SDP and H.245 Data Structures

Tom-PT Taylor taylor at NORTELNETWORKS.COM
Mon May 10 21:52:45 EDT 1999


This message is to aid discussion of the structure of the
LocalTerminationDescriptor and RemoteTerminationDescriptor components of the
Add, Modify, and Move commands.  In our protocol draft, the local and remote
termination descriptors actually represent two directions of transmission,
inwards to the MG and outwards from the MG respectively.  Each requires its
own media description.

The two proposals for media description are to use structures modelled on
the media description components of SDP (RFC 2327), or to use structures
modelled on the H.245 OpenLogicalChannel (OLC) and OpenLogicalChannelAck
commands.  This note shows details of each structure.

I start with the SDP structure.  The complete set of headers in the media
description is described in RFC 2327 as follows.  Asterisks denote optional
elements.

Media description
   m=<media> <port> <transport> <fmt list>
   i=* (media title)
   c=* (connection information - optional if included at session-level)
   b=* (bandwidth information)
   k=* (encryption key)
   a=* (zero or more media attribute lines)


Here is an example of an SDP media description:
                m=video 49232 RTP/AVP 98
        i=Pictures at an exhibition
        c=IN IP4 134.134.157.81
        a=rtpmap:98 L16/16000/2

Now here is the structure of a media description which would be consistent
with the H.245 OLC and OLCAck structures.  I say "consistent with" because
it is structured like the OLC and OLCAck, but contains only the information
which would result from their exchange.  Indentation is used to show data
structures within data structures.  The tags used are the names of the
various components in their ASN.1 description.  The comments following the
"--" show the mapping to SDP.

H245_channel_descriptor=
   logicalChannelNumber= <value in range 1...65535> -- unique to H.245
   dataType= <medium (implicit tag)> <codec description> -- m= and a=
headers
   h2250LogicalChannelParameters=
      <non-standardized parameters> -- SDP X- extensions
      <RTP or T.120 sessionID> -- not settable by SDP
      <associated sessionID> -- (for lip synch) -- not settable by SDP
      <mediaChannel> -- transport address -- c= header
      <mediaGuaranteedDelivery> -- TRUE for TCP transport -- m= header
      <mediaControlChannel> -- RTCP address/port (optional) -- implicit in
SDP
      <mediaControlGuaranteedDelivery> -- TRUE for TCP transport -- not in
SDP
      <silenceSuppression> -- not in SDP -- part of RTP profile?
      <destination> -- designates final destination if connection is to MCU
but
                           stream is intended for a particular terminal --
not in SDP
      <RTP payload type> -- m= header, possibly augmented by a= header(s)
      <transportCapability> -- RSVP or ATM QOS parameters -- not in SDP
                                      -- lower-layer transport -- c= header
      <redundancy encoding specification> -- m= header
      <source> -- designates originating terminal if working through MCU
                     -- not in SDP
   <logicalChannelDependency> -- which channel this one depends on
          if hierarchical encoding used -- implicit in SDP by order of port
          numbers in m= header
   separateStack=
         -- address for setting up ATM connection -- c= header
         -- alternatively, information for joining T.120 conference -- a=
header?
    encryptionSynch=
         -- key for media encoding -- k= header

  One small step in the discussion ...

Tom Taylor
E-mail: taylor at nortelnetworks.com  (internally Tom-PT Taylor)
Tel.: +1 613 736 0961     (ESN 396-1490)
FAX: same number by prior arrangement (manual answer).



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