|
Network Working Group Request for Comments: 2705 Category: Informational |
M. Arango RSL COM A. Dugan I. Elliott Level3 Communications C. Huitema Telcordia S. Pickett Vertical Networks October 1999 |
This memo provides information for the Internet community. It does not specify an Internet standard of any kind. Distribution of this memo is unlimited.
Copyright © The Internet Society (1999). All Rights Reserved.
This document is being published for the information of the community. It describes a protocol that is currently being deployed in a number of products. Implementers should be aware of developments in the IETF Megaco Working Group and ITF-T SG16 who are currently working on a potential successor to this protocol.
This document describes an application programming interface and a corresponding protocol (MGCP) for controlling Voice over IP (VoIP) Gateways from external call control elements. MGCP assumes a call control architecture where the call control "intelligence" is outside the gateways and handled by external call control elements.
The document is structured in 6 main sections:
1. Introduction
1.1. Relation with the H.323 standards
1.2. Relation with the IETF standards
1.3. Definitions
2. Media Gateway Control Interface
2.1. Model and naming conventions
2.1.1. Types of endpoints
2.1.1.1. Digital channel (DS0)
2.1.1.2. Analog line
2.1.1.3. Annoucement server access point
2.1.1.4. Interactive Voice Response access point
2.1.1.5. Conference bridge access point
2.1.1.6. Packet relay
2.1.1.7. Wiretap access point
2.1.1.8. ATM "trunk side" interface
2.1.2. Endpoint identifiers
2.1.3. Calls and connections
2.1.3.1. Names of calls
2.1.3.2. Names of connections
2.1.3.3. Management of resources, attributes of
2.1.3.4. Special case of local connections
2.1.4. Names of Call Agents and other entities
2.1.5. Digit maps
2.1.6. Names of events
2.2. Usage of SDP
2.3. Gateway Control Commands
2.3.1. EndpointConfiguration
2.3.2. NotificationRequest
2.3.3. CreateConnection
2.3.4. ModifyConnection
2.3.5. DeleteConnection (from the Call Agent)
2.3.6. DeleteConnection (from the VoIP gateway)
2.3.7. DeleteConnection (multiple connections, from the 51
2.3.8. Audit Endpoint
2.3.9. Audit Connection
2.3.10. Restart in progress
2.4. Return codes and error codes
2.5. Reason Codes
3. Media Gateway Control Protocol
3.1. General description
3.2. Command Header
3.2.1. Command line
3.2.1.1. Coding of the requested verb
3.2.1.2. Transaction Identifiers
3.2.1.3. Coding of the endpoint identifiers and
3.2.1.4. Coding of the protocol version
3.2.2. Parameter lines
3.2.2.1. Response Acknowledgement
3.2.2.2. Local connection options
3.2.2.3. Capabilities
3.2.2.4. Connection parameters
3.2.2.5. Reason Codes
3.2.2.6. Connection mode
3.2.2.7. Coding of event names
3.2.2.8. RequestedEvents
3.2.2.9. SignalRequests
3.2.2.10. ObservedEvent
3.2.2.11. RequestedInfo
3.2.2.12. QuarantineHandling
3.2.2.13. DetectEvents
3.2.2.14. EventStates
3.2.2.15. RestartMethod
3.2.2.16. Bearer Information
3.3. Format of response headers
3.4. Formal syntax description of the protocol
3.5. Encoding of the session description
3.5.1. Usage of SDP for an audio service
3.5.2. Usage of SDP in a network access service
3.5.3. Usage of SDP for ATM connections
3.5.4. Usage of SDP for local connections
3.6. Transmission over UDP
3.6.1. Providing the At-Most-Once functionality
3.6.2. Transaction identifiers and three ways handshake. 92
3.6.3. Computing retransmission timers
3.6.4. Piggy backing
3.6.5. Provisional responses
4. States, failover and race conditions
4.1. Basic Asumptions
4.2. Security, Retransmission, and Detection of Lost
4.3. Race conditions
4.3.1. Quarantine list
4.3.2. Explicit detection
4.3.3. Ordering of commands, and treatment of disorder .104
4.3.4. Fighting the restart avalanche
4.3.5. Disconnected Endpoints
1. A "disconnected" timer is initialized to a random value, .107
2. The gateway then waits for either the end of this timer, .107
3. When the "disconnected" timer elapses, when a command is .107
4. If the "disconnected" procedure still left the endpoint ..107
5. Security requirements
5.1. Protection of media connections
6. Event packages and end point types
6.1. Basic packages
6.1.1. Generic Media Package
6.1.2. DTMF package
6.1.3. MF Package
6.1.4. Trunk Package
6.1.5. Line Package
6.1.6. Handset emulation package
6.1.7. RTP Package
6.1.8. Network Access Server Package
6.1.9. Announcement Server Package
6.1.10. Script Package
6.2. Basic endpoint types and profiles
7. Versions and compatibility
7.1. Differences between version 1.0 and draft 0.5
7.2. Differences between draft-04 and draft-05
7.3. Differences between draft-03 and draft-04
7.4. Differences between draft-02 and draft-03
7.5. Differences between draft-01 and draft-02
7.6. The making of MGCP from IPDC and SGCP
7.7. Changes between MGCP and initial versions of SGCP
8. Security Considerations
9. Acknowledgements
10. References
11. Authors' Addresses
12. Appendix A: Proposed "MoveConnection" command
12.1. Proposed syntax modification
13. Full Copyright Statement
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:
MGCP assumes a call control architecture where the call control "intelligence" is outside the gateways and handled by external call control elements. The MGCP assumes that these call control elements, or Call Agents, will synchronize with each other to send coherent commands to the gateways under their control. MGCP does not define a mechanism for synchronizing Call Agents. MGCP is, in essence, a master/slave protocol, where the gateways are expected to execute commands sent by the Call Agents. In consequence, this document specifies in great detail the expected behavior of the gateways, but
only specify those parts of a call agent implementation, such as timer management, that are mandated for proper operation of the protocol.
MGCP assumes a connection model where the basic constructs are endpoints and connections. Endpoints are sources or sinks of data and could be physical or virtual. Examples of physical endpoints are:
An 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:
For point-to-point connections the endpoints of a connection could be in separate gateways or in the same gateway.
MGCP is designed as an internal protocol within a distributed system that appears to the outside as a single VoIP gateway. This system is composed of a Call Agent, that may or may not be distributed over several computer platforms, and of a set of gateways, including at least one "media gateway" that perform the conversion of media signals between circuits and packets, and at least one "signalling gateway" when connecting to an SS7 controlled network. In a typical configuration, this distributed gateway system will interface on one side with one or more telephony (i.e. circuit) switches, and on the other side with H.323 conformant systems, as indicated in the following table:
___________________________________________________________________
| Functional| Phone | Terminating | H.323 conformant |
| Plane | switch | Entity | systems |
|___________|____________|_________________|_______________________|
| Signaling | Signaling | Call agent | Signaling exchanges |
| Plane | exchanges | | with the call agent |
| | through | | through H.225/RAS and|
| | SS7/ISUP | | H.225/Q.931. |
|___________|____________|_________________|_______________________|
| | | | Possible negotiation |
| | | | of logical channels |
| | | | and transmission |
| | | | parameters through |
| | | | H.245 with the call |
| | | | agent. |
|___________|____________|_________________|_______________________|
| | | Internal | |
| | | synchronization| |
| | | through MGCP | |
|___________|____________|_________________|_______________________|
| Bearer | Connection| Telephony | Transmission of VOIP |
| Data | through | gateways | data using RTP |
| Transport | high speed| | directly between the |
| Plane | trunk | | H.323 station and the|
| | groups | | gateway. |
|___________|____________|_________________|_______________________|
In the MGCP model, the gateways focus on the audio signal translation function, while the Call Agent handles the signaling and call processing functions. As a consequence, the Call Agent implements the "signaling" layers of the H.323 standard, and presents itself as an "H.323 Gatekeeper" or as one or more "H.323 Endpoints" to the H.323 systems.
