11.1 Introduction

11.1 Introduction

UDP is a simple, datagram-oriented, transport layer protocol: each output operation by a process produces exactly one UDP datagram, which causes one IP datagram to be sent. This is different from a stream-oriented protocol such as TCP where the amount of data written by an application may have little relationship to what actually gets sent in a single IP datagram.

Figure 11.1 shows the encapsulation of a UDP datagram as an IP datagram.

Figure 11.1. UDP encapsulation.

RFC 768 [Postel 1980] is the official specification of UDP.

UDP provides no reliability: it sends the datagrams that the application writes to the IP layer, but there is no guarantee that they ever reach their destination. Given this lack of reliability, we are tempted to think we should avoid UDP and always use a reliable protocol such as TCP. After we describe TCP in Chapter 17 we'll return to this topic and see what types of applications can utilize UDP.

The application needs to worry about the size of the resulting IP datagram. If it exceeds the network's MTU (Section 2.8), the IP datagram is fragmented . This applies to each network that the datagram traverses from the source to the destination, not just the first network connected to the sending host. (We defined this as the path MTU in Section 2.9.) We examine IP fragmentation in Section 11.5.