The Internet protocol suite like many protocol suites can be viewed as a set of layers. Each layer solves a set of problems involving the transmission of data, and provides a well-defined service to the upper layer protocols based on using services from some lower layers. Upper layers are logically closer to the user and deal with more abstract data, relying on lower layer protocols to translate data into forms that can eventually be physically transmitted. The original TCP/IP reference model consisted of four layers, but has evolved into a five-layer model.

The OSI model describes a fixed, seven-layer stack for networking protocols. Comparisons between the OSI model and TCP/IP can give further insight into the significance of the components of the IP suite. The OSI model with its increased numbers of layers provides for more flexibility. Both the OSI and the TCP/IP models are standards and application developers will often implement solutions without strict adherence to proposed division of labor within the standard while providing for functionality within the application suite. This separation of practice from theory often leads to confusion.

The Internet protocol suite came from work done by DARPA in the early 1970s. After building the pioneering ARPANET, DARPA started work on a number of other data transmission technologies. In 1972, Robert E. Kahn was hired at the DARPA Information Processing Technology Office, where he worked on both satellite packet networks and ground-based radio packet networks, and recognized the value of being able to communicate across them. In the spring of 1973, Vinton Cerf, the developer of the existing ARPANET Network Control Program (NCP) protocol, joined Kahn to work on open-architecture interconnection models with the goal of designing the next protocol for the ARPANET.

By the summer of 1973, Kahn and Cerf had soon worked out a fundamental reformulation, where the differences between network protocols were hidden by using a common internet work protocol, and instead of the network being responsible for reliability, as in the ARPANET, the hosts became responsible. (Cerf credits Hubert Zimmerman and Louis Pouzin [designer of the CYCLADES network] with important influences on this design.)

With the role of the network reduced to the bare minimum, it became possible to join almost any networks together, no matter what their characteristics were, thereby solving Kahn's initial problem. (One popular saying has it that TCP/IP, the eventual product of Cerf and Kahn's work, will run over "two tin cans and a string", and it has in fact been implemented using homing pigeons.) A computer called a gateway (later changed to router to avoid confusion with other types of gateway) is provided with an interface to each network, and forwards packets back and forth between them.

The idea was worked out in more detailed form by Cerf's networking research group at Stanford from 1973-74. (The early networking work at Xerox PARC, which produced the PARC Universal Packet protocol suite, much of which was contemporaneous, was also a significant technical influence; people moved between the two.)

DARPA then contracted with BBN Technologies, Stanford University, and the University College London to develop operational versions of the protocol on different hardware platforms. Four versions were developed: TCP v1, TCP v2, a split into TCP v3 and IP v3 in the spring of 1978, and then stability with TCP/IP v4 the standard protocol still in use on the Internet today.

In 1975, a two-network TCP/IP communications test was performed between Stanford and University College London (UCL). In November, 1977, a three-network TCP/IP test was conducted between the U.S., UK, and Norway. Between 1978 and 1983, several other TCP/IP prototypes were developed at multiple research centers. A full switchover to TCP/IP on the ARPANET took place January 1, 1983.

In March 1982,the US Department of Defense made TCP/IP the standard for all military computer networking. In 1985, the Internet Architecture Board held a three day workshop on TCP/IP for the computer industry, attended by 250 vendor representatives, helping popularize the protocol and leading to its increasing commercial use. On November 9, 2005 Kahn and Cerf were presented with the Presidential Medal of Freedom for their contribution to American culture.

Transmission Control Protocol

The Transmission Control Protocol (TCP) is one of the core protocols of the Internet protocol suite, often simply referred to as TCP/IP. Using TCP, applications on networked hosts can create connections to one another, over which they can exchange streams of data using Stream Sockets. The protocol guarantees reliable and in-order delivery of data from sender to receiver. TCP also distinguishes data for multiple connections by concurrent applications (e.g., Web server and e-mail server) running on the same host.

