5G/NR  - NAS  

 

 

 

PDU Session Establishment in a Nutshell

  • PDU stands for Packet Data Unit. PDU Session Establishment is the process of establishing a data path between the UE and the 5G core network.
  • A PDU session is a logical connection between the UE and a data network, such as the internet or a private network. It is used to carry user data and can support different types of services, such as voice, video, and data.
  • The UE initiates the PDU Session Establishment process by sending a request to the 5G core network. The request includes information about the type of service that the UE wants to use, and the type of traffic.
  • Once the PDU session has been established, the UE can use it to send and receive data. The 5G core network manages the resources used by the PDU session to ensure that the network is used efficiently and that the UE receives the appropriate QoS.
  • PDU Session Establishment is a key component of 5G networks, as it enables the efficient and secure transport of data between the UE and the network
  • This is equivalent to PDN Setup process in LTE. If you have a good understandins on PDN setup process in LTE, it would be easy to get the picture of PDU Session Establishment

PDU Session Establishment in Detail

PDU Session Establishement is equivalent to PDN Establishment in LTE. Main purpose of this process is to assign UE IP and inform the UE of various the IP address of various corenetwork components (application layer component). With respect of IP allocation, PDU session establishment correponds to PDN connectivity procedure in LTE. Another important role of PDU Session Establishment is to establish QoS Flow. The QoS procedure will be explained in detail in this note (QoS / QFI / QRI).  

As mentioned above, main functionality of PDU Session Establishment will be treated in a few separate notes. In this note, I will focus mostly on the message structure of PDU Session Establishment signaling.

Before you move on

In this note, I would mostly focus on the two NAS messages between UE and the Network (especially AMF) and contents of the two messages, but there are a lot of other network components and interactions among those components happen. It is difficult to describe about every components, interactions and the functionalities in single page. So I wrote a few separate notes that are related to the functionalities of PDU Establishment process as listed below. Depending on your own interest and needs, follow through these notes in your own order.. but whatever you pick as the first note, you would need to go through all of these notes as you go deeper into this subject (i.e, PDU establishment process).

  • The key network component that is in charge of PDU establishment process is SMF and SMF perform interactions with other components to complete this process. If you want to understand overall procedure on what's happening on core network side for PDU establishment process, check out the note on SMF first.
  • In terms of protocol, PDU Session is managed and controlled by 5GSM (5G Session Management). So it would be good to have detailed understanding on 5GSM functionality. Check out the note of 5GSM.
  • Since PDU establishment is related to so many different functionality, it would be hard to grasp the big picture of it. It would be helpful to categorize the functionality of PDU session into a couple of group as follows.
  • In between this NAS message and Core Network, there is an interface protocol (N2, NGAP) is involved. Check out this note for the details.

Signaling Sequence

According to 24.501 - 6.4.1.2 UE-requested PDU session establishment procedure initiation goes as follows.

< Case A > Normal PDU Establishment

Direction

Message

UE Timer

NW Timer

UE -> NW(SMF)

ULInformationTransfer + UL NAS Transport

  + PDU SESSION ESTABLISHMENT REQUEST

T3580 Start

 

UE <- NW(SMF)

DLInformationTransfer + DL NAS Transport

  + PDU SESSION ESTABLISHMENT ACCEPT

T3582 Stop

 

< Case B > No PDU Session Establishment Accept

Direction

Message

UE Timer

NW Timer

UE -> NW(SMF)

ULInformationTransfer + UL NAS Transport

  + PDU SESSION ESTABLISHMENT REQUEST

T3580 Start

 

UE <- NW(SMF)

No PDU SESSION ESTABLISMENT ACCEPT

T3582 Exire

 

< Case C > PDU SESSION ESTABLISHMENT REJECT

Direction

Message

UE Timer

NW Timer

UE -> NW(SMF)

ULInformationTransfer + UL NAS Transport

  + PDU SESSION ESTABLISHMENT REQUEST

T3580 Start

 

UE <- NW(SMF)

DLInformationTransfer + DL NAS Transport

  + PDU SESSION ESTABLISHMENT REJECT

T3582 Stop

 

Following is a table listing various reject cause and the expected UE behavior based on 24.501-6.4.1.4, 6.4.1.7 and 24.501-AnnexA. I put only high level and most critical (most critical to me) in the 'Expected UE behavior' column. There are much more futher details specified in the specification.

Reject Cause

Expected UE Behavior

#8 operator determined barring;

(24.501-6.4.1.4)

 

#26 insufficient resources

(24.501-6.4.1.4,6.4.1.7)

The SMF may include a Back-off timer value IE in the PDU SESSION ESTABLISHMENT REJECT message when the 5GSM cause value #26"insufficient resources" is included in the PDU SESSION ESTABLISHMENT REJECT message. If the 5GSM cause value is #26"insufficient resources" and the PDU SESSION ESTABLISHMENT REQUEST message was received from a UE configured for high priority access in selected PLMN or the request type is set to "initial emergency request" or "existing emergency PDU session", the network shall not include a Back-off timer value IE.

