I believe that there is a problem with the forwarding section
of the Scoped Addressing Architecture document (section 9).
The current draft assumes that a next hop will be selected
based only on the destination address of the packet, but I
believe that it is necessary to take both the destination and
source addresses into account to choose the correct conceptual
routing table.
Consider the following example:
(Cost 1)
+--------R1-----------------+
+------------------|-----+ | +------------|------------------+
| SITE A | | | | SITE B | H1 |
| Link1________.___|___ | | | Link2______|_______|_ |
| | | +---------------+ +--+ |
+--------------|---------+ | Link3____|_____________| | |
| | //<=== Cost >=2
| | Link4_____.________.___. | |
| | | | | | |
| | | H2 +--+ |
| +-----------|-------------------+
| |
+--------------R2--------------+
(Cost 1)
There are two sites, SITE A and Site B. SITE A has one link, Link1.
SITE B has three links: Link2, Link3 and Link4. H1 is a host on
Link2, and H2 is a host on Link4. R1 is a site-boundary router
with interfaces on Link1(SITE A), Link2(SITE B) and Link3 (SITE B),
and the cost for traversing this router is always 1. R2 is another
site-boundary router, connecting Link1(SITE A) and Link4(SITE B),
and the cost for traversing R2 is also 1. There is additional routing
infrastructure inside SITE B which connects Link3 and Link4 at a
cost greater than or equal to 2.
H1 sends a packet to H2 with the following addresses:
Destination: Global address of H2
Source: Site Local address of H1
Although this packet can (and should) reach its destination, it will
not do so using the process described in section 9 of the Scoped
Addressing Architecture.
o The zone of the destination address is determined by the
scope of the address and arrival interface of the packet.
The next-hop interface is chosen by looking up the
destination address in a (conceptual) routing table
specific to that zone. That routing table is restricted
to refer only to interfaces belonging to that zone.
The zone of the destination address is Global. So, we will look in
the global table to determine how to route to H2. The shortest
(or cheapest) next-hop will be chosen -- router R2, through SITE A.
o After the next-hop interface is chosen, the zone of the
source address is considered. As with the destination
address, the zone of the source address is determined by
the scope of the address and arrival interface of the
packet. If transmitting the packet on the chosen next-
hop interface would cause the packet to leave the zone of
the source address, i.e., cross a zone boundary of the
scope of the source address, then the packet is discarded
and an ICMP Destination Unreachable message [RFC 2463]
with Code 2 ("beyond scope of source address") is sent to
the source of the packet.
However, the site local (to SITE B) source address was used, so we
will discard the packet before sending it to SITE A.
Preferably, we would choose a site-local route to send this packet,
even if it might result in a longer (or more expense) routing path.
This could be achieved by using the scope of both the destination
and source addresses to determine which conceptual routing table to
use for packet forwarding.
A router should look at both the source and destination addresses
and determine which is of the lesser scope. The lesser scope
(site-local in my example) will then be used to select the conceptual
routing table. In this instance, the global address would be
looked up in the site-local routing table, and an appropriate next
hop could be chosen (if it exists) to route the packet to the
global destination address using a site-local path.
Thoughts?
Margaret
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