Subject: NHRP question

From: Yakov Rekhter <[email-masked]>
Date: Wed, 19 Nov 97 07:12:07 PST

Folks,

>From the latest NHRP I-D:

   If an NBMA technology offers a group, an anycast, or a multicast
   addressing feature then the NHC may be configured with such an
   address which would be assigned to the appropriate NHSs.  

Can all the NHSs within an ATM network be assigned the same anycast
(multicast) address ? And if not, then what are the constraints ?

Yakov.

---

From: Joel Halpern <[email-masked]>
Date: Wed, 19 Nov 1997 10:40:13 -0500 (EST)

> (multicast) address ? And if not, then what are the constraints ?
Yes, all NHS can (and are expected to) be assigned the same anycast
address.  The registration forwarding rules will take care of getting the
registrations to the right place.

Yours,
Joel M. Halpern                         [email-masked]
Newbridge Networks Inc.

---

From: Albert Manfredi <[email-masked]>
Date: Wed, 19 Nov 1997 11:12:11 -0500

> (multicast) address ? And if not, then what are the constraints ?

The answer should be yes, but perhaps this doesn't really matter?
What you want is to find your NHS, I assume. But this presumes that
you're using IP over ATM, which must mean that you're sending an
NHRP Resolution Request over the routed path. Which requires that
the NHS be your default router or any other _local_ router your
routed path might take you over. No?

Bert
manfredi@#arl.bna.boeing.com

---

From: Yakov Rekhter <[email-masked]>
Date: Wed, 19 Nov 97 08:32:01 PST

Bert,

> routed path might take you over. No?

When an NHC sends an NHRP Request to an anycast (multicast) address,
can some other NHS that isn't the NHC's local router receive the Request ?

Yakov.

---

From: Albert Manfredi <[email-masked]>
Date: Wed, 19 Nov 1997 11:55:02 -0500

> can some other NHS that isn't the NHC's local router receive the Request ?

Yakov,

Anycast goes to the topologically closest entity with that anycast
address. However, in this case, I think that's not how it works.
Maybe I'm all wet, but what happens with NHRP is that the request
goes out using IP routing. In principle, the request might even be
redirected to some other router than your default router. And NHRP
only works if each of these IP routers on your ATM infrastructure
are NHSs.

For example, let's say your default router is an NHS, but the router
your NHRP Request is redirected to is not an NHS. NHRP won't work.

So I don't see that you depend on ATM anycast addressing to find the
correct NHS. I think you simply depend on perfectly ordinary IP
techniques (default router, ICMP redirects).

Bert
[email-masked]

---

From: Yakov Rekhter <[email-masked]>
Date: Wed, 19 Nov 97 09:23:43 PST

Joel,

> registrations to the right place.

So, when an NHC sends an NHRP Request for a particular target
destination address D, and this Request is sent using NBMA anycast
(multicast) address that is assigned to all the NHSs, and D is reachable 
via more than one egress router (NHS), would it be possible for all 
these egress routers (NHSs) to receive the Request, and then send 
their Replies back to the NHC ?

Yakov.

---

From: Joel Halpern <[email-masked]>
Date: Wed, 19 Nov 1997 12:34:45 -0500 (EST)

No, the request does not go to all routers:
1) ANYCAST only conects you to one place.
2) The ANYCAST mechanism is only used during registration.  After it has
completed, you will have a VC to an NHS serving your LAG.  You will
send NHRP requests to that NHS.  He will send you responses.   Your
registration information will be propagated (via SCSP) to all other NHS
serving your LAG, so that any NHS serving the LAG can answer queries for
you.
3) Any NHS serving the LAG may also connect to you to forward data packets
it receives.

Yours,

---

From: Yakov Rekhter <[email-masked]>
Date: Wed, 19 Nov 97 09:43:10 PST

Joel,

> serving your LAG, so that any NHS serving the LAG can answer queries for

The text I was referring to originally doesn't say anything about
constraining the use of anycast/multicast addresses to
registration only, but talks about NHRP Requests in general:

   If an NBMA technology offers a group, an anycast, or a multicast
   addressing feature then the NHC may be configured with such an
   address which would be assigned to the appropriate NHSs.  The NHC
   might then submit NHRP Resolution Requests to such an address,
   eliciting a response from one or more NHSs, depending on the response
   strategy selected. 

Yakov.

---

From: Albert Manfredi <[email-masked]>
Date: Wed, 19 Nov 1997 16:16:15 -0500

Yakov,

About the anycast question, anycast itself only goes to one place.
For example, anycast could be used in an RFC 1577 situation to find
your default router. But I don't think it should be made to apply in
NHRP in any more special way than that. After all, your NHS would
typically be your default router, so using anycast in that sense
seems a good fit.

As to the greater issue of which NHSs an NHC should use, here's the
text from draft-ietf-rolc-nhrp-12.txt that I think applies:

4. Configuration

   Next Hop Clients

     An NHC connected to an NBMA subnetwork MAY be configured with
the
     Protocol address(es) and NBMA address(es) of its NHS(s).  The
     NHS(s) will likely also represent the NHC's default or peer
     routers, so their NBMA addresses may be obtained from the NHC's
     existing configuration.  If the NHC is attached to several
     subnetworks (including logical NBMA subnetworks), the NHC
should
     also be configured to receive routing information from its
NHS(s)
     and peer routers so that it can determine which internetwork
layer
     networks are reachable through which subnetworks.

My reading of this is very simple. These NHSs are the local routers
you would use in any other IP framework, except in this ATM example
they also provide another function (they are used to eventually
translate this routed path into an ATM shortcut).

However, if you want to concentrate on the anycast role itself,
_that_ role is only to find your default router. But just as you do
in any other IP environment, the first hop router you end up using
in a given IP session might not be the default router at all. If
you're starting off knowing only the IP Destination Address, it
makes sense that the first hop router be found using IP routing
criteria.

The added complication about a client connected to several different
networks is basically nothing more than a client which is configured
as a router. In that instance, the client itself must worry about
reachability and distance to the destination network, and pick the
shortest path, which it would do based on OSPF or RIP, rather than
anything strictly related to anycast.

I think these considerations hold even if the client is not ATM, but
one or more of the client's immediate routers are ATM.

I don't know if this answers your concerns or if I'm missing the
boat entirely.

Bert

---

From: Eric W Gray <[email-masked]>
Date: Thu, 20 Nov 1997 07:12:34 -0500

Joel,

	Alternatively, groups of NHSs may be assigned different anycast
addresses - is that not true?

-- 
Eric

---

From: Eric W Gray <[email-masked]>
Date: Thu, 20 Nov 1997 07:26:00 -0500

Yakov,

	Even if a MCS is used to distribute NHC->NHS messages, so what?

	In that case, appropriate NHSs would consist of the same NHSs
which would be sharing registrations anyways.  And it's certainly not
hard for the NHSs to figure out autonomously which NHS should respond.

	Even if more than one NHS responds, the NHC should be able to
pick any response (such as the first one) and just go with it.  What
is the problem you're trying to get at?

-- 
Eric

---

From: Eric W Gray <[email-masked]>
Date: Thu, 20 Nov 1997 07:32:27 -0500

Bert,

	You're correct about "topologically closest entity" - but the
"topology" used is ATM.  For this reason, anycast addresses may need 
to be scoped (from the NHC's perspective) such that any one anycast
address known to the NHC can only map to (the set of) NHS(s) that are
"L3 toplogically" close to the NHC.  From the ATM topological point-
of-view, the closest NHS may be "L3 topologically remote" to the NHC.

-- 
Eric