Subject: Re: Raise initial congestion window size / speedup slow start?

From: Hagen Paul Pfeifer <[email-masked]>
Date: Thu, 15 Jul 2010 00:36:33 +0200

* Ed W | 2010-07-14 23:05:31 [+0100]:

>Initial cwnd was changed (increased) in the past (rfc3390) and the
>RFC claims that studies then suggested that the benefits were all
>positive. Some reasonably smart people have suggested that it might
>be time to review the status quo again so it doesn't seem completely
>obvious that the current number is optimal?

Do you cite "An Argument for Increasing TCP's Initial Congestion Window"?
People at google stated that a CWND of 10 seems to be fair in their
measurements. 10 because the test setup was equipped with a reasonable large
link capacity? Do they analyse their modification in environments with a small
BDP (e.g. multihop MANET setup, ...)? I am curious, but We will see what
happens if TCPM adopts this.

>That RFC is a subtle read - it appears to give more specific guidance
>on what to do in certain situations, but I'm not sure I see that it
>improves slow start convergence speed for my situation (large RTT)?
>Would you mind highlighting the new bits for those of us a bit newer
>to the subject?

The objection/hint was more of general nature - not specific for larger RTTs.
Environments with larger RTTs are disadvantaged because TCP is ACK clocked.
Half-truth statement for my part because RTT fairness is and was an issue at
the development of new congestion control algorithms: BIC, CUBIC and friends.

>>Partial local issues can already be "fixed" via route specific ip options -
>>see initcwnd.
>
>Oh, excellent.  This seems like exactly what I'm after.  (Thanks
>Stephen Hemminger!)

Great, you are welcome! ;-)

Hagen