On 7/18/11 5:25 AM, Karl Auer wrote:
I'm puzzled by something in RFC1981, which discusses PMTUD and IPv6.
It contains these two paragraphs towards the end of Section 4:
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].
What does that mean - "insert a Fragment header"? And it may not be
*required* to reduce the size of subsequent packets, but does that mean
it *can* if it wants to? And isn't inserting a whole new header a bit
counterproductive if someone is telling your your packets are too big
already? All in all I have this strong feeling I'm missing something
obvious. Any clues gratefully received.
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,
Erik
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