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

[Erik Nordmark <Erik.Nordmark@sun.com>]

> 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.

Yes, but as I said in some other email, this
notion of adjacency is a function of how "far" the host can
move while it retains information about the old linkid.
And since it can move between different operator's networks
in milliseconds, clearly you would need to make sure that
all operators that cover the same geography use distinct link IDs.

Furthermore, I think there are some performance benenefits for a host
to be able to cache information (prefixes, default routers, DHCP leases, DNS
info, all subject to their respective lifetimes) for links it has recently
visited. Thus it might make sense to look at the case when the linkID
is retained for at least a few minutes.

> 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.

What can't information be cached about a linkID longer than 47 seconds?


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

I don't see how using the link type helps; my employer can run a
802.11 network and the cafe downstairs runs a separate 802.11 network (with
overlapping coverage). Thus we still need to worry about multiple operators 
not picking conflicting link IDs.


> 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.

I think there isn't any harm in treating the linkIDs for different interfaces
as different, but I don't see how it helps disambiguate things due to
different providers (that can overlap) for 802.11 as well as for GPRS.

   Erik