Subject: Re: network device and tcp-flow packet ordering

From: Vernon Schryver <[email-masked]>
Date: Sat, 10 Jun 2000 10:06:11 -0600 (MDT)

> From: Craig Partridge <[email-masked]>
>
> There's no RFC that says it.
>
> However, Jon Bennett, Nick Shectman and I published a paper showing how
> packet reordering really trashes TCP performance in IEEE/ACM Trans. on
> Networking last year.

I've a real life endorsement of that thought.  Before PPP MP appeared
(long sad tale of IETF politics suppressed), I shipped what I call
BF&I Multilink (brute force and ignorance) and what others call "round
robin" or "load sharing."  MP spends about 5 bytes per packet to maintain
packet order so that protocols that don't tolerate re-ordering will work
over bundles of PPP links.  BF&I Multilink simply sends IP packets to the
shortest queue or round-robin, and so re-orders some TCP segments.  The
results of that with BSD TCP code are signifant retransmissions due to
duplicate Acks, and a 10-20% reduction in throughput compared to the
obvious estimate or to MP.

I suspect there are similar real life reasons why Cisco tries so hard to
maintain associations between flows and individual routes when load
balancing amoung routes.

Vernon Schryver    [email-masked]

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From: Craig Partridge <[email-masked]>
Date: Sat, 10 Jun 2000 16:56:58 -0400

In message <[email-masked]>, Vernon Schryver writes
:

>I've a real life endorsement of that thought....

Just a small followup -- the paper I wrote with Bennett and Shectman
used real-life examples too.  We found a point in the Internet that
massively reordered under heavy load and caught a large number of
TCP traces that sufferred reordering.  So we could see what happened
when data was reordered, when acks were reordered, and when both
were reordered.  In IEEE/ACM Trans. on Networking, Vol 7, No 6 (Dec '99).

Thanks!

Craig

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From: Fred Baker <[email-masked]>
Date: Mon, 12 Jun 2000 13:01:25 -0700

>maintain associations between flows and individual routes when load

hmmm. Yup, that thought has occurred to us.