[Xen-users] domU ignores MTU

[Christoph Fritsche <chr.fr@xxxxxxx>]

Hi list,

I got a wired network problem with a DNATed domU. During a TCP session 
the domU tries to send juboframes which get lost on the path.
I'm using almost stock Debian etch with Xen 3.2.1 from backports.

tcpdump says:
17:50:27.595327 IP (tos 0x0, ttl  64, id 48632, offset 0, flags [DF], 
proto: TCP (6), length: *2948*) 172.31.0.12.80 > 85.239.122.249.1213: . 
115841:118737(2896) ack 119 win 181 <nop,nop,timestamp 1187477 3076040081>
The TCP handshake has committed a segemt size of 1460. In ifconfig the 
MTU shows up as 1500.
The domU is linked with dom0 by a internal bridge and dom0 acts as gateway.

It looks like the frame makes it through the dom0 and gets dropped 
somewhere on the path, probably the next router. The retransmit is done 
with a proper size. Some packets later the same behavior occurs again 
resulting in an overall poor network performance.
Is there a way, other than ifconfig, to set the MTU, or force Xen not to 
send juboframes?
Thanks for your thoughts on this issue.
Christoph





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