how many port pairs in a call/conference?

grm at ZURICH.IBM.COM grm at ZURICH.IBM.COM
Wed Jun 24 05:58:37 EDT 1998


OK, I thought these were easy questions but maybe people wonder 'what does
he mean by RTP Session?' Let me rephrase:

(A) How many (RTP port, RTCP port) pairs does an H.323 endpoint allocate in
a two-party call with two-way audio?

(B) How many (RTP port, RTCP port) pairs does an H.323 endpoint allocate in
an n-party conference with everybody sending and receiving audio and using
distributed multi-unicast?

Marcel Graf
IBM Zurich Research Lab

> I'm trying to find an answer to the question (A) "how many RTP
> Sessions are there for a two-party call with two-way audio?", and to
> the related question (B) "how many RTP Sessions are there for an
> n-party conference with each participant being an audio Source and an
> audio Sink and using distributed multi-unicast?".
>
> What I already know is that, assuming unidirectional Logical
> Channels, in the first case there are two Logical Channels, and in
> the second case there are n*(n - 1) Logical Channels.
>
> H.245 says about the sessionID: "The sessionID is a unique RTP or
> T.120 Session Identifier in the conference. It is used by the
> transmitter to refer to the session to which the logical channel
> applies. Only the master can create the session identification. By
> convention, there are three primary sessions. The first primary
> session with a session identification of 1 is the audio session, the
> second primary session with a session identification of 2 is the
> video session, and the third primary session with a session
> identification of 3 is the data session."
>
> Can I interpret this paragraph as saying that in case (A) there is
> only one RTP Session, and in case (B) there is only one RTP Session,
> too?
>
> If that is correct, then the following paragraph from H.323 section
> 6.2.8.2 "Logical channel signalling" comes into play: "If a
> corresponding reverse channel is opened for a given existing RTP
> session (identified by the RTP sessionID), the mediaControlChannel
> transport addresses exchanged by the OpenLogicalChannel process shall
> be identical to those used for the forward channel. Should a
> collision occur where both ends attempt to establish conflicting RTP
> sessions at the same time, the master endpoint shall reject the
> conflicting attempt as described in Recommendation H.245. The
> rejected OpenLogicalChannel attempt may then be retried at a later
> time."
>
> What type of conflict does this paragraph refer to? Given there is a
> single RTP Session, and (multi-)unicast is used, each endpoint
> chooses the RTP and RTCP port it is listening on independently of the
> others. (The paragraph can't be talking about multicast, because it
> is the MCs responsibility to allocate multicast addresses.)
>
> I also made the following observation: The "primary session
> convention" established in the cited H.245 paragraph seems to be
> contradicted by the samples shown in H.323 Appendix I "Sample MC to
> Terminal Communication Mode Command" which uses the sessionIDs 2 and
> 3 differently. Maybe the convention is only meant for two-party calls?
>
>
> Marcel Graf
> IBM Zurich Research Lab



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