Subject: Use of same interface id on multiple interfaces (RFC 2373)

From: Murali Krishna Ch <[email-masked]>
Date: Wed, 26 Jul 2000 16:55:03 +0530

Hi
Can we use the same Interface id for multiple interfaces?
If so what are ISSUES, involved in it?
For example, if i have one Ethernet interface and 'n' PPP interfaces, 
is it right to use the same linklocal address constructed from Mac Address for **ALL** the 'n'  PPP links?
How does it effect the global uniqueness of the address?
Thanks & Regards,
murali.

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From: Steve Deering <[email-masked]>
Date: Wed, 26 Jul 2000 14:19:50 -0700

>How does it effect the global uniqueness of the address?

Murali,

Yes, a node may use the same link-local address on more than attached
link, and the same site-local address on more than one attached site.
Link-local and site-local addresses are required to be unique only
within the set of interfaces attached to a given link or site, respectively.

The main consequence is that, within a node, any non-global address
must be accompanied by a zone identifier (or an indication of "default
zone") to identify the link or site to which it pertains.

See draft-ipngwg-scoping-arch-01.txt.

Steve

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From: Margaret Wasserman <[email-masked]>
Date: Mon, 31 Jul 2000 15:26:10 -0400

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