Re: [RAM] revised draft proposed definitions

    > From: Russ White <riw@cisco.com>

    >> Ok, but a locator does NOT denote reachability. Thus, a locator is a
    >> string of bits that indicates a topological location.

    > I think we should, perhaps, leave reachability out? It seems like both
    > a locator and and id indicate some element of reachability (?).

Russ, reachability is a very different kind of thing, more an operational
property (i.e. a property of a name in particular circumstances in an
functioning system) , rather than a semantic one (i.e. an inherent property
of a particular name).

To give some examples, let's use 'IPvN address', which has relatively well
understood properties (since differing definitions are a real problem in this
discussion), but has the same "indicates a topological location" property as
"locator". Assume I have a host H at a site S, which is connected to the rest
of the Internet via router R; H, S and R all have an IPvN addresses.

For the first example, if the cable which connects H to the rest of the world
is unplugged, it still has a correct and authorized IPvN address, but it is
no longer reachable (from anywhere). For a slightly more complex example,
consider the situation, but this time the link connecting R and the rest of
the world is unplugged. In this case, H is still reachable from hosts inside
S, but not from the rest of the world.

So we can see that "reachability" is not a property of an address, but a
property of a tuple, (source, reachable_destination). Moreover, depending on
the actual network connectivity, and the contents of the routing tables,
reachability of a particular (source, reachable_destination) tuple can vary
over time.

So, yes, because reachabilty is an operational property, and not an inherent
semantic property, it definitely should be placed in a separate category
(although any definitions document might useful explain the above).

	Noel

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