The MVS™ socket APIs provide a standard interface to the transport and internetwork layer interfaces of TCP⁄IP. They support three socket types: stream, datagram, and raw. Stream and datagram sockets interface to the transport layer protocols, and raw sockets interface to the network layer protocols. All three socket types are described here for background purposes. While CICS® supports stream and datagram sockets, stream sockets provide the most reliable form of data transfer offered by TCP/IP.

Stream sockets transmit data between TCP/IP hosts that are already connected to one another. Data is transmitted in a continuous stream; in other words, there are no record length or newline character boundaries between data. Communicating processes ¹ must agree on a scheme to ensure that both client and server have received all data. One way of doing this is for the sending process to send the length of the data, followed by the data itself. The receiving process reads the length and then loops, accepting data until all of it has been transferred.

In TCP/IP terminology, the stream socket interface defines a reliable connection-oriented service. In this context, the word reliable means that data is sent without error or duplication and is received in the same order as it is sent. Flow control is built in to avoid data overruns.

The datagram socket interface defines a connectionless service. Datagrams are sent as independent packets. The service provides no guarantees; data can be lost or duplicated, and datagrams can arrive out of order. The size of a datagram is limited to the size that can be sent in a single transaction (currently the default is 8192 and the maximum is 65507). No disassembly and reassembly of packets is performed by TCP/IP.

The raw socket interface allows direct access to lower layer protocols, such as IP and Internet Control Message Protocol (ICMP). This interface is often used for testing new protocol implementations.