Subject: Re: Doubt regarding the MANAGING THE RTO TIMER

From: Mark Allman <[email-masked]>
Date: Mon, 10 Dec 2001 13:12:47 -0500

(Chunking through old messages...)

> In my opinion to avoid spurious RTOs TCP should collect RTT
> samples more frequently than once per window, 

Why?  Can you explain a bit more?

Vern and I found that collecting more samples does not seem to help
the accuracy of the timer.  See our 1999 SIGCOMM paper:

    Mark Allman, Vern Paxson.  \textit{On Estimating End-to-End
    Network Path Properties}.  ACM SIGCOMM, September 1999.
    http://roland.grc.nasa.gov/~mallman/papers/estimation.ps

allman

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From: "Andrei Gurtov" <[email-masked]>
Date: Tue, 11 Dec 2001 03:59:46 +0200

> the accuracy of the timer.  See our 1999 SIGCOMM paper:

I know this paper well, and I agree that based on backbone traces you have
been using there probably isn't much benefit of frequent RTT samples.
However, I think that if traces would be collected at some wireless gateway,
the result might have been different. The environment I was looking at
included slow overbuffered links with occasional delay spikes. On slow links
the queueing delay can double between two adjacent RTT samples if they are
collected once per window. This type of situation was the motivation for VJ
to increase the RTTvar coefficient from 2 to 4. Still, experimenting in NS
with TCP without timestamps I was getting spurios timeouts after even a
slight delay spike or during fast recovery. A technical report with results
and traces to back-up this will be available shortly. Hopefully, we will
have an interesting discussion.

Thanks,
Andrei