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TECHNICAL SPECIFICATION
5G;
5G System;
Restoration procedures
(3GPP TS 23.527 version 15.2.0 Release 15)
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3GPP TS 23.527 version 15.2.0 Release 15 1 ETSI TS 123 527 V15.2.0 (2019-04)
Reference
RTS/TSGC-0423527vf20
Keywords
5G
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3GPP TS 23.527 version 15.2.0 Release 15 2 ETSI TS 123 527 V15.2.0 (2019-04)
Intellectual Property Rights
Essential patents
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Foreword
This Technical Specification (TS) has been produced by ETSI 3rd Generation Partnership Project (3GPP).
The present document may refer to technical specifications or reports using their 3GPP identities, UMTS identities or
GSM identities. These should be interpreted as being references to the corresponding ETSI deliverables.
The cross reference between GSM, UMTS, 3GPP and ETSI identities can be found under
.
Modal verbs terminology
In the present document "shall", "shall not", "should", "should not", "may", "need not", "will", "will not", "can" and
"cannot" are to be interpreted as described in clause 3.2 of the ETSI Drafting Rules (Verbal forms for the expression of
provisions).
"must" and "must not" are NOT allowed in ETSI deliverables except when used in direct citation.
ETSI
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3GPP TS 23.527 version 15.2.0 Release 15 3 ETSI TS 123 527 V15.2.0 (2019-04)
Contents
Intellectual Property Rights . 2
Foreword . 2
Modal verbs terminology . 2
Foreword . 5
1 Scope . 6
2 References . 6
3 Definitions and abbreviations . 6
3.1 Definitions . 6
3.2 Abbreviations . 6
4 Restoration Procedures related to the N4 Interface . 6
4.1 General . 6
4.2 N4 Failure and Restart Detection . 7
4.3 UPF Restoration Procedures. 7
4.3.1 General . 7
4.3.2 Restoration Procedure for PSA UPF Restart . 7
4.3.3 Restoration Procedure for PSA UPF Failure without Restart . 7
4.3.4 Restoration Procedure for Intermediate UPF Restart . 7
4.3.5 Restoration Procedure for Intermediate UPF Failure without Restart . 8
4.4 SMF Restoration Procedures . 8
4.4.1 General . 8
4.4.2 Restoration Procedure for SMF Restart . 8
4.4.3 Restoration Procedure for SMF Failure without Restart . 8
4.5 N4 path failure . 8
5 Restoration Procedures related to the User Plane Interfaces N3 and N9 . 8
5.1 General . 8
5.2 User Plane Failure Detection . 9
5.2.1 Loss of GTP-U contexts . 9
5.2.2 User Plane Path Failure . 9
5.3 Restoration Procedures upon Loss of GTP-U contexts . 9
5.3.1 General . 9
5.3.2 Procedure for GTP-U Error Indication received from 5G-AN . 9
5.3.2.1 Principles . 9
5.3.3 Procedure for GTP-U Error Indication received from UPF . 10
5.3.3.1 GTP-U Error Indication received by 5G-AN . 10
5.3.3.2 GTP-U Error Indication received by another UPF . 10
5.4 Restoration Procedures upon User Plane Path Failure . 10
6 Restoration Procedures related to Service-Based Interfaces . 11
6.1 General . 11
6.2 NF (NF Service) Failure and Restart Detection using the NRF . 11
6.2.1 General . 11
6.2.2 NF (NF Service) Failure . 11
6.2.3 NF (NF Service) Restart . 13
6.3 NF Service Producer Restart Detection using direct signalling between NFs . 15
6.3.1 General . 15
6.3.2 NF Service Producer Restart . 15
6.4 NF Service Consumer Restart Detection using direct signalling between NFs . 16
6.4.1 General . 16
6.4.2 NF Service Consumer Restart . 17
6.5 NF Service Instance Reselection . 18
7 Restoration Procedures related to Public Warning System (PWS) . 18
7.1 General . 18
7.2 PWS operation failure in NG-RAN . 18
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3GPP TS 23.527 version 15.2.0 Release 15 4 ETSI TS 123 527 V15.2.0 (2019-04)
7.3 PWS operation restart in NG-RAN . 18
Annex A (informative): Change history . 20
History . 21
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Foreword
This Technical Specification has been produced by the 3rd Generation Partnership Project (3GPP).
The contents of the present document are subject to continuing work within the TSG and may change following formal
TSG approval. Should the TSG modify the contents of the present document, it will be re-released by the TSG with an
identifying change of release date and an increase in version number as follows:
Version x.y.z
where:
x the first digit:
1 presented to TSG for information;
2 presented to TSG for approval;
3 or greater indicates TSG approved document under change control.
y the second digit is incremented for all changes of substance, i.e. technical enhancements, corrections,
updates, etc.
z the third digit is incremented when editorial only changes have been incorporated in the document.
