6.2 mtu now limits size of incomming packet

[Stephen Clark]

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.

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