[Nml-wg] source/sink versus ingress/egress (was: Example topology of Automated GOLE)

[Freek Dijkstra]

A somewhat related issue on source/sink. I don't think this was ever
written down.

Freek Dijkstra wrote:

> The question at hand is basically how to describe the following (with
> apologies with my poor ASCII art skills)
> 
> port A    link X    port B    link Y    port C
>   O------------------>O------------------>O
> 
[...]
> In the NML schema it is currently defined as:
> 
>  link X
>     relation=source
>         port A
>     relation=sink
>         port B
>  link Y
>     relation=source
>         port B
>     relation=sink
>         port C

For the record, an alternative way to describe this is making the ports
leading instead of the links:

 port A
     relation=egress
         link X
 port B
     relation=ingress
         link X
     relation=egress
         link Y
 port C
     relation=ingress
         link Y

Technically I think these are equivalent: the provide a directed
relation between links and ports. Which one is syntactically better
depends what is more common, a one-to-many or a many-to-one relation
between ports and links.

For the circuits, this is often a one-to-one relation.
Since we implemented cross-connects as links, VLANs are likely also
described as some kind of "link", but one with multiple sources and
sinks. Hence, there is a one to many relation from link to port.
This means that the source and sink relation we have now is more easy to
convey in XML than the alternative ingress and egress relation.

I personally think the source/sink stuff is still the best alternative
we have.

Regards,
Freek