Re: [rtcweb] A proposal for FEC

[Magnus Westerlund <[e-mail]>]

On 2014-05-19 09:21, Justin Uberti wrote:
> RFC 5109 defines a basic XOR-based scheme that should be useful in
> certain cases (e.g. high RTT). The concern expressed today about this
> not working in BUNDLE situations is addressed in RFC 5956, S 4.3
> <http://tools.ietf.org/html/rfc5956#section-4.3>, using ssrc-group to
> allow SSRC multiplexing, and this is endorsed by Unified Plan, S 3.3
> <http://tools.ietf.org/html/draft-roach-mmusic-unified-plan-00#page-16>.
> 
> I would like to see ULPFEC/5109 move forward as a baseline FEC mechanism
> for 1.0. We can look into other options for future versions of WebRTC.
> 

I think adding ULPFEC in WebRTC is reasonable, however despite what RFC
5956 says, we do have a spec issue with the following part of RFC 5109
that will be required to be overridden:

Section 7.2:

   Synchronization Source (SSRC): The SSRC value SHALL be the same as
   the SSRC value of the media stream it protects.

Section 14.1:

   The SSRC of the FEC stream MUST
   be set to that of the protected payload stream.

   So the FEC
   stream and the payload stream SHOULD be sent through two separate RTP
   session, and multiplexing them by payload type into one single RTP
   session SHOULD be avoided.  In addition, the FEC and the payload MUST
   NOT be multiplexed by SSRC into one single RTP session since they
   always have the same SSRC.

>From my perspective this override should be done in a separate document
so that also others can use it and not being RTCWEB specific.

Cheers

Magnus Westerlund

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