6.2 mtu now limits size of incomming packet

[Stephen Clark]

Sten Daniel Soersdal wrote:

>Stephen Clark wrote:
>  
>
>>Sten Daniel Soersdal wrote:
>>
>>    
>>
>>>Stephen Clark wrote:
>>> 
>>>
>>>      
>>>
>>>>Hello,
>>>>
>>>>Did something change in 6.2? If my mtu size on rl0 is 1280 it won't
>>>>accept a larger incomming packet.
>>>>
>>>>kernel: rl0: discard oversize frame (ether type 800 flags 3 len 1514 
>>>>        
>>>>
>>>>>max
>>>>>          
>>>>>
>>>>1294)
>>>>  
>>>>        
>>>>
>>>That is what to be expected.
>>>Incoming interface must have mtu set to the same mtu as all other 
>>>hosts on the same L2 network. If mtu is set to the same as all other 
>>>hosts, then it is impossible to receive a frame that is too large 
>>>(assuming everything works).
>>>
>>> 
>>>
>>>      
>>>
>>>>I don't think it worked this way in the past.
>>>>
>>>>Won't this affect pmtud?
>>>>  
>>>>        
>>>>
>>>Incoming interface must have its mtu set to large enough to receive 
>>>the frame. Outgoing interface, on the other hand, can be lower.
>>>
>>>For pmtud to work you need to be able to receive packets on an 
>>>interface with sufficiently set mtu, but the exitting interface can 
>>>have a lower mtu configured. Thus the router can accept the incoming 
>>>packet but may drop and notify on a frame that is too large to exit 
>>>the outgoing interface (assuming DF is set).
>>>
>>> 
>>>
>>>      
>>>
>>>>man page for ifconfig says mtu limits size of "transmission" not 
>>>>reception.
>>>>
>>>>   "mtu n   Set the maximum transmission unit of the interface to n, 
>>>>default
>>>>           is interface specific."
>>>>  
>>>>        
>>>>
>>>Perhaps the man author considered reception to be implied?
>>>
>>>In any case, enforcing this on incoming packets is correct behavior.
>>>
>>> 
>>>
>>>      
>>>
>>But shouldn't an icmp be generated back to the system sending the packet 
>>that is
>>being dropped? This is not happening. So the connection stalls.
>>
>>client mtu 1500 <-> |rl0 mtu 1500 FreeBSD Router rl1 mtu 1280| <-> some 
>>host on internet
>>client sends syn saying i can do mss=1460
>>host sends syn saying i can do mss=1460
>>host tries to send packet of 1460 it get silently dropped. connection 
>>stalls.
>>
>>    
>>
>
>You are trying to lower the mtu on the wrong end.
>As i said, all hosts on the same L2 needs to share the same mtu.
>The router that forwarded you that packet is obviously not using the 
>same mtu (otherwise it would not be able to forward it to you).
>Either you need to lower that routers local interface mtu or you need to 
>raise your hosts mtu to match that of the router.
>Because ALL hosts on the same L2 network need to have the same mtu.
>
>  
>
I don't disagree - my issue is that this is new behavior in FreeBSD 6.x 
that did not exist in
FreeBSD 4.x.

However as many have said in thread :


>From RFC-1122, and memorialized on the working group coffee cup on my
bookshelf:

	Be liberal in what you accept, and
	conservative in what you send

(attributed to RFC-791, but paraphrased; also in RFC-793; for those
who don't recognize them, these are the original IP and TCP specs.)


>> The ability to receive packets larger than mtu was not accidental.
>> This should be fixed, if it is, as is suggested, a deliberate change.
>  
>

I'd be happy to see the change undone as well.  I (well, our test
group) found this change in a similar way, and it didn't agree with
our previous usage.




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