Subject: [dhcwg] DHCPv6 on link with no router?

From: Erik Nordmark <[email-masked]>
Date: Tue, 16 Mar 2004 16:19:11 -0800 (PST)

I don't know if this has come up with before so here it goes...

Suppose I want to use DHCPv6 to configure the hosts on a standadlone 
link i.e. a link which is not connected to any other link; hence
there is no need for a router on the link.

The DHCP server can hand out addresses to the hosts, but DHCPv6 doesn't
appear to have a way to say "this prefix is on-link" hence, in
the absense of that information in Router Advertisement messages,
the hosts will not know that they are all on link.

Did I get that right, or am I missing something?

An obvious fix would be to have a device on the link which 
sends RAs with the on-link prefix specified.

  Erik

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From: Greg Daley <[email-masked]>
Date: Wed, 17 Mar 2004 13:02:46 +1100

Hi Erik,

> sends RAs with the on-link prefix specified.

I'd guess that the Router Lifetime would be zero then.

I can't really see a problem except the router authorization/
delegation issue. If the policy is such that hosts have tried
router discovery first before undertaking DHC, then I'd guess
that they've already determined that there's no other routers.

Is it really a DHC issue or an ND issue?

If it is, is it better to have an information option in DHCP
indicating this otherwise?

Greg

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From: "Bernie Volz" <[email-masked]>
Date: Tue, 16 Mar 2004 21:52:57 -0500

In this case, what need is there for anything but the link local
addresses as there is no way to communicate with anyone off-link - no
router. DHCPv6 may still be used for "other" configuration parameters if
they may be useful.

- Bernie

-----Original Message-----
From: [email-masked] [mailto:[email-masked]] On Behalf Of
Erik Nordmark
Sent: Tuesday, March 16, 2004 7:19 PM
To: [email-masked]
Cc: [email-masked]
Subject: [dhcwg] DHCPv6 on link with no router?

Suppose I want to use DHCPv6 to configure the hosts on a standadlone 
link i.e. a link which is not connected to any other link; hence there
is no need for a router on the link.

The DHCP server can hand out addresses to the hosts, but DHCPv6 doesn't
appear to have a way to say "this prefix is on-link" hence, in the
absense of that information in Router Advertisement messages, the hosts
will not know that they are all on link.

  Erik

---

From: Greg Daley <[email-masked]>
Date: Wed, 17 Mar 2004 16:19:49 +1100

Hi Bernie,

This is just a guess:

> router. DHCPv6 may still be used for "other" configuration parameters if

Some applications may be interested in having long term bindings
to stable non-locally scoped ip addresses for local communication
(like NFS).

In the case that the network is intermittently connected or at least
the  router is (perhaps it is a LEO satellite??) you may wish to
still have some network prefix advertisement or allocation.

The current set of specifications for globally unique non
provider aggregatable addresses in IPv6 may meet the
stability requirement for such addressing.
Alternatively, global addresses sub-delegated within an
organization may have this sort of application in some
environments

Greg

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From: Erik Nordmark <[email-masked]>
Date: Wed, 17 Mar 2004 09:35:50 -0800 (PST)

> router. DHCPv6 may still be used for "other" configuration parameters if

The issue is that hosts might be multihomed and dealing with link-locals
on a multihomed host complicates the applications; being able
to use global address is much easier.

An example configuration where this might be useful is a 
load balancer in front of a bunch of (web) servers, with no router (hence
no direct IP path) from the Internet to the web servers.
In IPv4 the web servers might have net 10 addresses and the load
balancer effectively doing NAT (while this is from far the only type of
LB setup there are other cases when "backend" networks use private IPv4
addresses).

In some cases the web servers then talk out the "back" to a second tier
of servers running application logic, NFS, or databases. And those subnets
might use private IPv4 addresses as well. Thus the web servers could be
multihomed to multiple IPv4 private subnets.

Being able to assign addresses to the above "internal" subnets
from the site's global pool will remove the management complexity of 
dealing with private IPv4 addresses, without having the applications on
the multihomed servers have to deal with IPv6 link-local addresses.

