SHOULD trigger a
   "Response Acknowledgement" response to be sent back to the endpoint.
   The Response Acknowledgement" response will then include the
   transaction-id of the response it acknowledges in the response
   header.  Note that, for backwards compatibility, entities cannot
   depend on receiving such a "response acknowledgement", however it is
   strongly RECOMMENDED to support this behavior, as excessive delays in
   case of packet loss as well as excessive retransmissions may occur
   otherwise.



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   Receipt of a "Response Acknowledgement" response is subject to the
   same time-out and retransmission strategies and procedures as
   responses to commands, i.e., the sender of the final response will
   retransmit it if a "Response Acknowledgement" is not received in
   time.  For backwards compatibility, failure to receive a "response
   acknowledgement" SHOULD NOT affect the roundtrip time estimates for
   subsequent commands, and furthermore MUST NOT lead to the endpoint
   becoming "disconnected".  The "Response Acknowledgment" response is
   never acknowledged.

4. States, Failover and Race Conditions

   In order to implement proper call signaling, the Call Agent must keep
   track of the state of the endpoint, and the gateway must make sure
   that events are properly notified to the Call Agent.  Special
   conditions exist when the gateway or the Call Agent are restarted:
   the gateway must be redirected to a new Call Agent during "failover"
   procedures, the Call Agent must take special action when the gateway
   is taken offline, or restarted.

4.1 Failover Assumptions and Highlights

   The following protocol highlights are important to understanding Call
   Agent fail-over mechanisms:

   * Call Agents are identified by their domain name (and optional
     port), not their network addresses, and several addresses can be
     associated with a domain name.

   * An endpoint has one and only one Call Agent associated with it at
     any given point in time.  The Call Agent associated with an
     endpoint is the current value of the "notified entity".  The
     "notified entity" determines where the gateway will send it's
     commands.  If the "notified entity" does not include a port number,
     the default Call Agent port number (2727) is assumed.

   * NotifiedEntity is a parameter sent by the Call Agent to the gateway
     to set the "notified entity" for the endpoint.

   * The "notified entity" for an endpoint is the last value of the
     NotifiedEntity parameter received for this endpoint.  If no
     explicit NotifiedEntity parameter has ever been received, the
     "notified entity" defaults to a provisioned value.  If no value was
     provisioned or an empty NotifiedEntity parameter was provided (both
     strongly discouraged) thereby making the "notified entity" empty,
     the "notified entity" is set to the source address of the last
     non-audit command for the endpoint.  Thus auditing will not change
     the "notified entity".



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   * Responses to commands are sent to the source address of the
     command, regardless of the current "notified entity".  When a
     Notify message needs to be piggybacked with the response, the
     datagram is still sent to the source address of the new command
     received, regardless of the current "notified entity".

   The ability for the "notified entity" to resolve to multiple network
   addresses, allows a "notified entity" to represent a Call Agent with
   multiple physical interfaces on it and/or a logical Call Agent made
   up of multiple physical systems.  The order of network addresses when
   a DNS name resolves to multiple addresses is non-deterministic so
   Call Agent fail-over schemes MUST NOT depend on any order (e.g., a
   gateway MUST be able to send a "Notify" to any of the resolved
   network addresses).  On the other hand, the system is likely to be
   most efficient if the gateway sends commands to the interface with
   which it already has a current association.  It is RECOMMENDED that
   gateways use the following algorithm to achieve that goal:

   * If the "notified entity" resolves to multiple network addresses,
     and the source address of the request is one of those addresses,
     that network address is the preferred destination address for
     commands.

   * If on the other hand, the source address of the request is not one
     of the resolved addresses, the gateway must choose one of the
     resolved addresses for commands.

   * If the gateway fails to contact the network address chosen, it MUST
     try the alternatives in the resolved list as described in Section
     4.3.

   If an entire Call Agent becomes unavailable, the endpoints managed by
   that Call Agent will eventually become "disconnected".  The only way
   for these endpoints to become connected again is either for the
   failed Call Agent to become available, or for a backup call agent to
   contact the affected endpoints with a new "notified entity".

   When a backup Call Agent has taken over control of a group of
   endpoints, it is assumed that the failed Call Agent will communicate
   and synchronize with the backup Call Agent in order to transfer
   control of the affected endpoints back to the original Call Agent.
   Alternatively, the failed Call Agent could simply become the backup
   Call Agent.








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   We should note that handover conflict resolution between separate
   CA's is not in place - we are relying strictly on the CA's knowing
   what they are doing and communicating with each other (although
   AuditEndpoint can be used to learn about the current "notified
   entity").  If this is not the case, unexpected behavior may occur.

   Note that as mentioned earlier, the default "notified entity" is
   provisioned and may include both domain name and port.  For small
   gateways, provisioning may be done on a per endpoint basis.  For much
   larger gateways, a single provisioning element may be provided for
   multiple endpoints or even for the entire gateway itself.  In either
   case, once the gateway powers up, each endpoint MUST have its own
   "notified entity", so provisioned values for an aggregation of
   endpoints MUST be copied to the "notified entity" for each endpoint
   in the aggregation before operation proceeds.  Where possible, the
   RestartInProgress command on restart SHOULD be sent to the
   provisioned "notified entity" based on an aggregation that allows the
   "all of" wild-card to be used.  This will reduce the number of
   RestartInProgress messages.

   Another way of viewing the use of "notified entity" is in terms of
   associations between gateways and Call Agents.  The "notified entity"
   is a means to set up that association, and governs where the gateway
   will send commands to.  Commands received by the gateway however may
   come from any source.  The association is initially provisioned with
   a provisioned "notified entity", so that on power up
   RestartInProgress and persistent events that occur prior to the first
   NotificationRequest from Call Agents will be sent to the provisioned
   Call Agent.  Once a Call Agent makes a request, however it may
   include the NotifiedEntity parameter and set up a new association.
   Since the "notified entity" persists across calls, the association
   remains intact until a new "notified entity" is provided.

4.2 Communicating with Gateways

   Endpoint names in gateways include a local name indicating the
   specific endpoint and a domain name indicating the host/gateway where
   the endpoint resides.  Gateways may have several interfaces for
   redundancy.

   In gateways that have routing capability, the domain name may resolve
   to a single network address with internal routing to that address
   from any of the gateway's interfaces.  In others, the domain name may
   resolve to multiple network addresses, one for each interface.  In
   the latter case, if a Call Agent fails to contact the gateway on one
   of the addresses, it MUST try the alternates.





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4.3 Retransmission, and Detection of Lost Associations:

   The media gateway control protocol is organized as a set of
   transactions, each of which is composed of a command and a response,
   commonly referred to as an acknowledgement.  The MGCP messages, being
   carried over UDP, may be subject to losses.  In the absence of a
   timely response, commands are retransmitted.  MGCP entities MUST keep
   in memory a list of the responses that they sent to recent
   transactions, i.e., a list of all the responses they sent over the
   last T-HIST seconds, and a list of the transactions that have not yet
   finished executing.

   The transaction identifiers of incoming commands are compared to the
   transaction identifiers of the recent responses.  If a match is
   found, the MGCP entity does not execute the transaction, but simply
   repeats the response.  If a match to a previously responded to
   transaction is not found, the transaction identifier of the incoming
   command is compared to the list of transactions that have not yet
   finished executing.  If a match is found, the MGCP entity does not
   execute the transaction again, but SHOULD simply send a provisional
   response - a final response will be provided when the execution of
   the command is complete (see Section 3.5.6 for further detail).

   The repetition mechanism is used to guard against four types of
   possible errors:

   * transmission errors, when for example a packet is lost due to noise
     on a line or congestion in a queue,

   * component failure, when for example an interface to a Call Agent
     becomes unavailable,

   * Call Agent failure, when for example an entire Call Agent becomes
     unavailable,

   * failover, when a new Call Agent is "taking over" transparently.

   The elements should be able to derive from the past history an
   estimate of the packet loss rate due to transmission errors.  In a
   properly configured system, this loss rate should be very low,
   typically less than 1%.  If a Call Agent or a gateway has to repeat a
   message more than a few times, it is very legitimate to assume that
   something other than a transmission error is occurring.  For example,
   given a loss rate of 1%, the probability that 5 consecutive
   transmission attempts fail is 1 in 100 billion, an event that should
   occur less than once every 10 days for a Call Agent that processes
   1,000 transactions per second.  (Indeed, the number of
   retransmissions that is considered excessive should be a function of



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   the prevailing packet loss rate.)  We should note that the "suspicion
   threshold", which we will call "Max1", is normally lower than the
   "disconnection threshold", which we will call "Max2".  Max2 MUST be
   set to a larger value than Max1.

   The MGCP retransmission algorithm is illustrated in the Figure below
   and explained further in the following:












































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      Command issued: N=0, T=0
              |
              |  +------------ retransmission: N++ <--------------+
              |  |                                                |
              |  |     if T <= T-Max then                         |
              |  |      transmission                              |
              |  |  +-- to new address, <-+<----------------------|--+
              |  |  |       N=0           |                       |  |
              V  V  V                     |                       |  |
          +-----------+                   |                       |  |
      +-->| awaiting  |- new Call Agent ->+   +------------+      |  |
      |   |  response |--- timer elapsed  --->| T > T-Max ?|      |  |
      |   +-----------+                       +------------+      ^  ^
      |          |                             |    |             |  |
      |          v             +-----(yes)-----+   (no)           |  |
      |      (response         |                    |             |  |
      |       received)        |              +------------+      |  |
      |          |             |              | N >= Max1 ?|-(no)>+  |
      |          v             |              +------------+      ^  ^
      |      +--------+        |                    |             |  |
      +<(no)-| final ?|        |                  (yes)           |  |
      ^      +--------+        |                    |             |  |
      |          |             |     (if first address & N=Max1,  |  |
      |          v             |      or last address & N=Max2    |  |
      |        (yes)           |               check DNS)         |  |
      |          |             |                    |             |  |
      |          v             V           +---------------+      |  |
      |        (end)           |           |more addresses?|(yes)-|->+
      |                        |           +---------------+      |
      |                        |                    |             ^
      |                        |                  (no)            |
      |                        |                    |             |
      |                        |              +------------+      |
      |                        |              | N >= Max2 ?|(no)--+
      |                        |              +------------+
      |                        |                    |
      |                        |                  (yes)
      |                        |                    |
      |                        |            +----------------+
      |                        +----------->| T >= 2*T-HIST ?|
      |                                     +----------------+
      |                                       |       |
      |                                     (no)    (yes)
      +---------------<-----------------------+       |
                                                      v
                                                (disconnected)





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   A classic retransmission algorithm would simply count the number of
   successive repetitions, and conclude that the association is broken
   after re-transmitting the packet an excessive number of times
   (typically between 7 and 11 times). In order to account for the
   possibility of an undetected or in-progress "failover", we modify the
   classic algorithm as follows:

   * We require that the gateway always checks for the presence of a new
     Call Agent.  It can be noticed either by:

     - receiving a command where the NotifiedEntity points to the new
       Call Agent, or

     - receiving a redirection response pointing to a new Call Agent.

     If a new Call Agent is detected, the gateway MUST start
     retransmitting outstanding commands for the endpoint(s) redirected
     to that new Call Agent.  Responses to new or old commands are still
     transmitted to the source address of the command.

   * Prior to any retransmission, it is checked that the time elapsed
     since the sending of the initial datagram is no greater than T-MAX.
     If more than T-MAX time has elapsed, then retransmissions MUST
     cease.  If more than 2*T-HIST has elapsed, then the endpoint
     becomes disconnected.

   * If the number of repetitions for this Call Agent is equal to
     "Max1", and its domain name was not resolved recently (e.g., within
     the last 5 seconds or otherwise provisioned), and it is not in the
     process of being resolved, then the gateway MAY actively query the
     domain name server in order to detect the possible change of the
     Call Agent interfaces.  Note that the first repetition is the
     second transmission.

   * The gateway may have learned several IP addresses for the call
     agent.  If the number of repetitions for this IP address is greater
     than or equal to "Max1" and lower than "Max2", and there are more
     addresses that have not been tried, then the gateway MUST direct
     the retransmissions to alternate addresses.  Also, receipt of
     explicit network notifications such as, e.g., ICMP network, host,
     protocol, or port unreachable SHOULD lead the gateway to try
     alternate addresses (with due consideration to possible security
     issues).








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   * If there are no more interfaces to try, and the number of
     repetitions for this address is Max2, then the gateway SHOULD
     contact the DNS one more time to see if any other interfaces have
     become available, unless the domain name was resolved recently
     (e.g., within the last 5 seconds or otherwise provisioned), or it
     is already in the process of being resolved.  If there still are no
     more interfaces to try, the gateway is then disconnected and MUST
     initiate the "disconnected" procedure (see Section 4.4.7).

   In order to automatically adapt to network load, MGCP specifies
   exponentially increasing timers.  If the initial timer is set to 200
   milliseconds, the loss of a fifth retransmission will be detected
   after about 6 seconds.  This is probably an acceptable waiting delay
   to detect a failover.  The repetitions should continue after that
   delay not only in order to perhaps overcome a transient connectivity
   problem, but also in order to allow some more time for the execution
   of a failover - waiting a total delay of 30 seconds is probably
   acceptable.

   It is however important that the maximum delay of retransmissions be
   bounded.  Prior to any retransmission, it is checked that the time
   (T) elapsed since the sending of the initial datagram is no greater
   than T-MAX.  If more than T-MAX time has elapsed, retransmissions
   MUST cease.  If more than 2*T-HIST time has elapsed, the endpoint
   becomes disconnected.  The value T-MAX is related to the T-HIST
   value:  the T-HIST value MUST be greater than or equal to T-MAX plus
   the maximum propagation delay in the network.

   The default value for T-MAX is 20 seconds.  Thus, if the assumed
   maximum propagation delay is 10 seconds, then responses to old
   transactions would have to be kept for a period of at least 30
   seconds.  The importance of having the sender and receiver agree on
   these values cannot be overstated.

   The default value for Max1 is 5 retransmissions and the default value
   for Max2 is 7 retransmissions.  Both of these values may be altered
   by the provisioning process.

   The provisioning process MUST be able to disable one or both of the
   Max1 and Max2 DNS queries.

4.4 Race Conditions

   MGCP deals with race conditions through the notion of a "quarantine
   list" and through explicit detection of desynchronization, e.g., for
   mismatched hook state due to glare for an endpoint.





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   MGCP does not assume that the transport mechanism will maintain the
   order of commands and responses.  This may cause race conditions,
   that may be obviated through a proper behavior of the Call Agent.
   (Note that some race conditions are inherent to distributed systems;
   they would still occur, even if the commands were transmitted in
   strict order.)

   In some cases, many gateways may decide to restart operation at the
   same time.  This may occur, for example, if an area loses power or
   transmission capability during an earthquake or an ice storm.  When
   power and transmission are reestablished, many gateways may decide to
   send "RestartInProgress" commands simultaneously, leading to very
   unstable operation.

4.4.1 Quarantine List

   MGCP controlled gateways will receive "notification requests" that
   ask them to watch for a list of "events".  The protocol elements that
   determine the handling of these events are the "Requested Events"
   list, the "Digit Map", the "Quarantine Handling", and the "Detect
   Events" list.

   When the endpoint is initialized, the requested events list only
   consists of persistent events for the endpoint, and the digit map is
   assumed empty.  At this point, the endpoint MAY use an implicit
   NotificationRequest with the reserved RequestIdentifier zero ("0") to
   detect and report a persistent event, e.g., off-hook.  A pre-existing
   off-hook condition MUST here result in the off-hook event being
   generated as well.

   The endpoint awaits the reception of a NotificationRequest command,
   after which the gateway starts observing the endpoint for occurrences
   of the events mentioned in the list, including persistent events.

   The events are examined as they occur.  The action that follows is
   determined by the "action" parameter associated with the event in the
   list of requested events, and also by the digit map.  The events that
   are defined as "accumulate" or "accumulate according to digit map"
   are accumulated in a list of events, the events that are marked as
   "accumulate according to the digit map" will additionally be
   accumulated in the "current dial string".  This will go on until one
   event is encountered that triggers a notification which will be sent
   to the current "notified entity".

   The gateway, at this point, will transmit the Notify command and will
   place the endpoint in a "notification" state.  As long as the
   endpoint is in this notification state, the events that are to be
   detected on the endpoint are stored in a "quarantine" buffer (FIFO)



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   for later processing.  The events are, in a sense, "quarantined".
   All events that are specified by the union of the RequestedEvents
   parameter and the most recently received DetectEvents parameter or,
   in the absence of the latter, all events that are referred to in the
   RequestedEvents, SHALL be detected and quarantined, regardless of the
   action associated with the event.  Persistent events are here viewed
   as implicitly included in RequestedEvents.  If the quarantine buffer
   reaches the capacity of the endpoint, a Quarantine Buffer Overflow
   event (see Appendix B) SHOULD be generated (when this event is
   supported, the endpoint MUST ensure it has capacity to include the
   event in the quarantine buffer).  Excess events will now be
   discarded.

   The endpoint exits the "notification state" when the response
   (whether success or failure) to the Notify command is received.  The
   Notify command may be retransmitted in the "notification state", as
   specified in Section 3.5 and 4.  If the endpoint is or becomes
   disconnected (see Section 4.3) during this, a response to the Notify
   command will never be received.  The Notify command is then lost and
   hence no longer considered pending, yet the endpoint is still in the
   "notification state".  Should that occur, completion of the
   disconnected procedure specified in Section 4.4.7 SHALL then lead the
   endpoint to exit the "notification state".

   When the endpoint exits the "notification state" it resets the list
   of observed events and the "current dial string" of the endpoint to a
   null value.

   Following that point, the behavior of the gateway depends on the
   value of the QuarantineHandling parameter in the triggering
   NotificationRequest command:

   If the Call Agent had specified, that it expected at most one
   notification in response to the notification request command, then
   the gateway SHALL simply keep on accumulating events in the
   quarantine buffer until it receives the next notification request
   command.

   If, however, the gateway is authorized to send multiple successive
   Notify commands, it will proceed as follows.  When the gateway exits
   the "notification state", it resets the list of observed events and
   the "current dial string" of the endpoint to a null value and starts
   processing the list of quarantined events, using the already received
   list of requested events and digit map.  When processing these
   events, the gateway may encounter an event which triggers a Notify
   command to be sent.  If that is the case, the gateway can adopt one
   of the two following behaviors:




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   * it can immediately transmit a Notify command that will report all
     events that were accumulated in the list of observed events until
     the triggering event, included, leaving the unprocessed events in
     the quarantine buffer,

   * or it can attempt to empty the quarantine buffer and transmit a
     single Notify command reporting several sets of events (in a single
     list of observed events) and possibly several dial strings.  The
     "current dial string" is reset to a null value after each
     triggering event.  The events that follow the last triggering event
     are left in the quarantine buffer.

   If the gateway transmits a Notify command, the endpoint will reenter
   and remain in the "notification state" until the acknowledgement is
   received (as described above).  If the gateway does not find a
   quarantined event that triggers a Notify command, it places the
   endpoint in a normal state.  Events are then processed as they come,
   in exactly the same way as if a Notification Request command had just
   been received.

