Re: [rtcweb] RTP Usage: Is RTP Retransmission REQUIRED or RECOMMENDED
Stefan Hakansson LK <stefan.lk.hakansson@ericsson.com> Fri, 06 July 2012 07:08 UTC
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Date: Fri, 06 Jul 2012 09:08:59 +0200
From: Stefan Hakansson LK <stefan.lk.hakansson@ericsson.com>
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Subject: Re: [rtcweb] RTP Usage: Is RTP Retransmission REQUIRED or RECOMMENDED
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On 07/05/2012 07:10 AM, Randell Jesup wrote: > On 7/4/2012 10:23 AM, Stefan Hakansson LK wrote: >> On 07/02/2012 11:51 PM, Randell Jesup wrote: >>> >>>> Again, this is about making _support_ for retransmission a requirement, >>>> not _use_. >>> >>> >>> If you don't mandate use, what does mandating "support" buy you? An >>> implementation (see above about PSTN gateways) might always decline to >>> use retransmission, so what does MUST (REQUIRE) buy you? I say SHOULD >>> (RECOMMEND), with a statement that it's strongly recommended it be >>> supported if video is supported, especially for non-single-purpose >>> devices (i.e. browsers). >>> >>> In either case, you need to think about who decides to use re-xmit or >>> not, what influences that, and if the app has visibility into this. >>> Someone could as an rtx-time=0 to the SDP. Or not include one, but >>> never actually retransmit a packet regardless of NACKs - it could just >>> repair via IDR (or other means). >>> >>> So here's a real problem related to retransmission: since the request is >>> not specific and negotiated (it's normally a generic NACK), the receiver >>> who sent the NACK has no idea if it will get a retransmission, an IDR >>> (or IDR slice), another form of repair (pframe/slice against >>> known-decoded frame - rpsi/etc), or nothing at all (NACK lost). So, it >>> doesn't know if it should freeze the video or play with errors. It has >>> to decide if it *thinks* the sender will use retransmission, or if it >>> doesn't, does the receiver want to freeze video until an IDR or other >>> repair is done, which might be a while. >> >> Isn't retransmission negotiated using "rtx", "atp" and so on? > > Support for it is. However, negotiating support for it doesn't solve > the problem of not knowing how the sender will respond to a NACK (or > SLI/RPSI/etc). Generally, the sender has to make the decision, but the > receiver has to guess. > >> >> One way of removing the uncertainty would be to require that all >> browsers support RTP retransmission: This way, any RTP endpoint >> sending a NACK to a browser would know that that packet would be >> retransmitted (given that congestion does not prohibit, or NACK packet >> is lost etc.). > > Um, no. Just because both sides support retransmission does not mean > that a NACK will generate a retransmission. Retransmission only makes > sense in some instances, and there's no guarantee that retransmission > will be possible (for example, the requested packet might have timed out > and been thrown away). All I was trying to say was that mandating support for retransmission would remove one uncertainty. But I agree that there are situations when there still would be no retransmission (and the receiver would not know why). But I guess you're also saying that mandating it would not be helpful since the sender decides, and could in principle have support for retransmission, but being set up/implemented/tuned in such a way that it (almost) never retransmits. I didn't think about that; and I don't know how big that risk is. > >> >> If we have this in place, I'm confident that implementors will quickly >> develop strategies on how and when to _use_ retransmission in the best >> way. There has already been input made on its usefulness. > > Sure, and they will anyways. RECOMMENDED or REQUIRED, it really doesn't > matter to the implementation. For that matter, even if REQUIRED there's > nothing that says it has to be offered or accepted. Even if that were > mandated, the implementation could accept but with a 0-length rtx-time. > So it's really unclear what REQUIRED buys you that RECOMMENDED doesn't. > >> >> Maybe the problem with legacy not always supporting retransmission is >> not that big. First of all, legacy is to a large extent audio only >> (e.g. PSTN), and no one is arguing for retransmission there. Secondly, >> I fail to see that this situation would be improved in any way if >> browsers are only recommended (and not required) to support RTP >> retransmission. >> >> So I think we should require rtcweb capable browsers to support RTP >> retransmission. >> >>> >>> We also need to think about when a receiver should use SLI or RPSI. PLI >>> can also be used and should be if the other options don't negotiate (so >>> it's important to offer it). Certainly it's more work to keep track of >>> the contents of each packet (slices, etc), especially in things like >>> H.264 NALU aggregation packets, than it is to just respond to an SLI. >>> >>> That said, my previous systems just used NACK and let the sender decide >>> the correct repair, but I wasn't leveraging slices and wasn't using >>> retransmission, so there was only minor complexity (looking at possible >>> reference frames). > >
