Subject: Re: Global Prefixes in IPv6

From: Scott Bradner <[email-masked]>
Date: Mon, 26 Jun 2000 22:02:39 -0400 (EDT)

> Does anyone support the use of Global prefixes being used on more than
> one link?  I beleive it is not permitted by our standards.

the ability to support this is part of neighbor discovery but much 
other would would have to be done to make it work

Scott
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From: Jim Bound <[email-masked]>
Date: Mon, 26 Jun 2000 22:53:36 -0400

Scott,

>other would would have to be done to make it work

Where in ND?  I have looked at that and the Addr Arch spec.

But we need to think about the ramifications here.

If a global prefix can be advertrised on multiple links and as we have
already stated in the IPng WG the link-local adddresses may not be
globally unique?  We have just said IPv6 cannot support Global
Addresses.  This means Ipv6 is a lie?????

I suggest strongly any who believe this is possible defend it, show us
in the specs where this is permitted, and I believe very important.

This means we need special routing code for global addresses?
I can go on and on..........

regards,
/jim

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From: Steve Deering <[email-masked]>
Date: Mon, 26 Jun 2000 22:58:35 -0700

>Where in ND?  I have looked at that and the Addr Arch spec.

ND provides a way to tell the hosts not to assume that all nodes
within the same subnet prefix are on the same link, and therefore
to initially use the router to reach another host with the same
subnet prefix rather than doing a neighbor solicitation.  The
purpose of that capability is to support subnets than span more
than one link (both for globally-unique and site-local subnets).

>Addresses.

What the heck are you talking about?  Of course IPv6 can support
Global Addresses -- that's what all of deployed IPv6 today is using,
and they do seem to work.

If you are concerned about routing advertisement messages carrying
information about subnets that span multiple links, they still go
hop-by-hop, one link at a time, so non-globally-unique link-local
addresses can be used to carry them from router to router.  The trick
to making multi-link subnets work is not to advertise the subnet prefix
among the routers within the subnet, but rather to advertise individual
node addresses (i.e., "host routes") for all nodes within the subnet.
Those then get aggregated into a subnet route for advertisement across
links that are *not* part of the multi-link subnet.

>  This means Ipv6 is a lie?????

That's quite a leap.  We've never claimed that multi-link subnet are a
feature of IPv6, so where is the lie?  Perhaps if anyone ever gets around
to designing the rest of the necessary pieces and implements them,
we'll discover some unexpected, fatal flaw.  Until then, we won't know
absolutely for sure.  But no one's lying about any of this.

>in the specs where this is permitted, and I believe very important.

Why do you believe it is "very important"?  If it's very important to
support multi-link subnets, why have you not brought this up years ago,
and why has no one else complained about their absence?

(By the way, you'll see that multi-link subnets is one of the topics in
the draft new ipngwg charter that Bob posted on Friday, in the category
of stuff that we would welcome contributions on.)

>This means we need special routing code for global addresses?

No.  Regular old routing code works fine for global addresses.

>I can go on and on..........

I know.

Steve

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From: Steve Deering <[email-masked]>
Date: Mon, 26 Jun 2000 23:20:10 -0700

Correcting a possibly-confusing typo in my message:

>The purpose of that capability [in ND] is to support subnets
>than span more than one link 
 ^^^^
 that

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From: Jim Bound <[email-masked]>
Date: Tue, 27 Jun 2000 09:59:45 -0400

steve,

I know all that about ND.  For some reason we are passing each other in
this conversation.  Also I have no issue of prefixes spanning multiple
links unless the following happens.

Node A on link1 has fe35::1 as global address.

AND

Node B on link2 has fe35::1 as global address.

Because when the prefix spanned multiple links (link1 and link2) Node A
and B have the same link-local address 64bit ID in this case Unicast
Aggregate Format for this discussion.

They end up with the same global address meaning they are not global by
my definition.

Oh yeah.  My assumption of a global address may be different than yours?

So ND would have to make sure also that the link-local is unique across
the multiple links.  That could be difficult.

/jim

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From: Steve Deering <[email-masked]>
Date: Tue, 27 Jun 2000 07:30:05 -0700

At 9:59 AM -0400 6/27/00, Jim Bound wrote:
>Also I have no issue of prefixes spanning multiple

Part of the (not-yet-written-down) work of supporting multi-link subnets is
making sure that the routers that connect a multi-link subnet ensure that
DAD messages are relayed/proxied to all links in that subnet (or in some
other way detect and prevent duplicate interface IDs being used within the
subnet).

In theory, it would be OK for link-local IPv6 addresses to be duplicated
on different links within the subnet (because they would still be scoped
to a single link);  only the site-local and global addresses would
have to be unique across the subnet.  But the practice of doing DAD only
on the link-local address and then auto-generating the larger-scope
addresses from that means we'd really want to just do DAD-relaying for
all address types.

>They end up with the same global address meaning they are not global by

Right, that has to be prevented.

>Oh yeah.  My assumption of a global address may be different than yours?

It sounds like it's the same.  You just didn't explain your specific
concern well enough for me to understand before.

>the multiple links.  That could be difficult.

Could be, but I don't think so.

One thing we could have done to make this work more cleanly (i.e.,
eliminate the need for special-case DAD-relay function in the routers
in multi-link subnets) is to define a "subnet-local" multicast scope
(greater than or equal to link-local), and then require DAD to use
subnet-local multicast.  But it's not worth trying to change that now,
and the DAD-relay idea should suffice

You still didn't explain what you think is Very Important about any of
this.

Steve

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From: Jim Bound <[email-masked]>
Date: Tue, 27 Jun 2000 10:47:44 -0400

Steve,

OK I am on board here.  I agree it would be a nice feature to support
this but I also think we need to make sure its backwards compatible with
all the ND code out there now ok.  Thats first what is very important
and I did not state this yet.

