PDU Session Modification is the 5GSM procedure used to change an already established PDU session. It may be initiated by the UE or by the network. Typical changes include QoS rule updates, QoS flow changes, session-AMBR updates, mapped EPS bearer context changes, or other session-related parameters.

• Signaling Sequence
• Message Structure
  • Example : PDU Session Modification Request
  • Example : PDU Session Modification Command
  • Example : PDU Session Modification Complete
  • Example : PDU Session Modification Reject

Signaling Sequence

Message Structure

The exact content depends on the change being made. Common optional IEs include 5GSM cause, session-AMBR, QoS rules, QoS flow descriptions, mapped EPS bearer contexts, PDU address and extended protocol configuration options.

Example : PDU Session Modification Request

The checked Amarisoft MME logs did not contain this procedure, so the following is a constructed full-message example.

[NAS] UL 5GSM: PDU session modification request
        Protocol discriminator = 0x2e (5GS Session Management)
        PDU session identity = 1
        Procedure transaction identity = 52
        Message type = 0xc9 (PDU session modification request)
        5GSM cause = 0x1a (Insufficient resources)
        Requested QoS rules:
          QoS rule 1:
            QoS rule identifier = 1
            Rule operation code = example
            QFI = 1
                

Example : PDU Session Modification Command

The following constructed example shows a network-triggered QoS update.

[NAS] DL 5GSM: PDU session modification command
        Protocol discriminator = 0x2e (5GS Session Management)
        PDU session identity = 1
        Procedure transaction identity = 52
        Message type = 0xcb (PDU session modification command)
        Authorized QoS rules:
          QoS rule 1:
            QoS rule identifier = 1
            Rule operation code = 3 (modify existing QoS rule and add packet filters)
            DQR = 1
            Number of packet filters = 1
            Packet filter identifier = 15
              Packet filter direction = 3 (bidirectional)
              Match-all
            QoS rule precedence = 255
            QFI = 1
        Authorized QoS flow descriptions:
          QoS flow description 1:
            QFI = 1
            Operation code = 3 (modify existing QoS flow description)
            5QI = 9
                

Example : PDU Session Modification Complete

The following constructed example shows the UE acknowledgement after applying the modification.

[NAS] UL 5GSM: PDU session modification complete
        Protocol discriminator = 0x2e (5GS Session Management)
        PDU session identity = 1
        Procedure transaction identity = 52
        Message type = 0xcc (PDU session modification complete)
                

Example : PDU Session Modification Reject

The following constructed example shows the UE rejecting a modification command.

[NAS] UL 5GSM: PDU session modification command reject
        Protocol discriminator = 0x2e (5GS Session Management)
        PDU session identity = 1
        Procedure transaction identity = 52
        Message type = 0xcd (PDU session modification command reject)
        5GSM cause = 0x1a (Insufficient resources)