NAS 5G UE to SMF via AMF 3GPP TS 24.501

5G NAS - PDU Session Establishment Request

PDU Session Establishment Request is the 5GSM message the UE sends to request creation of a new PDU session for user-plane connectivity toward a DNN on a selected slice.

Message Fact Sheet

Protocol	nas	Network	5g
Spec	3GPP TS 24.501	Spec Section	8.3.1
Direction	UE to SMF via AMF	Message Type	5GSM signaling

Full message name	5G NAS - PDU Session Establishment Request
Protocol	NAS
Technology	5G
Direction	UE to SMF via AMF
Interface	N1
Signaling bearer / channel	NAS signaling / Usually carried inside UL NAS Transport on the access side
Typical trigger	The UE or upper layers need IP, Ethernet, or unstructured connectivity for data service, IMS, enterprise access, or another application flow.
Main purpose	Starts UE-requested PDU session setup by carrying the requested session type, SSC mode, DNN, slice context, and optional configuration parameters the network needs to build the session.
Main specification	3GPP TS 24.501, 8.3.1
Release added	Release 15
Procedures where used	5G PDU Session Establishment, VoNR Registration, Enterprise or internet data-session setup
Related timers	T3580
Related cause values	Request type, PDU session type, SSC mode

What is PDU Session Establishment Request in simple terms?

PDU Session Establishment Request is the 5GSM message the UE sends to request creation of a new PDU session for user-plane connectivity toward a DNN on a selected slice.

Starts UE-requested PDU session setup by carrying the requested session type, SSC mode, DNN, slice context, and optional configuration parameters the network needs to build the session.

Why this message matters

PDU Session Establishment Request is the UE asking the 5G core to create a usable data session, such as internet or IMS connectivity.

Where this message appears in the call flow

5G PDU Session Establishment

Call flow position: Starting 5GSM message for a UE-requested session establishment flow.

Typical state: UE is already registered and is asking the core network to create a new session.

Preconditions:

The UE has valid 5G registration context.
The requested slice and service context are available or can be requested.
A new or reusable PDU Session ID is available for the request.

Next likely message: PDU Session Establishment Accept, Reject, or a session-authentication branch

VoNR Registration

Call flow position: Often appears after registration when the UE requests the IMS PDU session used for voice services.

Typical state: UE is already registered and is moving into application or service-specific connectivity.

Preconditions:

Registration completed successfully.
IMS-related DNN, slice, and policy context are valid.

Next likely message: PDU Session Establishment Accept

Call flow position

Previous message(s): Registration Complete, UL NAS Transport, Allowed NSSAI and registration context already available

Next message(s): PDU Session Establishment Accept, PDU Session Establishment Reject, PDU Session Authentication Command

Message direction and transport

Sender and receiver: UE to SMF via AMF

Interface: N1

Domain: Core-side session management signaling with access-side NAS transport dependency

Signaling bearer: NAS signaling

Logical channel: Usually carried inside UL NAS Transport on the access side

Transport / encapsulation: 5GSM NAS message transported end-to-end between UE and SMF through AMF mediation

Security context: Normally sent after registration and NAS security establishment, so engineers usually expect a protected NAS transport path rather than an initial unsecured exchange.

Message Structure Overview

PDU Session Establishment Request is a 5GSM NAS message, so engineers read it as a 24.501 information-element structure instead of ASN.1.
Operationally, the message says what session the UE wants, on which slice or DNN, with what IP or Ethernet model, and with what continuity expectations.
In traces, the most useful first checks are PDU Session ID, PTI, request type, DNN, S-NSSAI, PDU session type, and SSC mode.

Structure Syntax

This message is not typically analyzed as ASN.1 on the wire. Engineers usually inspect it as a NAS or protocol field decode rather than an ASN.1 tree.

This is a NAS 5GSM message defined by TS 24.501 information elements rather than ASN.1 syntax.

Decoded Message Dump

PDU Session Establishment Request
  Extended Protocol Discriminator: 5G Session Management
  PDU Session ID: 10
  PTI: 1
  Message Type: PDU Session Establishment Request
  Integrity Protection Maximum Data Rate: full data rate
  PDU Session Type: IPv4v6
  SSC Mode: SSC mode 1
  Request Type: initial request
  5GSM Capability:
    Reflective QoS: supported
    Multi-homed IPv6 PDU session: not supported
  Maximum Number of Supported Packet Filters: 16
  S-NSSAI:
    SST: 1
    SD: 0x112233
  DNN: ims
  Extended Protocol Configuration Options:
    DNS IPv4 address request
    DNS IPv6 address request

How to read this dump

Start with PDU Session ID and PTI because they are the fastest way to correlate the request with the later 5GSM response.
Check Request Type, PDU Session Type, and SSC Mode together because they define the kind of session the UE is trying to create.
DNN and S-NSSAI are often the decisive fields in slice or subscription-related failures.
Extended Protocol Configuration Options often explain why the request is accepted but later service behavior is still incomplete.

