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Re: Non-Last Small IPv6 Fragments

Simon Hobson <linux@thehobsons.co.uk> Mon, 14 January 2019 16:01 UTC

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Subject: Re: Non-Last Small IPv6 Fragments
From: Simon Hobson <linux@thehobsons.co.uk>
In-Reply-To: <4373c8d2-b36f-39c7-3591-1263af0f155b@foobar.org>
Date: Mon, 14 Jan 2019 16:00:55 +0000
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Nick Hilliard <nick@foobar.org> wrote:

> No intermediate node is going to fragment anything other than the last frame to 8 bytes, or at least if it does, the network is so severely crippled that fragmentation is the least of its problems.  It would be fair to assume that if host B receives a stream of fragments where the fragment size is unfeasibly small, then the fragmentation was done by host A.

What if ...
An intermediate node gets a packet inbound of N bytes, but the MTU downstream only allows it to send N-8. If it doesn't have other packets* to degragment/merge with first, then it's only option is to send two smaller packets downstream - and one logical method would be to send N-8 followed by 8 bytes.

* And bear in mind that there is no rules that says the packets before or after this one have to be carried by the same route - hence (one reason) why nodes need to handle out of order fragments.


Fernando Gont <fgont@si6networks.com> wrote:

>> Step 1: As an addendum to RFC8200, state that there is no rule that non-last fragments must be at least 1280.
> 
> Isn't that implicit?

It may be, but (as already mentioned) we have a significant implementation where it would seem the devs didn't see that.

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