On 2012-01-04 08:02, Dan Wing wrote: > ... >>> and the current IPv6 specification also allows PTB < 1280, >>> http://tools.ietf.org/html/rfc2460#section-5 says: >>> >>> In response to an IPv6 packet that is sent to an IPv4 destination >>> (i.e., a packet that undergoes translation from IPv6 to IPv4), the >>> originating IPv6 node may receive an ICMP Packet Too Big message >>> reporting a Next-Hop MTU less than 1280. In that case, the IPv6 >> node >>> is not required to reduce the size of subsequent packets to less >> than >>> 1280, but must include a Fragment header in those packets so that >> the >>> IPv6-to-IPv4 translating router can obtain a suitable >> Identification >>> value to use in resulting IPv4 fragments. Note that this means >> the >>> payload may have to be reduced to 1232 octets (1280 minus 40 for >> the >>> IPv6 header and 8 for the Fragment header), and smaller still if >>> additional extension headers are used. >> Exactly. And my question was about whether the "atomic fragments" that >> were found in the wild were the result of translators, or of IPv6 >> networks that "violate" the standard and do not support an MTU of >= >> 1280. > > Dunno. > > I am only trying to point out that IPv6 hosts need to handle receiving > ICMP packet-too-big of less than 1280, because we are going to see > more stateless IPv6/IPv4 translators. If IPv6 hosts don't handle > ICMP packet-too-big of less than 1280, those IPv4/IPv6 translators > won't work with sub-1280 MTU IPv4 paths. > > And Ran has pointed out other deployments where sub-1280 MTUs are > being used on IPv6. (An aside comment: I wonder if those networks > can use LFI (link fragmentation and interleaving), which allows > preserving the layer 3 MTU and should also provide the smaller > packets needed by the layer 1 or 2 network). > > So, I don't think we can just wish away packet-too-big < 1280.
Sadly, that seems to be true unless we make a much more radical change, because of translators. Brian -------------------------------------------------------------------- IETF IPv6 working group mailing list ipv6@ietf.org Administrative Requests: https://www.ietf.org/mailman/listinfo/ipv6 --------------------------------------------------------------------
