Internet Engineering Task Force M. Arango
Internet Draft SUN Microsystems
Document: draft-andreasen-mgcp-rfc2705bis-00.txt A. Dugan
Category: Informational I. Elliott
Level3 Communications
C. Huitema
Microsoft
S. Pickett
Vertical Networks
F. Andreasen
B. Foster
R. Kumar
Cisco Systems
January 16, 2001
Media Gateway Control Protocol (MGCP)
Version 1.0bis
Status of this Memo
This document is an Internet-Draft and is in full conformance with
all provisions of Section 10 of RFC2026 [1].
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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 8 main sections:
* The introduction presents the basic assumptions and the relation
to other protocols such as H.323, RTSP, SAP or SIP.
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* 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:
Endpoint Configuration, Notification 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.
* A section describing race conditions and fail-over
considerations.
* The security section presents the security requirement of MGCP,
and its usage of IP security services (IPSEC).
* A section describing the extension mechanisms using packages.
* The description of the changes made in combining SGCP 1.1 and
IPDC to create MGCP 1.0 and the enhancements and clarifications
introduced in RFC 2705bis.
Comments on this document should be sent to the MGCP mailing list at
"mgcp@pulver.com" and/or the authors.
Table of Contents
1. INTRODUCTION......................................................6
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..................................11
2.1. Model and Naming Conventions..................................11
2.1.1. Types of Endpoints........................................11
2.1.1.1. Digital Channel (DS0).................................12
2.1.1.2. Analog Line...........................................12
2.1.1.3. Announcement Server Access Point......................13
2.1.1.4. Interactive Voice Response Access Point...............13
2.1.1.5. Conference Bridge Access Point........................14
2.1.1.6. Packet Relay..........................................14
2.1.1.7. Wiretap Access Point..................................14
2.1.1.8. ATM "trunk side" Interface............................15
2.1.2. Endpoint Identifiers......................................15
2.1.3. Calls and Connections.....................................17
2.1.3.1. Names of Calls........................................19
2.1.3.2. Names of Connections..................................20
2.1.3.3. Management of Resources, Attributes of Connections....20
2.1.3.4. Special Case of Local Connections.....................22
2.1.4. Names of Call Agents and Other Entities...................23
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2.1.5. Digit Maps................................................24
2.1.6. Packages..................................................26
2.1.7. Events and Signals........................................27
2.2. Usage of SDP..................................................31
2.3. Gateway Control Commands......................................31
2.3.1. Overview of Commands.......................................31
2.3.2. EndpointConfiguration......................................33
2.3.3. NotificationRequest.......................................34
2.3.4. Notify....................................................40
2.3.5. CreateConnection...........................................41
2.3.6. ModifyConnection..........................................45
2.3.7. DeleteConnection (from the Call Agent)....................47
2.3.8. DeleteConnection (from the gateway).......................51
2.3.9. DeleteConnection (multiple connections, from the Call Agent)
..................................................................51
2.3.10. AuditEndpoint............................................52
2.3.11. AuditConnection..........................................56
2.3.12. RestartInProgress........................................57
2.4. Return Codes and Error Codes..................................59
2.5. Reason Codes..................................................62
2.6. Use of Local Connection Options and Connection Descriptors.....62
2.7. Resource Reservations..........................................64
3. MEDIA GATEWAY CONTROL PROTOCOL...................................66
3.1. General Description...........................................66
3.2. Command Header................................................66
3.2.1. Command Line..............................................67
3.2.1.1. Coding of the Requested Verb..........................67
3.2.1.2. Transaction Identifiers...............................68
3.2.1.3. Coding of the Endpoint Identifiers and Entity Names...68
3.2.1.4. Coding of the Protocol Version........................69
3.2.2. Parameter Lines...........................................69
3.2.2.1. Response Acknowledgement..............................72
3.2.2.2. Local Connection Options..............................72
3.2.2.3. Capabilities..........................................75
3.2.2.4. Connection Parameters.................................76
3.2.2.5. Reason Codes..........................................78
3.2.2.6. Connection Mode.......................................78
3.2.2.7. Coding of Event Names.................................78
3.2.2.8. RequestedEvents.......................................79
3.2.2.9. SignalRequests........................................81
3.2.2.10. ObservedEvents.......................................81
3.2.2.11. RequestedInfo........................................81
3.2.2.12. QuarantineHandling...................................82
3.2.2.13. DetectEvents.........................................82
3.2.2.14. EventStates..........................................82
3.2.2.15. RestartMethod........................................83
3.2.2.16. Bearer Information...................................83
3.3. Format of response headers....................................83
3.3.1. CreateConnection Response..................................85
3.3.2. ModifyConnection Response..................................86
3.3.3. DeleteConnection Response..................................87
3.3.4. NotificationRequest Response...............................87
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3.3.5. Notify Response............................................87
3.3.6. AuditEndpoint Response.....................................87
3.3.7. AuditConnection Response...................................88
3.3.8. RestartInProgress Response.................................88
3.4. Encoding of the Session Description (SDP).....................89
3.4.1. Usage of SDP for an Audio Service.........................90
3.4.2. Usage of SDP for LOCAL Connections........................90
3.5. Transmission over UDP.........................................91
3.5.1. Providing the At-Most-Once Functionality..................91
3.5.2. Transaction Identifiers and Three Ways Handshake..........92
3.5.3. Computing Retransmission Timers...........................93
3.5.4. Piggy Backing.............................................94
3.5.5. Provisional Responses.....................................95
4. STATES, FAILOVER AND RACE CONDITIONS.............................97
4.1. Failover Assumptions and Highlights...........................97
4.2. Communicating with Gateways...................................99
4.3. Retransmission, and Detection of Lost Associations:...........99
4.4. Race Conditions..............................................103
4.4.1. Quarantine List..........................................103
4.4.2. Explicit Detection.......................................107
4.4.3. Transactional Semantics..................................108
4.4.4. Ordering of Commands, and Treatment of Disorder..........109
4.4.5. Fighting the Restart Avalanche...........................110
4.4.6. Disconnected Endpoints...................................112
5. SECURITY REQUIREMENTS...........................................114
5.1. Protection of Media Connections..............................114
6. PACKAGES.........................................................116
6.1 BearerInformation..............................................116
6.2 LocalConnectionOptions.........................................117
6.3 ExtensionParameters............................................117
6.4 ConnectionModes................................................118
6.5 Events and Signals.............................................118
6.5.1 Default and Reserved Events................................120
6.6 Actions........................................................121
6.7 DigitMapLetters................................................121
6.8 ConnectionParameters...........................................122
6.9 RestartMethods.................................................122
6.10 Reason Codes..................................................122
6.11 Return Codes..................................................123
7. VERSIONS AND COMPATIBILITY......................................124
7.1. Differences between RFC 2705bis and RFC 2705.................124
7.2. Differences between Version 1.0 and draft-0.5................126
7.3. Differences between draft-04 and draft-05....................127
7.4. Differences between draft-03 and draft-04....................127
7.5. Differences between draft-02 and draft-03....................127
7.6. Differences between draft-01 and draft-02....................127
7.7. The making of MGCP from IPDC and SGCP........................128
7.8. Changes between MGCP and Initial Versions of SGCP............128
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8. SECURITY CONSIDERATIONS.........................................131
9. ACKNOWLEDGEMENTS................................................132
10. REFERENCES.....................................................133
11. AUTHORS' ADDRESSES.............................................135
APPENDIX A: FORMAL SYNTAX DESCRIPTION OF THE PROTOCOL...............137
APPENDIX B: IANA CONSIDERATIONS.....................................144
B.1. Packages......................................................144
B.2. Local Connection Options......................................144
APPENDIX C: MODE INTERACTIONS.......................................145
APPENDIX D: EXAMPLE COMMAND ENCODINGS...............................147
D.1. NotificationRequest...........................................147
D.2. Notify........................................................147
D.3. CreateConnection..............................................148
D.4. ModifyConnection..............................................150
D.5. DeleteConnection (from the Call Agent)........................151
D.6. DeleteConnection (from the gateway)...........................151
D.7. DeleteConnection (multiple connections from the Call Agent)...151
D.8. AuditEndpoint.................................................152
D.9. AuditConnection...............................................153
D.10. RestartInProgress............................................154
APPENDIX E: ENDPOINT NAMING CONVENTIONS.............................156
E.1. Analog Access Line Endpoints..................................156
E.2. Digital Trunks................................................156
E.3. Virtual Endpoints.............................................156
APPENDIX F: EXAMPLE CALL FLOWS......................................158
F.1. Restart.......................................................158
F.1.1. Residential Gateway Restart...............................158
F.1.2. Call Agent Restart........................................161
F.2. Connection Creation...........................................162
F.2.1 Residential Gateway to Residential Gateway................163
F.3 Connection Deletion...........................................168
F.3.1 Residential Gateway to Residential Gateway................168
FULL COPYRIGHT STATEMENT............................................171
ACKNOWLEDGEMENT.....................................................172
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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.
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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 "signaling
gateway" when connecting to an SS7 controlled network. In a typical
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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), RFC 2327,
* the Session Announcement Protocol (SAP),
* the Session Initiation Protocol (SIP),
* the Real Time Streaming Protocol (RTSP), RFC 2326.
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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 syn- | |
| | | chronization | |
| | | 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
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Trunk: A communication channel between two switching systems. E.g.,
a DS0 on a T1 or E1 line.
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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 functionality. Our analysis, so far, has led us to
isolate the following basic endpoint types:
* Digital channel (DS0),
* Analog line,
* Announcement 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
endpoints.
This list is not final. 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.
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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 signaling. This is
the case for example of SS7 "F" links, or ISDN "D" channels. Media
gateways that support these signaling functions shall be able to
send and receive the signaling 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 signaling, such as "MF R2".
Media gateways that support these signaling functions shall be able
to detect and produce the corresponding signals, such as for example
"wink" or "A", according to the event signaling 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.
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+-------
+---------------+|
(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 way calling".
2.1.1.3. Announcement Server Access Point
An announcement server endpoint provides access 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 signaling 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 one way, or
"half duplex" -- the announcement server is not expected to listen
to the audio signals from the connection.
2.1.1.4. Interactive Voice Response Access Point
An Interactive Voice Response (IVR) endpoint provides access 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
signaling 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
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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.
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Connections to an wiretap endpoint are typically one-way, 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,
Endpoint names will be of the form:
local-endpoint-name@domain-name
where domain-name is an absolute domain-name as defined in RFC 1034
and includes a host portion, thus an example domain-name could be:
mygateway.whatever.net
Also, domain-name may be an IP-address on the form defined for
domain name in RFC 821, thus another example could be
[192.168.1.2]
Both IPv4 and IPv6 addresses can be specified, however use of IP
addresses as endpoint identifiers is generally discouraged.
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The local endpoint name is case-insensitive. The syntax of the local
endpoint name is hierarchical, where the least specific component of
the name is the leftmost term, and the most specific component is
the rightmost term. The syntax depends on the type of endpoint being
named and MAY start with a term that identifies the endpoint type.
The local endpoint name MUST adhere to the following naming rules:
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 delimiters ("/", "@"), 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 local endpoint name 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". This generally refers to all endpoints configured for
service, regardless of their actual service state, i.e. in
service or out of service. 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". This generally only
refers to endpoints that are in service. The description of a
specific command may add further criteria for selection within
the general rules given here.
Note, that wild-cards may be applied to more than one term in which
case they shall be evaluated from left to right. For example, if we
have the endpoint names "a/1", "a/2", "b/1", and "b/2", then "$/*"
will evaluate to either "a/1, a/2", or "b/1, b/2". However, "*/$"
may evaluate to "a/1, b/1", "a/1, b/2", "a/2, b/1", or "a/2, b/2".
The use of mixed wild-cards is considered error prone and is
consequently discouraged.
If the Media Gateway controls multiple physical gateways, the first
term of the naming MUST identify the physical gateway containing the
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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.
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 signaling 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
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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:
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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. Call identifiers are hexadecimal
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strings, which are created by the Call Agent. The maximum length of
call identifiers is 32 characters.
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. From the gateways perspective, the Call
identifier is thus unique. 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. Connection identifiers are
treated in MGCP as hexadecimal 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.). The maximum length of a connection identifier is 32
characters.
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 (referred to as the remote session
description).
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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 values, or no value at all. This allows
various implementations to implement various levels 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 lets 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.
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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 call to an Interactive Voice-Response unit,
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* 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.
Call Agent names consist of two parts, similar to endpoint names.
Semantically, the local portion of the name does not exhibit any
internal structure. An example Call Agent name is:
ca1@ca.whatever.net
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
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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 a "NotifiedEntity" parameter is sent with an
"empty" value, the "notified entity" for the endpoint will be set to
empty. 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 corresponds 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]xxx|8xxxxxxx|#xxxxxxx|*xx|91xxxxxxxxxx|9011x.T)
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The formal syntax of the digit map is described by the DigitMap rule
in the formal syntax description of the protocol (Appendix A)
support for basic digit map letters is mandatory while support for
extension digit map letters is optional. A gateway receiving a digit
map with an extension digit map letter not supported SHOULD return
error code 537 (unknown digit map extension).
A Digit-Map, according to this syntax, is defined either by a (case
insensitive) "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 over which the gateway
will attempt to find a shortest possible match. 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
3) If the result is under-qualified (partially matches at least one
entry in the digit map and doesn't completely match another
entry), do nothing further.
If the result matches an entry, 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 occurs when the dial
string does not match any of the alternatives. Unexpected timers,
for example, can cause "impossible matches". Both perfect matches
and impossible matches trigger notification of the accumulated
digits.
The following example illustrates the above. Assume we have the
digit map:
(xxxxxxx|x11)
and a current dial string of "41". Given the input "1" the current
dial string becomes "411". We have a partial match with "xxxxxxx",
but a complete match with "x11", and hence we send "411" to the Call
Agent.
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.
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* 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 inter-
digit 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 dial
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.
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. Packages
MGCP is a modular and extensible protocol, however with
extensibility comes the need to manage, identify, and name the
extensions. This is achieved by the concept of packages, which are
simply well-defined groupings of extensions. For example, one
package may support a certain group of events and signals, e.g. off-
hook and ringing, for analog access lines. Another package may
support another group of events and signals for analog access lines
or another type of endpoint such as video. One or more packages may
exist for a given endpoint.
MGCP allows the following types of extensions to be defined in a
package:
* BearerInformation
* LocalConnectionOptions
* ExtensionParameters
* ConnectionModes
* Events
* Signals
* Actions
* DigitMapLetters
* ConnectionParameters
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* RestartMethods
* ReasonCodes
* Return codes
each of which will be explained in more detail below. The rules for
defining each of these extensions in a package are described in
Section 6.
With the exception of DigitMapLetters a package defines a separate
namespace for each type of extension by adding the package name as a
prefix to the extension, i.e.:
package-name/extension
Thus the package-name is followed by a slash ("/") and the name of
the extension.
An endpoint supporting one or more packages may define one of those
packages as the default package for the endpoint. Use of the package
name for events and signals in the default package for an endpoint
is optional, however it is recommended to always include the package
name. All other extensions, except DigitMapLetter, defined in the
package must include the package-name when referring to the
extension.
Package names are case insensitive strings of letters, hyphens and
digits, with the restriction that hyphens shall never be the first
or last character in a name. Examples of package names are "D", "T",
and "XYZ". Package names are not case sensitive names such as
"XYZ", "xyz", and "xYz" are equal.
Package definitions will be provided in other documents and with
package names and extensions names registered with IANA. For more
details, refer to section 6.
Implementers can gain experience by using experimental packages. The
name of an experimental package must start with the two characters
"x-"; the IANA shall not register package names that start with
these characters or the characters "x+" which are reserved. A
gateway that receives a command referring to an unsupported package
MUST return an error (error code 518 unsupported package, is
recommended).
2.1.7. Events and Signals
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) by including the name of the event in a
RequestedEvents parameter (in a NotificationRequest command - see
later).
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A Call Agent may also request certain signals to be applied to an
endpoint (e.g. dial-tone) by supplying the name of the event in a
SignalRequests parameter.
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. Event names are case insensitive strings of letters,
hyphens and digits, with the restriction that hyphens shall never be
the first or last character in a name. Some event codes may need to
be parameterized with additional data, which is accomplished by
adding the parameters between a set of parentheses. Event names are
not case sensitive - values such as "hu", "Hu", "HU" or "hU" should
be considered equal.
Examples of event names can be "hu" (off hook or "hang-up"
transition), "hf" (flash hook) or "0" (the digit zero).
The package name is optional for events in the default package for
an endpoint, however it is recommended to always include the package
name. If the package name is excluded from the event name, the
default package name for that endpoint is assumed. For example, for
an analog access line which has the line package as a default with
dial-tone ("dl") as one of the events in that package, the following
two event names are equal:
l/dl
and
dl
For any other non-default packages that are associated with that
endpoint-type, (such as the generic package for an analog access
endpoint-type for example), the package name MUST be included with
the event name. Again, unconditional inclusion of the package name
is recommended.
Digits, or letters, are supported in some packages, notably "DTMF".
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. Also, extensions may
define use of other letters. 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
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A, B, C, D, T or X. (Such names should not contain the special signs
"*", "#", "/" or "$".)
A Call Agent may often have to ask a gateway to detect a group of
events. Two conventions can be used to denote such groups:
* The wildcard convention can be used to detect any event belonging
to a package, or a given event in many packages, or any event in
any package supported by the gateway.
* The regular expression Range notation can be used to detect a
range of digits.
The star sign (*) can be used as a wildcard instead of a package
name, and the keyword "all" can be used as a wildcard instead of an
event name:
* A name such as "foo/all" denotes all events in package "foo".
* A name such as "*/bar" denotes the event "bar" in any package
supported by the gateway.
* The names "*" or "*/all" denote all events supported by the
gateway.
The Call Agent can ask a gateway to detect a set of digits or
letters either by individually describing those letters, or by using
the "range" notation defined in the syntax of digit strings. For
example, the Call Agent can:
Use the letter "x" to denote "any letter or digit". Use the notation
"[0-9#]" to denote the digits 0 to 9 and the pound sign.
In some cases, Call Agents will request the gateway to generate or
detect events on connections rather than on the endpoint itself. For
example, gateways may be asked to provide a ringback tone on a
connection. When an event shall be applied on a connection, the name
of the connection is added to the name of the event, using an "at"
sign (@) as a delimiter, as in:
G/rt@0A3F58
where G is the name of the package and rt is the name of the event.
The wildcard character "*" (star) can be used to denote "all
connections". When this convention is used, the gateway will
generate or detect the event on all the connections that are
connected to the endpoint. An example of this convention could be:
R/qa@*
where R is the name of the package and qa is the name of the event.
The wildcard character "$" 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 connection
creation or modification command. When this convention is used, the
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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.
Signals are divided into different types depending on their
behavior:
* On/off (OO): Once applied, these signals last until they
are turned off. This can only happen as the result of a new
SignalRequests where the signal is turned off (see later).
