Sierra Wireless DART 200 CDPD Modem manual Slip Multicast Address Selection, TCP PAD Keep-Alive

DART 200 CDPD Modem User’s Guide F DART AT Command Set

Table F-4. Commands beginning with asterisk (*) (cont.)

*G	SLIP Multicast Address Selection
	Permits the user to specify whether the multicast NEI or the modem’s active point-to-point NEI is
	passed, as the multicast data packet’s destination address, across the SLIP interface to the user’s
	TCP/IP stack in the MAS. This capability is needed to support certain TCP/IP stacks that can only
	receive non-broadcast packets sent to a specific IP destination address, such as the point-to-point NEI.
	Format:
	AT*G0	Pass received multicast packets across the SLIP interface to the TCP/IP stack
		unchanged.
	AT*G1	Replace the IP destination address in the received multicast packets with the
		active point-to-point NEI before passing them across the SLIP interface to the
		TCP/IP stack (default).
	Result codes:
	OK	Valid argument
	ERROR	Invalid argument
*K	TCP PAD Keep-Alive
	Provides a facility to allow modems supporting TCP applications to time out, or keep-alive a TCP
	session with no current activity.

This capability can work in transmit only mode, receive only mode or in both modes. It makes use of a keep-alive character (S register 85) and a keep-alive time-out value (S register 86) to implement the function. The timer is restarted whenever any data is sent or received. The default time-out value is 120 minutes.

If the timer expires in transmit or transmit/receive mode pending data or the keep-alive character is forwarded. For a half open connection, this transmission eventually causes the connection to close because the other end is not able to provide a TCP acknowledgment for receipt of the keep-alive data packet.

If the timer expires in receive or transmit/receive mode the modem switches to command mode and an ATH command is issued to close the connection. While in receive mode incoming keep-alive packets are discarded only if the incoming keep-alive character matches the value in S-Register 85.

For one way keep-alives (transmit only at one end and receive only at the other) set the transmit time- out value as less than the receive time-out value to ensure proper operation. For keep-alives in both directions (transmit/receive) set the time-out values at each end as equal. The keep-alive algorithm has a small amount of hysteresis built in to cause the transmit time-out to occur ahead of the receive time- out in order to maintain the link.

If there is not a DART on both ends of a session using the keep-alive function, the application at the host computer end must handle keep-alive character generation and/or discarding.

This facility is primarily used with remote modems in telemetry applications to allow them to time out if the host side of the session terminates abnormally (crashes). Without this facility, the DART ends up with a half-open TCP connection requiring a trip to the remote location to reset the modem with a power cycle. The setting for this is *K2 with S86 set to a value in the 3 to 5 minute range.

Sierra Wireless, Inc. F-20