IPv6 MTUs smaller than 1280 bytes?

[Ralph Droms (rdroms)]

... At least not for future devices.  There is significant demand for IPv6 over 802.15.4 devices already deployed.



On Sep 13, 2010, at 6:18 PM, "Fred Baker" <fred at cisco.com> wrote:

> 
> 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.
>> 
>