Subject: Re: [homenet] [v6ops] default LAN routing protocol for IPv6 CE router

From: "Brzozowski, John" <[email-masked]>
Date: Tue, 2 Aug 2011 21:53:00 +0000

Nice migration to HOMENET. ;)

On 8/1/11 2:58 PM, "Ole Troan" <[email-masked]> wrote:

>[v6ops in bcc]
>
>> On Sun, Jul 31, 2011 at 10:38, Brzozowski, John
>><[email-masked]> wrote:
>> The outer most router supports DHCPv6 PD (LAN side).  It would have to
>>be
>> seeded with adequate address space to serve up IA_PD (and possibly
>>IA_NA)
>> for downstream routers.  Based on conversations to date I assume there
>> would be interest in downstream routers also support and being able to
>> delegate prefixes?
>> 
>> There are two issues I see with DHCPv6 PD to distribute prefixes inside
>>the home:
>> 
>> 1. Prefix delegation only really works in a tree topology, so when the
>>user plugs things in wrong (e.g., plugs a LAN port into another LAN
>>port, or creates a loop) it won't work. Having a routing protocol will
>>allow things to work regardless of how the user plugs things in.
>
>prefix delegation with DHCP can be made to work in arbitrary topologies.
[jjmb] agreed.
> 
>
>> 2. We already use PD between the home network and the ISP. If we use it
>>inside the home as well, then a home router getting a prefix via DHCPv6
>>PD will not know whether it is connected to the ISP or not, and thus if
>>it should turn on the firewall or not.
>
>PD (or not) as a mechanism to detect a network boundary is probably not
>reliable or secure enough.
>
>> Both of these issues can be solved by using an IGP.
>
>3. DHCP PD does not handle the multi-home / multi-prefix case. an IGP
>based solution would.
[jjmb] can you elaborate on this?  Why not?
>
>the requirement to have an IGP is somewhat orthogonal to the problem of
>prefix assignment. the autonomous properties of a flooding mechanism as
>opposed to a request/reply mechanism is the key in my opinion.
>
>cheers,
>Ole
>

---

From: Ole Troan <[email-masked]>
Date: Wed, 3 Aug 2011 08:48:09 +0200

John,

[...]

> [jjmb] can you elaborate on this?  Why not?

in the PD based solution for a self configuring mechanism, the Border Router would be the PD delegating router for the home network. that's the router receiving the delegated prefix from the ISP. downstream routers will use this router as a DHCP server for their PD requests.

what happens if you have multiple border routers? a downstream requesting router can only use one DHCP server, and there isn't as far as I know any protocol mechanism for multiple DHCP servers to synchronize and exchange information between themselves.

in the autonomous case where each border router injects its own understanding of what the site prefix is, into the home, this is not a problem. you'll get one prefix for each ISP.

cheers,
Ole

---

From: Philip Homburg <[email-masked]>
Date: Wed, 03 Aug 2011 16:00:18 +0200

In your letter dated Wed, 3 Aug 2011 08:48:09 +0200 you wrote:
>what happens if you have multiple border routers? a downstream =
>requesting router can only use one DHCP server, and there isn't as far =
>as I know any protocol mechanism for multiple DHCP servers to =
>synchronize and exchange information between themselves.

Section 17.1.3 of RFC-3315 allows a client to select multiple Advertisements.

Section 3.1 of RFC-3769 says: "The mechanism should allow for delegation
of more than one prefix to the customer."

Where does RFC-3633 say that you can have just one prefix or one DHCP server?

I do want to note that in my experience, the current IPv6 model for routing
is a bit incomplete when it comes to dealing with multiple upstreams if
those ISPs employ ingress filtering. In most current systems, you have to
resport to policy routing to send packets in the right direction.

I created a simply RIPv6 extension to make at least the routing part work
automatically but there are some other issues as well.

---

From: "Brzozowski, John" <[email-masked]>
Date: Thu, 4 Aug 2011 05:02:39 +0000

>>synchronize and exchange information between themselves.
[jjmb] there has recently been working started in dhc for DHCPv6 failover
which requires DHCPv6 servers to exchange information.
>Where does RFC-3633 say that you can have just one prefix or one DHCP
[jjmb] there is none as far as I know.

---

From: "Brzozowski, John" <[email-masked]>
Date: Thu, 4 Aug 2011 05:09:28 +0000

On 8/3/11 2:48 AM, "Ole Troan" <[email-masked]> wrote:

>in the PD based solution for a self configuring mechanism, the Border
>Router would be the PD delegating router for the home network. that's the
>router receiving the delegated prefix from the ISP. downstream routers
>will use this router as a DHCP server for their PD requests.
>
>what happens if you have multiple border routers? a downstream requesting
>router can only use one DHCP server, and there isn't as far as I know any
>protocol mechanism for multiple DHCP servers to synchronize and exchange
>information between themselves.
[jjmb] when you say synchronizing are referring to failover or simply a
means to coordinate which server is doing what?  Regardless, you are
correct there is not such mechanism, however, dhc has recently started
work on failover for DHCPv6.  If preference is the goal one server could
use a higher DHCPv6 preference than the other.  There would still have to
be a negotiation mechanism.
>
>in the autonomous case where each border router injects its own
>understanding of what the site prefix is, into the home, this is not a
>problem. you'll get one prefix for each ISP.
[jjmb] but does this solve the problem?  These prefixes will both be
active simultaneously?  If yes, this can also be done using DHCPv6 no?

---

From: Ole Troan <[email-masked]>
Date: Thu, 4 Aug 2011 09:36:35 +0200

John,

[...]

> active simultaneously?  If yes, this can also be done using DHCPv6 no?

if you are multi-homed, then that will result in multiple prefixes, one from each ISP.
if you have one CPE connected to each ISP, then downstream routers will if they were using some form of hierarchical DHCP PD, have to request prefixes from each CPE. as Philip pointed out, there is nothing in the DHCP spec prohibiting a requesting router to choose all servers that advertise themselves, instead of just one. I don't think that's current DHCP client behaviour though.

cheers,
Ole

---

From: "Brzozowski, John" <[email-masked]>
Date: Thu, 4 Aug 2011 20:12:09 +0000

On 8/4/11 3:36 AM, "Ole Troan" <[email-masked]> wrote:

>if you have one CPE connected to each ISP, then downstream routers will
>if they were using some form of hierarchical DHCP PD, have to request
>prefixes from each CPE. as Philip pointed out, there is nothing in the
>DHCP spec prohibiting a requesting router to choose all servers that
>advertise themselves, instead of just one. I don't think that's current
>DHCP client behaviour though.
[jjmb] in this case each edge CPE router will offer a unique prefix.
Unless the downstream routers issue separate DHCPv6 requests I imagine
only one would be accepted by the downstream router at any time.  The only
thing that comes to mind to allow one prefix from being selected over
another (which is non-deterministic) is DHCPv6 preference.  If the second
CPE uses something less than 255 this would increase the chances the
prefix from the first CPE router would be selected.