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 latter three specifications are in fact alternative signaling standards that allow for the transmission of a session description to an interested party. SAP is used by multicast session managers to distribute a multicast session description to a large group of recipients, SIP is used to invite an individual user to take part in a point-to-point or unicast session, RTSP is used to interface a server that provides real time data. In all three cases, the session description is described according to SDP; when audio is transmitted, it is transmitted through the Real-time Transport Protocol, RTP.
The distributed gateway systems and MGCP will enable PSTN telephony users to access sessions set up using SAP, SIP or RTSP. The Call Agent provides for signaling conversion, according to the following table:
_____________________________________________________________________
| Functional| Phone | Terminating | IETF conforming systems|
| Plane | switch | Entity | |
|___________|____________|_________________|_________________________|
| Signaling | Signaling | Call agent | Signaling exchanges |
| Plane | exchanges | | with the call agent |
| | through | | through SAP, SIP or |
| | SS7/ISUP | | RTSP. |
|___________|____________|_________________|_________________________|
| | | | Negotiation of session |
| | | | description parameters |
| | | | through SDP (telephony |
| | | | gateway terminated but |
| | | | passed via the call |
| | | | agent to and from the |
| | | | IETF conforming system)|
|___________|____________|_________________|_________________________|
| | | Internal | |
| | | synchronization| |
| | | through MGCP | |
|___________|____________|_________________|_________________________|
| Bearer | Connection| Telephony | Transmission of VoIP |
| Data | through | gateways | data using RTP, |
| Transport | high speed| | directly between the |
| Plane | trunk | | remote IP end system |
| | groups | | and the gateway. |
|___________|____________|_________________|_________________________|
The SDP standard has a pivotal status in this architecture. We will see in the following description that we also use it to carry session descriptions in MGCP.
Trunk: A communication channel between two switching systems. E.g., a DS0 on a T1 or E1 line.
The interface functions provide for connection control and endpoint control. Both use the same system model and the same 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.
In the introduction, we presented several classes of gateways. Such classifications, however, can be misleading. Manufacturers can arbitrarily decide to provide several types of services in a single packaging. A single product could well, for example, provide some trunk connections to telephony switches, some primary rate connections and some analog line interfaces, thus sharing the characteristics of what we described in the introduction as "trunking", "access" and "residential" gateways. MGCP does not make assumptions about such groupings. We simply assume that media gateways support collections of endpoints. The type of the endpoint determines its functionalities. Our analysis, so far, has led us to isolate the following basic endpoint types:
* Digital channel (DS0), * Analog line, * Annoucement server access point, * Interactive Voice Response access point, * Conference bridge access point, * Packet relay, * Wiretap access point, * ATM "trunk side" interface.
In this section, we will develop the expected behavior of such end points.
This list is not limitative. There may be other types of endpoints defined in the future, for example test endpoint that could be used to check network quality, or frame-relay endpoints that could be used to managed audio channels multiplexed over a frame-relay virtual circuit.
Digital channels provide an 8Khz*8bit service. Such channels are found in trunk and ISDN interfaces. They are typically part of digital multiplexes, such as T1, E1, T3 or E3 interfaces. Media gateways that support such channels are capable of translating the digital signals received on the channel, which may be encoded according to A or mu-law, using either the complete set of 8 bits or only 7 of these bits, into audio packets. When the media gateway also supports a NAS service, the gateway shall be capable of receiving either audio-encoded data (modem connection) or binary data (ISDN connection) and convert them into data packets.
+-------
+------------+|
(channel) ===|DS0 endpoint| -------- Connections
+------------+|
+-------
Media gateways should be able to establish several connections between the endpoint and the packet networks, or between the endpoint and other endpoints in the same gateway. The signals originating from these connections shall be mixed according to the connection "mode", as specified later in this document. The precise number of connections that an endpoint support is a characteristic of the gateway, and may in fact vary according with the allocation of resource within the gateway.
In some cases, digital channels are used to carry signalling. This is the case for example of SS7 "F" links, or ISDN "D" channels. Media gateways that support these signalling functions shall be able to send and receive the signalling packets to and from a call agent, using the "back haul" procedures defined by the SIGTRAN working group of the IETF. Digital channels are sometimes used in conjunction with channel associated signalling, such as "MF R2". Media gateways that support these signalling functions shall be able to detect and produce the corresponding signals, such as for example "wink" or "A", according to the event signalling and reporting procedures defined in MGCP.
Analog lines can be used either as a "client" interface, providing service to a classic telephone unit, or as a "service" interface, allowing the gateway to send and receive analog calls. When the media gateway also supports a NAS service, the gateway shall be capable of receiving audio-encoded data (modem connection) and convert them into data packets.
+-------
+---------------+|
(line) ===|analog endpoint| -------- Connections
+---------------+|
+-------
Media gateways should be able to establish several connections between the endpoint and the packet networks, or between the endpoint and other endpoints in the same gateway. The audio signals originating from these connections shall be mixed according to the connection "mode", as specified later in this document. The precise number of connections that an endpoint support is a characteristic of the gateway, and may in fact vary according with the allocation of resource within the gateway. A typical gateway should however be able to support two or three connections per endpoint, in order to provide services such as "call waiting" or "three ways calling".
An announcement server endpoint provides acces to an announcement service. Under requests from the call agent, the announcement server will "play" a specified announcement. The requests from the call agent will follow the event signalling and reporting procedures defined in MGCP.
+----------------------+
| Announcement endpoint| -------- Connection
+----------------------+
A given announcement endpoint is not supposed to support more than one connection at a time. If several connections were established to the same endpoint, then the same announcements would be played simultaneously over all the connections.
Connections to an announcement server are typically oneway, or "half duplex" -- the announcement server is not expected to listen the audio signals from the connection.
An Interactive Voice Response (IVR) endpoint provides acces to an IVR service. Under requests from the call agent, the IVR server will "play" announcements and tones, and will "listen" to responses from the user. The requests from the call agent will follow the event signalling and reporting procedures defined in MGCP.
+-------------+
| IVR endpoint| -------- Connection
+-------------+
A given IVR endpoint is not supposed to support more than one connection at a time. If several connections were established to the same endpoint, then the same tones and announcements would be played simultaneously over all the connections.
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.
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
+---------------------+ |
+-------
A wiretap access point provides access to a wiretap service, providing either a recording or a life playback of a connection.
+-----------------+
| Wiretap endpoint| -------- Connection
+-----------------+
A given wiretap endpoint is not supposed to support more than one connection at a time. If several connections were established to the same endpoint, then the recording or playback would mix the audio signals received on this connections.
Connections to an wiretap endpoint are typically oneway, or "half duplex" -- the wiretap server is not expected to signal its presence in a call.
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.
Endpoints identifiers have two components that both are case insensitive:
The syntax of the local name depends on the type of endpoint being named. However, the local name for each of these types is naturally hierarchical, beginning with a term which identifies the physical gateway containing the given endpoint and ending in a term which specifies the individual endpoint concerned. With this in mind, the following rules for construction and interpretation of the Entity Name field for these entity types MUST be supported:
1) The individual terms of the naming path MUST be separated by a single slash ("/", ASCII 2F hex).
2) The individual terms are character strings composed of letters, digits or other printable characters, with the exception of characters used as delimitors ("/", "@"), characters used for wildcarding ("*", "$") and white spaces.
3) Wild-carding is represented either by an asterisk ("*") or a dollar sign ("$") for the terms of the naming path which are to be wild-carded. Thus, if the full naming path looks like
term1/term2/term3
then the Entity Name field looks like this depending on which terms are wild-carded:
*/term2/term3 if term1 is wild-carded
term1/*/term3 if term2 is wild-carded
term1/term2/* if term3 is wild-carded
term1/*/* if term2 and term3 are wild-carded,
etc.
In each of these examples a dollar sign could have appeared instead of an asterisk.
4) A term represented by an asterisk is to be interpreted as: "use ALL values of this term known within the scope of the Media Gateway". A term represented by a dollar sign is to be interpreted as: "use ANY ONE value of this term known within the scope of the Media Gateway". The description of a specific command may add further criteria for selection within the general rules given here.
