Re: [tsvwg] Prague requirements survey

Bob Briscoe <ietf@bobbriscoe.net> Tue, 02 March 2021 11:03 UTC

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To: Ingemar Johansson S <ingemar.s.johansson@ericsson.com>, "De Schepper, Koen (Nokia - BE/Antwerp)" <koen.de_schepper@nokia-bell-labs.com>, tsvwg IETF list <tsvwg@ietf.org>
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From: Bob Briscoe <ietf@bobbriscoe.net>
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Date: Tue, 2 Mar 2021 11:03:30 +0000
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Archived-At: <https://mailarchive.ietf.org/arch/msg/tsvwg/uJo6kLaYtI2maBw4Hr1fLz87Zfg>
Subject: Re: [tsvwg] Prague requirements survey
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Ingemar,
inline [BB3]

On 24/02/2021 16:01, Ingemar Johansson S wrote:
>
> Hi
>
> Inline [IJ3]
>
> /Ingemar
>
> *From:* Bob Briscoe <ietf@bobbriscoe.net>
> *Sent:* den 23 februari 2021 17:50
> *To:* Ingemar Johansson S <ingemar.s.johansson@ericsson.com>om>; De 
> Schepper, Koen (Nokia - BE/Antwerp) 
> <koen.de_schepper@nokia-bell-labs.com>om>; tsvwg IETF list <tsvwg@ietf.org>
> *Subject:* Re: [tsvwg] Prague requirements survey
>
> Ingemar, - see inline [BB2]
>
> On 23/02/2021 08:45, Ingemar Johansson S wrote:
>
>     Hi Bob
>
>     Please see inline [IJ2]
>
>     *From:* Bob Briscoe <in@bobbriscoe.net> <mailto:in@bobbriscoe.net>
>     *Sent:* den 22 februari 2021 21:35
>     *To:* Ingemar Johansson S <ingemar.s.johansson@ericsson.com>
>     <mailto:ingemar.s.johansson@ericsson.com>; De Schepper, Koen
>     (Nokia - BE/Antwerp) <koen.de_schepper@nokia-bell-labs.com>
>     <mailto:koen.de_schepper@nokia-bell-labs.com>; tsvwg IETF list
>     <tsvwg@ietf.org> <mailto:tsvwg@ietf.org>
>     *Subject:* Re: [tsvwg] Prague requirements survey
>
>     Ingemar,
>
>     On 10/02/2021 09:09, Ingemar Johansson S wrote:
>
>         Hi
>
>         Please see inline
>
>         /Ingemar
>
>         *From:* De Schepper, Koen (Nokia - BE/Antwerp)
>         <koen.de_schepper@nokia-bell-labs.com>
>         <mailto:koen.de_schepper@nokia-bell-labs.com>
>         *Sent:* den 8 februari 2021 14:37
>         *To:* Ingemar Johansson S <ingemar.s.johansson@ericsson.com>
>         <mailto:ingemar.s.johansson@ericsson.com>; tsvwg IETF list
>         <tsvwg@ietf.org> <mailto:tsvwg@ietf.org>
>         *Subject:* RE: Prague requirements survey
>
>         Hi Ingemar,
>
>         Thanks for your contributions. I linked your doc to the
>         https://l4steam.github.io/#prague-requirements-compliance
>         <https://protect2.fireeye.com/v1/url?k=ae271499-f1bc2db2-ae275402-866038973a15-a6d18987ee63d9e2&q=1&e=4b98878d-2d51-48b6-9518-92696e72501d&u=https%3A%2F%2Fl4steam.github.io%2F%23prague-requirements-compliance>
>         web page (and will do so for others).
>
>         [IJ] OK, great!
>
>         I didn’t see any issues or objections mentioned to the current
>         requirements as specified in the draft. Does this mean you
>         think they are all reasonable, valid and feasible?
>
>         [IJ] There is a slight possibility that I misunderstood parts
>         of the layout. Even though SCReAM is only partially compliant
