RFC 887 (rfc887) - Page 2 of 16
Resource Location Protocol
Alternative Format: Original Text Document
RFC 887 December 1983 Resource Location Protocol 2. Resource Naming Although the resource location problem can, in most cases, be reduced to the problem of finding a host which implements a given Internet based protocol, locating only a particular lowest level Internet protocol (i.e. one assigned a protocol number for transport using IP) is not completely sufficient. Many significant network services and resources are provided through higher level protocols which merely utilize the various lower level protocols for their own transport purposes (e.g. the FTP protocol [2] employs the TCP protocol [4] for its lower level transport). Conceptually, this protocol nesting may even be carried out to arbitrary levels. Consequently, the Resource Location Protocol names a resource by the protocol number assigned to its lowest level Internet transport protocol and by a variable length protocol/resource specific identifier. For example, the UDP based Echo Protocol can be named by its assigned protocol number (17) and its assigned 16-bit "well-known" port number (7). Alternatively, the Internet Control Message Protocol [5] (lacking any higher level client protocols) would be named simply by its assigned protocol number (1) and an empty protocol specific identifier. On the other hand, some as yet undefined resource protocol (provided via say TCP), might be named by the assigned protocol number (6), its 16-bit "well-known" TCP port number, and then some additional fixed or variable length identifier specific to that TCP port. In general, the components of the protocol/resource specific identifier are defined to be the "natural" quantities used to successively de-multiplex the protocol at each next highest protocol level. See section 5 for some sample assignments. 3. Protocol Summary The Resource Location Protocol is a simple request/reply procedure. The querying host constructs a list of resources which it would like to locate and sends a request message on the network. A request message may be sent either to a particular IP address and host or, on networks which provide broadcast address capability, to the IP address which refers to all hosts on that network (see [7]). For example, a host attempting to locate a domain name server might construct a request containing the resource name [17, 53] (referring to the Domain Name Server protocol provided at "well-known" UDP port 53) and then broadcast that request on its local network. Each receiving host examines the list of resources named in the request packet, determines which of the resources it provides, and returns a reply message to the querying host in confirmation. The receiving host determines whether or not it provides a resource by successive decomposition of the resource name until either the name is exhausted or it encounters a component which is not supported. In the previous Accetta
