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Versions: 00 01 02 03 04 05 06 07 RFC 2885

Internet Engineering Task Force                        Fernando Cuervo
INTERNET DRAFT                                         Nortel Networks
February 21, 2000                                           Bryan Hill
Expires August 21, 2000                                Gotham Networks
<draft-ietf-megaco-protocol-07.txt>                       Nancy Greene
                                                       Nortel Networks
                                                     Christian Huitema
                                                Telcordia Technologies
                                                       Abdallah Rayhan
                                                       Nortel Networks
                                                           Brian Rosen
                                                               Marconi
                                                           John Segers
                                                   Lucent Technologies


                            Megaco Protocol




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
Force (IETF), its areas, and its working groups.  Note that other groups
may also distribute working documents as Internet-Drafts.

Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time.  It is inappropriate to use Internet- Drafts as reference material
or to cite them other than as "work in progress."

The list of current Internet-Drafts can be accessed at
http://www.ietf.org/ietf/1id-abstracts.txt

The list of Internet-Draft Shadow Directories can be accessed at
http://www.ietf.org/shadow.html.

This document will expire in July 2000.









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1.  SCOPE .....................................................  8
2.  REFERENCES ................................................  8
   2.1.  Normative references .................................  8
   2.2.  Informative references ............................... 10
3.  DEFINITIONS ............................................... 11
4.  ABBREVIATIONS ............................................. 12
5.  CONVENTIONS ............................................... 12
6.  CONNECTION MODEL .......................................... 12
   6.1.  Contexts ............................................. 15
      6.1.1.  Context Attributes and Descriptors .............. 16
      6.1.2.  Creating, Deleting and Modifying Contexts ....... 16
   6.2.  Terminations ......................................... 16
      6.2.1.  Termination Dynamics ............................ 17
      6.2.2.  TerminationIDs .................................. 17
      6.2.3.  Packages ........................................ 18
      6.2.4.  Termination Properties and Descriptors .......... 18
      6.2.5.  Root Termination ................................ 20
7.  COMMANDS .................................................. 21
   7.1.  Descriptors .......................................... 22
      7.1.1.  Specifying Parameters ........................... 22
      7.1.2.  Modem Descriptor ................................ 23
      7.1.3.  Multiplex Descriptor ............................ 23
      7.1.4.  Media Descriptor ................................ 23
      7.1.5.  Termination State Descriptor .................... 24
      7.1.6.  Stream Descriptor ............................... 24
      7.1.7.  LocalControl Descriptor ......................... 25
      7.1.8.  Local and Remote Descriptors .................... 26
      7.1.9.  Events Descriptor ............................... 29
      7.1.10.  EventBuffer Descriptor ......................... 31
      7.1.11.  Signals Descriptor ............................. 31
      7.1.12.  Audit Descriptor ............................... 33
      7.1.13.  ServiceChange Descriptor ....................... 34
      7.1.14.  DigitMap Descriptor ............................ 34
      7.1.15.  Statistics Descriptor .......................... 39
      7.1.16.  Packages Descriptor ............................ 39
      7.1.17.  ObservedEvents Descriptor ...................... 39
      7.1.18.  Topology Descriptor ............................ 39
   7.2.  Command Application Programming Interface ............ 42
      7.2.1.  Add ............................................. 42
      7.2.2.  Modify .......................................... 44
      7.2.3.  Subtract ........................................ 44
      7.2.4.  Move ............................................ 45
      7.2.5.  AuditValue ...................................... 46
      7.2.6.  AuditCapabilities ............................... 48
      7.2.7.  Notify .......................................... 49
      7.2.8.  ServiceChange ................................... 49
      7.2.9.  Manipulating and Auditing Context Attributes .... 53
      7.2.10.  Generic Command Syntax ......................... 54



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   7.3.  Command Error Codes .................................. 54
8.  TRANSACTIONS .............................................. 56
   8.1.  Common Parameters .................................... 57
      8.1.1.  Transaction Identifiers ......................... 57
      8.1.2.  Context Identifiers ............................. 57
   8.2.  Transaction Application Programming Interface ........ 58
      8.2.1.  TransactionRequest .............................. 58
      8.2.2.  TransactionReply ................................ 58
      8.2.3.  TransactionPending .............................. 59
   8.3.  Messages ............................................. 60
9.  TRANSPORT ................................................. 60
   9.1.  Ordering of Commands ................................. 61
   9.2.  Protection against Restart Avalanche ................. 62
10.  SECURITY CONSIDERATIONS .................................. 63
   10.1.  Protection of Protocol Connections .................. 63
   10.2.  Interim AH scheme ................................... 64
   10.3.  Protection of Media Connections ..................... 65
11.  MG-MGC CONTROL INTERFACE ................................. 65
   11.1.  Multiple Virtual MGs ................................ 66
   11.2.  Cold Start .......................................... 67
   11.3.  Negotiation of Protocol Version ..................... 67
   11.4.  Failure of an MG .................................... 68
   11.5.  Failure of an MGC ................................... 68
12.  PACKAGE DEFINITION ....................................... 69
   12.1.  Guidelines for defining packages .................... 70
      12.1.1.  Package ........................................ 70
      12.1.2.  Properties ..................................... 71
      12.1.3.  Events ......................................... 71
      12.1.4.  Signals ........................................ 72
      12.1.5.  Statistics ..................................... 72
      12.1.6.  Procedures ..................................... 72
   12.2.  Guidelines to defining  Properties, Statistics and .. 72
   12.3.  Lists ............................................... 73
   12.4.  Identifiers ......................................... 73
   12.5.  Package Registration ................................ 73
13.  IANA CONSIDERATIONS ...................................... 73
   13.1.  Packages ............................................ 73
   13.2.  Error Codes ......................................... 74
   13.3.  ServiceChange Reasons ............................... 74
ANNEX A BINARY ENCODING OF THE PROTOCOL (NORMATIVE) ........... 76
   A.1.  Coding of wildcards .................................. 76
   A.2.  ASN.1 syntax specification ........................... 78
   A.3.  Digit maps and path names ............................ 93
ANNEX B TEXT ENCODING OF THE PROTOCOL (NORMATIVE) ............. 94
   B.1.  Coding of wildcards .................................. 95
   B.2.  ABNF specification ................................... 95
ANNEX C TAGS FOR MEDIA STREAM PROPERTIES (NORMATIVE) ..........106



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   C.1.  General Media Attributes .............................107
   C.2.  Mux Properties .......................................107
   C.3.  General Bearer Properties ............................107
   C.4.  General ATM Properties ...............................108
   C.5.  Frame Relay ..........................................108
   C.6.  IP ...................................................109
   C.7.  ATM AAL2 .............................................109
   C.8.  ATM AAL1 .............................................109
   C.9.  Bearer Capabilities ..................................110
   C.10.  AAL5 Properties .....................................111
   C.11.  SDP Equivalents .....................................112
   C.12.  H.245 ...............................................112
ANNEX D TRANSPORT OVER IP (NORMATIVE) .........................112
   D.1.  Transport over IP/UDP using Application Level ........112
      D.1.1.  Providing At-Most-Once Functionality ............113
      D.1.2.  Transaction identifiers and three-way handshake  113
         D.1.2.1.  Transaction identifiers ....................113
         D.1.2.2.  Three-way handshake ........................114
      D.1.3.  Computing retransmission timers .................114
      D.1.4.  Provisional responses ...........................115
      D.1.5.  Repeating Requests, Responses and ...............116
   D.2.  using TCP ............................................117
      D.2.1.  Providing the At-Most-Once functionality ........117
      D.2.2.  Transaction identifiers and three way handshake  118
      D.2.3.  Computing retransmission timers .................118
      D.2.4.  Provisional responses ...........................118
      D.2.5.  Ordering of commands ............................118
ANNEX E BASIC PACKAGES ........................................118
   E.1.  Generic ..............................................118
      E.1.1.  Properties ......................................119
      E.1.2.  Events ..........................................119
      E.3.1.  Properties ......................................122
      E.3.2.  Events ..........................................123
      E.3.3.  Signals .........................................123
      E.3.4.  Statistics ......................................123
      E.3.5.  Procedures ......................................123
   E.4.  Tone Detection Package ...............................123
      E.4.1.  Properties ......................................124
      E.4.2.  Events ..........................................124
      E.4.3.  Signals .........................................125
      E.4.4.  Statistics ......................................125
      E.4.5.  Procedures ......................................125
   E.5.  Basic DTMF Generator Package .........................125
      E.5.1.  Properties ......................................126
      E.5.2.  Events ..........................................126



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      E.5.3.  Signals .........................................126
      E.5.4.  Statistics ......................................127
      E.5.5.  Procedures ......................................127
   E.6.  DTMF detection Package ...............................127
      E.6.1.  Properties ......................................128
      E.6.2.  Events ..........................................128
      E.6.3.  Signals .........................................128
      E.6.4.  Statistics ......................................129
      E.6.5.  Procedures ......................................129
   E.7.  Call Progress Tones Generator Package ................129
      E.7.1.  Properties ......................................129
      E.7.2.  Events ..........................................129
      E.7.3.  Signals .........................................129
      E.7.4.  Statistics ......................................130
      E.7.5.  Procedures ......................................130
   E.8.  Call Progress Tones Detection Package ................130
      E.8.1.  Properties ......................................130
      E.8.2.  Events ..........................................130
      E.8.3.  Signals .........................................130
      E.8.4.  Statistics ......................................131
      E.8.5.  Procedures ......................................131
   E.9.  Analog Line Supervision Package ......................131
      E.9.1.  Properties ......................................131
      E.9.2.  Events ..........................................131
      E.9.3.  Signals .........................................132
      E.9.4.  Statistics ......................................132
      E.9.5.  Procedures ......................................132
   E.10.  Basic Continuity Package ............................132
      E.10.1.  Properties .....................................133
      E.10.2.  Events .........................................133
      E.10.3.  Signals ........................................133
      E.10.4.  Statistics .....................................133
      E.10.5.  Procedures .....................................134
   E.11.  Network Package .....................................134
      E.11.1.  Properties .....................................134
      E.11.2.  Events .........................................134
      E.11.3.  Signals ........................................135
      E.11.4.  Statistics .....................................135
      E.11.5.  Procedures .....................................136
   E.12.  RTP Package .........................................136
      E.12.1.  Properties .....................................136
      E.12.2.  Events .........................................136
      E.12.3.  Signals ........................................137
      E.12.4.  Statistics .....................................137
      E.12.5.  Procedures .....................................137
   E.13.  TDM Circuit  Package ................................137
      E.13.1.  Properties .....................................138
      E.13.2.  Events .........................................138



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      E.13.3.  Signals ........................................138
      E.13.4.  Statistics .....................................138
      E.13.5.  Procedures .....................................138
APPENDIX A      EXAMPLE CALL FLOWS (INFORMATIVE) ..............139
    A.1.  Residential Gateway to Residential Gateway Call .....139
       A.1.1.  Programming Residential GW Analog Line .........139
       A.1.2. Collecting Originator Digits ....................141












































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TABLE OF FIGURES
Figure 1 Example of MEGACOH.248 Connection Model................13
Figure 2 Example Call Waiting Scenario / Alerting Applied to T1.14
Figure 3 Example Call Waiting Scenario / Answer by T1...........15
Figure 4 Example topologies......................... ...........41
Figure 5 Transactions, Actions and Commands.....................56













































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1.  SCOPE

MEGACO defines the protocols used between elements of a physically
decomposed multimedia gateway.  There are no functional differences from
a system view between a decomposed gateway, with distributed sub-
components potentially on more than one physical device, and a monol-
ithic gateway such as described in H.246. This document does not define
how gateways, multipoint control units or integrated voice response
units (IVRs) work.  Instead it creates a general framework that is suit-
able for these applications.

Packet network interfaces may include IP, ATM or possibly others.  The
interfaces will support a variety of SCN signalling systems, including
tone signalling, ISDN, ISUP, QSIG, and GSM. National variants of these
signalling systems will be supported where applicable.

The protocol definition in this document is common text with ITU Recom-
mendation H.248.

2.  REFERENCES

2.1.  Normative references

ITU-T Recommendation H.225.0 (1998): "Call Signalling Protocols and
Media Stream Packetization for Packet Based Multimedia Communications
Systems".

ITU-T Recommendation H.235 (02/98): "Security and encryption for H-
Series (H.323 and other H.245-based) multimedia terminals".

ITU-T Recommendation H.245 (1998): "Control Protocol for Multimedia Com-
munication".

ITU-T Recommendation H.323 (1998): "Packet Based Multimedia Communica-
tion Systems".

ITU-T Recommendation I.363.1 (08/96), "B-ISDN ATM Adaptation Layer
specification: Type 1 AAL".

ITU-T Recommendation I.363.2 (09/97), "B-ISDN ATM Adaptation Layer
specification: Type 2 AAL".

ITU-T Recommendation I.366.1 (06/98), "Segmentation and Reassembly Ser-
vice Specific Convergence Sublayer for the AAL type 2".

ITU-T Recommendation I.366.2 (02/99), "AAL type 2 service specific con-
vergence sublayer for trunking".




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ITU-T Recommendation I.371 (08/96), "Traffic control and congestion con-
trol in B- ISDN".

ITU-T Recommendation Q.763 (09/97), "Signalling System No. 7 - ISDN user
part formats and codes".

ITU-T Recommendation Q.765, "Signalling System No. 7 - Application tran-
sport mechanism".

ITU-T Recommendation Q.931 (05/98): "Digital Subscriber Signalling Sys-
tem No.  1 (DSS 1) - ISDN User-Network Interface Layer 3 Specification
for Basic Call Control".

ITU-T Recommendation Q.2630.1 (1999), "AAL Type 2 Signalling Protocol
(Capability Set 1)".

ITU-T Recommendation Q.2931 (10/95), "Broadband Integrated Services
Digital Network (B-ISDN) - Digital Subscriber Signalling System No. 2
(DSS 2) - User- Network Interface (UNI) - Layer 3 specification for
basic call/connection control".

ITU-T Recommendation Q.2941.1 (09/97), "Digital Subscriber Signalling
System No. 2 - Generic Identifier Transport".

ITU-T Recommendation Q.2961 (10/95), "Broadband integrated services
digital network (B-ISDN) - Digital subscriber signalling system no.2
(DSS 2) - additional traffic parameters".

ITU-T Recommendation Q.2961.2 (06/97), "Digital subscriber signalling
system No. 2 - Additional traffic parameters: Support of ATM transfer
capability in the broadband bearer capability information element."

ITU-T Recommendation X.213 (11/1995), "Information technology - Open
System Interconnection - Network service definition plus Amendment 1
(08/1997), Addition of the Internet protocol address format identifier".

ITU-T Recommendation V.76 (08/96), "Generic multiplexer using V.42
LAPM-based procedures".

ITU-T Recommendation X.680 (1997): "Information technology-Abstract Syn-
tax Notation One (ASN.1): Specification of basic notation".

ITU-T Recommendation H.246 (1998), "Interworking of H-series multimedia
terminals with H-series multimedia terminals and voice/voiceband termi-
nals on GSTN and ISDN".

RFC 1006, "ISO Transport Service on top of the TCP, Version 3", Marshall
T. Rose, Dwight E. Cass, May 1987.



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

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

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

RFC 2406, "IP Encapsulating Security Payload (ESP)", S. Kent, R. Atkin-
son, November 1998.

2.2.  Informative references

ITU-T Recommendation E.180/Q.35 (1998): "Technical characteristics of
tones for the telephone service".

CCITT Recommendation G.711 (1988), "Pulse Code Modulation (PCM) of voice
frequencies".

ITU-T Recommendation H.221 (05/99),"Frame structure for a 64 to 1920
kbit/s channel in audiovisual teleservices".

ITU-T Recommendation H.223 (1996), "Multiplexing protocol for low bit
rate multimedia communication".

ITU-T Recommendation Q.724 (1988): "Signalling procedures".

RFC 768, "User Datagram Protocol", J.Postel, August 1980.

RFC 791, "Internet protocol", J.Postel, September 1981.

RFC 793, "TRANSMISSION CONTROL PROTOCOL", J.Postel, September 1981.

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

RFC 1890, "RTP Profile for Audio and Video Conferences with Minimal Con-
trol", H. Schulzrinne, January 1996.

RFC 2401, "Security Architecture for the Internet Protocol", S. Kent, R.
Atkinson, November 1998.

RFC 2543, " SIP: Session Initiation Protocol", M. Handley, H.
Schulzrinne, E. Schooler, J. Rosenberg, March 1999.

RFC 2460, "Internet Protocol, Version 6 (IPv6) Specification", S. Deer-
ing, R. Hinden, December 1998.



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3.  DEFINITIONS

Access Gateway: A type of gateway that provides a User to Network Inter-
face (UNI) such as ISDN.

Descriptor: A syntactic element of the protocol that groups related pro-
perties.  For instance, the properties of a media flow on the MG can be
set by the MGC by including the appropriate descriptor in a command.

Media Gateway (MG): The media gateway converts media provided in one
type of network to the format required in another type of network. For
example, a MG could terminate bearer channels from a switched circuit
network (e.g., DS0s) and media streams from a packet network (e.g., RTP
streams in an IP network).  This gateway may be capable of processing
audio, video and T.120 alone or in any combination, and will be capable
of full duplex media translations. The MG may also play audio/video mes-
sages and performs other IVR functions, or may perform media conferenc-
ing.

Media Gateway Controller (MGC): Controls the parts of the call state
that pertain to connection control for media channels in a MG.

Multipoint Control Unit (MCU): An entity that controls the setup and
coordination of a multi- user conference that typically includes pro-
cessing of audio, video and data.

Residential Gateway: A gateway that interworks an analogue line to a
packet network. A residential gateway typically contains one or two
analogue lines and is located at the customer premises.

SCN FAS Signalling Gateway: This function contains the SCN Signalling
Interface that terminates SS7, ISDN or other signalling links where the
call control channel and bearer channels are collocated in the same phy-
sical span.

SCN NFAS Signalling Gateway: This function contains the SCN Signalling
Interface that terminates SS7 or other signalling links where the call
control channels are separated from bearer channels.

Stream: Bidirectional media or control flow received/sent by a media
gateway as part of a call or conference.

Trunk: A communication channel between two switching systems such as a
DS0 on a T1 or E1 line.

Trunking Gateway: A gateway between SCN network and packet network that
typically terminates a large number of digital circuits.




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4.  ABBREVIATIONS

This recommendation defines the following terms.
ATM             Asynchronous Transfer Mode
BRI             Basic Rate Interface
CAS             Channel Associated Signalling
DTMF            Dual Tone Multi-Frequency
FAS             Facility Associated Signalling
GW              GateWay
IANA            Internet Assigned Numbers Authority
IP              Internet Protocol
ISUP            ISDN User Part
MG              Media Gateway
MGC             Media Gateway Controller
NFAS            Non-Facility Associated Signalling
PRI             Primary Rate Interface
PSTN            Public Switched Telephone Network
QoS             Quality of Service
RTP             Real-time Transport Protocol
SCN             Switched Circuit Network
SG              Signalling Gateway
SS7             Signalling System No. 7

5.  CONVENTIONS

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC2119.

6.  CONNECTION MODEL

The connection model for the protocol describes the logical entities, or
objects, within the Media Gateway that can be controlled by the Media
Gateway Controller.  The main abstractions used in the connection model
are Terminations and Contexts.

A Termination sources and/or sinks one or more streams.  In a multimedia
conference, a Termination can be multimedia and sources or sinks multi-
ple media streams.  The media stream parameters, as well as modem, and
bearer parameters are encapsulated within the Termination.

A Context is an association between a collection of Terminations. There
is a special type of Context, the null Context, which contains all Ter-
minations that are not associated to any other Termination.  For
instance, in a decomposed access gateway, all idle lines are represented
by Terminations in the null Context.

Following is a graphical depiction of these concepts.  The diagram of



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Figure 1 gives several examples and is not meant to be an all-inclusive
illustration.  The asterisk box in each of the Contexts represents the
logical association of Terminations implied by the Context.


          +------------------------------------------------------+
          |Media Gateway                                         |
          | +-------------------------------------------------+  |
          | |Context                          +-------------+ |  |
          | |                                 | Termination | |  |
          | |                                 |-------------| |  |
          | |  +-------------+             +->| SCN Bearer  |<---+->
          | |  | Termination |   +-----+   |  |   Channel   | |  |
          | |  |-------------|   |     |---+  +-------------+ |  |
        <-+--->| RTP Stream  |---|  *  |                      |  |
          | |  |             |   |     |---+  +-------------+ |  |
          | |  +-------------+   +-----+   |  | Termination | |  |
          | |                              |  |-------------| |  |
          | |                              +->| SCN Bearer  |<---+->
          | |                                 |   Channel   | |  |
          | |                                 +-------------+ |  |
          | +-------------------------------------------------+  |
          |                                                      |
          |                                                      |
          |                    +------------------------------+  |
          |                    |Context                       |  |
          |  +-------------+   |              +-------------+ |  |
          |  | Termination |   | +-----+      | Termination | |  |
          |  |-------------|   | |     |      |-------------| |  |
        <-+->| SCN Bearer  |   | |  *  |------| SCN Bearer  |<---+->
          |  |   Channel   |   | |     |      |   Channel   | |  |
          |  +-------------+   | +-----+      +-------------+ |  |
          |                    +------------------------------+  |
          |                                                      |
          |                                                      |
          | +-------------------------------------------------+  |
          | |Context                                          |  |
          | |  +-------------+                +-------------+ |  |
          | |  | Termination |   +-----+      | Termination | |  |
          | |  |-------------|   |     |      |-------------| |  |
        <-+--->| SCN Bearer  |---|  *  |------| SCN Bearer  |<---+->
          | |  |   Channel   |   |     |      |   Channel   | |  |
          | |  +-------------+   +-----+      +-------------+ |  |
          | +-------------------------------------------------+  |
          | ___________________________________________________  |
          +------------------------------------------------------+
                Figure 1: Example of MEGACO Connection Model




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The example below shows an example of one way to accomplish a call-
waiting scenario in a decomposed access gateway, illustrating the relo-
cation of a Termination between Contexts. Terminations T1 and T2 belong
to Context C1 in a two-way audio call.  A second audio call is waiting
for T1 from Termination T3.  T3 is alone in Context C2.  T1 accepts the
call from T3, placing T2 on hold.  This action results in T1 moving into
Context C2, as shown below.


          +------------------------------------------------------+
          |Media Gateway                                         |
          | +-------------------------------------------------+  |
          | |Context C1                                       |  |
          | |  +-------------+                +-------------+ |  |
          | |  | Term. T2    |   +-----+      | Term. T1    | |  |
          | |  |-------------|   |     |      |-------------| |  |
        <-+--->| RTP Stream  |---|  *  |------| SCN Bearer  |<---+->
          | |  |             |   |     |      |   Channel   | |  |
          | |  +-------------+   +-----+      +-------------+ |  |
          | +-------------------------------------------------+  |
          |                                                      |
          | +-------------------------------------------------+  |
          | |Context C2                                       |  |
          | |                                 +-------------+ |  |
          | |                    +-----+      | Term. T3    | |  |
          | |                    |     |      |-------------| |  |
          | |                    |  *  |------| SCN Bearer  |<---+->
          | |                    |     |      |   Channel   | |  |
          | |                    +-----+      +-------------+ |  |
          | +-------------------------------------------------+  |
          +------------------------------------------------------+
        Figure 2 Example Call Waiting Scenario / Alerting Applied to T1



















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          +------------------------------------------------------+
          |Media Gateway                                         |
          | +-------------------------------------------------+  |
          | |Context C1                                       |  |
          | |  +-------------+                                |  |
          | |  | Term. T2    |   +-----+                      |  |
          | |  |-------------|   |     |                      |  |
        <-+--->| RTP Stream  |---|  *  |                      |  |
          | |  |             |   |     |                      |  |
          | |  +-------------+   +-----+                      |  |
          | +-------------------------------------------------+  |
          |                                                      |
          | +-------------------------------------------------+  |
          | |Context C2                                       |  |
          | |  +-------------+                +-------------+ |  |
          | |  | Term. T1    |   +-----+      | Term. T3    | |  |
          | |  |-------------|   |     |      |-------------| |  |
        <-+--->| SCN Bearer  |---|  *  |------| SCN Bearer  |<---+->
          | |  |   Channel   |   |     |      |   Channel   | |  |
          | |  +-------------+   +-----+      +-------------+ |  |
          | +-------------------------------------------------+  |
          +------------------------------------------------------+
        Figure 3. Example Call Waiting Scenario / Answer by T1


6.1.  Contexts

A Context is an association between a number of Terminations.  The Con-
text describes the topology (who hears/sees whom) and the media mixing
and/or switching parameters if more than two Terminations are involved
in the association.

There is a special Context called the null Context. It contains Termina-
tions that are not associated to any other Termination.  Terminations in
the null Context can have their parameters examined or modified, and may
have events detected on them.

In general, an Add command is used to add Terminations to Contexts.  If
the MGC does not specify an existing Context to which the Termination is
to be added, the MG creates a new Context.  A Termination may be removed
from a Context with a Subtract command, and a Termination may be moved
from one Context to another with a Move command. A Termination SHALL
exist in only one Context at a time.

The maximum number of Terminations in a Context is a MG property. Media
gateways that offer only point-to-point connectivity might allow at most
two Terminations per Context. Media gateways that support multipoint



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conferences might allow three or more terminations per Context.

6.1.1.  Context Attributes and Descriptors

The attributes of Contexts are:

*    ContextID.

*    The topology (who hears/sees whom).  The topology of a Context
     describes the flow of media between the Terminations within a Con-
     text.  In contrast, the mode of a Termination (send/receive/...)
     describes the flow of the media at the ingress/egress of the media
     gateway.

*    The priority is used for a context in order to provide the MG with
     information about a certain precedence handling for a context. The
     MGC can also use the priority to control autonomously the traffic
     precedence in the MG in a smooth way in certain situations (e.g.
     restart), when a lot of contexts must be handled simultaneously.

*    An indicator for an emergency call is also provided to allow a
     preference handling in the MG.

6.1.2.  Creating, Deleting and Modifying Contexts

The protocol can be used to (implicitly) create Contexts and modify the
parameter values of existing Contexts.  The protocol has commands to add
Terminations to Contexts, subtract them from Contexts, and to move Ter-
minations between Contexts.  Contexts are deleted implicitly when the
last remaining Termination is subtracted or moved out.

6.2.  Terminations

A Termination is a logical entity on a MG that sources and/or sinks
media and/or control streams.  A Termination is described by a number of
characterizing Properties, which are grouped in a set of Descriptors
that are included in commands. Terminations have unique identities (Ter-
minationIDs), assigned by the MG at the time of their creation.

Terminations representing physical entities have a semi-permanent
existence.  For example, a Termination representing a TDM channel might
exist for as long as it is provisioned in the gateway.  Terminations
representing ephemeral information flows, such as RTP flows, would usu-
ally exist only for the duration of their use.

Ephemeral Terminations are created by means of an Add command.  They are
destroyed by means of a Subtract command.  In contrast, when a physical
Termination is Added to or Subtracted from a Context, it is taken from



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or to the null Context, respectively.

Terminations may have signals applied to them.  Signals are MG generated
media streams such as tones and announcements as well as line signals
such as hookswitch.  Terminations may be programmed to detect Events,
the occurrence of which can trigger notification messages to the MGC, or
action by the MG.  Statistics may be accumulated on a Termination.
Statistics are reported to the MGC upon request (by means of the Audit-
Value command, see section 7.2.5) and when the Termination is taken out
of the call it is in.

Multimedia gateways may process multiplexed media streams.  For example,
Recommendation H.221 describes a frame structure for multiple media
streams multiplexed on a number of digital 64 kbit/s channels.  Such a
case is handled in the connection model in the following way.  For every
bearer channel that carries part of the multiplexed streams, there is a
Termination.  The Terminations that source/sink the digital channels are
connected to a separate Termination called the multiplexing Termination.
This Termination describes the multiplex used (e.g. how the H.221 frames
are carried over the digital channels used).  The MuxDescriptor is used
to this end. If multiple media are carried, this Termination contains
multiple StreamDescriptors. The media streams can be associated with
streams sourced/sunk by other Terminations in the Context.

Terminations may be created which represent multiplexed bearers, such as
an ATM AAL2. When a new multiplexed bearer is to be created, an ephem-
eral termination is created in a context established for this purpose.
When the termination is subtracted, the multiplexed bearer is destroyed.

6.2.1.  Termination Dynamics

The protocol can be used to create new Terminations and to modify pro-
perty values of existing Terminations.  These modifications include the
possibility of adding or removing events and/or signals.  The Termina-
tion properties, and events and signals are described in the ensuing
sections. An MGC can only release/modify terminations and the resources
that the termination represents which it has previously seized via,
e.g., the Add command.

6.2.2.  TerminationIDs

Terminations are referenced by a TerminationID, which is an arbitrary
schema chosen by the MG.

TerminationIDs of physical Terminations are provisioned in the Media
Gateway. The TerminationIDs may be chosen to have structure.  For
instance, a TerminationID may consist of trunk group and a trunk within
the group.



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A wildcarding mechanism using two types of wildcards can be used with
TerminationIDs.  The two wildcards are ALL and CHOOSE.  The former is
used to address multiple Terminations at once, while the latter is used
to indicate to a media gateway that it must select a Termination satis-
fying the partially specified TerminationID.  This allows, for instance,
that a MGC instructs a MG to choose a circuit within a trunk group.

When ALL is used in the TerminationID of a command, the effect is ident-
ical to repeating the command with each of the matching TerminationIDs.
Since each of these commands may generate a response, the size of the
entire response may be large.  If individual responses are not required,
a wildcard response may be requested.  In such a case, a single response
is generated, which contains the UNION of all of the individual
responses which otherwise would have been generated, with duplicate
values suppressed.  Wildcard response may be particularly useful in the
Audit commands.

The encoding of the wildcarding mechanism is detailed in Annexes A and
B.

6.2.3.  Packages

Different types of gateways may implement Terminations that have widely
differing characteristics.  Variations in Terminations are accommodated
in the protocol by allowing Terminations to have optional Properties,
Events, Signals and Statistics implemented by MGs.

In order to achieve MG/MGC interoperability, such options are grouped
into Packages, and a Termination realizes a set of such Packages.  More
information on definition of packages can be found in section 12.  An
MGC can audit a Termination to determine which Packages it realizes.

Properties, Events, Signals and Statistics defined in Packages, as well
as parameters to them, are referenced by identifiers (Ids).  Identifiers
are scoped. For each package, PropertyIds, EventIds, SignalIds, Statis-
ticsIds and ParameterIds have unique name spaces and the same identifier
may be used in each of them.  Two PropertyIds in different packages may
also have the same identifier, etc.

