From owner-freebsd-current@FreeBSD.ORG Sat Mar 3 22:51:05 2007 Return-Path: X-Original-To: freebsd-current@freebsd.org Delivered-To: freebsd-current@freebsd.org Received: from mx1.freebsd.org (mx1.freebsd.org [69.147.83.52]) by hub.freebsd.org (Postfix) with ESMTP id 07FBD16A402 for ; Sat, 3 Mar 2007 22:51:05 +0000 (UTC) (envelope-from andre@freebsd.org) Received: from c00l3r.networx.ch (c00l3r.networx.ch [62.48.2.2]) by mx1.freebsd.org (Postfix) with ESMTP id 728F913C4B7 for ; Sat, 3 Mar 2007 22:51:04 +0000 (UTC) (envelope-from andre@freebsd.org) Received: (qmail 2665 invoked from network); 3 Mar 2007 22:23:22 -0000 Received: from c00l3r.networx.ch (HELO [127.0.0.1]) ([62.48.2.2]) (envelope-sender ) by c00l3r.networx.ch (qmail-ldap-1.03) with SMTP for ; 3 Mar 2007 22:23:22 -0000 Message-ID: <45E9FBDF.9050606@freebsd.org> Date: Sat, 03 Mar 2007 23:51:11 +0100 From: Andre Oppermann User-Agent: Thunderbird 1.5.0.9 (Windows/20061207) MIME-Version: 1.0 To: Andrew Gallatin 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> In-Reply-To: <17897.60107.576150.627357@grasshopper.cs.duke.edu> Content-Type: text/plain; charset=ISO-8859-1; format=flowed Content-Transfer-Encoding: 7bit Cc: Peter Jeremy , freebsd-current@freebsd.org Subject: Re: excessive TCP duplicate acks? X-BeenThere: freebsd-current@freebsd.org X-Mailman-Version: 2.1.5 Precedence: list List-Id: Discussions about the use of FreeBSD-current List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , X-List-Received-Date: Sat, 03 Mar 2007 22:51:05 -0000 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