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RFC2705 - Media Gateway Control Protocol (MGCP) Version 1.0

王朝other·作者佚名  2008-05-31
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Network Working Group M. Arango

Request for Comments: 2705 RSL COM

Category: Informational A. Dugan

I. Elliott

Level3 Communications

C. Huitema

Telcordia

S. Pickett

Vertical Networks

October 1999

Media Gateway Control Protocol (MGCP)

Version 1.0

Status of this Memo

This memo provides information for the Internet community. It does

not specify an Internet standard of any kind. Distribution of this

memo is unlimited.

Copyright Notice

Copyright (C) The Internet Society (1999). All Rights Reserved.

IESG NOTE:

This document is being published for the information of the

community. It describes a protocol that is currently being deployed

in a number of prodUCts. Implementers should be aware of

developments in the IETF Megaco Working Group and ITF-T SG16 who are

currently working on a potential successor to this protocol.

Abstract

This document describes an application programming interface and a

corresponding protocol (MGCP) for controlling Voice over IP (VoIP)

Gateways from external call control elements. MGCP assumes a call

control architecture where the call control "intelligence" is outside

the gateways and handled by external call control elements.

The document is structured in 6 main sections:

* The introduction presents the basic assumptions and the relation

to other protocols such as H.323, RTSP, SAP or SIP.

* The interface section presents a conceptual overview of the MGCP,

presenting the naming conventions, the usage of the session

description protocol SDP, and the procedures that compose MGCP:

Notifications Request, Notification, Create Connection, Modify

Connection, Delete Connection, AuditEndpoint, AuditConnection and

RestartInProgress.

* The protocol description section presents the MGCP encodings,

which are based on simple text formats, and the transmission

procedure over UDP.

* The security section presents the security requirement of MGCP,

and its usage of IP security services (IPSEC).

* The event packages section provides an initial definition of

packages and event names.

* The description of the changes made in combining SGCP 1.1 and IPDC

to create MGCP 1.0.

Table of Contents

1. Introduction .............................................. 5

1.1. Relation with the H.323 standards .................... 7

1.2. Relation with the IETF standards ..................... 8

1.3. Definitions .......................................... 9

2. Media Gateway Control Interface ........................... 9

2.1. Model and naming conventions. ........................ 10

2.1.1. Types of endpoints .............................. 10

2.1.1.1. Digital channel (DS0) ...................... 11

2.1.1.2. Analog line ................................ 11

2.1.1.3. Annoucement server Access point ............ 12

2.1.1.4. Interactive Voice Response access point .... 12

2.1.1.5. Conference bridge access point ............. 13

2.1.1.6. Packet relay ............................... 13

2.1.1.7. Wiretap access point ....................... 14

2.1.1.8. ATM "trunk side" interface. ................ 14

2.1.2. Endpoint identifiers ............................ 15

2.1.3. Calls and connections ........................... 17

2.1.3.1. Names of calls ............................. 20

2.1.3.2. Names of connections ....................... 20

2.1.3.3. Management of resources, attributes of ..... 20

2.1.3.4. Special case of local connections .......... 23

2.1.4. Names of Call Agents and other entities ......... 23

2.1.5. Digit maps ...................................... 24

2.1.6. Names of events ................................. 26

2.2. Usage of SDP ......................................... 29

2.3. Gateway Control Commands ............................. 30

2.3.1. EndpointConfiguration ........................... 32

2.3.2. NotificationRequest ............................. 33

2.3.3. CreateConnection ................................ 38

2.3.4. ModifyConnection ................................ 44

2.3.5. DeleteConnection (from the Call Agent) .......... 46

2.3.6. DeleteConnection (from the VoIP gateway) ........ 51

2.3.7. DeleteConnection (multiple connections, from the 51

2.3.8. Audit Endpoint .................................. 52

2.3.9. Audit Connection ................................ 55

2.3.10. Restart in progress ............................ 56

2.4. Return codes and error codes. ........................ 58

2.5. Reason Codes ......................................... 61

3. Media Gateway Control Protocol ............................ 61

3.1. General description .................................. 62

3.2. Command Header ....................................... 62

3.2.1. Command line .................................... 62

3.2.1.1. Coding of the requested verb ............... 63

3.2.1.2. Transaction Identifiers .................... 63

3.2.1.3. Coding of the endpoint identifiers and ..... 64

3.2.1.4. Coding of the protocol version ............. 65

3.2.2. Parameter lines ................................. 65

3.2.2.1. Response Acknowledgement ................... 68

3.2.2.2. Local connection options ................... 68

3.2.2.3. Capabilities ............................... 70

3.2.2.4. Connection parameters ...................... 71

3.2.2.5. Reason Codes ............................... 72

3.2.2.6. Connection mode ............................ 73

3.2.2.7. Coding of event names ...................... 73

3.2.2.8. RequestedEvents ............................ 74

3.2.2.9. SignalRequests ............................. 76

3.2.2.10. ObservedEvent ............................. 76

3.2.2.11. RequestedInfo ............................. 76

3.2.2.12. QuarantineHandling ........................ 77

3.2.2.13. DetectEvents .............................. 77

3.2.2.14. EventStates ............................... 77

3.2.2.15. RestartMethod ............................. 78

3.2.2.16. Bearer Information ........................ 78

3.3. Format of response headers ........................... 78

3.4. Formal syntax description of the protocol ............ 81

3.5. Encoding of the session description .................. 86

3.5.1. Usage of SDP for an audio service ............... 86

3.5.2. Usage of SDP in a network access service ........ 87

3.5.3. Usage of SDP for ATM connections ................ 90

3.5.4. Usage of SDP for local connections .............. 91

3.6. Transmission over UDP ................................ 91

3.6.1. Providing the At-Most-Once functionality ........ 91

3.6.2. Transaction identifiers and three ways handshake. 92

3.6.3. Computing retransmission timers ................. 93

3.6.4. Piggy backing ................................... 94

3.6.5. Provisional responses ........................... 94

4. States, failover and race conditions. ..................... 95

4.1. Basic Asumptions ..................................... 95

4.2. Security, Retransmission, and Detection of Lost ...... 96

4.3. Race conditions ...................................... 99

4.3.1. Quarantine list ................................. 99

4.3.2. EXPlicit detection ..............................103

4.3.3. Ordering of commands, and treatment of disorder .104

4.3.4. Fighting the restart avalanche ..................105

4.3.5. Disconnected Endpoints ..........................107

1. A "disconnected" timer is initialized to a random value, .107

2. The gateway then waits for either the end of this timer, .107

3. When the "disconnected" timer elapses, when a command is .107

4. If the "disconnected" procedure still left the endpoint ..107

5. Security requirements .....................................108

5.1. Protection of media connections ......................109

6. Event packages and end point types ........................109

6.1. Basic packages .......................................110

6.1.1. Generic Media Package ...........................110

6.1.2. DTMF package ....................................112

6.1.3. MF Package ......................................113

6.1.4. Trunk Package ...................................114

6.1.5. Line Package ....................................116

6.1.6. Handset emulation package .......................119

6.1.7. RTP Package .....................................120

6.1.8. Network Access Server Package ...................121

6.1.9. Announcement Server Package .....................122

6.1.10. Script Package .................................122

6.2. Basic endpoint types and profiles ....................123

7. Versions and compatibility ................................124

7.1. Differences between version 1.0 and draft 0.5 ........124

7.2. Differences between draft-04 and draft-05 ............125

7.3. Differences between draft-03 and draft-04 ............125

7.4. Differences between draft-02 and draft-03 ............125

7.5. Differences between draft-01 and draft-02 ............126

7.6. The making of MGCP from IPDC and SGCP ................126

7.7. Changes between MGCP and initial versions of SGCP ....126

8. Security Considerations ...................................128

9. Acknowledgements ..........................................128

10. References ................................................129

11. Authors' Addresses ........................................130

12. Appendix A: Proposed "MoveConnection" command .............132

12.1. Proposed syntax modification ........................133

13. Full Copyright Statement ..................................134

1. Introduction

This document describes an abstract application programming interface

and a corresponding protocol (MGCP) for controlling Telephony

Gateways from external call control elements called media gateway

controllers or call agents. A telephony gateway is a network element

that provides conversion between the audio signals carried on

telephone circuits and data packets carried over the Internet or over

other packet networks. Example of gateways are:

* Trunking gateways, that interface between the telephone network

and a Voice over IP network. Such gateways typically manage a

large number of digital circuits.

* Voice over ATM gateways, which operate much the same way as voice

over IP trunking gateways, except that they interface to an ATM

network.

* Residential gateways, that provide a traditional analog (RJ11)

interface to a Voice over IP network. Examples of residential

gateways include cable modem/cable set-top boxes, xDSL devices,

broad-band wireless devices

* Access gateways, that provide a traditional analog (RJ11) or

digital PBX interface to a Voice over IP network. Examples of

access gateways include small-scale voice over IP gateways.

* Business gateways, that provide a traditional digital PBX

interface or an integrated "soft PBX" interface to a Voice over IP

network.

* Network Access Servers, that can attach a "modem" to a telephone

circuit and provide data access to the Internet. We expect that,

in the future, the same gateways will combine Voice over IP

services and Network Access services.

* Circuit switches, or packet switches, which can offer a control

interface to an external call control element.

MGCP assumes a call control architecture where the call control

"intelligence" is outside the gateways and handled by external call

control elements. The MGCP assumes that these call control elements,

or Call Agents, will synchronize with each other to send coherent

commands to the gateways under their control. MGCP does not define a

mechanism for synchronizing Call Agents. MGCP is, in essence, a

master/slave protocol, where the gateways are expected to execute

commands sent by the Call Agents. In consequence, this document

specifies in great detail the expected behavior of the gateways, but

only specify those parts of a call agent implementation, such as

timer management, that are mandated for proper operation of the

protocol.

MGCP assumes a connection model where the basic constructs are

endpoints and connections. Endpoints are sources or sinks of data and

could be physical or virtual. Examples of physical endpoints are:

* An interface on a gateway that terminates a trunk connected to a

PSTN switch (e.g., Class 5, Class 4, etc.). A gateway that

terminates trunks is called a trunk gateway.

* An interface on a gateway that terminates an analog POTS

connection to a phone, key system, PBX, etc. A gateway that

terminates residential POTS lines (to phones) is called a

residential gateway.

An example of a virtual endpoint is an audio source in an audio-

content server. Creation of physical endpoints requires hardware

installation, while creation of virtual endpoints can be done by

software.

Connections may be either point to point or multipoint. A point to

point connection is an association between two endpoints with the

purpose of transmitting data between these endpoints. Once this

association is established for both endpoints, data transfer between

these endpoints can take place. A multipoint connection is

established by connecting the endpoint to a multipoint session.

Connections can be established over several types of bearer networks:

* Transmission of audio packets using RTP and UDP over a TCP/IP

network.

* Transmission of audio packets using AAL2, or another adaptation

layer, over an ATM network.

* Transmission of packets over an internal connection, for example

the TDM backplane or the interconnection bus of a gateway. This is

used, in particular, for "hairpin" connections, connections that

terminate in a gateway but are immediately rerouted over the

telephone network.

For point-to-point connections the endpoints of a connection could be

in separate gateways or in the same gateway.

1.1. Relation with the H.323 standards

MGCP is designed as an internal protocol within a distributed system

that appears to the outside as a single VoIP gateway. This system is

composed of a Call Agent, that may or may not be distributed over

several computer platforms, and of a set of gateways, including at

least one "media gateway" that perform the conversion of media

signals between circuits and packets, and at least one "signalling

gateway" when connecting to an SS7 controlled network. In a typical

configuration, this distributed gateway system will interface on one

side with one or more telephony (i.e. circuit) switches, and on the

other side with H.323 conformant systems, as indicated in the

following table:

___________________________________________________________________

Functional Phone Terminating H.323 conformant

Plane switch Entity systems

_______________________________________________________________

Signaling Signaling Call agent Signaling exchanges

Plane exchanges with the call agent

through through H.225/RAS and

SS7/ISUP H.225/Q.931.

_______________________________________________________________

Possible negotiation

of logical channels

and transmission

parameters through

H.245 with the call

agent.

_______________________________________________________________

Internal

synchronization

through MGCP

_______________________________________________________________

Bearer Connection Telephony Transmission of VOIP

Data through gateways data using RTP

Transport high speed directly between the

Plane trunk H.323 station and the

groups gateway.

_______________________________________________________________

In the MGCP model, the gateways focus on the audio signal translation

function, while the Call Agent handles the signaling and call

processing functions. As a consequence, the Call Agent implements the

"signaling" layers of the H.323 standard, and presents itself as an

"H.323 Gatekeeper" or as one or more "H.323 Endpoints" to the H.323

systems.

1.2. Relation with the IETF standards

While H.323 is the recognized standard for VoIP terminals, the IETF

has also produced specifications for other types of multi-media

applications. These other specifications include:

* the Session Description Protocol (SDP), RFC2327,

* the Session Announcement Protocol (SAP),

* the Session Initiation Protocol (SIP),

* the Real Time Streaming Protocol (RTSP), RFC2326.

The latter three specifications are in fact alternative signaling

standards that allow for the transmission of a session description to

an interested party. SAP is used by multicast session managers to

distribute a multicast session description to a large group of

recipients, SIP is used to invite an individual user to take part in

a point-to-point or unicast session, RTSP is used to interface a

server that provides real time data. In all three cases, the session

description is described according to SDP; when audio is transmitted,

it is transmitted through the Real-time Transport Protocol, RTP.

The distributed gateway systems and MGCP will enable PSTN telephony

users to access sessions set up using SAP, SIP or RTSP. The Call

Agent provides for signaling conversion, according to the following

table:

_____________________________________________________________________

Functional Phone Terminating IETF conforming systems

Plane switch Entity

_________________________________________________________________

Signaling Signaling Call agent Signaling exchanges

Plane exchanges with the call agent

through through SAP, SIP or

SS7/ISUP RTSP.

_________________________________________________________________

Negotiation of session

description parameters

through SDP (telephony

gateway terminated but

passed via the call

agent to and from the

IETF conforming system)

_________________________________________________________________

Internal

synchronization

through MGCP

_________________________________________________________________

Bearer Connection Telephony Transmission of VoIP

Data through gateways data using RTP,

Transport high speed directly between the

Plane trunk remote IP end system

groups and the gateway.

_________________________________________________________________

The SDP standard has a pivotal status in this architecture. We will

see in the following description that we also use it to carry session

descriptions in MGCP.

1.3. Definitions

Trunk: A communication channel between two switching systems. E.g., a

DS0 on a T1 or E1 line.

2. Media Gateway Control Interface

The interface functions provide for connection control and endpoint

control. Both use the same system model and the same naming

conventions.

2.1. Model and naming conventions

The MGCP assumes a connection model where the basic constructs are

endpoints and connections. Connections are grouped in calls. One or

more connections can belong to one call. Connections and calls are

set up at the initiative of one or several Call Agents.

2.1.1. Types of endpoints

In the introduction, we presented several classes of gateways. Such

classifications, however, can be misleading. Manufacturers can

arbitrarily decide to provide several types of services in a single

packaging. A single product could well, for example, provide some

trunk connections to telephony switches, some primary rate

connections and some analog line interfaces, thus sharing the

characteristics of what we described in the introduction as

"trunking", "access" and "residential" gateways. MGCP does not make

assumptions about such groupings. We simply assume that media

gateways support collections of endpoints. The type of the endpoint

determines its functionalities. Our analysis, so far, has led us to

isolate the following basic endpoint types:

* Digital channel (DS0),

* Analog line,

* Annoucement server access point,

* Interactive Voice Response access point,

* Conference bridge access point,

* Packet relay,

* Wiretap access point,

* ATM "trunk side" interface.

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

points.

This list is not limitative. There may be other types of endpoints

defined in the future, for example test endpoint that could be used

to check network quality, or frame-relay endpoints that could be used

to managed audio channels multiplexed over a frame-relay virtual

circuit.

2.1.1.1. Digital channel (DS0)

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

found in trunk and ISDN interfaces. They are typically part of

digital multiplexes, such as T1, E1, T3 or E3 interfaces. Media

gateways that support such channels are capable of translating the

digital signals received on the channel, which may be encoded

according to A or mu-law, using either the complete set of 8 bits or

only 7 of these bits, into audio packets. When the media gateway

also supports a NAS service, the gateway shall be capable of

receiving either audio-encoded data (modem connection) or binary data

(ISDN connection) and convert them into data packets.

+-------

+------------+

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

+------------+

+-------

Media gateways should be able to establish several connections

between the endpoint and the packet networks, or between the endpoint

and other endpoints in the same gateway. The signals originating

from these connections shall be mixed according to the connection

"mode", as specified later in this document. The precise number of

connections that an endpoint support is a characteristic of the

gateway, and may in fact vary according with the allocation of

resource within the gateway.

In some cases, digital channels are used to carry signalling. This

is the case for example of SS7 "F" links, or ISDN "D" channels.

Media gateways that support these signalling functions shall be able

to send and receive the signalling packets to and from a call agent,

using the "back haul" procedures defined by the SIGTRAN working group

of the IETF. Digital channels are sometimes used in conjunction with

channel associated signalling, such as "MF R2". Media gateways that

support these signalling functions shall be able to detect and

produce the corresponding signals, such as for example "wink" or "A",

according to the event signalling and reporting procedures defined in

MGCP.

2.1.1.2. Analog line

Analog lines can be used either as a "client" interface, providing

service to a classic telephone unit, or as a "service" interface,

allowing the gateway to send and receive analog calls. When the

media gateway also supports a NAS service, the gateway shall be

capable of receiving audio-encoded data (modem connection) and

convert them into data packets.

+-------

+---------------+

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

+---------------+

+-------

Media gateways should be able to establish several connections

between the endpoint and the packet networks, or between the endpoint

and other endpoints in the same gateway. The audio signals

originating from these connections shall be mixed according to the

connection "mode", as specified later in this document. The precise

number of connections that an endpoint support is a characteristic of

the gateway, and may in fact vary according with the allocation of

resource within the gateway. A typical gateway should however be

able to support two or three connections per endpoint, in order to

provide services such as "call waiting" or "three ways calling".

2.1.1.3. Annoucement server access point

An announcement server endpoint provides acces to an announcement

service. Under requests from the call agent, the announcement server

will "play" a specified announcement. The requests from the call

agent will follow the event signalling and reporting procedures

defined in MGCP.

+----------------------+

Announcement endpoint -------- Connection

+----------------------+

A given announcement endpoint is not supposed to support more than

one connection at a time. If several connections were established to

the same endpoint, then the same announcements would be played

simultaneously over all the connections.

Connections to an announcement server are typically oneway, or "half

duplex" -- the announcement server is not expected to listen the

audio signals from the connection.

2.1.1.4. Interactive Voice Response access point

An Interactive Voice Response (IVR) endpoint provides acces to an IVR

service. Under requests from the call agent, the IVR server will

"play" announcements and tones, and will "listen" to responses from

the user. The requests from the call agent will follow the event

signalling and reporting procedures defined in MGCP.

+-------------+

IVR endpoint -------- Connection

+-------------+

A given IVR endpoint is not supposed to support more than one

connection at a time. If several connections were established to the

same endpoint, then the same tones and announcements would be played

simultaneously over all the connections.

2.1.1.5. Conference bridge access point

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

conference.

+-------

+--------------------------+

Conference bridge endpoint -------- Connections

+--------------------------+

+-------

Media gateways should be able to establish several connections

between the endpoint and the packet networks, or between the endpoint

and other endpoints in the same gateway. The signals originating

from these connections shall be mixed according to the connection

"mode", as specified later in this document. The precise number of

connections that an endpoint support is a characteristic of the

gateway, and may in fact vary according with the allocation of

resource within the gateway.

2.1.1.6. Packet relay

A packet relay endpoint is a specific form of conference bridge, that

typically only supports two connections. Packets relays can be found

in firewalls between a protected and an open network, or in

transcoding servers used to provide interoperation between

incompatible gateways, for example gateways that do not support

compatible compression algorithms, or gateways that operate over

different transmission networks such as IP and ATM.

+-------

+---------------------+

Packet relay endpoint 2 connections

+---------------------+

+-------

2.1.1.7. Wiretap access point

A wiretap access point provides access to a wiretap service,

providing either a recording or a life playback of a connection.

+-----------------+

Wiretap endpoint -------- Connection

+-----------------+

A given wiretap endpoint is not supposed to support more than one

connection at a time. If several connections were established to the

same endpoint, then the recording or playback would mix the audio

signals received on this connections.

Connections to an wiretap endpoint are typically oneway, or "half

duplex" -- the wiretap server is not expected to signal its presence

in a call.

2.1.1.8. ATM "trunk side" interface.

ATM "trunk side" endpoints are typically found when one or several

ATM permanent virtual circuits are used as a replacement for the

classic "TDM" trunks linking switches. When ATM/AAL2 is used,

several trunks or channels are multiplexed on a single virtual

circuit; each of these trunks correspond to a single endpoint.

+-------

+------------------+

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

+------------------+

+-------

Media gateways should be able to establish several connections

between the endpoint and the packet networks, or between the endpoint

and other endpoints in the same gateway. The signals originating

from these connections shall be mixed according to the connection

"mode", as specified later in this document. The precise number of

connections that an endpoint support is a characteristic of the

gateway, and may in fact vary according with the allocation of

resource within the gateway.

2.1.2. Endpoint identifiers

Endpoints identifiers have two components that both are case

insensitive:

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

* a local name within that gateway,

The syntax of the local name depends on the type of endpoint being

named. However, the local name for each of these types is naturally

hierarchical, beginning with a term which identifies the physical

gateway containing the given endpoint and ending in a term which

specifies the individual endpoint concerned. With this in mind, the

following rules for construction and interpretation of the Entity

Name field for these entity types MUST be supported:

1) The individual terms of the naming path MUST be separated by a

single slash ("/", ASCII 2F hex).

2) The individual terms are character strings composed of letters,

digits or other printable characters, with the exception of

characters used as delimitors ("/", "@"), characters used for

wildcarding ("*", "$") and white spaces.

3) Wild-carding is represented either by an asterisk ("*") or a

dollar sign ("$") for the terms of the naming path which are to be

wild-carded. Thus, if the full naming path looks like

term1/term2/term3

then the Entity Name field looks like this depending on which

terms are wild-carded:

*/term2/term3 if term1 is wild-carded

term1/*/term3 if term2 is wild-carded

term1/term2/* if term3 is wild-carded

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

etc.

In each of these examples a dollar sign could have appeared

instead of an asterisk.

4) A term represented by an asterisk is to be interpreted as: "use

ALL values of this term known within the scope of the Media

Gateway". A term represented by a dollar sign is to be

interpreted as: "use ANY ONE value of this term known within the

scope of the Media Gateway". The description of a specific

command may add further criteria for selection within the general

rules given here.

If the Media Gateway controls multiple physical gateways, the first

term of the naming MUST identify the physical gateway containing the

desired entity. If the Media Gateway controls only a single physical

gateway, the first term of the naming string MAY identify that

physical gateway, depending on local practice. A local name that is

composed of only a wildcard character refers to either all (*) or any

($) endpoints within the media gateway.

In the case of trunking gateways, endpoints are trunk circuits

linking a gateway to a telephone switch. These circuits are typically

grouped into a digital multiplex, that is connected to the gateway by

a physical interface. Such circuits are named in three contexts:

* In the ISUP protocol, trunks are grouped into trunk groups,

identified by the SS7 point codes of the switches that the group

connects. Circuits within a trunk group are identified by a

circuit number (CIC in ISUP).

* In the gateway configuration files, physical interfaces are

typically identified by the name of the interface, an arbitrary

text string. When the interface multiplexes several circuits,

individual circuits are typically identified by a circuit number.

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

The Call Agents use configuration databases to map ranges of circuit

numbers within an ISUP trunk group to corresponding ranges of

circuits in a multiplex connected to a gateway through a physical

interface. The gateway will be identified, in MGCP, by a domain name.

