Re: TCP acking too fast

From: Mika Liljeberg (
Date: Sun Oct 14 2001 - 02:05:33 EST

"David S. Miller" wrote:
> You need to post for us a tcpdump trace of a connection you feel
> exhibits bad behavior.
> Otherwise we can do nothing but guess, effectively your statistics
> aren't helpful at all if we have no idea what is happening on the
> wire.

Fair enough, chalk it down to lack of sleep addling my brain. I also
forgot to mention my kernel version, which is 2.4.10-ac10.

I've attached a fragment of tcpdump output from the middle of steady
state transfer. Looking at the dump, it seems that most arriving
segments have the PSH bit set. This leads me to believe that the
transfer is mostly application limited at the sender side.

For some reason, this causes the receiver to ack every segment
immediately (which is not suggested by the spec as far as I know). I'm
guessing that this is some kind of optimization for HTTP (i.e., avoid
Nagle on the last runt segment by acking pushed segments immediately).
However, this seems to produce less than desirable behaviour on sender
limited bulk transfers.

However, despite appearances, I can't seem to find the bit of code that
tests the PSH flag for immediate ack. Still sleepy, I guess.



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This archive was generated by hypermail 2b29 : Mon Oct 15 2001 - 21:00:53 EST