6.4.3 UE-requested PDU session release procedure

24.5013GPPNon-Access-Stratum (NAS) protocol for 5G System (5GS)Release 17Stage 3TS

6.4.3.1 General

The purpose of the UE-requested PDU session release procedure is to enable by the UE to request a release of a PDU session.

The UE is allowed to initiate the PDU session release procedure even if the timer T3396 is running.

The UE is allowed to initiate the PDU session release procedure even if the timer T3584 is running.

The UE is allowed to initiate the PDU session release procedure even if the timer T3585 is running.

The UE is allowed to initiate the PDU session release procedure even if the UE is outside the LADN service area.

6.4.3.2 UE-requested PDU session release procedure initiation

In order to initiate the UE-requested PDU session release procedure, the UE shall create a PDU SESSION RELEASE REQUEST message.

The UE may set the 5GSM cause IE of the PDU SESSION RELEASE REQUEST message to indicate the reason for releasing the PDU session.

The 5GSM cause IE typically indicates one of the following 5GSM cause values:

#26 insufficient resources;

#36 regular deactivation;

#44 Semantic errors in packet filter(s);

#45 Syntactical error in packet filter(s);

#83 Semantic error in the QoS operation;

#84 Syntactical error in the QoS operation;

#96 Invalid mandatory information.

The UE shall allocate a PTI value currently not used and shall set the PTI IE of the PDU SESSION RELEASE REQUEST message to the allocated PTI value.

The UE shall transport the PDU SESSION RELEASE REQUEST message and the PDU session ID, using the NAS transport procedure as specified in subclause 5.4.5, and the UE shall start timer T3582 (see example in figure 6.4.3.2.1).

If the UE is releasing the PDU session due to:

a) errors in QoS operations or packet filters; or

b) the number of the authorized QoS rules, the number of the packet filters, or the number of the authorized QoS flow descriptions associated with the PDU session have reached the maximum number supported by the UE,

the UE shall include the 5GSM cause IE in the PDU SESSION RELEASE REQUEST message as described in subclauses 6.3.2.4 and 6.4.1.3.

Figure 6.4.3.2.1: UE-requested PDU session release procedure

6.4.3.3 UE-requested PDU session release procedure accepted by the network

Upon receipt of a PDU SESSION RELEASE REQUEST message and a PDU session ID, if the SMF accepts the request to release the PDU session, and shall perform the network-requested PDU session release procedure as specified in subclause 6.3.3.

6.4.3.4 UE-requested PDU session release procedure not accepted by the network

Upon receipt of a PDU SESSION RELEASE REQUEST message, if the SMF does not accept the request to release the PDU session, the SMF shall create a PDU SESSION RELEASE REJECT message.

The SMF shall set the 5GSM cause IE of the PDU SESSION RELEASE REJECT message to indicate the reason for rejecting the PDU session release.

The 5GSM cause IE typically indicates one of the following SM cause values:

#35 PTI already in use; or

#43 Invalid PDU session identity; or

#95 – 111: protocol errors.

The SMF shall send the PDU SESSION RELEASE REJECT message.

Upon receipt of a PDU SESSION RELEASE REJECT message and a PDU session ID, using the NAS transport procedure as specified in subclause 5.4.5, the UE shall stop timer T3582, release the allocated PTI value and locally release the PDU session.

6.4.3.5 Abnormal cases in the UE

The following abnormal cases can be identified:

a) Expiry of timer T3582.

The UE shall, on the first expiry of the timer T3582, retransmit the PDU SESSION RELEASE REQUEST message and the PDU session information which was transported together with the initial transmission of the PDU SESSION RELEASE REQUEST message and shall reset and start timer T3582. This retransmission is repeated four times, i.e. on the fifth expiry of timer T3582, the UE shall abort the procedure, release the allocated PTI, perform a local release of the PDU session, and perform the registration procedure for mobility and periodic registration update by sending a REGISTRATION REQUEST message including the PDU session status IE over each access that user plane resources have been established if the PDU session is an MA PDU session, or over the access the PDU session is associated with if the PDU session is a single access PDU session.

b) Collision of UE-requested PDU session release procedure and network-requested PDU session modification procedure.

When the UE receives a PDU SESSION MODIFICATION COMMAND message during the UE-requested PDU session release procedure, and the PDU session indicated in PDU SESSION MODIFICATION COMMAND message is the PDU session that the UE had requested to release, the UE shall ignore the PDU SESSION MODIFICATION COMMAND message and proceed with the PDU session release procedure.

c) Collision of UE-requested PDU session release procedure and network-requested PDU session release procedure.

When the UE receives a PDU SESSION RELEASE COMMAND message with the PTI IE set to "No procedure transaction identity assigned" during the UE-requested PDU session release procedure, the PDU session indicated in the PDU SESSION RELEASE COMMAND message is the same as the PDU session that the UE requests to release:

– if the Access type IE is included in the PDU SESSION RELEASE COMMAND message and the PDU session is an MA PDU session and having user-plane resources established on the access different from the access indicated in the Access type IE in the PDU SESSION RELEASE COMMAND message, the UE shall proceed both the UE-requested PDU session release procedure and network-requested PDU session release procedure; or

– otherwise, the UE shall abort the UE-requested PDU session release procedure and shall stop the timer T3582 and proceed with the network-requested PDU session release procedure.

d) Receipt of an indication that the 5GSM message was not forwarded due to routing failure

Upon receiving an indication that the 5GSM message was not forwarded due to routing failure along with a PDU SESSION RELEASE REQUEST message with the PDU session ID IE set to the same value as the PDU session ID that was sent by the UE, the UE shall stop timer T3582, abort the procedure, release the allocated PTI, perform a local release of the PDU session, and perform the registration procedure for mobility and periodic registration update by sending a REGISTRATION REQUEST message including the PDU session status IE over each access that user plane resources have been established if the PDU session is an MA PDU session, or over the access the PDU session is associated with if the PDU session is a single access PDU session.

e) PDU session release signalling restricted due to service area restriction

The UE may delay the release of the PDU session until the UE is not restricted by service area restrictions, or it may release the allocated PTI, perform a local release of the PDU session, and include the PDU session status IE over each access that user plane resources have been established if the PDU session is an MA PDU session, or over the access the PDU session is associated with if the PDU session is a single access PDU when performing the next registration procedure.

f) Collision of UE-requested PDU session release procedure and N1 NAS signalling connection release

The UE may immediately retransmit the PDU SESSION RELEASE REQUEST message and stop, reset and restart timer T3582, if the following conditions apply:

1) The original UE-requested PDU session release procedure was initiated over an existing N1 NAS signalling connection;

2) the previous transmission of the PDU SESSION RELEASE REQUEST message was not initiated due to timer T3582 expiry; and

3) no 5GSM message related to the PDU session (e.g. PDU SESSION RELEASE REJECT message) or indication that the 5GSM message was not forwarded (see item d) was received after the PDU SESSION RELEASE REQUEST message was transmitted.

6.4.3.6 Abnormal cases on the network side

The following abnormal cases can be identified:

a) PDU session inactive for the received PDU session identity.

If the PDU session ID in the PDU SESSION RELEASE REQUEST message belongs to any PDU session in state PDU SESSION INACTIVE in the SMF, the SMF shall send the PDU SESSION RELEASE REJECT message to the UE with the 5GSM cause #43 "Invalid PDU session identity".