TCP/IP is based on a four-layer reference model. All protocols that belong to the TCP/IP protocol suite are located in the top three layers of this model.
As shown in the following illustration, each layer of the TCP/IP model corresponds to one or more layers of the seven-layer Open Systems Interconnection (OSI) reference model proposed by the International Standards Organization (ISO).
The types of services performed and protocols used at each layer within the TCP/IP model are described in more detail in the following table.
| Layer | Description | Protocols |
|---|---|---|
| Application | Defines TCP/IP application protocols and how host programs interface with transport layer services to use the network. | HTTP, Telnet, FTP, TFTP, SNMP, DNS, SMTP, X Windows, other application protocols |
| Transport | Provides communication session management between host computers. Defines the level of service and status of the connection used when transporting data. | TCP, UDP, RTP |
| Internet | Packages data into IP datagrams, which contain source and destination address information that is used to forward the datagrams between hosts and across networks. Performs routing of IP datagrams. | IP, ICMP, ARP, RARP |
| Network interface | Specifies details of how data is physically sent through the network, including how bits are electrically signaled by hardware devices that interface directly with a network medium, such as coaxial cable, optical fiber, or twisted-pair copper wire. | Ethernet, Token Ring, FDDI, X.25, Frame Relay, RS-232, v.35 |
For more information about ARP, IP, ICMP, IGMP, UDP, and TCP, see Understanding TCP/IP.
Note
The OSI reference model is not specific to TCP/IP. It was developed by the ISO in the late 1970s as a framework for describing all functions required of an open interconnected network. It is a widely known and accepted reference model in the data communications field and is used here only for comparison purposes.
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