shim design vs mobility/renumbering

[Pekka Savola <pekkas@netcore.fi>]

Hi,

This may be an obvious thing, but possibly worth bringing up 
explicitly..

When I read the specs, the obvious architectural approach taken by 
the shim6 approach -- not defining a separate identified name space -- 
has most implications in particular on renumbering and mobility.

In particular, especially with mobility, the locators are expected to 
become obsolete in a relatively frequent basis.  As the ULID is tied 
to the locators, this has a couple of implications:

 1) what to do when the locator used for ULID has been removed from
    the node (permanently)?

 2) in particular, what to do in the circumstances are such that
    the locators get assigned to other hosts?

 3) specifically, then what if the host wants to communicate with the
    host which was given its previous locator, if the locator is
    still used as ULID?

For a site multihoming solution it might be able to assume the apps 
could continue using the old ULID as long as it works.

However, for mobility ("frequent renumbering") I doubt this would be 
sufficient.  But having to change the ULID would defeat the purpose of 
IP mobility, so that's unacceptable.  The sessions can't be changed to 
use a different ULID either (without changes to the transport 
protocols), so that's a no-go.

So, from the mobility perspective, a solution like HIP is better. 
Shim6 could provide the same functionality, but only if the ULIDs 
would always use a unique, non-renumbering idenftifier (a ULA?).  But 
if those weren't routable, then the shim6 context would have to be 
exchanged up front.

So, this brings two main issues/observations:

 a) is the assumption 1) good enough for site multihoming?  and if
    not, what else would be needed?

 b) while shim6 could maybe be extended to also be a mobility
    protocol, mobility support seems to have very different
    requirements (e.g., about the identifier name space, when to
    exchange the identifiers) that ruling that out at this
    point seems justified.

--
Pekka Savola                 "You each name yourselves king, yet the
Netcore Oy                    kingdom bleeds."
Systems. Networks. Security. -- George R.R. Martin: A Clash of Kings