Re: [MMUSIC] Review of draft-ietf-mmusic-sdp-simulcast-08

Bo Burman <bo.burman@ericsson.com> Tue, 04 July 2017 00:06 UTC

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From: Bo Burman <bo.burman@ericsson.com>
To: Cullen Jennings <fluffy@iii.ca>, "Multiparty Multimedia Session Control Discussion List" <mmusic@ietf.org>
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Thread-Topic: Review of draft-ietf-mmusic-sdp-simulcast-08
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Subject: Re: [MMUSIC] Review of draft-ietf-mmusic-sdp-simulcast-08
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Cullen,

Thank you for good and constructive comments! Please see my responses below.
Just posted an -09 that hopefully addresses most if not all of your comments.

Cheers,
/Bo (as individual)



> -----Original Message-----
> From: Cullen Jennings [mailto:fluffy@iii.ca]
> Sent: den 15 maj 2017 19:00
> To: Multiparty Multimedia Session Control Discussion List <mmusic@ietf.org>
> Cc: Bo Burman <bo.burman@ericsson.com>
> Subject: Review of draft-ietf-mmusic-sdp-simulcast-08
> 
> 
> Overall the actually protocol described by the spec seems fine with a few very small technical details. However, it is a bit
> challenging to understand the basic overview of how it works -  I have a few suggestions that I think would be not much
> work that would really help people get up to speed by moving some of the later example text to closer to where the
> overview section is now.
> 
> 
> Technical Issues -----------
> 
> Issue 1: Pause in an SDP re-offer
> 
> The pause in section 6.3.4 seems problematic. If A is sending a re-offer to B, there is no real way to know the pause state
> of a stream B is sending to A at that point without glare issues. Saying A has to send something that matches a state on B
> is not really possible to implement.
[BoB] Agree. This is clearly flawed and must be changed.

> 
> I think this should be changed to say A sets the pause flags in the SDP for RIDs A is sending to match what A currently
> thinks the state of them is, and A sets the the pause flags for the RIDs A is receiving to what A wished they were. The
> way that B processes the offer and creates the answer as well as the handling of the answer by A would be the same as
> the initial offer/answer.
[BoB] OK. SDP information for send direction RIDs becomes a snapshot in time at a session modification and should therefore be taken as informational. A further consequence should reasonably be that, for send direction RID that uses initially paused streams, RFC 7728 signaling is already present in the ongoing session and takes precedence in case of any ambiguity or conflict.
Agree for receive direction RID. The only difference from an initial offer would be if a receive direction RID is actually already paused by A (through RFC 7728 signaling) at the time of sending the updated SDP offer. The (minor) difference to an initial offer is then that pausing a receive direction RID in the initial offer would instead always cause the SDP answerer (RTP sender) to put the stream in _unsolicited_ pause (if the SDP answerer is accepting to pause).

> 
> Given the timing issues of RTCP near at the start of the call, if we don't make this change, it may take significant time to
> un-pause a particular stream at the start of the call. The above change allows that to happen so that if a user joined a call
> that perhaps had a paused presentation stream and wanted to un-pause that right away, they could.
[BoB] OK. Makes sense.

> 
> 
> Issue 2: More than one simulcast line
> 
> Some SDP stacks to not preserve the order of a=lines. I have no idea if this is a bug in them or not. I think it would be
> better to phrase this spec as each m= section MUST have at most one a=simulcast line. 
[BoB] OK

> The current phrasing of receiver
> ignores all but first just begs for people to put proprietary stuff in a second simulcast line.
[BoB] In that case, I assume it is better to say that simulcast is ignored entirely (removed in answer) for a media description with multiple a=simulcast lines?

> 
> 
> Issue 3: Relating simulcast streams using PT
> 
> When the PT are unique, they can be used instead of RID. Obviously, if they are not unique they can't be used, but when
> they are, they often result in being able to display the video sooner. Often RID/MID will not be included in every RTP
> packet because of the bandwidth usage and are instead sent periodically. Being able to join a conference and start
> displaying stuff right away is nice when possible
> 
> I think section 6.5 needs to be updated to be clear that "RTP "Simulcast streams MUST be related on RTP level through
> RID." still means it is fine for the RTP receiver to use the PT of the RTP packet and if that uniquely maps to a RID in the
> SDP, use that for the relation.
[BoB] OK. That was already possible since before, but to be entirely clear I'll add a note saying that.