Important Information Elements

IE	Required	Description
`PDU Session ID`	Yes	Identifies the session being requested and is the first thing engineers correlate with later accept, reject, modify, and release messages.
`PTI`	Yes	Procedure Transaction Identity used to match the request with the resulting 5GSM response.
`Request type`	Yes	Shows what kind of establishment is being requested, such as an initial new session or an existing-session-related request.
`PDU session type`	Yes	Tells the network whether the UE wants IPv4, IPv6, IPv4v6, Ethernet, or Unstructured connectivity.
`SSC mode`	Yes	Defines the requested session continuity behavior and strongly affects mobility and anchor expectations.
`DNN`	Optional	Requested data network name, often crucial in enterprise, IMS, and internet access troubleshooting.
`S-NSSAI`	Optional	Indicates the slice context the UE wants the session to use when slice selection matters.
`Extended protocol configuration options`	Optional	Carries configuration requests such as DNS or MTU-related parameters that often matter in service bring-up issues.
`5GSM capability`	Optional	Advertises UE capability aspects relevant to session handling, packet filters, and advanced session features.

Detailed field explanation

`PDU Session ID`

Identifies the session being requested and is the first thing engineers correlate with later accept, reject, modify, and release messages.

Presence: Required

`PTI`

Procedure Transaction Identity used to match the request with the resulting 5GSM response.

Presence: Required

`Request type`

Shows what kind of establishment is being requested, such as an initial new session or an existing-session-related request.

Presence: Required

`PDU session type`

Tells the network whether the UE wants IPv4, IPv6, IPv4v6, Ethernet, or Unstructured connectivity.

Presence: Required

`SSC mode`

Defines the requested session continuity behavior and strongly affects mobility and anchor expectations.

Presence: Required

`DNN`

Requested data network name, often crucial in enterprise, IMS, and internet access troubleshooting.

Presence: Optional

`S-NSSAI`

Indicates the slice context the UE wants the session to use when slice selection matters.

Presence: Optional

`Extended protocol configuration options`

Carries configuration requests such as DNS or MTU-related parameters that often matter in service bring-up issues.

Presence: Optional

`5GSM capability`

Advertises UE capability aspects relevant to session handling, packet filters, and advanced session features.

Presence: Optional

What to check in logs and traces

Confirm the UE is already successfully registered before this request appears.
Verify the request is carried in the expected NAS transport path and protected as expected for the scenario.
Check whether the PDU Session ID and PTI are reused incorrectly from another ongoing procedure.
Inspect DNN, S-NSSAI, and Request Type first when the network rejects the session.
Correlate the request with the following PDU Session Establishment Accept, Reject, or Authentication Command.
If the request is for IMS or a service slice, compare the chosen DNN and S-NSSAI with registration-time Allowed NSSAI.

Common Issues and Troubleshooting

PDU Session Establishment Request is sent but the UE gets a reject.

Likely cause: The requested DNN, slice, session type, or subscription context does not match what the network allows.

What to inspect: Check DNN, S-NSSAI, request type, and the returned 5GSM cause in the reject.

Next step: Correlate with registration context, Allowed NSSAI, and subscriber policy.

The request is visible but no response comes back.

Likely cause: The message may not be forwarded correctly, the session procedure may be colliding with another transaction, or the core may drop it before response.

What to inspect: Check UL NAS Transport, AMF/SMF signaling continuity, PTI correlation, and timer T3580 behavior.

Next step: Move from UE-side decode to transport and core-side procedure tracing.

The session establishes but the service still does not work correctly.

Likely cause: The session may be up with incomplete EPCO handling, wrong DNN, wrong slice, or unexpected QoS or address assignment.

What to inspect: Compare the request with the later Accept message, especially DNN, S-NSSAI, PDU address, and QoS-related context.

Next step: Continue analysis into PDU Session Establishment Accept and QoS flow setup.

Detailed explanation

5G NAS - PDU Session Establishment Request Explained

PDU Session Establishment Request is the 5GSM message the UE sends when it wants the 5G core to create a new data session. In simple terms, this is the message that asks the network for usable connectivity such as internet access, IMS connectivity for voice, or enterprise access on a specific slice.

For beginners, the simple meaning is: the UE is asking the network to open a data session.
For experienced engineers, this is one of the most useful NAS messages for checking the requested DNN, S-NSSAI, PDU Session Type, SSC Mode, and the transaction identifiers that tie the request to later accept or reject handling.

What is PDU Session Establishment Request in simple terms?

The UE is already registered in 5G. Now it needs a real service session for data. To do that, it sends PDU Session Establishment Request.

This message tells the network:

which session the UE wants to create
what kind of connectivity it needs
which DNN it wants to reach
which slice it wants to use if slice selection matters
what session-continuity behavior it prefers

Without this message, registration alone does not create a normal user-plane data session.

Why this message matters

This message matters because it is the real starting point of 5G session management from the UE side.

Engineers come back to it constantly because it explains:

why a session was accepted or rejected
why IMS or enterprise service works on one device but not another
why a slice-specific session fails even though registration succeeded
why the network created a session with different characteristics than expected

It is also the point where many apparent “data problems” begin. If the request itself is wrong, later QoS or user-plane troubleshooting often goes in the wrong direction.

