Hi Andrea,
Thanks for the detailed definition. Please see my comments below.
On Fri, Sep 30, 2005 at 03:40:43PM -0400, Andrea Francini wrote:
Hi Yoshihiro,
I definitely don't mean to contradict what I wrote yesterday. I still think of
the PoA as a link endpoint.
Your comment rightly brings up the necessity of providing a clear definition of
"link" since link and PoA are tightly inter-related.
With a generic definition of PoA as a link endpoint, defining "L2 PoA", "L3
PoA", and "MIH PoA" implies corresponding definitions of "L2 link", "L3 link",
and "MIH link".
I assume from now on that a layer-agnostic notion of link is accepted and that
"link" is not strictly a Layer-2 notion. The group can debate if this is a valid
assumption. If not (i.e., the group prefers to assign a strong L2 flavor to
"link"), we can find a better term (e.g., "connection", or "relationship") and
base on the new term both the generic and the specific definitions of PoA. In
this latter case, "link" would be synonymous of "L2 connection" (or "L2
relationship", or whatever other term the group may identify).
I can think of the following generic definition for a layer-agnostic link:
"Communication relationship for the exchange of messages between adjacent peer
protocol entities."
Where:
"Peer protocol entities" always belong to the same protocol layer (e.g., L2, L3,
MIH).
"Adjacent" emphasizes that there is no other interposed peer entity between the
ones that terminate the link (e.g., there cannot be another L3 entity between
the endpoints of an L3 link; if such entity is present, there are two and not
one L3 links). This does not prevent a link from having more than two endpoints:
in a multicast link, for example, all endpoints are adjacent to each other and
none of them is necessary to enable connectivity between others.
The layer-specific definitions easily follow:
L2 link: "Communication relationship for the exchange of L2 messages between
adjacent L2 entities."
L3 link: "Communication relationship for the exchange of L3 messages between
adjacent L3 entities."
MIH link: "Communication relationship for the exchange of MIH messages between
adjacent MIH entities."
Having the notions of "L2 link", "L3 link", and "MIH link" in place, the PoA
definitions I previously proposed can easily be mapped as follows:
L2 PoA: network-side endpoint of L2 link involving the UE
L3 PoA: network-side endpoint of L3 link involving the UE
MIH PoA: network-side endpoint of MIH link involving the UE
As for identifying the endpoint entity as part of a network node:
The L2 PoA is an L2 interface on the network node, identified by an L2 address.
The L3 PoA is an L3 interface on the network node, identified by an L3 address
(on a router, the same physical interface can co-locate L2 and L3 interfaces).
By definition, this will make any IP neighbor (a host and a router) on
the same IP link an L3 PoA. But what is the meaning of defining L3
node as PoA from handover perspective while I would be mostly
interested in choosing an L2 link to attach?
The MIH PoA is an MIH interface on the network node, i.e., an interface (either
L2 or L3) with which the MIH function of the network node is registered for any
of the MIH services. When referring to both transport and MIH capabilities of
the interface, we may have an "L2 MIH PoA" or an "L3 MIH PoA".
Similar question: What is the meaning of defining MIH as PoA from
handover perspective while I would be mostly interested in choosing an
L2 link to attach?
The main purpose of the endpoint vs. node distinction in the PoA definition is
to avoid ambiguities when the same network node can terminate multiple links and
present for each of them different capabilities and behaviors (i.e., MIH
capability can be activated on one interface and not on another, or the node can
be a hybrid L2/L3 box with both L2 ports and L3 ports). Defining the PoA with
respect to a specific link (or connection) brings the focus of the PoA
definition on the functionality that the corresponding UE can obtain from that
point in the network, without requiring any unnecessary assumptions on the
overall nature of the network node that includes it.
While I am sure that the wording for the definitions I am proposing can be
dramatically improved, I am convinced of the absolute necessity to single out
the respective entities and provide clear definitions for each of them.
Thanks,
Andrea
Regards,
Yoshihiro Ohba