On Jul 28, 2007, at 2:05 AM, John Heffner wrote:
The difficult problem is the router's behavior. If a subnet is running a mixed MTU, it's not clear what that router's interface MTU to that subnet should be. It would be nice for it to forward the largest packets that it can support; however, to be compliant with the spec it must generate ICMP PTB messages for any packets that are too large to be delivered.
here's an interesting gotcha. Macs run a 1000/100/10 Ethernet interface and run a 1500 byte MTU/MRU regardless. The router can correctly observe that it is 1 GBPS and send the jumbogram, the switch can support the jumbogram, and have the Mac not accept the packet.
Hence, there are variations of this that are beyond the router's knowledge.
I would suggest that the router respond with the ICMP when the packet is too big for the configured interface MTU and not try to predict the end system's or the switch's behavior. 4821 encourages the sender to use the largest segment size that it can verify delivery of. Leave that to the end system.
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