[Nml-wg] Use case: broadcast network (single layer)

[Freek Dijkstra]

Hi all,

Yet another single layer use case (sorry to procrastinate on the multi 
layer use case, I want to make sure that single layer is tackled before 
diving in the hard stuff).

Let's start with a single bidirectional link (= 2 unidirectional links) 
between two nodes:

          ---------->
   Node A             Node B
          <----------

Each Node here has two (unidirectional!) interfaces:
Node A hasPort a_out and a_in
Node B hasPort b_out and b_in

The relations here are:
   Node A       hasPort      a_out
   Node A       hasPort      a_in
   Node B       hasPort      b_out
   Node B       hasPort      b_in
   a_out        source       Link A-B
   b_in         sink         Link A-B
   b_out        source       Link B-A
   a_in         sink         Link B-A

So far, so good.

Since we want to model Ethernet, I think we should be able to model 
broadcast networks. Imagine an old-fashioned LAN:

   Node A              Node B
      |                   |
    --+-----+-------------+---+--
            |                 |
         Node C            Node D

Again, each Node here has two (unidirectional!) interfaces:
Node A hasPort a_out and a_in
Node B hasPort b_out and b_in
Node C hasPort c_out and c_in
Node D hasPort d_out and d_in

Of course each *_out is the egress port, with each *_in is the ingress port.

Now how do I describe this LAN? (A) As a single link, or (B) as multiple 
links?

Let's first try (A) as a single link:

   Node A       hasPort      a_out
   Node A       hasPort      a_in
   Node B       hasPort      b_out
   Node B       hasPort      b_in
   Node C       hasPort      c_out
   Node C       hasPort      c_in
   Node D       hasPort      d_out
   Node D       hasPort      d_in
   a_out        source       MyFirstLAN
   b_out        source       MyFirstLAN
   c_out        source       MyFirstLAN
   d_out        source       MyFirstLAN
   a_in         sink         MyFirstLAN
   b_in         sink         MyFirstLAN
   c_in         sink         MyFirstLAN
   d_in         sink         MyFirstLAN

But is this correct? Semantically, this would imply that if a_out is 
sending data, it is also received by a_in.

Let's also try to describe this (B) as multiple links:

   Node A       hasPort      a_out
   Node A       hasPort      a_in
   Node B       hasPort      b_out
   Node B       hasPort      b_in
   Node C       hasPort      c_out
   Node C       hasPort      c_in
   Node D       hasPort      d_out
   Node D       hasPort      d_in
   a_out        source       LAN_A
   b_in         sink         LAN_A
   c_in         sink         LAN_A
   d_in         sink         LAN_A
   b_out        source       LAN_B
   a_in         sink         LAN_B
   c_in         sink         LAN_B
   d_in         sink         LAN_B
   c_out        source       LAN_C
   a_in         sink         LAN_C
   b_in         sink         LAN_C
   d_in         sink         LAN_C
   d_out        source       LAN_D
   a_in         sink         LAN_D
   b_in         sink         LAN_D
   c_in         sink         LAN_D
   MyFirstLAN   elements     {LAN_A, LAN_B, LAN_C, LAN_D}

This later description is closer to the concept of Unidirectional Link 
as is currently in the schema. The schema further allows grouping to a 
(Bidirectional) Link. That's what I added on the last line.

What are your preferences?

I see problems with both concepts.

Regards,
Freek