Re: [MMUSIC] [rtcweb] Working Group Last Call: draft-ietf-rtcweb-jsep-19.txt - Appendix B

Magnus Westerlund <magnus.westerlund@ericsson.com> Sun, 26 March 2017 15:45 UTC

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To: Peter Thatcher <pthatcher@google.com>, Ted Hardie <ted.ietf@gmail.com>, "rtcweb@ietf.org" <rtcweb@ietf.org>, Sean Turner <sean@sn3rd.com>, Cullen Jennings <fluffy@cisco.com>, "mmusic (E-mail)" <mmusic@ietf.org>
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From: Magnus Westerlund <magnus.westerlund@ericsson.com>
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Date: Sun, 26 Mar 2017 10:44:51 -0500
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Subject: Re: [MMUSIC] [rtcweb] Working Group Last Call: draft-ietf-rtcweb-jsep-19.txt - Appendix B
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Den 2017-03-24 kl. 16:46, skrev Peter Thatcher:
> To address your first two issues, I have made two PRs for JSEP:
>
> https://github.com/rtcweb-wg/jsep/pull/627
>
> https://github.com/rtcweb-wg/jsep/pull/628
>
>
> Please take a look and see what you think.
>

See other thread and on these PRs.

>
>
> On the issue of being a packet-based algorithm, I think the existing
> text addresses that fairly well:
>
>       Applications can implement RTP stacks in many different ways.
>       The algorithm below details one way that demultiplexing can be
>       accomplished, but is not meant to be prescriptive about exactly
>       how an RTP stack needs to be implemented. Applications MAY use
>       any algorithm that achieves equivalent results to those described
>       in the algorithm below.
>

Yes, that is what I required for being okay with this. And as I said, I 
accept that is looks like it does, but I would very much have preferred 
another way of expressing it, that is RTP stream centric.

>
> On the issue of future extensions of RTCP, I feell confident that the
> authors of future extensions can write their text in a way that works
> well with this algorithm if it's necessary.

Most likely, but there can clearly be cases which doesn't match existing 
patterns. And I think part of my issue here is there might exist an 
expectation that an RTP/RTCP extension also needs to update BUNDLE. 
Which I think is unfortunate.


Cheers

Magnus

>
> On Fri, Mar 24, 2017 at 3:49 AM Magnus Westerlund
> <magnus.westerlund@ericsson.com <mailto:magnus.westerlund@ericsson.com>>
> wrote:
>
>     Hi,
>
>     This email is specifically concerning Appendix B and I CC MMUSIC as the
>     text is being transfered to BUNDLE.
>
>     First Issues that I think needs to be addressed.
>
>     1. PT based mapping:
>
>            If the packet's SSRC is in the incoming SSRC mapping table, check
>            that the packet's PT matches a PT included on the associated "m="
>            line.  If so, route the packet to that associated "m=" line and
>            stop; otherwise drop the packet and stop.
>
>     I think the text should be clearer on the limitation of PT based mapping
>     versus MID based, that following this algorithm, an SSRC is not possible
>     to move from one media description (m=) to another after the initial
>     mapping. This probably belong to the earlier discussion of PT based
>     mapping, but the above quote is the reason for that limitation.
>
>     2. RTCP BYE handling:
>
>         If the packet is of type BYE, it indicates that the RTP streams
>            referenced in the packet are ending.  Therefore, for each SSRC
>            indicated in the packet that is found in the incoming SSRC table,
>            first deliver a copy of the packet to the "m=" line associated
>            with that SSRC, but then remove the entry for that SSRC from the
>            incoming SSRC table.
>
>     This has been discussed on the list, but I want to reinforce that the
>     more reasonable action is to mark the entry for removal, and then after
>     a short timeout (some seconds), allowing any re-ordered or delayed
>     packet to arrive and be processed, remove the entry.
>
>     3. I think it is time that this text is moved into bundle and removed
>     from JSEP document.
>
>
>     Then I have a reservation against this text. I call it a reservation as
>     it is something I do not require a text change for, but which I wished
>     was addressed. But at the same time I have been unable to properly
>     engage and work on an updated text that helps address the issue.
>
>     My reservation is that the text is written from a very RTP/RTCP packet
>     centric way, and from the perspective of a highly integrated
>     implementation. I would much have preferred that it was written in an
>     RTP stream centric way and from the perspective of a modularized RTP
>     stack implementation with some abstract API between the higher layers
>     consuming the RTP streams and handling any RTCP feedback messages. That
>     way the focus could have been on when to create and update the RTP
>     stream to higher layer "m=" association. RTCP handling is unfortunately
>     very dependent on what API model one has, and is therefore tricky to
>     describe as it is dependent on the API, and what it provides versus
>     hides.
>
>     The risk with the text as it is currently written is that the
>     implementers of WebRTC endpoints that uses generic RTP/RTCP
>     implementation modules may have significant issues with mapping the
>     current text to the API they will see.
>
>     The second aspect of the text which may be problematic is how future
>     proofing of it in regards to RTCP. If new RTCP feedback messages,
>     Extended Reports (XR) or other new RTCP functionality is introduced it
>     may be challenging to map that to the new messages. However, I thank the
>     authors for especially improving this aspect in regards to RTCP feedback
>     messages.
>
>     Cheers
>
>     Magnus Westerlund
>
>     ----------------------------------------------------------------------
>     Media Technologies, Ericsson Research
>     ----------------------------------------------------------------------
>     Ericsson AB                 | Phone  +46 10 7148287
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-- 

Magnus Westerlund

----------------------------------------------------------------------
Media Technologies, Ericsson Research
----------------------------------------------------------------------
Ericsson AB                 | Phone  +46 10 7148287
Färögatan 6                 | Mobile +46 73 0949079
SE-164 80 Stockholm, Sweden | mailto: magnus.westerlund@ericsson.com
----------------------------------------------------------------------