The TCP/IP reference model is widely used. This chapter gave an overview of the model and describes some of the protocols that are part of the specification. The transport protocols that currently exist have certain aspects that make them appropriate for different styles of data transmission. These protocols are reviewed in chapter 3. T/TCP is detailed in chapter 4. It is a more reliable protocol than UDP and more efficient than TCP and fits into the gap between the two protocols in figure 2.2.