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Weird MTU and TCP retranmission problem

  • From: Sam Stickland
  • Date: Fri Oct 22 10:42:15 2004


I haven't seen anything like this before, so I'm hoping someone here could enlighten me.

We have a customer that has taken a single co-located server from us. They can download large files from this server to any machine, except the Mac OS X machines at the end of their shared leased line at their office premises. Windows and OS9 machines at this site can download these files fine, as can Mac OS X machines at the end of consumer ADSL lines (offsite).

Downloads just stall shortly after starting, which initially appears to an MTU problem. Lowering the MTU on the affect Mac OS X machines fails to solve the problem until the MTU is set to 100 (yes 100) bytes. Strange that the windows machines don't have this problem. These affected Mac OS X machines don't experience this problem if the exact same files are downloaded from a different server in the same datacentre, behind the same router.

An ethereal dump of a failed download shows the following:

1.873772 Mac -> Server TCP [TCP Dup ACK 203#32] 7798 > http [ACK] Seq=786 Ack=106053 Win=65535 Len=0 TSV=4064715650 TSER=2370598
1.874145 Server -> Mac HTTP Continuation
1.885515 Mac -> Server TCP [TCP Dup ACK 203#33] 7798 > http [ACK] Seq=786 Ack=106053 Win=65535 Len=0 TSV=4064715650 TSER=2370598
1.885889 Server -> Mac HTTP Continuation
1.897384 Mac -> Server TCP [TCP Dup ACK 203#34] 7798 > http [ACK] Seq=786 Ack=106053 Win=65535 Len=0 TSV=4064715650 TSER=2370598
1.897758 Server -> Mac HTTP Continuation
1.909627 Mac -> Server TCP [TCP Dup ACK 203#35] 7798 > http [ACK] Seq=786 Ack=106053 Win=65535 Len=0 TSV=4064715650 TSER=2370598
1.921996 Mac -> Server TCP [TCP Dup ACK 203#36] 7798 > http [ACK] Seq=786 Ack=106053 Win=65535 Len=0 TSV=4064715650 TSER=2370598
1.933865 Mac -> Server TCP [TCP Dup ACK 203#37] 7798 > http [ACK] Seq=786 Ack=106053 Win=65535 Len=0 TSV=4064715650 TSER=2370598

which leads me unsure as to whether the server is failing to receive the ACKs (hence the ACK retransmission) or whether the Mac is failing to receive the next packet so is retransmitting what it believes to be a lost ACK for the last packet?

The server is an HP Proliant running Windows 2003, setup and installed by HP. It's running the built-in windows firewall (ICF), but the effects are the same if this is disabled.

Any suggestions of where to continue to look would be very much appreciated.

Sam




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