Subject: Re: Fwd: IPv6 Scoped Addresses and Routing Protocols

From: "Steven M. Bellovin" <[email-masked]>
Date: Mon, 10 Jun 2002 07:43:18 -0400

In message <[email-masked]>, Hiroki Ishibashi writes:
>Steven M. Bellovin wrote:
>

>>I'm still confused.  If a packet arrives on a site-enabled interface, 
>>addressed to multicast address AllSPFRouters, and with protocol number 
>>89 (OSPF), to which process is it delivered?  Does something actually 
>>peek inside the packet to see if it's advertising global or site-local 
>>addresses when making the dispatching decision?
>
>"2.4.  Explicit support for multiple instances per link" in RFC2740 
>is the one to be used to identify a process (instance) to which packet
>are delivered.  Not necessary to peek into LSAs for the dispatching
>decision.  
>

I must be missing something; I still don't understand.

Let's look at a simple case, a router with two physical interfaces with 
one attached to the site backbone, and one feeding a departmental LAN.  
I'm perfectly willing to assume multiple logical interfaces on either 
physical interface.

Let's consider the backbone interface.  It's going to be seeing two 
types of routing advertisement, site-local and global.  When an 
incoming OSPF packet arrives, it's addressed to the same multicast 
address in either case.  To which logical interface does it belong?

		--Steve Bellovin, http://www.research.att.com/~smb (me)
		http://www.wilyhacker.com ("Firewalls" book)

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From: Derek Fawcus <[email-masked]>
Date: Mon, 10 Jun 2002 16:57:57 +0100

> address in either case.  To which logical interface does it belong?

It looks as if the intended use of this Instance ID is to have multiple
groups of OSPF routers running on the same link,  _which know nothing
about each other_.  It doesn't look as if it's intended that any
particular router would ever have a given interface assigned more than
one Instance ID.

Mind that said,  it would be possible to do this,  with one instance
ID for globals and one for site locals,  but it would require some
form of receive demultiplexing to deliver the appropriate packets to
the appropriate 'instance' of OSPF running locally.  This would obviouly
require peeking into the OSPF header,  but not into the LSAs.

e.g. a unix method - have a listener process on protocol 89,  it peeks
     at the instance ID,  and demuxes the packets to the appropriate
     local OSPF process.  For Tx the OSPF process can send direct to the
     interface.

DF

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From: Bill Fenner <[email-masked]>
Date: Mon, 10 Jun 2002 13:00:50 -0700

An example that may be a little too contrived:

			      H1
			      |
                              |//
                      B-------A
                     /      // \
		    /          |
		    |          |
		    C          D
		    |          |
		    |          |
		    \       \\ /
		     E--------F
			      |\\
			      |
			      H2

A,B,C,E,F are in one site (on the left); A, D, and F are in another site (or
no site).  H1 is connected to A, and H2 is connected to F.

It's fairly clear how to allow A and F to not advertise site-local
addresses across the boundary (at least, as long as it coincides with a
routing protocol boundary, e.g. OSPF area).  However, if H1 knows that
H2 is witchin the same site, H1 is allowed to use its site-local source
address to send to H2's global address.  However, along the H1-A-D-F-H2
path, A would have to drop the packet because it has a site-local address
and is trying to cross a site boundary.

Although this example is a bit contrived, it comes close to describing
the topology at AT&T, with Research split between the coasts and both
a private link (e.g. the internal site link) and links to the AT&T
internal network (which is still organizational-internal, so it's
reasonable to speak an IGP).

  Bill

---

From: Derek Fawcus <[email-masked]>
Date: Mon, 10 Jun 2002 16:57:57 +0100

DF