Subject: RE: question about draft-ietf-ipv6-scoping-arch-02.txt

From: "Ghanwani, Anoop" <[email-masked]>
Date: Mon, 30 Aug 2004 11:20:27 -0700

> >   "Note that even with the decision that unicast site-local
addresses
> >    are deprecated, the above procedure still applies to link-local
> >    addresses.  Thus, if a router receives a packet with a link-local
> >    destination address that is not one of the router's own
link-local
> >    addresses on the arrival link, the router is expected to try to
> >    forward the packet to the destination on that link (subject to
> >    successful determination of the destination's link-layer address
via
> >    the Neighbor Discovery protocol [9]).  The forwarded packet may
be
> >    transmitted back out the arrival interface, or out any other
> >    interface attached to the same link."
> >
> > Where would we have a situation where something like this would be
> > necessary?  If it's addressing a special case, a reference would 
> > be useful.
> 
> One possible scenario I can think of is some special link where hosts
> do not want to or cannot perform link-layer address resolution for
> their neighbors and simply send all packets to the default router.
> The router is supposed to know the link-layer address of the
> destination (or know how to get the link-layer address), and forwards
> the packet to the destination.  If the destination is a link-local
> address, the packet is likely to be sent back to the incoming link.

Is this behavior supported by the IPv6 spec?
 
> Of course, I admit this is a very artificial example.  Perhaps others
> can provide a more realistic scenario.  In any event, my understanding
> of the above description comes from the view that it's better to
> provide maximum flexibility unless we have a strong reason not to do
> so.

I found it very confusing to see that this draft thinks that it
is OK for a router to forward a packet with a link-local 
destination address, and more so that it considers it 
acceptable to send the packet out the interface that it 
received the packet on.

This behavior is in conflict with Section 2.5.8 of RFC 2373 which 
states:  "Routers must not forward any packets with link-local 
source or destination addresses to other links."

I was wondering what the original motivation for the text was.  
It's good to keep things flexible, but when there's a conflict
between specs, it would be helpful to provide clarification.

Anoop

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From: Brian Haberman <[email-masked]>
Date: Mon, 30 Aug 2004 14:38:57 -0400

> Is this behavior supported by the IPv6 spec?

Such behavior is useful for the multi-link subnet scenario.  In that 
situation,
the "IP" layer sees a single interface, but it is constructed from 
multiple physical
links.  The router must proxy the NDP messages in order to connect all 
the nodes
on that subnet.

> between specs, it would be helpful to provide clarification.

That text was crafted for the multi-link subnet scenario.  Take a look 
at
http://www.ietf.org/internet-drafts/draft-thaler-ipv6-ndproxy-02.txt

Regards,
Brian

---

From: Fred Templin <[email-masked]>
Date: Mon, 30 Aug 2004 22:49:20 -0700 (PDT)

"Ghanwani, Anoop" <[email-masked]> wrote:
This behavior is in conflict with Section 2.5.8 of RFC 2373 which 
states: "Routers must not forward any packets with link-local 
source or destination addresses to other links."

Coming from one who found out the hard way, the key terms here
are: "routers" and "links" and the way one interprets them within
a given context is key to understanding why there is no conflict.
 
Fred L. Templin
[email-masked]

---

From: JINMEI Tatuya / 神明達哉<[email-masked]>
Date: Tue, 31 Aug 2004 16:42:39 +0900

>>>>> On Mon, 30 Aug 2004 11:20:27 -0700, 
>>>>> "Ghanwani, Anoop" <[email-masked]> said:

> Is this behavior supported by the IPv6 spec?

I'm not really sure which part you are referring to by "this
behavior", but regarding the following part:

   hosts

I believe the following part of RFC2461 (in Section 3.2) implies the
scenario:

     non-broadcast multiple access (NBMA)
                    - Redirect, Neighbor Unreachability Detection and
                      next-hop determination should be implemented as
                      described in this document.  Address resolution,
                      and the mechanism for delivering Router
                      Solicitations and Advertisements on NBMA links is
                      not specified in this document.  Note that if
                      hosts support manual configuration of a list of
                      default routers, hosts can dynamically acquire the
                      link-layer addresses for their neighbors from
                      Redirect messages.

Regarding the behavior of the router that forwards packets to a
link-local destination back to the same link, I don't know of any
other documents that explicitly allows the behavior.  But this is
exactly the reason why we clarified that in
draft-ietf-ipv6-scoping-arch.  And I'm 100% sure that this behavior is
the intended one by (one of) the original designer(s) of IPv6, Steve
Deering.  (Perhaps you can google his words on this at the ML archive
or something)
 
> source or destination addresses to other links."

I see we've already understood this does not conflict in this thread
by other posts, so I won't make further comments on it.

					JINMEI, Tatuya
					Communication Platform Lab.
					Corporate R&D Center, Toshiba Corp.
					[email-masked]

p.s. as you may have noticed, the IESG has just approved this draft as
a proposed standard.  I personally don't think the discussion in this
thread requires further changes on the draft before publication, and
so I'm going to work on final editorial stuff with the RFC editor.  If
I miss something critical, however, please let me know ASAP.