[rtcweb] AVPF vs AVP

Magnus Westerlund <magnus.westerlund@ericsson.com> Wed, 14 September 2011 15:06 UTC

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Date: Wed, 14 Sep 2011 17:08:55 +0200
From: Magnus Westerlund <magnus.westerlund@ericsson.com>
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Subject: [rtcweb] AVPF vs AVP
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There has been this long thread with the subject partially containing
"AVPF". I want to clarify something in this context around AVPF. Rather
than the SRTP question.

An end-point that is AVPF compliant is in fact interoperable with an AVP
one as long as the trr-int parameter is set reasonably large. A
parameter value of 1.5-5 seconds (I would recommend 3s) will ensure that
they are in fact compatible. This avoids the risk of any side timing out
the other if the AVP side is using the default 5 s minimum interval.

Based on this one could in fact have the RTCWEB nodes always use AVPF
for RTP/RTCP Behavior. The AVPF feedback messages are explicitly
negotiated and will only be used when agreed on.

This leaves us with any signaling incompatibilities when talking to a
legacy device. If one don't want to use cap-neg I see two directions to go:

1) RTCWEB end-point will always signal AVPF or SAVPF. I signalling
gateway to legacy will change that by removing the F to AVP or SAVP.

2) RTCWEB end-point will always use AVPF but signal it as AVP. It will
detect the AVPF capabilities of the other end-point based on the
signaling of the feedback events intended to be used.

I think 1) is cleaner for the future. 2) might be more pragmatic.

In both cases I believe there are methods for negotiating a lower
trr-int than some AVP fallback value to preserve interoperability.

However, this still don't resolve the question if the "S" should be in
front of the RTP profile indicator or not. But it might help by removing
the F or not in the profile.


Magnus Westerlund

Multimedia Technologies, Ericsson Research EAB/TVM
Ericsson AB                | Phone  +46 10 7148287
Färögatan 6                | Mobile +46 73 0949079
SE-164 80 Stockholm, Sweden| mailto: magnus.westerlund@ericsson.com