Network Working Group F. Cuervo Request for Comments: 2885 N. Greene Category: Standards Track Nortel Networks C. Huitema Microsoft Corporation A. Rayhan Nortel Networks B. Rosen Marconi J. Segers Lucent Technologies August 2000 Megaco Protocol version 0.8 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 is common text with Recommendation H.248 as redetermined in Geneva, February 2000. It must be read in conjunction with the Megaco Errata, RFC 2886. A merged document presenting the Megaco protocol with the Errata incorporated will be available shortly. The protocol presented in this document meets the requirements for a media gateway control protocol as presented in RFC 2805. Cuervo, et al. Standards Track [Page 1] RFC 2885 Megaco Protocol August 2000 TABLE OF CONTENTS 1. SCOPE..........................................................6 2. REFERENCES.....................................................6 2.1 Normative references..........................................6 2.2 Informative references........................................8 3. DEFINITIONS....................................................9 4. ABBREVIATIONS.................................................10 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......................................20 7. COMMANDS......................................................20 7.1 Descriptors..................................................21 7.1.1 Specifying Parameters.................................21 7.1.2 Modem Descriptor......................................22 7.1.3 Multiplex Descriptor..................................22 7.1.4 Media Descriptor......................................23 7.1.5 Termination State Descriptor..........................23 7.1.6 Stream Descriptor.....................................24 7.1.7 LocalControl Descriptor...............................24 7.1.8 Local and Remote Descriptors..........................25 7.1.9 Events Descriptor.....................................28 7.1.10 EventBuffer Descriptor...............................31 7.1.11 Signals Descriptor...................................31 7.1.12 Audit Descriptor.....................................32 7.1.13 ServiceChange Descriptor.............................33 7.1.14 DigitMap Descriptor..................................33 7.1.15 Statistics Descriptor................................38 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...................................................43 7.2.2 Modify................................................44 7.2.3 Subtract..............................................45 7.2.4 Move..................................................46 7.2.5 AuditValue............................................47 7.2.6 AuditCapabilities.....................................48 7.2.7 Notify................................................49 7.2.8 ServiceChange.........................................50 Cuervo, et al. Standards Track [Page 2] RFC 2885 Megaco Protocol August 2000 7.2.9 Manipulating and Auditing Context Attributes..........54 7.2.10 Generic Command Syntax...............................54 7.3 Command Error Codes..........................................55 8. TRANSACTIONS..................................................56 8.1 Common Parameters............................................58 8.1.1 Transaction Identifiers...............................58 8.1.2 Context Identifiers...................................58 8.2 Transaction Application Programming Interface................58 8.2.1 TransactionRequest....................................59 8.2.2 TransactionReply......................................59 8.2.3 TransactionPending....................................60 8.3 Messages.....................................................61 9. TRANSPORT.....................................................61 9.1 Ordering of Commands.........................................62 9.2 Protection against Restart Avalanche.........................63 10. SECURITY CONSIDERATIONS......................................64 10.1 Protection of Protocol Connections..........................64 10.2 Interim AH scheme...........................................65 10.3 Protection of Media Connections.............................66 11. MG-MGC CONTROL INTERFACE....................................66 11.1 Multiple Virtual MGs........................................67 11.2 Cold Start..................................................68 11.3 Negotiation of Protocol Version.............................68 11.4 Failure of an MG............................................69 11.5 Failure of an MGC...........................................69 12. PACKAGE DEFINITION...........................................70 12.1 Guidelines for defining packages............................71 12.1.1 Package..............................................71 12.1.2 Properties...........................................72 12.1.3 Events...............................................72 12.1.4 Signals..............................................73 12.1.5 Statistics...........................................73 12.1.6 Procedures...........................................73 12.2 Guidelines to defining Properties, Statistics and Parameters to Events and Signals.......................................73 12.3 Lists.......................................................74 12.4 Identifiers.................................................74 12.5 Package Registration........................................74 13. IANA CONSIDERATIONS.........................................74 13.1 Packages....................................................74 