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3GPP TS 23.527 version 15.2.0 Release 15 6 ETSI TS 123 527 V15.2.0 (2019-04)
1 Scope
The present document defines the restoration procedures in the 5G System.
2 References
The following documents contain provisions which, through reference in this text, constitute provisions of the present
document.
- References are either specific (identified by date of publication, edition number, version number, etc.) or
non-specific.
- For a specific reference, subsequent revisions do not apply.
- For a non-specific reference, the latest version applies. In the case of a reference to a 3GPP document (including
a GSM document), a non-specific reference implicitly refers to the latest version of that document in the same
Release as the present document.
[1] 3GPP TR 21.905: "Vocabulary for 3GPP Specifications".
[2] 3GPP TS 23.007: "Restoration procedures".
[3] 3GPP TS 29.281: "General Packet Radio System (GPRS) Tunneling Protocol User Plane (GTPv1-
U)".
[4] 3GPP TS 29.244: "Interface between the Control Plane and the User Plane Nodes; Stage 3".
[5] 3GPP TS 23.502:"Procedures for the 5G System; Stage 2"
[6] 3GPP TS 29.518: "5G System; Access and Mobility Management Service; Stage 3".
[7] 3GPP TS 29.510: "5G System; Network Function Repository Services; Stage 3".
[8] 3GPP TS 23.041: "Technical realization of Cell Broadcast Service (CBS)".
3 Definitions and abbreviations
3.1 Definitions
For the purposes of the present document, the terms and definitions given in 3GPP TR 21.905 [1] and the following
apply. A term defined in the present document takes precedence over the definition of the same term, if any, in 3GPP
TR 21.905 [1].
3.2 Abbreviations
For the purposes of the present document, the abbreviations given in 3GPP TR 21.905 [1] and the following apply. An
abbreviation defined in the present document takes precedence over the definition of the same abbreviation, if any, in
3GPP TR 21.905 [1].
F-SEID Fully Qualified SEID
PFCP Packet Forwarding Control Protocol
PSA PDU Session Anchor
4 Restoration Procedures related to the N4 Interface
4.1 General
This clause specifies the procedures supported in the 5G System to detect and handle failures affecting the N4 interface.
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4.2 N4 Failure and Restart Detection
Across PFCP based interfaces an SMF and UPF shall utilize the PFCP Heartbeat Request and Heartbeat Response
messages or PFCP messages containing the Recovery Time Stamp to detect a peer PFCP failure or restart as described
in clause 19A of 3GPP TS 23.007 [2].
4.3 UPF Restoration Procedures
4.3.1 General
When a UPF fails, all its Session contexts and PFCP associations affected by the failure become invalid and may be
deleted.
4.3.2 Restoration Procedure for PSA UPF Restart
If F-TEID allocation is performed in the UPF, the UPF shall ensure that previously used F-TEID values are not
immediately reused after a UPF restart, in order to avoid inconsistent TEID allocation throughout the network and to
enable the restoration of PFCP sessions affected by the failure. How this is ensured is implementation specific.
The UPF shall not send an Error indication message for a configurable period after an UPF restart when the UPF
receives a G-PDU not matching any PDRs.
During or immediately after an UPF Restart, the UPF shall place a local UPF Recovery Time Stamp value in all
Heartbeat Request/Response and PFCP Association Setup Request/Response Messages.
Immediately after the re-establishment of a PFCP association between the SMF and the UPF, the SMF may start
restoring PFCP sessions in the UPF.
4.3.3 Restoration Procedure for PSA UPF Failure without Restart
Procedures for PSA UPF failure without restart are implementation specific.
4.3.4 Restoration Procedure for Intermediate UPF Restart
The SMF will receive the UPF recovery time stamps in the PFCP association setup Request/Response and the PFCP
heartbeat requests/responses.
After an Intermediate UPF restart, the PFCP association between the SMF(s) and the Intermediate UPF has to be re-
established.
The restoration of the PFCP sessions may start immediately after the PFCP association setup procedure:
- if the restoration is supported in the SMF on a proactive basis, the SMF may start re-establishing PFCP sessions
matching any PDRs.
- if the restoration is supported in the SMF on a reactive basis:
- the SMF shall establish an PFCP session with a wildcarded PDR to instruct the Intermediate UPF to forward
G-PDU packets which are not matching any other PDRs to the SMF (to a F-TEID uniquely assigned in the
SMF for this PFCP-u tunnel);
- upon receipt of G-PDUs from this PFCP-u tunnel, the SMF shall then check if it has an active session for
each received G-PDU packet:
- if so, the SMF shall perform the PFCP Session establishment procedures to re-establish the corresponding
PFCP sessions in the Intermediate UPF;
- otherwise the SMF shall generate a GTP-U Error Indication with a destination address set to the source IP
address of the received G-PDU, and send it to the Intermediate UPF. The Intermediate UPF shall forward
this GTP-U Error Indication transparently. The SMF shall delete the G-PDU after the check for active
sessions.
