>Yes, but is it _correct_?
>That is, is PMTU discovery the one and only application, ever, of using the DF
>flag?
>(Meaning: are we really allowed to fragment a DF packet anyway?)

	We are not allowed to fragment a packet with DF bit.
	See RFC791, page 8.

>>    An internet datagram can be marked "don't fragment."  Any internet
>>    datagram so marked is not to be internet fragmented under any
>>    circumstances.  If internet datagram marked don't fragment cannot be
>>    delivered to its destination without fragmenting it, it is to be
>>    discarded instead.

itojun