If the Media Gateway controls multiple physical gateways, the first term of the naming MUST identify the physical gateway containing the desired entity. If the Media Gateway controls only a single physical gateway, the first term of the naming string MAY identify that physical gateway, depending on local practice. A local name that is composed of only a wildcard character refers to either all (*) or any ($) endpoints within the media gateway.
In the case of trunking gateways, endpoints are trunk circuits linking a gateway to a telephone switch. These circuits are typically grouped into a digital multiplex, that is connected to the gateway by a physical interface. Such circuits are named in three contexts:
The Call Agents use configuration databases to map ranges of circuit numbers within an ISUP trunk group to corresponding ranges of circuits in a multiplex connected to a gateway through a physical interface. The gateway will be identified, in MGCP, by a domain name. The local name will be structured to encode both the name of the physical interface, for example X35V3+A4, and the circuit number within the multiplex connected to the interface, for example 13. The circuit number will be separated from the name of the interface by a fraction bar, as in:
X35V3+A4/13
Other types of endpoints will use different conventions. For example, in gateways were physical interfaces by construction only control one circuit, the circuit number will be omitted. The exact syntax of such names should be specified in the corresponding server specification.
Connections are created on the call agent on each endpoint that will be involved in the "call." In the classic example of a connection between two "DS0" endpoints (EP1 and EP2), the call agents controlling the end points will establish two connections (C1 and C2):
+---+ +---+
(channel1) ===|EP1|--(C1)--... ...(C2)--|EP2|===(channel2)
+---+ +---+
Each connection will be designated locally by a connection identifier, and will be characterized by connection attributes.
When the two endpoints are located on gateways that are managed by the same call agent, the creation is done via the three following steps:
1) The call agent asks the first gateway to "create a connection" on the first endpoint. The gateway allocates resources to that connection, and respond to the command by providing a "session description." The session description contains the information necessary for a third party to send packets towards the newly created connection, such as for example IP address, UDP port, and packetization parameters.
2) The call agent then asks the second gateway to "create a connection" on the second endpoint. The command carries the "session description" provided by the first gateway. The gateway allocates resources to that connection, and respond to the command by providing its own "session description."
3) The call agent uses a "modify connection" command to provide this second "session description" to the first endpoint. Once this is done, communication can proceed in both directions.
When the two endpoints are located on gateways that are managed by the different call agents, these two call agents shall exchange information through a call-agent to call-agent signalling protocol, in order to synchronize the creation of the connection on the two endpoints.
Once established, the connection parameters can be modified at any time by a "modify connection" command. The call agent may for example instruct the gateway to change the compression algorithm used on a connection, or to modify the IP address and UDP port to which data should be sent, if a connection is "redirected."
The call agent removes a connection by sending to the gateway a "delete connection" command. The gateway may also, under some circumstances, inform a gateway that a connection could not be sustained.
The following diagram provides a view of the states of a connection, as seen from the gateway:
Create connection
received
|
V
+-------------------+
|resource allocation|-(failed)-+
+-------------------+ |
| (connection refused)
(successful)
|
v
+----------->+
| |
| +-------------------+
| | remote session |
| | description |----------(yes)--------+
| | available ? | |
| +-------------------+ |
| | |
| (no) |
| | |
| +-----------+ +------+
| +--->| half open |------> Delete <-------| open |<----------+
| | | (wait) | Connection |(wait)| |
| | +-----------+ received +------+ |
| | | | | |
| | Modify Connection | Modify Connection |
| | received | received |
| | | | | |
| | +--------------------+ | +--------------------+ |
| | |assess modification | | |assess modification | |
| | +--------------------+ | +--------------------+ |
| | | | | | | |
| |(failed) (successful) | (failed) (successful) |
| | | | | | | |
| +<---+ | | +-------------+-------+
| | |
+<-------------------+ |
|
+-----------------+
| Free connection |
| resources. |
| Report. |
+-----------------+
|
V
One of the attributes of each connection is the "call identifier."
Calls are identified by unique identifiers, independent of the underlying platforms or agents. These identifiers are created by the Call Agent. They are treated in MGCP as unstructured octet strings.
Call identifiers are expected to be unique within the system, or at a minimum, unique within the collection of Call Agents that control the same gateways. When a Call Agent builds several connections that pertain to the same call, either on the same gateway or in different gateways, these connections that belong to the same call share the same call-id. This identifier can then be used by accounting or management procedures, which are outside the scope of MGCP.
Connection identifiers are created by the gateway when it is requested to create a connection. They identify the connection within the context of an endpoint. They are treated in MGCP as unstructured octet strings. The gateway should make sure that a proper waiting period, at least 3 minutes, elapses between the end of a connection that used this identifier and its use in a new connection for the same endpoint. (Gateways may decide to use identifiers that are unique within the context of the gateway.)
Many types of resources will be associated to a connection, such as specific signal processing functions or packetization functions. Generally, these resources fall in two categories:
1) Externally visible resources, that affect the format of "the bits on the network" and must be communicated to the second endpoint involved in the connection.
2) Internal resources, that determine which signal is being sent over the connection and how the received signals are processed by the endpoint.
The resources allocated to a connection, and more generally the handling of the connection, are chosen by the gateway under instructions from the call agent. The call agent will provide these instructions by sending two set of parameters to the gateway:
1) The local directives instruct the gateway on the choice of resources that should be used for a connection,
2) When available, the "session description" provided by the other end of the connection.
The local directives specify such parameters as the mode of the connection (e.g. send only, send-receive), preferred coding or packetization methods, usage of echo cancellation or silence suppression. (A detailed list can be found in the specification of the LocalConnectionOptions parameter of the CreateConnection command.) For each of these parameters, the call agent can either specify a value, a range of value, or no value at all. This allow various implementations to implement various level of control, from a very tight control where the call agent specifies minute details of the connection handling to a very loose control where the call agent only specifies broad guidelines, such as the maximum bandwidth, and let the gateway choose the detailed values.
Based on the value of the local directives, the gateway will determine the resources allocated to the connection. When this is possible, the gateway will choose values that are in line with the remote session description - but there is no absolute requirement that the parameters be exactly the same.
Once the resource have been allocated, the gateway will compose a "session description" that describes the way it intends to receive packets. Note that the session description may in some cases present a range of values. For example, if the gateway is ready to accept one of several compression algorithm, it can provide a list of these accepted algorithms.
Local Directives
(from call agent 1)
|
V
+-------------+
| resources |
| allocation |
| (gateway 1) |
+-------------+
| |
V |
Local |
Parameters V
| Session
| Description Local Directives
| | (from call agent 2)
| +---> Transmission----+ |
| (CA to CA) | |
| V V
| +-------------+
| | resources |
| | allocation |
| | (gateway 2) |
| +-------------+
| | |
| | V
| | Local
| | Parameters
| Session
| Description
| +---- Transmission<---+
| | (CA to CA)
V V
+-------------+
| modification|
| (gateway 1) |
+-------------+
|
V
Local
Parameters
-- Information flow: local directives & session descriptions --
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:
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."
The media gateway control protocol has been designed to allow the implementation of redundant Call Agents, for enhanced network reliability. This means that there is no fixed binding between entities and hardware platforms or network interfaces.
Reliability can be improved by the following precautions:
In addition to the indirection provided by the use of domain names and the DNS, the concept of "notified entity" is central to reliability and fail-over in MGCP. The "notified entity" for an endpoint is the Call Agent currently controlling that endpoint. At any point in time, an endpoint has one, and only one, "notified entity" associated with it, and when the endpoint needs to send a command to the Call Agent, it MUST send the command to the current "notified entity" for which endpoint(s) the command pertains. Upon startup, the "notified entity" MUST be set to a provisioned value. Most commands sent by the Call Agent include the ability to explicitly name the "notified entity" through the use of a "NotifiedEntity" parameter. The "notified entity" will stay the same until either a new "NotifiedEntity" parameter is received or the endpoint reboots. If the "notified entity" for an endpoint is empty or has not been set explicitly, the "notified entity" will then default to the source address of the last connection handling command or notification request received for the endpoint. Auditing will thus not change the "notified entity."
