25.3 UDP Echo Server Using SIGIO

24.1 Introduction

Many transport layers have the concept of out-of- band data, which is sometimes called expedited data . The idea is that something important occurs at one end of a connection and that end wants to tell its peer quickly. By "quickly" we mean that this notification should be sent before any "normal" (sometimes called "in-band") data that is already queued to be sent, and should be sent regardless of any flow control or blocking issues. That is, out-of-band data is considered higher priority than normal data. Instead of using two connections between the client and server, out-of-band data is mapped onto the existing connection.

Unfortunately, once we get beyond the general concepts and down to the real world, almost every transport layer has a different implementation of out-of-band data. As an extreme example, UDP has no implementation of out-of-band data. In this chapter, we will focus on TCP's model of out-of-band data, provide numerous small examples of how it is handled by the sockets API, and describe how it is used by applications like telnet , rlogin , and FTP. Other than remote interactive applications like these, it is rare to find any use for out-of-band data.