Date: Fri, 25 Jan 2008 15:19:54 -0700
From: Yinghui Yao <Yinghui.Yao@alcatel-lucent.com>
Subject: [users:00491] Re: RFC 1981 undersized PMTU test
To: users@tahi.org
Message-Id: <479A608A.4020300@alcatel-lucent.com>
In-Reply-To: <20080125135823.V57419@moby.pdsea.f5net.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> <20080125135823.V57419@moby.pdsea.f5net.com>
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Thanks Mark. It now makes sense. Appreciate it.

Yinghui Yao
Alcatel-Lucent

Mark Atkinson wrote:
>
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>
> On 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 Atkinson
> m.atkinson@f5.com
> (!wired)?(coffee++):(wired);
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