[e2e] Spurious Timeouts, Fact or Fake?

Jasleen Kaur jasleen at cs.unc.edu
Wed Aug 3 09:42:03 PDT 2011


You might find our paper (below) interesting, in which we analyze traces 
of nearly 3 million Internet transfers to study the performance of TCP 
loss detection and recovery mecahnisms. Among other things, it also logs 
the occurence of spurious timeouts on a per-OS basis.

S. Rewaskar, J. Kaur, and F.D. Smith, "A Performance Study of Loss 
Detection/Recovery in Real-world TCP Implementations,"** 
<http://www.cs.unc.edu/%7Ejasleen/papers/icnp07.pdf> in Proceedings of 
the IEEE International Conference on Network Protocols (ICNP'07), 
Beijing, China, Oct 2007.



On 8/3/2011 10:08 AM, Detlef Bosau wrote:
> During the recent past, this list has seen quite some few posts 
> regarding TCP RTT measurement.
> Now, first of all, I was interested in how often RTT measurments shall 
> be made and how they can be made. A particular concern is Karn's 
> Algorithm,
> because to my understanding, the consequence of Karn's algorithm is 
> that RTT measurements obtained by a single RTT timer can be taken only 
> when a sender has no outstanding duplicate packets.
> Perhaps, I'm wrong here.
> However, from what I've read so far, it is not yet completely clear, 
> how often RTT measurements should be made. The alternatives discussed 
> so fare are:
> - once round,
> - each packet.
> While the latter appears appealing to me, particularly when 
> implemented with time stamps (RFC 1323), which overcomes the problems 
> discussed by Karn & Partridge regarding the problem of packets being 
> sent more than once, some literature indicates problems with the SRTT 
> estimator when time stamps are in use.
> Now, the whole discussion is somewhat confusing to me.
> 1.: Spurious Timeouts are confusing to me, because spurious timeouts 
> (i.e. a packet which is well successfully transmitted, however the ACK 
> does not reach the sender on time) are basically expected by Edges 
> paper and the literature based upon this. However, there are papers 
> around, which put the mere existence of spurious timeouts in question, 
> e.g.
> author = "Francesco Vacirca and Thomas Ziegler and Eduard Hasenleithner",
> title="{TCP Spurious Timeout estimation in
> an operational GPRS/UMTS network}",
> month="May",
> year="2005",
> journal = "Forschungszentrum Telekommunikation Wien
> Technical Report
> FTW-TR-2005-008"
> }
> , while others give detailed recommendations how to deal with spurious 
> timeouts in practical implementations, e.g.
> http://tools.ietf.org/search/draft-allman-rto-backoff-02
> However, to me the problem seems closely coupled to the underlying 
> question whether or not we can estimate the expectation and variance 
> of the RTT in a TCP session. Edge requires the according stochastic 
> process to be weakly stationary. In other words: In a TCP session, 
> once having started and being run for some settling time, the 
> observerd RTT shall be, at least roughly, identically distributed.
> This distribution should be subject to only very slow and very rare 
> change, if at all.
> And accourding to RFC 2988, we can obtain SRTT and RTTVAR by RTT 
> samples using the well known EWMA estimators for this purpose.
> So, my questions are:
> 1.: How often shall RTTM be made?
> 2.: Is it reasonable to assume "weakly stationary" RTTs as done by Edge?
> 3.: Are the EWMA filters from RFC 2988 satisfactory, particularly are 
> these sufficiently generic to yield reasonable results for an 
> arbitrary TCP session?
> One could summarize these to the question: Do we obtain RTO in a 
> reasonable way? And when we talk about spurious timeouts, are we 
> talking about spurious timeouts - or are we talking about shortcomings 
> of the SRTT and RTTVAR estimators here?
> I'm somewhat confused here at the moment. And I would appreciate any 
> enlightenment ;-)
> Detlef

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