Introduction to CUPS

CUPS is an architecture that gains popularity in the mobile core. The architecture was first adopted in the 4G Long Term Evolution (LTE) mobile core and later became the standard architecture for the 5G core. In 2019, Broadband Forum (BBF) began a BNG CUPS project and subsequently standardized the BNG CUPS architecture in TR-459, TR-459.2, and TR-459.3. BBF continues to define new standardized features and solutions contributed by service providers and vendors.

Multi-Access Gateway

Nokia offers the Multi-Access Gateway (MAG) solution with an access-agnostic core that supports both broadband wireline and fixed wireless use case. Today, the MAG solution can be deployed in a virtualized network function (VNF) environment or a cloud-native (CN) environment. The CN version of the MAG is called the containerized MAG controller (cMAG-c). The cMAG-c supports the wireline use case, as defined by BBF.

The following figure shows a high-level CUPS architecture as defined in TR-459. The interfaces between the CP and the UP are:
  • state control interface (SCi)
  • control packet redirect interface (CPRi)
  • management interface (Mi)
Figure 1. Broadband wireline CUPS architecture

BBF defines BNG CUPS as a disaggregated BNG (DBNG) where the CP of the DBNG is known as the DBNG-CP and the UP of the DBNG is known as the DBNG-UP. The DBNG-CP in the context of this document is the Nokia cMAG-c. The following terms are used interchangeably in the industry:

  • DBNG and BNG CUPS
  • user plane (UP) and data plane (DP)

Control plane overview

The Nokia cMAG-c is equivalent to the BBF defined DBNG-CP. It provides subscriber management functions including authentication, authorization, and accounting (AAA), IP address management, and policy management. The cMAG-c has northbound control interfaces (NCIs) to communicate with external systems such as remote authentication dial-in user service (RADIUS) servers and policy services. The cMAG-c uses standard interfaces defined in TR-459 to interact with the UP like programing data traffic forwarding rules.

Note: Subscriber routing, including the IGP or BGP routing control plane, is located on the UP only. CUPS is only applicable to the subscriber management function within the UP.

User plane overview

The Nokia 7750 SR platforms are equivalent to the BBF defined DBNG-UP, and are also referred to as MAG-u. The UP is responsible for subscriber data traffic forwarding. The UP performs the following key functions:
  • traffic management, including filtering and multicast replication
  • policy enforcement, such as QoS and charging
  • subscriber statistics collection
The UP installs the subscriber forwarding rules communicated by the CP through the SCi.
Note: For the CUPS architecture, the subscriber routing function, which includes the IGP and BGP routing control and packet forwarding, remains entirely on the MAG-u. The cMAG-c can install subscriber routes on the UP but is not involved in how these are exposed to the routing control function.

cMAG-c interfaces

The Nokia cMAG-c supports BBF defined standard CUPS interfaces.

State control interface

The state control interface (SCi) is responsible for programming subscriber data forwarding rules on the UP. The CUPS solution uses the 3GPP packet forwarding control protocol (PFCP) protocol as specified in 3GPP TS 29.244. 3GPP designed the PFCP to be fully extensible. To accommodate the broadband wireline use case, BBF extended the PFCP information elements (IEs) and published the extension in TR-459. Nokia extended the PFCP IE set to offer additional feature sets.

For wireline access, the SCi programs, at minimum, the following types of rules:

  • control packet redirection rules – bidirectional rules to redirect subscriber wireline control packets, via the UP, from the residential gateway (RG) to the CP and from the CP to the RG
  • subscriber data forwarding rules – bidirectional rules to forward subscriber traffic between the access interface and the network interface

Control packet redirect interface

The purpose of the control packet redirect interface (CPRi) is to tunnel the control packets of the RG that are received on the UP to the CP and from the CP to the UP and back to the RG. The tunnel encapsulation is the GPRS tunneling protocol - user plane (GTP-U) as specified in 3GPP TS 29.244. GTP-U is extended with an optional network service header (NSH) as defined in RFC 8300 to carry UP-specific metadata such as the ingress port. There are two types of CPRi tunnels:
  • per-session CPRi tunnels
  • common CPRi tunnels
The SCi installs each CPRi tunnel on the UP.
Note: The terms "CPRi" and "IBCP" (the Nokia in-band control plane) are used interchangeably; both indicate the same interface and function.

Management interface

The cMAG-c uses the management interface (Mi) to program the MAG-u for everything that is not related to subscriber forwarding rules. BBF TR-459 has no specifications for this interface.