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Internet Engineering Task Force                          Mauricio Arango
INTERNET DRAFT                                                   RSL COM
July 29, 1999                                               Andrew Dugan
Expires January 29, 2000                           Level3 Communications
<draft-huitema-megaco-mgcp-v1-02.txt>                      Isaac Elliott
                                                   Level3 Communications
                                                       Christian Huitema
                                                               Telcordia
                                                           Scott Pickett
                                                       Vertical Networks


                 Media Gateway Control Protocol (MGCP)
             Mauricio Arango, Andrew Dugan, Isaac Elliott,
                    Christian Huitema, Scott Pickett
                              Version 1.0




Status of this document

This document is an Internet-Draft and is in full conformance with all
provisions of Section 10 of RFC2026.

Internet-Drafts are working documents of the Internet Engineering Task
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Abstract

This document describes an application programming interface and a
corresponding protocol (MGCP) for controlling Voice over IP (VoIP) Gate-
ways from external call control elements. MGCP assumes a call control
architecture where the call control "intelligence" is outside the



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gateways and handled by external call control elements.

The document is structured in 6 main sections:

*    The introduction presents the basic assumptions and the relation to
     other protocols such as H.323, RTSP, SAP or SIP.

*    The interface section presents a conceptual overview of the MGCP,
     presenting the naming conventions, the usage of the session
     description protocol SDP, and the procedures that compose MGCP:
     Notifications Request, Notification, Create Connection, Modify Con-
     nection, Delete Connection, AuditEndpoint, AuditConnection and Res-
     tartInProgress.

*    The protocol description section presents the MGCP encodings, which
     are based on simple text formats, and the transmission procedure
     over UDP.

*    The security section presents the security requirement of MGCP, and
     its usage of IP security services (IPSEC).

*    The event packages section provides an initial definition of pack-
     ages and event names.

*    The description of the changes made in combining SGCP 1.1 and IPDC
     to create MGCP 0.1.

























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     Table of Contents ........................................... Page

     1.  Introduction ..............................................  6
        1.1.  Relation with the H.323 standards ....................  8
        1.2.  Relation with the IETF standards .....................  9
        1.3.  Definitions .......................................... 10
     2.  Media Gateway Control Interface ........................... 10
        2.1.  Model and naming conventions. ........................ 11
           2.1.1.  Types of endpoints .............................. 11
              2.1.1.1.  Digital channel (DS0) ...................... 11
              2.1.1.2.  Analog line ................................ 12
              2.1.1.3.  Annoucement server access point ............ 13
              2.1.1.4.  Interactive Voice Response access point .... 13
              2.1.1.5.  Conference bridge access point ............. 14
              2.1.1.6.  Packet relay ............................... 14
              2.1.1.7.  Wiretap access point ....................... 15
              2.1.1.8.  ATM "trunk side" interface. ................ 15
           2.1.2.  Endpoint identifiers ............................ 15
           2.1.3.  Calls and connections ........................... 17
              2.1.3.1.  Names of calls ............................. 20
              2.1.3.2.  Names of connections ....................... 20
              2.1.3.3.  Management of resources, attributes of ..... 20
              2.1.3.4.  Special case of local connections .......... 23
           2.1.4.  Names of Call Agents and other entities ......... 23
           2.1.5.  Digit maps ...................................... 25
           2.1.6.  Names of events ................................. 26
        2.2.  Usage of SDP ......................................... 30
        2.3.  Gateway Control Commands ............................. 30
           2.3.1.  EndpointConfiguration ........................... 32
           2.3.2.  NotificationRequest ............................. 34
           2.3.3.  CreateConnection ................................ 39
           2.3.4.  ModifyConnection ................................ 46
           2.3.5.  DeleteConnection (from the Call Agent) .......... 47
           2.3.6.  DeleteConnection (from the VoIP gateway) ........ 51
           2.3.7.  DeleteConnection (multiple connections, from the  52
           2.3.8.  Audit Endpoint .................................. 53
           2.3.9.  Audit Connection ................................ 56
           2.3.10.  Restart in progress ............................ 57
        2.4.  Return codes and error codes. ........................ 59
        2.5.  Reason Codes ......................................... 61
     3.  Media Gateway Control Protocol ............................ 62
        3.1.  General description .................................. 62
        3.2.  Command Header ....................................... 63
           3.2.1.  Command line .................................... 63
              3.2.1.1.  Coding of the requested verb ............... 64
              3.2.1.2.  Transaction Identifiers .................... 64
              3.2.1.3.  Coding of the endpoint identifiers and ..... 65
              3.2.1.4.  Coding of the protocol version ............. 66



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           3.2.2.  Parameter lines ................................. 66
              3.2.2.1.  Response Acknowledgement ................... 69
              3.2.2.2.  Local connection options ................... 69
              3.2.2.3.  Capabilities ............................... 70
              3.2.2.4.  Connection parameters ...................... 72
              3.2.2.5.  Reason Codes ............................... 73
              3.2.2.6.  Connection mode ............................ 74
              3.2.2.7.  Coding of event names ...................... 75
              3.2.2.8.  RequestedEvents ............................ 76
              3.2.2.9.  SignalRequests ............................. 77
              3.2.2.10.  ObservedEvent ............................. 77
              3.2.2.11.  RequestedInfo ............................. 78
              3.2.2.12.  QuarantineHandling ........................ 78
              3.2.2.13.  DetectEvents .............................. 78
              3.2.2.14.  EventStates ............................... 78
              3.2.2.15.  RestartMethod ............................. 79
              3.2.2.16.  Bearer Information ........................ 79
        3.3.  Format of response headers ........................... 79
        3.4.  Formal syntax description of the protocol ............ 81
        3.5.  Encoding of the session description .................. 86
           3.5.1.  Usage of SDP for an audio service ............... 87
           3.5.2.  Usage of SDP in a network access service ........ 88
           3.5.3.  Usage of SDP for ATM connections ................ 91
           3.5.4.  Usage of SDP for local connections .............. 92
        3.6.  Transmission over UDP ................................ 92
           3.6.1.  Providing the At-Most-Once functionality ........ 92
           3.6.2.  Transaction identifiers and three ways handshake  93
           3.6.3.  Computing retransmission timers ................. 94
           3.6.4.  Piggy backing ................................... 95
           3.6.5.  Provisional responses ........................... 95
     4.  States, failover and race conditions. ..................... 96
        4.1.  Basic Asumptions ..................................... 96
        4.2.  Security, Retransmission, and Detection of Lost ...... 97
        4.3.  Race conditions ......................................100
           4.3.1.  Quarantine list .................................100
           4.3.2.  Explicit detection ..............................103
           4.3.3.  Ordering of commands, and treatment of disorder .104
           4.3.4.  Fighting the restart avalanche ..................105
           4.3.5.  Disconnected Endpoints ..........................107
     1.   A "disconnected" timer is initialized to a random value, .107
     2.   The gateway then waits for either the end of this timer, .107
     3.   When the "disconnected" timer elapses, when a command is .107
     4.   If the "disconnected" procedure still left the endpoint ..107
     5.  Security requirements .....................................108
        5.1.  Protection of media connections ......................109
     6.  Event packages and end point types ........................110
        6.1.  Basic packages .......................................110
           6.1.1.  Generic Media Package ...........................111



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           6.1.2.  DTMF package ....................................111
           6.1.3.  MF Package ......................................113
           6.1.4.  Trunk Package ...................................114
           6.1.5.  Line Package ....................................116
           6.1.6.  Handset emulation package .......................118
           6.1.7.  RTP Package .....................................120
           6.1.8.  Network Access Server Package ...................121
           6.1.9.  Announcement Server Package .....................122
           6.1.10.  Script Package .................................123
        6.2.  Basic endpoint types and profiles ....................123
     7.  Versions and compatibility ................................124
        7.1.  Differences between version 1.0 and draft 0.5 ........124
        7.2.  Differences between draft-04 and draft-05 ............125
        7.3.  Differences between draft-03 and draft-04 ............125
        7.4.  Differences between draft-02 and draft-03 ............125
        7.5.  Differences between draft-01 and draft-02 ............126
        7.6.  The making of MGCP from IPDC and SGCP ................126
        7.7.  Changes between MGCP and initial versions of SGCP ....126
     8.  Acknowledgements ..........................................128
     9.  References ................................................129
     10.  Authors' Addresses .......................................130
     11.  Appendix A: Proposed "MoveConnection" command ............132
        11.1.  Proposed syntax modification ........................133




























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1.  Introduction

This document describes an abstract application programming interface
and a corresponding protocol (MGCP) for controlling Telephony Gateways
from external call control elements called media gateway controllers or
call agents. A telephony gateway is 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.
Example of 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 gate-
     ways include cable modem/cable set-top boxes, xDSL devices, broad-
     band wireless devices

*    Access gateways, that provide a traditional analog (RJ11) or digi-
     tal 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 "intel-
ligence" is outside the gateways and handled by external call control
elements. The MGCP assumes that these call control elements, or Call
Agents, will synchronize with each other to send coherent commands to
the gateways under their control. 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 detail the
expected behavior of the gateways, but only specify those parts of a
call agent implementation, such as timer management, that are mandated



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for proper operation of the protocol.

MGCP assumes a connection model where the basic constructs are endpoints
and connections. Endpoints are sources or sinks of data and could 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 ter-
     minates trunks is called a trunk 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:

*    Transmission of audio packets using RTP and UDP over a TCP/IP net-
     work.

*    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 tele-
     phone 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 "signalling gateway" when con-
necting 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 con-
formant 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 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 Gate-
keeper" 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 applica-
tions. These other specifications include:

*    the Session Description Protocol (SDP), RFC 2327,

*    the Session Announcement Protocol (SAP),

*    the Session Initiation Protocol (SIP),

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

The latter three specifications are in fact alternative signaling stan-
dards that allow for the transmission of a session description to an
interested party. SAP is used by multicast session managers to distri-
bute 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       |                         |
|           |            |  synchronization|                         |
|           |            |  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.

2.  Media Gateway Control Interface

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





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2.1.  Model and naming conventions.

The MGCP assumes a connection model where the basic constructs are end-
points 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 several 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 arbi-
trarily decide to provide several types of services in a single packag-
ing.  A single product could well, for example, provide some trunk con-
nections 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 functionalities.  Our analysis, so
far, has led us to isolate the following basic endpoint types:

*    Digital channel (DS0),

*    Analog line,

*    Annoucement server access point,

*    Interactive Voice Response access point,

*    Conference bridge access point,

*    Packet relay,

*    Wiretap access point,

*    ATM "trunk side" interface.

In this section, we will develop the expected behavior of such end-
points.

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

2.1.1.1.  Digital channel (DS0)

Digital channels provide an 8Khz*8bit service.  Such channels are found



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in trunk and ISDN interfaces.  They are typically part of digital multi-
plexes, 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 or mu-law, using either
the complete set of 8 bits or only 7 of these bits, into audio packets.
When the media gateway also supports a 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 con-
nections shall be mixed according to the connection "mode", as specified
later in this document.  The precise number of connections that an end-
point support is a characteristic of the gateway, and may in fact vary
according with the allocation of resource within the gateway.

In some cases, digital channels are used to carry signalling.  This is
the case for example of SS7 "F" links, or ISDN "D" channels.  Media
gateways that support these signalling functions shall be able to send
and receive the signalling packets to and from a call agent, using the
"back haul" procedures defined by the SIGTRAN working group of the IETF.

Digital channels are sometimes used in conjunction with channel associ-
ated signalling, such as "MF R2".  Media gateways that support these
signalling functions shall be able to detect and produce the correspond-
ing signals, such as for example "wink" or "A", according to the event
signalling and reporting procedures defined in MGCP.

2.1.1.2.  Analog line

Analog lines can be used either as a "client" interface, providing ser-
vice 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 pack-
ets.







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                                         +-------
                        +---------------+|
              (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 speci-
fied later in this document.  The precise number of connections that an
endpoint support is a characteristic of the gateway, and may in fact
vary according with the allocation of resource within the gateway.  A
typical gateway should however be able to support two or three connec-
tions per endpoint, in order to provide services such as "call waiting"
or "three ways calling".

2.1.1.3.  Annoucement server access point

An announcement server endpoint provides acces to an announcement ser-
vice. Under requests from the call agent, the announcement server will
"play" a specified announcement.  The requests from the call agent will
follow the event signalling and reporting procedures defined in MGCP.

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


A given announcement endpoint is not supposed 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 simultane-
ously over all the connections.

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

2.1.1.4.  Interactive Voice Response access point

An Interactive Voice Response (IVR) endpoint provides acces to an IVR
service. Under requests from the call agent, the IVR server will "play"
announcements and tones, and will "listen" to responses from the user.
The requests from the call agent will follow the event signalling and
reporting procedures defined in MGCP.





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


A given IVR endpoint is not supposed 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 con-
nections shall be mixed according to the connection "mode", as specified
later in this document. The precise number of connections that an end-
point support is a characteristic of the gateway, and may in fact vary
according with the allocation of resource 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 algo-
rithms, or gateways that operate over different transmission networks
such as IP and ATM.

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





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2.1.1.7.  Wiretap access point

A wiretap access point provides access to a wiretap service, providing
either a recording or a life playback of a connection.

                  +-----------------+
                  | Wiretap endpoint| -------- Connection
                  +-----------------+


A given wiretap endpoint is not supposed to support more than one con-
nection at a time. If several connections were established to the same
endpoint, then the recording or playback would mix the audio signals
received on this connections.

Connections to an wiretap endpoint are typically oneway, or "half
duplex" -- the wiretap server is not expected to signal its presence in
a call.

2.1.1.8.  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 con-
nections shall be mixed according to the connection "mode", as specified
later in this document.  The precise number of connections that an end-
point support is a characteristic of the gateway, and may in fact vary
according with the allocation of resource within the gateway.

2.1.2.  Endpoint identifiers

Endpoints identifiers have two components that both are case insensi-
tive:

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



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*    a local name within that gateway,

The syntax of the local name depends on the type of endpoint being
named. However, the local name for each of these types is naturally
hierarchical, beginning with a term which identifies the physical gate-
way containing the given endpoint and ending in a term which specifies
the individual endpoint concerned. With this in mind,  the following
rules for construction and interpretation of the Entity Name field for
these entity types MUST be supported:

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

2)   The individual terms are character strings composed of letters,
     digits or other printable characters, with the exception of charac-
     ters used as delimitors ("/", "@"), 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 naming path looks like

             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.

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

If the Media Gateway controls multiple physical gateways, the first term
of the naming MUST identify the physical gateway containing the desired
entity.  If the Media Gateway controls only a single physical gateway,
the first term of the naming string MAY identify that physical gateway,



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

In the case of trunking gateways, endpoints are trunk circuits linking a
gateway to a telephone switch. These circuits are typically grouped into
a digital multiplex, that is connected to the gateway by a physical
interface. Such circuits are named in three contexts:

*    In the ISUP protocol, trunks are grouped into trunk groups, identi-
     fied by the SS7 point codes of the switches that the group con-
     nects. Circuits within a trunk group are identified by a circuit
     number (CIC in ISUP).

*    In the gateway configuration files, physical interfaces are typi-
     cally identified by the name of the interface, an arbitrary text
     string. When the interface multiplexes several circuits, individual
     circuits are typically identified by a circuit number.

*    In MGCP, the endpoints are identified by an endpoint identifier.

The Call Agents use configuration databases to map ranges of circuit
numbers within an ISUP trunk group to corresponding ranges of circuits
in a multiplex connected to a gateway through a physical interface. The
gateway will be identified, in MGCP, by a domain name. The local name
will be structured to encode both the name of the physical interface,
for example X35V3+A4, and the circuit number within the multiplex con-
nected to the interface, for example 13. The circuit number will be
separated from the name of the interface by a fraction bar, as in:

        X35V3+A4/13


Other types of endpoints will use different conventions. For example, in
gateways were physical interfaces by construction only control one cir-
cuit, the circuit number will be omitted. The exact syntax of such names
should be specified in the corresponding server specification.

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 end
points will establish two connections (C1 and C2):

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



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Each connection will be designated locally by a 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 con-
     nection, and respond 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
     packetization parameters.

2)   The call agent then asks the second gateway to "create a connec-
     tion" on the second endpoint.  The command carries the "session
     description" provided by the first gateway. The gateway allocates
     resources to that connection, and respond to the command by provid-
     ing its own "session description."

3)   The call agent 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 the
different call agents, these two call agents shall exchange information
through a call-agent to call-agent signalling protocol, in order to syn-
chronize the creation of the connection on the two endpoints.

Once 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 compression algorithm used on a con-
nection, 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 to the gateway a "delete
connection" command.  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."

Calls are identified by unique identifiers, independent of the underly-
ing platforms or agents. These identifiers are created by the Call
Agent. They are treated in MGCP as unstructured octet strings.

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. 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 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. They are treated in MGCP as unstructured octet strings.
The gateway should make sure that a proper waiting period, at least 3
minutes, elapses between the end of a connection that used this identif-
ier 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.)

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. Gen-
erally, 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.

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 set of parameters to the gateway:

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



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2)   When available, the "session description" provided by the other end
     of the connection.

The local directives specify such parameters as the mode of the connec-
tion (e.g. send only, 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.) For each of these parame-
ters, the call agent can either specify a value, a range of value, or no
value at all.  This allow various implementations to implement various
level of control, from a very tight control where the call agent speci-
fies minute details of the connection handling to a very loose control
where the call agent only specifies broad guidelines, such as the max-
imum bandwidth, and let the gateway choose the detailed values.

Based on the value of the local directives, the gateway will determine
the resources allocated 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 resource have been allocated, the gateway will compose a "ses-
sion description" that describes the way it intends to 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 algorithm, it can provide a list of these accepted algo-
rithms.
























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                 Local Directives
                (from call agent 1)
                        |
                        V
                 +-------------+
                 | resources   |
                 | allocation  |
                 | (gateway 1) |
                 +-------------+
                   |         |
                   V         |
                 Local       |
              Parameters     V
                   |      Session
                   |    Description               Local Directives
                   |         |                   (from call agent 2)
                   |         +---> Transmission----+      |
                   |                (CA to CA)     |      |
                   |                               V      V
                   |                           +-------------+
                   |                           | resources   |
                   |                           | 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 connec-
tions between endpoints that are located within the same gateway.  Exam-
ples of such connections may be:

*    Connecting a trunk line to a wiretap device,

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

*    Connecting a call to a Conferencing unit,

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

Local connections are much simpler to establish than network connec-
tions. 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 name of the
two endpoints that will be connected.  The command is essentially a
"Create Connection" command which includes the name of the second end-
point 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 imple-
mentation of redundant Call Agents, for enhanced network reliability.
This means that there is no fixed binding between entities and hardware
platforms or network interfaces.

