Re: [tcpm] Why draft-gomez-tcpm-ack-rate-request?
Bob Briscoe <in@bobbriscoe.net> Wed, 23 March 2022 16:51 UTC
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Date: Wed, 23 Mar 2022 16:51:20 +0000
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To: Gorry Fairhurst <gorry@erg.abdn.ac.uk>
Cc: tcpm IETF list <tcpm@ietf.org>, Jonathan Morton <chromatix99@gmail.com>
References: <3546045c-454c-611f-be50-a6de6bd6b03b@erg.abdn.ac.uk> <8956E182-4F45-414D-85D9-14A654A2398D@gmail.com> <7f55afcf-059c-4a6f-9733-2034bbcef760@erg.abdn.ac.uk>
From: Bob Briscoe <in@bobbriscoe.net>
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Subject: Re: [tcpm] Why draft-gomez-tcpm-ack-rate-request?
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Gorry, On 23/03/2022 13:25, Gorry Fairhurst wrote: > On 23/03/2022 14:06, Jonathan Morton wrote: >>> On 23 Mar, 2022, at 2:00 pm, Gorry Fairhurst <gorry@erg.abdn.ac.uk> >>> wrote: >>> >>> Thanks again for raising questions about ACK traffiuc - I'm quite >>> passionate we look again at AcK traffic volume and rate. >>> I've just reread and I'm still quite skeptical this is the right >>> solution - look forwrad to the talk. >>> >>> Why is this so important to use a TCP option to signal, rather than >>> just specify a better method? >>> >>> 1. An option increases the probability that someone might >>> block/change the option on path. >> This sounds like a counterproductive argument to me. I believe we >> should be trying to *reduce* the amount of ossification inherent to >> the interaction of IETF protocols with overzealous middleboxes. If >> this option is blocked en route, the transport will fall back to the >> typical delayed-ack behaviour, so there should be little if any harm. >> >>> 2. Any receiver that sees this option has anyway to decide how to >>> process ACKs - and mkight also need to deal with offload. >>> - The use-case for IOT might not need an option: >>> For receivers that see one segment/RTT then ACKing each segment >>> immediately seems reasonable. Sending 2 segments would make anyway >>> have this effect. >>> When would you not wish to send fewer ACKs? >> There are cases where the sender wants to receive more frequent acks >> in order to infer things about the network path, particularly for >> advanced congestion control algorithms. > Which IETF CC needs this? [BB] Chicken and egg: * CC is unilaterally deployed at one end. * Without a 2-ended protocol like TARR, no CC that needs a facility like TARR can be implemented. If we use a chicken and egg criterion like your question, we'll never get chickens or eggs. A Data Point: To use paced chirping at the sender, we had to manually disable del-ack at the receivers in order to work well during slow start (it works with del-ack, just not very well). Paced chirping was a research project, rather than an IETF CC, precisely because there is nothing like TARR out there. Chicken and egg. >> IoT senders would also benefit from the ability to operate >> efficiently with single-MSS congestion windows, which is most easily >> achieved by explicitly signalling this intent to the receiver. > That's my point - any system using a window of 1 MSS is a special case > easily noted, that does not need negotiated. [BB] The receiver doesn't know the RTT, so 'easily' is overstated. But your point has some validity. More generally, receiver heuristics are not a panacea... Another data point from paced-chirping: The receiver heuristic in Linux that disables delayed acks during slow start doesn't work with paced chirping, 'cos it is specific to the pattern of slow-start packets, which is precisely what we were trying to improve on with paced chirping (i.e. the Linux heuristic has ossified slow-start, which is why we need sthg like TARR to override it). Bob >> Without ARR, the only mechanism for this is the PSH flag, which is >> not defined with the required on-the-wire meaning, but instead >> indicates desired behaviour for the receiving socket API. >> >>> 3. The use cases for asymmetry or processinbg load might not need an >>> option: >>> I don't understand the motive here, in QUIC some have been using one >>> ACK every ~10 packets >>> in some implementations (wuth whatever caveat to do something >>> differenet for the first ~100 packets). >>> I'd like to argue this is enough "control" for most cases, and not >>> so much "ACK traffic" in many cases where that matters. >> There's about a 25:1 ratio between the size of a data-segment packet >> and an ack packet in IPv4 - and it's closer for IPv6. This makes it >> relatively easy for modern traffic patterns to cause congestion on >> the smaller direction of an asymmetric-capacity link to limit >> practically available capacity in the other direction, with today's >> delayed-ack specifications which effectively require one ack for >> every two data segments. >> >> Existing solutions to this problem are not at all elegant. One >> relies on a side-effect of current NIC hardware releasing batches of >> received packets, all of which may be acked as a unit. Another >> relies on some middlebox actively dropping acks when it decides that >> too many are passing through it - which is inefficient on several >> levels simultaneously. ARR puts some control over this back in the >> hands of the transport endpoints, and may even result in a reduction >> of power consumption and shared-medium contention. >> >> - Jonathan Morton > I'm not saying don't change the spec. - far from it. It's just I don't > see the case why this signalling is needed to change the way a > receiver works. > > Gorry > > _______________________________________________ > tcpm mailing list > tcpm@ietf.org > https://www.ietf.org/mailman/listinfo/tcpm -- ________________________________________________________________ Bob Briscoe http://bobbriscoe.net/
- [tcpm] Why draft-gomez-tcpm-ack-rate-request? Gorry Fairhurst
- Re: [tcpm] Why draft-gomez-tcpm-ack-rate-request? Jonathan Morton
- Re: [tcpm] Why draft-gomez-tcpm-ack-rate-request? Gorry Fairhurst
- Re: [tcpm] Why draft-gomez-tcpm-ack-rate-request? Jonathan Morton
- Re: [tcpm] Why draft-gomez-tcpm-ack-rate-request? Bob Briscoe
- Re: [tcpm] Why draft-gomez-tcpm-ack-rate-request? Bob Briscoe
- Re: [tcpm] Why draft-gomez-tcpm-ack-rate-request? Jonathan Morton
- Re: [tcpm] Why draft-gomez-tcpm-ack-rate-request? Bob Briscoe
- Re: [tcpm] Why draft-gomez-tcpm-ack-rate-request? Scheffenegger, Richard
- [tcpm] RFC 5690 and TARR draft (was Re: Why draft… Carles Gomez Montenegro