ETSI TS 124 577 V18.4.0 (2026-03)

5G; Aircraft-to-Everything (A2X) services in 5G System (5GS) protocol aspects; Stage 3 (3GPP TS 24.577 version 18.4.0 Release 18)

RTS/TSGC-0124577vi40

General Information

• Status: Not Published
• Technical Committee: 3GPP CT 1 - MM/CC/SM [lu] (formely CN1)
• Current Stage: 8 - Draft receipt by ETSI Secretariat
• Completion Date: 10-Mar-2026

Standards Content (Sample)

TECHNICAL SPECIFICATION
5G;
Aircraft-to-Everything (A2X) services in 5G System (5GS)
protocol aspects;
Stage 3
(3GPP TS 24.577 version 18.4.0 Release 18)

3GPP TS 24.577 version 18.4.0 Release 18 1 ETSI TS 124 577 V18.4.0 (2026-03)

Reference
RTS/TSGC-0124577vi40
Keywords
5G
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ETSI
3GPP TS 24.577 version 18.4.0 Release 18 2 ETSI TS 124 577 V18.4.0 (2026-03)
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ETSI
3GPP TS 24.577 version 18.4.0 Release 18 3 ETSI TS 124 577 V18.4.0 (2026-03)
Contents
Intellectual Property Rights . 2
Legal Notice . 2
Modal verbs terminology . 2
Foreword . 9
1 Scope . 11
2 References . 11
3 Definitions of terms, symbols and abbreviations . 12
3.1 Terms . 12
3.2 Abbreviations . 12
4 General description. 13
5 Provisioning of parameters for A2X configuration . 13
5.1 General . 13
5.2 Configuration and precedence of A2X configuration parameters . 13
5.2.1 General . 13
5.2.2 Precedence of A2X configuration parameters . 13
5.2.3 Configuration parameters for A2X communication over PC5 . 14
5.2.4 Configuration parameters for broadcast remote ID (BRID) . 15
5.2.5 Configuration parameters for direct detect and avoid (DDAA) . 15
5.2.6 Configuration parameters for direct C2 communication over PC5 . 16
5.2.6A Configuration parameters for ground based detect and avoid for an area (GBDAAA) . 16
5.2.7 Configuration parameters for A2X communication over Uu . 16
5.3 Procedures . 17
5.3.1 General . 17
5.3.2 UE-requested A2X policy provisioning procedure . 17
5.3.2.1 General . 17
5.3.2.2 UE-requested A2X policy provisioning procedure initiation . 18
5.3.2.3 UE-requested A2X policy provisioning procedure accepted by the network . 18
5.3.2.4 UE-requested A2X policy provisioning procedure not accepted by the network . 18
5.3.2.5 Abnormal cases on the network side . 18
5.3.2.6 Abnormal cases on the UE . 19
6 A2X communication . 19
6.1 A2X communication over PC5 . 19
6.1.1 General . 19
6.1.2 Unicast mode A2X communication over NR-PC5 . 19
6.1.2.1 Overview . 19
6.1.2.2 A2X PC5 unicast link establishment procedure . 20
6.1.2.2.1 General . 20
6.1.2.2.2 A2X PC5 unicast link establishment procedure initiation by initiating UE . 20
6.1.2.2.3 A2X PC5 unicast link establishment procedure accepted by the target UE . 23
6.1.2.2.4 A2X PC5 unicast link establishment procedure completion by the initiating UE . 24
6.1.2.2.5 A2X PC5 unicast link establishment procedure not accepted by the target UE . 25
6.1.2.2.6 Abnormal cases . 26
6.1.2.3 A2X PC5 unicast link modification procedure . 26
6.1.2.3.1 General . 26
6.1.2.3.2 A2X PC5 unicast link modification procedure initiated by initiating UE . 27
6.1.2.3.3 A2X PC5 unicast link modification procedure accepted by the target UE . 28
6.1.2.3.4 A2X PC5 unicast link modification procedure completion by the initiating UE. 29
6.1.2.3.5 A2X PC5 unicast link modification procedure not accepted by the target UE . 29
