Date: Fri, 25 Jan 2008 14:01:20 -0800 From: "Mark Atkinson" <m.atkinson@F5.com> Subject: [users:00490] Re: RFC 1981 undersized PMTU test To: <users@tahi.org> Message-Id: <20080125135823.V57419@moby.pdsea.f5net.com> In-Reply-To: <479A59CE.6010007@alcatel-lucent.com> References: <47746300.1020404@cn.fujitsu.com> <477851F3.9030402@cn.fujitsu.com> <20080101222240.r1dqh8wlc000gk4g@webmail.corp.redhat.com> <BDC75A1E-9BF3-4B92-9C17-0170BAB06005@tahi.org> <4797CE07.7050202@alcatel-lucent.com> <479A06AE.90004@alcatel-lucent.com> <20080125163725.GB14346@pingi.kke.suse.de> <479A35AE.9030602@redhat.com> <479A5770.6050101@alcatel-lucent.com> <479A59CE.6010007@alcatel-lucent.com> X-Mail-Count: 00490-----BEGIN PGP SIGNED MESSAGE-----Hash: SHA1On Fri, 25 Jan 2008, Yinghui Yao wrote:> Self_Test_1.4.9 PMTU test #9 seems to test RFC 1981 section 4 =statement>> "A node MUST NOT reduce its estimate of the Path MTU below the IPv6> minimum link MTU.>> Note: A node may receive a Packet Too Big message reporting a> next-hop MTU that is less than the IPv6 minimum link MTU. In that> case, the node is not required to reduce the size of subsequent> packets sent on the path to less than the IPv6 minimun link MTU,> but rather must include a Fragment header in those packets [IPv6-> SPEC]. ">> Could anyone please shed some light on why the RFC requires this. Why =should> a node pay attention to an malformed ICMP packet, let alone adding a =frag> header to a unfrag packet? Why? Appreciate it.RFC 2460 answers your question: 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.- ---Mark Atkinsonm.atkinson@f5.com(!wired)?(coffee++):(wired);-----BEGIN PGP SIGNATURE-----Version: GnuPG v2.0.4 (FreeBSD)iD8DBQFHmlwzrDN5kXnx8yYRAuuGAJ0T2bKX+M+ekkkcEqfVqpWRHymJNwCgpCK2VoeusNdU13gl8d6HRr3WGZ4==RppL-----END PGP SIGNATURE-----490_2.html (attatchment)(tag is disabled)