13.2 Error Codes.................................................75 13.3 ServiceChange Reasons.......................................76 ANNEX A: BINARY ENCODING OF THE PROTOCOL (NORMATIVE).............77 A.1 Coding of wildcards..........................................77 A.2 ASN.1 syntax specification...................................78 A.3 Digit maps and path names....................................94 ANNEX B TEXT ENCODING OF THE PROTOCOL (NORMATIVE)................95 B.1 Coding of wildcards..........................................95 Cuervo, et al. Standards Track [Page 3] RFC 2885 Megaco Protocol August 2000 B.2 ABNF specification...........................................95 ANNEX C TAGS FOR MEDIA STREAM PROPERTIES (NORMATIVE)............107 C.1 General Media Attributes....................................107 C.2 Mux Properties..............................................108 C.3 General bearer properties...................................109 C.4 General ATM properties......................................109 C.5 Frame Relay.................................................112 C.6 IP..........................................................113 C.7 ATM AAL2....................................................113 C.8 ATM AAL1....................................................114 C.9 Bearer Capabilities.........................................116 C.10 AAL5 Properties............................................123 C.11 SDP Equivalents............................................124 C.12 H.245......................................................124 ANNEX D TRANSPORT OVER IP (NORMATIVE)...........................125 D.1 Transport over IP/UDP using Application Level Framing.......125 D.1.1 Providing At-Most-Once Functionality.................125 D.1.2 Transaction identifiers and three-way handshake......126 D.1.2.1 Transaction identifiers....................126 D.1.2.2 Three-way handshake........................126 D.1.3 Computing retransmission timers......................127 D.1.4 Provisional responses................................128 D.1.5 Repeating Requests, Responses and Acknowledgements...128 D.2 using TCP..................................................130 D.2.1 Providing the At-Most-Once functionality..........130 D.2.2 Transaction identifiers and three way handshake...130 D.2.3 Computing retransmission timers...................131 D.2.4 Provisional responses.............................131 D.2.5 Ordering of commands..............................131 ANNEX E BASIC PACKAGES..........................................131 E.1 Generic.....................................................131 E.1.1 Properties...........................................132 E.1.2 Events...............................................132 E.1.3 Signals..............................................133 E.1.4 Statistics...........................................133 E.2 Base Root Package...........................................133 E.2.1 Properties...........................................134 E.2.2 Events...............................................135 E.2.3 Signals..............................................135 E.2.4 Statistics...........................................135 E.2.5 Procedures...........................................135 E.3 Tone Generator Package......................................135 E.3.1 Properties...........................................135 E.3.2 Events...............................................136 E.3.3 Signals..............................................136 E.3.4 Statistics...........................................136 E.3.5 Procedures...........................................136 E.4 Tone Detection Package......................................137 Cuervo, et al. Standards Track [Page 4] RFC 2885 Megaco Protocol August 2000 E.4.1 Properties...........................................137 E.4.2 Events...............................................137 E.4.3 Signals..............................................139 E.4.4 Statistics...........................................139 E.4.5 Procedures...........................................139 E.5 Basic DTMF Generator Package................................140 E.5.1 Properties...........................................140 E.5.2 Events...............................................140 E.5.3 Signals..............................................140 E.5.4 Statistics...........................................141 E.5.5 Procedures...........................................141 E.6 DTMF detection Package......................................141 E.6.1 Properties...........................................142 E.6.2 Events...............................................142 E.6.3 Signals..............................................143 E.6.4 Statistics...........................................143 E.6.5 Procedures...........................................143 E.7 Call Progress Tones Generator Package.......................143 E.7.1 Properties...........................................144 E.7.2 Events...............................................144 E.7.3 Signals..............................................144 E.7.4 Statistics...........................................145 E.7.5 Procedures...........................................145 E.8 Call Progress Tones Detection Package.......................145 E.8.1 Properties...........................................145 E.8.2 Events...............................................145 E.8.3 Signals..............................................145 E.8.4 Statistics...........................................145 E.8.5 