IPv6 MTUs smaller than 1280 bytes?

[Fernando Gont]

Hi, Ralph,

Does 802.15.4 predate 6lowpan? Put another way: maybe there are
deployments of IPv6 (no 6lowpan) over 802.15.4?

Thanks!

Kind regards,
Fernando




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.
> 
> - 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.
> 
> 

-- 
Fernando Gont
e-mail: fernando at gont.com.ar || fgont at acm.org
PGP Fingerprint: 7809 84F5 322E 45C7 F1C9 3945 96EE A9EF D076 FFF1