Subject: RE: [dhcwg] prefix length determination for DHCPv6

From: "Leino, Tammy" <[email-masked]>
Date: Fri, 10 Aug 2007 17:26:25 -0500

Thank you very much for the information John.

>>RAs could be configured to supply the length for prefixes valid on the link  Otherwise, there is no DHCPv6 option that can be used to supply this data.

What if the router is configured just to inform the node to use DHCPv6 to obtain an address and does not specify any prefix options in the RA?  The only place I see to specify a prefix length in an RA is in a prefix option.  Is there some other way an RA could relay a prefix length to a node?  If not, how does the client know the prefix length of the address the DHCP server is assigning?  It seems to me that the DHCPv6 server should be sending a prefix length with the address it assigns.  Is there a reason this was not included in the specification?

>>Regarding, your second question why would you need information about the router if it does not exist or is not configured to route?   Just looking for a little more information.

In implementing this RFC, I am not making assumptions that there will be an IPv6-configured router on link.  This does not mean there is not a router on link that can forward IPv6 messages, it just means there might not be a router on link that sends RAs.  Just because a node can forward IPv6 traffic does not mean it will necessarily be configured to send RAs.

I greatly value your advice and comments in this area.

Best Regards,
Tammy

-----Original Message-----
From: John Jason Brzozowski (JJMB) [mailto:[email-masked]]
Sent: Friday, August 10, 2007 6:14 PM
To: Leino, Tammy; [email-masked]
Subject: RE: [dhcwg] prefix length determination for DHCPv6

Hello Tammy,

RAs could be configured to supply the length for prefixes valid on the link  Otherwise, there is no DHCPv6 option that can be used to supply this data.

Regarding, your second question why would you need information about the router if it does not exist or is not configured to route?   Just looking for a little more information.

John
######################
John Jason Brzozowski, CISSP, RHCT
[email-masked]
484-994-6787
######################

-----Original Message-----
From: "Leino, Tammy" <[email-masked]>
To: [email-masked]
Sent: 08/10/07 17:59
Subject: [dhcwg] prefix length determination for DHCPv6

Hello,

In implementing RFC 3315, I see no option or field in the IA_NA option for a prefix length associated with an assigned IPv6 address.  How does the client determine the prefix length of the IPv6 address the server is assigning?

Furthermore, in the absence of a configured IPv6 router on the link, can the client use the DHCPv6 server to find a default gateway out of the local network?  If so, what RFC specifies this option?

Any help or referrals to additional RFCs would be much appreciated.

Best Regards,
Tammy Leino

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---

From: Ole Troan <[email-masked]>
Date: Sat, 11 Aug 2007 00:03:28 +0100

>>>RAs could be configured to supply the length for prefixes valid on the link
>>> Otherwise, there is no DHCPv6 option that can be used to supply this data.
>
> What if the router is configured just to inform the node to use DHCPv6
> to obtain an address and does not specify any prefix options in the
> RA?  The only place I see to specify a prefix length in an RA is in a
> prefix option.  Is there some other way an RA could relay a prefix
> length to a node?  If not, how does the client know the prefix length
> of the address the DHCP server is assigning?  It seems to me that the
> DHCPv6 server should be sending a prefix length with the address it
> assigns.  Is there a reason this was not included in the
> specification?

an address does not have a prefix length. a host needs to know the
on-link prefix for on-link determination. that's done with ND.

>>>Regarding, your second question why would you need information about the
>>> router if it does not exist or is not configured to route?  Just looking
>>> for a little more information.
>
> In implementing this RFC, I am not making assumptions that there will
> be an IPv6-configured router on link.  This does not mean there is not
> a router on link that can forward IPv6 messages, it just means there
> might not be a router on link that sends RAs.  Just because a node can
> forward IPv6 traffic does not mean it will necessarily be configured
> to send RAs.

then your only option is manual configuration. which doesn't seem
optimal. what's wrong with RFC2461 router discovery?

/ot

---

From: "Leino, Tammy" <[email-masked]>
Date: Fri, 10 Aug 2007 18:23:41 -0500

Thank you for your comments.

