More LinkIDs (was Re: [DNA] Couple of points ondraft-jinchoi-dna-soln-frame-00.txt)

[Greg Daley <Greg.Daley@eng.monash.edu.au>]

Hi Erik, 

----- Original Message -----
From: Erik Nordmark <Erik.Nordmark@sun.com>
Date: Sunday, August 1, 2004 3:49 pm
Subject: Re: [DNA] Couple of points on draft-jinchoi-dna-soln-frame-00.txt

> >   2.  We assign each link a locally unique Link Identifier.  
> 'Locally>       unique' means, no two adjacent links have the same 
> Link>       Identifier.
> > 
> > This would have issues if the hosts ever make a long jump 
> (skipping some
> > intermediate links) for any reason.
> 
> Yes, perhaps globally unique (or probabilistically unique is better).
> 
> The length of the jump is limited by the lifetime of the information
> that is retained. For prefixes RFC 2461 suggests a default lifetime of
> 30 days. Withing 30 days a host could presumably move to any link on
> the planet.

Well, we really only need to keep track of the
links which are immediately adjacent to 
the MN, so we can discard knowledge of 
all but the current link ID.

The lifetime of the link ID knowledge is then
tied to the mobility of the MN, rather than
the duration.

The chance of collision is also much smaller,
even if there's 100000 (even potentially) 
reachable networks, we're only interested in them.
We're not interested in any transitivity to these
networks' neighbours.

This is about 3.5*10^-5 collision probability
by birthday problem for a 48 bit (truly) random
identifier space.  Links with routers which are
incorrectly judged as reachable will time out
after about 47 seconds though (30 reachable +
5 delay + 12 NUD) seconds.

It's worth working out if the cost is low enough
not to worry about global uniqueness.

> So unless we make the link-ID advertisement in the Router 
> Advertisementshave a small lifetime (a few seconds), then I think 
> "adjancent links"
> in this context becomes the whole internet.

Certainly it should have short lifetime,
but this should be defined by new link ID 
reception, rather than duration, if my beliefs
expressed above are correct 

> Furthermore, physically close links will be operated by different 
> entities(e.g. the corporate WLAN, the GPRS network covering the 
> same area,
> the coffeeshop WLAN downstairs) and a host can quickly move between 
> them.But the different operators are not likely to coordinate their 
> assignmentof their link IDs.
> So this is another argument for needing globally 
> (probabilistically) unique
> link-IDs.

Now this is interesting.

I think that for particular link types we should
not correlate the link ID's seen between the 
interfaces.

This is because DNA is typically done on a per-interface basis (as is Router Discovery),
A router discovered on another technology's 
interface isn't automatically configured as
a router for every interface.

(although I think the default router list may be
common across interfaces).

I believe that the number spaces for a WLAN
and an Ethernet Interface on a host are thus
independent, since they're identified by a host unique interface index.

This isn't necessarily explicitly stated anywhere, 
but follows if we believe that DNA is per-interface,
and that interfaces are identifiable within the
host.

The WLANs may each be reachable from each other,
but the GPRS isn't from the WLAN's host interface
perspective.
 
Greg