QoS Flow / QFI / 5QI — What Is 5G QoS Actually Bound To?¶

Learning objectives¶

After reading this chapter, you will be able to:

• Explain, in the words of TS 23.501 §5.7.1, that 5G QoS is bound not to the PDU Session or the TEID, but to the QoS Flow
• Distinguish TEID / QFI / 5QI by "what each identifies" and "where each is carried"
• Explain the QoS characteristics referenced by a 5QI (Resource Type / Priority Level / PDB / PER, etc.) and the difference between GBR / Non-GBR / Delay-critical GBR
• Explain that one QoS decision is distributed in different forms to three places — UE (QoS rule), gNB (QoS profile), UPF (PDR / QER) — and that only the QFI is the common tag on N3
• Isolate a QFI mismatch by lining up the values on N1 / N2 / N4 / N3, and in Wireshark not misread a QFI value as a 5QI

Prerequisites¶

• QoS — the basics of the 5G QoS model (this chapter digs deeper and uses the same terms and structure)
• PFCP and PDR / FAR / QER / URR — where the QER fits in the UPF
• PDU Session Establishment deep dive — how N1 SM / N2 SM / N4 information is handed over
• Supplementary: N3 / QoS dictionary / SMF / PDR / PDI / F-TEID / SDF Filter / Precedence

Why — why did 5G stop doing QoS "per bearer"?¶

A single UE carries traffic of very different natures at the same time.

UE
├─ Voice              low delay and low loss come first
├─ Video              wants steady throughput
├─ Web                best effort is enough
└─ Background update  delay is acceptable

In 4G EPC, an EPS Bearer was set up for each kind of traffic that needed different QoS, and each bearer had its own GTP-U tunnel (TEID). The unit of QoS and the unit of the forwarding path were the same.

5GS separates them. The forwarding path is consolidated into one N3 tunnel per PDU Session, and packets flowing inside it carry a tag called the QFI; QoS differs per tag (TS 23.501 §5.7.1.1 / §5.7.1.6, sub-clause numbers to be confirmed). Adding or changing QoS no longer requires rebuilding the tunnel, and the radio side can flexibly bundle QoS Flows into DRBs.

Analogy: 4G is "running a separate truck for each grade of parcel"; 5G is "loading parcels of mixed grades onto one truck (N3 tunnel), each with a grade sticker (QFI)." The sticker number only says "which parcel group this is"; what the grade actually means (express or standard) is written in a separate ledger (5QI → QoS characteristics).

What — the QoS Flow model (TS 23.501 §5.7.1)¶

The QoS Flow is the finest unit of QoS differentiation¶

TS 23.501 §5.7.1.1 defines the following (summary; sub-clause numbers to be confirmed):

• The 5G QoS model is based on QoS Flows. The QoS Flow is the finest granularity of QoS differentiation in a PDU Session
• A QoS Flow is identified by a QFI (QoS Flow ID). User Plane traffic with the same QFI receives the same forwarding treatment
• The QFI is unique within a PDU Session. On N3 (and N9) it is carried in the encapsulation header
• The QFI may be dynamically assigned, or may be equal to the 5QI

That last point is what later causes "QFI vs 5QI" confusion.

A QoS Flow is represented by three parts¶

The same clause says that one QoS Flow is characterized by the following three:

flowchart TD
    SMF["SMF
decides QoS Flows and assigns QFIs"] -- "N1: QoS rules +
QoS flow descriptions" --> UE["UE
UL classification"]
    SMF -- "N2: QoS Flow Setup Request List
(QFI + QoS Flow Level QoS Parameters)" --> GNB["gNB
QoS Flow → DRB"]
    SMF -- "N4: PDR (QFI in PDI) /
QER (QFI to set in DL)" --> UPF["UPF
detection, enforcement, marking"]
    UE == "Radio: DRB (SDAP)" ==> GNB
    GNB == "N3: GTP-U + QFI" ==> UPF

A single decision made by the SMF is distributed to three places in different forms, and the only common key that ties the three together on the User Plane is the QFI. That is why, when QFIs disagree, one place ends up believing a packet belongs to a different QoS Flow.

