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
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The limited permissions granted above are perpetual and will not be
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