Signals of type OO are defined to be idempotent, thus multiple
requests to turn a given OO signal on (or off) are perfectly
valid and MUST NOT result in any errors. An On/Off signal could
be a visual message waiting indicator (VMWI). Once turned on, it
MUST NOT be turned off until explicitly instructed to by the Call
Agent, or as a result of an endpoint restart, i.e. these signals
will not turn off as a result the arrival of a requested event
* Time-out (TO): Once applied, these signals last until they
are either cancelled (by the occurrence of an event or by not
being included in a subsequent (possibly empty) list of signals),
or a signal-specific period of time has elapsed. A signal that
times out will generate an "operation complete" event. A TO
signal could be "ringback" timing out after 180 seconds. If an
event occurs prior to the 180 seconds, the signal will, by
default, be stopped (the "Keep signals active" action may
override this behavior). If the signal is not stopped, the signal
will time out, stop and generate an "operation complete" event,
about which the Call Agent may or may not have requested to be
notified. If the Call Agent has asked for the "operation
complete" event to be notified, the "operation complete" event
sent to the Call Agent will include the name(s) of the signal(s)
that timed out (note that if parameters were passed to the
signal, the parameters will not be reported). Time-out signals
have a default time-out value defined for them, which may be
altered by the provisioning process. Also, the time-out period
may be provided as a parameter to the signal. A value of zero
indicates that the time-out period is infinite. A TO signal that
fails after being started, but before having generated on
"operation complete" event will generate an "operation failure"
event which will include the name of the signal that timed out.
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* Brief (BR): The duration of these signals is normally so
short that they stop on their own. If a signal stopping event
occurs, or a new SignalRequests is applied, a currently active BR
signal will not stop. However, any pending BR signals not yet
applied will be cancelled. As an example, a brief tone could be
a DTMF digit. If the DTMF digit "1" is currently being played,
and a signal stopping event occurs, the "1" would finish playing.
Signal(s) generated on a connection will include the name of that
connection.
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 RFC 2327.
2.3. Gateway Control Commands
2.3.1. Overview of 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.
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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), "inactive"
(inactive), "loopback", "continuity test" (conttest), "network loop
back" (netwloop) or "network continuity test" (netwtest).
Media generated by the endpoint is sent on connections whose mode is
either "send only", "send/receive", or "conference". However, media
generated by applying a signal to a connection is always sent on the
connection, regardless of the mode.
The handling of the audio streams received on connections is
determined by the mode parameters:
* Audio streams received through connections in "receive",
"conference" or "send/receive" mode are mixed and sent to the
endpoint.
* Audio streams 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, an audio stream
received through connections in "conference" mode are forwarded
to all the other connections whose mode is "conference". The
details of this forwarding, e.g., RTP translator or mixer, etc.,
is outside the scope of this document.
Note that in order to detect events on a connection, the connection
must be in one of the modes "receive", "conference", or
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"send/receive". The event detection only applies to the incoming
media.
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 expects 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.
2.3.2. 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 of the endpoint in the gateway where
EndpointConfiguration executes. The "any of" wildcard convention
shall not be used. If the "all of" wildcard convention is used, the
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command applies to all the endpoints 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". The set of sub-parameters may be extended.
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.
2.3.3. 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 and instruct the gateway accordingly with a
ModifyConnection Command.
ReturnCode
<-- NotificationRequest(EndpointId,
[NotifiedEntity,]
[RequestedEvents,]
RequestIdentifier,
[DigitMap,]
[SignalRequests,]
[QuarantineHandling,]
[DetectEvents,]
[encapsulated EndpointConfiguration])
EndpointId is the identifier for the endpoint in the gateway where
the NotificationRequest. The "any of" wildcard MUST NOT be used.
NotifiedEntity is an optional parameter that specifies a new
"notified entity" for the endpoint.
RequestIdentifier is used to correlate this request with the
notifications that it triggers. It will be repeated in the
corresponding Notify command.
RequestedEvents is a list of events, possibly qualified by event
parameters, that the gateway is requested to detect and report. Such
events include, for example, fax tones, continuity tones, or on-hook
transition. Unless otherwise specified, events are detected on the
endpoint, however some events can be detected on a connection.
To each event is associate an action, which can be:
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* 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.
Support for Notify, Accumulate, Keep Signal(s) Active, Embedded
Notification Request, and Ignore is mandatory. Support for
Accumulate according to Digit Map is mandatory on any endpoint
capable of detecting DTMF. Support for any other action is optional.
The set of actions can be extended.
Some actions can be combined as shown in the table below, where "Y"
means the two actions can be combined, and "N" means they can not:
--------------------------------------------------------------
| | Notif | Swap | Accum | AccDi | KeSiA | EmbNo | Ignor |
|--------------------------------------------------------------|
| Notif | N | Y | N | N | Y | Y* | N |
| Swap | - | N | Y | N | N | N | Y |
| Accum | - | - | N | N | Y | Y | N |
| AccDi | - | - | - | N | Y | N | N |
| KeSiA | - | - | - | - | N | Y | Y |
| EmbNo | - | - | - | - | - | N | N |
| Ignor | - | - | - | - | - | - | N |
--------------------------------------------------------------
Note (*): The "Embedded Notification Request" can only be
combined with "Notify", if the gateway is allowed to issue
several Notify commands in response to a single Notification
request.
When more than two two are combined, each action must be combinable
with all the other actions as determined by the table above.
If a client receives a request with an invalid or unsupported action
or an illegal combination of actions, it MUST return an error to the
Call Agent (error code 523 unknown or illegal combination of
actions recommended).
When multiple actions are specified, e.g., "Keep signal(s) active"
and "Notify", the individual actions are assumed to occur
simultaneously.
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In addition to the RequestedEvents parameter specified in the
command, some MGCP packages may contain "persistent events".
Persistent events in a given package are always detected on an
endpoint that implement that package. 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 that need to be 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 "off-
hook" was a persistent event, "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 included in either list, it
will be ignored.
The Call Agent can send a NotificationRequest with an empty
RequestedEvents list to the gateway. The Call Agent can do so, for
example, to a gateway when it does not want to collect any more DTMF
digits. However, persistent events will still be detected and
notified.
The Swap Audio action can be used when a gateway handles more than
one 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,
RequestIdentifier, QuarantineHandling and DetectEvents. When the
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"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 endpoint 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
RequestedEvents 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
accumulated are found in between the list of digits. If the gateway
is requested to "accumulate according to digit map" and the gateway
currently does not have a digit map for the endpoint in question,
the gateway MUST return an error (error code 519 endpoint does not
have a digit map).
SignalRequests is a parameter that contains the set of signals that
the gateway is asked to apply. When multiple signals are specified,
the signals will be applied in parallel. Unless otherwise specified,
signals are applied to the endpoint, however some signals can be
applied to a connection. Signals are identified by their name, which
is an event name, and may be qualified by signal parameters. The
following are examples of signals:
* Ringing,
* Busy tone,
* Call waiting tone,
* Off hook warning tone,
* Ringback tones on a connection
Names and descriptions of signals are defined in the appropriate
package.
Signals are, by default, applied to endpoints. If a signal applied
to an endpoint results in the generation of a media stream (audio,
video, etc.), the media stream MUST NOT be forwarded on any
connection associated with that endpoint, regardless of the mode of
the connection. For example, if a call-waiting tone is applied to an
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endpoint involved in an active call, only the party using the
endpoint in question will hear the call-waiting tone. However,
individual signals may define a different behavior.
When a signal is applied to a connection that has received a
RemoteConnectionDescriptor the media stream generated by that signal
will be forwarded on the connection regardless of the current mode
of the connection. If a RemoteConnectionDescriptor has not been
received, the gateway MUST return an error (error code 527 missing
RemoteConnectionDescriptor).
When a (possibly empty) list of signal(s) is supplied, this list
completely replaces the current list of active time-out signals.
Currently active time-out signals that are not provided in the new
list MUST be stopped and the new signal(s) provided will now become
active. Currently active time-out signals that are provided in the
new list of signals MUST remain active without interruption, thus
the timer for such time-out signals will not be affected.
Consequently, there is currently no way to restart the timer for a
currently active time-out signal without turning the signal off
first. If the time-out signal is parameterized, the original set of
parameters will remain in effect, regardless of what values are
provided subsequently. A given signal MUST NOT appear more than once
in a SignalRequests.
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 detects 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 RequestedEvents and SignalRequests may 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 must return
an error (error code 518 - unsupported or unknown package
recommended). 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
must return an error(error code 522 no such event or signal
recommended).
The Call Agent can send a NotificationRequest whose requested signal
list is empty. It will do so for example when a time-out signal(s)
should stop.
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If signal(s) are desired to start as soon as a "looked-for" event
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
embedded NotificationRequest action allows the Call Agent to set up
a "mini-script" to be processed by the gateway immediately following
the detection of the associated event. Any SignalRequests specified
in the embedded NotificationRequest will start immediately.
Considerable care must be taken to prevent discrepancies between the
Call Agent and the gateway. However, long-term discrepancies should
not occur as new SignalRequests completely replaces the old list of
active time-out signals, and BR-type signals always stop on their
own. Limiting the number of On/Off-type signals is encouraged. It is
considered good practice for a Call Agent to occasionally turn on
all On/Off signals that should be on, and turn off all On/Off
signals that should be off.
The Ignore action can be used to ignore an event, e.g., to prevent a
persistent event from being notified. However, the synchronization
between the event and an active time-out signal will still occur by
default (e.g. a time-out dial-tone signal will stop when an off-hook
occurs even if off-hook was a requested event with action "Ignore").
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 and processed 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
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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.
2.3.4. Notify
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. 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
NotifiedEntity parameter of the NotificationRequest that triggered
this notification. The parameter is absent if there was no such
parameter in the triggering request. Regardless of the value of the
NotifiedEntity parameter, the notification MUST be sent to the
current "notified entity" for the endpoint.
RequestIdentifier is a 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. Persistent events will be viewed here as
if they had been included in the last NotificationRequest. When no
NotificationRequest has been received, the RequestIdentifier used
will be zero ("0").
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
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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. . It should be noted that digits are added
to the list of observed events as they are accumulated, irrespective
of whether they are accumulated according to the digit map or not.
For example, if a user enters the digits "1234" and some event E is
accumulated between the digits "3" and "4" being entered, the list
of observed events would be "1, 2, 3, E, 4". Events that were
detected on a connection will include the name of that connection.
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.
2.3.5. 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 parameter that identifies the call (or session) to which
this connection belongs. This parameter is, at a minimum, unique
within the collection of Call Agents that control the same gateways.
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
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identifier will be assigned by the gateway and its complete value
returned in the SpecificEndPointId parameter of the response. The
endpoint assigned must be in service and must not already have any
connections on it. The "all of" wildcard must not be used.
The NotifiedEntity is an optional parameter that specifies a new
"notified entity" for the endpoint.
LocalConnectionOptions is a structure used by the Call Agent to
direct the handling of the connection by the gateway. The fields
contained in a LocalConnectionOptions structure may include one or
more of the following:
* Codec compression algorithm: One or more codecs, listed in order
of preference. See Section 2.6 for details on the codec selection
process.
* Packetization period: A single millisecond value or a
range may be specified. The packetization period should not
contradict the specification of the codec compression algorithm.
If a codec is specified that has a frame size which is
inconsistent with the packetization period, and that codec is
selected, the gateway is authorized to use a packetization period
that is consistent with the frame size even if it is different
from that specified. In so doing, the gateway should choose a
packetization period as close to that specified as possible.
* Bandwidth: The allowable bandwidth excluding
lower level transports, but including header overhead at this
level, e.g., RTP header plus payload. The bandwidth specification
should not contradict the specification of codec compression
algorithm or packetization period. If a codec is specified, then
the gateway is authorized to use it, even if it results in the
usage of a larger bandwidth than specified. Any discrepancy
between the bandwidth and codec specification will not be
reported as an error.
* Type of Service: This indicates the class of service
to be used for this connection. When the Type of Service is not
specified, the gateway shall use a default or configured value.
* Usage of echo cancellation: By default, the telephony gateways
always perform echo cancellation. However, it may be 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.)
* Silence Suppression: 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
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requested) and "off" (when it is turned off.) The default is
"off".
* Gain Control: 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.
* RTP security: The Call agent can request the
gateway to enable encryption of the audio Packets. It does so by
providing a key specification, as specified in RFC 2327. By
default, encryption is not performed.
* Network Type: The Call Agent may instruct the
gateway to prepare the connection on a specified type of network.
If absent, the value is based on the network type of the gateway
being used.
* Resource reservation: The Call Agent may instruct the
gateway to use network resource reservation for the connection.
See Section 2.7 for details.
The Call Agent specifies the relevant fields it cares about in the
command and leaves the rest to the discretion of the gateway. For a
detailed list of local connection options included with this
specification refer to section 3.2.2.2. The set of local connection
options can be extended.
The Mode indicates the mode of operation for this side of the
connection. The basic modes are "send", "receive", "send/receive",
"conference", "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 Control
Commands", section 2.3. Some endpoints may not be capable of
supporting all modes. If the command specifies a mode that the
endpoint cannot support, an error shall be returned (error 517 -
unsupported mode recommended). Also, if a connection has not yet
received a RemoteConnectionDescriptor, an error MUST be returned if
the connection is attempted to be placed in any of the modes "send
only", "send/receive", "conference", or if a signal is to be applied
to the connection (error code 527 missing
RemoteConnectionDescriptor recommended). The set of modes can be
extended.
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 the
connection, e.g. IP address and port for the media, as defined in
SDP. The SpecificEndPointId is an optional parameter that identifies
the responding endpoint. It is returned when the EndpointId argument
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referred to an "any of" wildcard name and the command succeeded.
When a SpecificEndPointId is returned, the Call Agent shall use it
as the EndpointId value in successive commands referring to this
call.
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 "any of"
wildcard convention. If the SecondEndpointId is underspecified, the
second endpoint identifier will be assigned by the gateway and its
complete value returned in the SecondEndPointId parameter of the
response.
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. The second connection is established in
"send/receive" mode.
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, or 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, and SendOnly
don't make sense in this situation. They should be treated as
errors, and the command should be rejected (Error code 517 or 527).
The command may optionally contain an encapsulated Notification
Request command, which applies to the EndpointId, in which case a
RequestIdentifier parameter must be present, as well as, optionally,
other parameters of the NotificationRequest with the exception of
the EndpointId, which is not replicated. The encapsulated
NotificationRequest is executed simultaneously with the creation of
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the connection. For example, when the Call Agent wants to initiate a
call to a 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 RequestedEvents parameters for the off-hook
event, and the SignalRequests parameter for the ringing signal.
When these parameters are present, the creation and the
NotificationRequest should be synchronized, which means that both
should 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 NotificationRequest 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,
which applies to the EndpointId. When this command is present, the
parameters of the EndpointConfiguration command are included with
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. Note that both of these apply to the
EndpointId only.
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.
2.3.6. 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
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local connection descriptor 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, in addition to 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 conventions.
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. Thus, if,
for example, only the mode of the connection is changed, a
LocalConnectionDescriptor will not be returned. Note however, that
inclusion of LocalConnectionOptions in the command is not a
prerequisite for local connection parameter changes to occur. If a
connection parameter is omitted, (e.g. mode or silence suppression),
the old value of that parameter will be retained if possible. If a
parameter change necessitates a change in one or more unspecified
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parameters, the gateway is free to choose suitable values for the
unspecified parameters that must change. This can for instance
happen if the packetization period was not specified. If the new
codec supported the old packetization period, the value of this
parameter would not change, as a change would not be necessary.
However, if it did not support the old packetization period, it
would choose a suitable value.
The command may optionally contain an encapsulated Notification
Request command, in which case a RequestIdentifier parameter must be
present, as well as, optionally, other parameters of the
NotificationRequest with the exception of the EndpointId, which is
not replicated. 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 RequestedEvents
parameters, for the on-hook event, and an empty SignalRequests
parameter, to stop the provision of ringing tones.
When these parameters are present, the modification and the
NotificationRequest 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 included
with 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.
2.3.7. 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.
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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 case of IP multicast, connections can be deleted individually
and independently. However, in the unicast case where a connection
has two ends, a DeleteConnection command has to be sent to both
gateways involved in the connection. After the connection has been
deleted, media streams previously supported by the connection are no
longer available. Any media packets received for the old connection
are simply discarded and no new media packets for the stream are
sent.
After the connection has been deleted, any loopback that has been
requested for the connection should be cancelled.
In response to the DeleteConnection command, the gateway returns a
list of connection parameters that describe statistics for the
connection.
When the connection was for an Internet media stream, these
parameters are:
Number of packets sent:
The total number of media packets transmitted by the sender since
starting transmission on this connection. In the case of RTP, 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 ModifyConnection command. The value is zero if the
connection was always set in "receive only" mode and no signals
were applied to the connection.
Number of octets sent:
The total number of payload octets (i.e., not including header or
padding) transmitted in media packets by the sender since
starting transmission on this connection. In the case of RTP, 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 always set in "receive only" mode and
no signals were applied to the connection.
Number of packets received:
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The total number of media packets received by the sender since
starting reception on this connection. In the case of RTP, the
count includes packets received from different SSRC, if the
sender used several values. The value is zero if the connection
was always set in "send only" mode.
Number of octets received:
The total number of payload octets (i.e., not including header,
e.g. RTP, or padding) transmitted in media packets by the sender
since starting transmission on this connection. In the case of
RTP, the count includes packets received from different SSRC, if
the sender used several values. The value is zero if the
connection was always set in "send only" mode.
Number of packets lost:
The total number of media 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. For RTP. 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 always set in "send only" mode.
Interarrival jitter:
An estimate of the statistical variance of the media packet
interarrival time measured in milliseconds and expressed as an
unsigned integer. For RTP, 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 RFC 1889. The count includes packets received from
different SSRC, if the sender used several values. The value is
zero if the connection was always set in "send only" mode.
Average transmission delay:
An estimate of the network latency, expressed in milliseconds.
For RTP, 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. When the gateway's clock is not synchronized by
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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 RFC 1889.
When the connection was set up over a LOCAL interconnect, the
meaning of these parameters is defined as follows:
Number of packets sent:
Not significant may be omitted.
Number of octets sent:
The total number of payload octets transmitted over the local
connection.
Number of packets received:
Not significant may be omitted.
Number of octets received:
The total number of payload octets received over the connection.
Number of packets lost:
Not significant may be omitted. A value of zero is assumed.
Interarrival jitter:
Not significant may be omitted. A value of zero is assumed.
Average transmission delay:
Not significant may be omitted. A value of zero is assumed.
The set of connection parameters can be extended. Also, the meaning
may be further defined by other types of networks which may
furthermore elect to not return all, or even any, of the above
specified parameters.
The command may optionally contain an encapsulated Notification
Request command, in which case a RequestIdentifier parameter must be
present, as well as, optionally, other parameters of the
NotificationRequest with the exception of the EndpointId, which is
not replicated. 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 RequestedEvents parameters, for the off-hook
event, and an empty SignalRequests parameter.
When these parameters are present, the DeleteConnection and the
NotificationRequest should be synchronized, which means that both
should be accepted, or both refused.
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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 included
with 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.
2.3.8. DeleteConnection (from the 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,
ReasonCode,
Connection-parameters)
The EndpointId, in this form of the DeleteConnection command, MUST
be fully qualified. Wildcard conventions MUST NOT be used.
The ReasonCode is a text string starting with a numeric reason code
and optionally followed by a descriptive text string. The reason
code indicates the cause of the DeleteConnection. A list of reason
codes can be found in Section 2.5.
In addition to the call, endpoint and connection identifiers, the
gateway will also send the connection parameters that would have
been returned to the Call Agent in response to a DeleteConnection
command.
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.
