TCP/IP protocols

TCP/IP is a communication protocol used between physically separated computer systems. TCP/IP can be implemented on a wide variety of physical networks.

TCP/IP is a large family of protocols that is named after its two most important members, Transmission Control Protocol and Internet Protocol. Figure 1 shows the TCP/IP protocols used by CICS® ONC RPC in terms of the layered Open Systems Interconnection (OSI) model. For CICS users, who may be more accustomed to SNA, Figure 1 shows the SNA layers that correspond very roughly to the OSI layers.
Figure 1. TCP/IP protocols compared to the OSI and SNA models
This diagram shows the 7 layers of the OSI protocol, : 1: Physical, 2: Data Link,3: Network, 4: Transport, 5: Session, 6: Presentation, 7: Application. These are compared with the SNA layers: Physical, Data Link, Path Control, Transmission, Data Flow, Presentation and Application, and the TCP/IP layers; 1 and 2: subnetwork, 3: IP, 4: TCP or UDP, 5: empty, 6: XDR, 7: RPC.
The protocols used by TCP/IP are shown in Figure 1.
Internet Protocol (IP)
In terms of the OSI model, IP is a network-layer protocol. It provides a connectionless data transmission service, and supports both TCP and UDP. Data is transmitted link by link; an end-to-end connection is never set up during the call. The unit of data transmission is the datagram.
Transmission Control Protocol (TCP)
In terms of the OSI model, TCP is a transport-layer protocol. It provides a connection-oriented data transmission service between applications, that is, a connection is established before data transmission begins. TCP has more error checking that UDP.
User Datagram Protocol (UDP)
UDP is also a transport-layer protocol and is an alternative to TCP. It provides a connectionless data transmission service between applications. UDP has less error checking than TCP. If UDP users want to be able to respond to errors, the communicating programs must establish their own protocol for error handling. With high-quality transmission networks, UDP errors are of little concern.
XDR and ONC RPC correspond to the sixth and seventh OSI layers.
Sockets interface
The interface between the fourth and higher layers is the sockets interface. In some TCP/IP implementations, the sockets interface is the API that customers use to write their higher-level applications.