[DNA] Explicit link identifier

[Brett Pentland <brett.pentland@eng.monash.edu.au>]

Hello DNAers,

draft-pentland-dna-protocol3-00.txt includes a concept of a link
identifier prefix that is implicit.  Routers know the LinkID prefix
and make sure they include it in RAs just to ensure that there is
always overlap between the prefix sets included in RAs on the link.
Hosts don't because they are just looking for an RA with no previously
seen prefixes to indicate movement.

 From a few recent conversations there does seem to be some interest
in having an explicit link identifier.

One way to do this might be to have a flag in the Learned Prefix
Option (LPO) that indicates that Prefix 1 in the option is the
LinkID prefix.

For now only prefix LinkIDs would be considered, but if hosts take
note of the LinkID, and use it as a further check on whether it is
still at the old link even when there are no matching prefixes in
the RA, then a future extension could make use of non-prefix
LinkIDs by having another flag that indicates that the "prefix 1"
is in fact *not* a prefix.  i.e. It is the LinkID, but should not
be added to DNAHostPrefixList.  That extension would also need to
define how routers would select a LinkID and how they would reach
agreement on it amongst themselves.

Here is a suggested LPO format.  The change from the existing draft
is to shift the Prefix Len fields by 8 bits, and to add the two
flags to the new space.  The rest is the same, but the first prefix
field may be interpreted differently, depending on the flags.

      0                   1                   2                   3
      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |     Type      |    Length     |L|P| Reserved  | Prefix Len 1  |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |      ...      | Prefix Len N  |            Padding            |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                                                               |
     +                                                               +
     |                                                               |
     +                          Prefix 1                             +
     |                                                               |
     +                                                               +
     |                                                               |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                                                               |
     +                                                               +
     |                                                               |
     +                          Prefix 2                             +
     |                                                               |
     +                                                               +
     |                                                               |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     ~ ...
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                                                               |
     +                                                               +
     |                                                               |
     +                          Prefix N                             +
     |                                                               |
     +                                                               +
     |                                                               |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Some text would be needed to say how routers fill the flags, etc., and
what to do if the LinkID prefix is one that would normally be in a PIO.
Should we add a flag to the PIO format to indicate that that prefix is
the LinkID, or does that prefix just end up in both a PIO and the LPO?
There would also need to be some text in the Host Operations section
that explains how to interpret the LinkID since in the draft at present,
hosts don't know about the LinkID.

So what do folks think?  Is this a sensible way of including an
explicit identifier?  Is 128 bits enough for future non-prefix
identifiers?  Is this something worth pursuing?  If so, why?
Those that have expressed interest have said that having an explicit
identifier is important for mesh networks, but it would be useful to
hear more about that or if there are other reasons as well.

Cheers,
Brett.