Hi Dow,
I am ok defining things this way. To ensure we're on the same page,
this means that only the network part of an IP address (from the
perspective of the host) can ever be considered a locator, right?
I think we're aligned, but there are some nits that happen here because
the IP address again this overloaded use of both locator and identifier.
To demonstrate a nit: what's the network part of the IPv4 address? I
have a prefix 192.168.1/24 that points to a particular Ethernet. I also
have a host 192.168.1.100 that is NOT on that Ethernet. I establish
host routes everywhere I need access to this host. I'd claim that
192.168.1.100/32 is both a locator and identifier.
Such is the way of semantic confusion.
Also, since the mask that is in effect (for forwarding) at various
points in the topology differs, then the part of the IP address that we
can call a locator also differs at various points in the topology.
Similarly, I'd make the slightly different distinction that you simply
can't say what part of an IPv4 address is a locator without full
information about the particular point of attachment. Note that
whatever bits are relevant to the locator do NOT change over time or the
point where they are examined. The fact that the observer has
incomplete and insufficient information doesn't change the real
semantics of the bits.
Note that this is more clear in other proposals (e.g., GSE) where the
locator is a fixed number of bits.
In other words, if we say that:
(a) on an ethernet segment (that maps to only a single prefix in the IP
routing system), only the network part of the IP address of connected
hosts is considered to be a (L3?) locator, then
(b) a similar property exists closer to the "middle" of the
topology/routing system if aggregation is used.
In (b), more specific prefixes "hidden behind" the aggregate are
topologically flat from the perspective of processes that are only aware
of the less-specific, aggregate mask.
Thus, I disagree with this, as with the above. Aggregation in the
routing system is (and should always be) independent of the semantics of
the namespace. I think we want to be very careful here, because we
don't want to define a locator namespace and accidentally have prefixes
ending up in packet headers.
I think maybe you are accounting for all of this relative-ness with the
phrase "attachment point at that layer"?
Some, but not as much as you're thinking. I'm really trying to use the
simplest definition of PoA, such that you have to transition to another
layer to make forwarding progress.
Tony
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