How one PDU Session relates to QoS Flows¶

UE
└─ PDU Session ID = 1 (DNN = ims)
    ├─ N3 tunnel: UL TEID = 0x00000101 (UPF side) / DL TEID = 0x12345678 (gNB side)
    │
    ├─ QoS Flow  QFI = 1  → 5QI = 5   (IMS Signalling, Non-GBR)  ← flow pointed to by the default QoS rule
    └─ QoS Flow  QFI = 2  → 5QI = 1   (Conversational Voice, GBR)

UE
└─ PDU Session ID = 2 (DNN = internet)
    ├─ N3 tunnel: different TEIDs
    └─ QoS Flow  QFI = 1  → 5QI = 9   (Non-GBR)  ← a different session, so QFI=1 can be used again

(Values are fictitious examples. The meaning of each 5QI follows Table 5.7.4-1, described later.)

• The QFI is unique only within a PDU Session. QFI=1 in PDU Session ID = 1 and QFI=1 in PDU Session ID = 2 are unrelated.
• The QFI value does not need to match the 5QI value (in the example above, QFI=2 has 5QI=1).

What — telling TEID, QFI and 5QI apart¶

Outer IP (gNB ⇔ UPF)
UDP (dst 2152)
GTP-U  TEID = 0x12345678          ← which PDU Session's tunnel
 └─ Extension Header: PDU Session Container
      ├─ PDU Type = DL PDU SESSION INFORMATION
      └─ QFI = 2                  ← which QoS Flow within that session
Inner IP (the UE's packet)

The 5QI is nowhere in this packet  ← you only learn the 5QI of QFI=2 by looking at N2/N1 signaling

What — 5QI and QoS characteristics (TS 23.501 §5.7.3 / §5.7.4)¶

A 5QI is "a reference to QoS characteristics"¶

A 5QI is a scalar used as a reference to a set of QoS characteristics (TS 23.501 §5.7.2.1). Standardized 5QIs map one-to-one to the values in Table 5.7.4-1, so only the value is sent; the characteristics themselves are not signaled. Besides standardized 5QIs, there are 5QIs pre-configured in the AN and dynamically assigned 5QIs. Standardized and pre-configured 5QIs are sent as values only; only for a dynamically assigned 5QI are the characteristics signaled explicitly as part of the QoS profile (TS 23.501 §5.7.2.1 / §5.7.3.1; in NGAP, the Non-Dynamic 5QI Descriptor / Dynamic 5QI Descriptor CHOICE; TS 38.413 §9.3.1.12, to be confirmed).

(Sub-clause numbers can move between versions, so to be confirmed.)

Resource Type — GBR / Non-GBR / Delay-critical GBR¶

Representative standardized 5QIs (TS 23.501 Table 5.7.4-1)¶

This is only a small excerpt. Rows, notes and values in the table are added and revised in each Release, so always refer to Table 5.7.4-1 of the target Release for actual design (values to be confirmed).

• The Priority Level can override the 5QI default: if the QoS profile explicitly signals a Priority Level, a priority different from the standardized 5QI's default can be used (TS 23.501 §5.7.3.3, to be confirmed).
• A 5QI is "the kind of quality promise," not "a bit rate": how many Mbps are guaranteed (GFBR) and how far a flow may go (MFBR) are parameters separate from the 5QI (next chapter).

What — the default QoS Flow and QoS rules (UE side)¶

The "default QoS Flow" is the flow pointed to by the default QoS rule¶

The central concept in the 3GPP text is the default QoS rule. TS 23.501 §5.7.1.4 specifies the following (summary, to be confirmed):

• Every PDU Session has exactly one default QoS rule. It is sent to the UE at PDU Session establishment
• The default QoS rule is the only QoS rule that may contain a Packet Filter Set that allows all UL packets, and in that case it uses the lowest priority (highest precedence value)
• The QoS Flow associated with the default QoS rule remains established for the lifetime of the PDU Session

What practitioners call the "default QoS Flow" is this QoS Flow associated with the default QoS rule. Its role is close to the 4G Default Bearer, but "PDU Session = default flow" is not true.

Contents of a QoS rule (TS 24.501 §9.11.4.13)¶

In the NAS QoS rules IE, one QoS rule consists roughly of the following elements:

QoS rule
├─ QoS rule identifier (QRI)        ← identifies the QoS rule within the PDU Session
├─ Rule operation code              ← create / delete / modify ...
├─ DQR bit                          ← 1 = default QoS rule
├─ Number of packet filters
├─ Packet filter list
│   └─ Packet filter
│       ├─ Packet filter direction  ← UL only / DL only / bidirectional
│       ├─ Packet filter identifier
│       └─ Components               ← remote address, protocol, remote port, etc.
├─ QoS rule precedence              ← lower values are evaluated first
├─ Segregation bit
└─ QFI                              ← QoS Flow that carries UL packets matching this rule

(Excerpt of the main fields. For encoding details, see TS 24.501 §9.11.4.13.)

