MTU - wireshark showing length longer than.

[Ant Mitchell <ant () badmagic com>]

Hi,

I am new to wireshark and TCP diagnostics in general. I have a problem  
with a HP procurve switch that I am trying to diagnose.

I have removed the switch from my config, and now have an xserve  
plugged into a centos box, both running 10gb myricom cards.

Both ends have flow control enabled, and an MTU of 9000. I have double  
triple checked this.

I have been running netperf tests across the link, mostly from the  
xserve to the centos box, and I see wireshark report packet lengths  
way in excess of the mtu. The throughput I am getting is about 2900  
Mbps.

How is the packet length being reported so large ?, sometimes these  
large packets seem to get acked and sometimes they don't , resulting  
in lost segments.

The same large packets appear in tcpdump also, so its not a bug in  
wireshark.

What should I be looking at ?, how does the packet size get larger  
than the MTU ?. A typical size is 8960 for several packets, then 17960  
and 20456 and several inbetween.

Thanks

--
Ant

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