The Internet architecture can be described in terms of a layered framework called the Internet Layering Model that derived its origins from the International Standard Organization's (ISO) reference model. The Internet Layering Model consists of five layers as shown in Figure A-1: the physical layer, the data link layer, the network layer, the transport layer, and the application layer. Each layer has a well-defined set of functions and capabilities. The functionality of each layer is described. The actual protocols that provide the services for each layer are described later.