VOS3000 Session Timer: Complete Guide to SIP Keep-Alive Configuration
VOS3000 session timer is a critical mechanism for maintaining call stability and preventing “zombie calls” that consume system resources. Based on RFC 4028 specifications, the session timer functionality in VOS3000 2.1.9.07 ensures that active VoIP sessions are properly monitored while failed or hung calls are detected and cleaned up automatically. This comprehensive guide covers all session timer parameters, NAT keep-alive configuration, and troubleshooting procedures based on the official VOS3000 manual.
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Table of Contents
🔍 What is VOS3000 Session Timer?
Reference: VOS3000 2.1.9.07 Manual, Section 4.1.3 (Page 213)
The VOS3000 session timer implements the SIP Session Timer mechanism defined in RFC 4028. This protocol extension addresses a fundamental problem in SIP-based VoIP systems: the inability to detect when a call has failed at one endpoint while the other endpoint believes the call is still active. These “zombie calls” can persist indefinitely, consuming system resources, occupying call capacity, and causing billing discrepancies.
📊 The Zombie Call Problem
| 🚨 Scenario | ❌ Without Session Timer | ✅ With Session Timer |
|---|---|---|
| Endpoint Power Failure | Call remains “active” indefinitely in system | Session expires, call terminated cleanly |
| Network Disconnection | No notification, resources wasted | Refresh fails, session cleaned up |
| Device Crash | Zombie call persists for hours/days | Maximum session duration enforced |
| NAT Timeout | One-way audio, confused state | Session refresh detects failure |
| Billing Impact | Incorrect CDR duration, revenue loss | Accurate call termination timing |
⚙️ VOS3000 Session Timer Parameters Complete Reference
Reference: VOS3000 2.1.9.07 Manual, Section 4.3.5.2 (Pages 229-239)
VOS3000 provides a comprehensive set of session timer parameters that control how the softswitch monitors and maintains active SIP sessions. These parameters are configured in the System Parameters section and affect all SIP-based communications.
📊 Core Session Timer Parameters Table
| ⚙️ Parameter | 📊 Default | 📏 Range | 📝 Description | 📖 Manual Page |
|---|---|---|---|---|
| SS_SIP_SESSION_TTL | 600 | 60-86400 sec | Detecting SIP connected status interval (Session-Expires value) | 230 |
| SS_SIP_SESSION_UPDATE_SEGMENT | 2 | 2-10 | Divisor for refresh interval calculation (TTL/segment) | 230 |
| SS_SIP_SESSION_TIMEOUT_EARLY_HANGUP | 0 | 0-3600 sec | Terminate session before actual timeout (margin) | 230 |
| SS_SIP_NO_TIMER_REINVITE_INTERVAL | 7200 | 0-86400 sec | Maximum call duration for non-timer endpoints | 230 |
| SS_SIP_SESSION_MIN_SE | 90 | 90-3600 sec | Minimum session expires value per RFC 4028 | 231 |
📊 Session Timer Refresh Calculation
📐 Session Timer Refresh Interval Formula
Refresh Interval = SS_SIP_SESSION_TTL ÷ SS_SIP_SESSION_UPDATE_SEGMENT
| Example with Defaults: | 600 ÷ 2 = 300 seconds (5 minutes) |
| First Refresh Attempt: | At 5 minutes into the call |
| Session Expires If: | No response to refresh within TTL period |
📡 NAT Keep-Alive Configuration Deep Dive
Reference: VOS3000 2.1.9.07 Manual, Section 4.1.2 (Pages 212-213)
NAT (Network Address Translation) devices maintain binding tables that map internal private IP addresses to external public addresses. These bindings have a timeout period, typically ranging from 30 to 300 seconds depending on the device. When a binding expires without traffic, incoming calls cannot reach the endpoint behind NAT.
📊 NAT Keep-Alive Parameters Table
| ⚙️ Parameter | 📊 Default | 📏 Range | 📝 Function | 📖 Page |
|---|---|---|---|---|
| SS_SIP_NAT_KEEP_ALIVE_MESSAGE | HELLO | Text string | Content of NAT keep-alive UDP packet | 212 |
| SS_SIP_NAT_KEEP_ALIVE_PERIOD | 30 | 10-86400 sec | Interval between keep-alive transmissions | 212 |
| SS_SIP_NAT_KEEP_ALIVE_SEND_INTERVAL | 500 | 1-10000 ms | Delay between individual keep-alive packets in batch | 212 |
| SS_SIP_NAT_KEEP_ALIVE_SEND_ONE_TIME | 3000 | 1-10000 | Number of keep-alive packets sent per batch cycle | 212 |
🔄 How NAT Keep-Alive Works in VOS3000
VOS3000 NAT Keep-Alive Operation Flow: ======================================= SCENARIO: Endpoint behind NAT firewall ┌─────────────────────────────────────────────────────────────────────────────┐ │ │ │ ENDPOINT NAT DEVICE VOS3000 SERVER │ │ (192.168.1.100) (Public IP) (Softswitch) │ │ │ │ 1. REGISTER ───────────────────────────────────────────────────────────► │ │ (Via: 192.168.1.100) │ │ │ │ 2. VOS3000 Records: │ │ - Received IP: Public NAT IP │ │ - Received Port: NAT mapped port │ │ - Contact: Internal IP (via Contact header) │ │ │ │ 3. NAT BINDING TABLE: │ │ Internal: 192.168.1.100:5060 → External: PublicIP:45678 │ │ │ │ 4. KEEP-ALIVE MESSAGE (every 30 seconds): │ │ ◄───────────────────────────────────────────────────────────────────── │ │ UDP packet "HELLO" to PublicIP:45678 │ │ │ │ 5. NAT BINDING REFRESHED: │ │ - Timer resets to 30+ seconds │ │ - Binding remains active │ │ │ │ 6. INCOMING CALL: │ │ ◄───────────────────────────────────────────────────────────────────── │ │ INVITE reaches endpoint successfully! │ │ │ └─────────────────────────────────────────────────────────────────────────────┘ IMPORTANT: If SS_SIP_NAT_KEEP_ALIVE_MESSAGE is empty, keep-alive is DISABLED!