  Erik

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From: Erik Nordmark <[email-masked]>
Date: Wed, 17 Mar 2004 09:37:27 -0800 (PST)

> I'd guess that the Router Lifetime would be zero then.

Yep.

> Is it really a DHC issue or an ND issue?

Your comment raises an issue. Today ND says that when there is
no router on the link assume that all destinations are local.
But rfc2461bis is about to change that (since it causes some problems
during transition and there doesn't seem to be any utility for that rule).

Well, in the case of DHCP-configured addresses (i.e. not link local)
and no router it now seems like that feature has some use.

> indicating this otherwise?

A dhcpv6 option was what I started looking for in the RFC and didn't
find one. I guess we've been assuming that there are always at least one
router on each link.

  Erik

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From: Ted Lemon <[email-masked]>
Date: Wed, 17 Mar 2004 12:09:40 -0600

In the load balancer example you describe, what's the benefit in having 
no "router" on the network?   Even if the "router" just sends out RA 
messages and never forwards packets, it seems like that's the right way 
to handle this case.   I don't think a DHCPv6 client will even try to 
operate if it doesn't see an RA message.

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From: "Bernie Volz" <[email-masked]>
Date: Wed, 17 Mar 2004 13:24:46 -0500

Well, RFC 2462 says:

"If no routers are present, stateful autoconfiguration should be
invoked."

So, DHCP would be used.

I too agree with you Ted - a router (or something serving that function
in some limited capacity) should send out RAs if that is needed. Having
DHCP send prefix information just opens a can of worms - which should a
host believe if it receives ones from DHCP and a router's RAs. It is
best if things come from only one place. So, if you need prefixes
advertised, some entity on the network must send RAs.

- Bernie

-----Original Message-----
From: [email-masked] [mailto:[email-masked]] On Behalf Of
Ted Lemon
Sent: Wednesday, March 17, 2004 1:10 PM
To: Erik Nordmark
Cc: [email-masked]; Bernie Volz
Subject: Re: [dhcwg] DHCPv6 on link with no router?

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From: Ted Lemon <[email-masked]>
Date: Wed, 17 Mar 2004 14:00:32 -0600

> "If no routers are present, stateful autoconfiguration should be

Oops, didn't know that.   Thanks for pointing it out!

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From: Erik Nordmark <[email-masked]>
Date: Wed, 17 Mar 2004 13:58:34 -0800 (PST)

> advertised, some entity on the network must send RAs.

I think that was what I suggested in the first post on the subject.

It would probably imply that the same node(s) on the link which runs DHCP
(be it a relay or a server) that would send these RAs,
because otherwise you need to assign a (pair of) additional nodes
to send the RAs.
["A pair" to avoid a single point of failure.]

This removes the need for the host to try to resolve conflicting
information it would need from different sources (DHCP vs. RA),
but it still requires care in configuring these nodes to make sure
that the DHC addresses are consistent with the subnet prefix that
is advertised in the RA. I think the latter can be an implementation
issue on the DHCP agent boxes that sending the RAs.

 Erik

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From: Erik Nordmark <[email-masked]>
Date: Mon, 22 Mar 2004 21:59:20 -0800 (PST)

> (This is probably not a dhcp issue, but an ND issue) assuming we have
> agreed that the "on-link by default" rule should be removed from
> rfc2461bis, it should also note that we need to be careful that
> the on-link prefixes may need to be advertised.
> 
> Should we bring this to the ipv6 wg to help the 2461bis work?

It might be useful to add a note about this (that prefixes need to
be advertised on links that don't have routers but want to use
non-link local addresses) in 2461bis - perhaps in the list of changes
it could state this as an implication of changing the "on-link by default".

  Erik

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From: JINMEI Tatuya / 神明達哉 <[email-masked]>
Date: Mon, 22 Mar 2004 17:31:46 +0900

>>>>> On Wed, 17 Mar 2004 13:58:34 -0800 (PST), 
>>>>> Erik Nordmark <[email-masked]> said:

					JINMEI, Tatuya
					Communication Platform Lab.
					Corporate R&D Center, Toshiba Corp.
					[email-masked]