19.1 Introduction

The previous chapter dealt with the establishment and termination of TCP connections. We now examine the transfer of data using TCP.

Studies of TCP traffic, such as [Caceres et al. 1991], usually find that on a packet-count basis about half of all TCP segments contain bulk data (FTP, electronic mail, Usenet news) and the other half contain interactive data (Telnet and Rlogin, for example). On a byte-count basis the ratio is around 90% bulk data and 10% interactive, since bulk data segments tend to be full sized (normally 512 bytes of user data), while interactive data tends to be much smaller. (The above-mentioned study found that 90% of Telnet and Rlogin packets carry less than 10 bytes of user data.)

TCP obviously handles both types of data, but different algorithms come into play for each. In this chapter we'll look at interactive data transfer, using the Rlogin application. We'll see how delayed acknowledgments work and how the Nagle algorithm reduces the number of small packets across wide area networks. The same algorithms apply to Telnet. In the next chapter we'll look at bulk data transfer.