The local name will be structured to encode both the name of the

physical interface, for example X35V3+A4, and the circuit number

within the multiplex connected to the interface, for example 13. The

circuit number will be separated from the name of the interface by a

fraction bar, as in:

X35V3+A4/13

Other types of endpoints will use different conventions. For example,

in gateways were physical interfaces by construction only control one

circuit, the circuit number will be omitted. The exact syntax of such

names should be specified in the corresponding server specification.

2.1.3. Calls and connections

Connections are created on the call agent on each endpoint that will

be involved in the "call." In the classic example of a connection

between two "DS0" endpoints (EP1 and EP2), the call agents

controlling the end points will establish two connections (C1 and

C2):

+---+ +---+

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

+---+ +---+

Each connection will be designated locally by a connection

identifier, and will be characterized by connection attributes.

When the two endpoints are located on gateways that are managed by

the same call agent, the creation is done via the three following

steps:

1) The call agent asks the first gateway to "create a connection" on

the first endpoint. The gateway allocates resources to that

connection, and respond to the command by providing a "session

description." The session description contains the information

necessary for a third party to send packets towards the newly

created connection, such as for example IP address, UDP port, and

packetization parameters.

2) The call agent then asks the second gateway to "create a

connection" on the second endpoint. The command carries the

"session description" provided by the first gateway. The gateway

allocates resources to that connection, and respond to the command

by providing its own "session description."

3) The call agent uses a "modify connection" command to provide this

second "session description" to the first endpoint. Once this is

done, communication can proceed in both directions.

When the two endpoints are located on gateways that are managed by

the different call agents, these two call agents shall exchange

information through a call-agent to call-agent signalling protocol,

in order to synchronize the creation of the connection on the two

endpoints.

Once established, the connection parameters can be modified at any

time by a "modify connection" command. The call agent may for

example instruct the gateway to change the compression algorithm used

on a connection, or to modify the IP address and UDP port to which

data should be sent, if a connection is "redirected."

The call agent removes a connection by sending to the gateway a

"delete connection" command. The gateway may also, under some

circumstances, inform a gateway that a connection could not be

sustained.

The following diagram provides a view of the states of a connection,

as seen from the gateway:

Create connection

received

V

+-------------------+

resource allocation-(failed)-+

+-------------------+

(connection refused)

(successful)

v

+----------->+

+-------------------+

remote session

description ----------(yes)--------+

available ?

+-------------------+

(no)

+-----------+ +------+

+---> half open ------> Delete <------- open <----------+

(wait) Connection (wait)

+-----------+ received +------+

Modify Connection Modify Connection

received received

+--------------------+ +--------------------+

assess modification assess modification

+--------------------+ +--------------------+

(failed) (successful) (failed) (successful)

+<---+ +-------------+-------+

+<-------------------+

+-----------------+

Free connection

resources.

Report.

+-----------------+

V

2.1.3.1. Names of calls

One of the attributes of each connection is the "call identifier."

Calls are identified by unique identifiers, independent of the

underlying platforms or agents. These identifiers are created by the

Call Agent. They are treated in MGCP as unstructured octet strings.

Call identifiers are expected to be unique within the system, or at a

minimum, unique within the collection of Call Agents that control the

same gateways. When a Call Agent builds several connections that

pertain to the same call, either on the same gateway or in different

gateways, these connections that belong to the same call share the

same call-id. This identifier can then be used by accounting or

management procedures, which are outside the scope of MGCP.

2.1.3.2. Names of connections

Connection identifiers are created by the gateway when it is

requested to create a connection. They identify the connection within

the context of an endpoint. They are treated in MGCP as unstructured

octet strings. The gateway should make sure that a proper waiting

period, at least 3 minutes, elapses between the end of a connection

that used this identifier and its use in a new connection for the

same endpoint. (Gateways may decide to use identifiers that are

unique within the context of the gateway.)

2.1.3.3. Management of resources, attributes of connections

Many types of resources will be associated to a connection, such as

specific signal processing functions or packetization functions.

Generally, these resources fall in two categories:

1) Externally visible resources, that affect the format of "the bits

on the network" and must be communicated to the second endpoint

involved in the connection.

2) Internal resources, that determine which signal is being sent over

the connection and how the received signals are processed by the

endpoint.

The resources allocated to a connection, and more generally the

handling of the connection, are chosen by the gateway under

instructions from the call agent. The call agent will provide these

instructions by sending two set of parameters to the gateway:

1) The local directives instruct the gateway on the choice of

resources that should be used for a connection,

2) When available, the "session description" provided by the other

end of the connection.

The local directives specify such parameters as the mode of the

connection (e.g. send only, send-receive), preferred coding or

packetization methods, usage of echo cancellation or silence

suppression. (A detailed list can be found in the specification of

the LocalConnectionOptions parameter of the CreateConnection

command.) For each of these parameters, the call agent can either

specify a value, a range of value, or no value at all. This allow

various implementations to implement various level of control, from a

very tight control where the call agent specifies minute details of

the connection handling to a very loose control where the call agent

only specifies broad guidelines, such as the maximum bandwidth, and

let the gateway choose the detailed values.

Based on the value of the local directives, the gateway will

determine the resources allocated to the connection. When this is

possible, the gateway will choose values that are in line with the

remote session description - but there is no absolute requirement

that the parameters be exactly the same.

Once the resource have been allocated, the gateway will compose a

"session description" that describes the way it intends to receive

packets. Note that the session description may in some cases present

a range of values. For example, if the gateway is ready to accept

one of several compression algorithm, it can provide a list of these

accepted algorithms.

Local Directives

(from call agent 1)

V

+-------------+

resources

allocation

(gateway 1)

+-------------+

V

Local

Parameters V

Session

Description Local Directives

(from call agent 2)

+---> Transmission----+

(CA to CA)

V V

+-------------+

resources

allocation

(gateway 2)

+-------------+

V

Local

Parameters

Session

Description

+---- Transmission<---+

(CA to CA)

V V

+-------------+

modification

(gateway 1)

+-------------+

V

Local

Parameters

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

2.1.3.4. Special case of local connections

Large gateways include a large number of endpoints which are often of

different types. In some networks, we may often have to set-up

connections between endpoints that are located within the same

gateway. Examples of such connections may be:

* Connecting a trunk line to a wiretap device,

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

* Connecting a call to a Conferencing unit,

* Routing a call from on endpoint to another, something often

described as a "hairpin" connection.

Local connections are much simpler to establish than network

connections. In most cases, the connection will be established

through some local interconnecting device, such as for example a TDM

bus.

When two endpoints are managed by the same gateway, it is possible to

specify the connection in a single command that conveys the name of

the two endpoints that will be connected. The command is essentially

a "Create Connection" command which includes the name of the second

endpoint in lieu of the "remote session description."

2.1.4. Names of Call Agents and other entities

The media gateway control protocol has been designed to allow the

implementation of redundant Call Agents, for enhanced network

reliability. This means that there is no fixed binding between

entities and hardware platforms or network interfaces.

Reliability can be improved by the following precautions:

* Entities such as endpoints or Call Agents are identified by their

domain name, not their network addresses. Several addresses can be

associated with a domain name. If a command or a response cannot

be forwarded to one of the network addresses, implementations

should retry the transmission using another address.

* Entities may move to another platform. The association between a

logical name (domain name) and the actual platform are kept in the

domain name service. Call Agents and Gateways should keep track of

the time-to-live of the record they read from the DNS. They should

query the DNS to refresh the information if the time to live has

expired.

In addition to the indirection provided by the use of domain names

and the DNS, the concept of "notified entity" is central to

reliability and fail-over in MGCP. The "notified entity" for an

endpoint is the Call Agent currently controlling that endpoint. At

any point in time, an endpoint has one, and only one, "notified

entity" associated with it, and when the endpoint needs to send a

command to the Call Agent, it MUST send the command to the current

"notified entity" for which endpoint(s) the command pertains. Upon

startup, the "notified entity" MUST be set to a provisioned value.

Most commands sent by the Call Agent include the ability to

explicitly name the "notified entity" through the use of a

"NotifiedEntity" parameter. The "notified entity" will stay the same

until either a new "NotifiedEntity" parameter is received or the

endpoint reboots. If the "notified entity" for an endpoint is empty

or has not been set explicitly, the "notified entity" will then

default to the source address of the last connection handling command

or notification request received for the endpoint. Auditing will thus

not change the "notified entity."

2.1.5. Digit maps

The Call Agent can ask the gateway to collect digits dialed by the

user. This facility is intended to be used with residential gateways

to collect the numbers that a user dials; it may also be used with

trunking gateways and access gateways alike, to collect the access

codes, credit card numbers and other numbers requested by call

control services.

An alternative procedure is for the gateway to notify the Call Agent

of the dialed digits, as soon as they are dialed. However, such a

procedure generates a large number of interactions. It is preferable

to accumulate the dialed numbers in a buffer, and to transmit them in

a single message.

The problem with this accumulation approach, however, is that it is

hard for the gateway to predict how many numbers it needs to

accumulate before transmission. For example, using the phone on our

desk, we can dial the following numbers:

_______________________________________________________

0 Local operator

00 Long distance operator

xxxx Local extension number

8xxxxxxx Local number

#xxxxxxx Shortcut to local number at

other corporate sites

*xx Star services

91xxxxxxxxxx Long distance number

9011 + up to 15 digits International number

_____________________________________________________

The solution to this problem is to load the gateway with a digit map

that correspond to the dial plan. This digit map is expressed using a

syntax derived from the Unix system command, egrep. For example, the

dial plan described above results in the following digit map:

(0T 00T[1-7]xxx8xxxxxxx#xxxxxxx*xx91xxxxxxxxxx9011x.T)

The formal syntax of the digit map is described by the DigitMap rule

in the formal syntax description of the protocol (section 3.4). A

Digit-Map, according to this syntax, is defined either by a "string"

or by a list of strings. Each string in the list is an alternative

numbering scheme, specified either as a set of digits or timers, or

as regular expression. A gateway that detects digits, letters or

timers will:

1) Add the event parameter code as a token to the end of an internal

state variable called the "current dial string"

2) Apply the current dial string to the digit map table, attempting a

match to each regular expression in the Digit Map in lexical order

3) If the result is under-qualified (partially matches at least one

entry in the digit map), do nothing further.

If the result matches, or is over-qualified (i.e. no further digits

could possibly produce a match), send the current digit string to the

Call Agent. A match, in this specification, can be either a "perfect

match," exactly matching one of the specified alternatives, or an

impossible match, which occur when the dial string does not match any

of the alternative. Unexpected timers, for example, can cause

"impossible matches." Both perfect matches and impossible matches

trigger notification of the accumulated digits.

Digit maps are provided to the gateway by the Call Agent, whenever

the Call Agent instructs the gateway to listen for digits.

2.1.6. Names of events

The concept of events and signals is central to MGCP. A Call Agent

may ask to be notified about certain events occurring in an endpoint,

e.g. off-hook events, and a call agent may request certain signals

to be applied to an endpoint, e.g. dial-tone.

Events and signals are grouped in packages within which they share

the same namespace which we will refer to as event names in the

following. Packages are groupings of the events and signals

supported by a particular type of endpoint. For instance, one package

may support a certain group of events and signals for analog access

lines, and another package may support another group of events and

signals for video lines. One or more packages may exist for a given

endpoint-type.

Event names are case insensitive and are composed of two logical

parts, a package name and an event name. Both names are strings of

letters, hyphens and digits, with the restriction that hyphens shall

never be the first or last characters in a name. Package or event

names are not case sensitive - values such as "hu", "Hu", "HU" or

"hU" should be considered equal.

Examples of package names are "D" (DTMF), "M" (MF), "T" (Trunk) or

"L" (Line). Examples of event names can be "hu" (off hook or "hang-

up" transition), "hf" (flash hook) or "0" (the digit zero).

In textual representations, the package name, when present, is

separated from the event name by a slash ("/"). The package name is

in fact optional. Each endpoint-type has a default package associated

with it, and if the package name is excluded from the event name, the

default package name for that endpoint-type is assumed. For example,

for an analog access line, the following two event names are equal:

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

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

line.

This document defines a basic set of package names and event names.

Additional package names and event names can be registered with the

IANA. A package definition shall define the name of the package, and

the definition of each event belonging to the package. The event

definition shall include the precise name of the event (i.e., the

code used in MGCP), a plain text definition of the event, and, when

appropriate, the precise definition of the corresponding signals, for

example the exact frequencies of audio signal such as dial tones or

DTMF tones.

In addition, implementers can gain experience by using experimental

packages. The names of experimental packages must start with the two

characters "x-"; the IANA shall not register package names that start

with these characters.

Digits, or letters, are supported in many packages, notably "DTMF"

and "MF". Digits and letters are defined by the rules "Digit" and

"Letter" in the definition of digit maps. This definition refers to

the digits (0 to 9), to the asterisk or star ("*") and orthotrope,

number or pound sign ("#"), and to the letters "A", "B", "C" and "D",

as well as the timer indication "T". These letters can be combined in

"digit string" that represent the keys that a user punched on a dial.

In addition, the letter "X" can be used to represent all digits, and

the sign "$" can be used in wildcard notations. The need to easily

express the digit strings has a consequence on the form of event

names:

An event name that does not denote a digit should always contain at

least one character that is neither a digit, nor one of the letters

A, B, C, D, T or X. (Such names should not contain the special

signs "*", "#", "/" or "$".)

A Call Agent may often have to ask a gateway to detect a group of

events. Two conventions can be used to denote such groups:

* The wildcard convention can be used to detect any event belonging

to a package, or a given event in many packages, or event any

event in any package supported by the gateway.

* The regular expression Range notation can be used to detect a

range of digits.

The star sign (*) can be used as a wildcard instead of a package

name, and the keyWord "all" can be used as a wildcard instead of an

event name:

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

A name such as "*/bar" denotes the event "bar" in any package

supported by the gateway

The names "*" or "*/all" denote all events supported by the

gate way.

The call agent can ask a gateway to detect a set of digits or letters

either by individually describing those letters, or by using the

"range" notation defined in the syntax of digit strings. For example,

the call agent can:

Use the letter "x" to denote "any letter or digit."

Use the notation "[0-9#]" to denote the digits 0 to 9 and the pound

sign.

In some cases, Call Agents will request the gateway to generate or

detect events on connections rather than on the end point itself.

For example, gateways may be asked to provide a ringback tone on a

connection. When an event shall be applied on a connection, the name

of the connection is added to the name of the event, using an "at"

sign (@) as a delimiter, as in:

G/rt@0A3F58

The wildcard character "*" (star) can be used to denote "all

connections". When this convention is used, the gateway will generate

or detect the event on all the connections that are connected to the

endpoint. An example of this convention could be:

R/qa@*

The wildcard character "$" can be used to denote "the current

connection." It should only be used by the call agent, when the event

notification request is "encapsulated" within a command creation or

modification command. When this convention is used, the gateway will

generate or detect the event on the connection that is currently

being created or modified. An example of this convention is:

G/rt@$

The connection id, or a wildcard replacement, can be used in

conjunction with the "all packages" and "all events" conventions.

For example, the notation:

*/all@*

can be used to designate all events on all connections.

Events and signals are described in packages. The package description

must provide, for each events, the following informations:

* The description of the event and its purpose, which should mean

the actual signal that is generated by the client (i.e., xx ms FSK

tone) as well as the resulting user observed result (i.e., MW

light on/off).

* The detailed characteristics of the event, such as for example

frequencies and amplitude of audio signals, modulations and

repetitions,

* The typical and maximum duration of the event.

Signals are divided into different types depending on their behavior:

* On/off (OO) Once applied, these signals last forever until they

are turned off. This may happen either as the result of an event

or a new SignalRequests (see later).

* Time-out (TO) Once applied, these signals last until they are

either turned off (by an event or SignalRequests) or a signal

specific period of time has elapsed. Depending on package

specifications, a signal that times out may generate an "operation

complete" event.

* Brief (BR) The duration of these signals is so short, that they

stop on their own. If an event occurs the signal will not stop,

however if a new SignalRequests is applied, the signal will stop.

(Note: this point should be debated. One could make a case that

events such as strings of DTMF digits should in fact be allowed to

complete.)

TO signals are normally used to alert the endpoints' users, to

signal them that they are expected to perform a specific action,

such as hang down the phone (ringing). Transmission of these

signals should typically be interrupted as soon as the first of

the requested events has been produced.

Package descriptions should describe, for all signals, their type

(OO, TO, BR). They should also describe the maximum duration of

the TO signals.

2.2. Usage of SDP

The Call Agent uses the MGCP to provision the gateways with the

description of connection parameters such as IP addresses, UDP port

and RTP profiles. These descriptions will follow the conventions

delineated in the Session Description Protocol which is now an IETF

proposed standard, documented in RFC2327.

SDP allows for description of multimedia conferences. This version

limits SDP usage to the setting of audio circuits and data access

circuits. The initial session descriptions contain the description

of exactly one media, of type "audio" for audio connections, "nas"

for data access.

2.3. Gateway Control Commands

This section describes the commands of the MGCP. The service consists

of connection handling and endpoint handling commands. There are nine

commands in the protocol:

* The Call Agent can issue an EndpointConfiguration command to a

gateway, instructing the gateway about the coding characteristics

expected by the "line-side" of the endpoint.

* The Call Agent can issue a NotificationRequest command to a

gateway, instructing the gateway to watch for specific events such

as hook actions or DTMF tones on a specified endpoint .

* The gateway will then use the Notify command to inform the Call

Agent when the requested events occur.

* The Call Agent can use the CreateConnection command to create a

connection that terminates in an "endpoint" inside the gateway.

* The Call Agent can use the ModifyConnection command to change the

parameters associated to a previously established connection.

* The Call Agent can use the DeleteConnection command to delete an

existing connection. The DeleteConnection command may also be used

by a gateway to indicate that a connection can no longer be

sustained.

* The Call Agent can use the AuditEndpoint and AuditConnection

commands to audit the status of an "endpoint" and any connections

associated with it. Network management beyond the capabilities

provided by these commands are generally desirable, e.g.

information about the status of the gateway. Such capabilities are

expected to be supported by the use of the Simple Network

Management Protocol (SNMP) and definition of a MIB which is

outside the scope of this specification.

* The Gateway can use the RestartInProgress command to notify the

Call Agent that the gateway, or a group of endpoints managed by

the gateway, is being taken out of service or is being placed back

in service.

These services allow a controller (normally, the Call Agent) to

instruct a gateway on the creation of connections that terminate in

an "endpoint" attached to the gateway, and to be informed about

events occurring at the endpoint. An endpoint may be for example:

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

gateway,

* A specific announcement handled by an announcement server.

Connections are grouped into "calls". Several connections, that may

or may not belong to the same call, can terminate in the same

endpoint . Each connection is qualified by a "mode" parameter, which

can be set to "send only" (sendonly), "receive only" (recvonly),

"send/receive" (sendrecv), "conference" (confrnce), "data",

"inactive" (inactive), "loopback", "continuity test" (conttest),

"network loop back" (netwloop) or "network continuity test"

(netwtest).

The handling of the audio signals received on these connections is

determined by the mode parameters:

* Audio signals received in data packets through connections in

"receive", "conference" or "send/receive" mode are mixed and sent

to the endpoint.

* Audio signals originating from the endpoint are transmitted over

all the connections whose mode is "send", "conference" or

"send/receive."

* In addition to being sent to the endpoint, audio signals received

in data packets through connections in "conference" mode are

replicated to all the other connections whose mode is

"conference."

The "loopback" and "continuity test" modes are used during

maintenance and continuity test operations. There are two flavors of

continuity test, one specified by ITU and one used in the US. In the

first case, the test is a loopback test. The originating switch will

send a tone (the go tone) on the bearer circuit and expect the

terminating switch to loopback the circuit. If the originating switch

sees the same tone returned (the return tone), the COT has passed. If

not, the COT has failed. In the second case, the go and return tones

are different. The originating switch sends a certain go tone. The

terminating switch detects the go tone, it asserts a different return

tone in the backwards direction. When the originating switch detects

the return tone, the COT is passed. If the originating switch never

detects the return tone, the COT has failed.

If the mode is set to "loopback", the gateway is expected to return

the incoming signal from the endpoint back into that same endpoint.

This procedure will be used, typically, for testing the continuity of

trunk circuits according to the ITU specifications.

If the mode is set to "continuity test", the gateway is informed that

the other end of the circuit has initiated a continuity test

procedure according to the GR specification. The gateway will place

the circuit in the transponder mode required for dual-tone continuity

tests.

If the mode is set to "network loopback", the audio signals received

from the connection will be echoed back on the same connection.

If the mode is set to "network continuity test", the gateway will

process the packets received from the connection according to the

transponder mode required for dual-tone continuity test, and send the

processed signal back on the connection.

2.3.1. EndpointConfiguration

The EndpointConfiguration commands are used to specify the encoding

of the signals that will be received by the endpoint. For example,

in certain international telephony configurations, some calls will

carry mu-law encoded audio signals, while other will use A-law. The

Call Agent will use the EndpointConfiguration command to pass this

information to the gateway. The configuration may vary on a call by

call basis, but can also be used in the absence of any connection.

ReturnCode

<-- EndpointConfiguration( EndpointId,

BearerInformation)

EndpointId is the name for the endpoint in the gateway where

EndpointConfiguration executes, as defined in section 2.1.1. The

"any of" wildcard convention shall not be used. If the "all of"

wildcard convention is used, the command applies to all the endpoint

whose name matches the wildcard.

BearerInformation is a parameter defining the coding of the data

received from the line side. These information is encoded as a list

of sub-parameters. The only sub-parameter defined in this version of

the specification is the encoding method, whose values can be set to

"A-law" and "mu-law".

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

outcome of the command and consists of an integer number optionally

followed by commentary.

2.3.2. NotificationRequest

The NotificationRequest commands are used to request the gateway to

send notifications upon the occurrence of specified events in an

endpoint. For example, a notification may be requested for when a

gateway detects that an endpoint is receiving tones associated with

fax communication. The entity receiving this notification may decide

to use a different type of encoding method in the connections bound

to this endpoint.

ReturnCode

<-- NotificationRequest( EndpointId,

[NotifiedEntity,]

[RequestedEvents,]

RequestIdentifier,

[DigitMap,]

[SignalRequests,]

[QuarantineHandling,]

[DetectEvents,]

[encapsulated EndpointConfiguration])

EndpointId is the name for the endpoint in the gateway where

NotificationRequest executes, as defined in section 2.1.1.

NotifiedEntity is an optional parameter that specifies where the

notifications should be sent. When this parameter is absent, the

notifications should be sent to the originator of the

NotificationRequest.

RequestIdentifier is used to correlate this request with the

notifications that it triggers.

RequestedEvents is a list of events that the gateway is requested to

detect and report. Such events include, for example, fax tones,

continuity tones, or on-hook transition. To each event is associated

an action, which can be:

* Notify the event immediately, together with the accumulated list

of observed events,

* Swap audio,

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

* Accumulate according to Digit Map,

* Keep Signal(s) active,

* process the Embedded Notification Request,

* Ignore the event.

Some actions can be combined. In particular:

* The "swap audio" action can be combined with "Notify",

"Accumulate" and "Ignore."

* The "keep signal active" action can be combined with "Notify",

"Accumulate", "Accumulate according to Digit Map", "Ignore" and

"Embedded Notification Request."

* The "Embedded Notification Request" can be combined with

"Accumulate" and with "Keep signals active." It can also be

combined with Notify, if the gateway is allowed to issue several

Notify commands in response to a single Notification request.

In addition to the requestedEvents parameter specified in the

command, some profiles of MGCP have introduced the concept of

"persistent events." According to such profiles, the persistent event

list is configured in the endpoint, by means outside the scope of

MGCP. The basic MGCP specification does not specify any persistent

event.