TCP supports many of the Internet's most popular application protocols and resulting applications, including the World Wide Web, e-mail and Secure Shell.

In the Internet protocol suite, TCP is the intermediate layer between the Internet Protocol (IP) below it, and an application above it. Applications often need reliable pipe-like connections to each other, whereas the Internet Protocol does not provide such streams, but rather only best effort delivery (i.e., unreliable packets). TCP does the task of the transport layer in the simplified OSI model of computer networks. The other main transport-level Internet protocol is UDP.

Applications send streams of octets (8-bit bytes) to TCP for delivery through the network, and TCP divides the byte stream into appropriately sized segments (usually delineated by the maximum transmission unit (MTU) size of the data link layer of the network to which the computer is attached). TCP then passes the resulting packets to the Internet Protocol, for delivery through a network to the TCP module of the entity at the other end. TCP checks to make sure that no packets are lost by giving each packet a sequence number, which is also used to make sure that the data are delivered to the entity at the other end in the correct order. The TCP module at the far end sends back an acknowledgement for packets which have been successfully received; a timer at the sending TCP will cause a timeout if an acknowledgement is not received within a reasonable round-trip time (or RTT), and the (presumably lost) data will then be re-transmitted. The TCP checks that no bytes are damaged by using a checksum; one is computed at the sender for each block of data before it is sent, and checked at the receiver.

Internet Protocol

At the lowest level is IP (Internet Protocol), which defines the datagrams or packets that carry blocks of data from one node to another. The vast majority of today's Internet uses version four of the IP protocol (i.e. IPv4), and although IPv6 is standardised, it exists only as "islands" of connectivity, and there are many ISPs who don't have any IPv6 connectivity at all.

Next come TCP (Transmission Control Protocol) and UDP (User Datagram Protocol) - the protocols by which one host sends data to another. The former makes a virtual 'connection', which gives some level of guarantee of reliability. The latter is a best-effort, connectionless transport, in which data packets that are lost in transit will not be re-sent.

On top comes the application protocol. This defines the specific messages and data formats sent and understood by the applications running at each end of the communication.

Unlike older communications systems, the Internet protocol suite was designed to be independent of the underlying physical medium. Any communications network, wired or wireless, that can carry two-way digital data can carry Internet traffic. Thus, Internet packets flow through wired networks like copper wire, coaxial cable, and fibre optic, and through wireless networks like Wi-Fi. Together, all these networks, sharing the same protocols, form the Internet.

The Internet protocols originate from discussions within the Internet Engineering Task Force (IETF) and its working groups, which are open to public participation and review. These committees produce documents that are known as Request for Comments documents (RFCs). Some RFCs are raised to the status of Internet Standard by the IETF process.

Some of the most-used application protocols in the Internet protocol suite are DNS, POP3, IMAP, SMTP, HTTP, HTTPS and FTP.

All services on the Internet make use of defined application protocols. Of these, e-mail and the World Wide Web are among the best known, and other services are built upon these, such as mailing lists and blogs. There are many others that are necessary 'behind the scenes' and yet others that serve specialised requirements.

Some application protocols were not created out of the IETF process, but initially as part of proprietary commercial or private experimental systems. They became much more widely used and have now become de facto or actual standards in their own right. Examples of these include IRC chat rooms, and various instant messaging and peer-to-peer file sharing protocols.

Today, most commercial operating systems include and install the TCP/IP stack by default. For most users, there is no need to look for implementations. TCP/IP is included in all commercial Unix systems, Mac OS X, and all free-software Unix-like systems such as Linux distributions and BSD systems, as well as Microsoft Windows. Unique implementations include Lightweight TCP/IP, an open source stack designed for embedded systems and KA9Q NOS, a stack and associated protocols for amateur packet radio systems and personal computers connected via serial lines.

Reference: Wikipedia
This text is available under the terms GNU Free Documentation License.