Date: Sat, 03 Mar 2007 23:51:11 +0100 From: Andre Oppermann <andre@freebsd.org> To: Andrew Gallatin <gallatin@cs.duke.edu> Cc: Peter Jeremy <peterjeremy@optushome.com.au>, freebsd-current@freebsd.org Subject: Re: excessive TCP duplicate acks? Message-ID: <45E9FBDF.9050606@freebsd.org> In-Reply-To: <17897.60107.576150.627357@grasshopper.cs.duke.edu> References: <17850.13146.266196.499166@grasshopper.cs.duke.edu> <20070303000125.GA9918@turion.vk2pj.dyndns.org> <45E99060.3030404@freebsd.org> <17897.60107.576150.627357@grasshopper.cs.duke.edu>
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Andrew Gallatin wrote: > Andre Oppermann writes: > > This thing is really strange and difficult to debug. A look at the CVS history > > of tcp_input/output doesn't show any smoking gun. ACKs like these are totally > > pointless. There are three places able to cause ACKs: 1) tcp_input decides to > > call tcp_output [not the case here as there are no corresponding input packets > > to cause this]; 2) tcp_output has a unterminated loop somewhere causing it to > > spew the ACKs in rapid succession [unlikely as it holds the tcpcb lock and that > > would block inbound packets]; 3) tcp timers are misfiring or not properly dis- > > armed [here the logic in tcp_output may/should just ignore it and return w/o > > sending any packet]. > > When I was taking traces a few weeks ago, I remember having the > impression that the acks would start happening when the FreeBSD > sender didn't have enough space in the window to send any more > data, and would seemingly continue until the (Linux|Macosx) > receiver sent an ack which updated the window and allowed > the FreeBSD sender to send more data. Ok, that's an ambulance worth chasing. Lets see if it leads us to an accident. > Are you using a FreeBSD receiver? Both Linux and MacOSX ack far, far > less often than we do. Maybe a FreeBSD receiver acks too quickly > for you to see the bug? I'll try reproduce the bug against OSX. -- Andre
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