   A gateway may receive at any time a new Notification Request command
   for the endpoint, including the case where the endpoint is
   disconnected.  Activating an embedded Notification Request is here
   viewed as receiving a new Notification Request as well, except that
   the current list of ObservedEvents remains unmodified rather than
   being processed again.  When a new notification request is received
   in the notification state, the gateway SHALL ensure that the pending
   Notify is received by the Call Agent prior to a new Notify (note that
   a Notify that was lost due to being disconnected, is no longer
   considered pending).  It does so by using the "piggybacking"
   functionality of the protocol.  The messages will then be sent in a
   single packet to the current "notified entity".  The steps involved
   are the following:

   a) the gateway sends a response to the new notification request.

   b) the endpoint is then taken out of the "notification state" without
      waiting for the acknowledgement of the pending Notify command.

   c) a copy of the unacknowledged Notify command is kept until an
      acknowledgement is received.  If a timer elapses, the Notify will
      be retransmitted.

   d) If the gateway has to transmit a new Notify before the previous
      Notify(s) is acknowledged, it constructs a packet that piggybacks
      a repetition of the old Notify(s) and the new Notify (ordered by
      age with the oldest first).  This datagram will be sent to the
      current "notified entity".



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   f) Gateways that cannot piggyback several messages in the same
      datagram and hence guarantee in-order delivery of two (or more)
      Notify's SHALL leave the endpoint in the "notification" state as
      long as the last Notify is not acknowledged.















































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   The procedure is illustrated by the following diagram:

    +-------------------+
    | Processing Events |<--------------------------------------+
    +-------------------+                                       |
             |                                                  |
     Need to send NTFY                                          |
             |                                                  |
             v                                                  |
    +-------------------+                                       |
    | Outstanding NTFY  |---- No -------+                       |
    |                   |               |                       |
    +-------------------+               v                       |
             |                    +-----------+                 |
            Yes                   | Send NTFY |                 |
             |                    +-----------+                 |
             v                          |                       |
    +--------------------+              v                       |
    | Piggyback new NTFY |     +--------------------+           |
    | w. old outstanding |---->| Notification State |           |
    | NTFY(s)            |     +--------------------+           |
    +--------------------+       |               |              |
                             new RQNT        NTFY response      |
                             received        received           |
                                 |               |              |
                                 |               v              |
                                 |        +-------------+       |
                                 |        | Step mode ? |- No ->+
                                 |        +-------------+       ^
                                 |               |              |
                                 |              Yes             |
                                 |               |              |
                                 |               v              |
                                 |       +---------------+      |
                                 |       | Wait for RQNT |      |
                                 |       +---------------+      |
                                 |               |              |
                                 |         RQNT received        |
                                 |               |              |
                                 |               v              |
                                 |       +---------------+      |
                                 +------>| Apply RQNT and|----->+
                                         | send response |
                                         +---------------+







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   Gateways may also attempt to deliver the pending Notify prior to a
   successful response to the new NotificationRequest by using the
   "piggybacking" functionality of the protocol.  This was in fact
   required behavior in RFC 2705, however there are several
   complications in doing this, and the benefits are questionable.  In
   particular, the RFC 2705 mechanism did not guarantee in-order
   delivery of Notify's and responses to NotificationRequests in
   general, and hence Call Agents had to handle out-of-order delivery of
   these messages anyway.  The change to optional status is thus
   backwards compatible while greatly reducing complexity.

   After receiving the Notification Request command, the requested
   events list and digit map (if a new one was provided) are replaced by
   the newly received parameters, and the current dial string is reset
   to a null value.  Furthermore, when the Notification Request was
   received in the "notification state", the list of observed events is
   reset to a null value.  The subsequent behavior is conditioned by the
   value of the QuarantineHandling parameter.  The parameter may specify
   that quarantined events (and observed events which in this case is
   now an empty list), should be discarded, in which case they will be.
   If the parameter specifies that the quarantined (and observed) events
   are to be processed, the gateway will start processing the list of
   quarantined (and observed) events, using the newly received list of
   requested events and digit map (if provided).  When processing these
   events, the gateway may encounter an event which requires a Notify
   command to be sent.  If that is the case, the gateway will
   immediately transmit a Notify command that will report all events
   that were accumulated in the list of observed events until the
   triggering event, included leaving the unprocessed events in the
   quarantine buffer, and will enter the "notification state".

   A new notification request may be received while the gateway has
   accumulated events according to the previous notification request,
   but has not yet detected a notification-triggering events, i.e., the
   endpoint is not in the "notification state".  The handling of not-
   yet-notified events is determined, as with the quarantined events, by
   the quarantine handling parameter:

   * If the quarantine-handling parameter specifies that quarantined
     events shall be ignored, the observed events list is simply reset.

   * If the quarantine-handling parameter specifies that quarantined
     events shall be processed, the observed event list is transferred
     to the quarantined event list.  The observed event list is then
     reset, and the quarantined event list is processed.






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   Call Agents controlling endpoints in lockstep mode SHOULD provide the
   response to a successful Notify message and the new
   NotificationRequest in the same datagram using the piggybacking
   mechanism.

4.4.2 Explicit Detection

   A key element of the state of several endpoints is the position of
   the hook.  A race condition may occur when the user decides to go
   off-hook before the Call Agent has the time to ask the gateway to
   notify an off-hook event (the "glare" condition well known in
   telephony), or if the user goes on-hook before the Call Agent has the
   time to request the event's notification.

   To avoid this race condition, the gateway MUST check the condition of
   the endpoint before acknowledging a NotificationRequest.  It MUST
   return an error:

   1. If the gateway is requested to notify an "off-hook" transition
      while the phone is already off-hook, (error code 401 - phone off
      hook)

   2. If the gateway is requested to notify an "on-hook" or "flash hook"
      condition while the phone is already on-hook (error code 402 -
      phone on hook).

   Additionally, individual signal definitions can specify that a signal
   will only operate under certain conditions, e.g., ringing may only be
   possible if the phone is already off-hook.  If such prerequisites
   exist for a given signal, the gateway MUST return the error specified
   in the signal definition if the prerequisite is not met.

   It should be noted, that the condition check is performed at the time
   the notification request is received, whereas the actual event that
   caused the current condition may have either been reported, or
   ignored earlier, or it may currently be quarantined.

   The other state variables of the gateway, such as the list of
   RequestedEvents or list of requested signals, are entirely replaced
   after each successful NotificationRequest, which prevents any long
   term discrepancy between the Call Agent and the gateway.

   When a NotificationRequest is unsuccessful, whether it is included in
   a connection-handling command or not, the gateway MUST simply
   continue as if the command had never been received.  As all other
   transactions, the NotificationRequest MUST operate as an atomic
   transaction, thus any changes initiated as a result of the command
   MUST be reverted.



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   Another race condition may occur when a Notify is issued shortly
   before the reception by the gateway of a NotificationRequest.  The
   RequestIdentifier is used to correlate Notify commands with
   NotificationRequest commands thereby enabling the Call Agent to
   determine if the Notify command was generated before or after the
   gateway received the new NotificationRequest.  This is especially
   important to avoid deadlocks in "step" mode.

4.4.3 Transactional Semantics

   As the potential transaction completion times increase, e.g., due to
   external resource reservations, a careful definition of the
   transactional semantics becomes increasingly important.  In
   particular the issue of race conditions, e.g., as it relates to
   hook-state, must be defined carefully.

   An important point to consider is, that the status of a pre-condition
   (e.g., hook-state) may in fact change between the time a transaction
   starts and the time it either completes successfully (transaction
   commit) or fails.  In general, we can say that the successful
   execution of a transaction depends on one or more pre-conditions
   where the status of one or more of the pre-conditions may change
   dynamically between the transaction start and transaction commit.

   The simplest semantics for this is simply to require that all pre-
   conditions be met from the time the transaction is initiated until
   the transaction commits.  If any pre-condition is not met before the
   completion of the transaction, the transaction will also fail.

   As an example, consider a transaction that includes a request for the
   "off-hook" event.  When the transaction is initiated the phone is
   "on-hook" and this pre-condition is therefore met.  If the hook-state
   changes to "off-hook" before the transaction completes, the pre-
   condition is no longer met, and the transaction therefore immediately
   fails.

   Finally, we need to consider the point in time when a new transaction
   takes effect and endpoint processing according to an old transaction
   stops.  For example, assume that transaction T1 has been executed
   successfully and event processing is currently being done according
   to transaction T1.  Now we receive a new transaction T2 specifying
   new event processing (for example a CreateConnection with an
   encapsulated NotificationRequest).  Since we don't know whether T2
   will complete successfully or not, we cannot start processing events
   according to T2 until the outcome of T2 is known.  While we could
   suspend all event processing until the outcome of T2 is known, this
   would make for a less responsive system and hence SHOULD NOT be done.
   Instead, when a new transaction Ty is received and Ty modifies



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   processing according to an old transaction Tx, processing according
   to Tx SHOULD remain active for as long as possible, until a
   successful outcome of Ty is known to occur.  If Ty fails, then
   processing according to Tx will of course continue as usual.  Any
   changes incurred by Ty logically takes effect when Ty commits.  Thus,
   if the endpoint was in the notification state when Ty commits, and Ty
   contained a NotificationRequest, the endpoint will be taken out of
   the notification state when Ty commits.  Note that this is
   independent of whether the endpoint was in the notification state
   when Ty was initiated.  For example, a Notify could be generated due
   to processing according to Tx between the start and commit of Ty.  If
   the commit of Ty leads to the endpoint entering the notification
   state, a new NotificationRequest (Tz) is needed to exit the
   notification state.  This follows from the fact that transaction
   execution respects causal order.

   Another related issue is the use of wildcards, especially the "all
   of" wildcard, which may match more than one endpoint.  When a command
   is requested, and the endpoint identifier matches more than one
   endpoint, transactional semantics still apply.  Thus, the command
   MUST either succeed for all the endpoints, or it MUST fail for all of
   them.  A single response is consequently always issued.

4.4.4 Ordering of Commands, and Treatment of Misorder

   MGCP does not mandate that the underlying transport protocol
   guarantees in-order delivery of commands to a gateway or an endpoint.
   This property tends to maximize the timeliness of actions, but it has
   a few drawbacks.  For example:

   * Notify commands may be delayed and arrive at the Call Agent after
     the transmission of a new Notification Request command,

   * If a new NotificationRequest is transmitted before a previous one
     is acknowledged, there is no guarantee that the previous one will
     not be received and executed after the new one.

   Call Agents that want to guarantee consistent operation of the
   endpoints can use the following rules:

   1) When a gateway handles several endpoints, commands pertaining to
      the different endpoints can be sent in parallel, for example
      following a model where each endpoint is controlled by its own
      process or its own thread.

   2) When several connections are created on the same endpoint,
      commands pertaining to different connections can be sent in
      parallel.



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   3) On a given connection, there should normally be only one
      outstanding command (create or modify).  However, a
      DeleteConnection command can be issued at any time.  In
      consequence, a gateway may sometimes receive a ModifyConnection
      command that applies to a previously deleted connection.  Such
      commands will fail, and an error code MUST be returned (error code
      515 - incorrect connection-id, is RECOMMENDED).

   4) On a given endpoint, there should normally be only one outstanding
      NotificationRequest command at any time.  The RequestId parameter
      MUST be used to correlate Notify commands with the triggering
      notification request.

   5) In some cases, an implicitly or explicitly wildcarded
      DeleteConnection command that applies to a group of endpoints can
      step in front of a pending CreateConnection command.  The Call
      Agent should individually delete all connections whose completion
      was pending at the time of the global DeleteConnection command.
      Also, new CreateConnection commands for endpoints named by the
      wild-carding SHOULD NOT be sent until the wild-carded
      DeleteConnection command is acknowledged.

   6) When commands are embedded within each other, sequencing
      requirements for all commands must be adhered to.  For example a
      Create Connection command with a Notification Request in it must
      adhere to the sequencing requirements associated with both
      CreateConnection and NotificationRequest at the same time.

   7) AuditEndpoint and AuditConnection are not subject to any
      sequencing requirements.

   8) RestartInProgress MUST always be the first command sent by an
      endpoint as defined by the restart procedure.  Any other command
      or non-restart response (see Section 4.4.6), except for responses
      to auditing, MUST be delivered after this RestartInProgress
      command (piggybacking allowed).

   9) When multiple messages are piggybacked in a single packet, the
      messages are always processed in order.

   10) On a given endpoint, there should normally be only one
      outstanding EndpointConfiguration command at any time.

   Gateways MUST NOT make any assumptions as to whether Call Agents
   follow these rules or not.  Consequently gateways MUST always respond
   to commands, regardless of whether they adhere to the above rules or
   not.  To ensure consistent operation, gateways SHOULD behave as
   specified below when one or more of the above rules are not followed:



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   * Where a single outstanding command is expected (ModifyConnection,
     NotificationRequest, and EndpointConfiguration), but the same
     command is received in a new transaction before the old finishes
     executing, the gateway SHOULD fail the previous command.  This
     includes the case where one or more of the commands were
     encapsulated.  The use of error code 407 (transaction aborted) is
     RECOMMENDED.

   * If a ModifyConnection command is received for a pending
     CreateConnection command, the ModifyConnection command SHOULD
     simply be rejected.  The use of error code 400 (transient error) is
     RECOMMENDED.  Note that this situation constitutes a Call Agent
     programming error.

   * If a DeleteConnection command is received for a pending
     CreateConnection or ModifyConnection command, the pending command
     MUST be aborted.  The use of error code 407 (transaction aborted)
     is RECOMMENDED.

   Note, that where reception of a new command leads to aborting an old
   command, the old command SHOULD be aborted regardless of whether the
   new command succeeds or not.  For example, if a ModifyConnection
   command is aborted by a DeleteConnection command which itself fails
   due to an encapsulated NotificationRequest, the ModifyConnection
   command is still aborted.

4.4.5 Endpoint Service States

   As described earlier, endpoints configured for operation may be
   either in-service or out-of-service.  The actual service-state of the
   endpoint is reflected by the combination of the RestartMethod and
   RestartDelay parameters, which are sent with RestartInProgress
   commands (Section 2.3.12) and furthermore may be audited in
   AuditEndpoint commands (Section 2.3.10).

   The service-state of an endpoint affects how it processes a command.
   An endpoint in-service MUST process any command received, whereas an
   endpoint that is out-of-service MUST reject non-auditing commands,
   but SHOULD process auditing commands if possible.  For backwards
   compatibility, auditing commands for an out-of-service endpoint may
   alternatively be rejected as well.  Any command rejected due to an
   endpoint being out-of-service SHOULD generate error code 501
   (endpoint not ready/out-of-service).

   Note that (per Section 2.1.2), unless otherwise specified for a
   command, endpoint names containing the "any of" wildcard only refer
   to endpoints in-service, whereas endpoint names containing the "all
   of" wildcard refer to all endpoints, regardless of service state.



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   The above relationships are illustrated in the table below which
   shows the current service-states and gateway processing of commands
   as a function of the RestartInProgress command sent and the response
   (if any) received to it.  The last column also lists (in parentheses)
   the RestartMethod to be returned if audited:














































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    ------------------------------------------------------------------
   | Restart-  | Restart- |    2xx    | Service- |   Response to      |
   |    Method |    Delay | received ?|    State |   new command      |
   |------------------------------------------------------------------|
   | graceful  |   zero   |   Yes/No  |   In     | non-audit: 2xx     |
   |           |          |           |          | audit:     2xx     |
   |           |          |           |          |        (graceful)  |
   |-----------+----------+-----------+----------+--------------------|
   | graceful  | non-zero |   Yes/No  |   In*    | non-audit: 2xx     |
   |           |          |           |          | audit:     2xx     |
   |           |          |           |          |        (graceful)  |
   |-----------+----------+-----------+----------+--------------------|
   | forced    |   N/A    |   Yes/No  |   Out    | non-audit: 501     |
   |           |          |           |          | audit:     2xx     |
   |           |          |           |          |         (forced)   |
   |-----------+----------+-----------+----------+--------------------|
   | restart   |   zero   |    No     |   In     | non-audit: 2xx,405*|
   |           |          |           |          | audit:     2xx     |
   |           |          |           |          |         (restart)  |
   |-----------+----------+-----------+----------+--------------------|
   | restart   |   zero   |    Yes    |   In     | non-audit: 2xx     |
   |           |          |           |          | audit:     2xx     |
   |           |          |           |          |         (restart)  |
   |-----------+----------+-----------+----------+--------------------|
   | restart   | non-zero |    No     |   Out*   | non-audit: 501*    |
   |           |          |           |          | audit:     2xx     |
   |           |          |           |          |         (restart)  |
   |-----------+----------+-----------+----------+--------------------|
   | restart   | non-zero |    Yes    |   Out*   | non-audit: 501*    |
   |           |          |           |          | audit:     2xx     |
   |           |          |           |          |         (restart)  |
   |-----------+----------+-----------+----------+--------------------|
   | discon-   |   zero/  |    No     |   In     | non-audit: 2xx,    |
   |    nected | non-zero |           |          | audit:     2xx     |
   |           |          |           |          |      (disconnected)|
   |-----------+----------+-----------+----------+--------------------|
   | discon-   |   zero/  |    Yes    |   In     | non-audit: 2xx     |
   |    nected | non-zero |           |          | audit:     2xx     |
   |           |          |           |          |         (restart)  |
   |-----------+----------+-----------+----------+--------------------|
   | cancel-   |   N/A    |   Yes/No  |   In     | non-audit: 2xx     |
   |  graceful |          |           |          | audit:     2xx     |
   |           |          |           |          |         (restart)  |
    ------------------------------------------------------------------







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   Notes (*):

   * The three service-states marked with "*" will change after the
     expiration of the RestartDelay at which time an updated
     RestartInProgress command SHOULD be sent.

   * If the endpoint returns 2xx when the restart procedure has not yet
     completed, then in-order delivery MUST still be satisfied, i.e.,
     piggy-backing is to be used.  If instead, the command is not
     processed, 405 SHOULD be returned.

   * Following a "restart" RestartInProgress with a non-zero
     RestartDelay, error code 501 is only returned until the endpoint
     goes in-service, i.e., until the expiration of the RestartDelay.

4.4.6 Fighting the Restart Avalanche

   Let's suppose that a large number of gateways are powered on
   simultaneously.  If they were to all initiate a RestartInProgress
   transaction, the Call Agent would very likely be swamped, leading to
   message losses and network congestion during the critical period of
   service restoration.  In order to prevent such avalanches, the
   following behavior is REQUIRED:

   1) When a gateway is powered on, it MUST initiate a restart timer to
      a random value, uniformly distributed between 0 and a maximum
      waiting delay (MWD).  Care should be taken to avoid synchronicity
      of the random number generation between multiple gateways that
      would use the same algorithm.

   2) The gateway MUST then wait for either the end of this timer, the
      reception of a command from the Call Agent, or the detection of a
      local user activity, such as for example an off-hook transition on
      a residential gateway.

   3) When the timer elapses, when a command is received, or when an
      activity is detected, the gateway MUST initiate the restart
      procedure.

