Re: RFC 8201 Packet Too Big Processing

[Timothy Carlin <[e-mail]>]

Hi Fernando,

On Fri, Apr 10, 2020 at 1:14 PM Fernando Gont <[e-mail]> wrote:

> 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?
>
>
This is correct.  Since the ICMPv6 PTB < 1280, and invalid, we would expect
the PTB to be discarded, and subsequent packets (for that destination) to
remain unfragmented.


> Thanks,
> --
> Fernando Gont
> SI6 Networks
> e-mail: [e-mail]
> PGP Fingerprint: 6666 31C6 D484 63B2 8FB1 E3C4 AE25 0D55 1D4E 7492
>
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