This report describes the internal structure of facilities added to the 4.2BSD version of the UNIX operating system for the VAX, as modified in the 4.3BSD release. The system facilities provide a uniform user interface to networking within UNIX. In addition, the implementation introduces a structure for network communications which may be used by system implementors in adding new networking facilities. The internal structure is not visible to the user, rather it is intended to aid implementors of communication protocols and network services by providing a framework which promotes code sharing and minimizes implementation effort.

The reader is expected to be familiar with the C programming language and system interface, as described in the Berkeley Software Architecture Manual, 4.3BSD Edition [Joy86]. Basic understanding of network communication concepts is assumed; where required any additional ideas are introduced.

The remainder of this document provides a description of the system internals, avoiding, when possible, those portions which are utilized only by the interprocess communication facilities.