The Call Agent can ask the gateway to collect digits dialed by the user. This facility is intended to be used with residential gateways to collect the numbers that a user dials; it may also be used with trunking gateways and access gateways alike, to collect the access codes, credit card numbers and other numbers requested by call control services.
An alternative procedure is for the gateway to notify the Call Agent of the dialed digits, as soon as they are dialed. However, such a procedure generates a large number of interactions. It is preferable to accumulate the dialed numbers in a buffer, and to transmit them in a single message.
The problem with this accumulation approach, however, is that it is hard for the gateway to predict how many numbers it needs to accumulate before transmission. For example, using the phone on our desk, we can dial the following numbers:
_______________________________________________________
| 0 | Local operator |
| 00 | Long distance operator |
| xxxx | Local extension number |
| 8xxxxxxx | Local number |
| #xxxxxxx | Shortcut to local number at|
| | other corporate sites |
| *xx | Star services |
| 91xxxxxxxxxx | Long distance number |
| 9011 + up to 15 digits| International number |
|________________________|_____________________________|
The solution to this problem is to load the gateway with a digit map that correspond to the dial plan. This digit map is expressed using a syntax derived from the Unix system command, egrep. For example, the dial plan described above results in the following digit map:
(0T| 00T|[1-7]xxx|8xxxxxxx|#xxxxxxx|*xx|91xxxxxxxxxx|9011x.T)
The formal syntax of the digit map is described by the DigitMap rule in the formal syntax description of the protocol (section 3.4). A Digit-Map, according to this syntax, is defined either by a "string" or by a list of strings. Each string in the list is an alternative numbering scheme, specified either as a set of digits or timers, or as regular expression. A gateway that detects digits, letters or timers will:
1) Add the event parameter code as a token to the end of an internal state variable called the "current dial string"
2) Apply the current dial string to the digit map table, attempting a match to each regular expression in the Digit Map in lexical order
3) If the result is under-qualified (partially matches at least one entry in the digit map), do nothing further.
If the result matches, or is over-qualified (i.e. no further digits could possibly produce a match), send the current digit string to the Call Agent. A match, in this specification, can be either a "perfect match," exactly matching one of the specified alternatives, or an impossible match, which occur when the dial string does not match any of the alternative. Unexpected timers, for example, can cause "impossible matches." Both perfect matches and impossible matches trigger notification of the accumulated digits.
Digit maps are provided to the gateway by the Call Agent, whenever the Call Agent instructs the gateway to listen for digits.
The concept of events and signals is central to MGCP. A Call Agent may ask to be notified about certain events occurring in an endpoint, e.g. off-hook events, and a call agent may request certain signals to be applied to an endpoint, e.g. dial-tone.
Events and signals are grouped in packages within which they share the same namespace which we will refer to as event names in the following. Packages are groupings of the events and signals supported by a particular type of endpoint. For instance, one package may support a certain group of events and signals for analog access lines, and another package may support another group of events and signals for video lines. One or more packages may exist for a given endpoint-type.
Event names are case insensitive and are composed of two logical parts, a package name and an event name. Both names are strings of letters, hyphens and digits, with the restriction that hyphens shall never be the first or last characters in a name. Package or event names are not case sensitive - values such as "hu", "Hu", "HU" or "hU" should be considered equal.
Examples of package names are "D" (DTMF), "M" (MF), "T" (Trunk) or "L" (Line). Examples of event names can be "hu" (off hook or "hang- up" transition), "hf" (flash hook) or "0" (the digit zero).
In textual representations, the package name, when present, is separated from the event name by a slash ("/"). The package name is in fact optional. Each endpoint-type has a default package associated with it, and if the package name is excluded from the event name, the default package name for that endpoint-type is assumed. For example, for an analog access line, the following two event names are equal:
l/dl dial-tone in the line package for an analog access line.
dl dial-tone in the line package (default) for an analog access
line.
This document defines a basic set of package names and event names. Additional package names and event names can be registered with the IANA. A package definition shall define the name of the package, and the definition of each event belonging to the package. The event definition shall include the precise name of the event (i.e., the code used in MGCP), a plain text definition of the event, and, when appropriate, the precise definition of the corresponding signals, for example the exact frequencies of audio signal such as dial tones or DTMF tones.
In addition, implementers can gain experience by using experimental packages. The names of experimental packages must start with the two characters "x-"; the IANA shall not register package names that start with these characters.
Digits, or letters, are supported in many packages, notably "DTMF" and "MF". Digits and letters are defined by the rules "Digit" and "Letter" in the definition of digit maps. This definition refers to the digits (0 to 9), to the asterisk or star ("*") and orthotrope, number or pound sign ("#"), and to the letters "A", "B", "C" and "D", as well as the timer indication "T". These letters can be combined in "digit string" that represent the keys that a user punched on a dial. In addition, the letter "X" can be used to represent all digits, and the sign "$" can be used in wildcard notations. The need to easily express the digit strings has a consequence on the form of event names:
An event name that does not denote a digit should always contain at least one character that is neither a digit, nor one of the letters A, B, C, D, T or X. (Such names should not contain the special signs "*", "#", "/" or "$".)
A Call Agent may often have to ask a gateway to detect a group of events. Two conventions can be used to denote such groups:
The star sign (*) can be used as a wildcard instead of a package name, and the keyword "all" can be used as a wildcard instead of an event name:
A name such as "foo/all" denotes all events in package "foo"
A name such as "*/bar" denotes the event "bar" in any package
supported by the gateway
The names "*" or "*/all" denote all events supported by the
gate way.
The call agent can ask a gateway to detect a set of digits or letters either by individually describing those letters, or by using the "range" notation defined in the syntax of digit strings. For example, the call agent can:
Use the letter "x" to denote "any letter or digit."
Use the notation "[0-9#]" to denote the digits 0 to 9 and the pound
sign.
In some cases, Call Agents will request the gateway to generate or detect events on connections rather than on the end point itself. For example, gateways may be asked to provide a ringback tone on a connection. When an event shall be applied on a connection, the name of the connection is added to the name of the event, using an "at" sign (@) as a delimiter, as in:
G/rt@0A3F58
The wildcard character "*" (star) can be used to denote "all connections". When this convention is used, the gateway will generate or detect the event on all the connections that are connected to the endpoint. An example of this convention could be:
R/qa@*
The wildcard character "$" can be used to denote "the current connection." It should only be used by the call agent, when the event notification request is "encapsulated" within a command creation or modification command. When this convention is used, the gateway will generate or detect the event on the connection that is currently being created or modified. An example of this convention is:
G/rt@$
The connection id, or a wildcard replacement, can be used in conjunction with the "all packages" and "all events" conventions. For example, the notation:
*/all@*
can be used to designate all events on all connections.
Events and signals are described in packages. The package description must provide, for each events, the following informations:
Signals are divided into different types depending on their behavior:
TO signals are normally used to alert the endpoints' users, to signal them that they are expected to perform a specific action, such as hang down the phone (ringing). Transmission of these signals should typically be interrupted as soon as the first of the requested events has been produced.
Package descriptions should describe, for all signals, their type (OO, TO, BR). They should also describe the maximum duration of the TO signals.
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.
SDP allows for description of multimedia conferences. This version limits SDP usage to the setting of audio circuits and data access circuits. The initial session descriptions contain the description of exactly one media, of type "audio" for audio connections, "nas" for data access.
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:
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:
Connections are grouped into "calls". Several connections, that may or may not belong to the same call, can terminate in the same endpoint . Each connection is qualified by a "mode" parameter, which can be set to "send only" (sendonly), "receive only" (recvonly), "send/receive" (sendrecv), "conference" (confrnce), "data", "inactive" (inactive), "loopback", "continuity test" (conttest), "network loop back" (netwloop) or "network continuity test" (netwtest).
The handling of the audio signals received on these connections is determined by the mode parameters:
The "loopback" and "continuity test" modes are used during maintenance and continuity test operations. There are two flavors of continuity test, one specified by ITU and one used in the US. In the first case, the test is a loopback test. The originating switch will send a tone (the go tone) on the bearer circuit and expect the terminating switch to loopback the circuit. If the originating switch sees the same tone returned (the return tone), the COT has passed. If not, the COT has failed. In the second case, the go and return tones are different. The originating switch sends a certain go tone. The terminating switch detects the go tone, it asserts a different return tone in the backwards direction. When the originating switch detects the return tone, the COT is passed. If the originating switch never detects the return tone, the COT has failed.
