TCP/IP First-Step [Electronic resources] نسخه متنی

Chapter 9. Best-Effort Delivery: It's Now or Never!

What You Will Learn

After reading this chapter, you should be able to answer the following questions:

Which of the seven layers in the OSI reference model does UDP occupy?

What is the relationship between TCP and UDP?

What is meant by the term best effort?

What is meant by the term connectionless?

What is the difference between TCP port numbers and UDP port numbers?

What are the functions performed by UDP on a source machine?

What are the functions performed by UDP on a destination machine?

If TCP guarantees that IP packets get delivered safely to their destination, why on earth would you ever need an inferior alternative? The answer lies in exploring the myth that every packet must be delivered. Different applications have different performance requirements. Traditional applications such as online transaction processing, batch updates, or file transfers absolutely require a guaranteed delivery of data. As you saw in the preceding chapter, that sometimes requires some packets to be retransmitted so that you can be assured of their successful delivery.

Other types of applications have radically different network performance requirements. You see, the time required to perform reliable delivery can be quite detrimental to other types of applications. Timeliness is everything for some applications. Quite literally, data arriving late is worthless! That might seem a bit odd or extreme, but it's true. Applications that have such strong requirements for timeliness of data delivery, such as some voice and video applications, rely on UDP. For those applications, it is truly now or never for incoming packets of data.

After having walked you through some of the complexities and sophisticated features of TCP last chapter, you will likely find UDP a refreshing change of pace! This chapter explains what best effort really means and show you how UDP does its best to get data there on time. You see the similarities between TCP and UDP, as well as the many differences.