The UNIX system: The virtual protocol machine

The UNIX™ operating system Virtual Protocol Machine (VPM) is a package of software tools that allows a wide variety of link-level data communications protocols to be implemented efficiently in a high-level language. The resulting protocol implementations are independent of the particular communications hardware, the host machine architecture, and the host operating system, and therefore can be ported easily from one hardware/software environment to another. An extension to VPM, the Common Synchronous Interface (CSI), provides similar benefits for the higher-level protocol software that runs in the UNIX system host. The implementations of VPM use Programmable Communications Devices (PCDs) to off load the link-level communications processing from the host CPU. A high-level language protocol description is translated by a protocol compiler that runs on the host machine. The resulting module is then loaded into the PCD and executed. The other components of VPM are a transparent protocol driver that allows user processes to interact directly with a link-level protocol implementation, a realtime trace capability to facilitate debugging, and several utility programs. VPM has been implemented on several different PCDs and several types of host computers. VPM-based protocol implementations can be ported with little or no change from one VPM implementation to another. VPM and CSI greatly reduce host system overhead while producing maximum communications throughput. A number of different higher-level protocols and their link-level counterparts have been implemented in the UNIX system using CSI and VPM; among them are X.25, 3270 emulation, a synchronous terminal interface, and a facility for remote job entry to IBM hosts.