Re: [AVTCORE] Question on multi-party RTT handling (draft-ietf-avtcore-multi-party-rtt-mix-02)
Gunnar Hellström <gunnar.hellstrom@omnitor.se> Wed, 27 May 2020 17:39 UTC
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From: Gunnar Hellström <gunnar.hellstrom@omnitor.se>
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Subject: Re: [AVTCORE] Question on multi-party RTT handling (draft-ietf-avtcore-multi-party-rtt-mix-02)
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Hi Yong, Den 2020-05-27 kl. 17:32, skrev Yong Xin: > > Hi Gunnar, > > Agreed, the mixer performance requirement of 5 simultaneous text > sources with less than 500ms delay should be sufficient. BTW, do you > expect to send the BCP document > https://tools.ietf.org/html/draft-hellstrom-avtcore-multi-party-rtt-solutions-00 > to IESG for approval around the same time (Feb/2021) as the standard > track document > https://tools.ietf.org/html/draft-ietf-avtcore-multi-party-rtt-mix-02? > The .....-solutions document has not yet gone through the procedure to be adopted as a working group draft. So, there is no current plan for its completion date. I have included a few words about the performance requirements in next version of draft-ietf-avtcore-multi-party-rtt-mix <https://tools.ietf.org/html/draft-ietf-avtcore-multi-party-rtt-mix-02> now. Are there any specific part of draft-hellstrom-avtcore-multi-party-rtt-solutions <https://tools.ietf.org/html/draft-hellstrom-avtcore-multi-party-rtt-solutions-00> that you see value of to see published? Regards Gunnar > Thanks, > > Yong > > *From:* Gunnar Hellström <gunnar.hellstrom@ghaccess.se> > *Sent:* Tuesday, May 26, 2020 11:01 PM > *To:* Yong Xin <Yong.Xin@radisys.com> > *Cc:* avt@ietf.org > *Subject:* Re: Question on multi-party RTT handling > (draft-ietf-avtcore-multi-party-rtt-mix-02) > > Thanks Yong for feedback, > > We discussed performance requirements initially. I have checked the > requirements from regulation outside of the emergency services. There, > there is only a requirement to identify the source and be able to > switch source smoothly in an ordely way between users taking turns > properly. > > In practice I know that there will be some simultaneous typing, at > least for very brief indications, like "I want to say something" or > "that suits me". So, good switching performance without much extra > delay between 5 simultaneous typing users as the documented goal in > draft-hellstrom-avtcore-multi-party-rtt-solutions-00 seems to be a > safe and maybe a bit high requirement. > > With the intended use for emergency services, the situation is similar > there. I expect eager users in emergency to sometimes type longer > explanations while they get instructions. But instead fewer extra > persons jumping in with comments. So also there, switching without too > much extra delay between a maximum of 5 simultaneous senders seem to > be a safe and a bit high requirement. > > Regards > > Gunnar > > Den 2020-05-26 kl. 19:12, skrev Yong Xin: > > Thanks Gunnar for the clarification and suggested change in next > revision – they all look good to me > > Regards, > > Yong > > *From:* Gunnar Hellström <gunnar.hellstrom@ghaccess.se> > <mailto:gunnar.hellstrom@ghaccess.se> > *Sent:* Saturday, May 23, 2020 6:24 AM > *To:* Yong Xin <Yong.Xin@radisys.com> <mailto:Yong.Xin@radisys.com> > *Cc:* avt@ietf.org <mailto:avt@ietf.org> > *Subject:* Re: Question on multi-party RTT handling > (draft-ietf-avtcore-multi-party-rtt-mix-02) > > > > > The e-mail below is from an external source. Please do not open > attachments or click links from an unknown or suspicious origin. > > Hi Yong, please see inline, > > Den 2020-05-23 kl. 01:31, skrev Yong Xin: > > Thanks for the quick response. The latest spec does > address my concern. I have some follow-up questions: > > * The new payload format “text/rex” can be used with or > without redundancy. When redundancy is used, mixer has > to use the same redundancy level when transmitting > texts from multiple sources. If the different party in > the same conference has negotiated a different > redundancy level, the mixer has to pick the lowest > level to use, right? > > No. There are two sides of this answer: > > The mixer should do separate mixing for each recipient, using > the redundancy level agreed with each recipient. This is also > because the users do not want to see their own transmitted > text being received from the mixer. The own text is displayed > locally by the endpoints. If the recipient does not support > "text/rex", the mixer also need to do the mixing for > multi-party unaware endpoints using the "text/red" format > described in section 13.2. > > */[YX] Understood, the text mixer is providing N-1 mixing, > similar to audio mixing, so the user never receive their own > transmitted text from the mixer./* > > */[GH] /*Yes. I see I have that specified for the multi-party > unaware mixing in 13.2.2 by this sentence: "Text received from a > participant SHOULD NOT be included in transmission to that > participant." I suggest I include a similar sentence in 13.1 for > the multi-party aware case. > > And the mixer must recover from loss in reception from each > source and create a queue of clean text from each source > before composing the packets for transmission. The mixer > cannot just resend received packet contents with redundancy, > because the recovery mechanism requires the sequence number > gaps for loss detection, and the mixer must create its own > sequence number series in the transmission. > > */[YX] I agree what you said here. I think I’m little confused > when reading the following paragraph in section 3 of the spec. > Let me put an example, there’s a 3-party conference and all > participants (A, B, C) are conference-aware RTT terminals and > support text/rex packet format. User A, B, C negotiates > different redundancy level 2, 3, 1 respectively. When mixer > transmitting text (source from B & C) to user A, what is the > number of redundant generations should be used by the mixer in > the transmitted packet? Is it 2 or 1? /* > > [GH] It is 1. I see my wording is confusing. I suggest to improve > the yellow sentence fron the paragraph below to read: "/ItSHOULD > be set to the minimum of the number declared by the two parties in > the SDP exchange."