Subject: Re: Regarding SDP Media level description

From: Arjun RC <[email-masked]>
Date: Thu, 15 Apr 1999 18:39:21 +0530 (IST)

Hi,
This brings me to another question: 
In SIP, after  A sends an INVITE to B and B responds with an OK, must
media communication start immediately ? Or is there a provision in which B
can send a provisional OK to A saying its got an INVITE but cannot
establish a media session as it does not yet have enough info to do so
(something analogous to the setupacknowledge in h323?) ? If so, then the
UAS could probably send this back and the sender knows that he must issue
further INVITEs to establish a media comm.

Cheers
ARC

On Thu, 15 Apr -1, eubpepe wrote:

eu> But what exactly should a UAS send as SDP in a 200 OK response to an
eu> INVITE without a media descriptor? Should it send all it's capabilities
eu> for all types of sessions? And even if the H323-SIP gateway sends the
eu> media descriptor in the ACK how will the UAS know what media to expect?
eu> Should it listen for all media types it supports?
eu> Suppose for instance that the UAS sends an SDP with mediadescriptors for
eu> both audio and video, and the H323-SIP gw sends an ACK with only audio.
eu> The UAS cannot be sure the gw won't send video?
eu> Peter Peldan
eu> Ellemtel Utvecklings AB
eu> 

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---

From: Jonathan Rosenberg <[email-masked]>
Date: Fri, 16 Apr 1999 08:58:48 -0400

Arjun RC wrote:
> media communication start immediately ? 

I think the spec says that B shouldn't send audio until it has received
the ACK; however many people don't do this, and allow A to receive media
right away. This is useful for in-band tones and progress messages.
We've had some discussion on this list about this issue (183 responses
and the like)... A can send media to B as soon as the 200 OK comes.

Or is there a provision in which B
> (something analogous to the setupacknowledge in h323?) ?

This is the purpose of 100 Trying.

 If so, then the
> further INVITEs to establish a media comm.

No; if B isn't ready to receive media yet, but wants to signal the
caller that the INVITE was received, it sends a 100 Trying (or any
provisional response, in fact). Once its ready, it then sends a 200 OK.

-Jonathan R.

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