The important thing to remember about the TCP/IP protocol stack is that each layer plays a role in the overall communication process. Each layer invokes services that are necessary for that layer to perform its role. As an outgoing transmission passes down through the stack, each layer includes a bundle of relevant information called a header along with the actual data. The little data package containing the header and the data then becomes the data that is repackaged at the next lower level with the next lower layer’s header. This process is shown in Figure 2.3. The reverse process occurs when data is received on the destination computer. As the data moves up through the stack, each layer unpacks the corresponding header and uses the information.
Figure 2.3 At each layer, the data is repackaged with that layer’s header.
As the data moves down through the stack, the effect is a little like the nested Russian wooden dolls you might have seen; the innermost doll is enclosed in another doll, which is then enclosed in another doll, and so on. At the receiving end, the data packages are unpacked, one by one, as the data climbs back up the protocol stack. The Internet layer on the receiving machine uses the information in the Internet layer header. The Transport layer uses the information in the Transport layer header. At each layer, the package of data takes a form that provides the necessary information to the corresponding layer on the receiving machine. Because each layer is responsible for different functions, the form of the basic data package is very different at each layer.
The data packet looks different at each layer, and at each layer it goes by a different name. The names for the data packages created at each layer are as follows:
- The data package created at the Application layer is called a message.
- The data package created at the Transport layer, which encapsulates the Application layer message, is called a segment if it comes from the Transport layer’s TCP protocol. If the data package comes from the Transport layer’s User Datagram Protocol (UDP) protocol, it is called a datagram.
- The data package at the Internet layer, which encapsulates the Transport layer segment, is called a datagram.
- The data package at the Network Access layer, which encapsulates and may subdivide the datagram, is called a frame. This frame is then turned into a bitstream at the lowest sublayer of the Network Access layer.
You learn more about the data packages for each layer in later hours.