X.25 Link Configuration for TCP/IP

Several X.25 link configuration parameters are important when transporting IP data.

Local X.25 Address

NetcomRouter uses the Calling Address field of an X.25 Incoming Call packet it receives to look up the matching IP address in the table of X.25 and IP addresses (/etc/x25hosts). This is done for security reasons. While anyone can configure their system to act as an IP address that you communicate with, the X.25 addresses are selected by the network provider. Attempts to place a fake X.25 address in a call request result in the call being cleared by the network.

Normally, public packet-switched networks fill in the calling X.25 address (the address of the system originating the call) for you. However, if NetcomRouter is used with point-to-point X.25 connections, such as in a testing lab or direct dial environment, or if the X.25 network does not add the correct calling address to outgoing calls, then a few options are available:

You can configure the X.25 link to supply your own calling address in outgoing X.25 calls. To do this, use the dna field of the link configuration file. For example:
```
 
dna 12345678
 
```
If you are connecting point-to-point, the number can be any arbitrary value, as long as the other system you are testing with also has this address as part of its address equivalence table (/etc/x25hosts).
Tell the software to ignore the X.25 calling address and obtain the IP address from the incoming data packets. This option is available by setting default entries in the /etc/x25hosts as described in Identifying Hosts That Call You.
There is an algorithm for calculating X.25 addresses from IP addresses. This is generally used on military (DDN) networks and can be turned on using the xinetconf value xinetDDNmode. Using this algorithm, the driver will automatically determine the X.25 address for outgoing calls. Unfortunately, the algorithm does not provide a one-to-one mapping. This means that the IP address of the system placing a call to you cannot be determined from the calling X.25 address. Rather, the IP address is determined by inspecting the first IP datagram on the virtual circuit.

Operation with Dial-up X.25 (X.32)

If NetcomRouter is to place a telephone network Dial-up call to obtain an X.25 link, you need to configure the X.25 link for Dial-up operation. Dial-up operation is not really different than that of other X.25 traffic. The X.25 driver provides a pool of virtual circuits, some of which may be used for the purposes of delivering TCP/IP data. X.32 is configured with the dial_defn field of the link configuration file. See the description of X.32 configuration in the Administrator's Guide for detailed field format information.

Also, keep in mind that, as for other X.32 operation, T21, is used to time out the dialing process. In cases where multiple numbers are tried until success, the T21 value must be large enough to allow for these retries.

Revision 1.1 (January 2002)