Virtualization - May 2023 - Re: [PATCH] virtio_net: set default mtu to 1500 when 'Device maximum MTU' bigger than 1500

On 6 May 2023 09:56:35 BST, Hao Chen <chenh at yusur.tech>
wrote:
>In the current code, if the maximum MTU supported by the virtio net hardware
is 9000, the default MTU of the virtio net driver will also be set to 9000. When
sending packets through "ping -s 5000", if the peer router does not
support negotiating a path MTU through ICMP packets, the packets will be
discarded.
That router is just plain broken, and it's going to break all kinds of
traffic. Hacking the virtio-net MTU is only a partial workaround.

Surely the correct fix here is to apply percussive education to whatever idiot
thought it was OK to block ICMP. Not to hack the default MTU of one device to
the lowest common denominator.

On Sun, May 07, 2023 at 10:31:34AM +0100, David Woodhouse
wrote:
> 
> 
> On 6 May 2023 09:56:35 BST, Hao Chen <chenh at yusur.tech> wrote:
> >In the current code, if the maximum MTU supported by the virtio net
hardware is 9000, the default MTU of the virtio net driver will also be set to
9000. When sending packets through "ping -s 5000", if the peer router
does not support negotiating a path MTU through ICMP packets, the packets will
be discarded.
> 
> That router is just plain broken, and it's going to break all kinds of
traffic. Hacking the virtio-net MTU is only a partial workaround.
> 
> Surely the correct fix here is to apply percussive education to whatever
idiot thought it was OK to block ICMP. Not to hack the default MTU of one device
to the lowest common denominator.
Yea I don't understand what does path MTU have to do with it.
MTU has to be set the same for all endpoints on LAN, that's
a fundamental assumption that ethernet makes. Going outside LAN
all best are off.


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MST