92. TCP/IP

TCP/IP is a name given to the collection (or suite) of networking protocols that have been used to construct the global Internet. The protocols are also referred to as the DoD (dee-oh-dee) or Arpanet protocol suite because their early development was funded by the Advanced Research Projects Agency (ARPA) of the US Department of Defense (DoD). The TCP/IP name is taken from two of the fundamental protocols in the collection, IP and TCP. Other core protocols in the suite are UDP and ICMP. These protocols work together to provide a basic networking framework that is used by many different application protocols, each tuned to achieving a particular goal. TCP/IP protocols are not used only on the Internet. They are also widely used to build private networks, called internets (spelled with a small 'i'), that may or may not be connected to the global Internet (spelled with a capital 'I'). An internet that is used exclusively by one organization is sometimes called an intranet. All of the protocols in the TCP/IP suite are defined by documents called Requests for Comments (RFC's). An important difference between TCP/IP RFC's and other (say, IEEE or ITU) networking standards is that RFC's are freely available online.

RFC's can be composed and submitted for approval by anyone. Standards RFC's are often the product of many weeks or months of discussion between interested parties designated as working groups, during which time drafts of the proposed RFC are continually updated and made available for comment. These discussions typically take place on open mailing lists which welcome input from all quarters. The RFC approval process is managed by the Internet Engineering Steering Group (IESG) based on recommendations from the Internet Engineering Task Force (IETF) which is a prime mover in the formation of working groups focused on strategic TCP/IP issues. You can find out more about IESG and IETF activities from the IETF home page. Not all RFC's specify TCP/IP standards. Some RFC's contain background information, some provide hints for managing an internet, and some document protocol weaknesses in the hope that they might be addressed by future standards and some are entirely humorous.

Transmission Control Protocol (TCP) provides a reliable byte-stream transfer service between two endpoints on an internet. TCP depends on IP to move packets around the network on its behalf. IP is inherently unreliable, so TCP protects against data loss, data corruption, packet reordering and data duplication by adding checksums and sequence numbers to transmitted data and, on the receiving side, sending back packets that acknowledge the receipt of data. Before sending data across the network, TCP establishes a connection with the destination via an exchange of management packets. The connection is destroyed, again via an exchange of management packets, when the application that was using TCP indicates that no more data will be transferred. In OSI terms, TCP is a Connection-Oriented Acknowledged Transport protocol.

Internet Protocol (IP) is the central, unifying protocol in the TCP/IP suite. It provides the basic delivery mechanism for packets of data sent between all systems on an internet, regardless of whether the systems are in the same room or on opposite sides of the world. All other protocols in the TCP/IP suite depend on IP to carry out the fundamental function of moving packets across the internet. In terms of the OSI networking model, IP provides a Connectionless Unacknowledged Network Service, which means that its attitude to data packets can be characterized as "send and forget". IP does not guarantee to actually deliver the data to the destination, nor does it guarantee that the data will be delivered undamaged, nor does it guarantee that data packets will be delivered to the destination in the order in which they were sent by the source, nor does it guarantee that only one copy of the data will be delivered to the destination.