🔧 VOS3000 Session Timer Configuration Guide
📍 Navigation to System Parameters
| Step | Navigation Path | Action |
|---|---|---|
| 1 | Operation management | Click main menu |
| 2 | Softswitch management | Select softswitch node |
| 3 | Additional settings | Right-click → Additional settings |
| 4 | System parameter tab | Find session timer parameters |
| 5 | Modify values | Edit desired parameters |
| 6 | Apply changes | Click OK to save |
⚙️ Recommended Configuration by Deployment Type
| 🏢 Scenario | ⏱️ SESSION_TTL | 🔄 SEGMENT | 🚫 NO_TIMER_INTERVAL | 📡 NAT_PERIOD |
|---|---|---|---|---|
| Standard VoIP Wholesale | 600 (10 min) | 2 | 0 (disabled) | 30 sec |
| Call Center Operations | 900 (15 min) | 3 | 14400 (4 hrs) | 20 sec |
| Mobile/Unstable Networks | 300 (5 min) | 2 | 3600 (1 hr) | 15 sec |
| Enterprise PBX | 1200 (20 min) | 2 | 28800 (8 hrs) | 30 sec |
| High-Security Environment | 180 (3 min) | 2 | 1800 (30 min) | 10 sec |
📊 Session Timer Message Flow Diagram
VOS3000 Session Timer - Complete Call Flow with Refresh: ========================================================= CALLER VOS3000 CALLEE │ │ │ │ 1. INVITE │ │ │ Session-Expires: 600 │ │ │ Min-SE: 90 │ │ │──────────────────────────────►│ │ │ │ 2. INVITE (forwarded) │ │ │ Session-Expires: 600 │ │ │──────────────────────────────►│ │ │ │ │ │ 3. 200 OK │ │ │ Session-Expires: 600 │ │ │◄──────────────────────────────│ │ 4. 200 OK │ │ │ Session-Expires: 600 │ │ │◄──────────────────────────────│ │ │ │ │ │ 5. ACK │ │ │──────────────────────────────►│ 6. ACK │ │ │──────────────────────────────►│ │ │ │ │ ═════════════════════════════════════════ │ │ ║ CALL ACTIVE - AUDIO FLOWING ║ │ │ ═════════════════════════════════════════ │ │ │ │ │ [5 minutes into call] │ │ │ │ │ │ 7. UPDATE (session refresh) │ │ │ Session-Expires: 600 │ │ │◄──────────────────────────────│ │ │ 8. 200 OK │ │ │ Session-Expires: 600 │ │ │──────────────────────────────►│ │ │ │ 9. UPDATE (session refresh) │ │ │──────────────────────────────►│ │ │ 10. 200 OK │ │ │◄──────────────────────────────│ │ │ │ │ ═════════════════════════════════════════ │ │ ║ SESSION REFRESHED SUCCESSFULLY ║ │ │ ═════════════════════════════════════════ │ │ │ │ │ [If refresh fails] │ │ │ │ │ │ 11. BYE (session timeout) │ │ │◄──────────────────────────────│ 12. BYE (session timeout) │ │ │──────────────────────────────►│ │ │ │ │ CDR: Termination Reason = "Session Timeout" │ │ │ │
🚨 Session Timer Troubleshooting Guide
📊 Common Problems and Solutions
| 🚨 Symptom | 🔍 Root Cause | ✅ Solution | 📖 Reference |
|---|---|---|---|
| Calls drop at exactly 30 seconds | NAT binding timeout, not session timer | Enable NAT keep-alive, reduce period to 15-20s | Page 212 |
| Calls drop at 5-minute intervals | Session refresh failing | Check if endpoint supports re-INVITE/UPDATE | Page 213 |
| “422 Session Interval Too Small” error | Session-Expires below minimum | Increase SS_SIP_SESSION_MIN_SE or TTL | Page 231 |
| No incoming calls after idle period | NAT binding expired | Verify NAT keep-alive is enabled and working | Page 212 |
| Re-INVITE rejected with 491 | Glare condition (simultaneous re-INVITEs) | Normal – VOS3000 will retry automatically | Page 213 |
| Zombie calls still occurring | Session timer not negotiated | Check NO_TIMER_REINVITE_INTERVAL setting | Page 230 |
🔧 Debug Trace Analysis for Session Timer
