Session Timers

Feature Summary and Revision History

Summary Data

Table 1. Summary Data

Applicable Product(s) or Functional Area

AMF

Applicable Platform(s)

SMI

Feature Default Setting

Enabled - Always-on

Related Documentation

Not Applicable

Revision History

Table 2. Revision History

Revision Details

Release

The following enhancements were introduced:

  • Non-3GPP timer configurations

2022.01.0

First introduced.

2021.04.0

Feature Description

AMF supports the following timers:

  • T3502 (t3502): It operates in the 5GMM-DEREGISTERED and 5GMM-REGISTERED states. AMF provides this timer value to UE in the Registration Accept and Registration Reject messages.

  • T3512 (t3512): It operates in the 5GMM-REGISTERED state. AMF provides this timer value to UE in the Registration Accept message.

  • T3513 (t3513): It operates in the 5GMM-REGISTERED state. It starts when the Paging procedure is initiated (with default paging algorithm) and stops when the Paging procedure ends (with the reception of paging response).

  • T3522 (t3522): It operates in the 5GMM-DEREGISTERED-INITIATED state. It starts with the transmission of Deregistration Request message and stops after receiving Deregistration Accept message.

  • T3550 (t3550): It operates in the 5GMM-COMMON-PROCEDURE-INITIATED state. It starts with the transmission of Registration Accept message and stops after receiving the Registration Complete message.

  • T3555 (t3555): It operates in the 5GMM-REGISTERED state. It starts with the transmission of Configuration Update Command message with the ACK bit set in the Configuration Update Indication IE. Stops with the Configuration Update complete message

  • T3560 (t3560): It operates in the 5GMM-COMMON-PROCEDURE-INITIATED state. It starts with the transmission of Authentication Request message and Security Mode Command. Stops after receiving the following messages:

    • Authentication Response

    • Authentication Failure

    • Security Mode Complete

    • Security Mode Reject

  • T3570 (t3570): It operates in the 5GMM-REGISTERED state. It starts with the transmission of Identity Request message and stops after receiving the Identity Response message

  • UE Context Transfer (context-transfer-guard): AMF uses this timer to keep the individual UE Context resources until the timer expires. AMF starts this timer when UeRegStatusUpdateReqData message contains transferStatus as TRANSFERRED. Upon expiry, it clears the PDUs locally.

  • Tidle (tidle): When the UE moves to the CONNECTED state, tidle timer is started and it's reset when any signalling occurs for the subscriber.

    On expiry of tidle timer, AMF checks:

    • If the UE Configuration Update is enabled and if new configuration is available to send to the UE, AMF triggers the UE Config Update Command to UE and resets the tidle timer.

    • If the UE Configuration Update isn’t enabled or there’s no configuration update to send the to UE, the UE is moved to the IDLE state. AMF triggers the Context Release Command towards the gNB and the SM Context Update towards the SMF accordingly.

  • HO Supervisory (ho-supervisory): It supervises PDU responses from SMF during N2, N26, and Xn handovers.

  • Tidt (tidt): It starts after four minutes of T3512 timer expiry. The subscriber gets Deregistered implicitly upon this timer expiry.

  • Tn2 (tn2): It functions in AMF-initiated N2 messages, specifically for the AMF that waits for the response.

  • Tpurge (tpurge): It starts when the Tidt timer expires. AMF sends a request to the UDM to Deregister (purge) the UE from the UDM for 3GPP access upon this timer expiry.

  • Procedural Timeout (proc-timeout): It starts when AMF receives Registration Request. After expiry, AMF sends the Registration Reject message to the UE.

For information on the timer configurations, refer to Feature Configuration.

How it Works

This section describes how this feature works.

Call Flows

This section describes the key call flows for the AMF timers.

T3502 Call Flow

This section describes the T3502 timer call flow.

Figure 1. T3502 Timer Call Flow
Table 3. T3502 Timer Call Flow Description

Step

Description

1

The UE sends the Registration Request to the AMF.

The UE starts the T3502 timer.

2

The UE receives a response of reject or accept from the AMF.

3

The UE performs the 5G MM registration procedure.

4

The UE starts the T3502 timer and the registration process during one of the following:

  • Registration failure

  • Attempt counter equals 5

5

The UE sends the Registration Request to the AMF and stops the T3502 timer.

