GUAMI / AMF Region / AMF Set / AMF Pointer — How AMFs Are Identified and Selected¶

Learning objectives¶

After reading this chapter, you will be able to:

• Break the GUAMI down into MCC / MNC / AMF Region ID / AMF Set ID / AMF Pointer and explain each field's bit length and role
• Explain the AMF Region → AMF Set → AMF hierarchy and the nuance that "an AMF Pointer may point to one or more AMFs"
• Explain how NG-RAN selects an AMF using the GUAMI / 5G-S-TMSI / Relative AMF Capacity
• Check the Served GUAMI List / Relative AMF Capacity of the NG Setup Response in Wireshark and compare them with the network design
• Isolate an "extremely uneven UE distribution between AMFs" by checking RAN, NGAP, GUAMI, AMF Set and AMF in order

Prerequisites¶

• SUPI / SUCI / 5G-GUTI — that 5G-GUTI = GUAMI + 5G-TMSI
• Registration — the procedure in which an AMF is first selected
• Supplementary: AMF / N2 / N14 / NRF / Identifier Dictionary

Why — why does an AMF need an "address"?¶

With only one AMF, the gNB can send everything there without thinking. In commercial networks, however, many AMFs are deployed for scale, redundancy and load distribution. This raises the following problems.

• When a UE comes back from idle, we want to deliver it to the AMF that holds its context
• When an AMF fails or is removed, we want to switch to an AMF that can take over the same role
• We want to distribute new UEs in proportion to each AMF's capacity
• For a UE that has moved to another AMF, we want the new AMF to identify the old AMF and retrieve the context

To achieve this, we need an "address" that identifies AMFs hierarchically. That is the GUAMI (Globally Unique AMF Identifier).

Analogy: The GUAMI is an address of the form "country (PLMN) → prefecture (AMF Region) → branch (AMF Set) → counter number (AMF Pointer)." Because the counter's address is printed on the numbered ticket (5G-GUTI), the usher (gNB) can guide a returning customer (UE) to the same counter. Even if that counter is closed, another counter in the same branch (AMF Set) can help, because they share the paperwork (UE context).

What — the structure of the GUAMI¶

Definition and bit lengths¶

The GUAMI is defined in TS 23.501 §5.9.4 and TS 23.003 §2.10.1 as follows.

<GUAMI>          = <MCC><MNC><AMF Identifier>
<AMF Identifier> = <AMF Region ID><AMF Set ID><AMF Pointer>

The AMF Identifier is 24 bits in total (TS 23.003 §2.10.1).

The Region → Set → AMF hierarchy¶

flowchart TD
    PLMN["PLMN 001-01"] --> R1["AMF Region ID = 0x01"]
    PLMN --> R2["AMF Region ID = 0x02"]
    R1 --> S1["AMF Set ID = 1
(unique within Region 0x01)"]
    R1 --> S2["AMF Set ID = 2"]
    R2 --> S3["AMF Set ID = 1
(unique within Region 0x02;
different from Set 1 of Region 0x01)"]
    S1 --> P1["AMF Pointer = 1"]
    S1 --> P2["AMF Pointer = 2"]
    S2 --> P3["AMF Pointer = 1"]
    S3 --> P4["AMF Pointer = 1"]

The values in the figure are a fictitious example using the test PLMN (001-01). Because an AMF Set ID is unique only within its Region, identifying an AMF Set globally requires the combination PLMN + Region ID + Set ID.

What are the AMF Region and the AMF Set?¶

• An AMF Region consists of one or more AMF Sets. Despite the name "Region," it need not be a physical geographic division; it is decided logically by the operator's network design (how Regions are carved out is operation dependent. The specification states that "an AMF Region consists of one or more AMF Sets" and does not require it to be a geographic division. TS 23.501 §5.21.2.1 / §5.9.4, to be confirmed).
• An AMF Set is a group of AMFs that serve the same area (group of TAs) and the same network slices (TS 23.501 §5.21.2.1, to be confirmed). The AMF Set is designed as a unit in which another AMF in the Set can take over a UE (by sharing UE context via a UDSF, or by transferring context between AMFs). Whether and how context is shared is configuration dependent (under TS 23.501 §5.21.2; sub-clause to be confirmed).

The GUAMI inside the 5G-GUTI¶

5G-GUTI
┌──────────────────────────────────────────┬──────────────┐
│                  GUAMI                   │   5G-TMSI    │
│  MCC │ MNC │ Region(8) │ Set(10) │ Ptr(6) │    (32)      │
│            points to the AMF             │ UE in the AMF│
└──────────────────────────────────────────┴──────────────┘

As long as the UE holds a 5G-GUTI, the network can look up "the AMF(s) that last served this UE" from the identifier. This is the starting point for AMF selection and context transfer.