   The restart procedure simply requires the endpoint to guarantee that
   the first

   * non-audit command, or

   * non-restart response (i.e., error codes other than 405, 501, and
     520) to a non-audit command





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   that the Call Agent sees from this endpoint is a "restart"
   RestartInProgress command.  The endpoint is free to take full
   advantage of piggybacking to achieve this.  Endpoints that are
   considered in-service will have a RestartMethod of "restart", whereas
   endpoints considered out-of-service will have a RestartMethod of
   "forced" (also see Section 4.4.5).  Commands rejected due to an
   endpoint not yet having completed the restart procedure SHOULD use
   error code 405 (endpoint "restarting").

   The restart procedure is complete once a success response has been
   received.  If an error response is received, the subsequent behavior
   depends on the error code in question:

   * If the error code indicates a transient error (4xx), then the
     restart procedure MUST be initiated again (as a new transaction).

   * If the error code is 521, then the endpoint is redirected, and the
     restart procedure MUST be initiated again (as a new transaction).
     The 521 response MUST have included a NotifiedEntity which then is
     the "notified entity" towards which the restart is initiated.  If
     it did not include a NotifiedEntity, the response is treated as any
     other permanent error (see below).

   * If the error is any other permanent error (5xx), and the endpoint
     is not able to rectify the error, then the endpoint no longer
     initiates the restart procedure on its own (until
     rebooted/restarted) unless otherwise specified.  If a command is
     received for the endpoint, the endpoint MUST initiate the restart
     procedure again.

   Note that if the RestartInProgress is piggybacked with the response
   (R) to a command received while restarting, then retransmission of
   the RestartInProgress does not require piggybacking of the response
   R.  However, while the endpoint is restarting, a resend of the
   response R does require the RestartInProgress to be piggybacked to
   ensure in-order delivery of the two.

   Should the gateway enter the "disconnected" state while carrying out
   the restart procedure, the disconnected procedure specified in
   Section 4.4.7 MUST be carried out, except that a "restart" rather
   than "disconnected" message is sent during the procedure.

   Each endpoint in a gateway will have a provisionable Call Agent,
   i.e., "notified entity", to direct the initial restart message
   towards.  When the collection of endpoints in a gateway is managed by
   more than one Call Agent, the above procedure MUST be performed for
   each collection of endpoints managed by a given Call Agent.  The
   gateway MUST take full advantage of wild-carding to minimize the



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   number of RestartInProgress messages generated when multiple
   endpoints in a gateway restart and the endpoints are managed by the
   same Call Agent.  Note that during startup, it is possible for
   endpoints to start out as being out-of-service, and then become in-
   service as part of the gateway initialization procedure.  A gateway
   may thus choose to send first a "forced" RestartInProgress for all
   its endpoints, and subsequently a "restart" RestartInProgress for the
   endpoints that come in-service.  Alternatively, the gateway may
   simply send "restart" RestartInProgress for only those endpoints that
   are in-service, and "forced" RestartInProgress for the specific
   endpoints that are out-of-service.  Wild-carding MUST still be used
   to minimize the number of messages sent though.

   The value of MWD is a configuration parameter that depends on the
   type of the gateway.  The following reasoning can be used to
   determine the value of this delay on residential gateways.

   Call agents are typically dimensioned to handle the peak hour traffic
   load, during which, in average, 10% of the lines will be busy,
   placing calls whose average duration is typically 3 minutes.  The
   processing of a call typically involves 5 to 6 MGCP transactions
   between each endpoint and the Call Agent.  This simple calculation
   shows that the Call Agent is expected to handle 5 to 6 transactions
   for each endpoint, every 30 minutes on average, or, to put it
   otherwise, about one transaction per endpoint every 5 to 6 minutes on
   average.  This suggest that a reasonable value of MWD for a
   residential gateway would be 10 to 12 minutes.  In the absence of
   explicit configuration, residential gateways should adopt a value of
   600 seconds for MWD.

   The same reasoning suggests that the value of MWD should be much
   shorter for trunking gateways or for business gateways, because they
   handle a large number of endpoints, and also because the usage rate
   of these endpoints is much higher than 10% during the peak busy hour,
   a typical value being 60%.  These endpoints, during the peak hour,
   are thus expected to contribute about one transaction per minute to
   the Call Agent load.  A reasonable algorithm is to make the value of
   MWD per "trunk" endpoint six times shorter than the MWD per
   residential gateway, and also inversely proportional to the number of
   endpoints that are being restarted.  For example MWD should be set to
   2.5 seconds for a gateway that handles a T1 line, or to 60
   milliseconds for a gateway that handles a T3 line.









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4.4.7 Disconnected Endpoints

   In addition to the restart procedure, gateways also have a
   "disconnected" procedure, which MUST be initiated when an endpoint
   becomes "disconnected" as described in Section 4.3.  It should here
   be noted, that endpoints can only become disconnected when they
   attempt to communicate with the Call Agent.  The following steps MUST
   be followed by an endpoint that becomes "disconnected":

   1. A "disconnected" timer is initialized to a random value, uniformly
      distributed between 1 and a provisionable "disconnected" initial
      waiting delay (Tdinit), e.g., 15 seconds.  Care MUST be taken to
      avoid synchronicity of the random number generation between
      multiple gateways and endpoints that would use the same algorithm.

   2. The gateway then waits for either the end of this timer, the
      reception of a command for the endpoint from the Call Agent, or
      the detection of a local user activity for the endpoint, such as
      for example an off-hook transition.

   3. When the "disconnected" timer elapses for the endpoint, when a
      command is received for the endpoint, or when local user activity
      is detected for the endpoint, the gateway initiates the
      "disconnected" procedure for the endpoint - if a disconnected
      procedure was already in progress for the endpoint, it is simply
      replaced by the new one.  Furthermore, in the case of local user
      activity, a provisionable "disconnected" minimum waiting delay
      (Tdmin) MUST have elapsed since the endpoint became disconnected
      or the last time it ended the "disconnected" procedure in order to
      limit the rate at which the procedure is performed.  If Tdmin has
      not passed, the endpoint simply proceeds to step 2 again, without
      affecting any disconnected procedure already in progress.

   4. If the "disconnected" procedure still left the endpoint
      disconnected, the "disconnected" timer is then doubled, subject to
      a provisionable "disconnected" maximum waiting delay (Tdmax),
      e.g., 600 seconds, and the gateway proceeds with step 2 again
      (using a new transaction-id).

   The "disconnected" procedure is similar to the restart procedure in
   that it simply states that the endpoint MUST send a RestartInProgress
   command to the Call Agent informing it that the endpoint was
   disconnected.  Furthermore, the endpoint MUST guarantee that the
   first non-audit message (non-audit command or response to non-audit
   command) that the Call Agent sees from this endpoint MUST inform the
   Call Agent that the endpoint is disconnected (unless the endpoint
   goes out-of-service).  When a command (C) is received, this is
   achieved by sending a piggy-backed datagram with a "disconnected"



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   RestartInProgress command and the response to command C to the source
   address of command C as opposed to the current "notified entity".
   This piggy-backed RestartInProgress is not automatically
   retransmitted by the endpoint but simply relies on fate-sharing with
   the piggy-backed response to guarantee the in-order delivery
   requirement.  The Call Agent still sends a response to the piggy-
   backed RestartInProgress, however, as usual, the response may be
   lost.  In addition to the piggy-backed RestartInProgress command, a
   new "disconnected" procedure is triggered by the command received.
   This will lead to a non piggy-backed copy (i.e., same transaction) of
   the "disconnected" RestartInProgress command being sent reliably to
   the current "notified entity".

   When the Call Agent learns that the endpoint is disconnected, the
   Call Agent may then for instance decide to audit the endpoint, or
   simply clear all connections for the endpoint.  Note that each such
   "disconnected" procedure will result in a new RestartInProgress
   command, which will be subject to the normal retransmission
   procedures specified in Section 4.3.  At the end of the procedure,
   the endpoint may thus still be "disconnected".  Should the endpoint
   go out-of-service while being disconnected, it SHOULD send a "forced"
   RestartInProgress message as described in Section 2.3.12.

   The disconnected procedure is complete once a success response has
   been received.  Error responses are handled similarly to the restart
   procedure (Section 4.4.6).  If the "disconnected" procedure is to be
   initiated again following an error response, the rate-limiting timer
   considerations specified above still apply.

   Note, that if the RestartInProgress is piggybacked with the response
   (R) to a command received while being disconnected, then
   retransmission of this particular RestartInProgress does not require
   piggybacking of the response R.  However, while the endpoint is
   disconnected, resending the response R does require the
   RestartInProgress to be piggybacked with the response to ensure the
   in-order delivery of the two.

   If a set of disconnected endpoints have the same "notified entity",
   and the set of endpoints can be named with a wildcard, the gateway
   MAY replace the individual disconnected procedures with a suitably
   wildcarded disconnected procedure instead.  In that case, the Restart
   Delay for the wildcarded "disconnected" RestartInProgress command
   SHALL be the Restart Delay corresponding to the oldest disconnected
   procedure replaced.  Note that if only a subset of these endpoints
   subsequently have their "notified entity" changed and/or are no
   longer disconnected, then that wildcarded disconnected procedure can
   no longer be used.  The remaining individual disconnected procedures
   MUST then be resumed again.



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   A disconnected endpoint may wish to send a command (besides
   RestartInProgress) while it is disconnected.  Doing so will only
   succeed once the Call Agent is reachable again, which raises the
   question of what to do with such a command meanwhile.  At one
   extreme, the endpoint could drop the command right away, however that
   would not work very well when the Call Agent was in fact available,
   but the endpoint had not yet completed the "disconnected" procedure
   (consider for example the case where a NotificationRequest was just
   received which immediately resulted in a Notify being generated).  To
   prevent such scenarios, disconnected endpoints SHALL NOT blindly drop
   new commands to be sent for a period of T-MAX seconds after they
   receive a non-audit command.

   One way of satisfying this requirement is to employ a temporary
   buffering of commands to be sent, however in doing so, the endpoint
   MUST ensure, that it:

   * does not build up a long queue of commands to be sent,

   * does not swamp the Call Agent by rapidly sending too many commands
     once it is connected again.

   Buffering commands for T-MAX seconds and, once the endpoint is
   connected again, limiting the rate at which buffered commands are
   sent to one outstanding command per endpoint is considered acceptable
   (see also Section 4.4.8, especially if using wildcards).  If the
   endpoint is not connected within T-MAX seconds, but a "disconnected"
   procedure is initiated within T-MAX seconds, the endpoint MAY
   piggyback the buffered command(s) with that RestartInProgress.  Note,
   that once a command has been sent, regardless of whether it was
   buffered initially, or piggybacked earlier, retransmission of that
   command MUST cease T-MAX seconds after the initial send as described
   in Section 4.3.

   This specification purposely does not specify any additional behavior
   for a disconnected endpoint.  Vendors MAY for instance choose to
   provide silence, play reorder tone, or even enable a downloaded wav
   file to be played.

   The default value for Tdinit is 15 seconds, the default value for
   Tdmin, is 15 seconds, and the default value for Tdmax is 600 seconds.










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4.4.8 Load Control in General

   The previous sections have described several MGCP mechanisms to deal
   with congestion and overload, namely:

   * the UDP retransmission strategy which adapts to network and call
     agent congestion on a per endpoint basis,

   * the guidelines on the ordering of commands which limit the number
     of commands issued in parallel,

   * the restart procedure which prevents flooding in case of a restart
     avalanche, and

   * the disconnected procedure which prevents flooding in case of a
     large number of disconnected endpoints.

   It is however still possible for a given set of endpoints, either on
   the same or different gateways, to issue one or more commands at a
   given point in time.  Although it can be argued, that Call Agents
   should be sized to handle one message per served endpoint at any
   given point in time, this may not always be the case in practice.
   Similarly, gateways may not be able to handle a message for all of
   its endpoints at any given point in time.  In general, such issues
   can be dealt with through the use of a credit-based mechanism, or by
   monitoring and automatically adapting to the observed behavior.  We
   opt for the latter approach as follows.

   Conceptually, we assume that Call Agents and gateways maintain a
   queue of incoming transactions to be executed.  Associated with this
   transaction queue is a high-water and a low-water mark.  Once the
   queue length reaches the high-water mark, the entity SHOULD start
   issuing 101 provisional responses (transaction queued) until the
   queue length drops to the low-water mark.  This applies to new
   transactions as well as to retransmissions.  If the entity is unable
   to process any new transactions at this time, it SHOULD return error
   code 409 (processing overload).

   Furthermore, gateways SHOULD adjust the sending rate of new commands
   to a given Call Agent by monitoring the observed response times from
   that Call Agent to a *set* of endpoints.  If the observed smoothed
   average response time suddenly rises significantly over some
   threshold, or the gateway receives a 101 (transaction queued) or 409
   (overload) response, the gateway SHOULD adjust the sending rate of
   new commands to that Call Agent accordingly.  The details of the
   smoothing average algorithm, the rate adjustments, and the thresholds
   involved are for further study, however they MUST be configurable.




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   Similarly, Call Agents SHOULD adjust the sending rate of new
   transactions to a given gateway by monitoring the observed response
   times from that gateway for a *set* of endpoints.  If the observed
   smoothed average response time suddenly rises significantly over some
   threshold, or the Call Agent receives a 101 (transaction queued) or
   409 (overloaded), the Call Agent SHOULD adjust the sending rate of
   new commands to that gateway accordingly.  The details of the
   smoothing average algorithm, the rate adjustments, and the thresholds
   involved are for further study, however they MUST be configurable.

5. Security Requirements

   Any entity can send a command to an MGCP endpoint.  If unauthorized
   entities could use the MGCP, they would be able to set-up
   unauthorized calls, or to interfere with authorized calls.  We expect
   that MGCP messages will always be carried over secure Internet
   connections, as defined in the IP security architecture as defined in
   RFC 2401, using either the IP Authentication Header, defined in RFC
   2402, or the IP Encapsulating Security Payload, defined in RFC 2406.
   The complete MGCP protocol stack would thus include the following
   layers:

                -------------------------------
               |              MGCP             |
               |-------------------------------|
               |              UDP              |
               |-------------------------------|
               |          IP security          |
               | (authentication or encryption)|
               |-------------------------------|
               |              IP               |
               |-------------------------------|
               |       transmission media      |
                -------------------------------

   Adequate protection of the connections will be achieved if the
   gateways and the Call Agents only accept messages for which IP
   security provided an authentication service.  An encryption service
   will provide additional protection against eavesdropping, thus
   preventing third parties from monitoring the connections set up by a
   given endpoint.

   The encryption service will also be requested if the session
   descriptions are used to carry session keys, as defined in SDP.







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   These procedures do not necessarily protect against denial of service
   attacks by misbehaving gateways or misbehaving Call Agents.  However,
   they will provide an identification of these misbehaving entities,
   which should then be deprived of their authorization through
   maintenance procedures.

5.1 Protection of Media Connections

   MGCP allows Call Agent to provide gateways with "session keys" that
   can be used to encrypt the audio messages, protecting against
   eavesdropping.

   A specific problem of packet networks is "uncontrolled barge-in".
   This attack can be performed by directing media packets to the IP
   address and UDP port used by a connection.  If no protection is
   implemented, the packets will be decoded and the signals will be
   played on the "line side".

   A basic protection against this attack is to only accept packets from
   known sources, however this tends to conflict with RTP principles.
   This also has two inconveniences:  it slows down connection
   establishment and it can be fooled by source spoofing:

   * To enable the address-based protection, the Call Agent must obtain
     the source address of the egress gateway and pass it to the ingress
     gateway.  This requires at least one network round trip, and leaves
     us with a dilemma:  either allow the call to proceed without
     waiting for the round trip to complete, and risk for example
     "clipping" a remote announcement, or wait for the full round trip
     and settle for slower call-set-up procedures.

   * Source spoofing is only effective if the attacker can obtain valid
     pairs of source and destination addresses and ports, for example by
     listening to a fraction of the traffic.  To fight source spoofing,
     one could try to control all access points to the network.  But
     this is in practice very hard to achieve.

   An alternative to checking the source address is to encrypt and
   authenticate the packets, using a secret key that is conveyed during
   the call set-up procedure.  This will not slow down the call set-up,
   and provides strong protection against address spoofing.

6. Packages

   As described in Section 2.1.6, packages are the preferred way of
   extending MGCP.  In this section we describe the requirements
   associated with defining a package.




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   A package MUST have a unique package name defined.  The package name
   MUST be registered with the IANA, unless it starts with the
   characters "x-" or "x+" which are reserved for experimental packages.
   Please refer to Appendix C for IANA considerations.

   A package MUST also have a version defined which is simply a non-
   negative integer.  The default and initial version of a package is
   zero, the next version is one, etc.  New package versions MUST be
   completely backwards compatible, i.e., a new version of a package
   MUST NOT redefine or remove any of the extensions provided in an
   earlier version of the package.  If such a need arises, a new package
   name MUST be used instead.

   Packages containing signals of type time-out MAY indicate if the "to"
   parameter is supported for all the time-out signals in the package as
   well as the default rounding rules associated with these (see Section
   3.2.2.4).  If no such definition is provided, each time-out signal
   SHOULD provide these definitions.

   A package defines one or more of the following extensions:

   * Actions

   * BearerInformation

   * ConnectionModes

   * ConnectionParameters

   * DigitMapLetters

   * Events and Signals

   * ExtensionParameters

   * LocalConnectionOptions

   * ReasonCodes

   * RestartMethods

   * Return codes

   For each of the above types of extensions supported by the package,
   the package definition MUST contain a description of the extension as
   defined in the following sections.  Please note, that package
   extensions, just like any other extension, MUST adhere to the MGCP
   grammar.



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6.1 Actions

   Extension Actions SHALL include:

   * The name and encoding of the extension action.

   * If the extension action takes any action parameters, then the name,
     encoding, and possible values of those parameters.

   * A description of the operation of the extension action.

   * A listing of the actions in this specification the extension can be
     combined with.  If such a listing is not provided, it is assumed
     that the extension action cannot be combined with any other action
     in this specification.

   * If more than one extension action is defined in the package, then a
     listing of the actions in the package the extension can be combined
     with.  If such a listing is not provided, it is assumed that the
     extension action cannot be combined with any other action in the
     package.

   Extension actions defined in two or more different packages SHOULD
   NOT be used simultaneously, unless very careful consideration to
   their potential interaction and side-effects has been given.

6.2 BearerInformation

   BearerInformation extensions SHALL include:

   * The name and encoding of the BearerInformation extension.

   * The possible values and encoding of those values that can be
     assigned to the BearerInformation extension.

   * A description of the operation of the BearerInformation extension.
     As part of this description the default value (if any) if the
     extension is omitted in an EndpointConfiguration command MUST be
     defined.  It may be necessary to make a distinction between the
     default value before and after the initial application of the
     parameter, for example if the parameter retains its previous value
     once specified, until explicitly altered.  If default values are
     not described, then the extension parameter simply defaults to
     empty in all EndpointConfiguration commands.

   Note that the extension SHALL be included in the result for an
   AuditEndpoint command auditing the BearerInformation.




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6.3 ConnectionModes

   Extension Connection Modes SHALL include:

   * The name and encoding of the extension connection mode.

   * A description of the operation of the extension connection mode.

   * A description of the interaction a connection in the extension
     connection mode will have with other connections in each of the
     modes defined in this specification.  If such a description is not
     provided, the extension connection mode MUST NOT have any
     interaction with other connections on the endpoint.

