Subject: [dhcwg] Prefix delegation

From: Bud Millwood <[email-masked]>
Date: Wed, 20 Jun 2007 15:16:44 +0200

I'm browsing through RFC 3633, and trying to wrap my head around exactly who 
has what responsibilities with Prefix Delegation, and I'd appreciate if 
someone could clarify these basic questions:

If an intermediate router is doing prefix delegation, can that router query a 
DHCP server for a set of prefixes that it can delegate?

Can a DHCP server directly support PD without the intermediate router being 
involved? Is there any reason PD cannot be handled directly by the DHCP 
server?

- Bud

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From: "Brzozowski, John" <[email-masked]>
Date: Wed, 20 Jun 2007 09:52:42 -0400

hello bud,

I believe the answer is both  are technically viable.  dhcp-pd can certainly be handled without the intermediate router being involved with prefix acquisition, however, the intermediate router or some portion of the underlying network will require information about the delegated prefix to ensure routability.

john
========================================= 
John Jason Brzozowski (CISSP, RHCT)
Comcast Corporation
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-----Original Message-----
From: Bud Millwood [mailto:[email-masked]]
Sent: Wed 6/20/2007 9:16 AM
To: [email-masked]
Subject: [dhcwg] Prefix delegation

- Bud

---

From: Ralph Droms <[email-masked]>
Date: Wed, 20 Jun 2007 10:10:52 -0400

Responses in line...

> DHCP server for a set of prefixes that it can delegate?

Well, assuming that the intermediate router is running a DHCP server to do
PD, there is no formal definition for "hierarchical" PD from a central DHCP
server to multiple subsidiary DHCP servers.  However, there's no reason a
subsidiary server couldn't request, say, a /40 from the central server and
then delegate /48s to requesting routers.

> involved? Is there any reason PD cannot be handled directly by the DHCP

I think this model is exactly what is described in RFC 3633.  The
"delegating router" runs a DHCP server and delegates directly to the
requesting router.

What RFC 3633 doesn't talk about is PD through a relay agetn.  There are
routing issues that need to be solved.

> - Bud

- Ralph

---

From: Bud Millwood <[email-masked]>
Date: Wed, 20 Jun 2007 16:26:59 +0200

Thanks to both of you for the responses. It seems there's really no need for a 
formal definition of heirarchical PD as long as the delegating router can be 
configured to ask the next upstream delegating router (or central server) for 
a prefix.

> > Can a DHCP server directly support PD without the intermediate router
> > being involved? Is there any reason PD cannot be handled directly by the
> > DHCP server?
>

So this means that until the routing issues are solved, a given router cannot 
(should not?) be both a delegating router and a relay agent?

And if you want to centrally manage all delegated prefixes from a non-router:

  - The DHCP server can delegate directly to a CPE router
 - The DHCP server can delegate to a delegating router that's upstream from 
the CPE router

Sound right?

- Bud

---

From: "Durand, Alain" <[email-masked]>
Date: Wed, 20 Jun 2007 11:03:36 -0400

> From: Ralph Droms [mailto:[email-masked]] 
>
> What RFC 3633 doesn't talk about is PD through a relay agetn. 
> There are routing issues that need to be solved.

This is an important issue that need to be solved if we ever want
prefix delegation to be deployed in a large scale.

  - Alain.

---

From: "Brzozowski, John" <[email-masked]>
Date: Wed, 20 Jun 2007 11:09:36 -0400

cannot is probably too strong I think it can be done, there might be some limitations.

one more note regarding the latter scenario.  in this case if dhcpv6 is used to delegate a prefix to a delegating router the dhcp server will only ever have knowledge of the delegated prefix not the sub-delegates.  depending on your use case this might not be an issue for some I suspect it would.

john

-----Original Message-----
From: Bud Millwood [mailto:[email-masked]]
Sent: Wed 6/20/2007 10:26 AM
To: [email-masked]
Subject: Re: [dhcwg] Prefix delegation

Sound right?

- Bud

---

From: Bud Millwood <[email-masked]>
Date: Wed, 20 Jun 2007 16:50:20 +0200

On Wednesday 20 June 2007 17:09, Brzozowski, John wrote:

> one more note regarding the latter scenario.  in this case if dhcpv6 is
> used to delegate a prefix to a delegating router the dhcp server will only
> ever have knowledge of the delegated prefix not the sub-delegates. 
> depending on your use case this might not be an issue for some I suspect it
> would.

Sure, but I figure that if you're delegating to a delegator, then that's 
exactly the behavior you want - the DHCP server relenquishes control of the 
entire prefix to the delegating router.

- Bud

---

From: "Durand, Alain" <[email-masked]>
Date: Wed, 20 Jun 2007 11:27:11 -0400

> Sure, but I figure that if you're delegating to a delegator, 
> then that's exactly the behavior you want - the DHCP server 
> relenquishes control of the entire prefix to the delegating router.

Bud,

There are two things you need to consider:
a) the delegation of a prefix
b) the accounting/reporting of the same

If you do sub-delegation, you may have a hard time to do b)

   - Alain.

---

From: Bud Millwood <[email-masked]>
Date: Wed, 20 Jun 2007 17:01:20 +0200

> > There are routing issues that need to be solved.