If the PDU session cannot be established due to resource unavailability in the UPF, the SMF shall include the 5GSM cause value #26 "insufficient resources" in the 5GSM cause IE of the PDU SESSION ESTABLISHMENT REJECT message.

5G access network cannot forward the message:

If the SMF determines based on content of the n2SmInfo attribute specified in 3GPP TS 29.502 that the DL NAS TRANSPORT message carrying the PDU SESSION ESTABLISHMENT ACCEPT was not forwarded to the UE by the 5G access network, then the SMF shall reject the PDU session establishment procedure with the 5GSM cause set to #26 "insufficient resources" in the PDU SESSION ESTABLISHMENT REJECT message.

#27 missing or unknown DNN

(24.501-6.4.1.4)

 

#28 unknown PDU session type

(24.501-6.4.1.4)

If the PDU SESSION ESTABLISHMENT REQUEST message includes a PDU session type IE set to "IPv6", and the subscription, the SMF configuration, or both, support none of "IPv4" and "IPv6" PDU session types for the requested DNN, the SMF shall include the 5GSM cause value #28 "unknown PDU session type" in the 5GSM cause IE of the PDU SESSION ESTABLISHMENT REJECT message.

If the PDU SESSION ESTABLISHMENT REQUEST message includes a PDU session type IE set to "IPv4", and the subscription, the SMF configuration, or both, support none of "IPv4" and "IPv6" PDU session types for the requested DNN, the SMF shall include the 5GSM cause value #28 "unknown PDU session type" in the 5GSM cause IE of the PDU SESSION ESTABLISHMENT REJECT message.

If the PDU SESSION ESTABLISHMENT REQUEST message includes a PDU session type IE set to "IPv4v6", and the subscription, the SMF configuration, or both, support none of "IPv4v6", "IPv4" and "IPv6" PDU session types for the requested DNN, the SMF shall include the 5GSM cause value #28 "unknown PDU session type" in the 5GSM cause IE of the PDU SESSION ESTABLISHMENT REJECT message.

If the PDU SESSION ESTABLISHMENT REQUEST message includes a PDU session type IE set to "Unstructured" or "Ethernet", and the subscription, the SMF configuration, or both, do not support the PDU session type for the requested DNN, the SMF shall include the 5GSM cause value #28 "unknown PDU session type" in the 5GSM cause IE of the PDU SESSION ESTABLISHMENT REJECT message.

#29 user authentication or authorization failed

(24.501-6.4.1.4,6.4.1.7)

If the DN authentication of the UE was performed and completed unsuccessfully, the SMF shall include the 5GSM cause value #29 "user authentication or authorization failed" in the 5GSM cause IE of the PDU SESSION ESTABLISHMENT REJECT message and shall set the EAP message IE of the PDU SESSION ESTABLISHMENT REJECT message to an EAP-failure message as specified in IETF RFC 3748, provided by the DN.

The information for the PDU session authentication and authorization by the external DN in PDU DN request container is not compliant with local policy and user's subscription data If the PDU session being established is a non-emergency PDU session, the PDU session authentication and authorization by the external DN is required due to local policy and user's subscription data and the information for the PDU session authentication and authorization by the external DN in PDU DN request container is not compliant with local policy and user's subscription data, the SMF shall reject the PDU session establishment request including the 5GSM cause #29 "user authentication or authorization failed", in the PDU SESSION ESTABLISHMENT REJECT message.

#31 request rejected, unspecified

(24.501-6.4.1.7)

If the received request type is "initial emergency request" and there is already another emergency PDU session for the UE, the SMF shall reject the PDU SESSION ESTABLISHMENT REQUEST message with 5GSM cause #31 "request rejected, unspecified" or release locally the existing emergency PDU session and proceed the new PDU SESSION ESTABLISHMENT REQUEST message

#32 service option not supported

(24.501-6.4.1.4)

 

#33 requested service option not subscribed

(24.501-6.4.1.4)

 

#35 PTI already in use

(24.501-6.4.1.4)

 

#38 network failure

(24.501-6.4.1.4)

 

#46 out of LADN service area

(24.501-6.4.1.4)

If the UE requests a PDU session establishment for an LADN when the UE is located outside of the LADN service area, the SMF shall include the 5GSM cause value #46 "out of LADN service area" in the 5GSM cause IE of the PDU SESSION ESTABLISHMENT REJECT message.

#50 PDU session type IPv4 only allowed

(24.501-6.4.1.4)

If the PDU SESSION ESTABLISHMENT REQUEST message includes a PDU session type IE set to "IPv6", and the subscription, the SMF configuration, or both, are limited to IPv4 only for the requested DNN, the SMF shall include the 5GSM cause value #50 "PDU session type IPv4 only allowed" in the 5GSM cause IE of the PDU SESSION ESTABLISHMENT REJECT message.