   Extension connection modes SHALL NOT be included in the list of modes
   in a response to an AuditEndpoint for Capabilities, since the package
   will be reported in the list of packages.

6.4 ConnectionParameters

   Extension Connection Parameters SHALL include:

   * The name and encoding of the connection parameter extension.

   * The possible values and encoding of those values that can be
     assigned to the connection parameter extension.

   * A description of how those values are derived.

   Note that the extension connection parameter MUST be included in the
   result for an AuditConnection command auditing the connection
   parameters.

6.5 DigitMapLetters

   Extension Digit Map Letters SHALL include:

   * The name and encoding of the extension digit map letter(s).

   * A description of the meaning of the extension digit map letter(s).

   Note that extension DigitMapLetters in a digit map do not follow the
   normal naming conventions for extensions defined in packages.  More
   specifically the package name and slash ("/") will not be part of the
   extension name, thereby forming a flat and limited name space with
   potential name clashing.





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   Therefore, a package SHALL NOT define a digit map letter extension
   whose encoding has already been used in another package.  If two
   packages have used the same encoding for a digit map letter
   extension, and those two packages are supported by the same endpoint,
   the result of using that digit map letter extension is undefined.

   Note that although an extension DigitMapLetter does not include the
   package name prefix and slash ("/") as part of the extension name
   within a digit map, the package name prefix and slash are included
   when the event code for the event that matched the DigitMapLetter is
   reported as an observed event.  In other words, the digit map just
   define the matching rule(s), but the event is still reported like any
   other event.

6.6 Events and Signals

   The event/signal definition SHALL include the precise name of the
   event/signal (i.e., the code used in MGCP), a plain text definition
   of the event/signal, and, when appropriate, the precise definition of
   the corresponding events/signals, for example the exact frequencies
   of audio signals such as dial tones or DTMF tones.

   The package description MUST provide, for each event/signal, the
   following information:

   * The description of the event/signal and its purpose, which SHOULD
     include the actual signal that is generated by the client (e.g., xx
     ms FSK tone) as well as the resulting user observed result (e.g.,
     Message Waiting light on/off).

   The event code used for the event/signal.

   * The detailed characteristics of the event/signal, such as for
     example frequencies and amplitude of audio signals, modulations and
     repetitions.  Such details may be country specific.

   * The typical and maximum duration of the event/signal if applicable.

   * If the signal or event can be applied to a connection (across a
     media stream), it MUST be indicated explicitly.  If no such
     indication is provided, it is assumed that the signal or event
     cannot be applied to a connection.

   For events, the following MUST be provided as well:

   * An indication if the event is persistent.  By default, events are
     not persistent - defining events as being persistent is discouraged
     (see Appendix B for a preferred alternative).  Note that persistent



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     events will automatically trigger a Notify when they occur, unless
     the Call Agent explicitly instructed the endpoint otherwise.  This
     not only violates the normal MGCP model, but also assumes the Call
     Agent supports the package in question.  Such an assumption is
     unlikely to hold in general.

   * An indication if there is an auditable event-state associated with
     the event.  By default, events do not have auditable event-states.

   * If event parameters are supported, it MUST be stated explicitly.
     The precise syntax and semantics of these MUST then be provided
     (subject to the grammar provided in Appendix A).  It SHOULD also be
     specified whether these parameters apply to RequestedEvents,
     ObservedEvents, DetectEvents and EventStates.  If not specified
     otherwise, it is assumed that:

     * they do not apply to RequestedEvents,

     * they do apply to ObservedEvents,

     * they apply in the same way to DetectEvents as they do to
       RequestedEvents for a given event parameter,

     * they apply in the same way to EventStates as they do to
       ObservedEvents for a given event parameter.

   * If the event is expected to be used in digit map matching, it
     SHOULD explicitly state so.  Note that only events with single
     letter or digit parameter codes can do this.  See Section 2.1.5 for
     further details.

   For signals, the following MUST be provided as well:

   * The type of signal (OO, TO, BR).

   * Time-Out signals SHOULD have an indication of the default time-out
     value.  In some cases, time-out values may be variable (if
     dependent on some action to complete such as out-pulsing digits).

   * If signal parameters are supported, it MUST be stated explicitly.
     The precise syntax and semantics of these MUST then be provided
     (subject to the grammar provided in Appendix A).

   * Time-Out signals may also indicate whether the "to" parameter is
     supported or not as well as what the rounding rules associated with
     them are.  If omitted from the signal definition, the package-wide
     definition is assumed (see Section 6).  If the package definition
     did not specify this, rounding rules default to the nearest non-



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     zero second, whereas support for the "to" parameter defaults to
     "no" for package version zero, and "yes" for package versions one
     and higher.

   The following format is RECOMMENDED for defining events and signals
   in conformance with the above:

    ------------------------------------------------------------------
   | Symbol  |   Definition               |  R  |   S     Duration    |
   |---------|----------------------------|-----|---------------------|
   |         |                            |     |                     |
   |         |                            |     |                     |
    ------------------------------------------------------------------

   where:

   * Symbol indicates the event code used for the event/signal, e.g.,
     "hd".

   * Definition gives a brief definition of the event/signal

   * R contains an "x" if the event can be detected or one or more of
     the following symbols:

     - "P" if the event is persistent.

     - "S" if the events is an event-state that may be audited.

     - "C" if the event can be detected on a connection.

   * S contains one of the following if it is a signal:

     - "OO" if the signal is On/Off signal.

     - "TO" if the signal is a Time-Out signal.

     - "BR" if the signal is a Brief signal.

   * S also contains:

     - "C" if the signal can be applied on a connection.

   The table SHOULD then be followed by a more comprehensive description
   of each event/signal defined.







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6.6.1 Default and Reserved Events

   All packages that contain Time-Out type signals contain the operation
   failure ("of") and operation complete ("oc") events, irrespective of
   whether they are provided as part of the package description or not.
   These events are needed to support Time-Out signals and cannot be
   overridden in packages with Time-Out signals.  They MAY be extended
   if necessary, however such practice is discouraged.

   If a package without Time-Out signals does contain definitions for
   the "oc" and "of" events, the event definitions provided in the
   package MAY over-ride those indicated here.  Such practice is however
   discouraged and is purely allowed to avoid potential backwards
   compatibility problems.

   It is considered good practice to explicitly mention that the two
   events are supported in accordance with their default definitions,
   which are as follows:

    ------------------------------------------------------------------
   | Symbol  |   Definition               |  R  |   S     Duration    |
   |---------|----------------------------|-----|---------------------|
   | oc      | Operation Complete         |  x  |                     |
   | of      | Operation Failure          |  x  |                     |
    ------------------------------------------------------------------

   Operation complete (oc):  The operation complete event is generated
   when the gateway was asked to apply one or several signals of type TO
   on the endpoint or connection, and one or more of those signals
   completed without being stopped by the detection of a requested event
   such as off-hook transition or dialed digit.  The completion report
   should carry as a parameter the name of the signal that came to the
   end of its live time, as in:

      O: G/oc(G/rt)

   In this case, the observed event occurred because the "rt" signal in
   the "G" package timed out.

   If the reported signal was applied on a connection, the parameter
   supplied will include the name of the connection as well, as in:

      O: G/oc(G/rt@0A3F58)

   When the operation complete event is requested, it cannot be
   parameterized with any event parameters.  When the package name is
   omitted (which is discouraged) as part of the signal name, the
   default package is assumed.



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   Operation failure (of):  The operation failure event is generated
   when the endpoint was asked to apply one or several signals of type
   TO on the endpoint or connection, and one or more of those signals
   failed prior to timing out.  The completion report should carry as a
   parameter the name of the signal that failed, as in:

      O: G/of(G/rt)

   In this case a failure occurred in producing the "rt" signal in the
   "G" package.

   When the reported signal was applied on a connection, the parameter
   supplied will include the name of the connection as well, as in:

      O: G/of(G/rt@0A3F58)

   When the operation failure event is requested, event parameters can
   not be specified.  When the package name is omitted (which is
   discouraged), the default package name is assumed.

6.7 ExtensionParameters

   Extension parameter extensions SHALL include:

   * The name and encoding of the extension parameter.

   * The possible values and encoding of those values that can be
     assigned to the extension parameter.

   * For each of the commands defined in this specification, whether the
     extension parameter is Mandatory, Optional, or Forbidden in
     requests as well as responses.  Note that extension parameters
     SHOULD NOT normally be mandatory.

   * A description of the operation of the extension parameter.  As part
     of this description the default value (if any) if the extension is
     omitted in a command MUST be defined.  It may be necessary to make
     a distinction between the default value before and after the
     initial application of the parameter, for example if the parameter
     retains its previous value once specified, until explicitly
     altered.  If default values are not described, then the extension
     parameter simply defaults to empty in all commands.

   * Whether the extension can be audited in AuditEndpoint and/or
     AuditConnection as well as the values returned.  If nothing is
     specified, then auditing of the extension parameter can only be
     done for AuditEndpoint, and the value returned SHALL be the current
     value for the extension.  Note that this may be empty.



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6.8 LocalConnectionOptions

   LocalConnectionOptions extensions SHALL include:

   * The name and encoding of the LocalConnectionOptions extension.

   * The possible values and encoding of those values that can be
     assigned to the LocalConnectionOptions extension.

   * A description of the operation of the LocalConnectionOptions
     extension.  As part of this description the following MUST be
     specified:

     - The default value (if any) if the extension is omitted in a
       CreateConnection command.

     - The default value if omitted in a ModifyConnection command.  This
       may be to simply retain the previous value (if any) or to apply
       the default value.  If nothing is specified, the current value is
       retained if possible.

     - If Auditing of capabilities will result in the extension being
       returned, then a description to that effect as well as with what
       possible values and their encoding (note that the package itself
       will always be returned).  If nothing is specified, the extension
       SHALL NOT be returned when auditing capabilities.

   Also note, that the extension MUST be included in the result for an
   AuditConnection command auditing the LocalConnectionOptions.

6.9 Reason Codes

   Extension reason codes SHALL include:

   * The number for the reason code.  The number MUST be in the range
     800 to 899.

   * A description of the extension reason code including the
     circumstances that leads to the generation of the reason code.
     Those circumstances SHOULD be limited to events caused by another
     extension defined in the package to ensure the recipient will be
     able to interpret the extension reason code correctly.

   Note that the extension reason code may have to be provided in the
   result for an AuditEndpoint command auditing the reason code.






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6.10 RestartMethods

   Extension Restart Methods SHALL include:

   * The name and encoding for the restart method.

   * A description of the restart method including the circumstances
     that leads to the generation of the restart method.  Those
     circumstances SHOULD be limited to events caused by another
     extension defined in the package to ensure the recipient will be
     able to interpret the extension restart method correctly.

   * An indication of whether the RestartDelay parameter is to be used
     with the extension.  If nothing is specified, it is assumed that it
     is not to be used.  In that case, RestartDelay MUST be ignored if
     present.

   * If the restart method defines a service state, the description MUST
     explicitly state and describe this.  In that case, the extension
     restart method can then be provided in the result for an
     AuditEndpoint command auditing the restart method.

6.11 Return Codes

   Extension Return Codes SHALL include:

   * The number for the extension return code.  The number MUST be in
     the range 800 to 899.

   * A description of the extension return code including the
     circumstances that leads to the generation of the extension return
     code.  Those circumstances SHOULD be limited to events caused by
     another extension defined in the package to ensure the recipient
     will be able to interpret the extension return code correctly.

7. Versions and Compatibility

7.1 Changes from RFC 2705

   RFC 2705 was issued in October 1999, as the last update of draft
   version 0.5.  This updated document benefits from further
   implementation experience.  The main changes from RFC 2705 are:

   * Contains several clarifications, editorial changes and resolution
     of known inconsistencies.

   * Firmed up specification language in accordance with RFC 2119 and
     added RFC 2119 conventions section.



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   * Clarified behavior of mixed wild-carding in endpoint names.

   * Deleted naming requirement about having first term identify the
     physical gateway when the gateway consists of multiple physical
     gateways.  Also added recommendations on wild-carding naming usage
     from the right only, as well as mixed wildcard usage.

   * Clarified that synonymous forms and values for endpoint names are
     not freely interchangeable.

   * Allowed IPv6 addresses in endpoint names.

   * Clarified Digit Map matching rules.

   * Added missing semantics for symbols used in digit maps.

   * Added Timer T description in Digit Maps.

   * Added recommendation to support digit map sizes of at least 2048
     bytes per endpoint.

   * Clarified use of wildcards in several commands.

   * Event and Signal Parameters formally defined for events and
     signals.

   * Persistent events now allowed in base MGCP protocol.

   * Added additional detail on connection wildcards.

   * Clarified behavior of loopback, and continuity test connection
     modes for mixing and multiple connections in those modes.

   * Modified BearerInformation to be conditional optional in the
     EndpointConfiguration command.

   * Clarified "swap audio" action operation for one specific scenario
     and noted that operation for other scenarios is undefined.

   * Added recommendation that all implementations support PCMU encoding
     for interoperability.

   * Changed Bandwidth LocalConnectionOptions value from excluding to
     including overhead from the IP layer and up for consistency with
     SDP.

   * Clarified that mode of second connection in a CreateConnection
     command will be set to "send/receive".



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   * Type of service default changed to zero.

   * Additional detail on echo cancellation, silence suppression, and
     gain control.  Also added recommendation for Call Agents not to
     specify handling of echo cancellation and gain control.

   * Added requirement for a connection to have a
     RemoteConnectionDescriptor in order to use the "network loopback"
     and "network continuity test" modes.

   * Removed procedures and specification for NAS's (will be provided as
     package instead).

   * Removed procedures and specification for ATM (will be provided as
     package instead).

   * Added missing optional NotifiedEntity parameter to the
     DeleteConnection (from the Call Agent) MGCI command.

   * Added optional new MaxMGCPDatagram RequestedInfo code for
     AuditEndpoint to enable auditing of maximum size of MGCP datagrams
     supported.

   * Added optional new PackageList RequestedInfo code for AuditEndpoint
     to enable auditing of packages with a package version number.
     PackageList parameter also allowed with return code 518
     (unsupported package).

   * Added missing attributes in Capabilities.

   * Clarified that at the expiration of a non-zero restart delay, an
     updated RestartInProgress should be sent.  Also clarified that a
     new NotifiedEntity can only be returned in response to a
     RestartInProgress command.

   * Added Response Acknowledgement response (return code 000) and
     included in three-way handshake.

   * ResponseAck parameter changed to be allowed in all commands.

   * Added return codes 101, 405, 406, 407, 409, 410, 503, 504, 505,
     506, 507, 508, 509, 533, 534, 535, 536, 537, 538, 539, 540, 541,
     and defined return codes in range 800-899 to be package specific
     return codes.  Additional text provided for some return codes and
     additional detail on how to handle unknown return codes added.

   * Added reason code 903, 904, 905 and defined reason codes 800-899 to
     be package specific reason codes.



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   * Added section clarifying codec negotiation procedure.

   * Clarified that resource reservation parameters in a
     ModifyConnection command defaults to the current value used.

   * Clarified that connection mode is optional in ModifyConnection
     commands.

   * Corrected LocalConnectionDescriptor to be optional in response to
     CreateConnection commands (in case of failure).

   * Clarified that quoted-strings are UTF-8 encoded and
     interchangeability of quoted strings and unquoted strings.

   * Clarified that Transaction Identifiers are compared as numerical
     values.

   * Clarified bit-ordering for Type Of Service LocalConnectionOptions.

   * Clarified the use of RequestIdentifier zero.

   * Added example sections for commands, responses, and some call
     flows.

   * Corrected usage of and requirements for SDP to be strictly RFC 2327
     compliant.

   * Added requirement that all MGCP implementations must support MGCP
     datagrams up to at least 4000 bytes.  Also added new section on
     Maximum Datagram Size, Fragmentation and reassembly.

   * Generalized piggybacking retransmission scheme to only state
     underlying requirements to be satisfied.

   * Clarified the section on computing retransmission timers.

   * Clarified operation of long-running transactions, including
     provisional responses, retransmissions and failures.

   * Enhanced description of provisional responses and interaction with
     three-way handshake.

   * Enhanced description of fail-over and the role of "notified
     entity".  An empty "notified entity" has been allowed, although
     strongly discouraged.






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   * Clarified retransmission procedure and removed "wrong key"
     considerations from it.  Also fixed inconsistencies between Max1
     and Max2 retransmission boundaries and the associated flow diagram.

   * Updated domain name resolution for retransmission procedure to
     incur less overhead when multiple endpoints are retransmitting.

   * Removed requirement for in-order delivery of NotificationRequests
     response and Notify commands.  Notify commands are still delivered
     in-order.

   * Clarified that activating an embedded Notification Request does not
     clear the list of ObservedEvents.

   * Defined interactions between disconnected state and notification
     state.

   * Added section on transactional semantics.

   * Defined gateway behavior when multiple interacting transactions are
     received.

   * Additional details provided on service states.  Clarified
     relationship between endpoint service states, restart methods, and
     associated processing of commands.

   * Clarified operation for transitioning from "restart procedure" to
     "disconnected state".

   * Allowed auditing commands and responses to bypass the "restart" and
     "disconnected" procedures.

   * Clarified operation of "disconnected procedure" and in particular
     the operation of piggy-backed "disconnected" RestartInProgress
     messages.

   * Added option to aggregate "disconnected" RestartInProgress messages
     under certain conditions to reduce message volume.

   * Defined additional behavior for endpoints wishing to send commands
     while in the "disconnected" state.

   * Added new section on Load Control in General which includes two new
     error codes (101 and 409) to handle overload.

   * Deleted the "Proposed MoveConnection command".





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   * Removed packages from protocol specification (will be provided in
     separate documents instead).

   * Package concept formally extended to be primary extension mechanism
     now allowing extensions for the following to be defined in packages
     as well:

     - BearerInformation

     - LocalConnectionOptions

     - ExtensionParameters

     - Connection Modes

     - Actions

     - Digit Map Letters

     - Connection Parameters

     - Restart Methods

     - Reason Codes

     - Return Codes

   * Requirements and suggested format for package definitions added.

   * Defined "operation complete" and "operation failure" events to be
     automatically present in packages with Time-Out signals.

   * Deleted list of differences that were prior to RFC 2705.

   * Added Base Package to deal with quarantine buffer overflow,
     ObservedEvents overflow, embedded NotificationRequest failure, and
     to enable events to be requested persistently.  A new "Message"
     command is included as well.

   * IANA registration procedures for packages and other extensions
     added.

   * Updated grammar to fix known errors and support new extensions in a
     backwards compatible manner.  Added new (optional) PackageList and
     MaxMGCPDatagram for auditing.  Changed explicit white space rules
     in some productions to make grammar more consistent.

   * Connection Mode interaction table added.



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   * Added additional detail on virtual endpoint naming conventions.
     Also added suggested gateway endpoint convention and a "Range
     Wildcard" option to the Endpoint Naming Conventions.

8. Security Considerations

   Security issues are discussed in section 5.

9. Acknowledgements

   Special thanks are due to the authors of the original MGCP 1.0
   specification:  Mauricio Arango, Andrew Dugan, Isaac Elliott,
   Christian Huitema, and Scott Picket.

