Re: [tcpm] Linux doesn’t implement RFC3465
Yuchung Cheng <ycheng@google.com> Mon, 02 August 2021 22:03 UTC
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From: Yuchung Cheng <ycheng@google.com>
Date: Mon, 02 Aug 2021 15:02:52 -0700
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To: Mark Allman <mallman@icir.org>
Cc: Vidhi Goel <vidhi_goel@apple.com>, Neal Cardwell <ncardwell@google.com>, Extensions <tcpm@ietf.org>
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Subject: Re: [tcpm] Linux doesn’t implement RFC3465
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The fact is that Linux CC has long moved to infinite L since 2013* On Mon, Aug 2, 2021 at 11:03 AM Mark Allman <mallman@icir.org> wrote: > > > “This document RECOMMENDS using mechanisms like Pacing to control > > how many bytes are sent to the network at a point of time. But if > > it is not possible to implement pacing, an implementation MAY > > implicitly pace their traffic by applying a limit L to the > > increase in congestion window per ACK during slow start. In > > modern stacks, acknowledgments are aggregated for various reason, > > CPU optimization, reducing network load etc. Hence it is common > > for a sender to receive an aggregated ACK that acknowledges more > > than 2 segments. For example, a stack that implements GRO could > > aggregate packets up to 64Kbytes or ~44 segments before passing > > on to the TCP layer and this would result in a single ACK to be > > generated by the TCP stack. Given that an initial window of 10 > > packets in current deployments has been working fine, the draft > > makes a recommendation to set L=10 during slow start. This would > > mean that with every ACK, we are probing for a new capacity by > > sending 10 packets in addition to the previously discovered > > capacity. Implementations MAY choose to set a lower limit if they > > believe an increase of 10 is too aggressive." > > > > Does this sound like what we would like to say? > > Not really, IMO. I think a few things here ... > > - I agree that if pacing is in play that we don't need to worry > about an L. > > - I think the above L=10 reasoning is at best pretty weak. Just > because IW=10 works OK once / connection does not mean > continually sending "10 more" will work out OK. It may. It may > not. But, the above sort of coupling between IW-10 and L=10 > seems highly tenuous without any sort of data. > > - The real issue with picking a number is that it is so hard to > reason about because the behavior All Depends. E.g., consider > something like IW=10. We know that will allow 10 or fewer > segments to be pumped into the network when the connection > starts. That's pretty easy to reason about / understand. But, > with L=10 we might have bursts anywhere from 2 packets to > cwnd+10 packets on every ACK---depending on how the ACKs are > stretched. And, cwnd isn't a constant. So, if the idea is to > somehow limit bursting then sometimes we're limiting to X and > others to Y and still others to Z. It's an inconsistent mess. > Making L something arbitrary without evidence seems like a bad > path to me. > > - Of course, by making L=10 a MAY we're effectively saying "no L, > anywhere" anyways. If we're going to define an L it should be a > SHOULD unless there is pacing. > > - Somewhat related to the above, it isn't clear what "10 more" is > more than in qualitative terms, as well. Say an ACK rolls in > that covers 10 packets. How were those packets sent? Were they > sent a back-to-back burst so "10 more" is in fact a back-to-back > burst that is 10 more than previously (or, 2x)? Or, did we send > those 10 packets in 5 little bursts of 2 packets each so that > "10 more" is actually 18 more than the previous burst size---a > 10x increase? Of course, it could be BOTH for the same ACK! > I.e., the segments were sent 2 at a time and aggregated > somewhere in the middle. This lack of clarity again makes the > choice of L feel pretty arbitrary. > > I agree we can elide L if pacing is in place. But, twiddling with L > by feel is crude and will produce an inconsistent approach to bursts > that doesn't seem to me to be particularly helpful because we don't > really grok the implications. > > allman >
