Subject: Re: New Version Notification for draft-bonica-6man-frag-deprecate-00.txt

From: Ray Hunter <[email-masked]>
Date: Sat, 29 Jun 2013 08:12:38 +0200

With all this talk of 1280 as the minimum *link* MTU, I think it's easy
to forget that 2460 states:

>   A node must be able to accept a fragmented packet that, after
>   reassembly, is as large as 1500 octets.  A node is permitted to
>  accept fragmented packets that reassemble to more than 1500 octets.
>   An upper-layer protocol or application that depends on IPv6
>   fragmentation to send packets larger than the MTU of a path should
>   not send packets larger than 1500 octets unless it has assurance that
>   the destination is capable of reassembling packets of that larger
>   size.

So there was a very clear expectation that the *application level API*
could always rely on being able to transmit as much data as it wanted as
long as it didn't exceed a *1500* octet packet, and that this would
always be reliably transmitted end to end. Anyone not transmitting this
data across their network is currently ignoring a must in 2460.

We should not forget the effect that changing such an important API
parameter will have. I think Brian Carpenter has certainly got that in
his suggestion.

But perhaps some additional limits on the sanity of fragments might
allow us to maintain a minimum 1500 octet limit at the API level, whilst
allowing middleware boxes to process them correctly? [ I do realise that
(nested) tunnels will always be problematic, no matter where you set any
MTU limit, but that's nothing new and we have ways of dealing with that]

regards,
RayH