#51 PDU session type IPv6 only allowed

(24.501-6.4.1.4)

If the PDU SESSION ESTABLISHMENT REQUEST message includes a PDU session type IE set to "IPv4", and the subscription, the SMF configuration, or both, are limited to IPv6 only for the requested DNN, the SMF shall include the 5GSM cause value #51 "PDU session type IPv6 only allowed" in the 5GSM cause IE of the PDU SESSION ESTABLISHMENT REJECT message.

#54 PDU session does not exist

(24.501-6.4.1.7)

UE-requested PDU session establishment with request type "existing PDU session" or "existing emergency PDU session" for a PDU session that does not exist:

If the SMF receives a PDU SESSION ESTABLISHMENT REQUEST message with request type set to "existing PDU session" or "existing emergency PDU session", and the SMF does not have any information about that PDU session, then the SMF shall reject the PDU session establishment procedure with the 5GSM cause set to #54 "PDU session does not exist" in the PDU SESSION ESTABLISHMENT REJECT message.

#67 insufficient resources for specific slice and DNN

(24.501-6.4.1.4)

The SMF may include a Back-off timer value IE in the PDU SESSION ESTABLISHMENT REJECT message when the 5GSM cause value #67 "insufficient resources for specific slice and DNN" is included in the PDU SESSION ESTABLISHMENT REJECT message. If the 5GSM cause value is #67 "insufficient resources for specific slice and DNN" and the PDU SESSION ESTABLISHMENT REQUEST message was received from a UE configured for high priority access in selected PLMN or the request type is "initial emergency request" or "existing emergency PDU session", the network shall not include a Back-off timer value IE.

#68 not supported SSC mode

(24.501-6.4.1.4)

If the PDU SESSION ESTABLISHMENT REQUEST message contains the SSC mode IE indicating an SSC mode not supported by the subscription, the SMF configuration, or both of them, and the SMF decides to rejects the PDU session establishment, the SMF shall include the 5GSM cause value #68 "not supported SSC mode" in the 5GSM cause IE and the SSC modes allowed by SMF in the Allowed SSC mode IE of the PDU SESSION ESTABLISHMENT REJECT message.

#69 insufficient resources for specific slice

(24.501-6.4.1.4)

The SMF may include a Back-off timer value IE in the PDU SESSION ESTABLISHMENT REJECT message when the 5GSM cause #69 "insufficient resources for specific slice" is included in the PDU SESSION ESTABLISHMENT REJECT message. If the 5GSM cause value is #69 "insufficient resources for specific slice" and the PDU SESSION ESTABLISHMENT REQUEST message was received from a UE configured for high priority access in selected PLMN or the request type is "initial emergency request" or "existing emergency PDU session", the network shall not include a Back-off timer value IE.

#70 missing or unknown DNN in a slice

(24.501-6.4.1.4)

 

#82 maximum data rate per UE for user-plane integrity protection is too low

(24.501-6.4.1.4)

In 3GPP access, if the operator's configuration requires user-plane integrity protection for the PDU session and, the maximum data rate per UE for user-plane integrity protection supported by the UE for uplink or the maximum data rate per UE for user-plane integrity protection supported by the UE for downlink, or both, are lower than required by the operator's configuration, the SMF shall include the 5GSM cause value #82 "maximum data rate per UE for user-plane integrity protection is too low" in the 5GSM cause IE of the PDU SESSION ESTABLISHMENT REJECT message.

#95 111 protocol errors

 

Message Structure

In this section the overall message structure and functionalities of important IE (Information Element) will be explained.

The overall structure of PDU Session related message is as follows :

NOTE : The sample log clips described in this note is from Amarisoft callbox.

If you want to see the contents of full log with Amarisoft Log viewer, go to LogAnalysis section and click on 'Sample Log' in this tutorial of Amarisoft TechAcademy.

PDU Session Establishment Request

When UE needs to establish a PDU session, it sends to AMF "PDU Session Establishment Request" message containing the information as listed below and AMF forward it to an SMF.

NOTE : This message is conveyed to AMF via NGAP message 'UL NAS Transport' The message is encapsulated in 'N1 SM information' container within  'UL NAS Transport'. AMF transfer the contents of  'N1 SM information' to SMF .

Example 01 >

This is a sample message from the log provided by Amarisoft.  

{
  message c1: ulInformationTransfer: {
    criticalExtensions ulInformationTransfer: {
      dedicatedNAS-Message '7E02FF096300027E006701001E2E0509C1FFFF93A17B001380000A00000
                                      200000100000C00000300000D00120581250403696D73'H
    }
  }
}

 

The decoded contents of the above NAS message is as below :


    Message: UL NAS transport

    Data:

