rfc2885.txt

著名的RFC文档,其中有一些文档是已经翻译成中文的的.

      ServiceChange command.  The MGC may also use ServiceChange to
      instruct the MG to take a Termination or group of Terminations in
      or out of service.

   These commands are detailed in sections 7.2.1 through 7.2.8

7.1 Descriptors

   The parameters to a command are termed Descriptors. A Descriptor
   consists of a name and a list of items. Some items may have values.
   Many Commands share common Descriptors.  This subsection enumerates
   these Descriptors.  Descriptors may be returned as output from a
   command.  Parameters and parameter usage specific to a given Command
   type are described in the subsection that describes the Command.

7.1.1 Specifying Parameters

   Command parameters are structured into a number of descriptors. In
   general, the text format of descriptors is
   DescriptorName=<someID>{parm=value, parm=value_.}.





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   Parameters may be fully specified, over-specified or under-specified:

   1. Fully specified parameters have a single, unambiguous value that
      the command initiator is instructing the command responder to use
      for the specified parameter.

   2. Under-specified parameters, using the CHOOSE value, allow the
      command responder to choose any value it can support.

   3. Over-specified parameters have a list of potential values.  The
      list order specifies the command initiator's order of preference
      of selection.  The command responder chooses one value from the
      offered list and returns that value to the command initiator.

   Unspecified mandatory parameters (i.e. mandatory parameters not
   specified in a descriptor) result in the command responder retaining
   the previous value for that parameter.  Unspecified optional
   parameters result in the command responder using the default value of
   the parameter.  Whenever a parameter is underspecified or
   overspecified, the descriptor containing the value chosen by the
   responder is included as output from the command.

   Each command specifies the TerminationId the command operates on.
   This TerminationId may be "wildcarded".  When the TerminationId of a
   command is wildcarded, the effect shall be as if the command was
   repeated with each of the TerminationIds matched.

7.1.2 Modem Descriptor

   The Modem descriptor specifies the modem type and parameters, if any,
   required for use in e.g. H.324 and text conversation.  The descriptor
   includes the following modem types: V.18, V.22, V.22bis, V.32,
   V.32bis, V.34, V.90, V.91, Synchronous ISDN, and allows for
   extensions.  By default, no modem descriptor is present in a
   Termination.

7.1.3 Multiplex Descriptor

   In multimedia calls, a number of media streams are carried on a
   (possibly different) number of bearers.  The multiplex descriptor
   associates the media and the bearers. The descriptor includes the
   multiplex type:

    . H.221
    . H.223,
    . H.226,
    . V.76,
    . Possible Extensions



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   and a set of TerminationIDs representing the multiplexed inputs, in
   order.  For example:

       Mux = H.221{ MyT3/1/2, MyT3/2/13, MyT3/3/6, MyT3/21/22}

7.1.4 Media Descriptor

   The Media Descriptor specifies the parameters for all the media
   streams.  These parameters are structured into two descriptors, a
   Termination State Descriptor, which specifies the properties of a
   termination that are not stream dependent, and one or more Stream
   Descriptors each of which describes a single media stream.

   A stream is identified by a StreamID.  The StreamID is used to link
   the streams in a Context that belong together. Multiple streams
   exiting a termination shall be synchronized with each other.  Within
   the Stream Descriptor, there are up to three subsidiary descriptors,
   LocalControl, Local, and Remote. The relationship between these
   descriptors is thus:

   Media Descriptor
        TerminationStateDescriptor
        Stream Descriptor
                LocalControl Descriptor
                Local Descriptor
                Remote Descriptor

   As a convenience a LocalControl, Local, or Remote descriptor may be
   included in the Media Descriptor without an enclosing Stream
   descriptor.  In this case, the StreamID is assumed to be 1.

7.1.5 Termination State Descriptor

   The Termination State Descriptor contains the ServiceStates property,
   the EventBufferControl property and properties of a termination
   (defined in Packages) that are not stream specific.

   The ServiceStates property describes the overall state of the
   termination (not stream-specific).  A Termination can be in one of
   the following states: "test", "out of service", or "in service".  The
   "test" state indicates that the termination is being tested. The
   state "out of service" indicates that the termination cannot be used
   for traffic.  The state "in service" indicates that a termination can
   be used or is being used for normal traffic.  "in service" is the
   default state.






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   Values assigned to Properties may be simple values
   (integer/string/enumeration) or may be underspecified, where more
   than one value is supplied and the MG may make a choice:
    .  Alternative Values: multiple values in a list, one of which must
       be selected
    .  Ranges: minimum and maximum values, any value between min and max
       must be selected, boundary values included
    .  Greater Than/Less Than: value must be greater/less than specified
       value
    .  CHOOSE Wildcard: the MG chooses from the allowed values for the
       property

   The EventBufferControl property  specifies whether events are
   buffered following detection of an event in the Events Descriptor, or
   processed immediately.  See section 7.1.9 for details.