   We also want to thank the many reviewers who provided advice on the
   design of SGCP and then MGCP, notably Sankar Ardhanari, Francois
   Berard, David Auerbach, Bob Biskner, David Bukovinsky, Charles Eckel,
   Mario Edini, Ed Guy, Barry Hoffner, Jerry Kamitses, Oren Kudevitzki,
   Rajesh Kumar, Troy Morley, Dave Oran, Jeff Orwick, John Pickens, Lou
   Rubin, Chip Sharp, Paul Sijben, Kurt Steinbrenner, Joe Stone, and
   Stuart Wray.

   The version 0.1 of MGCP was heavily inspired by the "Internet
   Protocol Device Control" (IPDC) designed by the Technical Advisory
   Committee set up by Level 3 Communications.  Whole sets of text were
   retrieved from the IP Connection Control protocol, IP Media Control
   protocol, and IP Device Management.  The authors wish to acknowledge
   the contribution to these protocols made by Ilya Akramovich, Bob
   Bell, Dan Brendes, Peter Chung, John Clark, Russ Dehlinger, Andrew
   Dugan, Isaac Elliott, Cary FitzGerald, Jan Gronski, Tom Hess, Geoff
   Jordan, Tony Lam, Shawn Lewis, Dave Mazik, Alan Mikhak, Pete
   O'Connell, Scott Pickett, Shyamal Prasad, Eric Presworsky, Paul
   Richards, Dale Skran, Louise Spergel, David Sprague, Raj Srinivasan,
   Tom Taylor and Michael Thomas.

10. References

   [1]  Bradner, S., "The Internet Standards Process -- Revision 3", BCP
        9, RFC 2026, October 1996.

   [2]  Bradner, S., "Key words for use in RFCs to Indicate Requirement
        Levels", BCP 14, RFC 2119, March 1997.

   [3]  Schulzrinne, H., Casner, S., Frederick, R. and V. Jacobson,
        "RTP:  A Transport Protocol for Real-Time Applications", RFC
        1889, January 1996.





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   [4]  Schulzrinne, H., "RTP Profile for Audio and Video Conferences
        with Minimal Control", RFC 1890, January 1996.

   [5]  Handley, M. and V. Jacobson, "SDP: Session Description
        Protocol", RFC 2327, April 1998.

   [6]  Handley, M., Perkins, C. and E. Whelan, "Session Announcement
        Protocol", RFC 2974, October 2000.

   [7]  Rosenberg, J., Camarillo, G., Johnston, A., Peterson, J.,
        Sparks, R., Handley, M., Schulzrinne, H. and E. Schooler,
        "Session Initiation Protocol (SIP)", RFC 3261, June 2002.

   [8]  Schulzrinne, H., Rao, A. and R. Lanphier, "Real Time Streaming
        Protocol (RTSP)", RFC 2326, April 1998.

   [9]  ITU-T, Recommendation Q.761, "FUNCTIONAL DESCRIPTION OF THE ISDN
        USER PART OF SIGNALING SYSTEM No. 7", (Malaga-Torremolinos,
        1984; modified at Helsinki, 1993).

   [10] ITU-T, Recommendation Q.762, "GENERAL FUNCTION OF MESSAGES AND
        SIGNALS OF THE ISDN USER PART OF SIGNALING SYSTEM No. 7",
        (MalagaTorremolinos, 1984; modified at Helsinki, 1993).

   [11] ITU-T, Recommendation H.323 (02/98), "PACKET-BASED MULTIMEDIA
        COMMUNICATIONS SYSTEMS".

   [12] ITU-T, Recommendation H.225, "Call Signaling Protocols and Media
        Stream Packetization for Packet Based Multimedia Communications
        Systems".

   [13] ITU-T, Recommendation H.245 (02/98), "CONTROL PROTOCOL FOR
        MULTIMEDIA COMMUNICATION".

   [14] Kent, S. and R. Atkinson, "Security Architecture for the
        Internet Protocol", RFC 2401, November 1998.

   [15] Kent, S. and R. Atkinson, "IP Authentication Header", RFC 2402,
        November 1998.

   [16] Kent, S. and R. Atkinson, "IP Encapsulating Security Payload
        (ESP)", RFC 2406, November 1998.

   [17] Crocker, D. and P. Overell, "Augmented BNF for Syntax
        Specifications: ABNF", RFC 2234, November 1997.

   [18] Stevens, W. Richard, "TCP/IP Illustrated, Volume 1, The
        Protocols", Addison-Wesley, 1994.



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   [19] Allman, M., Paxson, V. "On Estimating End-to-End Network Path
        Properties", Proc. SIGCOMM'99, 1999.

   [20] Yergeau, F., "UTF-8, a transformation format of ISO 10646", RFC
        2279, January 1998.

   [21] Braden, R., "Requirements for Internet Hosts -- Communication
        Layers", STD 3, RFC 1122, October 1989.

   [22] Bellcore, "LSSGR: Switching System Generic Requirements for Call
        Control Using the Integrated Services Digital Network User Part
        (ISDNUP)", GR-317-CORE, Issue 2, December 1997.

   [23] Narten, T., and Alvestrand H., "Guidelines for Writing an IANA
        Considerations Section in RFCs", RFC 2434, October 1998.




































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Appendix A: Formal Syntax Description of the Protocol

   In this section, we provide a formal description of the protocol
   syntax, following the "Augmented BNF for Syntax Specifications"
   defined in RFC 2234.  The syntax makes use of the core rules defined
   in RFC 2234, Section 6.1, which are not included here.  Furthermore,
   the syntax follows the case-sensitivity rules of RFC 2234, i.e., MGCP
   is case-insensitive (but SDP is not).  It should be noted, that ABNF
   does not provide for implicit specification of linear white space and
   MGCP messages MUST thus follow the explicit linear white space rules
   provided in the grammar below.  However, in line with general
   robustness principles, implementers are strongly encouraged to
   tolerate additional linear white space in messages received.

MGCPMessage = MGCPCommand / MGCPResponse

MGCPCommand = MGCPCommandLine 0*(MGCPParameter) [EOL *SDPinformation]

MGCPCommandLine = MGCPVerb 1*(WSP) transaction-id 1*(WSP)
                        endpointName 1*(WSP) MGCPversion EOL

MGCPVerb = "EPCF" / "CRCX" / "MDCX" / "DLCX" / "RQNT"
         / "NTFY" / "AUEP" / "AUCX" / "RSIP" / extensionVerb

extensionVerb  = ALPHA 3(ALPHA / DIGIT) ; experimental starts with X

transaction-id = 1*9(DIGIT)

endpointName      = LocalEndpointName "@" DomainName
LocalEndpointName = LocalNamePart 0*("/" LocalNamePart)
LocalNamePart     = AnyName / AllName / NameString
AnyName           = "$"
AllName           = "*"
NameString        = 1*(range-of-allowed-characters)
; VCHAR except "$", "*", "/", "@"
range-of-allowed-characters  = %x21-23 / %x25-29 / %x2B-2E
                             / %x30-3F / %x41-7E

DomainName = 1*255(ALPHA / DIGIT / "." / "-")    ; as defined
           / "#" number                          ; in RFC 821
           / "[" IPv4address / IPv6address "]"   ; see RFC 2373

; Rewritten to ABNF from RFC 821
number =  1*DIGIT

;From RFC 2373
IPv6address = hexpart [ ":" IPv4address ]
IPv4address = 1*3DIGIT "." 1*3DIGIT "." 1*3DIGIT "." 1*3DIGIT



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; this production, while occurring in RFC2373, is not referenced
; IPv6prefix  = hexpart "/" 1*2DIGIT
hexpart = hexseq / hexseq "::" [ hexseq ] / "::" [ hexseq ]
hexseq  = hex4 *( ":" hex4)
hex4    = 1*4HEXDIG

MGCPversion = "MGCP" 1*(WSP) 1*(DIGIT) "." 1*(DIGIT)
                            [1*(WSP) ProfileName]
ProfileName = VCHAR *( WSP / VCHAR)

MGCPParameter = ParameterValue EOL

; Check infoCode if more parameter values defined
; Most optional values can only be omitted when auditing
ParameterValue = ("K"  ":" 0*(WSP)  [ResponseAck])
               / ("B"  ":" 0*(WSP)  [BearerInformation])
               / ("C"  ":" 0*(WSP)  CallId)
               / ("I"  ":" 0*(WSP)  [ConnectionId])
               / ("N"  ":" 0*(WSP)  [NotifiedEntity])
               / ("X"  ":" 0*(WSP)  [RequestIdentifier])
               / ("L"  ":" 0*(WSP)  [LocalConnectionOptions])
               / ("M"  ":" 0*(WSP)  ConnectionMode)
               / ("R"  ":" 0*(WSP)  [RequestedEvents])
               / ("S"  ":" 0*(WSP)  [SignalRequests])
               / ("D"  ":" 0*(WSP)  [DigitMap])
               / ("O"  ":" 0*(WSP)  [ObservedEvents])
               / ("P"  ":" 0*(WSP)  [ConnectionParameters])
               / ("E"  ":" 0*(WSP)  ReasonCode)
               / ("Z"  ":" 0*(WSP)  [SpecificEndpointID])
               / ("Z2" ":" 0*(WSP)  SecondEndpointID)
               / ("I2" ":" 0*(WSP)  SecondConnectionID)
               / ("F"  ":" 0*(WSP)  [RequestedInfo])
               / ("Q"  ":" 0*(WSP)  QuarantineHandling)
               / ("T"  ":" 0*(WSP)  [DetectEvents])
               / ("RM" ":" 0*(WSP)  RestartMethod)
               / ("RD" ":" 0*(WSP)  RestartDelay)
               / ("A"  ":" 0*(WSP)  [Capabilities])
               / ("ES" ":" 0*(WSP)  [EventStates])
               / ("PL" ":" 0*(WSP)  [PackageList])    ; Auditing only
               / ("MD" ":" 0*(WSP)  MaxMGCPDatagram)  ; Auditing only
               / (extensionParameter ":" 0*(WSP) [parameterString])

; A final response may include an empty ResponseAck
ResponseAck =  confirmedTransactionIdRange
               *( ","  0*(WSP) confirmedTransactionIdRange )

confirmedTransactionIdRange = transaction-id ["-" transaction-id]




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BearerInformation = BearerAttribute 0*("," 0*(WSP) BearerAttribute)
BearerAttribute   = ("e" ":" BearerEncoding)
                  / (BearerExtensionName [":" BearerExtensionValue])
BearerExtensionName  = PackageLCOExtensionName
BearerExtensionValue = LocalOptionExtensionValue
BearerEncoding = "A" / "mu"

CallId = 1*32(HEXDIG)

; The audit request response may include a list of identifiers
ConnectionId = 1*32(HEXDIG) 0*("," 0*(WSP) 1*32(HEXDIG))
SecondConnectionID = ConnectionId

NotifiedEntity = [LocalName "@"] DomainName [":" portNumber]
LocalName  = LocalEndpointName            ; No internal structure

portNumber = 1*5(DIGIT)

RequestIdentifier = 1*32(HEXDIG)

LocalConnectionOptions = LocalOptionValue 0*(WSP)
                           0*("," 0*(WSP) LocalOptionValue 0*(WSP))
LocalOptionValue = ("p"  ":" packetizationPeriod)
                 / ("a"  ":" compressionAlgorithm)
                 / ("b"  ":" bandwidth)
                 / ("e"  ":" echoCancellation)
                 / ("gc" ":" gainControl)
                 / ("s"  ":" silenceSuppression)
                 / ("t"  ":" typeOfService)
                 / ("r"  ":" resourceReservation)
                 / ("k"  ":" encryptiondata)
                 / ("nt" ":" ( typeOfNetwork /
                                    supportedTypeOfNetwork))
                 / (LocalOptionExtensionName
                         [":" LocalOptionExtensionValue])

Capabilities    =  CapabilityValue 0*(WSP)
                     0*("," 0*(WSP) CapabilityValue 0*(WSP))
CapabilityValue = LocalOptionValue
                / ("v" ":" supportedPackages)
                / ("m" ":" supportedModes)

PackageList     = pkgNameAndVers 0*("," pkgNameAndVers)
pkgNameAndVers  = packageName ":" packageVersion
packageVersion  = 1*(DIGIT)

packetizationPeriod  = 1*4(DIGIT) ["-" 1*4(DIGIT)]
compressionAlgorithm = algorithmName 0*(";" algorithmName)



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algorithmName        = 1*(SuitableLCOCharacter)
bandwidth            = 1*4(DIGIT) ["-" 1*4(DIGIT)]
echoCancellation     = "on" / "off"
gainControl          = "auto" / ["-"] 1*4(DIGIT)
silenceSuppression   = "on" / "off"
typeOfService        = 1*2(HEXDIG)     ; 1 hex only for capabilities
resourceReservation  = "g" / "cl" / "be"

;encryption parameters are coded as in SDP (RFC 2327)
;NOTE: encryption key may contain an algorithm as specified in RFC 1890
encryptiondata = ( "clear" ":" encryptionKey )
               / ( "base64" ":" encodedEncryptionKey )
               / ( "uri" ":" URItoObtainKey )
               / ( "prompt" ) ; defined in SDP, not usable in MGCP!

encryptionKey = 1*(SuitableLCOCharacter) / quotedString
; See RFC 2045
encodedEncryptionKey = 1*(ALPHA / DIGIT / "+" / "/" / "=")
URItoObtainKey = 1*(SuitableLCOCharacter) / quotedString

typeOfNetwork = "IN" / "ATM" / "LOCAL" / OtherTypeOfNetwork
; Registered with IANA - see RFC 2327
OtherTypeOfNetwork     = 1*(SuitableLCOCharacter)
supportedTypeOfNetwork = typeOfNetwork *(";" typeOfNetwork)

supportedModes    = ConnectionMode 0*(";" ConnectionMode)

supportedPackages = packageName 0*(";" packageName)

packageName = 1*(ALPHA / DIGIT / HYPHEN) ; Hyphen neither first or last

LocalOptionExtensionName = VendorLCOExtensionName
                         / PackageLCOExtensionName
                         / OtherLCOExtensionName
VendorLCOExtensionName   = "x" ("+"/"-") 1*32(SuitableExtLCOCharacter)
PackageLCOExtensionName  = packageName "/"
                            1*32(SuitablePkgExtLCOCharacter)
; must not start with "x-" or "x+"
OtherLCOExtensionName    = 1*32(SuitableExtLCOCharacter)

LocalOptionExtensionValue = (1*(SuitableExtLCOValChar)
                                                    / quotedString)
                              *(";" (1*(SuitableExtLCOValChar)
                                                      / quotedString))

;Note: No "data" mode.
ConnectionMode = "sendonly" / "recvonly" / "sendrecv"
               / "confrnce" / "inactive" / "loopback"



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               / "conttest" / "netwloop" / "netwtest"
               / ExtensionConnectionMode
ExtensionConnectionMode = PkgExtConnectionMode
PkgExtConnectionMode    = packageName "/" 1*(ALPHA / DIGIT)

RequestedEvents = requestedEvent 0*("," 0*(WSP) requestedEvent)
requestedEvent  = (eventName ["(" requestedActions ")"])
                / (eventName "(" requestedActions ")"
                                       "(" eventParameters ")" )
eventName = [(packageName / "*") "/"]
                (eventId / "all" / eventRange
                                        / "*" / "#") ; for DTMF
                              ["@" (ConnectionId / "$" / "*")]
eventId = 1*(ALPHA / DIGIT / HYPHEN)   ; Hyphen neither first nor last
eventRange = "[" 1*(DigitMapLetter / (DIGIT "-" DIGIT) /
                        (DTMFLetter "-" DTMFLetter)) "]"
DTMFLetter = "A" / "B" / "C" / "D"

requestedActions = requestedAction 0*("," 0*(WSP) requestedAction)
requestedAction  = "N" / "A" / "D" / "S" / "I" / "K"
                 / "E" "(" EmbeddedRequest ")"
                 / ExtensionAction
ExtensionAction  = PackageExtAction
PackageExtAction = packageName "/" Action ["(" ActionParameters ")"]
Action           = 1*ALPHA
ActionParameters = eventParameters        ; May contain actions

;NOTE: Should tolerate different order when receiving, e.g., for NCS.
EmbeddedRequest = (      "R" "(" EmbeddedRequestList ")"
                    ["," 0*(WSP) "S" "(" EmbeddedSignalRequest ")"]
                    ["," 0*(WSP) "D" "(" EmbeddedDigitMap ")"]      )
                / (      "S" "(" EmbeddedSignalRequest ")"
                    ["," 0*(WSP) "D" "(" EmbeddedDigitMap ")"] )
                / (      "D" "(" EmbeddedDigitMap ")" )

EmbeddedRequestList   = RequestedEvents
EmbeddedSignalRequest = SignalRequests
EmbeddedDigitMap = DigitMap

SignalRequests   = SignalRequest 0*("," 0*(WSP) SignalRequest )
SignalRequest    = eventName [ "(" eventParameters ")" ]

eventParameters  = eventParameter 0*("," 0*(WSP) eventParameter)
eventParameter   = eventParameterValue
                 / eventParameterName "=" eventParameter
                 / eventParameterName "(" eventParameters ")"
eventParameterString = 1*(SuitableEventParamCharacter)
eventParameterName   = eventParameterString



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eventParameterValue  = eventParameterString / quotedString

DigitMap           = DigitString  / "(" DigitStringList ")"
DigitStringList    = DigitString 0*( "|" DigitString )
DigitString        = 1*(DigitStringElement)
DigitStringElement = DigitPosition ["."]
DigitPosition      = DigitMapLetter / DigitMapRange
; NOTE "X" is now included
DigitMapLetter     = DIGIT / "#" / "*" / "A" / "B" / "C" / "D" / "T"
                   / "X" / ExtensionDigitMapLetter
ExtensionDigitMapLetter = "E" / "F" / "G" / "H" / "I" / "J" / "K"
                        / "L" / "M" / "N" / "O" / "P" / "Q" / "R"
                        / "S" / "U" / "V" / "W" / "Y" / "Z"
; NOTE "[x]" is now allowed
DigitMapRange = "[" 1*DigitLetter "]"
DigitLetter   = *((DIGIT "-" DIGIT) / DigitMapLetter)

ObservedEvents = SignalRequests

EventStates    = SignalRequests

ConnectionParameters = ConnectionParameter
                        0*( "," 0*(WSP) ConnectionParameter )

ConnectionParameter  = ( "PS" "=" packetsSent )
                     / ( "OS" "=" octetsSent )
                     / ( "PR" "=" packetsReceived )
                     / ( "OR" "=" octetsReceived )
                     / ( "PL" "=" packetsLost )
                     / ( "JI" "=" jitter )
                     / ( "LA" "=" averageLatency )
                     / ( ConnectionParameterExtensionName
                              "=" ConnectionParameterExtensionValue )
packetsSent     = 1*9(DIGIT)
octetsSent      = 1*9(DIGIT)
packetsReceived = 1*9(DIGIT)
octetsReceived  = 1*9(DIGIT)
packetsLost     = 1*9(DIGIT)
jitter          = 1*9(DIGIT)
averageLatency  = 1*9(DIGIT)