Reliability can be improved by the following precautions:

*    Entities such as endpoints or Call Agents are identified by their
     domain name, not their network addresses. Several addresses can be
     associated with a domain name. If a command or a response cannot be
     forwarded to one of the network addresses, implementations should
     retry the transmission using another address.

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




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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 end-
point has one, and only one, "notified entity" associated with it, and
when the endpoint needs to send a command to the Call Agent, it MUST
send the command to the current "notified entity" for which endpoint(s)
the command pertains. 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 end-
point reboots. If the "notified entity" for an endpoint is empty or has
not been set explicitly, the "notified entity" will then default to the
source address of the last connection handling command or notification
request received for the endpoint. Auditing will thus not change the
"notified entity."


































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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 col-
lect the numbers that a user dials; it may also be used with trunking
gateways and access gateways alike, to collect the access codes, credit
card numbers and other numbers requested by call control services.

An alternative procedure is for the gateway to notify the Call Agent of
the dialed digits, 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 mes-
sage.

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 load the gateway with a digit map
that 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 (section 3.4).  A Digit-
Map, according to this syntax, is defined either by a "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 regular
expression. A gateway that detects digits, letters or timers will:

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



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2)   Apply the current dial string to the digit map table, attempting a
     match to each regular expression in the Digit Map in lexical order

3)   If the result is under-qualified (partially matches at least one
     entry in the digit map), do nothing further.

If the result matches, or is over-qualified (i.e. no further digits
could possibly produce a match), send the current digit string to the
Call Agent. A match, in this specification, can be either a "perfect
match," exactly matching one of the specified alternatives, or an impos-
sible match, which occur when the dial string does not match any of the
alternative. Unexpected timers, for example, can cause "impossible
matches."  Both perfect matches and impossible matches trigger notifica-
tion of the accumulated digits.

Digit maps are provided to the gateway by the Call Agent, whenever the
Call Agent instructs the gateway to listen for digits.

2.1.6.  Names of events

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, and a call agent may request certain signals to be
applied to an endpoint, e.g. dial-tone.

Events and signals are grouped in packages within which they share the
same namespace which we will refer to as event names in the following.
Packages are groupings of the events and signals supported by a particu-
lar type of endpoint. For instance, one package may support a certain
group of events and signals for analog access lines, and another package
may support another group of events and signals for video lines. One or
more packages may exist for a given endpoint- type.

Event names are case insensitive and are composed of two logical parts,
a package name and an event name. Both names are strings of letters,
hyphens and digits, with the restriction that hyphens shall never be the
first or last characters in a name. Package or event names are not case
sensitive - values such as "hu", "Hu", "HU" or "hU" should be considered
equal.

Examples of package names are "D" (DTMF), "M" (MF), "T" (Trunk) or "L"
(Line). Examples of event names can be "hu" (off hook or "hang-up" tran-
sition), "hf" (flash hook) or "0" (the digit zero).

In textual representations, the package name, when present, is separated
from the event name by a slash ("/").  The package name is in fact
optional. Each endpoint-type has a default package associated with it,
and if the package name is excluded from the event name, the default



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package name for that endpoint-type is assumed. For example, for an ana-
log access line, the following two event names are equal:

l/dl dial-tone in the line package for an analog access line.

dl   dial-tone in the line package (default) for an analog access line.

This document defines a basic set of package names and event names.
Additional package names and event names can be registered with the
IANA. A package definition shall define the name of the package, and the
definition of each event belonging to the package. The event definition
shall include the precise name of the event (i.e., the code used in
MGCP), a plain text definition of the event, and, went appropriate, the
precise definition of the corresponding signals, for example the exact
frequencies of audio signal such as dial tones or DTMF tones.

In addition, implementers can gain experience by using experimental
packages. The names of experimental packages must start with the two
characters "x-"; the IANA shall not register package names that start
with these characters.

Digits, or letters, are supported in many packages, notably "DTMF", "MF"
and "pulse". 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 represent the keys that a user punched on a dial. In addi-
tion, the letter "X" can be used to represent all digits, and the sign
"$" can be used in wildcard notations. The need to easily express the
digit strings has a consequence on the form of event names:

     An event name that does not denote a digit should 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 should not contain the special
     signs "*", "#", "/" or "$".)

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 wildcard convention can be used to detect any event belonging
     to a package, or a given event in many packages, or event 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,



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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 sup-
     ported by the gateway
     The names "*" or "*/all" denote all events supported by the gate-
     way.

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 "any letter or digit."
     Use the notation "[0-9#]" to denote the digits 0 to 9 and the pound
     sign.

In some cases, Call Agents will request the gateway to generate or
detect events on connections rather than on the end point itself.  For
example, gateways may be asked to provide a ringback tone on a connec-
tion.  When an event shall be applied on a connection, the name of the
connection is added to the name of the event, using an "at" sign (@) as
a delimiter, as in:

     G/rt@0A3F58

The wildcard character "*" (star) can be used to denote "all connec-
tions". When this convention is used, the gateway will generate or
detect the event on all the connections that are connected to the end-
point. An example of this convention could be:

     R/qa@*

The wildcard character "$" can be used to denote "the current connec-
tion." It should onbly be used by the call agent, when the event notifi-
cation request is "encapsulated" within a command creation or modifica-
tion 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@$

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@*



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can be used to designate all events on all connections.

Events and signals are described in packages. The package description
must provide, for each events, the following informations:

*    The description of the event and its purpose, which should mean the
     actual signal that is generated by the client (i.e., xx ms FSK
     tone) as well as the resulting user observed result (i.e., MW light
     on/off).

*    The detailed characteristics of the event, such as for example fre-
     quencies and amplitude of audio signals, modulations and repeti-
     tions,

*    The typical and maximum duration of the event.

Signals are divided into different types depending on their behavior:

*    On/off (OO)
     Once applied, these signals last forever until they are turned off.
     This may happen either as the result of an event or a new SignalRe-
     quests (see later).

*    Time-out (TO)
     Once applied, these signals last until they are either turned off
     (by an event or SignalRequests) or a signal specific period of time
     has elapsed. Depending on package specifications, a signal that
     times out may generate an "operation complete" event.

*    Brief (BR)
     The duration of these signals is so short, that they stop on their
     own. If an event occurs the signal will not stop, however if a new
     SignalRequests is applied, the signal will stop. (Note: this point
     should be debated.  One could make a case that events such as
     strings of DTMF digits should in fact be allowed to complete.)

TO signals are normally used to alert the endpoints' users, to signal
them that they are expected to perform a specific action, such as hang
down the phone (ringing). Transmission of these signals should  typi-
cally be interrupted as soon as the first of the requested events has
been produced.

Package descriptions should describe, for all signals, their type (OO,
TO, BR). They should also describe the maximum duration of the TO sig-
nals.






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2.2.  Usage of SDP

The Call Agent uses the MGCP to provision the gateways with the descrip-
tion 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.

SDP allows for description of multimedia conferences. This version lim-
its SDP usage to the setting of audio circuits and data access circuits.
The initial session descriptions contain the description of exactly one
media, of type "audio" for audio connections, "nas" for data access.

2.3.  Gateway Control Commands

This section describes the commands of the MGCP. The service consists of
connection handling and endpoint handling commands. There are nine com-
mands 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 gate-
     way, 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 to 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 sus-
     tained.

*    The Call Agent can use the AuditEndpoint and AuditConnection com-
     mands to audit the status of an "endpoint" and any connections
     associated with it. Network management beyond the capabilities pro-
     vided by these commands are generally desirable, e.g. information
     about the status of the gateway. 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



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     specification.

*    The Gateway can use the RestartInProgress command to notify the
     Call Agent that the gateway, or 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 grouped into "calls". 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" (sen-
drecv), "conference" (confrnce), "data", "inactive" (inactive), "loop-
back" , "continuity test" (conttest), "network loop back" (netwloop) or
"network continuity test" (netwtest).

The handling of the audio signals received on these connections is
determined by the mode parameters:

*    Audio signals received in data packets through connections in
     "receive", "conference" or "send/receive" mode are mixed and sent
     to the endpoint.

*    Audio signals originating from the endpoint are transmitted over
     all the connections whose mode is "send", "conference" or
     "send/receive."

*    In addition to being sent to the endpoint, audio signals received
     in data packets through connections in "conference" mode are repli-
     cated to all the other connections whose mode is "conference."

The "loopback" and "continuity test" modes are used during maintenance
and continuity test operations. 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 expect the terminating switch to
loopback the circuit. 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



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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. 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.

If the mode is set to "network continuity test", the gateway will pro-
cess the packets received from the connection according to the tran-
sponder mode required for dual-tone continuity test, and send the pro-
cessed signal back on the connection.

2.3.1.  EndpointConfiguration

The EndpointConfiguration commands are used to specify the encoding of
the signals that will be received by the endpoint.  For example, in cer-
tain international telephony configurations, some calls will carry mu-
law encoded audio signals, while other will use A-law.  The Call Agent
will 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
            <-- EndpointConfiguration( EndpointId,
                                       BearerInformation)


EndpointId is the name for the endpoint in the gateway where End-
pointConfiguration executes, as defined in section 2.1.1.  The "any of"
wildcard convention shall not be used.  If the "all of" wildcard conven-
tion is used, the command applies to all the endpoint whose name matches
the wildcard.

BearerInformation is a parameter defining the coding of the data
received from the line side.  These information is encoded as a list of
sub-parameters.  The only sub-parameter defined in this version of the



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specification is the encoding method, whose values can be set to "A-law"
and "mu-law".

ReturnCode is a parameter returned by the gateway. It indicates the out-
come of the command and consists of an integer number optionally fol-
lowed by commentary.













































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2.3.2.  NotificationRequest

The NotificationRequest commands are 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 decide to use a different
type of encoding method in the connections bound to this endpoint.

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


EndpointId is the name for the endpoint in the gateway where Notifica-
tionRequest executes, as defined in section 2.1.1.

NotifiedEntity is an optional parameter that specifies where the notifi-
cations should be sent. When this parameter is absent, the notifications
should be sent to the originator of the NotificationRequest.

RequestIdentifier is used to correlate this request with the notifica-
tions that it triggers.

RequestedEvents is a list of events that the gateway is requested to
detect and report. Such events include, for example, fax tones, con-
tinuity tones, or on-hook transition.  To each event is associated an
action, which can be:

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

*    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,



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*    Ignore the event.

Some actions can be combined.  In particular:

*    The "swap audio" action can be combined with "Notify", "Accumulate"
     and "Ignore."

*    The "keep signal active" action can be combined with "Notify",
     "Accumulate", "Accumulate according to Digit Map", "Ignore" and
     "Embedded Notification Request."

*    The "Embedded Notification Request" can be combined with "Accumu-
     late" and with "Keep signals active." It can also be combined with
     Notify, if the gateway is allowed to issue several Notify commands
     in response to a single Notification request.

In addition to the requestedEvents parameter specified in the command,
some profiles of MGCP have introduced the concept of "persistent
events." According to such profiles, the persistent event list is con-
figured in the endpoint, by means outside the scope of MGCP. The basic
MGCP specification does not specify any persistent event.

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. 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 per-
formed.

Non-persistent events are those events explicitly included in the
RequestedEvents list. The (possibly empty) list of requested events com-
pletely 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 then 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 "Ignore off-hook" was specified, and a new request without
any off-hook instructions were received, the default "Notify off-hook"
operation then would be restored. A given event MUST NOT appear more
than once in a RequestedEvents.

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

The Swap Audio action can be used when a gateway handles more than one



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active connection on an endpoint. This will be the case for three-way
calling, call waiting, and possibly other feature scenarios. In order to
avoid the round-trip to the Call Agent when just changing which connec-
tion is attached to the audio functions of the endpoint, the Notifica-
tionRequest can map an event (usually hook flash, but could be some
other event) to a local function swap audio, which selects the "next"
connection in a round robin fashion. If there is only one connection,
this action is effectively a no-op.

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, SignalRe-
quests and a new digit map as well. The semantics of the embedded Noti-
ficationRequest is as if a new NotificationRequest was just received
with the same NotifiedEntity, and RequestIdentifier. When the "Embedded
NotificationRequest" is activated, the "current dial string" will be
cleared; the list of observed events and the quarantine buffer will be
unaffected.

MGCP implementations shall be able to support at least one level of
embedding.  An embedded NotificationRequest that respects this limita-
tion shall not contain another Embedded NotificationRequest.

DigitMap is an optional parameter that allows the Call Agent to provi-
sion the gateways 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 expli-
citly or through a previous command, if the RequestedEvent parameters
contain an request to "accumulate according to the digit map." The col-
lection of these digits will result in a digit string. The digit string
is initialized to a null string upon reception of the NotificationRe-
quest, so that a subsequent notification only returns the digits that
were collected after this request. Digits that were accumulated accord-
ing 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 be
accumulated may be found in between the list of digits.

SignalRequests is a parameter that contains the set of signals that the
gateway is asked to apply to the endpoint , such as, for example ring-
ing, or continuity tones. Signals are identified by their name, which is
an event name, and may be qualified by parameters.

The action triggered by the SignalRequests is synchronized with the col-
lection of events specified in the RequestedEvents parameter. For exam-
ple, if the NotificationRequest mandates "ringing" and the event request
ask to look for an "off-hook" event, the ringing shall stop as soon as
the gateway detect an off hook event. The formal definition is that the
generation of all "Time Out" signals shall stop as soon as one of the



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requested events is detected, unless the "Keep signals active" action is
associated to the specified event.

The specific definition of actions that are requested via these Signal-
Requests, such as the duration of and frequency of a DTMF digit, is out-
side the scope of MGCP. This definition may vary from location to loca-
tion and hence from gateway to gateway.

The RequestedEvents and SignalRequests refer to the same event defini-
tions. 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 event packages that are supported by that end
point.  Each package specifies a list of events and actions 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 shall return an error. When the event name is not qualified by
a package name, the default package name for the end point is assumed.
If the event name is not registered in this default package, the gateway
shall return an error.

The Call Agent can send a NotificationRequest whose requested signal
list is empty. It will do so for example when tone generation should
stop.

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 han-
dling options:

*    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 notifica-
     tion (step by step), or multiple notifications (loop), in response
     to this request (the default is exactly 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 but not yet notified when the com-
mand is received.

DetectEvents is an optional parameter that specifies a list of events
that the gateway is requested to detect during the quarantine period.
When this parameter is absent, the events that should be detected in the
quarantine period are those listed in the last received DetectEvents



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list.  In addition, te gateway should also detect the events specified
in the request list, including those for which the "ignore" action is
specified.

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

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 inserted after 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 out-
come of the command and consists of an integer number optionally fol-
lowed by commentary. .NH 3 Notifications

Notifications are sent via the Notify command and are sent by the gate-
way when the observed events occur.

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


EndpointId is the name for the endpoint in the gateway which is issuing
the Notify command, as defined in section 2.1.1. The identifier should
be a fully qualified endpoint identifier, including the domain name of
the gateway.  The local part of the name shall not use the wildcard con-
vention.

NotifiedEntity is an optional parameter that identifies the entity to
which the notifications is sent. This parameter is equal to the last
received value of the NotifiedEntity parameter.  The parameter is absent
if there was no such parameter in the triggering request. The notifica-
tion is sent to the "current notified entity" or, if no such entity was
ever specified, to the address from which the request was received.

RequestIdentifier is parameter that repeats the RequestIdentifier



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parameter of the NotificationRequest that triggered this notification.
It is used to correlate this notification with the request that trig-
gered it.

ObservedEvents is a list of events that the gateway detected. A single
notification may report a list of events that will be reported in the
order in which they were detected. The list may only contain the iden-
tification of events that were requested in the RequestedEvents parame-
ter 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
detection or provided a final match in the digit map.

ReturnCode is a parameter returned by the call agent. It indicates the
outcome of the command and consists of an integer number optionally fol-
lowed by commentary.

2.3.3.  CreateConnection

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

            ReturnCode,
            ConnectionId,
            [SpecificEndPointId,]
            [LocalConnectionDescriptor,]
            [SecondEndPointId,]
            [SecondConnectionId]
            <--- 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 globally unique parameter that identifies the call (or ses-
sion) to which this connection belongs. Connections that belong to the
same call share the same call-id. The call-id can be used to identify
calls for reporting and accounting purposes. It does not affect the han-
dling of connections by the gateway.



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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 "anyone" wildcard convention. If
the endpoint is underspecified, the endpoint identifier will be assigned
by the gateway and its complete value returned in the SpecificEndPointId
parameter of the response.

The NotifiedEntity is an optional parameter that specifies where the
Notify or DeleteConnection commands should be sent. If the parameter is
absent, the Notify or DeleteConnection commands should be sent to the
last received Notified Entity, or to originator of the CreateConnection
command if no Notified Entity was ever received for the end point.

LocalConnectionOptions is a parameter used by the Call Agent to direct
the handling of the connection by the gateway.  The fields contained in
LocalConnectionOptions are the following:

*    Encoding Method,

*    Packetization period,

*    Bandwidth,

*    Type of Service,

*    Usage of echo cancellation,

*    Usage of silence suppression or voice activity detection,

*    Usage of signal level adaptation and noise level reduction, or
     "gain control."

*    Usage of reservation service,

*    Usage of RTP security,

*    Type of network used to carry the connection.

This set of field can be completed by vendor specific optional or manda-
tory extensions. The encoding of the first three fields, when they are
present, will be compatible with the SDP and RTP profiles:

*    The encoding method shall be specified by using one or several
     valid encoding names, as defined in the RTP AV Profile or
     registered with the IANA.

*    The packetization period is encoded as either the length of time in



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     milliseconds represented by the media in a packet, as specified in
     the "ptime" parameter of SDP, or as a range value, specifying both
     the minimum and maximum acceptable packetization periods.

*    The bandwidth is encoded as either a single value or a range,
     expressed as an integer number of kilobit per seconds.

For each of the first three fields, the Call Agent has three options:

*    It may state exactly one value, which the gateway will then use for
     the connection,

*    It may provide a loose specification, such as a list of allowed
     encoding methods or a range of packetization periods,

*    It may simply provide a bandwidth indication, leaving the choice of
     encoding method and packetization period to the gateway.

The bandwidth specification shall not contradict the specification of
encoding methods and packetization period. If an encoding method is
specified, then the gateway is authorized to use it, even if it results
in the usage of a larger bandwidth than specified.

The LocalConnectionOptions parameter may be absent in the case of a data
call.

The Type of Service specifies the class of service that will be used for
the connection. When the connection is transmitted over an IP network,
the parameters encodes the 8-bit type of service value parameter of the
IP header. When the Type of Service is not specified, the gateway shall
use a default or configured value.

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 that should 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. 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 "net-
     work continuity test" mode (if a 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"



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     mode.

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

By default, the telephony gateways always perform echo cancellation.
However, it is 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 telephony gateways may perform gain control, in order to adapt the
level of the signal.  However, it is necessary, for example for 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 default is to not perform gain control, which is equivalent
to specifying a gain of 0 decibels.

