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McGraw-Hill Dictionary of Scientific & Technical Terms, 6E, Copyright © 2003 by The McGraw-Hill Companies, Inc.


Transmission Control Protocol over Internet Protocol.

The de facto standard Ethernet protocols incorporated into 4.2BSD Unix. TCP/IP was developed by DARPA for internetworking and encompasses both network layer and transport layer protocols. While TCP and IP specify two protocols at specific protocol layers, TCP/IP is often used to refer to the entire DoD protocol suite based upon these, including telnet, FTP, UDP and RDP.

See also ICMP, SMTP, SNMP.
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(Transmission Control Protocol/Internet Protocol) The most widely used communications protocol. TCP/IP prepares and forwards data packets over a network such as Ethernet. Developed in the 1970s under contract from the U.S. Department of Defense, TCP/IP was invented by Vinton Cerf and Bob Kahn. This de facto Unix standard is the protocol of the Internet and the global standard for local and wide area networks, the major exception being the traditional networks of the telephone companies. However, telephone companies that deploy voice over IP (VoIP) networks are, in fact, using TCP/IP as well (see VoIP).

TCP/IP is commonly referred to as just "IP," which is the network layer of the protocol (see illustration below); thus, the terms "TCP/IP network" and "IP network" are synonymous.

Reliable and Unreliable Modes of Delivery
The TCP/IP suite provides two transport methods. TCP ensures that data arrive intact and complete, while UDP just transmits packets. TCP is used for data that must arrive in perfect form, and UDP is used for real-time applications such as voice over IP (VoIP) and video calling, where there is no time to retransmit erroneous or dropped packets.

IP Makes It Routable
TCP/IP is a routable protocol, and the IP network layer in TCP/IP provides this capability. The header prefixed to an IP packet contains not only source and destination addresses of the host computers, but source and destination addresses of the networks they reside in. Data transmitted using TCP/IP can be sent to multiple networks within an organization or around the globe via the Internet, the world's largest TCP/IP network.

The IP Address Identifies Everything
Every node in a TCP/IP network requires an IP address (an "IP") which is either permanently assigned or dynamically assigned (see IP address and DHCP). For an explanation of how the various layers in TCP/IP work, see TCP/IP abc's and OSI model. For a conceptual picture, see communications protocol. See protocol stack, TCP/IP port, DNS and IP on Everything.

The TCP/IP Stack
The TCP or UDP transport layer 4 sends packets to IP network layer 3, which adds its own header and delivers a "datagram" to a data link layer 2 protocol such as Ethernet, ATM or SONET. See datagram.
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