How — how the GUAMI is used¶

1. NG Setup — the AMF announces its "address" to the gNB¶

In the NG Setup procedure (TS 38.413 §8.7.1), in which the gNB and the AMF establish N2, the AMF notifies the gNB of the GUAMIs it serves.

sequenceDiagram
    participant gNB
    participant AMF
    gNB->>AMF: NG Setup Request (Global RAN Node ID, Supported TA List, Default Paging DRX, etc.)
    AMF-->>gNB: NG Setup Response (AMF Name, Served GUAMI List, Relative AMF Capacity, PLMN Support List)
    Note over gNB: keeps a GUAMI ⇔ AMF (TNL association) table
and each AMF's relative capacity
    AMF-->>gNB: AMF Configuration Update (changes to Served GUAMI List / Relative AMF Capacity)
    gNB-->>AMF: AMF Configuration Update Acknowledge

The messages are defined in TS 38.413 §9.2.6.1 (NG Setup Request) / §9.2.6.2 (NG Setup Response), and changes are notified with AMF Configuration Update (§8.7.3).

2. AMF selection by the gNB¶

When a UE connects, the gNB (NG-RAN) selects an AMF along the following lines (TS 23.501 §6.3.5).

flowchart TD
    Start["RRC connection from the UE
(RRCSetupRequest / RRCSetupComplete)"] --> Q1{"Does the UE present a 5G-S-TMSI
or GUAMI (registeredAMF)?"}
    Q1 -- "Yes" --> Q2{"Is the AMF for that GUAMI
connected and available?"}
    Q2 -- "Yes" --> A1["Select that AMF
(return to the AMF holding the context)"]
    Q2 -- "No" --> A2["Select another AMF in the same AMF Set
(or the backup AMF)"]
    Q1 -- "No (new UE)" --> A3["Narrow candidates by Requested NSSAI, etc.,
select in proportion to Relative AMF Capacity"]

• When the UE has an existing identifier — The gNB identifies the previous AMF from the 5G-S-TMSI, assembled from the ng-5G-S-TMSI-Part1 in the RRCSetupRequest and the ng-5G-S-TMSI-Part2 in the RRCSetupComplete (Part2 = the upper 9 bits [the upper 9 bits of the AMF Set ID]; Part1 = the lower 39 bits [the least significant bit of the AMF Set ID, the AMF Pointer, and the 5G-TMSI]; TS 23.003 §2.10.1, TS 38.331 §6.2.2), or from the registeredAMF (the AMF Identifier and, where needed, the PLMN) in the RRCSetupComplete (TS 38.331 §6.2.2). Because the 5G-S-TMSI contains the AMF Set ID and the AMF Pointer, the short form alone is enough to identify the AMF (this assumes a configuration in which Set ID + Pointer is unique among the AMFs the gNB connects to; operation dependent).
• For a new UE — The UE is distributed with a probability proportional to each AMF's Relative AMF Capacity (weight factor) (TS 23.501 §5.19.3 AMF Load Balancing; clause number to be confirmed).

3. AMF failure and removal — GUAMI switchover¶

When an AMF is removed in a planned way, that AMF can notify the gNB of an Unavailable GUAMI List with AMF Status Indication (TS 38.413 §8.7.6). On a sudden failure the AMF itself may be unable to send it; how NG-RAN detects the failure and how other AMFs in the same Set handle it depend on TS 23.501 AMF Management (under §5.21.2; sub-clause to be confirmed) and on the implementation. For subsequent signaling of UEs with that GUAMI, the gNB is expected to select the AMF indicated by the Backup AMF Name notified in the NG Setup Response / AMF Configuration Update (or in the Unavailable GUAMI Item of the AMF Status Indication), or another AMF in the same AMF Set (TS 38.413 §8.7.6; TS 23.501 §5.21.2.2 / §5.21.2.3, sub-clauses to be confirmed. The exact selection method is NG-RAN implementation dependent).

sequenceDiagram
    participant UE
    participant gNB
    participant AMF1 as AMF-1 (to be removed)
    participant AMF2 as AMF-2 (same Set / backup)
    AMF1->>gNB: AMF Status Indication (Unavailable GUAMI List = GUAMI-1)
    Note over gNB: records GUAMI-1 as unavailable
    UE->>gNB: RRC connection (5G-S-TMSI → indicates GUAMI-1)
    gNB->>AMF2: Initial UE Message (to the backup AMF / an AMF in the same Set)
    Note over AMF2: can continue serving the UE if context
is shared within the Set (configuration dependent)

This is why the AMF Set matters as the unit of redundancy.

4. AMF re-allocation — when the first AMF was not appropriate¶

If the initial AMF selected by the gNB turns out not to support the UE's requested slices, etc., AMF re-allocation moves the UE to another AMF (or AMF Set) (TS 23.502 §4.2.2.2.3). There are two ways: the initial AMF forwards directly to the target AMF, or the NGAP Reroute NAS Request (TS 38.413 §8.6.5) tells the gNB where to resend the NAS message by specifying an AMF Set ID. In the latter, the AMF Set is the unit of forwarding.

