[DNA] L2 link and L3 link

[JinHyeock Choi <athene@sait.samsung.co.kr>]

I think we'd better clarify the definition of 'link'. 

The current definition of 'link' has room for ambiguity. For example, it's 
not clear what it means to say that 'two hosts are at the same link'. 
There is subtle difference between L2 link and L3 link. 

If several Ethernet segments are bridged together, they form a link. If a host 
moves from one Ethernet segment to the another, it hardly notice a change. 
There is little need  to differentiate L2 link and L3 link.

But if 802.11 APs and an Ethernet segment are bridged to form a link, it's 
more complex. Assume there are two APs and a router on an Ethernet 
segment as below. 

                                 +----+ 
                                 | AR | 
                                 +----+                    
                                   | 
                             +-----+--------+ 
                             |                       | 
                          +----+         +----+ 
                          |AP 1|         |AP 2| 
                          +----+         +----+       

                          +----+         +----+ 
                          | H 1|            |H 2| 
                          +----+         +----+        

Each AP forms a different BSS. And if a host moves from AP1 to AP2, it will 
detect distinct link-layer change.

Also assume host 1 is attached to AP1 and host 2 is attached to AP2. I think 
it's reasonable to say that they are at the different L2 link. But they are at the 
same (L3) link. 

So currently there are ambiguities. Sometimes a person write that 'A host changed 
a link' even when a host only has changed its AP and still remains at the same 
link/ subnet. 

We'd better clarify this lest there should be unnecessary confusion and arguments.