HP X.25/9000 Programmer's Guide

Chapter 3 45

Establishing and Terminating a Socket Connection

Using Permanent Virtual Circuits

Permanent virtual circuits (PVCs) are provided by a network provider

and are set up during X.25 conﬁguration. Refer to your X.25/9000 User’s

Guide for details on conﬁguring PVCs.

A PVC is similar to a “leased line” in that it is always connected and

terminates at a single destination. By contrast, an SVC can terminate at

different destinations, depending on the call. A PVC can be used by only

one socket at a time.

Although a PVC can be RESET, so that all data on the connection is

discarded, it cannot be cleared like an SVC. A PVC is connected as long

as X.25 level 3 is active.

A socket is bound to a PVC with the ioctl(X25_SETUP_PVC) call. Unlike

setting up an SVC, the listen(), connect(), and accept() system calls

are not used to establish a connection over a PVC. The close() call

removes the binding between a socket and the circuit of a PVC, and also

sends a RESET REQUEST packet out on the circuit. Data is cleared

from the circuit, but the circuit is not destroyed.

The ioctl() calls associated with sending and receiving CALL

REQUEST and CALL ACCEPTED packets and the ﬁelds within them,

such as ioctl(X25_RD_USER_DATA), ioctl(X25_WR_USER_DATA),

ioctl(X25_WR_FACILITIES), and ioctl(X25_RD_FACILITIES)have no

effect on a PVC.

Preparing a PVC for Use

A PVC is connected from the moment the programmatic access interface

is active. However, until a socket is bound to the PVC, X.25 discards all

packets received over the PVC and a RESET REQUEST packet is sent.

There is no implicit client/server relationship in establishing a PVC

connection. Actions taken on either end of a PVC are essentially the

same.

The ioctl(X25_SETUP_PVC) call binds a socket to a PVC. The

application ﬁrst creates the socket, then calls ioctl(X25_SETUP_PVC).

The logical channel identiﬁer (lci) of the PVC (which you can get from

your system administrator) must be known before the

ioctl(X25_SETUP_PVC) is issued.

36960-90061.bk Page 45 Friday, November 10, 2000 3:42 PM