[rtcweb] Codec control and SDP offer/answer frequency

Christer Holmberg <christer.holmberg@ericsson.com> Wed, 22 August 2012 20:47 UTC

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From: Christer Holmberg <christer.holmberg@ericsson.com>
To: "rtcweb@ietf.org" <rtcweb@ietf.org>
Date: Wed, 22 Aug 2012 22:47:01 +0200
Thread-Topic: Codec control and SDP offer/answer frequency
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Subject: [rtcweb] Codec control and SDP offer/answer frequency
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In Vancouver two proposals for codec were presented, one based on COP and one based on the SDP imageattr attribute. The result from the straw poll was that people preferred to move forward with the SDP based solution.

When Harald presented his poker game example, he mentioned that there could be changes with a few seconds interval.

For my clarification, does that mean that a UA would send SDP offer/answers every few second???

If so, I don't think that would create good results.

First, it would cause heavy load on every entity that has to parse the SDP, as they need to parse the whole message body, and then figure out what has changed.

Second, due to race conditions rules, SIP transaction rules etc, and *other* usages of SDP offer/answer within the session, there is a high risk that SDP offers (not only those related to codec control) would be rejected. Offer/answer is a rather "heavy" mechanism, and I don't think it's intended to be used in the suggested way. Normally you would send one (or a few) offer/answer(s) during call establishment, and between zero and a few during the session.

Or, did I missunderstand something?