Where this message appears in the call flow

A simplified successful UE-requested session setup path is:

UE completes registration
UE sends PDU Session Establishment Request
Network processes DNN, slice, policy, and session requirements
Network returns PDU Session Establishment Accept or Reject

Mini sequence:

UE                              gNB / AMF / SMF
|                                   |
|--- UL NAS Transport ------------->|
|    PDU Session Establishment Req  |
|<-- DL NAS Transport --------------|
|    PDU Session Establishment Acc  |

In many traces, the access-side NAS wrapper is visible first, but the important inner payload is the 5GSM session-establishment request itself.

Transport characteristics

sender: UE
receiver: SMF through the AMF path
interface: N1
access-side transport: usually inside UL NAS Transport
signaling class: 5GSM
security expectation: normally carried after registration and NAS security setup

This is not an RRC message. It depends on RRC and NGAP to move through the access network, but the actual procedure meaning comes from NAS session management.

What PDU Session Establishment Request means operationally

Operationally, PDU Session Establishment Request is the UE’s attempt to turn a registered mobility context into a real service session.

This is where engineers verify:

which PDU Session ID the UE chose
which PTI tracks the transaction
which DNN the UE asked for
which S-NSSAI the UE wants to use
whether the UE requested IPv4, IPv6, IPv4v6, Ethernet, or another session model
which SSC mode the UE expects

That is why this message is often the root cause of later service failures even when registration, authentication, and paging all look healthy.

Important Information Elements

IE	Why it matters
`PDU Session ID`	Identifies the requested session and ties the procedure to later responses.
`PTI`	Correlates the request with the matching 5GSM transaction response.
`Request Type`	Explains what kind of session request is being made.
`PDU Session Type`	Shows whether the UE is asking for IPv4, IPv6, IPv4v6, Ethernet, or Unstructured service.
`SSC Mode`	Indicates the requested session continuity model.
`DNN`	Often decisive in internet, IMS, enterprise, and private-network troubleshooting.
`S-NSSAI`	Shows the requested slice context.
`EPCO`	Helps explain later address, DNS, and service-configuration behavior.

Example message dump

PDU Session Establishment Request
  Extended Protocol Discriminator: 5G Session Management
  PDU Session ID: 10
  PTI: 1
  Message Type: PDU Session Establishment Request
  Integrity Protection Maximum Data Rate: full data rate
  PDU Session Type: IPv4v6
  SSC Mode: SSC mode 1
  Request Type: initial request
  5GSM Capability:
    Reflective QoS: supported
    Multi-homed IPv6 PDU session: not supported
  Maximum Number of Supported Packet Filters: 16
  S-NSSAI:
    SST: 1
    SD: 0x112233
  DNN: ims
  Extended Protocol Configuration Options:
    DNS IPv4 address request
    DNS IPv6 address request

How to read this dump

Read PDU Session ID and PTI first so you can track the correct transaction.
Then read Request Type, PDU Session Type, and SSC Mode together because they define what kind of session the UE wants.
Check DNN and S-NSSAI next because they often explain subscription, slice, or service mismatch problems.
Finally inspect EPCO and capability fields when the session comes up but service behavior still looks incomplete.

What to check in logs

confirm the UE had a valid registration context before the request
verify the request is carried in the expected protected NAS path
inspect PDU Session ID and PTI for transaction mismatch or reuse
check DNN, S-NSSAI, PDU Session Type, and SSC Mode
correlate the request with the following Accept, Reject, or Authentication Command
compare the request with slice and policy context learned earlier during registration

FAQ

What is PDU Session Establishment Request in 5G?

It is the 5GSM NAS message the UE sends to ask the 5G core to create a new PDU session for data connectivity.

Who sends PDU Session Establishment Request?

The UE sends it, and it is handled as a 5GSM message toward the SMF through the AMF.

Does this message create internet access?

Often yes. It can request an internet, IMS, enterprise, or other service session depending on the DNN and slice context.

What fields matter most in troubleshooting?

PDU Session ID, PTI, Request Type, PDU Session Type, DNN, S-NSSAI, and SSC Mode are usually the first fields to inspect.

What normally comes after PDU Session Establishment Request?

The usual next message is PDU Session Establishment Accept or Reject, although some scenarios may branch into session authentication first.

Summary

PDU Session Establishment Request is the 5GSM message the UE sends to request creation of a new PDU session for user-plane connectivity toward a DNN on a selected slice.

Advanced Engineer Notes

In real traces, DNN and S-NSSAI mismatches are often more important than the outer message type when debugging failed session setup.
PTI and PDU Session ID must be tracked together, especially when multiple 5GSM procedures overlap.
For IMS and enterprise traffic, a successful request still needs to be checked against later QoS and policy outcomes.

LTE / 5G / Variant Comparison

Compared with Registration Request

Registration Request establishes UE mobility-management context in 5GMM, while PDU Session Establishment Request creates a specific user-plane session in 5GSM after registration already exists.

Related message pages

Related Procedures

Related Tools

Use This Reference in Practice

Decode this message with the 3GPP Decoder, inspect the related message database, or open the matching call flow to see where this signaling step fits in the full procedure.