If a persistent event is not included in the list of RequestedEvents,

and the event occurs, the event will be detected anyway, and

processed like all other events, as if the persistent event had been

requested with a Notify action. Thus, informally, persistent events

can be viewed as always being implicitly included in the list of

RequestedEvents with an action to Notify, although no glare

detection, etc., will be performed.

Non-persistent events are those events explicitly included in the

RequestedEvents list. The (possibly empty) list of requested events

completely replaces the previous list of requested events. In

addition to the persistent events, only the events specified in the

requested events list will be detected by the endpoint. If a

persistent event is included in the RequestedEvents list, the action

specified will then replace the default action associated with the

event for the life of the RequestedEvents list, after which the

default action is restored. For example, if "Ignore off-hook" was

specified, and a new request without any off-hook instructions were

received, the default "Notify off-hook" operation then would be

restored. A given event MUST NOT appear more than once in a

RequestedEvents.

The gateway will detect the union of the persistent events and the

requested events. If an event is not specified in either list, it

will be ignored.

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

one active connection on an endpoint. This will be the case for

three-way calling, call waiting, and possibly other feature

scenarios. In order to avoid the round-trip to the Call Agent when

just changing which connection is attached to the audio functions of

the endpoint, the NotificationRequest can map an event (usually hook

flash, but could be some other event) to a local function swap audio,

which selects the "next" connection in a round robin fashion. If

there is only one connection, this action is effectively a no-op.

If signal(s) are desired to start when an event being looked for

occurs, the "Embedded NotificationRequest" action can be used. The

embedded NotificationRequest may include a new list of

RequestedEvents, SignalRequests and a new digit map as well. The

semantics of the embedded NotificationRequest is as if a new

NotificationRequest was just received with the same NotifiedEntity,

and RequestIdentifier. When the "Embedded NotificationRequest" is

activated, the "current dial string" will be cleared; the list of

observed events and the quarantine buffer will be unaffected.

MGCP implementations shall be able to support at least one level of

embedding. An embedded NotificationRequest that respects this

limitation shall not contain another Embedded NotificationRequest.

DigitMap is an optional parameter that allows the Call Agent to

provision the gateways with a digit map according to which digits

will be accumulated. If this optional parameter is absent, the

previously defined value is retained. This parameter must be defined,

either explicitly or through a previous command, if the

RequestedEvent parameters contain an request to "accumulate according

to the digit map." The collection of these digits will result in a

digit string. The digit string is initialized to a null string upon

reception of the NotificationRequest, so that a subsequent

notification only returns the digits that were collected after this

request. Digits that were accumulated according to the digit map are

reported as any other accumulated event, in the order in which they

occur. It is therefore possible that other events be accumulated may

be found in between the list of digits.

SignalRequests is a parameter that contains the set of signals that

the gateway is asked to apply to the endpoint, such as, for example

ringing, or continuity tones. Signals are identified by their name,

which is an event name, and may be qualified by parameters.

The action triggered by the SignalRequests is synchronized with the

collection of events specified in the RequestedEvents parameter. For

example, if the NotificationRequest mandates "ringing" and the event

request ask to look for an "off-hook" event, the ringing shall stop

as soon as the gateway detect an off hook event. The formal

definition is that the generation of all "Time Out" signals shall

stop as soon as one of the requested events is detected, unless the

"Keep signals active" action is associated to the specified event.

The specific definition of actions that are requested via these

SignalRequests, such as the duration of and frequency of a DTMF

digit, is out side the scope of MGCP. This definition may vary from

location to location and hence from gateway to gateway.

The RequestedEvents and SignalRequests refer to the same event

definitions. In one case, the gateway is asked to detect the

occurrence of the event, and in the other case it is asked to

generate it. The specific events and signals that a given endpoint

can detect or perform are determined by the list of event packages

that are supported by that end point. Each package specifies a list

of events and actions that can be detected or performed. A gateway

that is requested to detect or perform an event belonging to a

package that is not supported by the specified endpoint shall return

an error. When the event name is not qualified by a package name, the

default package name for the end point is assumed. If the event name

is not registered in this default package, the gateway shall return

an error.

The Call Agent can send a NotificationRequest whose requested signal

list is empty. It will do so for example when tone generation should

stop.

The optional QuarantineHandling parameter specifies the handling of

"quarantine" events, i.e. events that have been detected by the

gateway before the arrival of this NotificationRequest command, but

have not yet been notified to the Call Agent. The parameter provides

a set of handling options:

* whether the quarantined events should be processed or discarded

(the default is to process them.)

* whether the gateway is expected to generate at most one

notification (step by step), or multiple notifications (loop), in

response to this request (the default is exactly one.)

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

We should note that the quarantine-handling parameter also governs

the handling of events that were detected but not yet notified when

the command is received.

DetectEvents is an optional parameter that specifies a list of events

that the gateway is requested to detect during the quarantine period.

When this parameter is absent, the events that should be detected in

the quarantine period are those listed in the last received

DetectEvents list. In addition, the gateway should also detect the

events specified in the request list, including those for which the

"ignore" action is specified.

Some events and signals, such as the in-line ringback or the quality

alert, are performed or detected on connections terminating in the

end point rather than on the endpoint itself. The structure of the

event names allow the Call Agent to specify the connection (or

connections) on which the events should be performed or detected.

The command may carry an encapsulated EndpointConfiguration command,

that will apply to the same endpoint. When this command is present,

the parameters of the EndpointConfiguration command are inserted

after the normal parameters of the NotificationRequest, with the

exception of the EndpointId, which is not replicated.

The encapsulated EndpointConfiguration command shares the fate of the

NotificationRequest command. If the NotificationRequest is rejected,

the EndpointConfiguration is not executed.

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

outcome of the command and consists of an integer number optionally

followed by commentary. .NH 3 Notifications

Notifications are sent via the Notify command and are sent by the

gateway when the observed events occur.

ReturnCode

<-- Notify( EndpointId,

[NotifiedEntity,]

RequestIdentifier,

ObservedEvents)

EndpointId is the name for the endpoint in the gateway which is

issuing the Notify command, as defined in section 2.1.1. The

identifier should be a fully qualified endpoint identifier, including

the domain name of the gateway. The local part of the name shall not

use the wildcard convention.

NotifiedEntity is an optional parameter that identifies the entity to

which the notifications is sent. This parameter is equal to the last

received value of the NotifiedEntity parameter. The parameter is

absent if there was no such parameter in the triggering request. The

notification is sent to the "current notified entity" or, if no such

entity was ever specified, to the address from which the request was

received.

RequestIdentifier is parameter that repeats the RequestIdentifier

parameter of the NotificationRequest that triggered this

notification. It is used to correlate this notification with the

request that triggered it.

ObservedEvents is a list of events that the gateway detected. A

single notification may report a list of events that will be reported

in the order in which they were detected. The list may only contain

the identification of events that were requested in the

RequestedEvents parameter of the triggering NotificationRequest. It

will contain the events that were either accumulated (but not

notified) or treated according to digit map (but no match yet), and

the final event that triggered the detection or provided a final

match in the digit map.

ReturnCode is a parameter returned by the call agent. It indicates

the outcome of the command and consists of an integer number

optionally followed by commentary.

2.3.3. CreateConnection

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

ReturnCode,

ConnectionId,

[SpecificEndPointId,]

[LocalConnectionDescriptor,]

[SecondEndPointId,]

[SecondConnectionId]

<--- CreateConnection(CallId,

EndpointId,

[NotifiedEntity,]

[LocalConnectionOptions,]

Mode,

[{RemoteConnectionDescriptor

SecondEndpointId}, ]

[Encapsulated NotificationRequest,]

[Encapsulated EndpointConfiguration])

A connection is defined by its endpoints. The input parameters in

CreateConnection provide the data necessary to build a gateway's

"view" of a connection.

CallId is a globally unique parameter that identifies the call (or

session) to which this connection belongs. Connections that belong to

the same call share the same call-id. The call-id can be used to

identify calls for reporting and accounting purposes. It does not

affect the handling of connections by the gateway.

EndpointId is the identifier for the connection endpoint in the

gateway where CreateConnection executes. The EndpointId can be

fully-specified by assigning a value to the parameter EndpointId in

the function call or it may be under-specified by using the "anyone"

wildcard convention. If the endpoint is underspecified, the endpoint

identifier will be assigned by the gateway and its complete value

returned in the SpecificEndPointId parameter of the response.

The NotifiedEntity is an optional parameter that specifies where the

Notify or DeleteConnection commands should be sent. If the parameter

is absent, the Notify or DeleteConnection commands should be sent to

the last received Notified Entity, or to originator of the

CreateConnection command if no Notified Entity was ever received for

the end point.

LocalConnectionOptions is a parameter used by the Call Agent to

direct the handling of the connection by the gateway. The fields

contained in LocalConnectionOptions are the following:

* Encoding Method,

* Packetization period,

* Bandwidth,

* Type of Service,

* Usage of echo cancellation,

* Usage of silence suppression or voice activity detection,

* Usage of signal level adaptation and noise level reduction, or

"gain control."

* Usage of reservation service,

* Usage of RTP security,

* Type of network used to carry the connection.

This set of field can be completed by vendor specific optional or

mandatory extensions. The encoding of the first three fields, when

they are present, will be compatible with the SDP and RTP profiles:

* The encoding method shall be specified by using one or several

valid encoding names, as defined in the RTP AV Profile or

registered with the IANA.

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

in milliseconds represented by the media in a packet, as specified

in the "ptime" parameter of SDP, or as a range value, specifying

both the minimum and maximum acceptable packetization periods.

* The bandwidth is encoded as either a single value or a range,

expressed as an integer number of kilobit per seconds.

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

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

for the connection,

* It may provide a loose specification, such as a list of allowed

encoding methods or a range of packetization periods,

* It may simply provide a bandwidth indication, leaving the choice

of encoding method and packetization period to the gateway.

The bandwidth specification shall not contradict the specification of

encoding methods and packetization period. If an encoding method is

specified, then the gateway is authorized to use it, even if it

results in the usage of a larger bandwidth than specified.

The LocalConnectionOptions parameter may be absent in the case of a

data call.

The Type of Service specifies the class of service that will be used

for the connection. When the connection is transmitted over an IP

network, the parameters encodes the 8-bit type of service value

parameter of the IP header. When the Type of Service is not

specified, the gateway shall use a default or configured value.

The gateways can be instructed to perform a reservation, for example

using RSVP, on a given connection. When a reservation is needed, the

call agent will specify the reservation profile that should be used,

which is either "controlled load" or "guaranteed service." The

absence of reservation can be indicated by aSKINg for the "best

effort" service, which is the default value of this parameter. When

reservation has been asked on a connection, the gateway will:

* start emitting RSVP "PATH" messages if the connection is in

"send-only", "send-receive", "conference", "network loop back" or

"network continuity test" mode (if a remote connection descriptor

has been received,)

* start emitting RSVP "RESV" messages as soon as it receives "PATH"

messages if the connection is in "receive-only", "send-receive",

"conference", "network loop back" or "network continuity test"

mode.

The RSVP filters will be deduced from the characteristics of the

connection. The RSVP resource profiles will be deduced from the

connection's bandwidth and packetization period.

By default, the telephony gateways always perform echo cancellation.

However, it is necessary, for some calls, to turn off these

operations. The echo cancellation parameter can have two values,

"on" (when the echo cancellation is requested) and "off" (when it is

turned off.)

The telephony gateways may perform gain control, in order to adapt

the level of the signal. However, it is necessary, for example for

modem calls, to turn off this function. The gain control parameter

may either be specified as "automatic", or as an explicit number of

decibels of gain. The default is to not perform gain control, which

is equivalent to specifying a gain of 0 decibels.

The telephony gateways may perform voice activity detection, and

avoid sending packets during periods of silence. However, it is

necessary, for example for modem calls, to turn off this detection.

The silence suppression parameter can have two values, "on" (when the

detection is requested) and "off" (when it is turned off.) The

default is "off."

The Call agent can request the gateway to enable encryption of the

audio Packets. It does so by providing an key specification, as

specified in RFC2327. By default, encryption is not used.

The Call Agent may instruct the gateway to prepare the connection on

a specified type of network. The type of network is encoded as in

the "connection-field" parameter of the SDP standard. Possible

values are IN (Internet), ATM and LOCAL. The parameter is optional;

if absent, the network is determined by the type of gateway.

RemoteConnectionDescriptor is the connection descriptor for the

remote side of a connection, on the other side of the IP network. It

includes the same fields as in the LocalConnectionDescriptor, i.e.

the fields that describe a session according to the SDP standard.

This parameter may have a null value when the information for the

remote end is not known yet. This occurs because the entity that

builds a connection starts by sending a CreateConnection to one of

the two gateways involved in it. For the first CreateConnection

issued, there is no information available about the other side of the

connection. This information may be provided later via a

ModifyConnection call. In the case of data connections (mode=data),

this parameter describes the characteristics of the data connection.

The SecondEndpointId can be used instead of the

RemoteConnectionDescriptor to establish a connection between two

endpoints located on the same gateway. The connection is by

definition a local connection. The SecondEndpointId can be fully-

specified by assigning a value to the parameter SecondEndpointId in

the function call or it may be under-specified by using the "anyone"

wildcard convention. If the secondendpoint is underspecified, the

second endpoint identifier will be assigned by the gateway and its

complete value returned in the SecondEndPointId parameter of the

response.

Mode indicates the mode of operation for this side of the connection.

The mode are "send", "receive", "send/receive", "conference", "data",

"inactive", "loopback", "continuity test", "network loop back" or

"network continuity test." The expected handling of these modes is

specified in the introduction of the "Gateway Handling Function"

section. Some end points may not be capable of supporting all modes.

If the command specifies a mode that the endpoint cannot support, and

error shall be returned.

The gateway returns a ConnectionId, that uniquely identifies the

connection within one endpoint, and a LocalConnectionDescriptor,

which is a session description that contains information about

addresses and RTP ports, as defined in SDP. The

LocalConnectionDescriptor is not returned in the case of data

connections. The SpecificEndPointId is an optional parameter that

identifies the responding endpoint. It can be used when the

EndpointId argument referred to a "any of" wildcard name. When a

SpecificEndPointId is returned, the Call Agent should use it as the

EndpointId value is successive commands referring to this call.

When a SecondEndpointId is specified, the command really creates two

connections that can be manipulated separately through

ModifyConnection and DeleteConnection commands. The response to the

creation provides a SecondConnectionId parameter that identifies the

second connection.

After receiving a "CreateConnection" request that did not include a

RemoteConnectionDescriptor parameter, a gateway is in an ambiguous

situation. Because it has exported a LocalConnectionDescriptor

parameter, it can potentially receive packets. Because it has not yet

received the RemoteConnectionDescriptor parameter of the other

gateway, it does not know whether the packets that it receives have

been authorized by the Call Agent. It must thus navigate between two

risks, i.e. clipping some important announcements or listening to

insane data. The behavior of the gateway is determined by the value

of the Mode parameter:

* If the mode was set to ReceiveOnly, the gateway should accept the

voice signals and transmit them through the endpoint.

* If the mode was set to Inactive, Loopback, Continuity Test, the

gateway should refuse the voice signals.

* If the mode was set to Network Loopback or Network Continuity

Test, the gateway should perform the expected echo or Response.

Note that the mode values SendReceive, Conference, Data and SendOnly

don't make sense in this situation. They should be treated as errors,

and the command should be rejected (Error code 517).

The command may optionally contain an encapsulated Notification

Request command, in which case a RequestIdentifier parameter will be

present, as well as, optionally, the RequestedEvents DigitMap,

SignalRequests, QuarantineHandling and DetectEvents parameters. The

encapsulated NotificationRequest is executed simultaneously with the

creation of the connection. For example, when the Call Agent wants to

initiate a call to an residential gateway, it should:

* ask the residential gateway to prepare a connection, in order to

be sure that the user can start speaking as soon as the phone goes

off hook,

* ask the residential gateway to start ringing,

* ask the residential gateway to notify the Call Agent when the

phone goes off-hook.

This can be accomplished in a single CreateConnection command, by

also transmitting the RequestedEvent parameters for the off hook

event, and the SignalRequest parameter for the ringing signal.

When these parameters are present, the creation and the

NotificationRequests should be synchronized, which means that

bothshould be accepted, or both refused. In our example, the

CreateConnection may be refused if the gateway does not have

sufficient resources, or cannot get adequate resources from the local

network access, and the off-hook Notification-Request can be refused

in the glare condition, if the user is already off-hook. In this

example, the phone should not ring if the connection cannot be

established, and the connection should not be established if the user

is already off hook.

The NotifiedEntity parameter, if present, applies to both the

CreateConnection and the NotificationRequest command. It defines the

new "notified entity" for the endpoint.

The command may carry an encapsulated EndpointConfiguration command,

that will apply to the same endpoint. When this command is present,

the parameters of the EndpointConfiguration command are inserted

after the normal parameters of the CreateConnection with the

exception of the EndpointId, which is not replicated. The

EndpointConfiguration command may be encapsulated together with an

encapsulated NotificationRequest command.

The encapsulated EndpointConfiguration command shares the fate of the

CreateConnection command. If the CreateConnection is rejected, the

EndpointConfiguration is not executed.

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

outcome of the command and consists of an integer number optionally

followed by commentary.

2.3.4. ModifyConnection

This command is used to modify the characteristics of a gateway's

"view" of a connection. This "view" of the call includes both the

local connection descriptors as well as the remote connection

descriptor.

ReturnCode,

[LocalConnectionDescriptor]

<--- ModifyConnection(CallId,

EndpointId,

ConnectionId,

[NotifiedEntity,]

[LocalConnectionOptions,]

[Mode,]

[RemoteConnectionDescriptor,]

[Encapsulated NotificationRequest,]

[Encapsulated EndpointConfiguration])

The parameters used are the same as in the CreateConnection command,

with the addition of a ConnectionId that identifies the connection

within the endpoint. This parameter is returned by the

CreateConnection function, as part of the local connection

descriptor. It uniquely identifies the connection within the context

of the endpoint.

The EndpointId should be a fully qualified endpoint identifier. The

local name shall not use the wildcard convention.

The ModifyConnection command can be used to affect parameters of a

connection in the following ways:

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

the RemoteConnectionDescriptor.

* Activate or deactivate the connection, by changing the value of

the Mode parameter. This can occur at any time during the

connection, with arbitrary parameter values.

* Change the sending parameters of the connection, for example by

switching to a different coding scheme, changing the packetization

period, or modifying the handling of echo cancellation.

Connections can only be activated if the RemoteConnectionDescriptor

has been provided to the gateway. The receive only mode, however, can

be activated without the provision of this descriptor.

The command will only return a LocalConnectionDescriptor if the local

connection parameters, such as RTP ports, were modified. (Usage of

this feature is actually for further study.)

The command may optionally contain an encapsulated Notification

Request command, in which case a RequestIdentifier parameter will be

present, as well as, optionnally, the RequestedEvents DigitMap,

SignalRequests, QuarantineHandling and DetectEvents parameters. The

encapsulated NotificationRequest is executed simultaneously with the

modification of the connection. For example, when a connection is

accepted, the calling gateway should be instructed to place the

circuit in send-receive mode and to stop providing ringing tones.

This can be accomplished in a single ModifyConnection command, by

also transmitting the RequestedEvent parameters, for the on hook

event, and an empty SignalRequest parameter, to stop the provision of

ringing tones.

When these parameters are present, the modification and the

NotificationRequests should be synchronized, which means that both

should be accepted, or both refused. The NotifiedEntity parameter,

if present, applies to both the ModifyConnection and the

NotificationRequest command.

The command may carry an encapsulated EndpointConfiguration command,

that will apply to the same endpoint. When this command is present,

the parameters of the EndpointConfiguration command are inserted

after the normal parameters of the ModifyConnection with the

exception of the EndpointId, which is not replicated. The

EndpointConfiguration command may be encapsulated together with an

encapsulated NotificationRequest command.

The encapsulated EndpointConfiguration command shares the fate of the

ModifyConnection command. If the ModifyConnection is rejected, the

EndpointConfiguration is not executed.

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

outcome of the command and consists of an integer number optionally

followed by commentary.

2.3.5. DeleteConnection (from the Call Agent)

This command is used to terminate a connection. As a side effect, it

collects statistics on the execution of the connection.

ReturnCode,

Connection-parameters

<-- DeleteConnection(CallId,

EndpointId,

ConnectionId,

[Encapsulated NotificationRequest,]

[Encapsulated EndpointConfiguration])

The endpoint identifier, in this form of the DeleteConnection

command, shall be fully qualified. Wildcard conventions shall not be

used.

In the general case where a connection has two ends, this command has

to be sent to both gateways involved in the connection. Some

connections, however, may use IP multicast. In this case, they can be

deleted individually.

After the connection has been deleted, any loopback that has been

requested for the connection should be cancelled. When all

connections to an endpoint have been deleted, that endpoint should be

placed in inactive mode.

In response to the DeleteConnection command, the gateway returns a

list of parameters that describe the status of the connection. These

parameters are:

Number of packets sent:

The total number of RTP data packets transmitted by the sender since

starting transmission on this connection. The count is not reset if

the sender changes its synchronization source identifier (SSRC, as

defined in RTP), for example as a result of a Modify command. The

value is zero if the connection was set in "receive only" mode.

Number of octets sent:

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

padding) transmitted in RTP data packets by the sender since starting

transmission on this connection. The count is not reset if the sender

changes its SSRC identifier, for example as a result of a

ModifyConnection command. The value is zero if the connection was set

in "receive only" mode.

Number of packets received:

The total number of RTP data packets received by the sender since

starting reception on this connection. The count includes packets

received from different SSRC, if the sender used several values. The

value is zero if the connection was set in "send only" mode.

Number of octets received:

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

padding) transmitted in RTP data packets by the sender since starting

transmission on this connection. The count includes packets received

from different SSRC, if the sender used several values. The value is

zero if the connection was set in "send only" mode.

Number of packets lost:

The total number of RTP data packets that have been lost since the

beginning of reception. This number is defined to be the number of

packets expected less the number of packets actually received, where

the number of packets received includes any which are late or

duplicates. The count includes packets received from different SSRC,

if the sender used several values. Thus packets that arrive late are

not counted as lost, and the loss may be negative if there are

duplicates. The count includes packets received from different SSRC,

if the sender used several values. The number of packets expected is

defined to be the extended last sequence number received, as defined

next, less the initial sequence number received. The count includes

packets received from different SSRC, if the sender used several

values. The value is zero if the connection was set in "send only"

mode. This parameter is omitted if the connection was set in "data"

mode.

Interarrival jitter:

An estimate of the statistical variance of the RTP data packet

interarrival time measured in milliseconds and expressed as an

unsigned integer. The interarrival jitter J is defined to be the mean

deviation (smoothed absolute value) of the difference D in packet

spacing at the receiver compared to the sender for a pair of packets.

Detailed computation algorithms are found in RFC1889. The count

includes packets received from different SSRC, if the sender used

several values. The value is zero if the connection was set in "send

only" mode. This parameter is omitted if the connection was set in

"data" mode.

Average transmission delay:

An estimate of the network latency, expressed in milliseconds. This

is the average value of the difference between the NTP timestamp

indicated by the senders of the RTCP messages and the NTP timestamp

of the receivers, measured when this messages are received. The

average is oBTained by summing all the estimates, then dividing by

the number of RTCP messages that have been received. This parameter

is omitted if the connection was set in "data" mode.

When the gateway's clock is not synchronized by NTP, the latency

value can be computed as one half of the round trip delay, as

measured through RTCP.

When the gateway cannot compute the one way delay or the round trip

delay, the parameter conveys a null value.

For a detailed definition of these variables, refer to RFC1889.

When the connection was set up over an ATM network, the meaning of

these parameters may change:

Number of packets sent: The total number of ATM cells transmitted

since starting transmission on this connection.