Procedures...........................................146 E.9 Analog Line Supervision Package.............................146 E.9.1 Properties...........................................146 E.9.2 Events...............................................146 E.9.3 Signals..............................................147 E.9.4 Statistics...........................................148 E.9.5 Procedures...........................................148 E.10 Basic Continuity Package...................................148 E.10.1 Properties..........................................148 E.10.2 Events..............................................148 E.10.3 Signals.............................................149 E.10.4 Statistics..........................................150 E.10.5 Procedures..........................................150 E.11 Network Package............................................150 E.11.1 Properties..........................................150 E.11.2 Events..............................................151 E.11.3 Signals.............................................152 E.11.4 Statistics..........................................152 E.11.5 Procedures..........................................153 E.12 RTP Package...............................................153 Cuervo, et al. Standards Track [Page 5] RFC 2885 Megaco Protocol August 2000 E.12.1 Properties..........................................153 E.12.2 Events..............................................153 E.12.3 Signals.............................................153 E.12.4 Statistics..........................................153 E.12.5 Procedures..........................................154 E.13 TDM Circuit Package........................................154 E.13.1 Properties..........................................155 E.13.2 Events..............................................155 E.13.3 Signals.............................................155 E.13.4 Statistics..........................................156 E.13.5 Procedures..........................................156 APPENDIX A EXAMPLE CALL FLOWS (INFORMATIVE).....................157 A.1 Residential Gateway to Residential Gateway Call.............157 A.1.1 Programming Residential GW Analog Line Terminations for Idle Behavior..............................................157 A.1.2 Collecting Originator Digits and Initiating Termination ...........................................................159 Authors' Addresses..............................................168 Full Copyright Statement........................................170 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 recommendation does not define how gateways, multipoint control units or integrated 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 ITU-T Recommendation H.225.0 (1998): "Call Signalling Protocols and Media Stream Packetization for Packet Based Multimedia Communications Systems". Cuervo, et al. Standards Track [Page 6] RFC 2885 Megaco Protocol August 2000 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 Communication". ITU-T Recommendation H.323 (1998): "Packet Based Multimedia Communication 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.363.5 (08/96), "B-ISDN ATM Adaptation Layer specification: Type 5 AAL". ITU-T Recommendation I.366.1 (06/98), "Segmentation and Reassembly Service Specific Convergence Sublayer for the AAL type 2". ITU-T Recommendation I.366.2 (02/99), "AAL type 2 service specific convergence sublayer for trunking". ITU-T Recommendation I.371 (08/96), "Traffic control and congestion control 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 transport mechanism". ITU-T Recommendation Q.931 (05/98): "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 (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". Cuervo, et al. Standards Track [Page 7] RFC 2885 Megaco Protocol August 2000 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 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", RFC 1006, May 1987. Crocker, D. and P. Overell, "Augmented BNF for Syntax Specifications: ABNF", RFC 2234, November 1997. Handley, M. and V. Jacobson, "SDP: Session Description Protocol", RFC 2327, April 1998. Kent, S. and R. Atkinson, "IP Authentication Header", RFC 2402, November 1998. Kent, S. and R. Atkinson, "IP Encapsulating Security Payload (ESP)", RFC 2406, 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". Cuervo, et al. Standards Track [Page 8] RFC 2885 Megaco Protocol August 2000 ITU-T Recommendation H.223 (1996), "Multiplexing protocol for low bit rate multimedia communication". ITU-T Recommendation Q.724 (1988): "Signalling procedures". Postel, J., "User Datagram Protocol", STD 6, RFC 768, August 1980. Postel, J., "Internet protocol", STD 5, RFC 791, September 1981. Postel, J., "TRANSMISSION CONTROL PROTOCOL", STD 7, RFC 793, September 1981. Simpson, W., "The Point-to-Point Protocol", STD 51, RFC 1661, July 1994. Schulzrinne, H., Casner, S., Frederick, R. and V. Jacobson, "RTP: A Transport Protocol for Real-Time Applications", RFC 1889, January 1996. Schulzrinne, H., "RTP Profile for Audio and Video Conferences with Minimal Control", RFC 1890, January 1996. Kent, S. and R. Atkinson, "Security Architecture for the Internet Protocol", RFC 2401, November 1998. Deering, S. and R. Hinden, "Internet Protocol, Version 6 (IPv6) Specification", RFC 2460, December 1998. Handley, M., Schulzrinne, H., Schooler, E. and J. Rosenberg, "SIP: Session Initiation Protocol", RFC 2543, March 1999. Greene, N., Ramalho, M. and B. Rosen, "Media Gateway control protocol architecture and requirements", RFC 2805, 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 Cuervo, et al. Standards Track [Page 9] RFC 2885 Megaco Protocol August 2000 (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 performs 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 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 Cuervo, et al. Standards Track [Page 10] RFC 2885 Megaco Protocol August 2000 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 In this recommendation, "shall" refers to a mandatory requirement, while "should" refers to a suggested but optional feature or procedure. The term "may" refers to an optional course of action without expressing a preference. 