> 
> 
> Editorial ----------------
> 
> 
> The use cases and requirements go for awhile and the first thing that starts to explain how this works is bullet point point
> 4 in the overview section which says
> 
>  o  The codec configuration for a simulcast stream is expressed
>       through use of separately specified RTP payload format
>       restrictions [I-D.ietf-mmusic-rid] with an associated RTP-level
>       identification mechanism [I-D.ietf-avtext-rid] to identify which
>       RTP payload format restrictions an RTP stream adheres to.  This
>       complements and effectively extends simulcast stream
>       identification and configuration possibilities that could be
>       provided by using only SDP formats as identifier.  Use of multiple
>       RTP streams with the same (non-redundancy) media type in the
>       context of a single media source, where those RTP streams are
>       using different RtpStreamId, is a strong but not totally
>       unambiguous indication of those RTP streams being part of a
>       simulcast.
> 
> Have a skim of the draft up to that point and ask yourself how much sense it would make it you get to this point?  Or how
> much you would understand about how the mechahims of this draft actually worked before you got to the ABNF shortly
> after this. I suspect that you will likely come to the conclusion that it's a bit hard to understand the big picture of how it
> works.
[BoB] OK, the above text is probably way too detailed to provide any benefit to the reader compared to just continue reading.

> 
> I really can't make head or tails of the paragraph I quoted above but the draft would make more sense to me if we
> removed that, along with rest of section 5. 
[BoB] To be clear, do you mean remove the entire section 5, or just the two last bullets? You mention below having an "overview" section.

> From the bullet point above, the draft goes straight into the ABNF for the new
> stuff. I think it would be easier to understand if we: Moved section 4 to appendix,
[BoB] OK, no problem.

> Greatly shortened section 3.
[BoB] Given the amount of discussion we have had to understand what "simulcast" is and why it is useful, I think the reader would benefit from this background. I'm not inclined to remove any of the use cases or motivations, but I'll consider shortening the text.

> Then add
> an overview that starts with showing the offer and answer from 6.6.1. and explaining how the only thing this draft adds is
> the a=simulcast line. 
[BoB] OK. I'd assume that the examples from 6.6.1 are a bit too detailed and could be simplified a bit in such high-level overview? I'll at least start something to discuss more around.

> Explain the one m line means one source concept. Then show how simulcast line allows sending the
> 1;2 RIDs. Then explain how alternatives work. From there jump into the details. I don't think this would be much new text
> and it would not change how anything works, just clear up the explanation.
[BoB] Yes, that would work.

> 
> Section 6.1. and 6.2 are confusing because they are written as if they are not for offer/answer SDP. I think it would be
> better to state up front as that this was offer/answer SDP only and write theses sections to be clear about if they are
> referring to offers or answers when talking about SDP. 
[BoB] The intent is clearly that both 6.1 and 6.2 should describe aspects of a=simulcast that are generally true, independent from offer and answer. Any text there that are applicable only to offer or answer is a mistake and should be moved into those sections.

> As a specific example, the pause discussion on page 13 might be
> correct for answers but looks less correct for offers.
[BoB] I find nothing in that text that does not apply both to offer and answer. The text was written to reflect the view of the sender of the SDP, in terms of stream directions, which should probably be more clearly stated. If that clarification does not help, could you please elaborate?

> 
> The SCID is really confusing in the draft. The draft is never fully clear about if this is a RID or not it calls them "identical too"
> but that hard to see if they are different but same value or work the same way or something else. I think we should
> remove the term SCID from the draft and just use RID. 
[BoB] I have no problem aligning identifiers, but "RID" does not exist. We once thought it would be called "RID", similar to "MID" from BUNDLE, but it didn't happen. "RID" also does not exist in draft-ietf-avtext-rid, where the information element ended up being called "RtpStreamId", both for the SDES Item and the RTP Header Extension carrying that SDES Item. The identifier on an "a=rid" line is called "rid-id" in draft-ietf-mmusic-rid, which is the very same value used on an "a=simulcast" line, so I assume this draft should be changed to be consistent with that naming. 

> Similarly, using RtpStreamID is confusing. I think we should just
> refer to that as the header that carries the RID.
[BoB] Formally, RtpStreamId is the name of the SDES Item in both RTCP and RTP header extension. It can be seen as the "container" that carries RID, but I don't think we can use RID as a name, as said above.

> 
> The example on the top of page 11 makes no sense without enough of the SDP to see the rids and m lines etc and needs
> to be broken apart to be a Offer example followed by the Answer back to that offer.
[BoB] OK, I expect that if any of the information around that example should be kept, it will likely best go into the new overview section you propose above.

> 
> NIT - define SFM on first use
[BoB] OK

> 
> 
>