    Protocol discriminator = 0x7e (5GS Mobility Management)
    Security header = 0x2 (Integrity protected and ciphered)
    Auth code = 0xff096300
    Sequence number = 0x02
    Protocol discriminator = 0x7e (5GS Mobility Management)
    Security header = 0x0 (Plain 5GS NAS message, not security protected)
    Message type = 0x67 (UL NAS transport)
    Payload container type = 1 (N1 SM information)
    Payload container:
      Protocol discriminator = 0x2e (5GS Session Management)
      PDU session identity = 5
      Procedure transaction identity = 9
      Message type = 0xc1 (PDU session establishment request)
      Integrity protection maximum data data:
        Maximum data rate per UE for user-plane integrity protection 
                                         for uplink = 0xff (Full data rate)
        Maximum data rate per UE for user-plane integrity protection 
                                         for downlink = 0xff (Full data rate)
      PDU session type = 0x3 (IPv4v6)
      SSC mode = 0x1 (1)
      Extended protocol configuration options:
        Ext = 1
        Configuration protocol = 0
        Protocol ID = 0x000a (IP address allocation via NAS signalling)
        Data =
        Protocol ID = 0x0002 (IM CN Subsystem Signaling Flag)
        Data =
        Protocol ID = 0x0001 (P-CSCF IPv6 Address Request)
        Data =
        Protocol ID = 0x000c (P-CSCF IPv4 Address Request)
        Data =
        Protocol ID = 0x0003 (DNS Server IPv6 Address Request)
        Data =
        Protocol ID = 0x000d (DNS Server IPv4 Address Request)
        Data =
    PDU session ID = 5
    Request type = 0x1 (initial request)
    DNN = "ims"

PDU Session Establishment Accept

Example 01 > PDU Address, DNN,QoS, No Slice Info

This is a sample message from the log provided by Amarisoft.

 

{
  message c1: dlInformationTransfer: {
    rrc-TransactionIdentifier 0,
    criticalExtensions dlInformationTransfer: {
      dedicatedNAS-Message '7E0206FC019B027E0054431081C176589E9EBFCD7490B34CBFBFE56
                            B450981C176589E9EBFCD7446404791118261539040490100'H
    }
  }
}

 

    Protocol discriminator = 0x7e (5GS Mobility Management)
    Security header = 0x2 (Integrity protected and ciphered)
    Auth code = 0x66f143b0
    Sequence number = 0x03
    Protocol discriminator = 0x7e (5GS Mobility Management)
    Security header = 0x0 (Plain 5GS NAS message, not security protected)
    Message type = 0x68 (DL NAS transport)
    Payload container type = 1 (N1 SM information)
    Payload container:
      Protocol discriminator = 0x2e (5GS Session Management)
      PDU session identity = 5
      Procedure transaction identity = 9
      Message type = 0xc2 (PDU session establishment accept)
      Selected PDU session type = 0x3 (IPv4v6)
      Selected SSC mode = 0x1 (1)
      Authorized QoS rules:
        QoS rule 1:
          QoS rule identifier = 1
          Rule operation code = 1 (create new QoS rule)
          DQR = 1 (the QoS rule is the default QoS rule)
          Number of packet filters = 1
          Packet filter identifier = 15
            Packet filter direction = 3 (bidirectional)
            Match-all
          QoS rule precedence = 255
          QFI = 1
      Session AMBR:
        Session-AMBR for downlink = 3000000 kbps
        Session-AMBR for uplink = 1000000 kbps
      PDU address:
        PDU session type = 3 (IPv4v6)
        IPv6 = ::2001:468:3000:1
        IPv4 = 192.168.4.2
      Authorized QoS flow descriptions:
        QoS flow description 1:
          QFI = 1
          Operation code = 1 (create new QoS flow description)
          E = 1 (parameters list is included)
          Number of parameters = 1
          5QI = 5
      Extended protocol configuration options:
        Ext = 1
        Configuration protocol = 0
        Protocol ID = 0x000d (DNS Server IPv4 Address)
        Data = 8.8.8.8
        Protocol ID = 0x0003 (DNS Server IPv6 Address)
        Data = 2001:4860:4860:0:0:0:0:8888
        Protocol ID = 0x000c (P-CSCF IPv4 Address)
        Data = 192.168.4.1
        Protocol ID = 0x0001 (P-CSCF IPv6 Address)
        Data = 2001:468:3000:1:0:0:0:0
        Protocol ID = 0x0002 (IM CN Subsystem Signaling Flag)
        Data =
      DNN = "ims.mnc001.mcc001.gprs"
      PDU session ID = 5

Example 02 > PDU Address, DNN,QoS, Slice Info

    Protocol discriminator = 0x2e (5GS Session Management)

    PDU session identity = 1

    Procedure transaction identity = 5

    Message type = 0xc2 (PDU session establishment accept)

    Selected PDU session type = 0x1 (IPv4)
    Selected SSC mode = 0x1 (1)
    Authorized QoS rules:

      QoS rule 1:

        QoS rule identifier = 1

        Rule operation code = 1 (create new QoS rule)

        DQR = 1 (the QoS rule is the default QoS rule)

        Number of packet filters = 1

        Packet filter identifier = 15

          Packet filter direction = 3 (bidirectional)

          Match-all

        QoS rule precedence = 255

        QFI = 1

    Session AMBR:

      Session-AMBR for downlink = 5000 Mbps

      Session-AMBR for uplink = 2000000 kbps

    5GSM cause = 0x32 (PDU session type IPv4 only allowed)

