INTERRUPT Packet

A application process sends an INTERRUPT packet to transmit out-of-band data to the remote process.

Figure A-5 INTERRUPT Packet

The access to the fields in the INTERRUPT packet is described below:

General Format Identifier

Indicates the packet sequence count (modulo 8). This field cannot be read or written by an X.25 application.

Logical Channel Identifier

Associates a logical channel number with the SVC. This field is controlled by the subsystem. It can be read with the ioctl(X25_RD_LCI).

Packet Type Identifier

Indicates the kind of packet. It cannot be read or written directly, but through system calls, the transmission and arrival can be effected.

To read this field on an incoming packet:recv(MSG_OOB) returns a buffer the second byte of which (buf[1]) con tains an indication that an INTERRUPT packet has arrived (OOB_INTERRUPT).

To write this field on an outgoing packet:Use the send(MSG_OOB) system call to transmit an INTERRUPT packet.

Interrupt User Data

Contains data associated with the INTERRUPT packet.

To read this field on an incoming packet:When the SIGURG signal handler is called, issue a recv() system call with the MSG_OOB flag set.

To write this field on an outgoing packet:Use the send() system call with the MSG_OOB flag set to transmit an INTER RUPT packet and specify the interrupt user data field.