[rbridge] PS Issue #11 -> now ARCH issue #6

[Joe Touch]

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Gray, Eric wrote:
> --> 
> --> #11 internal/external, inside/outside terminology
> -->     there is one objection to these terms as topological;
> -->     the draft intends to define them sufficiently
> -->     does this need to be addressed further?
> 
> This is not a Problem Statement Issue because these terms are
> defined in draft-touch-trill-rbridge-arch, rather than in
> draft-touch-trill-prob.  Consequently, this is an issue with
> the Architectural document, if not precisely an architectural
> issue.

Yup - needs to be moved to the other doc. Let's call this ARCH issue #6
then (see header).

> The draft definitions are not sufficiently clear on the current 
> iteration.
> 
> The definitions offered there were - 
> 
> -> o  External (to a campus): describes communication on the non-rbridge 
> ->    components of an Ethernet link subnet, notably not between 
> ->    rbridges or between non-rbridges and rbridges.

The last part is a bug i.e., 'or between non-rbridges and rbridges'. It
should be omitted.

> ->
> -> ... [snip] ...
> -> 
> -> o  Internal (to a campus): describes communication between rbridge 
> ->    devices; this communication may traverse external devices, but is 
> ->    still considered internal. 
>
> Clearer wording was suggested on this list some time ago.

The previous proposal had problems:

> External links/interfaces are those that do not connect to a
> cooperating RBridge; internal links/interfaces are those that
> do. 

what does "connect" mean?
	a) physically connect (probably not)
	b) attached to the same bridged segment (won't work - see below)
	b) communicate directly
		(that's the spirit of what I ended up using)

what is being connected?
	the above lists only one end ('to a cooperating rbridge')

why is 'cooperating' required?

Consider the following (all rbridge and bridges shown; hosts connect to
all devices):

               r1----r2------b1
               |     |       |
               |     |       |
               r3----r4------b2


is r2-b1 internal or external?

I would think it is external, since the HELLO would prevent a tunnel
being created on r2-b1-b2-r4

However, this definition makes them internal.

In particular, this definition makes all links/interfaces internal if
they connect to an 'external' bridged segment that reconnects to the
rbridge elsewhere.

> Whether or not non-Bridge devices exist on - or are part 
> of - a link between cooperating RBridges is irrelevant.  The
> existing attempt to somehow include the possibility of other
> devices as part of a link between RBridges in the definition
> of internal and external is only confusing.
> 
> I believe that the confusion stems from an attempt to allow
> for required communications between RBridges and other types
> of devices.  This confusion is one strong indication of why
> no such communications should be required.

Rbridges need to talk to bridges at some point; otherwise there's no
ingress or egress ;-)

Joe
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