    PDU address:

      SI6LLA = 0

      PDU session type = 1 (IPv4)

      IPv4 = 192.168.3.2

    S-NSSAI:

      Length of S-NSSAI contents = 1 (SST)

      SST = 0x01

    Mapped EPS bearer contexts:

      Mapped EPS bearer context 1:

        EPS bearer identity = 5

        Operation code = 1 (create new EPS bearer)

        E = 1 (parameters list is included)

        Number of EPS parameters = 2

        Mapped EPS QoS parameters:

          QCI = 9

        APN-AMBR:

          APN-AMBR for downlink = 4864000000 bits

          APN-AMBR for uplink = 1792000000 bits

    Authorized QoS flow descriptions:

      QoS flow description 1:

        QFI = 1

        Operation code = 1 (create new QoS flow description)

        E = 1 (parameters list is included)

        Number of parameters = 2

        5QI = 9

        EPS bearer identity = 5

    Extended protocol configuration options:

      Ext = 1

      Configuration protocol = 0

      Protocol ID = 0x8021 (IPCP)

      Data = 03 00 00 0a 81 06 08 08 08 08

      Protocol ID = 0x000d (DNS Server IPv4 Address)

      Data = 8.8.8.8

    DNN = "internet.mnc001.mcc001.gprs"

UL NAS Transport

    UL NAS transport (24.501 - 8.2.10.1)

      Extended protocol discriminator (24.501 - 9.2)

      Security header type (24.501 - 9.3)

      UL NAS TRANSPORT message identity (24.501 - 9.7)

      Payload container type (24.501 - 9.11.3.40)

      Payload container  (24.501 - 9.11.3.39)

      PDU session ID  (24.501 - 9.11.3.41)

      Old PDU session ID (24.501 - 9.11.3.41)

      Request Type (24.501 - 9.11.3.47)

      S-NSSAI (24.501 - 9.11.2.8)

      DNN (24.501 - 9.11.2.1B)

      Additional information  (24.501 - 9.11.2.1)

      MA PDU session information (24.501 - 9.11.3.31A)

      Release assistance indication (24.501 - 9.11.3.46A)

DL NAS Transport

    DL NAS transport (24.501 - 8.2.11.1)

      Extended protocol discriminator (24.501 - 9.2)

      Security header type (24.501 - 9.3)

      DL NAS TRANSPORT message identity (24.501 - 9.7)

      Payload container type (24.501 - 9.11.3.40)

      Payload container  (24.501 - 9.11.3.39)

      PDU session ID  (24.501 - 9.11.3.41)

      Additional information  (24.501 - 9.11.2.1)

      5GMM cause (24.501 - 9.11.3.2)

      Back-off timer value (24.501 - 9.11.2.5)

5GSM capability

This is an information element(IE) is to convey information about the supported features and capabilities of the User Equipment (UE) or the network during the PDU session establishment procedure. This allows the network to establish a PDU session that is optimized for the UE's features and capabilities, ensuring a seamless and efficient user experience. If the UE's capabilities are not compatible with the network's requirements or the requested service, the network may reject the PDU session establishment with an appropriate 5GSM cause value.

    RqoS

      0 Reflective QoS not supported

      1 Reflective QoS supported

    Multi-homed IPv6 PDU session (MH6-PDU)

      0 Multi-homed IPv6 PDU session not supported

      1 Multi-homed IPv6 PDU session supported

Brief description of the configuration listed above are :

    Reflective QoS support:

      0: Reflective QoS not supported - Indicates that the UE does not support reflective QoS, which means that the UE requires explicit signaling from the network to determine the QoS parameters.

      1: Reflective QoS supported - Specifies that the UE supports reflective QoS, allowing it to derive the QoS parameters from the network without explicit signaling.  

    Multi-homed IPv6 PDU session support (MH6-PDU):

      0: Multi-homed IPv6 PDU session not supported - Indicates that the UE does not support multi-homed IPv6 PDU sessions, which means it cannot maintain multiple IPv6 addresses and multiple paths for a single PDU session.

      1: Multi-homed IPv6 PDU session supported - Specifies that the UE supports multi-homed IPv6 PDU sessions, allowing it to maintain multiple IPv6 addresses and multiple paths for a single PDU session, improving resilience and load balancing.

5GSM cause

This is is an information element(IE) used in 5G PDU Establishment messages and other 5G Session Management procedures to convey information about the success or failure of a specific procedure. It provides a cause code that indicates the reason behind the outcome of the procedure, whether it's successful, rejected, or encounters an error.

