Re: RFC 8201 Packet Too Big Processing

[Bob Hinden <[e-mail]>]

Tim,

RFC8201 is very clear on this, from Section 4:

   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.  A
   node must not reduce its estimate of the Path MTU below the IPv6
   minimum link MTU on receipt of a Packet Too Big message.

Bob



> On Apr 20, 2020, at 9:10 AM, Timothy Winters <[e-mail]> wrote:
> 
> Hi Erik,
> 
> So I think the confusion comes from this line in 8021.
> 
> "The recommendation in the previous bullet ensures that there are
>       no longer any valid reasons for ICMPv6 PTB error messages
>       reporting an MTU value smaller than the minimum IPv6 MTU
>       (1280 bytes).  IPv6 nodes should therefore be updated to ignore
>       ICMPv6 PTB error messages reporting an MTU smaller than 1280 bytes
>       as invalid."
> 
> So devices should ignore ICMPv6 PTB if smaller then 1280 based on this reading.
> 
> ~Tim
> 
> On Sat, Apr 18, 2020 at 7:38 PM Erik Kline <[e-mail]> wrote:
> If the host downgrades the path mtu to 1280 (and no lower), that seems fine to me.
> 8201 section 3:
> 
> """
>    Alternatively, the node may elect to end the discovery process by
>    ceasing to send packets larger than the IPv6 minimum link MTU.
> """
> 
> And later on:
> 
> """
>    The node then uses the value in the MTU field in the Packet Too Big
>    message as a tentative PMTU value or the IPv6 minimum link MTU if
>    that is larger, and compares the tentative PMTU to the existing PMTU.
> """
> 
> I think the "it" that is to be discarded is likely not clear.  If a node discards the whole packet then no new MTU information can be learned.  Rather, the "it" is the MTU value in the PTB and discarding /that/ and using IPv6 MTU instead seems perfectly reasonable to me and allowed by the text..
> 
> On Fri, Apr 10, 2020 at 1:17 PM Timothy Carlin <[e-mail]> wrote:
> On Fri, Apr 10, 2020 at 2:33 PM Timothy Carlin <[e-mail]> wrote:
> 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.
> 
> Indications are that this remains broken as of 5.3.0-050300-generic.  Let me know if you want me to try another version.
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