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Date: Fri, 25 Jun 2004 10:14:18 +1000
From: Tim Robbins <tjr@freebsd.org>
To: "Wilkinson, Alex" <alex.wilkinson@dsto.defence.gov.au>
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cc: Paul Saab <ps@freebsd.org>
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Subject: Re: HEADS UP: SACK committed to HEAD
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On Fri, Jun 25, 2004 at 09:16:38AM +0930, Wilkinson, Alex wrote:
> erm .... what is SACK ? I'm assuming ACK is 'acknowledgment ?
> Quick summary anyone ?

>From RFC 2018:

   TCP may experience poor performance when multiple packets are lost
   from one window of data.   With the limited information available
   from cumulative acknowledgments, a TCP sender can only learn about a
   single lost packet per round trip time.  An aggressive sender could
   choose to retransmit packets early, but such retransmitted segments
   may have already been successfully received.

   A Selective Acknowledgment (SACK) mechanism, combined with a
   selective repeat retransmission policy, can help to overcome these
   limitations.  The receiving TCP sends back SACK packets to the sender
   informing the sender of data that has been received. The sender can
   then retransmit only the missing data segments.


Tim