    5GSM cause IEI

    Cause value

      Operator determined barring

      Insufficient resources

      Missing or unknown DNN

      Unknown PDU session type

      User authentication or authorization failed

      Request rejected, unspecified

      Service option not supported

      Requested service option not subscribed

      Service option temporarily out of order

      PTI already in use

      Regular deactivation

      Network failure

      Reactivation requested

      Invalid PDU session identity

      Semantic errors in packet filter(s)

      Syntactical error in packet filter(s)

      Out of LADN service area

      PTI mismatch

      PDU session type IPv4 only allowed

      PDU session type IPv6 only allowed

      PDU session does not exist

      Insufficient resources for specific slice and DNN

      Not supported SSC mode

      Insufficient resources for specific slice

      Missing or unknown DNN in a slice

      Invalid PTI value

      Maximum data rate per UE for user-plane integrity protection is too low

      Semantic error in the QoS operation

      Syntactical error in the QoS operation

      Invalid mapped EPS bearer identity

      Semantically incorrect message

      Invalid mandatory information

      Message type non-existent or not implemented

      Message type not compatible with the protocol state

      Information element non-existent or not implemented

      Conditional IE error

      Message not compatible with the protocol state

      Protocol error, unspecified

Always-on PDU session

An always-on PDU session is to provides a continuous data path between the UE and the network to make it sure that the connection remains active even when there is no user data being transmitted.

This kind of connection can be established by PDU Session Esbalishment process as shown below.

Direction

Message

UE Timer

NW Timer

UE -> NW(SMF)

PDU SESSION ESTABLISHMENT REQUEST

with Always-on PDU session requested

T3580 Start

 

UE <- NW(SMF)

PDU SESSION ESTABLISHMENT ACCEPT

with Always-on PDU session indication

T3582 Stop

 

      0 Always-on PDU session not requested

      1 Always-on PDU session requested

Use cases of Always-on PDU session

There are many cases of use cases for this type of connection. Some of them are listed below.

  • Mission-critical services: In some industries, such as public safety, healthcare, or critical infrastructure, maintaining an always-on connection is vital to ensure the reliable functioning of critical systems and services.
  • Reduced connection setup time: An always-on PDU session minimizes the need for establishing and releasing connections repeatedly when data transfers are needed, thus reducing the connection setup time and providing a more seamless user experience.
  • Network registration and maintenance: Always-on PDU sessions help maintain the UE's registration with the network and support the exchange of control plane messages, such as location updates, tracking area updates, and registration requests.
  • Timely delivery of notifications and incoming data: When an always-on PDU session is established, the network can promptly deliver notifications, such as incoming calls or messages, to the UE. This is particularly important for services with real-time requirements, like voice calls or instant messaging.
  • IoT devices and applications: In IoT (Internet of Things) scenarios, devices often require an always-on connection to the network to ensure continuous monitoring, reporting, and control of connected sensors, actuators, or devices. An always-on PDU session enables the network to maintain a persistent connection with these devices and support efficient data transfer with minimal latency.

How to disable Always-on Session after it is enabled ?

Even though always-on-PDU-session has obvious advantage and use cases, there is some drawback as well. This type of Session can consume more resources and battery life on the UE due to the constant connection. Due to the drawback, UE or Network may want to disable the feature. There are several ways to disable this type of session and some of the ways are listed below.

  • PDU Modification : This allows the User Equipment (UE) or the network to modify the attributes of an existing PDU session without having to release and re-establish the session. This modification process can be described as follows:
    • Initiate the PDU Modification Request: Either the UE or the network initiates the PDU Modification procedure by sending a PDU Modification Request message. This message should include the updated PDU session configuration, specifically indicating that the always-on PDU session indication should be changed.
    • Process the PDU Modification Request:: The receiving party (either the UE or the network, depending on who initiated the request) processes the PDU Modification Request and evaluates whether the requested changes can be applied to the existing PDU session.
    • Send the PDU Modification Complete message:: If the requested changes are acceptable and can be applied to the PDU session, the receiving party sends a PDU Modification Complete message back to the initiator. This message confirms that the requested changes, including the removal of the always-on PDU session indication, have been successfully applied to the PDU session
  • Release and Reconnec: UE or Network initiate the release of the current session and reconnect the session as follows.
    • Release the current PDU session:: UE or the network may initiate a PDU Session Release procedure to terminate the existing always-on PDU session. This process involves exchanging messages between the UE and the network to indicate the intention to release the PDU session.
    • Re-establish a new PDU session without the always-on indication:: The UE sends a PDU Session Establishment Request message to the network without including the always-on PDU session indication. This request informs the network that the UE does not want to establish an always-on PDU session for this particular session.
    • Complete the PDU Session Establishment process:: The network processes the PDU Session Establishment Request and sends a PDU Session Establishment Accept message back to the UE. This message confirms the establishment of a new PDU session without the always-on PDU session indication.  

EAP message

This is defined in RFC 3784.

Extended protocol configuration options

    Length of extended protocol configuration options contents

    Extended protocol configuration options contents (24.008-10.5.6.3)

    < 24.008-Figure 10.5.136: Protocol configuration options information element >

GPRS Timer

The GPRS timer is used in various cellular network procedures, such as the PDU (Protocol Data Unit) Establishment process, to manage and control the duration of specific events or actions.

    GPRS Timer IEI

    Timer Value

    Unit

      value is incremented in multiples of 2 seconds

      value is incremented in multiples of 1 minute

      value is incremented in multiples of decihours

      value indicates that the timer is deactivated.

