Communications Server

Network Administration Guide

Chapter 9. Planning for TN5250 Server

The TN5250 server function enables you to configure your network as shown in Figure 32 or Figure 33.

Figure 32. Communications Server Configured as a TN5250 Server with Wide Area SNA Network


Figure 33. Communications Server Configured as a TN5250 Server with Wide Area TCP/IP Network


The TN5250 server function supports:

Refer to Quick Beginnings for instructions on how to configure a TN5250 server.

Supported Client Workstations under TN5250 Server

TN5250 server supports any TN5250 client that is fully compliant with RFC 1205.


This section provides more information about some of the features you can configure for TN5250 server.

Changing the Default Port Number

When you configure the TN5250 server support, a default port is defined with port number 23, but other Telnet applications, such as the TN3270E server support, may also use this port. If other Telnet applications are running using port 23, you must use another port.

If you change the port number, avoid numbers that you know are used by other applications. If two applications use the same port number, one of the applications will fail.

If you change the port number, use a number greater than 1 024. Numbers less than 1 024 are reserved. For more information about reserved port numbers, see the following location on the Internet:

At the time of publication of this document, the most recent RFC for assigned numbers is RFC 1700.

Notify TN5250 client users when you change the port number, because they will have to configure their emulator applications to match.

Managing System Traffic

There are two ways to control how often unused connections are disconnected: keepalive processing and automatic logoff.

By default, TN5250 server does not use keepalive processing. If you use keepalive processing, you can choose either NOP or timing mark.

Timing mark processing causes more traffic on the system than NOP processing, but frees unused connections more quickly.

If you choose automatic logoff, the server disconnects any session that has no traffic for the specified period. Traffic from keepalive processing does not keep the connection open; data must be sent to or from the host.

If your client emulators are configured to do keepalive processing, you might want to turn it off at the server, and if keepalive processing is done at the server, you might want to turn it off at the client to reduce network traffic.

Specifying Access to AS/400s with Multiple Ports

Communications Server enables you to configure more than one AS/400, and enables you to configure more than one port for the TN5250 server to listen for incoming connections. When you configure a new TN5250 server port, you can specifiy the AS/400 to be associated with the port. Incoming TCP/IP connections received on that port by the TN5250 server will access the AS/400 associated with that port.

Notify TN5250 client users which port numbers to configure for their emulator applications to connect to specific AS/400s.

IP Filtering

Communications Server enables you to configure TN5250 filters to specifiy which TCP/IP clients can connect into the server. Clients can be specified using individual IP addresses, IP subnetworks, or TCP/IP host names or domain names.

You can also configure an AS/400 with a filter to specify a different AS/400 than the one associated with the port. This can be used to direct the clients associated with a filter to a specific AS/400.

More than one AS/400 can be specified in a filter. If a client connection attempt to the first AS/400 specified is unsuccessful, connection to other AS/400s specified in the filter are attempted.

Secure Sockets Layer (SSL) Support

When you configure a new TN5250 server port, you can specify that the port be used for secure connections. More than one port can be specified as secure.

To enable security, Communications Server provides a Key Management utility to generate the certificate and keys required by SSL. Refer to Chapter 10, Planning for Secure Sockets Layer-based Security for more information.

When you have configured and enabled security, TN5250 clients supporting SSL Version 3 can connect into a secure port number and establish secure connections.

Configuring the SNA Network

You need to configure your SNA network to allow the TN5250 server to connect to the AS/400s. Refer to Quick Beginnings for information on configuring your SNA network for the TN5250 server.

Load Balancing of TN5250 Servers

Communications Server also supports load balancing of TN5250 clients across multiple TN5250 servers. Refer to Chapter 11, Planning for Load Balancing for more information.

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