2.3.9. 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
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can be used to delete all connections that relate to a Call for an
endpoint:
ReturnCode,
<-- DeleteConnection(CallId,
EndpointId)
The EndpointId, in this form of the DeleteConnection command, MUST
NOT use the "any of" wildcard. All connections for the endpoint(s)
with the CallId specified will be deleted. Note that the command
will succeed if there were no connections with the CallId specified,
as long as the EndpointId was valid. However, if EndpointId is
invalid, the command will fail. The command does not return any
individual statistics or call parameters.
It can also be used to delete all connections that terminate in a
given endpoint:
ReturnCode,
<-- DeleteConnection(EndpointId)
The EndpointId, in this form of the DeleteConnection command, MUST
NOT use the "any of" wildcard. Again, the command succeeds even if
there were no connections on the endpoint(s).
Finally, Call Agents can take advantage of the hierarchical
structure of endpoint names 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 of" convention shall not be used.
For example, if endpoint 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, 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.
2.3.10. AuditEndpoint
The AuditEndPoint command can be used by the Call Agent to find out
the status of a given endpoint.
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ReturnCode,
EndPointIdList|{
[RequestedEvents,]
[QuarantineHandling,]
[DigitMap,]
[SignalRequests,]
[RequestIdentifier,]
[NotifiedEntity,]
[ConnectionIdentifiers,]
[DetectEvents,]
[ObservedEvents,]
[EventStates,]
[BearerInformation,]
[RestartMethod,]
[RestartDelay,]
[ReasonCode,]
[Capabilities]}
<-- AuditEndPoint(EndpointId,
[RequestedInfo])
The EndpointId identifies the endpoint that is being audited. The
"any of" wildcard convention MUST NOT be used.
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 shall
return the list of endpoint identifiers that match the wildcard in
the EndPointIdList parameter, which is simply one or more
SpecificEndpointIds (each supplied separately). In the case where
the "all of" wild card is used, RequestedInfo should not be included
(if it is included, it must be ignored). Note that the use of the
"all of" wildcard can potentially generate a large EndPointIdList.
If the the resulting EndPointIdList is considered too large, the
gateway returns an error (error code 555 - response too large, is
recommended)
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,
QuarantineHandling, NotifiedEntity, ConnectionIdentifiers,
DetectEvents, ObservedEvents, EventStates, BearerInformation,
RestartMethod, RestartDelay, ReasonCode, and Capabilities.
The list may be extended by extension parameters. The response will
in turn include information about each of the items for which
auditing info was requested. If an endpoint is queried about a
parameter it does not understand, the endpoint MUST NOT generate an
error; instead the parameter MUST be omitted from the response:
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* RequestedEvents: The current value of RequestedEvents the
endpoint is using including the action(s) associated with each
event. Persistent events are included in the list.
* DigitMap: the digit map the endpoint is currently using. The
parameter will be empty if the endpoint does not have a digit
map.
* 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. If
QuarantineHandling was not included, or no notification request
has been received, the default parameters will be returned.
* 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. Note that the
definition of the individual events will state if the event in
question has an auditable state associated with it. Only
persistent events and events from the last RequestedEvents can be
included in this list.
* BearerInformation: the value of the last received
BearerInformation parameter for this endpoint. The parameter will
be empty if the endpoint has not received a BearerInformation
parameter.
* RestartMethod: "restart" if the endpoint is in-service and
operation is normal. Otherwise the value of the restart method
parameter in the last RestartInProgress command issued by the
endpoint. Note that a "disconnected" endpoint will thus only
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report "disconnected" as long as it actually is disconnected, and
"restart" will be reported once it is no longer disconnected.
Similarly, "cancel-graceful" should not be reported, but
"graceful" might.
* 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 ReasonCode 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 normal.
* Capabilities: the capabilities for the endpoint similar to the
LocalConnectionOptions parameter and including event packages and
connection modes. Extensions may be included as well. If an
endpoint is queried about a capability it does not understand,
the endpoint MUST NOT generate an error; instead the parameter
MUST be omitted from the response. 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.
- Gain Control: Whether gain control is supported or not
- Type of Service: Whether type of service is supported or not.
- Resource Reservation: Whether resource reservation is
supported or not
- Security: Whether media encryption is supported or not.
- Type of network: The type(s) of network supported
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- 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.
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.
2.3.11. AuditConnection
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 AuditConnection response 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.
Note this is the same as the "notified entity" for the endpoint.
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* LocalConnectionOptions, the LocalConnectionOptions that was
supplied for the connection. Note that default parameters omitted
from the LocalConnectionOptions will not be included.
* Mode, the current mode of the connection.
* RemoteConnectionDescriptor, the RemoteConnectionDescriptor that
was supplied to the gateway for the connection.
* LocalConnectionDescriptor, the LocalConnectionDescriptor the
gateway supplied for the connection.
* ConnectionParameters, the current values 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.
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.
2.3.12. RestartInProgress
The RestartInProgress command is used by the gateway to signal that
an endpoint, or a group of endpoints, is taken in or out of service.
ReturnCode,
[NotifiedEntity]
<-- RestartInProgress(EndPointId,
RestartMethod,
[RestartDelay,]
[ReasonCode])
The EndPointId identifies the endpoint(s) 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. The
following 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.
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* A "forced" restart method indicates that the specified endpoints
are taken abruptly 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", i.e. the endpoints
will be in service. 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 endpoints are
still in service.
The list of restart methods may be extended.
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. To mitigate the effects of a gateway IP
address change as a result of a re-boot, the Call Agent MAY wish to
either flush its DNS cache for the gateway's domain name or resolve
the gateway's domain name by querying the DNS regardless of the TTL
of a current resource record for the restarted gateway.
The optional reason code parameter indicates 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 4.4.5 - 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 4.4.6. The "restart
delay" parameter MUST NOT be used with the "forced" restart method.
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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 so that
after a reboot, the default Call Agent will be the notified entity
for the 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. This
should only be done with error code 521 (endpoint redirected).
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 the following ranges of values
have been defined:
* values between 000 and 099 indicate a response acknowledgement,
* 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.
* values between 800 and 899 are package specific response codes.
A broad description of transient errors (4XX error codes) versus
permanent errors (5XX error codes) is as follows:
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* If a Call Agent receives a transient error, there is the
expectation of the possibility that a future similar request will
be honored by the endpoint. In some cases, this may require some
state change in the environment of the endpoint (e.g. hook state
as in the case of error codes 401 or 402; resource availability
as in the case of error code 403, or bandwidth availability as in
the case of error code 404).
* Permanent errors (error codes 500 to 599) indicate one or more
permanent conditions either due to protocol error or
incompatibility between the endpoint and the Call Agent, or
because of some error condition over which the Call Agent has no
control. Examples are protocol errors, requests for endpoint
capabilities that do not exist, errors on interfaces associated
with the endpoint, missing or incorrect information in the
request or any number of other conditions which will simply not
disappear with time.
The values that have been already defined are listed in the
following list:
000 Response Acknowledgement
100 The transaction is currently being executed. An actual
completion message will follow on later.
200 The requested transaction was executed normally. This return
code may be used for a successful response to any command.
250 The connection was deleted. This return code may only be used
for a successful response to a DeleteConnection command.
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
405 The transaction could not be executed, because the endpoint is
"restarting".
500 The transaction could not be executed, because the endpoint is
unknown.
501 The transaction could not be executed, because the endpoint is
not ready. This includes the case where the endpoint is out of
service.
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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.
511 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". In most cases this would be a transient error,
in which case, error code 405 should be used instead.
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. In cases where this is a transient
error, error code 404 should be used instead.
527 Missing RemoteConnectionDescriptor
528 Incompatible protocol version
529 Internal hardware failure
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530 CAS signaling protocol error.
531 Failure of a grouping of trunks (e.g. facility failure).
532 Unsupported value(s) in LocalConnectionOptions
533 Response too large
534 Codec negotiation failure
535 Packetization period not supported
536 Unknown or unsupported RestartMethod
537 Unknown or unsupported digit map extension
540 Per endpoint connection limit exceeded
The set of return codes may be extended in a future version of the
protocol. Implementations that receive a return code equal to or
larger than 300 which they do not understand, should assume the
return code indicates an error.
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 normal. (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)
903 QoS resource reservation was lost
The set of reason codes can be extended.
2.6. Use of Local Connection Options and Connection Descriptors
As indicated previously, the normal sequence in setting up a bi-
directional connection involves at least 3 steps:
1) The Call Agent asks the first gateway to "create a connection" on
an endpoint. The gateway allocates resources to that connection,
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and responds to the command by providing a "session description"
(referred to as its LocalConnectionDescriptor). The session
description contains the information necessary for another party
to send packets towards the newly created connection.
2) The Call Agent then asks the second gateway to "create a
connection" an endpoint. The command carries the "session
description" provided by the first gateway (now referred to as
the RemoteConnectionDescriptor). The gateway allocates resources
to that connection, and responds to the command by providing its
own "session description" (LocalConnectionDescriptor).
3) The Call Agent uses a "modify connection" command to provide this
second "session description" (now referred to as the
RemoteConnectionDescriptor ) to the first endpoint. Once this is
done, communication can proceed in both directions.
When the Call Agent issues a Create or Modify Connection command,
there are thus three parameters that determine the media supported
by that connection:
* LocalConnectionOptions: Supplied by the Call Agent to
control the media parameters used by the gateway for the
connection. When supplied, the gateway must conform to these
media parameters until either the connection is deleted, or a
ModifyConnection command is received.
* RemoteConnectionDescriptor: Supplied by the Call Agent to convey
the media parameters supported by the other side of the
connection. When supplied, the gateway must conform to these
media parameters until either the connection is deleted, or a
ModifyConnection command is received.
* LocalConnectionDescriptor: Supplied by the gateway to the Call
Agent to convey the media parameters it supports for the
connection. When supplied, the gateway must honor the media
parameters until either the connection is deleted, or the gateway
issues a new LocalConnectionDescriptor.
In determining which codec(s) to provide in the
LocalConnectionDescriptor, there are three lists of codecs that a
gateway needs to consider:
* A list of codecs provided in the LocalConnectionOptions (either
explicitly or implicitly).
* A list of codecs in the RemoteConnectionDescriptor
* An internal list of codecs that the gateway can support for the
connection. A gateway may support one or more codecs for a given
connection.
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Codec selection (incl. all relevant media parameters) can then be
described by the following steps:
1. An approved list of codecs is formed by taking the intersection
of the internal list of codecs and codecs allowed by the
LocalConnectionOptions. If LocalConnectionOptions were not
provided, the approved list of codecs thus contains the internal
list of codecs.
2. If the approved list of codecs is empty, a codec negotiation
failure has occurred and an error response is generated (error
code 534 codec negotiation failure is recommended)
3. Otherwise, a negotiated list of codecs is formed by taking the
intersection of the approved list of codecs and codecs allowed by
the RemoteConnectionDescriptor. If a RemoteConnectionDescriptor
was not provided, the negotiated list of codecs thus contains the
approved list of codecs.
4. If the negotiated list of codecs is empty, a codec negotiation
failure has occurred and an error response is generated (error
code 534 codec negotiation failure is recommended)
5. Otherwise, codec negotiation has succeeded, and the negotiated
list of codecs is returned in the LocalConnectionDescriptor.
Note that both LocalConnectionOptions and the
RemoteConnectionDescriptor can contain a list of codecs ordered by
preference. When both are supplied, the gateway should adhere to the
preferences provided in the LocalConnectionOptions.
2.7. Resource Reservations
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.
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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.
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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 nine commands:
* EndpointConfiguration
* CreateConnection
* ModifyConnection
* DeleteConnection
* NotificationRequest
* Notify
* AuditEndpoint
* AuditConnection
* RestartInProgress
The first five 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,
optionally, 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 sections 3.2.1.2 and 3.3).
An ABNF grammar for MGCP is provided in Appendix A.
3.2. Command Header
The command header is composed of:
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* 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(s) that should execute the command (in
notifications or restarts, the name of the endpoint(s) 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. However, MGCP entities must be able to
parse messages with additional white space characters.
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 |
-----------------------------
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The transaction identifier is encoded as a string of up to 9 decimal
digits. In the command line, 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 RFC 821, although with
some syntactic restrictions on the local part of the name. In these
addresses, the domain name identifies the system where the endpoint
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is attached, while the left side identifies a specific endpoint or
entity 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 a notified entity 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 Call Agent port
(2727) 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 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
An entity that receives a command with a protocol version it does
not support, MUST respond with an error (error code 528
Incompatible Protocol Version).
3.2.2. Parameter Lines
Parameter lines are composed of a parameter name, which in most
cases is composed of one or two characters, followed by a colon,
optional white space(s) and the parameter value. The parameters that
can be present in commands are defined in the following table:
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------------------------------------------------------------------
|Parameter name | Code| Parameter value |
|----------------------|------|------------------------------------|
|ResponseAck | K | See description |
|BearerInformation | B | See description |
|CallId | C | Hexadecimal string, max. 32 chars.|
|ConnectionId | I | Hexadecimal string, max. 32 chars.|
|NotifiedEntity | N | An identifier, in RFC 821 format, |
| | | composed of an arbitrary string |
| | | and of the domain name of the |
| | | requesting entity, possibly com- |
| | | pleted by a port number, as in: |
| | | Call-agent@ca.example.net:5234 |
|RequestIdentifier | X | Hexadecimal string, max. 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 | A string with a 3 digit integer |
| | | optionally followed by a set of |
| | | arbitrary characters |
|SpecificEndPointId | Z | An identifier, in RFC 821 format, |
| | | composed of an arbitrary string, |
| | | followed by an "@" followed by |
| | | the domain name of the gateway to |
| | | which this endpoint is attached. |
|SecondEndpointId | 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.
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------------------------------------------------------------------
| 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 | O | 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 | O | 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.
The set of parameters can be extended in two different ways:
* Package Extension Parameters (preferred)
* Vendor Extension Parameters
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Package Extension Parameters are defined in packages which provides
the following benefits:
* a registration mechanism (IANA) for the package name.
* a separate name space for the parameters.
* a convenient grouping of the extensions.
* a simple way to determine support for them through auditing.
The package extension mechanism is the preferred extension method.
Vendor extension parameters can be used 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-Flower: 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.
Note that vendor extension parameters use an unmanaged name space,
which implies a potential for name clashing. Vendors are
consequently encouraged to include some vendor specific string, e.g.
vendor name, in their vendor extensions.
3.2.2.1. Response Acknowledgement
The response acknowledgement attribute is used to managed the "at-
most-once" facility described in Section 3.5. 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
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The local connection options describe the operational parameters
that the Call Agent suggests to the gateway in connection handling
commands. These include:
* The preferred type of codec, 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.
For RTP, codecs shall be specified by using encoding names
defined in the RTP AV Profile [4] or its replacement, or by
encoding names registered with the IANA.
* 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 (as specified for the
"ptime parameter for SDP).
* 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 type of service parameter, encoded as the keyword "t",
followed by a colon and the value encoded as two hexadecimal
digits. When the connection is transmitted over an IP network,
the parameters encode the 8-bit type of service value parameter
of the IP header.
* 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 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 (RFC 2327).
* The type of network, encoded as the keyword "nt" followed by a
colon and the type of network encoded as the keyword "IN"
(internet), "ATM", "LOCAL" (for a local connection), or possibly
another type of network registered with the IANA as per SDP (RFC
2327).
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* 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 encoding of the first three fields, when they are present, will
be compatible with the SDP and RTP profiles. Note that 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
The set of Local Connection Options attributes can be extended in
three different ways:
* Package Extension attributes (preferred)
* Vendor Extension attributes
* Other Extension attributes
Package Extension Local Connection Options attributes are defined in
packages which provides the following benefits:
* A registration mechanism (IANA) for the package name.
* A separate name space for the attributes.
* A convenient grouping of the extensions.
* A simple way to determine support for them through auditing.
The package extension mechanism is the preferred extension method.
Vendor extension attributes are composed of an attribute name,
followed by a colon and by an attribute value. The attribute name
must start with the two characters "x+", for a mandatory extension,
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).
Note that vendor extension attributes use an unmanaged name space,
which implies a potential for name clashing. Vendors are
consequently encouraged to include some vendor specific string, e.g.
vendor name, in their vendor extensions.
Finally, for backwards compatibility with some existing
implementations, MGCP allows for other extension attributes as well
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(see grammar). Note however, that these attribute extensions do not
provide the package extension attribute benefits. Use of this
mechanism for new extensions is strongly discouraged.
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 Capability
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 Service:
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".
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Encryption Key:
Encoding any value indicates support for encryption. Default is
no support which is implied by omitting the parameter.
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 the endpoint(s) 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.
Lack of support for a capability can also be indicated by excluding
the parameter from the capability set.
Since Local Connection Options can be extended, the list of
capability parameters can also be extended. Individual extensions
may define how they are reported as capabilities. If no such
definition is provided, the following defaults apply:
* Package Extension attributes: The individual attributes are not
reported. Instead, the name of the package is simply reported in
the list of supported packages.
* Vendor Extension attributes: The name of the attribute is
reported without any value.
* Other Extension attributes: The name of the attribute is reported
without any value.
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:
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-----------------------------------------------------------------
| 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. |
-----------------------------------------------------------------
The set of connection parameters can be extended in two different
ways:
* Package Extension Parameters (preferred)
* Vendor Extension Parameters
Package Extension Connection Parameters are defined in packages
which provides the following benefits:
* A registration mechanism (IANA) for the package name.
* A separate name space for the parameters.
* A convenient grouping of the extensions.
* A simple way to determine support for them through auditing.
The package extension mechanism is the preferred extension method.
Vendor extension parameters names are composed of the string "X-"
followed by a two or more letters extension parameter name.
Call agents that receive unrecognized package or vendor connection
parameter 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
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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.
The set of reason codes can be extended through packages.
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. |
--------------------------------------------------------
The set of connection modes can be extended through packages.
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. Events and signals may be qualified by parameters defined
for the event/signal. The parameter name "!" (exclamation point) is
reserved for future use for both events and signals
Each signal has one of the following signal-types associated with
it: On/Off (OO), Time-out (TO), or Brief (BR). (These signal types
are specified in the package definitions, and are not present in the
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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
In addition to "!", "+" and "-", the signal parameter "to" is
reserved as well. It can be used with Time-Out signals to override
the default time-out value for the current request. A decimal value
in milliseconds will be supplied. The individual signal and/or
package definition must indicate if this parameter is supported for
one or more TO signals in the package. All new package and TO signal
definitions are strongly encouraged to support the "to" signal
parameter.
The following example illustrates how the "to" parameter can be used
to apply a signal for 6 seconds:
rg(to=6000)
rg(to(6000))
The following are 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 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 package. |
| R/qa@* | The quality alert event in all |
| | connections |
| R/rt@$ | Ringback on current connection |
---------------------------------------------------------
3.2.2.8. RequestedEvents
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The RequestedEvents 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 packages that 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 starts 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. Implementers are encouraged
to accept such encodings, but they should not generate them.)
If the RequestedEvents is not present, the parameter will be set to
a null value. If the SignalRequests is not present, the parameter
will be set to a null value. If the DigitMap is absent, the current
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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 separated by commas and enclosed within
parenthesis, as in:
S: adsi("123456 Francois Gerard")
S: ann(no-such-number, 1234567)
When a quoted-string is provided, the string itself is UTF-8 encoded
(RFC 2279).
When several signals are requested, their codes are separated by a
comma, as in:
S: adsi("123456 Your friend"), rg
Please refer to Section 3.2.2.7 for additional detail on signal
parameters.