NOTE 1: The UPF can filter the G-PDU packets with same target F-TEID and send only one such G-PDU to the
Intermediate SMF.
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The Intermediate UPF shall not send any Error indication messages for a configurable period after an Intermediate UPF
restart when the Intermediate UPF receives G-PDU not matching any PDRs.
NOTE 2: If restoration on a reactive basis is used, the period needs to be longer than the time required by the SMF
to detect the UPF restart, to establish the PFCP association and provision the wildcarded PDR. Otherwise,
the period needs to be longer than the time required by the SMF to restore all the PFCP sessions on a
proactive basis.
4.3.5 Restoration Procedure for Intermediate UPF Failure without Restart
Procedures for Intermediate UPF failure without restart are implementation specific.
4.4 SMF Restoration Procedures
4.4.1 General
When a SMF fails, all its PDU session contexts and PFCP associations affected by the failure may become invalid and
may be deleted.
If F-TEID allocation is performed in the SMF, the SMF should ensure as far as possible that previously used F-TEID
values are not immediately reused after a SMF restart, in order to avoid inconsistent TEID allocation throughout the
network.
NOTE: This is to ensure that F-TEIDs are not reused until earlier PDU sessions using them are released.
4.4.2 Restoration Procedure for SMF Restart
During or immediately after a SMF Restart, the SMF shall place local SMF-C Recovery Time Stamp value in all
Heartbeat Request/Response and PFCP Association Setup Request/Response Messages.
The UPF will receive the SMF recovery time stamps in PFCP Association Setup Request/Response and PFCP heartbeat
requests/responses.
When a UPF detects that the SMF has restarted (as specified in subclause 4.2), the UPF shall delete all session contexts
and the PFCP association context affected by the restart that it may have stored. When the UPF receives a GTP-U PDU
not matching any PDRs, it shall discard the GTP-U PDU and return a GTP error indication to the originating node (e.g.
other UPF, gNB or N3IWF).
4.4.3 Restoration Procedure for SMF Failure without Restart
When a UPF detects that a SMF is not reachable for a preconfigured time, the UPF shall delete all the session contexts
and the PFCP association context affected by the SMF failure that it may have stored.
4.5 N4 path failure
If the N4 path to the UPF is down, the SMF should handle this as an UPF Failure without Restart, see subclause 4.3.3.
If the N4 path to the SMF is down, the UPF should handle this as a SMF Failure without Restart, see subclause 4.4.3.
5 Restoration Procedures related to the User Plane
Interfaces N3 and N9
5.1 General
This clause specifies the procedures supported in the 5G System to detect and handle failures affecting the user plane
interfaces N3 and N9
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5.2 User Plane Failure Detection
5.2.1 Loss of GTP-U contexts
A GTP-U entity may lose its GTP-U contexts upon a failure or restart.
When a GTP-U node receives a G-PDU for which no corresponding GTP-U tunnel exists, the GTP-U node shall discard
the G-PDU and return a GTP-U Error Indication to the sending node, as specified in subclause 7.3.1 of
3GPP TS 29.281 [3].
The receipt of a GTP-U Error Indication is an indication for the sending GTP-U entity that the peer GTP-U entity
cannot receive any more user plane traffic on the corresponding GTP-U tunnel.
5.2.2 User Plane Path Failure
A GTP-U entity may detect a user plane path failure by using GTP-U Echo Request and Echo Response messages, as
specified in subclause 20.3.1 of 3GPP TS 23.007 [2].
5.3 Restoration Procedures upon Loss of GTP-U contexts
5.3.1 General
The following subclauses specify the behaviour of the different network entities when receiving a GTP-U Error
Indication.
5.3.2 Procedure for GTP-U Error Indication received from 5G-AN
5.3.2.1 Principles
UE 5G-AN AMF SMF UPF
PDU session with activated User Plane connection
GTP-U failure
1. G-PDU
2. GTP-U Error Indication
3. N4 Session Report Req/Rsp
4. N4 Session Modification Reqt/Rsp
5. Namf_Communication_N1N2MessageTransfer Req/Rsp
6. PDU Session Resource Release Command
PDU Session Resource Release Response
7. Nsmf_PDUSession_UpdateSMContext Req/Rsp
8. Network Triggered Service Request
Downlink data
Figure 5.3.2.1-1: GTP-U Error Indication from 5G-AN
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1. The user plane connection of an existing PDU session is activated. Downlink G-PDUs are sent towards the 5G-
AN.