Number of octets sent:

The total number of payload octets transmitted in ATM cells.

Number of packets received:

The total number of ATM cells received since starting reception on

this connection.

Number of octets received:

The total number of payload octets received in ATM cells.

Number of packets lost:

Should be determined as the number of cell losts, or set to zero

if the adaptation layer does not enable the gateway to assess

losses.

Interarrival jitter:

Should be understood as the interarrival jitter between ATM cells.

Average transmission delay:

The gateway may not be able to assess this parameter over an ATM

network. It could simply report a null value.

When the connection was set up over an LOCAL interconnect, the

meaning of these parameters is defined as follows:

Number of packets sent:

Not significant.

Number of octets sent:

The total number of payload octets transmitted over the local

connection.

Number of packets received:

Not significant.

Number of octets received:

The total number of payload octets received over the connection.

Number of packets lost:

Not significant. A value of zero is assumed.

Interarrival jitter:

Not significant. A value of zero is assumed.

Average transmission delay:

Not significant. A value of zero is assumed.

The standard set of connection parameters can be extended by the

creation of extension parameters.

The command may optionally contain an encapsulated Notification

Request command, in which case a RequestIdentifier parameter will be

present, as well as, optionnally, the RequestedEvents DigitMap,

SignalRequests, QuarantineHandling and DetectEvents parameters. The

encapsulated NotificationRequest is executed simultaneously with the

deletion of the connection. For example, when a user hang-up is

notified, the gateway should be instructed to delete the connection

and to start looking for an off hook event.

This can be accomplished in a single DeleteConnection command, by

also transmitting the RequestedEvent parameters, for the off hook

event, and an empty SignalRequest parameter.

When these parameters are present, the DeleteConnection and the

NotificationRequests should be synchronized, which means that both

should be accepted, or both refused.

The command may carry an encapsulated EndpointConfiguration command,

that will apply to the same endpoint. When this command is present,

the parameters of the EndpointConfiguration command are inserted

after the normal parameters of the DeleteConnection with the

exception of the EndpointId, which is not replicated. The

EndpointConfiguration command may be encapsulated together with an

encapsulated NotificationRequest command.

The encapsulated EndpointConfiguration command shares the fate of the

DeleteConnection command. If the DeleteConnection is rejected, the

EndpointConfiguration is not executed.

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

outcome of the command and consists of an integer number optionally

followed by commentary.

2.3.6. DeleteConnection (from the VoIP gateway)

In some circumstances, a gateway may have to clear a connection, for

example because it has lost the resource associated with the

connection, or because it has detected that the endpoint no longer is

capable or willing to send or receive voice. The gateway terminates

the connection by using a variant of the DeleteConnection command:

ReturnCode,

<-- DeleteConnection( CallId,

EndpointId,

ConnectionId,

Reason-code,

Connection-parameters)

In addition to the call, endpoint and connection identifiers, the

gateway will also send the call's parameters that would have been

returned to the Call Agent in response to a DeleteConnection command.

The reason code indicates the cause of the disconnection.

ReturnCode is a parameter returned by the call agent. It indicates

the outcome of the command and consists of an integer number

optionally followed by commentary.

2.3.7. DeleteConnection (multiple connections, from the Call Agent)

A variation of the DeleteConnection function can be used by the Call

Agent to delete multiple connections at the same time. The command

can be used to delete all connections that relate to a Call for an

endpoint:

ReturnCode,

<-- DeleteConnection( CallId,

EndpointId)

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

given endpoint:

ReturnCode,

<-- DeleteConnection( EndpointId)

Finally, Call Agents can take advantage of the hierarchical naming

structure of endoints to delete all the connections that belong to a

group of endpoints. In this case, the "local name" component of the

EndpointID will be specified using the "all value" wildcarding

convention. The "any value" convention shall not be used. For

example, if endpoints names are structured as the combination of a

physical interface name and a circuit number, as in "X35V3+A4/13",

the Call Agent may replace the circuit number by a wild card

character "*", as in "X35V3+A4/*". This "wildcard" command instructs

the gateway to delete all the connections that where attached to

circuits connected to the physical interface "X35V3+A4".

After the connections have been deleted, the endpoint should be

placed in inactive mode. Any loopback that has been requested for the

connections should be cancelled.

This command does not return any individual statistics or call

parameters.

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

outcome of the command and consists of an integer number optionally

followed by commentary.

2.3.8. Audit Endpoint

The AuditEndPoint command can be used by the Call Agent to find out

the status of a given endpoint.

ReturnCode,

EndPointIdList{

[RequestedEvents,]

[DigitMap,]

[SignalRequests,]

[RequestIdentifier,]

[NotifiedEntity,]

[ConnectionIdentifiers,]

[DetectEvents,]

[ObservedEvents,]

[EventStates,]

[BearerInformation,]

[RestartReason,]

[RestartDelay,]

[ReasonCode,]

[Capabilities]}

<--- AuditEndPoint(EndpointId,

[RequestedInfo])

The EndpointId identifies the endpoint that is being audited. The

"all of" wildcard convention can be used to start auditing of a group

of endpoints. If this convention is used, the gateway should return

the list of endpoint identifiers that match the wildcard in the

EndPointIdList parameter. It shall not return any parameter specific

to one of these endpoints.

When a non-wildcard EndpointId is specified, the (possibly empty)

RequestedInfo parameter describes the information that is requested

for the EndpointId specified. The following endpoint info can be

audited with this command:

RequestedEvents, DigitMap, SignalRequests, RequestIdentifier,

NotifiedEntity, ConnectionIdentifiers, DetectEvents, ObservedEvents,

EventStates, RestartReason, RestartDelay, ReasonCode, and

Capabilities.

The response will in turn include information about each of the items

for which auditing info was requested:

* RequestedEvents: The current value of RequestedEvents the endpoint

is using including the action associated with each event.

Persistent events are included in the list.

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

* SignalRequests: A list of the; Time-Out signals that are currently

active, On/Off signals that are currently "on" for the endpoint

(with or without parameter), and any pending Brief signals. Time-

Out signals that have timed-out, and currently playing Brief

signals are not included.

* RequestIdentifier, the RequestIdentifier for the last Notification

Request received by this endpoint (includes NotificationRequest

encapsulated in Connection handling primitives). If no

notification request has been received, the value zero will be

returned.

* QuarantineHandling, the QuarantineHandling for the last

NotificationRequest received by this endpoint.

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

quarantine mode.

* NotifiedEntity, the current notified entity for the endpoint.

* ConnectionIdentifiers, the list of ConnectionIdentifiers for all

connections that currently exist for the specified endpoint.

* ObservedEvents: the current list of observed events for the

endpoint.

* EventStates: For events that have auditable states associated with

them, the event corresponding to the state the endpoint is in,

e.g., off-hook if the endpoint is off-hook. The definition of the

individual events will state if the event in question has an

auditable state associated with it.

* BearerInformation: the value of the last received

BearerInformation parameter for this endpoint.

* RestartReason: the value of the restart reason parameter in the

last RestartInProgress command issued by the endpoint, "restart"

indicating a fully functional endpoint.

* RestartDelay: the value of the restart delay parameter if a

RestartInProgress command was issued by the endpoint at the time

of the response, or zero if the command would not include this

parameter.

* ReasonCode:the value of the Reason-Code parameter in the last

RestartInProgress or DeleteConnection command issued by the

gateway for the endpoint, or the special value 000 if the

endpoint's state is nominal.

* The capabilities for the endpoint similar to the

LocalConnectionOptions parameter and including event packages and

connection modes. If there is a need to specify that some

parameters, such as e.g., silence suppression, are only compatible

with some

* codecs, then the gateway will return several capability sets:

Compression Algorithm: a list of supported codecs. The rest of

the parameters will apply to all codecs specified in this list.

Packetization Period: A single value or a range may be

specified.

Bandwidth: A single value or a range corresponding to the range

for packetization periods may be specified (assuming no silence

suppression).

Echo Cancellation: Whether echo cancellation is supported or

not.

Silence Suppression: Whether silence suppression is supported

or not.

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

Event Packages: A list of event packages supported. The first

event package in the list will be the default package.

Modes: A list of supported connection modes.

The Call Agent may then decide to use the AuditConnection command to

obtain further information about the connections.

If no info was requested and the EndpointId refers to a valid

endpoint, the gateway simply returns a positive acknowledgement.

If no NotifiedEntity has been specified in the last

NotificationRequest, the notified entity defaults to the source

address of the last NotificationRequest command received for this

connection.

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

outcome of the command and consists of an integer number optionally

followed by commentary.

2.3.9. Audit Connection

The AuditConnection command can be used by the Call Agent to retrieve

the parameters attached to a connection:

ReturnCode,

[CallId,]

[NotifiedEntity,]

[LocalConnectionOptions,]

[Mode,]

[RemoteConnectionDescriptor,]

[LocalConnectionDescriptor,]

[ConnectionParameters]

<--- AuditConnection(EndpointId,

ConnectionId,

RequestedInfo)

The EndpointId parameter specifies the endpoint that handles the

connection. The wildcard conventions shall not be used.

The ConnectionId parameter is the identifier of the audited

connection, within the context of the specified endpoint.

The (possibly empty) RequestedInfo describes the information that is

requested for the ConnectionId within the EndpointId specified. The

following connection info can be audited with this command:

CallId, NotifiedEntity, LocalConnectionOptions, Mode,

RemoteConnectionDescriptor, LocalConnectionDescriptor,

ConnectionParameters

The AuditConnectionResponse will in turn include information about

each of the items auditing info was requested for:

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

* NotifiedEntity, the current notified entity for the Connection.

* LocalConnectionOptions, the LocalConnectionOptions that was

supplied for the connection.

* Mode, the current mode of the connection.

* RemoteConnectionDescriptor, the RemoteConnectionDescriptor that

was supplied to the gateway for the connection.

* LocalConnectionDescriptor, the LocalConnectionDescriptor the gate-

way supplied for the connection.

* ConnectionParameters, the current value of the connection

parameters for the connection.

If no info was requested and the EndpointId is valid, the gateway

simply checks that the connection exists, and if so returns a

positive acknowledgement.

If no NotifiedEntity has been specified for the connection, the

notified entity defaults to the source address of the last connection

handling command received for this connection.

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

outcome of the command and consists of an integer number optionally

followed by commentary.

2.3.10. Restart in progress

The RestartInProgress command is used by the gateway to signal that

An endpoint, or a group of endpoint, is taken in or out of service.

ReturnCode,

[NotifiedEntity]

<------- RestartInProgress ( EndPointId,

RestartMethod,

[RestartDelay,]

[Reason-code])

The EndPointId identifies the endpoint that are taken in or out of

service. The "all of" wildcard convention may be used to apply the

command to a group of endpoint, such as for example all endpoints

that are attached to a specified interface, or even all endpoints

that are attached to a given gateway. The "any of" wildcard

convention shall not be used.

The RestartMethod parameter specified the type of restart. Three

values have been defined:

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

will Be taken out of service after the specified delay. The

established connections are not yet affected, but the Call Agent

should refrain to establish new connections, and should try to

gracefully tear down the existing connections.

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

are taken abruptely out of service. The established connections,

if any, are lost.

* A "restart" method indicates that service will be restored on the

endpoints after the specified "restart delay." There are no

connections that are currently established on the endpoints.

* A "disconnected" method indicates that the endpoint has become

disconnected and is now trying to establish connectivity. The

"restart delay" specifies the number of seconds the endpoint has

been disconnected. Established connections are not affected.

* A "cancel-graceful" method indicates that a gateway is canceling a

previously issued "graceful" restart command.

The optional "restart delay" parameter is expressed as a number of

seconds. If the number is absent, the delay value should be

considered null. In the case of the "graceful" method, a null delay

indicates that the call agent should simply wait for the natural

termination of the existing connections, without establishing new

connections. The restart delay is always considered null in the case

of the "forced" method.

A restart delay of null for the "restart" method indicates that

service has already been restored. This typically will occur after

gateway startup/reboot.

The optional reason code parameter the cause of the restart.

Gateways SHOULD send a "graceful" or "forced" RestartInProgress

message as a courtesy to the Call Agent when they are taken out of

service, e.g., by being shutdown, or taken out of service by a

network management system, although the Call Agent cannot rely on

always receiving such messages. Gateways MUST send a "restart"

RestartInProgress message with a null delay to their Call Agent when

they are back in service according to the restart procedure specified

in Section 4.3.4 - Call Agents can rely on receiving this message.

Also, gateways MUST send a "disconnected" RestartInProgress message

to their current "notified entity" according to the "disconnected"

procedure specified in Section 4.3.5. The "restart delay" parameter

MUST NOT be used with the "forced" restart method.

The RestartInProgress message will be sent to the current notified

entity for the EndpointId in question. It is expected that a default

Call Agent, i.e., notified entity, has been provisioned for each

endpoint so, after a reboot, the default Call Agent will be the

notified entity for each endpoint. Gateways should take full

advantage of wild- carding to minimize the number of

RestartInProgress messages generated when multiple endpoints in a

gateway restart and the endpoints are managed by the same Call Agent.

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

outcome of the command and consists of an integer number optionally

followed by commentary.

A NotifiedEntity may additionally be returned with the response from

the Call Agent:

* If the response indicated success (return code 200 - transaction

executed), the restart procedure has completed, and the

NotifiedEntity returned is the new "notified entity" for the

endpoint(s).

* If the response from the Call Agent indicated an error, the

restart procedure is not yet complete, and must therefore be

initiated again. If a NotifiedEntity parameter was returned, it

then specifies the new "notified entity" for the endpoint(s),

which must consequently be used when retrying the restart

procedure.

2.4. Return codes and error codes.

All MGCP commands are acknowledged. The acknowledgment carries a

return code, which indicates the status of the command. The return

code is an integer number, for which four ranges of values have been

defined:

* values between 100 and 199 indicate a provisional response,

* values between 200 and 299 indicate a successful completion,

* values between 400 and 499 indicate a transient error,

* values between 500 and 599 indicate a permanent error.

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

list:

100 The transaction is currently being executed. An actual

completion message will follow on later.

200 The requested transaction was executed normally.

250 The connection was deleted.

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

401 The phone is already off hook

402 The phone is already on hook

403 The transaction could not be executed, because the endpoint does

not have sufficient resources at this time

404 Insufficient bandwidth at this time

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

unknown.

01 The transaction could not be executed, because the endpoint is

not ready.

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

not have sufficient resources

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

was detected.

11 The transaction could not be executed, because the command

contained an unrecognized extension.

512 The transaction could not be executed, because the gateway is

not equipped to detect one of the requested events.

513 The transaction could not be executed, because the gateway is

not equipped to generate one of the requested signals.

514 The transaction could not be executed, because the gateway

cannot send the specified announcement.

515 The transaction refers to an incorrect connection-id (may have

been already deleted)

516 The transaction refers to an unknown call-id.

517 Unsupported or invalid mode.

518 Unsupported or unknown package.

519 Endpoint does not have a digit map.

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

"restarting".

521 Endpoint redirected to another Call Agent.

522 No such event or signal.

523 Unknown action or illegal combination of actions

524 Internal inconsistency in LocalConnectionOptions

525 Unknown extension in LocalConnectionOptions

526 Insufficient bandwidth

527 Missing RemoteConnectionDescriptor

528 Incompatible protocol version

529 Internal hardware failure

530 CAS signaling protocol error.

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

2.5. Reason Codes

Reason-codes are used by the gateway when deleting a connection to

inform the Call Agent about the reason for deleting the connection.

They may also be used in a RestartInProgress command, to inform the

gateway of the Restart's reason. The reason code is an integer

number, and the following values have been defined:

000 Endpoint state is nominal. (This code is used only in response

to audit requests.)

900 Endpoint malfunctioning

901 Endpoint taken out of service

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

3. Media Gateway Control Protocol

The MGCP implements the media gateway control interface as a set of

transactions. The transactions are composed of a command and a

mandatory response. There are eight types of command:

* CreateConnection

* ModifyConnection

* DeleteConnection

* NotificationRequest

* Notify

* AuditEndpoint

* AuditConnection

* RestartInProgress

The first four commands are sent by the Call Agent to a gateway. The

Notify command is sent by the gateway to the Call Agent. The gateway

may also send a DeleteConnection as defined in 2.3.6. The Call Agent

may send either of the Audit commands to the gateway. The Gateway

may send a RestartInProgress command to the Call Agent.

3.1. General description

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

a session description.

All responses are composed of a Response header, optionally followed

by a session description.

Headers and session descriptions are encoded as a set of text lines,

separated by a carriage return and line feed character (or,

optionnally, a single line-feed character). The headers are separated

from the session description by an empty line.

MGCP uses a transaction identifier to correlate commands and

responses. The transaction identifier is encoded as a component of

the command header and repeated as a component of the response header

(see section 3.2.1, 3.2.1.2 and 3.3).

3.2. Command Header

The command header is composed of:

* A command line, identifying the requested action or verb, the

transaction identifier, the endpoint towards which the action is

requested, and the MGCP protocol version,

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

a parameter value.

Unless otherwise noted or dictated by other referenced standards,

each component in the command header is case insensitive. This goes

for verbs as well as parameters and values, and all comparisons MUST

treat upper and lower case as well as combinations of these as being

equal.

3.2.1. Command line

The command line is composed of:

* The name of the requested verb,

* The identification of the transaction,

* The name of the endpoint that should execute the command (in

notifications or restarts, the name of the endpoint that is

issuing the command),

* The protocol version.

These four items are encoded as strings of printable ASCII

characters, separated by white spaces, i.e. the ASCII space (0x20) or

tabulation (0x09) characters. It is recommended to use exactly one

ASCII space separator.

3.2.1.1. Coding of the requested verb

The verbs that can be requested are encoded as four letter upper or

lower case ASCII codes (comparisons should be case insensitive) as

defined in the following table:

______________________________

Verb Code

____________________________

EndpointConfiguration EPCF

CreateConnection CRCX

ModifyConnection MDCX

DeleteConnection DLCX

NotificationRequest RQNT

Notify NTFY

AuditEndpoint AUEP

AuditConnection AUCX

RestartInProgress RSIP

____________________________

The transaction identifier is encoded as a string of up to 9 decimal

digits. In the command lines, it immediately follows the coding of

the verb.

New verbs may be defined in further versions of the protocol. It may

be necessary, for experimentation purposes, to use new verbs before

they are sanctioned in a published version of this protocol.

Experimental verbs should be identified by a four letter code

starting with the letter X, such as for example XPER.

3.2.1.2. Transaction Identifiers

MGCP uses a transaction identifier to correlate commands and

responses. A gateway supports two separate transaction identifier

name spaces:

a transaction identifier name space for sending transactions, and

a transaction identifier name space for receiving transactions.

At a minimum, transaction identifiers for commands sent to a given

gateway MUST be unique for the maximum lifetime of the transactions

within the collection of Call Agents that control that gateway. Thus,

regardless of the sending Call Agent, gateways can always detect

duplicate transactions by simply examining the transaction

identifier. The coordination of these transaction identifiers between

Call Agents is outside the scope of this specification though.

Transaction identifiers for all commands sent from a given gateway

MUST be unique for the maximum lifetime of the transactions

regardless of which Call Agent the command is sent to. Thus, a Call

Agent can always detect a duplicate transaction from a gateway by the

combination of the domain-name of the endpoint and the transaction

identifier.

The transaction identifier is encoded as a string of up to nine

decimal digits. In the command lines, it immediately follows the

coding of the verb.

Transaction identifiers have values between 1 and 999999999. An MGCP

entity MUST NOT reuse a transaction identifier more quickly than

three minutes after completion of the previous command in which the

identifier was used.

3.2.1.3. Coding of the endpoint identifiers and entity names

The endpoint identifiers and entity names are encoded as case

insensitive e-mail addresses, as defined in RFC821. In these

addresses, the domain name identifies the system where the endpoint

is attached, while the left side identifies a specific endpoint on

that system.

Examples of such addresses can be:

______________________________________________________________________

hrd4/56@gw23.example.net Circuit number 56 in

interface "hrd4" of the Gateway 23

of the "Example" network

Call-agent@ca.example.net Call Agent for the

"example" network

Busy-signal@ann12.example.net The "busy signal" virtual

endpoint in the announcement

server number 12.

____________________________________________________________________

The name of notified entities is expressed with the same syntax, with

the possible addition of a port number as in:

Call-agent@ca.example.net:5234

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

be used.

3.2.1.4. Coding of the protocol version

The protocol version is coded as the key word MGCP followed by a

white space and the version number, and optionally followed by a

profile name.. The version number is composed of a major version,

coded by a decimal number, a dot, and a minor version number, coded

as a decimal number. The version described in this document is

version 1.0.

The profile name, if present, is represented by a white-space

separated strings of visible (printable) characters extending to the

end of the line. Profile names may be defined for user communities

who want to apply restrictions or other profiling to MGCP.

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

MGCP 1.0

3.2.2. Parameter lines

Parameter lines are composed of a parameter name, which in most cases

is composed of a single upper case character, followed by a colon, a

white space and the parameter value. The parameter that can be

present in commands are defined in the following table:

_______________________________________________________________________

Parameter name Code Parameter value

___________________________________________________________________

ResponseAck K see description

BearerInformation B see description

CallId C Hexadecimal string, at most 32 chars.

ConnectionId I Hexadecimal string, at most 32 chars.

NotifiedEntity N An identifier, in RFC821 format,

composed of an arbitrary string and

of the domain name of the requesting

entity, possibly completed by a port

number, as in:

Call-agent@ca.example.net:5234

RequestIdentifier X Hexadecimal string, at most 32 chars.

LocalConnectionOptions L See description

Connection Mode M See description

RequestedEvents R See description

SignalRequests S See description

DigitMap D A text encoding of a digit map

ObservedEvents O See description

ConnectionParameters P See description

ReasonCode E An arbitrary character string

SpecificEndpointID Z An identifier, in RFC821 format,

composed of an arbitrary string,

followed by an "@" followed by the

domain name of the gateway to which

this endpoint is attached.

Second Endpoint ID Z2 Endpoint Id.

SecondConnectionId I2 Connection Id.

RequestedInfo F See description

QuarantineHandling Q See description

DetectEvents T See Description

RestartMethod RM See description

RestartDelay RD A number of seconds, encoded as

a decimal number

EventStates ES See description

Capabilities A See description

___________________________________________________________________

RemoteConnection RC Session Description

Descriptor

LocalConnection LC Session Description

Descriptor

___________________________________________________________________

The parameters are not necessarily present in all commands. The

following table provides the association between parameters and

commands. The letter M stands for mandatory, O for optional and F for

forbidden.

___________________________________________________________________

Parameter name EP CR MD DL RQ NT AU AU RS

CF CX CX CX NT FY EP CX IP

_________________________________________________________

ResponseAck O O O O O O O O O

BearerInformation M O O O O F F F F

CallId F M M O F F F F F

ConnectionId F F M O F F F M F

RequestIdentifier F O+ O+ O+ M M F F F

LocalConnection F O O F F F F F F

Options

Connection Mode F M M F F F F F F

RequestedEvents F O O O O* F F F F

SignalRequests F O O O O* F F F F

NotifiedEntity F O O O O O F F F

ReasonCode F F F O F F F F O

ObservedEvents F F F F F M F F F

DigitMap F O O O O F F F F

Connection F F F O F F F F F

parameters

Specific Endpoint ID F F F F F F F F F

Second Endpoint ID F O F F F F F F F

RequestedInfo F F F F F F M M F

QuarantineHandling F O O O O F F F F

DetectEvents F O O O O F F F F

EventStates F F F F F F F F F

RestartMethod F F F F F F F F M

RestartDelay F F F F F F F F O

SecondConnectionID F F F F F F F F F

Capabilities F F F F F F F F F

_________________________________________________________

RemoteConnection F O O F F F F F F

Descriptor

LocalConnection F F F F F F F F F

Descriptor

_________________________________________________________

Note (+) that the RequestIdentifier parameter is optional in

connection creation, modification and deletion commands, but that it

becomes mandatory if the command contains an encapsulated

notification request.