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 turned off.) The default is "off."

The Call agent can request the gateway to enable encryption of the audio
Packets.  It does so by providing an key specification, as specified in
RFC 2327. By default, encryption is not used.

The Call Agent may instruct the gateway to prepare the connection on a
specified type of network.  The type of network is encoded as in the
"connection-field" parameter of the SDP standard.  Possible values are
IN (Internet), ATM and LOCAL. The parameter is optional; if absent, the
network is determined by the type of gateway.

RemoteConnectionDescriptor is the connection descriptor for the remote
side of a connection, on the other side of the IP network. It includes
the same fields as in the LocalConnectionDescriptor, i.e. the fields
that describe a session according to the SDP standard. This parameter
may have a null value when the information for the remote end is not
known yet. This occurs because the entity that builds a connection
starts by sending a CreateConnection to one of the two gateways involved
in it. For the first CreateConnection issued, there is no information
available about the other side of the connection. This information may
be provided later via a ModifyConnection call. In the case of data con-
nections (mode=data), this parameter describes the characteristics of
the data connection.

The SecondEndpointId can be used instead of the



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RemoteConnectionDescriptor to establish a connection between two end-
points located on the same gateway.  The connection is by definition a
local connection. The SecondEndpointId can be fully-specified by assign-
ing a value to the parameter SecondEndpointId in the function call or it
may be under-specified by using the "anyone" wildcard convention. If the
secondendpoint is underspecified, the second endpoint identifier will be
assigned by the gateway and its complete value returned in the Secon-
dEndPointId parameter of the response.

Mode indicates the mode of operation for this side of the connection.
The mode are "send", "receive", "send/receive", "conference", "data",
"inactive", "loopback", "continuity test", "network loop back" or "net-
work continuity test." The expected handling of these modes is specified
in the introduction of the "Gateway Handling Function" section. Some end
points may not be capable of supporting all modes.  If the command
specifies a mode that the endpoint cannot support, and error shall be
returned.

The gateway returns a ConnectionId, that uniquely identifies the connec-
tion within one endpoint , and a LocalConnectionDescriptor, which is a
session description that contains information about addresses and RTP
ports, as defined in SDP. The LocalConnectionDescriptor is not returned
in the case of data connections. The SpecificEndPointId is an optional
parameter that identifies the responding endpoint. It can be used when
the EndpointId argument referred to a "any of" wildcard name. When a
SpecificEndPointId is returned, the Call Agent should use it as the End-
pointId value is successive commands referring to this call.

When a SecondEndpointId is specified, the command really creates two
connections that can be manipulated separately through ModifyConnection
and DeleteConnection commands.  The response to the creation provides a
SecondEndpointId parameter that identifies the second connection.

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 parame-
ter, 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 should accept the
     voice signals and transmit them through the endpoint.

*    If the mode was set to Inactive, Loopback, Continuity Test, the
     gateway should refuse the voice signals.



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*    If the mode was set to Network Loopback or Network Continuity Test,
     the gateway should perform the expected echo or Response.

Note that the mode values SendReceive, Conference, Data and SendOnly
don't make sense in this situation. They should be treated as errors,
and the command should be rejected (Error code 517).

The command may optionally contain an encapsulated Notification Request
command, in which case a RequestIdentifier parameter will be present, as
well as, optionnally, the RequestedEvents DigitMap, SignalRequests,
QuarantineHandling and DetectEvents parameters. The encapsulated  Noti-
ficationRequest is executed simultaneously with the creation of the con-
nection. For example, when the Call Agent wants to initiate a call to an
residential gateway, it should:

*    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 RequestedEvent parameters for the off hook event, and
the SignalRequest parameter for the ringing signal.

When these parameters are present, the creation and the NotificationRe-
quests should be synchronized, which means that both should be accepted,
or both 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 Notification-
Request can be refused in the glare condition, if the user is already
off-hook. In this example, the phone should not ring if the connection
cannot be established, and the connection should not be established if
the user is already off hook.

The NotifiedEntity parameter, if present, applies to both the CreateCon-
nection and the NotificationRequest command. It defines the new "noti-
fied 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 inserted after the
normal parameters of the CreateConnection with the exception of the End-
pointId, which is not replicated. The EndpointConfiguration command may
be encapsulated together with an encapsulated NotificationRequest



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command.

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

ReturnCode is a parameter returned by the gateway. It indicates the out-
come of the command and consists of an integer number optionally fol-
lowed by commentary.










































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2.3.4.  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 connec-
tion descriptors as well as the remote connection descriptor.

           ReturnCode,
           [LocalConnectionDescriptor]
            <--- 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 is returned by the CreateConnection
function, as part of the local connection descriptor. It uniquely iden-
tifies the connection within the context of the endpoint.

The EndpointId should be a fully qualified endpoint identifier.  The
local name shall not use the wildcard convention.

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

*    Provide information about the other end of the connection, through
     the RemoteConnectionDescriptor.

*    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.

*    Change the sending parameters of the connection, for example by
     switching to a different coding scheme, changing the packetization
     period, or modifying the handling of echo cancellation.

Connections can only be 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. (Usage of this



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feature is actually for further study.)

The command may optionally contain an encapsulated Notification Request
command, in which case a RequestIdentifier parameter will be present, as
well as, optionnally, the RequestedEvents DigitMap, SignalRequests,
QuarantineHandling and DetectEvents parameters. The encapsulated  Noti-
ficationRequest 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 RequestedEvent parameters, for the on hook event, and
an empty SignalRequest parameter, to stop the provision of ringing
tones.

When these parameters are present, the modification and the Notifica-
tionRequests should be synchronized, which means that both should be
accepted, or both refused.  The NotifiedEntity parameter, if present,
applies to both the ModifyConnection and the NotificationRequest com-
mand.

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 inserted after the
normal parameters of the ModifyConnection with the exception of the End-
pointId, which is not replicated. The EndpointConfiguration command may
be encapsulated together with an encapsulated NotificationRequest com-
mand.

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

ReturnCode is a parameter returned by the gateway. It indicates the out-
come of the command and consists of an integer number optionally fol-
lowed by commentary.

2.3.5.  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,
             Connection-parameters
             <-- DeleteConnection(CallId,
                                  EndpointId,
                                  ConnectionId,



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                                  [Encapsulated NotificationRequest,]
                                  [Encapsulated EndpointConfiguration])


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

In the general case where a connection has two ends, this command has to
be sent to both gateways involved in the connection. Some connections,
however, may use IP multicast. In this case, they can be deleted indivi-
dually.

After the connection has been deleted, any loopback that has been
requested for the connection should be cancelled. When all connections
to an endpoint have been deleted, that endpoint should be placed in
inactive mode.

In response to the DeleteConnection command, the gateway returns a list
of parameters that describe the status of the connection. These parame-
ters are:

Number of packets sent:

The total number of RTP data packets transmitted by the sender since
starting transmission on this connection. 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 Modify command. The value is zero
if the connection was set in "receive only" mode.

Number of octets sent:

The total number of payload octets (i.e., not including header or pad-
ding) transmitted in RTP data packets by the sender since starting
transmission on this connection. The count is not reset if the sender
changes its SSRC identifier, for example as a result of a ModifyConnec-
tion command. The value is zero if the connection was set in "receive
only" mode.

Number of packets received:

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

Number of octets received:

The total number of payload octets (i.e., not including header or



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padding) transmitted in RTP data packets by the sender since starting
transmission on this connection. The count includes packets received
from different SSRC, if the sender used several values. The value is
zero if the connection was set in "send only" mode.

Number of packets lost:

The total number of RTP data packets that have been lost since the
beginning of reception. This number is defined to be the number of pack-
ets expected less the number of packets actually received, where the
number of packets received includes any which are late or duplicates.
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 ini-
tial 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 set in "send only" mode. This parameter is omitted if
the connection was set in "data" mode.

Interarrival jitter:

An estimate of the statistical variance of the RTP data packet interar-
rival time measured in milliseconds and expressed as an unsigned
integer. 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 computa-
tion 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 set in "send only" mode. This param-
eter is omitted if the connection was set in "data" mode.

Average transmission delay:

An estimate of the network latency, expressed in milliseconds. 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 this messages are received. The average is
obtained by summing all the estimates, then dividing by the number of
RTCP messages that have been received. This parameter is omitted if the
connection was set in "data" mode.
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.



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

When the connection was set up over an ATM network, the meaning of these
parameters may change:

     Number of packets sent:
     The total number of ATM cells transmitted since starting transmis-
     sion on this connection.

Number of octets sent:
     The total number of payload octets transmitted in ATM cells.

Number of packets received:
     The total number of ATM cells received since starting reception on
     this connection.

Number of octets received:
     The total number of payload octets received in ATM cells.

Number of packets lost:
     Should be determined as the number of cell losts, or set to zero if
     the adaptation layer does not enable the gateway to assess losses.

Interarrival jitter:
     Should be understood as the interarrival jitter between ATM cells.

Average transmission delay:
     The gateway may not be able to assess this parameter over an ATM
     network.  It could simply report a null value.

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

     Number of packets sent:
     Not significant.

Number of octets sent:
     The total number of payload octets transmitted over the local con-
     nection.

Number of packets received:
     Not significant.

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

Number of packets lost:
     Not significant.  A value of zero is assumed.



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Interarrival jitter:
     Not significant.  A value of zero is assumed.

Average transmission delay:
     Not significant.  A value of zero is assumed.

The standard set of connection parameters can be extended by the crea-
tion of extension parameters.

The command may optionally contain an encapsulated Notification Request
command, in which case a RequestIdentifier parameter will be present, as
well as, optionnally, the RequestedEvents DigitMap, SignalRequests,
QuarantineHandling and DetectEvents parameters. The encapsulated  Noti-
ficationRequest is executed simultaneously with the deletion of the con-
nection. 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.

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

When these parameters are present, the DeleteConnection and the Notifi-
cationRequests should be synchronized, which means that both should be
accepted, or both 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 inserted after the
normal parameters of the DeleteConnection with the exception of the End-
pointId, which is not replicated. The EndpointConfiguration command may
be encapsulated together with an encapsulated NotificationRequest com-
mand.

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

ReturnCode is a parameter returned by the gateway. It indicates the out-
come of the command and consists of an integer number optionally fol-
lowed by commentary.

2.3.6.  DeleteConnection (from the VoIP gateway)

In some 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 voice. The gateway terminates the connection



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by using a variant of the DeleteConnection command:

            ReturnCode,
            <-- DeleteConnection( CallId,
                                  EndpointId,
                                  ConnectionId,
                                  Reason-code,
                                  Connection-parameters)


In addition to the call, endpoint and connection identifiers, the gate-
way will also send the call's parameters that would have been returned
to the Call Agent in response to a DeleteConnection command. The reason
code indicates the cause of the disconnection.

ReturnCode is a parameter returned by the call agent. It indicates the
outcome of the command and consists of an integer number optionally fol-
lowed by commentary.

2.3.7.  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. The command can
be used to delete all connections that relate to a Call for an endpoint:

            ReturnCode,
            <-- DeleteConnection( CallId,
                                  EndpointId)


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

            ReturnCode,
            <-- DeleteConnection( EndpointId)

Finally, Call Agents can take advantage of the hierarchical naming
structure of endoints 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 value" wildcarding conven-
tion. The "any value" convention shall not be used.  For example, if
endpoints names are structured as the combination of a physical inter-
face name and a circuit number, as in "X35V3+A4/13", the Call Agent may
replace the circuit number by a wild card character "*", as in
"X35V3+A4/*".  This "wildcard" command instructs the gateway to delete
all the connections that where attached to circuits connected to the
physical interface "X35V3+A4".




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After the connections have been deleted, the endpoint should be placed
in inactive mode. Any loopback that has been requested for the connec-
tions should be cancelled.

This command does not return any individual statistics or call parame-
ters.

ReturnCode is a parameter returned by the gateway. It indicates the out-
come of the command and consists of an integer number optionally fol-
lowed by commentary.

2.3.8.  Audit Endpoint

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

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


The EndpointId identifies the endpoint that is being audited. The "all
of" wildcard convention can be used to start auditing of a group of end-
points. If this convention is used, the gateway should return the list
of endpoint identifiers that match the wildcard in the EndPointIdList
parameter. It shall not return any parameter specific to one of these
endpoints.

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,



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     NotifiedEntity, ConnectionIdentifiers, DetectEvents, Obser-
     vedEvents, EventStates, RestartReason, RestartDelay, ReasonCode,
     and Capabilities.

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

*    RequestedEvents: The current value of RequestedEvents the endpoint
     is using including the action associated with each event. Per-
     sistent events are included in the list.

*    DigitMap: the digit map the endpoint is currently using.

*    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 sig-
     nals are not included.

*    RequestIdentifier, the RequestIdentifier for the last Notification-
     Request received by this endpoint (includes NotificationRequest
     encapsulated in Connection handling primitives). If no notification
     request has been received, the value zero will be returned.

*    NotifiedEntity, the current notified entity for the endpoint.

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

*    DetectEvents, the list of events that are currently detected in
     quarantine mode.

*    ObservedEvents: the current list of observed events for the end-
     point.

*    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. The definition of the
     individual events will state if the event in question has an audit-
     able state associated with it.

*    RestartReason: the value of the restart reason parameter in the
     last RestartInProgress command issued by the endpoint, "restart"
     indicating a fully functional endpoint.

*    RestartDelay: the value of the  restart delay parameter if a Res-
     tartInProgress command was issued by the endpoint at the time of
     the response, or zero if the command would not include this



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     parameter.

*    ReasonCode:the value of the Reason-Code parameter in the last Res-
     tartInProgress or DeleteConnection command issued by the gateway
     for the endpoint, or the special value 000 if the endpoint's state
     is nominal.

*    The capabilities for the endpoint similar to the LocalConnectionOp-
     tions parameter and including event packages and connection modes.
     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 sets:

          Compression Algorithm: a list of supported codecs. The rest of
          the parameters will apply to all codecs specified in this
          list.

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

          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.

          Silence Suppression: Whether silence suppression is supported
          or not.

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

          Event Packages: A list of event packages supported. The first
          event 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,
the gateway simply returns a positive acknowledgement.

If no NotifiedEntity has been specified in the last NotificationRequest,
the notified entity defaults to the source address of the last Notifica-
tionRequest command received for this connection.

ReturnCode is a parameter returned by the gateway. It indicates the



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outcome of the command and consists of an integer number optionally fol-
lowed by commentary.

2.3.9.  Audit Connection

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]
                        <--- AuditConnection(EndpointId,
                                         ConnectionId,
                                         RequestedInfo)


The EndpointId parameter specifies the endpoint that handles the connec-
tion. 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 fol-
lowing connection info can be audited with this command:

     CallId, NotifiedEntity, LocalConnectionOptions, Mode, RemoteConnec-
     tionDescriptor, LocalConnectionDescriptor, ConnectionParameters

The AuditConnectionResponse 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.

*    LocalConnectionOptions, the LocalConnectionOptions that was sup-
     plied for the connection.

*    Mode, the current mode of the connection.

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



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*    LocalConnectionDescriptor, the LocalConnectionDescriptor the gate-
     way supplied for the connection.

*    ConnectionParameters, the current value of the connection parame-
     ters 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 ack-
nowledgement.

If no NotifiedEntity has been specified for the connection, the notified
entity defaults to the source address of the last connection handling
command received for this connection.

ReturnCode is a parameter returned by the gateway. It indicates the out-
come of the command and consists of an integer number optionally fol-
lowed by commentary.

2.3.10.  Restart in progress

The RestartInProgress command is used by the gateway to signal that An
endpoint, or a group of endpoint, is taken in or out of service.

           ReturnCode,
           [NotifiedEntity]
                 <------- RestartInProgress ( EndPointId,
                                              RestartMethod,
                                              [RestartDelay,]
                                              [Reason-code])


The EndPointId identifies the endpoint that are taken in or out of ser-
vice.  The "all of" wildcard convention may be used to apply the command
to a group of endpoint, 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 specified the type of restart.  Three values
have been defined:

*    A "graceful" restart method indicates that the specified endpoints
     will Be taken out of service after the specified delay. The esta-
     blished connections are not yet affected, but the Call Agent should
     refrain to establish new connections, and should try to gracefully
     tear down the existing connections.

*    A "forced" restart method indicates that the specified endpoints



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     are taken abruptely 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." There are no connec-
     tions that are currently established on the endpoints.

*    A "disconnected" method indicates that the endpoint has become
     disconnected and is now trying to establish connectivity. The "res-
     tart 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 optional "restart delay" parameter is expressed as a number of
seconds. If the number is absent, the delay value should be considered
null.  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" method.

A restart delay of null for the "restart" method indicates that service
has already been restored. This typically will occur after gateway
startup/reboot.

The optional reason code parameter the cause of the restart.

Gateways SHOULD send a "graceful" or "forced" RestartInProgress message
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, although the Call Agent cannot rely on always receiving such
messages. Gateways MUST send a "restart" RestartInProgress message with
a null delay to their Call Agent when they are back in service according
to the restart procedure specified in Section 4.3.4 - Call Agents can
rely on receiving this message. Also, gateways MUST send a "discon-
nected" RestartInProgress message to their current "notified entity"
according to the "disconnected" procedure specified in Section 4.3.5.
The "restart delay" parameter MUST NOT be used with the "forced" restart
method.

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 for each end-
point so, after a reboot, the default Call Agent will be the notified
entity for each 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



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managed by the same Call Agent.

ReturnCode is a parameter returned by the gateway. It indicates the out-
come of the command and consists of an integer number optionally fol-
lowed by commentary.

A NotifiedEntity may additionally be returned with the response from the
Call Agent:

*    If the response indicated success (return code 200 - transaction
     executed), the restart procedure has  completed, and the Noti-
     fiedEntity returned is the new "notified entity" for the
     endpoint(s).

*    If the response from the Call Agent indicated an error, the restart
     procedure is not yet complete, and must therefore be initiated
     again. If a NotifiedEntity parameter was returned, it then speci-
     fies the new "notified entity" for the endpoint(s), which must con-
     sequently be used when retrying the restart procedure.

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 four ranges of values have been defined:

*    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.

The values that have been already defined are listed in the following
list:

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

200  The requested transaction was executed normally.

250  The connection was deleted.

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

401  The phone is already off hook




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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

500  The transaction could not be executed, because the endpoint is unk-
     nown.

501  The transaction could not be executed, because the endpoint is not
     ready.

502  The transaction could not be executed, because the endpoint does
     not have sufficient resources

510  The transaction could not be executed, because a protocol error was
     detected.

511  The transaction could not be executed, because the command con-
     tained an unrecognized extension.

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.