6.2.4.  Termination Properties and Descriptors

Terminations have properties.  The properties have unique PropertyIDs.
Most properties have default values.  When a Termination is created,
properties get their default values, unless the controller specifically
sets a different value.  The default value of a property of a physical
Termination can be changed by setting it to a different value when the
Termination is in the null Context.  Every time such a Termination
returns to the null Context, the values of its properties are reset to



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this default value.

There are a number of common properties for Terminations and properties
specific to media streams. The common properties are also called the
termination state properties.  For each media stream, there are local
properties and properties of the received and transmitted flows.

Properties not included in the base protocol are defined in Packages.
These properties are referred to by a name consisting of the PackageName
and a PropertyId.  Most properties have default values described in the
Package description. Properties may be read-only or read/write. The pos-
sible values of a property may be audited, as can their current values.
For properties that are read/write, the MGC can set their values.  A
property may be declared as "Global" which has a single value shared by
all terminations realizing the package.  Related properties are grouped
into descriptors for convenience.

When a Termination is Added to a Context, the value of its read/write
properties can be set by including the appropriate descriptors as param-
eters to the Add command.  Properties not mentioned in the command
retain their prior values.  Similarly, a property of a Termination in a
Context may have its value changed by the Modify command.  Properties
not mentioned in the Modify command retain their prior values. Proper-
ties may also have their values changed when a Termination is moved from
one Context to another as a result of a Move command.  In some cases,
descriptors are returned as output from a command.  The following table
lists all of the possible Descriptors and their use.  Not all descrip-
tors are legal as input or output parameters to every command.


_________________________________________________________________________
|Descriptor Name   |Description                                         |
|__________________|____________________________________________________|
|Modem             |Identifies modem type and properties when           |
|                  |applicable                                          |
|__________________|____________________________________________________|
|Mux               |Describes multiplex type for multimedia terminations|
|                  |(e.g. H.221, H.223, H.225.0) and Terminations       |
|                  |forming the input mux.                              |
|__________________|____________________________________________________|
|Media             |A list of media stream specifications (see 7.1.4)   |
|__________________|____________________________________________________|
|TerminationState  |Properties of a Termination (which can be defined in|
|                  |Packages) that are not stream specific.             |
|__________________|____________________________________________________|
|Stream            |A list of remote/local/localControl descriptors for |
|                  |a single stream                                     | |__________________|____________________________________________________|
|Local             |Contains properties that specify the media flows    |



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|                  |that MG receives from the remote entity.            |
|__________________|____________________________________________________|
|                  |                                                    |
|Remote            |Contains properties that specify the media flows    |
|                  |that the MG sends to the remote entity.             | |__________________|____________________________________________________|
|LocalControl      |Contains properties (which can be defined in        |
|                  |packages) that are of interest between the MG and   |
|                  |the MGC                                             | |__________________|____________________________________________________|
|Events            |Describes events to be detected by the MG and what  |
|                  |to do when an event is detected                     | |__________________|____________________________________________________|
|EventBuffer       |Describes events to be detected by the MG when Event|
|                  |Buffering is active                                 | |__________________|____________________________________________________|
|Signals           |Describes signals and/or actions to be applied (e.g.|
|                  |Busy Tone) to the Terminations                      | |__________________|____________________________________________________|
|Audit             |In Audit commands, identifies which information is  |
|                  |desired                                             |
|__________________|____________________________________________________|
|Packages          |In AuditValue, returns a list of Packages realized  |
|                  |by a Termination                                    | |__________________|____________________________________________________|
|DigitMap          |Instructions for handling DTMF tones at the MG      |
|__________________|____________________________________________________|
|ServiceChange     |In ServiceChange, what, why service change occurred,|
|                  |etc.                                                |
|__________________|____________________________________________________|
|ObservedEvents    |In Notify or AuditValue, report of events observed  |
|__________________|____________________________________________________|
|Statistics        |In Subtract and Audit, Report of Statistics kept on |
|                  |a Termination.                                      | |__________________|____________________________________________________|


6.2.5.  Root Termination

Occasionally, a command must refer to the entire gateway, rather than a
termination within it.  A special TerminationID, "Root" is reserved for
this purpose.  Packages may be defined on Root. Root thus may have pro-
perties and events (signals  are not appropriate for root).  Accord-
ingly, the root TerminationID may appear in:

*    a Modify command - to change a property or set an event

*    a Notify command - to report an event

*    an AuditValue return - to examine the values of properties imple-
     mented on root

*    an AuditCapability - to determine what properties of root are
     implemented a ServiceChange - to declare the gateway in or out of
     service Any other use of the root TerminationID is an error.



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7.  COMMANDS

The protocol provides commands for manipulating the logical entities of
the protocol connection model, Contexts and Terminations.  Commands pro-
vide control at the finest level of granularity supported by the proto-
col.  For example, Commands exist to add Terminations to a Context,
modify Terminations, subtract Terminations from a Context, and audit
properties of Contexts or Terminations. Commands provide for complete
control of the properties of Contexts and Terminations.  This includes
specifying which events a Termination is to report, which
signals/actions are to be applied to a Termination and specifying the
topology of a Context (who hears/sees whom).

Most commands are for the specific use of the Media Gateway Controller
as command initiator in controlling Media Gateways as command
responders.  The exceptions are the Notify and ServiceChange commands:
Notify is sent from Media Gateway to Media Gateway Controller, and Ser-
viceChange may be sent by either entity.  Below is an overview of the
commands; they are explained in more detail in section 7.2.

1.   Add. The Add command adds a termination to a context.  The Add com-
     mand on the first Termination in a Context is used to create a Con-
     text.

2.   Modify. The Modify command modifies the properties, events and sig-
     nals of a termination.

3.   Subtract. The Subtract command disconnects a Termination from its
     Context and returns statistics on the Termination's participation
     in the Context.  The Subtract command on the last Termination in a
     Context deletes the Context.

4.   Move. The Move command atomically moves a Termination to another
     context.

5.   AuditValue. The AuditValue command returns the current state of
     properties, events,  signals and statistics of Terminations.

6.   AuditCapabilities. The AuditCapabilities command returns all the
     possible values for Termination properties, events and signals
     allowed by the Media Gateway.

7.   Notify. The Notify command allows the Media Gateway to inform the
     Media Gateway Controller of the occurrence of events in the Media
     Gateway.

8.   ServiceChange. The ServiceChange Command allows the Media Gateway
     to notify the Media Gateway Controller that a Termination or group



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     of Terminations is about to be taken out of service or has just
     been returned to service.   ServiceChange is also used by the MG to
     announce its availability to an MGC (registration), and to notify
     the MGC of impending or completed restart of the MG.  The MGC may
     announce a handover to the MG by sending it a ServiceChange com-
     mand.  The MGC may also use ServiceChange to instruct the MG to
     take a Termination or group of Terminations in or out of service.
     These commands are detailed in sections 7.2.1 through 7.2.8

7.1.  Descriptors

The parameters to a command are termed Descriptors. A Descriptor con-
sists of a name and a list of items. Some items may have values.  Many
Commands share common Descriptors.  This subsection enumerates these
Descriptors.  Descriptors may be returned as output from a command.
Parameters and parameter usage specific to a given Command type are
described in the subsection that describes the Command.

7.1.1.  Specifying Parameters

Command parameters are structured into a number of descriptors. In gen-
eral, the text format of descriptors is
DescriptorName=<someID>{parm=value, parm=value....}

Parameters may be fully specified, over-specified or under-specified:

1.   Fully specified parameters have a single, unambiguous value that
     the command initiator is instructing the command responder to use
     for the specified parameter.

2.   Under-specified parameters, using the CHOOSE value, allow the com-
     mand responder to choose any value it can support.

3.   Over-specified parameters have a list of potential values.  The
     list order specifies the command initiator's order of preference of
     selection.  The command responder chooses one value from the
     offered list and returns that value to the command initiator.
     Unspecified mandatory parameters (i.e. mandatory parameters not
     specified in a descriptor) result in the command responder retain-
     ing the previous value for that parameter.  Unspecified optional
     parameters result in the command responder using the default value
     of the parameter. Whenever a parameter is underspecified or over-
     specified, the descriptor containing the value chosen by the
     responder is included as output from the command.

Each command specifies the TerminationId the command operates on.  This
TerminationId may be "wildcarded".  When the TerminationId of a command
is wildcarded, the effect shall be as if the command was repeated with



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each of the TerminationIds matched.

7.1.2.  Modem Descriptor

The Modem descriptor specifies the modem type and parameters, if any,
required for use in e.g. H.324 and text conversation.  The descriptor
includes the following modem types: V.18, V.22, V.22bis, V.32, V.32bis,
V.34, V.90, V.91, Synchronous ISDN, and allows for extensions.  By
default, no modem descriptor is present in a Termination.

7.1.3.  Multiplex Descriptor

In multimedia calls, a number of media streams are carried on a (possi-
bly different) number of bearers.  The multiplex descriptor associates
the media and the bearers. The descriptor includes the multiplex type:

*    H.221

*    H.223,

*    H.226,

*    V.76,

*    Possible Extensions and a set of TerminationIDs representing the
     multiplexed inputs, in order.  For example:

Mux = H.221{ MyT3/1/2, MyT3/2/13, MyT3/3/6, MyT3/21/22}

7.1.4.  Media Descriptor

The Media Descriptor specifies the parameters for all the media streams.
These parameters are structured into two descriptors, a Termination
State Descriptor, which specifies the properties of a termination that
are not stream dependent, and one or more Stream Descriptors each of
which describes a single media stream.

A stream is identified by a StreamID.  The StreamID is used to link the
streams in a Context that belong together. Multiple streams exiting a
termination shall be synchronized with each other. Within the Stream
Descriptor, there are up to three subsidiary descriptors, LocalControl,
Local, and Remote. The relationship between these descriptors is thus:

Media Descriptor
        TerminationStateDescriptor
        Stream Descriptor
                LocalControl Descriptor
                Local Descriptor



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                Remote Descriptor


As a convenience a LocalControl, Local, or Remote descriptor may be
included in the Media Descriptor without an enclosing Stream descriptor.
In this case, the StreamID is assumed to be 1.

7.1.5.  Termination State Descriptor

The Termination State Descriptor contains the ServiceStates property,
the EventBufferControl property and properties of a termination (defined
in Packages) that are not stream specific.

The ServiceStates property describes the overall state of the termina-
tion (not stream-specific).  A Termination can be in one of the follow-
ing states: "test", "out of service", or "in service".  The "test" state
indicates that the termination is being tested. The state "out of ser-
vice" indicates that the termination cannot be used for traffic.  The
state "in service" indicates that a termination can be used or is being
used for normal traffic.  "in service" is the default state.

Values assigned to Properties may be simple values
(integer/string/enumeration) or may be underspecified, where more than
one value is supplied and the MG may make a choice:

*    Alternative Values - multiple values in a list, one of which must
     be selected

*    Ranges - minimum and maximum values, any value between min and max
     must be selected, boundary values included

*    Greater Than/Less Than - value must be greater/less than specified
     value

*    CHOOSE Wildcard - the MG chooses from the allowed values for the
     property The EventBufferControl property  specifies whether events
     are buffered following detection of an event in the Events Descrip-
     tor, or processed immediately.  See section 7.1.9 for details.

7.1.6.  Stream Descriptor

A Stream descriptor specifies the parameters of a single bi-directional
stream.  These parameters are structured into three descriptors: one
that contains termination properties specific to a stream and one each
for local and remote flows. The Stream Descriptor includes a StreamID
which identifies the stream.  Streams are created by specifying a new
StreamID on one of the terminations in a Context. A stream is deleted by
setting empty Local and Remote descriptors for the stream with



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ReserveGroup and ReserveValue in LocalControl set to "false" on all ter-
minations in the context that previously supported that stream.

StreamIDs are of local significance between MGC and MG and they are
assigned by the MGC. Within a context, StreamID is a means by which to
indicate which media flows are interconnected:  streams with the same
StreamID are connected.

If a termination is moved from one context to another, the effect on the
context to which the termination is moved is the same as in the case
that a new termination were added with the same StreamIDs as the moved
termination.

7.1.7.  LocalControl Descriptor

The LocalControl Descriptor contains the Mode property, the ReserveGroup
and ReserveValue properties and properties of a termination (defined in
Packages) that are stream specific, and are of interest between the MG
and the MGC.  Values of properties may be underspecified as in section
7.1.1.

The allowed values for the mode property are send-only, receive-only,
send/receive, inactive and loop-back.  "Send" and "receive" are with
respect to the exterior of the context, so that, for example, a stream
set to mode=sendonly does not pass received media into the context.
Signals and Events are not affected by mode.  The boolean-valued Reserve
properties, ReserveValue and ReserveGroup, of a Termination indicate
what the MG is expected to do when it receives a  local and/or remote
descriptor.

If the value of a Reserve property is True, the MG SHALL reserve
resources for all alternatives specified in the local and/or remote
descriptors for which it currently has resources available.  It SHALL
respond with the alternatives for which it reserves resources.  If it
cannot not support any of the alternatives, it SHALL respond with a
reply to the MGC that contains empty local and/or remote descriptors.

If the value of a Reserve property is False, the MG SHALL choose one of
the alternatives specified in the local descriptor (if present) and one
of the alternatives specified in the remote descriptor (if present).  If
the MG has not yet reserved resources to support the selected alterna-
tive, it SHALL reserve the resources.  If, on the other hand, it already
reserved resources for the Termination addressed (because of a prior
exchange with ReserveValue and/or ReserveGroup equal to True), it SHALL
release any excess resources it reserved previously. Finally, the MG
shall send a reply to the MGC containing the alternatives for the local
and/or remote descriptor that it selected.  If the MG does not have suf-
ficient resources to support any of the alternatives specified, is SHALL



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respond with error 510 (insufficient resources).

The default value of ReserveValue and ReserveGroup is False.

A new setting of the LocalControl Descriptor completely replaces the
previous setting of that descriptor in the MG.  Thus to retain informa-
tion from the previous setting the MGC must include that information in
the new setting.  If the MGC wishes to delete some information from the
existing descriptor, it merely resends the descriptor (in a Modify com-
mand) with the unwanted information stripped out.

7.1.8.  Local and Remote Descriptors

The MGC uses Local and Remote descriptors to reserve and commit MG
resources for media decoding and encoding for the given Stream(s) and
Termination to which they apply.  The MG includes these descriptors in
its response to indicate what it is actually prepared to support.  The
MG SHALL include additional properties and their values in its response
if these properties are mandatory yet not present in the requests made
by the MGC (e.g., by specifying detailed video encoding parameters where
the MGC only specified the payload type).

Local refers to the media received by the MG and Remote refers to the
media sent by the MG.

When text encoding the protocol, the descriptors consist of session
descriptions as defined in SDP (RFC2327).  In session descriptions sent
from the MGC to the MG, the following exceptions to the syntax of RFC
2327 are allowed:

*    the "s=", "t=" and "o=" lines are optional,

*    the use of CHOOSE is allowed in place of a single parameter value,
     and

*    the use of alternatives is allowed in place of a single parameter
     value.

*    When multiple session descriptions are provided in one descriptor,
     the "v=" lines are required as delimiters; otherwise they are
     optional in session descriptions sent to the MG.  Implementations
     shall accept session descriptions that are fully conformant to
     RFC2327. When binary encoding the protocol the descriptor consists
     of groups of properties (tag-value pairs) as specified in Annex C.
     Each such group may contain the parameters of a session descrip-
     tion.

Below, the semantics of the local and remote descriptors are specified



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in detail.  The specification consists of two parts.  The first part
specifies the interpretation of the contents of the descriptor.  The
second part specifies the actions the MG must take upon receiving the
local and remote descriptors.  The actions to be taken by the MG depend
on the values of the ReserveValue and ReserveGroup properties of the
LocalControl descriptor.

Either the local or the remote descriptor or both may be

*    unspecified (i.e., absent),

*    empty,

*    underspecified through use of CHOOSE in a property value,

*    fully specified, or

*    overspecified through presentation of multiple groups of properties
     and possibly multiple property values in one or more of these
     groups.  Where the descriptors have been passed from the MGC to the
     MG, they are interpreted according to the rules given in section
     7.1.1, with the following additional comments for clarification:

(a)  An unspecified Local or Remote descriptor is considered to be a
     missing mandatory parameter.  It requires the MG to use whatever
     was last specified for that descriptor.  It is possible that there
     was no previously-specified value, in which case the descriptor
     concerned is ignored in further processing of the command.

(b)  An empty Local (Remote) descriptor in a message from the MGC signi-
     fies a request to release any resources reserved for the media flow
     received (sent).

(c)  If multiple groups of properties are present in a Local or Remote
     descriptor or multiple values within a group, the order of prefer-
     ence is descending.

(d)  Underspecified or overspecified properties within a group of pro-
     perties sent by the MGC are requests for  the MG to choose one or
     more values which it can support for each of those properties.  In
     case of an overspecified property, the list of values is in des-
     cending order of preference.

Subject to the above rules, subsequent action depends on the values of
the ReserveValue and ReserveGroup properties in LocalControl. If
ReserveGroup is true, the MG reserves the resources required to support
any of the requested property group alternatives that it can currently
support.  If ReserveValue is true, the MG reserves the resources



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required to support any of the requested property value alternatives
that it can currently support.

NOTE - If a Local or Remote descriptor contains multiple groups of pro-
perties, and ReserveGroup is true, then the MG is requested to reserve
resources so that it can decode or encode the media stream according to
any of the alternatives.  For instance, if the Local descriptor contains
two groups of properties, one specifying packetized G.711 A-law audio
and the other G.723.1 audio, the MG reserves resources so that it can
decode one audio stream encoded in either G.711 A-law format or G.723.1
format.  The MG does not have to reserve resources to decode two audio
streams simultaneously, one encoded in G.711 A-law and one in G.723.1.
The intention for the use of  ReserveValue is analogous.  If Reserve-
Group is true or ReserveValue is true, then the following rules apply.

*    If the MG has insufficient resources to support all alternatives
     requested by the MGC and the MGC requested resources in both Local
     and Remote,  the MG should reserve resources to support at least
     one alternative each within Local and Remote.

*    If the MG has insufficient resources to support at least one alter-
     native  within a Local (Remote) descriptor received from the MGC,
     it shall return an empty Local (Remote) in response.

*    In its response to the MGC, when the MGC included Local and Remote
     descriptors, the MG SHALL include Local and Remote descriptors for
     all groups of properties and property values it reserved resources
     for.  If the MG is incapable of supporting at least one of the
     alternatives within the Local (Remote) descriptor received from the
     MGC, it SHALL return an empty Local (Remote) descriptor.

*    If the Mode property of the LocalControl descriptor is RecvOnly or
     SendRecv, the MG must be prepared to receive media encoded accord-
     ing to any of the alternatives included in its response to the MGC.
     If ReserveGroup is False and ReserveValue is false, then the MG
     SHOULD apply the following rules to resolve Local and Remote to a
     single alternative each:

*    The MG chooses the first alternative in Local for which it is able
     to support at least one alternative in Remote.

*    If the MG is unable to support at least one Local and one Remote
     alternative, it returns Error 510 (Insufficient Resources).

*    The MG returns its selected alternative in each of Local and
     Remote.  A new setting of a Local or Remote Descriptor completely
     replaces the previous setting of that descriptor in the MG.  Thus
     to retain information from the previous setting the MGC must



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     include that information in the new setting.  If the MGC wishes to
     delete some information from the existing descriptor, it merely
     resends the descriptor (in a Modify command) with the unwanted
     information stripped out.

7.1.9.  Events Descriptor

The EventsDescriptor parameter contains a RequestIdentifier and a list
of events that the Media Gateway is requested to detect and report.  The
RequestIdentifier is used to correlate the request with the notifica-
tions that it may trigger.  Requested events include, for example, fax
tones, continuity test results, and on-hook and off-hook transitions.

Each event in the descriptor contains the Event name, an optional
streamID, an optional KeepActive flag, and optional parameters.  The
Event name consists of a Package Name (where the event is defined) and
an EventID. The ALL wildcard may be used for the EventID, indicating
that all events from the specified package have to be detected.  The
default streamID is 0, indicating that the event to be detected is not
related to a particular media stream.  Events can have parameters.  This
allows a single event description to have some variation in meaning
without creating large numbers of individual events.  Further event
parameters are defined in the package.

The default action of the MG, when it detects an event in the Events
Descriptor, is to send a Notify command to the MG.  Any other action is
for further study.

If the value of the EventBufferControl property equals LockStep, follow-
ing detection of such an event, normal handling of events is suspended.
Any event which is subsequently detected and occurs in the EventBuffer
Descriptor is added to the end of the EventBuffer (a FIFO queue), along
with the time that it was detected.   The MG SHALL wait for a new
EventsDescriptor to be loaded.  A new EventsDescriptor can be loaded
either as the result of receiving a command with a new EventsDescriptor,
or by activating an embedded EventsDescriptor.

If EventBufferControl equals Off, the MG continues processing based on
the active EventsDescriptor.

In the case that an embedded EventsDescriptor being activated, the MG
continues event processing based on the newly activated EventsDescriptor
(Note - for purposes of EventBuffer handling, activation of an embedded
EventsDescriptor is equivalent to receipt of a new EventsDescriptor).

When the MG receives a command with a new EventsDescriptor, one or more
events may have been buffered in the EventBuffer in the MG. The value of
EventBufferControl then determines how the MG treats such buffered



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events.

Case 1

If EventBufferControl = LockStep  and the MG receives a new
EventsDescriptor it will check the FIFO EventBuffer and take the follow-
ing actions:

1.   If the EventBuffer is empty, the MG waits for detection of events
     based on the new EventsDescriptor.

2.   If the EventBuffer is non-empty, the MG processes the FIFO queue
     starting with the first event:

-    If the event in the queue is in the events listed in the new
     EventsDescriptor, the default action of the MG is to send a Notify
     command to the MGC and remove the event from the EventBuffer. Any
     other action is for further study.  The time stamp of the Notify
     shall be the time the event was actually

*    detected.  The MG then waits for a new EventsDescriptor. While
     waiting for a new EventsDescriptor, any events matching the
     EventsBufferDescriptor will be placed in the EventBuffer and the
     event processing will repeat from step 1.

*    If the event is not in the new EventsDescriptor, the MG SHALL dis-
     card the event and repeat from step 1.

Case 2

If EventBufferControl equals Off and the MG receives a new
EventsDescriptor, it processes new events with the new EventsDescriptor.

If the MG receives a command instructing it to set the value of
EventBufferControl to Off, all events in the EventBuffer SHALL be dis-
carded.

The MG may report several events in a single Transaction as long as this
does not unnecessarily delay the reporting of individual events.

For procedures regarding transmitting the Notify command, refer to the
appropriate annex for specific transport considerations.

The default value of EventBufferControl is Off.

Note - Since the EventBufferControl property is in the TerminationSta-
teDescriptor, the MG might receive a command that changes the EventBuf-
ferControl property and does not include an EventsDescriptor.



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Normally, detection of an event shall cause any active signals to stop.
When KeepActive is specified in the event, the MG shall not interrupt
any signals active on the Termination on which the event is detected.

An event can include an Embedded Signals descriptor and/or an Embedded
Events Descriptor which, if present, replaces the current Signals/Events
descriptor when the event is detected.  It is possible, for example, to
specify that the dial-tone Signal be generated when an off-hook Event is
detected, or that the dial-tone Signal be stopped when a digit is
detected.  A media gateway controller shall not send EventsDescriptors
with an event both marked KeepActive and containing an embedded Sig-
nalsDescriptor.

Only one level of embedding is permitted.  An  embedded EventsDescriptor
SHALL NOT contain another embedded EventsDescriptor; an embedded
EventsDescriptor may contain an embedded SignalsDescriptor.

An EventsDescriptor received by a media gateway replaces any previous
Events Descriptor. Event notification in process shall complete, and
events detected after the command containing the new EventsDescriptor
executes, shall be processed according to the new EventsDescriptor.

7.1.10.  EventBuffer Descriptor

The EventBuffer Descriptor contains a list of events, with their parame-
ters if any, that the MG is requested to detect and buffer when
EventBufferControl equals LockStep (see 7.1.9).

7.1.11.  Signals Descriptor

A SignalsDescriptor is a parameter that contains the set of signals that
the Media Gateway is asked to apply to a Termination. A SignalsDescrip-
tor contains a number of signals and/or sequential signal lists.  A Sig-
nalsDescriptor may contain zero signals and sequential signal lists.
Support of sequential signal lists is optional.

Signals are defined in packages.  Signals shall be named with a Package
name (in which the signal is defined) and a SignalID.  No wildcard shall
be used in the SignalID.  Signals that occur in a SignalsDescriptor have
an optional StreamID parameter (default is 0, to indicate that the sig-
nal is not related to a particular media stream), an optional signal
type (see below), an optional duration and possibly parameters defined
in the package that defines the signal.  This allows a single signal to
have some variation in meaning, obviating the need to create large
numbers of individual signals.  Finally, the optional parameter
"notifyCompletion" allows a MGC to indicate that it wishes to be  noti-
fied when the signal finishes playout.  When the MGC enables the signal
completion event (see section E.1.2) in an Events Descriptor, that event



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is detected whenever a signal terminates and "notifyCompletion" for that
signal is set to TRUE. The signal completion event of section E.1.2 has
a parameter that indicates how the signal terminated: it played to com-
pletion, it was interrupted by an event, it was halted because a new
SignalsDescriptor arrived, or the signal did not complete for some other
reason.

The duration is an integer value that is expressed in hundredths of a
second.

There are three types of signals:

*    on/off - the signal lasts until it is turned off,

*    timeout - the signal lasts until it is turned off or a specific
     period of time elapses,

*    brief - the signal duration is so short that it will stop on its
     own unless a new signal is applied that causes it to stop; no
     timeout value is needed.

If the signal type is specified in a SignalsDescriptor, it overrides the
default signal type (see Section 12.1.4). If duration is specified for
an on/off signal, it SHALL be ignored.

A sequential signal list consists of a signal list identifier, a
sequence of signals to be played sequentially, and a signal type.  Only
the trailing element of the sequence of signals in a sequential signal
list may be an on/off signal.  If the trailing element of the sequence
is an on/off signal, the signal type of the sequential signal list shall
be on/off as well.  If the sequence of signals in a sequential signal
list contains signals of type timeout and the trailing element is not of
type on/off, the type of the sequential signal list SHALL be set to
timeout.  The duration of a sequential signal list with type timeout is
the sum of the durations of the signals it contains.  If the sequence of
signals in a sequential signal list contains only signals of type brief,
the type of the sequential signal list SHALL be set to brief.  A signal
list is treated as a single signal of the specified type when played
out.

Multiple signals and sequential signal lists in the same SignalsDescrip-
tor shall be played simultaneously.

Signals are defined as proceeding from the termination towards the exte-
rior of the Context unless otherwise specified in a package.  When the
same Signal is applied to multiple Terminations within one Transaction,
the MG should consider using the same resource to generate these Sig-
nals.



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Production of a Signal on a Termination is stopped by application of a
new SignalsDescriptor, or detection of an Event on the Termination (see
section 7.1.9).

A new SignalsDescriptor replaces any existing SignalsDescriptor.  Any
signals applied to the Termination not in the replacement descriptor
shall be stopped, and new signals are applied, except as follows.  Sig-
nals present in the replacement descriptor and containing the KeepActive
flagshall be continued if they are currently playing and have not
already completed.  If a replacement signal descriptor contains a signal
that is not currently playing and contains the KeepActive flag, that
signal SHALL be ignored.  If the replacement descriptor contains a
sequential signal list with the same identifier as the existing descrip-
tor, then


*    the signal type and sequence of signals in the sequential signal
     list in the replacement descriptor shall be ignored, and

*    the playing of the signals in the sequential signal list in the
     existing descriptor shall not be interrupted.

7.1.12.  Audit Descriptor

The Audit Descriptor specifies what information is to be audited.  The
Audit Descriptor specifies the list of descriptors to be returned.
Audit may be used in any command to force the return of a descriptor
even if the descriptor in the command was not present, or had no under-
specified parameters.  Possible items in the Audit Descriptor are:

                            ________________
                           | Modem          |
                           |________________|
                           | Mux            |
                           |________________|
                           | Events         |
                           |________________|
                           | Media          |
                           |________________|
                           | Signals        |
                           |________________|
                           | ObservedEvents |
                           |________________|
                           | DigitMap       |
                           |________________|
                           | Statistics     |
                           |________________|
                           | Packages       |



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                           |_______________|
                           | EventBuffer   |
                           |_______________|


Audit may be empty, in which case, no descriptors are returned.  This is
useful in Subtract, to inhibit return of statistics, especially when
using wildcard.

7.1.13.  ServiceChange Descriptor

The ServiceChangeDescriptor contains the following parameters:

*    ServiceChangeMethod

*    ServiceChangeReason

*    ServiceChangeAddress

*    ServiceChangeDelay

*    ServiceChangeProfile

*    ServiceChangeVersion

*    ServiceChangeMGCId

*    TimeStamp

See section 7.2.8

7.1.14.  DigitMap Descriptor

A DigitMap is a dialing plan resident in the Media Gateway used for
detecting and reporting digit events received on a Termination.  The
DigitMap Descriptor contains a DigitMap name and the DigitMap to be
assigned.  A digit map may be preloaded into the MG by management action
and referenced by name in an EventsDescriptor, may be defined dynami-
cally and subsequently referenced by name, or the actual digitmap itself
may be specified in the EventsDescriptor. It is permissible for a digit
map completion event within an Events Descriptor to refer by name to a
DigitMap which is defined by a DigitMap Descriptor within the same com-
mand, regardless of the transmitted order of the respective descriptors.

DigitMaps defined in a DigitMapDescriptor can occur in any of the stan-
dard Termination manipulation Commands of the protocol.  A DigitMap,
once defined, can be used on all Terminations specified by the (possibly
wildcarded) TerminationID in such a command. DigitMaps defined on the



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root Termination are global and can be used on every Termination in the
MG, provided that a DigitMap with the same name has not been defined on
the given Termination. When a DigitMap is defined dynamically in a
DigitMap Descriptor:


*    A new DigitMap is created by specifying a name that is not yet
     defined.  The value shall be present.

*    A DigitMap value is updated by supplying a new value for a name
     that is already defined.  Terminations presently using the digitmap
     shall continue to use the old definition; subsequent EventsDescrip-
     tors specifying the name, including any EventsDescriptor in the
     command containing the DigitMap descriptor, shall use the new one.

