IPv6 MTUs smaller than 1280 bytes?

Fred Baker fred at cisco.com
Mon Sep 13 15:43:26 CEST 2010

On Sep 13, 2010, at 6:32 AM, Gert Doering wrote:

> Hi,
> On Mon, Sep 13, 2010 at 06:26:21AM -0700, Fred Baker wrote:
>>> Yes - which is, specifically, why I'm asking whether technology is out
>>> there today that has "small MTU" limitations but at the same time does not
>>> provide L2 (L2.5) segmentation and reassembly.
>> IEEE 802.15.4-2006 (133 octets, part of which is an ethernet-like
>> header and trailer) is an example. In March, 802.15.4g added options
>> to increase the size to 2047 bytes and include a four byte CRC,
>> which is a good thing, but up until recently we have been looking
>> at very small frames.
> 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.

>> To be honest, I'm not sure what L2, apart from ATM, *does* provide L2 
>> segmentation and reassembly. Certainly not anything from CCITT 
> X.25 :-)

Well, not really. X.25 PLP isn't link layer; that's the LAPB layer. But you are correct that X.25's PLP can provide what amounts to an octet stream for its user, which may be an octet stream for the upper layer's entire transmission or only for payload messages. 

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