Subject: Re: [tcpm] Proposing ACKNOW bit in TCP header

From: Michael Scharf <[email-masked]>
Date: Thu, 28 Dec 2006 14:23:03 +0100

On Tue, 21 Nov 2006 at 15:41:04, Lloyd Wood wrote:
> >FWIW, Linux has had ABC for quite some time now as well.  First it was 
> >enabled by default, but no it's disabled now due to the performance 
> >problems in lots of applications which seem to rely on earlier 
> >behaviour.
> 
> Do you have a pointer to information on the problems with applications?
> 
> I'd been presuming that ABC is universally a good thing, even though its use blocks the count-the-acks-not-the-bytes sender hole that allows simple ack-splitting intercept-one-ack-send-two 'TCP acceleration', which isn't outlined in RFC3135, but is implemented in Cisco IOS:
> http://www.cisco.com/en/US/products/ps6441/products_feature_guide09186a0080683d9d.html

Performance problems might also occur with typical signaling
applications, e. g., in IP telephony platforms:

We have performed measurements on the end-to-end delay of small-sized
signaling messages for the case that a (small) percentage of messages
get lost, i. e., head-of-line blocking effects occur. We have studied
this effect both for TCP and SCTP with Linux kernel 2.6.15, and we
have compared the measurement results to the theoretical optimum.

We found that using ABC in Linux TCP results in significantly higher
latencies. Some of our findings can be found in the following
presentation from IEEE Globecom 2006:

  http://www.ikr.uni-stuttgart.de/~scharf/globecom2006_scharf_kiesel.pdf

Details on the theoretical model can be found in the corresponding paper:

  Michael Scharf, Sebastian Kiesel: "Head-of-Line Blocking in TCP and
  SCTP: Analysis and Measurements", Proc. IEEE Globecom, Nov. 2006

Note that TCP performs significantly worse than SCTP in our setup.

Best regards,

Michael Scharf and Sebastian Kiesel

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