517  Unsupported or invalid mode.

518  Unsupported or unknown package.

519  Endpoint does not have a digit map.

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

521  Endpoint redirected to another Call Agent.

522  No such event or signal.



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523  Unknown action or illegal combination of actions

524  Internal inconsistency in LocalConnectionOptions

525  Unknown extension in LocalConnectionOptions

526  Insufficient bandwidth

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).

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 gateway
of the Restart's reason. The reason code is an integer number, and the
following values have been defined:

000  Endpoint state is nominal. (This code is used only in response to
     audit requests.)

900  Endpoint malfunctioning

901  Endpoint taken out of service

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

















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3.  Media Gateway Control Protocol

The 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 eight types of command:

*    CreateConnection

*    ModifyConnection

*    DeleteConnection

*    NotificationRequest

*    Notify

*    AuditEndpoint

*    AuditConnection

*    RestartInProgress

The first four 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 2.3.6. The Call Agent may
send either of the Audit commands to the gateway.  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
a session description.

Headers and session descriptions are encoded as a set of text lines,
separated by a carriage return and line feed character (or, optionnally,
a single line-feed character). The headers are separated from the ses-
sion description 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 section
3.2.1, 3.2.1.2 and 3.3).





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3.2.  Command Header

The command header is composed of:

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

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

Unless otherwise noted or dictated by other referenced standards, each
component in the command header is case insensitive. This goes for verbs
as well as parameters and values, and 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 that should execute the command (in notif-
     ications or restarts, the name of the endpoint 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.

















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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 should 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 lines, 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 should 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 gate-
way MUST be unique for the maximum lifetime of the transactions within
the collection of Call Agents that control that gateway. Thus, regard-
less of the sending Call Agent, gateways can always detect duplicate
transactions by simply examining the transaction identifier. The coordi-
nation 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 999999999. 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 insensi-
tive e-mail addresses, as defined in RFC 821. In these addresses, the
domain name identifies the system where the endpoint is attached, while
the left side identifies a specific endpoint on that system.






























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Examples of such addresses can be:


 ______________________________________________________________________
| 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 notified entities is expressed with the same syntax, with
the possible addition of a port number as in:

     Call-agent@ca.example.net:5234

In case the port number is omitted, the default MGCP port (2427) will be
used.

3.2.1.4.  Coding of the protocol version

The protocol version is coded as the key word 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 0.1.

The profile name, if present, is represented by a 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


3.2.2.  Parameter lines

Parameter lines are composed of a parameter name, which in most cases is
composed of a single upper case character, followed by a colon, a white
space and the parameter value. The parameter that can be present in com-
mands are defined in the following table:




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_______________________________________________________________________
|Parameter name        |  Code|  Parameter value                      |
|______________________|______|_______________________________________|
|ResponseAck           |   K  |  see description                      |
|BearerInformation     |   B  |  see description                      |
|CallId                |   C  |  Hexadecimal string, at most 32 chars.|
|ConnectionId          |   I  |  Hexadecimal string, at most 32 chars.|
|NotifiedEntity        |   N  |  An identifier, in RFC 821 format,    |
|                      |      |  composed of an arbitrary string and  |
|                      |      |  of the domain name of the requesting |
|                      |      |  entity, possibly completed by a port |
|                      |      |  number, as in:                       |
|                      |      |   Call-agent@ca.example.net:5234      |
|RequestIdentifier     |   X  |  Hexadecimal string, at most 32 chars.|
|LocalConnectionOptions|   L  |  See description                      |
|Connection Mode       |   M  |  See description                      |
|RequestedEvents       |   R  |  See description                      |
|SignalRequests        |   S  |  See description                      |
|DigitMap              |   D  |  A text encoding of a digit map       |
|ObservedEvents        |   O  |  See description                      |
|ConnectionParameters  |   P  |  See description                      |
|ReasonCode            |   E  |  An arbitrary character string        |
|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.           |
|Second Endpoint ID    |   Z2 |  Endpoint Id.                         |
|SecondConnectionId    |   I2 |  Connection Id.                       |
|RequestedInfo         |   F  |  See description                      |
|QuarantineHandling    |   Q  |  See description                      |
|DetectEvents          |   T  |  See Description                      |
|RestartMethod         |   RM |  See description                      |
|RestartDelay          |   RD |  A number of seconds, encoded as      |
|                      |      |  a decimal number                     |
|EventStates           |   ES |  See description                      |
|Capabilities          |   A  |  See description                      |
|______________________|______|_______________________________________|
|RemoteConnection      |   RC |  Session Description                  |
|Descriptor            |      |                                       |
|LocalConnection       |   LC |  Session Description                  |
|Descriptor            |      |                                       |
|______________________|______|_______________________________________|


The parameters are not necessarily present in all commands. The follow-
ing table provides the association between parameters and commands. The
letter M stands for mandatory, O for optional and F for forbidden.



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  ___________________________________________________________________
 | Parameter name      |  EP|  CR|  MD|  DL|  RQ|  NT|  AU|  AU|  RS|
 |                     |  CF|  CX|  CX|  CX|  NT|  FY|  EP|  CX|  IP|
 |_____________________|____|____|____|____|____|____|____|____|____|
 | ResponseAck         |  O |  O |  O |  O |  O |  O |  O |  O |  O |
 | BearerInformation   |  M |  O |  O |  O |  O |  F |  F |  F |  F |
 | CallId              |  F |  M |  M |  O |  F |  F |  F |  F |  F |
 | ConnectionId        |  F |  F |  M |  O |  F |  F |  F |  M |  F |
 | RequestIdentifier   |  F |  O |  O |  O |  M |  M |  F |  F |  F |
 | LocalConnection     |  F |  O |  O |  F |  F |  F |  F |  F |  F |
 | Options             |    |    |    |    |    |    |    |    |    |
 | Connection Mode     |  F |  M |  M |  F |  F |  F |  F |  F |  F |
 | RequestedEvents     |  F |  O |  O |  O |  O*|  F |  F |  F |  F |
 | SignalRequests      |  F |  O |  O |  O |  O*|  F |  F |  F |  F |
 | NotifiedEntity      |  F |  O |  O |  O |  O |  O |  F |  F |  F |
 | ReasonCode          |  F |  F |  F |  O |  F |  F |  F |  F |  O |
 | ObservedEvents      |  F |  F |  F |  F |  F |  M |  F |  F |  F |
 | DigitMap            |  F |  O |  O |  O |  O |  F |  F |  F |  F |
 | Connection          |  F |  F |  F |  O |  F |  F |  F |  F |  F |
 | parameters          |    |    |    |    |    |    |    |    |    |
 | Specific Endpoint ID|  F |  F |  F |  F |  F |  F |  F |  F |  F |
 | Second Endpoint ID  |  F |  O |  F |  F |  F |  F |  F |  F |  F |
 | RequestedInfo       |  F |  F |  F |  F |  F |  F |  M |  M |  F |
 | QuarantineHandling  |  F |  O |  O |  O |  O |  F |  F |  F |  F |
 | DetectEvents        |  F |  O |  O |  O |  O |  F |  F |  F |  F |
 | EventStates         |  F |  F |  F |  F |  F |  F |  F |  F |  F |
 | RestartMethod       |  F |  F |  F |  F |  F |  F |  F |  F |  M |
 | RestartDelay        |  F |  F |  F |  F |  F |  F |  F |  F |  O |
 | SecondConnectionID  |  F |  O |  F |  F |  F |  F |  F |  F |  F |
 | Capabilities        |  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          |    |    |    |    |    |    |    |    |    |
 |_____________________|____|____|____|____|____|____|____|____|____|


Note (*) that the RequestedEvents and SignalRequests parameters are
optional in the NotificationRequest. If these parameters are omitted,
the corresponding lists will be considered empty.

If implementers need to experiment with new parameters, for example when
developing a new application of MGCP, they should identify these parame-
ters by names that start with the string "X-" or "X+", such as for exam-
ple:

             X-FlowerOfTheDay: Daisy



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Parameter names that start with "X+" are critical parameter extensions.
An MGCP entity that receives a critical parameter extension that it can-
not understand should refuse to execute the command.  It should respond
with an error code 511 (Unrecognized extension).

Parameter names that start with "X-" are non critical parameter exten-
sions. An MGCP entity that receives a non critical parameter extension
that it cannot understand can safely ignore that parameter.

3.2.2.1.  Response Acknowledgement

The response acknowledgement attribute is used to managed the "at-most-
once" facility described in the "transmission over UDP" section.  It
contains a comma separated list of "confirmed transaction-id ranges".

Each "confirmed transaction-id ranges" 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 response acknowledgement is:

        K: 6234-6255, 6257, 19030-19044


3.2.2.2.  Local connection options

The local connection options describe the operational parameters that
the Call Agent suggests to the gateway. These parameters are:

*    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 an hyphen.

*    The preferred type of compression algorithm, 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.

*    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 an hyphen.

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




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*    The gain control parameter, encoded as the keyword "gc", followed
     by a colon 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", fol-
     lowed by a colon and the value "on" or "off".

*    The type of service parameter, encoded as the keyword "t", followed
     by a colon and the value encoded as two hexadecimal digits.

*    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" of SDP
     (RFC 2327).

*    The type of network, encoded as the keyword "nt" followed by a
     colon and the type of network encoded as the keyword "IN", "ATM" or
     "LOCAL".

Each of the parameters is optional. When several parameters are present,
the values are separated by a comma.

Examples of connection descriptors are:

             L: p:10, a:G.711
             L: p:10, a:G.711;G.726-32
             L: p:10-20, b: 64
             L: b:32-64, e:off


These set of attributes may be extended by extension attributes.  Exten-
sion attributes are composed of an attribute name, followed by a semi-
colon and by an attribute value. The attribute name should start by the
two characters "x+", for a mandatory extensions, or "x-", for a non man-
datory extension.  If a gateway receives a mandatory extension attribute
that it does not recognize, it should reject the command with an error
code 525 (Unknown extension in LocalConnectionOptions).

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. In addi-
tion, capabilities can also contain a list of supported packages, and a



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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
     LocalConnectionOptions parameters, one for each set of codecs.

Packetization Period:
     A range may be specified.

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 for this codec, "off" other-
     wise. The default is support.

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

Gain Control:
     "0" if gain control is not supported.  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 sup-
     port.

Resource Reservation:
     The parameter indicates the reservation services that are sup-
     ported, 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.

Type of network:
     The keyword "nt", followed by a colon and a semicolon separated



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     list of supported network types.  This parameter is optional.

Event Packages
     The event packages supported by this endpoint encoded as the key-
     word "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 that end-
     point.

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.

3.2.2.4.  Connection parameters

Connection parameters are encoded as a string of type and value pairs,
where the type is a either 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 encoded from each other by a
comma.

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 not received on the          |
  |                     |      |  connection, as deduced from       |
  |                     |      |  gaps in the 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.   |
  |_____________________|______|____________________________________|

Extension parameters names are composed of the string "X-" followed by a



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two letters extension parameter name.  Call agents that received
unrecognized extensions shall silently ignore these extensions.

An example of connection parameter encoding is:

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


3.2.2.5.  Reason Codes

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

     900 Endpoint malfunctioning

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



































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3.2.2.6.  Connection mode

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.                  |
       | M: data    |  The gateway should use the circuit      |
       |            |  for network access for data             |
       |            |  (e.g., PPP, SLIP, etc.).                |
       |____________|__________________________________________|






















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3.2.2.7.  Coding of event names

Event names are composed of an optional package name, separated by a
slash (/) from the name of the actual event.  The event name can option-
ally be followed by an at sign (@) and the identifier of a connection on
which the event should be observed. Event names are used in the
RequestedEvents, SignalRequests and ObservedEvents parameter.

Each signal has one of the following signal-types associated with:
On/Off (OO), Time-out (TO), Brief (BR). (These signal types are speci-
fied 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
on:

     vmwi(+), vmwi

The following are valid examples of event names:

      ____________________________________________________________
     | L/hu        |   on-hook transition, in the line package   |
     | F/0         |   digit 0 in the MF package                 |
     | fh          |   Flash-hook, assuming that the line package|
     |             |   is a default package for the end point.   |
     | 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 star sign can be used to denote "all connections", and
the dollar sign can be used to denote the "current" connection.  The
following are valid examples of such notations:

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




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3.2.2.8.  RequestedEvents

The RequestedEvent parameter provides the list of events that have been
requested. The event codes are described in the previous section.

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 vari-
ous 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.

The digit-map action can only be specified for the digits, letters and
interdigit timers in the MF and DTMF packages, or in other packages that
would define the encoding of digits and timers.

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

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


In the case of the "enable" action, the embedded notification request
parameters are encoded as a list of up to three parameter groups,
separated by commas.  Each group start by a one letter identifier, fol-
lowed by a list of parameters enclosed between parenthesis.  The first
optional parameter group, identified by the letter "R", is the enabled
value of the RequestedEvents parameter.  The second optional group,
identified by the letter "S", is the enabled value of the SignalRequests
parameter.  The third optional group, identified by the letter "D", is
the enabled value of the DigitMap. (Note that some existing implementa-
tion may encode these three components in a different order.)



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

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


3.2.2.9.  SignalRequests

The SignalRequests parameter provides the name of the signals that have
been requested. Each signal is identified by a name, as indicated in the
previous section.

Several signals, such as for example announcement or ADSI display, can
be qualified by additional parameters:

*    the name and parameters of the announcement,

*    the string that should be displayed.

These parameters will be encoded as a set of UTF8 character strings,
spearated by comams and enclosed within parenthesis, as in:
     S: adsi("123456 Francois Gerard")
     S: ann(no-such-number, 1234567)

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

        S: asdi(123456 Your friend), rg


3.2.2.10.  ObservedEvent

The observed event parameters provides the list of events that have been
observed. The event codes are the same as those used in the Notifica-
tionRequest. Events that have been accumulated according to the digit
map may be grouped in a single string; they should be reported as lists
of isolated events if other events where detected during the digit accu-
mulation. Examples of observed actions are:

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





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3.2.2.11.  RequestedInfo

The RequestedInfo parameter contains a comma separated list of parameter
codes, as defined in the "Parameter lines" section.  For example, if one
wants to audit the value of the NotifiedEntity, RequestIdentifier,
RequestedEvents, SignalRequests, DigitMap, QuarantineHandling and Detec-
tEvents parameters, The value of the RequestedInfo parameter will be:

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

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

        F:A


3.2.2.12.  QuarantineHandling

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

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

*    The keyword "step" or "loop" to indicate whether exactly at most
     one notification is expected, or whether multiple notifications are
     allowed. If neither step or loop is present, step is assumed.  The
     following values are valid examples:

             Q:loop
             Q:process
             Q:discard,loop


3.2.2.13.  DetectEvents

The DetectEvent parameter is encoded as a comma separated list of
events, such as for example:

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

It should be noted, that no actions can be associated with the events.

3.2.2.14.  EventStates

The EventStates parameter is encoded as a comma separated list of
events, such as for example:



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     ES: hu

It should be noted, that no actions can be associated with the events.

3.2.2.15.  RestartMethod

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

        RM:restart


3.2.2.16.  Bearer Information

The values of the bearer informations are encoded as a comma separated
list of attributes, represented by an attribute name, separated by a
colon from an attribute value.

The only attribute that is defined is the "encoding" (code "e"), whose
defined values are "A" (A-law) and "mu" (mu-law).

An example of bearer information encoding is:

        B: e:mu


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 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, the transaction
identifier, and an optional commentary preceded by a white space.

The following table describe 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.






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  ___________________________________________________________________
 | Parameter name      |  EP|  CR|  MD|  DL|  RQ|  NT|  AU|  AU|  RS|
 |                     |  CF|  CX|  CX|  CX|  NT|  FY|  EP|  CX|  IP|
 |_____________________|____|____|____|____|____|____|____|____|____|
 | ResponseAck         |  F |  F |  F |  F |  F |  F |  F |  F |  F |
 | BearerInformation   |  F |  F |  F |  F |  F |  F |  O |  F |  F |
 | CallId              |  F |  F |  F |  F |  F |  F |  F |  O |  F |
 | ConnectionId        |  F |  O*|  F |  F |  F |  F |  F |  O |  F |
 | RequestIdentifier   |  F |  F |  F |  F |  F |  F |  O |  F |  F |
 | LocalConnection     |  F |  F |  F |  F |  F |  F |  O |  O |  F |
 | Options             |    |    |    |    |    |    |    |    |    |
 | Connection Mode     |  F |  F |  F |  F |  F |  F |  F |  O |  F |
 | RequestedEvents     |  F |  F |  F |  F |  F |  F |  O |  F |  F |
 | SignalRequests      |  F |  F |  F |  F |  F |  F |  O |  F |  F |
 | NotifiedEntity      |  F |  F |  F |  F |  F |  F |  F |  F |  O |
 | ReasonCode          |  F |  F |  F |  F |  F |  F |  O |  F |  F |
 | ObservedEvents      |  F |  F |  F |  F |  F |  F |  O |  F |  F |
 | DigitMap            |  F |  F |  F |  F |  F |  F |  O |  F |  F |
 | Connection          |  F |  F |  F |  O |  F |  F |  F |  O |  F |
 | Parameters          |    |    |    |    |    |    |    |    |    |
 | Specific Endpoint ID|  F |  O |  F |  F |  F |  F |  F |  F |  F |
 | RequestedInfo       |  F |  F |  F |  F |  F |  F |  O |  F |  F |
 | QuarantineHandling  |  F |  F |  F |  F |  F |  F |  O |  F |  F |
 | DetectEvents        |  F |  F |  F |  F |  F |  F |  O |  F |  F |
 | EventStates         |  F |  F |  F |  F |  F |  F |  O |  F |  F |
 | RestartMethod       |  F |  F |  F |  F |  F |  F |  O |  F |  F |
 | RestartDelay        |  F |  F |  F |  F |  F |  F |  O |  F |  F |
 | Capabilities        |  F |  F |  F |  F |  F |  F |  O |  F |  F |
 | SecondConnectionId  |  F |  O |  F |  F |  F |  F |  F |  F |  F |
 | SecondEndpointID    |  F |  O |  F |  F |  F |  F |  F |  F |  F |
 |_____________________|____|____|____|____|____|____|____|____|____|
 | LocalConnection     |  F |  M |  O |  F |  F |  F |  F |  O*|  F |
 | Descriptor          |  F |    |    |    |    |    |    |    |    |
 | RemoteConnection    |  F |  F |  F |  F |  F |  F |  F |  O*|  F |
 | Descriptor          |    |    |    |    |    |    |    |    |    |
 |_____________________|____|____|____|____|____|____|____|____|____|


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

In the case of a DeleteConnection message, the response line is followed
by a Connection Parameters parameter, as defined in section 3.2.2.2.



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A LocalConnectionDescriptor should be transmitted with a positive
response (code 200) to a CreateConnection. It may 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.

