We discuss gateway queueing algorithms and their role in controlling congestion in datagram networks. A fair queueing algorithm, based on an earlier suggestion by Nagle, is proposed. Analysis and simulations are used to compare this algorithm to other congestion control schemes. We find that fair queueing provides several important advantages over the usual first-come-first-serve queueing algorithm: fair allocation of bandwidth, lower delay for sources using less than their full share of bandwidth, and protection from ill-behaved sources.

Operating system abstractions do not always reach high enough for direct use by a language or applications designer. The gap is filled by language-specific runtime environments, which become more complex for richer languages (CommonLisp needs more than C+ +, which needs more than C). But language-specific environments inhibit integrated multi-lingual programming, and also make porting hard (for instance, because of operating system dependencies). To help solve these problems, we have built the Portable Common Runtime (PCR), a language-independent and operating-system-independent base for modern languages. PCR offers four interrelated facilities: storage management (including universal garbage collection), symbol binding (including static and dynamic linking and loading), threads (lightweight processes), and low-level I/O (including network sockets). PCR is “common” because these facilities simultaneously support programs in several languages. PCR supports C. Cedar, Scheme, and CommonLisp intercalling and runs pre-existing C and CommonLisp (Kyoto) binaries. PCR is “portable” because it uses only a small set of operating system features. The PCR source code is available for use by other researchers and developers.

Cedar is the name for both a language and an environment in use in the Computer Science Laboratory at Xerox PARC since 1980. The Cedar language is a superset of Mesa, the major additions being garbage collection and runtime types. Neither the language nor the environment was originally intended to be portable, and for many years ran only on Dmachines at PARC and a few other locations in Xerox. We recently re-implemented the language to make it portable across many different architectures. Our strategy was, first, to use machine-dependent C code as an intermediate language, second, to create a languageindependent layer known as the Portable Common Runtime, and third, to write a relatively large amount of Cedar-specific runtime code in a subset of Cedar itself. By treating C as an intermediate code we are able to achieve reasonably fast compilation, very good eventual machine code, and all with relatively small programmer effort. Because Cedar is a much richer language than C, there were numerous issues to resolve in performing an efficient translation and in providing reasonable debugging. These strategies will be of use to many other porters of high-level languages who may wish to use C as an assembler language without giving up either ease of debugging or high performance. We present a brief description of the Cedar language, our portability strategy for the compiler and runtime, our manner of making connections to other languages and the Unix* operating system, and some measures of the performance of our "Portable Cedar".