SIP OPTIONS
OPTIONS is the SIP method used to query the capabilities of a remote endpoint — or, more commonly, as a keepalive 'ping' to check whether a SIP peer is still alive. It is the standard way carriers and PBXs probe each other for liveness.
Common usage: keepalive
Asterisk pjsip qualify_frequency=30 sends an OPTIONS every 30 seconds. If no 200 OK arrives within qualify_timeout seconds, the AOR is marked unreachable. Same pattern in Kamailio (dispatcher module), FreeSWITCH (ping param), and most carrier SBCs.
Capabilities query
The receiving endpoint replies with 200 OK and headers like:
Allow: INVITE, ACK, BYE, CANCEL, OPTIONS, MESSAGE, INFO, REGISTER, NOTIFY, SUBSCRIBE, REFER Accept: application/sdp Supported: replaces, 100rel, timer
This tells the caller which methods, content-types, and SIP extensions are supported.
Don't sleep on OPTIONS
Carriers monitor OPTIONS reachability for failover and reporting. If your PBX stops responding to OPTIONS (e.g. firewall drops them), the carrier may stop routing inbound traffic to you within minutes. Make sure SIP OPTIONS is not silently filtered.
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