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 ContentsVOS3000 Session Timer: Complete Guide to SIP Keep-Alive Configuration What is VOS3000 Session Timer? The Zombie Call Problem VOS3000 Session Timer Parameters Complete Reference Core Session Timer Parameters Table Session Timer Refresh Calculation Session Timer Refresh Interval Formula NAT Keep-Alive Configuration Deep Dive NAT Keep-Alive Parameters Table How NAT Keep-Alive Works in VOS3000 VOS3000 Session Timer Configuration Guide Navigation to System Parameters Recommended Configuration by Deployment Type Session Timer Message Flow Diagram Session Timer Troubleshooting Guide Common Problems and Solutions Debug Trace Analysis for Session Timer Session Timer vs NAT Keep-Alive Comparison VOS3000 Installation and Support Services Related Resources Frequently Asked Questions about VOS3000 Session TimerWhat happens if an endpoint doesn’t support session timer?Why are my calls dropping exactly at 30 seconds?What is the difference between re-INVITE and UPDATE for session refresh?How do I calculate the optimal session timer refresh interval?Can session timer prevent billing fraud? Get Expert VOS3000 Session Timer Support Need Professional VOS3000 Setup Support?
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 TimerEndpoint Power FailureCall remains “active” indefinitely in systemSession expires, call terminated cleanlyNetwork DisconnectionNo notification, resources wastedRefresh fails, session cleaned upDevice CrashZombie call persists for hours/daysMaximum session duration enforcedNAT TimeoutOne-way audio, confused stateSession refresh detects failureBilling ImpactIncorrect CDR duration, revenue lossAccurate 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 PageSS_SIP_SESSION_TTL60060-86400 secDetecting SIP connected status interval (Session-Expires value)230SS_SIP_SESSION_UPDATE_SEGMENT22-10Divisor for refresh interval calculation (TTL/segment)230SS_SIP_SESSION_TIMEOUT_EARLY_HANGUP00-3600 secTerminate session before actual timeout (margin)230SS_SIP_NO_TIMER_REINVITE_INTERVAL72000-86400 secMaximum call duration for non-timer endpoints230SS_SIP_SESSION_MIN_SE9090-3600 secMinimum session expires value per RFC 4028231
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 callSession 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 PageSS_SIP_NAT_KEEP_ALIVE_MESSAGEHELLOText stringContent of NAT keep-alive UDP packet212SS_SIP_NAT_KEEP_ALIVE_PERIOD3010-86400 secInterval between keep-alive transmissions212SS_SIP_NAT_KEEP_ALIVE_SEND_INTERVAL5001-10000 msDelay between individual keep-alive packets in batch212SS_SIP_NAT_KEEP_ALIVE_SEND_ONE_TIME30001-10000Number of keep-alive packets sent per batch cycle212
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
StepNavigation PathAction1Operation managementClick main menu2Softswitch managementSelect softswitch node3Additional settingsRight-click → Additional settings4System parameter tabFind session timer parameters5Modify valuesEdit desired parameters6Apply changesClick OK to save
Recommended Configuration by Deployment Type
Scenario SESSION_TTL SEGMENT NO_TIMER_INTERVAL NAT_PERIODStandard VoIP Wholesale600 (10 min)20 (disabled)30 secCall Center Operations900 (15 min)314400 (4 hrs)20 secMobile/Unstable Networks300 (5 min)23600 (1 hr)15 secEnterprise PBX1200 (20 min)228800 (8 hrs)30 secHigh-Security Environment180 (3 min)21800 (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 ReferenceCalls drop at exactly 30 secondsNAT binding timeout, not session timerEnable NAT keep-alive, reduce period to 15-20sPage 212Calls drop at 5-minute intervalsSession refresh failingCheck if endpoint supports re-INVITE/UPDATEPage 213“422 Session Interval Too Small” errorSession-Expires below minimumIncrease SS_SIP_SESSION_MIN_SE or TTLPage 231No incoming calls after idle periodNAT binding expiredVerify NAT keep-alive is enabled and workingPage 212Re-INVITE rejected with 491Glare condition (simultaneous re-INVITEs)Normal – VOS3000 will retry automaticallyPage 213Zombie calls still occurringSession timer not negotiatedCheck NO_TIMER_REINVITE_INTERVAL settingPage 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:1234567890@192.168.1.100:5060 SIP/2.0
Via: SIP/2.0/UDP 10.0.0.1:5060;branch=z9hG4bK12345
From: ;tag=abc123
To:
Call-ID: a84b4c76e66710@example.com
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:1234567890@192.168.1.100:5060 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-AlivePrimary PurposeDetect failed calls, prevent zombie sessionsMaintain NAT bindings for incoming callsRFC StandardRFC 4028 (SIP Session Timer)NAT traversal best practicesProtocol UsedSIP re-INVITE or UPDATE messagesUDP packets or SIP messagesWhen ActiveDuring active call (after 200 OK)While endpoint is registeredDirectionBidirectional (negotiated refresh)Server to endpoint (unidirectional)Default Interval600 seconds (10 minutes)30 secondsFailure ResultCall terminated, CDR updatedIncoming calls may failEndpoint Support RequiredYes (RFC 4028 compliance)No (transparent to endpoint)
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Service Description IncludesVOS3000 InstallationComplete server setupOS, VOS3000, Database, SecuritySession Timer ConfigurationOptimize for your environmentNAT handling, Timer tuningTechnical Support24/7 remote assistanceTroubleshooting, 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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