3.2.2.10. ObservedEvents
The observed events parameter 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, however such
practice is discouraged; they should be reported as lists of
isolated events if other events where detected during the digit
accumulation. Examples of observed events 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
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The RequestedInfo parameter contains a comma separated list of
parameter codes, as defined in Section 3.2.2. 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
Note that extension parameters in general can be audited as well.
The individual extension will define the auditing operation.
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 DetectEvents 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
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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
The set of restart methods can be extended through packages.
3.2.2.16. Bearer Information
The values of the bearer information 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
The set of bearer information attributes may be extended through
packages.
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 a 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, and the
transaction identifier. Response codes defined in packages (8xx) are
followed by white space, a slash ("/") and the package name. All
response codes may furthermore be followed by optional commentary
preceded by a white space.
The following table describes 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. Note that the table
only reflects the default for responses that have not defined any
other behavior. If a response is received with a parameter that is
either not understood or marked as forbidden, the offending
parameter is simply ignored.
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------------------------------------------------------------------
| 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 | 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 | O*| O | F | F | F | F | O*| F |
| Descriptor | | | | | | | | | |
| RemoteConnection | F | F | F | F | F | F | F | O*| F |
| Descriptor | | | | | | | | | |
------------------------------------------------------------------
Notes (*):
* The ResponseAck parameter MUST NOT be used with any other
responses than a final response issued after a provisional
response for the transaction in question. In that case, the
presence of the ResponseAck parameter should trigger a Response
Acknowledgement any ResponseAck values provided will be ignored.
* In the case of a CreateConnection message, the response line is
followed by a Connection-Id parameter and a
LocalConnectionDescriptor. It may also be followed a Specific-
Endpoint-Id parameter, if the creation request was sent to a
wildcarded Endpoint-Id. The connection-Id and
LocalConnectionDescriptor parameter are marked as optional in the
Table. In fact, they are mandatory with all positive responses,
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when a connection was created, and forbidden when the response is
negative, when no connection as created.
* 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
o=- 25678 753849 IN IP4 128.96.41.1
s=-
c=IN IP4 128.96.41.1
t=0 0
m=audio 1296 RTP/AVP 0
v=0
o=- 33343 346463 IN IP4 128.96.63.25
s=-
c=IN IP4 128.96.63.25
t=0 0
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.
The response parameters are described for each of the commands in
the following.
3.3.1. CreateConnection Response
In the case of a CreateConnection message, the response line is
followed by a Connection-Id parameter with a successful response
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(code 200). A LocalConnectionDescriptor is furthermore transmitted
with a positive response. The LocalConnectionDescriptor is encoded
as a "session description", as defined by SDP (RFC 2327). It is
separated from the response header by an empty line, e.g.:
200 1204 OK
I: FDE234C8
v=0
o=- 25678 753849 IN IP4 128.96.41.1
s=-
c=IN IP4 128.96.41.1
t=0 0
m=audio 3456 RTP/AVP 96
a=rtpmap:96 G726-32/8000
When a provisional response has been issued previously, the final
response may furthermore contain the Response Acknowledgement
parameter:
200 1204 OK
K:
I: FDE234C8
v=0
o=- 25678 753849 IN IP4 128.96.41.1
s=-
c=IN IP4 128.96.41.1
t=0 0
m=audio 3456 RTP/AVP 96
a=rtpmap:96 G726-32/8000
The final response should then be acknowledged by a Response
Acknowledgement:
000 1204
3.3.2. ModifyConnection Response
In the case of a successful ModifyConnection message, the response
line is followed by a LocalConnectionDescriptor, if the modification
resulted in a modification of the session parameters (e.g., changing
only the mode of a connection does not alter the session
parameters). The LocalConnectionDescriptor is encoded as a "session
description", as defined by SDP. It is separated from the response
header by an empty line.
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200 1207 OK
v=0
o=- 25678 753849 IN IP4 128.96.41.1
s=-
c=IN IP4 128.96.41.1
t=0 0
m=audio 3456 RTP/AVP 0
When a provisional response has been issued previously, the final
response may furthermore contain the Response Acknowledgement
parameter as in:
200 1207 OK
K:
The final response should then be acknowledged by a Response
Acknowledgement:
000 1207 OK
3.3.3. DeleteConnection Response
Depending on the variant of the DeleteConnection message, the
response line may be followed by a Connection Parameters parameter
line, as defined in Section 3.2.2.4.
250 1210 OK
P: PS=1245, OS=62345, PR=780, OR=45123, PL=10, JI=27,LA=48
3.3.4. NotificationRequest Response
A successful NotificationRequest response does not include any
additional response parameters.
3.3.5. Notify Response
A successful Notify response does not include any additional
response parameters.
3.3.6. AuditEndpoint Response
In the case of a successful AuditEndPoint the response line may be
followed by information for each of the parameters requested - each
parameter will appear on a separate line. Parameters for which no
value currently exists, e.g., digit map, will still be provided.
Each local endpoint name "expanded" by a wildcard character will
appear on a separate line using the "SpecificEndPointId" parameter
code, e.g.:
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200 1200 OK
Z: aaln/1@rgw.whatever.net
Z: aaln/2@rgw.whatever.net
or:
200 1200 OK
A: a:PCMU;G728, p:10-100, e:on, s:off, t:1, v:L,
m:sendonly;recvonly;sendrecv;inactive
A: a:G729A; p:30-90, e:on, s:on, t:1, v:L,
m:sendonly;recvonly;sendrecv;inactive;confrnce
Note: The carriage return for Capabilities shown above is present
for formatting reasons only. It is not permissible in a real command
encoding.
3.3.7. AuditConnection Response
In the case of a successful AuditConnection, the response may be
followed by information for each of the parameters requested.
Parameters for which no value currently exists will still be
provided. Connection descriptors will always appear last and each
will be preceded by an empty line, as for example:
200 1203 OK
C: A3C47F21456789F0
N: [128.96.41.12]
L: p:10, a:PCMU;G728
M: sendrecv
P: PS=622, OS=31172, PR=390, OR=22561, PL=5, JI=29, LA=50
v=0
o=- 4723891 7428910 IN IP4 128.96.63.25
s=-
c=IN IP4 128.96.63.25
t=0 0
m=audio 1296 RTP/AVP 96
a=rtpmap:96 G726-32/8000
If both a local and a remote connection descriptor are provided, the
local connection descriptor will be the first of the two. 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.
3.3.8. RestartInProgress Response
A successful RestartInProgress response does not include any
additional response parameters.
The response to a RestartInProgress may include the name of another
Call Agent to contact, for instance when the Call Agent redirects
the endpoint to another Call Agent as in:
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521 1204 Redirect
N: CA-1@whatever.net
3.4. Encoding of the Session Description (SDP)
The session description (SDP) 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 general description and explanation of SDP parameters can be
found in RFC 2327. In particular, it should be noted that the
* Origin ("o="),
* Session Name ("s="), and
* Time active ("t=")
are all mandatory in RFC 2327. While they are of little use to MGCP,
they must be provided in conformance with RFC 2327 nevertheless. The
following suggests values to be used for each of the fields, however
the reader is encouraged to consult RFC 2327 for details:
Origin
o = <username> <session id> <version> <network type> <address type>
<address>
* The username should be set to hyphen ("-").
* The session id is recommended to be an NTP timestamp as suggested
in RFC 2327.
* The version is a version number that must increment with each
change to the SDP. A counter initialized to zero or an NTP
timestamp as suggested in RFC 2327 is recommended.
* The network type defines the type of network. For RTP sessions
the network type should be "IN".
* The address type defines the type of address. For RTP sessions
the address type should be "IP4" (or "IP6").
* The address should be the same address as provided in the
connection information ("c=") field.
Session Name
s = <session name>
The session name should be hyphen ("-").
Time active
t = <start time> <stop time>
* The start time may be set to zero.
* The stop time should be set to zero.
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Each of the three fields can be ignored upon reception.
To further accommodate the extensibility principles of MGCP,
implementations are encouraged to support the PINT "a=require"
attribute - please refer to RFC 2848 for further details.
The usage of SDP actually depends on the type of session that is
being established. Below we describe usage of SDP for an audio
service using the RTP/AVP profile [4], or the LOCAL interconnect
defined in this document. In case of any conflicts between what is
described below and SDP (RFC 2327), the SDP specification takes
precedence.
3.4.1. Usage of SDP for an Audio Service
In a telephony gateway, we only have to describe sessions that use
exactly one media, audio. The usage of SDP for this is
straightforward and described in detail in RFC 2327.
The following is an example of an RFC 2327 conformant session
description for an audio connection:
v=0
o=- A7453949499 0 IN IP4 128.96.41.1
s=-
c=IN IP4 128.96.41.1
t=0 0
m=audio 3456 RTP/AVP 0 96
a=rtpmap:96 G726-32/8000
3.4.2. 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 connection and media parameters will be used as
follows:
* 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 local 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 the RTP AVP profile(RTP payload numbers).
The type of encoding should normally be set to 0 (PCMU).
A session-level attribute identifying the connection may furthermore
be present. This enables endpoints to support multiple LOCAL
connections. Use of this attribute is optional and indeed
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unnecessary for endpoints that only support a single LOCAL
connection. The attribute is defined as follows:
a=MGCPlocalcx:<ConnectionID>
The MGCP Local Connection attribute is a session level only case-
insensitive attribute that identifies the MGCP LOCAL connection,
on the endpoint identified in the connection information, to
which the SDP applies. The ConnectionID is a hexadecimal string
containing at most 32 characters. The ConnectionID itself is
case-insensitive. The MGCP Local Connection attribute is not
subject to the charset attribute.
An example of a LOCAL session description could be:
v=0
o=- A7453949499 0 LOCAL EPN X35V3+A4/13
s=-
c=LOCAL EPN X35V3+A4/13
t=0 0
a=MGCPlocalcx:FDE234C8
m=audio 0 LOCAL 0
Note that the MGCP Local Connection attribute is specified at the
session level and that it could have been omitted in case only a
single LOCAL connection per endpoint is supported.
3.5. 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 the
response may or may not arrive from the same address as the command
was sent to.
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.
3.5.1. 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
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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
transactions. If a match is found, the MGCP entity does not execute
the transaction, which is simply ignored.
The procedure uses a long timer value, noted T-HIST 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 T-HIST 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 ". For transactions that are
acknowledged through this attribute, the gateway shall keep a copy
of the transaction-id (as opposed to the entire transaction
response) for T-HIST 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.
3.5.2. 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 Section 4. 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" for final
responses received provisional responses must not be confirmed. A
given response should not be confirmed in two separate messages.
MGCP entities may choose to delete the copies of the responses (but
not the transaction-id) to transactions whose id is included in
"confirmed transaction-id ranges" received in the Response
Confirmation messages (command or response). They should silently
discard further commands from that entity when the transaction-id
falls within these ranges, and the response was issued less than T-
HIST seconds ago.
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Entities should exercise due caution when acknowledging responses.
In particular, a response should only be acknowledged if the
response acknowledgement is sent to the same entity as the
corresponding command (i.e. the command whose response is being
acknowledged) was sent to.
Likewise, entities should not blindly accept a response
acknowledgement for a given response. However it is considered safe
to accept a response acknowledgement for a given response, when that
response acknowledgement is sent by the same entity as the command
that generated that response.
It should be noted, that use of response acknowledgements in
commands is optional (as opposed to the required Response
Acknowledgement response following a provisional response). The
benefit of using it is that it reduces overall memory consumption.
However, in order to avoid large messages, implementations should
not generate large response acknowledgement lists. One strategy is
to manage responses to commands on a per endpoint basis. A command
for an endpoint can confirm a response to an older command for that
same endpoint. Responses to commands with wildcarded endpoint names
can be confirmed selectively with due consideration to message
sizes, or alternatively simply not be acknowledged. Care must be
taken to not confirm the same response twice or a response that is
more than T-HIST seconds old.
The "confirmed transaction-id ranges" values shall not be used if
more than T-HIST seconds have elapsed since the entity issued its
last response to the other entity, or when an entity 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 union of the "confirmed transaction-id
ranges" received in recent messages shall be retained.
3.5.3. 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. At a minimum, a retransmission strategy
involving exponential backoff must be implemented. One possibility
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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, RTO, in TCP, is set to the sum of the
average delay plus N times the average deviation, where N is a
constant. 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 T-HIST seconds. A suggested maximum
value for RTO is 4 seconds.
After any retransmission, the MGCP entity should do the following:
* It should double the estimated value of the acknowledgement delay
for this transaction, T-DELAY.
* It should compute a random value, uniformly distributed between
0.5 T-DELAY and T-DELAY.
* It should set the retransmission timer (RTO) 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.
Note that the estimators AAD and ADEV should not be updated for
transactions that involve retransmissions. Also, the first new
transmission following a successful retransmission should use the
RTO for that last retransmission. If this transmission succeeds
without any retransmissions, the AAD and ADEV estimators are updated
and RTO is determined as usual again. See e.g. [18] for further
details.
3.5.4. Piggy Backing
There are cases when a Call Agent will want to send several messages
at the same time to the same gateways, and vice versa. When several
MGCP messages have to be sent in the same UDP packet, they should be
separated by a line of text that contains a single dot, as in for
example:
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200 2005 OK
DLCX 1244 card23/21@tgw-7.example.net MGCP 1.0
C: A3C47F21456789F0
I: FDE234C8
The piggy-backed messages should be processed exactly as if they had
been received in several simultaneous messages. However, if a
message (command or response) needs to be retransmitted, the entire
datagram must be retransmitted, not just the missing message.
Furthermore, if a (new) datagram piggy-backs a currently outstanding
command with a new command, retransmission of the old datagram with
the outstanding command should cease. For example, assume command C1
was sent in datagram D1, and later commands C1 and C2 are sent
(piggy-backed) in datagram D2. Retransmission of D1 should then
cease. This will prevent endpoints from inadvertently becoming
disconnected when a backup call agent has taken over.
The individual messages in the datagram must be processed in order
starting with the first message, and each command must be responded
to. Errors encountered in a message that was piggybacked must not
affect any of the other messages received in that packet each
message is processed on its own.
3.5.5. 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 should send a provisional response, with response
code 100. As a guideline, a transaction that requires external
communication to complete, e.g. network resource reservation, should
issue a provisional response. Furthermore entities should send this
response if they receive a repetition of a transaction that is still
being executed.
Pure transactional semantics would imply, that provisional responses
should not return any other information than the fact that the
transaction is currently executing, however an optimistic approach
allowing some information to be returned enables a reduction in the
delay that would otherwise be incurred in the system.
In order to reduce the delay in the system, it is recommended to
include a connection identifier and session description in a
provisional response to the CreateConnection command. If a session
description should be returned by the ModifyConnection command, the
session description should be included in the provisional response
here as well. If the transaction completes successfully, the
information returned in the provisional response MUST be repeated in
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the final response. It is considered a protocol error not to repeat
this information or to change any of the previously supplied
information in a successful response. If the transaction fails, an
error code is returned - the information returned previously is no
longer valid.
A currently executing CreateConnection or ModifyConnection
transaction MUST be cancelled if a DeleteConnection command for the
endpoint is received. In that case, a response for the cancelled
transaction SHOULD still be returned automatically, and a response
for the cancelled transaction MUST be returned if a retransmission
of the cancelled transaction is detected.
MGCP entities that receive a provisional response shall switch to a
longer repetition timer (LONGTRAN-TIMER) for that transaction.
When the transaction finishes execution, the final response is sent
and the by now obsolete provisional response is deleted. In order to
ensure rapid detection of a lost final response, final responses
issued after provisional responses for a transaction should be
acknowledged.
The endpoint should therefore include an empty "ResponseAck"
parameter in those, and only those, final responses. The presence of
the "ResponseAck" parameter in the final response should trigger a
"Response Acknowledgement" response to be sent back to the endpoint.
The Response Acknowledgement" response will then include the
transaction-id of the response it acknowledges in the response
header. Note, that for backwards compatibility, entities can not
depend on receiving such a "response acknowledgement", however it is
strongly recommended to support this behavior, as excessive delays
in case of packet loss as well as excessive retransmissions may
occur otherwise.
Receipt of a "Response Acknowledgement" response is subject to the
same time-out and retransmission strategies and procedures as
responses to commands, i.e., the sender of the final response will
retransmit it if a "Response Acknowledgement" is not received in
time. For backwards compatibility, failure to receive a "response
acknowledgement" should not affect the roundtrip time estimates for
subsequent commands, and furthermore should not lead to the endpoint
becoming "disconnected". The "Response Acknowledgment " response is
never acknowledged.
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4. States, Failover and Race Conditions.
In order to implement proper call signaling, 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. Failover Assumptions and Highlights
The following protocol highlights are important to understanding
Call Agent fail-over mechanisms:
* Call Agents are identified by their domain name (and optional
port), not their network addresses, and several addresses can be
associated with a domain name.
* An endpoint has one and only one Call Agent associated with it at
any given point in time. The Call Agent associated with an
endpoint is the current value of the "notified entity". The
"notified entity" determines where the gateway will send it's
commands. If the "notified entity" does not include a port
number, the default Call Agent port number (2727) is assumed.
* NotifiedEntity is a parameter sent by the Call Agent to the
gateway to set the "notified entity" for the endpoint.
* The "notified entity" for an endpoint is the last value of the
NotifiedEntity parameter received for this endpoint. If no
explicit NotifiedEntity parameter has ever been received, the
"notified entity" defaults to a provisioned value. If no value
was provisioned or an empty NotifiedEntity parameter was provided
(both strongly discouraged) thereby making the "notified entity"
empty, the "notified entity" is set to the source address of the
last non-audit command for the endpoint. Thus auditing will not
change the "notified entity".
* Responses to commands are sent to the source address of the
command, regardless of the current "notified entity". When a
Notify message needs to be piggy-backed with the response, the
datagram is still sent to the source address of the new command
received, regardless of the current "notified entity".
The ability for the "notified entity" to resolve to multiple network
addresses, allows a "notified entity" to represent a Call Agent with
multiple physical interfaces on it and/or a logical Call Agent made
up of multiple physical systems. The order of network addresses when
a DNS name resolves to multiple addresses is non-deterministic so
that Call Agent fail-over schemes should not depend on that order
(e.g. a gateway should be able to send a "Notify" to any of the
resolved network addresses). On the other hand, the system is likely
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to be most efficient if the gateway sends commands to the interface
with which it already has a current association. Gateways may use
the following algorithm to achieve that goal:
* if the "notified entity" resolves to multiple network addresses,
and the source address of the request is one of those addresses,
that network address is the preferred destination address for new
commands.
* If on the other hand, the source address of the request is not
one of the resolved addresses, the gateway must choose one of the
resolved addresses.
* If the gateway fails to contact the network address chosen, it
must try the alternatives in the resolved list as described in
the retransmission section below.
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 with a new "notified entity".
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.
Alternatively, the failed Call Agent could simply become the backup
Call Agent.
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)
Note that as mentioned earlier, the default "notified entity" may be
provisioned and may include both domain name and port. For small
gateways, provisioning may be done on a per endpoint basis. For much
larger gateways, a single provisioning element may be provided for
multiple endpoints or even for the entire gateway itself. In either
case, once the gateway powers up, each endpoint must have its own
"notified entity" so that provisioned values for an aggregation of
endpoints must be copied to the "notified entity" for each endpoint
in the aggregation before operation proceeds. Where possible, the
RestartInProgress command on restart should be sent to the
provisioned "notified entity" based on an aggregation that allows
the "all of" wild-card to be used.