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 descrip-
tion, as in:

        200 1203 OK
        C: A3C47F21456789F0
        N: [128.96.41.12]
        L: p:10, a:G.711;G.726-32
        M: sendrecv
        P: PS=1245, OS=62345, PR=780, OR=45123, PL=10, JI=27,LA=48

        v=0
        c=IN IP4 128.96.41.1
        m=audio 1296 RTP/AVP 0

        v=0
        c=IN IP4 128.96.63.25
        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 con-
nection descriptor will appear with the SDP protocol version field only.

3.4.  Formal syntax description of the protocol

In this section, we provided a formal description of the protocol syn-
tax, following the "Augmented BNF for Syntax Specifications" defined in
RFC 2234.

MGCPMessage = MGCPCommand / MGCPResponse

MGCPCommand = MGCPCommandLine 0*(MGCPParameter) [EOL *SDPinformation]

MGCPCommandLine = MGCPVerb 1*(WSP) <transaction-id> 1*(WSP)
                        <endpointName> 1*(WSP) MGCPversion EOL




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MGCPVerb = "EPCF" / "CRCX" / "MDCX" / "DLCX" / "RQNT"
         / "NTFY" / "AUEP" / "AUCX" / "RSIP" / extensionVerb

extensionVerb = "X" 3(ALPHA / DIGIT)

transaction-id = 1*9(DIGIT)

endpointName =  localEndpointName "@" DomainName
LocalEndpointName = LocalNamePart 0*("/" LocalNamePart)
LocalNamePart = AnyName / AllName / NameString
AnyName = "$"
AllNames = "*"
NameString = 1*(range-of-allowed-characters)
DomainName = 1*256(ALPHA / DIGIT / "." / "-") ; as defined in RFC 821

MGCPversion = "MGCP" 1*(WSP) 1*(DIGIT) "." 1*(DIGIT) [1*(WSP) ProfileName]
ProfileName = 1*(range-of-allowed-characters)

MGCPParameter = ParameterValue EOL

ParameterValue = ("K" ":" <ResponseAck>) /
                 ("B" ":" <BearerInformation>) /
                 ("C" ":" <CallId>) /
                 ("I" ":" <ConnectionId>) /
                 ("N" ":" <NotifiedEntity>) /
                 ("X" ":" <RequestIdentifier>) /
                 ("L" ":" <LocalConnectionOptions>) /
                 ("M" ":" <ConnectionMode>) /
                 ("R" ":" <RequestedEvents>) /
                 ("S" ":" <SignalRequests>) /
                 ("D" ":" <DigitMap>) /
                 ("O" ":" <ObservedEvents>) /
                 ("P" ":" <ConnectionParameters>) /
                 ("E" ":" <ReasonCode>) /
                 ("Z" ":" <SpecificEndpointID>) /
                 ("Z2" ":" <SecondEndpointID>) /
                 ("I2" ":" <SecondConnectionID>) /
                 ("F" ":" <RequestedInfo>) /
                 ("Q" ":" <QuarantineHandling>) /
                 ("T" ":" <DetectEvents>) /
                 ("RM" ":" <RestartMethod>) /
                 ("RD" ":" <RestartDelay>) /
                 ("A" ":" <Capabilities>) /
                 ("ES" ":" <EventStates>) /
                     (extensionParameter ":" <parameterString>)

ResponseAck =  confirmedTransactionIdRange
               *[ ","  confirmedTransactionIdRange



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confirmedTransactionIdRange = 1*9DIGIT [ "-" 1*9DIGIT ]

BearerInformation = BearerAttribute 0*("," 0*WSP BearerAttribute)
BearerAttribute = ("e" ":" <BearerEncoding>)
BearerEncoding = "A" / "mu"

CallId = 1*32(HEXDIG)

// The audit request response may include a list of identifiers
ConnectionId = [1*32(HEXDIG) 0*("," 1*32(HEXDIG))]
SecondConnectionID = ConnectionId

NotifiedEntity = [LocalName "@"] DomainName [":" portNumber]
LocalName = 1*32(suitableCharacter)
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" ":" <encryptionmethod>[":"<encryptionKey>])
                 / ("nt" ":" <typeOfNetwork> )
                 / (localOptionExtensionName ":" localOptionExtensionValue)

Capabilities = [ CapabilityValue 0*(WSP)
                 0*("," 0*(WSP) CapabilityValue 0*(WSP)) ]


CapabilityValue = LocalOptionValue
                / ("v" ":" <supportedPackages>)
                / ("m" ":" <supportedModes> )


packetizationPeriod = 1*4(DIGIT)["-" 1*4(DIGIT)]
compressionAlgorithm = algorithmName 0*(";" algorithmName)
algorithmName = 1*32(SuitableCharacter)
bandwidth = 1*4(DIGIT)["-" 1*4(DIGIT)]
echoCancellation = "on" / "off"
gainControl = "auto" | ["-"]1*4(DIGIT)
silenceSuppression = "on" / "off"



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typeOfService = 2HEXDIG
resourceReservation = "g" / "cl" / "be"

;encryption parameters are coded as in SDP (RFC 2327)
encryptiondata = ( "clear" ":" <encryptionKey> )
               / ( "base64" ":" <encodedEncryptionKey> )
               / ( "uri" ":" <URItoObtainKey> )
               / ( "prompt" ) ; defined in SDP, not usable in MGCP!
encryptionKey = 1*(SuitableCharacter / SP)
encodedEncryptionKey = 1*(ALPHA / DIGIT / "+" / "/" / "=")
URItoObtainKey = 1*(SuitableCharacter) / quotedString

typeOfNetwork = "IN" / "ATM" / "LOCAL"
supportedModes= ConnectionMode 0*(";" ConnectionMode)
supportedPackages = packageName 0*(";" packageName)



localOptionExtensionName = "x" ("+"/"-") 1*32(SuitableCharacter)
localOptionExtensionValue = 1*32(SuitableCharacter) / quotedString


ConnectionMode = "sendonly" / "recvonly" / "sendrecv" /
                 "confrnce" / "inactive" / "loopback" /
                 "conttest" / "netwloop" / "netwtest" / "data"

RequestedEvents = [requestedEvent 0*("," 0*(WSP) requestedEvent)]
requestedEvent = eventName [ "(" requestedActions ")" ]

eventName = [ (packageName / "*") "/" ] (eventId / "all" / eventRange)
            [ "@" (ConnectionId / "$" / "*") ]
packageName = 1*(ALPHA|DIGIT|HYPHEN)
eventId = 1*(SuitableCharacter)
eventRange = "[" 1*(DIGIT / DTMFLetter / "*" / "#" /
                        (DIGIT "-" DIGIT)/(DTMFLetter "-" DTMFLetter)) "]"

requestedActions = requestedAction 0*("," 0*(WSP) requestedAction)
requestedAction = "N" / "A" / "D" / "S" / "I" / "K" /
                  "E" "(" EmbeddedRequest ")"

EmbeddedRequest =   (      "R" "(" EmbeddedRequestList ")"
                      ["," "S" "(" EmbeddedSignalRequest ")" ]
                      ["," "D" "(" EmbeddedDigitMap ")" ] )
                /   (      "S" "(" EmbeddedSignalRequest ")"
                      ["," "D" "(" EmbeddedDigitMap ")" ] )
                /   (      "D" "(" EmbeddedDigitMap ")" )

EmbeddedRequestList = RequestedEvents



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EmbeddedSignalRequest = SignalRequests
EmbeddedDigitMap = DigitMap

SignalRequests = [ SignalRequest 0*("," 0*(WSP) SignalRequest ]
SignalRequest = eventName [ "(" eventParameters ")" ]
eventParameters = eventParameter 0*("," 0*(WSP) eventParameter)
eventParameter = eventParameterString / quotedString
eventParameterString = 1*(SuitableCharacter)

DigitMap = DigitString  / "(" DigitStringList ")"
DigitStringList = DigitString 0*( "|" DigitString )
DigitString = 1*(DigitStringElement)
DigitStringElement = DigitPosition ["."]
DigitPosition = DigitMapLetter | DigitMapRange
DigitMapLetter = DIGIT / "#" / "*" / "A" / "B" / "C" / "D" / "T"
DigitMapRange =  "x" / "[" 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 = "X" "-" 2*ALPHA
ConnectionParameterExtensionValue = 1*9(DIGIT)

ReasonCode = 3DIGIT [SPACE 1*(%x20-7E)]

SpecificEndpointID = endpointName
SecondEndpointID = endpointName

RequestedInfo = [infoCode 0*("," infoCode)]



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infoCode = "B" / "C" / "I" / "N" / "X" / "L" / "M" /
           "R" / "S" / "D" / "O" / "P" / "E" / "Z" /
           "Q" / "T" / "RC" / "LC" / "A" / "ES" / "RM" / "RD"

QuarantineHandling = loopControl / processControl /
              (loopControl "," processControl )
loopControl = "step" / "loop"
processControl = "process" / "discard"

DetectEvents = [eventName 0*("," eventName)]

RestartMethod = "graceful" / "forced" / "restart" / "disconnected"

RestartDelay = 1*6(DIGIT)

extensionParameter = "X" ("-"/"+") 1*6(ALPHA / DIGIT)
parameterString = 1*(%x20-7F)

MGCPResponse = MGCPResponseLine 0*(MGCPParameter)
                [EOL *SDPinformation]

MGCPResponseLine = (<responseCode> 1*(WSP) <transaction-id>
                          [1*(WSP) <responseString>] EOL)
responseCode = 3DIGIT
responseString = *(%x20-7E)

SuitableCharacter= DIGIT / ALPHA / "+" / "-" / "_" / "&" /
                   "!" / "'" / "|" / "=" / "#" / "?" / "/" /
                   "." / "$" / "*" / ";" / "@" / "[" / "]" /
                   "^" / "`" / "{" / "}" / "~"

quotedString = DQUOTE visibleString
                 0*(quoteEscape visibleString) DQUOTE
quoteEscape = DQUOTE DQUOTE
visibleString = (%x00-21 / %x23-FF)
EOL = CRLF / LF

SDPinformation = ;See RFC 2327


3.5.  Encoding of the session description

The session description is encoded in conformance with the session
description protocol, SDP. MGCP implementations are expected to be fully
capable of parsing any conformant SDP message, and should send session
descriptions that strictly conform to the SDP standard. The usage of SDP
actually depends on the type of session that is being, as specified in
the "mode" parameter:



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*    if the mode is set to "data", the session description describes the
     configuration of a data access service.

*    if the mode is set to any other value, the session description is
     for an audio service.

For an audio service, the gateway will consider the information provided
in SDP for the "audio" media. For a data service, the gateway will con-
sider the information provided for the "network-access" media.

3.5.1.  Usage of SDP for an audio service

In a telephony gateway, we only have to describe sessions that use
exactly one media, audio. The parameters of SDP that are relevant for
the telephony application are:
     At the session description level:

     *    The IP address of the remote gateway (in commands) or of the
          local gateway (in responses), or multicast address of the
          audio conference, encoded as an SDP "connection data" parame-
          ter. This parameter specifies the IP address that will be used
          to exchange RTP packets.

     For the audio media:

     *    Media description field (m) specifying the audio media, the
          transport port used for receiving RTP packets by the remote
          gateway (commands) or by the local gateway (responses) , the
          RTP/AVP transport, and the list of formats that the gateway
          will accept. This list should normally always include the code
          0 (reserved for G.711).

     *    Optionally, RTPMAP attributes that define the encoding of
          dynamic audio formats,

     *    Optionally, a packetization period (packet time) attribute
          (Ptime) defining the duration of the packet,

     *    Optionally, an attribute defining the type of connection (sen-
          donly, recvonly, sendrecv, inactive). Note that this attribute
          does not have a direct relation with the "Mode" parameter of
          MGCP.  In fact, the SDP type of connection will most of the
          time be set to "sendrecv", regardless of the value used by
          MGCP.  Other values will only be used rarely, for example in
          the case of information or announcement servers that need to
          establish one way connections.

     *    The IP address of the remote gateway (in commands) or of the



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          local gateway (in responses), if it is not present at the ses-
          sion level.
     An example of SDP specification for an audio connection could be:

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


There is a request, in some environments, to use the MGCP to negotiate
connections that will use other transmission channels than RTP over UDP
and IP. This will be detailed in an extension to this document.

3.5.2.  Usage of SDP in a network access service

The parameters of SDP that are relevant for a data network access appli-
cation are:
     For the data media:

     *    Media description field (m) specifying the network access
          media, identified by the code "m=nas/xxxx", where "xxxx"
          describes the access control method that should be used for
          parametrizing the network access, as specified below. The
          field may also specify the port that should be used for con-
          tacting the server, as specified in the SDP syntax.

     *    Connection address parameter (c=) specifying the address, or
          the domain name, of the server that implement the access con-
          trol method. This parameter may also be specified at the ses-
          sion level.

     *    Optionally, a bearer type attribute (a=bearer:) describing the
          type of data connection to be used, including the modem type.

     *    Optionally, a framing type attribue (a=framing:) describing
          the type of framing that will be used on the channel.

     *    Optionally, attributes describing the called number
          (a=dialed:), the number to which the call was delivered
          (a=called:) and the calling number (a=dialing:).

     *    Optionally, attributes describing the range of addresses that
          could be used by the dialup client on its LAN (a=subnet:).

     *    Optionally, an encryption key, encoded as specified in the SDP
          protocol(k=).




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The connection address shall be encoded as specified in the SDP stan-
dard. It will be used in conjunction with the port specified in the
media line to access a server, whose type will one of:

       __________________________________________________________
      | Method name|  Method description                        |
      |____________|____________________________________________|
      | radius     |  Authentication according                  |
      |            |  to the Radius protocol.                   |
      | tacacs     |  Authentication according                  |
      |            |  to the TACACS+ protocol.                  |
      | diameter   |  Authentication according                  |
      |            |  to the Diameter protocol.                 |
      | l2tp       |  Level 2 tunneling protocol.               |
      |            |  The address and port are those of the LNS.|
      | login      |  Local login. (There is normally           |
      |            |  no server for that method.)               |
      | none       |  No authentication required.               |
      |            |  (The call was probably vetted             |
      |            |  by the Call Agent.)                       |
      |____________|____________________________________________|


If needed, the gateway may use the key specified in the announcement to
access the service. That key, in particular, may be used for the estab-
lishment of an L2TP tunnel.

























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The bearer attribute is composed of a bearer name and an optional exten-
sion.  The bearer type specifies the type of modulation (modem name) or,
in the case of digital connections, the type of ISDN service (8 bits, 7
bits).  When an extension is present, it is separated from the bearer
name by a single slash (/).  The valid values of the bearer attribute
are defined in the following table:

  ____________________________________________________________________
 | Type of bearer description      |  Example of values              |
 |_________________________________|_________________________________|
 | ITU modem standard              |  V.32, V.34, V.90.              |
 | ITU modem standard qualified    |  v.90/3com,                     |
 | by a manufacturer name          |  v.90/rockwell,                 |
 |                                 |  v.90/xxx                       |
 | Well known modem types          |  X2, K56flex                    |
 | ISDN transparent access, 64 kbps|  ISDN64                         |
 | ISDN64 + V.110                  |  ISDN64/V.110                   |
 | ISDN64 + V.120                  |  ISDN64/V.120                   |
 | ISDN transparent access, 56 kbps|  ISDN56                         |
 | Informal identification         |  (Requires coordination between |
 |                                 |  the Call Agent and the gateway)|
 |_________________________________|_________________________________|


The valid values of the framing attribute are defined in the following
table:

           _________________________________________________
          | Type of framing description|  Example of values|
          |____________________________|___________________|
          | PPP, asynchronous framing  |  ppp-asynch       |
          | PPP, HDLC framing          |  ppp-hdlc         |
          | SLIP, asynchronous         |  slip             |
          | Asynchronous, no framing   |  asynch           |
          |____________________________|___________________|


The network access authentication parameter provides instructions on the
access control that should be exercized for the data call. This optional
attribute is encoded as:

        "a=subnet:" <network type> <address type>
           <connection address> "/" <prefix length>


Where the parameters "network type", "address type", and "connection
address" are formatted as defined for the connection address parameter
(c=) in SDP, and where the "prefix length" is a decimal representation



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of the number of bits in the prefix.

Examples of SDP announcement for the network access service could be:

        v=0
        m=nas/radius
        c=IN IP4 radius.example.net
        a=bearer:v.34
        a=framing:ppp-asynch
        a=dialed:18001234567
        a=called:12345678901
        a=dialing:12340567890

        v=0
        m=nas/none
        c=IN IP4 128.96.41.1
        a=subnet:IN IP4 123.45.67.64/26
        a=bearer:isdn64
        a=framing:ppp-sync
        a=dialed:18001234567
        a=dialing:2345678901

        v=0
        c=IN IP4 access.example.net
        m=nas/l2tp
        k=clear:some-shared-secret
        a=bearer:v.32
        a=framing:ppp-asynch
        a=dialed:18001234567
        a=dialing:2345678901


3.5.3.  Usage of SDP for ATM connections

The specification of the SDP payload for ATM connections will be
described in a companion document, "Usage of MGCP to control Voice over
ATM gateways." The following text is indicative.

The SDP payload will specify:

*    That the connection is to be established over an ATM interface,
     using the "c=" parameter of SDP to specify an address in the ATM
     family, the ATM addressing variant (NSAP, UNI, E.164) and the ATM
     address.

*    The "m=audio" parameter will specify the audio encoding and, if
     needed, the VPI and VCI.




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*    Additional attributes parameters (a=) will be used to specify the
     ATM coding variants, such as the type of adaptation layer and the
     error correction or loss compenmsation algorithms.

An example of SDP payload for an ATM connection could be:

        v=0
        c=ATM NSAP 47.0091.8100.0000.0060.3e64.fd01.0060.3e64.fd01.fe
        m=audio 5/1002 ATM/AVP G.711u
        a=connection_type:AAL2


3.5.4.  Usage of SDP for local connections

When MGCP is used to set up internal connections within a single gate-
way, the SDP format is used to encode the parameters of that connection.
The following parameters will be used:

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

*    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 RTP (RTP payload numbers.) The type of encoding should normally
     be set to 0 (G.711).

An example of local SDP payload could be:

        v=0
        c=LOCAL EPN X35V3+A4/13
        m=audio 0 LOCAL 0


3.6.  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 of the commands.

When no port is specified for the endpoint, the commands should be sent
to the default MGCP port, 2427.

3.6.1.  Providing the At-Most-Once functionality

MGCP messages, being carried over UDP, may be subject to losses. In the



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absence of a timely response, commands are repeated. 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" func-
tionality.

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 transaction identifiers of incom-
ing 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. The remaining commands
will be compared to the list of current transaction. If a match is
found, the MGCP entity does not execute the transaction, which is simply
ignored.

