Re: [MMUSIC] pause/resume in draft-ietf-mmusic-sdp-simulcast
Bo Burman <bo.burman@ericsson.com> Wed, 04 September 2019 10:30 UTC
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From: Bo Burman <bo.burman@ericsson.com>
To: Matthew Kaufman <mkaufman@bluejeans.com>, "mmusic@ietf.org" <mmusic@ietf.org>
Thread-Topic: pause/resume in draft-ietf-mmusic-sdp-simulcast
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Date: Wed, 04 Sep 2019 10:30:27 +0000
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Subject: Re: [MMUSIC] pause/resume in draft-ietf-mmusic-sdp-simulcast
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Hi Matthew, Long overdue and with apologies for not realizing this was unanswered and therefore not responding earlier, please see my comments inline below /Bo (as individual) From: mmusic <mmusic-bounces@ietf.org> On Behalf Of Matthew Kaufman Sent: den 10 juli 2019 01:09 To: mmusic@ietf.org Subject: [MMUSIC] pause/resume in draft-ietf-mmusic-sdp-simulcast Issue 1 - Shouldn't stream pause/resume be required? If I'm a SFM and you are sending me multiple simulcast streams and I decide that nobody is watching one of them, it is wasteful of bandwidth and power (CPU) for you to continue sending a stream I don't want. Likewise if I'm a client and you're sending me simulcast streams directly. I believe that for this reason, pause/resume should be mandatory in this draft. All of the below issues rely on having pause/resume messaging for their successful resolution. [BoB] This makes a whole lot of sense to me, but I seem to remember that this was proposed and discussed before during the development of this draft and was rejected for some reason that I cannot remember or find any record of right now. I however also dont think it would be too hard to require simulcast and pause instead of having pause implicitly included in a simulcast requirement. Furthermore, I believe that we must require that TMMBR 0 NOT be used for pause of simulcast streams, because of the relevant note in section 5.5 of RFC 7728. (See Issue 3 below) [BoB] Fully agree that RFC 5104 TMMBR 0 is a crippled way of pausing that not only impacts the device sending TMMBR 0 but also impacts other conference participants by preventing the media sender from making intelligent choices that would be possible if RFC 7728 PAUSE was used instead. IMHO, the only motivation for allowing TMMBR 0 as pause is to provide a limited pause service to devices that implements RFC 5104 but not RFC 7728. That can be feasible in some cases, e.g. if many devices have TMMBR 0 support but none has PAUSE support, but in many other cases the downsides with TMMBR 0 outweighs much of the benefits. Issue 2 - What to send in the media (or signaling) layer when bitrate adaptation says there's not enough bandwidth to send? If I'm a SFM and I'm receiving your three streams that you are sending (ex., 180P, 480P, 720P all with temporal scaling) and you decide that you can't send the 720P any more, how can I tell the difference between "sender ran out of bandwidth" and "sender is broken"? In particular, if I have subscribers to your 720P stream, and you ran out of bandwidth, I need to know ASAP to start forwarding them the 480P stream instead. Thus an indication that you have paused, preferably in the media stream itself, would be useful... and furthermore, it would be nice to know why you've chosen to do that. This can be (largely) solved by requiring senders to use the PAUSED indication for this case, and calling that out explicitly in section 7.1 (or elsewhere) of the draft (though as a receiver, it would be nice to know the difference between the various reasons a sender might have appeared to me to have unilaterally decided to pause) [BoB] Agree that it makes a lot of sense that a media sender announces a unilateral pause, be it in a simulcast case or not, using PAUSED or TMMBN 0 (section 5.6 of RFC 7728 <https://tools.ietf.org/html/rfc7728#section-5.6> ). Section 7.1 already says that the sender can pause streams that can no longer be sent, if the media sender has pause capability. You want this can to be a MUST (or a SHOULD)? At this very late point in the publication process, I doubt we can do changes that are not essential corrections or omissions. Do you consider current text broken? Could you also elaborate on why knowing the reason for an unilateral pause would matter to the SFM? The sender could be limited in bandwidth, in processing resources, by a broken codec instance, or from something else. I may be naïve but if 720P stops arriving for some reason and 480P is still available, isnt it just a matter for the SFM of accepting the fact and forwarding what is available? Issue 3 - What to send when bitrate adaptation says there's not enough bandwidth to send *and* pause has been requested for a lower bandwidth stream? If I'm a SFM and I'm receiving your three streams (ex., 180P, 480P, and 720P) and I note that my viewers are only watching your 720P it would save bandwidth and CPU to send you a pause for the 180P and 480P streams. Once I request that of you, and these are paused, what are you allowed to do when bandwidth adaptation determines that there isn't sufficient bandwidth to send the 720P? Can you unilaterally un-pause the 480P stream in order to keep sending video, or are you forced to stop the 720P and hope the SFM guesses properly that it needs to request un-pause of the 480P? Likewise, what if the lower resolution streams were signaled as initially paused? Can the sender unilaterally un-pause one in order to use it instead of a higher-bandwidth choice? I believe the answer here lies in RFC 7728, where an RTP sender may due to local considerations choose to resume at any time, [BoB] Agree. however because TMMBR 0 prohibits this, TMMBR 0 must not be used to pause simulcast streams that a remote sender may need to begin using in order to bandwidth-adapt. [BoB] If the SFM used TMMBR 0 to pause the lower resolution streams, both the SFM and the media sender would also know that they cannot be unilaterally resumed. The SFM would likely also get a hint from the media stream itself that the 720P stream is in trouble; serious bitrate reduction from the sender and/or sudden increase of packet loss and/or sudden increase of packet jitter. The SFM should then be capable to realize that it has the option to either continue receiving this troublesome 720P, or actively pause the 720P and choose to resume one of the 480P or 180P instead. Also, what mandates what is negotiated as a 720P to stay as a 720P in case theres serious transport trouble? Couldnt the media sender decide to decrease the resolution at some point of the bitrate adaptation process? I agree that some of the transport adaptation considerations ends up in the SFM when using TMMBR 0, which would otherwise and probably better (using RFC 7728 PAUSE) remain in the media sender, but it doesnt seem entirely undoable. This required behavior (unilateral unpause if necessary) should be noted in section 7.1 of the draft. [BoB] While clearly sensible and desirable behavior, I doubt this can be seen as more than an optimization / clarification, which may not warrant text changes at this late point in the publication process. As one of the authors of -simulcast Im however open to make such clarifications if the IESG permits and the WG so decides. Matthew Kaufman
- [MMUSIC] pause/resume in draft-ietf-mmusic-sdp-si… Matthew Kaufman
- Re: [MMUSIC] pause/resume in draft-ietf-mmusic-sd… Bernard Aboba
- Re: [MMUSIC] pause/resume in draft-ietf-mmusic-sd… Magnus Westerlund
- Re: [MMUSIC] pause/resume in draft-ietf-mmusic-sd… Bo Burman