3.8 Summary

We began this chapter by studying the services that a transport layer protocol can provide to network applications. At one extreme, the transport layer protocol can be very simple and offer a no-frills service to applications, providing only the multiplexing/demultiplexing function for communicating processes. The Internet's UDP protocol is an example of such a no-frills (and no-thrills, from the persective of someone interested in networking) transport-layer protocol. At the other extreme, a transport layer protocol can provide a variety of guarantees to applications, such as reliable delivery of data, delay guarantees and bandwidth guarantees. Nevertheless, the services that a transport protocol can provide are often constrained by the service model of the underlying network-layer protocol. If the network layer protocol cannot provide delay or bandwidth guarantees to transport-layer segments, then the transport layer protocol cannot provide delay or bandwidth guarantees for the messages sent between processes.

We learned in Section 3.4 that a transport layer protocol can provide reliable data transfer even if the underlying network layer is unreliable. We saw that providing reliable data transfer has many subtle points, but that the task can be accomplished by carefully combining acknowledgments, timers, retransmissions and sequence numbers.

Although we covered reliable data transfer in this chapter, we should keep in mind that reliable data transfer can be provided by link, network, transport or application layer protocols. Any of upper four layers of the protocol stack can implement acknowledgments, timers, retransmissions and sequence numbers and provide reliable data transfer to the layer above. In fact, over the years, engineers and computer scientists have independently designed and implemented link, network, transport and application layer protocols that provide reliable data transfer (although many of these protocols have quietly disappeared).

In Section 3.5 we took a close look at TCP, the Internet's connection-oriented and reliable transport-layer protocol. We learned that TCP is complex, involving connection management, flow control, round-trip time estimation, as well as reliable data transfer. In fact, TCP is actually more complex that we made it out to be -- we intentionally did not discuss a variety of TCP patches fixes, and improvements that are widely implemented in various versions of TCP. All of this complexity, however, is hidden from the network application. If a client on one host wants to reliably send data to a server on another host, it simply opens a TCP socket to the server and then pumps data into that socket. The client-server application is oblivious to all of TCP's complexity.

In Section 3.6 we examined congestion control from a broad perspective, and in Section 3.7 we showed how TCP implements congestion control. We learned that congestion is imperative for the well-being of the network. Without congestion control, a network can easily become grid locked, with little or no data being transported end-to-end. In Section 3.7 we learned that TCP implements an end-to-end congestion control mechanism that additively increases its transmission rate when the TCP connection's path is judged to be congestion-free, and nultiplicatively decreases its transmission rate when loss occurs. This mechanism also strives to give each TCP connection passing through a congested link an equal share of the link bandwidth. We also examined in some depth the impact of TCP connection establishment and slow start on latency. We observed that in many important scenarios, connection establishment and slow start significantly contribute to end-to-end delay. We emphasize once more that TCP congestion control has evolved over the years, remains an area of intensive research, and will likely continue to evolve in the upcoming years.

In Chapter 1 we said that a computer network can be partitioned into the "network edge" and the "network core". The network edge covers everything that happens in the end systems. Having now covered the application layer and the transport layer, our discussion of the network edge is now complete. It is time to explore the network core! This journey begins in the next chapter, where we'll study the network layer, and continues into Chapter 5, where we'll study the link layer.