*    A DigitMap is deleted by supplying an empty value for a name that
     is already defined. Terminations presently using the digitmap shall
     continue to use the old definition.

The collection of digits according to a DigitMap may be protected by
three timers, viz. a start timer (T), short timer (S), and long timer
(L).

1.   The start timer (T) is used prior to any digits having been dialed.

2.   If the Media Gateway can determine that at least one more digit is
     needed for a digit string to match any of the allowed patterns in
     the digit map, then the interdigit timer value should be set to a
     long (L) duration (e.g. 16 seconds).

3.   If the digit string has matched one of the patterns in a digit map,
     but it is possible that more digits could be received which would
     cause a match with a different pattern, then instead of reporting
     the match immediately, the MG must apply the short timer (S) and
     wait for more digits.

The timers are configurable parameters to a DigitMap.  The Start timer
is started at the beginning of every digit map use, but can be overrid-
den.

The formal syntax of the digit map is described by the DigitMap rule in
the formal syntax description of the protocol (see Annex A and Annex B).
A DigitMap, according to this syntax, is defined either by a string or
by a list of strings. Each string in the list is an alternative event
sequence, specified either as a sequence of digit map symbols or as a
regular expression of digit map symbols.  These digit map symbols, the
digits "0" through "9" and letters "A" through a maximum value depending
on the signalling system concerned, but never exceeding "K", correspond



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to specified events within a package which has been designated in the
Events Descriptor on the termination to which the digit map is being
applied.  (The mapping between events and digit map symbols is defined
in the documentation for packages associated with channel-associated
signalling systems such as DTMF, MF, or R2.  Digits "0" through "9" MUST
be mapped to the corresponding digit events within the signalling system
concerned.  Letters should be allocated in logical fashion, facilitating
the use of range notation for alternative events.)

The letter "x" is used as a wildcard, designating any event correspond-
ing to symbols in the range "0"-"9".  The string may also contain expli-
cit ranges and, more generally, explicit sets of symbols, designating
alternative events any one of which satisfies that position of the digit
map. Finally, the dot symbol "." stands for zero or more repetitions of
the event selector (event, range of events, set of alternative events,
or wildcard) that precedes it.  As a consequence of the third timing
rule above, inter-event timing while matching the dot symbol uses the
short timer by default.

In addition to these event symbols, the string may contain "S" and "L"
inter-event timing specifiers and the "Z" duration modifier.  "S" and
"L" respectively indicate that the MG should use the short (S) timer or
the long (L) timer for subsequent events, over-riding the timing rules
described above. A timer specifier following a dot specifies inter-event
timing for all events matching the dot as well as for subsequent events.
If an explicit timing specifier is in effect in one alternative event
sequence, but none is given in any other candidate alternative, the
timer value set by the explicit timing specifier must be used.  If all
sequences with explicit timing controls are dropped from the candidate
set, timing reverts to the default rules given above. Finally, if con-
flicting timing specifiers are in effect in different alternative
sequences, the results are undefined.

A "Z" designates a long duration event: placed in front of the symbol(s)
designating the event(s) which satisfy a given digit position, it indi-
cates that that position is satisfied only if the duration of the event
exceeds the long-duration threshold.  The value of this threshold is
assumed to be provisioned in the MG. A digit map is active while the
events descriptor which invoked it is active and it has not completed.
A digit map completes when:

*    a timer has expired, or

*    an alternative event sequence has been matched and no other alter-
     native event sequence in the digit map could be matched through
     detection of an additional event (unambiguous match), or

*    an event has been detected such that a match to a complete



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     alternative event sequence of the digit map will be impossible no
     matter what additional events are received.

Upon completion, a digit map completion event as defined in the package
providing the events being mapped into the digit map shall be generated.
At that point the digit map is deactivated. Subsequent events in the
package are processed as per the currently active event processing
mechanisms.

Pending completion, successive events shall be processed according to
the following rules:

1.   The "current dial string", an internal variable, is initially
     empty.  The set of candidate alternative event sequences includes
     all of the alternatives specified in the digit map.

2.   At each step, a timer is set to wait for the next event, based
     either on the default timing rules given above or on explicit tim-
     ing specified in one or more alternative event sequences. If the
     timer expires and a member of the candidate set of alternatives is
     fully satisfied, a timeout completion with full match is reported.
     If the timer expires and part or none of any candidate alternative
     is satisfied, a timeout completion with partial match is reported.

3.   If an event is detected before the timer expires, it is mapped to a
     digit string symbol and provisionally added to the end of the
     current dial string.  The duration of the event (long or not long)
     is noted if and only if this is relevant in the current symbol
     position (because at least one of the candidate alternative event
     sequences includes the "Z" modifier at this position in the
     sequence).

4.   The current dial string is compared to the candidate alternative
     event sequences.  If and only if a sequence expecting a long-
     duration event at this position is matched (i.e. the event had long
     duration and met the specification for this position), then any
     alternative event sequences not specifying a long duration event at
     this position are discarded, and the current dial string is modi-
     fied by inserting a "Z" in front of the symbol representing the
     latest event. Any sequence expecting a long-duration event at this
     position but not matching the observed event is discarded from the
     candidate set.   If alternative event sequences not specifying a
     long duration event in the given position remain in the candidate
     set after application of the above rules, the observed event dura-
     tion is treated as irrelevant in assessing matches to them.  If
     exactly one candidate remains, a completion event is generated
     indicating an unambiguous match.  If no candidates remain, the
     latest event is removed from the current dial string and a



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     completion event is generated indicating full match if one of the
     candidates from the previous step was fully satisfied before the
     latest event was detected, or partial match otherwise.  The event
     removed from the current dial string will then be reported as per
     the currently active event processing mechanisms.

5.   If no completion event is reported out of step 5 (because the can-
     didate set still contains more than one alternative event
     sequence), processing returns to step 2.  A digit map is activated
     whenever a new event descriptor is applied to the termination or
     embedded event descriptor is activated, and that event descriptor
     contains a digit map completion event which itself contains a digit
     map parameter.  Each new activation of a digit map begins at step 1
     of the above procedure, with a clear current dial string.  Any pre-
     vious contents of the current dial string from an earlier activa-
     tion are lost.  While the digit map is activated, detection is
     enabled for all events defined in the package containing the speci-
     fied digit map completion event.  Normal event behaviour (e.g.
     stopping of signals unless the digit completion event has the
     KeepActive flag enabled) continues to apply for each such event
     detected, except that the events in the package containing the
     specified digit map completion event other than the completion
     event itself are not individually notified.  Note that if a package
     contains a digit map completion event, then an event specification
     consisting of the package name with a wildcarded ItemID (Property
     Name) will activate a digit map if the event includes a digit map
     parameter.  Regardless of whether a digit map is activated, this
     form of event specification will cause the individual events to be
     reported to the MGC as they are detected.

As an example, consider the following dial plan:

 _____________________________________________________________________
| 0                        |  Local operator                         |
| 00                       |  Long distance operator                 |
| xxxx                     |  Local extension number(starts with 1-7)|
| 8xxxxxxx                 |  Local number                           |
| #xxxxxxx                 |  Off-site extension                     |
| *xx                      |  Star services                          |
| 91xxxxxxxxxx             |  Long distance number                   |
| 9011 + up to 15 digits   |  International number                   |
|__________________________|_________________________________________|


If the DTMF detection package described in Annex E (section E.6) is used
to collect the dialled digits, then the dialling plan shown above
results in the following digit map:




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  (0| 00|[1-7]xxx|8xxxxxxx|Fxxxxxxx|Exx|91xxxxxxxxxx|9011x.)

7.1.15.  Statistics Descriptor

The Statistics parameter provides information describing the status and
usage of a Termination during its existence within a specific Context.
There is a set of standard statistics kept for each termination where
appropriate (number of octets sent and received for example).  The par-
ticular statistical properties that are reported for a given Termination
are determined by the Packages realized by the Termination.  By default,
statistics are reported when the Termination is Subtracted from the Con-
text.  This behavior can be overridden by including an empty Audit-
Descriptor in the Subtract command.  Statistics may also be returned
from the AuditValue command, or any Add/Move/Modify command using the
Audit descriptor.

Statistics are cumulative; reporting Statistics does not reset them.
Statistics are reset when a Termination is Subtracted from a Context.

7.1.16.  Packages Descriptor

Used only with the AuditValue command, the PackageDescriptor returns a
list of Packages realized by the Termination.

7.1.17.  ObservedEvents Descriptor

ObservedEvents is supplied with the Notify command to inform the MGC of
which event(s) were detected.  Used with the AuditValue command, the
ObservedEventsDescriptor returns events in the event buffer which have
not been Notified. ObservedEvents contains the RequestIdentifier of the
EventsDescriptor that triggered the notification, the event(s) detected
and the detection time(s). Detection times are reported with a precision
of hundredths of a second.  Time is expressed in UTC.

7.1.18.  Topology Descriptor

A topology descriptor is used to specify flow directions between termi-
nations in a Context. Contrary to the descriptors in previous sections,
the topology descriptor applies to a Context instead of a Termination.
The default topology of a Context is that each termination's transmis-
sion is received by all other terminations.  The Topology Descriptor is
optional to implement.

The Topology Descriptor occurs before the commands in an action.  It is
possible to have an action containing only a Topology Descriptor, pro-
vided that the context to which the action applies already exists.

A topology descriptor consists of a sequence of triples of the form (T1,



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T2, association). T1 and T2 specify Terminations within the Context,
possibly using the ALL or CHOOSE wildcard.  The association specifies
how media flows between these two Terminations as follows.


*    (T1, T2, isolate) means that the Terminations matching T2 do not
     receive media from the Terminations matching T1, nor vice versa.

*    (T1, T2, oneway) means that the Terminations that match T2 receive
     media from the Terminations matching T1, but not vice versa.  In
     this case use of the ALL wildcard such that there are Terminations
     that match both T1 and T2 is not allowed.

*    (T1, T2, bothway) means that the Terminations matching T2 receive
     media from the Terminations matching T1, and vice versa.  In this
     case it is allowed to use wildcards such that there are Termina-
     tions that match both T1 and T2.  However, if there is a Termina-
     tion that matches both, no loopback is introduced; loopbacks are
     created by setting the TerminationMode.

CHOOSE wildcards may be used in T1 and T2 as well, under the following
restrictions:

*    the action (see section 8) of which the topology descriptor is part
     contains an Add command in which a CHOOSE wildcard is used;

*    if a CHOOSE wildcard occurs in T1 or T2, then a partial name SHALL
     NOT be specified.  The CHOOSE wildcard in a topology descriptor
     matches the TerminationID that the MG assigns in the first Add com-
     mand that uses a CHOOSE wildcard in the same action.  An existing
     Termination that matches T1 or T2 in the Context to which a Termi-
     nation is added, is connected to the newly added Termination as
     specified by the topology descriptor. The default association when
     a termination is not mentioned in the Topology descriptor is both-
     way (if T3 is added to a context with T1 and T2 with topology
     (T3,T1,oneway) it will be connected bothway to T2).

The figure below and the table following it show some examples of the
effect of including topology descriptors in actions.  In these examples
it is assumed that the topology descriptors are applied in sequence.











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               Context 1           Context 2           Context 3
         +------------------+  +------------------+  +------------------+
         |      +----+      |  |      +----+      |  |      +----+      |
         |      | T2 |      |  |      | T2 |      |  |      | T2 |      |
         |      +----+      |  |      +----+      |  |      +----+      |
         |       ^  ^       |  |          ^       |  |          ^       |
         |       |  |       |  |          |       |  |          |       |
         |    +--+  +--+    |  |          +---+   |  |          +--+    |
         |    |        |    |  |              |   |  |             |    |
         |    v        v    |  |              v   |  |             |    |
         | +----+    +----+ |  | +----+    +----+ |  | +----+    +----+ |
         | | T1 |<-->| T3 | |  | | T1 |<-->| T3 | |  | | T1 |<-->| T3 | |
         | +----+    +----+ |  | +----+    +----+ |  | +----+    +----+ |
         +------------------+  +------------------+  +------------------+
          1. No Topology Desc.  2. T1, T2 Isolate     3. T3, T2 oneway
               Context 1           Context 2           Context 3


         +------------------+  +------------------+  +------------------+
         |      +----+      |  |      +----+      |  |      +----+      |
         |      | T2 |      |  |      | T2 |      |  |      | T2 |      |
         |      +----+      |  |      +----+      |  |      +----+      |
         |          |       |  |          ^       |  |       ^  ^       |
         |          |       |  |          |       |  |       |  |       |
         |          +--+    |  |          +---+   |  |    +--+  +--+    |
         |             |    |  |              |   |  |    |        |    |
         |             v    |  |              v   |  |    v        v    |
         | +----+    +----+ |  | +----+    +----+ |  | +----+    +----+ |
         | | T1 |<-->| T3 | |  | | T1 |<-->| T3 | |  | | T1 |<-->| T3 | |
         | +----+    +----+ |  | +----+    +----+ |  | +----+    +----+ |
         +------------------+  +------------------+  +------------------+
          1. T2, T3 oneway      2. T2, T3 bothway     3. T1, T2 bothway

                            Figure 4: Example topologies
















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        _______________________________________________________________________
        |Topology |  Description                                               |
        |_________|____________________________________________________________|
        |1        |No topology descriptors.  When no topology descriptors are  |
        |         |included, all terminations have a both way connection to all|
        |         |other terminations.                                         |
        |_________|____________________________________________________________|
        |2        |T1, T2, Isolate.  Removes the connection between T1 and T2. |
        |         |T3 has a both way connection with both T1 and T2.           |
        |_________|____________________________________________________________|
        |3        |T3, T2, oneway.  A oneway connection from T3 to T2 (i.e. T2 |
        |         |receives media flow from T3).  A bothway connection between |
        |         |T1 and T3.                                                  |
        |_________|____________________________________________________________|
        |4        |T2, T3, oneway.  A oneway connection between T2 to T3.      |
        |         |T1 and T3 remain bothway connected                          |
        |_________|____________________________________________________________|
        |5        |T2, T3 bothway.  T2 is bothway connected to T3.             |
        |         |This results in the same as 2.                              |
        |_________|____________________________________________________________|
        |6        |T1, T2 bothway. (T2, T3 bothway and T1,T3 bothway may be    |
        |         |implied or explicit).  terminations have a bothway          |
        |_________|____________________________________________________________|


A oneway connection must implemented in such a way that the other Termi-
nations in the Context are not aware of the change in topology.

7.2.  Command Application Programming Interface

Following is an Application Programming Interface (API) describing the
Commands of the protocol.  This API is shown to illustrate the Commands
and their parameters and is not intended to specify implementation (e.g.
via use of blocking function calls).  It describes the input parameters
in parentheses after the command name and the return values in front of
the Command. This is only for descriptive purposes; the actual Command
syntax and encoding are specified in later subsections.  All parameters
enclosed by square brackets ([. . . ]) are considered optional.

7.2.1.  Add


TerminationID
[,MediaDescriptor]
[,ModemDescriptor]
[,MuxDescriptor]
[,EventsDescriptor]
[,SignalsDescriptor]



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[,DigitMapDescriptor]
[,ObservedEventsDescriptor]
[,EventBufferDescriptor]
[,StatisticsDescriptor]
[,PackagesDescriptor]
     Add( TerminationID
          [, MediaDescriptor]
          [, ModemDescriptor]
          [, MuxDescriptor]
          [, EventsDescriptor]
          [, SignalsDescriptor]
          [, DigitMapDescriptor]
          [, AuditDescriptor]
         )

The TerminationID specifies the termination to be added to the Context.
The Termination is either created, or taken from the null Context.  For
an existing Termination, the TerminationID would be specific.  For a
Termination that does not yet exist, the TerminationID is specified as
CHOOSE  in the command. The new TerminationID will be returned.  Wild-
cards may be used in an Add, but such usage would be unusual.  If the
wildcard matches more than one TerminationID, all possible matches are
attempted, with results reported for each one.  The order of attempts
when multiple TerminationIDs match is not specified.

The optional MediaDescriptor describes all media streams.

The optional ModemDescriptor and MuxDescriptor specify a modem and mul-
tiplexer if applicable. For convenience, if a Multiplex Descriptor is
present in an Add command and lists any Terminations that are not
currently in the Context, such Terminations are added to the context as
if individual Add commands listing the Terminations were invoked.  If an
error occurs on such an implied Add, error 471 - Implied Add for Multi-
plex failure shall be returned and further processing of the command
shall cease.

The EventsDescriptor parameter is optional.  If present, it provides the
list of events that should be detected on the Termination.

The SignalsDescriptor parameter is optional.  If present, it provides
the list of signals that should be applied to the Termination.

The DigitMapDescriptor parameter is optional.  If present, defines a
DigitMap definition that may be used in an EventsDescriptor.

The AuditDescriptor is optional.  If present, the command will return
descriptors as specified in the AuditDescriptor.




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All descriptors that can be modified could be returned by MG if a param-
eter was underspecified or overspecified.  ObservedEvents, Statistics,
and Packages, and the EventBuffer Descriptors are returned only if
requested in the AuditDescriptor.  Add SHALL NOT be used on a Termina-
tion with a serviceState of "OutofService".

7.2.2.  Modify


TerminationID
[,MediaDescriptor]
[,ModemDescriptor]
[,MuxDescriptor]
[,EventsDescriptor]
[,SignalsDescriptor]
[,DigitMapDescriptor]
[,ObservedEventsDescriptor]
[,EventBufferDescriptor]
[,StatisticsDescriptor]
[,PackagesDescriptor]
     Modify( TerminationID
          [, MediaDescriptor]
          [, ModemDescriptor]
          [, MuxDescriptor]
          [, EventsDescriptor]
          [, SignalsDescriptor]
          [, DigitMapDescriptor]
          [, AuditDescriptor]
         )

The TerminationID may be specific if a single Termination in the Context
is to be modified.  Use of wildcards in the TerminationID may be
appropriate for some operations. If the wildcard matches more than one
TerminationID, all possible matches are attempted, with results reported
for each one.  The order of attempts when multiple TerminationIDs match
is not specified. The CHOOSE option is an error, as the Modify command
may only be used on existing Terminations.

The remaining parameters to Modify are the same as those to Add.  Possi-
ble return values are the same as those to Add.

7.2.3.  Subtract

The Subtract Command disconnects a Termination from its Context and
returns statistics on the Termination's participation in the Context.


TerminationID



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[,MediaDescriptor]
[,ModemDescriptor]
[,MuxDescriptor]
[,EventsDescriptor]
[,SignalsDescriptor]
[,DigitMapDescriptor]
[,ObservedEventsDescriptor]
[,EventBufferDescriptor]
[,StatisticsDescriptor]
[,PackagesDescriptor]
     Subtract(TerminationID
          [, AuditDescriptor]
         )


TerminationID in the input parameters represents the Termination that is
being subtracted.  The TerminationID may be specific or may be a wild-
card value indicating that all (or a set of related) Terminations in the
Context of the Subtract Command are to be subtracted. If the wildcard
matches more than one TerminationID, all possible matches are attempted,
with results reported for each one.  The order of attempts when multiple
TerminationIDs match is not specified. The CHOOSE option is an error, as
the Subtract command may only be used on existing Terminations.  ALL may
be used as the ContextID as well as the TerminationId in a Subtract,
which would have the effect of deleting all contexts, deleting all
ephemeral terminations, and returning all physical terminations to Null
context.

By default, the Statistics parameter is returned to report information
collected on the Termination or Terminations specified in the Command.
The information reported applies to the Termination's or Terminations'
existence in the Context from which it or they are being subtracted.

The AuditDescriptor is optional.  If present, the command will return
descriptors as specified in the AuditDescriptor.   Possible return
values are the same as those to Add.

When a provisioned Termination is Subtracted from a context, its pro-
perty values shall revert to:

*    the default value, if specified for the property and not overridden
     by provisioning,

*    otherwise, the provisioned value.

7.2.4.  Move

The Move Command moves a Termination to another Context from its current



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Context in one atomic operation.  The Move command is the only command
that refers to a Termination in a Context different from that to which
the command is applied.  The Move command shall not be used to move Ter-
minations to or from the null Context.


TerminationID
[,MediaDescriptor]
[,ModemDescriptor]
[,MuxDescriptor]
[,EventsDescriptor]
[,SignalsDescriptor]
[,DigitMapDescriptor]
[,ObservedEventsDescriptor]
[,EventBufferDescriptor]
[,StatisticsDescriptor]
[,PackagesDescriptor]
     Move( TerminationID
          [, MediaDescriptor]
          [, ModemDescriptor]
          [, MuxDescriptor]
          [, EventsDescriptor]
          [, SignalsDescriptor]
          [, DigitMapDescriptor]
          [, AuditDescriptor]
         )


The TerminationID specifies the Termination to be moved.  It may be
wildcarded.  If the wildcard matches more than one TerminationID, all
possible matches are attempted, with results reported for each one.  The
order of attempts when multiple TerminationIDs match is not specified.
By convention, the Termination is subtracted from its previous Context.
The Context to which the Termination is moved is indicated by the target
ContextId in the Action.  If the last remaining Termination is moved out
of a Context, the Context is deleted.

The remaining descriptors are processed as in the Modify Command.  The
AuditDescriptor with the Statistics option, for example, would return
statistics on the Termination just prior to the Move.  Possible descrip-
tors returned from Move are the same as for Add.  Move SHALL NOT be used
on a Termination with a serviceState of "OutofService".

7.2.5.  AuditValue

The AuditValue Command returns the current values of properties, events,
signals and statistics associated with Terminations.




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TerminationID
[,MediaDescriptor]
[,ModemDescriptor]
[,MuxDescriptor]
[,EventsDescriptor]
[,SignalsDescriptor]
[,DigitMapDescriptor]
[,ObservedEventsDescriptor]
[,EventBufferDescriptor]
[,StatisticsDescriptor]
[,PackagesDescriptor]
        AuditValue(TerminationID,
                AuditDescriptor
        )


TerminationID may be specific or wildcarded. If the wildcard matches
more than one TerminationID, all possible matches are attempted, with
results reported for each one.  The order of attempts when multiple Ter-
minationIDs match is not specified. If a wildcarded response is
requested, only one command return is generated, with the contents con-
taining the union of the values of all Terminations matching the wild-
card.  This convention may reduce the volume of data required to audit a
group of Terminations.  Use of CHOOSE is an error.

The appropriate descriptors, with the current values for the Termina-
tion, are returned from AuditValue.  Values appearing in multiple
instances of a descriptor are defined to be alternate values supported,
with each parameter in a descriptor considered independent.

ObservedEvents returns a list of events in the EventBuffer, Packages-
Descriptor returns a list of packages realized by the Termination.
DigitMapDescriptor returns the name or value of the current DigitMap for
the Termination.  DigitMap requested in an AuditValue command with Ter-
minationID ALL returns all DigitMaps in the gateway.  Statistics returns
the current values of all statistics being kept on the Termination.
Specifying an empty Audit Descriptor results in only the TerminationID
being returned.  This may be useful to get a list of TerminationIDs when
used with wildcard.

AuditValue results depend on the Context, viz. specific, null, or wild-
carded.  The TerminationID may be specific, or wildcarded.

The following illustrates other information that can be obtained with
the Audit Command:

________________________________________________________________________
|ContextID   |TerminationID|  Information Obtained                     |



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|Specific    |  wildcard   |Audit of matching Terminations in a Context|
|Specific    |  specific   |Audit of a single Termination in a Context |
|Null        |  Root       |Audit of Media Gateway state and events    |
|Null        |  wildcard   |Audit of all matching Terminations in the  |
|            |             |  Null context                             |
|Null        |  specific   |Audit of a single Termination outside of   |
|            |             |any Context                                |
|All         |  wildcard   |Audit of all matching Terminations and the |
|            |             |Context to which they are associated       |
|All         |  Root       |  List of all ContextIds                   |
|____________|_____________|___________________________________________|


7.2.6.  AuditCapabilities

The AuditCapabilities Command returns the possible values of properties,
events, signals and statistics associated with Terminations.

TerminationID
[,MediaDescriptor]
[,ModemDescriptor]
[,MuxDescriptor]
[,EventsDescriptor]
[,SignalsDescriptor]
[,ObservedEventsDescriptor]
[,EventBufferDescriptor]
[,StatisticsDescriptor]
        AuditCapabilities(TerminationID,
                AuditDescriptor)


The appropriate descriptors, with the possible values for the Termina-
tion are returned from AuditCapabilities.  Descriptors may be repeated
where there are multiple possible values.  values.  If a wildcarded
response is requested, only one command return is generated, with the
contents containing the union of the values of all Terminations matching
the wildcard.  This convention may reduce the volume of data required to
audit a group of Terminations.

Interpretation of what capabilities are requested for various values of
ContextID and TerminationID is the same as in AuditValue.

The EventsDescriptor returns the list of possible events on the Termina-
tion together with the list of all possible values for the
EventsDescriptor Parameters.  The SignalsDescriptor returns the list of
possible signals that could be applied to the Termination together with
the list of all possible values for the Signals Parameters.  Statis-
ticsDescriptor returns the names of the statistics being kept on the



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termination.  ObservedEventsDescriptor returns the names of active
events on the termination.  DigitMap and Packages are not legal in
AuditCapability

7.2.7.  Notify

The Notify Command allows the Media Gateway to notify the Media Gateway
Controller of events occurring within the Media Gateway.

        Notify(TerminationID,
               ObservedEventsDescriptor,
            [ErrorDescriptor])

The TerminationID parameter specifies the Termination issuing the Notify
Command.  The TerminationID shall be a fully qualified name.

The ObservedEventsDescriptor contains the RequestID and a list of events
that the Media Gateway detected in the order that they were detected.
Each event in the list is accompanied by parameters associated with the
event and an indication of the time that the event was detected. Pro-
cedures for sending Notify commands with RequestID equal to 0 are for
further study.

 Notify Commands with RequestID not equal to 0 shall occur only as the
result of detection of an event specified by an Events Descriptor which
is active on the termination concerned.  The RequestID returns the
RequestID parameter of the EventsDescriptor that triggered the Notify
Command.  It is used to correlate the notification with the request that
triggered it.  The events in the list must have been requested via the
triggering EventsDescriptor or embedded events descriptor unless the
RequestID is 0 (which is for further study).

7.2.8.  ServiceChange

The ServiceChange Command allows the Media Gateway to notify the Media
Gateway Controller that a Termination or group of Terminations is about
to be taken out of service or has just been returned to service.   The
Media Gateway Controller may indicate that Termination(s) shall be taken
out of or returned to service.  The Media Gateway may notify the MGC
that the capability of a Termination has changed.  It also allows a MGC
to hand over control of a MG to another MGC.

TerminationID,
[ServiceChangeDescriptor]
        ServiceChange(TerminationID,
                ServiceChangeDescriptor
        )




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The TerminationID parameter specifies the Termination(s) that are taken
out of or returned to service.  Wildcarding of Termination names is per-
mitted, with the exception that the CHOOSE mechanism shall not be used.
Use of the "Root" TerminationID indicates a ServiceChange affecting the
entire Media Gateway.

The ServiceChangeDescriptor contains the following parameters as
required:

*    ServiceChangeMethod

*    ServiceChangeReason

*    ServiceChangeDelay

*    ServiceChangeAddress

*    ServiceChangeProfile

*    ServiceChangeVersion

*    ServiceChangeMgcId

*    TimeStamp

The ServiceChangeMethod parameter specifies the type of ServiceChange
that will or has occurred:

1)   Graceful - indicates that the specified Terminations will be taken
     out of service after the specified ServiceChangeDelay; established
     connections are not yet affected, but the Media Gateway Controller
     should refrain from establishing new connections and should attempt
     to gracefully tear down existing connections. The MG should set
     termination serviceState at the expiry of ServiceChangeDelay or the
     removal of the termination from an active context (whichever is
     first), to "out of service".

2)   Forced - indicates that the specified Terminations were taken
     abruptly out of service and any established connections associated
     with them were lost. The MGC is responsible for cleaning up the
     context (if any) with which the failed termination is associated.
     At a minimum the termination shall be subtracted from the context.
     The termination serviceState should be "out of service".

3)   Restart - indicates that service will be restored on the specified
     Terminations after expiration of the ServiceChangeDelay. The ser-
     viceState should be set  to "inService" upon expiry of Servi-
     ceChangeDelay.



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4)   Disconnected - always applied with the Root TerminationID, indi-
     cates that the MG lost communication with the MGC, but it was sub-
     sequently restored.  Since MG state may have changed, the MGC may
     wish to use the Audit command to resynchronize its state with the
     MG's.

5)   Handoff - sent from the MGC to the MG, this reason indicates that
     the MGC is going out of service and a new MGC association must be
     established. Sent from the MG to the MGC, this indicates that the
     MG is attempting to establish a new association in accordance with
     a Handoff received from the MGC with which it was previously asso-
     ciated.

6)   Failover - sent from MG to MGC to indicate the primary MG is out of
     service and a secondary MG is taking over.

7)   Another value whose meaning is mutually understood between the MG
     and the MGC.  The ServiceChangeReason parameter specifies the rea-
     son why the ServiceChange has or will occur.  It consists of an
     alphanumeric token (IANA registered) and an explanatory string.

The optional ServiceChangeAddress parameter specifies the address (e.g.,
IP port number for IP networks) to be used for subsequent communica-
tions.  It can be specified in the input parameter descriptor or the
returned result descriptor.  ServiceChangeAddress and ServiceChangeMgcId
parameters must not both be present in the ServiceChangeDescriptor or
the ServiceChangeResultDescriptor.  The serviceChangeAddress provides an
address to be used within the context of the association currently being
negotiated, while the ServiceChangeMgcId provides an alternate address
where the MG should seek to establish another association.

The optional ServiceChangeDelay parameter is expressed in seconds.  If
the delay is absent or set to zero, the delay value should be considered
to be null.  In the case of a "graceful" ServiceChangeMethod, a null
delay indicates that the Media Gateway Controller should wait for the
natural removal of existing connections and should not establish new
connections.  .  For "graceful" only, a null delay means the MG must not
set serviceState "out of service" until the termination is in the null
context.

The optional ServiceChangeProfile parameter specifies the Profile (if
any) of the protocol supported.  The ServiceChangeProfile includes the
version of the profile supported.

The optional ServiceChangeVersion parameter contains the protocol ver-
sion and is used if protocol version negotiation occurs (see section
11.3).




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The optional TimeStamp parameter specifies the actual time as kept by
the sender.  It can be used by the responder to determine how its notion
of time differs from that of its correspondent. TimeStamp is sent with a
precision of hundredths of a second, and is expressed in UTC.

The optional Extension parameter may contain any value whose meaning is
mutually understood by the MG and MGC.

