Problem
=======

Given a Port identifier, a NSA wants to figure out in which Topology this Port is located, and what the NSA is for this Port identifier.

Use cases
=========

* ...
* NORDUnet: split network in two parts with other Topology ID
* SURFnet: move a Port from testbed to production network
* ...

Assumptions
===========

The following proposals all make the following assumption:
* Port and Topology identifiers MUST use urn:ogf:network identifiers.

Proposal 1 (Henrik)
===================

          string rewrite               lookup in full table
Port ID ------------------> Topology ID ------------------> NSA URL
        (extract prefix and            (build by distribute
      add the word 'Topology')      reachability between peers)

Limitation of proposal 1:
* Port can't be placed in another Topology later
* Max 1 Topology ID per prefix (though it's possible to work around by further interpretation of the URN)
* All NSA need database with all Topology IDs
* Adds limitations on the syntax of URN:ogf:network identifiers (organisations can't freely pick their Topology ID)

Proposal 2 (Freek)
==================

             extract                   lookup service
Port ID ------------------> prefix ---------------------> NSA URL
                                  \ 
                                   \--------------------> Topology ID
                                   (lookup by prefix in
                                   global lookup service)
                                
Limitation of proposal 2:
* Port can't be placed in another Topology later
* max 1 NSA per prefix      (though 1 organisation can have multiple prefixes)
* max 1 Topology per prefix (though 1 organisation can have multiple prefixes)


Proposal 3 (Freek)
==================

             extract               lookup service                       lookup service
Port ID ----------------> prefix --------------------> Lookup Service -------------------> NSA URL
                                 (lookup by prefix in                \
                                global lookup service)                \------------------> Topology ID
                                                                   (lookup by Port ID in 
                                                                    home lookup service)

Note: this can be combined in 1 step if the global lookup service extracts the Port ID itself, and redirects to the home lookup service.

Limitation of proposal 3:
* Requires one lookup per Port ID, rather than one per prefix or per Topology.

Advantage of proposal 3:
* Easier to later extend for different (non urn:ogf:network) identifier, since only the global lookup service (gLS) needs to be updated, not all NSA.