IP allocation in NR process is very similar to LTE IP Allocation process at high level. It is done by a pair of NAS signaling (PDU Session Establishment Request and PDU Session Establishment Accept) as it is done by a pair of NAS message (PDN Connectivity Request and Activate Default/Dedicated EPS Bearer Context Request) in LTE.

Following is overall signaling procedure for IP allocation in 5G. For the details of these messages refer to 5GSM page.

PDU Session Establishment Request

The first step for the IP allocation is for UE to make a request to the network to allocate an IP. In case of LTE, network allocate at least one IP to the UE through default EPS bearer setup process, but in NR there is no default IP allocation. So UE should request IP allocation. The signaling message used to request IP allocation is PDU session establishment request (it is equivalent to PDN Connectivity Request in LTE)

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

The way UE indicates the request for IP allocation is similar to the case of LTE. As shown in the example, UE specifies the type of IP (IPv4, IPv6 etc) via the IE PDU sesssion type and specifies the protocols for the IP via PCO(Protocol Configuration Options).

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

Once the core network (SMF) recieves the IP allocation request from UE, the network allocates the necessary resources and configurations with other network components and notifies about the final result (i.e, allocated IP) to the UE via PDU session Establsihment accept.

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

Reference

[1] 3GPP SA2 architecture and functions for 5G mobile communication system

[2] 5G beyond radio access: a flatter sliced network

[3] End to End Network Slicing