Another way of viewing the use of "notified entity" is in terms of
associations between gateways and Call Agents. The "notified entity"
is a means to set up that association. The association is initially
provisioned with a provisioned "notified entity", so that on power
up Restart-in-Progress and persistent events that occur prior to the
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first NotificationRequest from Call Agents will be sent to the
provisioned Call Agent. Once a Call Agent makes a request, however
it may include the NotifiedEntity parameter and set up a new
association. Since the "notified entity" persists across calls, the
association remains intact until a new "notified entity" is
provided.
4.2. Communicating with Gateways
Endpoint names in gateways include a local name indicating the
specific endpoint and a domain name indicating the host/gateway
where the endpoint resides. Large gateways may have several
interfaces for redundancy.
In gateways that have routing capability, the domain name may
resolve to a single network address with internal routing to that
address from any of the gateway's interfaces. In others, the domain
name may resolve to multiple network addresses, one for each
interface. In the latter case, if a Call Agent fails to contact the
gateway on one of the addresses, it should try the alternates.
4.3. 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 T-HIST 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. If a match to a previously responded to
transaction is not found, the transaction identifier of the incoming
command is compared to the list of transactions that have not yet
finished executing If a match is found, the MGCP entity does not
execute the transaction, which is simply ignored - a final response
will be provided when the execution of the command is complete. In
the mean time, a provisional response should be issued.
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,
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* 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 we will call "Max2", which should be set to a
larger value.
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Command issued: N=0, T=0
|
transmission: N++
| +------------ retransmission: N++ -------------+
| | |
| | transmission |
| | +---to new address -+<----------------------|--+
| | | N=0 | | |
V V V | | |
+-----------+ | | |
| awaiting |- new Call Agent ->+ +------------+ | |
| response |-- timer elapsed --->| T > T-Max ?| | |
+-----------+ +------------+ ^ |
| | | | |
| +----(yes)-----+ (no) | |
response | | | |
received | +------------+ | |
| | | N > Max1 ? |-(no)+ |
| | +------------+ ^ |
v | | | |
(end) | (yes) | |
| | | |
| (if first address & N=Max1, | |
| or last address & N=Max2 | |
| check DNS) | |
| | | |
| +---------------+ | |
| |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 command where the NotifiedEntity points to the new
Call Agent, or
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- receiving a redirection response pointing to a new Call Agent.
If a new Call Agent is detected, the gateway starts
retransmitting outstanding commands for the endpoint(s)
redirected to that new agent. Responses to new or old commands
are still transmitted to the source address of the command.
* 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 t
i
m
e
has elapsed, the endpoint becomes
disconnected.
* 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 for this IP address is larger
than "Max1" and lower than "Max2", and there are more addresses
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 for this address 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 and
must initiate the "disconnected" procedure.
The above 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
(T) 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 T-HIST
value: the
T-HIST value must be greater than or equal to T-MAX plus the maximum
propagation delay in the network.
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The default value for T-MAX is 20 seconds. Thus, if the assumed
maximum propagation delay is 10 seconds, then responses to old
transactions must be kept for a period of at least 30 seconds. The
importance of having the sender and receiver agree on these values
cannot be overstated.
The default value for Max1 is 5 retransmissions and the default
value for Max2 is 7 retransmissions. Both of these values may be
altered by the provisioning process.
4.4. Race Conditions
MGCP deals with race conditions through the notion of a "quarantine
list" and through explicit detection of desynchronization e.g. for
mismatched hook state due to glare for an endpoint.
MGCP does not assume that the transport mechanism will maintain the
order of commands 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.
4.4.1. 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 only
consists of persistent events for the endpoint, and the digit map is
assumed empty. After reception of a command, the gateway starts
observing the endpoint for occurrences of the events mentioned in
the list, including persistent events.
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 "accumulate according to digit map"
are accumulated in a list of events, the events that are marked as
"accumulate according to the digit map" will additionally be
accumulated in the "current dial string". This will go on until one
event is encountered that triggers a Notification which will be sent
to the current "notified entity".
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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 DetectEvents parameter or,
in the absence of the latter, all events that are referred to in the
RequestedEvents, shall be detected and quarantined, regardless of
the action associated to the event. Persistent events are here being
viewed as implicitly included in RequestedEvents.
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 triggering
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 endpoint will reenter
and 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
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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 packet to the source of the new
NotificationRequest, regardless of 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 Notify command.
c) a copy of the unacknowledged Notify command is kept until an
acknowledgement is received. If a timer elapses, the Notify will
be retransmitted (to the source of the new notification request),
in a packet that will also carry a repetition of the
acknowledgement of the new notification request.
d) if the acknowledgement to the new notification request 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 new notification request (step a).
e) if the gateway has to transmit a new 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. This datagram will be sent to the current "notified
entity".
f) Gateways that cannot piggyback several messages in the same
datagram should elect to leave the endpoint in the "notification"
state as long as the last Notify is not acknowledged.
The procedure is illustrated by the following diagram:
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+-------------------+
| Processing Events |<----------------------------------+
+-------------------+ |
| |
Need to send NTFY |
| |
v |
+-------------------+ |
| Outstanding NTFY |---- No -------+ |
| + RQNT response ? | | |
+-------------------+ v |
| +-----------+ |
Yes | Send NTFY | |
| +-----------+ |
v | |
+--------------------+ v |
| Piggyback new NTFY | +--------------------+ |
| w. old outstanding |---->| Notification State | |
| datagram | +--------------------+ |
+--------------------+ | | |
new RQNT NTFY response |
received received |
| | |
+-------------+ v |
| +-------------+ |
v +----| Step mode ? | |
+-----------------------+ | +-------------+ |
| Piggyback new RQNT | | | |
| response with *new* | No Yes |
| outstandding datagram | | | |
+-----------------------+ | v |
| | +---------------+ |
| | | Wait for RQNT | |
| | +---------------+ |
| | | |
| | RQNT received |
| | | |
| | v |
| | +------------+ |
| | | Apply RQNT |--->+
| | +------------+ |
v v |
+----------------->+--------------------->+
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 current 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, and 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 and
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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 events 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.
4.4.2. 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, (error code 401 phone off
hook)
2. If the gateway is requested to notify an "on-hook" or "flash
hook" condition while the phone is already on-hook (error code
402 phone on hook).
Additionally, individual signal definitions can specify that a
signal will only operate under certain conditions, e.g., ringing may
only be possible if the phone is already off-hook. If such
prerequisites exist for a given signal, the gateway MUST return the
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error specified in the signal definition if the prerequisite is not
met.
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
RequestedEvents 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 thereby enabling the Call Agent to
determine if the Notify command was generated before or after the
gateway received the new NotificationRequest. This is especially
important to avoid deadlocks in "step" mode.
4.4.3. Transactional Semantics
As the potential transaction completion times increase, e.g. due to
external resource reservations, a careful definition of the
transactional semantics becomes increasingly important. In
particular the issue of race conditions, specifically as it relates
to hook-state must be defined carefully.
An important point to consider is, that the hook-state may in fact
change between the time a transaction is initiated and the time it
completes. More generally, we may say that the successful completion
of a transaction depends on one or more pre-conditions where one or
more of the pre-conditions may change dynamically during the
execution of the transaction.
The simplest semantics for this is simply to require that all pre-
conditions MUST be met from the time the transaction is initiated
until the transaction completes. Thus, if any of the preconditions
change during the execution of the transaction, the transaction MUST
fail. Furthermore, as soon as the transaction is initiated, all new
events are quarantined. When the outcome of the transaction is
known, all quarantined events are then processed.
As an example, consider a transaction that includes a request for
the "off-hook" event. When the transaction is initiated the phone is
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"on-hook" and this pre-condition is therefore met. If the hook-state
changes to "off-hook" before the transaction completes, the pre-
condition is no longer met, and the transaction therefore
immediately fails. The "off-hook" event will now be stored in the
"quarantine" buffer which then gets processed.
Another related issue is the use of wildcards, especially the "all
of" wildcard which may match more than one endpoint. When a command
is requested, and the endpoint identifier matches more than one
endpoint, transactional semantics still apply. Thus, the command
must either succeed for all the endpoints, or it must fail for all
of them. A single response is consequently always issued.
4.4.4. 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
endpoints 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 will fail, 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
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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, except for responses to auditing, 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.
10) On a given endpoint, there should normally be only one
outstanding EndpointConfiguration command at any time.
Gateways must not make any assumptions as to whether Call Agents
follow the rules or not. Consequently gateways must always respond
to commands, regardless of whether they adhere to the above rules or
not.
Where a single outstanding command is expected (ModifyConnection,
NotificationRequest, and EndpointConfiguration), but the same
command is received in a new transaction before the old finishes
executing, the gateway may choose to fail the previous command
(recommended) or continue executing the commands in order for the
affected endpoint(s) and connection(s). In the former case, the use
of error code 400 (transient error) is recommended.
4.4.5. 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
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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 non-audit message (command or non-501 response to commands
other than auditing) 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. Endpoints that are considered in-service
will have a restartMethod of "restart", where as endpoints
considered out-of-service will have a restartMethod of "forced".
Endpoints that are out of service should respond with error code 501
(not ready) to non-audit commands if possible.
Should the gateway enter the "disconnected" state while carrying out
the restart procedure, the disconnected procedure specified in
Section 4.4.6 must be carried out, except that a "restart" rather
than "disconnected" message is sent during the procedure.
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. Note that during
startup, it is possible for endpoints to start out as being out-of-
service, and then become in-service as part of the gateway
initialization procedure. A gateway may thus choose to send first a
"forced" RestartInProgress for all its endpoints, and subsequently a
"restart" RestartInProgress for the endpoints that come in-service.
Alternatively, the gateway may simply send "restart"
RestartInProgress for only those endpoints that are in-service, and
"forced" RestartInProgress for the specific endpoints that are out-
of-service. Wild-carding MUST still be used to minimize the number
of messages sent though.
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.
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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 endpoint 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 thus 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.
4.4.6. 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 4.3. 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
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"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
non-audit message (command or response to command other than
auditing) 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.
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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 RFC 2401, using either the IP Authentication Header,
defined in RFC 2402, or the IP Encapsulating Security Payload,
defined in RFC 2406. 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".
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A basic protection against this attack is to only accept packets
from known sources, however this tends to conflict with RTP
principles. 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 source address 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 and 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 provide strong protection against address spoofing.
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6. Packages
As described in Section 2.1.6, packages are the preferred way of
extending MGCP. In this section we describe the requirements
associated with defining a package.
A package must have a unique package name defined. The package name
can be registered with the IANA, as long as it does not start with
the characters "x-" or "x+" which are reserved for experimental
packages. Please refer to Appendix B for IANA considerations.
A package can define one or more of the following extensions:
* BearerInformation
* LocalConnectionOptions
* ExtensionParameters
* ConnectionModes
* Events
* Signals
* Actions
* DigitMapLetters
* ConnectionParameters
* RestartMethods
* ReasonCodes
* Return codes
For each of the above types of extensions supported by the package,
the package definition must contain a description of the extension
as defined in the following sections. Please note, that package
extensions, just like any other extension, must adhere to the MGCP
grammar.
6.1 BearerInformation
BearerInformation extensions shall include:
* The name and encoding of the BearerInformation extension.
* The possible values and encoding of those values that can be
assigned to the BearerInformation extension.
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* A description of the operation of the BearerInformation
extension. As part of this description the default value (if any)
if the extension is omitted in an EndpointConfiguration command
must be defined. It may be necessary to make a distinction
between the default value before and after the initial
application of the parameter, for example if the parameter
retains its previous value once specified, until explicitly
altered. If default values are not described, then the extension
parameter simply defaults to empty in all EndpointConfiguration
commands.
Note that the extension must be included in the result for an
AuditEndpoint command auditing the BearerInformation.
6.2 LocalConnectionOptions
LocalConnectionOptions extensions shall include:
* The name and encoding of the LocalConnectionOptions extension.
* The possible values and encoding of those values that can be
assigned to the LocalConnectionOptions extension.
* A description of the operation of the LocalConnectionOptions
extension. As part of this description the following must be
specified:
* The default value (if any) if the extension is omitted in a
CreateConnection command.
* The default value if omitted in a ModifyConnection command. This
may be to simply retain the previous value (if any) or clear it.
If nothing is specified, the value is cleared.
* If Auditing of capabilities will result in the extension being
returned, then a description to that effect as well as with what
possible values and their encoding (note that the package itself
will be always be returned). If nothing is specified, the
extension will not be returned when auditing capabilities.
Also note, that the extension must be included in the result for an
AuditConnection command auditing the LocalConnectionOptions.
6.3 ExtensionParameters
Extension parameter extensions shall include
* The name and encoding of the extension parameter.
* The possible values and encoding of those values that can be
assigned to the extension parameter.
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* For each of the commands defined in this specification, whether
the extension parameter is Mandatory, Optional, or Forbidden in
requests as well as responses. Note that extension parameters
should normally not be mandatory.
* A description of the operation of the extension parameter. As
part of this description the default value (if any) if the
extension is omitted in a command must be defined. It may be
necessary to make a distinction between the default value before
and after the initial application of the parameter, for example
if the parameter retains its previous value once specified, until
explicitly altered. If default values are not described, then the
extension parameter simply defaults to empty in all commands.
* Whether the extension can be audited in AuditEndpoint and/or
AuditConnection as well as the values returned. If nothing is
specified, then auditing of the extension parameter can only be
done for AuditEndpoint, and the value returned will be the
current value for the extension. Note that this may be empty.
6.4 ConnectionModes
Extension Connection Modes shall include:
* The name and encoding of the extension connection mode.
* A description of the operation of the extension connection mode.
* A description of the interaction a connection in the extension
connection mode will have with other connections in each of the
modes defined in this specification. If such a description is not
provided, the extension connection mode will not have any
interaction with other connections on the endpoint.
Extension connection modes are not included in the list of modes in
a response to an AuditEndpoint for Capabilities since the event
package will be reported in the list of event packages.
6.5 Events and Signals
The event/signal definition shall include the precise name of the
event/signal (i.e., the code used in MGCP), a plain text definition
of the event/signal, and, when appropriate, the precise definition
of the corresponding events/signals, for example the exact
frequencies of audio signal such as dial tones or DTMF tones.
The package description must provide, for each event/signal, the
following information:
* The description of the event/signal 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).
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* The detailed characteristics of the event/signal, such as for
example frequencies and amplitude of audio signals, modulations
and repetitions,
* The typical and maximum duration of the event/signal if
applicable.
* An indication of whether the signal or event can be applied to a
connection (across a media stream).
For events that are not signals, the following must be provided as
well:
* An indication if the event is persistent. By default, events are
not persistent.
* An indication if there is an auditable event-state associated
with the event. By default, events do not have auditable event-
states.
For signals that are not events, the following must be provided as
well:
* The type of signal (OO, TO, BR). Time-Out signals should have an
indication of the default time-out value. In some cases, time-out
values may be variable (if dependent on some action to complete
such as out-pulsing digits).
The following format is recommended for defining events and signals
in conformance with the above:
------------------------------------------------------------------
| Symbol | Definition | R | S Duration |
|---------|----------------------------|-----|---------------------|
| | | | |
| | | | |
------------------------------------------------------------------
where:
* Symbol indicates the event code used for the event/signal, e.g.
"hd".
* Definition gives a brief definition of the event/signal
* R contains an "x" if the event can be detected or one or more of
the following symbols:
- "P" if the event is persistent.
- "S" if the events is an event-state that may be audited.
- "C" if the event can be detected on a connection.
* S contains one of the following if it is a signal
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- "OO" if the signal is On/Off signal.
- "TO" if the signal is a Time-Out signal.
- "BR" if the signal is a Brief signal.
The table should then be followed by a more comprehensive
description of each event/signal defined.
6.5.1 Default and Reserved Events
All packages that contain Time-Out type signals contain the
operation fail ("of") and operation complete ("oc") events,
irrespective of whether they are provided as part of the package
description or not. These events are needed to support Time-Out
signals and can not be overridden in packages with Time-Out signals.
If a package without Time-Out signals does contain definitions for
the "oc" and "of" events, the event definitions provided in the
package may over-ride those indicated here. Such practice is however
discouraged and is purely allowed to avoid potential backwards
compatibility problems.
It is however considered good practice to explicitly mention that
the two events are supported in accordance with their default
definitions, which are as follows:
------------------------------------------------------------------
| Symbol | Definition | R | S Duration |
|---------|----------------------------|-----|---------------------|
| oc | Operation Complete | x | |
| of | Operation Failure | x | |
------------------------------------------------------------------
Operation complete (oc): The operation complete event is generated
when the gateway was asked to apply one or several signals of type
TO on the endpoint, and one or more of those signals 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 a parameter the name of the signal that came to the end of its
live time, as in:
O: G/oc(G/rt)
In this case, the observed event occurred because the "rt" signal in
the "G" package timed out.
If the reported signal was applied on a connection, the parameter
supplied will include the name of the connection as well, as in:
O: G/oc(G/rt@0A3F58)
When the operation complete event is requested, it cannot be
parameterized with any event parameters. When the package name is
omitted as part of the signal name, the default package is assumed.
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Operation failure (of): The operation failure event may be
generated when the endpoint was asked to apply one or several
signals of type TO on the endpoint, and one or more of those signals
failed prior to timing out. The completion report may carry as a
parameter the name of the signal that failed, as in:
O: G/of(G/rt)
In this case a failure occurred in producing the "rt" signal in the
"G" package.
When the reported signal was applied on a connection, the parameter
supplied will include the name of the connection as well, as in:
O: G/of(G/rt@0A3F58)
When the operation failure event is requested, event parameters can
not be specified. When the package name is omitted, the default
package name is assumed.
6.6 Actions
Extension Actions shall include:
* The name and encoding of the extension action.
* If the extension action takes any action parameters, then the
name, encoding, and possible values of those parameters.
* A description of the operation of the extension action.
* A listing of the actions in this specification the extension can
be combined with. If such a listing is not provided, the
extension action cannot be combined with any other action in this
specification.
* If more than one extension action is defined in the package, then
a listing of the actions in the package the extension can be
combined with. If such a listing is not provided, the extension
action cannot be combined with any other action in the package.
Extension actions defined in two or more different packages should
not be used simultaneously, unless very careful consideration to
their potential interaction and side-effects has been given.
6.7 DigitMapLetters
Extension Digit Map Letters shall include:
* The name and encoding of the extension digit map letter(s).
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* A description of the meaning of the extension digit map
letter(s).
Note that extension DigitMapLetters do not follow the normal naming
conventions for extensions defined in packages. More specifically
the package name and slash ("/") will not be part of the extension
name, thereby forming a flat and limited name space with potential
name clashing.
A package shall therefore not define a digit map letter extension
whose encoding has already been used in another package. If two
packages have used the same encoding for a digit map letter
extension, and those two packages are supported by the same
endpoint, the result of using that digit map letter extension is
undefined.
6.8 ConnectionParameters
Extension Connection Parameters shall include:
* The name and encoding of the connection parameter extension.
* The possible values and encoding of those values that can be
assigned to the connection parameter extension.
* A description of how those values are derived.
Note that the extension connection parameter must be included in the
result for an AuditConnection command auditing the connection
parameters.
6.9 RestartMethods
Extension Restart Methods shall include:
* The name and encoding for the restart method.
* A description of the restart method including the circumstances
that leads to the generation of the restart method. Those
circumstances should be limited to events caused by another
extension defined in the package to ensure the recipient will be
able to interpret the extension restart method correctly.
