Ports

A port is used to differentiate among different applications using the same network interface. It is an additional qualifier used by the system software to get data to the correct application. Physically, a port is a 16-bit integer. Some ports are reserved for particular applications; they are labeled as well-known ports.

In the client/server model, the server provides a resource by listening for clients on a particular port. Some applications, such as FTP, SMTP, and Telnet, are standardized protocols and listen on a well-known port. Such standardized applications use the same port number on all TCP/IP hosts. For your client/server applications, however, you need a way to assign port numbers to represent the services you intend to provide. An easy way to define services and their ports is to enter them into data set hlq.ETC.SERVICES. In C, the programmer uses the getservbyname() function to determine the port for a particular service. Should the port number for a particular service change, only the hlq.ETC.SERVICES data set needs to be modified.

Note: Note that hlq is the high-level qualifier. z/OS® Communications Server ships with a default hlq of TCPIP. Use this default or override it using the DATASETPREFIX statement in the PROFILE.TCPIP and TCPIP.DATA configuration files. TCP/IP is shipped with data set hlq.ETC.SERVICES that contains the well-known services of FTP, SMTP, and Telnet. Data set hlq.ETC.SERVICES is described in the z/OS Communications Server: IP Configuration Reference.

A socket program in an IP host identifies itself to the underlying TCP/IP protocol layers by port number.

A port number is a 16-bit integer ranging from 0 to 65535. A port number uniquely identifies this application to the protocol underlying this TCP/IP host (TCP, UDP, or IP). Other applications in the TCP/IP network can contact this application by way of reference to the port number on this specific IP host.

Figure 1 shows the port concept.

Figure 1. The port concept

Diagram that shows two application processes with their specific port numbers although they share the same protocol, IP address, and hardware address.

Both server applications and client applications have port numbers. A server application uses a specific port number to uniquely identify this server application. The port number can be reserved to a particular server, so no other process ever uses it. In an IBM® TCP/IP Services environment, you can do this using the PORT statement in the hlq.PROFILE.TCP/IP configuration data set. When the server application initializes, it uses the bind() socket call to identify its port number. A client application must know the port number of a server application in order to contact it.

Because advance knowledge of the client's port number is not needed, a client often leaves it to TCP/IP to assign a free port number when the client issues the connect() socket call to connect to a server. Such a port number is called an ephemeral port number; this means it is a port number with a short life. The selected port number is assigned to the client for the duration of the connection, and is then made available to other processes. It is the responsibility of the TCP/IP software to ensure that a port number is assigned to only one process at a time.

Well-known official Internet port numbers are in the range of 0 - 255. See http://www.iana.org/assignments/multicast-addresses for details. In addition, port numbers in the range of 256 - 1023 are reserved for other well-known services. Port numbers in the range of 1024 - 65535, which are neither in-use nor reserved, are used when TCP/IP automatically assigns port numbers to client programs that do not use specific port numbers.

Figure 2 shows port number assignments.

Figure 2. Port number assignments

Port numbers 0 - 255 are assigned to official internet services, 256 - 1023 to other well-known services, and 1024 - 65535 to ephemeral ports.

Before you select a port number for your server application, consult the hlq.ETC.SERVICES data set. This data set is used to assign port numbers to server applications. The server application can use socket call getservbyname() to retrieve the port number assigned to a given server name. Add the names of your server applications to this data set and use socket call getservbyname(). With this technique, you avoid hard coding the port number into your server program. The client program must know the port number of the server on the server host. There is no socket call to obtain that information from the server host. To compensate, synchronize the contents of data sets ETC.SERVICES on all TCP/IP hosts in your network. Client application can then use the getservbyname() socket call to query its local ETC.SERVICES data set for the port number of the server. Use this technique to develop your own local well-known services.