Date: Tue, 8 Mar 2005 13:46:46 +0100 From: Daniel Hartmeier <daniel@benzedrine.cx> To: Charles Sprickman <spork@fasttrackmonkey.com> Cc: freebsd-net@freebsd.org Subject: Re: FreeBSD 4.x and OS-X tcp performance Message-ID: <20050308124646.GF26999@insomnia.benzedrine.cx> In-Reply-To: <20050308101633.GC26999@insomnia.benzedrine.cx> References: <Pine.OSX.4.61.0503041726460.5816@oof.local> <Pine.OSX.4.61.0503041841060.5816@oof.local> <20050305024850.GA96307@wjv.com> <Pine.OSX.4.61.0503061628260.5816@oof.local> <20050307090802.GR26999@insomnia.benzedrine.cx> <Pine.OSX.4.61.0503071358310.5816@oof.local> <20050307204825.GY26999@insomnia.benzedrine.cx> <Pine.OSX.4.61.0503080150550.5816@oof.local> <20050308101633.GC26999@insomnia.benzedrine.cx>
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According to RFC 793 (the original TCP specification), the client may (even should) wait at least one second before retransmitting any segment. However, RFC 2001 describes Fast Retransmission, where the third acknowledgment for the same segment should be interpreted as an indication of packet loss, and cause an immediate retransmission (without waiting for the timeout of at least one second). I'd have expected Mac OS X to both implement this and enable it by default, but maybe I'm wrong. There's a sysctl net.inet.tcp.newreno which defaults to 0, and which seems to affect things. If you google, you find patches like http://www.opendarwin.org/~fkr/xnu/mach_kernel-to-517.7.21-SACK.diff which doesn't just contain SACK code, but possibly fixes fast retransmissions when newreno is not set. I'm not sure if disabling the newreno sysctl should disable fast retransmissions, or whether that's a bug. So, the Mac OS X client is not wrong in honouring RFC 793 alone. It just suffers badly from any packet loss. For you, the more relevant question is why there's 1.2% packet loss between the Mac OS X client and FreeBSD server, even when connected directly with crossover. Daniel
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