If the mode is set to "loopback", the gateway is expected to return the incoming signal from the endpoint back into that same endpoint. This procedure will be used, typically, for testing the continuity of trunk circuits according to the ITU specifications.
If the mode is set to "continuity test", the gateway is informed that the other end of the circuit has initiated a continuity test procedure according to the GR specification. The gateway will place the circuit in the transponder mode required for dual-tone continuity tests.
If the mode is set to "network loopback", the audio signals received from the connection will be echoed back on the same connection.
If the mode is set to "network continuity test", the gateway will process the packets received from the connection according to the transponder mode required for dual-tone continuity test, and send the processed signal back on the connection.
The EndpointConfiguration commands are used to specify the encoding of the signals that will be received by the endpoint. For example, in certain international telephony configurations, some calls will carry mu-law encoded audio signals, while other will use A-law. The Call Agent will use the EndpointConfiguration command to pass this information to the gateway. The configuration may vary on a call by call basis, but can also be used in the absence of any connection.
ReturnCode
<-- EndpointConfiguration( EndpointId,
BearerInformation)
EndpointId is the name for the endpoint in the gateway where EndpointConfiguration executes, as defined in section 2.1.1. The "any of" wildcard convention shall not be used. If the "all of" wildcard convention is used, the command applies to all the endpoint whose name matches the wildcard.
BearerInformation is a parameter defining the coding of the data received from the line side. These information is encoded as a list of sub-parameters. The only sub-parameter defined in this version of the specification is the encoding method, whose values can be set to "A-law" and "mu-law".
ReturnCode is a parameter returned by the gateway. It indicates the outcome of the command and consists of an integer number optionally followed by commentary.
The NotificationRequest commands are used to request the gateway to send notifications upon the occurrence of specified events in an endpoint. For example, a notification may be requested for when a gateway detects that an endpoint is receiving tones associated with fax communication. The entity receiving this notification may decide to use a different type of encoding method in the connections bound to this endpoint.
ReturnCode
<-- NotificationRequest( EndpointId,
[NotifiedEntity,]
[RequestedEvents,]
RequestIdentifier,
[DigitMap,]
[SignalRequests,]
[QuarantineHandling,]
[DetectEvents,]
[encapsulated EndpointConfiguration])
EndpointId is the name for the endpoint in the gateway where NotificationRequest executes, as defined in section 2.1.1.
NotifiedEntity is an optional parameter that specifies where the
notifications should be sent. When this parameter is absent, the
notifications should be sent to the originator of the
NotificationRequest.
RequestIdentifier is used to correlate this request with the notifications that it triggers.
RequestedEvents is a list of events that the gateway is requested to detect and report. Such events include, for example, fax tones, continuity tones, or on-hook transition. To each event is associated an action, which can be:
Some actions can be combined. In particular:
In addition to the requestedEvents parameter specified in the command, some profiles of MGCP have introduced the concept of "persistent events." According to such profiles, the persistent event list is configured in the endpoint, by means outside the scope of MGCP. The basic MGCP specification does not specify any persistent event.
If a persistent event is not included in the list of RequestedEvents, and the event occurs, the event will be detected anyway, and processed like all other events, as if the persistent event had been requested with a Notify action. Thus, informally, persistent events can be viewed as always being implicitly included in the list of RequestedEvents with an action to Notify, although no glare detection, etc., will be performed.
Non-persistent events are those events explicitly included in the RequestedEvents list. The (possibly empty) list of requested events completely replaces the previous list of requested events. In addition to the persistent events, only the events specified in the requested events list will be detected by the endpoint. If a persistent event is included in the RequestedEvents list, the action specified will then replace the default action associated with the event for the life of the RequestedEvents list, after which the default action is restored. For example, if "Ignore off-hook" was specified, and a new request without any off-hook instructions were received, the default "Notify off-hook" operation then would be restored. A given event MUST NOT appear more than once in a RequestedEvents.
The gateway will detect the union of the persistent events and the requested events. If an event is not specified in either list, it will be ignored.
The Swap Audio action can be used when a gateway handles more than one active connection on an endpoint. This will be the case for three-way calling, call waiting, and possibly other feature scenarios. In order to avoid the round-trip to the Call Agent when just changing which connection is attached to the audio functions of the endpoint, the NotificationRequest can map an event (usually hook flash, but could be some other event) to a local function swap audio, which selects the "next" connection in a round robin fashion. If there is only one connection, this action is effectively a no-op.
If signal(s) are desired to start when an event being looked for
occurs, the "Embedded NotificationRequest" action can be used. The
embedded NotificationRequest may include a new list of
RequestedEvents, SignalRequests and a new digit map as well. The
semantics of the embedded NotificationRequest is as if a new
NotificationRequest was just received with the same NotifiedEntity,
and RequestIdentifier. When the "Embedded NotificationRequest" is
activated, the "current dial string" will be cleared; the list of
observed events and the quarantine buffer will be unaffected.
MGCP implementations shall be able to support at least one level of embedding. An embedded NotificationRequest that respects this limitation shall not contain another Embedded NotificationRequest.
DigitMap is an optional parameter that allows the Call Agent to
provision the gateways with a digit map according to which digits
will be accumulated. If this optional parameter is absent, the
previously defined value is retained. This parameter must be defined,
either explicitly or through a previous command, if the
RequestedEvent parameters contain an request to "accumulate according
to the digit map." The collection of these digits will result in a
digit string. The digit string is initialized to a null string upon
reception of the NotificationRequest, so that a subsequent
notification only returns the digits that were collected after this
request. Digits that were accumulated according to the digit map are
reported as any other accumulated event, in the order in which they
occur. It is therefore possible that other events be accumulated may
be found in between the list of digits.
SignalRequests is a parameter that contains the set of signals that the gateway is asked to apply to the endpoint, such as, for example ringing, or continuity tones. Signals are identified by their name, which is an event name, and may be qualified by parameters.
The action triggered by the SignalRequests is synchronized with the collection of events specified in the RequestedEvents parameter. For example, if the NotificationRequest mandates "ringing" and the event request ask to look for an "off-hook" event, the ringing shall stop as soon as the gateway detect an off hook event. The formal definition is that the generation of all "Time Out" signals shall stop as soon as one of the requested events is detected, unless the "Keep signals active" action is associated to the specified event.
The specific definition of actions that are requested via these SignalRequests, such as the duration of and frequency of a DTMF digit, is out side the scope of MGCP. This definition may vary from location to location and hence from gateway to gateway.
The RequestedEvents and SignalRequests refer to the same event definitions. In one case, the gateway is asked to detect the occurrence of the event, and in the other case it is asked to generate it. The specific events and signals that a given endpoint can detect or perform are determined by the list of event packages that are supported by that end point. Each package specifies a list of events and actions that can be detected or performed. A gateway that is requested to detect or perform an event belonging to a package that is not supported by the specified endpoint shall return an error. When the event name is not qualified by a package name, the default package name for the end point is assumed. If the event name is not registered in this default package, the gateway shall return an error.
The Call Agent can send a NotificationRequest whose requested signal list is empty. It will do so for example when tone generation should stop.
The optional QuarantineHandling parameter specifies the handling of "quarantine" events, i.e. events that have been detected by the gateway before the arrival of this NotificationRequest command, but have not yet been notified to the Call Agent. The parameter provides a set of handling options:
When the parameter is absent, the default value is assumed.
We should note that the quarantine-handling parameter also governs the handling of events that were detected but not yet notified when the command is received.
DetectEvents is an optional parameter that specifies a list of events that the gateway is requested to detect during the quarantine period. When this parameter is absent, the events that should be detected in the quarantine period are those listed in the last received DetectEvents list. In addition, the gateway should also detect the events specified in the request list, including those for which the "ignore" action is specified.
Some events and signals, such as the in-line ringback or the quality alert, are performed or detected on connections terminating in the end point rather than on the endpoint itself. The structure of the event names allow the Call Agent to specify the connection (or connections) on which the events should be performed or detected.
