Missing 5GMM Capability in Registration Request

User reported tests of OAI UE simulator with free5GC core network:

OAI UE is encoding the Registration Request incorrectly.

In the attached packet trace file, frame 69 contains the offending packet:

UplinkNASTransport
    protocolIEs: 4 items
        Item 0: id-AMF-UE-NGAP-ID
        Item 1: id-RAN-UE-NGAP-ID
        Item 2: id-NAS-PDU
            ProtocolIE-Field
                id: id-NAS-PDU (38)
                criticality: reject (0)
                value
                    NAS-PDU: 7e04f370e7b3007e005e7700096557547698103214f371001d7e004119000d0100f1100000000007505501002e08e060000000000000
                        Non-Access-Stratum 5GS (NAS)PDU
                            Security protected NAS 5GS message
                            Plain NAS 5GS Message
                                Extended protocol discriminator: 5G mobility management messages (126)
                                0000 .... = Spare Half Octet: 0
                                .... 0000 = Security header type: Plain NAS message, not security protected (0)
                                Message type: Security mode complete (0x5e)
                                5GS mobile identity
                                NAS message container
                                    Element ID: 0x71
                                    Length: 29
                                    Non-Access-Stratum 5GS (NAS)PDU
                                        Plain NAS 5GS Message
                                            Extended protocol discriminator: 5G mobility management messages (126)
                                            0000 .... = Spare Half Octet: 0
                                            .... 0000 = Security header type: Plain NAS message, not security protected (0)
                                            Message type: Registration request (0x41)
                                            5GS registration type
                                            NAS key set identifier
                                            5GS mobile identity
                                            UE security capability
        Item 3: id-UserLocationInformation

The Registration Request was created by the UE and relayed by the OAI gNB simulator to the free5GC AMF. According to TS 124 501 https://www.etsi.org/deliver/etsi_ts/124500_124599/124501/15.02.01_60/ts_124501v150201p.pdf :

Section 8.2.6.1: In a Registration Request, the 5GMM capability field is "optional".
Section 8.2.6.3 5GMM capability: The UE shall include this IE, unless the UE performs a periodic registration updating procedure.

My interpretation of the word "shall" is, this IE must be included in the initial Registration Request, but may be omitted during registration updating. However, OAI UE is not including this IE in the initial Registration Request. This subsequently caused free5GC AMF to reject the registration:

    2024-12-12T20:22:31.213235728Z [36m [INFO][AMF][Gmm][amf_ue_ngap_id:RU:1,AU:1(3GPP)][supi:SUPI:imsi-001017005551000] [0mHandle InitialRegistration
    2024-12-12T20:22:31.218202767Z [36m [INFO][AMF][Gmm][amf_ue_ngap_id:RU:1,AU:1(3GPP)][supi:SUPI:imsi-001017005551000] [0mSend Registration Reject
    2024-12-12T20:22:31.218278008Z [36m [INFO][AMF][Ngap][amf_ue_ngap_id:RU:1,AU:1(3GPP)][ran_addr:172.25.199.19:60528] [0mSend Downlink Nas Transport
    2024-12-12T20:22:31.218903925Z [31m [ERRO][AMF][Gmm] [0mCapability5GMM is nil
    2024-12-12T20:22:31.218930484Z [36m [INFO][AMF][Gmm] [0mHandle event[ContextSetup Fail], transition from [ContextSetup] to [Deregistered]
    2024-12-12T20:23:34.836264722Z [36m [INFO][AMF][Ngap][ran_addr:172.25.199.19:60528] [0mHandle UEContextReleaseRequest

After that the OAI UE crashes:

[0m[NAS]   [UE 0] Received NAS_DOWNLINK_DATA_IND: length 25 , buffer 0x7f466c0028e0
[0m[NAS]   [UE 0] Received NAS_DOWNLINK_DATA_IND: length 11 , buffer 0x7f466c002df0
[0m [1;31m[NAS]   Received Registration reject cause: Illegal_UE
[0mUE threads created by 7
TYPE <CTRL-C> TO TERMINATE
kgnb : b8 a2 3b ef 03 4c 9a f4 7f 02 c3 f9 c4 bb 31 0e 43 6c 7d f1 d1 1c 0b c3 5a 0d f6 c0 7c 20 fa 2d
kausf:4b 5e 6d 8b 1c 5 ff 6 9a ab 0 ed cf b6 20 50 3c a4 5c 11 3c 4d 9 2f 36 12 41 a6 64 34 bb b1
kseaf:f0 6c bc 0 b6 b6 63 d6 99 40 11 6e b ef d1 5 25 d1 f0 d0 48 de b 42 ee fb 51 94 60 30 a 44
kamf:3 79 b3 3e 24 f3 14 89 72 32 49 77 ed 8d 2f c7 4c 6 17 fe 79 27 47 20 74 94 a5 15 d9 f5 16 9f
knas_int: 6 60 8b 64 e1 ce 59 5f 67 7c d5 3 1a 66 27 18
knas_enc: 85 dd ae 95 21 b0 28 b7 2 4d e6 94 2a 63 97 9c
mac f3 70 e7 b3