ConnectionParameterExtensionName = VendorCPExtensionName
                                 /    PackageCPExtensionName
VendorCPExtensionName  = "X" "-" 2*ALPHA
PackageCPExtensionName = packageName "/" CPName
CPName = 1*(ALPHA / DIGIT / HYPHEN)
ConnectionParameterExtensionValue = 1*9(DIGIT)




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MaxMGCPDatagram = 1*9(DIGIT)

ReasonCode = 3DIGIT
             [1*(WSP) "/" packageName]     ; Only for 8xx
             [WSP 1*(%x20-7E)]

SpecificEndpointID = endpointName
SecondEndpointID   = endpointName

RequestedInfo = infoCode 0*("," 0*(WSP) infoCode)

infoCode = "B" / "C" / "I" / "N" / "X" / "L" / "M" / "R" / "S"
         / "D" / "O" / "P" / "E" / "Z" / "Q" / "T" / "RC" / "LC"
         / "A" / "ES" / "RM" / "RD" / "PL" / "MD" / extensionParameter

QuarantineHandling = loopControl / processControl
                   / (loopControl "," 0*(WSP) processControl )
loopControl    = "step" / "loop"
processControl = "process" / "discard"

DetectEvents = SignalRequests

RestartMethod = "graceful" / "forced" / "restart" / "disconnected"
              / "cancel-graceful" / extensionRestartMethod
extensionRestartMethod = PackageExtensionRM
PackageExtensionRM     = packageName "/" 1*32(ALPHA / DIGIT / HYPHEN)
RestartDelay = 1*6(DIGIT)

extensionParameter = VendorExtensionParameter
                   / PackageExtensionParameter
                   / OtherExtensionParameter
VendorExtensionParameter  = "X" ("-"/"+") 1*6(ALPHA / DIGIT)
PackageExtensionParameter = packageName "/"
                            1*32(ALPHA / DIGIT / HYPHEN)
; must not start with "x-" or x+"
OtherExtensionParameter   = 1*32(ALPHA / DIGIT / HYPHEN)

;If first character is a double-quote, then it is a quoted-string
parameterString = (%x21 / %x23-7F) *(%x20-7F) ; first and last must not
                                              ; be white space
                    / quotedString

MGCPResponse = MGCPResponseLine 0*(MGCPParameter)
                                        *2(EOL *SDPinformation)

MGCPResponseLine = responseCode 1*(WSP) transaction-id
                        [1*(WSP) "/" packageName]    ; Only for 8xx
                             [WSP responseString] EOL



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responseCode = 3DIGIT
responseString = *(%x20-7E)

SuitablePkgExtLCOCharacter =  SuitableLCOCharacter

SuitableExtLCOCharacter = DIGIT / ALPHA / "+" / "-" / "_" / "&"
              / "!" / "'" / "|" / "=" / "#" / "?"
              / "." / "$" / "*" /       "@" / "[" / "]"
              / "^" / "`" / "{" / "}" / "~"

SuitableLCOCharacter   = SuitableExtLCOCharacter / "/"

SuitableExtLCOValChar  = SuitableLCOCharacter / ":"

; VCHAR except """, "(", ")", ",", and "="
SuitableEventParamCharacter = %x21 / %x23-27 / %x2A-2B
                            / %x2D-3C / %x3E-7E

; NOTE: UTF8 encoded
quotedString  = DQUOTE 0*(quoteEscape / quoteChar) DQUOTE
quoteEscape   = DQUOTE DQUOTE
quoteChar = (%x00-21 / %x23-FF)

EOL = CRLF / LF

HYPHEN = "-"

; See RFC 2327 for proper SDP grammar instead.
SDPinformation = SDPLine CRLF *(SDPLine CRLF)        ; see RFC 2327
SDPLine        = 1*(%x01-09 / %x0B / %x0C / %x0E-FF) ; for proper def.





















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Appendix B: Base Package

   Package name: B
   Version: 0

   The MGCP specification defines a base package which contains a set of
   events and extension parameters that are of general use to the
   protocol.  Although not required, it is highly RECOMMENDED to support
   this package as it provides important functionality for the base
   protocol.

B.1 Events

   The table below lists the events:

    ------------------------------------------------------------------
   | Symbol  |   Definition               |  R  |   S     Duration    |
   |---------|----------------------------|-----|---------------------|
   | enf(##) | embedded RQNT failure      |  x  |                     |
   | oef     | observed events full       |  x  |                     |
   | qbo     | quarantine buffer overflow |  x  |                     |
    ------------------------------------------------------------------

   The events are defined as follows:

   Embedded NotificationRequest failure (enf):
     The Embedded NotificationRequest Failure (enf) event is generated
     when an embedded Notification Request failure occurs.  When the
     event is requested, it should be as part of the Embedded
     NotificationRequest itself.  When the event is reported, it may be
     parameterized with an error code (see Section 2.4) detailing the
     error that occurred.  When requested, it cannot be parameterized.

   Observed events full (oef):
     The event is generated when the endpoint is unable to accumulate
     any more events in the list of ObservedEvents.  If this event
     occurs, and it is not used to trigger a Notify, subsequent events
     that should have been added to the list will be lost.

   Quarantine buffer overflow (qbo):
     The event is generated when the quarantine buffer overflows and one
     or more events have been lost.









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B.2 Extension Parameters

B.2.1 PersistentEvents

   PersistentEvents:  A list of events that the gateway is requested to
   detect and report persistently.  The parameter is optional but can be
   provided in any command where the DetectEvents parameter can be
   provided.  The initial default value of the parameter is empty.  When
   the parameter is omitted from a command, it retains its current
   value.  When the parameter is provided, it completely replaces the
   current value.  Providing an event in this list, is similar (but
   preferable) to defining that particular event as being persistent.
   The current list of PersistentEvents will implicitly apply to the
   current as well as subsequent NotificationRequests, however no glare
   detection etc. will be performed (similarly to DetectEvents).  If an
   event provided in this list is included in a RequestedEvents list,
   the action and event parameters used in the RequestedEvents will
   replace the action and event parameters associated with the event in
   the PersistentEvents list for the life of the RequestedEvents list,
   after which the PersistentEvents action and event parameters are
   restored.  Events with event states requested through this parameter
   will be included in the list of EventStates if audited.

   PersistentEvents can also be used to detect events on connections.
   Use of the "all connections" wildcard is straightforward, whereas
   using PersistentEvents with one or more specific connections must be
   considered carefully.  Once the connection in question is deleted, a
   subsequent NotificationRequest without a new PersistentEvents value
   will fail (error code 515 - incorrect connection-id, is RECOMMENDED),
   as it implicitly refers to the deleted connection.

   The parameter generates the relevant error codes from the base
   protocol, e.g., error code 512 if an unknown event is specified.

   The PersistentEvents parameter can be audited, in which case it will
   return its current value.  Auditing of RequestedEvents is not
   affected by this extension, i.e., events specified in this list are
   not automatically reported when auditing RequestedEvents.

   The parameter name for PersistentEvents is "PR" and it is defined by
   the production:

     PersistentEvents = "PR" ":" 0*WSP  [RequestedEvents]








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   The following example illustrates the use of the parameter:

     B/PR: L/hd(N), L/hf(N), L/hu(N), B/enf, B/oef, B/qbo

   which instructs the endpoint to persistently detect and report off-
   hook, hook-flash, and on-hook.  It also instructs the endpoint to
   persistently detect and report Embedded Notification Request failure,
   Observed events full, and Quarantine buffer overflow.

B.2.2 NotificationState

   NotificationState is a RequestedInfo parameter that can be audited
   with the AuditEndpoint command.  It can be used to determine if the
   endpoint is in the notification state or not.

   The parameter is forbidden in any command.  In responses, it is a
   valid response parameter for AuditEndpoint only.

   It is defined by the following grammar:

     NotificationState        = "NS" ":" 0*WSP NotificationStateValue
     NotificationStateValue   = "ns" / "ls" / "o"

   It is requested as part of auditing by including the parameter code
   in RequestedInfo, as in:

     F: B/NS

   The response parameter will contain the value "ns" if the endpoint is
   in the "notification state", the value "ls" if the endpoint is in the
   "lockstep state" (i.e., waiting for an RQNT after a response to a
   NTFY has been received when operating in "step" mode), or the value
   "o" otherwise, as for example:

     B/NS: ns

B.3 Verbs

   MGCP packages are not intended to define new commands, however an
   exception is made in this case in order to add an important general
   capability currently missing, namely the ability for the gateway to
   send a generic message to the Call Agent.

   The definition of the new command is:

          ReturnCode
          <-- Message(EndpointId
                         [, ...])



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   EndpointId is the name for the endpoint(s) in the gateway which is
   issuing the Message command.  The identifier MUST be a fully
   qualified endpoint identifier, including the domain name of the
   gateway.  The local part of the endpoint name MUST NOT use the "any
   of" wildcard.

   The only parameter specified in the definition of the Message command
   is the EndpointId, however, it is envisioned that extensions will
   define additional parameters to be used with the Message command.
   Such extensions MUST NOT alter or otherwise interfere with the normal
   operation of the basic MGCP protocol.  They may however define
   additional capabilities above and beyond that provided by the basic
   MGCP protocol.  For example, an extension to enable the gateway to
   audit the packages supported by the Call Agent could be defined,
   whereas using the Message command as an alternative way of reporting
   observed events would be illegal, as that would alter the normal MGCP
   protocol behavior.

   In order to not interfere with normal MGCP operation, lack of a
   response to the Message command MUST NOT lead the endpoint to become
   disconnected.  The endpoint(s) MUST be prepared to handle this
   transparently and continue normal processing unaffected.

   If the endpoint(s) receive a response indicating that the Call Agent
   does not support the Message command, the endpoint(s) MUST NOT send a
   Message command again until the current "notified entity" has
   changed.  Similarly, if the endpoint(s) receive a response indicating
   that the Call Agent does not support one or more parameters in the
   Message command, the endpoint(s) MUST NOT send a Message command with
   those parameters again until the current "notified entity" has
   changed.

   The Message command is encoded as MESG, as shown in the following
   example:

      MESG 1200 aaln/1@rgw.whatever.net MGCP 1.0















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Appendix C: IANA Considerations

C.1 New MGCP Package Sub-Registry

   The IANA has established a new sub-registry for MGCP packages under
   http://www.iana.org/assignments/mgcp-packages.

   Packages can be registered with the IANA according to the following
   procedure:

   The package MUST have a unique string name which MUST NOT start with
   the two characters "x-" or "x+".

   The package title, name, and version (zero assumed by default) MUST
   be registered with IANA as well as a reference to the document that
   describes the package.  The document MUST have a stable URL and MUST
   be contained on a public web server.

   Packages may define one or more Extension Digit Map Letters, however
   these are taken from a limited and flat name space.  To prevent name
   clashing, IANA SHALL NOT register a package that defines an Extension
   Digit Map Letter already defined in another package registered by
   IANA.  To ease this task, such packages SHALL contain the line
   "Extension Digit Map Letters:  " followed by a list of the Extension
   Digit Map Letters defined in the package at the beginning of the
   package definition.

   A contact name, e-mail and postal address for the package MUST be
   provided.  The contact information SHALL be updated by the defining
   organization as necessary.

   Finally, prior to registering a package, the IANA MUST have a
   designated expert [23] review the package. The expert reviewer will
   send e-mail to the IANA on the overall review determination.

C.2 New MGCP Package

   This document defines a new MGCP Base Package in Appendix B, which
   has been registered by IANA.

C.3 New MGCP LocalConnectionOptions Sub-Registry

   The IANA has established a new sub-registry for MGCP
   LocalConnectionOptions under http://www.iana.org/assignments/mgcp-
   localconnectionoptions.






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   Packages are the preferred extension mechanism, however for backwards
   compatibility, local connection options beyond those provided in this
   specification can be registered with IANA.  Each such local
   connection option MUST have a unique string name which MUST NOT start
   with "x-" or "x+".  The local connection option field name and
   encoding name MUST be registered with IANA as well as a reference to
   the document that describes the local connection option.  The
   document MUST have a stable URL and MUST be contained on a public web
   server.

   A contact name, e-mail and postal address for the local connection
   option MUST be provided.  The contact information SHALL be updated by
   the defining organization as necessary.

   Finally, prior to registering a LocalConnectionOption, the IANA MUST
   have a designated expert [23] review the LocalConnectionOption. The
   expert reviewer will send e-mail to the IANA on the overall review
   determination.

Appendix D: Mode Interactions

   An MGCP endpoint can establish one or more media streams.  These
   streams are either incoming (from a remote endpoint) or outgoing
   (generated at the handset microphone).  The "connection mode"
   parameter establishes the direction and generation of these streams.
   When there is only one connection to an endpoint, the mapping of
   these streams is straightforward; the handset plays the incoming
   stream over the handset speaker and generates the outgoing stream
   from the handset microphone signal, depending on the mode parameter.

   However, when several connections are established to an endpoint,
   there can be many incoming and outgoing streams.  Depending on the
   connection mode used, these streams may interact differently with
   each other and the streams going to/from the handset.

   The table below describes how different connections SHALL be mixed
   when one or more connections are concurrently "active".  An active
   connection is here defined as a connection that is in one of the
   following modes:

   *  "send/receive"
   *  "send only"
   *  "receive only"
   *  "conference"

   Connections in "network loopback", "network continuity test", or
   "inactive" modes are not affected by connections in the "active"
   modes.  The Table uses the following conventions:



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   *  Ai is the incoming media stream from Connection A
   *  Bi is the incoming media stream from Connection B
   *  Hi is the incoming media stream from the Handset Microphone
   *  Ao is the outgoing media stream to Connection A
   *  Bo is the outgoing media stream to Connection B
   *  Ho is the outgoing media stream to the Handset earpiece
   *  NA indicates no stream whatsoever (assuming there are no signals
      applied on the connection)

   "netw" in the following table indicates either "netwloop" or
   "netwtest" mode.

     -------------------------------------------------------------
    |       |               Connection A Mode                     |
    |       |-----------------------------------------------------
    |       |sendonly|recvonly|sendrecv|confrnce|inactive|  netw  |
    |-------|-----------------------------------------------------|
    | |Send | Ao=Hi  | Ao=NA  | Ao=Hi  | Ao=Hi  | Ao=NA  | Ao=Ai  |
    |C|only | Bo=Hi  | Bo=Hi  | Bo=Hi  | Bo=Hi  | Bo=Hi  | Bo=Hi  |
    |o|     | Ho=NA  | Ho=Ai  | Ho=Ai  | Ho=Ai  | Ho=NA  | Ho=NA  |
    |n|-----------------------------------------------------------
    |n|recv |        |Ao=NA   |Ao=Hi   |Ao=Hi   | Ao=NA  | Ao=Ai  |
    |e|only |        |Bo=NA   |Bo=NA   |Bo=NA   | Bo=NA  | Bo=NA  |
    |c|     |        |Ho=Ai+Bi|Ho=Ai+Bi|Ho=Ai+Bi| Ho=Bi  | Ho=Bi  |
    |t|-----------------------------------------------------------|
    |i|send |        |        |Ao=Hi   |Ao=Hi   | Ao=NA  | Ao=Ai  |
    |o|recv |        |        |Bo=Hi   |Bo=Hi   | Bo=Hi  | Bo=Hi  |
    |n|     |        |        |Ho=Ai+Bi|Ho=Ai+Bi| Ho=Bi  | Ho=Bi  |
    | |-----------------------------------------------------------|
    |B|conf |        |        |        |Ao=Hi+Bi| Ao=NA  | Ao=Ai  |
    | |rnce |        |        |        |Bo=Hi+Ai| Bo=Hi  | Bo=Hi  |
    |M|     |        |        |        |Ho=Ai+Bi| Ho=Bi  | Ho=Bi  |
    |o|-----------------------------------------------------------|
    |d|Inac |        |        |        |        | Ao=NA  | Ao=Ai  |
    |e|tive |        |        |        |        | Bo=NA  | Bo=NA  |
    | |     |        |        |        |        | Ho=NA  | Ho=NA  |
    | |-----------------------------------------------------------|
    | |netw |        |        |        |        |        | Ao=Ai  |
    | |     |        |        |        |        |        | Bo=Bi  |
    | |     |        |        |        |        |        | Ho=NA  |
     -------------------------------------------------------------

   If there are three or more "active" connections they will still
   interact as defined in the table above with the outgoing media
   streams mixed for each interaction (union of all streams).  If
   internal resources are used up and the streams cannot be mixed, the
   gateway MUST return an error (error code 403 or 502, not enough
   resources, are RECOMMENDED).



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Appendix E: Endpoint Naming Conventions

   The following sections provide some RECOMMENDED endpoint naming
   conventions.

E.1 Analog Access Line Endpoints

   The string "aaln", should be used as the first term in a local
   endpoint name for analog access line endpoints.  Terms following
   "aaln" should follow the physical hierarchy of the gateway so that if
   the gateway has a number of RJ11 ports, the local endpoint name could
   look like the following:

      aaln/#

   where "#" is the number of the analog line (RJ11 port) on the
   gateway.

   On the other hand, the gateway may have a number of physical plug-in
   units, each of which contain some number of RJ11 ports, in which
   case, the local endpoint name might look like the following:

      aaln/<unit #>/#

   where <unit #> is the number of the plug in unit in the gateway and
   "#" is the number of the analog line (RJ11 port) on that unit.
   Leading zeroes MUST NOT be used in any of the numbers ("#") above.

E.2 Digital Trunks

   The string "ds" should be used for the first term of digital
   endpoints with a naming convention that follows the physical and
   digital hierarchy such as:

      ds/<unit-type1>-<unit #>/<unit-type2>-<unit #>/.../<channel #>

   where:  <unit-type> identifies the particular hierarchy level.  Some
   example values of <unit-type> are:  "s", "su", "oc3", "ds3", "e3",
   "ds2", "e2", "ds1", "e1" where "s" indicates a slot number and "su"
   indicates a sub-unit within a slot.  Leading zeroes MUST NOT be used
   in any of the numbers ("#") above.

   The <unit #> is a decimal number which is used to reference a
   particular instance of a <unit-type> at that level of the hierarchy.
   The number of levels and naming of those levels is based on the
   physical hierarchy within the media gateway.





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E.3 Virtual Endpoints

   Another type of endpoint is one that is not associated with a
   physical interface (such as an analog or digital endpoint).  This
   type of endpoint is called a virtual endpoint and is often used to
   represent some DSP resources that gives the endpoint some capability.
   Examples are announcement, IVR or conference bridge devices.  These
   devices may have multiple instances of DSP functions so that a
   possible naming convention is:

      <virtual-endpoint-type>/<endpoint-#>

   where <virtual-endpoint-type> may be some string representing the
   type of endpoint (such as "ann" for announcement server or "cnf" for
   conference server) and <endpoint-#> would identify a particular
   virtual endpoint within the device.  Leading zeroes MUST NOT be used
   in the number ("#") above.  If the physical hierarchy of the server
   includes plug-in DSP cards, another level of hierarchy in the local
   endpoint name may be used to describe the plug in unit.

   A virtual endpoint may be created as the result of using the "any of"
   wildcard.  Similarly, a virtual endpoint may cease to exist once the
   last connection on the virtual endpoint is deleted.  The definition
   of the virtual endpoint MUST detail both of these aspects.