Note that the extension restart method may have to be provided in
the result for an AuditEndpoint command auditing the restart method.
6.10 Reason Codes
Extension reason codes shall include:
* The number for the reason code. The number must be in the range
800 to 899.
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* A description of the extension reason code including the
circumstances that leads to the generation of the reason code.
Those circumstances should be limited to events caused by another
extension defined in the package to ensure the recipient will be
able to interpret the extension reason code correctly.
Note that the extension reason code may have to be provided in the
result for an AuditEndpoint command auditing the reason code.
6.11 Return Codes
Extension Return Codes shall include:
* The number for the extension return code. The number must be in
the range 800 to 899.
* A description of the extension return code including the
circumstances that leads to the generation of the extension
return code. Those circumstances should be limited to events
caused by another extension defined in the package to ensure the
recipient will be able to interpret the extension return code
correctly.
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7. Versions and Compatibility
7.1. Differences between RFC 2705bis and RFC 2705
RFC 2705 was issued in October 1999, as the last update of draft
version 0.5. RFC 2705bis benefits from further implementation
experience. The main differences between RFC 2705 and RFC 2705bis
are:
* Contains several clarifications, editorial changes and resolution
of known inconsistencies.
* Clarified behavior of mixed wild-carding.
* Allowed IPv6 addresses in endpoint-names.
* Clarified Digit Map matching rules.
* Added Timer T description in Digit Maps.
* Clarified use of wildcards in several commands.
* Event and Signal Parameters formally defined for events and
signals.
* Persistent events now allowed in base MGCP protocol.
* Clarified that mode of second connection in a CreateConnection
command will be set to "send/receive".
* Removed procedures and specification for NAS's (will be provided
as package instead).
* Removed procedures and specification for ATM (will be provided as
package instead).
* Added missing attributes in Capabilities.
* Added Response Acknowledgement response (return code 000) and
included in three-way handshake.
* ResponseAck parameter changed to be allowed in all commands.
* Added return codes 405, 532-537, and 540, and defined return
codes in range 800-899 to be package specific return codes.
* Added reason code 903, and defined reason codes 800-899 to be
package specific reason codes.
* Added section clarifying codec negotiation procedure.
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* Corrected Connection Mode to be optional in ModifyConnection
commands.
* Corrected LocalConnectionDescriptor to be optional in response to
CreateConnection commands.
* Corrected usage of and requirements for SDP to be strictly RFC
2327 compliant.
* Added example sections for commands, responses, and some call
flows.
* Enhanced description of provisional responses and interaction
with three-way handshake.
* Enhanced description of fail-over and the role of "notified
entity". An empty "notified entity" has been allowed, although
strongly discouraged.
* Clarified retransmission procedure and removed "wrong key"
considerations from it.
* Clarified the section on computing retransmission timers.
* Added section on transaction semantics.
* Clarified relationship between endpoint service state and
RestartMethods for restarts and auditing.
* Clarified operation for transitioning from "restart procedure" to
"disconnected state".
* Allowed auditing commands and responses to bypass the "restart"
and "disconnected" procedures.
* Deleted the "Proposed MoveConnection command".
* Removed packages from protocol specification (will be provided in
separate documents instead).
* Package concept formally extended to be primary extension
mechanism now allowing extensions for the following to be defined
in packages as well:
- BearerInformation
- LocalConnectionOptions
- ExtensionParameters
- Connection Modes
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- Actions
- Digit Map Letters
- Connection Parameters
- Restart Methods
- Reason Codes
- Return Codes
* Requirements and suggested format for package definitions added.
* Defined "operation complete" and "operation failure" events to be
automatically present in packages with Time-Out signals.
* IANA registration procedures for packages and other extensions
added.
* Updated grammar to fix known errors and support new extensions in
a backwards compatible manner.
* Connection Mode interaction table added.
7.2. 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.
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* 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.
7.3. Differences between draft-04 and draft-05
Differences are minor: corrected the copyright statement, and
corrected a bug in the formal description.
7.4. 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.
7.5. 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".
* Proposed a "create two connections" command, as an appendix.
7.6. 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.
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* 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 implementers,
* 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.
7.7. The making of MGCP from IPDC and SGCP
MGCP version 0.1 results from the fusion of the SGCP and IPDC
proposals.
7.8. 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:
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* Extension parameters (X-??:)
* Error Code 511 (Unrecognized extension).
* All event codes can be used in RequestedEvents, SignalRequests
and ObservedEvents 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.
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* 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.
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8. Security Considerations
Security issues are discussed in section 5.
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9. Acknowledgements
We want to thank here the many reviewers who provided us with advice
on the design of SGCP and then MGCP, notably Sankar Ardhanari,
Francois Berard, David Auerbach, Bob Biskner, David Bukovinsky,
Charles Eckel, 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.
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10. References
[1] Bradner, S., "The Internet Standards Process -- Revision 3",
BCP 9, RFC 2026, October 1996.
[2] Bradner, S., "Key words for use in RFCs to Indicate Requirement
Levels", BCP 14, RFC 2119, March 1997
[3] Schulzrinne, H., Casner, S., Frederick, R. and V. Jacobson,
"RTP: A Transport Protocol for Real-Time Applications", RFC
1889, January 1996.
[4] Schulzrinne, H., "RTP Profile for Audio and Video Conferences
with Minimal Control", RFC 1890, January 1996.
[5] Handley, M and V. Jacobson, "SDP: Session Description
Protocol", RFC 2327, April 1998.
[6] Handley, M., "SAP - Session Announcement Protocol", Work in
Progress.
[7] Handley, M., Schulzrinne, H. and E. Schooler, "Session
Initiation Protocol (SIP)", RFC 2543, March 1999.
[8] Schulzrinne, H., Rao, A. and R. Lanphier, "Real Time Streaming
Protocol (RTSP)", RFC 2326, April 1998.
[9] ITU-T, Recommendation Q.761, "FUNCTIONAL DESCRIPTION OF THE
ISDN USER PART OF SIGNALING SYSTEM No. 7", (Malaga-
Torremolinos, 1984; modified at Helsinki, 1993).
[10] ITU-T, Recommendation Q.762, "GENERAL FUNCTION OF MESSAGES AND
SIGNALS OF THE ISDN USER PART OF SIGNALING SYSTEM No. 7",
(MalagaTorremolinos, 1984; modified at Helsinki, 1993).
[11] ITU-T, Recommendation H.323 (02/98), "PACKET-BASED MULTIMEDIA
COMMUNICATIONS SYSTEMS".
[12] ITU-T, Recommendation H.225, "Call Signaling Protocols and
Media Stream Packetization for Packet Based Multimedia
Communications Systems".
[13] ITU-T, Recommendation H.245 (02/98), "CONTROL PROTOCOL FOR
MULTIMEDIA COMMUNICATION".
[14] Kent, S. and R. Atkinson, "Security Architecture for the
Internet Protocol", RFC 2401, November 1998.
[15] Kent, S. and R. Atkinson, "IP Authentication Header", RFC 2402,
November 1998.
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[16] Kent, S. and R. Atkinson, "IP Encapsulating Security Payload
(ESP)", RFC 2406, November 1998.
[17] Crocker, D. and P. Overell, "Augmented BNF for Syntax
Specifications: ABNF", RFC 2234, November 1997.
[18] PacketCableTM Network-Based call Signaling Specification, PKT-
SP-EC-MGCP-I02-991201.
[19] Yergeau, F., "UTF-8, a transformation format of ISO 10646", RFC
2279, January 1998.
[20] Braden, R., "Requirements for Internet Hosts -- Communication
Layers", RFC 1122, October 1989.
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11. Authors' Addresses
Mauricio Arango
901 San Antonio Road, UMPK15-214
Palo Alto, CA 94303
Phone:
Email: Mauricio.Arango@sun.com
Andrew Dugan
Level3 Communications
1025 Eldorado Blvd
Broomfield, CO 80021
Phone: +1 720 888 2983
EMail: andrew.dugan@level3.com
Isaac Elliott
Level3 Communications
1025 Eldorado Blvd., Bldg 4000
Broomfield, CO 80021
Phone: +1 720 888 6763
EMail: ike.elliott@level3.com
Christian Huitema
Microsoft Corporation
One Microsoft Way
Redmond, WA 98052-6399
Phone:
EMail: huitema@microsoft.com
Scott Pickett
Vertical Networks
1148 East Arques Ave
Sunnyvale, CA 94086
Phone: +1 408 585 3200
EMail: ScottP@vertical.com
Flemming Andreasen
Cisco Systems
499 Thornall Street, 8th Floor
Edison, NJ 08837
Phone: +1 732 452 1667
EMail: fandreas@cisco.com
Bill Foster
Cisco Systems
771 Alder Drive
Milpitas, CA 95035
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Phone: +1 408 527 8791
EMail: bfoster@cisco.com
Rajesh Kumar
Cisco Systems
3850 Zanker Road
San Jose, CA 95134
Phone: +1 408 527 0811
Email: rkumar@cisco.com
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Appendix A: Formal Syntax Description of the Protocol
In this section, we provide a formal description of the protocol
syntax, following the "Augmented BNF for Syntax Specifications"
defined in RFC 2234. It should be noted, that ABNF does not provide
for implicit specification of linear white space and MGCP messages
must thus follow the explicit linear white space rules provided in
the grammar below. However, in line with general robustness
principles, implementors are strongly encouraged to tolerate
additional linear white space in messages received.
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 = "$"
AllName = "*"
NameString = 1*(range-of-allowed-characters)
; VCHAR except "$", "*", "/", "@"
range-of-allowed-characters = %x21-23 / %x25-29 / %2B-2E
/ %30-3F / %x41-7E
DomainName = 1*256(ALPHA / DIGIT / "." / "-") ; as defined
/ "#" <number> / "[" <dotnum> "]" ; in RFC 821
/ "[" Ipv6address "]" ; see RFC 2372
MGCPversion = "MGCP" 1*(WSP) 1*(DIGIT) "." 1*(DIGIT)
[1*(WSP) ProfileName]
ProfileName = VCHAR *( WSP / VCHAR) ; Was range-of-allowed-characters
MGCPParameter = ParameterValue EOL
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; Check infoCode if more parameter values defined
; Most optional values can only be omitted when auditing
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])
; A final response may include an empty ResponseAck
ResponseAck = confirmedTransactionIdRange
*( "," confirmedTransactionIdRange )
confirmedTransactionIdRange = 1*9DIGIT ["-" 1*9DIGIT]
BearerInformation = BearerAttribute 0*("," 0*WSP BearerAttribute)
BearerAttribute = ("e" ":" <BearerEncoding>)
/ (BearerExtensionName [":"BearerExtensionValue])
BearerExtensionName = PackageLCOExtensionName
BearerExtensionValue = LocalOptionExtensionValue
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 = LocalEndpointName ; No internal structure
; was 1*32(suitableCharacter)
portNumber = 1*5(DIGIT)
RequestIdentifier = 1*32(HEXDIG)
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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" ":" <encryptiondata>)
/ ("nt" ":" ( <typeOfNetwork> /
<supportedTypeOfNetwork> )
/ (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(SuitableLCOCharacter)
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 (RFC 2327)
;NOTE that encryption key may contain an algorithm
;as specified in RFC 1890
encryptiondata = ( "clear" ":" <encryptionKey> )
/ ( "base64" ":" <encodedEncryptionKey> )
/ ( "uri" ":" <URItoObtainKey> )
/ ( "prompt" ) ; defined in SDP, not usable in MGCP!
; was 1*(SuitableCharacter / SP)
encryptionKey = 1*(SuitableLCOCharacter) / quotedString
; See RFC 2045
encodedEncryptionKey = 1*(ALPHA / DIGIT / "+" / "/" / "=")
URItoObtainKey = 1*(SuitableLCOCharacter) / quotedString
typeOfNetwork = "IN" / "ATM" / "LOCAL" / OtherTypeOfNetwork
; Registered with IANA - see RFC 2327
OtherTypeOfNetwork = 1*(SuitableLCOCharacter)
supportedTypeOfNetwork = typeOfNetwork *(";" typeOfNetwork)
supportedModes = ConnectionMode 0*(";" ConnectionMode)
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supportedPackages = packageName 0*(";" packageName)
packageName = 1*(ALPHA / DIGIT / HYPHEN) ; Hyphen neither first or last
localOptionExtensionName = VendorLCOExtensionName
/ PackageLCOExtensionName
/ OtherLCOExtensionname
VendorLCOExtensionName = "x" ("+"/"-") 1*32(SuitableExtLCOCharacter)
PackageLCOExtensionName = packageName "/"
1*32(SuitablePkgExtLCOCharacter)
; must not start with "x-" or "x+"
OtherLCOExtensionName = 1*32(SuitableExtLCOCharacter)
localOptionExtensionValue = (1*32(SuitableLCOCharacter) / quotedString)
*(";" *WSP (1*32(SuitableLCOCharacter)
/ quotedString)))
;Note: No "data" mode.
ConnectionMode = "sendonly" / "recvonly" / "sendrecv"
/ "confrnce" / "inactive" / "loopback"
/ "conttest" / "netwloop" / "netwtest"
/ ExtensionConnectionMode
ExtensionConnectionMode = PkgExtConnectionMode
PkgExtConnectionMode = packageName "/" 1*(ALPHA / DIGIT)
RequestedEvents = requestedEvent 0*("," 0*(WSP) requestedEvent)
requestedEvent = (eventName ["(" requestedActions ")"])
/ (eventName "(" requestedActions ")"
"(" eventParameters ")" )
eventName = [(packageName / "*") "/"]
(eventId / "all" / eventRange)
["@" (ConnectionId / "$" / "*")]
; Was 1*(SuitableCharacter)
eventId = 1*(ALPHA / DIGIT / HYPHEN)
; Hyphen neither first or last
eventRange = "[" 1*(DigitMapLetter / (DIGIT "-" DIGIT) /
(DTMFLetter "-" DTMFLetter)) "]"
DTMFLetter = "A" / "B" / "C" / "D" ; Was undefined
requestedActions = requestedAction 0*("," 0*(WSP) requestedAction)
requestedAction = "N" / "A" / "D" / "S" / "I" / "K"
/ "E" "(" EmbeddedRequest ")"
/ ExtensionAction
ExtensionAction = PackageExtAction
PackageExtAction = packageName "/" Action ["(" ActionParameters ")"]
Action = 1*ALPHA
ActionParameters = eventParameters ; May contain actions
;NOTE: Should tolerate different order when receiving, e.g. for NCS.
EmbeddedRequest = ( "R" "(" EmbeddedRequestList ")"
["," "S" "(" EmbeddedSignalRequest ")"]
["," "D" "(" EmbeddedDigitMap ")"] )
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/ ( "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 = eventParameterValue
/ eventParameterName "=" eventParameter
/ eventParameterName "(" eventParameters ")"
eventParameterString = 1*(SuitableEventParamCharacter)
eventParameterName = eventParameterString
eventParameterValue = eventParameterString / quotedString
DigitMap = DigitString / "(" DigitStringList ")"
DigitStringList = DigitString 0*( "|" DigitString )
DigitString = 1*(DigitStringElement)
DigitStringElement = DigitPosition ["."]
DigitPosition = DigitMapLetter / DigitMapRange
; NOTE "X" is now included
DigitMapLetter = DIGIT / "#" / "*" / "A" / "B" / "C" / "D" / "T"
/ "X" / ExtensionDigitMapLetter
ExtensionDigitMapLetter = "E" / "F" / "G" / "H" / "I" / "J" / "K"
/ "L" / "M" / "N" / "O" / "P" / "Q" / "R"
/ "S" / "U" / "V" / "W" / "Y" / "Z"
; NOTE "[x]" is now allowed
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)
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packetsLost = 1*9(DIGIT)
jitter = 1*9(DIGIT)
averageLatency = 1*9(DIGIT)
ConnectionParameterExtensionName = VendorCPExtensionName
/ PackageCPExtensionName
VendorCPExtensionName = "X" "-" 2*ALPHA
PackageCPExtensionName = packageName "/" CPName
CPName = 1*(ALPHA / DIGIT / HYPHEN)
ConnectionParameterExtensionValue = 1*9(DIGIT)
ReasonCode = 3DIGIT
[1*(WSP) "/" packageName] ; Only for 8xx
[WSP 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" / extensionParameter
QuarantineHandling = loopControl / processControl
/ (loopControl "," processControl )
loopControl = "step" / "loop"
processControl = "process" / "discard"
DetectEvents = eventName 0*("," eventName)
RestartMethod = "graceful" / "forced" / "restart" / "disconnected"
/ "cancel-graceful" / extensionRestartMethod
extensionRestartMethod = PackageExtensionRM
PackageExtensionRM = packageName "/" 1*32(ALPHA / DIGIT / HYPHEN)
RestartDelay = 1*6(DIGIT)
extensionParameter = VendorExtensionParameter
/ PackageExtensionParameter
/ OtherExtensionParameter
VendorExtensionParameter = "X" ("-"/"+") 1*6(ALPHA / DIGIT)
PackageExtensionParameter = packagename "/"
1*32(ALPHA / DIGIT / HYPHEN)
; must not start with "x-" or x+"
OtherExtensionParameter = 1*32(ALPHA / DIGIT / HYPHEN)
parameterString = 1*(%x20-7F) ; First and last must not be white space
MGCPResponse = MGCPResponseLine 0*(MGCPParameter) [EOL *SDPinformation]
MGCPResponseLine = <responseCode> 1*(WSP) <transaction-id>
[1*(WSP) "/" packageName] ; Only for 8xx
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[WSP <responseString>] EOL
responseCode = 3DIGIT
responseString = *(%x20-7E)
SuitableCharacter = DIGIT / ALPHA / "+" / "-" / "_" / "&"
/ "!" / "'" / "|" / "=" / "#" / "?" / "/"
/ "." / "$" / "*" / ";" / "@" / "[" / "]"
/ "^" / "`" / "{" / "}" / "~"
SuitablePkgExtLCOCharacter = SuitableLCOCharacter
SuitableExtLCOCharacter = DIGIT / ALPHA / "+" / "-" / "_" / "&"
/ "!" / "'" / "|" / "=" / "#" / "?" /
/ "." / "$" / "*" / "@" / "[" / "]"
/ "^" / "`" / "{" / "}" / "~"
SuitableLCOCharacter = SuitableExtLCOCharacter / "/"
SuitableEventCharacter = DIGIT / ALPHA / "+" / "-" / "_" / "&"
/ "!" / "'" / "|" / "#" / "?" / "/"
/ "." / "$" / "*" / ";" / "@" / "[" / "]"
/ "^" / "`" / "{" / "}" / "~"
; VCHAR except """, "(", ")", and ","
SuitableEventParamCharacter = %x21 / %x23-27 / %x2A-2B / %x2D-7E
; NOTE: UTF8 encoded
quotedString = DQUOTE visibleString
0*(quoteEscape visibleString) DQUOTE
quoteEscape = DQUOTE DQUOTE
visibleString = (%x00-21 / %x23-FF)
EOL = CRLF / LF
HYPHEN = "-"
SDPinformation = ;See RFC 2327
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Appendix B: IANA Considerations
B.1. Packages
Packages can be registered with the IANA according to the following
procedure:
The package must have a unique string name which must not start with
the two characters "x-" or "x+".
The package name must be registered with IANA as well as a reference
to the document that describes the package. The document must have a
stable URL and must be contained on public web server.