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. 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. Cuervo, et al. Standards Track [Page 11] RFC 2885 Megaco Protocol August 2000 +------------------------------------------------------+ |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 Cuervo, et al. Standards Track [Page 12] RFC 2885 Megaco Protocol August 2000 Figure 1 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 Cuervo, et al. Standards Track [Page 13] RFC 2885 Megaco Protocol August 2000 +------------------------------------------------------+ |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. Cuervo, et al. Standards Track [Page 14] RFC 2885 Megaco Protocol August 2000 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. Cuervo, et al. Standards Track [Page 15] RFC 2885 Megaco Protocol August 2000 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 AAL2. 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. Cuervo, et al. Standards Track [Page 16] RFC 2885 Megaco Protocol August 2000 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. 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, Cuervo, et al. Standards Track [Page 17] RFC 2885 Megaco Protocol August 2000 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. 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 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 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. Cuervo, et al. Standards Track [Page 18] RFC 2885 Megaco Protocol August 2000 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 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. Cuervo, et al. Standards Track [Page 19] RFC 2885 Megaco Protocol August 2000 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 and events (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 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. Cuervo, et al. Standards Track [Page 20] RFC 2885 Megaco Protocol August 2000 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. 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={parm=value, parm=value_.}. Cuervo, et al. Standards Track [Page 21] RFC 2885 Megaco Protocol August 2000 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. Unspecified mandatory parameters (i.e. mandatory parameters not specified in a descriptor) result in the command responder retaining 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 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 Cuervo, et al. Standards Track [Page 22] RFC 2885 Megaco Protocol August 2000 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. Cuervo, et al. Standards Track [Page 23] RFC 2885 Megaco Protocol August 2000 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. Cuervo, et al. Standards Track [Page 24] RFC 2885 Megaco Protocol August 2000 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. 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 Cuervo, et al. Standards Track [Page 25] RFC 2885 Megaco Protocol August 2000 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 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: Cuervo, et al. Standards Track [Page 26] RFC 2885 Megaco Protocol August 2000 (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. Cuervo, et al. Standards Track [Page 27] RFC 2885 Megaco Protocol August 2000 . 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 or SendRecv, 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. Cuervo, et al. Standards Track [Page 28] RFC 2885 Megaco Protocol August 2000 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, following 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 events. Case 1 If EventBufferControl = LockStep and the MG receives a new EventsDescriptor it will check the FIFO EventBuffer and take the following actions: 1. If the EventBuffer is empty, the MG waits for detection of events based on the new EventsDescriptor. Cuervo, et al. Standards Track [Page 29] RFC 2885 Megaco Protocol August 2000 2. If the EventBuffer is non-empty, the MG processes the FIFO queue starting with the first event: a) 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. b) If the event is not in the new EventsDescriptor, the MG SHALL discard 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 discarded. 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 TerminationStateDescriptor, the MG might receive a command that changes the EventBufferControl property and does not include an EventsDescriptor. 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 Cuervo, et al. Standards Track [Page 30] RFC 2885 Megaco Protocol August 2000 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 SignalsDescriptor. 