Do you mean that there's a problem if there are *any* relay agents between a 
server and a delegating router?

> prefix delegation to be deployed in a large scale.

I think I'm missing something here. If a CPE router is a delegating router, 
and any intermediate routers are *not* delegating routers, a central DHCP 
server can delegate prefixes to the CPE routers, right? Is there a routing 
problem with this configuration?

- Bud

---

From: Ole Troan <[email-masked]>
Date: Wed, 20 Jun 2007 16:31:12 +0100

> prefix delegation to be deployed in a large scale.

this draft explores the solution space:
http://tools.ietf.org/html/draft-stenberg-pd-route-maintenance-00

/ot

---

From: John Jason Brzozowski <[email-masked]>
Date: Wed, 20 Jun 2007 11:33:29 -0400

Yes, I just wanted to make sure that was clear.  It is an important point
that will not work in some deployments.

John

---

From: Bud Millwood <[email-masked]>
Date: Wed, 20 Jun 2007 17:07:42 +0200

On Wednesday 20 June 2007 17:01, Bud Millwood wrote:
> Do you mean that there's a problem if there are *any* relay agents between
> a server and a delegating router?
>

Scratch that, I think I got my terminology wrong. If a CPE router requests a 
prefix, it's not considered a "delegating router", right? The CPE router is 
then just a router with a DHCP server that happens to have been configured 
using prefix delegation.

So I think I mean, if a CPE router requests a prefix through a relay agent, 
and a central DHCP server delegates that prefix to the CPE router, then: Is 
there a routing problem with the relay agent, or anywhere else in this 
scenario?

(Thanks for the link Ole)

- Bud

---

From: Ralph Droms <[email-masked]>
Date: Wed, 20 Jun 2007 11:37:31 -0400

Bud - there's a simple (although perhaps only partial) solution for the
routing issue: choose appropriate aggregation so all of the prefixes
delegated through a given relay agent belong to a single prefix.  Now
configure the router on which the relay agent runs to inject routing
information for the aggregated prefix.

+---+
|RR1+----|
+---+    |    +-----------+       +----------+
+---+    |    |Relay agent|       |   DHCP   |
|RR2+----+----+  Router   +--...--+  Server  |
+---+    |    +-----------+       +----------+
+---+    |  dead:beef:ca00::/40
|RR3|----|
+---+              

In this picture, the router is statically configured to advertise
dead:beef:ca00::/40 and the DHCP server is configured to delegate prefixes
from dead:beef:ca00::/40 to the requesting routers RR*.  So, when the DHCP
server delegates dead:beef:ca01::/48 to RR1, the routing info for the
delegated prefix is advertised by the aggregating prefix from the router.

Routing between the relay agent/router and the RRs can be established
through RAAN and DHCPv6 leasequery.

And this only works if its OK to restrict the prefixes assigned to the RRs
to a single aggregating prefix.  That restriction may not be acceptable for
some ISPs.

- Ralph

---

From: Ole Troan <[email-masked]>
Date: Wed, 20 Jun 2007 16:41:01 +0100

> problem with this configuration?

not necessarily. someone has to inject a route for the delegated prefix
into the routing system. the PD spec is silent on this.  it would
typically either be the requesting router or the first-hop router
(delegating router or DHCP relay) from the perspective on the requesting
router. you can of course do this in a hierarchical manner too as you
suggest.

/ot

---

From: "Durand, Alain" <[email-masked]>
Date: Wed, 20 Jun 2007 11:44:33 -0400

> I think I'm missing something here. If a CPE router is a 
> delegating router, and any intermediate routers are *not* 
> delegating routers, a central DHCP server can delegate 
> prefixes to the CPE routers, right? Is there a routing 

Not really. You have a problem if the delegating router is the
first hop router after the CPE. 

   - Alain.

---

From: "Durand, Alain" <[email-masked]>
Date: Wed, 20 Jun 2007 11:47:31 -0400

> Bud - there's a simple (although perhaps only partial) 
> solution for the routing issue: choose appropriate 
> aggregation so all of the prefixes delegated through a given 
> relay agent belong to a single prefix.  Now configure the 
> router on which the relay agent runs to inject routing 
>[...]
> And this only works if its OK to restrict the prefixes 
> assigned to the RRs to a single aggregating prefix.  That 
> restriction may not be acceptable for some ISPs.

A clear limitation to this model is what happen if, at some point
down the road, you need to 'split' the router, eg for capacity issue...
You cannot keep the agregate in place.

   - Alain.

---

From: Ralph Droms <[email-masked]>
Date: Wed, 20 Jun 2007 12:21:58 -0400

Right, and the constraint restricts customer mobility between aggregating
routers (relay agent/router in my picture) if you want to give the customer
a static prefix assignment and avoid customer renumbering.

- Ralph

---

From: Bud Millwood <[email-masked]>
Date: Wed, 20 Jun 2007 21:33:27 +0200

> information for the aggregated prefix.

Aha - this was the *only* scenario I considered viable with PD.

Thanks for the input, all of you. I think I'll ice this for the time being, 
and take a closer look at it some other time.

- Bud