Active Names are organized hierarchically into namespaces, analogous to domains in the Domain Naming System (DNS) and directories in a UNIX file system. Names within a namespace can be, in turn, namespaces (subdomains in DNS or subdirectories in UNIX); they can also be terminal leaves in the naming tree (machines in DNS, files in UNIX). Each namespace has a program associated with it that is responsible for interpreting that portion of the namespace; this program is free to interpret the names within the namespace in any fashion it wants. A root namespace interprets all names. Each namespace has an owner with the right to determine the program bound to the namespace. The client, by default, is the owner of root. This allows the client to install a program to mediate how its names will be translated. For example, a PDA could install a root program to take whatever is returned by lower level name spaces and compress any images to fit the screen size [23].
To illustrate how delegation works, consider how our system implements the WWW namespace to support per-service naming and transport. Traditionally, users type web requests that specify a specific transport protocol (e.g., ``http'') along with the service name (e.g., ``cnn.com''). But the transport used to communicate with a service should not be the concern of the end-user. In our framework, users simply name the service they wish to contact, and services specify the transport for names they control via the hierarchical namespace delegation mechanism. In particular, the root namespace sends web requests to the WWW-root Active Name program, which implements the WWW-root namespace. For bootstrapping (and by default), WWW-root delegates incoming requests to a series of Active Name programs that implement the default HTTP caching and transport protocol. But under the Active Names paradigm, rather than delegate resolution of all names to HTTP-default, the WWW-root namespace has the right to delegate portions of the WWW namespace to other Active Name programs according to any policy it chooses. The WWW-root's policy is to set these mappings as follows: the response for a request to a URL may include in its MIME header a directive specifying an Active Name program to be invoked for subsequent requests for which that URL is a prefix. For example, the reply to a request to www.cs.utexas.edu/home/smith can delegate the www.cs.utexas.edu/home/smith/active/* namespace, but it cannot delegate the www.cs.utexas.edu/home/jones/* namespace, the www.cs.utexas.edu/* namespace or the www.cs.duke.edu/home/smith/* namespace.