   When a <virtual-endpoint-type> creates and deletes virtual endpoints
   automatically, there will be cases where no virtual endpoints exist
   at the time a RestartInProgress command is to be issued.  In such
   cases, the gateway SHOULD simply use the "all of" wildcard in lieu of
   any specific <endpoint-#> as in, e.g.:

     ann/*@mygateway.whatever.net

   If the RestartInProgress command refers to all endpoints in the
   gateway (virtual or not), the <virtual-endpoint-id> can be omitted as
   in, e.g.:

     *@mygateway.whatever.net

   Commands received by the gateway will still have to refer to an
   actual endpoint (possibly created by that command by use of the "any
   of" wildcard) in order for the command to be processed though.









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E.4 Media Gateway

   MGCP only defines operation on endpoints in a media gateway.  It may
   be beneficial to define an endpoint that represents the gateway
   itself as opposed to the endpoints managed by the gateway.
   Implementations that wish to do so should use the local endpoint name
   "mg" (for media gateway) as in:

     mg@mygateway.whatever.net

   Note that defining such an endpoint does not change any of the
   protocol semantics, i.e., the "mg" endpoint and other endpoints
   (e.g., digital trunks) in the gateway are still independent endpoints
   and MUST be treated as such.  For example, RestartInProgress commands
   MUST still be issued for all endpoints in the gateway as usual.

E.5 Range Wildcards

   As described in Section 2.1.2, the MGCP endpoint naming scheme
   defines the "all of" and "any of" wildcards for the individual terms
   in a local endpoint name.  While the "all of" wildcard is very useful
   for reducing the number of messages, it can by definition only be
   used when we wish to refer to all instances of a given term in the
   local endpoint name.  Furthermore, in the case where a command is to
   be sent by the gateway to the Call Agent, the "all of" wildcard can
   only be used if all of the endpoints named by it have the same
   "notified entity".  Implementations that prefer a finer-grained
   wildcarding scheme can use the range wildcarding scheme described
   here.

   A range wildcard is defined as follows:

   RangeWildcard    = "[" NumericalRange *( "," NumericalRange ) "]"
   NumericalRange   = 1*(DIGIT) [ "-" 1*(DIGIT) ]

   Note that white space is not permitted.  Also, since range wildcards
   use the character "[" to indicate the start of a range, the "["
   character MUST NOT be used in endpoint names that use range
   wildcards.  The length of a range wildcard SHOULD be bounded to a
   reasonably small value, e.g., 128 characters.

   Range wildcards can be used anywhere an "all of" wildcard can be
   used.  The semantics are identical for the endpoints named.  However,
   it MUST be noted, that use of the range wildcarding scheme requires
   support on both the gateway and the Call Agent.  Therefore, a gateway
   MUST NOT assume that it's Call Agent supports range wildcarding and
   vice versa.  In practice, this typically means that both the gateway
   and Call Agent will need to be provisioned consistently in order to



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   use range wildcards.  Also, if a gateway or Call Agent using range
   wildcards receives an error response that could indicate a possible
   endpoint naming problem, they MUST be able to automatically revert to
   not using range wildcards.

   The following examples illustrates the use of range wildcards:

      ds/ds1-1/[1-12]
      ds/ds1-1/[1,3,20-24]
      ds/ds1-[1-2]/*
      ds/ds3-1/[1-96]

   The following example illustrates how to use it in a command:

      RSIP 1204 ds/ds3-1/[1-96]@tgw-18.whatever.net MGCP 1.0
      RM: restart
      RD: 0

Appendix F: Example Command Encodings

   This appendix provides examples of commands and responses shown with
   the actual encoding used.  Examples are provided for each command.
   All commentary shown in the commands and responses is optional.

F.1 NotificationRequest

   The first example illustrates a NotificationRequest that will ring a
   phone and look for an off-hook event:

      RQNT 1201 aaln/1@rgw-2567.whatever.net MGCP 1.0
      N: ca@ca1.whatever.net:5678
      X: 0123456789AC
      R: l/hd(N)
      S: l/rg

   The response indicates that the transaction was successful:

      200 1201 OK

   The second example illustrates a NotificationRequest that will look
   for and accumulate an off-hook event, and then provide dial-tone and
   accumulate digits according to the digit map provided.  The "notified
   entity" is set to "ca@ca1.whatever.net:5678", and since the
   SignalRequests parameter is empty (it could have been omitted as
   well), all currently active TO signals will be stopped.  All events
   in the quarantine buffer will be processed, and the list of events to
   detect in the "notification" state will include fax tones in addition
   to the "requested events" and persistent events:



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      RQNT 1202 aaln/1@rgw-2567.whatever.net MGCP 1.0
      N: ca@ca1.whatever.net:5678
      X: 0123456789AC
      R: L/hd(A, E(S(L/dl),R(L/oc, L/hu, D/[0-9#*T](D))))
      D: (0T|00T|#xxxxxxx|*xx|91xxxxxxxxxx|9011x.T)
      S:
      Q: process
      T: G/ft

   The response indicates that the transaction was successful:

      200 1202 OK

F.2 Notify

   The example below illustrates a Notify message that notifies an off-
   hook event followed by a 12-digit number beginning with "91".  A
   transaction identifier correlating the Notify with the
   NotificationRequest it results from is included.  The command is sent
   to the current "notified entity", which typically will be the actual
   value supplied in the NotifiedEntity parameter, i.e.,
   "ca@ca1.whatever.net:5678" - a failover situation could have changed
   this:

      NTFY 2002 aaln/1@rgw-2567.whatever.net MGCP 1.0
      N: ca@ca1.whatever.net:5678
      X: 0123456789AC
      O: L/hd,D/9,D/1,D/2,D/0,D/1,D/8,D/2,D/9,D/4,D/2,D/6,D/6

   The Notify response indicates that the transaction was successful:

      200 2002 OK

F.3 CreateConnection

   The first example illustrates a CreateConnection command to create a
   connection on the endpoint specified.  The connection will be part of
   the specified CallId.  The LocalConnectionOptions specify that G.711
   mu-law will be the codec used and the packetization period will be 10
   ms.  The connection mode will be "receive only":

      CRCX 1204 aaln/1@rgw-2567.whatever.net MGCP 1.0
      C: A3C47F21456789F0
      L: p:10, a:PCMU
      M: recvonly






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   The response indicates that the transaction was successful, and a
   connection identifier for the newly created connection is therefore
   included.  A session description for the new connection is included
   as well - note that it is preceded by an empty line.

      200 1204 OK
      I: FDE234C8

      v=0
      o=- 25678 753849 IN IP4 128.96.41.1
      s=-
      c=IN IP4 128.96.41.1
      t=0 0
      m=audio 3456 RTP/AVP 0

   The second example illustrates a CreateConnection command containing
   a notification request and a RemoteConnectionDescriptor:

      CRCX 1205 aaln/1@rgw-2569.whatever.net MGCP 1.0
      C: A3C47F21456789F0
      L: p:10, a:PCMU
      M: sendrecv
      X: 0123456789AD
      R: L/hd
      S: L/rg

      v=0
      o=- 25678 753849 IN IP4 128.96.41.1
      s=-
      c=IN IP4 128.96.41.1
      t=0 0
      m=audio 3456 RTP/AVP 0

   The response indicates that the transaction failed, because the phone
   was already off-hook.  Consequently, neither a connection-id nor a
   session description is returned:

      401 1205 Phone off-hook

   Our third example illustrates the use of the provisional response and
   the three-way handshake.  We create another connection and
   acknowledge the previous response received by using the response
   acknowledgement parameter:








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      CRCX 1206 aaln/1@rgw-2569.whatever.net MGCP 1.0
      K: 1205
      C: A3C47F21456789F0
      L: p:10, a:PCMU
      M: inactive

      v=0
      o=- 25678 753849 IN IP4 128.96.41.1
      s=-
      c=IN IP4 128.96.41.1
      t=0 0
      m=audio 3456 RTP/AVP 0

   A provisional response is returned initially:

      100 1206 Pending
      I: DFE233D1

      v=0
      o=- 4723891 7428910 IN IP4 128.96.63.25
      s=-
      c=IN IP4 128.96.63.25
      t=0 0
      m=audio 3456 RTP/AVP 0

   A little later, the final response is received:

      200 1206 OK
      K:
      I: DFE233D1

      v=0
      o=- 4723891 7428910 IN IP4 128.96.63.25
      s=-
      c=IN IP4 128.96.63.25
      t=0 0
      m=audio 3456 RTP/AVP 0

   The Call Agent acknowledges the final response as requested:

      000 1206

   and the transaction is complete.








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F.4 ModifyConnection

   The first example shows a ModifyConnection command that simply sets
   the connection mode of a connection to "send/receive" - the "notified
   entity" is set as well:

      MDCX 1209 aaln/1@rgw-2567.whatever.net MGCP 1.0
      C: A3C47F21456789F0
      I: FDE234C8
      N: ca@ca1.whatever.net
      M: sendrecv

   The response indicates that the transaction was successful:

      200 1209 OK

   In the second example, we pass a session description and include a
   notification request with the ModifyConnection command.  The endpoint
   will start playing ring-back tones to the user:

      MDCX 1210 aaln/1@rgw-2567.whatever.net MGCP 1.0
      C: A3C47F21456789F0
      I: FDE234C8
      M: recvonly
      X: 0123456789AE
      R: L/hu
      S: G/rt

      v=0
      o=- 4723891 7428910 IN IP4 128.96.63.25
      s=-
      c=IN IP4 128.96.63.25
      t=0 0
      m=audio 3456 RTP/AVP 0

   The response indicates that the transaction was successful:

      200 1206 OK

F.5 DeleteConnection (from the Call Agent)

   In this example, the Call Agent simply instructs the gateway to
   delete the connection "FDE234C8" on the endpoint specified:

      DLCX 1210 aaln/1@rgw-2567.whatever.net MGCP 1.0
      C: A3C47F21456789F0
      I: FDE234C8




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   The response indicates success, and that the connection was deleted.
   Connection parameters for the connection are therefore included as
   well:

      250 1210 OK
      P: PS=1245, OS=62345, PR=780, OR=45123, PL=10, JI=27, LA=48

F.6 DeleteConnection (from the gateway)

   In this example, the gateway sends a DeleteConnection command to the
   Call Agent to instruct it that a connection on the specified endpoint
   has been deleted.  The ReasonCode specifies the reason for the
   deletion, and Connection Parameters for the connection are provided
   as well:

      DLCX 1210 aaln/1@rgw-2567.whatever.net MGCP 1.0
      C: A3C47F21456789F0
      I: FDE234C8
      E: 900 - Hardware error
      P: PS=1245, OS=62345, PR=780, OR=45123, PL=10, JI=27, LA=48

   The Call Agent sends a success response to the gateway:

      200 1210 OK

F.7 DeleteConnection (multiple connections from the Call Agent)

   In the first example, the Call Agent instructs the gateway to delete
   all connections related to call "A3C47F21456789F0" on the specified
   endpoint:

      DLCX 1210 aaln/1@rgw-2567.whatever.net MGCP 1.0
      C: A3C47F21456789F0

   The response indicates success and that the connection(s) were
   deleted:

      250 1210 OK

   In the second example, the Call Agent instructs the gateway to delete
   all connections related to all of the endpoints specified:

      DLCX 1210 aaln/*@rgw-2567.whatever.net MGCP 1.0

   The response indicates success:

      250 1210 OK




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F.8 AuditEndpoint

   In the first example, the Call Agent wants to learn what endpoints
   are present on the gateway specified, hence the use of the "all of"
   wild-card for the local portion of the endpoint-name:

      AUEP 1200 *@rgw-2567.whatever.net MGCP 1.0

   The gateway indicates success and includes a list of endpoint names:

      200 1200 OK
      Z: aaln/1@rgw-2567.whatever.net
      Z: aaln/2@rgw-2567.whatever.net

   In the second example, the capabilities of one of the endpoints is
   requested:

      AUEP 1201 aaln/1@rgw-2567.whatever.net MGCP 1.0
      F: A

   The response indicates success and the capabilities as well.  Two
   codecs are supported, however with different capabilities.
   Consequently two separate capability sets are returned:

      200 1201 OK
      A: a:PCMU, p:10-100, e:on, s:off, v:L;S, m:sendonly;
               recvonly;sendrecv;inactive;netwloop;netwtest
      A: a:G729, p:30-90, e:on, s:on, v:L;S, m:sendonly;
               recvonly;sendrecv;inactive;confrnce;netwloop

   Note that the carriage return in the Capabilities lines are shown for
   formatting reasons only - they are not permissible in a real
   implementation.

   In the third example, the Call Agent audits several types of
   information for the endpoint:

      AUEP 2002 aaln/1@rgw-2567.whatever.net MGCP 1.0
      F: R,D,S,X,N,I,T,O,ES












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   The response indicates success:

      200 2002 OK
      R: L/hu,L/oc(N),D/[0-9](N)
      D:
      S: L/vmwi(+)
      X: 0123456789B1
      N: [128.96.41.12]
      I: 32F345E2
      T: G/ft
      O: L/hd,D/9,D/1,D/2
      ES: L/hd

   The list of requested events contains three events.  Where no package
   name is specified, the default package is assumed.  The same goes for
   actions, so the default action - Notify - must therefore be assumed
   for the "L/hu" event.  The omission of a value for the "digit map"
   means the endpoint currently does not have a digit map.  There are
   currently no active time-out signals, however the OO signal "vmwi" is
   currently on and is consequently included - in this case it was
   parameterized, however the parameter could have been excluded.  The
   current "notified entity" refers to an IP-address and only a single
   connection exists for the endpoint.  The current value of
   DetectEvents is "G/ft", and the list of ObservedEvents contains the
   four events specified.  Finally, the event-states audited reveals
   that the phone was off-hook at the time the transaction was
   processed.

F.9 AuditConnection

   The first example shows an AuditConnection command where we audit the
   CallId, NotifiedEntity, LocalConnectionOptions, Connection Mode,
   LocalConnectionDescriptor, and the Connection Parameters:

      AUCX 2003 aaln/1@rgw-2567.whatever.net MGCP 1.0
      I: 32F345E2
      F: C,N,L,M,LC,P














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   The response indicates success and includes information for the
   RequestedInfo:

      200 2003 OK
      C: A3C47F21456789F0
      N: ca@ca1.whatever.net
      L: p:10, a:PCMU
      M: sendrecv
      P: PS=395, OS=22850, PR=615, OR=30937, PL=7, JI=26, LA=47

      v=0
      o=- 4723891 7428910 IN IP4 128.96.63.25
      s=-
      c=IN IP4 128.96.63.25
      t=0 0
      m=audio 1296 RTP/AVP 0

   In the second example, we request to audit RemoteConnectionDescriptor
   and LocalConnectionDescriptor:

      AUCX 1203 aaln/2@rgw-2567.whatever.net MGCP 1.0
      I: FDE234C8
      F: RC,LC

   The response indicates success, and includes information for the
   RequestedInfo.  In this case, no RemoteConnectionDescriptor exists,
   hence only the protocol version field is included for the
   RemoteConnectionDescriptor:

      200 1203 OK

      v=0
      o=- 4723891 7428910 IN IP4 128.96.63.25
      s=-
      c=IN IP4 128.96.63.25
      t=0 0
      m=audio 1296 RTP/AVP 0

      v=0

F.10 RestartInProgress

   The first example illustrates a RestartInProgress message sent by an
   gateway to inform the Call Agent that the specified endpoint will be
   taken out-of-service in 300 seconds:






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      RSIP 1200 aaln/1@rgw-2567.whatever.net MGCP 1.0
      RM: graceful
      RD: 300

   The Call Agent's response indicates that the transaction was
   successful:

      200 1200 OK

   In the second example, the RestartInProgress message sent by the
   gateway informs the Call Agent, that all of the gateway's endpoints
   are being placed in-service in 0 seconds, i.e., they are currently in
   service.  The restart delay could have been omitted as well:

      RSIP 1204 *@rgw-2567.whatever.net MGCP 1.0
      RM: restart
      RD: 0

   The Call Agent's response indicates success, and furthermore provides
   the endpoints in question with a new "notified entity":

      200 1204 OK
      N: CA-1@whatever.net

   Alternatively, the command could have failed with a new "notified
   entity" as in:

      521 1204 OK
      N: CA-1@whatever.net

   In that case, the command would then have to be retried in order to
   satisfy the "restart procedure", this time going to Call Agent "CA-
   1@whatever.net".

Appendix G: Example Call Flows

   The message flow tables in this section use the following
   abbreviations:

   * rgw = Residential Gateway

   * ca  = Call Agent

   * n+  = step 'n' is repeated one or more times







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   Note that any use of upper and lower case within the text of the
   messages is to aid readability and is not in any way a requirement.
   The only requirement involving case is to be case insensitive at all
   times.

G.1 Restart

G.1.1 Residential Gateway Restart

   The following table shows a message sequence that might occur when a
   call agent (ca) is contacted by two independent residential gateways
   (rgw1 and rgw2) which have restarted.

                  Table F.1: Residential Gateway Restart

 ---------------------------------------------------------------------
|step#|    usr1    |    rgw1    |     ca     |    rgw2    |    usr2   |
|=====|============|============|============|============|===========|
|  1  |            |    rsip -> |            |            |           |
|     |            |            | <- ack     |            |           |
|-----|------------|------------|------------|------------|-----------|
|  2  |            |            | <- auep    |            |           |
|     |            |     ack -> |            |            |           |
|-----|------------|------------|------------|------------|-----------|
|  3+ |            |            | <- rqnt    |            |           |
|     |            |     ack -> |            |            |           |
|-----|------------|------------|------------|------------|-----------|
|  4  |            |            |            | <- rsip    |           |
|     |            |            |     ack -> |            |           |
|-----|------------|------------|------------|------------|-----------|
|  5  |            |            |    auep -> |            |           |
|     |            |            |            | <- ack     |           |
|-----|------------|------------|------------|------------|-----------|
|  6+ |            |            |    rqnt -> |            |           |
|     |            |            |            | <- ack     |           |
 ---------------------------------------------------------------------

   Step 1 - RestartInProgress (rsip) from rgw1 to ca

   rgw1 uses DNS to determine the domain name of ca and send to the
   default port of 2727.  The command consists of the following:

      rsip 1 *@rgw1.whatever.net mgcp 1.0
      rm: restart







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   The "*" is used to inform ca that all endpoints of rgw1 are being
   restarted, and "restart" is specified as the restart method.  The
   Call Agent "ca" acknowledges the command with an acknowledgement
   message containing the transaction-id (in this case 1) for the
   command.  It sends the acknowledgement to rgw1 using the same port
   specified as the source port for the rsip.  If none was indicated, it
   uses the default port of 2727.

      200 1 ok

   A response code is mandatory.  In this case, "200", indicates "the
   requested transaction was executed normally".  The response string is
   optional.  In this case, "ok" is included as an additional
   description.

   Step 2 - AuditEndpoint (auep) from ca to rgw1

   The command consists of the following:

      auep 153 *@rgw1.whatever.net mgcp 1.0

   The "*" is used to request audit information from rgw1 of all its
   endpoints.  rgw1 acknowledges the command with an acknowledgement
   message containing the transaction-id (in this case 153) of the
   command, and it includes a list of its endpoints.  In this example,
   rgw1 has two endpoints, aaln/1 and aaln/2.

