[DNA] Revised terminology section for DNAv4

[Bernard Aboba <aboba@internaut.com>]

Here is the revised terminology section for DNAv4 (ARP terms left out for
brevity).  Comments welcome.

DHCP client
     A DHCP client or "client" is an Internet host using the Dynamic
     Host Configuration protocol (DHCP) [RFC2131] to obtain
     configuration parameters such as a network address.

DHCP server
     A DHCP server or "server" is an Internet host that returns
     configuration parameters to DHCP clients.

Attachment Point
     The link endpoint on the link to which the host is currently
     connected.

Link
     A communication facility or medium over which network nodes can
     communicate.  Each link is associated with a minimum of two
     endpoints.  Each link endpoint has a unique link-layer identifier.

Link Down
     An event provided by the link layer that signifies a state change
     associated with the interface no longer being capable of
     communicating IP packets.

Link Layer
     Conceptual layer of control or processing logic that is responsible
     for maintaining control of the data link.  The data link layer
     functions provide an interface between the higher-layer logic and
     the data link.  The link layer is the layer immediately below IP.

Link Up
     An event provided by the link layer that signifies a state change
     associated with the interface becoming capable of communicating IP
     packets.

Most Likely Attachment Network (MLAN)
     The attached network heuristically determined by the host to be
     most likely, based on hints.

Routable address
     In this specification, the term "routable address" refers to any
     address other than an IPv4 Link-Local address.  This includes
     private addresses as specified in [RFC1918].

Valid address
     In this specification, the term "valid address" refers to either a
     static IPv4 address, or an address assigned via DHCPv4 which has
     not been relinquished, and whose lease has not yet expired.