  • Timer Value: This represents the actual duration of the timer.
  • Unit: This indicates the unit of time (e.g., seconds, minutes, decihours) used to interpret the timer value.

If the timer value indicates that the timer is deactivated, it means that the specific event or action associated with the timer is not active or applicable in the current context.

Mapped EPS bearer contexts

"Mapped EPS bearer contexts" indicates the mapping of existing EPS (Evolved Packet System) bearers from a 4G LTE network to 5G PDU sessions when a UE transitions from the 4G network to the 5G network. This mapping ensures that the ongoing data sessions and services are seamlessly continued during the network transition.

During the PDU Session Establishment process in a 5G network, the UE and the network may include the "Mapped EPS bearer contexts" information in the PDU Session Establishment Request and PDU Session Establishment Accept messages, respectively. This information indicates which EPS bearer contexts from the 4G LTE network are being mapped to the new 5G PDU session and the corresponding QoS flow identifiers.

    Length of Mapped EPS bearer contexts contents

    Mapped EPS bearer context 1

      EPS bearer identity

      Length of Mapped EPS bearer context

      Number of EPS parameters

      Ebit

      Operation code

      EPS parameters list

      EPS parmeter identifier 1

      Length of EPS parameter contents 1

      EPS parameter contents 1

      EPS parmeter identifier 2

      Length of EPS parameter contents 2

      EPS parameter contents 2

      ...

      EPS parmeter identifier n

      Length of EPS parameter contents n

      EPS parameter contents n

    Mapped EPS bearer context 2

      EPS bearer identity

      Length of Mapped EPS bearer context

      Number of EPS parameters

      Ebit

      Operation code

      EPS parameters list

      EPS parmeter identifier 1

      Length of EPS parameter contents 1

      EPS parameter contents 1

      EPS parmeter identifier 2

      Length of EPS parameter contents 2

      EPS parameter contents 2

      ...

      EPS parmeter identifier n

      Length of EPS parameter contents n

      EPS parameter contents n

    ...

    Mapped EPS bearer context N

      EPS bearer identity

      Length of Mapped EPS bearer context

      Number of EPS parameters

      Ebit

      Operation code

      EPS parameters list

      EPS parmeter identifier 1

      Length of EPS parameter contents 1

      EPS parameter contents 1

      EPS parmeter identifier 2

      Length of EPS parameter contents 2

      EPS parameter contents 2

      ...

      EPS parmeter identifier n

      Length of EPS parameter contents n

      EPS parameter contents n

PDU address (24.501 - 9.11.4.10)

    PDU address IEI

    PDU session type value

      IPv4

      IPv6

      IPv4v6

    PDU address information

      IPv4 Address (4 Byte)

      IPv6 Address (8 Byte)

      IPv4v6 Address (12 Byte)

PDU session type

"PDU session type" indicates the type of data traffic that will be transmitted over the PDU session between UE and the 5G Core Network (5GC). The PDU session type determines the characteristics of the PDU session, such as the IP version used for the data traffic and the type of data bearers required.

The PDU session type is negotiated between the UE and the 5GC during the PDU Session Establishment process. The UE indicates its supported PDU session types in the PDU Session Establishment Request message, and the network selects the appropriate PDU session type based on the UE's capabilities, network configuration, and specific use case. Once the PDU session type is agreed upon, the network assigns IP addresses and allocates resources accordingly, enabling the UE to send and receive data over the established PDU session.

    IPv4

    IPv6

    IPv4v6

    Unstructured

    Ethernet

    reserved

QoS rules

    QoS rule 1

      QoS rule identifier

      Length of QoS rule

      Number of packet filters

      DQR bit

        0 the QoS rule is not the default QoS rule.

        1 the QoS rule is the default QoS rule.

      Rule operation code

        Reserved

        Create new QoS rule

        Delete existing QoS rule

        Modify existing QoS rule and add packet filters

        Modify existing QoS rule and replace all packet filters

        Modify existing QoS rule and delete packet filters

        Modify existing QoS rule without modifying packet filters

        Reserved

      Packet filter list

      QoS rule precedence

      QoS flow identifier (QFI)

    QoS rule 2

      QoS rule identifier

      Length of QoS rule

      Number of packet filters

      DQR bit

        0 the QoS rule is not the default QoS rule.

        1 the QoS rule is the default QoS rule.

      Rule operation code

        Reserved

        Create new QoS rule

        Delete existing QoS rule

        Modify existing QoS rule and add packet filters

        Modify existing QoS rule and replace all packet filters

        Modify existing QoS rule and delete packet filters

        Modify existing QoS rule without modifying packet filters

        Reserved

      Packet filter list

      QoS rule precedence

      QoS flow identifier (QFI)

    ....

Request Type (24.501-9.11.3.47)

The purpose of the Request type information element is to indicate the type of the 5GSM message.

