On 27-jul-2007, at 14:10, Matt Mathis wrote:
And as John Heffner noted, the community can decide to evade this problem. All we have to do is declare it to be a bug to be trying to do jumbo from a non-4821 host. Then RFC 4821, 1191 and 1981 are definitely sufficient.
I don't think that's a workable approach. For ethernet, using packets larger than 1500 bytes is illegal, strictly speaking, but there are no limits on other link technologies. So if I get some FDDI equipment in a garage sale or connect to the internet over ATM, SONET or even PPP, I will / can have an MTU larger than 1500 bytes without breaking any specification. Retroactively making this illegal would be a curious move to say the least.
Also, this is something that will only lead to deploment problems, because there is no way to know if someone who is sending you large packets is implementing RFC 4821, using "illegal" jumboframes without RFC 4821 or is simply running old 1500+ gear. Right now, you can be pretty sure that it's not the first case, because RFC 4821 isn't in wide use, unless I'm mistaken. So at what point do we get to assume that it is?
A better approach is to exchange MTU information at the neighbor discovery / ARP stage. That way, you're not breaking anything that your correspondent may be depending on.
I'm not sure how jumbo ARPs work. My initial thoughts for backporting the stuff in my draft to IPv4 was to use neighbor discovery for IPv4 addresses, but later it occurred to me that it's easier to do regular ARP and let that trigger the sending of a new message type to announce the supported MTU.
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