TCP-IP Protocol Layer

Figure 1-1 shows the TCP IP protocol suite in relationship to the OSI reference model.1 The network interface layer, which corresponds to the OSI physical and information link layers, isn't truly element with the specification. Nonetheless, it has become a de facto layer either as shown in Figure 1-1 or as separate physical and information link layers. It truly is described in this section with regards to the OSI physical and information link layers.

1 The OSI protocol suite itself has become, with some uncommon exceptions, a relic of early World-wide-web history. Its current contribution to networking technology appears to be mainly restricted to the usefulness of its reference model in illustrating modular protocol suites to networking studentsand, naturally, the IS-IS routing protocol nonetheless widely applied 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 Layers will be communicating. Officially, the protocols of this layer fall within 4 categories that with each other describe all aspects of physical media:

Electrical/optical protocols describe signal characteristics for example voltage or photonic levels, bit timing, encoding, and signal shape.
Mechanical protocols are specifications for example the dimensions of a connector or the metallic makeup of a wire.
Functional protocols describe what something does. By way of example, "Request to Send" could be the functional description of pin four of an EIA-232-D connector.
Procedural protocols describe how something is accomplished. By way of example, a binary 1 is represented on an EIA-232-D lead as a voltage extra unfavorable than three volts.

The information link layer contains the protocols that manage the physical layer: how the medium is accessed and shared, how devices on the medium are identified, and how information 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 mainly accountable for enabling the routing of information across logical network paths by defining a packet format and an addressing format. This layer is, naturally, the one with which this book is most concerned.

The host-to-host layer, corresponding to the OSI transport layer, specifies the protocols that manage the internet layer, significantly as the information link layer controls the physical layer. Each the host-to-host and information link layers can define such mechanisms as flow and error manage. The difference is the fact that whilst data-link protocols manage targeted traffic on the information linkthe physical medium connecting two devicesthe transport layer controls targeted traffic on the logical linkthe end-to-end connection of two devices whose logical connection traverses a series of information links.

The application layer corresponds to the OSI session, presentation, and application layers. Though some routing protocols for example Border Gateway Protocol (BGP) and routing Data Protocol (RIP) reside at this layer,2 one of the most frequent services with the application layer deliver the interfaces by which user applications access the network.

2 BGP is an application layer protocol since it makes use of TCP to transport its messages, and RIP since it makes use of UDP for the same purposes. Other routing protocols for example OSPF are mentioned to operate at the online layer since they encapsulate their messages straight into IP packets.

A function frequent to the protocol suite of Figure 1-1 and any other protocol suite is multiplexing in between layers. Lots of applications may well use a service at the host-to-host layer, and several services at the host-to-host layer may well use the internet layer. Numerous protocol suites (IP, IPX, and AppleTalk, as an example) can share a physical link through frequent data-link protocols.