• QRI and QFI are different things. Several QoS rules can point to the same QFI (multiple sets of packet filters tied to one QoS Flow).
• The QoS Flow-side parameters (5QI, plus GFBR / MFBR for a GBR flow, etc.) are delivered per QFI to the UE in a separate IE, QoS flow descriptions (TS 24.501 §9.11.4.12). In PDU SESSION ESTABLISHMENT ACCEPT they are carried as Authorized QoS rules / Authorized QoS flow descriptions (TS 24.501 §8.3.2).

UL traffic classification¶

The UE evaluates a UL packet against the packet filters of the QoS rules in increasing order of precedence value, and sends it with the QFI of the first matching rule. If no rule matches, the UL packet is discarded (TS 23.501 §5.7.1.7, to be confirmed).

UL packet: TCP dst port 443
  ├─ QoS rule QRI=2  precedence=10   filter: UDP remote port 5004  → no match
  ├─ QoS rule QRI=3  precedence=20   filter: TCP remote port 443   → MATCH → QFI = 3
  └─ QoS rule QRI=1  precedence=255  DQR=1 (match-all)              (not evaluated)

Because the match-all default QoS rule acts as the final catch-all, "matches nothing and is discarded" normally does not happen. If the design does not put a match-all filter in the default QoS rule, UL packets that match no rule are dropped by the UE.

How — how QoS Flows are created and where they are distributed¶

Call Flow (the part embedded in PDU Session Establishment)¶

A QoS Flow is not created by a standalone procedure; it is established, added or modified within PDU Session Establishment (TS 23.502 §4.3.2.2.1) or PDU Session Modification (TS 23.502 §4.3.3.2).

sequenceDiagram
    participant UE
    participant gNB
    participant AMF
    participant SMF
    participant PCF
    participant UPF
    UE->>AMF: PDU Session Establishment Request (UL NAS Transport)
    AMF->>SMF: Nsmf_PDUSession_CreateSMContext
    opt Dynamic PCC
        SMF->>PCF: Npcf_SMPolicyControl_Create
        PCF-->>SMF: SM Policy Decision (PCC Rules, Session Rules)
    end
    Note over SMF: Bind PCC Rules to QoS Flows (QoS Flow binding)
assign QFIs, build QoS rules / QoS profiles / QERs
    SMF->>UPF: PFCP Session Establishment (Create PDR / QER)
    UPF-->>SMF: PFCP Session Establishment Response
    SMF->>AMF: Namf_Communication_N1N2MessageTransfer
(N1: Accept + QoS rules / N2: QoS Flow Setup Request List)
    AMF->>gNB: NGAP PDU Session Resource Setup Request
    Note over gNB: Decide QoS Flow → DRB (RAN implementation)
    gNB->>UE: RRC Reconfiguration (SDAP config + NAS Accept)
    gNB-->>AMF: PDU Session Resource Setup Response
(DL QoS Flow per TNL Information / QoS Flow Failed to Setup List)
    AMF->>SMF: Nsmf_PDUSession_UpdateSMContext
    SMF->>UPF: PFCP Session Modification (gNB F-TEID into DL FAR)
    Note over UE,UPF: From now on, UL is classified by the UE with QoS rules, and DL gets the QER QFI from the UPF

Signal Flow — what carries QoS Flow information¶

When QoS Flows are added to or modified in an existing PDU Session, N2 uses the QoS Flow Add or Modify Request List of PDU Session Resource Modify Request (TS 38.413 §8.2.3), and N1 uses PDU SESSION MODIFICATION COMMAND (TS 24.501 §8.3.9, to be confirmed).

Where does the SMF's default QoS come from?¶

The 5QI / ARP of the QoS Flow pointed to by the default QoS rule start from the UDM subscription data (subscribed default QoS) and, when a PCF is used, are decided by the PCF (the authorized default QoS in the Session Rule) (TS 23.501 §5.7.2.7, to be confirmed). In configurations without a PCF, the UDM values are used as they are or changed by SMF local configuration (the change policy is implementation dependent). The relationship between the PCF and SM, and PCC Rule details, are covered in PCF / SM Policy Association / PCC Rule.