The command may carry an encapsulated EndpointConfiguration command, that will apply to the same endpoint. When this command is present, the parameters of the EndpointConfiguration command are inserted after the normal parameters of the NotificationRequest, with the exception of the EndpointId, which is not replicated.
The encapsulated EndpointConfiguration command shares the fate of the NotificationRequest command. If the NotificationRequest is rejected, the EndpointConfiguration is not executed.
ReturnCode is a parameter returned by the gateway. It indicates the outcome of the command and consists of an integer number optionally followed by commentary. .NH 3 Notifications
Notifications are sent via the Notify command and are sent by the gateway when the observed events occur.
ReturnCode
<-- Notify( EndpointId,
[NotifiedEntity,]
RequestIdentifier,
ObservedEvents)
EndpointId is the name for the endpoint in the gateway which is issuing the Notify command, as defined in section 2.1.1. The identifier should be a fully qualified endpoint identifier, including the domain name of the gateway. The local part of the name shall not use the wildcard convention.
NotifiedEntity is an optional parameter that identifies the entity to which the notifications is sent. This parameter is equal to the last received value of the NotifiedEntity parameter. The parameter is absent if there was no such parameter in the triggering request. The notification is sent to the "current notified entity" or, if no such entity was ever specified, to the address from which the request was received.
RequestIdentifier is parameter that repeats the RequestIdentifier
parameter of the NotificationRequest that triggered this
notification. It is used to correlate this notification with the
request that triggered it.
ObservedEvents is a list of events that the gateway detected. A
single notification may report a list of events that will be reported
in the order in which they were detected. The list may only contain
the identification of events that were requested in the
RequestedEvents parameter of the triggering NotificationRequest. It
will contain the events that were either accumulated (but not
notified) or treated according to digit map (but no match yet), and
the final event that triggered the detection or provided a final
match in the digit map.
ReturnCode is a parameter returned by the call agent. It indicates the outcome of the command and consists of an integer number optionally followed by commentary.
This command is used to create a connection between two endpoints.
ReturnCode,
ConnectionId,
[SpecificEndPointId,]
[LocalConnectionDescriptor,]
[SecondEndPointId,]
[SecondConnectionId]
<--- CreateConnection(CallId,
EndpointId,
[NotifiedEntity,]
[LocalConnectionOptions,]
Mode,
[{RemoteConnectionDescriptor |
SecondEndpointId}, ]
[Encapsulated NotificationRequest,]
[Encapsulated EndpointConfiguration])
A connection is defined by its endpoints. The input parameters in CreateConnection provide the data necessary to build a gateway's "view" of a connection.
CallId is a globally unique parameter that identifies the call (or session) to which this connection belongs. Connections that belong to the same call share the same call-id. The call-id can be used to identify calls for reporting and accounting purposes. It does not affect the handling of connections by the gateway.
EndpointId is the identifier for the connection endpoint in the gateway where CreateConnection executes. The EndpointId can be fully-specified by assigning a value to the parameter EndpointId in the function call or it may be under-specified by using the "anyone" wildcard convention. If the endpoint is underspecified, the endpoint identifier will be assigned by the gateway and its complete value returned in the SpecificEndPointId parameter of the response.
The NotifiedEntity is an optional parameter that specifies where the Notify or DeleteConnection commands should be sent. If the parameter is absent, the Notify or DeleteConnection commands should be sent to the last received Notified Entity, or to originator of the CreateConnection command if no Notified Entity was ever received for the end point.
LocalConnectionOptions is a parameter used by the Call Agent to direct the handling of the connection by the gateway. The fields contained in LocalConnectionOptions are the following:
This set of field can be completed by vendor specific optional or mandatory extensions. The encoding of the first three fields, when they are present, will be compatible with the SDP and RTP profiles:
For each of the first three fields, the Call Agent has three options:
The bandwidth specification shall not contradict the specification of encoding methods and packetization period. If an encoding method is specified, then the gateway is authorized to use it, even if it results in the usage of a larger bandwidth than specified.
The LocalConnectionOptions parameter may be absent in the case of a data call.
The Type of Service specifies the class of service that will be used for the connection. When the connection is transmitted over an IP network, the parameters encodes the 8-bit type of service value parameter of the IP header. When the Type of Service is not specified, the gateway shall use a default or configured value.
The gateways can be instructed to perform a reservation, for example using RSVP, on a given connection. When a reservation is needed, the call agent will specify the reservation profile that should be used, which is either "controlled load" or "guaranteed service." The
absence of reservation can be indicated by asking for the "best effort" service, which is the default value of this parameter. When reservation has been asked on a connection, the gateway will:
The RSVP filters will be deduced from the characteristics of the connection. The RSVP resource profiles will be deduced from the connection's bandwidth and packetization period.
By default, the telephony gateways always perform echo cancellation. However, it is necessary, for some calls, to turn off these operations. The echo cancellation parameter can have two values, "on" (when the echo cancellation is requested) and "off" (when it is turned off.)
The telephony gateways may perform gain control, in order to adapt the level of the signal. However, it is necessary, for example for modem calls, to turn off this function. The gain control parameter may either be specified as "automatic", or as an explicit number of decibels of gain. The default is to not perform gain control, which is equivalent to specifying a gain of 0 decibels.
The telephony gateways may perform voice activity detection, and avoid sending packets during periods of silence. However, it is necessary, for example for modem calls, to turn off this detection. The silence suppression parameter can have two values, "on" (when the detection is requested) and "off" (when it is turned off.) The default is "off."
The Call agent can request the gateway to enable encryption of the audio Packets. It does so by providing an key specification, as specified in RFC 2327. By default, encryption is not used.
The Call Agent may instruct the gateway to prepare the connection on a specified type of network. The type of network is encoded as in the "connection-field" parameter of the SDP standard. Possible values are IN (Internet), ATM and LOCAL. The parameter is optional; if absent, the network is determined by the type of gateway.
RemoteConnectionDescriptor is the connection descriptor for the
remote side of a connection, on the other side of the IP network. It
includes the same fields as in the LocalConnectionDescriptor, i.e.
the fields that describe a session according to the SDP standard.
This parameter may have a null value when the information for the
remote end is not known yet. This occurs because the entity that
builds a connection starts by sending a CreateConnection to one of
the two gateways involved in it. For the first CreateConnection
issued, there is no information available about the other side of the
connection. This information may be provided later via a
ModifyConnection call. In the case of data connections (mode=data),
this parameter describes the characteristics of the data connection.
The SecondEndpointId can be used instead of the
RemoteConnectionDescriptor to establish a connection between two
endpoints located on the same gateway. The connection is by
definition a local connection. The SecondEndpointId can be fully-
specified by assigning a value to the parameter SecondEndpointId in
the function call or it may be under-specified by using the "anyone"
wildcard convention. If the secondendpoint is underspecified, the
second endpoint identifier will be assigned by the gateway and its
complete value returned in the SecondEndPointId parameter of the
response.
Mode indicates the mode of operation for this side of the connection. The mode are "send", "receive", "send/receive", "conference", "data", "inactive", "loopback", "continuity test", "network loop back" or "network continuity test." The expected handling of these modes is specified in the introduction of the "Gateway Handling Function" section. Some end points may not be capable of supporting all modes. If the command specifies a mode that the endpoint cannot support, and error shall be returned.
The gateway returns a ConnectionId, that uniquely identifies the
connection within one endpoint, and a LocalConnectionDescriptor,
which is a session description that contains information about
addresses and RTP ports, as defined in SDP. The
LocalConnectionDescriptor is not returned in the case of data
connections. The SpecificEndPointId is an optional parameter that
identifies the responding endpoint. It can be used when the
EndpointId argument referred to a "any of" wildcard name. When a
SpecificEndPointId is returned, the Call Agent should use it as the
EndpointId value is successive commands referring to this call.
When a SecondEndpointId is specified, the command really creates two
connections that can be manipulated separately through
ModifyConnection and DeleteConnection commands. The response to the
creation provides a SecondConnectionId parameter that identifies the
second connection.
