Subject: [AVTCORE] RTP Retransmission and RID

From: Magnus Westerlund <[email-masked]>
Date: Tue, 4 Dec 2018 09:23:31 +0000

WG,

We are currently having a discussion on the RTCWeb WG mailing list
regarding RID and RTP Retransmission (RTX) using RFC 4588. In that
discussion I think there are three things worth noting.

1) That RFC4588 handling of RTP Header Extensions may actually needs
specific rules for the RID related header extensions (RtpStreamId and
RepairedRtpStreamId) that are defined in
https://datatracker.ietf.org/doc/draft-ietf-avtext-rid/. The reason is
that the reconstructed packet clearly needs to have its RtpStreamId
header extension if the original packet had it. Secondly to enable that
one actually explicitly indicates the particular repair RTP stream that
carries the RTX relation to its source stream using the RID mechanism
one need to add a RepairedRtpStreamId header extension to the repair
packet also in some cases like stream start or at certain session
participant changes.

2) There was discussion if for media sources that has only a single
source RTP stream if that binding between source and repair RTP streams
are accomplished using MIDs in SDES items or header extensions?

3) As the current late binding mechanism has some shortcomings, like not
allowing NACKing multiple streams that not yet have an established
binding as well as not handling dynamic rebinding well there is the
question if there should be an updated recommendation for how one should
accomplish the binding between repair and source RTP streams for the RFC
4588 mechanism? I do note that RID is not capable of addressing the case
with separate RTP sessions for source and repair as RID has a scope that
can't reach over multiple RTP sessions.

So how do we handle this, especially 1 appears to need a resolution.

Cheers 

Magnus Westerlund 

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