Architecting SIP OPTIONS Polling Strategies for Sub-Second Trunk Failure Detection

How-To-Articles
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Architecting SIP OPTIONS Polling Strategies for Sub-Second Trunk Failure Detection

What This Guide Covers

  • Architecting a high-frequency health monitoring strategy for SIP Trunks using SIP OPTIONS pings.
  • Implementing sub-second failure detection and automatic rerouting in Genesys Cloud BYOC.
  • Designing a resilient polling hierarchy that avoids false positives while maintaining maximum uptime.

Prerequisites, Roles & Licensing

  • Licensing: Genesys Cloud CX 1/2/3 with BYOC Cloud or BYOC Premise.
  • Infrastructure: A SIP-compliant Session Border Controller (SBC) or Carrier Gateway.
  • Permissions:
    • Telephony > Trunk > Add/Edit
    • Admin > Network > External IP Configuration

The Implementation Deep-Dive

1. The Strategy: The Heartbeat of Voice

Standard SIP Trunking often relies on TCP/TLS timeouts or 4xx/5xx responses to detect failures. However, if a carrier has a “Silent Failure” (black hole), your agents could be stuck on a broken trunk for 30+ seconds before the system gives up.

The Strategy:

  1. The Poller: The SBC (or Genesys Cloud) sends a SIP OPTIONS message to the peer every few seconds.
  2. The Timeout: If no 200 OK is received within a short threshold (e.g., 500ms), the trunk is marked as “Unstable.”
  3. The Threshold: If multiple pings fail in a row, the trunk is taken “Out of Service” (OOS).

2. Configuring sub-second Polling in Genesys Cloud

Genesys Cloud allows you to configure “Edge Health Check” settings for your trunks.

The Implementation:

  1. Navigate to Admin > Telephony > Trunks.
  2. Select your External SIP Trunk.
  3. Under SIP Health Check, configure:
    • Interval: 2 seconds (The minimum native setting).
    • Retry Count: 2.
    • Timeout: 500ms.
  4. The Logic: If two consecutive pings fail to respond within 500ms, the trunk is disabled. Total detection time: 2.5 seconds.
  5. Architectural Reasoning: While “sub-second” in a single ping is possible, you need the retry to prevent a single lost UDP packet from causing a massive trunk flap.

3. Implementing Advanced Proxy-Side Polling (Sub-Second)

For true sub-second detection (under 1 second total), you must use a SIP Proxy (like Kamailio or OpenSIPS) as a front-end for your carriers.

The Implementation:

  1. Configure Kamailio’s dispatcher module.
  2. The Config:
    modparam("dispatcher", "ds_probing_threshold", 1)
modparam("dispatcher", "ds_probing_mode", 1)
modparam("dispatcher", "ds_ping_interval", 1)
  3. The Workflow: The proxy sends pings every 1 second. If the carrier fails to respond to the first ping, the proxy immediately routes the next incoming call to the secondary carrier.
  4. The Benefit: This provides a “Zero-Interrupt” experience for the caller, as the failover happens before the invite is even sent to the primary (failed) carrier.

4. Designing a Multi-Carrier Failover Cascade

Polling is only useful if you have a place to go when the primary trunk fails.

The Strategy:

  1. Active-Active: Send 50% of traffic to Carrier A and 50% to Carrier B.
  2. The “Tombstone” Logic: When Proxy A marks Carrier A as “Down,” it stores a “Tombstone” record in Redis with a 60-second TTL.
  3. The Recovery: The proxy continues to probe the “Down” carrier. Once it receives 5 successful pings in a row, it clears the tombstone and restores the carrier to the rotation.
  4. The Trap: “Flapping.” If a carrier is unstable, you don’t want it coming back online every 5 seconds. Implement Exponential Backoff for the recovery interval.

Validation, Edge Cases & Troubleshooting

Edge Case 1: The “False Positive” Storm

Failure Condition: A temporary network congestion event causes 3 pings to drop, taking all trunks offline even though the voice path is fine.
Solution: Use Dual-Path Polling. Poll the carrier over two different ISP paths. Only take the trunk offline if both paths fail.

Edge Case 2: OPTIONS Message Size

Failure Condition: Some older carriers reject SIP OPTIONS messages if they are too large or contain certain headers.
Solution: Keep the OPTIONS message “Lean.” Strip unnecessary Supported or User-Agent headers to ensure it fits within a single MTU (1500 bytes) to avoid fragmentation issues.

Edge Case 3: Regional “Ghost” Failures

Failure Condition: Carrier A is up in London but down in Frankfurt. Your global proxy in London incorrectly thinks everything is fine.
Solution: Implement Distributed Polling Nodes. Run small polling agents in each AWS region and aggregate their status into a global health table.

Official References