After receiving a "CreateConnection" request that did not include a RemoteConnectionDescriptor parameter, a gateway is in an ambiguous situation. Because it has exported a LocalConnectionDescriptor parameter, it can potentially receive packets. Because it has not yet received the RemoteConnectionDescriptor parameter of the other gateway, it does not know whether the packets that it receives have been authorized by the Call Agent. It must thus navigate between two risks, i.e. clipping some important announcements or listening to insane data. The behavior of the gateway is determined by the value of the Mode parameter:
Note that the mode values SendReceive, Conference, Data and SendOnly don't make sense in this situation. They should be treated as errors, and the command should be rejected (Error code 517).
The command may optionally contain an encapsulated Notification Request command, in which case a RequestIdentifier parameter will be present, as well as, optionally, the RequestedEvents DigitMap, SignalRequests, QuarantineHandling and DetectEvents parameters. The encapsulated NotificationRequest is executed simultaneously with the creation of the connection. For example, when the Call Agent wants to initiate a call to an residential gateway, it should:
This can be accomplished in a single CreateConnection command, by also transmitting the RequestedEvent parameters for the off hook event, and the SignalRequest parameter for the ringing signal.
When these parameters are present, the creation and the
NotificationRequests should be synchronized, which means that
bothshould be accepted, or both refused. In our example, the
CreateConnection may be refused if the gateway does not have
sufficient resources, or cannot get adequate resources from the local
network access, and the off-hook Notification-Request can be refused
in the glare condition, if the user is already off-hook. In this
example, the phone should not ring if the connection cannot be
established, and the connection should not be established if the user
is already off hook.
The NotifiedEntity parameter, if present, applies to both the CreateConnection and the NotificationRequest command. It defines the new "notified entity" for the endpoint.
The command may carry an encapsulated EndpointConfiguration command, that will apply to the same endpoint. When this command is present, the parameters of the EndpointConfiguration command are inserted after the normal parameters of the CreateConnection with the exception of the EndpointId, which is not replicated. The EndpointConfiguration command may be encapsulated together with an encapsulated NotificationRequest command.
The encapsulated EndpointConfiguration command shares the fate of the CreateConnection command. If the CreateConnection is rejected, the EndpointConfiguration is not executed.
ReturnCode is a parameter returned by the gateway. It indicates the outcome of the command and consists of an integer number optionally followed by commentary.
This command is used to modify the characteristics of a gateway's "view" of a connection. This "view" of the call includes both the local connection descriptors as well as the remote connection descriptor.
ReturnCode,
[LocalConnectionDescriptor]
<--- ModifyConnection(CallId,
EndpointId,
ConnectionId,
[NotifiedEntity,]
[LocalConnectionOptions,]
[Mode,]
[RemoteConnectionDescriptor,]
[Encapsulated NotificationRequest,]
[Encapsulated EndpointConfiguration])
The parameters used are the same as in the CreateConnection command,
with the addition of a ConnectionId that identifies the connection
within the endpoint. This parameter is returned by the
CreateConnection function, as part of the local connection
descriptor. It uniquely identifies the connection within the context
of the endpoint.
The EndpointId should be a fully qualified endpoint identifier. The local name shall not use the wildcard convention.
The ModifyConnection command can be used to affect parameters of a connection in the following ways:
Connections can only be activated if the RemoteConnectionDescriptor has been provided to the gateway. The receive only mode, however, can be activated without the provision of this descriptor.
The command will only return a LocalConnectionDescriptor if the local connection parameters, such as RTP ports, were modified. (Usage of this feature is actually for further study.)
The command may optionally contain an encapsulated Notification Request command, in which case a RequestIdentifier parameter will be present, as well as, optionnally, the RequestedEvents DigitMap, SignalRequests, QuarantineHandling and DetectEvents parameters. The
encapsulated NotificationRequest is executed simultaneously with the modification of the connection. For example, when a connection is accepted, the calling gateway should be instructed to place the circuit in send-receive mode and to stop providing ringing tones.
This can be accomplished in a single ModifyConnection command, by also transmitting the RequestedEvent parameters, for the on hook event, and an empty SignalRequest parameter, to stop the provision of ringing tones.
When these parameters are present, the modification and the
NotificationRequests should be synchronized, which means that both
should be accepted, or both refused. The NotifiedEntity parameter,
if present, applies to both the ModifyConnection and the
NotificationRequest command.
The command may carry an encapsulated EndpointConfiguration command, that will apply to the same endpoint. When this command is present, the parameters of the EndpointConfiguration command are inserted after the normal parameters of the ModifyConnection with the exception of the EndpointId, which is not replicated. The EndpointConfiguration command may be encapsulated together with an encapsulated NotificationRequest command.
The encapsulated EndpointConfiguration command shares the fate of the ModifyConnection command. If the ModifyConnection is rejected, the EndpointConfiguration is not executed.
ReturnCode is a parameter returned by the gateway. It indicates the outcome of the command and consists of an integer number optionally followed by commentary.
This command is used to terminate a connection. As a side effect, it collects statistics on the execution of the connection.
ReturnCode,
Connection-parameters
<-- DeleteConnection(CallId,
EndpointId,
ConnectionId,
[Encapsulated NotificationRequest,]
[Encapsulated EndpointConfiguration])
The endpoint identifier, in this form of the DeleteConnection command, shall be fully qualified. Wildcard conventions shall not be used.
In the general case where a connection has two ends, this command has to be sent to both gateways involved in the connection. Some connections, however, may use IP multicast. In this case, they can be deleted individually.
After the connection has been deleted, any loopback that has been requested for the connection should be cancelled. When all connections to an endpoint have been deleted, that endpoint should be placed in inactive mode.
In response to the DeleteConnection command, the gateway returns a list of parameters that describe the status of the connection. These parameters are:
Number of packets sent:
The total number of RTP data packets transmitted by the sender since starting transmission on this connection. The count is not reset if the sender changes its synchronization source identifier (SSRC, as defined in RTP), for example as a result of a Modify command. The value is zero if the connection was set in "receive only" mode.
Number of octets sent:
The total number of payload octets (i.e., not including header or padding) transmitted in RTP data packets by the sender since starting transmission on this connection. The count is not reset if the sender changes its SSRC identifier, for example as a result of a ModifyConnection command. The value is zero if the connection was set in "receive only" mode.
Number of packets received:
The total number of RTP data packets received by the sender since starting reception on this connection. The count includes packets received from different SSRC, if the sender used several values. The value is zero if the connection was set in "send only" mode.
Number of octets received:
The total number of payload octets (i.e., not including header or padding) transmitted in RTP data packets by the sender since starting transmission on this connection. The count includes packets received from different SSRC, if the sender used several values. The value is zero if the connection was set in "send only" mode.
Number of packets lost:
The total number of RTP data packets that have been lost since the beginning of reception. This number is defined to be the number of packets expected less the number of packets actually received, where the number of packets received includes any which are late or duplicates. The count includes packets received from different SSRC, if the sender used several values. Thus packets that arrive late are not counted as lost, and the loss may be negative if there are duplicates. The count includes packets received from different SSRC, if the sender used several values. The number of packets expected is defined to be the extended last sequence number received, as defined next, less the initial sequence number received. The count includes packets received from different SSRC, if the sender used several values. The value is zero if the connection was set in "send only" mode. This parameter is omitted if the connection was set in "data" mode.
Interarrival jitter:
An estimate of the statistical variance of the RTP data packet interarrival time measured in milliseconds and expressed as an unsigned integer. The interarrival jitter J is defined to be the mean deviation (smoothed absolute value) of the difference D in packet spacing at the receiver compared to the sender for a pair of packets. Detailed computation algorithms are found in RFC 1889. The count includes packets received from different SSRC, if the sender used several values. The value is zero if the connection was set in "send only" mode. This parameter is omitted if the connection was set in "data" mode.
Average transmission delay:
An estimate of the network latency, expressed in milliseconds. This
is the average value of the difference between the NTP timestamp
indicated by the senders of the RTCP messages and the NTP timestamp
of the receivers, measured when this messages are received. The
average is obtained by summing all the estimates, then dividing by
the number of RTCP messages that have been received. This parameter
is omitted if the connection was set in "data" mode.
