12.7.3 Fast Select User Data

The fast select facility is handled in the following way.

12.7.3.1 Calling Side

To send fast select data, fast_select_type must be set to the proper value (with the X25_SET_FACILITY ioctl) before connect is called (see the section "12.7.1 Facility Specification and Negotiation"of this chapter for more information). Using the CONN_DB structure, a calling DTE can specify a user data field up to 102 bytes (including the optional protocol identifier). If 102 bytes of call user data are not enough for the current fast select message, use the X25_WR_USER_DATA ioctl before calling connect to pass the additional user data. The user data specified in connect will precede this additional user data. To write user data:

typedef struct user_data_db_s {
 u_char      datalen;
 u_char       data[MAX_USER_DATA];
 } USER_DATA_DB;
 int s, error;
 USER_DATA_DB user_data;
 error = ioctl(s, X25_WR_USER_DATA, &user_data)

Here, MAX_USER_DATA is 124.

If connect returns -1 and errno is EFASTDATA, the remote side has cleared the call by sending a Clear Indication packet with up to 32 bytes (1980) or 128 bytes (1984) of user data. At this time, the user can read the user data in the Clear Indication packet with calls to the X25_RD_USER_DATA ioctl until the returned datalen in USER_DATA_DB structure is 0 or less than MAX_USER_DATA, then close the socket with close.

To read user data:

USER_DATA_DB user_data;
 int s, error;
 error = ioctl(s, X25_RD_USER_DATA, &user_data);

If connect returns 0, it indicates that the connection has been set up successfully. If the connection is over an interface that supports 1984 X.25, the remote user may have sent user data in the Call Accepted packet. (This will happen only if the initiator of the connection has specified fast select with no restriction on response.) Thus the initiating user must repeatedly read any user data using the X25_RD_USER_DATA ioctl until the returned length in the USER_DATA_DB structure is less than MAX_USER_DATA.

When a call is cleared after being connected, the Clear Indication packet may contain user data if the interface supports 1984 X.25 and fast select is in effect for that call. Either the initiator of the connection or the responder can send user data in the Clear Request packet. Thus when a call with fast select is cleared by the remote user, user data must be read in the same way as for the other cases.

For 1980 X.25 interfaces, if the connection was accepted by the remote user, the Call Accepted and Clear Request packets will not have any user data; the only time that the Clear Request can have user data is when a fast select call is cleared immediately (this is detectable by means of the EFASTDATA error return).

12.7.3.2 Called Side

To receive a fast select incoming call, the called side must specify either FAST_ACPT_CLR or FAST_CLR_ONLY as the value for fast_select_type using the X25_SET_FACILITY ioctl, before issuing the listen call.

If the Incoming Call has the fast select facility, it will be accepted only if the listener has specified fast select. The incoming call will also be accepted if it does not have the fast select facility and the listener has specified FAST_ACPT_CLR.

The call will be rejected if there are more than 16 bytes of user data, and the called side has either not specified the fast select facility at all, or has specified FAST_OFF (which is equivalent to not specifying fast select).

After accept returns, the called side may use the X25_GET_FACILITY ioctl to determine the type of fast select in effect. For example, if the called side has specified FAST_ACPT_CLR and the calling side has specified FAST_CLR_ONLY, after accept returns, the type of fast select in effect will be FAST_CLR_ONLY. If fast select is indicated, the called side can read the user data that was received in the Call Request by looking at the CONN_DB structure returned by accept. If more than 102 bytes of user data were received, the extra bytes can be read with the X25_RD_USER_DATA ioctl.

The X25_WR_USER_DATA ioctl can be used to specify user data to be sent back in the response to the fast select Call Request. To write more than MAX_USER_DATA bytes of user data, a second X25_WR_USER_DATA ioctl can be used to append the additional data after that from the first X25_WR_USER_DATA ioctl (total length of all user data may not exceed 128 bytes).

If the type of fast select in effect is FAST_CLR_ONLY, the called side can only clear the fast select call by closing the socket (which causes the user data specified by X25_WR_USER_DATA to be sent in the Clear Request). If the type of fast select in effect after accept returns is FAST_ACPT_CLR, the called side has the option, after writing the reply message with the X25_WR_USER_DATA ioctl, of either sending a Clear Request packet with close or sending a Call Accepted packet with the X25_SEND_CALL_ACPT ioctl and thereby entering the normal data transfer state.

int news, error;
 error = ioctl(news, X25_SEND_CALL_ACPT);

When the value in effect is FAST_CLR_ONLY, the called side can only close the socket with the close system call after writing the reply message.

FAST_OFF is the type of fast select that will be in effect when the listener has specified FAST_ACPT_CLR and the incoming call does not have the fast select facility. Even in this case, the listener must use the X25_SEND_CALL_ACPT ioctl to put the connection into normal data transfer state.

In the current release (and not in SunNet X.25 7.0), the listen socket should not be closed until after the incoming fast select call has been either cleared (with close) or accepted (with X25_SEND_CALL_ACPT).