Re: [tcpm] TCP tuning
Mark Allman <mallman@icir.org> Mon, 15 March 2010 21:16 UTC
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From: Mark Allman <mallman@icir.org>
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Cc: tcpm@ietf.org, Michael Welzl <michawe@ifi.uio.no>
Subject: Re: [tcpm] TCP tuning
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[These are re-sends from long ago... they never made it to the list. But, that seems to have been worked out, now. --allman] > " (2.4) Whenever RTO is computed, if it is less than 1 second then the > RTO SHOULD be rounded up to 1 second. > > Traditionally, TCP implementations use coarse grain clocks to > measure the RTT and trigger the RTO, which imposes a large > minimum value on the RTO. Research suggests that a large > minimum RTO is needed to keep TCP conservative and avoid > spurious retransmissions [AP99]. Therefore, this > specification requires a large minimum RTO as a conservative > approach, while at the same time acknowledging that at some > future point, research may show that a smaller minimum RTO is > acceptable or superior." > > Is it time to reconsider a smaller minimum RTO as well? One problem with the minimum is we don't know what to set it at [*]. The finding in [AP99] is that as you reduce the min you take more spurious timeouts. The tradeoff seemed fundamental to us. But, given our methodology (trace driven simulation) we could not really tell the impact of the spurious timeouts. So, we just sought to minimize them in the spec. I know some TCP implementations are being pretty aggressive with the min these days. But, it is not at all clear to me whether this is a net win or net lose. I.e., is the win in terms of peppiness to needed retransmits still a win when it is played against the consequences of a spurious transmit (i.e., setting cwnd = 1)? Perhaps there is a sweet spot that is < 1sec whereby this tradeoff better balances and we should be using that. But, it'd take a big experiment to show that, I think. [*] Maybe there is work that I just don't know about and if so I'd like to be pointed at it. Another thing I have wondered about from time to time .... Much like Early Retransmit could we have some sort of early RTO when the cwnd size is small? When the window size is small it seems that the downside of making a bad rexmt decision is not as bad as when the cwnd is large so it might pay in terms of responsiveness to RTO sooner. Also, as the cwnd gets larger we have better loss recovery techniques that should come into play that can repair lots of loss without involving the RTO and so perhaps by being overly aggressive in the RTO we just hurt ourselves. Finally, as the cwnd gets bigger that means we are transferring more data. And, the longer a connection goes the less important the fine-grained responsiveness matters. Just a random thought ... allman
- [tcpm] TCP tuning Lars Eggert
- Re: [tcpm] TCP tuning Adam Langley
- Re: [tcpm] TCP tuning Michael Welzl
- Re: [tcpm] TCP tuning SCHARF, Michael
- Re: [tcpm] TCP tuning Stefanos Harhalakis
- Re: [tcpm] TCP tuning Kacheong Poon
- Re: [tcpm] TCP tuning Joe Touch
- Re: [tcpm] TCP tuning Kacheong Poon
- Re: [tcpm] TCP tuning Joe Touch
- Re: [tcpm] TCP tuning Michael Welzl
- Re: [tcpm] TCP tuning Joe Touch
- Re: [tcpm] TCP tuning Michael Welzl
- Re: [tcpm] TCP tuning Joe Touch
- Re: [tcpm] TCP tuning Adam Langley
- Re: [tcpm] TCP tuning Joe Touch
- Re: [tcpm] TCP tuning Jerry Chu
- Re: [tcpm] TCP tuning Michael Welzl
- Re: [tcpm] TCP tuning Michael Welzl
- Re: [tcpm] TCP tuning Joe Touch
- Re: [tcpm] TCP tuning Jerry Chu
- Re: [tcpm] TCP tuning Jerry Chu
- Re: [tcpm] TCP tuning Joe Touch
- Re: [tcpm] TCP tuning Jerry Chu
- Re: [tcpm] TCP tuning John Heffner
- Re: [tcpm] TCP tuning Jerry Chu
- Re: [tcpm] TCP tuning Jerry Chu
- Re: [tcpm] TCP tuning Biswas, Anumita
- Re: [tcpm] TCP tuning Jerry Chu
- Re: [tcpm] TCP tuning John Heffner
- Re: [tcpm] TCP tuning Jerry Chu
- Re: [tcpm] TCP tuning Biswas, Anumita
- Re: [tcpm] TCP tuning Michael Welzl
- Re: [tcpm] TCP tuning Jerry Chu
- Re: [tcpm] TCP tuning Murali Bashyam
- Re: [tcpm] TCP tuning Michael Welzl
- Re: [tcpm] TCP tuning John Heffner
- Re: [tcpm] TCP tuning Joe Touch
- Re: [tcpm] TCP tuning Joe Touch
- Re: [tcpm] TCP tuning Joe Touch
- Re: [tcpm] TCP tuning Joe Touch
- Re: [tcpm] TCP tuning Joe Touch
- Re: [tcpm] TCP tuning Jerry Chu
- Re: [tcpm] TCP tuning Marco Mellia
- Re: [tcpm] TCP tuning Joe Touch
- Re: [tcpm] TCP tuning Joe Touch
- Re: [tcpm] TCP tuning Jerry Chu
- Re: [tcpm] TCP tuning Joe Touch
- Re: [tcpm] TCP tuning Jerry Chu
- Re: [tcpm] TCP tuning rick jones
- Re: [tcpm] TCP tuning Joe Touch
- Re: [tcpm] TCP tuning Mike Belshe
- Re: [tcpm] TCP tuning rick jones
- Re: [tcpm] TCP tuning rick jones
- Re: [tcpm] TCP tuning Joe Touch
- Re: [tcpm] TCP tuning Alexander Zimmermann
- Re: [tcpm] TCP tuning Kacheong Poon
- Re: [tcpm] TCP tuning Kacheong Poon
- Re: [tcpm] TCP tuning SCHARF, Michael
- Re: [tcpm] TCP tuning SCHARF, Michael
- Re: [tcpm] TCP tuning Andrew Yourtchenko
- Re: [tcpm] TCP tuning Joe Touch
- Re: [tcpm] TCP tuning Jerry Chu
- Re: [tcpm] TCP tuning rick jones
- Re: [tcpm] TCP tuning Kacheong Poon
- Re: [tcpm] TCP tuning Joe Touch
- Re: [tcpm] TCP tuning SCHARF, Michael
- Re: [tcpm] TCP tuning rick jones
- Re: [tcpm] TCP tuning Joe Touch
- Re: [tcpm] TCP tuning Joe Touch
- Re: [tcpm] TCP tuning Jerry Chu
- Re: [tcpm] TCP tuning Joe Touch
- Re: [tcpm] TCP tuning Mike Belshe
- Re: [tcpm] TCP tuning Joe Touch
- Re: [tcpm] TCP tuning Mark Allman
- Re: [tcpm] TCP tuning Mark Allman
- Re: [tcpm] TCP tuning Mark Allman
- Re: [tcpm] TCP tuning Mark Allman
- Re: [tcpm] TCP tuning Mark Allman
- Re: [tcpm] TCP tuning Joe Touch