3. Prerequisites

3.1. Network architecture

Being only one component of the general 5GS architecture, UPF won’t do much without at least one SMF instance controlling it.

Since the Virtual Service Router focuses on UPF exclusively, other network functions (SMF in particular) must be fulfilled by external components. Their configuration is out of the scope of this documentation.

Because N3/N4/N6/N9 are all IPv4/IPv6-based, addresses and routes must be configured on the underlying interfaces to reach the various networks and network functions of the topology.

Note

UPF is not a router. It does not simply route PDUs according to IPv4/IPv6 routing tables but steers them between the relevant links (interfaces) according to rules issued by SMF.

Therefore, only directly connected networks and gateways for UPF’s own traffic are typically configured on it.

Below diagram illustrates the IP configuration and PDU traffic flow with a Virtual Service Router UPF configured as a basic PSA.

_images/upf-psa-detail.svg

UPF-PSA IP configuration. eth1 handles GTP-U-encapsulated DL traffic over N3, eth0 handles PFCP traffic with SMF over N4 and eth2 handles bare IP traffic over N6 (which typically comprises Internet traffic). Linux-style network interface names are used here for clarity.

So in essence:

  1. Virtual Service Router UPF is configured to reach RAN, SMF and DN over IP; N3/N4/N6 roles are derived from the chosen network interfaces.

  2. To prevent bare IP datagrams coming from N6 from being misrouted or dropped altogether instead of entering the GTP-U tunnel over N3 (i.e. DL-bound traffic), an appropriate IP route injecting the relevant traffic in a virtual GTP network interface is configured in advance (here represented by 10.60.0.1 via gtp0).

    Note

    This step is specific to PSA scenarios as bare IP traffic must be routed somehow.

  3. Virtual Service Router UPF is configured to handle PFCP traffic over eth0.

  4. SMF initiates a PFCP session with UPF once the latter is ready.

  5. SMF then initiates a new PDU session, causing UPF to automatically set up the N3 GTP-U tunnel and configure internal steering.

  6. PDU traffic then flows in both UL and DL directions according to the rules provided by SMF.

3.2. Host requirements

A physical machine, VM or container dimensioned for the maximum number of simultaneous PDU sessions to handle with respect to their memory footprint.

See also