Development of the 4-layer TCP/IP model started before work began on the 7-layer OSI model. As it turned out, TCP/IP had too much momentum to be overtaken by the OSI model or any of the other competing network models. The TCP/IP model is now the dominant protocol suite that is used on today’s Internet. The TCP/IP model is also referred to as the TCP/IP protocol suite, the Internet protocol suite, the TCP/IP stack, and the DoD model.
Development of the OSI model began in the late 1970s, and the model was published in 1984. Although the OSI model was never actually implemented, the theory that it embodies has influenced the continual development and maturation of TCP/IP. TCP/IP was developed in a rather ad hoc fashion. The timeline includes the following:
- ARPANET, the precursor to the Internet, was developed in the late 1960s.
- In May of 1974, Kahn and Cerf published a paper titled “A Protocol for Packet Network Intercommunication,” which described the early ideas of what would become the TCP/IP model.
- In March of 1982, the US Department of Defense declared TCP/IP as the standard for all military computer networking.
- On January 1, 1983 (known as “Flag Day”), the ARPANET switched from the old networking protocol, NCP, to TCP/IP.
TCP/IP includes not only TCP and IP, but also specifications for other protocols, such as UDP, ICMP, and so on. It also includes common applications such as electronic mail, terminal emulation, and file transfer. TCP/IP embodies every aspect of modern network communications that use IP at the network layer. TCP operates at the transport layer of the OSI and TCP/IP models and is responsible for making sure that the data that the source device sends arrives at its destination. IP operates at the network layer of the OSI model (Internet layer of the TCP/IP model) and is responsible for the transmission of data. It does not do any error correction itself. The figure below provides a comparison of the OSI and TCP/IP models.
In the late 1970s through the 1980s, no networking protocol was dominant. There were many, including IBM SNA, DEC DECnet, Apple AppleTalk, Banyan Vines, and Novell IPX. Due to many historical circumstances and compared to all the competitors, TCP/IP has gained momentum, and has become the de facto standard in the industry.
Take a closer look at the four layers of the TCP/IP model:
- Link layer: This layer is also known as the network access layer and is the equivalent of both the physical and data link layers of the OSI model. It deals with components such as cables, connectors, and network cards, like OSI Layer 1. Like Layer 2 of the OSI model, the link layer of the TCP/IP model is concerned with hardware addresses.
- Internet layer: This layer aligns directly with Layer 3 of the OSI model. You may also know this layer as the network layer. It routes data from the source to the destination by defining the packet and the addressing scheme, moving data between the link and transport layers, routing packets of data to remote hosts, and performing fragmentation and reassembly of data packets. The Internet layer is where IP operates.
- Transport layer: This layer is directly aligned with Layer 4 of the OSI model: It is the core of the TCP/IP architecture. It is the layer where TCP and UDP operate. This layer provides communication services directly to the application processes that are running on network hosts.
- Application layer: This layer corresponds to Layers 5, 6, and 7 of the OSI model. It provides applications for file transfer, network troubleshooting, and Internet activities. It also supports network APIs, which allow programs that have been created for a particular operating system to access the network.
Variants of both the TCP/IP model and IP itself have been developed. IPv4 was the basis of the DoD standard in 1982. Later IPv6 was developed to deal with many of the shortcomings of IPv4. The industry is currently working through a long transition period between IPv4 and IPv6.
The example in the figure below illustrates encapsulation and de-encapsulation in light of the TCP/IP model. It shows what happens when you use a web browser to go to a site on the Internet.
Your web browser is an application that operates at the application layer. After you enter an address in the address bar, the browser passes data (an HTTP “GET” request) to the application layer. When the application layer passes the data to the transport layer, the transport layer may split the data into segments (if the amount of data is deemed large enough). The transport layer adds a TCP header to the segment, encapsulating it in TCP. If there are multiple segments, TCP sequences them so the data stream can be reassembled when it reaches its destination. The segment is then passed to the Internet layer, where it receives an IP header to encapsulate it as an IP packet. The IP header contains source and destination IP addresses, which will enable the data to be properly routed to the destination. The Internet layer may also break a large packet into smaller fragments, then the fragments are reassembled at the Internet layer at the destination system. When the IP packet reaches the link layer, it is encapsulated in an Ethernet frame, which contains the hardware, or MAC, addresses of the source and destination computers. The frame is then transmitted in the form of bits onto the physical network.