IPv6 MTUs smaller than 1280 bytes?

[Fred Baker]

On Sep 13, 2010, at 7:39 AM, Fernando Gont wrote:

> Hi, Fred,
> 
>>> Thanks.  So how do people adapt IPv6 to 802.15.4-2006?
>> 
>> They're using PMTU. On the local side you can know that it is
>> 802.15.4 and set a TCP MSS very small, but unless one side does that
>> the other has no way to detect the problem apart from PMTU.
> 
> Just double checking: So... these link layers do not support MTUs of
> 1280 bytes?
> 
> e.g., what if the flow does not implement PMTUD?
> 
> FWIW, I'm just trying to figure out if, when receiving an ICMP PTB that
> advertises a Next-Hop MTU smaller than 1280, it is really safe to *not*
> fragment the original packet in fragments of (at most) the advertised MTU.
> 
> If there are link layers that do not support an MTU of 1280 bytes then,
> despite of what RFC 2460 requires, one may need to be more careful in
> this case, as sticking to 1280-byte packets may result in
> interoperability problems.

duh. :-)

If you get a message back telling you to fragment to 50 bytes, I'd suggest you do so.