Separating Abstraction from Implementation in Communication Network Design

Datagrams and virtual circuits are not disjoint conceptual models for data communication, but rather inhabitants of a wide design space containing many other viable networking solutions. Many design choices often closely associated with these two communication styles can be decoupled from the datagram and virtual circuit abstractions, and combined to form new and effective network implementations. This paper examines several key elements of network architecture. For each element, it shows how certain characteristics often thought to differentiate datagrams and virtual circuits are independent of these two concepts and form a multi-valued spectrum of design choices. This discussion is motivated by the current drive to design a new generation of high-speed wide-area networks, and the observation that this effort would benefit from a more systematic evaluation of existing and future network design alternatives. Introduction Datagrams and virtual circuits are widely used data communication paradigms. The two terms have been defined many ways. A useful analogy is drawn in [Tan81] and [Wec80], where datagram service is compared to the common postal service, and virtual circuit service is compared to ordinary telephone service. We define them in a similar fashion as follows: Like letters flowing through the postal system, datagrams are individually addressed and delivered items of information. No guarantees are made regarding the ordering or reliability of datagrams. Like voice conversations in the telephone system, virtual circuits must be explicitly set up and torn down before and after information is delivered. Virtual circuit setup consists of a user host requesting a connection to a specified destination address, and the network acknowledging that the connection has been established. In addition, virtual circuit information is delivered in the same order in which it was sent, although not necessarily free of errors. In the remainder of this paper, this is our exclusive definition for datagrams and virtual circuits. It consists of the least common denominator of the many properties that are hhhhhhhhhhhhhhhhhh This work was supported in part by AT&T Bell Laboratories and the International Computer Science Institute.