IPv6 MTUs smaller than 1280 bytes?

Fred Baker fred at cisco.com
Mon Sep 13 18:18:12 CEST 2010

On Sep 13, 2010, at 9:00 AM, Ralph Droms wrote:

> 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.

OK, that's true. Of course, if the March change to 802.15.4g stays, that (and 6lowpan?) may no longer be needed.

> - 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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