Note (*) that the RequestedEvents and SignalRequests parameters are

optional in the NotificationRequest. If these parameters are omitted,

the corresponding lists will be considered empty.

If implementers need to experiment with new parameters, for example

when developing a new application of MGCP, they should identify these

parameters by names that start with the string "X-" or "X+", such as

for example:

X-FlowerOfTheDay: Daisy

Parameter names that start with "X+" are critical parameter

extensions. An MGCP entity that receives a critical parameter

extension that it cannot understand should refuse to execute the

command. It should respond with an error code 511 (Unrecognized

extension).

Parameter names that start with "X-" are non critical parameter

extensions. An MGCP entity that receives a non critical parameter

extension that it cannot understand can safely ignore that parameter.

3.2.2.1. Response Acknowledgement

The response acknowledgement attribute is used to managed the "at-

most-once" facility described in the "transmission over UDP" section.

It contains a comma separated list of "confirmed transaction-id

ranges".

Each "confirmed transaction-id ranges" is composed of either one

decimal number, when the range includes exactly one transaction, or

two decimal numbers separated by a single hyphen, describing the

lower and higher transaction identifiers included in the range.

An example of response acknowledgement is:

K: 6234-6255, 6257, 19030-19044

3.2.2.2. Local connection options

The local connection options describe the operational parameters that

the Call Agent suggests to the gateway. These parameters are:

* The packetization period in milliseconds, encoded as the keyword

"p", followed by a colon and a decimal number. If the Call Agent

specifies a range of values, the range will be specified as two

decimal numbers separated by an hyphen.

* The preferred type of compression algorithm, encoded as the

keyword "a", followed by a colon and a character string. If the

Call Agent specifies a list of values, these values will be

separated by a semicolon.

* The bandwidth in kilobits per second (1000 bits per second),

encoded as the keyword "b", followed by a colon and a decimal

number. If the Call Agent specifies a range of values, the range

will be specified as two decimal numbers separated by an hyphen.

* The echo cancellation parameter, encoded as the keyword "e",

followed by a colon and the value "on" or "off".

* The gain control parameter, encoded as the keyword "gc", followed

by a colon a value which can be either the keyword "auto" or a

decimal number (positive or negative) representing the number of

decibels of gain.

* The silence suppression parameter, encoded as the keyword "s",

followed by a colon and the value "on" or "off".

* The type of service parameter, encoded as the keyword "t",

followed by a colon and the value encoded as two hexadecimal

digits.

* The resource reservation parameter, encoded as the keyword "r",

followed by a colon and the value "g" (guaranteed service), "cl"

(controlled load) or "be" (best effort).

* The encryption key, encoded as the keyword "k" followed by a colon

and a key specification, as defined for the parameter "K" of SDP

(RFC2327).

* The type of network, encoded as the keyword "nt" followed by a

colon and the type of network encoded as the keyword "IN", "ATM"

or "LOCAL".

Each of the parameters is optional. When several parameters are

present, the values are separated by a comma.

Examples of connection descriptors are:

L: p:10, a:PCMU

L: p:10, a:G726-32

L: p:10-20, b:64

L: b:32-64, e:off

These set of attributes may be extended by extension attributes.

Extension attributes are composed of an attribute name, followed by a

semi-colon and by an attribute value. The attribute name should start

by the two characters "x+", for a mandatory extensions, or "x-", for

a non mandatory extension. If a gateway receives a mandatory

extension attribute that it does not recognize, it should reject the

command with an error code 525 (Unknown extension in

LocalConnectionOptions).

3.2.2.3. Capabilities

Capabilities inform the Call Agent about endpoints' capabilities when

audited. The encoding of capabilities is based on the Local

Connection Options encoding for the parameters that are common to

both. In addition, capabilities can also contain a list of supported

packages, and a list of supported modes.

The parameters used are:

*

A list of supported codecs. The following parameters will apply to

all codecs specified in this list. If there is a need to specify

that some parameters, such as e.g. silence suppression, are only

compatible with some codecs, then the gateway will return several

LocalConnectionOptions parameters, one for each set of codecs.

Packetization Period:

A range may be specified.

Bandwidth:

A range corresponding to the range for packetization periods may

be specified (assuming no silence suppression). If absent, the

values will be deduced from the codec type.

Echo Cancellation:

"on" if echo cancellation is supported for this codec, "off"

otherwise. The default is support.

Silence Suppression:

"on" if silence suppression is supported for this codec, "off"

otherwise. The default is support.

Gain Control:

"0" if gain control is not supported. The default is support.

Type of Service:

The value "0" indicates no support for type of service, all other

values indicate support for type of service. The default is

support.

Resource Reservation:

The parameter indicates the reservation services that are

supported, in addition to best effort. The value "g" is encoded

when the gateway supports both the guaranteed and the controlled

load service, "cl" when only the controlled load service is

supported. The default is "best effort."

Encryption Key:

Encoding any value indicates support for encryption. Default is

no support.

Type of network:

The keyword "nt", followed by a colon and a semicolon separated

list of supported network types. This parameter is optional.

Event Packages

The event packages supported by this endpoint encoded as the

keyword "v", followed by a colon and a character string. If a list

of values is specified, these values will be separated by a

semicolon. The first value specified will be the default package

for that endpoint.

Modes

The modes supported by this endpoint encoded as the keyword "m",

followed by a colon and a semicolon-separated list of supported

connection modes for this endpoint.

3.2.2.4. Connection parameters

Connection parameters are encoded as a string of type and value

pairs, where the type is a either letter identifier of the parameter

or an extension type, and the value a decimal integer. Types are

separated from value by an `=' sign. Parameters are encoded from each

other by a comma.

The connection parameter types are specified in the following table:

__________________________________________________________________

Connection parameter Code Connection parameter

name value

_______________________________________________________________

Packets sent PS The number of packets that

were sent on the connection.

Octets sent OS The number of octets that

were sent on the connection.

Packets received PR The number of packets that

were received on the connection.

Octets received OR The number of octets that

were received on the connection.

Packets lost PL The number of packets that

were not received on the

connection, as deduced from

gaps in the sequence number.

Jitter JI The average inter-packet arrival

jitter, in milliseconds,

expressed as an integer number.

Latency LA Average latency, in milliseconds,

expressed as an integer number.

_______________________________________________________________

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

by a two letters extension parameter name. Call agents that received

unrecognized extensions shall silently ignore these extensions.

An example of connection parameter encoding is:

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

3.2.2.5. Reason Codes

Reason codes are three-digit numeric values. The reason code is

optionally followed by a white space and commentary, e.g.:

900 Endpoint malfunctioning

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

3.2.2.6. Connection mode

The connection mode describes the mode of operation of the

connection. The possible values are:

________________________________________________________

Mode Meaning

______________________________________________________

M: sendonly The gateway should only send packets

M: recvonly The gateway should only receive packets

M: sendrecv The gateway should send

and receive packets

M: confrnce The gateway should place

the connection in conference mode

M: inactive The gateway should neither

send nor receive packets

M: loopback The gateway should place

the circuit in loopback mode.

M: conttest The gateway should place

the circuit in test mode.

M: netwloop The gateway should place

the connection in network loopback mode.

M: netwtest The gateway should place

the connection in network

continuity test mode.

M: data The gateway should use the circuit

for network access for data

(e.g., PPP, SLIP, etc.).

______________________________________________________

3.2.2.7. Coding of event names

Event names are composed of an optional package name, separated by a

slash (/) from the name of the actual event. The event name can

optionally be followed by an at sign (@) and the identifier of a

connection on which the event should be observed. Event names are

used in the RequestedEvents, SignalRequests and ObservedEvents

parameter.

Each signal has one of the following signal-types associated with:

On/Off (OO), Time-out (TO), Brief (BR). (These signal types are

specified in the package definitions, and are not present in the

messages.) On/Off signals can be parameterized with a "+" to turn

the signal on, or a "-" to turn the signal off. If an on/off signal

is not parameterized, the signal is turned on. Both of the following

will turn the vmwi signal on:

vmwi(+), vmwi

The following are valid examples of event names:

____________________________________________________________

L/hu on-hook transition, in the line package

F/0 digit 0 in the MF package

fh Flash-hook, assuming that the line package

is a default package for the end point.

G/rt@0A3F58 Ring back signal on

connection "0A3F58".

__________________________________________________________

In addition, the range and wildcard notation of events can be used,

instead of individual names, in the RequestedEvents and DetectEvents

parameters. The star sign can be used to denote "all connections",

and the dollar sign can be used to denote the "current" connection.

The following are valid examples of such notations:

__________________________________________________________

M/[0-9] Digits 0 to 9 in the MF package

fh Flash-hook, assuming that the line package

is a default package for the end point.

[0-9*#A-D] All digits and letters in the DTMF

packages (default for endpoint).

T/$ All events in the trunk packages.

R/qa@* The quality alert event in all

connections

R/rt@$ Ringback on current connection

________________________________________________________

3.2.2.8. RequestedEvents

The RequestedEvent parameter provides the list of events that have

been requested. The event codes are described in the previous

section.

Each event can be qualified by a requested action, or by a list of

actions. The actions, when specified, are encoded as a list of

keywords, enclosed in parenthesis and separated by commas. The codes

for the various actions are:

______________________________________

Action Code

____________________________________

Notify immediately N

Accumulate A

Treat according to digit map D

Swap S

Ignore I

Keep Signal(s) active K

Embedded Notification Request E

____________________________________

When no action is specified, the default action is to notify the

event. This means that, for example, ft and ft(N) are equivalent.

Events that are not listed are ignored.

The digit-map action can only be specified for the digits, letters

and interdigit timers in the MF and DTMF packages, or in other

packages that would define the encoding of digits and timers.

The requested list is encoded on a single line, with event/action

groups separated by commas. Examples of RequestedEvents encoding are:

R: hu(N), hf(S,N)

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

In the case of the "enable" action, the embedded notification request

parameters are encoded as a list of up to three parameter groups,

separated by commas. Each group start by a one letter identifier,

followed by a list of parameters enclosed between parenthesis. The

first optional parameter group, identified by the letter "R", is the

enabled value of the RequestedEvents parameter. The second optional

group, identified by the letter "S", is the enabled value of the

SignalRequests parameter. The third optional group, identified by

the letter "D", is the enabled value of the DigitMap. (Note that some

existing implementation may encode these three components in a

different order.)

If the RequestedEvents is not present, the parameter will be set to a

null value. If the SignalRequest is not present, the parameter will

be set to a null value. If the DigitMap is absent, the current value

should be used. The following are valid examples of embedded

requests:

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

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

3.2.2.9. SignalRequests

The SignalRequests parameter provides the name of the signals that

have been requested. Each signal is identified by a name, as

indicated in the previous section.

Several signals, such as for example announcement or ADSI display,

can be qualified by additional parameters:

* the name and parameters of the announcement,

* the string that should be displayed.

These parameters will be encoded as a set of UTF8 character strings,

spearated by comams and enclosed within parenthesis, as in:

S: adsi("123456 Francois Gerard")

S: ann(no-such-number, 1234567)

When several signals are requested, their codes are separated by a

comma, as in:

S: asdi(123456 Your friend), rg

3.2.2.10. ObservedEvent

The observed event parameters provides the list of events that have

been observed. The event codes are the same as those used in the

NotificationRequest. Events that have been accumulated according to

the digit map may be grouped in a single string; they should be

reported as lists of isolated events if other events where detected

during the digit accumulation. Examples of observed actions are:

O: L/hu

O: 8295555T

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

O: L/hf, L/hf, L/hu

3.2.2.11. RequestedInfo

The RequestedInfo parameter contains a comma separated list of

parameter codes, as defined in the "Parameter lines" section. For

example, if one wants to audit the value of the NotifiedEntity,

RequestIdentifier, RequestedEvents, SignalRequests, DigitMap,

QuarantineHandling and DetectEvents parameters, The value of the

RequestedInfo parameter will be:

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

The capabilities request, in the AuditEndPoint command, is encoded by

the keyword "A", as in:

F:A

3.2.2.12. QuarantineHandling

The quarantine handling parameter contains a list of comma separated

keywords:

* The keyword "process" or "discard" to indicate the treatment of

quarantined events. If neither process or discard is present,

process is assumed.

* The keyword "step" or "loop" to indicate whether exactly at most

one notification is expected, or whether multiple notifications

are allowed. If neither step or loop is present, step is assumed.

The following values are valid examples:

Q:loop

Q:process

Q:discard,loop

3.2.2.13. DetectEvents

The DetectEvent parameter is encoded as a comma separated list of

events, such as for example:

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

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

events.

3.2.2.14. EventStates

The EventStates parameter is encoded as a comma separated list of

events, such as for example:

ES: hu

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

events.

3.2.2.15. RestartMethod

The RestartMethod parameter is encoded as one of the keywords

"graceful", "forced", "restart", "disconnected" or "cancel-graceful"

as for example:

RM:restart

3.2.2.16. Bearer Information

The values of the bearer informations are encoded as a comma

separated list of attributes, represented by an attribute name,

separated by a colon from an attribute value.

The only attribute that is defined is the "encoding" (code "e"),

whose defined values are "A" (A-law) and "mu" (mu-law).

An example of bearer information encoding is:

B: e:mu

3.3. Format of response headers

The response header is composed of a response line, optionally

followed by headers that encode the response parameters.

An example of response header could be:

200 1203 OK

The response line starts with the response code, which is a three

digit numeric value. The code is followed by a white space, the

transaction identifier, and an optional commentary preceded by a

white space.

The following table describe the parameters whose presence is

mandatory or optional in a response header, as a function of the

command that triggered the response. The letter M stands for

mandatory, O for optional and F for forbidden.

___________________________________________________________________

Parameter name EP CR MD DL RQ NT AU AU RS

CF CX CX CX NT FY EP CX IP

_________________________________________________________

ResponseAck F F F F F F F F F

BearerInformation F F F F F F O F F

CallId F F F F F F F O F

ConnectionId F O* F F F F F F F

RequestIdentifier F F F F F F O F F

LocalConnection F F F F F F O O F

Options

Connection Mode F F F F F F F O F

RequestedEvents F F F F F F O F F

SignalRequests F F F F F F O F F

NotifiedEntity F F F F F F F F O

ReasonCode F F F F F F O F F

ObservedEvents F F F F F F O F F

DigitMap F F F F F F O F F

Connection F F F O F F F O F

Parameters

Specific Endpoint ID F O F F F F F F F

RequestedInfo F F F F F F F F F

QuarantineHandling F F F F F F O F F

DetectEvents F F F F F F O F F

EventStates F F F F F F O F F

RestartMethod F F F F F F O F F

RestartDelay F F F F F F O F F

Capabilities F F F F F F O F F

SecondConnectionId F O F F F F F F F

SecondEndpointID F O F F F F F F F

_________________________________________________________

LocalConnection F M O F F F F O* F

Descriptor

RemoteConnection F F F F F F F O* F

Descriptor

_________________________________________________________

In the case of a CreateConnection message, the response line is

followed by a Connection-Id parameter. It may also be followed a

Specific-Endpoint-Id parameter, if the creation request was sent to a

wildcarded Endpoint-Id. The connection-Id parameter is marked as

optional in the Table. In fact, it is mandatory with all positive

responses, when a connection was created, and forbidden when the

response is negative, when no connection as created.

In the case of a DeleteConnection message, the response line is

followed by a Connection Parameters parameter, as defined in section

3.2.2.2.

A LocalConnectionDescriptor should be transmitted with a positive

response (code 200) to a CreateConnection. It may be transmitted in

response to a ModifyConnection command, if the modification resulted

in a modification of the session parameters. The

LocalConnectionDescriptor is encoded as a "session description," as

defined in section 3.4. It is separated from the response header by

an empty line.

When several session descriptors are encoded in the same response,

they are encoded one after each other, separated by an empty line.

This is the case for example when the response to an audit connection

request carries both a local session description and a remote session

description, as in:

200 1203 OK

C: A3C47F21456789F0

N: [128.96.41.12]

L: p:10, a:PCMU;G726-32

M: sendrecv

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

v=0

c=IN IP4 128.96.41.1

m=audio 1296 RTP/AVP 0

v=0

c=IN IP4 128.96.63.25

m=audio 1296 RTP/AVP 0 96

a=rtpmap:96 G726-32/8000

In this example, according to the SDP syntax, each description starts

with a "version" line, (v=...). The local description is always

transmitted before the remote description. If a connection descriptor

is requested, but it does not exist for the connection audited, that

connection descriptor will appear with the SDP protocol version field

only.

3.4. Formal syntax description of the protocol

In this section, we provided a formal description of the protocol

syntax, following the "Augmented BNF for Syntax Specifications"

defined in RFC2234.

MGCPMessage = MGCPCommand / MGCPResponse

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

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

<endpointName> 1*(WSP) MGCPversion EOL

MGCPVerb = "EPCF" / "CRCX" / "MDCX" / "DLCX" / "RQNT"

/ "NTFY" / "AUEP" / "AUCX" / "RSIP" / extensionVerb

extensionVerb = "X" 3(ALPHA / DIGIT)

transaction-id = 1*9(DIGIT)

endpointName = localEndpointName "@" DomainName

LocalEndpointName = LocalNamePart 0*("/" LocalNamePart)

LocalNamePart = AnyName / AllName / NameString

AnyName = "$"

AllNames = "*"

NameString = 1*(range-of-allowed-characters)

DomainName = 1*256(ALPHA / DIGIT / "." / "-") ; as defined in RFC821

MGCPversion = "MGCP" 1*(WSP) 1*(DIGIT) "." 1*(DIGIT)

[1*(WSP) ProfileName]

ProfileName = 1*(range-of-allowed-characters)

MGCPParameter = ParameterValue EOL

ParameterValue = ("K" ":" 0*WSP <ResponseAck>) /

("B" ":" 0*WSP <BearerInformation>) /

("C" ":" 0*WSP <CallId>) /

("I" ":" 0*WSP <ConnectionId>) /

("N" ":" 0*WSP <NotifiedEntity>) /

("X" ":" 0*WSP <RequestIdentifier>) /

("L" ":" 0*WSP <LocalConnectionOptions>) /

("M" ":" 0*WSP <ConnectionMode>) /

("R" ":" 0*WSP <RequestedEvents>) /

("S" ":" 0*WSP <SignalRequests>) /

("D" ":" 0*WSP <DigitMap>) /

("O" ":" 0*WSP <ObservedEvents>) /

("P" ":" 0*WSP <ConnectionParameters>) /

("E" ":" 0*WSP <ReasonCode>) /

("Z" ":" 0*WSP <SpecificEndpointID>) /

("Z2" ":" 0*WSP <SecondEndpointID>) /

("I2" ":" 0*WSP <SecondConnectionID>) /

("F" ":" 0*WSP <RequestedInfo>) /

("Q" ":" 0*WSP <QuarantineHandling>) /

("T" ":" 0*WSP <DetectEvents>) /

("RM" ":" 0*WSP <RestartMethod>) /

("RD" ":" 0*WSP <RestartDelay>) /

("A" ":" 0*WSP <Capabilities>) /

("ES" ":" 0*WSP <EventStates>) /

(extensionParameter ":" 0*WSP <parameterString>)

ResponseAck = confirmedTransactionIdRange

*[ "," confirmedTransactionIdRange ]

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

BearerInformation = BearerAttribute 0*("," 0*WSP BearerAttribute)

BearerAttribute = ("e" ":" <BearerEncoding>)

BearerEncoding = "A" / "mu"

CallId = 1*32(HEXDIG)

// The audit request response may include a list of identifiers

ConnectionId = 1*32(HEXDIG) 0*("," 1*32(HEXDIG))

SecondConnectionID = ConnectionId

NotifiedEntity = [LocalName "@"] DomainName [":" portNumber]

LocalName = 1*32(suitableCharacter)

portNumber = 1*5(DIGIT)

RequestIdentifier = 1*32(HEXDIG)

LocalConnectionOptions = [ LocalOptionValue 0*(WSP)

0*("," 0*(WSP) LocalOptionValue 0*(WSP)) ]

LocalOptionValue = ("p" ":" <packetizationPeriod> )

/ ("a" ":" <compressionAlgorithm> )

/ ("b" ":" <bandwidth> )

/ ("e" ":" <echoCancellation> )

/ ("gc" ":" <gainControl> )

/ ("s" ":" <silenceSuppression> )

/ ("t" ":" <typeOfService> )

/ ("r" ":" <resourceReservation> )

/ ("k" ":" <encryptionmethod>[":"<encryptionKey>])

/ ("nt" ":" <typeOfNetwork> )

/ (localOptionExtensionName ":"

/ localOptionExtensionValue)

Capabilities = [ CapabilityValue 0*(WSP)

0*("," 0*(WSP) CapabilityValue 0*(WSP)) ]

CapabilityValue = LocalOptionValue

/ ("v" ":" <supportedPackages>)

/ ("m" ":" <supportedModes> )

packetizationPeriod = 1*4(DIGIT)["-" 1*4(DIGIT)]

compressionAlgorithm = algorithmName 0*(";" algorithmName)

algorithmName = 1*32(SuitableCharacter)

bandwidth = 1*4(DIGIT)["-" 1*4(DIGIT)]

echoCancellation = "on" / "off"

gainControl = "auto" / ["-"]1*4(DIGIT)

silenceSuppression = "on" / "off"

typeOfService = 2HEXDIG

resourceReservation = "g" / "cl" / "be"

;encryption parameters are coded as in SDP (RFC2327)

encryptiondata = ( "clear" ":" <encryptionKey> )

/ ( "base64" ":" <encodedEncryptionKey> )

/ ( "uri" ":" <URItoObtainKey> )

/ ( "prompt" ) ; defined in SDP, not usable in MGCP!

encryptionKey = 1*(SuitableCharacter / SP)

encodedEncryptionKey = 1*(ALPHA / DIGIT / "+" / "/" / "=")

URItoObtainKey = 1*(SuitableCharacter) / quotedString

typeOfNetwork = "IN" / "ATM" / "LOCAL"

supportedModes= ConnectionMode 0*(";" ConnectionMode)

supportedPackages = packageName 0*(";" packageName)

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

localOptionExtensionValue = 1*32(SuitableCharacter) / quotedString

ConnectionMode = "sendonly" / "recvonly" / "sendrecv" /

"confrnce" / "inactive" / "loopback" /

"conttest" / "netwloop" / "netwtest" / "data"

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

requestedEvent = eventName [ "(" requestedActions ")" ]

eventName = [ (packageName / "*") "/" ] (eventId / "all" / eventRange)

[ "@" (ConnectionId / "$" / "*") ]

packageName = 1*(ALPHA / DIGIT / HYPHEN)

eventId = 1*(SuitableCharacter)

eventRange = "[" 1*(DIGIT / DTMFLetter / "*" / "#" /

(DIGIT "-" DIGIT)/(DTMFLetter "-"

DTMFLetter)) "]"

requestedActions = requestedAction 0*("," 0*(WSP) requestedAction)

requestedAction = "N" / "A" / "D" / "S" / "I" / "K" /

"E" "(" EmbeddedRequest ")"

EmbeddedRequest = ( "R" "(" EmbeddedRequestList ")"

["," "S" "(" EmbeddedSignalRequest ")" ]

["," "D" "(" EmbeddedDigitMap ")" ] )

/ ( "S" "(" EmbeddedSignalRequest ")"

["," "D" "(" EmbeddedDigitMap ")" ] )

/ ( "D" "(" EmbeddedDigitMap ")" )

EmbeddedRequestList = RequestedEvents

EmbeddedSignalRequest = SignalRequests

EmbeddedDigitMap = DigitMap

SignalRequests = [ SignalRequest 0*("," 0*(WSP) SignalRequest ) ]

SignalRequest = eventName [ "(" eventParameters ")" ]

eventParameters = eventParameter 0*("," 0*(WSP) eventParameter)

eventParameter = eventParameterString / quotedString

eventParameterString = 1*(SuitableCharacter)

DigitMap = DigitString / "(" DigitStringList ")"

DigitStringList = DigitString 0*( "" DigitString )

DigitString = 1*(DigitStringElement)

DigitStringElement = DigitPosition ["."]

