Re: [tsvwg] I-D Action: draft-ietf-tsvwg-ecn-l4s-id-12.txt
Ingemar Johansson S <ingemar.s.johansson@ericsson.com> Thu, 14 January 2021 15:26 UTC
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From: Ingemar Johansson S <ingemar.s.johansson@ericsson.com>
To: Bob Briscoe <ietf@bobbriscoe.net>, "\"koen.de_schepper@nokia.com\"" <koen.de_schepper@nokia.com>
CC: "tsvwg@ietf.org" <tsvwg@ietf.org>, Ingemar Johansson S <ingemar.s.johansson@ericsson.com>
Thread-Topic: [tsvwg] I-D Action: draft-ietf-tsvwg-ecn-l4s-id-12.txt
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Date: Thu, 14 Jan 2021 15:26:15 +0000
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Archived-At: <https://mailarchive.ietf.org/arch/msg/tsvwg/qMugQDscAbrZ1SgixkD21oGczbQ>
Subject: Re: [tsvwg] I-D Action: draft-ietf-tsvwg-ecn-l4s-id-12.txt
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Hi I have now re-read this draft with some more careful eyes. It is not for certain that the comments lead to any major changes in the draft, but nonetheless... I tried to look at this from a real time media congestion control perspective (RTP/UDP) and based on the experience I've had with SCReAM and L4S thus far. This can also serve as an indication how well the requirements in section 4.3 is fulfilled at the time of writing this email. /Ingemar ================= Section 4.3 : Prerequisite Congestion Response Quote "As well as responding to ECN markings, a scalable congestion control MUST react to packet loss in a way that will coexist safely with a TCP Reno congestion control " SCReAM backs off, on packet loss, but not with a factor 0.5. Rather it scales down the congestion window with a factor 0.8. The rationale is that the encoded video has a variable frame size and that gives some additional headroom to avoid that the larger frames build up a large queue. The outcome is still that SCReAM can coexist safely with Reno Quote "A scalable congestion control MUST implement monitoring in order to detect a likely non-L4S but ECN-capable AQM at the bottleneck." SCReAM use delay based congestion control. The estimated queue delay will be near zero when L4S bottlenecks are encountered. For cases with classic ECN queues in the network, the queue delay would be higher which means that it should be possible to detect non-L4S but ECN capable AQMs Quote "A scalable congestion control MUST eliminate RTT bias as much as possible in the range between the minimum likely RTT and typical RTTs expected in the intended deployment scenario " This has drawn less focus and needs to be evaluated and possibly addressed. It is here likely that the algorithms devised for Prague can be used Quote "A scalable congestion control SHOULD remain responsive to congestion when typical RTTs over the public Internet are significantly smaller because they are no longer inflated by queuing delay" The minimum congestion window is 3 MSS (configurable). SCReAM however implements packet pacing but SCReAMs performance in very low RTT deployments is not yet evaluated. Quote "A scalable congestion control SHOULD detect loss by counting in time-based units". SCReAM implements loss detection in time based units only. Quote "A scalable congestion control is expected to limit the queue caused by bursts of packets." SCReAM by default implements packet pacing. Lately however it has been experimented with microburst pacing wherein packets are transmitted in bursts with e.g. 2 or 5ms. The reason to this is that it can help to reduce power consumption in 5G phones. ================= Section 4.3 Filtering or Smoothing of ECN Feedback Quote "the specification of a particular L4S congestion control SHOULD describe how it smooths the L4S ECN signals fed back to it from the receiver" The RTCP feedback (https://tools.ietf.org/wg/avtcore/draft-ietf-avtcore-cc-feedback-message/ ) indicates CE mark for each packet, this means that no smoothing is applied on the CE marks. The L4S congestion response in the sender is similar to DCTCP and Prague. ================= Section 5.2 The Meaning of L4S CE Relative to Drop Quote "An L4S AQM SHOULD NOT smooth or filter out variations in the queue before signalling congestion." I believe the SHOULD NOT is reasonable. In some cases it may however be necessary to apply some limited smoothing. One example is where congestion is detected on the RLC layer in a 5G system. It I not always realistic to signal a marking decision up to the PDCP layer, where the IP packets are marked, for each packet that is scheduled for transmission. This because it can cause a too high internal signaling overhead in the radio base stations. A more realistic solution is to signal a marking probability up to the PDCP layer. Smoothing is applied to avoid the issues that the subsampling of the RLC queue delay can