Who wants SACK? (Re: was My planned work on networking stack)

Mark Allman mallman at icir.org
Wed Mar 10 11:38:42 PST 2004

> I looked for the paper I paraphrased, I'm pretty sure if was one by
> Sally Floyd.

I don't have the paper reference handy, but it's Sally's HighSpeed TCP
work.  I do happen to have a blurb on it sitting here that I think
captures it well...  Think of a network with an RTT of 100ms, a 1500
byte packet size and 10Gbps capacity.  That means the congestion window
needs to be 83,333 packets to fill the pipe (just by looking at the
delay*bandwidth product of the network).  And, to sustain this rate you
need at most one loss every 5,000,000,000 packets (from the TCP model).
That translates into about one loss every 100 minutes.  And, that seems
like a fairly large stretch.  (This actually might be laid out in

However, this is a bit off-topic from SACK.  Because this is all based
on the AIMD nature of TCP's congestion control, not really on whether
you employ SACK.  (The world would be worse than described above if you
didn't use SACK.  But, if you have to take at most one loss every 100
minutes that's pretty bad already.)

There is an experimental change to TCP's algorithms specified in
RFC3649 (that only applies when you are going quite fast).


Mark Allman -- ICIR -- http://www.icir.org/mallman/

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