HP X.25/9000 Programmer's Guide

96 Chapter6
Extended Features
Using Fast Select
specified in separate ioctl() calls or combined into one, but the protocol
ID must come first. Remember that the protocol ID length is not defined
by the CCITT. Developers of the two processes must agree on the number
of bits of the call user data field to use for the protocol ID before
connection establishment begins.
When the facilities and call user data fields have been specified for the
fast select call, the calling side issues a connection request with
connect() and waits for the response from the remote server.
Fast Select on the Called Side
The called side may respond in three ways:
• If fast select calls are not supported, the called side will return a
CLEAR REQUEST packet with no data.
• If fast select calls are supported, the called side may execute an
ioctl(X25_WR_USER_DATA) call to send data back to the calling side,
then return a CLEAR REQUEST packet with data by executing a
close() call. The called side can also return a call accept (with data)
by executing ioctl(X25_SEND_CALL_ACEPT). The called side may
return clear user data or call user data to the calling side only if the
called side is controlling call acceptance with
ioctl(X25_CALL_ACPT_APPROVAL).
• If fast select was indicated in the facilities field with no restrictions
on response, the called side may simply accept the call and read the
call user data with ioctl(X25_RD_USER_DATA). The connection is
then considered an ordinary virtual circuit (that is, fast select is no
longer in effect).
The maximum length for the user data field in the CLEAR REQUEST
packet is 128 bytes. The server may execute the
ioctl(X25_WR_USER_DATA) more than once to write the entire user data
field. If more than 128 bytes must be transmitted, the called side (server)
of the connection should respond with a CALL ACCEPTED packet and
transmit the entire response with normal DATA packets or else clear the
connection completely by calling close().
Details for fast select are defined in the CCITT X.25 Recommendations,
and X.25 configuration information is provided in your X.25/9000 User’s
Guide.
36960-90061.bk Page 96 Friday, November 10, 2000 3:42 PM