>         in many cases I believe that requirements are reasonable.
>
>         Interesting observation (related to the performance
>         optimization topic 1) that for the control packets “RTCP is
>         likely not using ECT(1)”. Why is this not likely? I assume
>         this will impact the performance? Do we need to recommend the
>         use of ECT(1) on RTCP packets in the draft?
>
>         [IJ] Upon first glance I don’t see that it is beneficial that
>         RTCP packets are ECT(1). But of course there is a possibility
>         that RTCP packets go into a queue with higher latency and that
>         may affect performance. So… perhaps it is reasonable that RTCP
>         packets are ECT(1) too, but these are then to be regarded as
>         non queue building as it can be hard to rate control RTCP.
>
>
>     [BB] My knowledge is outdated, but RTCP used to be rate controlled
>     in multicast to prevent implosion (that was in the early days on
>     shared multicast, not single source). Are you saying all rate
>     control mechanisms have been removed? Is the problem that there's
>     no feedback channel for congestion indications on feedback? In
>     2-way RTP at least, couldn't you infer the congestion of RTCP
>     datagrams from the ECN on data running alongside it? Or might they
>     be disjoint paths?
>
>     [IJ2] Yes. It would of course be possible to set ECT(1) on
>     RTCP/UDP/IP packets and determine how large share of these packets
>     that are marked. The problem is then that there is not (currently)
>     any RTCP feedback(on feedback) format to carry that information
>     back to the RTCP sender (==streaming client) to make some rate
>     control of the RTCP feedback possible.
>     The RTCP bandwidth is indeed limited, the rule of thumb is that it
>     should be less than 5% of the RTP media bitrate scaled down by the
>     number of receivers. SCReAM is just one receiver and the RTCP
>     bitrate is roughly 2% of the medial bitrate.
>
>
> [BB2] Maybe what I said above didn't make sense, so I'll try saying it 
> a different way...
> Imagine this bidirectional scenario where X and Y are sending r-t data 
> to each other:
>
> 1a. X --RTP--> Y
> 1b. X <-RTCP-- Y
> 2a. X <--RTP-- Y
> 2b. X --RTCP-> Y
>
> Why can't Y use the proportion of ECN on the arriving RTP stream (1a) 
> to infer the likely ECN on RTCP stream 2b? and
> Why can't X use the proportion of ECN on the arriving RTP stream (2a) 
> to infer the likely ECN on RTCP stream 1b?
> you
> Then I thought maybe one can't be sure that 1a and 2b are actually 
> traversing the same path.
> Similarly for 1b and 2a.
>
> [IJ3] RTP and RTCP is quite likely to traverse the same path, the 
> difference may of course be if RTP is L4S capable and RTCP is not, for 
> this reason it can make sense to have RTCP L4S capable as well. And 
> you are right, as they traverse the same path then the RTCP traffic is 
> equally likely to become marked as the RTP traffic. The question I 
> have is how one congestion control RTCP and if it is needed, it is 
> after all a small fraction of the media bitrate. Currently there is no 
> available mechanism for this in the IETF standards that cover RTCP (at 
> least it is unknown to me).
>

