Direction of Port-Link Relation
===============================

> * What is the direction and name of the relation between Port and Link?
>   https://forge.ogf.org/sf/go/artf6538 (second part)
>   https://forge.ogf.org/svn/repos/nml-examples/201203-vlans/compoundlink.xml
>   (second question at footer)

This one was brought up by Jeroen, and backed by Roman at OGF34.

Summary: We need to define a two relations between port and link: for traffic from port to link, and for traffic from 
link to port. (aka source and sink relations).

Proposal: Use a "isSource" and "isSink" relation to relate a Port to a Link.

Semantics:
  Port A --(isSource)-> Link X
   means: there is a traffic flow from Port A to Link X.
  Port B --(isSink)-> Link X
   means: there is a traffic flow from Link X to Port A.

Note: this is contrary to how we currently related Ports and Links:
  Link --(hasSource)-> Port
  Link --(hasSink)-> Port
The rationale is that this proposal provides a "hierarchy" of relations:
  Topology --> Node --> Port --> Link

Example:

XML:
<nml:Port id="urn:ogf:network:example.net:2012:PortA">
  <nml:Relation type="isSource">
    <nml:Link idRef="urn:ogf:network:example.net:2012:LinkX"/>
  </nml:Relation>
</nml:Port>
<nml:Port id="urn:ogf:network:example.net:2012:PortB">
  <nml:Relation type="isSink">
    <nml:Link idRef="urn:ogf:network:example.net:2012:LinkX"/>
  </nml:Relation>
</nml:Port>

RDF:
@prefix exnet <urn:ofg:network:example.net:2012> .
exnet:PortA  nml:isSource   exnet:LinkX
exnet:PortB  nml:isSink     exnet:LinkX