6.1.2.3.6 Abnormal cases at the initiating UE . 30
6.1.2.4 A2X PC5 unicast link release procedure . 30
6.1.2.4.1 General . 30
6.1.2.4.2 A2X PC5 unicast link release procedure initiation by initiating UE . 30
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3GPP TS 24.577 version 18.4.0 Release 18 4 ETSI TS 124 577 V18.4.0 (2026-03)
6.1.2.4.3 A2X PC5 unicast link release procedure accepted by the target UE . 31
6.1.2.4.4 A2X PC5 unicast link release procedure completion by the initiating UE . 31
6.1.2.4.5 Abnormal cases . 32
6.1.2.5 A2X PC5 unicast link identifier update procedure . 32
6.1.2.5.1 General . 32
6.1.2.5.2 A2X PC5 unicast link identifier update procedure initiation by initiating UE . 32
6.1.2.5.3 A2X PC5 unicast link identifier update procedure accepted by the target UE . 33
6.1.2.5.4 A2X PC5 unicast link identifier update procedure acknowledged by the initiating UE . 34
6.1.2.5.5 A2X PC5 unicast link identifier update procedure completion by the target UE . 34
6.1.2.5.6 A2X PC5 unicast link identifier update procedure not accepted by the target UE . 34
6.1.2.5.7 Abnormal cases . 35
6.1.2.6 A2X PC5 unicast link authentication procedure . 36
6.1.2.6.1 General . 36
6.1.2.6.2 A2X PC5 unicast link authentication procedure initiation by the initiating UE . 36
6.1.2.6.3 A2X PC5 unicast link authentication procedure accepted by the target UE . 37
6.1.2.6.4 A2X PC5 unicast link authentication procedure completion by the initiating UE . 38
6.1.2.6.5 A2X PC5 unicast link authentication procedure not accepted by the target UE . 38
6.1.2.6.5A A2X PC5 unicast link authentication procedure not accepted by the initiating UE . 38
6.1.2.6.6 Abnormal cases . 38
6.1.2.7 A2X PC5 unicast link security mode control procedure . 39
6.1.2.7.1 General . 39
6.1.2.7.2 A2X PC5 unicast link security mode control procedure initiation by the initiating UE . 39
6.1.2.7.3 A2X PC5 unicast link security mode control procedure accepted by the target UE . 41
6.1.2.7.4 A2X PC5 unicast link security mode control procedure completion by the initiating UE . 43
6.1.2.7.5 A2X PC5 unicast link security mode control procedure not accepted by the target UE . 43
6.1.2.7.6 Abnormal cases . 44
6.1.2.8 A2X PC5 unicast link keep-alive procedure . 44
6.1.2.8.1 General . 44
6.1.2.8.2 A2X PC5 unicast link keep-alive procedure initiation by the initiating UE . 45
6.1.2.8.3 A2X PC5 unicast link keep-alive procedure accepted by the target UE . 46
6.1.2.8.4 A2X PC5 unicast link keep-alive procedure completion by the initiating UE . 46
6.1.2.8.5 Abnormal cases . 46
6.1.2.9 Data transmission over A2X PC5 unicast link . 47
6.1.2.9.1 Transmission . 47
6.1.2.9.2 Procedure for UE to use provisioned radio resources for A2X communication over PC5 . 47
6.1.2.10 A2X PC5 unicast link re-keying procedure . 48
6.1.2.10.1 General . 48
6.1.2.10.2 A2X PC5 unicast link re-keying procedure initiation by the initiating UE . 48
6.1.2.10.3 A2X PC5 unicast link re-keying procedure accepted by the target UE . 49
6.1.2.10.4 A2X PC5 unicast link re-keying procedure completion by the initiating UE . 49
6.1.2.10.5 Abnormal cases at the initiating UE . 49
6.1.2.11 A2X PC5 unicast link security . 50
6.1.2.11.1 Overview . 50
6.1.2.11.2 Handling of A2X PC5 unicast security contexts . 50
6.1.2.11.3 Checking of PC5 signalling messages in the UE . 51