T3512 Call Flow

This section describes the T3512 timer call flow.

Figure 2. T3512 Timer Call Flow
Table 4. T3512 Timer Call Flow Description

Step

Description

1

The UE sends the Registration Request to the AMF.

2

The UE receives a response of reject or accept from the AMF.

The UE starts the T3512 timer.

3

The UE performs the 5G MM Registration procedure.

4

The UE starts the T3512 timer and the registration process during one of the following:

  • Registration failure

  • Attempt counter equals 5

5

The UE sends the Registration Request to the AMF and stops the T3512 timer.

T3522 Call Flow

This section describes the T3522 timer call flow.

Figure 3. T3522 Timer Call Flow
Table 5. T3522 Timer Call Flow Description

Step

Description

1, 2

The AMF sends the Deregistration Request to the UE and starts the T3522 timer.

3, 4

The AMF receives the Deregistration Response from the UE and stops the T3522 timer.

T3550 Call Flow

This section describes the T3550 call flow.

Figure 4. T3550 Timer Call Flow
Table 6. T3550 Timer Call Flow Description

Step

Description

1, 2

The AMF sends the Deregistration Request to the UE and receives the response.

3

The AMF starts the timer T3550 when temporary ID is allocated.

4, 5

AMF receives Deregistration Response from the UE and stops the T3550 timer.

T3555 Call Flow

This section describes the T3555 call flow.

Figure 5. T3555 Timer Call Flow
Table 7. T3555 Timer Call Flow Description

Step

Description

1, 2

The AMF sends the Configuration Update Command to the UE and starts the T3555 timer.

3, 4

The AMF receives the Configuration Update Complete from the UE and stops the T3555 timer.

T3560 Call Flow

This section describes the T3560 timer call flow.

Figure 6. T3560 Call Flow
Table 8. T3560 Timer Call Flow Description

Step

Description

1, 2

The AMF sends the Authentication Request to the UE and starts the T3560 timer.

3, 4

The AMF receives the Authentication Response from the UE and stops the T3560 timer.

T3570 Call Flow

This section describes the T3570 timer call flow.

Figure 7. T3570 Call Flow
Table 9. T3570 Timer Call Flow Description

Step

Description

1, 2

The AMF sends the Identity Request to the UE and starts the T3570 timer.

3, 4

The AMF receives the Identity Response from the UE and stops the T3570 timer.

Tidle Timer Call Flow

This section describes the Tidle timer call flow.

Tidle timer call flow is a sample call flow. Tidle timer expiry can happen post various signalling procedures, not just registration procedure.

Figure 8. Tidle Timer Call Flow
Table 10. Tidle Timer Call Flow Description

Step

Description

1

The UE sends the Registration Request to the AMF

2

The UE receives Registration Response from the AMF.
3

The UE sends the Registration Complete to the AMF.

When UE moves to CONNECTED state, the AMF starts the configured timer. The value ranges between 30 seconds to seven hours.

4

The AMF waits for the Tidle timer expiry.
5

The AMF sends the Context Release Command to the gNodeB.

6

The AMF sends the Context Update to the SMF.

Procedural Timer Call Flow

This section describes the procedural timer call flow.

Figure 9. Procedural Timer Call Flow
Table 11. Procedural Timer Call Flow Description

Step

Description

1

The AMF starts the Procedure timer upon receiving Registration Request per UE basis.

2

AMF starts the T3550 timer when it sends the Registration Accept to UE and stops after receiving the Registration Complete.

When the timer expires before sending Registration Accept towards the UE, the following actions takes place:

  • Clear context locally, on UDM and PCF.

  • Registration Reject procedure.

When Registration Accept to the UE is successful, AMF updates t3550 (retry and timeout) based on remaining time and retry count.

3

The UE sends Registration Complete to the AMF.

Standards Compliance

This feature complies with the following standards specification:

  • 3GPP TS 24.501 " Non-Access-Stratum (NAS) protocol for 5G System (5GS); Stage 3"

Feature Configuration

Configuring this feature involves the following steps:

  • 3GPP timer configurations—These commands support in configuring 3GPP timers. For more information, refer to Configuring the 3GPP Timers.

  • Non-3GPP timer configurations—These commands support in configuring non-3GPP timers. For more information, refer to Configuring the Non-3GPP Timers.

