Network Working Group F. Cuervo
Request for Comments: 3015 N. Greene
Obsoletes: 2885, 2886 A. Rayhan
Category: Standards Track Nortel Networks
C. Huitema
Microsoft Corporation
B. Rosen
Marconi
J. Segers
LUCent Technologies
November 2000
Megaco Protocol Version 1.0
Status of this Memo
This document specifies an Internet standards track protocol for the
Internet community, and requests discussion and suggestions for
improvements. Please refer to the current edition of the "Internet
Official Protocol Standards" (STD 1) for the standardization state
and status of this protocol. Distribution of this memo is unlimited.
Copyright Notice
Copyright (C) The Internet Society (2000). All Rights Reserved.
Abstract
This document defines the protocol used between elements of a
physically decomposed multimedia gateway, i.e. a Media Gateway and a
Media Gateway Controller. The document is common text with ITU-T
Recommendation H.248 and is a result of applying the changes in RFC
2886 to the text of RFC2885.
The protocol presented in this document meets the requirements for a
media gateway control protocol as presented in RFC2805.
TABLE OF CONTENTS
1. SCOPE........................................................ 6
2. REFERENCES................................................... 6
2.1 Normative references................................... 6
2.2 Informative references................................. 9
3. DEFINITIONS.................................................. 10
4. ABBREVIATIONS................................................ 11
5. CONVENTIONS.................................................. 11
6. CONNECTION MODEL............................................. 11
6.1 Contexts............................................... 14
6.1.1 Context Attributes and Descriptors........... 15
6.1.2 Creating, Deleting and Modifying Contexts.... 15
6.2 Terminations........................................... 15
6.2.1 Termination Dynamics......................... 16
6.2.2 TerminationIDs............................... 17
6.2.3 Packages..................................... 17
6.2.4 Termination Properties and Descriptors....... 18
6.2.5 Root Termination............................. 21
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............................. 24
7.1.5 Termination State Descriptor................. 24
7.1.6 Stream Descriptor............................ 25
7.1.7 LocalControl Descriptor...................... 26
7.1.8 Local and Remote Descriptors................. 27
7.1.9 Events Descriptor............................ 30
7.1.10 EventBuffer Descriptor...................... 32
7.1.11 Signals Descriptor.......................... 32
7.1.12 Audit Descriptor............................ 34
7.1.13 ServiceChange Descriptor.................... 35
7.1.14 DigitMap Descriptor......................... 36
7.1.15 Statistics Descriptor....................... 41
7.1.16 Packages Descriptor......................... 41
7.1.17 ObservedEvents Descriptor................... 42
7.1.18 Topology Descriptor........................ 42
7.2 Command Application Programming Interface.............. 45
7.2.1 Add.......................................... 46
7.2.2 Modify....................................... 47
7.2.3 SuBTract..................................... 48
7.2.4 Move......................................... 49
7.2.5 AuditValue................................... 50
7.2.6 AuditCapabilities............................ 52
7.2.7 Notify....................................... 53
7.2.8 ServiceChange................................ 54
7.2.9 Manipulating and Auditing Context Attributes. 58
7.2.10 Generic Command Syntax...................... 58
7.3 Command Error Codes.................................... 58
8. TRANSACTIONS................................................. 60
8.1 Common Parameters...................................... 62
8.1.1 Transaction Identifiers...................... 62
8.1.2 Context Identifiers.......................... 62
8.2 Transaction Application Programming Interface.......... 63
8.2.1 TransactionRequest........................... 63
8.2.2 TransactionReply............................. 63
8.2.3 TransactionPending........................... 65
8.3 Messages............................................... 65
9. TRANSPORT.................................................... 65
9.1 Ordering of Commands................................... 66
9.2 Protection against Restart Avalanche................... 67
10. SECURITY CONSIDERATIONS..................................... 68
10.1 Protection of Protocol Connections.................... 68
10.2 Interim AH scheme..................................... 69
10.3 Protection of Media Connections....................... 70
11. MG-MGC CONTROL INTERFACE................................... 71
11.1 Multiple Virtual MGs.................................. 71
11.2 Cold Start............................................ 72
11.3 Negotiation of Protocol Version....................... 72
11.4 Failure of an MG...................................... 73
11.5 Failure of an MGC..................................... 74
12. PACKAGE DEFINITION.......................................... 75
12.1 Guidelines for defining packages...................... 75
12.1.1 Package..................................... 76
12.1.2 Properties.................................. 76
12.1.3 Events...................................... 77
12.1.4 Signals..................................... 77
12.1.5 Statistics.................................. 77
12.1.6 Procedures.................................. 78
12.2 Guidelines to defining Properties, Statistics and
Parameters to Events and Signals........................... 78
12.3 Lists................................................. 78
12.4 Identifiers........................................... 78
12.5 Package Registration.................................. 79
13. IANA CONSIDERATIONS........................................ 79
13.1 Packages.............................................. 79
13.2 Error Codes........................................... 79
13.3 ServiceChange Reasons................................. 80
ANNEX A: BINARY ENCODING OF THE PROTOCOL (NORMATIVE)............ 80
A.1 Coding of wildcards.................................... 81
A.2 ASN.1 syntax specification............................. 82
A.3 Digit maps and path names.............................. 99
ANNEX B TEXT ENCODING OF THE PROTOCOL (NORMATIVE)...............100
B.1 Coding of wildcards....................................100
B.2 ABNF specification.....................................100
ANNEX C TAGS FOR MEDIA STREAM PROPERTIES (NORMATIVE)............112
C.1 General Media Attributes...............................113
C.2 Mux Properties.........................................114
C.3 General bearer properties..............................115
