VOS3000 SIP Session Timer: Complete Keep-Alive & Session Management Guide

VOS3000 SIP session timer is essential for maintaining reliable VoIP calls and preventing “zombie calls” that waste resources. By implementing RFC 4028 session timers and NAT keep-alive mechanisms, VOS3000 ensures that active calls are properly monitored and terminated calls are detected quickly. This comprehensive guide covers all session timer and keep-alive features based on official VOS3000 2.1.9.07 documentation.

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🔍 Understanding VOS3000 SIP Session Timer

Reference: VOS3000 2.1.9.07 Manual, Section 4.1.3 (Page 213)

The SIP Session Timer, defined in RFC 4028, provides a mechanism to detect failed calls that would otherwise remain “hung” in the system. Without session timers, calls that lose one-way audio or have endpoint failures may continue to exist in the system, consuming resources and potentially causing billing errors.

📊 Why Session Timers Matter

Problem	Without Session Timer	With Session Timer
Zombie Calls	Calls remain active indefinitely after endpoint failure	Failed endpoints detected, calls cleaned up
Resource Waste	System resources consumed by dead sessions	Resources freed when session expires
Billing Errors	Incorrect long-duration billing for dead calls	Accurate call termination timing
NAT Issues	NAT bindings expire causing call drops	Keep-alive maintains NAT bindings

⚙️ VOS3000 SIP Session Timer Parameters

Reference: VOS3000 2.1.9.07 Manual, Section 4.3.5.2 (Page 230-231)

📊 Core Session Timer Parameters

Parameter	Default	Range	Description
SS_SIP_SESSION_TTL	600	seconds	Detecting SIP connected status interval
SS_SIP_SESSION_UPDATE_SEGMENT	2	2-10	SIP timer re-INVITE/UPDATE interval segment
SS_SIP_SESSION_TIMEOUT_EARLY_HANGUP	0	seconds	Session timer early hangup before timeout
SS_SIP_NO_TIMER_REINVITE_INTERVAL	7200	seconds	Max conversation time for non-timer SIP caller

📐 How Session Timer Works (VOS3000 SIP Session Timer)

VOS3000 SIP Session Timer Operation:
================================

1. Call Establishment:
   - INVITE with Session-Expires header (if supported)
   - VOS3000 records session timer requirements

2. Session Refresh:
   - Re-INVITE or UPDATE sent at regular intervals
   - Interval = SS_SIP_SESSION_TTL / SS_SIP_SESSION_UPDATE_SEGMENT
   - Default: 600 / 2 = 300 seconds (5 minutes)

3. Session Monitoring:
   - If refresh fails, session is considered dead
   - Call is terminated after timeout
   - CDR updated with proper end reason

4. Non-Timer Endpoints:
   - For SIP endpoints without timer support
   - VOS3000 uses SS_SIP_NO_TIMER_REINVITE_INTERVAL
   - Default 7200 seconds (2 hours) maximum call duration

Example Flow with SS_SIP_SESSION_TTL = 600:
===========================================
Time 0:00   - Call established
Time 5:00   - Re-INVITE/UPDATE sent (refresh attempt)
Time 5:01   - 200 OK received (refresh successful)
Time 10:00  - Re-INVITE/UPDATE sent
Time 10:01  - 200 OK received
...continues for duration of call

If refresh fails:
Time 10:00  - Re-INVITE/UPDATE sent
Time 10:30  - No response (timeout)
Time 10:30  - Call terminated
Time 10:30  - CDR records "Session timeout"

📡 NAT Keep-Alive Configuration

Reference: VOS3000 2.1.9.07 Manual, Section 4.1.2 (Page 212-213)

NAT keep-alive ensures that NAT bindings remain active for devices behind NAT devices. Without proper keep-alive, incoming calls may fail because the NAT mapping has expired.

