Flash renumbering (was:Re: [DNA] DNA proposal issue 19 - was[Issue X]LinkID v.s. LandmarkPrefix)

[Sathya Narayanan <sathya@research.panasonic.com>]

Erik -

I reordered the text in your email (and changed the subject line) with my comments inline.

> But the problem certainly isn't specific to any particular proposal, 
> since they all rely on prefixes for link identification. And I suspect 
> that we have the same choices in terms of solutions independent of which 
> proposal we apply this to.
FWIW, I completely agree with you on this. IMHO, the synchronization complexity/issue is same/similar for all of these schemes.

> We could decide this is the administrators fault, and not do anything. 
> Or we could add a mechanism to attempt to detect this and recover 
> in less than 7 days.

Having said that, I am going to try to solve this problem using the Landmark scheme. Be warned that this is me just thinking out loud and you should remember how lost I get when I do that ;-)

> The worst case would be when P1 and P2 are assigned to link1, and P2 is 
> immediately reassigned to link3 (which had prefix P3 before), at about 
> the same time that the a host moves from link1 to link3.
> 
> In that case, any prefix based scheme for link identification (CPL, landmark, linkid, or anything else we can invent) can get into trouble, 
> by the host assuming that P1, P2, and P3 are assigned to link3.
It can be argued that the semantics of landmark exchange is not to imply all those prefixes are on the link - if the host did a landmark question using 'P3', then a 'yes' answer just means P3 is on the link (link3 in this case). Does that make sense? 

The negative side is that when a host is on a link with P4, P5, P6, sends a landmark question with P6 and gets a 'yes' answer, the host cannot say anything about the availability of P4 or P5. The host will have to do separate landmark questions for each of them or hope for a complete RA to be received. The choice of the landmark on the host side becomes crucial in doing DNA. Now, since G1 in the goals document is
"G1: DNA schemes should detect the identity of the currently attached link to ascertain the validity of the existing IP configuration.", if the chosen landmark-prefix corresponds to a non-local IPv6 address of the host, its validity of atleast one address can be ascertained.

Such a change in the semantics, I think, restricts the host from making assumptions about prefixes on link unless explicitly seen in RA messages continuously and hence can address the flash renumbering and early assignment problem.

What do you think?

-Sathya

> 
> Should this happen, then the host will assume that P1 is usable until 
> the last valid/preferred lifetimes it saw has expired. Since RFC 
> 2461 
> suggests a default preferred lifetime of 7 days, the host will try 
> to 
> use P1 as a source address for 7 days, even though no packets 
> returned 
> to P1 will make it back to the link to which the host is attached.
>