Subject: [Sipping] SDP fmtp video negotiation

From: "Erez Morabia" <[email-masked]>
Date: Sun, 20 Aug 2006 06:32:22 +0300

Hi,

I'm trying to find here a common base for the negotiation regarding the
different codecs on the video media side.

It seems that when you advertise fmtp, you declare what you are willing
to receive (decoder capabilities) and implicitly what you prefer to send
(except for some H264 exceptions).

The thumb rule is that when receiving an offer, and there is
intersection between the offerer and offeree capabilities, then video
channels can be opened in both directions. 

Is my observation correct?

Example:

Alice supports QCIF(MPI=2) Maximum-Bit=Rate=256Kbps

Bob supports CIF(MPI=1) QCIF(MPI=1) Maximum-Bit=Rate=128Kbps

Scenario 1:

Alice offers: QCIF(MPI=2) Maximum-Bit=Rate=256Kbps

Bob answers: QCIF(MPI=2) Maximum-Bit=Rate=128Kbps (or CIF(MPI=1)
QCIF(MPI=1) Maximum-Bit=Rate=128Kbps).

Is that a correct negotiation?

Scenario 2:

Bob offers CIF(MPI=1) QCIF(MPI=1) Maximum-Bit=Rate=128Kbps.

Alice answers: QCIF(MPI=2) Maximum-Bit=Rate=128Kbps (or QCIF(MPI=2)
Maximum-Bit=Rate=256Kbps).

Is that a correct negotiation?

In both scenarios, both incoming and outgoing channels should be opened
correctly with QCIF(MPI=2) Maximum-Bit=Rate=128Kbps.

BTW

The drafts/RFCs I'm using are:

H.261: draft-ietf-avt-rfc2032-bis-13.txt

H.263: draft-koskelainen-sdp263-01.txt (I'm not sure about that one,
does the H.263+ draft overrides it?)

H.263+: draft-ietf-avt-rfc2429-bis-09.txt

H.264: RFC 3984

Thanks,

Erez

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