Re: PMTU discovery is broken on kernel 3.7.1 for UDP sockets

["Yurij M. Plotnikov" <Yurij.Plotnikov@oktetlabs.ru>]

On 12/19/12 17:35, Ben Hutchings wrote:
> On Wed, 2012-12-19 at 17:10 +0400, Yurij M. Plotnikov wrote:
>    
>> On kernel 3.7.1 I get strange behaviour of IP_MTU_DISCOVER socket
>> option. The behaviour in case of IP_PMTUDISC_DO and IP_PMTUDISC_WANT
>> values of IP_MTU_DISCOVER socket option on SOCK_DGRAM socket are the
>> same and packet is always sent with "Don't Fragment" bit in case of
>> IP_PMTUDISC_WANT. Also, the value of IP_MTU socket option is not updated.
>>      
> You could try reverting:
>
> commit ee9a8f7ab2edf801b8b514c310455c94acc232f6
> Author: Steffen Klassert<steffen.klassert@secunet.com>
> Date:   Mon Oct 8 00:56:54 2012 +0000
>
>      ipv4: Don't report stale pmtu values to userspace
>
>      We report cached pmtu values even if they are already expired.
>      Change this to not report these values after they are expired
>      and fix a race in the expire time calculation, as suggested by
>      Eric Dumazet.
>
> Still, PMTU information is not supposed to expire for 10 minutes...
>
>    
With reverted commit there is no such problem on 3.7.1: IP_MTU is 
updated and DF is set only for the first packet in case of 
IP_PMTUDISC_WANT.
> [...]
>    
>> On eth2 on host_B and on eth1 on host_C change MTU from 1500 to 750.
>> Wait for a while.
>>
>> 9. send(6, lenght=1400) ->  1400 // the packet is sent with "Don't
>> Fragment" bit, tcpdump on eth1 on host_B shows it
>> 10. sleep(5);
>> 11. send(6, length=1400) ->  -1 with EMSGSIZE
>> 12. sleep(5);
>> 13. getsockopt(6,IP_MTU) ->  0 // Returns that MTU is 1500 once again. So
>> value is not updated.
>>      
> [...]
>
> What if you read this option immediately before the sleep(5)?
>    
It still returns that MTU is 1500.

Yurij.