Date: Tue, 09 Mar 2004 15:09:53 -0800 (PST) From: jhanna@shaw.ca To: Brad Knowles <brad.knowles@skynet.be> Cc: Kevin Oberman <oberman@es.net> Subject: Re: Who wants SACK? (Re: was My planned work on networking stack) Message-ID: <XFMail.20040309150953.jhanna@shaw.ca> In-Reply-To: <20040309214205.3EE2D5D07@ptavv.es.net>
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On 09-Mar-2004 Kevin Oberman wrote: >> 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. Radio links as well, with their latency and higher frame drop rates, can benefit considerably. Cell phones and such may account for a large amount of garden variety traffic as time goes on. jhanna@shaw.ca
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