A contact name, e-mail and postal address for the package must be
provided. The contact information shall be updated by the defining
organization as necessary.
B.2. Local Connection Options
Packages are the preferred extension mechanism, however for
backwards compatibility, local connection options beyond those
provided in this specification can be registered with IANA. Each
local connection option must have a unique string name which must
not start with "x-" or "x+". The name of the local connection option
must be registered with IANA as well as a reference to the document
that describes the local connection option. The document must have a
stable URL and must be contained on a public web server.
A contact name, e-mail and postal address for the local connection
option must be provided.
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Appendix C: Mode Interactions
An MGCP connection can establish one or more media streams. These
streams are either incoming (from a remote endpoint) or outgoing
(generated at the handset microphone). The "connection mode"
parameter establishes the direction and generation of these streams.
When there is only one connection to an endpoint, the mapping of
these streams is straightforward; the handset plays the incoming
stream over the handset speaker and generates the outgoing stream
from the handset microphone signal, depending on the mode parameter.
However, when several connections are established to an endpoint,
there can be many incoming and outgoing streams. Depending on the
connection mode used, these streams may interact differently with
each other and the streams going to/from the handset.
The table below describes how different connections should be mixed
when one or more connections are concurrently "active". An active
connection is here defined as a connection that is in one of the
following modes:
* "send/receive"
* "send only"
* "receive only"
* "conference"
Connections in "network loopback", "network continuity test", or
"inactive" modes are not affected by connections in the "active"
modes. The Table uses the following conventions:
* Ai is the incoming media stream from Connection A
* Bi is the incoming media stream from Connection B
* Hi is the incoming media stream from the Handset Microphone
* Ao is the outgoing media stream to Connection A
* Bo is the outgoing media stream to Connection B
* Ho is the outgoing media stream to the Handset earpiece
* NA indicates No Stream whatever
"netw" in the following table indicates either "netwloop" or
"netwtest" mode.
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-------------------------------------------------------------
| | Connection A Mode |
| |-----------------------------------------------------
| |sendonly|recvonly|sendrecv|confrnce|inactive| netw |
|-------|-----------------------------------------------------|
| |Send | Ao=Hi | Ao=NA | Ao=Hi | Ao=Hi | Ao=NA | Ao=Ai |
|C|only | Bo=Hi | Bo=Hi | Bo=Hi | Bo=Hi | Bo=Hi | Bo=Hi |
|o| | Ho=NA | Ho=Ai | Ho=Ai | Ho=Ai | Ho=NA | Ho=NA |
|n|-----------------------------------------------------------
|n|recv | | Ao=NA | Ao=Hi | Ao=Hi | Ao=NA | Ao=Ai |
|e|only | | Bo=NA | Bo=NA | Bo=NA | Bo=NA | Bo=NA |
|c| | |Ho=Ai+Bi|Ho=Ai+Bi|Ho=Ai+Bi| Ho=Bi | Ho=Bi |
|t|-----------------------------------------------------------|
|i|send | | | Ao=Hi | Ao=Hi | Ao=NA | Ao=Ai |
|o|recv | | | Bo=Hi | Bo=Hi | Bo=Hi | Bo=Hi |
|n| | | |Ho=Ai+Bi|Ho=Ai+Bi| Ho=Bi | Ho=Bi |
| |-----------------------------------------------------------|
|B|conf | | | |Ao=Hi+Bi| Ao=NA | Ao=Ai |
| |rnce | | | |Ho=Ai+Bi| Bo=Hi | Bo=Hi |
|M| | | | |Ho=Ai+Bi| Ho=Bi | Ho=Bi |
|o|-----------------------------------------------------------|
|d|Inac | | | | | Ao=NA | Ao=Ai |
|e|tive | | | | | Bo=NA | Bo=NA |
| | | | | | | Ho=NA | Ho=NA |
| |-----------------------------------------------------------|
| |netw | | | | | | Ao=Ai |
| | | | | | | | Bo=Bi |
| | | | | | | | Ho=NA |
-------------------------------------------------------------
If there are three or more "active" channels they will still
interact as defined in the table above with the outgoing media
streams mixed for each interaction. (Union of all streams) If
internal resources are used up and the streams cannot be mixed, the
gateway should return a resources Not available error.
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Appendix D: Example Command Encodings
This appendix provides examples of commands and responses shown with
the actual encoding used. Examples are provided for each command.
All commentary shown in the commands and responses is optional.
D.1. NotificationRequest
The first example illustrates a NotificationRequest that will ring a
phone and look for an off-hook event:
RQNT 1201 aaln/1@rgw-2567.whatever.net MGCP 1.0
N: ca@ca1.whatever.net:5678
X: 0123456789AC
R: l/hd(N)
S: g/rg
The response indicates that the transaction was successful:
200 1201 OK
The second example illustrates a NotificationRequest that will look
for and accumulate an off-hook event, and then provide dial-tone and
accumulate digits according to the digit map provided. The "notified
entity" is set to "ca@ca1.whatever.net:5678", and since the
SignalRequests parameter is empty (it could have been omitted as
well), all currently active TO signals will be stopped. All events
in the quarantine buffer will be processed, and the list of events
to detect in the "notification" state will include fax tones in
addition to the "requested events" and persistent events:
RQNT 1202 aaln/1@rgw-2567.whatever.net MGCP 1.0
N: ca@ca1.whatever.net:5678
X: 0123456789AC
R: hd(A, E(S(dl),R(oc, hu, [0-9#*T](D))))
D: (0T|00T|#xxxxxxx|*xx|91xxxxxxxxxx|9011x.T)
S:
Q: process
T: ft
The response indicates that the transaction was successful:
200 1202 OK
D.2. Notify
The example below illustrates a Notify message that notifies an off-
hook event followed by a 12-digit number beginning with "91". A
transaction identifier correlating the Notify with the
NotificationRequest it results from is included. The command is sent
to the current "notified entity", which typically will be the actual
value supplied in the NotifiedEntity parameter, i.e.,
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"ca@ca1.whatever.net:5678" a failover situation could have changed
this:
NTFY 2002 aaln/1@rgw-2567.whatever.net MGCP 1.0
N: ca@ca1.whatever.net:5678
X: 0123456789AC
O: hd,9,1,2,0,1,8,2,9,4,2,6,6
The Notify response indicates that the transaction was successful:
200 2002 OK
D.3. CreateConnection
The first example illustrates a CreateConnection command to create a
connection on the endpoint specified. The connection will be part of
the specified CallId. The LocalConnectionOptions specify that G.711
u-law will be the codec used and the packetization period will be 10
ms. The connection mode will be "receive only":
CRCX 1204 aaln/1@rgw-2567.whatever.net MGCP 1.0
C: A3C47F21456789F0
L: p:10, a:PCMU
M: recvonly
The response indicates that the transaction was successful, and a
connection identifier for the newly created connection is therefore
included. A session description for the new connection is included
as well note that it is preceded by an empty line.
200 1204 OK
I: FDE234C8
v=0
o=- 25678 753849 IN IP4 128.96.41.1
s=-
c=IN IP4 128.96.41.1
t=0 0
m=audio 3456 RTP/AVP 0
The second example illustrates a CreateConnection command containing
a notification request and a RemoteConnectionDescriptor:
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CRCX 1205 aaln/1@rgw-2569.whatever.net MGCP 1.0
C: A3C47F21456789F0
L: p:10, a:PCMU
M: sendrecv
X: 0123456789AD
R: hd
S: rg
v=0
o=- 25678 753849 IN IP4 128.96.41.1
s=-
c=IN IP4 128.96.41.1
t=0 0
m=audio 3456 RTP/AVP 0
The response indicates that the transaction failed, because the
phone was already off-hook. Consequently, neither a connection-id
nor a session description is returned:
401 1205 Phone off-hook
Our third example illustrates the use of the provisional response
and the three-way handshake. We create another connection this time
using dynamic quality of service and acknowledging the previous
response received:
CRCX 1206 aaln/1@rgw-2569.whatever.net MGCP 1.0
K: 1205
C: A3C47F21456789F0
L: p:10, a:PCMU
M: inactive
v=0
o=- 25678 753849 IN IP4 128.96.41.1
s=-
c=IN IP4 128.96.41.1
t=0 0
m=audio 3456 RTP/AVP 0
A provisional response is returned initially:
100 1206 Pending
I: DFE233D1
v=0
o=- 4723891 7428910 IN IP4 128.96.63.25
s=-
c=IN IP4 128.96.63.25
t=0 0
m=audio 3456 RTP/AVP 0
A little later, the final response is received:
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200 1206 OK
K:
I: DFE233D1
v=0
o=- 4723891 7428910 IN IP4 128.96.63.25
s=-
c=IN IP4 128.96.63.25
t=0 0
m=audio 3456 RTP/AVP 0
The Call Agent acknowledges the final response as requested:
000 1206
and the transaction is complete.
D.4. ModifyConnection
The first example shows a ModifyConnection command that simply sets
the connection mode of a connection to "send/receive" the
"notified entity" is set as well:
MDCX 1209 aaln/1@rgw-2567.whatever.net MGCP 1.0
C: A3C47F21456789F0
I: FDE234C8
N: ca@ca1.whatever.net
M: sendrecv
The response indicates that the transaction was successful:
200 1209 OK
In the second example, we pass a session description and include a
notification request with the ModifyConnection command. The endpoint
will start playing ring-back tones to the user until it detects
audio on the connection specified for the media start event:
MDCX 1210 aaln/1@rgw-2567.whatever.net MGCP 1.0
C: A3C47F21456789F0
I: FDE234C8
M: recvonly
X: 0123456789AE
R: hu, ma@FDE234C8
S: rt
v=0
o=- 4723891 7428910 IN IP4 128.96.63.25
s=-
c=IN IP4 128.96.63.25
t=0 0
m=audio 3456 RTP/AVP 0
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The response indicates that the transaction was successful:
200 1206 OK
D.5. DeleteConnection (from the Call Agent)
In this example, the Call Agent simply instructs the gateway to
delete the connection FDE234C8 on the endpoint specified:
DLCX 1210 aaln/1@rgw-2567.whatever.net MGCP 1.0
C: A3C47F21456789F0
I: FDE234C8
The response indicates success, and that the connection was deleted.
Connection parameters for the connection are therefore included as
well:
250 1210 OK
P: PS=1245, OS=62345, PR=780, OR=45123, PL=10, JI=27, LA=48
D.6. DeleteConnection (from the gateway)
In this example, the gateway sends a DeleteConnection command to the
Call Agent to instruct it that a connection on the specified
endpoint has been deleted. The ReasonCode specifies the reason for
the deletion, and Connection Parameters for the connection are
provided as well:
DLCX 1210 aaln/1@rgw-2567.whatever.net MGCP 1.0
C: A3C47F21456789F0
I: FDE234C8
E: 900 Hardware error
P: PS=1245, OS=62345, PR=780, OR=45123, PL=10, JI=27, LA=48
The Call Agent sends a success response to the gateway:
200 1210 OK
D.7. DeleteConnection (multiple connections from the Call Agent)
In the first example, the Call Agent instructs the gateway to delete
all connections related to call "A3C47F21456789F0" on the specified
endpoint:
DLCX 1210 aaln/1@rgw-2567.whatever.net MGCP 1.0
C: A3C47F21456789F0
The response indicates success and that the connection(s) were
deleted:
250 1210 OK
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In the second example, the Call Agent instructs the gateway to
delete all connections related to all of the endpoints specified:
DLCX 1210 aaln/*@rgw-2567.whatever.net MGCP 1.0
The response indicates success:
250 1210 OK
D.8. AuditEndpoint
In the first example, the Call Agent wants to learn what endpoints
are present on the gateway specified, hence the use of the "all of"
wild-card for the local portion of the endpoint-name:
AUEP 1200 *@rgw-2567.whatever.net MGCP 1.0
The gateway indicates success and includes a list of endpoint names:
200 1200 OK
Z: aaln/1@rgw-2567.whatever.net
Z: aaln/2@rgw-2567.whatever.net
In the second example, the capabilities of one of the endpoints is
requested:
AUEP 1201 aaln/1@rgw-2567.whatever.net MGCP 1.0 NCS 1.0
F: A
The response indicates success and the capabilities as well. Two
codecs are supported, however with different capabilities.
Consequently two separate capability sets are returned:
200 1201 OK
A: a:PCMU, p:10-100, e:on, s:off, v:L;S, m:sendonly;
recvonly;sendrecv;inactive;netwloop;netwtest
A: a:G729A, p:30-90, e:on, s:on, v:L;S, m:sendonly;
recvonly;sendrecv;inactive;confrnce;netwloop
Note that the carriage return in the Capabilities lines are shown
for formatting reasons only - they are not permissible in a real
implementation.
In the third example, the Call Agent audits all possible information
for the endpoint:
AUEP 2002 aaln/1@rgw-2567.whatever.net MGCP 1.0 NCS 1.0
F: R,D,S,X,N,I,T,O,ES
The response indicates success:
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200 2002 OK
R: L/hu,oc(N),[0-9](N)
D:
S: vmwi(+)
X: 0123456789B1
N: [128.96.41.12]
I: 32F345E2
T: ft
O: hd,9,1,2
ES: hd
The list of requested events contains three events. Where no package
name is specified, the default package is assumed. The same goes for
actions, so the default action Notify - must therefore be assumed
for the "L/hu" event. The omission of a value for the "digit map"
means the endpoint currently does not have a digit map. There are
currently no active time-out signals, however the OO signal "vmvi"
is currently on and is consequently included in this case it was
parameterized, however the parameter could have been excluded. The
current "notified entity" refers to an IP-address and only a single
connection exists for the endpoint. The current value of
DetectEvents is "ft", and the list of ObservedEvents contains the
four events specified. Finally, the event-states audited reveals
that the phone was off-hook at the time the transaction was
processed.
D.9. AuditConnection
The first example shows an AuditConnection command where we audit
the CallId, NotifiedEntity, LocalConnectionOptions, Connection Mode,
LocalConnectionDescriptor, and the Connection Parameters:
AUCX 2003 aaln/1@rgw-2567.whatever.net MGCP 1.0
I: 32F345E2
F: C,N,L,M,LC,P
The response indicates success and includes information for the
RequestedInfo:
200 2003 OK
C: A3C47F21456789F0
N: ca@ca1.whatever.net
L: p:10, a:PCMU
M: sendrecv
P: PS=395, OS=22850, PR=615, OR=30937, PL=7, JI=26, LA=47
v=0
o=- 4723891 7428910 IN IP4 128.96.63.25
s=-
c=IN IP4 128.96.63.25
t=0 0
m=audio 1296 RTP/AVP 0
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In the second example, we request to audit
RemoteConnectionDescriptor and LocalConnectionDescriptor:
AUCX 1203 aaln/2@rgw-2567.whatever.net MGCP 1.0 NCS 1.0
I: FDE234C8
F: RC,LC
The response indicates success, and includes information for the
RequestedInfo. In this case, no RemoteConnectionDescriptor exists,
hence only the protocol version field is included for the
RemoteConnectionDescriptor:
200 1203 OK
v=0
o=- 4723891 7428910 IN IP4 128.96.63.25
s=-
c=IN IP4 128.96.63.25
t=0 0
m=audio 1296 RTP/AVP 0
v=0
D.10. RestartInProgress
The first example illustrates a RestartInProgress message sent by an
gateway to inform the Call Agent that the specified endpoint will be
taken out of service in 300 seconds:
RSIP 1200 aaln/1@rgw-2567.whatever.net MGCP 1.0
RM: graceful
RD: 300
The Call Agents response indicates that the transaction was
successful:
200 1200 OK
In the second example, the RestartInProgress message sent by the
gateway informs the Call Agent, that all of the gateways endpoints
are being placed in service in 0 seconds, i.e., they are back in
service. The delay could have been omitted as well:
RSIP 1204 *@rgw-2567.whatever.net MGCP 1.0 NCS 1.0
RM: restart
RD: 0
The Call Agents response indicates success, and furthermore
provides the endpoints in question with a new "notified entity":
200 1204 OK
N: CA-1@whatever.net
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Alternatively, the command could have failed with a new "notified
entity" as in:
521 1204 OK
N: CA-1@whatever.net
In that case, the command would then have to be retried in order to
satisfy the "restart procedure", this time going to Call Agent "CA-
1@whatever.net".
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Appendix E: Endpoint Naming Conventions
The following sections provide some example endpoint naming
conventions.
E.1. Analog Access Line Endpoints
The string "aaln", MAY be used as the first term in a local endpoint
name for analog access line endpoints. Terms following "aaln" should
follow the physical hierarchy of the gateway so that if the gateway
has a number of RJ11 ports, the local endpoint name could look like
the following:
aaln/#
where "#" is the number of the analog line (RJ11 port) on the
gateway
On the other hand, the gateway may have a number of physical plug-in
units, each of which contain some number of RJ11 ports, in which
case, the local endpoint name might look like the following:
aaln/<unit #>/#
where <unit #> is the number of the plug in unit in the gateway and
"#" is the number of the analog line (RJ11 port) on that unit.
E.2. Digital Trunks
The string "ds" MAY be used for the first term of digital endpoints
with a naming convention that follows the physical and digital
hierarchy such as:
ds/<unit-type1>-<unit #>/<unit-type2>-<unit #>/.../<channel #>
where: <unit-type> identifies the particular hierarchy level. Some
example values of <unit-type> are: "s", "su", "oc3", "ds3", "e3",
"ds2", "e2", "ds1", "e1" where "s" indicates a slot number and "su"
indicates a sub-unit within a slot.
The <unit #> is a decimal number which is used to reference a
particular instance of a <unit-type> at that level of the hierarchy.
The number of levels and naming of those levels is based on the
physical hierarchy within the media gateway.
E.3. Virtual Endpoints
Another type of endpoint is one that is not associated with a
physical interface (such as an analog or digital endpoint). This
type of endpoint is called a virtual endpoint and is often used to
represent some DSP resources that gives the endpoint some
capability. Examples are announcement, IVR or conference bridge
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devices. These devices may have multiple instances of DSP functions
so that a possible naming convention is:
<virtual-endpoint-type>/<endpoint-#>
where < virtual-endpoint-type> may be some string representing the
type of endpoint (such as "ann" for announcement server or cnf for
conference server) and <endpoint-#> would identify a particular
endpoint within the device. If the physical hierarchy of the server
includes plug-in DSP cards, another level of hierarchy in the local
endpoint name may be used to describe the plug in unit.
A virtual endpoint may be created as the result of using the "any
of" wildcard. Similarly, a virtual endpoint may cease to exist once
the last connection on the virtual endpoint is deleted. The
definition of the virtual endpoint must detail both of these
aspects.
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Appendix F: Example Call Flows
The message flow tables in this section use the following
abbreviations:
* rgw = Residential Gateway
* ca = Call Agent
* n+ = step 'n' is repeated one or more times
Note that any use of upper and lower case within the text of the
messages is to aid readability and is not in any way a requirement.
The only requirement involving case is to be case insensitive at all
times.
F.1. Restart
F.1.1. Residential Gateway Restart
The following table shows a message sequence that might occur when a
call agent (ca) is contacted by two independent residential gateways
(rgw1 and rgw2) which have restarted.
