Date: Tue, 09 Mar 2004 13:42:05 -0800 From: "Kevin Oberman" <oberman@es.net> To: Brad Knowles <brad.knowles@skynet.be> Cc: freebsd-current@freebsd.org Subject: Re: Who wants SACK? (Re: was My planned work on networking stack) Message-ID: <20040309214205.3EE2D5D07@ptavv.es.net> In-Reply-To: Your message of "Tue, 09 Mar 2004 20:13:11 %2B0100." <p06002062bc73c73fc798@[10.0.1.4]>
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> Date: Tue, 9 Mar 2004 20:13:11 +0100 > From: Brad Knowles <brad.knowles@skynet.be> > Sender: owner-freebsd-current@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@es.net Phone: +1 510 486-8634
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