If I don't get a prefix length for an assigned address, I will not have
a network route for that prefix in my routing table.  When I send data
to an on-link node, all the data will go through the default router.
This will promote redirect messages for on-link destinations.  This will
get my packet to the destination in the end, but in a very ungraceful
way.  My routing table will become overpopulated with host routes that
could all have been covered by one network route if the DHCPv6 server
would tell me my prefix length.  Yes, the user should be sending data to
on-link nodes using the node's link-local address in most cases, but who
can tell what a user is going to do?

There is nothing wrong with router discovery.  I think it's a great
innovation, but my point is that there might not be a router configured
on link to sends RAs.  There might be a router on link to forward IPv6
packets, but the network implementer may be using DHCPv6 in lieu of RAs.

Best Regards,
Tammy

-----Original Message-----
From: Ole Troan [mailto:[email-masked]]
Sent: Friday, August 10, 2007 7:03 PM
To: Leino, Tammy
Cc: John Jason Brzozowski (JJMB); [email-masked]
Subject: Re: [dhcwg] prefix length determination for DHCPv6

/ot

---

From: "David W. Hankins" <[email-masked]>
Date: Fri, 10 Aug 2007 16:28:52 -0700

On Fri, Aug 10, 2007 at 05:26:25PM -0500, Leino, Tammy wrote:
> What if the router is configured just to inform the node to use DHCPv6 to obtain an address and does not specify any prefix options in the RA?

I think the expected behaviour is that the client would pass all
traffic to the RA.  That is, the clients all configure /128's, and
rather than talking to each other, talk to the router.  It would
have no subnets in its table, and so would not be able to speek to
neighbors except through the router(s).

This store and forward star topology may be desirable for example in
radio frequency networks, where you don't want to take on full ROSPF
or similar functions in clients but inter-client traffic is
unreliable.  The router would necessarily be positioned where all
clients could reliably communicate with it.

> Is there a reason this was not included in the specification?

The reason at the time I think was that IPv6 speakers were told to
assume 'on link by default' in the absence of a router
advertisement...so in the absence of a router you could still all
speak to each other without knowing the relevant prefix(es).  This
was found to have some flaws, and is currently not implemented by
most IPv6 stacks.

Anyway, since this has changed, it may be time to revisit this design.

I think more or less at IETF 69 we heard consensus that there should
be a prefix length option.  We didn't agree on the default router, in
which I think I was the only one who supported the idea?

> Just because a node can forward IPv6 traffic does not mean it will necessarily be configured to send RAs.

Why do you think this is the case?

-- 
Ash bugud-gul durbatuluk agh burzum-ishi krimpatul.
-- 
David W. Hankins	"If you don't do it right the first time,
Software Engineer		     you'll just have to do it again."
Internet Systems Consortium, Inc.		-- Jack T. Hankins
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---

From: "Leino, Tammy" <[email-masked]>
Date: Fri, 10 Aug 2007 18:41:44 -0500

Thank you for your insightful comments David.  I appreciate you taking
the time to respond to me.

I am glad to hear this.  Perhaps you can convince others of the value in
the default router option.

>Why do you think this is the case?

I do not see that the presence of RAs is necessary for an IPv6 network
to function.  Client nodes can obtain the information they need from
DHCPv6.  If DHCPv6 requires RAs, then I don't understand why RAs don't
perform all functionality of DHCPv6.  There is some overlapping
functionality, and then there is functionality that is unique to each
protocol.  Also, I'll fall back on the DoD again and quote their "either
RFC 2462 or RFC 3315" comment.

When I first began implementing the IPv6 protocol for our OS in 2001, I
could not understand why RAs did not contain all configuration
information that DHCPv6 provided.  Six years later, I am still as
perplexed :)  I have been holding off on DHCPv6 hoping you would beef up
RAs, but the time has come and customers need that configuration
information from DHCPv6, but they are also expecting DHCPv6 to assign
IPv6 addresses.

Best Regards,
Tammy

-----Original Message-----
From: David W. Hankins [mailto:[email-masked]]
Sent: Friday, August 10, 2007 7:29 PM
To: Leino, Tammy
Cc: [email-masked]
Subject: Re: [dhcwg] prefix length determination for DHCPv6

> What if the router is configured just to inform the node to use DHCPv6
to obtain an address and does not specify any prefix options in the RA?