Internet Core Protocols: The Definitive Guide

UDP Ports

UDP does very little. In fact, it does almost nothing, acting only as a very basic facilitator for applications to use when they need to send or receive datagrams on an IP network. In order to perform this task, UDP has to provide two basic services: it must provide a way for applications to send data over the IP software, and it must also provide a way to get data that it has received from IP back to the applications that need it.

These services are provided by a multiplexing component within the UDP software. Applications must register with UDP, allowing it to map incoming and outgoing messages to the appropriate application protocols themselves.

This multiplexing service is provided by 16-bit port numbers that are assigned to specific applications by UDP. When an application wishes to communicate with the network, it must request a port number from UDP (server applications such as TFTP will typically request a pre-defined, specific port number, while most client applications will use whatever port number they are given by UDP). UDP will then use these port numbers for all incoming and outgoing datagrams

This concept is illustrated in Figure 6.1. Each of the applications that are using UDP have allocated a dedicated port number from UDP, which they use for all incoming and outgoing data.

Figure 6.1. Application-level multiplexing with port numbers

When an application wishes ...