⚙️ NAT Keep-Alive Parameters (VOS3000 SIP Session Timer)

Parameter	Default	Range	Description
SS_SIP_NAT_KEEP_ALIVE_MESSAGE	HELLO	text	Content of NAT keep-alive message
SS_SIP_NAT_KEEP_ALIVE_PERIOD	30	10-86400 sec	NAT keep-alive message sending period
SS_SIP_NAT_KEEP_ALIVE_SEND_INTERVAL	500	ms	Interval between sending keep-alives
SS_SIP_NAT_KEEP_ALIVE_SEND_ONE_TIME	3000	count	Number of keep-alive messages per batch

📐 NAT Keep-Alive Operation

VOS3000 NAT Keep-Alive Mechanism:
==================================

Purpose:
========
When devices are behind NAT, the NAT device maintains a mapping table.
If no traffic passes through for a period (typically 30-300 seconds),
the NAT mapping expires, and incoming calls cannot reach the device.

How It Works:
=============
1. Device registers with VOS3000
2. VOS3000 records device IP and port
3. VOS3000 sends periodic keep-alive messages
4. Keep-alive traffic maintains NAT mapping
5. Incoming calls can reach the device

Configuration Example:
======================
SS_SIP_NAT_KEEP_ALIVE_MESSAGE = "HELLO"
SS_SIP_NAT_KEEP_ALIVE_PERIOD = 30 (seconds)

VOS3000 sends "HELLO" to registered devices every 30 seconds.

Important Notes:
================
- If SS_SIP_NAT_KEEP_ALIVE_MESSAGE is empty, keep-alive is disabled
- Period should be less than NAT device timeout (typically 60 seconds)
- For large deployments, adjust SEND_INTERVAL and SEND_ONE_TIME

Usage Scenarios:
================
1. Normal Registration: Device maintains registration via REGISTER
2. Non-REGISTER Devices: VOS3000 sends UDP keep-alive
3. Symmetric NAT: May require media proxy instead

🔧 Session Timer Configuration Guide (VOS3000 SIP Session Timer)

⚙️ Recommended Parameter Values

Scenario	SS_SIP_SESSION_TTL	SS_SIP_NO_TIMER_REINVITE_INTERVAL	NAT_KEEP_ALIVE_PERIOD
Standard VoIP	600 (10 min)	7200 (2 hours)	30 seconds
Call Center	900 (15 min)	14400 (4 hours)	20 seconds
Wholesale	600 (10 min)	0 (disabled)	30 seconds
Mobile/Unstable	300 (5 min)	3600 (1 hour)	15 seconds

🔧 Configuration Steps

Step-by-Step Session Timer Configuration:
==========================================

1. Navigate to System Parameters:
   Navigation > Operation management > Softswitch management
   > Additional settings > System parameter

2. Configure Session Timer:
   Find: SS_SIP_SESSION_TTL
   Set: 600 (or desired value in seconds)

3. Configure Update Segment:
   Find: SS_SIP_SESSION_UPDATE_SEGMENT
   Set: 2 (refresh interval = TTL/segment)

4. Configure NAT Keep-Alive:
   Find: SS_SIP_NAT_KEEP_ALIVE_MESSAGE
   Set: HELLO (or custom message)

   Find: SS_SIP_NAT_KEEP_ALIVE_PERIOD
   Set: 30 (seconds between keep-alives)

5. Apply Changes:
   Click Apply to save configuration

6. Verify Settings:
   Check CDR for session timeout behavior
   Monitor for 30-second call drops

Important: Changes require softswitch service restart
to take effect in some cases.

