Procedure: UE Context Transfer (NamfCommunication)

Mirrored documentation

Mirrored read-only from claudia-5gc @ v2.3.1. Edit upstream, not here.

Spec: TS 23.502 §4.2.2.2.2 step 6 / §4.2.2.2.3 (Registration with AMF change) · TS 29.518 §5.3.2 Status: 🟢 Implemented (server side — old/source AMF) Primary NF: AMF (old AMF — the Namf_Communication service producer) Other NFs involved: AMF (new AMF — the consumer), gNB, UE

Context

When a UE moves to a new AMF and identifies itself with a 5G-GUTI that was assigned by a different (old) AMF, the new AMF retrieves the UE’s MM context (including the NAS security context) and the list of established PDU session contexts from the old AMF before completing the (Mobility) Registration. This avoids re-authenticating the UE and re-running the full registration from scratch.

This task implements the producer (old AMF) side of Namf_Communication_UEContextTransfer: the inbound SBI server on namf-comm (port 8001) that answers the new AMF’s request.

POST /namf-comm/v1/ue-contexts/{ueContextId}/transfer

{ueContextId} is the UE identifier supplied by the new AMF — the 5G-GUTI the UE presented (5g-guti-<…>) or, where known, the SUPI (imsi-<digits>).

Sequence

sequenceDiagram
  participant UE
  participant gNB
  participant nAMF as New AMF (consumer)
  participant oAMF as Old AMF (producer)

  UE->>gNB: NAS Registration Request (Mobility, 5G-GUTI of old AMF)
  gNB->>nAMF: NGAP Initial UE Message (RegistrationRequest)
  Note over nAMF: GUTI points to another AMF → resolve old AMF (NRF / GUAMI)
  nAMF->>oAMF: POST /namf-comm/v1/ue-contexts/{ueContextId}/transfer<br/>UeContextTransferReqData {reason, accessType, regRequest}
  Note over oAMF: Locate UE by GUTI/SUPI<br/>(optionally validate integrity of regRequest)
  oAMF-->>nAMF: 200 OK UeContextTransferRspData {ueContext: mmContextList + sessionContextList}
  Note over oAMF: Mark context transferred (kept until RegistrationStatusUpdate)
  nAMF->>UE: NAS Security Mode Command / Registration Accept (reusing the security context)

IEs (key)

Request — UeContextTransferReqData (TS 29.518 §6.1.6.2.2)

IE	M/O	Notes
reason	M	TransferReason: INIT_REG, MOBI_REG, MOBI_REG_UE_VALIDATED
accessType	O	AccessType: 3GPP_ACCESS (default) / NON_3GPP_ACCESS
plmnId	O	Serving PLMN of the new AMF
regRequest	O	N1MessageContainer — the NAS Registration Request, so the old AMF can verify the integrity (MAC) of the message and confirm the requester is serving the genuine UE
supportedFeatures	O	Negotiated optional features

Response — UeContextTransferRspData (TS 29.518 §6.1.6.2.3)

IE	M/O	Notes
ueContext	M	UeContext — see below
supportedFeatures	O

UeContext (TS 29.571 §5.x / TS 29.518 §6.1.6.2.x), the fields populated here:

Field	Notes
supi	UE permanent identity (when authenticated)
mmContextList[]	One MmContext per access. Carries accessType, nasSecurityMode (integrityAlgorithm NIAx + cipheringAlgorithm NEAx), nasDownlinkCount, nasUplinkCount, ueSecurityCapability, and (dev) the kamf key material
sessionContextList[]	One PduSessionContext per established PDU session: pduSessionId, smContextRef, sNssai, dnn, accessType, smfInstanceId
pcfId	Serving PCF, when an association exists

Error scenarios to test

Scenario	HTTP	Cause (ProblemDetails)
UE context not found for the given GUTI/SUPI	404	CONTEXT_NOT_FOUND
reason IE missing	400	MANDATORY_IE_MISSING
Malformed JSON body	400	MANDATORY_IE_MISSING
Integrity check of regRequest fails (UE not validated)	403	INTEGRITY_CHECK_FAIL (future — integrity replay is best-effort in this build)

Post-transfer behaviour

• The old AMF marks the UE context as transferred (UEContext.Transferred = true) but keeps it. Per TS 23.502 §4.2.2.2.2 step 21, the context is only released when the new AMF confirms success via Namf_Communication_RegistrationStatusUpdate (not in scope here) or when the old AMF’s own implicit-detach timers expire.

Validation approach

• Functional (in-process, no UERANSIM): seed a UE context in a context.Manager, start the internal/sbi server, POST a transfer request, assert 200 + security context + session list, and assert the context is marked transferred. Error cases (404, 400) covered.
• Conformance: IE names / TransferReason enum / ProblemDetails causes checked against TS 29.518 §6.1.6 and TS 29.571 §5.2.7 (CONTEXT_NOT_FOUND, MANDATORY_IE_MISSING).
• mTLS + HTTP/2: server sets TLSConfig (with NextProtos: ["h2"]) before http2.ConfigureServer and requires + verifies client certs (TS 29.500 §4.4, TS 33.501 §13).