Subject: [AVT] comments on draft-ietf-avp-rtcp-feedback-01.txt

From: "Even ,Roni" <[email-masked]>
Date: Sun, 30 Dec 2001 17:28:35 +0200

Hi,
My understanding of the document is that the feedback mechanism is to allow
a feedback channel between the decoder and encoder to allow immediate repair
of media stream. After reading the document I have the following comments

1. In section 6.3 the text is "AVPF entities MUST include Generic INFO
messages along with any payload-specific ones in compound RTCP packets
(early as well as  regularly scheduled ones). "  This means that if an
encoder is capable of supporting "nack" it needs to support generic info.
The capability to support generic INFO is not explained in 4.2. In 6.2 it
says that "two general purpose transport layer feedback..." but three are
listed. In 6.2.3 it states that "The Generic INFO packet MUST only be used
in conjunction with an application-specific feedback message. " which
contradict the above text in 6.3.

2. The document leaves open the issue of what the senders should do with the
feedback. In the video implementations today we see two kind of feedbacks,
between send and receiver of media, commands and indications. My
understanding of the nack params (PLI and SLI) is that they are similar to
the commands, the preferred mode is for them to cause new intra picture or
MB. This params are for general video media and not for specific codec. The
indications for specific codec are left to the application layer feedback as
described in section 6.4. I suggest that the last sentence in 4.2 will be
changed to  "It is up to the recipients whether or not they send feedback
information and up to the sender(s) to make use of feedback provided unless
an action is required by the specific feedback message." The text in 6.3
will state that PLI and SLI will cause fast updates from the sender. This
will help with having a consistent response between sender and receiver.

3. In order to be able to use the messages in video switching scenarios we
need in 6.3 another payload specific message which is PIZ (Picture Freeze).
This is needed for completeness. Video switch involves freeze and fast
update and since we have here fast update it is better to have the full
support.

4. For information purpose, we have today in video codec specific
application the following indications used for resilience support, lost
picture and lost partial picture for H.263 annex U & W. Not decoded MB for
H.263 appendix I and video bad MB to let the encoder know that those MBs
cannot be used for prediction.

5. In section 4.3 the references to other sections like in the description
of pli, sli and rpsi are not correct.

Thanks

Roni Even

**********************************************
Roni Even
Polycom Network Systems
Tel: +972-3-9251200
Mobile: +972-55-481099
Fax: +972-3-9211571
Email: [email-masked]

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