6.1.2.12 PC5 QoS flow establishment over A2X PC5 unicast link. 51
6.1.2.13 PC5 QoS flow match over A2X PC5 unicast link . 52
6.1.3 Broadcast mode A2X communication over PC5 . 53
6.1.3.1 Overview . 53
6.1.3.2 Transmission of broadcast mode A2X communication over PC5 . 53
6.1.3.2.1 Initiation . 53
6.1.3.2.2 Transmission . 55
6.1.3.2.3 Procedure for UE to use provisioned radio resources for A2X communication over PC5 . 56
6.1.3.2.4 Privacy of A2X transmission over PC5 . 58
6.1.3.3 Reception of broadcast mode A2X communication over PC5 . 58
6.2 A2X communication over Uu . 59
6.2.1 General . 59
6.2.2 Transmission of A2X communication over Uu from UE to A2X application server . 59
6.2.3 Reception of A2X communication over Uu from UE to A2X application server . 60
6.2.4 Transmission of A2X communication over Uu from A2X application server to UE . 60
6.2.5 Reception of A2X communication over Uu from A2X application server to UE . 61
6.2.6 A2X application server discovery . 63
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6.2.6.1 A2X application server discovery using MBS . 67
6.2.6.1.1 General . 67
6.2.6.1.2 Procedure for A2X application server discovery using MBS . 67
6.2.7 A2X application server configuration . 67
6.2.8 A2X MBS parameter discovery . 68
7 Broadcast remote ID (BRID) . 69
7.1 General . 69
7.2 Procedures . 69
8 Direct detect and avoid (DDAA) over PC5 . 69
8.1 General . 69
8.2 Procedures . 70
8A Ground based detect and avoid for an area (GBDAAA) . 70
8A.1 General . 70
8A.2 Procedures . 70
9 Direct C2 communication over PC5 . 71
9.1 General . 71
9.2 Procedures . 71
9.2.1 C2 authorization procedure for direct C2 communication . 71
9.2.2 Direct C2 communication procedures over NR-PC5 . 71
10 Handling of unknown, unforeseen, and erroneous PC5 signalling protocol data . 72
10.1 General . 72
10.2 Message too short or too long . 72
10.2.1 Message too short . 72
10.2.2 Message too long . 72
10.3 Unknown or unforeseen message type . 72
10.4 Non-semantical mandatory information element errors . 73
10.5 Unknown and unforeseen IEs in the non-imperative message part . 73
10.5.1 IEIs unknown in the message . 73
10.5.2 Out of sequence IEs . 73
10.5.3 Repeated IEs . 73
10.6 Non-imperative message part errors . 73
10.6.1 General . 73
10.6.2 Syntactically incorrect optional IEs . 73
10.6.3 Conditional IE errors . 74
10.7 Messages with semantically incorrect contents . 74
11 Message functional definition and contents . 74
11.1 Overview . 74
11.2 A2X communication over PC5 signalling messages . 74
11.2.1 A2X Direct link establishment request . 74
11.2.1.1 Message definition . 74
11.2.1.2 Target user info . 75
11.2.1.3 Key establishment information container. 75
11.2.1.4 Nonce_1 . 75
11.2.1.5 MSB of K ID . 75
NRP-sess
11.2.1.6 K ID . 75
NRP
11.2.2 A2X Direct link establishment accept . 75
11.2.2.1 Message definition . 75
11.2.2.2 IP address configuration . 76
11.2.2.3 Link local IPv6 address . 76
11.2.3 A2X Direct link establishment reject . 76
11.2.3.1 Message definition . 76
11.2.4 A2X Direct link modification request . 77
11.2.4.1 Message definition . 77
11.2.5 A2X Direct link modification accept . 77
11.2.5.1 Message definition . 77
11.2.5.2 QoS flow descriptions . 77
11.2.6 A2X Direct link modification reject . 78
11.2.6.1 Message definition . 78
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3GPP TS 24.577 version 18.4.0 Release 18 6 ETSI TS 124 577 V18.4.0 (2026-03)
