netdev - Re: Question about LRO/GRO and TCP acknowledgements

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Message-ID: <20110612113004.79f48f40@konijn>
Date:	Sun, 12 Jun 2011 11:30:04 +0200
From:	Joris van Rantwijk <joris@...isvr.nl>
To:	Eric Dumazet <eric.dumazet@...il.com>
Cc:	netdev@...r.kernel.org
Subject: Re: Question about LRO/GRO and TCP acknowledgements

On 2011-06-12, Eric Dumazet <eric.dumazet@...il.com> wrote:
> > As far as I can see, current code will send just one ACK per
> > coalesced GRO bundle, while the sender expects one ACK per two
> > segments.

> One ACK carries an implicit ack for _all_ previous segments. If sender
> only 'counts' ACKs, it is a bit dumb...

It may be dumb, but it's what the RFCs recommend and it's what Linux
implements.

RFC 5681:
  "During slow start, a TCP increments cwnd by at most SMSS bytes for
   each ACK received that cumulatively acknowledges new data."

In Linux, each incoming ACK causes one call to tcp_cong_avoid(),
which causes one call to tcp_slow_start() - assuming the connection is
in slow start - which increases the congestion window by one MSS.
Am I mistaken?

Please note I'm talking about managing the congestion window.
Of course I agree that each ACK implicitly covers all previous segments
for the purpose of retransmission management. But congestion
management is a different story.

Joris.
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