Internet Core Protocols: The Definitive Guide

TCP Is a Reliable, Connection-Centric Transport Protocol

Remember that all of the transport-layer protocols (including TCP and UDP) use IP for their basic delivery services, and that IP is an unreliable protocol, providing no guarantees that datagrams or packets will reach their destination intact. It is quite possible for IP packets to get lost entirely (due to an untimely link failure on the network somewhere), or for packets to become corrupted (due to an overworked or buggy router), or for packets to get reordered as they cross different networks en route to the destination system, or for a myriad of other problems to crop up while packets are being bounced around the Internet.

For applications that need some sort of guarantee that data will arrive at its destination intact, this uncertainty is simply unacceptable. Electronic mail, TELNET, and other network applications are the basis of many mission-critical efforts, and as such they need some sort of guarantee that the data they transmit will arrive in its original form.

This reliability is achieved through the use of a virtual circuit that TCP builds whenever two applications need to communicate. As we discussed in Chapter 1, a TCP session is somewhat analogous to a telephone conversation in that it provides a managed, full-duplex, point-to-point communications circuit for application protocols to use. Whenever data needs to be sent between two TCP-based applications, a virtual circuit is established between the two TCP providers, ...