- [rtcweb] RTP Usage: Is RTP Retransmission REQUIRE… Magnus Westerlund
- Re: [rtcweb] RTP Usage: Is RTP Retransmission REQ… Fabio Pietrosanti (naif)
- Re: [rtcweb] RTP Usage: Is RTP Retransmission REQ… Bernard Aboba
- Re: [rtcweb] RTP Usage: Is RTP Retransmission REQ… Cameron Byrne
- Re: [rtcweb] RTP Usage: Is RTP Retransmission REQ… Colin Perkins
- Re: [rtcweb] RTP Usage: Is RTP Retransmission REQ… Fabio Pietrosanti (naif)
- Re: [rtcweb] RTP Usage: Is RTP Retransmission REQ… Roy, Radhika R CIV (US)
- Re: [rtcweb] RTP Usage: Is RTP Retransmission REQ… Fabio Pietrosanti (naif)
- Re: [rtcweb] RTP Usage: Is RTP Retransmission REQ… Roy, Radhika R CIV (US)
- Re: [rtcweb] RTP Usage: Is RTP Retransmission REQ… Colin Perkins
- Re: [rtcweb] RTP Usage: Is RTP Retransmission REQ… Cameron Byrne
- Re: [rtcweb] RTP Usage: Is RTP Retransmission REQ… Colin Perkins
- Re: [rtcweb] RTP Usage: Is RTP Retransmission REQ… Fabio Pietrosanti (naif)
- Re: [rtcweb] RTP Usage: Is RTP Retransmission REQ… Cameron Byrne
- Re: [rtcweb] RTP Usage: Is RTP Retransmission REQ… Fabio Pietrosanti (naif)
- Re: [rtcweb] RTP Usage: Is RTP Retransmission REQ… Roni Even
- Re: [rtcweb] RTP Usage: Is RTP Retransmission REQ… Magnus Westerlund
- Re: [rtcweb] RTP Usage: Is RTP Retransmission REQ… Michael Welzl
- Re: [rtcweb] RTP Usage: Is RTP Retransmission REQ… Roy, Radhika R CIV (US)
- Re: [rtcweb] RTP Usage: Is RTP Retransmission REQ… Roy, Radhika R CIV (US)
- Re: [rtcweb] RTP Usage: Is RTP Retransmission REQ… Magnus Westerlund
- Re: [rtcweb] RTP Usage: Is RTP Retransmission REQ… Roni Even
- Re: [rtcweb] RTP Usage: Is RTP Retransmission REQ… Roni Even
- Re: [rtcweb] RTP Usage: Is RTP Retransmission REQ… Roy, Radhika R CIV (US)
- Re: [rtcweb] RTP Usage: Is RTP Retransmission REQ… Cameron Byrne
- Re: [rtcweb] RTP Usage: Is RTP Retransmission REQ… Roy, Radhika R CIV (US)
- Re: [rtcweb] RTP Usage: Is RTP Retransmission REQ… Magnus Westerlund
- Re: [rtcweb] RTP Usage: Is RTP Retransmission REQ… Cullen Jennings
- Re: [rtcweb] RTP Usage: Is RTP Retransmission REQ… Roni Even
- Re: [rtcweb] RTP Usage: Is RTP Retransmission REQ… Roni Even
- Re: [rtcweb] RTP Usage: Is RTP Retransmission REQ… Magnus Westerlund
- Re: [rtcweb] RTP Usage: Is RTP Retransmission REQ… Magnus Westerlund
- Re: [rtcweb] RTP Usage: Is RTP Retransmission REQ… Magnus Westerlund
- Re: [rtcweb] RTP Usage: Is RTP Retransmission REQ… Stefan Holmer
- Re: [rtcweb] RTP Usage: Is RTP Retransmission REQ… John Leslie
- Re: [rtcweb] RTP Usage: Is RTP Retransmission REQ… Cullen Jennings
- Re: [rtcweb] RTP Usage: Is RTP Retransmission REQ… Colin Perkins
- Re: [rtcweb] RTP Usage: Is RTP Retransmission REQ… Cullen Jennings
- Re: [rtcweb] RTP Usage: Is RTP Retransmission REQ… Roy, Radhika R CIV (US)
- Re: [rtcweb] RTP Usage: Is RTP Retransmission REQ… Randell Jesup
- Re: [rtcweb] RTP Usage: Is RTP Retransmission REQ… Randell Jesup
- Re: [rtcweb] RTP Usage: Is RTP Retransmission REQ… Stefan Holmer
- Re: [rtcweb] RTP Usage: Is RTP Retransmission REQ… Roni Even
- Re: [rtcweb] RTP Usage: Is RTP Retransmission REQ… Roni Even
- Re: [rtcweb] RTP Usage: Is RTP Retransmission REQ… Randell Jesup
- Re: [rtcweb] RTP Usage: Is RTP Retransmission REQ… Stefan Holmer
- Re: [rtcweb] RTP Usage: Is RTP Retransmission REQ… Olle E. Johansson
- Re: [rtcweb] RTP Usage: Is RTP Retransmission REQ… Justin Uberti
- Re: [rtcweb] RTP Usage: Is RTP Retransmission REQ… Randell Jesup
- Re: [rtcweb] RTP Usage: Is RTP Retransmission REQ… Olle E. Johansson
- Re: [rtcweb] RTP Usage: Is RTP Retransmission REQ… Stefan Hakansson LK
- Re: [rtcweb] RTP Usage: Is RTP Retransmission REQ… Roman Shpount
- Re: [rtcweb] RTP Usage: Is RTP Retransmission REQ… Olle E. Johansson
- Re: [rtcweb] RTP Usage: Is RTP Retransmission REQ… Roman Shpount
- [rtcweb] 答复: RTP Usage: Is RTP Retransmission REQ… Lishitao
- Re: [rtcweb] RTP Usage: Is RTP Retransmission REQ… Randell Jesup
- Re: [rtcweb] RTP Usage: Is RTP Retransmission REQ… Martin Thomson
- Re: [rtcweb] RTP Usage: Is RTP Retransmission REQ… Olle E. Johansson
- Re: [rtcweb] RTP Usage: Is RTP Retransmission REQ… Martin Thomson
- Re: [rtcweb] RTP Usage: Is RTP Retransmission REQ… Stefan Hakansson LK
- [rtcweb] LTE using AM_RLC or UM_RLC mode for UDP? Harald Alvestrand
- Re: [rtcweb] LTE using AM_RLC or UM_RLC mode for … Göran Eriksson AP
- Re: [rtcweb] LTE using AM_RLC or UM_RLC mode for … Cameron Byrne