In this chapter we describe the implementation of the PVM software and the reasons behind the basic design decisions. The most important goals for PVM 3 are fault tolerance, scalability, heterogeneity, and portability. PVM is able to withstand host and network failures. It doesn't automatically recover an application after a crash, but it does provide polling and notification primitives to allow fault-tolerant applications to be built. The virtual machine is dynamically reconfigurable. This property goes hand in hand with fault tolerance: an application may need to acquire more resources in order to continue running once a host has failed. Management is as decentralized and localized as possible, so virtual machines should be able to scale to hundreds of hosts and run thousands of tasks. PVM can connect computers of different types in a single session. It runs with minimal modification on any flavor of Unix or an operating system with comparable facilities (multitasking, networkable). The programming interface is simple but complete, and any user can install the package without special privileges.
To allow PVM to be highly portable, we avoid the use of operating system and language features that would be be hard to retrofit if unavailable, such as multithreaded processes and asynchronous I/O. These exist in many versions of Unix, but they vary enough from product to product that different versions of PVM might need to be maintained. The generic port is kept as simple as possible, though PVM can always be optimized for any particular machine.
We assume that sockets are used for interprocess communication and that each host in a virtual machine group can connect directly to every other host via TCP  and UDP  protocols. The requirement of full IP connectivity could be removed by specifying message routes and using the pvmds to forward messages. Some multiprocessor machines don't make sockets available on the processing nodes, but do have them on the front-end (where the pvmd runs).