2. The 5G-AN returns a GTP-U Error Indication if it does not have a corresponding GTP-U context (see subclause
5.2).
3. Upon receipt of a GTP-U Error Indication, the UPF shall identify the related PFCP session and send an Error
Indication Report to the SMF, as specified in subclause 5.10 of 3GPP TS 29.244 [4].
4. For a GTP-U Error Indication received from a 5G-AN, the SMF shall modify the PFCP session to instruct the
UPF to buffer downlink packets.
5. If the user plane connection of the PDU session is seen as activated by the SMF, the SMF shall initiate an
Namf_Communication_N1N2MessageTransfer service operation to request the 5G-AN to release the PDU
session's resources, as specified in subclause 4.3.7 of 3GPP TS 23.502 [5].
6. Upon receipt of an Namf_Communication_N1N2MessageTransfer request to transfer the PDU Session Resource
Release Command, the AMF shall:
- proceed with the request, as specified in subclause 5.2.2.3.1 of 3GPP TS 29.518 [6], if the UE is in CM-
CONNECTED state for the Access Network Type associated to the PDU session;
- otherwise, reject the request with an error indicating that the UE is in CM-IDLE state for the Access Network
Type associated to the PDU session.
7. If the AMF sent a PDU Session Resource Release Command to the 5G-AN, the PDU session's resource
release is acknowledged to the SMF.
8. The SMF initiates the Network Triggered Service Request procedure specified in subclause 4.2.3.3 of
3GPP TS 23.502 [5], to re-activate the user plane connection of the PDU session.
5.3.3 Procedure for GTP-U Error Indication received from UPF
5.3.3.1 GTP-U Error Indication received by 5G-AN
Upon receipt of a GTP-U Error Indication, the 5G-AN shall proceed as follows:
- if the GTP-U Error Indication was received from an UPF for a NG-U tunnel other than an indirect forwarding
tunnel, the 5G-AN shall initiate a PDU Session Resource Notify procedure and release immediately the
resources of the PDU session for which the Error Indication was received;
- if the GTP-U Error Indication was received from a peer 5G-AN or UPF for a direct or indirect forwarding
tunnel, the 5G-AN may ignore the error indication or delete the forwarding tunnel context locally without
deleting the corresponding PDU session and bearers.
NOTE: The 5G-AN behaviour for dual connectivity is not described in this specification.
5.3.3.2 GTP-U Error Indication received by another UPF
Upon receipt of a GTP-U Error Indication, the UPF shall identify the related PFCP session and send an Error Indication
Report to the SMF, as specified in subclause 5.10 of 3GPP TS 29.244 [4].
Upon receipt of an Error Indication Report from the UPF, the SMF shall identify the PDU session for which the Error
Indication is received using the remote F-TEID included in the report.
For a GTP-U Error Indication received from another UPF, the SMF shall delete the PFCP session and PDU session,
unless the UPF from which the Error Indication was received is controlled by the same SMF and the SMF is able to
restore the user plane connectivity of the PDU session (e.g. Error Indication received from an Intermediate UPF
controlled by the same SMF).
5.4 Restoration Procedures upon User Plane Path Failure
Upon detecting a GTP-U user plane path failure as specified in subclause 5.2.2, the UPF shall report the user plane path
failure to the SMF, by sending a PFCP Node Report Request (see 3GPP TS 29.244 [4]) including a User Plane Path
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Failure Report with the IP address of the remote GTP-U peer(s) towards which a failure has been detected. The UPF
should also notify the GTP-U user plane path failure via the Operation and Maintenance system.
When the SMF receives the PFCP Node Report Request with a User Plane Path Failure Report, the SMF may:
- delete the PDU session contexts associated with the path in failure; or
- maintain the PDU session contexts associated with the path in failure during an operator configurable maximum
path failure duration. The SMF shall delete the PDU session contexts associated with the path in failure if the
path is still down when this duration expires.
NOTE 1: During transient path failures (e.g. path failures not exceeding few minutes at most), maintaining the PDU
session contexts associated with the peer's IP address enables the delivery of end user services (when the
path is re-established again) and this also avoids unnecessary signalling in the network for restoring those
PDU sessions.
NOTE 2: It is not intended to maintain PDU session contexts during long path failures (e.g. exceeding few minutes
at most) as this would imply undesirable effects like undue charging.
When deciding to delete the PDU session contexts associated with the path in failure, the SMF shall modify or delete
the affected PFCP sessions in the UPF.
NOTE 3: The SMF need to take care to smoothen the signalling load towards the UPF if a
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