Re: RFC2460 violation of RFC1122

Mark Andrews <marka@isc.org> Sat, 14 July 2012 08:02 UTC

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To: "Templin, Fred L" <Fred.L.Templin@boeing.com>
From: Mark Andrews <marka@isc.org>
References: <4FFD71D7.4070209@gmail.com> <9B57C850BB53634CACEC56EF4853FF653B6BF582@TK5EX14MBXW603.wingroup.windeploy.ntdev.microsoft.com> <4FFF29E2.6090909@viagenie.ca> <6.2.5.6.2.20120712152812.082ba6f8@resistor.net> <50001613.2090203@viagenie.ca> <6.2.5.6.2.20120713085321.095aaf60@resistor.net> <50004916.4000206@viagenie.ca> <E1829B60731D1740BB7A0626B4FAF0A65D8F4C8D63@XCH-NW-01V.nw.nos.boeing.com>
Subject: Re: RFC2460 violation of RFC1122
In-reply-to: Your message of "Fri, 13 Jul 2012 10:25:37 MST." <E1829B60731D1740BB7A0626B4FAF0A65D8F4C8D63@XCH-NW-01V.nw.nos.boeing.com>
Date: Sat, 14 Jul 2012 18:02:54 +1000
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In message <E1829B60731D1740BB7A0626B4FAF0A65D8F4C8D63@XCH-NW-01V.nw.nos.boeing
.com>, "Templin, Fred L" writes:
> Section 5 of RFC2460 states:
> 
>    "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."
> 
> RFC2460 therefore requires the IPv4 destination to be able to
> reassemble at least 1280 bytes minus 28 (since the translation
> from an IPv6 header plus fragment header to an IPv4 header
> incurs a 28 byte size reduction). However, section 3.3.2 of
> RFC1122 states:
> 
>          "We designate the largest datagram size that can be reassembled
>          by EMTU_R ("Effective MTU to receive"); this is sometimes
>          called the "reassembly buffer size".  EMTU_R MUST be greater
>          than or equal to 576, SHOULD be either configurable or
>          indefinite, and SHOULD be greater than or equal to the MTU of
>          the connected network(s)."
> 
> By assuming an EMTU_R of greater than 576 bytes, RFC2460
> is therefore in violation of RFC1122, which could lead to
> communication failures. How do we reconcile this?

You live with it.   Nobody has said that IPv6 to IPv4 translation
will work in all circumstances.

> Thanks - Fred
> fred.l.templin@boeing.com
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-- 
Mark Andrews, ISC
1 Seymour St., Dundas Valley, NSW 2117, Australia
PHONE: +61 2 9871 4742                 INTERNET: marka@isc.org