On 2011-07-20 03:15, Erik Nordmark wrote:
> On 7/19/11 4:26 AM, Karl Auer wrote:
>
>> Thanks Erik. I've read the explanation in RFC 2765, and sort of see the
>> motivation, but it doesn't help me understand exactly what the node
>> should be doing. That is, the "why" is slightly clearer, but the "what"
>> is not.
>>
>> What, exactly, is a node supposed to do when it receives a PTB< 1280
>> after it has sent an ordinary packet?
>>
>> a) fragment at 1280, regardless of the returned PMTU value
>
> Correct; fragment as if the path mtu was 1280.
> But in addition, insert a fragment header even for packets that don't
> require fragmentation.
It's always been my understanding that an interface sending IPv6 packets
MUST implement some (unspecified) form of framentation and reassembly
*below layer 3* if the link MTU is less than 1280. In other words a
PTB for a packet of length 1280 is an unrecoverable violation.
For example, if you're tunneling IPv6 over an IPv4 network whose PMTU (to
the other end of the tunnel) is, to take a random example, 576, the tunnel
end points could use IPv4 fragmentation and reassembly to provide a 1280 MTU
for the IPv6 traffic.
Of course this doesn't cover the NAT64 case; so what? NAT breaks many things.
We just have to rely on the real world, where the layer 2 link MTU is pretty
much always 1500 today.
Brian
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