When the gateway's clock is not synchronized by NTP, the latency
value can be computed as one half of the round trip delay, as
measured through RTCP.
When the gateway cannot compute the one way delay or the round trip
delay, the parameter conveys a null value.
For a detailed definition of these variables, refer to RFC 1889.
When the connection was set up over an ATM network, the meaning of these parameters may change:
Number of packets sent: The total number of ATM cells transmitted since starting transmission on this connection.
Number of octets sent:
The total number of payload octets transmitted in ATM cells.
Number of packets received:
The total number of ATM cells received since starting reception on
this connection.
Number of octets received:
The total number of payload octets received in ATM cells.
Number of packets lost:
Should be determined as the number of cell losts, or set to zero
if the adaptation layer does not enable the gateway to assess
losses.
Interarrival jitter:
Should be understood as the interarrival jitter between ATM cells.
Average transmission delay:
The gateway may not be able to assess this parameter over an ATM
network. It could simply report a null value.
When the connection was set up over an LOCAL interconnect, the meaning of these parameters is defined as follows:
Number of packets sent:
Not significant.
Number of octets sent:
The total number of payload octets transmitted over the local
connection.
Number of packets received:
Not significant.
Number of octets received:
The total number of payload octets received over the connection.
Number of packets lost:
Not significant. A value of zero is assumed.
Interarrival jitter:
Not significant. A value of zero is assumed.
Average transmission delay:
Not significant. A value of zero is assumed.
The standard set of connection parameters can be extended by the creation of extension parameters.
The command may optionally contain an encapsulated Notification Request command, in which case a RequestIdentifier parameter will be present, as well as, optionnally, the RequestedEvents DigitMap, SignalRequests, QuarantineHandling and DetectEvents parameters. The encapsulated NotificationRequest is executed simultaneously with the deletion of the connection. For example, when a user hang-up is notified, the gateway should be instructed to delete the connection and to start looking for an off hook event.
This can be accomplished in a single DeleteConnection command, by also transmitting the RequestedEvent parameters, for the off hook event, and an empty SignalRequest parameter.
When these parameters are present, the DeleteConnection and the NotificationRequests should be synchronized, which means that both should be accepted, or both refused.
The command may carry an encapsulated EndpointConfiguration command, that will apply to the same endpoint. When this command is present, the parameters of the EndpointConfiguration command are inserted after the normal parameters of the DeleteConnection with the exception of the EndpointId, which is not replicated. The EndpointConfiguration command may be encapsulated together with an encapsulated NotificationRequest command.
The encapsulated EndpointConfiguration command shares the fate of the DeleteConnection command. If the DeleteConnection is rejected, the EndpointConfiguration is not executed.
ReturnCode is a parameter returned by the gateway. It indicates the outcome of the command and consists of an integer number optionally followed by commentary.
In some circumstances, a gateway may have to clear a connection, for example because it has lost the resource associated with the connection, or because it has detected that the endpoint no longer is capable or willing to send or receive voice. The gateway terminates the connection by using a variant of the DeleteConnection command:
ReturnCode,
<-- DeleteConnection( CallId,
EndpointId,
ConnectionId,
Reason-code,
Connection-parameters)
In addition to the call, endpoint and connection identifiers, the gateway will also send the call's parameters that would have been returned to the Call Agent in response to a DeleteConnection command. The reason code indicates the cause of the disconnection.
ReturnCode is a parameter returned by the call agent. It indicates the outcome of the command and consists of an integer number optionally followed by commentary.
A variation of the DeleteConnection function can be used by the Call Agent to delete multiple connections at the same time. The command can be used to delete all connections that relate to a Call for an endpoint:
ReturnCode,
<-- DeleteConnection( CallId,
EndpointId)
It can also be used to delete all connections that terminate in a given endpoint:
ReturnCode,
<-- DeleteConnection( EndpointId)
Finally, Call Agents can take advantage of the hierarchical naming structure of endoints to delete all the connections that belong to a group of endpoints. In this case, the "local name" component of the EndpointID will be specified using the "all value" wildcarding convention. The "any value" convention shall not be used. For example, if endpoints names are structured as the combination of a physical interface name and a circuit number, as in "X35V3+A4/13",
the Call Agent may replace the circuit number by a wild card character "*", as in "X35V3+A4/*". This "wildcard" command instructs the gateway to delete all the connections that where attached to circuits connected to the physical interface "X35V3+A4".
After the connections have been deleted, the endpoint should be placed in inactive mode. Any loopback that has been requested for the connections should be cancelled.
This command does not return any individual statistics or call parameters.
ReturnCode is a parameter returned by the gateway. It indicates the outcome of the command and consists of an integer number optionally followed by commentary.
The AuditEndPoint command can be used by the Call Agent to find out the status of a given endpoint.
ReturnCode,
EndPointIdList|{
[RequestedEvents,]
[DigitMap,]
[SignalRequests,]
[RequestIdentifier,]
[NotifiedEntity,]
[ConnectionIdentifiers,]
[DetectEvents,]
[ObservedEvents,]
[EventStates,]
[BearerInformation,]
[RestartReason,]
[RestartDelay,]
[ReasonCode,]
[Capabilities]}
<--- AuditEndPoint(EndpointId,
[RequestedInfo])
The EndpointId identifies the endpoint that is being audited. The "all of" wildcard convention can be used to start auditing of a group of endpoints. If this convention is used, the gateway should return the list of endpoint identifiers that match the wildcard in the EndPointIdList parameter. It shall not return any parameter specific to one of these endpoints.
When a non-wildcard EndpointId is specified, the (possibly empty) RequestedInfo parameter describes the information that is requested for the EndpointId specified. The following endpoint info can be audited with this command:
RequestedEvents, DigitMap, SignalRequests, RequestIdentifier, NotifiedEntity, ConnectionIdentifiers, DetectEvents, ObservedEvents, EventStates, RestartReason, RestartDelay, ReasonCode, and Capabilities.
The response will in turn include information about each of the items for which auditing info was requested:
* ConnectionIdentifiers, the list of ConnectionIdentifiers for all
connections that currently exist for the specified endpoint.
Compression Algorithm: a list of supported codecs. The rest of the parameters will apply to all codecs specified in this list.
Packetization Period: A single value or a range may be specified.
Bandwidth: A single value or a range corresponding to the range for packetization periods may be specified (assuming no silence suppression).
Echo Cancellation: Whether echo cancellation is supported or not.
Silence Suppression: Whether silence suppression is supported or not.
Type of Service: Whether type of service is supported or not.
Event Packages: A list of event packages supported. The first event package in the list will be the default package.
Modes: A list of supported connection modes.
The Call Agent may then decide to use the AuditConnection command to obtain further information about the connections.
If no info was requested and the EndpointId refers to a valid endpoint, the gateway simply returns a positive acknowledgement.
If no NotifiedEntity has been specified in the last
NotificationRequest, the notified entity defaults to the source
address of the last NotificationRequest command received for this
connection.
ReturnCode is a parameter returned by the gateway. It indicates the outcome of the command and consists of an integer number optionally followed by commentary.
The AuditConnection command can be used by the Call Agent to retrieve the parameters attached to a connection:
ReturnCode,
[CallId,]
[NotifiedEntity,]
[LocalConnectionOptions,]
[Mode,]
[RemoteConnectionDescriptor,]
[LocalConnectionDescriptor,]
[ConnectionParameters]
<--- AuditConnection(EndpointId,
ConnectionId,
RequestedInfo)
The EndpointId parameter specifies the endpoint that handles the connection. The wildcard conventions shall not be used.
The ConnectionId parameter is the identifier of the audited connection, within the context of the specified endpoint.
The (possibly empty) RequestedInfo describes the information that is requested for the ConnectionId within the EndpointId specified. The following connection info can be audited with this command:
CallId, NotifiedEntity, LocalConnectionOptions, Mode,
RemoteConnectionDescriptor, LocalConnectionDescriptor,
ConnectionParameters
The AuditConnectionResponse will in turn include information about each of the items auditing info was requested for: