Re: [tcpm] Possible error in accurate-ecn
"Scheffenegger, Richard" <rs.ietf@gmx.at> Wed, 24 February 2021 10:03 UTC
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To: Bob Briscoe <research@bobbriscoe.net>, Mirja Kuehlewind <ietf@kuehlewind.net>
Cc: tcpm IETF list <tcpm@ietf.org>, Richard Scheffenegger <rscheff@gmx.at>, Yoshifumi Nishada <nishida@sfc.wide.ad.jp>
References: <47df9b8b-515e-d40d-3473-599b0a3e3876@bobbriscoe.net> <6031BE2B-4D33-426F-BA17-DDF15CF821DE@kuehlewind.net> <060c8bd8-d64b-3e46-7874-742e35e6d114@bobbriscoe.net> <221e58f3-ada0-c880-db72-d98af84fedb8@gmx.at> <bd6ab65d-ccd5-9fa9-58be-6d9fea4af870@bobbriscoe.net>
From: "Scheffenegger, Richard" <rs.ietf@gmx.at>
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Date: Wed, 24 Feb 2021 11:02:38 +0100
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Subject: Re: [tcpm] Possible error in accurate-ecn
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Hi Bob, unless there is ACK duplication going on, I don't see how the current text would cause any issues. The only aspect I would emphasize is, that in the 2nd case, "n" MUST NOT be as low as one, as that may cause a ACK-triggering-ACK when the path exhibits a pathological misconfiguration, where all packets get CE marked. As long as a smaller numer of ACKs ("n" >= 2) carry a sum of CE marks seen from received frames, any such pathological paths would possible trigger a few rounds of ACKs, but these will eventually die down, correct? Do you have a different scenario in mind? Richard Am 23.02.2021 um 01:12 schrieb Bob Briscoe: > Mirja, Richard, Yoshi, > > I just posted a new rev of AccECN for a number of other reasons, and > realized we hadn't converged on an answer to the tricky problem in this > old thread from Nov'20. For now, the text says the following (which I've > justified below, but I don't think it's the solution yet), and I've > asked Richard & Mirja if we (as co-authors) can act as a little design > team to thrash out this problem in the next few days. > > 3.2.2.5.1. Data Receiver Safety Procedures > > An AccECN Data Receiver: > > o SHOULD immediately send an ACK whenever a data packet marked CE > arrives after the previous packet was not CE. > > o MUST immediately send an ACK once 'n' CE marks have arrived since > the previous ACK, where 'n' SHOULD be 2 and MUST be in the range 2 > to 6 inclusive. > > > Justification for first bullet: The aim is to trigger an ACK on a > transition from non-CE to CE, but only when the latest packet is a data > packet. Cases where the previous packet was a CE-marked pure ACK and > therefore not ACK'd ought to be picked up by the 2nd bullet, not the > first. Similarly, if there's CE-marked packets (data or pure ACKs) that > haven't been ACKd yet, and the next data packet is not CE-marked, I > think the first bullet is still correct, and the second rule will pick > up cases with enough CE marks. > > The second bullet is more tricky. I've added Richard's suggesting of > flooring 'n' at 2. But made no other changes. That's deliberate, 'cos I > think it's sufficient to allow ACK CC to be added, but not to preclude > it. And I think the danger of being interpreted as DupACKs shouldn't > apply if there's no outstanding data. I know I'm wrong in those odd > cases I mentioned when new data might cross the ACKs in the other > direction. But I don't think that's going to cause too much harm. > > However, I'm probably very wrong. Somehow I feel we haven't got a good > model to check all the possibilities. > > > Bob > > > > On 17/11/2020 08:48, Scheffenegger, Richard wrote: >> >> See my suggestion; The critical requirement for the 2nd bullet point is, >> to have n > 1 for pure, CE-marked ACKs. The exponential decay, after how >> many ACKs triggered by CE-marked ACKs these ACK-for-ACK stop, strongly >> depends on "n", so the recommendation should be to use a higher value of >> n in the case where you only observe a stream of pure ACKs (with or >> without marks; only those marked would account against "n" though). >> >> In the case of data packets mixed in, you don't want to delay >> excessively, thus "n" may be choosen lower... >> >> Richard >> >> >> >> Am 10.11.2020 um 17:38 schrieb Bob Briscoe: >>> >>> Moving on to the second bullet... >>>>> The second bullet doesn't consider the possibility that the 'n'th CE >>>>> mark might arrive on a pure ACK. Then, the wording as it stands says >>>>> the Data Receiver MUST immediately ACK a pure ACK. I know TCP never >>>>> ACKs a pure ACK, but I'm not actually sure it does any harm to do so >>>>> in this case (it cannot cause an infinite loop of ACKs). However, >>>>> given it would be unorthodox, we maybe ought to rule it out by >>>>> rewording anyway? >>>> [MK] I think we also can leave this as it is because if that ’n’ >>>> packet is a pure ACK that still means that you have unacked data >>>> packets with CE marks and you should trigger an ACK for those packet >>>> now (rather than waiting for the delayed ACK timer to expire). However >>>> this case is less important. Also we should probably make sure that >>>> this doesn’t apply if there are only pure ACKs with CE marks, maybe by >>>> add “if unacknowledged data are outstanding” or something. >>> >>> [BB] By the end of your last para you start to realize that, if the n'th >>> CE mark arrives on pure ACK, it doesn't say anything about whether the >>> previous (n-1) CE marks were on data or pure ACKs. >>> >>> Consider another common scenario; a server delivering chunks of video >>> over a high BDP path (e.g. 500 packets per RTT), and going idle between >>> times (perhaps in response to an "exceeded high watermark" data packet >>> from the server). Now consider the possibility that the reverse path >>> bottleneck builds a queue during each chunk, and starts CE marking >>> towards the end of each chunk. Then, after the server stops sending a >>> chunk, it continues to see a couple of hundred CE-marked pure ACKs >>> arriving. >>> >>> However, by your proposed rule, it doesn't send any feedback, 'cos it >>> has no data to acknowledge. So the client cannot regulate its ACK >>> stream. That's just tough, 'cos the client will stop sending more pure >>> ACKs after it has received the last data, which is before any later >>> feedback from the server can reach it. >>> >>> However, if the client later sends a data packet that the server can >>> acknowledge (e.g. "a low watermark" message), the server's ACE counter >>> will have wrapped dozens of times, and the client won't be able to work >>> out how many. That doesn't matter if a few round trips have elapsed - >>> the congestion info will have become stale. But it does matter if it's >>> fairly soon. >>> >>> Instead, if we keep the text of the second bullet... if the server's 'n' >>> was 2, it would send a pure ACK for every other pure ACK it received. >>> But, what if these pure ACKs from the server were also CE-marked? Then >>> the client would feed back pure ACKs using its 'n'. This regression >>> would eventually die out, but I don't like it. >>> >>> This conversation about the second bullet presumes AccECN is being used >>> for ACK congestion control. I'm not comfortable with writing anything >>> specific about ACK congestion control into a draft on the standards >>> track. But I don't want to preclude AccECN being used for ACK CC. >>> >>> >>> I believe the second bullet needs something changed to limit the ACKing >>> of pure ACKs. However, before we put effort into wordsmithing, I'd like >>> to hear whether you agree with the general idea of not defining what to >>> do for ACK CC, but not precluding it. >>> >>> Cheers >>> >>> >>> >>> >>> Bob >>> >>>> >>>> Mirja >>>> >>>> >>>> >>>>> Thoughts anyone? >>>>> >>>>> >>>>> Bob >>>>> >>>>> -- >>>>> ________________________________________________________________ >>>>> Bob Briscoe >>>>> http://bobbriscoe.net/ >>>> _______________________________________________ >>>> tcpm mailing list >>>> tcpm@ietf.org >>>> https://www.ietf.org/mailman/listinfo/tcpm >>> >> >> _______________________________________________ >> tcpm mailing list >> tcpm@ietf.org >> https://www.ietf.org/mailman/listinfo/tcpm > > -- > ________________________________________________________________ > Bob Briscoehttp://bobbriscoe.net/ >
- [tcpm] Possible error in accurate-ecn Bob Briscoe
- Re: [tcpm] Possible error in accurate-ecn Mirja Kuehlewind
- Re: [tcpm] Possible error in accurate-ecn Bob Briscoe
- Re: [tcpm] Possible error in accurate-ecn Yoshifumi Nishida
- Re: [tcpm] Possible error in accurate-ecn Bob Briscoe
- Re: [tcpm] Possible error in accurate-ecn Yoshifumi Nishida
- Re: [tcpm] Possible error in accurate-ecn Scheffenegger, Richard
- Re: [tcpm] Possible error in accurate-ecn Scheffenegger, Richard
- Re: [tcpm] Possible error in accurate-ecn Scheffenegger, Richard
- Re: [tcpm] Possible error in accurate-ecn Mirja Kuehlewind
- Re: [tcpm] Possible error in accurate-ecn Bob Briscoe
- Re: [tcpm] Possible error in accurate-ecn Scheffenegger, Richard
- Re: [tcpm] Possible error in accurate-ecn Yoshifumi Nishida
- Re: [tcpm] Possible error in accurate-ecn Scheffenegger, Richard
- Re: [tcpm] Possible error in accurate-ecn Yoshifumi Nishida
- Re: [tcpm] Possible error in accurate-ecn Bob Briscoe
- Re: [tcpm] Possible error in accurate-ecn Yoshifumi Nishida
- [tcpm] Seeking WG opinions on ACKing ACKs with go… Bob Briscoe
- Re: [tcpm] Seeking WG opinions on ACKing ACKs wit… Martin Duke
- Re: [tcpm] Seeking WG opinions on ACKing ACKs wit… Scheffenegger, Richard
- Re: [tcpm] Seeking WG opinions on ACKing ACKs wit… Bob Briscoe
- Re: [tcpm] Seeking WG opinions on ACKing ACKs wit… Mirja Kuehlewind
- Re: [tcpm] Seeking WG opinions on ACKing ACKs wit… Martin Duke
- Re: [tcpm] Seeking WG opinions on ACKing ACKs wit… Martin Duke
- Re: [tcpm] Seeking WG opinions on ACKing ACKs wit… Vidhi Goel
- Re: [tcpm] Seeking WG opinions on ACKing ACKs wit… Yoshifumi Nishida
- Re: [tcpm] Seeking WG opinions on ACKing ACKs wit… Yoshifumi Nishida
- Re: [tcpm] Seeking WG opinions on ACKing ACKs wit… Scheffenegger, Richard
- Re: [tcpm] Seeking WG opinions on ACKing ACKs wit… Mirja Kuehlewind
- Re: [tcpm] Seeking WG opinions on ACKing ACKs wit… Mirja Kuehlewind
- Re: [tcpm] Seeking WG opinions on ACKing ACKs wit… Vidhi Goel
- Re: [tcpm] Seeking WG opinions on ACKing ACKs wit… Mirja Kuehlewind
- Re: [tcpm] Seeking WG opinions on ACKing ACKs wit… Scheffenegger, Richard
- Re: [tcpm] Seeking WG opinions on ACKing ACKs wit… Jonathan Morton
- Re: [tcpm] Seeking WG opinions on ACKing ACKs wit… Yoshifumi Nishida
- Re: [tcpm] Seeking WG opinions on ACKing ACKs wit… Scheffenegger, Richard
- Re: [tcpm] Seeking WG opinions on ACKing ACKs wit… Bob Briscoe
- Re: [tcpm] Seeking WG opinions on ACKing ACKs wit… Bob Briscoe
- Re: [tcpm] Seeking WG opinions on ACKing ACKs wit… Mirja Kuehlewind
- Re: [tcpm] Seeking WG opinions on ACKing ACKs wit… Yoshifumi Nishida
- Re: [tcpm] Seeking WG opinions on ACKing ACKs wit… Bob Briscoe
- Re: [tcpm] Seeking WG opinions on ACKing ACKs wit… Yoshifumi Nishida
- Re: [tcpm] Seeking WG opinions on ACKing ACKs wit… Scheffenegger, Richard
- Re: [tcpm] [EXTERNAL] Re: Seeking WG opinions on … Praveen Balasubramanian
- Re: [tcpm] [EXTERNAL] Re: Seeking WG opinions on … Vidhi Goel
- Re: [tcpm] [EXTERNAL] Re: Seeking WG opinions on … Jonathan Morton
- Re: [tcpm] [EXTERNAL] Re: Seeking WG opinions on … Praveen Balasubramanian
- Re: [tcpm] [EXTERNAL] Re: Seeking WG opinions on … Jonathan Morton
- Re: [tcpm] [EXTERNAL] Re: Seeking WG opinions on … Vidhi Goel
- Re: [tcpm] [EXTERNAL] Re: Seeking WG opinions on … Jonathan Morton
- Re: [tcpm] [EXTERNAL] Re: Seeking WG opinions on … Yoshifumi Nishida
- Re: [tcpm] [EXTERNAL] Re: Seeking WG opinions on … Bob Briscoe
- Re: [tcpm] [EXTERNAL] Re: Seeking WG opinions on … Christian Huitema
- Re: [tcpm] [EXTERNAL] Re: Seeking WG opinions on … Bob Briscoe
- Re: [tcpm] [EXTERNAL] Re: Seeking WG opinions on … Christian Huitema
- Re: [tcpm] [EXTERNAL] Re: Seeking WG opinions on … Neal Cardwell
- Re: [tcpm] [EXTERNAL] Re: Seeking WG opinions on … Bob Briscoe
- Re: [tcpm] [EXTERNAL] Re: Seeking WG opinions on … Ilpo Järvinen
- Re: [tcpm] [EXTERNAL] Re: Seeking WG opinions on … Neal Cardwell
- Re: [tcpm] [EXTERNAL] Re: Seeking WG opinions on … Bob Briscoe