GTPv2 Dictionary (GTPv2-C / N26)¶
A dictionary for looking up the key messages of the EPC control-plane protocol GTPv2-C (GTP-C v2). In the 5GC it is used on N26 (AMF⇔MME) for 4G/5G interworking. N26 is not an SBI (HTTP/2 + JSON) like N14; because the peer is the 4G/EPC MME, it speaks the EPC-native GTPv2-C.
The specification is TS 29.274 (GTPv2-C). The interworking / N26 procedures are specified in TS 23.501 / TS 23.502.
Related: Protocol Dictionary / N26 page / 4G-5G Interworking chapter / EPS Fallback chapter
GTPv2-C key messages¶
Focused on the ones that appear on N26 (AMF⇔MME) in the 5GC.
| Message | Direction | Purpose | Notes | Spec |
|---|---|---|---|---|
| Forward Relocation Request/Response | AMF⇔MME | Transfer the UE context in Handover-based inter-system mobility | Connected-mode 5G⇔4G transitions (in the N26-based Handover case; EPS Fallback, etc.). Equivalent to 4G S10 | TS 29.274 |
| Forward Relocation Complete Notification/Acknowledge | AMF⇔MME | Notify/acknowledge relocation completion | Synchronizes Handover completion | TS 29.274 |
| Context Request/Response | AMF⇔MME | Retrieve the UE context in idle-mode inter-system mobility | Context retrieval at TAU / registration update in idle mode. Equivalent to the 4G S10 (inter-MME) / S3 (SGSN⇔MME) transfer path | TS 29.274 |
| Context Acknowledge | AMF⇔MME | Acknowledge context retrieval | Acknowledgment to Context Response | TS 29.274 |
| Relocation Cancel Request/Response | AMF⇔MME | Cancel an in-progress relocation | Rollback on Handover failure | TS 29.274 |
| Echo Request/Response | AMF⇔MME | Liveness monitoring (path management) | Liveness check between nodes | TS 29.274 |
(The § clause numbers for each message are to be confirmed. GTPv2-C is specified in TS 29.274; its use on N26 is specified in TS 23.501 / TS 23.502.)
Limited to what N26 uses
The table above is limited to the procedures that appear on N26 (AMF⇔MME). GTPv2-C has many other messages used within the EPC on S11 (MME⇔SGW) and S5/S8 (SGW⇔PGW), such as the Create/Modify/Delete Session family, but those are EPC-side procedures, not 5GC reference points. Details on the EPC side are out of scope for this dictionary.
Mapping to 5GC / EPC reference points¶
| Reference point using GTPv2-C | Endpoints | Positioning |
|---|---|---|
| N26 | AMF ⇔ MME | 5GC⇔EPC inter-system interworking. Plays the S10-equivalent role across AMF⇔MME |
| S10 | MME ⇔ MME | Inter-MME context transfer within 4G/EPC (reference) |
| S11 | MME ⇔ SGW | Session control within 4G/EPC (reference; not a 5GC reference point) |
| S5/S8 | SGW ⇔ PGW | Core session control in 4G/EPC (reference; not a 5GC reference point) |
(N26 is specified in TS 23.501 / TS 23.502; GTPv2-C itself in TS 29.274. § clause numbers to be confirmed)
GTPv2-C is EPC-native, not SBI
N26 uses not the SBI (HTTP/2 + JSON) that 5GC NFs use among themselves, but the EPC control-plane protocol GTPv2-C (because the peer is the 4G/EPC MME). GTPv2-C generally runs over UDP (the exact destination port and lower transport specifics are to be confirmed). Note that GTPv2-C (control plane) is distinct from GTP-U (TS 29.281, N3/N9), which encapsulates the user plane (Protocol Dictionary).