On Mon, 2011-07-18 at 15:39 -0700, Erik Nordmark wrote: > The motivation for the fragment header insertion was to be able to > support stateless IPv6->IPv4 translators (with multi-path routing), such > as RFC 2765. > > Such a translator normally sets DF (don't fragment) in the IPv4 packets. > But should the IPv4 path MTU drop below 1300 (meaning that a translated > 1280 byte IPv6 packet wouldn't fit), then DF can't be set. In that case > a unique IP ID field is needed. The fragment header contains a unique ID > which will be used by the translator. > > I hope that helps clarify the motivation,
Thanks Erik. I've read the explanation in RFC 2765, and sort of see the motivation, but it doesn't help me understand exactly what the node should be doing. That is, the "why" is slightly clearer, but the "what" is not. What, exactly, is a node supposed to do when it receives a PTB < 1280 after it has sent an ordinary packet? a) fragment at 1280, regardless of the returned PMTU value b) fragment so as to produce fragments of sizes equal to or smaller than the returned PMTU value, but don't "remember" the returned PMTU value c) create a new packet that is identical to the original packet but has a fragment header inserted (i.e., forms a fragment sequence one fragment long, with More-Fragments=0 in the fragment header) d) fail to send the packet e) something else...? Only b) seems remotely useful, but it makes a mockery of the idea that a PMTU of 1280 is required for IPv6. Regards, K. -- ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Karl Auer ([email protected]) +61-2-64957160 (h) http://www.biplane.com.au/kauer/ +61-428-957160 (mob) GPG fingerprint: DA41 51B1 1481 16E1 F7E2 B2E9 3007 14ED 5736 F687 Old fingerprint: B386 7819 B227 2961 8301 C5A9 2EBC 754B CD97 0156
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