/ > > It can be discussed if the minimum of what the parties declared is > the best choice. It is quite common that SDP parameters declare > what the party wants to receive. In this case the number can be > decided by the party knowing the network conditions in the > network, or it can know it has decoding limitations and does not > want to receive more than a specific number of generations in the > packets. It may also have coding limitations, so that it cannot > create more generations than itself can receive. That made me > think that the resulting number of generations sent should be the > minimum of what the two parties in each connection declared. I can > change that to say that "It SHOULD be set to what the other party > declared". What is actually used in the stream is found by > dividing the number of data header entries with the number of > members in the CSRC list. > > This discussion is a bit theoretical, because the default is one > primary and two redundant generations, and there are rarely any > reason to deviate from that. > > *//* > > / The number of redundant generations of T140blocks to > include in/ > > / transmitted packets SHALL be deducted from the SDP > negotiation. It/ > > /SHOULD be set to the minimum of the number declared by the > receiver/ > > /and the transmitter//. The same number of redundant > generations MUST/ > > / be used for all sources in the transmissions. The number of/ > > / generations sent to a receiver SHALL be the same during > the whole/ > > / session unless it is modified by session renegotiation./ > > * But in case there’s one party has negotiated > “text/rex” without any redundancy level, does that > mean mixer has to turn of the redundancy for this > conference? Does mixer need to change the redundancy > level up and down dynamically as user joins or leaves > the conference? Does mixer need to send re-INVITE to > re-negotiate the redundancy level with other party > when such change happens? > > From the logic above, the answers on these questions are: no. > I realize that an explanation should be inserted in the > beginning of section 3. "Actions at transmission by the mixer" > to clarify that the source for transmission from the mixer is > clean text in separate queues regardless of which format or > protocol they used in the individual receptions. > > > > */[YX] This is related to the above question. Some > clarification in the spec would be helpful../* > > * In section 12, I noticed the 150cps recommendation is > still there and has been made as default value for the > new packet format, but the transmission interval is > back to 300ms (the recommended interval was 100ms in > the old spec). I guess with the new packet format, it > is not required to use the shorter transmission > interval any more. > > The transmission interval is mentioned in two paragraphs in > section 3. One saying that the default is 300 ms, the other > saying: > > "For multi-party operation, it is RECOMMENDED that the mixer > sends a packet to each receiver as soon as text has been > received from a source as long as the maximum number of > characters per second indicated by the recipient is not > exceeded, and also the number of packets sent per second to a > recipient is kept under a specified number.. This number > SHALL be 10 if no other limit is applied for the application. > The intention is to keep the latency introduced by the mixer low." > > This is intended to create a balance between low latency and > protection against bursty packet loss. Even if the latency > requirements from real-time text users are much lower than > from audio and video users, a low latency is appreciated, and > latency of over 2 seconds end-to-end creates conversation > problems. Therefore, this paragraph about when to transmit > will self-regulate to about 100 ms packet interval from about > 3 simultaneous typing sources. > > The 300 ms default assures that the remaining redundancy > transmissions will be sent even shortly after all sources have > stopped typing. > > */[YX] So are you saying the recommended transmission interval > in the new spec is still 100ms, and the 300ms is actually the > time that covers 3 transmissions (one primary transmission > plus 2 redundancy transmissions, assuming redundancy level 2), > is my understanding correct? I guess this is better clarified > in the new spec. The old spec is much clear in this definition. /* > > */[GH]/*The intention now is to not have a fixed interval, but > have it variable between 100 and 300 ms. This is in order to not > add delay by the mixer when it is possible to avoid it. > > So, when the mixer receives a packet, it checks for each party it > is sending to, if it already has sent 10 packets to that party > during the last second. If not, it sends the just received text > immediately on to that party together with whatever redundant text > there was to be sent. If it already had sent 10 packets, then this > text needs to wait in queue until 300 ms has passed from the > latest transmission and then sent together with any other new text > and the redundancy. Whoops, there was a logic error in that > algorithm. It can allow a burst of 10 packets sent to one party > with very short intervals, and then three more packets during the > same second with 300 ms interval. That causes a risk for loss of > text if a short burst loss happens when the 10 packets are sent. > And it results in 13 packets sent when