[Serdev] SIP UDP / dont fragment issue

[Benjamin Black]

My mistake for misreading his email: it is happening with UDP and I  
was referring to TCP.  Regardless, try disabling PMTUD to see if your  
problem is resolved.


Ben

On Jun 1, 2005, at 4:01 AM, Maxim Sobolev wrote:

> No, he is not quite correct. We are talking about UDP, not TCP.
> UDP doesn't have path MTU discovery mechanism, so that kernel
> should never set DF flag on UDP packets.
>
> -Maxim
>
> On Wed, Jun 01, 2005 at 11:22:53AM +0200, Martin Koenig wrote:
>
>> Hello Ben,
>>
>> you are correct, of course. The problem originates from a Router  
>> on the
>> way to the destination (Japan) that drops / does not reply with the
>> correct ICMP destination unreachable.
>>
>> Sorry for the disturbance,
>>
>> regards,
>> Martin
>>
>> Benjamin Black wrote:
>>
>>
>>> That's the kernel implementing TCP path MTU discovery.  The first  
>>> hop
>>> before the low MTU link should be sending back an ICMP message
>>> (fragmentation required/DF set) that includes the MTU of the  
>>> outgoing
>>> link.  So, the problem is not really SER, which doesn't control
>>> setting the DF bit, but the router in front of the slow link that
>>> isn't sending back the ICMP messages needed for PMTUD to function  
>>> (or
>>> perhaps you are filtering them).
>>>
>>> Try disabling PMTUD (I forget if this is the proper way under  
>>> current
>>> Linux systems):
>>>
>>>    echo 1 >/proc/sys/net/ipv4/ip_no_pmtu_disc
>>>
>>>
>>>
>>> Ben
>>>
>>> On May 28, 2005, at 5:22 PM, Martin Koenig wrote:
>>>
>>>
>>>> Hello,
>>>>
>>>>
>>>>
>>>>> Martin Koenig wrote:
>>>>>
>>>>>
>>>>>
>>>>>> Hello,
>>>>>>
>>>>>> we are having issues piping SIP to asia via a low MTU transit
>>>>>> point. The problem is that SER sets the "dont fragement" bit for
>>>>>> outgoing messages, which will then be dropped at the low MTU
>>>>>> transit because it may not fragment the too big packets.
>>>>>>
>>>>>> A quick search on the archive did not put up anything useful. Has
>>>>>> anyone ever had similar problems?
>>>>>>
>>>>>>
>>>>>
>>>>>
>>>>> I don't think that SER is responsible for that. Actually, if my
>>>>> memory serves BSD sockets API doesn't provide any portable way for
>>>>> the application to set DF flag. I suspect that either your
>>>>> operating system does it, or the problem is elsewhere.
>>>>>
>>>>> Setting DF on UDP packets is stupid thing to do since UDP doesn't
>>>>> have any path MTU discovery mechanism. Therefore, sending UDP with
>>>>> DF is a very good way to shoot yourself in the foot.
>>>>>
>>>>>
>>>>
>>>> This System is RH9. I verified with tcpdump that outgoing UDP has
>>>> dont fragment bit set. Argh.
>>>>
>>>> Is there anything that can be done about this issue except lowering
>>>> the MTU of the corresponding NIC?
>>>>
>>>> Regards,
>>>> Martin
>>>>
>>>> _______________________________________________
>>>> Serdev mailing list
>>>> serdev at lists.iptel.org
>>>> http://lists.iptel.org/mailman/listinfo/serdev
>>>>
>>>>
>>>>
>>>
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>>>
>>
>>
>>
>>
>
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