Re: RFC 8201 Packet Too Big Processing

[Fernando Gont <[e-mail]>]

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.

Thanks,
-- 
Fernando Gont
SI6 Networks
e-mail: [e-mail]
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