SIP Option Messages

SIP Option Messages can be used to dynamically track the state of a SIP trunk’s destination(s) by periodically sending Option Request Messages. A destination address is considered to be "out of service" if it fails to respond to an OPTIONS Request, if it sends a Service Unavailable (503) response or Request Timeout (408) response, or if a TCP or TLS connection cannot be established. When a destination is “out of service,” it will not be considered in the call routing logic, which can shorten call complete times. 

SIP Option Messages from the Peeredge Switches

The Peeredge SBC determines the availability of the customer’s SBCs/PBXs by periodically (typically every 60 seconds) sending SIP Option messages to the SBCs/PBXs. For SBC/PBX SIP Trunks direct to the Peeredge SBC, the Peeredge SBC will retransmit the Option message two more times if it fails to receive a valid response. If there is no response to all three consecutive SIP Option messages, the Peeredge SBC will consider the SIP Trunk group to be down. The Peeredge SBC will not forward SIP messages (except SIP Option messages) to a SIP Trunk group in a down status. Once the SIP Trunk group is considered down, two consecutive SIP 200OK responses to the SIP Option messages must be received to bring the SIP Trunk group back to an operational state. The Peeredge SBC does not check the source IP address for the SIP 200OK responses. For SBCs/PBXs with multiple IP addresses and multiple SIP Trunks, 46 Labs strongly recommends that the SBCs/PBXs be configured to use the same IP address in the SIP 200OK response that was sent in the SIP Option message.

SIP Option Messages to the Peeredge Switches

Sending SIP Option messages to the Peeredge SBC may reduce the call setup times for outbound calls in the event of certain partial service disruptions (i.e. a Peeredge SBC has failed, or a routing failure exists within the MPLS or Internet network). The Peeredge SBC will respond to SIP Option messages generated by the SBC/PBX(s). 46 Labs recommends the frequency of SIP Option messages to be set at 60 seconds or greater from each SBC/PBX and recommends the Max-Forwards header in the SIP Option message be set to a value of one. 

Recommendation

46 Labs recommends implementing SIP Option Messages between every customer SBC/PBX to each Peeredge SBC. We also recommend implementing SIP Option Messages between each Peeredge SBC to every customer SBC/PBX.