IPv6 MTUs smaller than 1280 bytes?

Fred Baker fred at cisco.com
Mon Sep 13 15:26:21 CEST 2010

On Sep 12, 2010, at 11:20 PM, Gert Doering wrote:
> Hi,
> On Mon, Sep 13, 2010 at 08:40:12AM +1200, Brian E Carpenter wrote:
>>> Technology that cannot do larger MTUs because of the way the technology
>>> is specified (imagine ATM without SAR at the VC endpoints) cannot be 
>>> fixed without changing standards, upgrading lots of different vendor
>>> implementations, etc.
>> What's supposed to happen then is that the logical link layer handles
>> fragmentation as a link layer (or maybe layer 2.5) function, so that
>> the IPv6 layer still sees at least 1280.
> 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.

To be honest, I'm not sure what L2, apart from ATM, *does* provide L2 segmentation and reassembly. Certainly not anything from CCITT (anything HDLC-based) or IEEE 802.

> Gert Doering
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