7.1.6 Stream Descriptor

   A Stream descriptor specifies the parameters of a single bi-
   directional stream.  These parameters are structured into three
   descriptors: one that contains termination properties specific to a
   stream and one each for local and remote flows. The Stream Descriptor
   includes a StreamID which identifies the stream.  Streams are created
   by specifying a new StreamID on one of the terminations in a Context.
   A stream is deleted by setting empty Local and Remote descriptors for
   the stream with ReserveGroup and ReserveValue in LocalControl set to
   "false" on all terminations in the context that previously supported
   that stream.

   StreamIDs are of local significance between MGC and MG and they are
   assigned by the MGC.  Within a context, StreamID is a means by which
   to indicate which media flows are interconnected:  streams with the
   same StreamID are connected.

   If a termination is moved from one context to another, the effect on
   the context to which the termination is moved is the same as in the
   case that a new termination were added with the same StreamIDs as the
   moved termination.

7.1.7 LocalControl Descriptor

   The LocalControl Descriptor contains the Mode property, the
   ReserveGroup and ReserveValue properties and properties of a
   termination (defined in Packages) that are stream specific, and are
   of interest between the MG and the MGC.  Values of properties may be
   underspecified as in section 7.1.1.





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   The allowed values for the mode property are send-only, receive-only,
   send/receive, inactive and loop-back.  "Send" and "receive" are with
   respect to the exterior of the context, so that, for example, a
   stream set to mode=sendonly does not pass received media into the
   context.  Signals and Events are not affected by mode.

   The boolean-valued Reserve properties, ReserveValue and ReserveGroup,
   of a Termination indicate what the MG is expected to do when it
   receives a  local and/or remote descriptor.

   If the value of a Reserve property is True, the MG SHALL reserve
   resources for all alternatives specified in the local and/or remote
   descriptors for which it currently has resources available.  It SHALL
   respond with the alternatives for which it reserves resources.  If it
   cannot not support any of the alternatives, it SHALL respond with a
   reply to the MGC that contains empty local and/or remote descriptors.

   If the value of a Reserve property is False, the MG SHALL choose one
   of the alternatives specified in the local descriptor (if present)
   and one of the alternatives specified in the remote descriptor (if
   present).  If the MG has not yet reserved resources to support the
   selected alternative, it SHALL reserve the resources.  If, on the
   other hand, it already reserved resources for the Termination
   addressed (because of a prior exchange with ReserveValue and/or
   ReserveGroup equal to True), it SHALL release any excess resources it
   reserved previously.  Finally, the MG shall send a reply to the MGC
   containing the alternatives for the local and/or remote descriptor
   that it selected.  If the MG does not have sufficient resources to
   support any of the alternatives specified, is SHALL respond with
   error 510 (insufficient resources).

   The default value of ReserveValue and ReserveGroup is False.

   A new setting of the LocalControl Descriptor completely replaces the
   previous setting of that descriptor in the MG.  Thus to retain
   information from the previous setting the MGC must include that
   information in the new setting.  If the MGC wishes to delete some
   information from the existing descriptor, it merely resends the
   descriptor (in a Modify command) with the unwanted information
   stripped out.

7.1.8 Local and Remote Descriptors

   The MGC uses Local and Remote descriptors to reserve and commit MG
   resources for media decoding and encoding for the given Stream(s) and
   Termination to which they apply.  The MG includes these descriptors
   in its response to indicate what it is actually prepared to support.
   The MG SHALL include additional properties and their values in its



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   response if these properties are mandatory yet not present in the
   requests made by the MGC (e.g., by specifying detailed video encoding
   parameters where the MGC only specified the payload type).

   Local refers to the media received by the MG and Remote refers to the
   media sent by the MG.

   When text encoding the protocol, the descriptors consist of session
   descriptions as defined in SDP (RFC2327).  In session descriptions
   sent from the MGC to the MG, the following exceptions to the syntax
   of RFC 2327 are allowed:

    .  the "s=", "t=" and "o=" lines are optional,
    .  the use of CHOOSE is allowed in place of a single parameter
       value, and
    .  the use of alternatives is allowed in place of a single parameter
       value.

   When multiple session descriptions are provided in one descriptor,
   the "v=" lines are required as delimiters; otherwise they are
   optional in session descriptions sent to the MG.  Implementations
   shall accept session descriptions that are fully conformant to
   RFC2327. When binary encoding the protocol the descriptor consists of
   groups of properties (tag-value pairs) as specified in Annex C.  Each
   such group may contain the parameters of a session description.

   Below, the semantics of the local and remote descriptors are
   specified in detail.  The specification consists of two parts.  The
   first part specifies the interpretation of the contents of the
   descriptor.  Th