On Mon, 7 Mar 2005, Daniel Hartmeier wrote:
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.
Correct. 192.168.0.40 is the OS-X box, home.manymonkeys.com (192.168.0.6) is the FreeBSD box. I sent a 1.6MB tar archive via ftp.
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.
Very interesting, thank you for that read of the tcpdump output. If you have the time, could you post back a few lines of the tcpdump with comments so that I might learn a little about what's going on? I don't have the best understanding of the intricacies of tcp...
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?
The tcpdump was run on the server (FBSD). Later today I will gladly do this again with a dump from each side.
Would be interesting to see a tcpdump of a connection from the same client (same OS) to a different server OS, which works fine.
I will do that as well from both ends. The other box will be OpenBSD 3.3 (anything beyond 3.3 panics on boot, so I'm stuck at 3.3).
For giggles I'll do a dump of a transfer via udp (NFS) between the problem boxes to see if any packet loss is happening...
Thanks very much,
Charles
Daniel
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