RTP: Microsoft x-pli

[Haakon Sporsheim]

Hi.

I'm trying to add support for Microsoft extension of RTCP feedback messages
for Picture Loss Indication (PLI). This is based upon the standard
payload-specific feedback message specified by RFC4585 [1], but has extra
Feedback Control Information (FCI) [2].

For sending RTCP feedback this is easily implemented by introducing a new
field for caps of the incoming RTP stream as given in the pt-map of
GstRTPSession. Currently there are two existing fields; "rtcp-fb-nack-pli"
and "rtcp-fb-ccm-fir" which enables respectively PLI and/or Full Intra
Request (FIR). My suggestion is to introduce "rtcp-fb-x-message-x-pli"
which enables sending of the Microsoft specific PLI. [3]

For receiving on the other hand I'm not so sure what to do. As far as I
know the rtcp feedback fields is not present in output caps, so I have no
way of knowing if I should expect a regular PLI or Microsoft PLI. Well, I
could obviously just check if there are FCI data and assume that if there
are 3x 32bit words, this must be MS PLI, but that sounds like a bad
solution. Read; hack.
So the question is, what do you think about actually checking caps of the
RTP being sent. If the "rtcp-fb-x-message-x-pli" is present then we enable
Microsoft PLI instead of regular PLI. Would this work out for everyone? So
to enable the Microsoft x-pli one should make sure that the RTP caps coming
in to GstRTPSession on the send_rtp_%u pad has the correct field.

Please let me know what you think. I'll work out a patch in the coming
days, and would be happy for any input as I go. Thanks!

Best regards,
Haakon Sporsheim
Pexip AS

[1] http://tools.ietf.org/html/rfc4585
[2]
http://download.microsoft.com/download/1/6/F/16F4E321-AA6B-4FA3-8AD3-E94C895A3C97/[MS-RTP].pdf
(section: 2.2.12.1)
[3] http://msdn.microsoft.com/en-us/library/hh659616(v=office.12).aspx
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