Operation

The X.25 NPSI server uses NPSI to set up X.25 virtual circuits as needed to carry traffic to and from remote X.25 equipment. The three main functional areas shown in Figure 1 are:

TCP/IP interface
NPSI interface
IP/X.25 address mapping

IP datagrams are transferred between TCP/IP and the X.25 NPSI server on a DLC path established when a TCPIP X25NPSI device is started. The transfer protocol is similar to that used with SNALINK, with the addition of a first-hop IP address passed by TCP/IP from the relevant Start of change BEGINROUTES statement End of change . The X.25 NPSI server uses the first hop IP address to look up an X.25 address in its destination table.

Communication with NPSI is by way of several SNA sessions. One control session is established at initialization for each MCH LU defined in a LINK statement in the X.25 NPSI server configuration data set. Commands to establish and terminate X.25 virtual circuit connections pass between the X.25 NPSI server and NPSI on the control session. See X.25 NPSI Host Programming for details of the control commands. As new virtual circuits are established, NPSI initiates new SNA sessions with the X.25 NPSI server application by means of VTAM® LOGON. IP datagrams are then exchanged with the remote equipment over the VC session until an idle timeout occurs or the VC is taken for another destination.

IP addresses are mapped to X.25 addresses by table lookup, or in the case of the DDN network, by a calculation described in RFC 1236. The X.25 NPSI server performs the lookup with the first-hop IP address on each datagram it receives from TCP/IP. The LINK and DEST entries defined in the X.25 NPSI server configuration data set are scanned in order from top to bottom to find a DEST with a matching IP address. After the DEST is found, the link it applies to is selected to carry the datagram, and the active virtual circuits on that link are scanned to find one with an X.25 address that matches the DEST. If such a VC is found, the datagram is queued for transmission on that VC; if none is found and there is a free VC, a new X.25 call is initiated; if all VCs on the link are in use, the least recently used connection is cleared, as long as it has been open for at least the minimum open time, and a new call is initiated. If no VC matches these conditions, the datagram is discarded.