Re: [rtcweb] AVPF vs AVP

Harald Alvestrand <harald@alvestrand.no> Thu, 15 September 2011 12:16 UTC

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Date: Thu, 15 Sep 2011 14:18:21 +0200
From: Harald Alvestrand <harald@alvestrand.no>
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To: Magnus Westerlund <magnus.westerlund@ericsson.com>
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Subject: Re: [rtcweb] AVPF vs AVP
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On 09/15/11 14:05, Magnus Westerlund wrote:
> On 2011-09-14 21:32, Harald Alvestrand wrote:
>> On 09/14/11 17:08, Magnus Westerlund wrote:
>>> Hi,
>>>
>>> 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.
>> If 2) is more pragmatic, the IETF should update the definition of AVP to
>> allow use of AVPF functionality.
> No, Harald that I think is a bad idea. The core of AVPF is the timing
> rule. From my perspective the important aspect is that one do implement
> the timing rules. Thus it is important we have the label of AVPF to
> indicate these rules.
But how do you "have" the label if you signal it as AVP??????
Is it a label that occurs in the spec only, and is undetectable on the wire?
That's the place where you lost me.
> Thus my proposal is really to say that an RTCWEB end-point SHALL
> implement and always use the AVPF timing rules. It might send feedback
> messages if those have been negotiated with a=rtcp-fb attribute.
>
>> The responses seem to indicate that in practice, this is already being
>> done (and this is a clear indication that the concept of "profile" in
>> RTP has failed to achieve its purpose).
> It works fine on the RTP level. But the signalling aspects has clearly
> been botched.
>
> cheers
>
> Magnus Westerlund
>
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