the limit was intended to > be 10. > > So, it is likely better to say in chapter 3: > > > " A "text/rex" transmitter SHOULD send packets distributed in time > as long as there is something (new or redundant T140blocks) to > transmit. The maximum transmission interval SHOULD then be 300 > ms. It is RECOMMENDED to send a packet to a receiver as soon as > new text to that receiver is available, as long as the time after > the latest sent packet to the same receiver is more than 100 ms, > and also the maximum character rate to the receiver is not > exceeded. The intention is to keep the latency low while keeping a > good protection against text loss in bursty packet loss conditions." > > The introduced delay for up to 16 simultaneously sending users > will be between 0 and 100 ms. I think it is good that a middlebox > like this does not introduce more latency. I hope we can have a > discussion if we instead get too high risk for text loss in case > of bursty packet loss. With intensive participants the coverage > for such loss is only 200-300 ms, while at low traffic it is > 600-900 ms. > > *//* > > However, this algorithm may make the protection against bursty > loss weaker than with a steady 300 ms interval. With between > 17 and 32 simultaneous typing users, the latency caused by the > mixer will be around 300 ms and then passes both regulatory > and human requirements. > > Now, even if it passes the requirements, 32 is a very > unrealistic number of simultaneous typing users. In audio > conferences it is only possible to perceive one source at a > time well. The benefit of enabling more is just for noticing > that someone else want to say something. Requirements for this > work are collected in > draft-hellstrom-avtcore-multi-party-rtt-solutions-00, and > there the performance requirements are set to be valid for up > to 5 simultaneously transmitting users and the delay caused by > the mixer to be less than 500 ms. I think we should design for > these figures. > > * And you mentioned these characteristics provide for > smooth flow of text with acceptable latency from at > least 32 sources simultaneously. Since the new packet > format can support up to 16 sources per packet, the > text from 32 sources will have be transmitted in turn. > If my calculation is correct, with 300ms transmission > interval and redundancy level 2, it will take 900ms > (one primary + 2 redundant) for mixer to switch from > first 16 sources to next 16 sources, so the delay is > about 900ms. Is this the acceptable latency in your mind? > > See discussions above. Maybe regulators need to say how many > simultaneous users the requirements are for. I think 5 is a > high and good figure even if the discussion above indicates 32 > to be possible. > */[YX] Yes I agree, at least for emergency type of service, I > don’t see a multi-party use case that requires more than 5 > parties /* > > [GH] I want to create a specification that is regarded sufficient > for all realistic use cases. The most commonly foreseen emergency > service use case has only two occsionally simultaneously > transmitting users. > > It is hard to imagine a use case with 5 simultaneously sending > participants, where it would be important for a user to be able to > read text from one source with less than one second delay. > > It does not happen for audio. As soon as two talks, the result is > usually not perceivable.(Except in the efforts you experience now > in Corona-times, with a choir of individual conference > participants trying to sing together.) > > With video, you cannot concentrate on what more than one does. But > the mixer can usually present up to 9 or 16 or so with maintained > temporal resolution but less spatial resolution. The receiver > selects one to perceive. > > One application that could cause many simultaneous sources of text > would be a conference with voice to text translations in many > languages for a large audience. That will require a user interface > at the receiving end that shows one selected text stream and hides > everything else. I imagine that there are other mechanisms for > that already. It cannot be the application we design for, but I > dont mind if it will be possible. > > This reasoning tells again that a goal of 5 simultaneous text > sources with less than 500 ms delay is high but maybe realistic in > some situation, and our solution that provides for 16 simultaneous > transmitting users and 0-100 ms delay meets the requirement well. > I do not think we will need to mention the 32 simultaneously > transmitting users still meeting the requirements. > > I will adjust the performance chapter and the introduction for this. > > [GH] Thanks. > > -- > > Gunnar Hellström > > GHAccess > > gunnar.hellstrom@ghaccess.se <mailto:gunnar.hellstrom@ghaccess.se> > > -- > Gunnar Hellström > GHAccess > gunnar.hellstrom@ghaccess.se <mailto:gunnar.hellstrom@ghaccess.se> > > _______________________________________________ > Audio/Video Transport Core Maintenance > avt@ietf.org > https://www.ietf.org/mailman/listinfo/avt -- + + + + + + + + + + + + + + Gunnar Hellström Omnitor gunnar.hellstrom@omnitor.se +46 708 204 288
- Re: [AVTCORE] Question on multi-party RTT handlin… Yong Xin
- [AVTCORE] Question on multi-party RTT handling (d… Yong Xin
- Re: [AVTCORE] Question on multi-party RTT handlin… Gunnar Hellström
- Re: [AVTCORE] Question on multi-party RTT handlin… Gunnar Hellström
- Re: [AVTCORE] Question on multi-party RTT handlin… Yong Xin
- Re: [AVTCORE] Question on multi-party RTT handlin… Gunnar Hellström
- Re: [AVTCORE] Question on multi-party RTT handlin… Yong Xin
- Re: [AVTCORE] Question on multi-party RTT handlin… Gunnar Hellström
- Re: [AVTCORE] Question on multi-party RTT handlin… Yong Xin
- Re: [AVTCORE] Question on multi-party RTT handlin… Gunnar Hellström
- Re: [AVTCORE] Question on multi-party RTT handlin… Yong Xin