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 parameters 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 SignalsDescriptor contains a number of signals and/or sequential signal lists. A SignalsDescriptor 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 signal 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 notified when the signal finishes playout. When the MGC enables the signal completion event (see section E.1.2) in an Events Descriptor, that event 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 Cuervo, et al. Standards Track [Page 31] RFC 2885 Megaco Protocol August 2000 completion, 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 SignalsDescriptor shall be played simultaneously. Signals are defined as proceeding from the termination towards the exterior 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 Signals. 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). Cuervo, et al. Standards Track [Page 32] RFC 2885 Megaco Protocol August 2000 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. Signals 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 descriptor, 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 underspecified parameters. Possible items in the Audit Descriptor are: Modem Mux Events Media Signals ObservedEvents DigitMap Statistics Packages 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 Cuervo, et al. Standards Track [Page 33] RFC 2885 Megaco Protocol August 2000 . 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 dynamically 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 command, regardless of the transmitted order of the respective descriptors. DigitMaps defined in a DigitMapDescriptor can occur in any of the standard 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 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 EventsDescriptors 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). Cuervo, et al. Standards Track [Page 34] RFC 2885 Megaco Protocol August 2000 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 overridden. 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 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 corresponding to symbols in the range "0"-"9". The string may also contain explicit 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" Cuervo, et al. Standards Track [Page 35] RFC 2885 Megaco Protocol August 2000 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 conflicting 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 indicates 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 alternative 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 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 timing specified in one or more alternative event sequences. If Cuervo, et al. Standards Track [Page 36] RFC 2885 Megaco Protocol August 2000 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 modified 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 duration is treated as irrelevant in assessing matches to them. 5. 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 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. 6. If no completion event is reported out of step 5 (because the candidate 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 previous contents of the current dial Cuervo, et al. Standards Track [Page 37] RFC 2885 Megaco Protocol August 2000 string from an earlier activation are lost. While the digit map is activated, detection is enabled for all events defined in the package containing the specified 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: (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 particular 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 Context. This behavior can be overridden by including an empty AuditDescriptor in the Subtract command. Statistics may also be returned from the AuditValue command, or any Add/Move/Modify command using the Audit descriptor. Cuervo, et al. Standards Track [Page 38] RFC 2885 Megaco Protocol August 2000 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 terminations 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 transmission 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, provided that the context to which the action applies already exists. A topology descriptor consists of a sequence of triples of the form (T1, 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 Cuervo, et al. Standards Track [Page 39] RFC 2885 Megaco Protocol August 2000 Terminations that match both T1 and T2. However, if there is a Termination 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 command that uses a CHOOSE wildcard in the same action. An existing Termination that matches T1 or T2 in the Context to which a Termination 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 bothway (if T3 is added to a context with T1 and T2 with topology (T3,T1,oneway) it will be connected bothway to T2). Cuervo, et al. Standards Track [Page 40] RFC 2885 Megaco Protocol August 2000 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. 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 | | | +----+ +----+ | | +----+ +----+ | | +----+ +----+ | +------------------+ +------------------+ +------------------+ 4. T2, T3 oneway 5. T2, T3 bothway 6. T1, T2 bothway Figure 4: A Sequence Of Example Topologies Cuervo, et al. Standards Track [Page 41] RFC 2885 Megaco Protocol August 2000 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. T1 and T2 have bothway connection to T3. 