But what was very important was we not have an issue where we don't have
Global Addresses for IPv6.  Mail on the DHCPv6 mailing list made
it sound like that is what was happening.  I had to make sure that was
misunderstood to support this feature.  IPv6 without global addresses
would be a bummer.

The key is that a solution will have to support what you relayed for DAD
in some manner across multiple links.  This new feature should be
transparent to protocols assuming IPv6 does support the concept of
Global Addresses even when prefixes span multiple links.

Hmmm.  If we make sure link-locals can be unique across multiple links
that also opens new opportunities elsewhere too !!!

Are there proposals on the table?

I would like to look at this worrying about backwards compatibility with
ND and Addrconf as they exist today too?

regards,
/jim

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From: Francis Dupont <[email-masked]>
Date: Tue, 27 Jun 2000 18:12:19 +0200

In your previous mail you wrote:

   ... unless the following happens.

   AND

   
=> then you concern is about duplicate global address detection only?

=> of course current ND DAD doesn't work in this case but a management
tool can easily fill the hole (for instance if we require nodes must
register new addresses in a central (ie with the multiple link scope)
database then it will be easy to detect duplicates. I'd like to use
DHCPv6 for that (then I am *not* happy with the last I-D which makes
this impossible, even this is not (yet) written down) but this function
is easy to implement if the way interface IDs are built does not imply
they are unique).

Regards

[email-masked]

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From: "Charles E. Perkins" <[email-masked]>
Date: Tue, 27 Jun 2000 09:19:41 -0700

Hello,

I think that this discussion came up in the first place
because Francis Dupont pointed out that DHCPv6 does not
specifically support multi-link subnets.

They aren't typically implemented as part of stateless
autoconfiguration, and we don't know how to support them
directly with DHCPv6, and DHCPv6 is already late by a year+.
So, I would like to avoid seeing DHCPv6 encumbered with
any requirement to support multi-link subnets, except
insofar as avoiding the obvious problem of assigning the
same IP address to two different DHCP clients on the same
subnet.  This latter problem does not exist in the current
spec.

I suggest that the existing DHCPv6 specification does not
need to deal with the issue at all, because ensuring
uniqueness is a problem for DAD, not DHCPv6.  Whatever
DAD does, will work as well for DHCPv6 as for stateless.

Regards,
Charlie P.

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From: "Charles E. Perkins" <[email-masked]>
Date: Tue, 27 Jun 2000 09:39:56 -0700

Hello Francis,

I saw your note after I had already written my previous note.

> is easy to implement if the way interface IDs are built does not imply

If you can contribute the necessary text to the DHCPv6 specification,
and if there is agreement that this problem needs to be fixed now,
then maybe that will make things different.  Until then, I would
hope to avoid this delay.  I don't see the easy solution.

Regards,
Charlie P.

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From: Alex Conta <[email-masked]>
Date: Tue, 27 Jun 2000 14:58:40 -0400

> other way detect and prevent duplicate interface IDs being used within the

It is a neat idea to  have a subnet span different type of links, in particular
when such a solution seem to apply so well in a  scenario that is  more and
more frequent in home or small office networking environments.

Since DHCPv6 seems to have a problem with this,  it would be good to have this
documented ASAP.

With the number of  IPv6 documents on continue growth, and with the delay
incurred with writing and posting a new I-D,  and since this problem occurs
only with non-unique interface IDs, wouldn't it be more efficient - in terms of
documenting the solution - to add to the specifications that document the
generation of such IDs a section that suggests the solution(s)?

Alex

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From: Jim Bound <[email-masked]>
Date: Tue, 27 Jun 2000 15:10:58 -0400

Francis,

If you want to discuss dhcpv6 do it on that list.

And link-local addresses are unique right now on a link.  When prefixes
span multiple links then they will be unique there.  There is no problem
I am done discussing this thread on IPv6 working group until we see work
for us to do.

thanks
/jim

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From: Jim Bound <[email-masked]>
Date: Tue, 27 Jun 2000 15:24:27 -0400

Alex,

Francis was misguided on the issue.  DHCPv6 has no problem.  When it
gets executed DHCPv6 will work.

thanks
/jim

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From: Bob Hinden <[email-masked]>
Date: Wed, 28 Jun 2000 10:50:30 -0700

[was Re: Global Prefixes in IPv6]

IPv6 includes some elements of support for multi-link subnets (i.e., a 
subnet that spans multiple independent links] but, like IPv4, does not 
currently specify a complete solution for multi-link subnet support.

I do not believe that adding multi-link subnet support to IPv6 is an 
important topic for the IPng working group.  We have a number of more 
important topics on our plate (e.g., completing multihoming support, scoped 
address architecture, etc.) that have higher priority.  There are other 
approaches to this (e.g., level 2 bridge/switches, mobile IPv6, etc.) and 
we shouldn't have any shortage of addresses to assign subnet prefixes to 
individual links.

I also do not think that the lack of multi-link subnet support in other 
IETF protocols (e.g., DHCPv6) should hold up progress on those protocols.

Bob

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From: Francis Dupont <[email-masked]>
Date: Thu, 29 Jun 2000 10:36:51 +0200

=> I still disagree. DHCPv6 recognizes links using prefixes (an address
and prefix length pair). This doesn't work with multi-link subnets then
I propose to add a line saying DHCPv6 doesn't support multi-link subnets
in the DHCPv6 I-D "non-goals" section (multi-link subnets don't exist
today and we need DHCPv6 for two years ago).

Regards

PS: but I agree with Jim that to find a full solution for this issue
is not the job of the DHCPv6 team but the job of multi-link subnets
researchers.
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