How — RAN side: QoS Flow → DRB (SDAP)¶

Based on the received QoS profile, the gNB maps QoS Flows to DRBs (Data Radio Bearers). This mapping is handled by the radio-side SDAP (Service Data Adaptation Protocol) layer (TS 37.324 §4.4 / §5.3, TS 38.300 §12, to be confirmed).

flowchart LR
    subgraph N3["N3 Tunnel (1 TEID)"]
        Q1["QFI 1
5QI 5"]
        Q2["QFI 2
5QI 1"]
        Q3["QFI 3
5QI 9"]
    end
    subgraph RAN["gNB (SDAP)"]
        D1["DRB 1"]
        D2["DRB 2"]
    end
    Q1 --> D1
    Q3 --> D1
    Q2 --> D2
    D1 --> UE["UE"]
    D2 --> UE

• The gNB decides which QoS Flow goes onto which DRB and how many DRBs to create (RAN implementation dependent). It can bundle multiple QoS Flows into one DRB (N:1). This differs from the 1:1 "E-RAB = DRB" relationship in 4G.
• The mapping is signaled to the UE in the RRC SDAP configuration (mapped QoS Flows, default DRB, etc.) (SDAP-Config in TS 38.331, clause number to be confirmed). In UL, packets of a QFI with no configured mapping are placed on the default DRB (TS 37.324 §5.2.1, to be confirmed).
• The SDAP header (when configured) also contains the QFI (TS 37.324 §6.2.2, to be confirmed). In other words, the QFI is carried in two different headers: radio (SDAP header) → N3 (PDU Session Container).

How — UPF side: PDR / QER and QFI marking¶

UL:  gNB ──GTP-U (TEID=0x00000101, QFI=2)──► UPF
       PDR #11  PDI: Source=Access, F-TEID=0x00000101, QFI=2   → MATCH
         └─ QER #21: Gate UL=OPEN, MBR/GBR ...

DL:  DN ──IP (dst=UE IP, UDP 5004)──► UPF
       PDR #12  PDI: Source=Core, UE IP, SDF Filter (UDP 5004)  → MATCH
         ├─ FAR #12: Outer Header Creation (gNB F-TEID)
         └─ QER #22: Gate DL=OPEN, MBR/GBR ..., QFI = 2       ← QFI to put on DL GTP-U

(Values are fictitious examples.)

• The 5QI itself is not in the PFCP QER. What the UPF needs to know is which packets (PDR), with which bit rates and gates (QER), and with which QFI (QER).
• The mapping table from 5QI / ARP to DSCP values, and the queues / algorithms used for rate control, are operator configuration and vendor implementation.
• For how to read PDRs / QERs themselves, see PFCP and PDR / FAR / QER / URR; for PDI matching conditions, see PDR / PDI / F-TEID / SDF Filter / Precedence.

Connecting it end to end¶

flowchart LR
    A["UE App"] --> B["QoS rules
evaluated in precedence order"]
    B --> C["QFI decided"]
    C --> D["SDAP
QFI → DRB"]
    D --> E["gNB"]
    E --> F["N3 GTP-U
TEID + QFI"]
    F --> G["UPF UL PDR
F-TEID + QFI"]
    G --> H["QER"]
    H --> I["N6 / DN"]

Packet Analysis (Wireshark)¶

Display Filter¶

Decode example (conceptual)¶

The following is a conceptual representation of the display structure, with fictitious values (capture needed).

NGAP PDUSessionResourceSetupRequest
 └─ PDUSessionResourceSetupListSUReq
     └─ PDUSessionResourceSetupItemSUReq
         ├─ pDUSessionID: 1
         ├─ pDUSessionNAS-PDU          ← contains PDU SESSION ESTABLISHMENT ACCEPT (may be ciphered)
         └─ PDUSessionResourceSetupRequestTransfer
             ├─ PDUSessionAggregateMaximumBitRate
             ├─ UL-NGU-UP-TNLInformation: 198.51.100.20 / TEID 0x00000101
             └─ QosFlowSetupRequestList
                 ├─ QosFlowSetupRequestItem
                 │   ├─ qosFlowIdentifier: 1
                 │   └─ qosFlowLevelQosParameters
                 │       ├─ qosCharacteristics: nonDynamic5QI → fiveQI: 5
                 │       └─ allocationAndRetentionPriority
                 └─ QosFlowSetupRequestItem
                     ├─ qosFlowIdentifier: 2
                     └─ qosFlowLevelQosParameters
                         ├─ qosCharacteristics: nonDynamic5QI → fiveQI: 1
                         ├─ allocationAndRetentionPriority
                         └─ gBR-QosInformation (MFBR / GFBR)

