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Date: Sun, 14 Oct 2001 16:05:11 +0200
From: Andi Kleen <ak@muc.de>
To: Mika Liljeberg <Mika.Liljeberg@welho.com>
Cc: Andi Kleen <ak@muc.de>, "David S. Miller" <davem@redhat.com>,
        linux-kernel@vger.kernel.org, kuznet@ms2.inr.ac.ru
Subject: Re: TCP acking too fast
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In-Reply-To: <3BC97BC5.9F341ACE@welho.com>; from Mika Liljeberg on Sun, Oct 14, 2001 at 01:49:25PM +0200
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On Sun, Oct 14, 2001 at 01:49:25PM +0200, Mika Liljeberg wrote:
> Andi Kleen wrote:
> > The only special case for PSH in RX left I can is in rcv_mss estimation,
> > where is assumes that a packet with PSH set is not full sized.
> 
> A packet without PSH should be full size. Assuming the sender implemets
> SWS avoidance correctly, this should be a safe enough assumption.

It's not guaranteed by any spec; just common behaviour from BSD derived
stacks. SWS avoidance does not say anything about PSH flags.


> 
> > On further
> > look the 2.4 tcp_measure_rcv_mss will never update rcv_mss for packets
> > which do have PSH set and in this case cause random ack behaviour depending
> > on the initial rcv_mss guess.
> > Not very nice; definitely violates the "be conservative what you accept"
> > rule. I'm not sure how to fix it, adding a fallback to every-two-packet-add
> > would pollute the fast path a bit.
> 
> You're right. As far as I can see, it's not necessary to set the
> TCP_ACK_PUSHED flag at all (except maybe for SYN-ACK). I'm just writing
> a patch to clean this up.

Setting it for packets >= rcv_mss looks useful to me to catch mistakes.
Better too many acks than to few.


-Andi
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