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
grm＠ZURICH.IBM.COM
Jim Toga