DigitPosition = DigitMapLetter / DigitMapRange

DigitMapLetter = DIGIT / "#" / "*" / "A" / "B" / "C" / "D" / "T"

DigitMapRange = "x" / "[" 1*DigitLetter "]"

DigitLetter ::= *((DIGIT "-" DIGIT ) / DigitMapLetter)

ObservedEvents = SignalRequests

EventStates = SignalRequests

ConnectionParameters = [ConnectionParameter

0*( "," 0*(WSP) ConnectionParameter )

ConnectionParameter = ( "PS" "=" packetsSent )

/ ( "OS" "=" octetsSent )

/ ( "PR" "=" packetsReceived )

/ ( "OR" "=" octetsReceived )

/ ( "PL" "=" packetsLost )

/ ( "JI" "=" jitter )

/ ( "LA" "=" averageLatency )

/ ( ConnectionParameterExtensionName "="

ConnectionParameterExtensionValue )

packetsSent = 1*9(DIGIT)

octetsSent = 1*9(DIGIT)

packetsReceived = 1*9(DIGIT)

octetsReceived = 1*9(DIGIT)

packetsLost = 1*9(DIGIT)

jitter = 1*9(DIGIT)

averageLatency = 1*9(DIGIT)

ConnectionParameterExtensionName = "X" "-" 2*ALPHA

ConnectionParameterExtensionValue = 1*9(DIGIT)

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

SpecificEndpointID = endpointName

SecondEndpointID = endpointName

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

infoCode = "B" / "C" / "I" / "N" / "X" / "L" / "M" /

"R" / "S" / "D" / "O" / "P" / "E" / "Z" /

"Q" / "T" / "RC" / "LC" / "A" / "ES" / "RM" / "RD"

QuarantineHandling = loopControl / processControl /

(loopControl "," processControl )

loopControl = "step" / "loop"

processControl = "process" / "discard"

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

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

RestartDelay = 1*6(DIGIT)

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

parameterString = 1*(%x20-7F)

MGCPResponse = MGCPResponseLine 0*(MGCPParameter)

[EOL *SDPinformation]

MGCPResponseLine = (<responseCode> 1*(WSP) <transaction-id>

[1*(WSP) <responseString>] EOL)

responseCode = 3DIGIT

responseString = *(%x20-7E)

SuitableCharacter= DIGIT / ALPHA / "+" / "-" / "_" / "&" /

"!" / "'" / "" / "=" / "#" / "?" / "/" /

"." / "$" / "*" / ";" / "@" / "[" / "]" /

"^" / "`" / "{" / "}" / "~"

quotedString = DQUOTE visibleString

0*(quoteEscape visibleString) DQUOTE

quoteEscape = DQUOTE DQUOTE

visibleString = (%x00-21 / %x23-FF)

EOL = CRLF / LF

SDPinformation = ;See RFC2327

3.5. Encoding of the session description

The session description is encoded in conformance with the session

description protocol, SDP. MGCP implementations are expected to be

fully capable of parsing any conformant SDP message, and should send

session descriptions that strictly conform to the SDP standard. The

usage of SDP actually depends on the type of session that is being,

as specified in the "mode" parameter:

* if the mode is set to "data", the session description describes

the configuration of a data access service.

* if the mode is set to any other value, the session description is

for an audio service.

For an audio service, the gateway will consider the information

provided in SDP for the "audio" media. For a data service, the

gateway will consider the information provided for the "network-

access" media.

3.5.1. Usage of SDP for an audio service

In a telephony gateway, we only have to describe sessions that use

exactly one media, audio. The parameters of SDP that are relevant for

the telephony application are:

At the session description level:

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

local gateway (in responses), or multicast address of the audio

conference, encoded as an SDP "connection data" parameter. This

parameter specifies the IP address that will be used to

exchange RTP packets.

For the audio media:

* Media description field (m) specifying the audio media, the

transport port used for receiving RTP packets by the remote

gateway (commands) or by the local gateway (responses), the

RTP/AVP transport, and the list of formats that the gateway

will accept. This list should normally always include the code

0 (reserved for PCMU).

* Optionally, RTPMAP attributes that define the encoding of

dynamic audio formats,

* Optionally, a packetization period (packet time) attribute

(Ptime) defining the duration of the packet,

* Optionally, an attribute defining the type of connection

(sendonly, recvonly, sendrecv, inactive). Note that this

attribute does not have a direct relation with the "Mode"

parameter of MGCP. In fact, the SDP type of connection will

most of the time be set to "sendrecv", regardless of the value

used by MGCP. Other values will only be used rarely, for

example in the case of information or announcement servers that

need to establish one way connections.

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

local gateway (in responses), if it is not present at the

session level.

An example of SDP specification for an audio connection could be:

v=0

c=IN IP4 128.96.41.1

m=audio 3456 RTP/AVP 0 96

a=rtpmap:96 G726-32/8000

There is a request, in some environments, to use the MGCP to

negotiate connections that will use other transmission channels than

RTP over UDP and IP. This will be detailed in an extension to this

document.

3.5.2. Usage of SDP in a network access service

The parameters of SDP that are relevant for a data network access

application are:

For the data media:

* Media description field (m) specifying the network access

media, identified by the code "m=nas/xxxx", where "xxxx"

describes the access control method that should be used for

parametrizing the network access, as specified below. The field

may also specify the port that should be used for contacting

the server, as specified in the SDP syntax.

* Connection address parameter (c=) specifying the address, or

the domain name, of the server that implement the access

control method. This parameter may also be specified at the

session level.

* Optionally, a bearer type attribute (a=bearer:) describing the

type of data connection to be used, including the modem type.

* Optionally, a framing type attribue (a=framing:) describing the

type of framing that will be used on the channel.

* Optionally, attributes describing the called number

(a=dialed:), the number to which the call was delivered

(a=called:) and the calling number (a=dialing:).

* Optionally, attributes describing the range of addresses that

could be used by the dialup client on its LAN (a=subnet:).

* Optionally, an encryption key, encoded as specified in the SDP

protocol(k=).

The connection address shall be encoded as specified in the SDP

standard. It will be used in conjunction with the port specified in

the media line to access a server, whose type will one of:

__________________________________________________________

Method name Method description

________________________________________________________

radius Authentication according

to the Radius protocol.

tacacs Authentication according

to the TACACS+ protocol.

diameter Authentication according

to the Diameter protocol.

l2tp Level 2 tunneling protocol.

The address and port are those of the LNS.

login Local login. (There is normally

no server for that method.)

none No authentication required.

(The call was probably vetted

by the Call Agent.)

________________________________________________________

If needed, the gateway may use the key specified in the announcement

to access the service. That key, in particular, may be used for the

establishment of an L2TP tunnel.

The bearer attribute is composed of a bearer name and an optional

extension. The bearer type specifies the type of modulation (modem

name) or, in the case of digital connections, the type of ISDN

service (8 bits, 7 bits). When an extension is present, it is

separated from the bearer name by a single slash (/). The valid

values of the bearer attribute are defined in the following table:

____________________________________________________________________

Type of bearer description Example of values

__________________________________________________________________

ITU modem standard V.32, V.34, V.90.

ITU modem standard qualified v.90/3com,

by a manufacturer name v.90/rockwell,

v.90/xxx

Well known modem types X2, K56flex

ISDN transparent access, 64 kbps ISDN64

ISDN64 + V.110 ISDN64/V.110

ISDN64 + V.120 ISDN64/V.120

ISDN transparent access, 56 kbps ISDN56

Informal identification (Requires coordination between

the Call Agent and the gateway)

__________________________________________________________________

The valid values of the framing attribute are defined in the

following table:

_________________________________________________

Type of framing description Example of values

_______________________________________________

PPP, asynchronous framing ppp-asynch

PPP, HDLC framing ppp-hdlc

SLIP, asynchronous slip

Asynchronous, no framing asynch

_______________________________________________

The network access authentication parameter provides instructions on

the access control that should be exercized for the data call. This

optional attribute is encoded as:

"a=subnet:" <network type> <address type>

<connection address> "/" <prefix length>

Where the parameters "network type", "address type", and "connection

address" are formatted as defined for the connection address

parameter (c=) in SDP, and where the "prefix length" is a decimal

representation of the number of bits in the prefix.

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

v=0

m=nas/radius

c=IN IP4 radius.example.net

a=bearer:v.34

a=framing:ppp-asynch

a=dialed:18001234567

a=called:12345678901

a=dialing:12340567890

v=0

m=nas/none

c=IN IP4 128.96.41.1

a=subnet:IN IP4 123.45.67.64/26

a=bearer:isdn64

a=framing:ppp-sync

a=dialed:18001234567

a=dialing:2345678901

v=0

c=IN IP4 access.example.net

m=nas/l2tp

k=clear:some-shared-secret

a=bearer:v.32

a=framing:ppp-asynch

a=dialed:18001234567

a=dialing:2345678901

3.5.3. Usage of SDP for ATM connections

The specification of the SDP payload for ATM connections will be

described in a companion document, "Usage of MGCP to control Voice

over ATM gateways." The following text is indicative.

The SDP payload will specify:

* That the connection is to be established over an ATM interface,

using the "c=" parameter of SDP to specify an address in the ATM

family, the ATM addressing variant (NSAP, UNI, E.164) and the ATM

address.

* The "m=audio" parameter will specify the audio encoding and, if

needed, the VPI and VCI.

* Additional attributes parameters (a=) will be used to specify the

ATM coding variants, such as the type of adaptation layer and the

error correction or loss compenmsation algorithms.

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

v=0 c=ATM NSAP

47.0091.8100.0000.0060.3e64.fd01.0060.3e64.fd01.fe m=audio

5/1002 ATM/AVP PCMU a=connection_type:AAL2

3.5.4. Usage of SDP for local connections

When MGCP is used to set up internal connections within a single

gateway, the SDP format is used to encode the parameters of that

connection. The following parameters will be used:

* The connection parameter (C=) will specify that the connection is

local, using the keyword "LOCAL" as network type space, the

keyword "EPN" (endpoint name) as address type, and the name of

the endpoint as the connection-address.

* The "m=audio" parameter will specify a port number, which will

always be set to 0, the type of protocol, always set to the

keyword LOCAL, and the type of encoding, using the same

conventions used for RTP (RTP payload numbers.) The type of

encoding should normally be set to 0 (PCMU).

An example of local SDP payload could be:

v=0

c=LOCAL EPN X35V3+A4/13

m=audio 0 LOCAL 0

3.6. Transmission over UDP

MGCP messages are transmitted over UDP. Commands are sent to one of

the IP addresses defined in the DNS for the specified endpoint . The

responses are sent back to the source address of the commands.

When no port is specified for the endpoint, the commands should be

sent:

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

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

3.6.1. Providing the At-Most-Once functionality

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

the absence of a timely response, commands are repeated. Most MGCP

commands are not idempotent. The state of the gateway would become

unpredictable if, for example, CreateConnection commands were

executed several times. The transmission procedures must thus

provide an "At-Most-Once" functionality.

MGCP entities are expected to keep in memory a list of the responses

that they sent to recent transactions and a list of the transactions

that are currently being executed. The transaction identifiers of

incoming commands are compared to the transaction identifiers of the

recent responses. If a match is found, the MGCP entity does not

execute the transaction, but simply repeats the response. The

remaining commands will be compared to the list of current

transaction. If a match is found, the MGCP entity does not execute

the transaction, which is simply ignored.

The procedure use a long timer value, noted LONG-TIMER in the

following. The timer should be set larger than the maximum duration

of a transaction, which should take into account the maximum number

of repetitions, the maximum value of the repetition timer and the

maximum propagation delay of a packet in the network. A suggested

value is 30 seconds.

The copy of the responses can be destroyed either LONG-TIMER seconds

after the response is issued, or when the gateway (or the call agent)

receives a confirmation that the response has been received, through

the "Response Acknowledgement attribute". For transactions that are

acknowledged through this attribute, the gateway shall keep a copy of

the transaction-id for LONG-TIMER seconds after the response is

issued, in order to detect and ignore duplicate copies of the

transaction request that could be produced by the network.

3.6.2. Transaction identifiers and three ways handshake

Transaction identifiers are integer numbers in the range from 0 to

999,999,999. Call-agents may decide to use a specific number space

for each of the gateways that they manage, or to use the same number

space for all gateways that belong to some arbitrary group. Call

agents may decide to share the load of managing a large gateway

between several independent processes. These processes will share

the same transaction number space. There are multiple possible

implementations of this sharing, such as having a centralized

allocation of transaction identifiers, or pre-allocating non-

overlapping ranges of identifiers to different processes. The

implementations must guarantee that unique transaction identifiers

are allocated to all transactions that originate from a logical call

agent, as defined in the "states, failover and race conditions"

section. Gateways can simply detect duplicate transactions by looking

at the transaction identifier only.

The Response Acknowledgement Attribute can be found in any command.

It carries a set of "confirmed transaction-id ranges."

MGCP gateways may choose to delete the copies of the responses to

transactions whose id is included in "confirmed transaction-id

ranges" received in the Response Confirmation messages. They should

silently discard further commands from that Call Agent when the

transaction-id falls within these ranges.

The "confirmed transaction-id ranges" values shall not be used if

more than LONG-TIMER seconds have elapsed since the gateway issued

its last response to that call agent, or when a gateway resumes

operation. In this situation, commands should be accepted and

processed, without any test on the transaction-id.

Commands that carry the "Response Acknowledgement attribute" may be

transmitted in disorder. The gateway shall retain the union of the

"confirmed transaction-id ranges" received in recent commands.

3.6.3. Computing retransmission timers

It is the responsibility of the requesting entity to provide suitable

time outs for all outstanding commands, and to retry commands when

time outs have been exceeded. Furthermore, when repeated commands

fail to be acknowledged, it is the responsibility of the requesting

entity to seek redundant services and/or clear existing or pending

connections.

The specification purposely avoids specifying any value for the

retransmission timers. These values are typically network dependent.

The retransmission timers should normally estimate the timer by

measuring the time spent between the sending of a command and the

return of a response. One possibility is to use the algorithm

implemented in TCP-IP, which uses two variables:

* the average acknowledgement delay, AAD, estimated through an

exponentially smoothed average of the observed delays,

* the average deviation, ADEV, estimated through an exponentially

smoothed average of the absolute value of the difference between

the observed delay and the current average

The retransmission timer, in TCP, is set to the sum of the average

delay plus N times the average deviation. In MGCP, the maximum value

of the timer should however be bounded, in order to guarantee that no

repeated packet will be received by the gateways after LONG-TIMER

seconds. A suggested maximum value is 4 seconds.

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

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

* It should compute a random value, uniformly distributed between

0.5 AAD and AAD

* It should set the retransmission timer to the sum of that random

value and N times the average deviation.

This procedure has two effects. Because it includes an exponentially

increasing component, it will automatically slow down the stream of

messages in case of congestion. Because it includes a random

component, it will break the potential synchronization between

notifications triggered by the same external event.

3.6.4. Piggy backing

There are cases when a Call Agent will want to send several messages

at the same time to the same gateways. When several MGCP messages

have to be sent in the same UDP packets, they should be separated by

a line of text that contain a single dot, as in for example:

200 2005 OK

DLCX 1244 card23/21@trgw-7.example.net MGCP 1.0

C: A3C47F21456789F0

I: FDE234C8

The piggy-backed messages should be processed exactly has if they had

been received in several simultaneous messages.

3.6.5. Provisional responses

Executing some transactions may require a long time. Long execution

times may interact with the timer based retransmission procedure.

This may result either in an inordinate number of retransmissions, or

in timer values that become too long to be efficient.

Gateways that can predict that a transaction will require a long

execution time may send a provisional response, with response code

100. They should send this response if they receive a repetition of

a transaction that is still being executed.

MGCP entities that receive a provisional response shall switch to a

longer repetition timer for that transaction.

4. States, failover and race conditions.

In order to implement proper call signalling, the Call Agent must

keep track of the state of the endpoint, and the gateway must make

sure that events are properly notified to the call agent. Special

conditions exist when the gateway or the call agent are restarted:

the gateway must be redirected to a new call agent during "failover"

procedures, the call agent must take special action when the gateway

is taken offline, or restarted.

4.1. Basic Asumptions

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

* Call Agents are identified by their domain name, not their network

addresses, and several addresses can be associated with a domain

name.

* An endpoint has one NotifiedEntity associated with it any given

point in time.

* The NotifiedEntity is the last value of the "NotifiedEntity"

parameter received for this endpoint (including wild-carded end-

point-names). If no explicit "NotifiedEntity" parameter has been

received, the "NotifiedEntity" defaults to the provisioned

NotifiedEntity value, or if no value was provisioned to the source

address of the last command received for the endpoint,

* Responses to commands are always sent to the source address of the

command, regardless of the NotifiedEntity.

* When the "notified entity" refers to a domain name that resolves

to multiple IP- address, endpoints are capable of switching

between different interfaces on the same logical call agent,

however they cannot switch to other (backup) call agent(s) on

their own. A backup call agent can however instruct them to

switch, either directly or indirectly.

* If an entire call agent becomes unavailable, the endpoints managed

by that call agent will eventually become "disconnected". The only

way for these endpoints to become connected again is either for

the failed call agent to become available, or for a backup call

agent to contact the affected endpoints.

* When a backup call agent has taken over control of a group of

endpoints, it is assumed that the failed call agent will

communicate and synchronize with the backup call agent in order to

transfer control of the affected endpoints back to the original

call agent (if that's even desired - maybe the failed call agent

should simply become the backup call agent now).

We should note that handover conflict resolution between separate

CA's is not in place - we are relying strictly on the CA's knowing

what they are doing and communicating with each other (although

AuditEndpoint can be used to learn about the current NotifiedEntity).

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

The media gateway control protocol is organized as a set of

transactions, each of which is composed of a command and a response,

commonly referred to as an acknowledgement. The MGCP messages, being

carried over UDP, may be subject to losses. In the absence of a

timely response, commands are repeated. MGCP entities are expected to

keep in memory a list of the responses that they sent to recent

transactions, i.e. a list of all the responses they sent over the

last LONG-TIMER seconds, and a list of the transactions that are

currently being executed.

The transaction identifiers of incoming commands are compared to the

transaction identifiers of the recent responses. If a match is found,

the MGCP entity does not execute the transaction, but simply repeats

the response. The remaining commands will be compared to the list of

current transaction. If a match is found, the MGCP entity does not

execute the transaction, which is simply ignored - a response will be

provided when the execution of the command is complete.

The repetition mechanism is used to guard against four types of

possible errors:

* transmission errors, when for example a packet is lost due to

noise on a line or congestion in a queue,

* component failure, when for example an interface to a call agent

becomes unavailable,

* call agent failure, when for example an entire call agent becomes

unavailable,

* failover, when a new call agent is "taking over" transparently.

The elements should be able to derive from the past history an

estimate of the packet loss rate due to transmission errors. In a

properly configured system, this loss rate should be kept very low,

typically less than 1%. If a call agent or a gateway has to repeat a

message more than a few times, it is very legitimate to assume that

something else than a transmission error is occurring. For example,

given a loss rate of 1%, the probability that 5 consecutive

transmission attempts fail is 1 in 100 billion, an event that should

occur less than once every 10 days for a call agent that processes

1,000 transactions per second. (Indeed, the number of repetition that

is considered excessive should be a function of the prevailing packet

loss rate.) We should note that the "suspicion threshold", which we

will call "Max1", is normally lower than the "disconnection

threshold", which should be set to a larger value.

Command issued: N=0

transmission: N++

+------------ retransmission: N++ -----------+

transmission

+---to new address -+<----------------------+

N=0

V V V

+-----------+

awaiting - new call agent ->+ +------------+

response --- timer elapsed ---> N > Max1 ? -(no)+

+-----------+ <----------+ +------------+ ^

+- wrong key? -+ (yes)

response received (if N=Max1,

or N=Max2

check DNS)

v

(end) +---------------+

more addresses?(yes)--+

+---------------+

(no)

+------------+

N > Max2 ? (no)-+

+------------+

(yes)

v

(disconnected)

A classic retransmission algorithm would simply count the number of

successive repetitions, and conclude that the association is broken

after re-transmitting the packet an excessive number of times

(typically between 7 and 11 times.) In order to account for the

possibility of an undetected or in-progress "failover", we modify the

classic algorithm as follows:

* We request that the gateway always checks for the presence of a

new call agent. It can be noticed either by

- receiving a valid multicast message announcing a failover, or

- receiving a command where the NotifiedEntity points to the new

call agent, or

- receiving a redirection response pointing to a new Call Agent.

If a new call agent is detected, the gateway starts transmitting

outstanding commands to that new agent. Responses to commands are

still transmitted to the source address of the command.

* we request that if the number of repetitions for this Call Agent

is larger than "Max1", that the gateway actively queries the name

server in order to detect the possible change of the call agent

interfaces.

* The gateway may have learned several IP addresses for the call

agent. If the number of repetitions is larger than "Max1" and

lower than "Max2", and there are more interfaces that have not

been tried, then the gateway should direct the retransmissions to

alternate addresses.

* If there are no more interfaces to try, and the number of

repetitions is Max2, then the gateway contacts the DNS one more

time to see if any other interface should have become available.

If not, the gateway is now disconnected.

The procedure will maximize the chances of detecting an ongoing

failover. It poses indeed two very specific problems, the potentially

long delays of a timer based procedure and the risk of confusion

caused by the use of cryptographic protections.

In order to automatically adapt to network load, MGCP specifies

exponentially increasing timers. If the initial timer is set to 200

milliseconds, the loss of a fifth retransmission will be detected

after about 6 seconds. This is probably an acceptable waiting delay

to detect a failover. The repetitions should continue after that

delay not only in order to perhaps overcome a transient connectivity

problem, but also in order to allow some more time for the execution

of a failover - waiting a total delay of 30 seconds is probably

acceptable.

It is however important that the maximum delay of retransmissions be

bounded. Prior to any retransmission, it is checked that the time

elapsed since the sending of the initial datagram is no greater than

T- MAX. If more than T-MAX time has elapsed, the endpoint becomes

disconnected. The value T-MAX is related to the LONG-TIMER value: the

LONG-TIMER value is obtained by adding to T-MAX the maximum

propagation delay in the network.

Another potential cause of connection failure would be the reception

of a "wrong key" message, sent by a call agent that could not

authenticate the command, presumably because it had lost the security

parameters of the association. Such messages are actually not

authorized in IPSEC, and they should in fact not be taken at face

value: an attacker could easily forge "wrong key" messages in order

to precipitate the loss of a control connection. The current

algorithm ignores these messages, which translates into a strict

reliance on timers. The algorithm could in fact be improved, maybe

by executing a check with the key server of the call agent after

"Max1" repetitions.

4.3. Race conditions

MGCP deals with race conditions through the notion of a "quarantine

list" and through explicit detection of desynchronization.

MGCP does not assume that the transport mechanism will maintain the

order of command and responses. This may cause race conditions, that

may be obviated through a proper behavior of the call agent. (Note

that some race conditions are inherent to distributed systems; they

would still occur, even if the commands were transmitted in strict

order.)

In some cases, many gateways may decide to restart operation at the

same time. This may occur, for example, if an area loses power or

transmission capability during an earthquake or an ice storm. When

power and transmission are reestablished, many gateways may decide to

send "RestartInProgress" commands simultaneously, leading to very

unstable operation.

