7 - Host-to-host (transport) layer protocols

Abstract

This chapter explains basic functions of the host-to-host layer. The host-to-host communications layer, also referred to as the service layer or as the transport layer in terms of the OSI model, is primarily responsible for ensuring end-to-end delivery of packets transmitted by the Internet protocol (IP). This additional reliability is needed to compensate for the lack of reliability in IP. There are only two relevant protocols residing in the host-to-host communications layer, namely TCP (transmission control protocol) and UDP (user datagram protocol). TCP is a connection-oriented protocol and is therefore reliable. TCP establishes a connection between two hosts before any data is transmitted. UDP makes use of the underlying IP protocol to deliver its datagrams. UDP is a “connectionless” or nonconnection-oriented protocol and does not require a connection to be established between two machines prior to data transmission. This chapter explains the basic operation of TCP and UDP and discusses the fundamental differences between TCP and UDP. It explains which protocol (TCP or UDP) should be used for a particular application and also explains the meaning of each field in the TCP and UDP headers.