Re: ICMPv6 question

2001:db8::f00

On 1 September 2017 at 09:07, Templin, Fred L <Fred.L.Templin@boeing.com> wrote:
> I have a question. If an IPV6 host with address 2001:db8::f00 assigned to an interface
> receives a packet with destination address 2001:db8::baa over that interface, what
> should it do? Drop the packet silently, or drop and return an ICMPv6 Destination
> Unreachable?
>

Hmm, at face value it appears to be a simple question and the answer
would be DU.

Looking at RFC4443, it limits generation of DUs to routers or the
packet source host:

"A Destination Unreachable message SHOULD be generated by a router, or
   by the IPv6 layer in the originating node, in response to a packet
   that cannot be delivered to its destination address for reasons other
   than congestion.  (An ICMPv6 message MUST NOT be generated if a
   packet is dropped due to congestion.)A Destination Unreachable
message SHOULD be generated by a router, or
   by the IPv6 layer in the originating node, in response to a packet
   that cannot be delivered to its destination address for reasons other
   than congestion.  (An ICMPv6 message MUST NOT be generated if a
   packet is dropped due to congestion.)"

Per RFC8200 (a.k.a. new RFC2460), we have these definitions for a
router and a host

   router       a node that forwards IPv6 packets not explicitly
                addressed to itself.  (See Note below.)

   host         any node that is not a router.  (See Note below.)

A host is therefore to ignore "IPv6 packets not explicitly addressed
to itself", which I think means a host does not generate a DU for
misdirected packets.

I was wondering about how this could happen and I think there could be
two possible causes,

- incorrect ND cache entry for 2001:db8::baa2001:db8::baa, somehow
with the link layer address of 2001:db8::f00

- host attached to one end of a point-to-point link, and the upstream
device (likely router) is blindly forwarding packets over the p2p link
assuming the destination is at the other end rather than performing ND
to discover and continue to verify the presence of the specific
addresses at the other end of the link (That's why I think ND should
be performed on p2p links. Even a /127 configured on one end of the
link doesn't guarantee the other address within the /127 is present.
If a router performed ND on p2p links, it could discover the DA for a
packet is not present and generate a DU for it.)

Regards,
Mark.

> Thanks - Fred
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