Subject: Re: [DNA] Confirming today's face-to-face meeting decisions

From: Sathya Narayanan <[email-masked]>
Date: Wed, 10 Nov 2004 21:49:05 -0500

> And given that we have two different styles to detect the link (an 
> explicit link identifier, compared to what I would characterize as 
> a 
> cookie or a landmark, where the host asks "does this landmark 
> still 
> exist" without an explicit linkid) we should express the need 
> based on 
> the property that a host should be able to determine whether it is 
> on 
> the same link or not from receiving a single message.
Yes. The advantage in using a 'Link Identifier' with a scope applicable to all the nodes in the network is that the host will be able to determine whether it is on the same link or not from receiving a single (RA) message. On the other hand, when such identifier is not available, the host can ask the question 'does this landmark still exist' everytime it has (good) reason to believe it may have changed link. 

Can we refer to the first scheme as 'link scoped identifier' and second as 'host scoped identifier' scheme? ;-)

The question to ask is, whether the cost of establishing the unique link identifier among all the nodes (routers) in the link is worth to avoid an extra 'does this landmark still exists' question and answer? 

Link scoped identifier scheme requires completely securing the link so that nobody can spoof a RA message or confuse the link identifier, while we (I am not sure about this) may not need complete security as long as a secure association between the host and the current AR is good in the host scoped identifier approach.

-Sathya

---

From: Erik Nordmark <[email-masked]>
Date: Wed, 10 Nov 2004 19:07:13 -0800

Sathya Narayanan wrote:

> Yes. The advantage in using a 'Link Identifier' with a scope
> applicable to all the nodes in the network is that the host will be
> able to determine whether it is on the same link or not from
> receiving a single (RA) message. On the other hand, when such
> identifier is not available, the host can ask the question 'does this
> landmark still exist' everytime it has (good) reason to believe it
> may have changed link.
> 
> Can we refer to the first scheme as 'link scoped identifier' and
> second as 'host scoped identifier' scheme? ;-)
> 
> The question to ask is, whether the cost of establishing the unique
> link identifier among all the nodes (routers) in the link is worth to
> avoid an extra 'does this landmark still exists' question and answer?

Indeed.

But I think that we shouldn't constrain our thinking about the latter to 
the case when only one router can answer the question about the 
landmark; one could have the routers share this so that any of them can 
answer the question.

> Link scoped identifier scheme requires completely securing the link
> so that nobody can spoof a RA message or confuse the link identifier,
> while we (I am not sure about this) may not need complete security as
> long as a secure association between the host and the current AR is
> good in the host scoped identifier approach.

Why would the RAs that carry linkids need to be secured any more than 
RAs that carry prefixes and default router information? I don't understand.

It might be that the Rtr-to-Rtr messages have different concerns than 
the RAs and other ND messages, but I haven't seen any semblance of a 
threat analysis that makes even this case. But that's separate from the 
RAs adverting linkids to hosts.

    Erik

---

From: JinHyeock Choi <[email-masked]>
Date: Thu, 11 Nov 2004 12:32:40 +0900

Sathya

> Can we refer to the first scheme as 'link scoped identifier' and second as 'host scoped identifier' scheme? ;-)

Landmark approach don't have to be host scoped. Many hosts can
share the same landmark.

> Link scoped identifier scheme requires completely securing the link so that nobody can spoof a RA message or confuse the link identifier, while we (I am not sure about this) may not need complete s…

Even with host scoped (?) approach, if there is only the secure
association between a host and the current AR, the host can't securely
detect that a link change has happened. The host can't receive a secure 
message confirming a link change when it actually happens.

JinHyeock

---

From: Brett Pentland <[email-masked]>
Date: Fri, 12 Nov 2004 01:42:24 +1100

> avoid an extra 'does this landmark still exists' question and answer?

There may be extra cost in making all the routers on a link agree on
a "link scoped identifier", but that is an out of band management cost.
Using "does this landmark still exist" has a cost during movement in
that there is no negative acknowledgement and we have to wait for a
timer to expire if the link is new and the landmark does not still
exist.

Brett.

---

From: Brett Pentland <[email-masked]>
Date: Fri, 12 Nov 2004 01:47:31 +1100

> landmark; one could have the routers share this so that any of them can 
I agree that the routers on a link should share information.  If they
do, then isn't this similar to the cost of negotiating an explicit link
identifier, but with the additional cost of maintaining the "landmark"
information for the link as prefixes come and go?

Brett.

---

From: Erik Nordmark <[email-masked]>
Date: Thu, 11 Nov 2004 07:13:30 -0800

> timer to expire if the link is new and the landmark does not still

While the timeout is the case in the current rrd draft, it isn't a 
fundamental property of landmark-based schemes.

For instance, in a landmark scheme the routers on the link can share the 
landmarks they have so that any router can respond. If you do this, a RS 
containing a landmark can be responded by any router, whether on the 
same link or not. If on the same link the router would respond "yes, 
this landmark is on this link" and if its it a different link the router 
would respond "no".

    Erik

---

From: Brett Pentland <[email-masked]>
Date: Fri, 12 Nov 2004 04:01:30 +1100

> this landmark is on this link" and if its it a different link the router 

Ok, fair enough.  It does require a little work from the routers to make
sure that they know that landmarks they have seen on the link are still
current, but that's certainly doable.

Brett.