VOS3000 Debug Trace - Session Timer Analysis: ============================================== Step 1: Enable Debug Trace Navigation: System → Debug trace Enable: Check "On" Set duration: 10-30 minutes Step 2: Look for Session Timer Headers in SIP Messages: ─────────────────────────────────────────────────────── INVITE sip:[email protected]:5060 SIP/2.0 Via: SIP/2.0/UDP 10.0.0.1:5060;branch=z9hG4bK12345 From: ;tag=abc123 To: Call-ID: [email protected] CSeq: 1 INVITE Contact: Session-Expires: 600;refresher=uac ← SESSION TIMER HEADER Min-SE: 90 ← MINIMUM SESSION EXPIRES Content-Type: application/sdp Content-Length: ... Step 3: Check 200 OK Response: ────────────────────────────── SIP/2.0 200 OK ... Session-Expires: 600;refresher=uac ← CONFIRMED SESSION TIMER ... Step 4: Look for Session Refresh Messages (UPDATE or re-INVITE): ──────────────────────────────────────────────────────────────── UPDATE sip:[email protected]:5060 SIP/2.0 ... Session-Expires: 600 ← REFRESHING SESSION ... Step 5: If No Session Timer Headers Found: ────────────────────────────────────────── - Endpoint does not support RFC 4028 - VOS3000 will use SS_SIP_NO_TIMER_REINVITE_INTERVAL - Maximum call duration will be enforced
📊 Session Timer vs NAT Keep-Alive Comparison
| 📊 Aspect | ⏱️ Session Timer | 📡 NAT Keep-Alive |
|---|---|---|
| Primary Purpose | Detect failed calls, prevent zombie sessions | Maintain NAT bindings for incoming calls |
| RFC Standard | RFC 4028 (SIP Session Timer) | NAT traversal best practices |
| Protocol Used | SIP re-INVITE or UPDATE messages | UDP packets or SIP messages |
| When Active | During active call (after 200 OK) | While endpoint is registered |
| Direction | Bidirectional (negotiated refresh) | Server to endpoint (unidirectional) |
| Default Interval | 600 seconds (10 minutes) | 30 seconds |
| Failure Result | Call terminated, CDR updated | Incoming calls may fail |
| Endpoint Support Required | Yes (RFC 4028 compliance) | No (transparent to endpoint) |
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| 📦 Service | 📝 Description | 💼 Includes |
|---|---|---|
| VOS3000 Installation | Complete server setup | OS, VOS3000, Database, Security |
| Session Timer Configuration | Optimize for your environment | NAT handling, Timer tuning |
| Technical Support | 24/7 remote assistance | Troubleshooting, Debug, Analysis |
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🔗 Related Resources
- 📖 VOS3000 SIP Call Flow Complete Guide
- 📖 VOS3000 ASR ACD Analysis Guide
- 📖 VOS3000 Codec G729 Transcoding
- 📖 VOS3000 RTP Media Configuration
- 📥 VOS3000 Downloads – Manual and Software
❓ Frequently Asked Questions about VOS3000 Session Timer
What happens if an endpoint doesn’t support session timer?
VOS3000 will use the SS_SIP_NO_TIMER_REINVITE_INTERVAL parameter to limit the maximum call duration. This ensures that zombie calls cannot persist indefinitely even when the endpoint doesn’t support RFC 4028. Set this value based on your business requirements (default is 7200 seconds or 2 hours).
Why are my calls dropping exactly at 30 seconds?
30-second call drops are almost always caused by NAT binding timeout, not session timer issues. The solution is to enable NAT keep-alive by setting SS_SIP_NAT_KEEP_ALIVE_MESSAGE to a value like “HELLO” and reducing SS_SIP_NAT_KEEP_ALIVE_PERIOD to 15-20 seconds. Also check if SIP ALG is enabled on your router (it should be disabled).
What is the difference between re-INVITE and UPDATE for session refresh?
Both methods can be used for session refresh. UPDATE is generally preferred because it doesn’t modify the SDP session parameters, while re-INVITE also renegotiates media. VOS3000 automatically selects the appropriate method based on endpoint capabilities and configuration.
How do I calculate the optimal session timer refresh interval?
The refresh interval equals SS_SIP_SESSION_TTL divided by SS_SIP_SESSION_UPDATE_SEGMENT. With defaults (600 ÷ 2 = 300 seconds), VOS3000 sends a refresh every 5 minutes. For mobile networks, consider 300 ÷ 2 = 150 seconds for faster failure detection.
Can session timer prevent billing fraud?
Session timer helps prevent zombie calls that could result in incorrect CDR durations, but it’s not a fraud prevention mechanism. For fraud protection, implement proper account limits, IP restrictions, and monitor for unusual calling patterns using VOS3000’s built-in reports.
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