IPv6 MTUs smaller than 1280 bytes?

Ralph Droms rdroms at cisco.com
Mon Sep 13 18:00:46 CEST 2010

Just to be clear - the 6lowpan adaptation layer for IPv6-over-802.15.4 does define its own fragmentation, which should appear to provide an MTU of 1280 to IPv6.

- Ralph

On Sep 13, 2010, at 5:58 PM 9/13/10, Fred Baker wrote:

> 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.

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