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). All terminations have a bothway connection to all other terminations. A oneway connection must implemented in such a way that the other Terminations 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. Cuervo, et al. Standards Track [Page 42] RFC 2885 Megaco Protocol August 2000 7.2.1 Add The Add Command adds a Termination to a Context. TerminationID [,MediaDescriptor] [,ModemDescriptor] [,MuxDescriptor] [,EventsDescriptor] [,SignalsDescriptor] [,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. Wildcards 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 multiplexer 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 Multiplex failure shall be returned and further processing of the command shall cease. Cuervo, et al. Standards Track [Page 43] RFC 2885 Megaco Protocol August 2000 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. All descriptors that can be modified could be returned by MG if a parameter 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 Termination with a serviceState of "OutofService". 7.2.2 Modify The Modify Command modifies the properties of a Termination. 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 Cuervo, et al. Standards Track [Page 44] RFC 2885 Megaco Protocol August 2000 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. Possible 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 [,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 wildcard 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. Cuervo, et al. Standards Track [Page 45] RFC 2885 Megaco Protocol August 2000 When a provisioned Termination is Subtracted from a context, its property 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 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 Terminations 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. Cuervo, et al. Standards Track [Page 46] RFC 2885 Megaco Protocol August 2000 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 descriptors 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. 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 TerminationIDs match is not specified. 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. Use of CHOOSE is an error. The appropriate descriptors, with the current values for the Termination, 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, PackagesDescriptor 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 TerminationID ALL returns all DigitMaps in the gateway. Statistics returns the current values of all statistics Cuervo, et al. Standards Track [Page 47] RFC 2885 Megaco Protocol August 2000 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 wildcarded. The TerminationID may be specific, or wildcarded. The following illustrates other information that can be obtained with the Audit Command: ContextID TerminationID Information Obtained 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] Cuervo, et al. Standards Track [Page 48] RFC 2885 Megaco Protocol August 2000 [,StatisticsDescriptor] AuditCapabilities(TerminationID, AuditDescriptor ) The appropriate descriptors, with the possible values for the Termination are returned from AuditCapabilities. Descriptors may be repeated where there are multiple possible 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 Termination 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. StatisticsDescriptor returns the names of the statistics being kept on the 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. Procedures for sending Notify commands with RequestID equal to 0 are for further study. Cuervo, et al. Standards Track [Page 49] RFC 2885 Megaco Protocol August 2000 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 ) The TerminationID parameter specifies the Termination(s) that are taken out of or returned to service. Wildcarding of Termination names is permitted, 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: Cuervo, et al. Standards Track [Page 50] RFC 2885 Megaco Protocol August 2000 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 serviceState should be set to "inService" upon expiry of ServiceChangeDelay. 4) Disconnected - always applied with the Root TerminationID, indicates that the MG lost communication with the MGC, but it was subsequently 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 associated. 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 reason 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 communications. It can be specified in the input parameter descriptor or the returned result descriptor. ServiceChangeAddress Cuervo, et al. Standards Track [Page 51] RFC 2885 Megaco Protocol August 2000 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 version and is used if protocol version negotiation occurs (see section 11.3). 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 Controllers. Acknowledgement of the ServiceChange Command completes the registration process. The MG may specify the transport ServiceChangeAddress to be used by the MGC for sending messages in the ServiceChangeAddress 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 destination for all subsequent transaction requests within the association. At the same time, as indicated in section 9, transaction replies and pending Cuervo, et al. Standards Track [Page 52] RFC 2885 Megaco Protocol August 2000 indications must be sent to the address from which the corresponding requests originated. This must be done even if it implies extra messaging 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 terminationID 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; . ServiceChangeProfile, if the responder wishes to negotiate the profile to be used for the association; . ServiceChangeVersion, if the responder wishes to negotiate the version 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 Cuervo, et al. Standards Track [Page 53] RFC 2885 Megaco Protocol August 2000 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 manipulated 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 attributes of a context. The contex