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Related Documentation

  • Understanding SIP ALG

Understanding Scaling Busy Lamp Field Support for the UDP-Based SIP ALG

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.

When a phone is registered to the IP PBX, the IP PBX notifies the phone of the state of the resource list. For example, if the resource list is huge, the body of the NOTIFY message will also be huge. Because the SIP ALG supports only 3000-byte SIP messages, it bypasses the huge NOTIFY message. If there are too many instances of BLF in the message body, the payload will not be changed and the gate will not be opened.

Starting with Junos OS Release 12.3X48-D15, the SIP ALG supports 65,000-byte SIP messages on the UDP protocol. In the scaling BLF application, if every instance is around 500 bytes, the SIP ALG supports 100 instances in one SIP UDP message.

BLF support for the UDP-based SIP ALG includes the following features:

  • The device can send and receive 65,000-byte SIP messages.
  • The SIP ALG can parse the 65,000-byte SIP messages and open the pinhole, if required.
  • The SIP ALG regenerates the new jumbo SIP message if NAT is configured and the payload is changed.

Related Documentation

  • Understanding SIP ALG

Modified: 2016-05-01