🚨 Common Session Timer Problems

📊 Problem Diagnosis Table (VOS3000 SIP Session Timer)

Symptom	Possible Cause	Solution
Calls drop at 30 seconds	NAT binding timeout, SIP ALG issue	Disable SIP ALG, increase NAT keep-alive
Calls drop at specific intervals	Session timer negotiation failure	Check session timer support, adjust TTL
No incoming calls after idle	NAT binding expired	Enable NAT keep-alive, reduce period
Session timer errors in trace	Endpoint doesn’t support RFC 4028	Use SS_SIP_NO_TIMER_REINVITE_INTERVAL
Re-INVITE rejected by endpoint	Endpoint doesn’t support re-INVITE	Try UPDATE method, check endpoint config

🔧 Troubleshooting Session Timer Issues

Session Timer Troubleshooting Checklist:
=========================================

1. Check Debug Trace:
   System > Debug trace > Enable
   Look for re-INVITE or UPDATE messages
   Check for 200 OK responses

2. Verify Endpoint Support:
   - Check if endpoint includes "timer" in Supported header
   - Look for Session-Expires in INVITE/200 OK
   - Verify endpoint responds to session refresh

3. Check NAT Configuration:
   - Verify NAT keep-alive is enabled
   - Check SS_SIP_NAT_KEEP_ALIVE_PERIOD
   - Monitor for NAT binding expiration

4. Analyze CDR:
   - Check termination reason for session timeouts
   - Look for patterns in call drop timing
   - Compare with session timer configuration

5. Test Different Scenarios:
   - Test calls from different networks
   - Test with different endpoints
   - Test with/without media proxy

Common Fixes:
=============
- Increase SS_SIP_SESSION_TTL for longer refresh intervals
- Reduce SS_SIP_NAT_KEEP_ALIVE_PERIOD for aggressive keep-alive
- Disable SIP ALG on routers
- Enable media proxy for NAT scenarios

📊 Session Timer vs NAT Keep-Alive (VOS3000 SIP Session Timer)

Understanding the difference between session timer and NAT keep-alive is important for proper configuration:

Aspect	Session Timer	NAT Keep-Alive
Purpose	Detect failed calls, prevent zombie calls	Maintain NAT bindings for incoming calls
Protocol	SIP re-INVITE/UPDATE	UDP packets or SIP messages
Direction	Both directions (refresh negotiation)	Server to client (keep binding active)
Default Interval	600 seconds (10 minutes)	30 seconds
When Active	During active call	During registration period
RFC Reference	RFC 4028	NAT traversal best practices

🔗 Related Resources (VOS3000 SIP Session Timer)

• 📖 VOS3000 One-Way Audio Troubleshooting
• 📖 VOS3000 Extended Firewall Configuration
• 📖 SIP ALG Problems Troubleshooting Guide
• 📖 VOS3000 Server Configuration Guide
• 📖 VOS3000 Downloads

❓ Frequently Asked Questions

What happens if both endpoints don’t support session timer?

VOS3000 uses SS_SIP_NO_TIMER_REINVITE_INTERVAL to limit maximum call duration. This prevents zombie calls even when endpoints don’t support RFC 4028. Set this value based on your business needs (default is 7200 seconds / 2 hours).

Why are my calls dropping at exactly 30 seconds?

30-second call drops are typically caused by NAT binding timeouts, not session timer issues. Check if SIP ALG is enabled on your router (should be disabled), and verify NAT keep-alive is configured correctly with a period less than 30 seconds.

Should I use re-INVITE or UPDATE for session refresh?

VOS3000 automatically negotiates the refresh method based on endpoint capabilities. UPDATE is generally preferred as it doesn’t affect SDP negotiation. Both methods work for session timer purposes – VOS3000 handles this automatically.

What is a good SS_SIP_SESSION_TTL value?

The default of 600 seconds (10 minutes) works well for most scenarios. For mobile or unstable networks, consider reducing to 300 seconds (5 minutes) for faster detection of failed calls. For stable enterprise environments, 900 seconds (15 minutes) reduces overhead.

How do I know if NAT keep-alive is working?

Enable debug trace and look for periodic messages matching your SS_SIP_NAT_KEEP_ALIVE_MESSAGE content (default “HELLO”). You should see these messages at intervals matching SS_SIP_NAT_KEEP_ALIVE_PERIOD.

📞 Get Expert Help with VOS3000 Session Timer

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