- [tcpm] Linux doesn’t implement RFC3465 Vidhi Goel
- Re: [tcpm] Linux doesn’t implement RFC3465 Neal Cardwell
- Re: [tcpm] Linux doesn’t implement RFC3465 Vidhi Goel
- Re: [tcpm] Linux doesn’t implement RFC3465 Lars Eggert
- Re: [tcpm] Linux doesn’t implement RFC3465 Vidhi Goel
- Re: [tcpm] Linux doesn’t implement RFC3465 Vidhi Goel
- Re: [tcpm] Linux doesn’t implement RFC3465 Mark Allman
- Re: [tcpm] Linux doesn’t implement RFC3465 Vidhi Goel
- Re: [tcpm] Linux doesn’t implement RFC3465 Yuchung Cheng
- Re: [tcpm] Linux doesn’t implement RFC3465 Neal Cardwell
- Re: [tcpm] Linux doesn’t implement RFC3465 Mark Allman
- Re: [tcpm] Linux doesn’t implement RFC3465 Neal Cardwell
- Re: [tcpm] Linux doesn’t implement RFC3465 Mark Allman
- Re: [tcpm] Linux doesn’t implement RFC3465 Neal Cardwell
- Re: [tcpm] Linux doesn’t implement RFC3465 Vidhi Goel
- Re: [tcpm] Linux doesn’t implement RFC3465 Yuchung Cheng
- Re: [tcpm] Linux doesn’t implement RFC3465 Vidhi Goel
- Re: [tcpm] Linux doesn’t implement RFC3465 Yuchung Cheng
- Re: [tcpm] Linux doesn’t implement RFC3465 Vidhi Goel
- Re: [tcpm] Linux doesn’t implement RFC3465 Yuchung Cheng
- Re: [tcpm] Linux doesn’t implement RFC3465 Neal Cardwell
- Re: [tcpm] Linux doesn’t implement RFC3465 Mark Allman
- Re: [tcpm] Linux doesn’t implement RFC3465 Yuchung Cheng
- Re: [tcpm] Linux doesn’t implement RFC3465 Yuchung Cheng
- Re: [tcpm] Linux doesn’t implement RFC3465 Mark Allman
- Re: [tcpm] Linux doesn’t implement RFC3465 Yuchung Cheng
- Re: [tcpm] Linux doesn’t implement RFC3465 David Lang
- Re: [tcpm] Linux doesn’t implement RFC3465 Neal Cardwell
- Re: [tcpm] Linux doesn’t implement RFC3465 Vidhi Goel
- Re: [tcpm] Linux doesn’t implement RFC3465 Neal Cardwell
- Re: [tcpm] Linux doesn’t implement RFC3465 Yuchung Cheng
- Re: [tcpm] [EXTERNAL] Re: Linux doesn’t implement… Praveen Balasubramanian
- Re: [tcpm] [EXTERNAL] Re: Linux doesn’t implement… Neal Cardwell
- Re: [tcpm] [EXTERNAL] Re: Linux doesn’t implement… Yuchung Cheng
- Re: [tcpm] [EXTERNAL] Re: Linux doesn’t implement… Yuchung Cheng
- Re: [tcpm] [EXTERNAL] Re: Linux doesn’t implement… Yuchung Cheng
- Re: [tcpm] [EXTERNAL] Re: Linux doesn’t implement… Vidhi Goel
- Re: [tcpm] [EXTERNAL] Re: Linux doesn’t implement… Neal Cardwell
- Re: [tcpm] [EXTERNAL] Re: Linux doesn’t implement… Yoshifumi Nishida
- Re: [tcpm] [EXTERNAL] Re: Linux doesn’t implement… Yuchung Cheng
- Re: [tcpm] [EXTERNAL] Re: Linux doesn’t implement… Neal Cardwell
- Re: [tcpm] Linux doesn’t implement RFC3465 Mark Allman
- Re: [tcpm] [EXTERNAL] Re: Linux doesn’t implement… Yoshifumi Nishida
- Re: [tcpm] [EXTERNAL] Re: Linux doesn’t implement… Vidhi Goel
- Re: [tcpm] [EXTERNAL] Re: Linux doesn’t implement… Yuchung Cheng
- Re: [tcpm] Linux doesn’t implement RFC3465 Yuchung Cheng
- Re: [tcpm] Linux doesn’t implement RFC3465 Mirja Kuehlewind
- Re: [tcpm] Linux doesn’t implement RFC3465 Martin Duke
- Re: [tcpm] Linux doesn’t implement RFC3465 Vidhi Goel
- Re: [tcpm] Linux doesn’t implement RFC3465 Martin Duke
- Re: [tcpm] Linux doesn’t implement RFC3465 Vidhi Goel
- Re: [tcpm] Linux doesn’t implement RFC3465 Michael Tuexen
- Re: [tcpm] Linux doesn’t implement RFC3465 Mark Allman