Re: RFC 8201 Packet Too Big Processing

[Timothy Carlin <[e-mail]>]

On Fri, Apr 10, 2020 at 1:27 PM Fernando Gont <[e-mail]> wrote:

> On 10/4/20 14:19, Timothy Carlin wrote:
> > Hi Fernando,
> >
> > On Fri, Apr 10, 2020 at 1:14 PM Fernando Gont <[e-mail]
> > <mailto:[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.
>
> Agreed. Unless I'm missing something, there's no point in doing that (at
> the end of the day, if the offending MTU was < 1280, fragmenting packets
> at 1280 will be of no use).
>
> Can you provide the exact kernel version, so I may try to take a look at
> the kernel code and figure out what's going on?
>
>
4.15.0-96-generic and 4.18.0-147 both seem to have this issue.