[Iccrg] Question on RFC 2988 - TCP Retransmission timer

[Lachlan Andrew]

Greetings Gorry,

On 18/10/2007, Gorry Fairhurst <gf at erg.abdn.ac.uk> wrote:
>
> I'm surprised that anyone would judge "200ms" as "safe" for general
> deployment, and would like to understand why they believe this is desirable.

TCP has a mechanism for determining the timeout, based on the mean RTT
and the mean deviation.  If the minimum RTO is 1s as per the RFC, this
value is carefully calculated, and then ignored, since 1s will be used
instead.

Nobody is saying that it is safe to have a rule like "use RTO=200ms".
However, it should be "safe" to trust the RTO calculations, unless
they predict a very small timeout.  If that is not the case, we should
IMO fix the RTO calculations, rather than simply ignoring them and
using a 1s minimum.

> I'd also wonder why tuning the min RTO is needed (except to recover from
> quirky packet loss in the few initial RTTs), since modern TCP does a
> pretty good job of recovering packets without needing timer tuning.

I'm not sure what you mean by "without needing timer tuning".  RTO
tuning has been part of TCP since the '80s.

Cheers,
Lachlan

-- 
Lachlan Andrew  Dept of Computer Science, Caltech
1200 E California Blvd, Mail Code 256-80, Pasadena CA 91125, USA
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