Re: [tcpm] Re: RFC 1323: Timestamps option
Matt Mathis <mathis@psc.edu> Wed, 30 May 2007 22:23 UTC
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Date: Wed, 30 May 2007 18:23:16 -0400
From: Matt Mathis <mathis@psc.edu>
To: David Borman <david.borman@windriver.com>
Subject: Re: [tcpm] Re: RFC 1323: Timestamps option
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Cc: TCP Maintenance and Minor Extensions WG <tcpm@ietf.org>, Joe Touch <touch@isi.edu>
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You are being more conservative than necessary: If I start out w/o timestamps, then I am assuming that I will be at low rate (or perhaps a short flow), and I don't need PAWS. I just need to be sure that PAWS is fully enabled before I would send the same sequence number within one MSL. As long as I meet this criteria, all prior segments are protected by either the MSL or as the "minus first" clock tick. Which raises another point, if I start without timestamps and just a few seconds later I am already 25% through the sequence space (think 10 Gigabit Ethernet), and I then try to turn on timestamps and the other end balks, what do I do? One answer would be to force the window down to RTT*4294967296/MSL, but people might think that is just a bit draconian. Furthermore, this solution has the nasty marketing problem that a correct highspeed stack is vastly slower than an incorrect implementation in some situations. Other suggestions? Ignore the problem? What happens today if you fail to request timestamps, and the path turns out to be a much faster than 100 Mb/s? Although permitting "late timestamps" has been on my todo list for years, I have come to think that a safer solution is to unconditionally request timestamps as long as the route of choice does not point to a directly connected compressing modem on a voice grade line. There are now several secondary control algorithms (such as the receiver side autotuning in Linux) that require timestamps for optimal operation, and thus provide motivation for (almost) unconditionally turning them on. Thanks, --MM-- ------------------------------------------- Matt Mathis http://www.psc.edu/~mathis Work:412.268.3319 Home/Cell:412.654.7529 ------------------------------------------- Evil is defined by mortals who think they know "The Truth" and use force to apply it to others. On Wed, 30 May 2007, David Borman wrote: > Hi Matt, > > On May 30, 2007, at 10:59 AM, Matt Mathis wrote: > > > I had assumed that you can "fake" paws as follows: once you receive > > a late > > timestamp negotiation (in either role), you treat any later > > untimestamped > > segments as though they bear the timstamp just before the earliest > > time stamp > > seen. I believe that this is optimal, given other assumptions. > > Yes, for PAWS you only need the clock to tick at least once per > sequence wrap, so as long as Timestamps are enabled before the first > wrap, PAWS should work just fine this way. > > > > > I think it also works well enough to treat any later untimestamped > > segments as though they bear the timestamp just before the the most > > recently > > received timestamp. This is actually the same if the timestamps do > > not > > advance during the negotiation RTT. (Well enough means that the > > failures > > are neither frequent nor serious). > > The problem is, how do you tell that it is a "later untimestamped" > segment, and not an old segment from before we enabled timestamps? > This would allow old packets from before we enabled timestamps to > arrive after the sequence wrap and be treated as OK, the very thing > that PAWS is trying to prevent. That's one of the reasons why once > you enable timestamps, you have to then include them in every packet > for the duration of the connection. > > -David Borman > _______________________________________________ tcpm mailing list tcpm@ietf.org https://www1.ietf.org/mailman/listinfo/tcpm
- [tcpm] RFC 1323: Timestamps option David Borman
- Re: [tcpm] RFC 1323: Timestamps option Sally Floyd
- Re: [tcpm] RFC 1323: Timestamps option Mark Allman
- Re: [tcpm] RFC 1323: Timestamps option David Borman
- Re: [tcpm] RFC 1323: Timestamps option Matt Mathis
- Re: [tcpm] RFC 1323: Timestamps option Fernando Gont
- Re: [tcpm] RFC 1323: Timestamps option David Borman
- Re: [tcpm] RFC 1323: Timestamps option David Malone
- Re: [tcpm] RFC 1323: Timestamps option Mark Allman
- Re: [tcpm] RFC 1323: Timestamps option Mark Allman
- Re: [tcpm] RFC 1323: Timestamps option Richard Wendland
- [tcpm] RE: RFC 1323: Timestamps option Mahdavi, Jamshid
- Re: [tcpm] RFC 1323: Timestamps option Kacheong Poon
- Re: [tcpm] RFC 1323: Timestamps option Gavin McCullagh
- [tcpm] Re: RFC 1323: Timestamps option David Borman
- Re: [tcpm] Re: RFC 1323: Timestamps option Joe Touch
- Re: [tcpm] Re: RFC 1323: Timestamps option David Borman
- Re: [tcpm] Re: RFC 1323: Timestamps option Matt Mathis
- Re: [tcpm] Re: RFC 1323: Timestamps option Joe Touch
- Re: [tcpm] Re: RFC 1323: Timestamps option David Borman
- Re: [tcpm] Re: RFC 1323: Timestamps option David Borman
- Re: [tcpm] Re: RFC 1323: Timestamps option Matt Mathis
- Re: [tcpm] Re: RFC 1323: Timestamps option Joe Touch