Re: [DNA] Prefix information for link identification in DNA

[JinHyeock Choi <jinchoe@gmail.com>]

Dear Brett

Thanks for your trouble advancing DNA again.

> Though it was in reference to CPL, Keith Moore raised an issue that is
> relevant to both of the link-identification solution proposals (LinkID
> and CompleteRA).  He thought that it is questionable to assume that:
>
>    1) Every network has a router.
>    2) you can name a network using one of its prefixes.
>
> (I hope that's right Keith - I pulled it out of the minutes)
>
> Both of the proposed solutions use prefix information for link
> identification, though neither relies on using a single prefix as
> the link name for eternity.  Both allow the prefix(es) used to
> change over time and are designed so that hosts can cope with
> those changes.  However both also rely on routers to distribute
> this information to hosts which falls apart if assumption 1)
> above is invalid.
>
> So if a link has no router and no prefixes except the link-local
> prefix (which will be the same for all links) we have a problem.
>
> I'm not sure what we can reasonably do at layer 3 if there is(are)
> no router(s) present on the link to help the hosts identify the
> link.  (Any ideas?)

DNA solution checks for link change to verify whether a host has valid
IP configuration so that the host can have Internet Connectivity
without disruption.

For example, a MIPv6 MN may use DNA procedure to determine that it's
CoA is no longer valid, so that it can quickly get a new CoA and send
a BU accordingly.

If we assume a link which doesn't have neither a router nor a (global)
prefix, I wonder whether a host can have Internet Connectivity in such
a link at all.

In general, for Internet Connectivity, i.e. to be able to send/
receive a packet to/ from off-link node, a hosts needs at least
1) a (partially) reachable default router
2) a valid (global) IP address

* In fact DNA tasks were originally to verify the above two
configurations as in the following first PS draft.

http://www.diffeo.com/drafts/draft-jinchoi-dna-dnav6-prob-00.txt

In a link without a router nor a prefix, I wonder which IP
configuration a host needs to change when it moves to a link. In fact,
in such a environment, it's not even clear to me either what
constitutes a link or DNA is needed at all.

So for the time being, IMHO, we'd better focus on a link with a router
and a prefix. I think that DNA is mostly needed in such a link.

> If there are routers present then I think that prefix information
> should be usable for identification.  If there are no global
> prefixes then maybe the solution can require that routers configure
> a ULA prefix (it might simply be used in an identifier and not
> advertised in a PIO)?

I wonder whether a host can have a global IP address in a link if
there is no global address assigned to the link.

> I guess I would like to find out:
>
> 1. Does anyone think we can come up with a reasonable solution at
>    layer 3 that can deal with links that have no routers?
>
> 1a. Should we?

I don't think we can design a meaningful DNA solution in such a link.

> 2. If we can't/won't, and thus we can make use of routers in
>    identifying other links, can anyone explain why prefix
>    information cannot/must not be used in some way for identifying
>    links?  (Note that using prefix information doesn't require
>    picking a single fixed prefix to act as the identifier)
>
> 2a. If it is *possible* to use prefix information in general to
>     identify links, then is there a good reason why we should
>     design an identification scheme that allows the use of non-
>     prefix information for link identification?

I think that it's ok to use a prefix for link identification.

Thanks in advance for your kind consideration.

Best Regards

JinHyeock