A ServiceChange Command specifying the "Root" for the TerminationID and
ServiceChangeMethod equal to Restart is a registration command by which
a Media Gateway announces its existence to the Media Gateway Controller.
The Media Gateway is expected to be provisioned with the name of one
primary and optionally some number of alternate Media Gateway Controll-
ers.    Acknowledgement of the ServiceChange Command completes the
registration process.  The MG may specify the transport ServiceChangeAd-
dress to be used by the MGC for sending messages in the ServiceChangeAd-
dress parameter in the input ServiceChangeDescriptor. The MG may specify
an address in the ServiceChangeAddress parameter of the ServiceChange
request, and the MGC may also do so in the ServiceChange reply.  In
either case, the recipient must use the supplied address as the destina-
tion for all subsequent transaction requests within the association.  At
the same time, as indicated in section 9, transaction replies and pend-
ing indications must be sent to the address from which the corresponding
rquests originated.  This must be done even if it implies extra messag-
ing because commands and responses cannot be packed together.  The
TimeStamp parameter shall be sent with a registration command and its
response.

The Media Gateway Controller may return an ServiceChangeMgcId parameter
that describes the Media Gateway Controller that should preferably be
contacted for further service by the Media Gateway.  In this case the
Media Gateway shall reissue the ServiceChange command to the new Media
Gateway Controller.   The Gateway specified in an ServiceChangeMgcId, if
provided, shall be contacted before any further alternate MGCs.  On a
HandOff message from MGC to MG, the ServiceChangeMgcId is the new MGC
that will take over from the current MGC.

The return from ServiceChange is empty except when the Root termina-
tionID is used.  In that case it includes the following parameters as
required:

*    ServiceChangeAddress, if the responding MGC wishes to specify an
     new destination for messages from the MG for the remainder of the
     association;

*    ServiceChangeMgcId, if the responding MGC does not wish to sustain
     an association with the MG;




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*    ServiceChangeProfile, if the responder wishes to negotiate the pro-
     file to be used for the association;

*    ServiceChangeVersion, if the responder wishes to negotiate the ver-
     sion of the protocol to be used for the association.

The following ServiceChangeReasons are defined.  This list may be
extended by an IANA registration as outlined in section 13.3


        900 Service Restored
        901 Cold Boot
        902 Warm Boot
        903 MGC Directed Change
        904 Termination malfunctioning
        905 Termination taken out of service
        906 Loss of lower layer connectivity (e.g. downstream
            sync)
        907 Transmission Failure
        908 MG Impending Failure
        909 MGC Impending Failure
        910 Media Capability Failure
        911 Modem Capability Failure
        912 Mux Capability Failure
        913 Signal Capability Failure
        914 Event Capability Failure
        915 State Loss


7.2.9.  Manipulating and Auditing Context Attributes

The commands of the protocol as discussed in the preceding sections
apply to terminations.  This section specifies how contexts are manipu-
lated and audited.

Commands are grouped into actions (see section 8).  An action applies to
one context.  In addition to commands, an action may contain context
manipulation and auditing instructions.

An action request sent to a MG may include a request to audit attributes
of a context.  An action may also include a request to change the attri-
butes of a context.

The context properties that may be included in an action reply are used
to return information to a MGC.  This can be information requested by an
audit of context attributes or details of the effect of manipulation of
a context.




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If a MG receives an action which contains both a request to audit con-
text attributes and a request to manipulate those attributes, the
response SHALL include the values of the attributes after processing the
manipulation request.

7.2.10.  Generic Command Syntax

The protocol can be encoded in a binary format or in a text format.
MGCs should support both encoding formats.  MGs may support both for-
mats.

The protocol syntax for the binary format of the protocol is defined in
Annex A.  Annex C specifies the encoding of the Local and Remote
descriptors for use with the binary format.

A complete ABNF of the text encoding of the protocol per RFC2234 is
given in Annex B.  SDP is used as the encoding of the Local and Remote
Descriptors for use with the text encoding as modified in section 7.1.8.

7.3.  Command Error Codes

Errors consist of an IANA registered error code and an explanatory
string.  Sending the explanatory string is optional.  Implementations
are encouraged to append diagnostic information to the end of the
string.

When a MG reports an error to a MGC, it does so in an error descriptor.
An error descriptor consists of an error code and optionally the associ-
ated explanatory string.

The identified error codes are:


        400 - Bad Request
        401 - Protocol Error
        402 - Unauthorized
        403 - Syntax Error in Transaction
        404 - Syntax Error in TransactionReply
        405 - Syntax Error in TransactionPending
        406 - Version Not Supported
        410 - Incorrect identifier
        411 - The transaction refers to an unknown ContextId
        412 - No ContextIDs available

        421 - Unknown action or illegal combination of actions
        422 - Syntax Error in Action
        430 - Unknown TerminationID
        431 - No TerminationID matched a wildcard



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        432 - Out of TerminationIDs or No TerminationID available
        433 - TerminationID is already in a Context
        440 - Unsupported or unknown Package
        441 - Missing RemoteDescriptor
        442 - Syntax Error in Command
        443 - Unsupported or Unknown Command
        444 - Unsupported or Unknown Descriptor
        445 - Unsupported or Unknown Property
        446 - Unsupported or Unknown Parameter
        447 - Descriptor not legal in this command
        448 - Descriptor appears twice in a command
        450 - No such property in this package
        451 - No such event in this package
        452 - No such signal in this package
        453 - No such statistic in this package
        454 - No such parameter value in this package
        455 - Parameter illegal in this Descriptor
        456 - Parameter or Property appears twice in this Descriptor
        461 - TransactionIDs in Reply do not match Request
        462 - Commands in Transaction Reply do not match commands in request
        463 - TerminationID of Transaction Reply does not match  request
        464 - Missing reply in Transaction Reply
        465 - TransactionID in Transaction Pending does not match any open
              request
        466 - Illegal Duplicate Transaction Request
        467 - Illegal Duplicate Transaction Reply
        471 - Implied Add for Multiplex failure

        500 - Internal Gateway Error
        501 - Not Implemented
        502 - Not ready.
        503 - Service Unavailable
        504 - Command Received from unauthorized entity
        505 - Command Received before Restart Response
        510 - Insufficient resources
        512 - Media Gateway unequipped to detect requested Event
        513 - Media Gateway unequipped to generate requested Signals
        514 - Media Gateway cannot send the specified announcement
        515 - Unsupported Media Type
        517 - Unsupported or invalid mode
        518 - Event buffer full
        519 - Out of space to store digit map
        520 - Media Gateway does not have a digit map
        521 - Termination is "ServiceChangeing"
        526 - Insufficient bandwidth
        529 - Internal hardware failure
        530 - Temporary Network failure
        531 - Permanent Network failure
        581 - Does Not Exist



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8.  TRANSACTIONS

Commands between the Media Gateway Controller and the Media Gateway are
grouped into Transactions, each of which is identified by a Transac-
tionID.  Transactions consist of one or more Actions.  An Action con-
sists of a series of Commands that are limited to operating within a
single Context.   Consequently each Action typically specifies a Contex-
tID.  However, there are two circumstances where a specific ContextID is
not provided with an Action.  One is the case of modification of a Ter-
mination outside of a Context.  The other is where the controller
requests the gateway to create a new Context.  Following is a graphic
representation of the Transaction, Action and Command relationships.


          +----------------------------------------------------------+
          | Transaction x                                            |
          |  +----------------------------------------------------+  |
          |  | Action 1                                           |  |
          |  | +---------+  +---------+  +---------+  +---------+ |  |
          |  | | Command |  | Command |  | Command |  | Command | |  |
          |  | |    1    |  |    2    |  |    3    |  |    4    | |  |
          |  | +---------+  +---------+  +---------+  +---------+ |  |
          |  +----------------------------------------------------+  |
          |                                                          |
          |  +----------------------------------------------------+  |
          |  | Action 2                                           |  |
          |  | +---------+                                        |  |
          |  | | Command |                                        |  |
          |  | |    1    |                                        |  |
          |  | +---------+                                        |  |
          |  +----------------------------------------------------+  |
          |                                                          |
          |  +----------------------------------------------------+  |
          |  | Action 3                                           |  |
          |  | +---------+  +---------+  +---------+              |  |
          |  | | Command |  | Command |  | Command |              |  |
          |  | |    1    |  |    2    |  |    3    |              |  |
          |  | +---------+  +---------+  +---------+              |  |
          |  +----------------------------------------------------+  |
          +----------------------------------------------------------+
                  Figure 5 Transactions, Actions and Commands


Transactions are presented as TransactionRequests.  Corresponding
responses to a TransactionRequest are received in a single reply, possi-
bly preceded by a number of TransactionPending messages (see section
8.2.3).




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Transactions guarantee ordered Command processing.  That is, Commands
within a Transaction are executed sequentially. Ordering of Transactions
is NOT guaranteed - transactions may be executed in any order, or simul-
taneously.

At the first failing Command in a Transaction, processing of the remain-
ing Commands in that Transaction stops.  If a command contains a wild-
carded TerminationID, the command is attempted with each of the actual
TerminationIDs matching the wildcard.  A response within the Transac-
tionReply is included for each matching TerminationID, even if one or
more instances generated an error.  If any TerminationID matching a
wildcard results in an error when executed, any commands following the
wildcarded command are not attempted.  Commands may be marked as
"Optional" which can override this behaviour -  if a command marked as
Optional results in an error, subsequent commands in the Transaction
will be executed.

A TransactionReply includes the results for all of the Commands in the
corresponding TransactionRequest.  The TransactionReply includes the
return values for the Commands that were executed successfully, and the
Command and error descriptor for any Command that failed. Transaction-
Pending is used to periodically notify the receiver that a Transaction
has not completed yet, but is actively being processed.

Applications SHOULD implement an application level timer per transac-
tion.  Expiration of the timer should cause a retransmission of the
request.  Receipt of a Reply should cancel the timer. Receipt of Pending
should restart the timer.

8.1.  Common Parameters


8.1.1.  Transaction Identifiers

Transactions are identified by a TransactionID, which is assigned by
sender and is unique within the scope of the sender.

8.1.2.  Context Identifiers

Contexts are identified by a ContextID, which is assigned by the Media
Gateway and is unique within the scope of the Media Gateway.  The Media
Gateway Controller shall use the ContextID supplied by the Media Gateway
in all subsequent Transactions relating to that Context.  The protocol
makes reference to a distinguished value that may be used by the Media
Gateway Controller when referring to a Termination that is currently not
associated with a Context, namely the null ContextID.

The CHOOSE wildcard is used to request that the Media Gateway create a



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new Context.  The MGC shall not use partially specified ContextIDs con-
taining the CHOOSE wildcard.  The MGC may use the ALL wildcard to
address all Contexts on the MG.

8.2.  Transaction Application Programming Interface

Following is an Application Programming Interface (API) describing the
Transactions of the protocol.  This API is shown to illustrate the Tran-
sactions and their parameters and is not intended to specify implementa-
tion (e.g. via use of blocking function calls).  It will describe the
input parameters and return values expected to be used by the various
Transactions of the protocol from a very high level.  Transaction syntax
and encodings are specified in later subsections.

8.2.1.  TransactionRequest

The TransactionRequest is invoked by the sender.  There is one Transac-
tion per request invocation.  A request contains one or more Actions,
each of which specifies its target Context and one or more Commands per
Context.

TransactionRequest(TransactionId {
       ContextID {Command ... Command},
                        . . .
       ContextID  {Command ... Command } })


The TransactionID parameter must specify a value for later correlation
with the TransactionReply or TransactionPending response from the
receiver.

The ContextID parameter must specify a value to pertain to all Commands
that follow up to either the next specification of a ContextID parameter
or the end of the TransactionRequest, whichever comes first.

The Command parameter represents one of the Commands mentioned in the
"Command Details" subsection titled "Application Programming Interface".

8.2.2.  TransactionReply

The TransactionReply is invoked by the receiver.  There is one reply
invocation per transaction. A reply contains one or more Actions, each
of which must specify its target Context and one or more Responses per
Context.

TransactionReply(TransactionID {
      ContextID { Response ... Response },
      . . .



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      ContextID { Response ... Response } })

The TransactionID parameter must be the same as that of the correspond-
ing TransactionRequest.

The ContextID parameter must specify a value to pertain to all Responses
for the action.  The ContextID may be specific or null.

Each of the Response parameters represents a return value as mentioned
in section 7.2, or an error descriptor if the command execution encoun-
tered an error. Commands after the point of failure are not processed
and, therefore, Responses are not issued for them.

An exception to this occurs if a command has been marked as optional in
the Transaction request. If the optional command  generates an error,
the transaction still continues to execute, so the Reply would, in this
case, have Responses after an Error.

If the receiver encounters an error in processing a ContextID, the
requested Action response will consist of the context ID and a single
error descriptor, 422 Syntax Error in Action.

If the receiver encounters an error such that it cannot determine a
legal Action, it will return a TransactionReply consisting of the Tran-
sactionID and a single error descriptor, 422 Syntax Error in Action. If
the end of an action cannot be reliably determined but one or more
Actions can be parsed, it will process them and then send 422 Syntax
Error in Action as the last action for the transaction.  If the receiver
encounters an error such that is cannot determine a legal Transaction,
it will return a TransactionReply with a null TransactionID and a single
error descriptor (403 Syntax Error in Transaction).

If the end of a transaction can not be reliably determined and one or
more Actions can be parsed, it will process them and then return 403
Syntax Error in Transaction as the last action reply for the transac-
tion.  If no Actions can be parsed, it will return 403 Syntax Error in
Transaction as the only reply

If the terminationID cannot be reliably determined it will send 442 Syn-
tax Error in Command as the action reply.

If the end of a command cannot be reliably determined it will return 442
Syntax Error in Transaction as the reply to the last action it can
parse.

8.2.3.  TransactionPending

The receiver invokes the TransactionPending.  A TransactionPending



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indicates that the Transaction is actively being processed, but has not
been completed.  It is used to prevent the sender from assuming the
TransactionRequest was lost where the Transaction will take some time to
complete.

TransactionPending(TransactionID { } )

The TransactionID parameter must be the same as that of the correspond-
ing TransactionRequest. A property of root (normalMGExecutionTime) is
settable by the MGC to indicate the interval within which the MGC
expects a response to any transaction from the MG.  Another property
(normalMGCExecutionTime) is settable by the MGC to indicate the interval
within which the MG should expects a response to any transaction from
the MGC.  Senders may receive more than one TransactionPending for a
command.  If a duplicate request is received when pending, the responder
may send a duplicate pending immediately, or continue waiting for its
timer to trigger another Transaction Pending.

8.3.  Messages

Multiple Transactions can be concatenated into a Message.  Messages have
a header, which includes the identity of the sender. The Message Iden-
tifier (MID) of a message is set to a provisioned name (e.g. domain
address/domain name/device name) of the entity transmitting the message.
Domain name is a suggested default.

Every Message contains a Version Number identifying the version of the
protocol the message conforms to.  Versions consist of one or two
digits, beginning with version 1 for the present version of the proto-
col.

The transactions in a message are treated independently.  There is no
order implied, there is no application or protocol acknowledgement of a
message.

9.  TRANSPORT

The transport mechanism for the protocol should allow the reliable tran-
sport of transactions between an MGC and MG. The transport shall remain
independent of what particular commands are being sent and shall be
applicable to all application states.  There are several transports
defined for the protocol, which are defined in normative Annexes to this
document.  Additional Transports may be defined as additional annexes in
subsequent editions of this document, or in separate documents.  For
transport of the protocol over IP, MGCs shall implement both TCP and
UDP/ALF, an MG shall implement TCP or UDP/ALF or both.

The MG is provisioned with a name or address (such as DNS name or IP



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address) of a primary and zero or more secondary MGCs (see section
7.2.8) that is the address the MG uses to send messages to the MGC. If
TCP or UDP is used as the protocol transport and the port to which the
initial ServiceChange request is to be sent is not otherwise known, that
request should be sent to the default port number for the protocol.
This port number is 2944 for text-encoded operation or 2945 for binary-
encoded operation, for either UDP or TCP.  The MGC receives the message
containing the ServiceChange request from the MG and can determine the
MG's address from it. As described in section 7.2.8, either the MG or
the MGC may supply an address in the ServiceChangeAddress parameter to
which subsequent transaction requests must be addressed, but responses
(including the response to the initial ServiceChange request) must
always be sent back to the address which was the source of the
corresponding request.

9.1.  Ordering of Commands

This document does not mandate that the underlying transport protocol
guarantees the sequencing of transactions sent to an entity.  This pro-
perty tends to maximize the timeliness of actions, but it has a few
drawbacks.  For example:

*    Notify commands may be delayed and arrive at the MGC after the
     transmission of a new command changing the EventsDescriptor

*    If a new command is transmitted before a previous one is ack-
     nowledged, there is no guarantee that prior command will be exe-
     cuted before the new one.

Media Gateway Controllers that want to guarantee consistent operation of
the Media Gateway may use the following rules.  These rules are with
respect to commands that are in different transactions.  Commands that
are in the same transaction are executed in order (see section 8).

1.   When a Media Gateway handles several Terminations, commands per-
     taining to the different Terminations may be sent in parallel, for
     example following a model where each Termination (or group of Ter-
     minations) is controlled by its own process or its own thread.

2.   On a Termination, there should normally be at most one outstanding
     command (Add or Modify or Move), unless the outstanding commands
     are in the same transaction. However, a Subtract command may be
     issued at any time.  In consequence, a Media Gateway may sometimes
     receive a Modify command that applies to a previously subtracted
     Termination.  Such commands should be ignored, and an error code
     should be returned.

3.   On a given Termination, there should normally be at most one



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     outstanding Notify command at any time.

4.   In some cases, an implicitly or explicitly wildcarded Subtract com-
     mand that applies to a group of Terminations may step in front of a
     pending Add command.  The Media Gateway Controller should individu-
     ally delete all Terminations for which an Add command was pending
     at the time of the global Subtract command.  Also, new Add commands
     for Terminations named by the wild-carding (or implied in a Multi-
     plex descriptor) should not be sent until the wild-carded Subtract
     command is acknowledged.

5.   AuditValue and AuditCapability are not subject to any sequencing.

6.   ServiceChange shall always be the first command sent by a MG as
     defined by the restart procedure. Any other command or response
     must be delivered after this ServiceChange command.

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

9.2.  Protection against Restart Avalanche

In the event that a large number of Media Gateways are powered on simul-
taneously and they were to all initiate a ServiceChange transaction, the
Media Gateway Controller would very likely be swamped, leading to mes-
sage losses and network congestion during the critical period of service
restoration. In order to prevent such avalanches, the following behavior
is suggested:

1.   When a Media Gateway is powered on, it should initiate a restart
     timer to a random value, uniformly distributed between 0 and a max-
     imum waiting delay (MWD). Care should be taken to avoid synchroni-
     city of the random number generation between multiple Media Gate-
     ways that would use the same algorithm.

2.   The Media Gateway should then wait for either the end of this timer
     or the detection of a local user activity, such as for example an
     off-hook transition on a residential Media Gateway.

3.   When the timer elapses, or when an activity is detected, the Media
     Gateway should initiate the restart procedure.

The restart procedure simply requires the MG to guarantee that the first
message that the Media Gateway Controller sees from this MG is a Servi-
ceChange message informing the Media Gateway Controller about the res-
tart

Note - The value of MWD is a configuration parameter that depends on the



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type of the Media Gateway. The following reasoning may be used to deter-
mine the value of this delay on residential gateways.

Media Gateway Controllers 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 Media Gateway Controller
transactions between each Media Gateway and the Media Gateway Con-
troller.  This simple calculation shows that the Media Gateway Con-
troller is expected to handle 5 to 6 transactions for each Termination,
every 30 minutes on average, or, to put it otherwise, about one transac-
tion per Termination every 5 to 6 minutes on average.  This suggests
that a reasonable value of MWD for a residential gateway would be 10 to
12 minutes.  In the absence of explicit configuration, residential gate-
ways should adopt a value of 600 seconds for MWD.

The same reasoning suggests that the value of MWD should be much shorter
for trunking gateways or for business gateways, because they handle a
large number of Terminations, and also because the usage rate of these
Terminations is much higher than 10% during the peak busy hour, a typi-
cal value being 60%.  These Terminations, during the peak hour, are this
expected to contribute about one transaction per minute to the Media
Gateway Controller load. A reasonable algorithm is to make the value of
MWD per "trunk" Termination six times shorter than the MWD per residen-
tial gateway, and also inversely proportional to the number of Termina-
tions 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.

10.  SECURITY CONSIDERATIONS

This section covers security when using the protocol in an IP environ-
ment.

10.1.  Protection of Protocol Connections

A security mechanism is clearly needed to prevent unauthorized entities
from using the protocol defined in this document for setting up unau-
thorized calls or interfering with authorized calls. The security
mechanism for the protocol when transported over IP networks is IPsec
[RFC2401 to RFC2411].

The AH header [RFC2402] affords data origin authentication, connection-
less integrity and optional anti-replay protection of messages passed
between the MG and the MGC. The ESP header [RFC2406] provides confiden-
tiality of messages, if desired. For instance, the ESP encryption ser-
vice should be requested if the session descriptions are used to carry
session keys, as defined in SDP.



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Implementations of the protocol defined in this document employing the
ESP header SHALL comply with section 5 of [RFC2406], which defines a
minimum set of algorithms for integrity checking and encryption. Simi-
larly, implementations employing the AH header SHALL comply with section
5 of [RFC2402], which defines a minimum set of algorithms for integrity
checking using manual keys.

Implementations SHOULD use IKE [RFC2409] to permit more robust keying
options. Implementations employing IKE SHOULD support authentication
with RSA signatures and RSA public key encryption.

10.2.  Interim AH scheme

Implementation of IPsec requires that the AH or ESP header be inserted
immediately after the IP header. This cannot be easily done at the
application level.  Therefore, this presents a deployment problem for
the protocol defined in this document where the underlying network
implementation does not support IPsec.

As an interim solution, an optional AH header is defined within the
MEGACO protocol header. The header fields are exactly those of the SPI,
SEQUENCE NUMBER and DATA fields as defined in [RFC2402]. The semantics
of the header fields are the same as the "transport mode" of [RFC2402],
except for the calculation of the Integrity Check value (ICV). In IPsec,
the ICV is calculated over the entire IP packet including the IP header.
This prevents spoofing of the IP addresses.  To retain the same func-
tionality, the ICV calculation should be performed across the entire
transaction prepended by a synthesized IP header consisting of a 32 bit
source IP address, a 32 bit destination address and an 16 bit UDP
encoded as 10 hex digits. When the interim AH mechanism is employed when
TCP is the transport Layer, the UDP Port above becomes the TCP port, and
all other operations are the same.

Implementations of the MEGACO protocol SHALL implement IPsec where the
underlying operating system and the transport network supports IPsec.
Implementations of the protocol using IPv4 SHALL implement the interim
AH scheme. However, this interim scheme SHALL NOT be used when the
underlying network layer supports IPsec. IPv6 implementations are
assumed to support IPsec and SHALL NOT use the interim AH scheme.

All implementations of the interim AH mechanism SHALL comply with sec-
tion 5 of [RFC2402] which defines a minimum set of algorithms for
integrity checking using manual keys.

The interim AH interim scheme does not provide protection against eaves-
dropping; thus forbidding third parties from monitoring the connections
set up by a given termination. Also, it does not provide protection
against replay attacks.  These procedures do not necessarily protect



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against denial of service attacks by misbehaving MGs or misbehaving
MGCs. However, they will provide an identification of these misbehaving
entities, which should then be deprived of their authorization through
maintenance procedures.

10.3.  Protection of Media Connections

The protocol allows the MGC to provide MGs 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 must be decompressed and the signals must 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 Descriptor.  This
has two inconveniences: it slows down connection establishment and it
can be fooled by source spoofing:

*    To enable the address-based protection, the MGC must obtain the
     remote session description of the egress MG and pass it to the
     ingress MG.  This requires at least one network roundtrip, 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 roundtrip
     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 not slow down the call set- up, and provides
strong protection against address spoofing.

11.  MG-MGC CONTROL INTERFACE

The control association between MG and MGC is initiated at MG cold
start, and announced by a ServiceChange message, but can be changed by
subsequent events, such as failures or manual service events.  While the
protocol does not have an explicit mechanism to support multiple MGCs
controlling a physical MG, it has been designed to support the multiple



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logical MG (within a single physical MG) that can be associated with
different MGCs.

11.1.  Multiple Virtual MGs

A physical Media Gateway may be partitioned into one or more Virtual
MGs.  A virtual MG consists of a set of statically partitioned physical
Terminations and/or sets of ephemeral Terminations.  A physical Termina-
tion is controlled by one MGC.  The model does not require that other
resources be statically allocated, just Terminations.  The mechanism for
allocating Terminations to virtual MGs is a management method outside
the scope of the protocol.  Each of the virtual MGs appears to the MGC
as a complete MG client.

A physical MG may have only one network interface, which must be shared
across virtual MGs. In such a case, the packet/cell side Termination is
shared.  It should be noted however, that in use, such interfaces
require an ephemeral instance of the Termination to be created per flow,
and thus sharing the Termination is straightforward.  This mechanism
does lead to a complication, namely that the MG must always know which
of its controlling MGCs should be notified if an event occurs on the
interface.

In normal operation, the Virtual MG will be instructed by the MGC to
create network flows (if it is the originating side), or to expect flow
requests (if it is the terminating side), and no confusion will arise.
However, if an unexpected event occurs, the Virtual MG must know what to
do with respect to the physical resources it is controlling.

If recovering from the event requires manipulation of a physical
interface's state, only one MGC should do so.  These issues are resolved
by allowing any of the MGCs to create EventsDescriptors to be notified
of such events, but only one MGC can have read/write access to the phy-
sical interface properties; all other MGCs have read-only access.  The
management mechanism is used to designate which MGC has read/write capa-
bility, and is designated the Master MGC.

Each virtual MG has its own Root Termination.  In most cases the values
for the properties of the Root Termination are independently settable by
each MGC.  Where there can only be one value, the parameter is read-only
to all but the Master MGC.

ServiceChange may only be applied to a Termination or set of Termina-
tions partitioned to the Virtual MG or created (in the case of ephemeral
Terminations) by that Virtual MG.






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11.2.  Cold Start

A MG is pre-provisioned by a management mechanism outside the scope of
this protocol with a Primary and (optionally) an ordered list of Secon-
dary MGCs.  Upon a cold start of the MG, it will issue a ServiceChange
command with a "Restart" method, on the Root Termination to its primary
MGC.  If the MGC accepts the MG, it will send a Transaction Accept, with
the ServiceChangeMgcId set to itself.  If the MG receives an Servi-
ceChangeMgcId not equal to the MGC it contacted, it sends a Servi-
ceChange to the MGC specified in the ServiceChangeMgcId. It continues
this process until it gets a controlling MGC to accept its registration,
or it fails to get a reply.  Upon failure to obtain a reply, either from
the Primary MGC, or a designated successor, the MG tries its pre-
provisioned Secondary MGCs, in order.  If the MG is unable to comply and
it has established a transport connection to the MGC, it should close
that connection.  In any event, it should reject all subsequent requests
from the MGC with Error 406 Version Not Supported.

It is possible that the reply to a ServiceChange with Restart will be
lost, and a command will be received by the MG prior to the receipt of
the ServiceChange response.  The MG shall issue error 505 - Command
Received before Restart Response.

11.3.  Negotiation of Protocol Version

The first ServiceChange command from an MG shall contain the version
number of the protocol supported by the MG in the ServiceChangeVersion
parameter. Upon receiving such a message, if the MGC supports only a
lower version, then the MGC shall send a ServiceChangeReply with the
lower version and thereafter all the messages between MG and MGC shall
conform to the lower version of the protocol.  If the MG is unable to
comply and it has established a transport connection to the MGC, it
should close that connection.  In any event, it should reject all subse-
quent requests from the MGC with Error 406 Version Not supported.

If the MGC supports a higher version than the MG but is able to support
the lower version proposed by the MG, it shall send a ServiceChangeReply
with the lower version and thereafter all the messages between MG and
MGC shall conform to the lower version of the protocol. If the MGC is
unable to comply, it shall reject the association, with Error 406 Ver-
sion Not Supported.

Protocol version negotiation may also occur at "handoff" and "failover"
ServiceChanges.

When extending the protocol with new versions, the following rules
should be followed.




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1.   Existing protocol elements, i.e., procedures, parameters, descrip-
     tor, property,  values, should not be changed unless a protocol
     error needs to be corrected or it becomes necessary to change the
     operation of the service that is being supported by the protocol.

2.   The semantics of a command, a parameter, descriptor, property,
     value should not be changed.

3.   Established rules for formatting and encoding messages and parame-
     ters should not be modified.

4.   When information elements are found to be obsolete they can be
     marked as not used. However, the identifier for that information
     element will be marked as reserved. In that way it can not be used
     in future versions.

11.4.  Failure of an MG

If a MG fails, but is capable of sending a message to the MGC, it sends
a ServiceChange with an appropriate method (graceful or forced) and
specifies the Root TerminationID.  When it returns to service, it sends
a ServiceChange with a "Restart" method.

Allowing the MGC to send duplicate messages to both MGs accommodates
pairs of MGs that are capable of redundant failover of one of the MGs.
Only the Working MG shall accept or reject transactions.  Upon failover,
the Primary MG sends a ServiceChange command with a "Failover" method
and a "MG Impending Failure" reason.  The MGC then uses the primary MG
as the active MG.  When the error condition is repaired, the Working MG
can send a "ServiceChange" with a "Restart" method.

11.5.  Failure of an MGC

If the MG detects a failure of its controlling MGC, it attempts to con-
tact the next MGC on its pre-provisioned list.  It starts its attempts
at the beginning (Primary MGC), unless that was the MGC that failed, in
which case it starts at its first Secondary MGC.  It sends a Servi-
ceChange message with a "Failover" method and a " MGC Impending Failure"
reason.

In partial failure, or manual maintenance reasons, an MGC may wish to
direct its controlled MGs to use a different MGC.  To do so, it sends a
ServiceChange method to the MG with a "HandOff" method, and its desig-
nated replacement in ServiceChangeMgcId. The MG should send a Servi-
ceChange message with a "Handoff" method and a "MGC directed change"
reason to the designated MGC.  If it fails to get a reply, or fails to
see an Audit command subsequently, it should behave as if its MGC
failed, and start contacting secondary MGCs.  If the MG is unable to



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establish a control relationship with any MGC, it shall wait a random
amount of time as described in section 9.2 and then start contacting its
primary, and if necessary, its secondary MGCs again.