11.2.7 A2X Direct link release request . 78
11.2.7.1 Message definition . 78
11.2.8 A2X Direct link release accept . 78
11.2.8.1 Message definition . 78
11.2.9 A2X Direct link identifier update request . 79
11.2.9.1 Message definition . 79
11.2.9.2 Source user info . 79
11.2.9.3 Source link local IPv6 address . 79
11.2.10 A2X Direct link identifier update accept . 79
11.2.10.1 Message definition . 79
11.2.10.2 Target user info . 80
11.2.10.3 Target link local IPv6 address . 80
11.2.10.4 Source user info . 80
11.2.10.5 Source link local IPv6 address . 80
11.2.11 A2X Direct link identifier update ack . 80
11.2.11.1 Message definition . 80
11.2.11.2 Target user info . 81
11.2.11.3 Target link local IPv6 address . 81
11.2.12 A2X Direct link identifier update reject . 81
11.2.12.1 Message definition . 81
11.2.13 A2X Direct link keepalive request . 81
11.2.13.1 Message definition . 81
11.2.13.2 Maximum inactivity period . 82
11.2.14 A2X Direct link keepalive response . 82
11.2.14.1 Message definition . 82
11.2.15 A2X Direct link authentication request . 82
11.2.15.1 Message definition . 82
11.2.16 A2X Direct link authentication response . 83
11.2.16.1 Message definition . 83
11.2.17 A2X Direct link authentication reject . 83
11.2.17.1 Message definition . 83
11.2.18 A2X Direct link authentication failure. 84
11.2.18.1 Message definition . 84
11.2.18.2 Key establishment information container. 84
11.2.19 A2X Direct link security mode command . 84
11.2.19.1 Message definition . 84
11.2.19.2 Nonce_2 . 85
11.2.19.3 LSB of KNRP-sess ID . 85
11.2.19.4 Key establishment information container. 85
11.2.19.5 MSBs of K ID . 85
NRP
11.2.19.6 UE PC5 unicast signalling security policy . 85
11.2.20 A2X Direct link security mode complete . 85
11.2.20.1 Message definition . 85
11.2.20.2 IP address configuration . 86
11.2.20.3 Link local IPv6 address . 86
11.2.20.4 LSBs of K ID . 86
NRP
11.2.21 A2X Direct link security mode reject . 86
11.2.21.1 Message definition . 86
11.2.22 A2X Direct link rekeying request . 87
11.2.22.1 Message definition . 87
11.2.22.2 Key establishment information container. 87
11.2.22.3 Nonce_1 . 87
11.2.22.4 MSB of KNRP-sess ID . 87
11.2.22.5 Re-authentication indication . 87
11.2.23 A2X Direct link rekeying response . 87
11.2.23.1 Message definition . 87
12 Information elements coding . 88
12.1 Overview . 88
12.2 General . 88
12.3 A2X communication over PC5 signalling information elements . 88
12.3.1 A2X PC5 signalling message type . 88
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3GPP TS 24.577 version 18.4.0 Release 18 7 ETSI TS 124 577 V18.4.0 (2026-03)
12.3.2 Sequence number . 89
12.3.3 A2X service identifier . 89
12.3.4 Application layer ID . 90
12.3.5 PC5 QoS flow descriptions . 90
12.3.6 IP address configuration . 97
12.3.7 Link local IPv6 address . 97
12.3.8 Link modification operation code . 97
12.3.9 PC5 signalling protocol cause . 98
12.3.10 Keep-alive counter . 99
12.3.11 Maximum inactivity period . 99
12.3.12 Layer-2 ID . 100
12.3.13 Nonce . 100
12.3.14 UE security capabilities . 101
12.3.15 UE PC5 unicast signalling security policy . 104
12.3.16 MSB of K ID . 104
NRP-sess
12.3.17 K ID . 105
NRP
12.3.18 Selected security algorithms . 105
12.3.19 LSB of K ID . 106
NRP-sess
12.3.20 MSBs of K ID . 106
NRP
12.3.21 LSBs of K I
...