Date: Thu, 28 Jul 2005 22:46:00 -0400 From: Chuck Swiger <cswiger@mac.com> To: Daniel O'Connor <doconnor@gsoft.com.au> Cc: freebsd-net@freebsd.org, julian@elischer.org Subject: Re: AltQ + ng_iface Message-ID: <42E99868.1080306@mac.com> In-Reply-To: <200507291115.06612.doconnor@gsoft.com.au> References: <200507290834.10268.doconnor@gsoft.com.au> <200507291035.46770.doconnor@gsoft.com.au> <42E98725.1020600@mac.com> <200507291115.06612.doconnor@gsoft.com.au>
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Daniel O'Connor wrote: > On Friday 29 July 2005 11:02, Chuck Swiger wrote: >> Either the "established" or the "tcpflags !syn,ack" keywords in a rule >> adding matching packets to a high-priority queue ought to do it...? Or >> perhaps you meant something more specific than just "TCP packets with >> TH_ACK" set? :-) > > Hmm, I guess you could make those skip the pipe.. > >> Anyway, I'm not convinced that trying to classify packets within an >> established TCP connection in order to place them on different queues is a >> really good idea, since you're quite likely to reorder the packets by doing >> so. I'd expect both latency and bandwidth of a TCP connection to suffer >> very noticably if more than 10% or so of the packets arrive out of order... > > The theory is that by prioritising outgoing ACKs you don't cause downstream > delays when your upstream is full. eg > http://www.benzedrine.cx/ackpri.html Ah. OK, it makes sense that delaying outgoing ACKs too much would slow things down. So you want to send dataless ACKs at a higher priority than generic big packets full of data, maybe via the "iplen" keyword with "established", look for packets smaller than ~100 bytes? My other thought on this is to wonder about window size and whether that was scaling properly up to a reasonable value, and whether both sides implement a sane network stack, or whether the other side was a windows box looking for quick responses and usage of SACK, rather than BSD (new-reno?) delayed ACKs... >> [ Hmm. I suppose that one could make an exception to the above >> generalization if URG was set, but the TCP stack already makes an effort to >> prioritize and deliver out-of-band urgent stuff as quickly as possible, >> anyway, right? ] > > Maybe, but it doesn't appear to do a particularly good job for a lot of > situations :) I guess. :-) Getting 25% of the hoped-for max performance under a problematic case isn't so horrible, either, but I suspect other factors were involved, too. A tcpdump would've been informative.... -- -Chuck
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