GTP-U (N3, DL)
 ├─ TEID: 0x12345678
 └─ PDU Session Container
     ├─ PDU Type: DL PDU SESSION INFORMATION (0)
     └─ QoS Flow Identifier: 2          ← not "5QI 2"; NGAP shows 5QI = 1

Order of checks¶

flowchart LR
    A["1. NGAP
build the QFI-to-5QI table"] --> B["2. NAS
QFI and packet filters
of QoS rules (if readable)"]
    B --> C["3. PFCP
QFI in PDI / QFI in QER"]
    C --> D["4. GTP-U
TEID + QFI of real packets"]

First build, in step 1, a table saying "in this PDU Session, QFI x = 5QI y", and then fit the values from steps 2–4 into it.

Trouble Shooting¶

Isolating a QFI mismatch¶

Because the QFI appears in four places (N1 / N2 / N4 / N3), the fastest method is to line up the four values side by side for the same PDU Session.

flowchart TD
    S["Expected QFI differs from QFI on N3"] --> D{"Direction?"}
    D -- "DL" --> Q["Compare the PFCP QER QFI
with the NGAP QFI"]
    Q -- "QER differs" --> F1["Fix QER generation in the SMF"]
    Q -- "QER is correct" --> P["Check which DL PDR matches
(Precedence / SDF Filter)"]
    D -- "UL" --> R["Check the QFI and packet filters
of the NAS QoS rules"]
    R -- "rules are correct" --> G["Check how the gNB maps DRB to QFI
and the QFI in the UL PDR"]
    R -- "rules differ" --> F2["Fix QoS rule generation in the SMF"]

Conversely, if the QFI on N3 is as expected but only the quality is wrong, the problem is not the QFI; suspect the QER's Gate / MBR / GBR, the gNB's DRB mapping and scheduling, and DSCP handling in the transport network.

Boundary between 3GPP specifications and vendor implementations¶

"Which queue / algorithm the UPF uses to honor the rate specified in the QER" and "which scheduler the gNB uses to meet the PDB" are outside the scope of 3GPP.

Comparison with EPC (4G)¶

• 5QI=1 corresponds to QCI=1 and 5QI=5 to QCI=5 (TS 23.501 §5.7.4). The correspondence of other values, and the details of how Priority Levels differ (e.g., 20 for 5QI 1 versus 2 for QCI 1), are to be confirmed.
• For interworking with EPC (N26), the QoS flow description includes the EPS bearer identity so that the UE learns the correspondence between QoS Flows and EPS Bearers (TS 24.501 §9.11.4.12, to be confirmed). See Interworking for details.

3GPP Specification¶

Summary¶

• 5G QoS is bound to the QoS Flow. A QoS Flow is identified by a QFI, and the QFI is unique only within a PDU Session
• TEID = tunnel, QFI = QoS Flow within a session, 5QI = kind of QoS characteristics. Only the TEID and QFI are carried in user packets; the 5QI exists only in signaling
• One PDU Session normally uses one N3 tunnel, with multiple QFIs flowing inside it (1 TEID : multiple QFIs)
• A 5QI references the Resource Type (GBR / Delay-critical GBR / Non-GBR), Priority Level, PDB, PER, etc. Bit-rate values are separate from the 5QI
• The "default QoS Flow" is the flow pointed to by the default QoS rule (DQR=1). The UE evaluates QoS rules in precedence order to decide the UL QFI
• One SMF decision is distributed to the UE (QoS rules) / gNB (QoS Flow Setup Request List) / UPF (PDR, QER), and the gNB decides QoS Flow → DRB with SDAP
• Isolate a QFI mismatch by lining up the values on N2 → N1 → N4 → N3. Do not read QFI = 7 in Wireshark as 5QI 7

Comprehension check¶

Next Step¶

• Bit-rate values and priority (GFBR / MFBR / Session-AMBR / UE-AMBR / ARP) → GFBR / MFBR / AMBR / ARP
• The policy that QoS starts from → PCF / SM Policy Association / PCC Rule
• The conditions with which the UPF detects QoS Flows → PDR / PDI / F-TEID / SDF Filter / Precedence / PFCP and PDR / FAR / QER / URR
• The procedures that carry QoS Flow information → PDU Session Establishment deep dive / PDU Session Modification / Release
• Review the basics → QoS / N3 / QoS dictionary / VoNR