A name-domain is a set of registered (mail) names. A name-domain specification resolves to a subordinate name-domain specification or to a terminal domain-dependent string. Hence, domain specification is extensible, permitting any number of registration levels. Name-domains model a global, logical, hierarchical addressing scheme. The model is logical, in that an address specification is related to name registration and is not necessarily tied to transmission path. The model's hierarchy is a directed graph, called an in-tree, such that there is a single path from the root of the tree to any node in the hierarchy. If more than one path actually exists, they are considered to be different addresses.
The root node is common to all addresses; consequently, it is not referenced. Its children constitute "top-level" name-domains. Usually, a service has access to its own full domain specification and to the names of all top-level name-domains.
The "top" of the domain addressing hierarchy -- a child of the root -- is indicated by the right-most field, in a domain specification. Its child is specified to the left, its child to the left, and so on.
Some groups provide formal registration services; these constitute name-domains that are independent logically of specific machines. In addition, networks and machines implicitly compose name-domains, since their membership usually is registered in name tables.
In the case of formal registration, an organization implements a (distributed) data base which provides an address-to-route mapping service for addresses of the form:
Note that "organization" is a logical entity, separate from any particular communication network.
A mechanism for accessing "organization" is universally available. That mechanism, in turn, seeks an instantiation of the registry; its location is not indicated in the address specification. It is assumed that the system which operates under the name "organization" knows how to find a subordinate registry. The registry will then use the "person" string to determine where to send the mail specification.
The latter, network-oriented case permits simple, direct, attachment-related address specification, such as:
Once the network is accessed, it is expected that a message will go directly to the host and that the host will resolve the user name, placing the message in the user's mailbox.