The procedure use a long timer value, noted LONG-TIMER in the following.
The timer should be set larger than the maximum duration of a transac-
tion, which should 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 can be destroyed either LONG-TIMER 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 attribute". For transactions that are ack-
nowledged through this attribute, the gateway shall keep a copy of the
transaction-id for LONG-TIMER 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.

3.6.2.  Transaction identifiers and three ways handshake

Transaction identifiers are integer numbers in the range from 0 to
999,999,999.  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 will share the same transaction
number space.  There are multiple possible implementations of this shar-
ing, 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 log-
ical call agent, as defined in the "states, failover and race condi-
tions" section. Gateways can simply detect duplicate transactions by
looking at the transaction identifier only.




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The Response Acknowledgement Attribute can be found in any command. It
carries a set of "confirmed transaction-id ranges."

MGCP gateways may choose to delete the copies of the responses to tran-
sactions whose id is included in "confirmed transaction-id ranges"
received in the Response Confirmation messages. They should silently
discard further commands from that Call Agent when the transaction-id
falls within these ranges.

The "confirmed transaction-id ranges" values shall not be used if more
than LONG-TIMER seconds have elapsed since the gateway issued its last
response to that call agent, or when a gateway resumes operation.  In
this situation, commands should be accepted and processed, without any
test on the transaction-id.

Commands that carry the "Response Acknowledgement attribute" may be
transmitted in disorder.  The gateway shall retain the union of the
"confirmed transaction-id ranges" received in recent commands.

3.6.3.  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 connections.

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 a
response. 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

The retransmission timer, in TCP, is set to the sum of the average delay
plus N times the average deviation. 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 LONG-TIMER seconds.  A
suggested maximum value is 4 seconds.

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



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*    It should double the estimated value of the average delay, AAD

*    It should compute a random value, uniformly distributed between 0.5
     AAD and AAD

*    It should set the retransmission timer to the sum of that random
     value and N times the average deviation.

This procedure has two effects. Because it includes an exponentially
increasing component, it will automatically slow down the stream of mes-
sages in case of congestion. Because it includes a random component, it
will break the potential synchronization between notifications triggered
by the same external event.

3.6.4.  Piggy backing

There are cases when a Call Agent will want to send several messages at
the same time to the same gateways.  When several MGCP messages have to
be sent in the same UDP packets, they should be separated by a line of
text that contain a single dot, as in for example:

         200 2005 OK
         .
         DLCX 1244 card23/21@trgw-7.example.net MGCP 0.1
         C: A3C47F21456789F0
         I: FDE234C8


The piggy-backed messages should be processed exactly has if they had
been received in several simultaneous messages.

3.6.5.  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 that can predict that a transaction will require a long execu-
tion time may send a provisional response, with response code 100.  They
should send this response if they receive a repetition of a transaction
that is still being executed.

MGCP entities that receive a provisional response shall switch to a
longer repetition timer for that transaction.






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4.  States, failover and race conditions.

In order to implement proper call signalling, 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.  Basic Asumptions

The support of "failover" is based on the following assumptions:

*    Call Agents are identified by their domain name, not their network
     addresses, and several addresses can be associated with a domain
     name.

*    An endpoint has one NotifiedEntity associated with it any given
     point in time.

*    The NotifiedEntity is the last value of the "NotifiedEntity" param-
     eter received for this endpoint (including wild-carded endpoint-
     names). If no explicit "NotifiedEntity" parameter has been
     received, the "NotifiedEntity" defaults to the provisioned Noti-
     fiedEntity value, or if no value was provisioned to the source
     address of the last command received for the endpoint,

*    Responses to commands are always sent to the source address of the
     command, regardless of the NotifiedEntity.

*    When the "notified entity" refers to a domain name that resolves to
     multiple IP- address, endpoints are capable of switching between
     different interfaces on the same  logical call agent, however they
     cannot switch to other (backup) call agent(s) on their own. A
     backup call agent can however instruct them to switch, either
     directly or indirectly.

*    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.

*    When a backup call agent has taken over control of a group of end-
     points, 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



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     (if that's even desired - maybe the failed call agent should simply
     become the backup call agent now).

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 NotifiedEntity).

4.2.  Security, Retransmission, and Detection of Lost Associations:

The media gateway control protocol is organized as a set of transac-
tions, 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 repeated. MGCP entities are expected to 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 LONG-TIMER seconds, and a
list of the transactions that are currently being executed.

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. The remaining commands will be compared to the list of current
transaction. If a match is found, the MGCP entity does not execute the
transaction, which is simply ignored - a response will be provided when
the execution of the command is complete.

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 con-
figured system, this loss rate should be kept 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 else 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



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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 repetition that is considered excessive should be a function
of 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 should be set to a larger value.

         Command issued: N=0
                 |
          transmission: N++
                 |  +------------ retransmission: N++ -----------+
                 |  |                                            |
                 |  |       transmission                         |
                 |  |  +---to new address -+<--------------------|--+
                 |  |  |        N=0        |                     |  |
                 V  V  V                   |                     |  |
           +-----------+                   |                     |  |
           | awaiting  |- new call agent ->+  +------------+     |  |
           |  response |--- timer elapsed --->| N > Max1 ? |-(no)+  |
           +-----------+ <----------+         +------------+     ^  |
                 |   |              |               |            |  |
                 |   +- wrong key? -+             (yes)          |  |
                 |                                  |            |  |
         response received                    (if N=Max1,        |  |
                 |                             or N=Max2         |  |
                 |                             check DNS)        |  |
                 v                                  |            |  |
               (end)                       +---------------+     |  |
                                           |more addresses?|(yes)|--+
                                           +---------------+     |
                                                    |            |
                                                  (no)           |
                                                    |            |
                                              +------------+     |
                                              | N > Max2 ? |(no)-+
                                              +------------+
                                                    |
                                                  (yes)
                                                    |
                                                    v
                                             (disconnected)

A classic retransmission algorithm would simply count the number of suc-
cessive 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:



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*    We request that the gateway always checks for the presence of a new
     call agent. It can be noticed either by

     -    receiving a valid multicast message announcing a failover, or

     -    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 starts transmitting
     outstanding commands to that new agent.  Responses to commands are
     still transmitted to the source address of the command.

*    we request that if the number of repetitions for this Call Agent is
     larger than "Max1", that the gateway actively queries the name
     server in order to detect the possible change of the call agent
     interfaces.

*    The gateway may have learned several IP addresses for the call
     agent. If the number of repetitions is larger than "Max1" and lower
     than "Max2", and there are more interfaces that have not been
     tried, then the gateway should direct the retransmissions to alter-
     nate addresses.

*    If there are no more interfaces to try, and the number of repeti-
     tions is Max2, then the gateway contacts the DNS one more time to
     see if any other interface should have become available. If not,
     the gateway is now disconnected.

The procedure will maximize the chances of detecting an ongoing fail-
over. It poses indeed two very specific problems, the potentially long
delays of a timer based procedure and the risk of confusion caused by
the use of cryptographic protections.

In order to automatically adapt to network load, MGCP specifies exponen-
tially increasing timers.  If the initial timer is set to 200 mil-
liseconds, 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 - wait-
ing 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
elapsed since the sending of the initial datagram is no greater than T-
MAX. If more than T-MAX time has elapsed, the endpoint becomes



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disconnected. The value T-MAX is related to the LONG-TIMER value: the
LONG-TIMER value is obtained by adding to T-MAX the maximum propagation
delay in the network.

Another potential cause of connection failure would be the reception of
a "wrong key" message, sent by a call agent that could not authenticate
the command, presumably because it had lost the security parameters of
the association.  Such messages are actually not authorized in IPSEC,
and they should in fact not be taken at face value: an attacker could
easily forge "wrong key" messages in order to precipitate the loss of a
control connection.  The current algorithm ignores these messages, which
translates into a strict reliance on timers.  The algorithm could in
fact be improved, maybe by executing a check with the key server of the
call agent after "Max1" repetitions.

4.3.  Race conditions

MGCP deals with race conditions through the notion of a "quarantine
list" and through explicit detection of desynchronization.

MGCP does not assume that the transport mechanism will maintain the
order of command 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 transmis-
sion capability during an earthquake or an ice storm.  When power and
transmission are reestablished, many gateways may decide to send "Res-
tartInProgress" commands simultaneously, leading to very unstable opera-
tion.

4.3.1.  Quarantine list

MGCP controlled gateways will receive "notification requests" that ask
them to watch for a list of "events."  The protocol elements that deter-
mine the handling of these events are the "Requested Events" list, the
"Digit Map" and the "Detect Events" list.

When the endpoint is initialized, the requested events list and the
digit map are empty.  After reception of a command, the gateway starts
observing the endpoint for occurrences of the events mentioned in the
list.

The events are examined as they occur. The action that follows is deter-
mined by the "action" parameter associated to the event in the list of
requested events, and also by the digit map.  The events that are



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defined as "accumulate" or "treat according to digit map" are accumu-
lated in a list of events, the events that are marked as "treated
according to the digit map" will additionally be accumulated in the
dialed string. This will go on until one event is encountered that
triggers a Notification to the "notified entity."

The gateway, at this point, will transmit the notification command and
will place the endpoint in a "notification" state. As long as the end-
point is in this notification state, the events that are to be detected
on the endpoint are stored in a "quarantine" buffer for later process-
ing.  The events are, in a sense, "quarantined." All events that are
specified by the union of the RequestedEvents parameter and the most
recently received DetectEvent parameter or, in the absence of the
latter, all events that are referred to in the RequestedEvents, should
be detected and quarantined, regardless of the action associated to the
event.

The endpoint exits the "notification state" when the acknowledgement of
the Notify  command is received. The Notify command may be retransmitted
in the "notification state", as specified in section 3.5. When the end-
point 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 notification request.  If the
Call Agent specified that it expected at most one notification in
response to the notification request command, then the gateway should
simply keep on accumulating events in the quarantine list until it
receives the next notification request command.

If the gateway is authorized to send multiple successive Notify com-
mands, it will proceed as follows.  When the gateway exits the "notifi-
cation 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 requires a Notify command to be sent. If that
is the case, the gateway can adopt one of the two following behaviors:

*    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 list,

*    or it can attempt to empty the quarantined list and transmit a sin-
     gle Notify command reporting several sets of events and possibly
     several dial strings. The dial string is reset to a null value
     after each triggering event. The events that follow the last



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     triggering event are left in the quarantine list.

If the gateway transmits a Notify command, the end point will remain in
the "notification state" until the acknowledgement is received. If the
gateway does not find a quarantined event that requests a Notify com-
mand, it places the end point in a normal state.  Events are then pro-
cessed 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 end point. When a new notification request is received in the notif-
ication state, the gateway shall ensure that the pending notification is
received by the Call Agent prior to a successful response to the new
NotificationRequest. It does so by using the "piggy-backing" functional-
ity of the protocol. The messages will then be sent in a single packet
to the source of the new NotificationRequest, regardless of respectively
the source and "notified entity" for the old and new command. The steps
involved are the following:

a)   the gateway builds a message that carries in a single packet a
     repetition of the old pending Notify command and the acknowledge-
     ment of the new notification request.

b)   the endpoint is then taken out of the "notification state" without
     waiting for the acknowledgement of the notification command.

c)   a copy of the unacknowledged Notify command command is kept until
     an acknowledgement is received.  If a timer elapses, the notifica-
     tion will be repeated, in a packet that will also carry a repeti-
     tion of the acknowledgement of the notification request.

d)   if the acknowledgement is lost, the Call Agent will retransmit the
     Notification Request.  The gateway will reply to this repetition by
     retransmitting in a single packet the unacknowledged Notify and the
     acknowledgement of the notification request.

e)   if the gateway has to transmit a Notify before the previous Notify
     is acknowledged, it should construct a packet that piggybacks a
     repetition of the old Notify, a repetition of the acknowledgement
     of the last notification request and the new Notify.

f)   Gateways that cannot piggyback several packets in the same message
     should elect to leave the endpoint in the "notification" state as
     long as the last notification is not acknowledged.

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 list of observed events and accumulated



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dial string are reset to a null value.  The behavior is conditioned by
the value of the QuarantineHandling parameter. The parameter may specify
that quarantined events, or previously observed events, should be dis-
carded, in which case they will be. If the parameter specifies that the
quarantined events should be processed, the gateway will start process-
ing the list of quarantined events or previously observed events, using
the newly received list of requested events and digit map. When process-
ing these events, the gateway may encounter an event which requires a
Notify command to be sent. If that is the case, the gateway will immedi-
ately 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 accumu-
lated events according to the previous notification requests, but has
not yet detected a notification-triggering events.  The handling of
not-yet-notified events is determined, as with the quarantined events,
by the quarantine handling parameters:

*    If the quarantine-handling parameter specifies that quarantined
     events shall be ignored, the observed event 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.

Call Agents SHOULD provide the response to a successful Notify message
and the new NotificationRequest in the same datagram using the piggy-
backing mechanism.

4.3.2.  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 should check the condition of
the endpoint before acknowledging a NotificationRequest. It should
return an error:

1-   If the gateway is requested to notify an "off hook" transition
     while the phone is already off hook,




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2-   If the gateway is requested to notify an "on hook" or "flash hook"
     condition while the phone is already on hook.

It should be noted, that the condition check is performed at the time
the notification request is received, where as 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
RequestedEvent 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 will simply continue as
if the command had never been received. As all other transactions, the
NotificationRequest should operate as an atomic transaction, thus any
changes initiated as a result of the command should be reverted.

Another race condition may occur when a Notify is issued shortly before
the reception by the gateway of a NotificationRequest. The RequestIden-
tifier is used to correlate Notify commands with NotificationRequest
commands.

4.3.3.  Ordering of commands, and treatment of disorder

MGCP does not mandate that the underlying transport protocol guarantees
the sequencing of commands sent to a gateway or an endpoint.  This pro-
perty tends to maximize the timeliness of actions, but it has a few draw
backs.  For example:

*    Notify commands may be delayed and arrive to 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 in second position.

Call Agents that want to guarantee consistent operation of the end
points 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 fol-
     lowing 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 outstand-
     ing command (create or modify).  However, a DeleteConnection com-
     mand can be issued at any time.  In consequence, a gateway may
     sometimes receive a ModifyConnection command that applies to a pre-
     viously deleted connection.  Such commands should be ignored, and
     an error code should be returned.

4)   On a given endpoint, there should normally be only one outstanding
     NotificationRequest command at any time.  The RequestId parameter
     should be used to correlate Notify commands with the triggering
     notification request.

5)   In some cases, an implicitly or explicitly wildcarded DeleteConnec-
     tion command that applies to a group of endpoints can step in front
     of a pending CreateConnection command.  The Call Agent should indi-
     vidually delete all connections whose completion was pending at the
     time of the global DeleteConnection command.  Also, new CreateCon-
     nection commands for endpoints named by the wild-carding cannot be
     sent until the wild-carded DeleteConnection command is ack-
     nowledged.

6)   When commands are embedded within each other, sequencing require-
     ments for all commands must be adhered to. For example a Create
     Connection command with a Notification Request in it must adhere to
     the sequencing for CreateConnection and NotificationRequest at the
     same time.

7)   AuditEndpoint and AuditConnection is not subject to any sequencing.

8)   RestartInProgress must always be the first command sent by an end-
     point as defined by the restart procedure. Any other command or
     response must be delivered after this RestartInProgress command
     (piggy-backing allowed).

9)   When multiple messages are piggy-backed in a single packet, the
     messages are always processed in order.

These rules do not affect the gateway, which should always respond to
commands.

4.3.4.  Fighting the restart avalanche

Let's suppose that a large number of gateways are powered on simultane-
ously.  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 restora-
tion. In order to prevent such avalanches, the following behavior is
suggested:



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1)   When a gateway is powered on, it should initiate a restart timer to
     a random value, uniformly distributed between 0 and a maximum wait-
     ing 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 should 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 should initiate the restart pro-
     cedure.

The restart procedure simply requires the endpoint to guarantee that the
first message (command or response) that the Call Agent sees from this
endpoint is a RestartInProgress message informing the Call Agent about
the restart. The endpoint is free to take full advantage of piggy-
backing to achieve this.

It is expected that each endpoint in a gateway will have a provisionable
Call Agent, i.e., "notified entity", to direct the initial restart mes-
sage 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 number of Res-
tartInProgress messages generated when multiple endpoints in a gateway
restart and the endpoints are managed by the same Call Agent.

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 end
point and the call agent.  This simple calculation shows that the call
agent is expected to handle 5 to 6 transactions for each end point,
every 30 minutes on average, or, to put it otherwise, about one transac-
tion per end point 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



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large number of endpoints, and also because the usage rate of these end-
points is much higher than 10% during the peak busy hour, a typical
value being 60%.  These endpoints, during the peak hour, are this
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.

4.3.5.  Disconnected Endpoints

In addition to the restart procedure, gateways also have a "discon-
nected" procedure, which is initiated when an endpoint becomes "discon-
nected" as described in Section 3.4.2. It should here be noted, that
endpoints can only become disconnected when they attempt to communicate
with the Call Agent. The following steps are followed by an endpoint
that becomes "disconnected":

1.   A "disconnected" timer is initialized to a random value, uniformly
     distributed between 0 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 multi-
     ple gateways and endpoints that would use the same algorithm.

2.   The gateway then waits for either the end of this timer, the recep-
     tion of a command 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, when a command is received,
     or when a local user activity is detected, the gateway initiates
     the "disconnected" procedure for the endpoint. In the case of local
     user activity, a provisionable "disconnected" minimum waiting delay
     (Tdmin) must furthermore have elapsed since the gateway became
     disconnected or the last time it initiated the "disconnected" pro-
     cedure in order to limit the rate at which the procedure is per-
     formed.

4.   If the "disconnected" procedure still left the endpoint discon-
     nected, the "disconnected" timer is then doubled, subject to a pro-
     visionable "disconnected" maximum waiting delay (Tdmax), e.g., 600
     seconds, and the gateway proceeds with step 2 again.

The "disconnected" procedure is similar to the restart procedure in that
it now simply states that the endpoint MUST send a RestartInProgress
command to the Call Agent informing it that the endpoint was



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disconnected and furthermore guarantee that the first message (command
or response) that the Call Agent now sees from this endpoint MUST be
this RestartInProgress command. The endpoint MUST take full advantage of
piggy-backing in achieving this. The Call Agent may then for instance
decide to audit the endpoint, or simply clear all connections for the
endpoint.

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.

5.  Security requirements

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 connec-
tions, 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 forbidding third parties from
monitoring the connections set up by a given endpoint

The encryption service will also be requested if the session descrip-
tions 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 pro-
cedures.

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 decompressed and the signals will be played on the "line
side".