cause. ================= Section 5.5 Limiting Packet Bursts from Links Supporting L4S AQMs Quote "Some take the attitude that there is no point reducing burst delay", perhaps can be reworded as "One may argue that ......" In general, 4G and 5G systems can generate packet bursts due to how they are designed. On the other hand, the use of larger sub carrier spacing (shorter time slots) will also reduce the burst delays. ================= Section 6.1 Open questions In addition to all the metrics, I believe that it can be good to also be able to record the ratio of marked packets (average, 95%, max?) . My sense is that endpoints that behave reasonably well will have marking ratios up to say 20% (guessing a bit)? If problems occur, then recording of L4S marking ratios can give an indication of e.g. issues with bloated access links are because of endpoints that implement poor or non-functional congestion control. ================= Section 6.3 Future Potential Although SCReAM appears to work reasonably well (more testing needed), I do believe that there is a lot of room for improvement as regards to how one stream low latency real time video from mostly "black box" encoders (with wildly varying frame sizes) over L4S capable access. > -----Original Message----- > From: tsvwg <tsvwg-bounces@ietf.org> On Behalf Of internet-drafts@ietf.org > Sent: den 16 november 2020 03:07 > To: i-d-announce@ietf.org > Cc: tsvwg@ietf.org > Subject: [tsvwg] I-D Action: draft-ietf-tsvwg-ecn-l4s-id-12.txt > > > A New Internet-Draft is available from the on-line Internet-Drafts > directories. > This draft is a work item of the Transport Area Working Group WG of the > IETF. > > Title : Identifying Modified Explicit Congestion > Notification (ECN) > Semantics for Ultra-Low Queuing Delay (L4S) > Authors : Koen De Schepper > Bob Briscoe > Filename : draft-ietf-tsvwg-ecn-l4s-id-12.txt > Pages : 58 > Date : 2020-11-15 > > Abstract: > This specification defines the identifier to be used on IP packets > for a new network service called low latency, low loss and scalable > throughput (L4S). L4S uses an Explicit Congestion Notification (ECN) > scheme that is similar to the original (or 'Classic') ECN approach. > 'Classic' ECN marking was required to be equivalent to a drop, both > when applied in the network and when responded to by a transport. > Unlike 'Classic' ECN marking, for packets carrying the L4S > identifier, the network applies marking more immediately and more > aggressively than drop, and the transport response to each mark is > reduced and smoothed relative to that for drop. The two changes > counterbalance each other so that the throughput of an L4S flow will > be roughly the same as a non-L4S flow under the same conditions. > Nonetheless, the much more frequent control signals and the finer > responses to them result in much more fine-grained adjustments, so > that ultra-low and consistently low queuing delay (typically sub- > millisecond on average) becomes possible for L4S traffic without > compromising link utilization. Thus even capacity-seeking (TCP-like) > traffic can have high bandwidth and very low delay at the same time, > even during periods of high traffic load. > > The L4S identifier defined in this document distinguishes L4S from > 'Classic' (e.g. TCP-Reno-friendly) traffic. It gives an incremental > migration path so that suitably modified network bottlenecks can > distinguish and isolate existing traffic that still follows the > Classic behaviour, to prevent it degrading the low queuing delay and > low loss of L4S traffic. This specification defines the rules that > L4S transports and network elements need to follow to ensure they > neither harm each other's performance nor that of Classic traffic. > Examples of new active queue management (AQM) marking algorithms and > examples of new transports (whether TCP-like or real-time) are > specified separately. > > > The IETF datatracker status page for this draft is: > https://datatracker.ietf.org/doc/draft-ietf-tsvwg-ecn-l4s-id/ > > There are also htmlized versions available at: > https://tools.ietf.org/html/draft-ietf-tsvwg-ecn-l4s-id-12 > https://datatracker.ietf.org/doc/html/draft-ietf-tsvwg-ecn-l4s-id-12 > > A diff from the previous version is available at: > https://www.ietf.org/rfcdiff?url2=draft-ietf-tsvwg-ecn-l4s-id-12 > > > Please note that it may take a couple of minutes from the time of submission > until the htmlized version and diff are available at tools.ietf.org. > > Internet-Drafts are also available by anonymous FTP at: > ftp://ftp.ietf.org/internet-drafts/ >
- [tsvwg] I-D Action: draft-ietf-tsvwg-ecn-l4s-id-1… internet-drafts
- Re: [tsvwg] I-D Action: draft-ietf-tsvwg-ecn-l4s-… Ingemar Johansson S
- Re: [tsvwg] I-D Action: draft-ietf-tsvwg-ecn-l4s-… Bob Briscoe