Subject: [dhcwg] dhcpv6-24: use of anycast

From: Thomas Narten <[email-masked]>
Date: Wed, 08 May 2002 10:59:29 -0400

Seems to me that the anycast text is a bit underspecified. It's also
not clear to me exactly what problem the anycast text is trying to
address.

> 13. Transmission of messages by a client
> 
>    Unless otherwise specified, a client sends DHCP messages to the
>    All_DHCP_Relay_Agents_and_Servers or the DHCP_Anycast address.

This is ambiguous. What should clients actually implement by default?
This document should make it clear what clients should implement. For
example try one and if it doesn't work, try the other? Leaving when to
use anycast to the implementor without guidelines will likely lead to
interoperability problems, with different implementations doing
different things.

>    If the client is attached to a link that supports multicast
>    transmission, the client sends DHCP messages to the
>    All_DHCP_Relay_Agents_and_Servers address.  If the client is

My understanding is that all IPv6 links support multicast by
definition. If the underlying network doesn't, it must be
emulated. Note that multicast is needed for Neighbor Discovery, which
all IPv6 nodes have to implement.

>    When a client sends a DHCP message to the DHCP_Anycast address, it
>    MUST use an address assigned to the interface for which the client
>    is interested in obtaining configuration, which is suitable for use
>    by the server in responding to the client, as the source address in
>    the header of the IP datagram.  See "Default Address Selection for
>    IPv6" [4] for more details.

Don't you also need to say that anycast can only be used if the client
already has a properly scoped address? It seems to me that anycast
isn't very useful for the *client*. I.e, let it always send to the
relay agent and let the relay agent deal with anycast vs. mcast.

Or, is this specifically restricted to the information-request
message? If so, might be good to say that.

In private mail, Ralph asked:

> Good question to ask...How about words to the effect of "if the link is 
> connected to an NBMA link, use anycast; otherwise, use multicast"?

What is an NBMA link? Who defines what that is? I'm not sure there is
a well-defined definition of NBMA link.

Thomas

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From: Ralph Droms <[email-masked]>
Date: Wed, 08 May 2002 11:07:35 -0400

Thomas - the use of anycast is to allow the use of DHCP on links that do 
not support multicast.  The intention is to use anycast just across the 
link to which the client is attached; a relay agent forward messages across 
the rest of the path to the server.

More comments in line.

>interoperability problems, with different implementations doing

Should read "send to All_DHCP_Relay_Agents_and_Servers unless link does not 
support multicast, in which case used DHCP_Anycast"

>all IPv6 nodes have to implement.

If all IPv6 links support multicast, then we don't need the anycast address
for DHCP.

>relay agent and let the relay agent deal with anycast vs. mcast.

We're extending the use of anycast to include direct communication
with the server, which is parallel to the use of unicast (under the
control of the server).

>message? If so, might be good to say that.

No, it's not intended to be specific to Information-request.

---

From: Thomas Narten <[email-masked]>
Date: Wed, 08 May 2002 11:31:12 -0400

> Thomas - the use of anycast is to allow the use of DHCP on links
> that do not support multicast.  The intention is to use anycast just
> across the link to which the client is attached; a relay agent
> forward messages across the rest of the path to the server.

OK. It was not clear to me that anycast was restricted to the first
hop.

But... If a link doesn't support IPv6 multicast, how will it support
Neighbor Discovery? I suspect there are only a few links that might
have this property. Do we understand what those link types are and
whether its important that DHC run over them?

> If all IPv6 links support multicast, then we don't need the anycast address

This would be my assumption. Do folks have examples of link types they
are concerned about?

Thomas

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From: Ole Troan <[email-masked]>
Date: Wed, 08 May 2002 16:46:17 +0100

> This would be my assumption. Do folks have examples of link types they

ATM, Frame Relay RFC2491), ISDN/PSTN, MPLS, 6to4, ISATAP.  if routers
use DHCP (e.g Prefix Delegation) then support is needed for the first
set of media. hosts connecting through an ISATAP cloud could very well
need DHCP I should think.

cheers,
Ole

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From: Thomas Narten <[email-masked]>
Date: Thu, 09 May 2002 08:04:16 -0400

> ATM,

See RFC 2491 and 2492. Multicast is supported. At least in the case of
SVC/PVC. Not clear that anyone uses anything else (i.e., pure NBMA
cloud).

> Frame Relay RFC2491)

Ditto.

> ISDN/PSTN

Always modeled as a p-2-p link, so mcast is trivially defined.

> MPLS

MPLS is a funny one. I don't think it is typically viewed as an NBMA
cloud. Also, MPLS is used within the core, by routers, and NOT at the
edges, so I'd think DHC is not really applicable in general here.

> 6to4

Not clear that this is applicable. The router defining 6to4 is an IPv4
box on the interface and uses DHCPv4. It does provide IPv6 support,
but this is explicitely in an environment where IPv6 is not supported
by the upstream. So DHCPv6 doesn't seem applicable; it it was, why
would one even be using 6to4?

> ISATAP.

Not clear this is applicable.

> if routers use DHCP (e.g Prefix Delegation) then support is needed
> for the first set of media. hosts connecting through an ISATAP cloud
> could very well need DHCP I should think.

This is not at all clear to me.

99% of the applicability of DHCv6 will be traditional media, i.e.,
Ethernets.

Another thought. The IPv6 over foo documents provide specifics for how
to run IPv6 over that media. In some cases, those documents can
override defaults for running IPv6 over a media in general. For
example, one can change the number of times one retransmits in
DAD. The default is 1, but a specific document can make that 0 or 2,
etc.

Perhaps one could take the approach that DHC uses link-local multicast
by default, unless the specific link-layer over FOO document says use
anycast. This is cleaner than saying "use anycast over NBMA" because
its clearly specified on a case-by-case basis.

Thomas

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From: Thomas Narten <[email-masked]>
Date: Wed, 15 May 2002 12:52:22 -0400

> the rest of the path to the server.

I've asked some more IPv6 people about whether they believe there are
(or will ever be) links that support IPv6, but not IPv6 multicast.

No one has argued that such links will exists or should be allowed to
exist.

Based on the above, I would like to request that all the anycast text
be removed from the document. It simply does not seem to be needed to
solve an actual problem.

Thomas