User`s guide

4: Application Interface
DC 900-1516D 75
4.3 DLI Session Interface
Protogate protocols designed for use on ICP2432 boards use a session-based method of
communicating between the client application program and the protocol software on
the ICP. This method of communication allows greater flexibility in connecting TCP/IP
sockets to individual ICP ports for the Freeway line of servers. Protogate’s Data Link
Interface (DLI) library uses this session-based interface on both the Freeway server and
embedded ICP boards. If you have previously used Simpact protocols on older ICP
boards, you will find that the session-based interface differs somewhat from the older
interface. As a rule, protocol image files that begin with
fw use the DLI session interface.
Inside a Freeway server, a program called
msgmux manages protocol sessions for all
applications. When you use Protogate’s standard device driver with a session-based pro-
tocol image, your VMS program must take over these session management functions as
described in the following subsections.
4.3.1 DLI Session Basics
A session is made up of a logical connection to an ICP protocol. A session simply defines
a single connection or “service” to an ICP protocol. A session is started by “attaching”
to an ICP port number using a specific access mode. Sessions have different access
modes depending on the protocol. Consult your protocol programmer’s guide for
details.
4.3.2 Use Of Node Numbers (DLI)
When using DLI sessions, all writes to the ICP are performed on nodes 1 and 2. All reads
are performed on nodes 3 to 126. When using multiple programs to access the same
ICP, different read nodes are used to direct packets to the correct program. The follow-
ing subsections describe the node numbers in more detail.