    Request type value

      initial request

      existing PDU session

      initial emergency request

      existing emergency PDU session

      modification request

      reserved

    Spare

    Request Type IEI

 

23.502 - 4.3.2.2.1 states

  • Initial request : the PDU Session Establishment is a request to establish a new PDU Session
  • Existing PDU Session : the request refers to an existing PDU Session switching between 3GPP access and non-3GPP access or to a PDU Session handover from an existing PDN connection in EPC. If the request refers to an existing PDN connection in EPC, the S-NSSAI is set as described in 23.501 - 5.15.7.2
  • (initial) emergency request : When Emergency service is required and an Emergency PDU Session is not already established, a UE shall initiate the UE Requested PDU Session Establishment procedure with a Request Type indicating "(initial) Emergency Request".
  • existing emergency PDU session : the request refers to an existing PDU Session for Emergency services switching between 3GPP access and non-3GPP access or to a PDU Session handover from an existing PDN connection for Emergency services

Session-AMBR (Aggregate Maximum Bit Rate) - (24.501-9.11.4.14)

    Session-AMBR IEI

      value is not used

      value is incremented in multiples of 1 Kbps

      value is incremented in multiples of 4 Kbps

      value is incremented in multiples of 16 Kbps

      value is incremented in multiples of 64 Kbps

      value is incremented in multiples of 256 kbps

      value is incremented in multiples of 1 Mbps

      value is incremented in multiples of 4 Mbps

      value is incremented in multiples of 16 Mbps

      value is incremented in multiples of 64 Mbps

      value is incremented in multiples of 256 Mbps

      value is incremented in multiples of 1 Gbps

      value is incremented in multiples of 4 Gbps

      value is incremented in multiples of 16 Gbps

      value is incremented in multiples of 64 Gbps

      value is incremented in multiples of 256 Gbps

      value is incremented in multiples of 1 Tbps

      value is incremented in multiples of 4 Tbps

      value is incremented in multiples of 16 Tbps

      value is incremented in multiples of 64 Tbps

      value is incremented in multiples of 256 Tbps

      value is incremented in multiples of 1 Pbps

      value is incremented in multiples of 4 Pbps

      value is incremented in multiples of 16 Pbps

      value is incremented in multiples of 64 Pbps

      value is incremented in multiples of 256 Pbps

    Length of Session-AMBR contents

    Unit for Session-AMBR for downlink

    Session-AMBR for downlink

    Unit for Session-AMBR for uplink

    Session-AMBR for uplink

SM PDU DN request container

    DN-specific identity : network access identifier (NAI) format according to IETF RFC 7542

S-NSSAI

    S-NSSAI IEI

    Length of S-NSSAI contents

      SST

      SST and mapped HPLMN SST

      SST and SD

      SST, SD and mapped HPLMN SST

      SST, SD, mapped HPLMN SST and mapped HPLMN SD

    SST(Slice/service type)

    SD(Slice differentiator)

    Mapped HPLMN SST

    Mapped HPLMN SD

SSC (Session and Service Continuity) mode

SSC (Session and Service Continuity) modes refer to different ways of handling mobility and data flow during a PDU session when the User Equipment (UE) moves between different access networks or between 4G and 5G networks.

    SSC mode 1

    SSC mode 2

    SSC mode 3

    unused; shall be interpreted as "SSC mode 1", if received by the network

    unused; shall be interpreted as "SSC mode 2", if received by the network

    unused; shall be interpreted as "SSC mode 3", if received by the network

     

  • SSC Mode 1: In this mode, the PDU session is anchored at the User Plane Function (UPF) in the 5G Core Network (5GC). When the UE moves between different access networks or between 4G and 5G networks, the data path is switched at the UPF. The UE maintains a single PDU session, and the data flow is continuous during the handover process. This mode provides seamless session and service continuity.
  • SSC Mode 2: In this mode, the UE establishes separate PDU sessions for different access networks or for 4G and 5G networks. When the UE moves between networks, the data flow is switched between the different PDU sessions at the UE. This mode does not provide seamless session and service continuity, as the UE needs to establish a new PDU session when moving to a different network, which may cause temporary disruption in the data flow.
  • SSC Mode 3: This mode is a combination of SSC Mode 1 and SSC Mode 2. The UE establishes separate PDU sessions for different access networks or for 4G and 5G networks, similar to SSC Mode 2. However, the data flow is switched at the UPF, like in SSC Mode 1, providing seamless session and service continuity. This mode is used when the UE moves between networks with different PDU session types, such as from a 5G non-roaming network to a 4G roaming network.

Payload container type (24.501-9.11.3.40)

The purpose of the Payload container type information element indicates type of payload included in the payload container information element. This type can be one of the followings :

    N1 SM information

    SMS

    LTE Positioning Protocol (LPP) message container

    SOR transparent container

    UE policy container

    UE parameters update transparent container

    Location services message container

    CIoT user data container

    Multiple payloads

MA PDU session information (24.501 - 9.11.3.31A)

The purpose of the MA PDU(Multi Access PDU) session information information element is to convey the MA-related information for the PDU session.

Currently there is only one value specified for this IE

    MA PDU session network upgrade is allowed

Reference

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