A basic protection against this attack is to only accept packets from
known sources, checking for example that the IP source address and UDP
source port match the values announced in the "remote session descrip-
tion."  But this has two inconveniences: it slows down connection estab-
lishment and it can be fooled by source spoofing:

*    To enable the address-based protection, the call agent must obtain
     the remote session description of the e-gress gateway and pass it
     to the in-gress 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 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 authen-
ticate the packets, using a secret key that is conveyed during the call
set-up procedure. This will no slow down the call set-up, and provides
strong protection against address spoofing.










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6.  Event packages and end point types

This section provides an initial definition of packages and event names.
More packages can be defined in additional documents.

6.1.  Basic packages

The list of basic packages includes the following:

               _________________________________________
              | Package                      |   name  |
              |______________________________|_________|
              | Generic Media Package        |   G     |
              | DTMF package                 |   D     |
              | MF Package                   |   M     |
              | Trunk Package                |   T     |
              | Line Package                 |   L     |
              | Handset Package              |   H     |
              | RTP Package                  |   R     |
              | Network Access Server Package|   N     |
              | Announcement Server Package  |   A     |
              | Script Package               |   Script|
              |______________________________|_________|


In the tables of events for each package, there are five columns:
     Symbol: the unique symbol used for the event
     Definition: a short description of the event

     R:   an x appears in this column is the event can be Requested by
          the call agent.

     S:   if nothing appears in this column for an event, then the event
          cannot be signaled on command by the call agent. Otherwise,
          the following symbols identify the type of event:

          OO   On/Off signal.  The signal is turned on until commanded
               by the call agent to turn it off, and vice versa.

          TO   Timeout signal.  The signal lasts for a given duration
               unless it is superseded by a new signal.

          BR   Brief signal.  The event has a short, known duration.

          Duration: specifies the duration of TO signals.






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6.1.1.  Generic Media Package

Package Name: G

The generic media package group the events and signals that can be
observed on several types of endpoints, such as trunking gateways,
access gateways or residential gateways.

 _____________________________________________________________________
| Symbol   |   Definition               |   R |   S      Duration    |
|__________|____________________________|_____|______________________|
| mt       |   Modem detected           |   x |                      |
| ft       |   Fax tone detected        |   x |                      |
| ld       |   Long duration connection |   x |                      |
| pat(###) |   Pattern ### detected     |   x |   OO                 |
| rt       |   Ringback tone            |     |   TO                 |
| rbk(###) |   ring back on connection  |     |   TO     180 seconds |
| cf       |   Confirm tone             |     |   BR                 |
| cg       |   Network Congestion tone  |     |   TO                 |
| it       |   Intercept tone           |     |   OO                 |
| pt       |   Preemption tone          |     |   OO                 |
| of       |   report failure           |   x |                      |
|__________|____________________________|_____|______________________|

The signals are defined as follow:

     The pattern definition can be used for specific algorithms such as
     answering machine detection, tone detection, and the like.

Ring back tone (rt)
     an Audible Ring Tone, a combination of two AC tones with frequen-
     cies of 440 and 480 Hertz and levels of -19 dBm each, to give a
     combined level of -16 dBm.  The cadence for Audible Ring Tone is 2
     seconds on followed by 4 seconds off. See GR- 506-CORE - LSSGR:
     SIGNALING, Section 17.2.5.

Ring back on connection
     A ring back tone, applied to the connection whose identifier is
     passed as a parameter.

The "long duration connection" is detected when a connection has been
established for more than 1 hour.

6.1.2.  DTMF package

Package name: D





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    _______________________________________________________________
   | Symbol |   Definition              |   R |   S      Duration |
   |________|___________________________|_____|___________________|
   | 0      |   DTMF 0                  |   x |   BR              |
   | 1      |   DTMF 1                  |   x |   BR              |
   | 2      |   DTMF 2                  |   x |   BR              |
   | 3      |   DTMF 3                  |   x |   BR              |
   | 4      |   DTMF 4                  |   x |   BR              |
   | 5      |   DTMF 5                  |   x |   BR              |
   | 6      |   DTMF 6                  |   x |   BR              |
   | 7      |   DTMF 7                  |   x |   BR              |
   | 8      |   DTMF 8                  |   x |   BR              |
   | 9      |   DTMF 9                  |   x |   BR              |
   | #      |   DTMF #                  |   x |   BR              |
   | *      |   DTMF *                  |   x |   BR              |
   | A      |   DTMF A                  |   x |   BR              |
   | B      |   DTMF B                  |   x |   BR              |
   | C      |   DTMF C                  |   x |   BR              |
   | D      |   DTMF D                  |   x |   BR              |
   | L      |   long duration indicator |   x |          2 seconds|
   | X      |   Wildcard, match         |   x |                   |
   |        |   any digit 0-9           |     |                   |
   | T      |   Interdigit timer        |   x |          4 seconds|
   | of     |   report failure          |   x |                   |
   |________|___________________________|_____|___________________|


The "interdigit timer" T is a digit input timer that can be used in two
ways:

*    When timer T is used with a digit map, the timer is not started
     until the first digit is entered, and the timer is restarted after
     each new digit is entered until either a digit map match or
     mismatch occurs. In this case, timer T functions as an inter-digit
     timer.

*    When timer T is used without a digit map, the timer is started
     immediately and simply cancelled (but not restarted) as soon as a
     digit is entered. In this case, timer T can be used as an inter-
     digit timer when overlap sending is used.

When used with a digit map, timer T takes on one of two values,
T(partial) or T(critical). When at least one more digit is required for
the digit string to match any of the patterns in the digit map, timer T
takes on the value T(partial), corresponding to partial dial timing. If
a timer is all that is required to produce a match, timer T takes on the
value T(critical) corresponding to critical timing. When timer T is used
without a digit map, timer T takes on the value T(critical).  The



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default value for T(partial) is 16 seconds and the default value for
T(critical) is 4 seconds. The provisioning process may alter both of
these.

The "long duration indicator" is observed when a DTMF signal is produced
for a duration larger than two seconds.  In this case, the gateway will
detect two successive events: first, when the signal has been recog-
nized, the DTMF signal, and then, 2 seconds later, the long duration
signal.

6.1.3.  MF Package

Package Name: M

        ________________________________________________________
       | Symbol |   Definition       |   R |   S      Duration |
       |________|____________________|_____|___________________|
       | 0      |   MF 0             |   x |   BR              |
       | 1      |   MF 1             |   x |   BR              |
       | 2      |   MF 2             |   x |   BR              |
       | 3      |   MF 3             |   x |   BR              |
       | 4      |   MF 4             |   x |   BR              |
       | 5      |   MF 5             |   x |   BR              |
       | 6      |   MF 6             |   x |   BR              |
       | 7      |   MF 7             |   x |   BR              |
       | 8      |   MF 8             |   x |   BR              |
       | 9      |   MF 9             |   x |   BR              |
       | X      |   Wildcard, match  |   x |                   |
       |        |   any digit 0-9    |     |                   |
       | T      |   Interdigit timer |   x |          4 seconds|
       | K0     |   MF K0 or KP      |   x |   BR              |
       | K1     |   MF K1            |   x |   BR              |
       | K2     |   MF K2            |   x |   BR              |
       | S0     |   MF S0 or ST      |   x |   BR              |
       | S1     |   MF S1            |   x |   BR              |
       | S2     |   MF S2            |   x |   BR              |
       | S3     |   MF S3            |   x |   BR              |
       | wk     |   Wink             |   x |   BR              |
       | wko    |   Wink off         |   x |   BR              |
       | is     |   Incoming seizure |   x |   OO              |
       | rs     |   Return seizure   |   x |   OO              |
       | us     |   Unseize circuit  |   x |   OO              |
       | of     |   report failure   |   x |                   |
       |________|____________________|_____|___________________|


The definition of the MF package events is as follow:




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Wink
     A transition from unseized to seized to unseized trunk states
     within a specified period.  Typical seizure period is 100-350
     msec.)

Incoming seizure
     Incoming indication of call attempt.

Return seizure:
     Seizure in response to outgoing seizure.

Unseize circuit:
     Unseizure of a circuit at the end of a call.

Wink off:
     A signal used in operator services trunks.  A transition from
     seized to unseized to seized trunk states within a specified period
     of 100-350 ms. (To be checked)

6.1.4.  Trunk Package

Package Name: T

 _____________________________________________________________________
| Symbol |   Definition                   |   R |   S      Duration  |
|________|________________________________|_____|____________________|
| co1    |   Continuity tone (single tone,|   x |   OO               |
|        |   or return tone)              |     |                    |
| co2    |   Continuity test (go tone,    |   x |   OO               |
|        |  in dual tone procedures)      |     |                    |
| lb     |   Loopback                     |     |   OO               |
| om     |   Old Milliwatt Tone (1000 Hz) |   x |   OO               |
| nm     |   New Milliwatt Tone (1004 Hz) |   x |   OO               |
| tl     |   Test Line                    |   x |   OO               |
| zz     |   No circuit                   |   x |   OO               |
| as     |   Answer Supervision           |   x |   OO               |
| ro     |   Reorder Tone                 |   x |   TO     30 seconds|
| of     |   report failure               |   x |                    |
| bl     |   Blocking                     |     |   OO               |
|________|________________________________|_____|____________________|


The definition of the trunk package signal events is as follow:

Continuity Tone (co1):
     A tone at 2010 + or - 30 Hz.

Continuity Test (co2):



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     A tone at the 1780 + or - 30 Hz.

Milliwatt Tones:
     Old Milliwatt Tone (1000 Hz), New Milliwatt Tone (1004 Hz)

Line Test:
     105 Test Line test progress tone (2225 Hz + or - 25 Hz at -10 dBm0
     + or -- 0.5dB).

No circuit:
     (that annoying tri-tone, low to high)

Answer Supervision:

Reorder Tone:
     Reorder tone is a combination of two AC tones with frequencies of
     480 and 620 Hertz and levels of -24 dBm each, to give a combined
     level of -21 dBm.  The cadence for Station Busy Tone is 0.25
     seconds on followed by 0.25 seconds off, repeating continuously.
     See GR-506-CORE - LSSGR: SIGNALING, Section 17.2.7.

Blocking:
     The call agent can place the circuit in a blocked state by applying
     the "bl(+)" signal to the endpoint.  It can unblock it by applying
     the "bl(-)" signal.

The continuity tones are used when the call agent wants to initiate a
continuity test. There are two types of tests, single tone and dual
tone. The Call agent is expected to know, through provisioning informa-
tion, which test should be applied to a given endpoint. For example, the
call agent that wants to initiate a single frequency test will send to
the gateway a command of the form:

        RQNT 1234 epx-t1/17@tgw2.example.net
        X: AB123FE0
        S: co1
        R: co1

If it wanted instead to initiate a dual-tone test, it would send the
command:

        RQNT 1234 epx-t1/17@tgw2.example.net
        X: AB123FE0
        S: co2
        R: co1

The gateway would send the requested signal, and in both cases would
look for the return of the 2010 Hz tone (co1).  When it detects that



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tone, it will send the corresponding  notification.

The tones are of type OO: the gateway will keep sending them until it
receives a new notification request.

6.1.5.  Line Package

Package Name: L

__________________________________________________________________________
|Symbol       |   Definition                 |   R |   S      Duration   |
|_____________|______________________________|_____|_____________________|
|adsi(string) |   adsi display               |     |   BR                |
|vmwi         |   visual message             |     |   OO                |
|             |   waiting indicator          |     |                     |
|hd           |   Off hook transition        |   x |                     |
|hu           |   On hook transition         |   x |                     |
|hf           |   Flash hook                 |   x |                     |
|aw           |   Answer tone                |   x |   OO                |
|bz           |   Busy tone                  |     |   TO     30 seconds |
|ci(ti,nu,na) |   Caller-id                  |     |   BR                |
|wt           |   Call Waiting tone          |     |   TO     30 seconds |
|wt1, wt2,    |   Alternative call           |     |                     |
|wt3, wt4     |   waiting tones              |     |                     |
|dl           |   Dial tone                  |     |   TO     16 seconds |
|mwi          |   Message waiting ind.       |     |   TO     16 seconds |
|nbz          |   Network busy               |   x |   OO                |
|             |   (fast cycle busy)          |     |                     |
|ro           |   Reorder tone               |     |  TO      30 seconds |
|rg           |   Ringing                    |     |   TO     180 seconds|
|r0, r1, r2,  |  Distinctive ringing         |     |   TO     180 seconds|
|r3, r4, r5,  |                              |     |                     |
|r6 or r7     |                              |     |                     |
|rs           |   Ringsplash                 |     |   BR                |
|p            |   Prompt tone                |   x |   BR                |
|e            |   Error tone                 |   x |   BR                |
|sl           |   Stutter dialtone           |     |   TO     16 seconds |
|v            |   Alerting Tone              |     |   OO                |
|y            |   Recorder Warning Tone      |     |   OO                |
|sit          |   SIT tone                   |     |                     |
|z            |   Calling Card Service Tone  |     |   OO                |
|oc           |   Report on completion       |   x |                     |
|ot           |   Off hook warning tone      |     |   TO     indefinite |
|s(###)       |   Distinctive tone pattern   |   x |   BR                |
|of           |   report failure             |   x |                     |
|_____________|______________________________|_____|_____________________|





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The definition of the tones is as follow:

Dial tone:
     A combined 350 + 440 Hz tone.

Visual Message Waiting Indicator
     The transmission of the VMWI messages will conform to the require-
     ments in Section 2.3.2, "On-hook Data Transmission Not Associated
     with Ringing" in TR-H- 000030 and the CPE guidelines in SR-TSV-
     002476. VMWI messages will only be sent from the SPCS when the line
     is idle. If new messages arrive while the line is busy, the VMWI
     indicator message will be delayed until the line goes back to the
     idle state. The CA should periodically refresh the CPE's visual
     indicator. See TR-NWT-001401 - Visual Message Waiting Indicator
     Generic Requirements; and GR- 30-CORE - Voiceband Data Transmission
     Interface.

Message waiting Indicator
     See GR-506-CORE, 17.2.3.

Alerting Tone:
     a 440 Hz Tone of 2 second duration followed by 1/2 second of tone
     every 10 seconds.

Ring splash
     Ringsplash, also known as "Reminder ring" is a burst of ringing
     that may be applied to the physical forwarding line (when idle) to
     indicate that a call has been forwarded and to remind the user that
     a CF subfeature is active.  In the US, it is defined to be a 0.5(-
     0,+0.1) second burst of power ringing. See TR- TSY-000586 - Call
     Forwarding Subfeatures.

Call waiting tone
     Call Waiting tone is defined in GR-506-CORE, 14.2. Call Waiting
     feature is defined in TR-TSY-000571. By defining "wt" as a TO sig-
     nal you are really defining the feature which seems wrong to me
     (given the spirit of MGCP), hence the definition of "wt" as a BR
     signal in ECS, per GR-506-CORE. Also, it turns out that there is
     actually four different call waiting tone patterns (see GR-506-
     CORE, 14.2) so we have wt1, wt2, wt3, wt4.

Caller Id (ci(time, number, name)):
     The caller-id event carries three parameters, the time of the call,
     the calling number and the calling name. Each of the three fields
     are optional, however each of the commas will always be included.
     See TR-NWT-001188, GR-30-CORE, and TR-NWT-000031.

Recorder Warning Tone:



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     1400 Hz of Tone of 0.5 second duration every 15 seconds.

SIT tone:
     used for indicating a line is out of service.

Calling Card Service Tone:
     60 ms of 941 + 1477 Hz and 940 ms of 350 + 440 Hz (dial tone),
     decaying exponentially with a time constant of 200 ms.

Distinctive tone pattern:
     where ### is any number between 000 and 999, inclusive.  Can be
     used for distinctive ringing, customized dial tone, etc.

Report on completion
     The report on completion event is detected when the gateway was
     asked to perform one or several signals of type TO on the endpoint,
     and when these signals were completed without being stopped by the
     detection of a requested event such as off-hook transition or
     dialed digit.  The completion report may carry as parameter the
     name of the signal that came to the end of its live time, as in:

             O: L/oc(L/dl)


Ring back on connection
     A ring back tone, applied to the connection wghose identifier is
     passed as a parameter.

We should note that many of these definitions vary from country to coun-
try.  The frequencies listed above are the one in use in North America.
There is a need to accommodate different tone sets in different coun-
tries, and there is still an ongoing debate on the best way to meet that
requirement:

*    One solution is to define different event packages specifying for
     example the German dialtone as "L-DE/DL".

*    Another solution is to use a management interface to specify on an
     end-point basis which frequency shall be associated to what tone.

6.1.6.  Handset emulation package

Package Name: H








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__________________________________________________________________________
|Symbol       |   Definition                 |   R |   S      Duration   |
|_____________|______________________________|_____|_____________________|
|adsi(string) |   adsi display               |   x |   BR                |
|tdd          |                              |     |                     |
|vmwi         |                              |     |                     |
|hd           |   Off hook transition        |   x |   OO                |
|hu           |   On hook transition         |   x |   OO                |
|hf           |   Flash hook                 |   x |   BR                |
|aw           |   Answer tone                |   x |   OO                |
|bz           |   Busy tone                  |   x |   OO                |
|wt           |   Call Waiting tone          |   x |   TO     30 seconds |
|dl           |   Dial tone (350 + 440 Hz)   |   x |   TO     120 seconds|
|nbz          |   Network busy               |   x |   OO                |
|             |   (fast cycle busy)          |     |                     |
|rg           |   Ringing                    |   x |   TO     30 seconds |
|r0, r1, r2,  |  Distinctive ringing         |   x |   TO     30 seconds |
|r3, r4, r5,  |                              |     |                     |
|r6 or r7     |                              |     |                     |
|p            |   Prompt tone                |   x |   BR                |
|e            |   Error tone                 |   x |   BR                |
|sdl          |   Stutter dialtone           |   x |   TO     16 seconds |
|v            |   Alerting Tone              |   x |   OO                |
|y            |   Recorder Warning Tone      |   x |   OO                |
|t            |   SIT tone                   |   x |                     |
|z            |   Calling Card Service Tone  |   x |   OO                |
|oc           |   Report on completion       |   x |                     |
|ot           |   Off hook warning tone      |   x |   OO                |
|s(###)       |   Distinctive tone pattern   |   x |   BR                |
|of           |   report failure             |   x |                     |
|_____________|______________________________|_____|_____________________|


The handset emulation package is an extension of the line package, to be
used when the gateway is capable of emulating a handset.  The difference
with the line package is that events such as "off hook" can be signalled
as well as detected.














