TCP/IP refers to two network protocols used on the Internet: Transmission Control Protocol and Internet Protocol. However, TCP and IP are only two protocols belonging to a much larger collection of protocols called the TCP/IP suite.

The TCP/IP suite of networking protocols connects various operating systems and network components. It provides a standard method for moving data between systems, and is used both on the Internet as well as in the world of private networking.

Protocols within the TCP/IP suite provide data transport for all services available to today's network user. Some of those services include

· Transmission of electronic mail

· File transfers

· Instant messaging

· Access to the World Wide Web

The Open Systems Interconnection (OSI) Reference Model

The OSI Reference Model was defined to standardize discussion of various technologies involved in networking. Its seven layers represent the architecture for data communication protocols. Each layer in the OSI model specifies a particular network function. The OSI model can be thought of as a stack with each layer lying on the one below it. The services that a given layer performs are defined by the protocols at that layer. Understanding the OSI model and each layer is very helpful in conceptualizing how the different parts of TCP/IP networks and applications interact. The seven layers of the OSI model stack are

· Layer 7 (Application). The highest layer of the OSI model. This layer defines the way applications interact with the network and between systems.

· Layer 6 (Presentation). Contains protocols that are part of the operating system. This layer defines how information is formatted for display. Data encryption and translation can occur at this layer.

· Layer 5 (Session). Coordinates communication between endpoints. Session state is maintained at this layer for security, logging, and administrative functions.

· Layer 4 (Transport). Controls the flow of data between systems, defines the structure of data in messages, and performs error checking. Web browser encryption commonly occurs at this layer.

· Layer 3 (Network). Defines protocols for routing data between systems. This is the layer where endpoint addressing occurs; it makes sure that data arrives at the correct destination.

· Layer 2 (Data-link or Network interface). Defines the rules for sending and receiving information from one node to another in local network environments (that is, LANs).

· Layer 1 (Physical or Media). Governs hardware connections and byte-stream encoding for transmission. It is the only layer that involves a physical transfer of information between network nodes.

TCP/IP was created prior to the development of the OSI reference model, and, although TCP/IP fits within the OSI architecture, not all OSI layers are relevant when talking about TCP/IP. With respect to TCP/IP, the most important OSI layers are the application, transport, network, and data-link. Each of these layers has specific protocols associated with them. Common protocols at these layers are

· Application: Hypertext Transfer Protocol (HTTP)

· Transport: Transmission Control Protocol (TCP)

· Network: Internet Protocol (IP)

· Data-link: Address Resolution Protocol (ARP)

These protocols can be divided into two types, network and application.

Network-Level Protocols

Network-level protocols manage the mechanics of data transfer, and are typically invisible to the end user. For example, the Internet Protocol (IP) provides packet delivery of information sent between the user and remote machines. It does this based on a variety of data, most notably the IP address of the two machines. Based on the IP address and other information, IP provides a "best-effort" service to route the information to its intended destination. Throughout this process, IP interacts with other network-level protocols engaged in data transport. Short of using network utilities (perhaps a sniffer or other device that reads IP datagrams), the user will never see IP's work on the network.

Application-Level Protocols

Unlike network-level protocols, application-level protocols are visible to the user. For example, Hypertext Transfer Protocol (HTTP) is an interactive protocol; you see the results of your connection and transfer as it's happening. (That information is presented in error messages and status reports on the transfer—for example, the number of bytes that have been transferred at any given moment).

The History of TCP/IP

In 1969, the Defense Advanced Research Projects Agency (DARPA) commissioned development of a network for its research centers. The chief concern was this network's capability to withstand a nuclear attack. If the Soviet Union launched a first strike, the network still had to facilitate communication. The design of this network had several other requisites, the most important of which was this: It had to operate independently of any centralized control. The prototype for this system (called ARPANET) was based in large part on research done in 1962 and 1963.

The original ARPANET worked well but was subject to periodic system crashes. Furthermore, long-term expansion of that network proved costly. A search was therefore initiated for a more reliable set of protocols; that search ended in the mid-1970s with the development of TCP/IP.

TCP/IP had two chief advantages over other protocols: It was lightweight and could be implemented at lower cost than the other choices then available. Based on these factors, TCP/IP became exceedingly popular. By 1983, TCP/IP was integrated into release 4.2 of Berkeley Software Distribution (BSD) UNIX. Its integration into commercial forms of UNIX soon followed, and TCP/IP was established as the Internet standard. It has remained so to this day.

Today, TCP/IP is used for many purposes, not just for Internet communication. For example, intranets are often built using TCP/IP. In such environments, TCP/IP can offer significant advantages over other networking protocols. For example, TCP/IP works on a wide variety of hardware and operating systems. Using TCP/IP, one can quickly and easily create a heterogeneous network that links Macs, IBM compatibles, Mainframes, Sun UNIX servers, MIPS machines, and so on. Each of these can communicate with its peers using a common protocol suite. For this reason, TCP/IP has remained extremely popular since its introduction.

The RFCs

The protocols of TCP/IP suite are usually defined by documents called Requests For Comments (RFCs). The RFC approval process is managed by the Internet Engineering Steering Group (IESG) based on recommendations from the Internet Engineering Task Force (IETF). RFCs can be composed and submitted by anyone. In addition, RFCs are unlike many other networking standards in that they are freely available online and are open to comment by anyone.

The IETF is primarily responsible for forming working groups focused on strategic TCP/IP issues. Standards RFCs are often the product of many months of discussion within these working groups, which are made up of people interested in a particular aspect of the Internet. The working groups often draft proposed RFCs and make them available for discussion. These discussions typically take place on mailing lists, which welcome input from any interested party. Not all RFCs specify TCP/IP standards. Some RFCs contain background information, some provide documentation and tips for managing a TCP/IP network, some document protocol weaknesses, and some are even completely humorous. The core RFCs that define the standards associated with TCP/IP networking are

· RFC 768: User Datagram Protocol

· RFC 791: Internet Protocol

· RFC 792: Internet Control Message Protocol

· RFC 793: Transmission Control Protocol

· RFC 1122: Requirements for Internet Hosts—Communication Layers

· RFC 1123: Requirements for Internet Hosts—Application and Support.

For more complete information about Internet protocols and associated RFCs, visit an RFC archive such as that provided by the IETF at http://www.ietf.org/rfc.html.

Implementations of TCP/IP

The de facto standard for TCP/IP implementations has been the 4.x BSD releases, and its code has been the starting point for many other implementations. There are numerous implementations available including a number of BSD derivatives such as FreeBSD, OpenBSD, and NetBSD. In addition, the UNIX-like operating system GNU/Linux includes source code for a TCP/IP implementation. Source code for non-UNIX implementations is also available. Packages for MS-DOS and Windows include WATTCP/WATT-32 and KA9Q.