No recommendation is made on how the MGCs involved in the Handoff main-
tain state information; this is considered to be out of scope of this
recommendation. The MGC and MG may take the following steps when Handoff
occurs.  When the MGC initiates a HandOff, the handover should be tran-
sparent to Operations on the Media Gateway.  Transactions can be exe-
cuted in any order, and could be in progress when the ServiceChange is
executed. Accordingly, commands in progress continue, transaction
replies are sent to the new MGC (after a new control association is
established), and the MG should expect outstanding transaction replies
from the new MGC.  No new messages shall be sent to the new MGC until
the control association is established.  Repeated transaction requests
shall be directed to the new MGC.  The MG shall maintain state on all
terminations and contexts.

It is possible that the MGC could be implemented in such a way that a
failed MGC is replaced by a working MGC where the identity of the new
MGC is the same as the failed one.  In such a case, ServiceChangeMgcId
would be specified with the previous value and the MG shall behave as if
the value was changed, and send a ServiceChange message, as above.

Pairs of MGCs that are capable of redundant failover can notify the con-
trolled MGs of the failover by the above mechanism.

12.  PACKAGE DEFINITION

The primary mechanism for extension is by means of Packages.  Packages
define additional Properties, Events, Signals and Statistics that may
occur on Terminations.

Packages defined by IETF will appear in separate RFCs.

Packages defined by ITU-T may appear in the relevant recommendations
(e.g. as annexes).

1.   A public document or a standard forum document, which can be refer-
     enced as the document that describes the package following the
     guideline above, should be specified.

2.   The document shall specify the version of the Package that it
     describes.

3.   The document should be available on a public web server and should
     have a stable URL. The site should provide a mechanism to provide
     comments and appropriate responses should be returned.



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12.1.  Guidelines for defining packages

Packages define Properties, Events, Signals, and Statistics.

Packages may also define new error codes according to the guidelines
given in section 13.2. This is a matter of documentary convenience: the
package documentation is submitted to IANA in support of the error code
registration. If a package is modified, it is unnecessary to provide
IANA with a new document reference in support of the error code unless
the description of the error code itself is modified.

Names of all such defined constructs shall consist of the PackageID
(which uniquely identifies the package) and the ID of the item (which
uniquely identifies the item in that package).  In the text encoding the
two shall be separated by a forward slash ("/") character. Example:
togen/playtone is the text encoding to refer to the play tone signal in
the tone generation package.

A Package will contain the following sections:

12.1.1.  Package

Overall description of the package, specifying:

        Package Name: only descriptive,
        PackageID:  Is an identifier
        Description:
        Version:
                A new version of a package can only add additional
                Properties, Events, Signals, Statistics and new possible
                values for an existing parameter described in the
                package. No deletions or modifications shall be allowed.
                A version is an integer in the range from 1 to 99.
        Extends (Optional):
                A package may extend an existing package. The version of
                the original package must be specified. When a package
                extends another package it shall only add additional
                Properties, Events, Signals, Statistics and new possible
                values for an existing parameter described in the original
                package. An extended package shall not redefine or
                overload a name defined in the original package.
                Hence, if package B version 1 extends package A version 1,
                version 2 of B will not be able to extend the A version 2
                if A version 2 defines a name already in B version 1.







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12.1.2.  Properties

Properties defined by the package, specifying:

        Property Name: only descriptive.
        PropertyID:  Is an identifier
        Description:
        Type: One of:
                String: UTF-8 string
                Integer: 4 byte signed integer
                Double: 8 byte signed integer
                Character: Unicode UTF-8 encoding of a single letter.
                        Could be more than one octet.
                Enumeration: One of a list of possible unique values (See 12.3)
                Sub-list: A list of several values from a list
                Boolean
        Possible Values:
        Defined in:
                Which descriptor the property is defined in.  LocalControl
                is for stream dependent properties. TerminationState is for
                stream independent properties.
        Characteristics: Read / Write or both, and (optionally), global:
                Indicates whether a property is read-only, or read-write,
                and if it is global.  If Global is omitted, the property
                is not global.  If a property is declared as global,
                the value of the property is shared by all terminations
                realizing the package.


12.1.3.  Events

Events defined by the package, specifying:

        Event name: only descriptive.
        EventID:  Is an identifier
        Description:
        EventsDescriptor Parameters:
                Parameters used by the MGC to configure the event,
                and found in the EventsDescriptor.  See section 12.2.
        ObservedEventsDescriptor Parameters:
                Parameters returned to the MGC in  Notify requests and in
                replies to command requests from the MGC that audit
                ObservedEventsDescriptor, and found in the
                ObservedEventsDescriptor.  See section 12.2.







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12.1.4.  Signals

Signals defined by the package, specifying:

        Signal Name: only descriptive.
        SignalID:  Is an identifier. SignalID is used in a
                        SignalsDescriptor
        Description
        SignalType: One of:
                        OO (On/Off)
                        TO (TimeOut)
                        BR (Brief)


Note:SignalType may be defined such that it is dependent on the
          value of one or more parameters. Signals that would be played
     with SignalType BR should have a default duration. The package has
     to define the default duration and signalType.

          Duration: in hundredths of seconds
          Additional Parameters: See section 12.2


12.1.5.  Statistics

Statistics defined by the package, specifying:

        Statistic name: only descriptive.
        StatisticID:  Is an identifier
        StatisticID is used in a StatisticsDescriptor
        Description
        Units: unit of measure, e.g. milliseconds, packets


12.1.6.  Procedures

Additional guidance on the use of the package.

12.2.  Guidelines to defining  Properties, Statistics and Parameters to
Events and Signals.

        Parameter Name: only descriptive
        ParameterID: Is an identifier
        Type: One of:
                String: UTF-8 octet string
                Integer: 4 octet signed integer
                Double: 8 octet signed integer
                Character: Unicode UTF-8 encoding of a single letter.



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                        Could be more than one octet.
                Enumeration: One of a list of possible unique values
                        (See 12.3)
                Sub-list: A list of several values from a list
                Boolean
        Possible values:
        Description:


12.3.  Lists

Possible values for parameters include enumerations.  Enumerations may
be defined in a list.  It is recommended that the list be IANA
registered so that packages that extend the list can be defined without
concern for conflicting names.

12.4.  Identifiers

Identifiers in text encoding shall be strings of up to 64 characters,
containing no spaces, starting with an alphanumeric character and con-
sisting of alphanumeric characters and / or digits, and possibly includ-
ing the special character underscore ("_").  Identifiers in binary
encoding are 2 octets long.  Both text and binary values shall be speci-
fied for each identifier, including identifiers used as values in
enumerated types.

12.5.  Package Registration

A package can be registered with IANA for interoperability reasons.  See
section 13 for IANA considerations.

13.  IANA CONSIDERATIONS

13.1.  Packages

The following considerations SHALL be met to register a package with
IANA:

1.   A unique string name, unique serial number and version number is
     registered for each package.  The string name is used with text
     encoding.  The serial number shall be used with binary encoding.
     Serial Numbers 60000-64565 are reserved for private use. Serial
     number 0 is reserved.

2.   A contact name, email and postal addresses for that contact shall
     be specified.  The contact information shall be updated by the
     defining organization as necessary.




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3.   A reference to a document that describes the package, which should
     be public: The document shall specify the version of the Package
     that it describes.  If the document is public, it should be located
     on a public web server and should have a stable URL. The site
     should provide a mechanism to provide comments and appropriate
     responses should be returned.

4.   Packages registered by other than recognized standards bodies shall
     have a minimum package name length of 8 characters

5.   All other package names are first come-first served if all other
     conditions are met

13.2.  Error Codes

The following considerations SHALL be met to register an error code with
IANA:

1.   An error number and a one line (80 character maximum) string is
     registered for each error.

2.   A complete description of the conditions under which the error is
     detected shall be included in a publicly available document.  The
     description shall be sufficiently clear to differentiate the error
     from all other existing error codes.

3.   The document should be available on a public web server and should
     have a stable URL.

4.   Error numbers registered by recognized standards bodies shall have
     3 or 4 character error numbers.

5.   Error numbers registered by all other organizations or individuals
     shall have 4 character error numbers.

6.   An error number shall not be redefined, nor modified except by the
     organization or individual that originally defined it, or their
     successors or assigns.

13.3.  ServiceChange Reasons

The following considerations SHALL be met to register service change
reason with IANA:

1.   A one phrase, 80-character maximum, unique reason code is
     registered for each reason.

2.   A complete description of the conditions under which the reason is



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     used is detected shall be included in a publicly available docu-
     ment.  The description shall be sufficiently clear to differentiate
     the reason from all other existing reasons.

3.   The document should be available on a public web server and should
     have a stable URL.

14.  CONTACT INFORMATION

IETF Editor
        Brian Rosen
        Marconi
        1000 FORE Drive
        Warrendale, PA  15086
        U.S.A.
        Phone: +1 724-742-6826
        Email: brosen@fore.com
ITU Editor
        John Segers
        Lucent Technologies
        Room HE 306
        Dept. Forward Looking Work
        P.O. Box 18, 1270 AA  Huizen
        Netherlands
        Phone: +31 35 687 4724
        Email: jsegers@lucent.com

Additional IETF Authors
        Fernando Cuervo
        Nortel Networks
        P.O. Box 3511 Stn C Ottawa, ON, K1Y 4H7
        Canada
        Email: cuervo@nortelnetworks.com

        Bryan Hill
        Gotham Networks
        15 Discovery Way
        Acton, MA 01720
        USA
        Phone: +1 978-263-6890
        Email: bhill@gothamnetworks.com

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



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

        Nancy Greene
        Nortel Networks
        P.O. Box 3511 Stn C
        Ottawa, ON, K1Y 4H7
        Canada
        Phone: +1 514-271-7221
        Email: ngreene@nortelnetworks.com

        Abdallah Rayhan
        Nortel Networks
        P.O. Box 3511 Stn C
        Ottawa, ON, K1Y 4H7
        Canada
        Phone: +1 613-763-9611
        Email: arayhan@nortelnetworks.com



ANNEX A BINARY ENCODING OF THE PROTOCOL (NORMATIVE)

This Annex specifies the syntax of messages using the notation defined
in ASN.1 [ITU-T Recommendation X.680 (1997): Information Technology -
Abstract Syntax Notation One (ASN.1) - Specification of basic nota-
tion.]. Messages shall be encoded for transmission by applying the basic
encoding rules specified in [ITU-T Recommendation X.690(1994) Informa-
tion Technology - ASN.1  Encoding Rules: Specification  of  Basic
Encoding Rules (BER)].

A.1.  Coding of wildcards

The use of wildcards ALL and CHOOSE is allowed in the protocol.  This
allows a MGC to partially specify Termination IDs and let the MG choose
from the values that conform to the partial specification.  Termination
IDs may encode a hierarchy of names.  This hierarchy is provisioned. For
instance, a TerminationID may consist of a trunk group, a trunk within
the group and a circuit.  Wildcarding must be possible at all levels.
The following paragraphs explain how this is achieved.

The ASN.1 description uses octet strings of up to 8 octets in length for
Termination IDs.  This means that Termination IDs consist of at most 64
bits.  A fully specified Termination ID may be preceded by a sequence of
wildcarding fields.  A wildcarding field is octet in length.  Bit 7 (the
most significant bit) of this octet specifies what type of wildcarding
is invoked:  if the bit value equals 1, then the ALL wildcard is used;
if the bit value if 0, then the CHOOSE wildcard is used. Bit 6 of the



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wildcarding field specifies whether the wildcarding pertains to one
level in the hierarchical naming scheme (bit value 0) or to the level of
the hierarchy specified in the wildcarding field plus all lower levels
(bit value 1).  Bits 0 through 5 of the wildcarding field specify the
bit position in the Termination ID at which the starts.

We illustrate this scheme with some examples.  In these examples, the
most significant bit in a string of bits appears on the left hand side.

Assume that Termination IDs are three octets long and that each octet
represents a level in a hierarchical naming scheme.  A valid Termination
ID is

          00000001 00011110 01010101.

Addressing ALL names with prefix 00000001 00011110 is done as follows:

          wildcarding field: 10000111

          Termination ID: 00000001 00011110 xxxxxxxx.

The values of the bits labeled "x" is irrelevant and shall be ignored by
the receiver.

Indicating to the receiver that is must choose a name with 00011110 as
the second octet is done as follows:

          wildcarding fields: 00010111 followed by 00000111

          Termination ID: xxxxxxxx 00011110 xxxxxxxx.  The first wild-
     card field indicates a CHOOSE wildcard for the level in the naming
     hierarchy starting at bit 23, the highest level in our assumed nam-
     ing scheme.  The second wildcard field indicates a CHOOSE wildcard
     for the level in the naming hierarchy starting at bit 7, the lowest
     level in our assumed naming scheme.

Finally, a CHOOSE-wildcarded name with the highest level of the name
equal to 00000001 is specified as follows:

          wildcard field: 01001111

          Termination ID: 0000001 xxxxxxxx xxxxxxxx .

Bit value 1 at bit position 6 of the first octet of the wildcard field
indicates that the wildcarding pertains to the specified level in the
naming hierarchy and all lower levels.

Context IDs may also be wildcarded.  In the case of Context IDs,



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however, specifying partial names is not allowed.  Context ID 0x0  SHALL
be used to indicate the NULL Context, Context ID 0xFFFFFFFE SHALL be
used to indicate a CHOOSE wildcard, and Context ID 0xFFFFFFFF SHALL be
used to indicate an ALL wildcard.

TerminationID 0xFFFFFFFFFFFFFFFF SHALL be used to indicate the ROOT Ter-
mination.

A.2.  ASN.1 syntax specification

This section contains the ASN.1 specification of the H.248 protocol syn-
tax.

NOTE In case a transport mechanism is used that employs application
     level framing, the definition of Transaction below changes.  Refer
     to the annex defining the transport mechanism for the definition
     that applies in that case.

NOTE The ASN.1 specification below contains a clause defining Termina-
     tionIDList as a sequence of TerminationIDs.  The length of this
     sequence SHALL be one.  The SEQUENCE OF construct is present only
     to allow future extensions.


     MEDIA-GATEWAY-CONTROL DEFINITIONS AUTOMATIC TAGS::=
     BEGIN


     MegacoMessage ::= SEQUENCE
     {
          authHeader  AuthenticationHeader OPTIONAL,
          mess Message
     }

     AuthenticationHeader ::= SEQUENCE
     {
          secParmIndex   SecurityParmIndex,
          seqNum            SequenceNum,
          ad           AuthData
     }

     SecurityParmIndex ::= OCTET STRING(SIZE(4))

     SequenceNum       ::= OCTET STRING(SIZE(4))

     AuthData          ::= OCTET STRING (SIZE (16..32))

     Message ::= SEQUENCE



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     {
          version        INTEGER(0..99),
     -- The version of the protocol defined here is equal to 1.
          mId          MId,-- Name/address of message originator
          messageBodyCHOICE
          {
          messageErrorErrorDescriptor,
          transactions SEQUENCE OF Transaction
          },
          ...
     }

     MId ::= CHOICE
     {
          ip4Address IP4Address,
          ip6AddressIP6Address,
          domainName   DomainName,
          deviceName   PathName,
     mtpAddressOCTET STRING(SIZE(2)),
      -- Addressing structure of mtpAddress:
      --        15                0
      --        |  PC        | NI |
      --           14 bits    2 bits
           ...
     }

     DomainName ::= SEQUENCE
     {
     name IA5String,
     -- The name starts with an alphanumeric digit followed by a
     -- sequence of alphanumeric digits, hyphens and dots.  No two
     -- dots shall occur consecutively.
     portNumberINTEGER(0..65535) OPTIONAL
     }

     IP4Address ::= SEQUENCE
     {
          addressOCTET STRING (SIZE(4)),
          portNumberINTEGER(0..65535) OPTIONAL
     }

     IP6Address ::= SEQUENCE
     {
     addressOCTET STRING (SIZE(16)),
          portNumberINTEGER(0..65535) OPTIONAL
     }

     PathName ::= IA5String(SIZE (1..64))



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     -- See section A.3

     Transaction ::= CHOICE
     {
          transactionRequest  TransactionRequest,
          transactionPending TransactionPending,
          transactionReply   TransactionReply,
          transactionResponseAckTransactionResponseAck,
               -- use of response acks is dependent on underlying transport
          ...
     }

     TransactionId ::= INTEGER(0..4294967295)  -- 32 bit unsigned integer

     TransactionRequest ::= SEQUENCE
     {
          transactionId TransactionId,
          actions       SEQUENCE OF ActionRequest,
          ...
     }

     TransactionPending ::= SEQUENCE
     {
          transactionId TransactionId,
          ...
     }

     TransactionReply ::= SEQUENCE
     {
          transactionId TransactionId,
          transactionResult CHOICE
          {
               transactionErrorErrorDescriptor,
               actionRepliesSEQUENCE OF ActionReply
          },
          ...
     }

     TransactionResponseAck ::= SEQUENCE
     {
          firstAckTransactionId,
          lastAckTransactionId OPTIONAL
     }

     ErrorDescriptor ::= SEQUENCE
     {
          errorCode ErrorCode,
          errorText ErrorText OPTIONAL



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     }

     ErrorCode ::= INTEGER(0..65535)
     -- See section 13 for IANA considerations w.r.t. error codes

     ErrorText ::= IA5String

     ContextID ::= INTEGER(0..4294967295)

     -- Context NULL Value: 0
     -- Context CHOOSE Value: 429467294 (0xFFFFFFFE)
     -- Context ALL Value: 4294967295 (0xFFFFFFFF)


     ActionRequest ::= SEQUENCE
     {
          contextIdContextID,
          contextRequestContextRequest OPTIONAL,
          contextAttrAuditReqContextAttrAuditRequest OPTIONAL,
          commandRequestsSEQUENCE OF CommandRequest
     }

     ActionReply ::= SEQUENCE
     {
          contextId ContextID,
          errorDescriptorErrorDescriptor OPTIONAL,
          contextReplyContextRequest OPTIONAL,
          commandReply SEQUENCE OF CommandReply
     }

     ContextRequest ::= SEQUENCE
     {
          priorityINTEGER(0..15) OPTIONAL,
          emergencyBOOLEAN OPTIONAL,
          topologyReqSEQUENCE OF TopologyRequest OPTIONAL,
          ...
     }

     ContextAttrAuditRequest ::= SEQUENCE
     {
     topologyNULL OPTIONAL,
          emergencyNULL OPTIONAL,
          priorityNULL OPTIONAL,
          ...
     }

     CommandRequest ::= SEQUENCE
     {



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          commandCommand,
          optionalNULL OPTIONAL,
          wildcardReturnNULL OPTIONAL,
          ...
     }

     Command ::= CHOICE
     {
          addReq AmmRequest,
          moveReq           AmmRequest,
          modReq            AmmRequest,
          -- Add, Move, Modify requests have the same parameters
          subtractReq       SubtractRequest,
          auditCapRequest   AuditRequest,
          auditValueRequest AuditRequest,
          notifyReq         NotifyRequest,
          serviceChangeReq  ServiceChangeRequest,
          ...
     }

     CommandReply ::= CHOICE
     {
          addReply           AmmsReply,
          moveReply          AmmsReply,
          modReply           AmmsReply,
          subtractReply      AmmsReply,
          -- Add, Move, Modify, Subtract replies have the same parameters
          auditCapReply      AuditReply,
          auditValueReply    AuditReply,
          notifyReply        NotifyReply,
          serviceChangeReply ServiceChangeReply,
          ...
     }

     TopologyRequest ::= SEQUENCE
     {
          terminationFrom      TerminationID,
          terminationTo      TerminationID,
          topologyDirection ENUMERATED
          {
          bothway(0),
          isolate(1),
          oneway(2)
          }
     }

     AmmRequest ::= SEQUENCE
     {



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          terminationID      TerminationIDList,
          mediaDescriptor    MediaDescriptor OPTIONAL,
          modemDescriptor    ModemDescriptor OPTIONAL,
          muxDescriptor      MuxDescriptor OPTIONAL,
          eventsDescriptor   EventsDescriptor OPTIONAL,
          eventBufferDescriptorEventBufferDescriptor OPTIONAL,
          signalsDescriptor  SignalsDescriptor OPTIONAL,
          digitMapDescriptor DigitMapDescriptor OPTIONAL,
          auditDescriptor    AuditDescriptor OPTIONAL,
          ...
     }

     AmmsReply ::= SEQUENCE
     {
          terminationID TerminationIDList,
          terminationAudit TerminationAudit OPTIONAL
     }

     SubtractRequest ::= SEQUENCE
     {
          terminationID   TerminationIDList,
          auditDescriptor AuditDescriptor OPTIONAL,
          ...
     }

     AuditRequest ::= SEQUENCE
     {
          terminationIDTerminationID,
          auditDescriptorAuditDescriptor,
          ...
     }

     AuditReply ::= SEQUENCE
     {
          terminationIDTerminationID,
          auditResultAuditResult
     }

     AuditResult ::= CHOICE
     {
          contextAuditResultTerminationIDList,
          terminationAuditResultTerminationAudit
     }



     AuditDescriptor ::= SEQUENCE
     {



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     auditTokenBIT STRING
     {
          muxToken(0), modemToken(1), mediaToken(2),
     eventsToken(3), signalsToken(4),
     digitMapToken(5), statsToken(6),
     observedEventsToken(7),
     packagesToken(8), eventBufferToken(9)
     } OPTIONAL,
          ...
     }

     TerminationAudit ::= SEQUENCE OF AuditReturnParameter

     AuditReturnParameter ::= CHOICE
     {
          errorDescriptorErrorDescriptor,
     mediaDescriptor MediaDescriptor,
          modemDescriptor ModemDescriptor,
          muxDescriptor MuxDescriptor,
          eventsDescriptor      EventsDescriptor,
          eventBufferDescriptorEventBufferDescriptor,
          signalsDescriptor     SignalsDescriptor,
          digitMapDescriptor      DigitMapDescriptor,
          observedEventsDescriptor ObservedEventsDescriptor,
          statisticsDescriptor     StatisticsDescriptor,
          packagesDescriptor       PackagesDescriptor,
          ...
     }

     NotifyRequest ::= SEQUENCE
     {
          terminationID            TerminationIDList,
          observedEventsDescriptor ObservedEventsDescriptor,
          errorDescriptorErrorDescriptor OPTIONAL,
          ...
     }

     NotifyReply ::= SEQUENCE
     {
          terminationID   TerminationIDList OPTIONAL,
          errorDescriptor ErrorDescriptor OPTIONAL,
          ...
     }

     ObservedEventsDescriptor ::= SEQUENCE
     {
          requestId        RequestID,
          observedEventLst SEQUENCE OF ObservedEvent



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     }

     ObservedEvent ::= SEQUENCE
     {
          eventNameEventName,
          streamIDStreamID OPTIONAL,
          eventParListSEQUENCE OF EventParameter,
          timeNotation  TimeNotation OPTIONAL
     }

     EventName ::= PkgdName

     EventParameter ::= SEQUENCE
     {
          eventParameterNameName,
          value            Value
     }

     ServiceChangeRequest ::= SEQUENCE
     {
          terminationID      TerminationIDList,
          serviceChangeParms ServiceChangeParm,
          ...
     }

     ServiceChangeReply ::= SEQUENCE
     {
          terminationID       TerminationIDList,
          serviceChangeResult ServiceChangeResult,
          ...
     }

     -- For ServiceChangeResult, no parameters are mandatory.  Hence the
     -- distinction between ServiceChangeParm and ServiceChangeResParm.

     ServiceChangeResult ::= CHOICE
     {
          errorDescriptor       ErrorDescriptor,
          serviceChangeResParms ServiceChangeResParm
     }

     WildcardField ::= OCTET STRING(SIZE(1))

     TerminationID ::= SEQUENCE
     {
          wildcardSEQUENCE OF WildcardField,
          idOCTET STRING(SIZE(1..8))
     }



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     -- See Section A.1 for explanation of wildcarding mechanism.
     -- Termination ID 0xFFFFFFFFFFFFFFFF indicates the ROOT Termination.

     TerminationIDList ::= SEQUENCE OF TerminationID

     MediaDescriptor ::= SEQUENCE
     {

          termStateDescrTerminationStateDescriptor OPTIONAL,
          streamsCHOICE
          {
          oneStreamStreamParms,
          multiStreamSEQUENCE OF StreamDescriptor
          },
          ...
     }

     StreamDescriptor ::= SEQUENCE
     {
          streamID               StreamID,
          streamParmsStreamParms
     }

     StreamParms ::= SEQUENCE
     {
          localControlDescriptor LocalControlDescriptor OPTIONAL,
          localDescriptor        LocalRemoteDescriptor OPTIONAL,
          remoteDescriptor       LocalRemoteDescriptor OPTIONAL,
          ...
     }

     LocalControlDescriptor ::= SEQUENCE
     {
          streamMode    StreamMode OPTIONAL,
          reserveBOOLEAN,
          propertyParms SEQUENCE OF PropertyParm,
          ...
     }

     StreamMode ::= ENUMERATED
     {
          sendOnly(0),
          recvOnly(1),
          sendRecv(2),
          inactive(3),
          loopBack(4),
          ...
     }



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     -- In PropertyParm, value is a SEQUENCE OF octet string.  When sent
     -- by an MGC the interpretation is as follows:
     -- empty sequence means CHOOSE
     -- one element sequence specifies value
     -- longer sequence means "choose one of the values"
     -- The relation field may only be selected if the value sequence
     -- has length 1.  It indicates that the MG has to choose a value
     -- for the property. E.g., x > 3 (using the greaterThan
     -- value for relation) instructs the MG to choose any value larger
     -- than 3 for property x.
     -- The range field may only be selected if the value sequence
     -- has length 2.  It indicates that the MG has to choose a value
     -- in the range between the first octet in the value sequence and
     -- the trailing octet in the value sequence, including the
     -- boundary values.
     -- When sent by the MG, only responses to an AuditCapability request
     -- may contain multiple values, a range, or a relation field.

     PropertyParm ::= SEQUENCE
     {
          name PkgdName,
          value SEQUENCE OF OCTET STRING,
          extraInfoCHOICE
          {
          relationRelation,
          rangeBOOLEAN
          } OPTIONAL

     }

     Name ::= OCTET STRING(SIZE(2))

     PkgdName ::= OCTET STRING(SIZE(4))
     -- represents Package Name (2 octets) plus Property Name (2 octets)
     -- To wildcard a package use 0xFFFF for first two octets, choose
     -- is not allowed. To reference native property tag specified in
     -- Annex C, use 0x0000 as first two octets.
     -- Wildcarding of Package Name is permitted only if Property Name is
     -- also wildcarded.