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6.1.7.  RTP Package

Package Name: R

  ____________________________________________________________________
 | Symbol  |   Definition                   |   R |   S      Duration|
 |_________|________________________________|_____|__________________|
 | UC      |   Used codec changed           |   x |                  |
 | SR(###) |   Sampling rate changed        |   x |                  |
 | JI(###) |   Jitter buffer size changed   |   x |                  |
 | PL(###) |   Packet loss exceeded         |   x |                  |
 | qa      |   Quality alert                |   x |                  |
 | co1     |   Continuity tone (single tone,|   x |   OO             |
 |         |   or return tone)              |     |                  |
 | co2     |   Continuity test (go tone,    |   x |   OO             |
 |         |  in dual tone procedures)      |     |                  |
 | of      |   report failure               |   x |                  |
 |_________|________________________________|_____|__________________|


Codec Changed:
     Codec changed to hexadecimal codec number enclosed in parenthesis,
     as in UC(15), to indicate the codec was changed to PCM mu-law.
     Codec Numbers are specified in RFC 1890, or in a new definition of
     the audio profiles for RTP that replaces this RFC.  Some implemen-
     tations of media gateways may not allow the codec to be changed
     upon command from the call agent.  codec changed to codec hexade-
     cimal ##.

Sampling Rate Changed:
     Sampling rate changed to decimal number in milliseconds enclosed in
     parenthesis, as in SR(20), to indicate the sampling rate was
     changed to 20 milliseconds.  Some implementations of media gateways
     may not allow the sampling rate to be changed upon command from a
     call agent.

Jitter Buffer Size Changed:
     When the media gateway has the ability to automatically adjust the
     depth of the jitter buffer for received RTP streams, it is useful
     for the media gateway controller to receive notification that the
     media gateway has automatically increased its jitter buffer size to
     accomodate increased or decreased variability in network latency.
     The syntax for requesting notification is "JI", which tells the
     media gateway that the controller wants notification of any jitter
     buffer size changes.  The syntax for notification from the media
     gateway to the controller is "JI(####)", where the #### is the new
     size of the jitter buffer, in milliseconds.




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Packet Loss Exceeded:
     Packet loss rate exceed the threshold of the specified decimal
     number of packets per 100,000 packets, where the packet loss number
     is contained in parenthesis.  For example, PL(10) indicates packets
     are being dropped at a rate of 1 in 10,000 packets.

Quality alert
     The packet loss rate or the combination of delay and jitter exceed
     a specified quality threshold.

The continuity tones are the same as those defined in the Trunk package.
They can be use in conjunction with the Network LoopBack or Network Con-
tinuity Test modes to test the continuity of an RTP circuit.

The "operation failure" code can be used to report problems such as the
loss of underlying connectivity.  The observed event can include as
parameter the reason code of the failure.

6.1.8.  Network Access Server Package

Package Name: N

      ____________________________________________________________
     | Symbol |   Definition             |   R |   S     Duration|
     |________|__________________________|_____|_________________|
     | pa     |  Packet arrival          |  x  |                 |
     | cbk    |  Call back request       |  x  |                 |
     | cl     |  Carrier lost            |  x  |                 |
     | au     |   Authorization succeeded|  x  |                 |
     | ax     |   Authorization denied   |  x  |                 |
     | of     |   Report failure         |  x  |                 |
     |________|__________________________|_____|_________________|


The packet arrival event is used to notify that at least one packet was
recently sent to an Internet address that is observed by an endpoint.
The event report includes the Internet address, in standard ASCII encod-
ing, between parenthesis:

        O: pa(192.96.41.1)


The call back event is used to notify that a call back has been
requested during the initial phase of a data connection. The event
report includes the identification of the user that should be called
back, between parenthesis:

        O: cbk(user25)



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6.1.9.  Announcement Server Package

Package Name: A

  ___________________________________________________________________
 | Symbol         |   Definition           |   R |   S      Duration|
 |________________|________________________|_____|__________________|
 | ann(url,parms) |   Play an announcement |     |   TO     variable|
 | oc             |   Report on completion |   x |                  |
 | of             |   Report failure       |   x |                  |
 |________________|________________________|_____|__________________|

The announcement action is qualified by an URL name and by a set of ini-
tial parameters as in for example:

        S: ann(http://scripts.example.net/all-lines-busy.au)

The "operation complete" event will be detected when the announcement is
played out. If the announcement cannot be played out, an operation
failure event can be returned.  The failure may be explained by a com-
mentary, as in:

        O: A/of(file not found)




























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6.1.10.  Script Package

Package Name: Script

     ______________________________________________________________
    | Symbol    |   Definition           |   R |   S  |   Duration|
    |___________|________________________|_____|______|___________|
    | java(url) |   Load a java script   |     |   TO |   variable|
    | perl(url) |   Load a perl script   |     |   TO |   variable|
    | tcl(url)  |   Load a TCL script    |     |   TO |   variable|
    | xml(url)  |   Load an XML script   |     |   TO |   variable|
    | oc        |   Report on completion |   x |      |           |
    | of        |   Report failure       |   x |      |           |
    |___________|________________________|_____|______|___________|

The "language" action define is qualified by an URL name and by a set of
initial parameters as in for example:

        S: script/java(http://scripts.example.net/credit-card.java,long,1234)


The current definition defines keywords for the most common languages.
More languages may be defined in further version of this documents.  For
each language, an API specification will describe how the scripts can
issue local "notificationRequest" commands, and receive the correspond-
ing notifications.

The script produces an output which consists of one or several text
string, separated by commas.  The text string are reported as a commen-
tary in the report on completion, as in for example:

        O: script/oc(21223456794567,9738234567)

The failure report may also return a string, as in:

        O: script/oc(21223456794567,9738234567)

The definition of the script environment and the specific actions in
that environment are for further study.

6.2.  Basic endpoint types and profiles

We define the following basic endpoint types and profiles:

*    Trunk gateway (ISUP)

*    Trunk gateway (MF)




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*    Network Access Server (NAS)

*    Combined NAS/VOIP gateway

*    Access Gateway

*    Residential Gateway

*    Announcement servers

These gateways are supposed to implement the following packages

      ___________________________________________________________
     | Gateway                    |   Supported packages        |
     |____________________________|_____________________________|
     | Trunk gateway (ISUP)       |   GM, DTMF, TK, RTP         |
     | Trunk gateway (MF)         |   GM, MF, DTMF, TK, RTP     |
     | Network Access Server (NAS)|   GM, MF, TK, NAS           |
     | Combined NAS/VOIP gateway  |   GM, MF, DTMF, TK, NAS, RTP|
     | Access Gateway (VOIP)      |   GM, DTMF, MF, RTP         |
     | Access Gateway (VOIP+NAS)  |   GM, DTMF, MF, NAS, RTP    |
     | Residential Gateway        |   GM, DTMF, Line, RTP       |
     | Announcement Server        |   ANN, RTP                  |
     |____________________________|_____________________________|


Advanced announcement servers may also support the Script package.

Advanced trunking servers may support the ANN package, the Script pack-
age, and in some cases the Line and Handset package as well.

7.  Versions and compatibility

7.1.  Differences between version 1.0 and draft 0.5

Draft 0-5 was issued in February 1999, as the last update of draft ver-
sion 0.1. Version 1.0 benefits from implementation experience, and also
aligns as much as possible with the CableLabs' NCS project. The main
differences between the February draft and version 1.0 are:

*    Specified more clearly that the encoding of three LocalConnec-
     tionOptions parameters, Encoding Method, Packetization Period and
     Bandwidth, shall follow the conventions laid out in SDP.

*    Specified how the quarantine handling parameter governs the han-
     dling of detected but not yet specified events.

*    Specified that unexpected timers or digits should trigger



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     transmission of the dialed string.

*    Removed the digit map syntax description from section 2.1.5 (it was
     redundant with section 3.4.)

*    Corrected miscellaneous bugs in the formal syntax description.

*    Aligned specification of commands with the CableLabs NCS specifica-
     tion.  This mostly affects the AuditEndpoint and RestartInProgress
     commands.

*    Aligned the handling of retransmission with the CableLabs NCS
     specification.

*    Added the provisional response return code and corresponding
     behavior description.

*    Added an optional reason code parameter to restart in progress.

*    Added the possibility to audit the restart method, restart delay
     and reason code.

7.2.  Differences between draft-04 and draft-05

Differences are minor: corrected the copyright statement, and corrected
a bug in the formal description.

7.3.  Differences between draft-03 and draft-04

Draft 04 corrects a number of minor editing mistakes that were pointed
out during the review of draft 03, issued on February 1.

7.4.  Differences between draft-02 and draft-03

The main differences between draft-02, issued in January 22 1998, and
draft 03 are:

*    Introduced a discussion on endpoint types,

*    Introduced a discussion of the connection set-up procedure, and of
     the role of connection parameters,

*    Introduced a notation of the connection identifier within event
     names,

*    Documented the extension procedure for the LocalConnectionOptions
     parameter and for the ConnectionParameters parameter,




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*    Introduced a three-way handshake procedure, using a ResponseAck
     parameter, in order to allow gateways to delete copies of old
     responses without waiting for a 30 seconds timer,

*    Expanded the security section to include a discussion of "uncon-
     trolled barge-in."

*    Propsed a "create two connections" command, as an appendix.

7.5.  Differences between draft-01 and draft-02

The main differences between draft-01, issued in November 1998, and
draft 02 are:

*    Added an ABNF description of the protocol.

*    Specification of an EndpointConfiguration command,

*    Addition of a "two endpoints" mode in the create connection com-
     mand,

*    Modification of the package wildcards from "$/$" to "*/all" at the
     Request of early implementors,

*    Revision of some package definitions to better align with external
     specifications.

*    Addition of a specification for the handling of "failover."

*    Revision of the section on race conditions.

7.6.  The making of MGCP from IPDC and SGCP

MGCP version 0.1 results from the fusion of the SGCP and IPDC proposals.

7.7.  Changes between MGCP and initial versions of SGCP

MGCP version 0.1 (which subsumes SGCP version 1.2) introduces the fol-
lowing changes from SGCP version 1.1:

*    Protocol name changed to MGCP.

*    Introduce a formal wildcarding structure in the name of endpoints,
     inspired from IPDC, and detailed the usage of wildcard names in
     each operation.

*    Naming scheme for events, introducing a package structure inspired
     from IPDC.



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*    New operations for audit endpoint, audit connection (requested by
     the Cablelabs) and restart (inspired from IPDC).

*    New parameter to control the behavior of the notification request.

*    Improved text on the detection and handling of race conditions.

*    Syntax modification for event reporting, to incorporate package
     names.

*    Definition of basic event packages (inspired from IPDC).

*    Incorporation of mandatory and optional extension parameters,
     inspired by IPDC.

SGCP version 1.1 introduces the following changes from version SGCP 1.0:

*    Extension parameters (X-??:)

*    Error Code 511 (Unrecognized extension).

*    All event codes can be used in RequestEvent, SignalRequest and
     ObservedEvent parameters.

*    Error Code 512 (Not equipped to detect requested event).

*    Error Code 513 (Not equipped to generate requested signal).

*    Error Code 514 (Unrecognized announcement).

*    Specific Endpoint-ID can be returned in creation commands.

*    Changed the code for the ASDI display from "ad" to "asdi" to avoid
     conflict with the digits A and D.

*    Changed the code for the answer tone from "at" to "aw" to avoid
     conflict with the digit A and the timer mark T

*    Changed the code for the busy tone from "bt" to "bz" to avoid con-
     flict with the digit B and the timer mark T

*    Specified that the continuity tone value is "co" (CT was
     incorrectly used in several instances; CT conflicts with .)

*    Changed the code for the dial tone from "dt" to "dl" to avoid con-
     flict with the digit D and the timer mark T

*    Added a code point for announcement requests.



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*    Added a code point for the "wink" event.

*    Set the "octet received" code in the "Connection Parameters" to
     "OR" (was set to RO, but then "OR" was used throughout all exam-
     ples.)

*    Added a "data" mode.

*    Added a description of SDP parameters for the network access mode
     (NAS).

*    Added four flow diagrams for the network access mode.

*    Incorporated numerous editing suggestions to make the description
     easier to understand. In particular, cleared the confusion between
     requests, queries, functions and commands.

*    Defined the continuity test mode as specifying a dual-tone tran-
     sponder, while the loopback mode can be used for a single tone
     test.

*    Added event code "OC", operation completed.

*    Added the specification of the "quarantine list", which clarifies
     the expected handling of events and notifications.

*    Added the specification of a "wildcard delete" operation.

8.  Acknowledgements

We want to thank here the many reviewers who provided us with advice on
the design of SGCP and then MGCP, notably Flemming Andreasen, Sankar
Ardhanari, Francois Berard, David Auerbach, Bob Biskner, David Bukovin-
sky, Barry Hoffner, Oren Kudevitzki, Dave Oran, Jeff Orwick, John Pick-
ens, Lou Rubin, Chip Sharp, Paul Sijben, Kurt Steinbrenner, Joe Stone
and Stuart Wray.

The version 0.1 of MGCP is 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 have been 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.



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9.  References

*    Schulzrinne, H., Casner, S., Frederick, R. and V. Jacobson, "RTP: A
     Transport Protocol for Real-Time Applications", RFC 1889, January
     1996.

*    Schulzrinne, H., "RTP Profile for Audio and Video Conferences with
     Minimal Control", RFC 1890, January 1996

*    Handley, M, Jacobson, V., "SDP: Session Description Protocol", RFC
     2327, April 1998.

*    Handley, M., "SAP - Session Announcement Protocol", Work in Pro-
     gress.

*    Handley, M., Schooler, E., and H. Schulzrinne, "Session Initiation
     Protocol (SIP)", Work in Progress.

*    Schulzrinne, H., Rao, A., and R. Lanphier, "Real Time Streaming
     Protocol (RTSP)", RFC 2326, April 1998.

*    ITU-T, Recommendation Q.761, "FUNCTIONAL DESCRIPTION OF THE ISDN
     USER PART OF SIGNALLING SYSTEM No. 7", (Malaga-Torremolinos, 1984;
     modified at Helsinki, 1993)

*    ITU-T, Recommendation Q.762, "GENERAL FUNCTION OF MESSAGES AND SIG-
     NALS OF THE ISDN USER PART OF SIGNALLING SYSTEM No. 7", (Malaga-
     Torremolinos, 1984; modified at Helsinki, 1993)

*    ITU-T, Recommendation H.323 (02/98), "PACKET-BASED MULTIMEDIA COM-
     MUNICATIONS SYSTEMS."

*    ITU-T, Recommendation H.225, "Call Signaling Protocols and Media
     Stream Packetization for Packet Based Multimedia Communications
     Systems."

*    ITU-T, Recommendation H.245 (02/98), "CONTROL PROTOCOL FOR MUL-
     TIMEDIA COMMUNICATION."

*    Kent, S., R. Atkinson, "Security Architecture for the Internet Pro-
     tocol." RFC 2401, November 1998.

*    Kent, S., R. Atkinson, "IP Authentication Header." RFC 2402,
     November 1998.

*    Kent, S., R. Atkinson, "IP Encapsulating Security Payload (ESP)."
     RFC 2406, November 1998.




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*    Crocker, D., P. Overell, "Augmented BNF for Syntax Specifications:
     ABNF", RFC 2234, November 1997.

10.  Authors' Addresses

                   Mauricio Arango
                   RSL COM Latin America
                   6300 N.W. 5th Way, Suite 100
                   Ft. Lauderdale, FL 33309

                   Phone: (954) 492-0913
                   Email: marango@rslcom.com

                   Andrew Dugan
                   Level3 Communications
                   1450 Infinite Drive
                   Louisville, CO 80027

                   Phone: (303)926 3123
                   Email: andrew.dugan@l3.com

                   Isaac Elliott
                   Level3 Communications
                   1450 Infinite Drive
                   Louisville, CO 80027

                   Phone: (303)926 3123
                   Email: ike.elliott@l3.com

                   Christian Huitema
                   Telcordia Technologies
                   MCC 1J236B
                   445 South Street
                   Morristown, NJ 07960
                   U.S.A.

                   Phone: +1 973-829-4266
                   EMail: huitema@research.telcordia.com

                   Scott Pickett
                   Vertical Networks
                   1148 East Arques Ave
                   Sunnyvale, CA 94086

                   Phone: (408) 523-9700 extension 200
                   Email: ScottP@vertical.com





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Further information is available on the SGCP web site:

        http://sgcp.bellcore.com
















































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11.  Appendix A: Proposed "MoveConnection" command

It has been proposed to create a new command, that would move an exist-
ing connection from one endpoint to another, on the same gateway.  This
command would be specially useful for handling certain call services,
such as call forwarding between endpoints served by the same gateway.

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


The parameters used are the same as in the ModifyConnection command,
with the addition of a SecondEndpointId that identifies the endpoint
towards which the connection is moved.

The EndpointId should be the fully qualified endpoint identifier of the
endpoint on which the connection has been created. The local name shall
not use the wildcard convention.

The SecondEndpointId shall be the endpoint identifier of the endpoint
towards which the connection has been created. The "any of" wildcard
convention can be used, but not the "all of" convention.  If the Secon-
dEndpointId parameter is unqualified, the gateway will choose a value,
that will be returned to the call agent as a response parameter.

The command will result in the "move" of the existing connection to the
second endpoint.  Depending on gateway implementations, the connection
identifier of the connection after the move may or may not be the same
as the connection identifier before the move.  If it is not the same,
the new value is returned as a response parameter.

The intent of the command is to effect a local relocation of the connec-
tion, without having to modify such transmission parameters as IP
addresses and port, and thus without forcing the call agent to signal
the change of parameters to the remote gateway, at the other end of the
connection.  However, gateway architectures may not always allow such
transparent moves.  For example, some architectures could allow specific



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IP addresses to different boards that handles specific group of end-
points.  If for any reason the transmission parameters have to be
changed as a result of the move, the new LocalConnectionDescriptor is
returned as a response parameter.

The LocalConnectionOptions, Mode, and RemoteConnectionDescriptor, when
present, are applied after the move.

The RequestedEvents, RequestIdentifier, DigitMap, SignalRequests,
QuarantineHandling and DetectEvents parameters are optional.  They can
be used by the Call Agent to transmit a NotificationRequest that is exe-
cuted simultaneously with the move of the connection. When these parame-
ters are present, the NotificationRequest applies to the second end-
point.

When these parameters are present, the move and the NotificationRequests
should be synchronized, which means that both should be accepted, or
both refused.  The NotifiedEntity parameter, if present, applies to both
the ModifyConnection and the NotificationRequest command.

The command may carry an encapsulated EndpointConfiguration command,
that will also apply to the second endpoint.  When this command is
present, the parameters of the EndpointConfiguration command are
inserted after the normal parameters of the MoveConnection with the
exception of the SecondEndpointId, which is not replicated. The End-
pointConfiguration command may be encapsulated together with an encapsu-
lated NotificationRequest command.

The encapsulated EndpointConfiguration command shares the fate of the
MoveConnection command.  If the MoveConnection is rejected, the End-
pointConfiguration is not executed.

11.1.  Proposed syntax modification

The only syntax modification necessary for the addition of the moveCon-
nection command is the addition of the keyword MOVE to the authorized
values in the MGCPVerb clause of the formal syntax.














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