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xen-users
RE: [Xen-users] domU sending frames larger than MTU
>
> I have a machine running a couple of domUs in a private (i.e. not
bridged
> with any real ethernet devices) virtual network where the host machine
is
> a router between the domUs and the physical network.
>
> I have been observing, via tcpdump, the following behaviour:
>
> 14:36:26.777823 IP 192.168.0.1.988 > 10.75.22.151.1022: .
> 6992256:6996300(4044) ack 1 win 362 <nop,nop,timestamp 58407492
129250939>
> 14:36:26.777922 IP 192.168.0.254 > 192.168.0.1: ICMP 10.75.22.151
> unreachable - need to frag (mtu 1500), length 556
>
> Is this a bug or intended behaviour? If intended, can I prevent it
> somehow?
>
It's intended behaviour to send packets that big, but it's a bug that it
doesn't work for you. The idea is that DomU sends big packets, and the
hardware adapter splits them up into MTU sized packets.
What is your DomU?
If it's windows running GPLPV, you can turn off the large send offload
in the adapter properties.
If it's linux (pv or hvm) you can use ethtool to disable the large send
offload, eg ethtool -K eth0 tso off
James
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