Understanding TCP/IP

TCP/IP (Transmission Control Protocol/Internet Protocol) is the basic communication language or protocol of the Internet. It can also be used as a communications protocol in a private network (either an intranet or an extranet). When you are set up with direct access to the Internet, your computer is provided with a copy of the TCP/IP program just as every other computer that you may send messages to or get information from also has a copy of TCP/IP.

TCP/IP is a two-layer program. The higher layer, Transmission Control Protocol (TCP), manages the assembling of a message or file into smaller packets that are transmitted over the Internet and received by a TCP layer that reassembles the packets into the original message. The lower layer, Internet Protocol (IP), handles the address part of each packet so that it gets to the right destination. Each gateway computer on the network checks this address to see where to forward the message. Even though some packets from the same message are routed differently than others, they'll be reassembled at the destination.

The original research was performed in the late 1960s and early 1970s by the Advanced Research Projects Agency (ARPA), which is the research arm of the US Department of Defense (DOD). The DOD wanted to build a network to connect a number of military sites. The key requirements for the network were as follows:

ARPA hired a firm called BBN to design the network. The prototype was a research network called ARPANET (first operational in 1972). This connected four university sites using a system described as a packet switching network.

Prior to this development, any two computers wanting to communicate had to open a direct channel (known as a circuit) and information was then sent. If this circuit were broken, the computers would stop communicating immediately, which the DOD specifically wanted to avoid.

One computer could forward information to another by using packet-switching, so it superseded circuit-switched networks. To ensure information reached the correct destination, each packet was addressed with a source and destination and the packet was then transferred using any available pathway to the destination computer.

It was divided into small chunks or packets (originally 1008 bits). Sending large chunks of information has always presented problems, often because the full message fails to reach its destination at the first attempt, and the whole message then has to be resent. The facilities within the new protocol to divide large messages into numerous small packets meant that a single packet could be resent if it was lost or damaged during transmission, rather than the whole message.

The new network was decentralized with no one computer controlling its operation where the packet switching protocol controlled most of the network operations.

TCP/IP is a very robust protocol and can automatically recover from any communication link failures. It re-routes data packets if transmission lines are damaged or if a computer fails to respond, utilizing any available network path. The figure below shows an example of an Internet system. A packet being sent from Network A to Network F may be sent via Network D (the quickest route). If this route becomes unavailable, the packet is routed using an alternate route (for example, A B C E F).

Once ARPANET was proven, the DOD built MILNET (Military Installation in US) and MINET (Military Installation in Europe). To encourage the wide adoption of TCP/IP, BBN and the University of California at Berkeley were funded by the US Government to implement the protocol in the Berkeley version of Unix. UNIX was given freely to US universities and colleges, allowing them to network their computers. Researchers at Berkeley developed a program interface to the network protocol called sockets and wrote many applications using this interface.

During the early 1980s, the National Science Foundation (NSF) used Berkeley TCP/IP to create the Computer Science Network (CSNET) to link US universities. They saw the benefit of sharing information between universities and ARPANET provided the infrastructure. Meanwhile, in 1974 a successor to ARPANET was developed named NSFNET. This was based on a backbone of six supercomputers into which many regional networks were allowed to connect.

The first stage in the commercial development of the Internet occurred in 1990 when a group of telecommunications and computer companies formed a non-profit making organization called Advanced Networks and Services (ANS). This organization took over NSFNET and allowed commercial organizations to connect to the system. The commercial Internet grew from these networks.

Why TCP/IP?

There are numerous reasons why TCP/IP was chosen to be the protocol that drives the Internet; some of these include:

TCP/IP as an Open Standard

TCP/IP and the Internet are inextricably linked. Although no organization owns the Internet or its technologies, a number of organizations are responsible for the development of the Internet and so TCP/IP.

Request for Comment (RFC)

TCP/IP standards are not developed by a committee but rather by consensus. The process for adopting Internet standards revolves around the production of documents called Request for Comments (RFCs). When an RFC is published, it is assigned the next available number. Anyone can submit a document for publication as an RFC. A technical expert or task force then reviews all documents.

A period of time for comments is followed by an updated draft that responds to these comments. After several drafts, the proposed standard is either accepted or rejected.

The standards for TCP/IP are published via these RFCs. Some RFCs describe network services or protocols and their implementation while others summarize policies.

An older RFC is never updated. If changes are required, a new RFC is published with a new number. It is always important to verify you have the most recent RFC on a particular topic.

Internet Network Information Center (InterNIC)

For the Internet to function successfully, a central organization must assume two major responsibilities:

InterNIC is responsible for ensuring the same IP address or domain name is not allocated twice. They partially delegate this responsibility by giving the Internet Service Providers (ISPs) a block of IP addresses that they can assign to customers.

The TCP/IP Protocol Suite

The figure below shows a comparison of the Open Systems Interconnection (OSI) model and the TCP/IP protocol suite. The TCP/IP protocol maps to a four layer conceptual model: application, transport, Internet and network interface. This model is referred to as the Internet Protocol Suite or the ARPA model. As shown below, each layer in the green Internet Protocol Suite corresponds to one or more layers of the white OSI model.

Network Interface
The network interface layer is the equivalent of the OSI physical and data link layers as it defines the host's connection to the network. This layer comprises the hardware and software involved in the interchange of frames between computers. The technologies used can be LAN-based (e.g. Ethernet) or WAN-based (e.g. ISDN)

Internet Layer
The network layer uses a number of protocols to ensure the delivery of packets. These are described below:

Application Layer
The Application layer is the layer at which many TCP/IP services (high level protocols) can be run (such as FTP, HTTP and SMTP). Two application programming interfaces (APIs) are commonly used within the TCP/IP environment:

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Understanding TCP/IP

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