TCP/IP Protocol Layers
Figure 1-1 shows the TCP/IP protocol suite in relationship to the OSI reference model. The network interface layer, which corresponds to the OSI physical and data link layers, is not actually part of the specification. However, it has become a de facto layer either as shown in Figure 1-1 or as separate physical and data link layers. It is described in this section in terms of the OSI physical and data link layers.
 The OSI protocol suite itself has become, with some rare exceptions, a relic of early Internet history. Its current contribution to networking technology seems to be mainly limited to the usefulness of its reference model in illustrating modular protocol suites to networking studentsand, of course, the IS-IS routing protocol still widely used in large service provider and carrier networks.
Figure 1-1. TCP/IP protocol suite.
The physical layer contains the protocols relating to the physical medium on which TCP/IP will be communicating. Officially, the protocols of this layer fall within four categories that together describe all aspects of physical media:
- Electrical/optical protocols describe signal characteristics such as voltage or photonic levels, bit timing, encoding, and signal shape.
- Mechanical protocols are specifications such as the dimensions of a connector or the metallic makeup of a wire.
- Functional protocols describe what something does. For example, “Request to Send” is the functional description of pin 4 of an EIA-232-D connector.
- Procedural protocols describe how something is done. For example, a binary 1 is represented on an EIA-232-D lead as a voltage more negative than 3 volts.
The data link layer contains the protocols that control the physical layer: how the medium is accessed and shared, how devices on the medium are identified, and how data is framed before being transmitted on the medium. Examples of data-link protocols are IEEE 802.3/Ethernet, Frame Relay, ATM, and SONET.
The internet layer, corresponding to the OSI network layer, is primarily responsible for enabling the routing of data across logical network paths by defining a packet format and an addressing format. This layer is, of course, the one with which this book is most concerned.
The host-to-host layer, corresponding to the OSI transport layer, specifies the protocols that control the internet layer, much as the data link layer controls the physical layer. Both the host-to-host and data link layers can define such mechanisms as flow and error control. The difference is that while data-link protocols control traffic on the data linkthe physical medium connecting two devicesthe transport layer controls traffic on the logical linkthe end-to-end connection of two devices whose logical connection traverses a series of data links.
The application layer corresponds to the OSI session, presentation, and application layers. Although some routing protocols such as Border Gateway Protocol (BGP) and routing Information Protocol (RIP) reside at this layer, the most common services of the application layer provide the interfaces by which user applications access the network.
 BGP is an application layer protocol because it uses TCP to transport its messages, and RIP because it uses UDP for the same purposes. Other routing protocols such as OSPF are said to operate at the internet layer because they encapsulate their messages directly into IP packets.
A function common to the protocol suite of Figure 1-1 and any other protocol suite is multiplexing between layers. Many applications might use a service at the host-to-host layer, and many services at the host-to-host layer might use the internet layer. Multiple protocol suites (IP, IPX, and AppleTalk, for example) can share a physical link via common data-link protocols.