[e2e] Spurious Timeouts, Fact or Fake?

Emmanuel Lochin emmanuel.lochin at gmail.com
Wed Aug 3 10:45:12 PDT 2011

Hi Detlef,

I think the study on spurious timeout you cite cannot be transposed to
the Internet.
If you look at :

Sharad Jaiswal, Gianluca Iannaccone, Christophe Diot, James F. Kurose,
Donald F. Towsley,
Measurement and classification of out-of-sequence packets in a tier-1
IP backbone.
IEEE/ACM Trans. Netw. (TON) 15(1):54-66 (2007)

the authors show that 40% of the links present in their dataset
effectively reorder
packets (might due to load balancing, multiple network paths, dynamic
route generation and link bonding).


On 3 August 2011 16:08, Detlef Bosau <detlef.bosau at web.de> 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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