Re: FreeBSD 4.x and OS-X tcp performance

From: Daniel Hartmeier (daniel_at_benzedrine.cx)
Date: 03/07/05

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    Date: Mon, 7 Mar 2005 10:08:02 +0100
To: Charles Sprickman <spork@fasttrackmonkey.com>

    On Sun, Mar 06, 2005 at 04:45:30PM -0500, Charles Sprickman wrote:

    > For fun I'm going to post a full tcpdump of an ftp session from one box to
    > the other, maybe someone can spot something there? It's attached and
    > bzip'd. It's a tcpdump of both hosts transferring a 1MB tarfile.

    I can only find an FTP control connection and _one_ data connection in
    that dump. Client 192.168.0.40 is uploading one file of about 1.6MB to
    server home.manymonkeys.com.

    There's a pattern in the dump, looks like a TCP problem. The client
    pushes data to the server. Every now and then, packets are lost. Mostly,
    the client retransmits normally. But ten times, it seems to ignore the
    server ACKing below a lost segment. It quickly gets several ACKs but
    only retransmits the lost segment after a full 1.4 seconds. This
    accounts for a total of 14.5 seconds of stalling the upload. The entire
    transfer is 15.02 seconds, so the 1.6MB are actually uploaded in 0.5
    seconds, and the stalling entirely accounts for the slow throughput.

    Looks like the client is at fault. There's window scaling, but with
    scale factors 0. No SACK. I think the client should retransmit earlier.

    Which OS is running on which of those hosts? Which host did you tcpdump
    on (or was it on a third machine, in between)? Could you get a tcpdump
    from both server and client simultanously for the same connection, so we
    can see where packets are lost, and get both peers' point of view?

    Would be interesting to see a tcpdump of a connection from the same
    client (same OS) to a different server OS, which works fine.

    Daniel
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