Table F.1: Residential Gateway Restart
---------------------------------------------------------------------
|step#| usr1 | rgw1 | ca | rgw2 | usr2 |
|=====|============|============|============|============|===========|
| 1 | | rsip -> | | | |
| | | | <- ack | | |
|-----|------------|------------|------------|------------|-----------|
| 2 | | | <- auep | | |
| | | ack -> | | | |
|-----|------------|------------|------------|------------|-----------|
| 3+ | | | <- rqnt | | |
| | | ack -> | | | |
|-----|------------|------------|------------|------------|-----------|
| 4 | | | | <- rsip | |
| | | | ack -> | | |
|-----|------------|------------|------------|------------|-----------|
| 5 | | | auep -> | | |
| | | | | <- ack | |
|-----|------------|------------|------------|------------|-----------|
| 6+ | | | rqnt -> | | |
| | | | | <- ack | |
---------------------------------------------------------------------
Step 1 - RestartInProgress (rsip) from rgw1 to ca
rgw1 uses DNS to determine the domain name of ca and send to the
default port of 2727. The command consists of the following:
rsip 0 *@rgw1.whatever.net mgcp 1.0
rm: restart
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The "*" is used to inform ca that all endpoints of rgw1 are being
restarted, and "restart" is specified as the restart method. The
Call Agent "ca" acknowledges the command with an acknowledgement
message containing the transaction id (in this case 0) for the
command. It sends the acknowledgement to rgw1 using the same port
specified in as the source port for the rsip. If none was indicated,
it uses the default port of 2727.
200 0 ok
A response code in mandatory. In this case, "200", indicates "the
requested transaction was executed normally". The response string is
optional. In this case, "ok" is included as an additional
description.
Step 2 - AuditEndpoint (auep) from ca to rgw1
The command consists of the following:
auep 153 *@rgw1.whatever.net mgcp 1.0
The "*" is used to request audit information from rgw1 of all its
endpoints. rgw1 acknowledges the command with an acknowledgement
message containing the id (in this case 153) of the command, and it
includes a list of its endpoints. In this example, rgw1 has two
endpoints, aaln/1 and aaln/2.
200 153 ok
Z: aaln/1@rgw1.whatever.net
Z: aaln/2@rgw1.whatever.net
Once it has the list of endpoint ids, ca may send individual
AuditEndpoint commands in which the "*" is replaced by the id of the
given endpoint. As its response, rgw1 would replace the endpoint id
list returned in the example with the endpoint capabilities of the
endpoint. This optional message exchange is not shown in this
example.
Step 3 - NotificationRequest (rqnt) from ca to each endpoint of rgw1
In this case, ca sends two rqnts, one for aaln/1:
rqnt 154 aaln/1@rgw1.whatever.net mgcp 1.0
r: l/hd(n)
x: 3456789a0
and a second for aaln/2:
rqnt 155 aaln/2@rgw1.whatever.net mgcp 1.0
r: l/hd(n)
x: 3456789a1
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Note that the in requested events parameter line, the event is fully
specified as "l/hd", i.e. with the package name, in order to avoid
any potential ambiguity. This is the recommended behavior. For the
sake of clarity, the action, which in this case is to Notify, is
explicitly specified by including the "(n)". If no action is
specified, Notify is assumed as the default regardless of the event.
If any other action is desired, it must be stated explicitly.
The expected response from rgw1 to these requests is an
acknowledgement from aaln/1 as follows:
200 154 ok
and from aaln/2:
200 155 ok
Step 4 RestartInProgress (rsip) from rgw2 to ca
rsip 0 *@rgw2.whatever.net mgcp 1.0
rm: restart
followed by the acknowledgement from ca:
200 0 ok
Step 5 - AuditEndpoint (auep) from ca to rgw2
auep 156 *@rgw2.whatever.net mgcp 1.0
followed by an acknowledgement from rgw2:
200 156 ok
z: aaln/1@rgw2.whatever.net
z: aaln/2@rgw2.whatever.net
Step 6 - NotificationRequest (rqnt) from ca to each endpoint of rgw2
rqnt 157 aaln/1@rgw2.whatever.net mgcp 1.0
r: l/hd(n)
x: 3456789a2
followed by:
rqnt 158 aaln/2@rgw2.whatever.net mgcp 1.0
r: l/hd(n)
x: 3456789a3
with rgw2 acknowledging for aaln/1:
200 157 ok
and for aaln/2:
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200 158 ok
F.1.2. Call Agent Restart
The following table shows the message sequence which occurs when a
call agent (ca) restarts. How it determines the address information
of the gateways, in this case rgw1 and rgw2, is not covered in this
document. For interoperability, it is recommended to provide the
ability to configure the call agent to send AUEP (*) to specific
addresses and ports.
Table F.2: Residential Gateway Restart
---------------------------------------------------------------------
| # | usr1 | rgw1 | ca | rgw2 | usr2 |
|===|=============|============|============|============|============|
| 1 | | | <- auep | | |
| | | ack -> | | | |
|---|-------------|------------|------------|------------|------------|
| 2+| | | <- rqnt | | |
| | | ack -> | | | |
|---|-------------|------------|------------|------------|------------|
| 3 | | | auep -> | | |
| | | | | <- ack | |
|---|-------------|------------|------------|------------|------------|
| 4+| | | rqnt -> | | |
| | | | | <- ack | |
---------------------------------------------------------------------
Step 1 - AuditEndpoint (auep) from ca to rgw1
The command consists of the following:
auep 0 *@rgw1.whatever.net mgcp 1.0
The "*" is used to request audit information from rgw1 of all its
endpoints. rgw1 acknowledges the command with an acknowledgement
message containing the transaction id (in this case 0) of the
command, and it includes a list of its endpoints. In this example,
rgw1 has two endpoints, aaln/1 and aaln/2.
200 0 ok
z: aaln/1@rgw1.whatever.net, aaln/2@rgw1.whatever.net
Once it has the list of endpoint ids, ca may send individual
AuditEndpoint commands in which the "*" is replaced by the id of the
given endpoint. As its response, rgw1 would replace the endpoint id
list returned in the example with the endpoint capabilities of the
endpoint. This optional message exchange is not shown in this
example.
Step 2 - NotificationRequest (rqnt) off-hook from ca to rgw1
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In this case, ca sends two rqnts, one for aaln/1:
rqnt 1 aaln/1@rgw1.whatever.net mgcp 1.0
r: l/hd(n)
x: 234567890
and a second for aaln/2:
rqnt 2 aaln/2@rgw1.whatever.net mgcp 1.0
r: l/hd(n)
x: 234567891
The expected response from rgw1 to these requests is an
acknowledgement from aaln/1 as follows:
200 1 ok
and from aaln/2:
200 2 ok
Step 3 - AuditEndpoint (auep) from ca to rgw2
auep 3 *@rgw2.whatever.net mgcp 1.0
followed by an acknowledgement from rgw2:
200 3 ok
z: aaln/1@rgw2.whatever.net, aaln/2@rgw2.whatever.net
Step 4 - NotificationRequest (rqnt) from ca to each endpoint of rgw2
rqnt 4 aaln/1@rgw2.whatever.net mgcp 1.0
r: l/hd(n)
x: 234567892
followed by:
rqnt 5 aaln/2@rgw2.whatever.net mgcp 1.0
r: l/hd(n)
x: 234567893
with rgw2 acknowledging for aaln/1:
200 4 ok
and for aaln/2:
200 5 ok
F.2. Connection Creation
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F.2.1 Residential Gateway to Residential Gateway
The following table shows the message sequence which occurs when a
user (usr1) makes a call through a residential gateway (rgw1) to a
user served by another residential gateway (rgw2). This example
illustrates the communication between the residential gateways and
the call agent (ca) only. The local name of the endpoints in this
example is aaln/1 for both gateways, and references within the
description of the steps to rgw1 and rgw2 can be assumed to refer to
aaln/1 of rgw1 and aaln/1 of rgw2. Note that this is only an
example and is not the only legal call scenario.
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Table F.3: Residential Gateway Connection Creation
---------------------------------------------------------------------
| # | usr1 | rgw1 | ca | rgw2 | usr2 |
|===|=============|============|============|============|============|
| 1 | offhook -> | ntfy -> | | | |
| | | | <- ack | | |
|---|-------------|------------|------------|------------|------------|
| 2 | <- dialtone | | <- rqnt | | |
| | | ack -> | | | |
|---|-------------|------------|------------|------------|------------|
| 3 | digits -> | ntfy -> | | | |
| | | | <- ack | | |
|---|-------------|------------|------------|------------|------------|
| 4 | | | <- rqnt | | |
| | | ack -> | | | |
|---|-------------|------------|------------|------------|------------|
| 5 | <- recvonly | | <- crcx | | |
| | | ack -> | | | |
|---|-------------|------------|------------|------------|------------|
| 6 | | | crcx -> | | sendrcv -> |
| | | | | <- ack | |
|---|-------------|------------|------------|------------|------------|
| 7 | <- recvonly | | <- mdcx | | |
| | | ack -> | | | |
|---|-------------|------------|------------|------------|------------|
| 8 | <- ringback | | <- rqnt | | |
| | | ack -> | | | |
|---|-------------|------------|------------|------------|------------|
| 9 | | | rqnt -> | | ringing -> |
| | | | | <- ack | |
|---|-------------|------------|------------|------------|------------|
|10 | | | | <- ntfy | <- offhook |
| | | | ack -> | | |
|---|-------------|------------|------------|------------|------------|
|11 | | | rqnt -> | | |
| | | | | <- ack | |
|---|-------------|------------|------------|------------|------------|
|12 | | | <- rqnt | | |
| | | ack -> | | | |
|---|-------------|------------|------------|------------|------------|
|13 | <- sendrcv | | <- mdcx | | |
| | | ack -> | | | |
---------------------------------------------------------------------
Step 1 - Notify (ntfy) offhook from rgw1 to ca
This ntfy is the result of usr1 going offhook and assumes ca had
previously sent an rqnt with RequestId 445678944 to rgw1
requesting notification in the event of an offhook:
ntfy 12 aaln/1@rgw1.whatever.net mgcp 1.0
o: l/hd
x: 445678944
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Acknowledgement from ca:
200 12 ok
Step 2 - Request Notification (rqnt) for digits from ca to rgw1
Request rgw1 to notify if on-hook and collect digits according to
the digit map, and to provide dialtone:
rqnt 1057 aaln/1@rgw1.whatever.net mgcp 1.0
r: l/hu(n), d/[0-9#*T](d)
s: l/dl
x: 445678945
d: 5xxx
Acknowledgement from rgw1:
200 1057 ok
Step 3 - Notify (ntfy) digits from rgw1 to ca
ntfy 13 aaln/1@rgw1.whatever.net mgcp 1.0
o: d/5, d/0, d/0, d/1
x: 445678945
Acknowledgement from ca:
200 13 ok
Step 4 - Request Notification (rqnt) from ca to rgw1
Request rgw1 to notify in the event of an on-hook transition:
rqnt 1058 aaln/1@rgw1.whatever.net mgcp 1.0
r: l/hu(n)
x: 445678946
Acknowledgement from rgw1:
200 1058 ok
Step 5 - Create Connection (crcx) from ca to rgw1
Request a new connection on rgw1 with the specified local connection
options, including 20 msec as the packetization period, G.711 u-law
as the codec, and receive only as the mode:
crcx 1059 aaln/1@rgw1.whatever.net mgcp 1.0
c: 9876543210abcdef
l: p:20, a:PCMU
m: recvonly
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It is recommended to support these connection options for the
purpose of interoperability. Acknowledgement from rgw1 that a new
connection, "456789fedcba5", has been created; followed by a blank
line and then the SDP parameters:
200 1059 ok
i: 456789fedcba5
v=0
o=- 23456789 98765432 IN IP4 192.168.5.7
s=-
c=IN IP4 192.168.5.7
t=0 0
m=audio 6058 RTP/AVP 0
Step 6 - Create Connection (crcx) from ca to rgw2
Request a new connection on rgw2. The request includes the session
description returned by rgw1 such that a two way connection can be
initiated:
crcx 2052 aaln/1@rgw2.whatever.net mgcp 1.0
c: 9876543210abcdef
l: p:20, a:PCMU
m: sendrecv
v=0
o=- 23456789 98765432 IN IP4 192.168.5.7
s=-
c=IN IP4 192.168.5.7
t=0 0
m=audio 6058 RTP/AVP 0
Acknowledgement from rgw2 that a new connection, "67890af54c9", has
been created; followed by a blank line and then the SDP parameters:
200 2052 ok
i: 67890af54c9
v=0
o=- 23456889 98865432 IN IP4 192.168.5.8
s=-
c=IN IP4 192.168.5.8
t=0 0
m=audio 6166 RTP/AVP 0
Step 7 - Modify Connection (mdcx) from ca to rgw1
Request rgw1 to modify the existing connection, "456789fedcba5", to
use the session description returned by rgw2 establishing a half
duplex connection which, though not used in this example, could be
used to provide usr1 with in band ringback tone, announcements, etc:
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mdcx 1060 aaln/1@rgw1.whatever.net mgcp 1.0
c: 9876543210abcdef
i: 456789fedcba5
l: p:20, a:PCMU
M: recvonly
v=0
o=- 23456889 98865432 IN IP4 192.168.5.8
s=-
c=IN IP4 192.168.5.8
t=0 0
m=audio 6166 RTP/AVP 0
Acknowledgement from rgw1:
200 1060 ok
Step 8 - Request Notification (rqnt) from ca for rgw1 to provide
ringback
Request rgw1 to notify in the event of an on-hook transition, and
also to provide ringback tone:
rqnt 1061 aaln/1@rgw1.whatever.net mgcp 1.0
r: l/hu(n)
s: g/rt
x: 445678947
Acknowledgement from rgw1:
200 1061 ok
Step 9 - Request Notification (rqnt) from ca to rgw2 to provide
ringing
Request rgw2 to continue to look for offhook and provide ringing:
rqnt 2053 aaln/1@rgw2.whatever.net mgcp 1.0
r: l/hd(n)
s: l/rg
x: 445678948
Acknowledgement from rgw2:
200 2053 ok
Step 10 - Notify (ntfy) offhook from rgw2 to ca
ntfy 27 aaln/1@rgw2.whatever.net mgcp 1.0
o: l/hd
x: 445678948
Acknowledgement from ca:
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200 27 ok
Step 11 - Request Notification (rqnt) of on-hook from ca to rgw2
rqnt 2054 aaln/1@rgw2.whatever.net mgcp 1.0
r: l/hu(n)
x: 445678949
Acknowledgement from rgw2:
200 2054 ok
Step 12 - Request Notification (rqnt) of on-hook from ca to rgw1
rqnt 1062 aaln/1@rgw1.whatever.net mgcp 1.0
r: l/hu(n)
x: 445678950
Acknowledgement from rgw1:
200 1062 ok
Step 13 - Modify Connection (mdcx) from ca to rgw1
Request rgw1 to modify the existing connection, "456789fedcba5", to
sendrecv such that a full duplex connection is initiated:
mdcx 1063 aaln/1@rgw1.whatever.net mgcp 1.0
c: 9876543210abcdef
i: 456789fedcba5
m: sendrecv
Acknowledgement from rgw1:
200 1063 ok
F.3 Connection Deletion
F.3.1 Residential Gateway to Residential Gateway
The following table shows the message sequence which occurs when a
user (usr2) initiates the deletion of an existing connection on a
residential gateway (rgw2) with a user served by another residential
gateway (rgw1). This example illustrates the communication between
the residential gateways and the call agent (ca) only. The local
name of the endpoints in this example is aaln/1 for both gateways,
and references within the description of the steps to rgw1 and rgw2
can be assumed to refer to aaln/1 of rgw1 and aaln/1 of rgw2.
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Table F.4: Residential Gateway Connection Deletion
---------------------------------------------------------------------
| # | usr1 | rgw1 | ca | rgw2 | usr2 |
|===|=============|============|============|============|============|
| 1 | | | | <- ntfy | <- on-hook |
| | | | ack -> | | |
|---|-------------|------------|------------|------------|------------|
| 2 | | | dlcx -> | | |
| | | | | <- ack | |
|---|-------------|------------|------------|------------|------------|
| 3 | | | <- dlcx | | |
| | | ack -> | | | |
|---|-------------|------------|------------|------------|------------|
| 4 | | | rqnt -> | | |
| | | | | <- ack | |
|---|-------------|------------|------------|------------|------------|
| 5 | on-hook -> | ntfy -> | | | |
| | | | <- ack | | |
|---|-------------|------------|------------|------------|------------|
| 6 | | | <- rqnt | | |
| | | ack -> | | | |
---------------------------------------------------------------------
Step 1 - Notify (ntfy) offhook from rgw1 to ca
This ntfy is the result of usr2 going on-hook and assumes that ca
had
previously sent an rqnt to rgw2 requesting notification in the event
of
an on-hook (see end of Connection Creation sequence):
ntfy 28 aaln/1@rgw2.whatever.net mgcp 1.0
o: l/hu
x: 445678949
Acknowledgement from ca:
200 28 ok
Step 2 - Delete Connection (dlcx) from ca to rgw2
Requests rgw2 to delete the connection "67890af54c9":
dlcx 2055 aaln/1@rgw1.whatever.net mgcp 1.0
c: 9876543210abcdef
i: 67890af54c9
Acknowledgement from rgw2. Note the response code of "250" meaning
"the connection was deleted":
250 2055 ok
Step 3 - Delete Connection (dlcx) from ca to rgw1
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Requests rgw1 to delete the connection "456789fedcba5":
dlcx 1064 aaln/1@rgw1.whatever.net mgcp 1.0
c: 9876543210abcdef
i: 456789fedcba5
Acknowledgement from rgw1:
250 1064 ok
Step 4 - NotificationRequest (rqnt) from ca to rgw2
Requests rgw2 to notify ca in the event of an offhook transition:
rqnt 2056 aaln/1@rgw2.whatever.net mgcp 1.0
r: l/hd(n)
x: 445678951
Acknowledgement from rgw2:
200 2056 ok
Step 5 - Notify (ntfy) on-hook from rgw1 to ca
Notify ca that usr1 at rgw1 went back on-hook:
ntfy 15 aaln/1@rgw1.whatever.net mgcp 1.0
o: l/hu
x: 445678950
Acknowledgement from ca:
200 15 ok
Step 6 - NotificationRequest (rqnt) offhook from ca to rgw1
Requests rgw1 to notify ca in the event of an offhook transition:
rqnt 1065 aaln/1@rgw1.whatever.net mgcp 1.0
r: l/hd(n)
x: 445678952
Acknowledgement from rgw1:
200 1065 ok
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Internet Draft MGCP 1.0bis January 2001
Full Copyright Statement
Copyright (C) The Internet Society (2001). All Rights Reserved.
This document and translations of it may be copied and furnished to
others, and derivative works that comment on or otherwise explain it
or assist in its implementation may be prepared, copied, published
and distributed, in whole or in part, without restriction of any
kind, provided that the above copyright notice and this paragraph
are included on all such copies and derivative works. However, this
document itself may not be modified in any way, such as by removing
the copyright notice or references to the Internet Society or other
Internet organizations, except as needed for the purpose of
developing Internet standards in which case the procedures for
copyrights defined in the Internet Standards process must be
followed, or as required to translate it into languages other than
English.
The limited permissions granted above are perpetual and will not be
revoked by the Internet Society or its successors or assigns.
This document and the information contained herein is provided on an
"AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING
TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING
BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION
HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF
MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.
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Internet Draft MGCP 1.0bis January 2001
Acknowledgement
Funding for the RFC Editor function is currently provided by the
Internet Society.
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