Re: RFC 8201 Packet Too Big Processing

[Timothy Carlin <[e-mail]>]

On Fri, Apr 10, 2020 at 2:20 PM Fernando Gont <[e-mail]> wrote:

> On 10/4/20 14:36, Timothy Carlin wrote:
> > On Fri, Apr 10, 2020 at 1:27 PM Fernando Gont <[e-mail]
> > <mailto:[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]>
> >      > <mailto:[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.
>
> Have you tried with newer kernels? e.g., I'm running 5.3.0-42-generic here.


 I have not.  These were from two relatively new distributions, but
apparently are lagging on the kernel version.  I'll try something newer.