     Relation ::= ENUMERATED
     {
          greaterThan(0),
          smallerThan(1),
          unequalTo(2),
          ...
     }




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     LocalRemoteDescriptor ::= SEQUENCE
     {
          propGrpsSEQUENCE OF PropertyGroup,
          ...
     }

     PropertyGroup ::= SEQUENCE OF PropertyParm

     TerminationStateDescriptor ::= SEQUENCE
     {
          propertyParms       SEQUENCE OF PropertyParm,
          eventBufferControl  EventBufferControl OPTIONAL,
          serviceState        ServiceState OPTIONAL,
          ...
     }

     EventBufferControl ::= ENUMERATED
     {
          Off(0),
          LockStep(1),
          ...
     }

     ServiceState ::= ENUMERATED
     {
          test(0),
          outOfSvc(1),
          inSvc(2),
           ...
     }

     MuxDescriptor   ::= SEQUENCE
     {
          muxType         MuxType,
          termList      SEQUENCE OF TerminationID,
          nonStandardDataNonStandardData OPTIONAL,
          ...
     }

     MuxType ::= ENUMERATED
     {
          h221(0),
          h223(1),
          h226(2),
          v76(3),
          ...
     }




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     StreamID ::= INTEGER(0..65535)  -- 16 bit unsigned integer

     EventsDescriptor ::= SEQUENCE
     {
          requestID  RequestID,
          eventList  SEQUENCE OF RequestedEvent
     }

     RequestedEvent ::= SEQUENCE
     {
          pkgdName    PkgdName,
          streamIDStreamID OPTIONAL,
          eventAction RequestedActions OPTIONAL,
          evParList   SEQUENCE OF EventParameter
     }

     RequestedActions ::= SEQUENCE
     {
          keepActiveBOOLEAN,
          eventDMEventDM OPTIONAL,
          secondEvent         SecondEventsDescriptor OPTIONAL,
          signalsDescriptor   SignalsDescriptor OPTIONAL,
          ...
     }


     EventDM ::= CHOICE
     {    digitMapNameDigitMapName,
          digitMapValueDigitMapValue
     }

     SecondEventsDescriptor ::= SEQUENCE
     {
          requestID RequestID,
          eventList SEQUENCE OF SecondRequestedEvent
     }

     SecondRequestedEvent ::= SEQUENCE
     {
          pkgdName    PkgdName,
          streamIDStreamID OPTIONAL,
          eventAction SecondRequestedActions OPTIONAL,
          evParList   SEQUENCE OF EventParameter
     }

     SecondRequestedActions ::= SEQUENCE
     {
          keepActiveBOOLEAN,



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          eventDMEventDM OPTIONAL,
          signalsDescriptor SignalsDescriptor OPTIONAL,
          ...
     }

     EventBufferDescriptor ::= SEQUENCE OF ObservedEvent

     SignalsDescriptor ::= SEQUENCE OF SignalRequest

     SignalRequest ::=CHOICE
     {
          signalSignal,
          seqSigListSeqSigList
     }

     SeqSigList ::= SEQUENCE
     {
          idINTEGER(0..65535),
          signalListSEQUENCE OF Signal
     }

     Signal ::= SEQUENCE
     {
          signalName SignalName,
          streamIDStreamID OPTIONAL,
          sigTypeSignalType OPTIONAL,
          durationINTEGER (0..65535) OPTIONAL,
          notifyCompletionBOOLEAN OPTIONAL,
          keepActiveBOOLEAN OPTIONAL,
          sigParList SEQUENCE OF SigParameter
     }

     SignalType ::= ENUMERATED
     {
          brief(0),
          onOff(1),
          timeOut(2),
          ...
     }

     SignalName ::= PkgdName

     SigParameter ::= SEQUENCE
     {
          sigParameterName Name,
          value            Value
     }




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     RequestID ::= INTEGER(0..4294967295)   -- 32 bit unsigned integer

     ModemDescriptor ::= SEQUENCE
     {
     mtl  SEQUENCE OF ModemType,
          mpl SEQUENCE OF PropertyParm,nonStandardData
          NonStandardData OPTIONAL
     }

     ModemType ::= ENUMERATED
     {
          v18(0),
          v22(1),
          v22bis(2),
          v32(3),
          v32bis(4),
          v34(5),
          v90(6),
          v91(7),
          synchISDN(8),
          ...
     }

     DigitMapDescriptor ::= SEQUENCE
     {
          digitMapName  DigitMapName,
          digitMapValue DigitMapValue
     }

     DigitMapName ::= Name

     DigitMapValue ::= SEQUENCE
     {
          startTimer INTEGER(0..99) OPTIONAL,
          shortTimer INTEGER(0..99) OPTIONAL,
          longTimer  INTEGER(0..99) OPTIONAL,
          digitMapBody IA5String
          -- See Section A.3 for explanation of digit map syntax
     }

     ServiceChangeParm ::= SEQUENCE
     {
          serviceChangeMethod  ServiceChangeMethod,
          serviceChangeAddressServiceChangeAddress OPTIONAL,
          serviceChangeVersionINTEGER(0..99) OPTIONAL,
          serviceChangeProfile ServiceChangeProfile OPTIONAL,
          serviceChangeReason  Value,
          serviceChangeDelay   INTEGER(0..4294967295) OPTIONAL,



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                                      -- 32 bit unsigned integer
          serviceChangeMgcId   MId OPTIONAL,
          timeStampTimeNotation OPTIONAL,
          nonStandardDataNonStandardData OPTIONAL,
     }

     ServiceChangeAddress ::= CHOICE
     {
          portNumberINTEGER(0..65535), -- TCP/UDP port number
          ip4Address IP4Address,
          ip6AddressIP6Address,
          domainName   DomainName,
          deviceName   PathName,
     mtpAddressOCTET STRING(SIZE(2)),
          ...
     }

     ServiceChangeResParm ::= SEQUENCE
     {
          serviceChangeMgcId MId OPTIONAL,
          serviceChangeAddressServiceChangeAddress OPTIONAL,
          serviceChangeVersionINTEGER(0..99) OPTIONAL,
     serviceChangeProfileServiceChangeProfile OPTIONAL
     }

     ServiceChangeMethod ::= ENUMERATED
     {
          failover(0),
          forced(1),
          graceful(2),
          restart(3),
          disconnected(4),
          handOff(5),
          ...
     }

     ServiceChangeProfile ::= SEQUENCE
     {
          profileName Name,
          version       INTEGER(0..99)
     }

     PackagesDescriptor ::= SEQUENCE OF PackagesItem

     PackagesItem ::= SEQUENCE
     {
     packageNameName,
     packageVersionINTEGER(0..99)



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     }

     StatisticsDescriptor ::= SEQUENCE OF StatisticsParameter

     StatisticsParameter ::= SEQUENCE
     {
          statName PkgdName,
          statValue Value
     }

     NonStandardData ::= SEQUENCE
     {
          nonStandardIdentifierNonStandardIdentifier,
          dataOCTET STRING
     }

     NonStandardIdentifier::= CHOICE
     {
          objectOBJECT IDENTIFIER,
          h221NonStandardH221NonStandard,
          experimentalIA5STRING(SIZE(8)),
      -- first two characters should be "X-" or "X+"
          ...
     }

     H221NonStandard ::= SEQUENCE
     {    t35CountryCodeINTEGER(0..255), -- country, as per T.35
          t35ExtensionINTEGER(0..255), -- assigned nationally
          manufacturerCodeINTEGER(0..65535), -- assigned nationally
          ...
     }

     TimeNotation ::= SEQUENCE
     {
          date IA5String(SIZE(8)),  -- yyyymmdd format
          time IA5String(SIZE(8))  -- hhmmssss format
     }

     Value ::= OCTET STRING


     END


A.3.  Digit maps and path names

>From a syntactic viewpoint, digit maps are strings with syntactic res-
trictions imposed upon them. The syntax of valid digit maps is specified



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in ABNF [RFC 2234].  The syntax for digit maps presented in this section
is for illustrative purposes only. The definition of digitMap in Annex B
takes precedence in the case of differences between the two.


digitMap = (digitString / LWSP "(" LWSP digitStringList LWSP ")"
LWSP)
digitStringList = digitString *( LWSP "/" LWSP digitString )
digitString = 1*(digitStringElement)
digitStringElement = digitPosition [DOT]
digitPosition = digitMapLetter / digitMapRange
digitMapRange = ("x" / LWSP "[" LWSP digitLetter LWSP "]" LWSP)
digitLetter = *((DIGIT "-" DIGIT) /digitMapLetter)
digitMapLetter = DIGIT               ;digits 0-9
    / %x41-4B / %x61-6B    ;a-k and A-K
                    / "L"       / "S"                   ;Inter-event timers
;(long, short)
                    / "Z"                               ;Long duration event
LWSP = *(WSP / COMMENT / EOL)
WSP = SP / HTAB
COMMENT = ";" *(SafeChar / RestChar / WSP) EOL
EOL = (CR [LF]) / LF
SP = %x20
HTAB = %x09
CR = %x0D
LF = %x0A
SafeChar = DIGIT / ALPHA / "+" / "-" / "&" / "!" / "_" / "/" /
"'" / "?" / "@" / "^" / "`" / "~" / "*" / "$" / "
"(" / ")" / "%" / "."
RestChar = ";" / "[" / "]" / "{" / "}" / ":" / "," / "#" /
                "<" / ">" / "=" / %x22
DIGIT = %x30-39                 ; digits 0 through 9
ALPHA = %x41-5A / %x61-7A       ; A-Z, a-z


A path name is also a string with syntactic restrictions imposed upon
it.  The ABNF production defining it is copied from Annex B.


PathName = NAME *(["/"] ["*"] ["@"] (ALPHA / DIGIT)) ["*"]
NAME = ALPHA *63(ALPHA / DIGIT / "_" )



ANNEX B TEXT ENCODING OF THE PROTOCOL (NORMATIVE)






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B.1.  Coding of wildcards

In a text encoding of the protocol, while TerminationIDs are arbitrary,
by judicious choice of names, the wildcard character, "*" may be made
more useful.  When the wildcard character is encountered, it will
"match" all TerminationIDs having the same previous and following char-
acters (if appropriate).  For example, if there were TerminationIDs of
R13/3/1, R13/3/2 and R13/3/3, the TerminationID R13/3/* would match all
of them.  There are some circumstances where ALL Terminations must be
referred to.  The TerminationID "*" suffices, and is referred to as ALL.
The CHOOSE TerminationID "$" may be used to signal to the MG that it has
to create an ephemeral Termination or select an idle physical Termina-
tion.

B.2.  ABNF specification

The protocol syntax is presented in ABNF according to RFC2234.


megacoMessage        = LWSP [authenticationHeader SEP ] message

authenticationHeader = AuthToken EQUAL SecurityParmIndex COLON
                       SequenceNum COLON AuthData

SecurityParmIndex    = "0x" 8(HEXDIG)
SequenceNum          = "0x" 8(HEXDIG)
AuthData             = "0x" 32*64(HEXDIG)

message            = MegacopToken SLASH Version SEP mId SEP messageBody

messageBody          = ( errorDescriptor / transactionList )

transactionList      = 1*( transactionRequest / transactionReply /
                       transactionPending)

transactionPending   = PendingToken EQUAL TransactionID LBRKT
RBRKT

transactionResponseAck = ResponseAckToken LBRKT transactionAck
                                        *(COMMA transactionAck) RBRKT
transactionAck = transactionID / (transactionID "-" transactionID)

transactionRequest   = TransToken EQUAL TransactionID LBRKT
                       actionRequest *(COMMA actionRequest) RBRKT

actionRequest        = CtxToken EQUAL ContextID LBRKT ((
                       contextRequest [COMMA  commandRequestList])
                       / commandRequestList) RBRKT



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contextRequest     = ((contextProperties [COMMA contextAudit])
                           / contextAudit)

contextProperties    = contextProperty *(COMMA contextProperty)

; at-most-once
contextProperty      = (topologyDescriptor / priority / EmergencyToken)

contextAudit       = ContextAuditToken LBRKT
                       contextAuditProperties *(COMMA
                       contextAuditProperties) RBRKT

; at-most-once
contextAuditProperties = ( TopologyToken / EmergencyToken /
                           PriorityToken )

commandRequestList= ["O-"] commandRequest *(COMMA ["O-"]commandRequest)

commandRequest       = ( ammRequest / subtractRequest / auditRequest /
                        notifyRequest / serviceChangeRequest)

transactionReply     = ReplyToken EQUAL TransactionID LBRKT
                       ( errorDescriptor / actionReplyList ) RBRKT

actionReplyList      = actionReply *(COMMA actionReply )

actionReply          = CtxToken EQUAL ContextID LBRKT
                       ( errorDescriptor / commandReply ) RBRKT

commandReply       = (( contextProperties [COMMA commandReplyList] ) /
                        commandReplyList )


commandReplyList     = commandReplys *(COMMA commandReplys )

commandReplys        = (serviceChangeReply / auditReply / ammsReply /
                        notifyReply )

;Add Move and Modify have the same request parameters
ammRequest           = (AddToken / MoveToken / ModifyToken ) EQUAL
                       TerminationID [LBRKT ammParameter *(COMMA
                       ammParameter) RBRKT]

;at-most-once
ammParameter         = (mediaDescriptor / modemDescriptor /
                        muxDescriptor / eventsDescriptor /
                        signalsDescriptor / digitMapDescriptor /
                        eventBufferDescriptor / auditDescriptor)



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ammsReply            = (AddToken / MoveToken / ModifyToken /
                        SubtractToken ) EQUAL TerminationID [ LBRKT
                        terminationAudit RBRKT ]

subtractRequest      =  ["W-"] SubtractToken EQUAL TerminationID
                        [ LBRKT auditDescriptor RBRKT]

auditRequest         = ["W-"] (AuditValueToken / AuditCapToken ) EQUAL
                        TerminationID LBRKT auditDescriptor RBRKT

auditReply           = (AuditValueToken / AuditCapToken )
                       ( contextTerminationAudit  / auditOther)

auditOther           = EQUAL TerminationID LBRKT
                       terminationAudit RBRKT

terminationAudit   = auditReturnParameter *(COMMA auditReturnParameter)

contextTerminationAudit = EQUAL CtxToken ( terminationIDList /
                       LBRKT errorDescriptor RBRKT )
;at-most-once except errorDescriptor
auditReturnParameter = (mediaDescriptor / modemDescriptor /
                        muxDescriptor / eventsDescriptor /
                        signalsDescriptor / digitMapDescriptor /
                      observedEventsDescriptor / eventBufferDescriptor /
                        statisticsDescriptor / packagesDescriptor /
                         errorDescriptor )

auditDescriptor      = AuditToken LBRKT [ auditItem
                       *(COMMA auditItem) ] RBRKT

notifyRequest        = NotifyToken EQUAL TerminationID
                       LBRKT ( observedEventsDescriptor
                             [ COMMA errorDescriptor ] ) RBRKT

notifyReply          = NotifyToken EQUAL TerminationID
                       [ LBRKT errorDescriptor RBRKT ]

serviceChangeRequest = ServiceChangeToken EQUAL TerminationID
                       LBRKT serviceChangeDescriptor RBRKT

serviceChangeReply   = ServiceChangeToken EQUAL TerminationID
                       [LBRKT (errorDescriptor /
                       serviceChangeReplyDescriptor) RBRKT]

errorDescriptor   = ErrorToken EQUAL ErrorCode
                    LBRKT [quotedString] RBRKT




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ErrorCode            = 1*4(DIGIT) ; could be extended

TransactionID        = UINT32

mId                  = (( domainAddress / domainName )
                       [":" portNumber]) / mtpAddress / deviceName

; ABNF allows two or more consecutive "." although it is meaningless
; in a domain name.
domainName           = "<" (ALPHA / DIGIT) *63(ALPHA / DIGIT / "-" /
                       ".") ">"
deviceName           = pathNAME

ContextID            = (UINT32 / "*" / "-" / "$")

domainAddress        = "[" (IPv4address / IPv6address) "]"
;RFC2373 contains the definition of IP6Addresses.
IPv6address          = hexpart [ ":" IPv4address ]
IPv4address          = V4hex DOT V4hex DOT V4hex DOT V4hex
V4hex                = 1*3(DIGIT) ; "0".."225"
; this production, while occurring in RFC2373, is not referenced
; IPv6prefix           = hexpart SLASH 1*2DIGIT
hexpart             = hexseq "::" [ hexseq ] / "::" [ hexseq ] / hexseq
hexseq               = hex4 *( ":" hex4)
hex4                 = 1*4HEXDIG

portNumber           = UINT16

; An mtp address is two octets long
mtpAddress           = MTPToken LBRKT octetString RBRKT

terminationIDList    = LBRKT TerminationID *(COMMA TerminationID) RBRKT

; Total length of pathNAME must not exceed 64 chars.
pathNAME             = ["*"] NAME *("/" / "*"/ ALPHA / DIGIT /"_" / "$"
)
                       ["@" pathDomainName ]

; ABNF allows two or more consecutive "." although it is meaningless
; in a path domain name.
pathDomainName       = (ALPHA / DIGIT / "*" )
                       *63(ALPHA / DIGIT / "-" / "*" / ".")

TerminationID        = "ROOT" / pathNAME / "$" / "*"

mediaDescriptor   = MediaToken LBRKT mediaParm *(COMMA mediaParm) RBRKT

; at-most-once per item



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; and either streamParm or streamDescriptor but not both
mediaParm            = (streamParm / streamDescriptor /
                        terminationStateDescriptor)

; at-most-once
streamParm           = ( localDescriptor / remoteDescriptor /
                        localControlDescriptor )

streamDescriptor     = StreamToken EQUAL StreamID LBRKT streamParm
                       *(COMMA streamParm) RBRKT

localControlDescriptor = LocalControlToken LBRKT localParm
                         *(COMMA localParm) RBRKT

; at-most-once per item
localParm            = ( streamMode / propertyParm / reservedValueMode
     / reservedGroupMode )

reservedValueMode          = ReservedValueToken EQUAL ( "ON" / "OFF" )
reservedGroupMode          = ReservedGroupToken EQUAL ( "ON" / "OFF" )

streamMode           = ModeToken EQUAL streamModes

streamModes          = (SendonlyToken / RecvonlyToken / SendrecvToken /
                       InactiveToken / LoopbackToken )

propertyParm         = pkgdName parmValue
parmValue            = (EQUAL alternativeValue/ INEQUAL VALUE)
alternativeValue     = ( VALUE / LSBRKT VALUE *(COMMA VALUE) RSBRKT  /
                       LSBRKT VALUE DOT DOT VALUE RSBRKT )

INEQUAL              = LWSP (">" / "<" / "#" ) LWSP
LSBRKT               = LWSP "[" LWSP
RSBRKT               = LWSP "]" LWSP

localDescriptor      = LocalToken LBRKT octetString RBRKT

remoteDescriptor     = RemoteToken LBRKT octetString RBRKT

eventBufferDescriptor= EventBufferToken LBRKT observedEvent
                       *( COMMA observedEvent ) RBRKT

eventBufferControl     = BufferToken EQUAL ( "OFF" / LockStepToken )

terminationStateDescriptor = TerminationStateToken LBRKT
            terminationStateParm *( COMMA terminationStateParm ) RBRKT

; at-most-once per item



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terminationStateParm =(propertyParm / serviceStates / eventBufferControl
)

serviceStates        = ServiceStatesToken EQUAL ( TestToken /
                       OutOfSvcToken / InSvcToken )

muxDescriptor        = MuxToken EQUAL MuxType  terminationIDList

MuxType              = ( H221Token / H223Token / H226Token / V76Token
                        extensionParameter )

StreamID             = UINT16
pkgdName             = (PackageName SLASH ItemID) ;specific item
                            / (PackageName SLASH "*") ;all events in package
                            / ("*" SLASH "*") ; all events supported by the MG
PackageName          = NAME
ItemID               = NAME

eventsDescriptor     = EventsToken EQUAL RequestID LBRKT
                       requestedEvent *( COMMA requestedEvent ) RBRKT

requestedEvent       = pkgdName [ LBRKT eventParameter
                       *( COMMA eventParameter ) RBRKT ]

; at-most-once each of KeepActiveToken , eventDM and eventStream
;at most one of either embedWithSig or embedNoSig but not both
;KeepActiveToken and embedWithSig must not both be present
eventParameter       = ( embedWithSig / embedNoSig / KeepActiveToken
   /eventDM / eventStream / eventOther )

embedWithSig         = EmbedToken LBRKT signalsDescriptor
     [COMMA embedFirst ] RBRKT
embedNoSig         = EmbedToken LBRKT embedFirst RBRKT

; at-most-once of each
embedFirst      = EventsToken EQUAL RequestID LBRKT
               secondRequestedEvent *(COMMA secondRequestedEvent) RBRKT

secondRequestedEvent = pkgdName [ LBRKT secondEventParameter
                       *( COMMA secondEventParameter ) RBRKT ]

; at-most-once each of embedSig , KeepActiveToken, eventDM or
; eventStream
; KeepActiveToken and embedSig must not both be present
secondEventParameter = ( EmbedSig / KeepActiveToken / eventDM /
                         eventStream / eventOther )

embedSig  = EmbedToken LBRKT signalsDescriptor RBRKT



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eventStream          = StreamToken EQUAL StreamID

eventOther           = eventParameterName parmValue

eventParameterName   = NAME

eventDM              = DigitMapToken ((EQUAL digitMapName ) /
                       (LBRKT digitMapValue RBRKT ))

signalsDescriptor    = SignalsToken LBRKT [ signalParm
                       *(COMMA signalParm)] RBRKT

signalParm           = signalList / signalRequest

signalRequest        = signalName [ LBRKT sigParameter
                       *(COMMA sigParameter) RBRKT ]

signalList           = SignalListToken EQUAL signalListId LBRKT
                       signalListParm *(COMMA signalListParm) RBRKT

signalListId         = UINT16

;exactly once signalType, at most once duration and every signal
;parameter
signalListParm       = signalRequest

signalName           = pkgdName
;at-most-once sigStream, at-most-once sigSignalType,
;at-most-once sigDuration, every signalParameterName at most once
sigParameter       = sigStream / sigSignalType / sigDuration / sigOther
                           / notifyCompletion / KeepActiveToken
sigStream            = StreamToken EQUAL StreamID
sigOther             = sigParameterName parmValue
sigParameterName     = NAME
sigSignalType        = SignalTypeToken EQUAL signalType
signalType           = (OnOffToken / TimeOutToken / BriefToken)
sigDuration          = DurationToken EQUAL UINT16
notifyCompletion     = NotifyCompletionToken EQUAL ("ON" / "OFF")

observedEventsDescriptor = ObservedEventsToken EQUAL RequestID
                   LBRKT observedEvent *(COMMA observedEvent) RBRKT

;time per event, because it might be buffered
observedEvent        = [ TimeStamp LWSP COLON] LWSP
                       pkgdName [ LBRKT observedEventParameter
                       *(COMMA observedEventParameter) RBRKT ]

;at-most-once eventStream, every eventParameterName at most once



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observedEventParameter = eventStream / eventOther

RequestID            = UINT32

modemDescriptor      = ModemToken (( EQUAL modemType) /
                       (LSBRKT modemType *(COMMA modemType) RSBRKT))
                       [ LBRKT NAME parmValue
                      *(COMMA NAME parmValue) RBRKT ]

; at-most-once
modemType            = (V32bisToken / V22bisToken / V18Token /
                        V22Token / V32Token / V34Token / V90Token /
                        V91Token / SynchISDNToken / extensionParameter)

digitMapDescriptor   = DigitMapToken EQUAL digitMapName
                       ( LBRKT digitMapValue RBRKT )
digitMapName         = NAME
digitMapValue        = ["T" COLON Timer COMMA] ["S" COLON Timer COMMA]
                       ["L" COLON Timer COMMA] digitMap
Timer                = 1*2DIGIT
digitMap = (digitString / LWSP "(" LWSP digitStringList LWSP ")" LWSP)
digitStringList      = digitString *( LWSP "|" LWSP digitString )
digitString          = 1*(digitStringElement)
digitStringElement   = digitPosition [DOT]
digitPosition        = digitMapLetter / digitMapRange
digitMapRange        = ("x" / LWSP "[" LWSP digitLetter LWSP "]" LWSP)
digitLetter          = *((DIGIT "-" DIGIT ) / digitMapLetter)
digitMapLetter       = DIGIT   ;Basic event symbols
                           / %x41-4B / %x61-6B ; a-k, A-K
                           / "L" / "S"   ;Inter-event timers (long, short)
                             "Z"         ;Long duration modifier

;at-most-once
auditItem            = ( MuxToken / ModemToken / MediaToken /
                        SignalsToken / EventBufferToken /
                        DigitMapToken / StatsToken / EventsToken /
                        ObservedEventsToken / PackagesToken )

serviceChangeDescriptor = ServicesToken LBRKT serviceChangeParm
                         *(COMMA serviceChangeParm) RBRKT

serviceChangeParm    = (serviceChangeMethod / serviceChangeReason /
                       serviceChangeDelay / serviceChangeAddress /
                       serviceChangeProfile / extension / TimeStamp /
                       serviceChangeMgcId / serviceChangeVersion )

serviceChangeReplyDescriptor = ServicesToken LBRKT
                       servChgReplyParm *(COMMA servChgReplyParm) RBRKT



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;at-most-once. Version is REQUIRED on first ServiceChange response
servChgReplyParm     = (serviceChangeAddress / serviceChangeMgcId /
                       serviceChangeProfile / serviceChangeVersion )
serviceChangeMethod  = MethodToken EQUAL (FailoverToken /
                       ForcedToken / GracefulToken / RestartToken /
                       DisconnectedToken / HandOffToken /
                       extensionParameter)

serviceChangeReason  = ReasonToken  EQUAL VALUE
serviceChangeDelay   = DelayToken   EQUAL UINT32
serviceChangeAddress = ServiceChangeAddressToken EQUAL VALUE
serviceChangeMgcId   = MgcIdToken   EQUAL mId
serviceChangeProfile = ProfileToken EQUAL NAME SLASH Version
serviceChangeVersion = VersionToken EQUAL Version
extension            = extensionParameter parmValue

packagesDescriptor   = PackagesToken LBRKT packagesItem
                       *(COMMA packagesItem) RBRKT

Version              = 1*2(DIGIT)
packagesItem         = NAME "-" UINT16

TimeStamp            = Date "T" Time ; per ISO 8601:1988
; Date = yyyymmdd
Date                 = 8(DIGIT)
; Time = hhmmssss
Time                 = 8(DIGIT)
statisticsDescriptor = StatsToken LBRKT statisticsParameter
                      *(COMMA statisticsParameter ) RBRKT

;at-most-once per item
statisticsParameter  = pkgdName EQUAL VALUE

topologyDescriptor   = TopologyToken LBRKT terminationA COMMA
                       terminationB COMMA topologyDirection RBRKT
terminationA         = TerminationID
terminationB         = TerminationID
topologyDirection    = BothwayToken / IsolateToken / OnewayToken

priority             = PriorityToken EQUAL UINT16

extensionParameter   = "X"  ("-" / "+") 1*6(ALPHA / DIGIT)

; octetString is used to describe SDP defined in RFC2327.
; Caution should be taken if CRLF in RFC2327 is used.
; To be safe, use EOL in this ABNF.
; Whenever "}" appears in SDP, it is escaped by "
octetString          = *(nonEscapeChar)



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nonEscapeChar        = ( "" / %x01-7C / %x7E-FF )
quotedString         = DQUOTE 1*(SafeChar / RestChar/ WSP) DQUOTE

UINT16               = 1*5(DIGIT)  ; %x0-FFFF
UINT32               = 1*10(DIGIT) ; %x0-FFFFFFFF

NAME                 = ALPHA *63(ALPHA / DIGIT / "_" )
VALUE                = quotedString / 1*(SafeChar)
SafeChar             = DIGIT / ALPHA / "+" / "-" / "&" /
                       "!" / "_" / "/" / "'" / "?" / "@" /
                       "^" / "`" / "~" / "*" / "$" / "
                       "(" / ")" / "%" / "|" / "."

EQUAL                = LWSP %x3D LWSP ; "="
COLON                = %x3A           ; ":"
LBRKT                = LWSP %x7B LWSP ; "{"
RBRKT                = LWSP %x7D LWSP ; "}"
COMMA                = LWSP %x2C LWSP ; ","
DOT                  = %x2E           ; "."
SLASH                = %x2F           ; "/"
ALPHA                = %x41-5A / %x61-7A ; A-Z / a-z
DIGIT                = %x30-39         ; 0-9
DQUOTE               = %x22            ; " (Double Quote)
HEXDIG               = ( DIGIT / "A" / "B" / "C" / "D" / "E" / "F" )
SP                   = %x20        ; space
HTAB                 = %x09        ; horizontal tab
CR                   = %x0D        ; Carriage return
LF                   = %x0A        ; linefeed
LWSP                 = *( WSP / COMMENT / EOL )
EOL                  = (CR [LF] / LF )
WSP                  = SP / HTAB ; white space
SEP                  = ( WSP / EOL / COMMENT) LWSP
COMMENT              = ";" *(SafeChar/ RestChar / WSP / %x22) EOL
RestChar             = ";" / "[" / "]" / "{" / "}" / ":" / "," / "#" /
                       "<" / ">" / "="


AddToken                   = ("Add"                   / "A")
AuditToken                 = ("Audit"                 / "AT")
AuditCapToken              = ("AuditCapability"       / "AC")
AuditValueToken            = ("AuditValue"            / "AV")
AuthToken                  = ("Authentication"        / "AU")
BothwayToken               = ("Bothway"               / "BW")
BriefToken                 = ("Brief"                 / "BR")
BufferToken                = ("Buffer"                / "BF")
CtxToken                   = ("Context"               / "C")
ContextAuditToken                  = ("ContextAudit"            / "CA")
DigitMapToken              = ("DigitMap"              / "DM")



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DiscardToken               = ("Discard"               / "DS")
DisconnectedToken          = ("Disconnected"          / "DC")
DelayToken                 = ("Delay"                 / "DL")
DurationToken              = ("Duration"              / "DR")
EmbedToken                 = ("Embed"                 / "EB")
EmergencyToken             = ("Emergency"             / "EM")
ErrorToken                 = ("Error"                 / "ER")
EventBufferToken           = ("EventBuffer"           / "EB")
EventsToken                = ("Events"                / "E")
FailoverToken              = ("Failover"              / "FL")
ForcedToken                = ("Forced"                / "FO")
GracefulToken              = ("Graceful"              / "GR")
H221Token                  = ("H221" )
H223Token                  = ("H223" )
H226Token                  = ("H226" )
HandOffToken               = ("HandOff"               / "HO")
InactiveToken              = ("Inactive"              / "IN")
IsolateToken               = ("Isolate"               / "IS")
InSvcToken                 = ("InService"             / "IV")
KeepActiveToken            = ("KeepActive"            / "KA")
LocalToken                 = ("Local"                 / "L")
LocalControlToken          = ("LocalControl"          / "O")
LockStepToken              = ("LockStep"                        / "SP")
LoopbackToken              = ("Loopback"              / "LB")
MediaToken                 = ("Media"                 / "M")
MegacopToken               = ("MEGACO"                / "!")
MethodToken                = ("Method"                / "MT")
MgcIdToken                 = ("MgcIdToTry"            / "MG")
ModeToken                  = ("Mode"                  / "MO")
ModifyToken                = ("Modify"                / "MF")
ModemToken                 = ("Modem"                 / "MD")
MoveToken                  = ("Move"                  / "MV")
MTPToken                   = ("MTP")
MuxToken                   = ("Mux"                   / "MX")
NotifyToken                = ("Notify"                / "N")
NotifyCompletionToken      = ("NotifyCompletion"        / "NC")
ObservedEventsToken        = ("ObservedEvents"        / "OE")
OnewayToken                = ("Oneway"                / "OW")
OnOffToken                 = ("OnOff"                 / "OO")
OutOfSvcToken              = ("OutOfService"          / "OS")
PackagesToken              = ("Packages"              / "PG")
PendingToken               = ("Pending"               / "PN")
PriorityToken              = ("Priority"              / "PR")
ProfileToken               = ("Profile"               / "PF")
ReasonToken                = ("Reason"                / "RE")
RecvonlyToken              = ("ReceiveOnly"           / "RC")
ReplyToken                 = ("Reply"                 / "P")
RestartToken               = ("Restart"               / "RS")



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RemoteToken                = ("Remote"                / "R")
ReservedGroupToken         = ("ReservedGroup"           / "RG")
ReservedValueToken         = ("ReservedValue"           / "RV")
SendonlyToken              = ("SendOnly"              / "SO")
SendrecvToken              = ("SendReceive"           / "SR")
ServicesToken              = ("Services"              / "SV")
ServiceStatesToken         = ("ServiceStates"         / "SI")
ServiceChangeToken         = ("ServiceChange"         / "SC")
ServiceChangeAddressToken  = ("ServiceChangeAddress"  / "AD")
SignalListToken            = ("SignalList"            / "SL")
SignalsToken               = ("Signals"               / "SG")
SignalTypeToken            = ("SignalType"            / "SY")
StatsToken                 = ("Statistics"            / "SA")
StreamToken                = ("Stream"                / "ST")
SubtractToken              = ("Subtract"              / "S")
SynchISDNToken             = ("SynchISDN"             / "SN")
TerminationStateToken      = ("TerminationState"      / "TS")
TestToken                  = ("Test"                  / "TE")
TimeOutToken               = ("TimeOut"               / "TO")
TopologyToken              = ("Topology"              / "TP")
TransToken                 = ("Transaction"           / "T")
ResponseAckToken                   = ("TransactionResponseAck"/ "K")
V18Token                   = ("V18")
V22Token                   = ("V22")
V22bisToken                = ("V22b")
V32Token                   = ("V32")
V32bisToken                = ("V32b")
V34Token                   = ("V34")
V76Token                   = ("V76")
V90Token                   = ("V90")
V91Token                   = ("V91")




ANNEX C TAGS FOR MEDIA STREAM PROPERTIES (NORMATIVE)

Parameters for Local descriptors and Remote descriptors are specified as
tag-value pairs if binary encoding is used for the protocol.  This annex
contains the property names (PropertyID), the tags (Property Tag), type
of the property (Type) and the values (Value).Values presented in the
Value field when the field contains references shall be regarded as
"information". The reference contains the normative values.  If a value
field does not contain a reference then the values in that field can be
considered as "normative".

Tags are given as hexadecimal numbers in this annex. When setting the
value of a property, a MGC may underspecify the value according to one



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of the mechanisms specified in section 7.1.1.

For type "enumeration" the value is represented by the value in brack-
ets, e.g., Send(0), Receive(1).

C.1.  General Media Attributes

________________________________________________________________________
|PropertyID   | Tag |  Type     |  Value                               |
|Media        |1001 |Enumeration|Audio(0), Video(1) ,Data(2),          |
|TransMode    |1002 |Enumeration|Send(0), Receive(1), Send&Receive(2)  |
|NumChan      |1003 |UINT       | 0-255                                |
|SamplingRate |1004 |UINT       | 0-2^32                               |
|Bitrate      |1005 |Integer    |(0..4294967295) Note-units of 100 bit |
|Acodec       |1006 |Octet str  |Audio Codec Type                      |
|Samplepp     |1007 |UINT       |Maximum samples/fr per packet:0-65535 |
|Silencesupp  |1008 |BOOLEAN    |Silence Suppression                   |
|Encrypttype  |1009 |Octet str  |Ref.: rec. H.245                      |
|Encryptkey   |100A |Octet str  |SIZE(0..65535) Encryption key         |
|Echocanc     |100B |Enumeration|Echo Canceller:Off(0),G.165(1),G168(2)|
|Gain         |100C |UINT       |Gain in db: 0-65535                   |
|Jitterbuff   |100D |UINT       |Jitter buffer size in ms: 0-65535     |
|PropDelay    |100E |UINT       |  Propagation Delay: 0..65535         |
|RTPpayload   |100F |integer    |Payload type in RTP Profile           |
|_____________|_____|___________|______________________________________|


C.2.  Mux Properties

_________________________________________________________________________
|PropertyID|  Tag       |  Type        |  Value                         |
|H.221     |  2001      |  Octet string|   H222LogicalChannelParameters |
|H223      |  2002      |  Octet string|   H223LogicalChannelParameters |
|V76       |  2003      |  Octet String|   V76LogicalChannelParameters  |
|H2250     |  2004      |  Octet String|   H2250LogicalChannelParameters|
|__________|____________|______________|________________________________|


C.3.  General Bearer Properties

 _____________________________________________________________________
| PropertyID|  Tag       |  Type       |  Value                      |
| Mediatx   |  3001      |  Enumeration|  Media Transport Type       |
| BIR       |  3002      |  4 OCTET    |  Value depends on transport |
| NSAP      |  3003      |  1-20 OCTETS|  Ref: ITU X.213 Annex A     |
|___________|____________|_____________|_____________________________|





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C.4.  General ATM Properties

   _________________________________________________________________
  | PropertyID|  Tag |  Type       |  Value                        |
  | AESA      |  4001|  20 OCTETS  |  ATM End System Address       |
  | VPVC      |  4002|  2x16b int  |  VPC-VCI                      |
  | SC        |  4003|  4 bits     |  Service Category             |
  | BCOB      |  4004|  5b integer |  Broadband Bearer Class       |
  | BBTC      |  4005|  octet      |  Broadband Transfer Capability|
  | ATC       |  4006|  Enumeration|  I.371 ATM Traffic Cap.       |
  | STC       |  4007|  2 bits     |  Susceptibility to clipping   |
  | UPCC      |  4008|  2 bits     |  User Plane Connection config |
  | PCR0      |  4009|  24b integer|  Peak Cell Rate CLP=0         |
  | SCR0      |  400A|  24b integer|  Sustainable Cell Rate CLP=0  |
  | MBS0      |  400B|  24b integer|  Maximum Burst Size CLP=0     |
  | PCR1      |  400C|  24b integer|  Peak Cell Rate CLP=0+1       |
  | SCR2      |  400D|  24b integer|  Sustain. Cell Rate CLP=0+1   |
  | MBS3      |  400E|  24b integer|  Maximum Burst Size CLP=0+1   |
  | BEI       |  400F|  Boolean    |  Best Effort Indicator        |
  | TI        |  4010|  Boolean    |  Tagging                      |
  | FD        |  4011|  Boolean    |  Frame Discard                |
  | FCDV      |  4012|  24b integer|  Forward P-P CDV              |
  | BCDV      |  4013|  24b integer|  Backward P-P CDV             |
  | FCLR0     |  4014|  8b integer |  Fwd Cell Loss Ratio CLP=0    |
  | BCLR0     |  4015|  8b integer |  Bkwd P-P CLR CLP=0           |
  | FCLR1     |  4016|  8b integer |  Fwd Cell Loss Ratio CLP=0+1  |
  | BCLR1     |  4017|  8b integer |  Bkwd P-P CLR CLP=0+1         |
  | FCDV      |  4018|  24b integer|  Fwd Cell Delay Variation     |
  | BCDV      |  4019|  24b integer|  Bkwd Cell Delay Variation    |
  | FACDV     |  401A|  24b integer|  Fwd Acceptable P-P-P CDV     |
  | BACDV     |  401B|  24b integer|  Bkwd Acceptable P-P CDV      |
  | FCCDV     |  401C|  24b integer|  Fwd Cumulative P-P CDV       |
  | BCCDV     |  401D|  24b integer|  Bkwd Cumulative P-P CDV      |
  | FCLR      |  401E|  8b integer |  Acceptable Fwd CLR           |
  | BCLR      |  401F|  8b integer |  Acceptable Bkwd CLR          |
  | EETD      |  4020|  16b integer|  End-to-end transit delay     |
  | Mediatx   |  4021|             |  AAL Type                     |
  | QosClass  |  4022|  Integer    |  0-4 Qos Class                |
  | AALtype   |  4023|  1 OCTET    |  AAL Type Reference           |
  |___________|______|_____________|_______________________________|


C.5.  Frame Relay








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 ______________________________________________________________________
| PropertyID|  Tag |  Type            |  Value                        |
| DLCI      |  5001|  Unsigned Integer|  Data link connection id      |
| CID       |  5002|  Unsigned Integer|  sub-channel id.              |
| SID       |  5003|  Unsigned Integer|  silence insertion descriptor |
| Payload   |  5004|  Unsigned Integer|  Primary Payload Type         |
|___________|______|__________________|_______________________________|


C.6.  IP

    ________________________________________________________________
   | PropertyID|  Tag       |  Type        |  Value                |
   | IPv4      |  6001      |  32 BITS     |  Ipv4Address          |
   | IPv6      |  6002      |  128 BITS    |  IPv6 Address         |
   | Port      |  6003      |  Unsigned Int|  Port                 |
   | Porttype  |  6004      |  Enumerated  |  TCP(0),UDP(1),SCTP(2)|
   | UDP       |  6004      |  Boolean     |                       |
   |___________|____________|______________|_______________________|


C.7.  ATM AAL2

_______________________________________________________________________________
|PropertyID|  Tag    |  Type         |  Value                                 |
|AESA      |  7001   |  20 OCTETS    |  AAL2 service endpoint address         |
|BIR       |  See C.3|  4 OCTETS     |  Served user generated reference       |
|ALC       |  7002   |  12 OCTETS    |  AAL2 link                             |
|SSCS      |  7003   |  8..14 OCTETS |  Service specific convergence sublayer |
|SUT       |  7004   |  1..254 octets|  Served user transport param           |
|TCI       |  7005   |  BOOLEAN      |  Test connection                       |
|Timer_CU  |  7006   |  32b integer  |  Timer-CU                              |
|MaxCPSSDU |  7007   |  8b integer   |  Max. Common Part Sublayer SDU         |
|SCLP      |  7008   |  Boolean      |  Set Cell Local PriorityLP bit         |
|EETR      |  7009   |  Boolean      |  End to End Timing Required            |
|CID       |  700A   |  8 bits       |  subchannel id                         |
|__________|_________|_______________|________________________________________|


C.8.  ATM AAL1











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________________________________________________________________________________
|PropertyID|  Tag          |  Type         |  Value                            |
|BIR       |  See Table C.3|  4 OCTETS     |  GIT(Generic Identifier Transport)|
|AAL1ST    |  8001         |  1 OCTET      |  AAL1 Subtype:                    |
|8002      |  1 OCTET      |  CBR Rate     |                                   |
|SCRI      |  8003         |  1 OCTET      |  Source Clock Frequency Recovery  |
|ECM       |  8004         |  1 OCTET      |  Error Correction Method          |
|SDTB      |  8005         |  16b integer  |  Structured Data Transfer Blcksize|
|PFCI      |  8006         |  8b integer   |  Partially filled cells identifier|
|EETR      |  See Table C.7|  See Table C.7|                                   |
|__________|_______________|_______________|___________________________________|

C.9.  Bearer Capabilities





































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________________________________________________________________________
|PropertyID   |  Tag |  Type       |  Value                            |
|TMR          |  9001|  1 OCTET    |  Transmission Medium Requirement  |
|TMRSR        |  9002|  1 OCTET    |  Trans. Medium Requirement Subrate|
|Contcheck    |  9003|  BOOLEAN    |  Continuity Check                 |
|ITC          |  9004|  5 BITS     |  Information Transfer Capability  |
|TransMode    |  9005|  2 BITS     |  Transfer Mode                    |
|TransRate    |  9006|  5 BITS     |  Transfer Rate                    |
|MULT         |  9007|  7 BITS     |  Rate Multiplier                  |
|USI          |  9008|  5 BITS     |  User Information Layer 1 Protocol|
|Syncasync    |  9009|  BOOLEAN    |  Synchronous-Asynchronous         |
|Userrate     |  900B|  5 BITS     |  User Rate Reference              |
|INTRATE      |  900C|  2 BITS     |  Intermediate Rate                |
|Nictx        |  900D|  BOOLEAN    |  Tx Network Independent Clock     |
|Nicrx        |  900E|  BOOLEAN    |  Rx Network independent clock     |
|Flowconttx   |  900F|  BOOLEAN    |  Tx Flow Control                  |
|Flowcontrx   |  9010|  BOOLEAN    |  Rx Flow control                  |
|Rateadapthdr |  9011|  BOOLEAN    |  Rate adapt header-no header      |
|Multiframe   |  9012|  BOOLEAN    |  Multiple frame estab.            |
|OPMODE       |  9013|  BOOLEAN    |  Mode of operation                |
|Llidnegot    |  9014|  BOOLEAN    |  Logical link identifier neg.     |
|Assign       |  9015|  BOOLEAN    |  Assignor-assignee                |
|Inbandneg    |  9016|  BOOLEAN    |  In-band or out-band negotiation  |
|Stopbits     |  9017|  2 BITS     |  Number of stop bits              |
|Databits     |  9018|  2 BIT      |  Number of data bits              |
|Parity       |  9019|  3 BIT      |  Parity information               |
|Duplexmode   |  901A|  BOOLEAN    |  Mode duplex                      |
|Modem        |  901B|  6 BIT      |  Modem Type                       |
|layer2prot   |  901C|  5 BIT      |  User info layer 2 protocol       |
|layer3prot   |  901D|  5 BIT      |  User info layer 3 protocol       |
|addlayer3prot|  901E|  OCTET      |  Addl User Info L3 protocol       |
|DialledN     |  901F|  30 OCTETS  |  Dialled Number                   |
|DiallingN    |  9020|  30 OCTETS  |  Dialling Number                  |
|ECHOCI       |  9021|  Enumeration|  Echo Control Information         |
|NCI          |  9022|  1 OCTET    |  Nature of Connection Indicators  |
|_____________|______|_____________|___________________________________|


C.10.  AAL5 Properties

 ______________________________________________________________________
| PropertyID|  Tag    |  Type       |  Value                          |
| FMSDU     |  A001   |  32b integer|  Forward Maximum CPCS-SDU Size: |
| BMSDU     |  A002   |  2b integer |  Backwards Maximum CPCS-SDU Size|
| SSCS      |  See C.7|  See C.7    |  See table C.                   |
| SC        |  See C.4|  See C.4    |  See table C.4                  |
|___________|_________|_____________|_________________________________|




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C.11.  SDP Equivalents

     ______________________________________________________________
    | PropertyID|  Tag |  Type  |  Value                          |
    | SDP_V     |  B001|  STRING|  Protocol Version               |
    | SDP_O     |  B002|  STRING|  Owner-creator and session ID   |
    | SDP_S     |  B003|  STRING|  Sesson name                    |
    | SDP_I     |  B004|  STRING|  Session identifier             |
    | SDP_U     |  B005|  STRING|  URI of descriptor              |
    | SDC_E     |  B006|  STRING|  email address                  |
    | SDP_P     |  B007|  STRING|  phone number                   |
    | SDP_C     |  B008|  STRING|  Connection information         |
    | SDP_B     |  B009|  STRING|  Bandwidth Information          |
    | SDP_Z     |  B00A|  STRING|  time zone adjustment           |
    | SDP_K     |  B00B|  STRING|  Encryption Key                 |
    | SDP_A     |  B00C|  STRING|  Zero or more session attributes|
    | SDP_T     |  B00D|  STRING|  Active Session Time            |
    | SDP_R     |  B00E|  STRING|  Zero or more repeat times      |
    |___________|______|________|_________________________________|


C.12.  H.245

________________________________________________________________________
|OLC   |  C001|  octet string|  H.245 OpenLogicalChannel structure.    |
|OLCack|  C002|  octet string|   H.245 OpenLogicalChannelAck structure.|
|OLCcnf|  C003|  octet string|   OpenLogicalChannelConfirm structure.  |
|OLCrej|  C004|  octet string|   OpenLogicalChannelReject structure.   |
|CLC   |  C005|  octet string|   CloseLogicalChannel structure.        |
|CLCack|  C006|  octet string|   CloseLogicalChannelAck structure.     |
|______|______|______________|_________________________________________|


ANNEX D TRANSPORT OVER IP (NORMATIVE)

D.1.  Transport over IP/UDP using Application Level Framing

Protocol messages defined in this document may be transmitted over UDP.
When no port is provided by the peer (see section 7.2.8), commands
should be sent to the default port number, 2944 for text-encoded opera-
tion or 2945 for binary-encoded operation.  Responses must be sent to
the address and port from which the corresponding commands were sent
except if the response is to a handoff or failover, in which case the
procedures of 11.5 apply.

Implementors using IP/UDP with ALF should be aware of the restrictions
of the MTU on the maximum message size.




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D.1.1.  Providing At-Most-Once Functionality

Messages, being carried over UDP, may be subject to losses. In the
absence of a timely response, commands are repeated. Most commands are
not idempotent.  The state of the MG would become unpredictable if, for
example, Add commands were executed several times.  The transmission
procedures shall thus provide an "At- Most-Once" functionality.

Peer protocol entities are expected to keep in memory a list of the
responses that they sent to recent transactions and a list of the tran-
sactions that are currently outstanding. The transaction identifier of
each incoming message is compared to the transaction identifiers of the
recent responses sent to the same MId. If a match is found, the entity
does not execute the transaction, but simply repeats the response. If no
match is found, the message will be compared to the list of currently
outstanding transactions. If a match is found in that list, indicating a
duplicate transaction, the entity does not execute the transaction (see
section 8.2.3 for procedures on sending TransactionPending).

The procedure uses a long timer value, noted LONG-TIMER in the follow-
ing.  The timer should be set larger than the maximum duration of a
transaction, 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 may be destroyed either LONG-TIMER seconds
after the response is issued, or when the entity receives a confirmation
that the response has been received, through the "Response Acknowledge-
ment parameter". For transactions that are acknowledged through this
parameter, the entity 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 pro-
duced by the network.

D.1.2.  Transaction identifiers and three-way handshake

D.1.2.1.  Transaction identifiers

Transaction identifiers are 32 bit integer numbers.  A Media Gateway
Controller may decide to use a specific number space for each of the MGs
that they manage, or to use the same number space for all MGs that
belong to some arbitrary group.  MGCs may decide to share the load of
managing a large MG between several independent processes.  These
processes will share the same transaction number space.  There are mul-
tiple possible implementations of this sharing, such as having a cen-
tralized allocation of transaction identifiers, or pre-allocating non-
overlapping ranges of identifiers to different processes.  The



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implementations shall guarantee that unique transaction identifiers are
allocated to all transactions that originate from a logical MGC (identi-
cal mId). MGs can simply detect duplicate transactions by looking at the
transaction identifier and mId only.

D.1.2.2.  Three-way handshake

The TransactionResponse Acknowledgement parameter can be found in any
message. It carries a set of "confirmed transaction-id ranges". Entities
may choose to delete the copies of the responses to transactions whose
id is included in "confirmed transaction-id ranges" received in the
transaction response messages. They should silently discard further com-
mands 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 MG issued its last
response to that MGC, or when a MG resumes operation.  In this situa-
tion, transactions should be accepted and processed, without any test on
the transaction-id.

Messages that carry the "Transaction Response Acknowledgement" parameter
may be transmitted in any order.  The entity shall retain the "confirmed
transaction-id ranges" receivedfor LONG-TIMER seconds.

In the binary encoding, if only the firstAck is present in a response
acknowledgement (see Annex A.2), only one transaction is acknowledged.
If both firstAck and lastAck are present, then the range of transactions
from firstAck to lastAck is acknowledged.  In the text encoding, a hor-
izontal dash is used to indicate a range of transactions being ack-
nowledged (see Annex B.2).

D.1.3.  Computing retransmission timers

It is the responsibility of the requesting entity to provide suitable
time outs for all outstanding transactions, and to retry transactions
when time outs have been exceeded. Furthermore, when repeated transac-
tions fail to be acknowledged, it is the responsibility of the request-
ing 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 value by
measuring the time spent between the sending of a command and the return
of a response.

Note - One possibility is to use the algorithm implemented in TCP-IP,
which uses two variables:



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*    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. The maximum value of the timer should how-
     ever be bounded for the protocol defined in this document, in order
     to guarantee that no repeated packet would be received by the gate-
     ways after LONG-TIMER seconds.  A suggested maximum value is 4
     seconds.

After any retransmission, the entity should do the following:

*    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.

D.1.4.  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. Entities that can
predict that a transaction will require a long execution time may send a
provisional response, "Transaction Pending".

Entities that receive a Transaction Pending shall switch to a different
repetition timer for repeating requests.  The root termination has a
property (ProvisionalResponseTimerValue), which can be set to the
requested maximum number of milliseconds between receipt of a command
and transmission of the TransactionPending response.  Upon receipt of a
final response, an immediate confirmation shall be sent, and normal
repetition timers shall be used thereafter.  Receipt of a Transaction
Pending after receipt of a reply shall be ignored.





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D.1.5.  Repeating Requests, Responses and Acknowledgements

The protocol is organized as a set of transactions, each of which is
composed request and a response, commonly referred to as an acknowledge-
ment.  The protocol messages, being carried over UDP, may be subject to
losses. In the absence of a timely response, transactions are repeated.
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 repetition mechanism is used to guard against three types of possi-
ble 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 entity
     becomes unavailable;

*    entity failure, when for example an entire entity become unavail-
     able.

The entities 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 Media Gateway Controller or a Media 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 five consecutive transmis-
sion attempts fail is 1 in 100 billion, an event that should occur less
than once every 10 days for a Media Gateway Controller 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.

A classic retransmission algorithm would simply count the number of suc-
cessive repetitions, and conclude that the association is broken after
retransmitting 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 so
that if the Media Gateway receives a valid ServiceChange message
announcing a failover, it will start transmitting outstanding commands
to that new MGC.  Responses to commands are still transmitted to the
source address of the command.




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In order to automatically adapt to network load, this document specifies
exponentially increasing timers.  If the initial timer is set to 200
milliseconds, the loss of a fifth retransmission will be detected after
about 6 seconds.  This is probably an acceptable waiting delay to detect
a failover. The repetitions should continue after that delay not only in
order to perhaps overcome a transient connectivity problem, but also in
order to allow some more time for the execution of a failover - waiting
a total delay of 30 seconds is probably acceptable.

It is, however, important that the maximum delay of retransmissions be
bounded.  Prior to any retransmission, it is checked that the time
elapsed since the sending of the initial datagram is no greater than T-
MAX. If more than T-MAX time has elapsed, the MG concludes that the MGC
has failed, and it begins its recovery process. When the MG establishes
a new control association, it can retransmit to the new MGC.  Alterna-
tively, a MG may use a ServiceChange with ServiceChangeMethod equal to
disconnected to inform the new MGC that the MG lost one or more transac-
tions.  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.

D.2.  using TCP

Protocol messages as defined in this document may be transmitted over
TCP.  When no port is specified by the other side (see section 7.2.8),
the commands should be sent to the default port. The defined protocol
has messages as the unit of transfer, while TCP is a stream-oriented
protocol.  TPKT, according to RFC1006 SHALL be used to delineate mes-
sages within the TCP stream.

In a transaction-oriented protocol, there are still ways for transaction
requests or responses to be lost.  As such, it is recommended that enti-
ties using TCP transport implement application level timers for each
request and each response, similar to those specified for application
level framing over UDP.

D.2.1.  Providing the At-Most-Once functionality

Messages, being carried over TCP, are not subject to transport losses,
but loss of a transaction request or its reply may nonetheless be noted
in real implementations. In the absence of a timely response, commands
are repeated. Most commands are not idempotent.  The state of the MG
would become unpredictable if, for example, Add commands were executed
several times.

To guard against such losses, it is recommended that entities follow the
procedures in section D.1.1




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D.2.2.  Transaction identifiers and three way handshake

For the same reasons, it is possible that transaction replies may be
lost even with a reliable delivery protocol such as TCP.  It is recom-
mended that entities follow the procedures in section D.1.2.2.

D.2.3.  Computing retransmission timers

With reliable delivery, the incidence of loss of a transaction request
or reply is expected to be very low.  Therefore, only simple timer
mechanisms are required. Exponential back-off algorithms should not be
necessary, although they could be employed where, as in an MGC, the code
to do so is already required, since MGCs must implement ALF/UDP as well
as TCP.

D.2.4.  Provisional responses

As with UDP, executing some transactions may require a long time. Enti-
ties that can predict that a transaction will require a long execution
time may send a provisional response, "Transaction Pending". They should
send this response if they receive a repetition of a transaction that is
still being executed.

Entities that receive a Transaction Pending shall switch to a longer
repetition timer for that transaction.

Entities shall retain Transactions and replies until they are confirmed.
The basic procedure of section D.1.4 should be followed, but simple
timer values should be sufficient. There is no need to send an immediate
confirmation upon receipt of a final response.

D.2.5.  Ordering of commands

TCP provides ordered delivery of transactions.  No special procedures
are required.  It should be noted that ALF/UDP allows sending entity to
modify its behavior under congestion, and in particular, could reorder
transactions when congestion is encountered.  TCP could not achieve the
same results.

ANNEX E BASIC PACKAGES

This Annex contains definitions of some packages for use with MEGACO.

E.1.  Generic

      PackageID: g (0x000e)
      Version: 1
      Extends: None



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      Description:
            Generic package for commonly encountered items


E.1.1.  Properties

None

E.1.2.  Events

Cause
      EventID:  cause (0x0001)
      Generic error event
        Event Descriptor Parameters:
              General Cause
                  ParameterID: Generalcause (0x0001)
                  This parameter groups the failures into six groups,
                  which the MGC may act upon.
                  Possible values:      Enumerated,
                        "NR" Normal Release (0x0001)
                              "UR" Unavailable Resources (0x0002)
                              "FT" Failure, Temporary (0x0003)
                              "FP" Failure, Permanent (0x0004)
                              "IW" Interworking Error (0x0005)
                              "UN" Unsupported (0x0006)
            Failure Cause
                      ParameterID: Failurecause (0x0002)
                  Possible Values:      OCTET STRING
                  Description: The Release Cause is the value generated
                       by the Released equipment, i.e. a released network
                       connection.  The concerned value is defined in the
                       appropriate bearer control protocol.
Signal Completion
        EventID: sc (0x0002)
        Indicates termination of one or more signals for which the
        notifyCompletion parameter was set to "ON".  For further
        procedural description, see sections 7.1.11, 7.1.17, and 7.2.7.

        ObservedEvents Descriptor parameters:
                Signal Identity
                        ParameterID:  SigID (0x0001)
                        This parameter identifies the signals which have terminated.
                        Type: list
                        Possible values: a list of signals and/or sequential
                        signal lists which have terminated.  A signal outside
                        of a sequential signal list shall be identified using
                        the pkgdName syntax without wildcarding.  An
                        individual signal inside of a sequential signal list



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                        shall be identified using the sequential signal list
                        syntax with the correct signal list     identifier,
                        enclosing the name of the specific signal which
                        terminated in pkgdName syntax.

                Termination Method
                        ParameterID:  Meth (0x0002)
                        Indicates the means by which the signal terminated.
                        Type: enumeration
                        Possible values:
                                "TO" (0x0001) Duration expired
                                "EV" (0x0002) Interrupted by event
                                "SD" (0x0003) Halted by new Signals Descriptor
                                "NC" (0x0004) Not completed, other cause





































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E.1.3.  Signals

        None

E.1.4.  Statistics

        None

E.2.  Base Root Package

        Base Root Package

              PackageID: root (0x000f)
              Version: 1
              Extends: None
              Description:
                    This package defines Gateway wide properties.

E.2.1.  Properties

        MaxNrOfContexts
                PropertyID: maxNumberOfContexts (0x0001)
              The value of this property gives the maximum number of
              contexts that can exist at any time.  The NULL context
              is not included in this number.
              Type: Double
              Possible values: 1 and up
        MaxTerminationsPerContext
              PropertyID: maxTerminationsPerContext (0x0002)
              The maximum number of allowed terminations in a context,
                    see section 6.1
              Type: Integer
              Possible Values: any integer
              Defined In: TerminationState
        normalMGExecutionTime
              PropertyId: normalMGExecutionTime (0x0003)
              Settable by the MGC to indicate the interval within which
                    the MGC expects a response to any transaction from
                    the MG (exclusive of network delay)
              Type: Integer
              Possible Values: any integer, represents milliseconds
        normalMGCExecutionTime
              PropertyId: normalMGCExecutionTime (0x0004)
              Settable by the MGC to indicate the interval within which
                    the MG should expects a response to any transaction
                    from the MGC (exclusive of network delay)



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              Type: Integer
              Possible Values: any integer, represents milliseconds
        ProvisionalResponseTimerValue
              PropertyId: ProvisionalResponseTimerValue (0x0005)
              Indicates the time within which to expect a Pending
                    Response if a Transaction cannot be completed.
                    Initially set to normalMGExecutionTime or
                    normalMGCExecutionTime as appropriate, plus network
                    delay, but may be lowered.

E.2.2.  Events

        None

E.2.3.  Signals

        None

E.2.4.  Statistics

        None

E.2.5.  Procedures

        None

E.3.  Tone Generator Package

        PackageID: tonegen (0x0001)
              Version: 1
              Extends: None
              Description:
                    This package defines signals to generate audio tones.
                    This package does not specify parameter values. It is
                    intended to be extendable. Generally, tones are defined
                    as an individual signal with a parameter, ind,
                    representing "interdigit" time delay, and a tone id to
                    be used with playtones.  A tone id should be kept
                    consistent with any tone generation for the same tone.
                    MGs are expected to be provisioned with the characteristics
                    of appropriate tones for the country in which the MG is located.


E.3.1.  Properties

None





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E.3.2.  Events

None

E.3.3.  Signals

Play tone

      SignalID: pt (0x0001)
      Plays audio tone over an audio channel
      Signal Type: Brief
      Duration: Provisioned
      Additional Parameters:
            Tone id list
                  ParameterID: tl       (0x0001)
                  Type: list of tone ids.
                  List of tones to be played in sequence.
                        The list SHALL contain one or more tone ids.
            Inter signal duration
                  ParameterID: ind (0x0002)
                  Type: integer.
                  Timeout between two consecutive tones in milliseconds


No tone ids are specified in this package. Packages that extend this
package can add possible values for tone id as well as adding individual
tone signals

E.3.4.  Statistics

None

E.3.5.  Procedures

None

E.4.  Tone Detection Package

PackageID: tonedet (0x0002)
      Version: 1
      Extends: None
      This Package defines events for audio tone detection.
            Tones are selected by name (tone id). MGs are expected
            to be provisioned with the characteristics of appropriate
            tones for the country in which the MG is located.


This package does not specify parameter values. It is intended to be



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extendable.

E.4.1.  Properties

None

E.4.2.  Events

Start tone detected
      EventID: std, 0x0001
      Detects the start of a tone. The characteristics of positive
            tone detection is implementation dependent.
      EventsDescriptor parameters:
            Tone id list
                  ParameterID: tl (0x0001)
                  Type:  list of tone ids
                  Possible values: The only tone id defined in this
                              package is "wild card" which is "*" in
                        text encoding and 0x0000 in binary.
                        Extensions to this package would add
                        possible values for tone id.
                        If tl is "wild card", any tone id is detected
      ObservedEventsDescriptor parameters:
            Tone id
                  ParameterID: tid (0x0003)
                  Type: Enumeration
                  Possible values: "wildcard" as defined above is the
                        only value defined in this package. Extensions
                        to this package would add additional possible
                        values for tone id
End tone detected
      EventID: etd, 0x0002
      Detects the end of a tone.
      EventDescriptor parameters:
            Tone id list
                  ParameterID: tl (0x0001)
                  Type: enumeration or list of enumerated types
                  Possible values: No possible values are specified
                        in this package.  Extensions to this package
                        would add possible values for tone id
      ObservedEventsDescriptor parameters:
            Tone id
                  ParameterID: tid (0x0003)
                  Type: Enumeration
                  Possible values: "wildcard" as defined above is the
                        only value defined in this package.
                        Extensions to this package would add possible
                        values for tone id



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            Duration
                  ParameterId: dur (0x0002)
                  Type: integer, in milliseconds
                  This parameter contains the duration of the tone
                        from first detection until it stopped.
Long tone detected
      EventID: ltd, 0x0003
      Detects that a tone has been playing for at least a certain
            amount of time
      EventDescriptor parameters:
            Tone id list
                  ParameterID: tl (0x0001)
                  Type: enumeration or list
                  Possible values: "wildcard" as defined above is the
                        only value defined in this package. Extensions
                        to this package would add possible values for
                        tone id
            Duration:
                  ParameterID: dur (0x0002)
                  Type: integer, duration to test against
                  Possible values: any legal integer, expressed in
                        milliseconds
      ObservedEventsDescriptor parameters:
            Tone id:
                  ParameterID: tid (0x0003)
                  Possible values: No possible values are specified
                  in this package.  Extensions to this package
                  would add possible values for tone id


E.4.3.  Signals

None

E.4.4.  Statistics

None

E.4.5.  Procedures

None

E.5.  Basic DTMF Generator Package

PackageID: dg (0x0003)
      Version: 1
      Extends: tonegen version 1
      This package defines the basic DTMF tones as signals and



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            extends the allowed values of parameter tl of playtone
            in tonegen.


E.5.1.  Properties

None

E.5.2.  Events

None

E.5.3.  Signals

dtmf character 0
      SignalID: d0 (0x0010)
      Generate DTMF 0 tone. The physical characteristic of DTMF 0
            is defined in the gateway.
      Signal Type: Brief
      Duration: Provisioned
      Additional Parameters:
            None
      Additional Values:
            d0 (0x0010) is defined as a toneid for playtone

The other dtmf characters are specified in exactly the same way.  A
table with all signal names and signal IDs is included.  Note that each
dtmf character is defined as both a signal and a toneid, thus extending
the basic tone generation package. Also note that dtmf SignalIds are
different from the names used in a digit map.

                    ________________________________
                     Signal Name        Signal ID
                     dtmf character 0   d1 (0x0010)
                     dtmf character 1   d1 (0x0011)
                     dtmf character 2   d2 (0x0012)
                     dtmf character 3   d3 (0x0013)
                     dtmf character 4   d4 (0x0014)
                     dtmf character 5   d5 (0x0015)
                     dtmf character 6   d6 (0x0016)
                     dtmf character 7   d7 (0x0017)
                     dtmf character 8   d8 (0x0018)
                     dtmf character 9   d9 (0x0019)
                     dtmf character *   ds (0x0020)
                     dtmf character #   do (0x0021)
                     dtmf character A   da (0x001a)
                     dtmf character B   db (0x001b)
                     dtmf character C   dc (0x001c)



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                   | dtmf character D|  dd (0x001d)|
                   |_________________|_____________|


E.5.4.  Statistics

None

E.5.5.  Procedures

None

E.6.  DTMF detection Package

PackageID: dd (0x0004)
      Version: 1
      Extends: tonedet version 1
      This package defines the basic DTMF tones detection.
            This Package extends the possible values of tone id
            in the "start tone detected" "end tone detected"
            and "long tone detected" events.

Additional tone id values are all tone ids described in package dg
(basic DTMF generator package).

The following table maps DTMF events to digit map symbols as described
in section 7.1.14.

                   _________________________________
                  | DTMF Event        | Symbol     |
                  | d0                |  "0"       |
                  | d1                |  "1"       |
                  | d2                |  "2"       |
                  | d3                |  "3"       |
                  | d4                |  "4"       |
                  | d5                |  "5"       |
                  | d6                |  "6"       |
                  | d7                |  "7"       |
                  | d8                |  "8"       |
                  | d9                |  "9"       |
                  | da                |  "A" or "a"|
                  | db                |  "B" or "b"|
                  | dc                |  "C" or "c"|
                  | dd                |  "D" or "d"|
                  | ds                |  "E" or "e"|
                  | do                |  "F" or "f"|
                  |___________________|____________|




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E.6.1.  Properties

None

E.6.2.  Events

DTMF digits
      EventIds are defined with the same names as the SignalIds
            defined in the table found in section E.5.3

DigitMap Completion Event
      EventID: ce, 0x0001
      Generated when a digit map completes as described in section
        7.1.14.

      EventsDescriptor parameters: digit map processing is activated
only if a digit map parameter is present, specifying a
digit map by name or by value.  Other parameters such as
a KeepActive flag or embedded Events or Signals Descriptors may be present..

      ObservedEventsDescriptor parameters:
            DigitString
                  ParameterID: ds (0x0001)
                  Type: string of digit map symbols (possibly empty)
                           returned as a quotedString
                  Possible values: any sequence of the characters "0" through
                     "9", "A" through "F", and the long duration modifier "L".
                  Description: the portion of the current dial string as
                    described in section 7.1.14 which matched part or all
                    of an alternative event sequence specified in the digit map.

            Termination Method
                  ParameterID:    Meth (0x0003)
                  Type: enumeration
                  Possible values:
                      "UM" (0x0001) Unambiguous match
                      "PM" (0x0002) Partial match, completion by timer expiry
                                  or unmatched event
                      "FM" (0x0003) Full match, completion by timer expiry
                                  or unmatched event
                  Description: indicates the reason for generation of the event.
                            See the procedure in section 7.1.14.


E.6.3.  Signals

None




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E.6.4.  Statistics

None

E.6.5.  Procedures

None

E.7.  Call Progress Tones Generator Package

      PackageID: cg, 0x0005
      Version: 1
      Extends: tonegen version 1
      This package defines the basic call progress tones as signals
            and extends the allowed values of the tl parameter of
            playtone in tonegen.


E.7.1.  Properties

None

E.7.2.  Events

None

E.7.3.  Signals

Dial Tone
      SignaID: dt (0x0030)
      Generate dial tone. The physical characteristic of dial tone
            is available in the gateway.
      Signal Type: Timeout
      Duration: Provisioned
      Additional Parameters:
            None
      Additional Values
            dt (0x0030) is defined as a tone id for playtone


The other tones of this package are defined in exactly the same way. A
table with all signal names and signal IDs is included.  Note that each
tone is defined as both a signal and a toneid, thus extending the basic
tone generation package.
                                 l | l.
Signal Name!Signal ID/tone id Dial Tone!dt (0x0030) Ringing Tone!rt
(0x0031) Busy Tone!bt (0x0032) Congestion Tone!ct (0x0033) Special
Information Tone!sit(0x0034) Warning Tone!wt (0x0035) Payphone



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Recognition Tone!pt (0x0036) Call Waiting Tone!cw (0x0037) Caller Wait-
ing Tone!cr (0x0038)


E.7.4.  Statistics

None

E.7.5.  Procedures

NOTE - The required set of tone ids corresponds to those defined in
Recommendation E.180/Q.35 [ITU-T Recommendation E.180/Q.35 (1998)].  See
E.180 for definition of the meanings of these tones.

E.8.  Call Progress Tones Detection Package

PackageID: cd (0x0006)
      Version: 1
      Extends: tonedet version 1
      This package defines the basic call progress detection tones.
            This Package extends the possible values of tone id
            in the "start tone detected", "end tone detected" and
            "long tone detected" events.
      Additional values
            tone id values are defined for start tone detected,
                  end tone detected and long tone detected with
                  the same values as those in package cg (call
                  progress tones generation package).


The required set of tone ids corresponds to Recommendation E.180/Q.35
[ITU-T Recommendation E.180/Q.35 (1998)].  See Recommendation E.180/Q.35
for definition of the meanings of these tones.

E.8.1.  Properties

none

E.8.2.  Events

Events are defined as in the call progress tones generator package (cg)
for the tones listed in the table of section E.7.3

E.8.3.  Signals

none





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E.8.4.  Statistics

none

E.8.5.  Procedures

none

E.9.  Analog Line Supervision Package

PackageID: al, 0x0009
      Version: 1
      Extends: None
      This package defines events and signals for an analog line.

E.9.1.  Properties

None

E.9.2.  Events

onhook
      EventID: on (0x0004)
      Detects handset going on hook. Whenever an events descriptor
        is activated that requests monitoring for an on-hook event
        and the line is already on-hook, then the MG immediately
        generate an on-hook event.
      EventDescriptor parameters
            None
      ObservedEventsDescriptor parameters
            None
offhook
      EventID: of (0x0005)
      Detects handset going off hook. Whenever an events descriptor
        is activated that requests monitoring for an off-hook event
        and the line is already off-hook, then the MG immediately
        generate an off-hook event.
      EventDescriptor parameters
            None
      ObservedEventsDescriptor parameters
            None
flashhook
      EventID: fl, 0x0006
      Detects handset flash. A flash occurs when an onhook is
        followed by an offhook between a minimum and
        maximum duration.
      EventDescriptor parameters
            Minimum duration



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                  ParameterID: mindur (0x0004)
                  Type: integer in milliseconds
                  Default value is provisioned
            Maximum duration
                  ParameterID: maxdur (0x0005)
                  Type: integer in milliseconds
                  Default value is provisioned
      ObservedEventsDescriptor parameters
            None


E.9.3.  Signals

ring
      SignalID: ri, 0x0002
      Applies ringing on the line
      Signal Type: TimeOut
      Duration: Provisioned
      Additional Parameters:
            Cadence
                  ParameterID: cad (0x0006)
                  Type: list of integers representing durations of
                        alternating on and off segments, constituting
                        a complete ringing cycle starting with an on.
                        Units in milliseconds
                  Default is fixed or provisioned.  Restricted function
                        MGs may ignore cadence values they are
                        incapable of generating.
            Frequency
                  ParameterID: freq (0x0007)
                  Type: integer in Hz
                  Default is fixed or provisioned.  Restricted function
                        MGs may ignore frequency  values they are
                        incapable of generating.


E.9.4.  Statistics

None

E.9.5.  Procedures

None

E.10.  Basic Continuity Package

PackageID: ct (0x000a)
      Version: 1



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      Extends: None
      This package defines events and signals for continuity test.
      The continuity test includes provision of either a loopback
      or transceiver functionality.


E.10.1.  Properties

None

E.10.2.  Events

Completion
      EventID: cmp, 0x0005
      This event detects test completion of continuity test.
      EventDescriptor parameters
            None
      ObservedEventsDescriptor parameters
            Result
                  ParameterID: res (0x0008)
                  Type: Enumeration
                  Possible values: success (0x0001), failure (0x0000)


E.10.3.  Signals

Continuity Test
      SignalID: ct (0x0003)
      Initiates sending of continuity test tone on the termination
        to which it is applied.
      Signal Type: Timeout
Default value is provisioned
      Additional Parameters:
          None
Respond
      SignalID: rsp (0x0004)
      The signal is used to respond to a continuity test
      See section E.5.10 for further explanation.
      Signal Type: TimeOut
      Default duration is provisioned
      Additional Parameters:
        None


E.10.4.  Statistics

None




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E.10.5.  Procedures

When a MGC wants to initiate a continuity test, it sends a command to
the MG containing a signals descriptor with the ct signal, and an events
descriptor containing the cmp event.  Upon reception of a command con-
taining the ct signal and cmp event, the MG initiates the continuity
test tone for the specified termination.  If the return tone is detected
before the signal times out, the cmp event shall be generated with the
value of the result parameter equal to success.  In all other cases, the
cmp event shall be generated with the value of the result parameter
equal to failure.  When a MGC wants the MG to respond to a continuity
test, it sends a command to the MG containing a signals descriptor with
the rsp signal.  Upon reception of a command with the rsp signal, the MG
awaits reception of the continuity test tone.  When the tone is received
before the rsp signal times out, the MG returns the applicable return
tone.  If the rsp signal times out, the MG removes the detection and the
return tone (if that was playing).  When a continuity test is performed
on a termination, no echo devices or codecs shall be active on that ter-
mination.  Performing voice path assurance as part of continuity testing
is provisioned by bilateral agreement between network operators.

E.11.  Network Package

      PackageID: nt (0x000b)
      Version: 1
      Extends: None
      This package defines properties of network terminations
            independent of network type.


E.11.1.  Properties

Maximum Jitter Buffer
      PropertyID: jit (0x0007)
      This property puts a maximum size on the jitter buffer.
      Type: integer in milliseconds
      Possible Values: This property is specified in milliseconds.
      Defined In: LocalControlDescriptor
      Characteristics: read/write



E.11.2.  Events

network failure
      EventID: netfail, 0x0005
      The termination generates this event upon detection of a
            failure due to external or internal network reasons.



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      EventDescriptor parameters
            None
      ObservedEventsDescriptor parameters
            cause
                  ParameterID: cs (0x0001)
                  Type: String
                  Possible values: any text string
                  This parameter may be included with the failure
                  event to provide diagnostic information on the
                  reason of failure.
quality alert
      EventID: qualert, 0x0006
      This property allows the MG to indicate a loss of quality
            of the network connection. The MG may do this by
            measuring packet loss, interarrival jitter, propogation
            delay and then indicating this using a percentage of
            quality loss.
      EventDescriptor parameters
            Threshold
                  ParameterId: th (0x0001)
                  Type: integer
                  Possible Values: threshold for percent of quality
                        loss measured, calculated based on a
                        provisioned method, that could take into
                        consideration packet loss, jitter, and delay
                        for example.  Event is triggered when
                        calculation exceeds the threshold.
      ObservedEventsDescriptor parameters
            Threshold
                  ParameterId: th (0x0001)
                  Type: integer
                  Possible Values: percent of quality loss measured,
                        calculated based on a provisioned method,
                        that could take into consideration packet loss,
                        jitter, and delay for example.


E.11.3.  Signals

none

E.11.4.  Statistics

Duration
      StatisticsID: dur (0x0001)
      Description: Provides duration of time the termination has
            been in the context.
      Type: Double, in milliseconds



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Octets Sent
      StatisticID: os (0x0002)
      Type: double
      Possible Values: any 64 bit integer
Octets Received
      StatisticID: or (0x0003)
      Type: double
      Possible Values: any 64 bit integer


E.11.5.  Procedures

none

E.12.  RTP Package

      PackageID: rtp (0x000c)
      Version: 1
      Extends: Network Package version 1
      This package is used to support packet based multimedia
            data transfer by means of the Real-time Transport Protocol
            (RTP) [RFC 1889].


E.12.1.  Properties

None

E.12.2.  Events

Payload Transition
      EventID: pltrans, 0x0001
      This event detects and notifies when there is a transition
      of the RTP payload format from one format to another.
      EventDescriptor parameters
            None
      ObservedEventsDescriptor parameters
            ParameterName: rtppayload
                  ParameterID: rtppltype, 0x01
                  Type: list of enumerated types.
                  Possible values: The encoding method shall be
                        specified by using one or several valid
                        encoding names, as defined in the RTP AV
                        Profile or registered with IANA.







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E.12.3.  Signals

None

E.12.4.  Statistics

Packets Sent
      StatisticID: ps (0x0004)
      Type: double
      Possible Values: any 64 bit integer
Packets Received
      StatisticID: pr (0x0005)
      Type: double
      Possible Values: any 64 bit integer
Packet Loss
      StatisticID: pl (0x0006)
      Describes the current rate of packet loss on an RTP stream,
      as defined in IETF RFC 1889. Packet loss is expressed as
      percentage value: number of packets lost in the interval
      between two reception reports, divided by the number of
      packets expected during that interval.
      Type: double
      Possible Values: a 32 bit whole number and a 32 bit fraction.
Jitter
      StatisticID: jit (0x0007)
      Requests the current value of the interarrival jitter
            on an RTP stream as defined in IETF RFC 1889.
            Jitter measures the variation in interarrival time
            for RTP data packets.
Delay
      StatisticID:delay (0x0008)
      Requests the current value of packet propagation delay
            expressed in timestamp units. Same as average latency.


E.12.5.  Procedures

none

E.13.  TDM Circuit  Package

PackageID: tdmc (0x000d)
      Version: 1
      Extends: Network Package version 1
      This package is used to support TDM circuit terminations.






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E.13.1.  Properties

Echo Cancellation
      PropertyID: ec (0x0008)
      By default, the telephony gateways always perform
        echo cancellation.
        However, it is necessary, for some calls, to turn
        off these operations.
      Type: boolean
      Possible Values:
            "on" (when the echo cancellation isrequested) and
            "off (when it is turned off.)
            The default "on".
      Defined In: LocalControlDescriptor
        Characteristics: read/write

Gain Control
      PropertyID: gain (0x000a)
      Gain control, or usage of of signal level adaptation and
            noise level reduction is used to adapt the level of
            the signal. However, it is necessary, for example
            for modem calls, to turn off this function.
      Type: enumeration (integer)
      Possible Values:
            The gain control parameter may either be specified
            as "automatic" (0xffffffff), or as an explicit number
            of decibels of gain (any other integer value).
            The default is provisioned in the MG.
      Defined In: LocalControlDescriptor
      Characteristics: read/write


E.13.2.  Events

none

E.13.3.  Signals

none

E.13.4.  Statistics

None

E.13.5.  Procedures

None




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APPENDIX A      EXAMPLE CALL FLOWS (INFORMATIVE)

All Megaco implementors must read the normative part of this document
carefully before implementing from it. No one should use the examples in
this section as stand-alone explanations of how to create protocol mes-
sages.

The examples in this section use SDP for encoding of the Local and
Remote stream descriptors. SDP is defined in RFC 2327. If there is any
discrepancy between the SDP in the examples, and RFC 2327, the RFC
should be consulted for correctness. Audio profiles used are those
defined in RFC 1890, and others registered with IANA. For example, G.711
A-law is called PCMA in the SDP, and is assigned profile 0. G.723 is
profile 4, and H263 is profile 34. See also http://www.isi.edu/in-
notes/iana/assignments/rtp-parameters

A.1.  Residential Gateway to Residential Gateway Call

This example scenario illustrates the use of the elements of the proto-
col to set up a Residential Gateway to Residential Gateway call over an
IP-based network.  For simplicity, this example assumes that both
Residential Gateways involved in the call are controlled by the same
Media Gateway Controller.

A.1.1.  Programming Residential GW Analog Line Terminations for Idle
Behavior

The following illustrates the API invocations from the Media Gateway
Controller and Media Gateways to get the Terminations in this scenario
programmed for idle behavior.  Both the originating and terminating
Media Gateways have idle AnalogLine Terminations programmed to look for
call initiation events (i.e.-offhook) by using the Modify Command with
the appropriate parameters.  The null Context is used to indicate that
the Terminations are not yet involved in a Context. The ROOT termination
is used to indicate the entire MG instead of a termination within the
MG.

In this example, MG1 has the IP address 124.124.124.222, MG2 is
125.125.125.111, and the MGC is 123.123.123.4. The default Megaco port
is 55555 for all three.

1.   An MG registers with an MGC using the ServiceChange command:

     MG1 to MGC:
     MEGACO/1 [124.124.124.222]
     Transaction = 9998 {
         Context = - {
             ServiceChange = ROOT {Services {



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                 Method=Restart,
                 ServiceChangeAddress=55555, Profile=ResGW/1}
             }
         }
     }


2.   The MGC sends a reply:

     MGC to MG1:
     MEGACO/1 [123.123.123.4]:55555
     Reply = 9998 {
        Context = - {ServiceChange = ROOT {
          Services {ServiceChangeAddress=55555, Profile=ResGW/1} } }
     }


3.   The MGC programs a Termination in the NULL context. The termina-
     tionId is A4444, the streamId is 1, the requestId in the Events
     descriptor is 2222. The   mId is the identifier of the sender of
     this message, in this case, it is the IP address and port
     [123.123.123.4]:55555. Mode for this stream is set to SendReceive.
     "al" is the analog line supervision package.

     MGC to MG1:
     MEGACO/1 [123.123.123.4]:55555
     Transaction = 9999 {
         Context = - {
             Modify = A4444 {
                 Media { Stream = 1 {
                          LocalControl {
                              Mode = SendReceive,
                              ds0/gain=2,  ; in dB,
                              ds0/ec=G165
                          },
                          Local {
     v=0
     c=IN IP4 $
     m=audio $ RTP/AVP 0
     a=fmtp:PCMU VAD=X-NNVAD ; special voice activity
                             ; detection algorithm
                          }
                      }
                 },
                 Events = 2222 {al/of}
             }
         }
     }



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The dialplan script could have been loaded into the MG previously.  Its
function would be to wait for the OffHook, turn on dialtone and start
collecting DTMF digits. However in this example, we use the digit map,
which is put into place after the offhook is detected (step 5 below).

Note that the embedded EventsDescriptor could have been used to combine
steps 3 and 4 with steps 8 and 9, eliminating steps 6 and 7.

4.   The MG1 accepts the Modify with this reply:

     MG1 to MGC:
     MEGACO/1 [124.124.124.222]:55555
     Reply = 9999 {
        Context = - {Modify = A4444}
     }


5.   A similar exchange happens between MG2 and the MGC, resulting in an
     idle Termination called A5555.

A.1.2. Collecting Originator Digits and Initiating Termination

The following builds upon the previously shown conditions.  It illus-
trates the transactions from the Media Gateway Controller and originat-
ing Media Gateway (MG1) to get the originating Termination (A4444)
through the stages of digit collection required to initiate a connection
to the terminating Media Gateway (MG2).

6.   MG1 detects an offhook event from User 1 and reports it to the
     Media Gateway Controller via the Notify Command.

     MG1 to MGC:
     MEGACO/1 [124.124.124.222]:55555
     Transaction = 10000 {
        Context = - {
            Notify = A4444 {ObservedEvents =2222 {
              19990729T22000000:al/of}}
        }
     }



7.   And the Notify is acknowledged.

     MGC to MG1:
     MEGACO/1 [123.123.123.4]:55555



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     Reply = 10000 {
         Context = - {Notify = A4444}
     }



8.   The MGC Modifies the termination to play dial tone, to look for
     digits according to Dialplan0 and to look for the on-hook event
     now.

     MGC to MG1:
     MEGACO/1 [123.123.123.4]:55555
     Transaction = 10001 {
         Context = - {
             Modify = A4444 {
                 Events = 2223 {
                     al/on, dd/ce {DigitMap=Dialplan0}
                 },
                 Signals {cg/dt},
                 DigitMap= Dialplan0{
     (0| 00|[1-7]xxx|8xxxxxxx|Fxxxxxxx|Exx|91xxxxxxxxxx|9011x.)}
             }
         }
     }



9.   And the Modify is acknowledged.

     MG1 to MGC:
     MEGACO/1 [124.124.124.222]:55555
     Reply = 10001 {
         Context = - {Modify = A4444}
     }



10.  Next, digits are accumulated by MG1 as they are dialed by User 1.
     Dialtone is stopped upon detection of the first digit. When an
     appropriate match is made of collected digits against the currently
     programmed Dialplan for A4444, another Notify is sent to the Media
     Gateway Controller.

     MG1 to MGC:
     MEGACO/1 [124.124.124.222]:55555
     Transaction = 10002 {
        Context = - {
            Notify = A4444 {ObservedEvents =2223 {



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              19990729T22010001:dd/ce{ds="916135551212",Meth=FM}}}
        }
     }



11.  And the Notify is acknowledged.

     MGC to MG1:
     MEGACO/1 [123.123.123.4]:55555
     Reply = 10002 {
         Context = - {Notify = A4444}
     }



12.  The controller then analyses the digits and determines that a con-
     nection needs to be made from MG1 to MG2. Both the TDM termination
     A4444, and an RTP termination are added to a new context in MG1.
     Mode is ReceiveOnly since Remote descriptor values are not yet
     specified. Preferred codecs are in the MGC's preferred order of
     choice.

     MGC to MG1:
     MEGACO/1 [123.123.123.4]:55555
     Transaction = 10003 {
         Context = $ {
            Add = A4444,
            Add = $ {
                Media {
                  Stream = 1 {
                       LocalControl {
                           Mode = ReceiveOnly,

                           nt/jit=40, ; in ms
                       },
                       Local {
     v=0
     c=IN IP4 $
     m=audio $ RTP/AVP 4
     a=ptime:30
     v=0
     c=IN IP4 $
     m=audio $ RTP/AVP 0
                       }
                  }
               }
            }



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         }
     }



NOTE - The MGC states its preferred parameter values as a series of sdp
blocks in  Local. The MG fills in the Local Descriptor in the Reply.


13.  MG1 acknowledges the new Termination and fills in the Local IP
     address and UDP port. It also makes a choice for the codec based on
     the MGC preferences in Local. MG1 sets the RTP port to 2222.

     MEGACO/1 [124.124.124.222]:55555
     Reply = 10003 {
        Context = 2000 {
           Add = A4444,
           Add=A4445{
              Media {
                  Stream = 1 {
                      Local {
     v=0
     c=IN IP4 124.124.124.222
     m=audio 2222 RTP/AVP 4
     a=ptime:30
     a=recvonly
                      } ; RTP profile for G.723 is 4
                  }
              }
           }
        }
     }



14.  The MGC will now associate A5555 with a new Context on MG2, and
     establish an RTP Stream (i.e, A5556 will be assigned), SendReceive
     connection through to the originating user, User 1. The MGC also
     sets ring on A5555.

     MGC to MG2:
     MEGACO/1 [123.123.123.4]:55555
     Transaction = 50003 {
         Context = $ {
            Add = A5555  { Media {
                 Stream = 1 {
                      LocalControl {Mode = SendReceive} }},
          Events=1234{al/of}



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                 Signals {al/ri}
                 },
            Add  = $ {Media {
                 Stream = 1 {
                      LocalControl {
                         Mode = SendReceive,
                         nt/jit=40 ; in ms
                      },
                      Local {
     v=0
     c=IN IP4 $
     m=audio $ RTP/AVP 4
     a=ptime:30
                      },
                      Remote {
     v=0
     c=IN IP4 124.124.124.222
     m=audio 2222 RTP/AVP 4
     a=ptime:30
                      } ; RTP profile for G.723 is 4
                  }
               }
           }
        }
     }




15.  This is acknowledged. The stream port number is different from the
     control port number. In this case it is 1111 (in the SDP).

     MG2 to MGC:
     MEGACO/1 [124.124.124.222]:55555
     Reply = 50003 {
        Context = 5000 {
           Add = A5555{},
           Add = A5556{
              Media {
                 Stream = 1 {
                     Local {
     v=0
     c=IN IP4 125.125.125.111
     m=audio 1111 RTP/AVP 4
     }
                 } ; RTP profile for G723 is 4
              }
            }



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        }
     }



16.  The above IPAddr and UDPport need to be given to MG1 now.

     MGC to MG1:
     MEGACO/1 [123.123.123.4]:55555
     Transaction = 10005 {
       Context = 2000 {
         Modify = A4444 {
           Signals {cg/rt}
         },
         Modify = A4445 {
            Media {
                 Stream = 1 {
                     Remote {
     v=0
     c=IN IP4 125.125.125.111
     m=audio 1111 RTP/AVP 4
                     }
                 } ; RTP profile for G723 is 4
             }
         }
       }
     }

     MG1 to MGC:
     MEGACO/1 [124.124.124.222]:55555
     Reply = 10005 {
        Context = 2000 {Modify = A4444, Modify = A4445}
     }



17.  The two gateways are now connected and User 1 hears the RingBack.
     The MG2 now waits until User2 picks up the receiver and then the
     two-way call is established.

     From MG2 to MGC:

     MEGACO/1 [125.125.125.111]:55555
     Transaction = 50005 {
        Context = 5000 {
            Notify = A5555 {ObservedEvents =1234 {
              19990729T22020002:al/of}}
        }



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     }

     From MGC to MG2:

     MEGACO/1 [123.123.123.4]:55555
     Reply = 50005 {
         Context = - {Notify = A5555}
     }

     From MGC to MG2:

     MEGACO/1 [123.123.123.4]:55555
     Transaction = 50006 {
        Context = 5000 {
           Modify = A5555 {
              Events = 1235 {al/on},
              Signals { } ; to turn off ringing
           }
        }
     }

     From MG2 to MGC:

     MEGACO/1 [125.125.125.111]:55555
     Reply = 50006 {
      Context = 5000 {Modify = A4445}
     }



18.  Change mode on MG1 to SendReceive, and stop the ringback.

     MGC to MG1:
     MEGACO/1 [123.123.123.4]:55555
     Transaction = 10006 {
        Context = 2000 {
           Modify = A4445 {
              Media {
                 Stream = 1 {
                    LocalControl {
                       Mode=SendReceive
                    }
                 }
              }
           },
           Modify = A4444 {
              Signals { }
           }



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        }
     }

     from MG1 to MGC:
     MEGACO/1 [124.124.124.222]:55555
     Reply = 10006 {
        Context = 2000 {Modify = A4445, Modify = A4444}}



19.  The MGC decides to Audit the RTP termination on MG2.

     MEGACO/1 [123.123.123.4]:55555
     Transaction = 50007 {
        Context = - {AuditValue = A5556{
           Audit{Media, DigitMap, Events, Signals, Packages, Statistics }}
        }
     }



20.  The MG2 replies. An RTP termination has no events nor signals, so
     these are left out in the reply .

     MEGACO/1 [125.125.125.111]:55555
     Reply = 50007 {
        Context = - {
            AuditValue = A5556 {
               Media {
                  Stream = 1 {
                      LocalControl { Mode = SendReceive,
                         nt/jit=40 },
                      Local {
     v=0
     c=IN IP4 125.125.125.111
     m=audio 1111 RTP/AVP  4
     a=ptime:30
                     },
                      Remote {
     v=0
     c=IN IP4 124.124.124.222
     m=audio 2222 RTP/AVP  4
     a=ptime:30
                      } } },
               Packages {nt-1, rtp-1},
               Statistics { rtp/ps=1200,  ; packets sent
                            nt/os=62300, ; octets sent
                            rtp/pr=700, ; packets received



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                            nt/or=45100, ; octets received
                            rtp/pl=0.2,  ; % packet loss
                            rtp/jit=20,
                            rtp/delay=40 } ; avg latency
            }
         }
     }



21.  When the MGC receives an onhook signal from one of the MGs, it
     brings down the call. In this example, the user at MG2 hangs up
     first.

     From MG2 to MGC:

     MEGACO/1 [125.125.125.111]:55555
     Transaction = 50008 {
        Context = 5000 {
            Notify = A5555 {ObservedEvents =1235 {
               19990729T24020002:al/on}
            }
        }
     }

     From MGC to MG2:

     MEGACO/1 [123.123.123.4]:55555
     Reply = 50008 {
         Context = - {Notify = A5555}
     }



22.  The MGC now sends both MGs a Subtract to take down the call. Only
     the subtracts to MG2 are shown here. Each termination has its own
     set of statistics that it gathers. An MGC may not need to request
     both to be returned. A5555 is a physical termination, and A5556 is
     an RTP termination.

     From MGC to MG2:

     MEGACO/1 [123.123.123.4]:55555
     Transaction = 50009 {
        Context = 5000 {
           Subtract = A5555 {Audit{Statistics}},
           Subtract = A5556 {Audit{Statistics}}
        }



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     }

     From MG2 to MGC:

     MEGACO/1 [125.125.125.111]:55555
     Reply = 50009 {
        Context = 5000 {
          Subtract = A5555 {
               Statistics {
                  nt/os=45123, ; Octets Sent
                  nt/dur=40 ; in seconds
                  }
            },
            Subtract = A5556 {
               Statistics {
                  rtp/ps=1245, ; packets sent
                  nt/os=62345, ; octets sent
                  rtp/pr=780, ; packets received
                  nt/or=45123, ; octets received
                  rtp/pl=10, ;  % packets lost
                  rtp/jit=27,
                  rtp/delay=48 ; average latency
               }
            }
        }
     }



23.  The MGC now sets up both MG1 and MG2 to be ready to detect the next
     off-hook event. See step 1. Note that this could be the default
     state of a termination in the null context, and if this were the
     case, no message need be sent from the MGC to the MG. Once a termi-
     nation returns to the null context, it goes back to the default
     termination values for that termination.
















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