4.3.1. Quarantine list

MGCP controlled gateways will receive "notification requests" that

ask them to watch for a list of "events." The protocol elements that

determine the handling of these events are the "Requested Events"

list, the "Digit Map" and the "Detect Events" list.

When the endpoint is initialized, the requested events list and the

digit map are empty. After reception of a command, the gateway

starts observing the endpoint for occurrences of the events mentioned

in the list.

The events are examined as they occur. The action that follows is

determined by the "action" parameter associated to the event in the

list of requested events, and also by the digit map. The events that

are defined as "accumulate" or "treat according to digit map" are

accumulated in a list of events, the events that are marked as

"treated according to the digit map" will additionally be accumulated

in the dialed string. This will go on until one event is encountered

that triggers a Notification to the "notified entity."

The gateway, at this point, will transmit the notification command

and will place the endpoint in a "notification" state. As long as the

endpoint is in this notification state, the events that are to be

detected on the endpoint are stored in a "quarantine" buffer for

later processing. The events are, in a sense, "quarantined." All

events that are specified by the union of the RequestedEvents

parameter and the most recently received DetectEvent parameter or, in

the absence of the latter, all events that are referred to in the

RequestedEvents, should be detected and quarantined, regardless of

the action associated to the event.

The endpoint exits the "notification state" when the acknowledgement

of the Notify command is received. The Notify command may be

retransmitted in the "notification state", as specified in section

3.5. When the endpoint exits the "notification state" it resets the

list of observed events and the "current dial string" of the endpoint

to a null value.

Following that point, the behavior of the gateway depends on the

value of The QuarantineHandling parameter in the notification

request. If the Call Agent specified that it expected at most one

notification in response to the notification request command, then

the gateway should simply keep on accumulating events in the

quarantine list until it receives the next notification request

command.

If the gateway is authorized to send multiple successive Notify

commands, it will proceed as follows. When the gateway exits the

"notification state", it resets the list of observed events and the

"current dial string" of the endpoint to a null value and starts

processing the list of quarantined events, using the already received

list of requested events and digit map. When processing these events,

the gateway may encounter an event which requires a Notify command to

be sent. If that is the case, the gateway can adopt one of the two

following behaviors:

* it can immediately transmit a Notify command that will report all

events that were accumulated in the list of observed events until

the triggering event, included, leaving the unprocessed events in

the quarantine list,

* or it can attempt to empty the quarantined list and transmit a

single Notify command reporting several sets of events and

possibly several dial strings. The dial string is reset to a null

value after each triggering event. The events that follow the last

triggering event are left in the quarantine list.

If the gateway transmits a Notify command, the end point will remain

in the "notification state" until the acknowledgement is received. If

the gateway does not find a quarantined event that requests a Notify

command, it places the end point in a normal state. Events are then

processed as they come, in exactly the same way as if a Notification

Request command had just been received.

A gateway may receive at any time a new Notification Request command

for the end point. When a new notification request is received in the

notification state, the gateway shall ensure that the pending

notification is received by the Call Agent prior to a successful

response to the new NotificationRequest. It does so by using the

"piggy-backing" functionality of the protocol. The messages will then

be sent in a single packetto the source of the new

NotificationRequest, regardless of respectively the source and

"notified entity" for the old and new command. The steps involved are

the following:

a) the gateway builds a message that carries in a single packet a

repetition of the old pending Notify command and the

acknowledgement of the new notification request.

b) the endpoint is then taken out of the "notification state" without

waiting for the acknowledgement of the notification command.

c) a copy of the unacknowledged Notify command command is kept until

an acknowledgement is received. If a timer elapses, the

notification will be repeated, in a packet that will also carry a

repetition of the acknowledgement of the notification request.

d) if the acknowledgement is lost, the Call Agent will retransmit the

Notification Request. The gateway will reply to this repetition

by retransmitting in a single packet the unacknowledged Notify and

the acknowledgement of the notification request.

e) if the gateway has to transmit a Notify before the previous Notify

is acknowledged, it should construct a packet that piggybacks a

repetition of the old Notify, a repetition of the acknowledgement

of the last notification request and the new Notify.

f) Gateways that cannot piggyback several packets in the same message

should elect to leave the endpoint in the "notification" state as

long as the last notification is not acknowledged.

After receiving the Notification Request command, the requested

events list and digit map (if a new one was provided) are replaced by

the newly received parameters, and the list of observed events and

accumulated dial string are reset to a null value. The behavior is

conditioned by the value of the QuarantineHandling parameter. The

parameter may specify that quarantined events, or previously observed

events, should be discarded, in which case they will be. If the

parameter specifies that the quarantined events should be processed,

the gateway will start processing the list of quarantined events or

previously observed events, using the newly received list of

requested events and digit map. When processing these events, the

gateway may encounter an event which requires a Notify command to be

sent. If that is the case, the gateway will immediately transmit a

Notify command that will report all events that were accumulated in

the list of observed events until the triggering event, included,

leaving the unprocessed events in the quarantine buffer, and will

enter the "notification state".

A new notification request may be received while the gateway has

accumulated events according to the previous notification requests,

but has not yet detected a notification-triggering events. The

handling of not-yet-notified events is determined, as with the

quarantined events, by the quarantine handling parameters:

* If the quarantine-handling parameter specifies that quarantined

events shall be ignored, the observed event list is simply reset.

* If the quarantine-handling parameter specifies that quarantined

events shall be processed, the observed event list is transferred

to the quarantined event list. The observed event list is then

reset, and the quarantined event list is processed.

Call Agents SHOULD provide the response to a successful Notify

message and the new NotificationRequest in the same datagram using

the piggy-backing mechanism.

4.3.2. Explicit detection

A key element of the state of several endpoints is the position of

the hook. A race condition may occur when the user decides to go

off-hook before the Call Agent has the time to ask the gateway to

notify an off hook event (the "glare" condition well known in

telephony), or if the user goes on-hook before the Call Agent has the

time to request the event's notification.

To avoid this race condition, the gateway should check the condition

of the endpoint before acknowledging a NotificationRequest. It should

return an error:

1- If the gateway is requested to notify an "off hook" transition

while the phone is already off hook,

2- If the gateway is requested to notify an "on hook" or "flash hook"

condition while the phone is already on hook.

It should be noted, that the condition check is performed at the time

the notification request is received, where as the actual event that

caused the current condition may have either been reported, or

ignored earlier, or it may currently be quarantined.

The other state variables of the gateway, such as the list of

RequestedEvent or list of requested signals, are entirely replaced

after each successful NotificationRequest, which prevents any long

term discrepancy between the Call Agent and the gateway.

When a NotificationRequest is unsuccessful, whether it is included in

a connection-handling command or not, the gateway will simply

continue as if the command had never been received. As all other

transactions, the NotificationRequest should operate as an atomic

transaction, thus any changes initiated as a result of the command

should be reverted.

Another race condition may occur when a Notify is issued shortly

before the reception by the gateway of a NotificationRequest. The

RequestIdentifier is used to correlate Notify commands with

NotificationRequest commands.

4.3.3. Ordering of commands, and treatment of disorder

MGCP does not mandate that the underlying transport protocol

guarantees the sequencing of commands sent to a gateway or an

endpoint. This property tends to maximize the timeliness of actions,

but it has a few draw backs. For example:

* Notify commands may be delayed and arrive to the call agent after

the transmission of a new Notification Request command,

* If a new NotificationRequest is transmitted before a previous one

is acknowledged, there is no guarantee that the previous one will

not be received in second position.

Call Agents that want to guarantee consistent operation of the end

points can use the following rules:

1) When a gateway handles several endpoints, commands pertaining to

the different endpoints can be sent in parallel, for example

following a model where each endpoint is controlled by its own

process or its own thread.

2) When several connections are created on the same endpoint,

commands pertaining to different connections can be sent in

parallel.

3) On a given connection, there should normally be only one

outstanding command (create or modify). However, a

DeleteConnection command can be issued at any time. In

consequence, a gateway may sometimes receive a ModifyConnection

command that applies to a previously deleted connection. Such

commands should be ignored, and an error code should be returned.

4) On a given endpoint, there should normally be only one outstanding

NotificationRequest command at any time. The RequestId parameter

should be used to correlate Notify commands with the triggering

notification request.

5) In some cases, an implicitly or explicitly wildcarded

DeleteConnection command that applies to a group of endpoints can

step in front of a pending CreateConnection command. The Call

Agent should individually delete all connections whose completion

was pending at the time of the global DeleteConnection command.

Also, new CreateConnection commands for endpoints named by the

wild-carding cannot be sent until the wild-carded DeleteConnection

command is acknowledged.

6) When commands are embedded within each other, sequencing

requirements for all commands must be adhered to. For example a

Create Connection command with a Notification Request in it must

adhere to the sequencing for CreateConnection and

NotificationRequest at the same time.

7) AuditEndpoint and AuditConnection is not subject to any

sequencing.

8) RestartInProgress must always be the first command sent by an

endpoint as defined by the restart procedure. Any other command or

response must be delivered after this RestartInProgress command

(piggy-backing allowed).

9) When multiple messages are piggy-backed in a single packet, the

messages are always processed in order.

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

commands.

4.3.4. Fighting the restart avalanche

Let's suppose that a large number of gateways are powered on

simultaneously. If they were to all initiate a RestartInProgress

transaction, the call agent would very likely be swamped, leading to

message losses and network congestion during the critical period of

service restoration. In order to prevent such avalanches, the

following behavior is suggested:

1) When a gateway is powered on, it should initiate a restart timer

to a random value, uniformly distributed between 0 and a maximum

waiting delay (MWD). Care should be taken to avoid synchronicity

of the random number generation between multiple gateways that

would use the same algorithm.

2) The gateway should then wait for either the end of this timer, the

reception of a command from the call agent, or the detection of a

local user activity, such as for example an off-hook transition on

a residential gateway.

3) When the timer elapses, when a command is received, or when an

activity is detected, the gateway should initiate the restart

procedure.

The restart procedure simply requires the endpoint to guarantee that

the first message (command or response) that the Call Agent sees from

this endpoint is a RestartInProgress message informing the Call Agent

about the restart. The endpoint is free to take full advantage of

piggy-backing to achieve this.

It is expected that each endpoint in a gateway will have a

provisionable Call Agent, i.e., "notified entity", to direct the

initial restart message towards. When the collection of endpoints in

a gateway is managed by more than one Call Agent, the above procedure

must be performed for each collection of endpoints managed by a given

Call Agent. The gateway MUST take full advantage of wild-carding to

minimize the number of RestartInProgress messages generated when

multiple endpoints in a gateway restart and the endpoints are managed

by the same Call Agent.

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

type of the gateway. The following ]reasoning can be used to

determine the value of this delay on residential gateways.

Call agents are typically dimensioned to handle the peak hour traffic

load, during which, in average, 10% of the lines will be busy,

placing calls whose average duration is typically 3 minutes. The

processing of a call typically involves 5 to 6 MGCP transactions

between each end point and the call agent. This simple calculation

shows that the call agent is expected to handle 5 to 6 transactions

for each end point, every 30 minutes on average, or, to put it

otherwise, about one transaction per end point every 5 to 6 minutes

on average. This suggest that a reasonable value of MWD for a

residential gateway would be 10 to 12 minutes. In the absence of

explicit configuration, residential gateways should adopt a value of

600 seconds for MWD.

The same reasoning suggests that the value of MWD should be much

shorter for trunking gateways or for business gateways, because they

handle a large number of endpoints, and also because the usage rate

of these endpoints is much higher than 10% during the peak busy hour,

a typical value being 60%. These endpoints, during the peak hour,

are this expected to contribute about one transaction per minute to

the call agent load. A reasonable algorithm is to make the value of

MWD per "trunk" endpoint six times shorter than the MWD per

residential gateway, and also inversely proportional to the number of

endpoints that are being restarted. for example MWD should be set to

2.5 seconds for a gateway that handles a T1 line, or to 60

milliseconds for a gateway that handles a T3 line.

4.3.5. Disconnected Endpoints

In addition to the restart procedure, gateways also have a

"disconnected" procedure, which is initiated when an endpoint becomes

"disconnected" as described in Section 3.4.2. It should here be

noted, that endpoints can only become disconnected when they attempt

to communicate with the Call Agent. The following steps are followed

by an endpoint that becomes "disconnected":

1. A "disconnected" timer is initialized to a random value, uniformly

distributed between 0 and a provisionable "disconnected" initial

waiting delay (Tdinit), e.g., 15 seconds. Care MUST be taken to

avoid synchronicity of the random number generation between

multiple gateways and endpoints that would use the same algorithm.

2. The gateway then waits for either the end of this timer, the

reception of a command from the call agent, or the detection of a

local user activity for the endpoint, such as for example an off-

hook transition.

3. When the "disconnected" timer elapses, when a command is received,

or when a local user activity is detected, the gateway initiates

the "disconnected" procedure for the endpoint. In the case of

local user activity, a provisionable "disconnected" minimum

waiting delay (Tdmin) must furthermore have elapsed since the

gateway became disconnected or the last time it initiated the

"disconnected" procedure in order to limit the rate at which the

procedure is performed.

4. If the "disconnected" procedure still left the endpoint

disconnected, the "disconnected" timer is then doubled, subject to

a provisionable "disconnected" maximum waiting delay (Tdmax),

e.g., 600 seconds, and the gateway proceeds with step 2 again.

The "disconnected" procedure is similar to the restart procedure in

that it now simply states that the endpoint MUST send a

RestartInProgress command to the Call Agent informing it that the

endpoint was disconnected and furthermore guarantee that the first

message (command or response) that the Call Agent now sees from this

endpoint MUST be this RestartInProgress command. The endpoint MUST

take full advantage of piggy-backing in achieving this. The Call

Agent may then for instance decide to audit the endpoint, or simply

clear all connections for the endpoint.

This specification purposely does not specify any additional behavior

for a disconnected endpoint. Vendors MAY for instance choose to

provide silence, play reorder tone, or even enable a downloaded wav

file to be played.

The default value for Tdinit is 15 seconds, the default value for

Tdmin, is 15 seconds, and the default value for Tdmax is 600 seconds.

5. Security requirements

If unauthorized entities could use the MGCP, they would be able to

set-up unauthorized calls, or to interfere with authorized calls. We

expect that MGCP messages will always be carried over secure Internet

connections, as defined in the IP security architecture as defined in

RFC2401, using either the IP Authentication Header, defined in RFC

2402, or the IP Encapsulating Security Payload, defined in RFC2406.

The complete MGCP protocol stack would thus include the following

layers:

________________________________

MGCP

_______________________________

UDP

_______________________________

IP security

(authentication or encryption)

_______________________________

IP

_______________________________

transmission media

_______________________________

Adequate protection of the connections will be achieved if the

gateways and the Call Agents only accept messages for which IP

security provided an authentication service. An encryption service

will provide additional protection against eavesdropping, thus

forbidding third parties from monitoring the connections set up by a

given endpoint

The encryption service will also be requested if the session

descriptions are used to carry session keys, as defined in SDP.

These procedures do not necessarily protect against denial of service

attacks by misbehaving gateways or misbehaving call agents. However,

they will provide an identification of these misbehaving entities,

which should then be deprived of their authorization through

maintenance procedures.

5.1. Protection of media connections

MGCP allows call agent to provide gateways with "session keys" that

can be used to encrypt the audio messages, protecting against

eavesdropping.

A specific problem of packet networks is "uncontrolled barge-in."

This attack can be performed by directing media packets to the IP

address and UDP port used by a connection. If no protection is

implemented, the packets will be decompressed and the signals will be

played on the "line side".

A basic protection against this attack is to only accept packets from

known sources, checking for example that the IP source address and

UDP source port match the values announced in the "remote session

description." But this has two inconveniences: it slows down

connection establishment and it can be fooled by source spoofing:

* To enable the address-based protection, the call agent must obtain

the remote session description of the e-gress gateway and pass it

to the in-gress gateway. This requires at least one network round

trip, and leaves us with a dilemma: either allow the call to

proceed without waiting for the round trip to complete, and risk

for example "clipping" a remote announcement, or wait for the full

round trip and settle for slower call-set-up procedures.

* Source spoofing is only effective if the attacker can obtain valid

pairs of source destination addresses and ports, for example by

listening to a fraction of the traffic. To fight source spoofing,

one could try to control all access points to the network. But

this is in practice very hard to achieve.

An alternative to checking the source address is to encrypt and

authenticate the packets, using a secret key that is conveyed during

the call set-up procedure. This will no slow down the call set-up,

and provides strong protection against address spoofing.

6. Event packages and end point types

This section provides an initial definition of packages and event

names. More packages can be defined in additional documents.

6.1. Basic packages

The list of basic packages includes the following:

_________________________________________

Package name

_______________________________________

Generic Media Package G

DTMF package D

MF Package M

Trunk Package T

Line Package L

Handset Package H

RTP Package R

Network Access Server Package N

Announcement Server Package A

Script Package Script

_______________________________________

In the tables of events for each package, there are five columns:

Symbol: the unique symbol used for the event

Definition: a short description of the event

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

the call agent.

S: if nothing appears in this column for an event, then the event

cannot be signaled on command by the call agent. Otherwise, the

following symbols identify the type of event:

OO On/Off signal. The signal is turned on until commanded by the

call agent to turn it off, and vice versa.

TO Timeout signal. The signal lasts for a given duration unless

it is superseded by a new signal.

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

Duration: specifies the duration of TO signals.

6.1.1. Generic Media Package

Package Name: G

The generic media package group the events and signals that can be

observed on several types of endpoints, such as trunking gateways,

access gateways or residential gateways.

_____________________________________________________________________

Symbol Definition R S Duration

_________________________________________________________________

mt Modem detected x

ft Fax tone detected x

ld Long duration connection x

pat(###) Pattern ### detected x OO

rt Ringback tone TO

rbk(###) ring back on connection TO 180 seconds

cf Confirm tone BR

cg Network Congestion tone TO

it Intercept tone OO

pt Preemption tone OO

of report failure x

_________________________________________________________________

The signals are defined as follows:

The pattern definition can be used for specific algorithms such as

answering machine detection, tone detection, and the like.

Ring back tone (rt)

an Audible Ring Tone, a combination of two AC tones with

frequencies of 440 and 480 Hertz and levels of -19 dBm each, to

give a combined level of -16 dBm. The cadence for Audible Ring

Tone is 2 seconds on followed by 4 seconds off. See GR- 506-CORE -

LSSGR: SIGNALING, Section 17.2.5.

Ring back on connection

A ring back tone, applied to the connection whose identifier is

passed as a parameter.

The "long duration connection" is detected when a connection has been

established for more than 1 hour.

6.1.2. DTMF package

Package name: D

_______________________________________________________________

Symbol Definition R S Duration

___________________________________________________________

0 DTMF 0 x BR

1 DTMF 1 x BR

2 DTMF 2 x BR

3 DTMF 3 x BR

4 DTMF 4 x BR

5 DTMF 5 x BR

6 DTMF 6 x BR

7 DTMF 7 x BR

8 DTMF 8 x BR

9 DTMF 9 x BR

# DTMF # x BR

* DTMF * x BR

A DTMF A x BR

B DTMF B x BR

C DTMF C x BR

D DTMF D x BR

L long duration indicator x 2 seconds

X Wildcard, match x

any digit 0-9

T Interdigit timer x 4 seconds

of report failure x

___________________________________________________________

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

two ways:

* When timer T is used with a digit map, the timer is not started

until the first digit is entered, and the timer is restarted after

each new digit is entered until either a digit map match or

mismatch occurs. In this case, timer T functions as an inter-digit

timer.

* When timer T is used without a digit map, the timer is started

immediately and simply cancelled (but not restarted) as soon as a

digit is entered. In this case, timer T can be used as an

interdigit timer when overlap sending is used.

When used with a digit map, timer T takes on one of two values,

T(partial) or T(critical). When at least one more digit is

required for the digit string to match any of the patterns in the

digit map, timer T takes on the value T(partial), corresponding to

partial dial timing. If a timer is all that is required to produce

a match, timer T takes on the value T(critical) corresponding to

critical timing. When timer T is used without a digit map, timer T

takes on the value T(critical). The default value for T(partial)

is 16 seconds and the default value for T(critical) is 4 seconds.

The provisioning process may alter both of these.

The "long duration indicator" is observed when a DTMF signal is

produced for a duration larger than two seconds. In this case,

the gateway will detect two successive events: first, when the

signal has been recognized, the DTMF signal, and then, 2 seconds

later, the long duration signal.

6.1.3. MF Package

Package Name: M

________________________________________________________

Symbol Definition R S Duration

____________________________________________________

0 MF 0 x BR

1 MF 1 x BR

2 MF 2 x BR

3 MF 3 x BR

4 MF 4 x BR

5 MF 5 x BR

6 MF 6 x BR

7 MF 7 x BR

8 MF 8 x BR

9 MF 9 x BR

X Wildcard, match x

any digit 0-9

T Interdigit timer x 4 seconds

K0 MF K0 or KP x BR

K1 MF K1 x BR

K2 MF K2 x BR

S0 MF S0 or ST x BR

S1 MF S1 x BR

S2 MF S2 x BR

S3 MF S3 x BR

wk Wink x BR

wko Wink off x BR

is Incoming seizure x OO

rs Return seizure x OO

us Unseize circuit x OO

of report failure x

____________________________________________________

The definition of the MF package events is as follows:

Wink

A transition from unseized to seized to unseized trunk states

within a specified period. Typical seizure period is 100-350

msec.)

Incoming seizure

Incoming indication of call attempt.

Return seizure:

Seizure in response to outgoing seizure.

Unseize circuit:

Unseizure of a circuit at the end of a call.

Wink off:

A signal used in operator services trunks. A transition from

seized to unseized to seized trunk states within a specified

period of 100-350 ms. (To be checked)

6.1.4. Trunk Package

Package Name: T

_____________________________________________________________________

Symbol Definition R S Duration

_________________________________________________________________

co1 Continuity tone (single tone, x OO

or return tone)

co2 Continuity test (go tone, x OO

in dual tone procedures)

lb Loopback OO

om Old Milliwatt Tone (1000 Hz) x OO

nm New Milliwatt Tone (1004 Hz) x OO

tl Test Line x OO

zz No circuit x OO

as Answer Supervision x OO

ro Reorder Tone x TO 30 seconds

of report failure x

bl Blocking OO

_________________________________________________________________

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

Continuity Tone (co1):

A tone at 2010 + or - 30 Hz.

Continuity Test (co2):

A tone at the 1780 + or - 30 Hz.

Milliwatt Tones:

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

Line Test:

105 Test Line test progress tone (2225 Hz + or - 25 Hz at -10 dBm0

+ or -- 0.5dB).

No circuit:

(that annoying tri-tone, low to high)

Answer Supervision:

Reorder Tone:

Reorder tone is a combination of two AC tones with frequencies of

480 and 620 Hertz and levels of -24 dBm each, to give a combined

level of -21 dBm. The cadence for Station Busy Tone is 0.25

seconds on followed by 0.25 seconds off, repeating continuously.

See GR-506-CORE - LSSGR: SIGNALING, Section 17.2.7.

Blocking:

The call agent can place the circuit in a blocked state by

applying the "bl(+)" signal to the endpoint. It can unblock it by

applying the "bl(-)" signal.

The continuity tones are used when the call agent wants to initiate a

continuity test. There are two types of tests, single tone and dual

tone. The Call agent is expected to know, through provisioning

information, which test should be applied to a given endpoint. For

example, the call agent that wants to initiate a single frequency

test will send to the gateway a command of the form:

RQNT 1234 epx-t1/17@tgw2.example.net

X: AB123FE0

S: co1

R: co1

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

command:

RQNT 1234 epx-t1/17@tgw2.example.net

X: AB123FE0

S: co2

R: co1

The gateway would send the requested signal, and in both cases would

look for the return of the 2010 Hz tone (co1). When it detects that

tone, it will send the corresponding notification.

The tones are of type OO: the gateway will keep sending them until it

receives a new notification request.