      200 153 ok
      Z: aaln/1@rgw1.whatever.net
      Z: aaln/2@rgw1.whatever.net

   Once it has the list of endpoint ids, ca may send individual
   AuditEndpoint commands in which the "*" is replaced by the id of the
   given endpoint.  As its response, rgw1 would replace the endpoint id
   list returned in the example with the info requested for the
   endpoint.  This optional message exchange is not shown in this
   example.

   Step 3 - NotificationRequest (rqnt) from ca to each endpoint of rgw1

   In this case, ca sends two rqnts, one for aaln/1:

      rqnt 154 aaln/1@rgw1.whatever.net mgcp 1.0
      r: l/hd(n)
      x: 3456789a0






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   and a second for aaln/2:

      rqnt 155 aaln/2@rgw1.whatever.net mgcp 1.0
      r: l/hd(n)
      x: 3456789a1

   Note that in the requested events parameter line, the event is fully
   specified as "l/hd", i.e., with the package name, in order to avoid
   any potential ambiguity.  This is the recommended behavior.  For the
   sake of clarity, the action, which in this case is to Notify, is
   explicitly specified by including the "(n)".  If no action is
   specified, Notify is assumed as the default regardless of the event.
   If any other action is desired, it must be stated explicitly.

   The expected response from rgw1 to these requests is an
   acknowledgement from aaln/1 as follows:

      200 154 ok

   and from aaln/2:

      200 155 ok

   Step 4 RestartInProgress (rsip) from rgw2 to ca

      rsip 0 *@rgw2.whatever.net mgcp 1.0
      rm: restart

   followed by the acknowledgement from ca:

      200 0 ok

   Step 5 - AuditEndpoint (auep) from ca to rgw2

      auep 156 *@rgw2.whatever.net mgcp 1.0

   followed by an acknowledgement from rgw2:

      200 156 ok
      z: aaln/1@rgw2.whatever.net
      z: aaln/2@rgw2.whatever.net

   Step 6 - NotificationRequest (rqnt) from ca to each endpoint of rgw2

      rqnt 157 aaln/1@rgw2.whatever.net mgcp 1.0
      r: l/hd(n)
      x: 3456789a2




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   followed by:

      rqnt 158 aaln/2@rgw2.whatever.net mgcp 1.0
      r: l/hd(n)
      x: 3456789a3

   with rgw2 acknowledging for aaln/1:

      200 157 ok

   and for aaln/2:

      200 158 ok

G.1.2 Call Agent Restart

   The following table shows the message sequence which occurs when a
   call agent (ca) restarts.  How it determines the address information
   of the gateways, in this case rgw1 and rgw2, is not covered in this
   document.  For interoperability, it is RECOMMENDED to provide the
   ability to configure the call agent to send AUEP (*) to specific
   addresses and ports.

                  Table F.2: Residential Gateway Restart

 ---------------------------------------------------------------------
| # |     usr1    |    rgw1    |     ca     |    rgw2    |     usr2   |
|===|=============|============|============|============|============|
| 1 |             |            | <- auep    |            |            |
|   |             |     ack -> |            |            |            |
|---|-------------|------------|------------|------------|------------|
| 2+|             |            | <- rqnt    |            |            |
|   |             |     ack -> |            |            |            |
|---|-------------|------------|------------|------------|------------|
| 3 |             |            |    auep -> |            |            |
|   |             |            |            | <- ack     |            |
|---|-------------|------------|------------|------------|------------|
| 4+|             |            |    rqnt -> |            |            |
|   |             |            |            | <- ack     |            |
 ---------------------------------------------------------------------

   Step 1 - AuditEndpoint (auep) from ca to rgw1

   The command consists of the following:

      auep 0 *@rgw1.whatever.net mgcp 1.0





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   The "*" is used to request audit information from rgw1 of all its
   endpoints.  rgw1 acknowledges the command with an acknowledgement
   message containing the transaction id (in this case 0) of the
   command, and it includes a list of its endpoints.  In this example,
   rgw1 has two endpoints, aaln/1 and aaln/2.

      200 0 ok
      z: aaln/1@rgw1.whatever.net
      z: aaln/2@rgw1.whatever.net

   Once it has the list of endpoint ids, ca may send individual
   AuditEndpoint commands in which the "*" is replaced by the id of the
   given endpoint.  As its response, rgw1 would replace the endpoint id
   list returned in the example with the info requested for the
   endpoint.  This optional message exchange is not shown in this
   example.

   Step 2 - NotificationRequest (rqnt) off-hook from ca to rgw1

   In this case, ca sends two rqnts, one for aaln/1:

      rqnt 1 aaln/1@rgw1.whatever.net mgcp 1.0
      r: l/hd(n)
      x: 234567890

   and a second for aaln/2:

      rqnt 2 aaln/2@rgw1.whatever.net mgcp 1.0
      r: l/hd(n)
      x: 234567891

   The expected response from rgw1 to these requests is an
   acknowledgement from aaln/1 as follows:

      200 1 ok

   and from aaln/2:

      200 2 ok

   Step 3 - AuditEndpoint (auep) from ca to rgw2

      auep 3 *@rgw2.whatever.net mgcp 1.0








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   followed by an acknowledgement from rgw2:

      200 3 ok
      z: aaln/1@rgw2.whatever.net
      z: aaln/2@rgw2.whatever.net

   Step 4 - NotificationRequest (rqnt) from ca to each endpoint of rgw2

      rqnt 4 aaln/1@rgw2.whatever.net mgcp 1.0
      r: l/hd(n)
      x: 234567892

   followed by:

      rqnt 5 aaln/2@rgw2.whatever.net mgcp 1.0
      r: l/hd(n)
      x: 234567893

   with rgw2 acknowledging for aaln/1:

      200 4 ok

   and for aaln/2:

      200 5 ok
G.2 Connection Creation

G.2.1 Residential Gateway to Residential Gateway

   The following table shows the message sequence which occurs when a
   user (usr1) makes a call through a residential gateway (rgw1) to a
   user served by another residential gateway (rgw2).  This example
   illustrates the communication between the residential gateways and
   the call agent (ca) only.  The local name of the endpoints in this
   example is aaln/1 for both gateways, and references within the
   description of the steps to rgw1 and rgw2 can be assumed to refer to
   aaln/1 of rgw1 and aaln/1 of rgw2.  Note that this is only an example
   and is not the only legal call scenario.













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            Table F.3: Residential Gateway Connection Creation

 ---------------------------------------------------------------------
| # |     usr1    |    rgw1    |     ca     |    rgw2    |     usr2   |
|===|=============|============|============|============|============|
| 1 |  offhook -> |    ntfy -> |            |            |            |
|   |             |            | <- ack     |            |            |
|---|-------------|------------|------------|------------|------------|
| 2 | <- dialtone |            | <- rqnt    |            |            |
|   |             |     ack -> |            |            |            |
|---|-------------|------------|------------|------------|------------|
| 3 |   digits -> |    ntfy -> |            |            |            |
|   |             |            | <- ack     |            |            |
|---|-------------|------------|------------|------------|------------|
| 4 |             |            | <- rqnt    |            |            |
|   |             |     ack -> |            |            |            |
|---|-------------|------------|------------|------------|------------|
| 5 | <- recvonly |            | <- crcx    |            |            |
|   |             |     ack -> |            |            |            |
|---|-------------|------------|------------|------------|------------|
| 6 |             |            |    crcx -> |            | sendrcv -> |
|   |             |            |            | <- ack     |            |
|---|-------------|------------|------------|------------|------------|
| 7 | <- recvonly |            | <- mdcx    |            |            |
|   |             |     ack -> |            |            |            |
|---|-------------|------------|------------|------------|------------|
| 8 | <- ringback |            | <- rqnt    |            |            |
|   |             |     ack -> |            |            |            |
|---|-------------|------------|------------|------------|------------|
| 9 |             |            |    rqnt -> |            | ringing -> |
|   |             |            |            | <- ack     |            |
|---|-------------|------------|------------|------------|------------|
|10 |             |            |            | <- ntfy    | <- offhook |
|   |             |            |     ack -> |            |            |
|---|-------------|------------|------------|------------|------------|
|11 |             |            |    rqnt -> |            |            |
|   |             |            |            | <- ack     |            |
|---|-------------|------------|------------|------------|------------|
|12 |             |            | <- rqnt    |            |            |
|   |             |     ack -> |            |            |            |
|---|-------------|------------|------------|------------|------------|
|13 | <- sendrcv  |            | <- mdcx    |            |            |
|   |             |     ack -> |            |            |            |
 ---------------------------------------------------------------------

   Step 1 - Notify (ntfy) offhook from rgw1 to ca





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   This ntfy is the result of usr1 going offhook and assumes ca had
   previously sent an rqnt with RequestId "445678944" to rgw1 requesting
   notification in the event of an offhook:

      ntfy 12 aaln/1@rgw1.whatever.net mgcp 1.0
      o: l/hd
      x: 445678944

   Acknowledgement from ca:

      200 12 ok

   Step 2 - Request Notification (rqnt) for digits from ca to rgw1

   Request rgw1 to notify if on-hook and collect digits according to the
   digit map, and to provide dialtone:

      rqnt 1057 aaln/1@rgw1.whatever.net mgcp 1.0
      r: l/hu(n), d/[0-9#*T](d)
      s: l/dl
      x: 445678945
      d: 5xxx

   Acknowledgement from rgw1:

      200 1057 ok

   Step 3 - Notify (ntfy) digits from rgw1 to ca

      ntfy 13 aaln/1@rgw1.whatever.net mgcp 1.0
      o: d/5, d/0, d/0, d/1
      x: 445678945

   Acknowledgement from ca:

      200 13 ok

   Step 4 - Request Notification (rqnt) from ca to rgw1

   Request rgw1 to notify in the event of an on-hook transition:

      rqnt 1058 aaln/1@rgw1.whatever.net mgcp 1.0
      r: l/hu(n)
      x: 445678946







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   Acknowledgement from rgw1:

      200 1058 ok

   Step 5 - Create Connection (crcx) from ca to rgw1

   Request a new connection on rgw1 with the specified local connection
   options, including 20 msec as the packetization period, G.711 mu-law
   as the codec, and receive only as the mode:

      crcx 1059 aaln/1@rgw1.whatever.net mgcp 1.0
      c: 9876543210abcdef
      l: p:20, a:PCMU
      m: recvonly

   Acknowledgement from rgw1 that a new connection, "456789fedcba5", has
   been created, followed by a blank line and then the SDP parameters:

      200 1059 ok
      i: 456789fedcba5

      v=0
      o=- 23456789 98765432 IN IP4 192.168.5.7
      s=-
      c=IN IP4 192.168.5.7
      t=0 0
      m=audio 6058 RTP/AVP 0

   Step 6 - Create Connection (crcx) from ca to rgw2

   Request a new connection on rgw2.  The request includes the session
   description returned by rgw1 such that a two way connection can be
   initiated:

      crcx 2052 aaln/1@rgw2.whatever.net mgcp 1.0
      c: 9876543210abcdef
      l: p:20, a:PCMU
      m: sendrecv

      v=0
      o=- 23456789 98765432 IN IP4 192.168.5.7
      s=-
      c=IN IP4 192.168.5.7
      t=0 0
      m=audio 6058 RTP/AVP 0






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   Acknowledgement from rgw2 that a new connection, "67890af54c9", has
   been created; followed by a blank line and then the SDP parameters:

      200 2052 ok
      i: 67890af54c9

      v=0
      o=- 23456889 98865432 IN IP4 192.168.5.8
      s=-
      c=IN IP4 192.168.5.8
      t=0 0
      m=audio 6166 RTP/AVP 0

   Step 7 - Modify Connection (mdcx) from ca to rgw1

   Request rgw1 to modify the existing connection, "456789fedcba5", to
   use the session description returned by rgw2 establishing a half
   duplex connection which, though not used in this example, could be
   used to provide usr1 with in band ringback tone, announcements, etc:

      mdcx 1060 aaln/1@rgw1.whatever.net mgcp 1.0
      c: 9876543210abcdef
      i: 456789fedcba5
      l: p:20, a:PCMU
      M: recvonly

      v=0
      o=- 23456889 98865432 IN IP4 192.168.5.8
      s=-
      c=IN IP4 192.168.5.8
      t=0 0
      m=audio 6166 RTP/AVP 0

   Acknowledgement from rgw1:

      200 1060 ok

   Step 8 - Request Notification (rqnt) from ca for rgw1 to provide
   ringback

   Request rgw1 to notify in the event of an on-hook transition, and
   also to provide ringback tone:

      rqnt 1061 aaln/1@rgw1.whatever.net mgcp 1.0
      r: l/hu(n)
      s: g/rt
      x: 445678947




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   Acknowledgement from rgw1:

      200 1061 ok

   Step 9 - Request Notification (rqnt) from ca to rgw2 to provide
   ringing

   Request rgw2 to continue to look for offhook and provide ringing:

      rqnt 2053 aaln/1@rgw2.whatever.net mgcp 1.0
      r: l/hd(n)
      s: l/rg
      x: 445678948

   Acknowledgement from rgw2:

      200 2053 ok

   Step 10 - Notify (ntfy) offhook from rgw2 to ca

      ntfy 27 aaln/1@rgw2.whatever.net mgcp 1.0
      o: l/hd
      x: 445678948

   Acknowledgement from ca:

      200 27 ok

   Step 11 - Request Notification (rqnt) of on-hook from ca to rgw2

      rqnt 2054 aaln/1@rgw2.whatever.net mgcp 1.0
      r: l/hu(n)
      x: 445678949

   Acknowledgement from rgw2:

      200 2054 ok

   Step 12 - Request Notification (rqnt) of on-hook from ca to rgw1

      rqnt 1062 aaln/1@rgw1.whatever.net mgcp 1.0
      r: l/hu(n)
      x: 445678950

   Acknowledgement from rgw1:

      200 1062 ok




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   Step 13 - Modify Connection (mdcx) from ca to rgw1

   Request rgw1 to modify the existing connection, "456789fedcba5", to
   sendrecv such that a full duplex connection is initiated:

      mdcx 1063 aaln/1@rgw1.whatever.net mgcp 1.0
      c: 9876543210abcdef
      i: 456789fedcba5
      m: sendrecv

   Acknowledgement from rgw1:

      200 1063 ok

G.3 Connection Deletion

G.3.1 Residential Gateway to Residential Gateway

   The following table shows the message sequence which occurs when a
   user (usr2) initiates the deletion of an existing connection on a
   residential gateway (rgw2) with a user served by another residential
   gateway (rgw1).  This example illustrates the communication between
   the residential gateways and the call agent (ca) only.  The local
   name of the endpoints in this example is aaln/1 for both gateways,
   and references within the description of the steps to rgw1 and rgw2
   can be assumed to refer to aaln/1 of rgw1 and aaln/1 of rgw2.

























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            Table F.4: Residential Gateway Connection Deletion

 ---------------------------------------------------------------------
| # |     usr1    |    rgw1    |     ca     |    rgw2    |     usr2   |
|===|=============|============|============|============|============|
| 1 |             |            |            | <- ntfy    | <- on-hook |
|   |             |            |     ack -> |            |            |
|---|-------------|------------|------------|------------|------------|
| 2 |             |            |    dlcx -> |            |            |
|   |             |            |            | <- ack     |            |
|---|-------------|------------|------------|------------|------------|
| 3 |             |            | <- dlcx    |            |            |
|   |             |     ack -> |            |            |            |
|---|-------------|------------|------------|------------|------------|
| 4 |             |            |    rqnt -> |            |            |
|   |             |            |            | <- ack     |            |
|---|-------------|------------|------------|------------|------------|
| 5 |  on-hook -> |    ntfy -> |            |            |            |
|   |             |            | <- ack     |            |            |
|---|-------------|------------|------------|------------|------------|
| 6 |             |            | <- rqnt    |            |            |
|   |             |     ack -> |            |            |            |
 ---------------------------------------------------------------------

   Step 1 - Notify (ntfy) offhook from rgw1 to ca

   This ntfy is the result of usr2 going on-hook and assumes that ca had
   previously sent an rqnt to rgw2 requesting notification in the event
   of an on-hook (see end of Connection Creation sequence):

      ntfy 28 aaln/1@rgw2.whatever.net mgcp 1.0
      o: l/hu
      x: 445678949

   Acknowledgement from ca:

      200 28 ok

   Step 2 - Delete Connection (dlcx) from ca to rgw2

   Requests rgw2 to delete the connection "67890af54c9":

      dlcx 2055 aaln/1@rgw1.whatever.net mgcp 1.0
      c: 9876543210abcdef
      i: 67890af54c9






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   Acknowledgement from rgw2.  Note the response code of "250" meaning
   "the connection was deleted":

      250 2055 ok

   Step 3 - Delete Connection (dlcx) from ca to rgw1

   Requests rgw1 to delete the connection "456789fedcba5":

      dlcx 1064 aaln/1@rgw1.whatever.net mgcp 1.0
      c: 9876543210abcdef
      i: 456789fedcba5

   Acknowledgement from rgw1:

      250 1064 ok

   Step 4 - NotificationRequest (rqnt) from ca to rgw2

   Requests rgw2 to notify ca in the event of an offhook transition:

      rqnt 2056 aaln/1@rgw2.whatever.net mgcp 1.0
      r: l/hd(n)
      x: 445678951

   Acknowledgement from rgw2:

      200 2056 ok

   Step 5 - Notify (ntfy) on-hook from rgw1 to ca

   Notify ca that usr1 at rgw1 went back on-hook:

      ntfy 15 aaln/1@rgw1.whatever.net mgcp 1.0
      o: l/hu
      x: 445678950

   Acknowledgement from ca:

      200 15 ok

   Step 6 - NotificationRequest (rqnt) offhook from ca to rgw1

   Requests rgw1 to notify ca in the event of an offhook transition:

      rqnt 1065 aaln/1@rgw1.whatever.net mgcp 1.0
      r: l/hd(n)
      x: 445678952



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   Acknowledgement from rgw1:

      200 1065 ok

Authors' Addresses

   Flemming Andreasen
   Cisco Systems
   499 Thornall Street, 8th Floor
   Edison, NJ 08837

   EMail: fandreas@cisco.com


   Bill Foster
   Cisco Systems
   771 Alder Drive
   Milpitas, CA 95035

   EMail: bfoster@cisco.com






























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Full Copyright Statement

   Copyright (C) The Internet Society (2003).  All Rights Reserved.

   This document and translations of it may be copied and furnished to
   others, and derivative works that comment on or otherwise explain it
   or assist in its implementation may be prepared, copied, published
   and distributed, in whole or in part, without restriction of any
   kind, provided that the above copyright notice and this paragraph are
   included on all such copies and derivative works.  However, this
   document itself may not be modified in any way, such as by removing
   the copyright notice or references to the Internet Society or other
   Internet organizations, except as needed for the purpose of
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   copyrights defined in the Internet Standards process must be
   followed, or as required to translate it into languages other than
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   The limited permissions granted above are perpetual and will not be
   revoked by the Internet Society or its successors or assigns.

   This document and the information contained herein is provided on an
   "AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING
   TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING
   BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION
   HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF
   MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.

Acknowledgement

   Funding for the RFC Editor function is currently provided by the
   Internet Society.



















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