Bad TCP performance with large MTU on 7-stable.

Steven Hartland killing at
Thu May 29 15:04:05 UTC 2008

Does the trace show the window size being negotiated correctly?

----- Original Message ----- 
From: "Arnaud Houdelette" <arnaud.houdelette at>

>I got really poor performance when I try to upload files to the box (via 
> pureFTP or Samba) when using jumbo frames somewhat  above  2k.
> uname -a :
> FreeBSD 7.0-STABLE FreeBSD 7.0-STABLE #4: Wed May 28 
> 17:45:14 CEST 2008     
> tzim at  amd64
> It seems that TCP acks are delayed for about a second, so the upload 
> rate nevers goes above 60kB/s. I can provide wireshark captures done on 
> the box uploading the files if necessary.
> The client is a Windows XP64 box, but I do not recall having the same 
> problem on 6.2. I did got about 100 MB/s for similar file transfers, but 
> I had to disable jumbo frames when I upgraded to 7.0-Release because of 
> some bug on txcsum/rxcsum on re(4) so I only noticed the issue recently.
> The txcsum bug doesn't seem to occur anymore, but now I have this 
> strange rate issue, with or without checksum offloading disabled.
> I also tried with tso disabled. Same results. Is it related to the re(4) 
> driver ? Or to the TCP stack ?

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