SIP ALGs: Scaling busy lamp field (BLF) support

Busy lamp field (BLF) is a light on an IP phone that indicates whether another extension connected to the same private branch exchange (PBX) is busy or not. You can manually configure the BLF by using a Web interface. When BLF is configured, the phone subscribes to a resource list available on the IP PBX to be notified of status information for other extensions. BLF works through the Session Initiation Protocol (SIP) and uses the SUBSCRIBE and NOTIFY messages. Usually, the phone is the subscriber and the IP PBX is the notifier.

The SIP ALG supports 65,000-byte SIP messages on the UDP protocol.