Re: RFC 8201 Packet Too Big Processing

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 <tjcarlin@iol.unh.edu> wrote:

> On Fri, Apr 10, 2020 at 2:33 PM Timothy Carlin <tjcarlin@iol.unh.edu>
> wrote:
>
>> On Fri, Apr 10, 2020 at 2:20 PM Fernando Gont <fgont@si6networks.com>
>> wrote:
>>
>>> On 10/4/20 14:36, Timothy Carlin wrote:
>>> > On Fri, Apr 10, 2020 at 1:27 PM Fernando Gont <fgont@si6networks.com
>>> > <mailto:fgont@si6networks.com>> wrote:
>>> >
>>> >     On 10/4/20 14:19, Timothy Carlin wrote:
>>> >      > Hi Fernando,
>>> >      >
>>> >      > On Fri, Apr 10, 2020 at 1:14 PM Fernando Gont
>>> >     <fgont@si6networks.com <mailto:fgont@si6networks.com>
>>> >      > <mailto:fgont@si6networks.com <mailto:fgont@si6networks.com>>>
>>> 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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