[BB3] This is OK then, isn't it? I think this means the media receiver 
can set ECT on RTCP packets, at least for 2-way media sessions. Because:
a) RTCP is a small fraction of the media bit-rate
b) the media receiver can tell if congestion rises
c) Even tho the media receiver currently has no code to regulate the 
RTCP stream, if congestion persists it will at least reduce its own 
parallel RTP stream even more to compensate.
d) There's always the fall-back that if RTCP congestion is not cleared, 
the queue will progress from ECN to drop and naturally thin out enough 
RTCP datagrams.

If RTCP congestion became a widespread problem in the future, code could 
be added to slow down the RTCP stream driven by congestion notifications 
in the parallel RTP media stream. Incremental deployment would be 
possible (I mean, the congestion info is already there, so each end 
could deploy unilaterally, without needing additional negotiation at 
session set up).


Bob

PS. Sry for delayed reply.


>
>
>
> Bob
>
>
>
>
>     Incidentally, the attitude being taken to ECT on TCP Pure ACKs in
>     AccECN is in the AccECN draft. Basically, the info is now there
>     for Ack CC if needed, but no need to use it currently.
>
>
>     Bob
>
>
>
>         Thanks,
>
>         Koen.
>
>         *From:* Ingemar Johansson S <ingemar.s.johansson@ericsson.com
>         <mailto:ingemar.s.johansson@ericsson.com>>
>         *Sent:* Monday, February 8, 2021 10:59 AM
>         *To:* De Schepper, Koen (Nokia - BE/Antwerp)
>         <koen.de_schepper@nokia-bell-labs.com
>         <mailto:koen.de_schepper@nokia-bell-labs.com>>; tsvwg IETF
>         list <tsvwg@ietf.org <mailto:tsvwg@ietf.org>>
>         *Cc:* Ingemar Johansson S <ingemar.s.johansson@ericsson.com
>         <mailto:ingemar.s.johansson@ericsson.com>>
>         *Subject:* RE: Prague requirements survey
>
>         Hi
>
>         Please find attached (hopefully) a Prague requirements survey
>         applied to SCReAM (RFC8298 std + running code)
>
>         Regards
>         Ingemar
>
>         *From:* tsvwg <tsvwg-bounces@ietf.org
>         <mailto:tsvwg-bounces@ietf.org>> *On Behalf Of *De Schepper,
>         Koen (Nokia - BE/Antwerp)
>         *Sent:* den 6 februari 2021 23:20
>         *To:* tsvwg IETF list <tsvwg@ietf.org <mailto:tsvwg@ietf.org>>
>         *Subject:* [tsvwg] Prague requirements survey
>
>         Hi all,
>
>         To get a better understanding on the level of consensus on the
>         Prague requirements, we prepared an overview document listing
>         the L4S-ID draft requirements specific to the CC (wider Prague
>         requirements), as a questionnaire towards potential CC
>         developers. If you are developing or have developed an L4S
>         congestion control, you can describe the status of your
>         ongoing development in the second last column. If you cannot
>         share status, or plan-to/would implement an L4S CC, you can
>         list what you would want to support (see feasible). In the
>         last column you can put any
>         description/limitations/remarks/explanations related to
>         evaluations, implementations and/or plans (will implement or
>         will not implement). Any expected or experienced issues and
>         any objections/disagreements to the requirement can be
>         explained and colored appropriately.
>
>         The document can be found on following link:
>         https://raw.githubusercontent.com/L4STeam/l4steam.github.io/master/PragueReqs/Prague_requirements_Compliance_and_Objections_template.docx
>         <https://protect2.fireeye.com/v1/url?k=d16bc960-8ef0f066-d16b89fb-86ee86bd5107-080c65bfd839440d&q=1&e=7dbb7494-67c3-4315-88a6-325f32e4e8b1&u=https%3A%2F%2Fraw.githubusercontent.com%2FL4STeam%2Fl4steam.github.io%2Fmaster%2FPragueReqs%2FPrague_requirements_Compliance_and_Objections_template.docx>
>
>         As an example I filled it for the Linux TCP-Prague
>         implementation on following link:
>         https://l4steam.github.io/PragueReqs/Prague_requirements_Compliance_and_Objections_Linux_TCP-Prague.docx
>         <https://protect2.fireeye.com/v1/url?k=f839c5f7-a7a2fcf1-f839856c-86ee86bd5107-29dabadc5d0e673d&q=1&e=7dbb7494-67c3-4315-88a6-325f32e4e8b1&u=https%3A%2F%2Fl4steam.github.io%2FPragueReqs%2FPrague_requirements_Compliance_and_Objections_Linux_TCP-Prague.docx>
>
>         Please send your filled document to the list (Not sure if an
>         attachment will work, so I assume you also need to store it
>         somewhere and send a link to it, or send to me directly).
>
>         We hope to collect many answers, understanding the position of
>         the different (potential) implementers and come faster to
>         consensus.
>
>         Thanks,
>
>         Koen.
>
>
>
>
>     -- 
>
>     ________________________________________________________________
>
>     Bob Briscoehttp://bobbriscoe.net/  <https://protect2.fireeye.com/v1/url?k=9e93ec25-c108d525-9e93acbe-86fc6812c361-6ce26fbc5b7d3fe4&q=1&e=9dde995b-c278-4be1-a612-c70ef0067fc2&u=http%3A%2F%2Fbobbriscoe.net%2F>
>
>
>
> -- 
> ________________________________________________________________
> Bob Briscoehttp://bobbriscoe.net/  <https://protect2.fireeye.com/v1/url?k=9c972e29-c30c1729-9c976eb2-86fc6812c361-770acfd11988f54b&q=1&e=67990d9c-a717-4edd-bfdc-2431e035aa8b&u=http%3A%2F%2Fbobbriscoe.net%2F>

-- 
________________________________________________________________
Bob Briscoe                               http://bobbriscoe.net/