Margaret,
Another piece of info that may be missing. If a non-global
scoped address (source or destination) is forwarded outside of its
zone, there is no way to determine where it belongs. The discussion
that Steve D. and I had was that Scope Exceeded message would indicate
to a sender that a source address was trying to be forwarded out of
its zone. I will try and build a table that specifies when the
Scope Exceeded message should be used.
Brian
Margaret Wasserman wrote:
>
> Hi Rich,
>
> >Your example also has the best route between H1 & H2 in site B, be to go
> >through site A. This seems really unlikely to occur in actual practice. You
> >seem to be implicitly assuming that the two sites are in a single IGP
> >routing domain.
>
> I am assuming that two sites _may_ be in the same routing domain, and
> that problems could occur in that situation. I don't believe that the
> current IPv6 specifications assume that a single routing domain will
> always be a single site.
>
> My example packet could easily occur in response to a packet sent from
> a host that doesn't have a site-local address (perhaps because it was
> configured via DHCP?). That host would need to use a global address to
> communicate with a site-local address, and the response would have
> a site-local source and a global destination.
>
> The ICMP issue that you have described would also exist when using
> multiple "conceptual" routing tables in the case of a partitioned
> site. For example:
>
> ==========================================================
> SITEA
> Host1 Host2
> | |
> ________|__.__._____ ____.______._|_____________
> Link1 | | | | Link2
> | | (down) |
> | +-----R1----+ |
> | |
> | |
> ===========|=====================|=========================
> SITEB | |
> +--------R1-----------+
>
> ===========================================================
>
> If a site-local packet (site-local destination and site-
> local source) is send from Host1 to Host2, it would
> probably be sent to the local router, R1.
>
> R1 would then use the scoped addressing rules to forward the
> packet. First, he would look at the destination address
> to determine that he should look in the site-local table.
> The site-local table would have no active route to Link2,
> so he would send a Destination Unreachable/No Route
> To Destination message, _not_ a Scope Exceeded message.
>
> In this case, it might actually be _useful_ for the sender
> to receive a Scope Exceeded message, because the sender might
> be able to switch to global communication. The sender may
> have had both site-local and global addresses available, but
> have chosen site-local communication. But, he doesn't get the
> Scope Exceeded message, so he can't act on it.
>
> However, if a mixed scope packet is sent (global destination
> and site-local source) from Host1 to Host2, R1 would look
> in the global table, determine that a route exists to Link2
> through R2, THEN discard the packet because of the site-local
> source. This would result in a Destination Unreachable/Scope
> Exceeded message. It seems unlikely, though, that Host1 would
> have sent a mixed-scope packet if he had a global address
> available for Host2, so the Scope Exceeded message is potentially
> _less_ useful in this situation.
>
> Margaret
>
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Brian Haberman
Nortel Networks
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