Test validating similar scenario can be found in MME_Tests.TC_ue_cell_reselect_geran_to_eutran in https://gitea.osmocom.org/ttcn3/osmo-ttcn3-hacks/src/branch/master/mme/MME_Tests.ttcn#L1545

Related test generated pcap: https://jenkins.osmocom.org/jenkins/job/ttcn3-mme-test-ogs/236/artifact/logs/mme-tester/MME_Tests.TC_ue_cell_reselect_geran_to_eutran.pcap.gz

@asdfuser check function guti_to_rai_ptmsi() in open5gs.git

I'm not totally sure, but I think the mapping in open5gs is the other way round.

In mme_gn_build_sgsn_context_request() you call guti_to_rai_ptmsi() which according to 3GPP TS 23.003 Ch.2.8.2.1.1 is the mapping used "when a UE moves from an MME to an SGSN. The SGSN may be either an S4 SGSN or a Gn/Gp SGSN."
(4G->2G)

However in the case where the MME requests an SGSN context the reverse is happening (2G->4G) so you'd need to apply the mapping from Ch. 2.8.2.2.3 (Mapping in the new MME) which is the reverse from Ch. 2.8.2.2.2.

I started modifying the mapping functions, will test them tomorrow:

/* 3GPP TS 23.003 2.8.2.1 Mapping GUTI from RAI, P-TMSI and P-TMSI signature (in the MME) */
void guti_from_rai_ptmsi(ogs_nas_eps_guti_t *nas_guti, const ogs_nas_rai_t *rai, mme_p_tmsi_t ptmsi, uint32_t ptmsi_sig)
{
    nas_guti->nas_plmn_id = rai->lai.nas_plmn_id;
    nas_guti->mme_gid = rai->lai.lac;
    nas_guti->mme_code = rai->rac;
    nas_guti->m_tmsi = 0xC0000000 | (ptmsi & 0x3f00ffff) | (ptmsi_sig & 0x00ff0000);
}

/* 3GPP TS 23.003 2.8.2.2 Mapping RAI and P-TMSI from GUTI (in the MME) */
void rai_ptmsi_from_guti(ogs_nas_rai_t *rai, mme_p_tmsi_t *ptmsi, const ogs_nas_eps_guti_t *nas_guti)
{
    rai->lai.nas_plmn_id = nas_guti->nas_plmn_id;
    rai->lai.lac = nas_guti->mme_gid;
    rai->rac = (nas_guti->m_tmsi >> 16) & 0xff;
    if (ptmsi)
        *ptmsi = (nas_guti->mme_code << 16) | (nas_guti->m_tmsi & 0x3f00ffff);
}

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