Computer Communications Software

Computer communications software is becoming increasingly important, as a result of the increasing deployment and use of computer networks. Stringent requirements of real-time processing, efciency, reliability, and inter-operability make design and development of such software extremely challenging. The development typically starts by identifying the services to be provided. A service is de ned by its functionality and its interface. The functionality may range from low-level tasks, such as retransmission of lost messages, to high-level applications, such as electronic mail. The interface describes the supported operations and their parameters. The development continues with the design of a protocol, which describes the messages that will be exchanged in an implementation of the service; the protocol speci es message format (e.g., message length, division into elds, and data encoding), timing (e.g., minimum and maximum intervals between messages in certain situations), and semantics (i.e., the meaning of each message). Finally, an implementation of the service is constructed. Service de nitions are often su ciently exible to allow many di erent implementations of the service using the same protocol. This allows each computer in a network to use an implementation optimized for its particular architecture. Since the implementations all follow the same protocol, they interact correctly to provide the service. Communication is possible only when all participants follow the same protocol, so standards are essential. A protocol architecture is a collection of protocols designed to be used together. The International Organization for Standardization (ISO) issued a standard for an in uential|though not widely used|protocol architecture, called the Open Systems Interconnection (OSI) Reference Model [1]. The Internet Activities Board issues standards for the protocols used on the Internet; collectively, these form the Internet Architecture or TCP/IP Architecture. Both of the standards just mentioned (and most other protocol standards) incorporate a classic design technique: layering. To help manage the complexity of writing, testing, and maintaining