On 23-jul-2007, at 18:46, Iljitsch van Beijnum wrote:
After the feedback this afternoon I read RFC 4821, which is about discovering the path MTU by probing rather than depending on ICMP messages.
[...]
This means that it would be possible for hosts implementing this mechanism to simply set the maximum MTU search size to their local non-standard MTU and everything will work.
However, for this to work reliably, it's necessary that a host only sends large packets if it actually supports RFC 4821. What could happen if that host A1 on subnet A where RFC 4821 in combination with a variable MTU is used by systems supporting jumboframes communicates with host B2 on subnet B where RFC 4821 isn't used, but the administrator has set a larger than 1500 byte MTU on all nodes on the subnet (the way jumboframes are done traditionally). If host B2 then sends a packet that is too large for the switch that host A1 connects to, the router on subnet A keeps sending packets that are too large into the layer 2 network and host B2 never recovers from the lack of reachability.
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