Connection management in transport protocols

Abstract

Transport protocols are designed to provide fully reliable communication between processes which must communicate over a less reliable medium such as a packet switching network (which may damage, lose, or duplicate packets, or deliver them out of order). This is typically accomplished by assigning a sequence number and checksum to each packet transmitted, and retransmitting any packets not positively acknowledged by the other side. The use of such mechanisms requires the maintenance of state information describing the progress of data exchange. The initialization and maintenance of this state information constitutes a connection between the two processes, provided by the transport protocol programs on each side of the connection. Since a connection requires significant resources, it is desirable to maintain a connection only while processes are communicating. This requires mechanisms for opening a connection when needed, and for closing a connection after ensuring that all user data have been properly exchanged. These connection management procedures form and the main subject of this paper. Mechanisms for establishing connections, terminating connections, recoverty from crashes or failures of either side, and for resynchronizing a connection are presented. Connection management functions are intimately involved in protocol reliability, and if not designed properly may result in deadlocks or old data eing erroneously delivered in place of current data. Some protocol modeling techniques useful in analyzing connection management are discussed, using verification of connection establishment as an example. The paper is based on experience with the Transmission control Protocol (TCP), and examples throughout the naper are taken from TCP. © 1978.