rfc2705.txt

著名的RFC文档,其中有一些文档是已经翻译成中文的的.

      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.




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RFC 2705         Media Gateway Control Protocol (MGCP)      October 1999


   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:






































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RFC 2705         Media Gateway Control Protocol (MGCP)      October 1999


             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





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RFC 2705         Media Gateway Control Protocol (MGCP)      October 1999


2.1.3.1.  Names of calls

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

   Calls are identified by unique identifiers, independent of the
   underlying platforms or agents. These identifiers are created by the
   Call Agent. They are treated in MGCP as unstructured octet strings.

   Call identifiers are expected to be unique within the system, or at a
   minimum, unique within the collection of Call Agents that control the
   same gateways. When a Call Agent builds several connections that
   pertain to the same call, either on the same gateway or in different
   gateways, these connections that belong to the same call share the
   same call-id.  This identifier can then be used by accounting or
   management procedures, which are outside the scope of MGCP.

2.1.3.2.  Names of connections

   Connection identifiers are created by the gateway when it is
   requested to create a connection. They identify the connection within
   the context of an endpoint. They are treated in MGCP as unstructured
   octet strings.  The gateway should make sure that a proper waiting
   period, at least 3 minutes, elapses between the end of a connection
   that used this identifier and its use in a new connection for the
   same endpoint.  (Gateways may decide to use identifiers that are
   unique within the context of the gateway.)

2.1.3.3.  Management of resources, attributes of connections

   Many types of resources will be associated to a connection, such as
   specific signal processing functions or packetization functions.
   Generally, these resources fall in two categories:

   1) Externally visible resources, that affect the format of "the bits
      on the network" and must be communicated to the second endpoint
      involved in the connection.

   2) Internal resources, that determine which signal is being sent over
      the connection and how the received signals are processed by the
      endpoint.

   The resources allocated to a connection, and more generally the
   handling of the connection, are chosen by the gateway under
   instructions from the call agent.  The call agent will provide these
   instructions by sending two set of parameters to the gateway:

   1) The local directives instruct the gateway on the choice of
      resources that should be used for a connection,



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RFC 2705         Media Gateway Control Protocol (MGCP)      October 1999


   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.























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RFC 2705         Media Gateway Control Protocol (MGCP)      October 1999


                 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 --








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RFC 2705         Media Gateway Control Protocol (MGCP)      October 1999


2.1.3.4.  Special case of local connections