C.4 General ATM properties.................................115
C.5 Frame Relay............................................118
C.6 IP.....................................................118
C.7 ATM AAL2...............................................119
C.8 ATM AAL1...............................................120
C.9 Bearer Capabilities....................................121
C.10 AAL5 Properties.......................................129
C.11 SDP Equivalents.......................................130
C.12 H.245.................................................131
ANNEX D TRANSPORT OVER IP (NORMATIVE)...........................131
D.1 Transport over IP/UDP using Application Level Framing..131
D.1.1 Providing At-Most-Once Functionality.........132
D.1.2 Transaction identifiers and three-way handshake
...................................................132
D.1.3 Computing retransmission timers..............133
D.1.4 Provisional responses........................134
D.1.5 Repeating Requests, Responses and
Acknowledgements...................................135
D.2 Using TCP..............................................136
D.2.1 Providing the At-Most-Once functionality.....137
D.2.2 Transaction identifiers and three way handshake
...................................................137
D.2.3 Computing retransmission timers..............137
D.2.4 Provisional responses........................137
D.2.5 Ordering of commands.........................138
ANNEX E BASIC PACKAGES (NORMATIVE)..............................138
E.1 Generic................................................138
E.1.1 Properties...................................138
E.1.2 Events.......................................138
E.1.3 Signals......................................140
E.1.4 Statistics...................................140
E.2 Base Root Package......................................140
E.2.1 Properties...................................140
E.2.2 Events.......................................142
E.2.3 Signals......................................142
E.2.4 Statistics...................................142
E.2.5 Procedures...................................142
E.3 Tone Generator Package.................................142
E.3.1 Properties...................................142
E.3.2 Events.......................................143
E.3.3 Signals......................................143
E.3.4 Statistics...................................143
E.3.5 Procedures...................................143
E.4 Tone Detection Package.................................144
E.4.1 Properties...................................144
E.4.2 Events.......................................144
E.4.3 Signals......................................146
E.4.4 Statistics...................................146
E.4.5 Procedures...................................146
E.5 Basic DTMF Generator Package...........................147
E.5.1 Properties...................................147
E.5.2 Events.......................................147
E.5.3 Signals......................................147
E.5.4 Statistics...................................148
E.5.5 Procedures...................................148
E.6 DTMF detection Package.................................148
E.6.1 Properties...................................149
E.6.2 Events.......................................149
E.6.3 Signals......................................150
E.6.4 Statistics...................................150
E.6.5 Procedures...................................150
E.7 Call Progress Tones Generator Package..................151
E.7.1 Properties...................................151
E.7.2 Events.......................................151
E.7.3 Signals......................................151
E.7.4 Statistics...................................152
E.7.5 Procedures...................................152
E.8 Call Progress Tones Detection Package..................152
E.8.1 Properties...................................152
E.8.2 Events.......................................153
E.8.3 Signals......................................153
E.8.4 Statistics...................................153
E.8.5 Procedures...................................153
E.9 Analog Line Supervision Package........................153
E.9.1 Properties...................................153
E.9.2 Events.......................................153
E.9.3 Signals......................................156
E.9.4 Statistics...................................157
E.9.5 Procedures...................................157
E.9.6 Error Code...................................157
E.10 Basic Continuity Package..............................157
E.10.1 Properties..................................157
E.10.2 Events......................................157
E.10.3 Signals.....................................158
E.10.4 Statistics..................................158
E.10.5 Procedures..................................159
E.11 Network Package.......................................159
E.11.1 Properties..................................159
E.11.2 Events......................................160
E.11.3 Signals.....................................161
E.11.4 Statistics..................................161
E.11.5 Procedures..................................162
E.12 RTP Package..........................................162
E.12.1 Properties..................................162
E.12.2 Events......................................162
E.12.3 Signals.....................................163
E.12.4 Statistics..................................163
E.12.5 Procedures..................................164
E.13 TDM Circuit Package...................................164
E.13.1 Properties..................................164
E.13.2 Events......................................165
E.13.3 Signals.....................................165
E.13.4 Statistics..................................165
E.13.5 Procedures..................................165
APPENDIX A EXAMPLE CALL FLOWS (INFORMATIVE).....................166
A.1 Residential Gateway to Residential Gateway Call........166
A.1.1 Programming Residential GW Analog Line
Terminations for Idle Behavior.....................166
A.1.2 Collecting Originator Digits and Initiating
Termination........................................168
AUTHORS ADDRESS.................................................178
FULL COPYRIGHT STATEMENT........................................179
1. SCOPE
This document defines the protocol 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 monolithic gateway such as described in H.246. This
document does not define how gateways, multipoint control units or
interactive voice response units (IVRs) work. Instead it creates a
general framework that is suitable 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-T
Recommendation H.248. It meets the requirements documented in RFC
2805.
2. REFERENCES
2.1 Normative references
ATM Forum (1994): "User-Network Interface, Version 4.0".
ITU-T Recommendation H.225.0: "Call Signalling Protocols and Media
Stream Packetization for Packet Based Multimedia Communications
Systems".
ITU-T Recommendation H.235: "Security and encryption for H-Series
(H.323 and other H.245-based) multimedia terminals".
ITU-T Recommendation H.245: "Control Protocol for Multimedia
Communication".
ITU-T Recommendation H.323: "Packet Based Multimedia Communication
Systems".
ITU-T Recommendation I.363.1, "B-ISDN ATM Adaptation Layer
specification: Type 1 AAL".
ITU-T Recommendation I.363.2, "B-ISDN ATM Adaptation Layer
specification: Type 2 AAL".
ITU-T Recommendation I.363.5, "B-ISDN ATM Adaptation Layer
specification: Type 5 AAL".
ITU-T Recommendation I.363.5, "B-ISDN ATM Adaptation Layer
specification: Type 5 AAL".
ITU-T Recommendation I.366.1, "Segmentation and Reassembly Service
Specific Convergence Sublayer for the AAL type 2".
ITU-T Recommendation I.366.2, "AAL type 2 service specific
convergence sublayer for trunking".
ITU-T Recommendation I.371, "Traffic control and congestion control
in B-ISDN".
ITU-T Recommendation Q.763, "Signalling System No. 7 - ISDN user part
formats and codes".
ITU-T Recommendation Q.765, "Signalling System No. 7 - Application
transport mechanism".
ITU-T Recommendation Q.931: "Digital Subscriber Signalling System No.
1 (DSS 1) - ISDN User-Network Interface Layer 3 Specification for
Basic Call Control".
ITU-T Recommendation Q.2630.1, "AAL Type 2 Signalling Protocol
(Capability Set 1)".
ITU-T Recommendation Q.2931, "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, "Digital Subscriber Signalling System
No. 2 - Generic Identifier Transport".
ITU-T Recommendation Q.2961, "Broadband integrated services digital
network (B-ISDN) - Digital subscriber signalling system no.2 (DSS 2)
- additional traffic parameters".
ITU-T Recommendation Q.2965.1, "Digital subscriber signalling system
No. 2 _ Support of Quality of Service classes."
ITU-T Recommendation Q.2965.2, "Digital subscriber signalling system
No. 2 _ Signalling of individual Quality of Service parameters."
ITU-T Recommendation Q.2961.2, "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, "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
Syntax 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 terminals on GSTN and ISDN".
Rose, M. and D. Cass, "ISO Transport Service on top of the TCP,
Version 3", RFC1006, May 1987.
Crocker, D. and P. Overell, "Augmented BNF for Syntax Specifications:
ABNF", RFC2234, November 1997.
Handley, M. and V. Jacobson, "SDP: Session Description Protocol", RFC
2327, April 1998.
Kent, S. and R. Atkinson, "IP Authentication Header", RFC2402,
November 1998.
Kent, S. and R. Atkinson, "IP Encapsulating Security Payload (ESP)",
RFC2406, 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".
Postel, J., "User Datagram Protocol", RFC768, August 1980.
Postel, J., "Internet protocol", RFC791, September 1981.
Postel, J., "TRANSMISSION CONTROL PROTOCOL", RFC793, September 1981.
Simpson, W., "The Point-to-Point Protocol", RFC1661, July 1994.
Schulzrinne, H., Casner, S., Frederick, R. and V. Jacobson, "RTP: A
Transport Protocol for Real-Time Applications", RFC1889, January
1996.
Schulzrinne, H., "RTP Profile for Audio and Video Audio and Video
Conferences with Minimal Control", RFC1890, January 1996.
Kent, S. and R. Atkinson, "Security Architecture for the Internet
Protocol", RFC2401, November 1998.
Deering, S. and R. Hinden, "Internet Protocol, Version 6 (IPv6)
Specification", RFC2460, December 1998.
Handley, M., Schulzrinne, H., Schooler, E. and J. Rosenberg, "SIP:
Session Initiation Protocol", RFC2543, March 1999.
Greene, N., Ramalho, M. and B. Rosen, "Media Gateway control protocol
architecture and requirements", RFC2805, April 1999.
3. DEFINITIONS
Access Gateway: A type of gateway that provides a User to Network
Interface (UNI) such as ISDN.
Descriptor: A syntactic element of the protocol that groups related
properties. 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 messages and perform other IVR functions, or may
perform media conferencing.
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
processing 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 physical 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.
4. ABBREVIATIONS
This recommendation defines the following terms.
ATM Asynchronous Transfer Mode
CAS Channel Associated Signalling
DTMF Dual Tone Multi-Frequency
FAS Facility Associated Signalling
GSM Global System for Mobile communications
GW GateWay
IANA Internet Assigned Numbers Authority
IP Internet Protocol
ISUP ISDN User Part
IVR Interactive Voice Response
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 multiple media streams. The media stream parameters, as well
as modem, and bearer parameters are encapsulated within the
Termination.
+------------------------------------------------------+
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 H.248 Connection Model
A Context is an association between a collection of Terminations.
There is a special type of Context, the null Context, which contains
all Terminations 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.
Figure 1 above is a graphical depiction of these concepts. The
diagram of 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.
The example below shows an example of one way to accomplish a call-
waiting scenario in a decomposed access gateway, illustrating the
relocation 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
+------------------------------------------------------+
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
Context 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
Terminations 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 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 Context. 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 Terminations 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 (TerminationIDs), 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 usually 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 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 AuditValue 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 AAL Type 2 bearer. When a new multiplexed bearer is to be
created, an ephemeral 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
property values of existing Terminations. These modifications
include the possibility of adding or removing events and/or signals.
The Termination 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.
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
satisfying 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
identical 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. For
instance, given a Termination Ta with properties p1=a, p2=b and
Termination Tb with properties p2=c, p3=d, a UNION response would
consist of a wildcarded TerminationId and the sequence of properties
p1=a, p2=b,c and p3=d. 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, StatisticsIds 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, which are
eXPlicitly defined in this standard or in a package (see Section 12)
or set by provisioning. If not provisioned otherwise, all
descriptors except TerminationState and LocalControl default to
empty/"no value" when a Termination is first created or returned to
the null Context. The default contents of the two exceptions are
described in sections 7.1.5 and 7.1.7.
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 possible 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
parameters 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. Properties 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 descriptors 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 that the 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 Termination.
DigitMap Defines patterns against which sequences of a
specified set of events are to be matched so
they can be reported as a group rather than
singly.
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 properties, events and statistics (signals are not appropriate
for root). Accordingly, 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 and
statistics implemented 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.
7. COMMANDS
The protocol provides commands for manipulating the logical entities
of the protocol connection model, Contexts and Terminations.
Commands provide control at the finest level of granularity supported
by the protocol. 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 ServiceChange 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
command on the first Termination in a Context is used to create a
Context.
2. Modify. The Modify command modifies the properties, events and
signals 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
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 command. 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
consists 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. In any such return of descriptor contents, an empty
descriptor is represented by its name unaccompanied by any list.
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
general, 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
command 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.
If a required descriptor other than the Audit descriptor is
unspecified (i.e., entirely absent) from a command, the previous
values set in that descriptor for that termination, if any, are
retained. A missing Audit descriptor is equivalent to an empty Audit
Descriptor. The behavior of the MG with respect to unspecified
parameters within a descriptor varies with the descriptor concerned,
as indicated in succeeding sections. Whenever a parameter is
underspecified or overspecified, 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 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
(possibly 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
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
termination (not stream-specific). A Termination can be in one of
the following states: "test", "out of service", or "in service". The
"test" state indicates that the termination is being tested. The
state "out of service" 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 Descriptor, 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 ReserveGroup and ReserveValue in LocalControl set to
"false" on all terminations 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 alternative, 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 sufficient resources to
support any of the alternatives specified, is SHALL respond with
error 510 (insufficient resources).
The default value of ReserveValue and ReserveGroup is False. More
information on the use of the two Reserve properties is provided in
section 7.1.8.
A new setting of the LocalControl Descriptor completely replaces the
previous setting of that descriptor in the MG. Thus to retain
information 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 command) 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 RFC2327 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 description.
Below, the semantics of the local and remote descriptors are
specified 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
signifies 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 preference
is descending.
(d) Underspecified or overspecified properties within a group of
properties 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
descending 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 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
properties, 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 ReserveGroup 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
alternative 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,
SendRecv, or Loopback, the MG must be prepared to receive media
encoded according 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 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 notifications 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
vari