5. The AMF Set in the SBI world¶

The AMF Set is not only an N2 concept. In the SBA (Service Based Architecture), the AMF publishes its AMF Set ID / AMF Region ID / GUAMI list in the NF profile it registers with the NRF (TS 29.510, AmfInfo type; clause number to be confirmed). Other NFs use this to search the NRF for "the AMF with this GUAMI" or "the AMFs of this AMF Set."

An AMF Set is also represented as an NF Set identifier in the following format (TS 23.003 §28.12).

set<AMF Set ID>.region<AMF Region ID>.amfset.5gc.mnc<MNC>.mcc<MCC>

flowchart LR
    subgraph RAN["RAN / NAS world"]
        G["GUAMI
Region / Set / Pointer"]
    end
    subgraph SBA["SBI world"]
        N["NF Profile (AmfInfo)
amfSetId / amfRegionId / guamiList"]
        F["NF Set ID
set..region..amfset.5gc..."]
    end
    G <-- "the same AMF Set in different notations" --> N
    N --- F

Packet Analysis (Wireshark)¶

Display filters¶

The numeric NGAP procedureCode values may be displayed differently depending on the dissector and version, so this page does not assert them. Checking message names in the GUI's protocol hierarchy is the reliable approach.

Decode example (conceptual)¶

The following is a conceptual representation of the display structure (capture needed). The values are a fictitious example with the test PLMN.

NGAP NGSetupResponse
 ├─ AMFName: amf-1.example
 ├─ ServedGUAMIList
 │   └─ ServedGUAMIItem
 │       ├─ GUAMI
 │       │   ├─ PLMN Identity: 001-01
 │       │   ├─ AMFRegionID: 0x01
 │       │   ├─ AMFSetID: 1
 │       │   └─ AMFPointer: 1
 │       └─ BackupAMFName: amf-2.example   (optional)
 ├─ RelativeAMFCapacity: 100
 └─ PLMNSupportList

What to look at¶

1. Served GUAMI List — whether the Region / Set / Pointer match the design document, and whether there are duplicate announcements not in the design (unintended duplicates of the same GUAMI)
2. Relative AMF Capacity — whether it has an unintended value (0 or extreme differences)
3. Whether there is an AMF Status Indication — whether a GUAMI has been made unavailable due to removal or failure
4. The GUAMI inside the UE's 5G-GUTI — "which AMF the UE is trying to return to." If the GUAMI does not exist, a 5G-GUTI from an old configuration may remain

Trouble Shooting — uneven load between AMFs¶

Suppose you see an extreme imbalance like this.

AMF-1 : UE 9000
AMF-2 : UE  100

Do not jump to "an AMF bug"; check from the outside in.

flowchart LR
    A["① N2 (SCTP) connection
gNB⇔each AMF"] --> B["② Is NG Setup
successful?"]
    B --> C["③ What the Served
GUAMI List announces"]
    C --> D["④ AMF Set / Region
configuration"]
    D --> E["⑤ Relative AMF
Capacity"]
    E --> F["⑥ UEs' existing
5G-GUTIs"]
    F --> G["⑦ RAN-side AMF
selection implementation"]

Boundary between 3GPP specifications and vendor implementations¶

Comparison with EPC (4G)¶

Mapping between the 5G-GUTI and the 4G GUTI (interworking)¶

In 4G⇔5G interworking, the 5G-GUTI and the 4G GUTI are mapped to each other (TS 23.003 §2.10.2, sub-clause to be confirmed). The bit lengths are designed to match.

When the UE presents a value mapped from a 4G GUTI, RRC distinguishes it with guami-Type (native / mapped) (TS 38.331 §6.2.2). For details, see 4G-5G Interworking.

3GPP Specification¶

Summary¶

• GUAMI = MCC + MNC + AMF Region ID (8) + AMF Set ID (10) + AMF Pointer (6). It represents "which AMF(s), in which Set, in which Region, of which PLMN"
• The AMF Set ID is unique within the Region, and the AMF Pointer points to one or more AMFs within the Set. One AMF may also have multiple GUAMIs
• The AMF announces its address and capacity to the gNB with the Served GUAMI List / Relative AMF Capacity in the NG Setup Response
• The gNB returns UEs to their previous AMF using the UE's 5G-S-TMSI / GUAMI, and distributes new UEs in proportion to Relative AMF Capacity
• On AMF failure or removal, UEs are taken over via the Unavailable GUAMI List / backup AMF / same AMF Set. The AMF Set is the unit of redundancy, designed for takeover by another AMF in the Set (how context is shared or transferred is configuration dependent)
• The AMF Set is a logical grouping in 3GPP and a different concept from a Kubernetes cluster, LB or DC

Comprehension check¶

Next Step¶

• Back to the identifier basics → SUPI / SUCI / 5G-GUTI
• The whole picture of N2 and NG Setup → NG Setup Procedure / N2 / NGAP Dictionary
• Context transfer between old and new AMFs → N14 / Mobility Registration Update
• Where the mapping to the 4G GUTI matters → 4G-5G Interworking
• The whole picture of the AMF as an NF → AMF