6.1.5. Line Package

Package Name: L

________________________________________________________________________

Symbol Definition R S Duration

___________________________________________________________________

adsi(string) adsi display BR

vmwi visual message OO

waiting indicator

hd Off hook transition x

hu On hook transition x

hf Flash hook x

aw Answer tone x OO

bz Busy tone TO 30 seconds

ci(ti,nu,na) Caller-id BR

wt Call Waiting tone TO 30 seconds

wt1, wt2, Alternative call

wt3, wt4 waiting tones

dl Dial tone TO 16 seconds

mwi Message waiting ind. TO 16 seconds

nbz Network busy x OO

(fast cycle busy)

ro Reorder tone TO 30 seconds

rg Ringing TO 180 seconds

r0, r1, r2, Distinctive ringing TO 180 seconds

r3, r4, r5,

r6 or r7

rs Ringsplash BR

p Prompt tone x BR

e Error tone x BR

sl Stutter dialtone TO 16 seconds

v Alerting Tone OO

y Recorder Warning Tone OO

sit SIT tone

z Calling Card Service Tone OO

oc Report on completion x

ot Off hook warning tone TO indefinite

s(###) Distinctive tone pattern x BR

of report failure x

___________________________________________________________________

The definition of the tones is as follows:

Dial tone:

A combined 350 + 440 Hz tone.

Visual Message Waiting Indicator

The transmission of the VMWI messages will conform to the

requirements in Section 2.3.2, "On-hook Data Transmission Not

Associated with Ringing" in TR-H-000030 and the CPE guidelines in

SR-TSV-002476. VMWI messages will only be sent from the SPCS when

the line is idle. If new messages arrive while the line is busy,

the VMWI indicator message will be delayed until the line goes

back to the idle state. The CA should periodically refresh the

CPE's visual indicator. See TR-NWT-001401 - Visual Message Waiting

Indicator Generic Requirements; and GR- 30-CORE - Voiceband Data

Transmission Interface.

Message waiting Indicator

See GR-506-CORE, 17.2.3.

Alerting Tone:

a 440 Hz Tone of 2 second duration followed by 1/2 second of tone

every 10 seconds.

Ring splash

Ringsplash, also known as "Reminder ring" is a burst of ringing

that may be applied to the physical forwarding line (when idle) to

indicate that a call has been forwarded and to remind the user

that a CF subfeature is active. In the US, it is defined to be a

0.5(-0,+0.1) second burst of power ringing. See TR-TSY-000586 -

Call Forwarding Subfeatures.

Call waiting tone

Call Waiting tone is defined in GR-506-CORE, 14.2. Call Waiting

feature is defined in TR-TSY-000571. By defining "wt" as a TO

signal you are really defining the feature which seems wrong to me

(given the spirit of MGCP), hence the definition of "wt" as a BR

signal in ECS, per GR-506-CORE. Also, it turns out that there is

actually four different call waiting tone patterns (see GR-506-

CORE, 14.2) so we have wt1, wt2, wt3, wt4.

Caller Id (ci(time, number, name)):

The caller-id event carries three parameters, the time of the

call, the calling number and the calling name. Each of the three

fields are optional, however each of the commas will always be

included. See TR-NWT-001188, GR-30-CORE, and TR-NWT-000031.

Recorder Warning Tone:

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

SIT tone:

used for indicating a line is out of service.

Calling Card Service Tone:

60 ms of 941 + 1477 Hz and 940 ms of 350 + 440 Hz (dial tone),

decaying exponentially with a time constant of 200 ms.

Distinctive tone pattern:

where ### is any number between 000 and 999, inclusive. Can be

used for distinctive ringing, customized dial tone, etc.

Report on completion

The report on completion event is detected when the gateway was

asked to perform one or several signals of type TO on the

endpoint, and when these signals were completed without being

stopped by the detection of a requested event such as off-hook

transition or dialed digit. The completion report may carry as

parameter the name of the signal that came to the end of its live

time, as in:

O: L/oc(L/dl)

Ring back on connection

A ring back tone, applied to the connection wghose identifier is

passed as a parameter.

We should note that many of these definitions vary from country to

country. The frequencies listed above are the one in use in North

America. There is a need to accommodate different tone sets in

different countries, and there is still an ongoing debate on the best

way to meet that requirement:

* One solution is to define different event packages specifying for

example the German dialtone as "L-DE/DL".

* Another solution is to use a management interface to specify on an

endpoint basis which frequency shall be associated to what tone.

6.1.6. Handset emulation package

Package Name: H

________________________________________________________________________

Symbol Definition R S Duration

___________________________________________________________________

adsi(string) adsi display x BR

tdd

vmwi

hd Off hook transition x OO

hu On hook transition x OO

hf Flash hook x BR

aw Answer tone x OO

bz Busy tone x OO

wt Call Waiting tone x TO 30 seconds

dl Dial tone (350 + 440 Hz) x TO 120 seconds

nbz Network busy x OO

(fast cycle busy)

rg Ringing x TO 30 seconds

r0, r1, r2, Distinctive ringing x TO 30 seconds

r3, r4, r5,

r6 or r7

p Prompt tone x BR

e Error tone x BR

sdl Stutter dialtone x TO 16 seconds

v Alerting Tone x OO

y Recorder Warning Tone x OO

t SIT tone x

z Calling Card Service Tone x OO

oc Report on completion x

ot Off hook warning tone x OO

s(###) Distinctive tone pattern x BR

of report failure x

___________________________________________________________________

The handset emulation package is an extension of the line package, to

be used when the gateway is capable of emulating a handset. The

difference with the line package is that events such as "off hook"

can be signalled as well as detected.

6.1.7. RTP Package

Package Name: R

____________________________________________________________________

Symbol Definition R S Duration

________________________________________________________________

UC Used codec changed x

SR(###) Sampling rate changed x

JI(###) Jitter buffer size changed x

PL(###) Packet loss exceeded x

qa Quality alert x

co1 Continuity tone (single tone, x OO

or return tone)

co2 Continuity test (go tone, x OO

in dual tone procedures)

of report failure x

________________________________________________________________

Codec Changed:

Codec changed to hexadecimal codec number enclosed in parenthesis,

as in UC(15), to indicate the codec was changed to PCM mu-law.

Codec Numbers are specified in RFC1890, or in a new definition of

the audio profiles for RTP that replaces this RFC. Some

implementations of media gateways may not allow the codec to be

changed upon command from the call agent. codec changed to codec

hexadecimal ##.

Sampling Rate Changed:

Sampling rate changed to decimal number in milliseconds enclosed

in parenthesis, as in SR(20), to indicate the sampling rate was

changed to 20 milliseconds. Some implementations of media

gateways may not allow the sampling rate to be changed upon

command from a call agent.

Jitter Buffer Size Changed:

When the media gateway has the ability to automatically adjust the

depth of the jitter buffer for received RTP streams, it is useful

for the media gateway controller to receive notification that the

media gateway has automatically increased its jitter buffer size

to accomodate increased or decreased variability in network

latency. The syntax for requesting notification is "JI", which

tells the media gateway that the controller wants notification of

any jitter buffer size changes. The syntax for notification from

the media gateway to the controller is "JI(####)", where the ####

is the new size of the jitter buffer, in milliseconds.

Packet Loss Exceeded:

Packet loss rate exceed the threshold of the specified decimal

number of packets per 100,000 packets, where the packet loss

number is contained in parenthesis. For example, PL(10) indicates

packets are being dropped at a rate of 1 in 10,000 packets.

Quality alert

The packet loss rate or the combination of delay and jitter exceed

a specified quality threshold.

The continuity tones are the same as those defined in the Trunk

package. They can be use in conjunction with the Network LoopBack or

Network Continuity Test modes to test the continuity of an RTP

circuit.

The "operation failure" code can be used to report problems such as

the loss of underlying connectivity. The observed event can include

as parameter the reason code of the failure.

6.1.8. Network Access Server Package

Package Name: N

____________________________________________________________

Symbol Definition R S Duration

________________________________________________________

pa Packet arrival x

cbk Call back request x

cl Carrier lost x

au Authorization succeeded x

ax Authorization denied x

of Report failure x

________________________________________________________

The packet arrival event is used to notify that at least one packet

was recently sent to an Internet address that is observed by an

endpoint. The event report includes the Internet address, in

standard ASCII encoding, between parenthesis:

O: pa(192.96.41.1)

The call back event is used to notify that a call back has been

requested during the initial phase of a data connection. The event

report includes the identification of the user that should be called

back, between parenthesis:

O: cbk(user25)

6.1.9. Announcement Server Package

Package Name: A

___________________________________________________________________

Symbol Definition R S Duration

_______________________________________________________________

ann(url,parms) Play an announcement TO variable

oc Report on completion x

of Report failure x

_______________________________________________________________

The announcement action is qualified by an URL name and by a set of

initial parameters as in for example:

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

The "operation complete" event will be detected when the announcement

is played out. If the announcement cannot be played out, an operation

failure event can be returned. The failure may be explained by a

commentary, as in:

O: A/of(file not found)

6.1.10. Script Package

Package Name: Script

______________________________________________________________

Symbol Definition R S Duration

_________________________________________________________

Java(url) Load a java script TO variable

perl(url) Load a perl script TO variable

tcl(url) Load a TCL script TO variable

XML(url) Load an XML script TO variable

oc Report on completion x

of Report failure x

_________________________________________________________

The "language" action define is qualified by an URL name and by a set

of initial parameters as in for example:

S: script/java(http://scripts.example.net/credit-

card.java,long,1234)

The current definition defines keywords for the most common

languages. More languages may be defined in further version of this

documents. For each language, an API specification will describe how

the scripts can issue local "notificationRequest" commands, and

receive the corresponding notifications.

The script produces an output which consists of one or several text

string, separated by commas. The text string are reported as a

commentary in the report on completion, as in for example:

O: script/oc(21223456794567,9738234567)

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

O: script/oc(21223456794567,9738234567)

The definition of the script environment and the specific actions in

that environment are for further study.

6.2. Basic endpoint types and profiles

We define the following basic endpoint types and profiles:

* Trunk gateway (ISUP)

* Trunk gateway (MF)

* Network Access Server (NAS)

* Combined NAS/VOIP gateway

* Access Gateway

* Residential Gateway

* Announcement servers

These gateways are supposed to implement the following packages

___________________________________________________________

Gateway Supported packages

_________________________________________________________

Trunk gateway (ISUP) GM, DTMF, TK, RTP

Trunk gateway (MF) GM, MF, DTMF, TK, RTP

Network Access Server (NAS) GM, MF, TK, NAS

Combined NAS/VOIP gateway GM, MF, DTMF, TK, NAS, RTP

Access Gateway (VOIP) GM, DTMF, MF, RTP

Access Gateway (VOIP+NAS) GM, DTMF, MF, NAS, RTP

Residential Gateway GM, DTMF, Line, RTP

Announcement Server ANN, RTP

_________________________________________________________

Advanced announcement servers may also support the Script package.

Advanced trunking servers may support the ANN package, the Script

package, and in some cases the Line and Handset package as well.

7. Versions and compatibility

7.1. Differences between version 1.0 and draft 0.5

Draft 0-5 was issued in February 1999, as the last update of draft

version 0.1. Version 1.0 benefits from implementation experience, and

also aligns as much as possible with the CableLabs' NCS project. The

main differences between the February draft and version 1.0 are:

* Specified more clearly that the encoding of three

LocalConnectionOptions parameters, Encoding Method, Packetization

Period and Bandwidth, shall follow the conventions laid out in

SDP.

* Specified how the quarantine handling parameter governs the

handling of detected but not yet specified events.

* Specified that unexpected timers or digits should trigger

transmission of the dialed string.

* Removed the digit map syntax description from section 2.1.5 (it

was redundant with section 3.4.)

* Corrected miscellaneous bugs in the formal syntax description.

* Aligned specification of commands with the CableLabs NCS

specification. This mostly affects the AuditEndpoint and

RestartInProgress commands.

* Aligned the handling of retransmission with the CableLabs NCS

specification.

* Added the provisional response return code and corresponding

behavior description.

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

* Added the possibility to audit the restart method, restart delay

and reason code.

7.2. Differences between draft-04 and draft-05

Differences are minor: corrected the copyright statement, and

corrected a bug in the formal description.

7.3. Differences between draft-03 and draft-04

Draft 04 corrects a number of minor editing mistakes that were

pointed out during the review of draft 03, issued on February 1.

7.4. Differences between draft-02 and draft-03

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

draft 03 are:

* Introduced a discussion on endpoint types,

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

the role of connection parameters,

* Introduced a notation of the connection identifier within event

names,

* Documented the extension procedure for the LocalConnectionOptions

parameter and for the ConnectionParameters parameter,

* Introduced a three-way handshake procedure, using a ResponseAck

parameter, in order to allow gateways to delete copies of old

responses without waiting for a 30 seconds timer,

* Expanded the security section to include a discussion of

"uncontrolled barge-in."

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

7.5. Differences between draft-01 and draft-02

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

draft 02 are:

* Added an ABNF description of the protocol.

* Specification of an EndpointConfiguration command,

* Addition of a "two endpoints" mode in the create connection

command,

* Modification of the package wildcards from "$/$" to "*/all" at the

Request of early implementors,

* Revision of some package definitions to better align with external

specifications.

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

* Revision of the section on race conditions.

7.6. The making of MGCP from IPDC and SGCP

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

proposals.

7.7. Changes between MGCP and initial versions of SGCP

MGCP version 0.1 (which subsumes SGCP version 1.2) introduces the

following changes from SGCP version 1.1:

* Protocol name changed to MGCP.

* Introduce a formal wildcarding structure in the name of endpoints,

inspired from IPDC, and detailed the usage of wildcard names in

each operation.

* Naming scheme for events, introducing a package structure inspired

from IPDC.

* New operations for audit endpoint, audit connection (requested by

the Cablelabs) and restart (inspired from IPDC).

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

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

* Syntax modification for event reporting, to incorporate package

names.

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

* Incorporation of mandatory and optional extension parameters,

inspired by IPDC.

SGCP version 1.1 introduces the following changes from version SGCP

1.0:

* Extension parameters (X-??:)

* Error Code 511 (Unrecognized extension).

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

ObservedEvent parameters.

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

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

* Error Code 514 (Unrecognized announcement).

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

* Changed the code for the ASDI display from "ad" to "asdi" to avoid

conflict with the digits A and D.

* Changed the code for the answer tone from "at" to "aw" to avoid

conflict with the digit A and the timer mark T

* Changed the code for the busy tone from "bt" to "bz" to avoid

conflict with the digit B and the timer mark T

* Specified that the continuity tone value is "co" (CT was

incorrectly used in several instances; CT conflicts with .)

* Changed the code for the dial tone from "dt" to "dl" to avoid

conflict with the digit D and the timer mark T

* Added a code point for announcement requests.

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

* Set the "octet received" code in the "Connection Parameters" to

"OR" (was set to RO, but then "OR" was used throughout all

examples.)

* Added a "data" mode.

* Added a description of SDP parameters for the network access mode

(NAS).

* Added four flow diagrams for the network access mode.

* Incorporated numerous editing suggestions to make the description

easier to understand. In particular, cleared the confusion between

requests, queries, functions and commands.

* Defined the continuity test mode as specifying a dual-tone

transponder, while the loopback mode can be used for a single tone

test.

* Added event code "OC", operation completed.

* Added the specification of the "quarantine list", which clarifies

the expected handling of events and notifications.

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

8. Security Considerations

Security issues are discussed in section 5.

9. Acknowledgements

We want to thank here the many reviewers who provided us with advice

on the design of SGCP and then MGCP, notably Flemming Andreasen,

Sankar Ardhanari, Francois Berard, David Auerbach, Bob Biskner, David

Bukovinsky, Jerry Kamitses, Oren Kudevitzki, Barry Hoffner, Troy

Morley, Dave Oran, Jeff Orwick, John Pickens, Lou Rubin, Chip Sharp,

Paul Sijben, Kurt Steinbrenner, Joe Stone and Stuart Wray.

The version 0.1 of MGCP is heavily inspired by the "Internet Protocol

Device Control" (IPDC) designed by the Technical Advisory Committee

set up by Level 3 Communications. Whole sets of text have been

retrieved from the IP Connection Control protocol, IP Media Control

protocol, and IP Device Management. The authors wish to acknowledge

the contribution to these protocols made by Ilya Akramovich, Bob

Bell, Dan Brendes, Peter Chung, John Clark, Russ Dehlinger, Andrew

Dugan, Isaac Elliott, Cary FitzGerald, Jan Gronski, Tom Hess, Geoff

Jordan, Tony Lam, Shawn Lewis, Dave Mazik, Alan Mikhak, Pete

O'Connell, Scott Pickett, Shyamal Prasad, Eric Presworsky, Paul

Richards, Dale Skran, Louise Spergel, David Sprague, Raj Srinivasan,

Tom Taylor and Michael Thomas.

10. References

* Schulzrinne, H., Casner, S., Frederick, R. and V. Jacobson, "RTP:

A Transport Protocol for Real-Time Applications", RFC1889,

January 1996.

* Schulzrinne, H., "RTP Profile for Audio and Video Conferences with

Minimal Control", RFC1890, January 1996.

* Handley, M and V. Jacobson, "SDP: Session Description Protocol",

RFC2327, April 1998.

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

Progress.

* Handley, M., Schulzrinne, H. and E. Schooler, "Session Initiation

Protocol (SIP)", RFC2543, March 1999.

* Schulzrinne, H., Rao, A. and R. Lanphier, "Real Time Streaming

Protocol (RTSP)", RFC2326, April 1998.

* ITU-T, Recommendation Q.761, "FUNCTIONAL DESCRIPTION OF THE ISDN

USER PART OF SIGNALLING SYSTEM No. 7", (Malaga-Torremolinos, 1984;

modified at Helsinki, 1993)

* ITU-T, Recommendation Q.762, "GENERAL FUNCTION OF MESSAGES AND

SIGNALS OF THE ISDN USER PART OF SIGNALLING SYSTEM No. 7",

(MalagaTorremolinos, 1984; modified at Helsinki, 1993)

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

COMMUNICATIONS SYSTEMS."

* ITU-T, Recommendation H.225, "Call Signaling Protocols and Media

Stream Packetization for Packet Based Multimedia Communications

Systems."

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

MULTIMEDIA COMMUNICATION."

* Kent, S. and R. Atkinson, "Security Architecture for the Internet

Protocol", RFC2401, November 1998.

* Kent, S. and R. Atkinson, "IP Authentication Header", RFC2402,

November 1998.

* Kent, S. and R. Atkinson, "IP Encapsulating Security Payload

(ESP)", RFC2406, November 1998.

* Crocker, D. and P. Overell, "Augmented BNF for Syntax

Specifications: ABNF", RFC2234, November 1997.

11. Authors' Addresses

Mauricio Arango

RSL COM Latin America

6300 N.W. 5th Way, Suite 100

Ft. Lauderdale, FL 33309

Phone: (954) 492-0913

EMail: marango@rslcom.com

Andrew Dugan

Level3 Communications

1450 Infinite Drive

Louisville, CO 80027

Phone: (303)926 3123

EMail: andrew.dugan@l3.com

Isaac Elliott

Level3 Communications

1450 Infinite Drive

Louisville, CO 80027

Phone: (303)926 3123

EMail: ike.elliott@l3.com

Christian Huitema

Telcordia Technologies

MCC 1J236B

445 South Street

Morristown, NJ 07960

U.S.A.

Phone: +1 973-829-4266

EMail: huitema@research.telcordia.com

Scott Pickett

Vertical Networks

1148 East Arques Ave

Sunnyvale, CA 94086

Phone: (408) 523-9700 extension 200

EMail: ScottP@vertical.com

Further information is available on the SGCP web site:

http://www.argreenhouse.com/SGCP/

12. Appendix A: Proposed "MoveConnection" command

It has been proposed to create a new command, that would move an

existing connection from one endpoint to another, on the same

gateway. This command would be specially useful for handling certain

call services, such as call forwarding between endpoints served by

the same gateway.

[SecondEndPointId,]

[ConnectionId,]

[LocalConnectionDescriptor]

<--- ModifyConnection(CallId,

EndpointId,

ConnectionId,

SecondEndPointId,

[NotifiedEntity,]

[LocalConnectionOptions,]

[Mode,]

[RemoteConnectionDescriptor,]

[Encapsulated NotificationRequest,]

[Encapsulated EndpointConfiguration])

The parameters used are the same as in the ModifyConnection command,

with the addition of a SecondEndpointId that identifies the endpoint

towards which the connection is moved.

The EndpointId should be the fully qualified endpoint identifier of

the endpoint on which the connection has been created. The local name

shall not use the wildcard convention.

The SecondEndpointId shall be the endpoint identifier of the endpoint

towards which the connection has been created. The "any of" wildcard

convention can be used, but not the "all of" convention. If the

SecondEndpointId parameter is unqualified, the gateway will choose a

value, that will be returned to the call agent as a response

parameter.

The command will result in the "move" of the existing connection to

the second endpoint. Depending on gateway implementations, the

connection identifier of the connection after the move may or may not

be the same as the connection identifier before the move. If it is

not the same, the new value is returned as a response parameter.

The intent of the command is to effect a local relocation of the

connection, without having to modify such transmission parameters as

IP addresses and port, and thus without forcing the call agent to

signal the change of parameters to the remote gateway, at the other

end of the connection. However, gateway architectures may not always

allow such transparent moves. For example, some architectures could

allow specific IP addresses to different boards that handles specific

group of endpoints. If for any reason the transmission parameters

have to be changed as a result of the move, the new

LocalConnectionDescriptor is returned as a response parameter.

The LocalConnectionOptions, Mode, and RemoteConnectionDescriptor,

when present, are applied after the move.

The RequestedEvents, RequestIdentifier, DigitMap, SignalRequests,

QuarantineHandling and DetectEvents parameters are optional. They

can be used by the Call Agent to transmit a NotificationRequest that

is executed simultaneously with the move of the connection. When

these parameters are present, the NotificationRequest applies to the

second endpoint.

When these parameters are present, the move and the

NotificationRequests should be synchronized, which means that both

should be accepted, or both refused. The NotifiedEntity parameter,

if present, applies to both the ModifyConnection and the

NotificationRequest command.

The command may carry an encapsulated EndpointConfiguration command,

that will also apply to the second endpoint. When this command is

present, the parameters of the EndpointConfiguration command are

inserted after the normal parameters of the MoveConnection with the

exception of the SecondEndpointId, which is not replicated. The End-

pointConfiguration command may be encapsulated together with an

encapsulated NotificationRequest command.

The encapsulated EndpointConfiguration command shares the fate of the

MoveConnection command. If the MoveConnection is rejected, the End-

pointConfiguration is not executed.

12.1. Proposed syntax modification

The only syntax modification necessary for the addition of the

moveConnection command is the addition of the keyword MOVE to the

authorized values in the MGCPVerb clause of the formal syntax.

13. Full Copyright Statement

Copyright (C) The Internet Society (1999). All Rights Reserved.

This document and translations of it may be copied and furnished to

others, and derivative works that comment on or otherwise explain it

or assist in its implementation may be prepared, copied, published

and distributed, in whole or in part, without restriction of any

kind, provided that the above copyright notice and this paragraph are

included on all such copies and derivative works. However, this

document itself may not be modified in any way, such as by removing

the copyright notice or references to the Internet Society or other

Internet organizations, except as needed for the purpose of

developing Internet standards in which case the procedures for

copyrights defined in the Internet Standards process must be

followed, or as required to translate it into languages other than

English.

The limited permissions granted above are perpetual and will not be

revoked by the Internet Society or its successors or assigns.

This document and the information contained herein is provided on an

"AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING

TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING

BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION

HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF

MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.

Acknowledgement

Funding for the RFCEditor function is currently provided by the

Internet Society.

 
 
 
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