Configuring the 3GPP Timers

To configure the GPP timers, use the following configuration:

config 
    amf-global 
        call-control-policy policy_name 
            timers timer_type { retry retry_count | value timeout_value } 
            end

NOTES:

  • timers timer_type retry retry_count —Specify the retry count.

  • timers timer_type value timeout_value —Specify the timeout value.

    For the timer_type, refer to the following table.

Table 12. 3GPP Timers and Values

Timer

Retry Count or Attempt Count

Timeout Value

tidt

Not Applicable

Must be an integer in the range of 0–35712000 seconds.

The default value is 3480 seconds.

ho-supervisory

Not Applicable

Must be an integer in the range of 100–10000 mill seconds.

The default value is 500 milliseconds.

tpurge

Not Applicable

Must be an integer in the range of 0–35712000 seconds.

The default value is 86400.

t3502

Not Applicable

Must be an integer in the range of 0–35712000 seconds.

The default value is 720 seconds.

t3512

Not Applicable

Must be an integer in the range of 0–35712000 seconds.

The default value is 3240 seconds.

t3513

This parameter defines the number of paging attempts.

Must be an integer in the range of 1–5.

The default value is 2.

Must be an integer in the range of 1–10 seconds.

The default value is 5 seconds.

t3522

This parameter defines the number of paging retries.

Must be an integer in the range of 0–5.

The default value is 4.

Must be an integer in the range of 0–30 seconds.

The default value is 6 seconds.

t3550

This parameter defines the number of paging retries.

Must be an integer in the range of 0–5.

The default value is 4.

Must be an integer in the range of 0–30 seconds.

The default value is 6 seconds.

t3555

This parameter defines the number of paging retries.

Must be an integer in the range of 0–5.

The default value is 4.

Must be an integer in the range of 0–30 seconds.

The default value is 6 seconds.

t3560

This parameter defines the number of paging retries.

Must be an integer in the range of 0–5.

The default value is 4.

Must be an integer in the range of 0–30 seconds.

The default value is 6 seconds.

t3570

This parameter defines the number of paging retries.

Must be an integer in the range of 0–5.

The default value is 4.

Must be an integer in the range of 0–30 seconds.

The default value is 6 seconds.

To configure the t3513 timer, use the following configuration: 

config 
    amf-global 
        call-control-policy policy_name 
            timers t3513  { attempts paging_attemps_count | value timeout_value } 
            end

NOTES:

  • timers t3513 attempts paging_attemps_count —Specify the number of paging attempts.

  • timers t3513 value timeout_value —Specify the t3513 timeout value.

    For the t3513 timer configuration values, refer to the 3GPP Timers and Values table.

Configuring the Non-3GPP Timers

To configure the Non-3GPP timers, use the following configuration:

config 
    amf-global 
        call-control-policy policy_name 
            timers timer_type value timer_value 
            end

NOTES:

  • timers timer_type value timer_value —Specify the timeout value.

    For the timer_type, refer to the following table.

Table 13. Non-3GPP Timers and Values

Timer

Timeout Value

context-transfer-guard

Must be an integer in the range of 0–35712000 seconds.

The default value is zero seconds.

proc-timeout ue-registration

Must be an integer in the range of 10–120 seconds.

tidle

Must be an integer in the range of 30–25200 seconds.

tn2

Must be an integer in the range of 0–35712000 seconds.

The default value is six seconds.

Configuring the IDLE Timer

To configure the IDLE timer, use the following configuration:

config 
    amf-global 
        call-control-policy policy_name 
            timers tidle value timeout_value 
            end

NOTES:

  • timers tidle value timeout_value —Specify the IDLE timeout value in seconds.

Configuration Example

The following is an example configuration.

config
   amf-global
      call-control-policy local
         timers tidle 2
         end

Configuring the Procedural Timer

To configure the Procedural timer for AMF, use the following configuration:

config 
    amf-global 
        timers proc-timeout ue-registration value timeout_value 
        end

NOTES:

  • timers procedure-timeout ue-registration value timeout_value —Specify the UE Registration procedure timeout value in seconds. Must be an integer in the range of 10-120 seconds.

Configuration Example

The following is an example configuration.

config
   amf-global
      call-control-policy local
         timers proc-timeout pdu-create value 20
         end