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Re: RFC 8201 Packet Too Big Processing

Fernando Gont <fgont@si6networks.com> Fri, 10 April 2020 17:14 UTC

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Subject: Re: RFC 8201 Packet Too Big Processing
To: Timothy Carlin <tjcarlin@iol.unh.edu>, 6man@ietf.org
References: <CAB-aFv8wVjcXB73wLrBupbq3XLdmdMWE9i-8+TwHfYQE6V52_w@mail.gmail.com>
From: Fernando Gont <fgont@si6networks.com>
Message-ID: <a878bb68-38a9-0c0e-0006-c7830122cdee@si6networks.com>
Date: Fri, 10 Apr 2020 14:14:34 -0300
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Hello, Tim,

On 10/4/20 14:07, Timothy Carlin wrote:
> Hello,
> 
> We've noticed during testing for RFC 8200 and 8201 that, for packets 
> larger than 1280, the Linux kernel is processing invalid Packet Too Big 
> messages that indicate an MTU less than 1280, and subsequently 
> fragmenting packets to a size of 1280. We've seen this with 4.15 and 4.18.
> 
> This is from Section 4 of RFC 8201:
> 
>  >   If a node receives a Packet Too Big message reporting a next-hop MTU
>  >   that is less than the IPv6 minimum link MTU, it must discard it.
> 
> Have others noticed this issue with Linux or other OSes?  I'll also note 
> that it correctly does not generate an atomic fragment if the packet is 
> less than 1280 bytes.

I'm trying to understand the scenario...

Host sends a packet of size > 1280
It receives an ICMPv6 PTB < 1280
And it retransmit the packet as a fragmented packet, where none of the 
fragments is larger than 1280 bytes?

Thanks,
-- 
Fernando Gont
SI6 Networks
e-mail: fgont@si6networks.com
PGP Fingerprint: 6666 31C6 D484 63B2 8FB1 E3C4 AE25 0D55 1D4E 7492

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