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

[Kevin Oberman]

> Date: Tue, 9 Mar 2004 20:13:11 +0100
> From: Brad Knowles <brad.knowles at skynet.be>
> Sender: owner-freebsd-current at freebsd.org
> 
> At 3:32 PM -0800 2004/03/08, Jeffrey Hsu wrote:
> 
> >  What Luigi says is absolutely correct.  It doesn't take a lot to
> >  get this done.  I've talked to a number of companies about implementing
> >  SACK for them and while there was interest, no one wanted to fund
> >  it all themselves, potentially for the benefit of their competitors.
> 
> 	Out of curiosity, can someone provide some pointers as to where 
> SACK really helps?  Is this just for high-speed WANs and doesn't help 
> on LANs, or is it useful in both contexts?  Also, at what 
> speeds/packet sizes does SACK start to become really useful?
> 
> 	I'm just wondering if there aren't a lot of people who could 
> benefit from something like this, only they don't know it.  If they 
> were to find out, it might help provide funding and other resources 
> to spur development.

Selective ACKnowledgment (SACK) allows acknowledgment of received
packets in a TCP window so that only the missing/damaged packet needs to
be re-transmitted. This is normally of little value on a LAN where ACKs
arrive quickly and windows are smaller and no use on slow circuits. On
fat pipes with latency and big windows it is a huge win as it allows you to
recover much faster from a packet drop. If you don't have SACK, you need
to re-transmit all of the packets in flight within the window while
with SACK, you need only retransmit the dropped packet(s). If you have a
10 or 20 MB window, this is a big deal.

Dynamic window sizing will make it of less significance in LANs as the
windows will not be very large.
-- 
R. Kevin Oberman, Network Engineer
Energy Sciences Network (ESnet)
Ernest O. Lawrence Berkeley National Laboratory (Berkeley Lab)
E-mail: oberman at es.net			Phone: +1 510 486-8634