Introduction to CUPS

The cloud-native version of the Multi-Access Gateway supports wireline use cases. The cMAG-c functions as the control plane in a CUPS architecture, managing user planes and using BBF-defined interfaces for subscriber and network configurations.

CUPS is a subscriber-focused architecture that was first adopted for mobile core. Initially, it appeared in the 4G long-term evolution (LTE) mobile core and later became the standard architecture for the 5G core. In 2019, the Broadband Forum (BBF) began standardization of the BNG CUPS architecture. Nokia was one of the pioneer contributors at BBF in BNG CUPS standardization, which led to the publication of TR-459 for broadband applications, TR-459.2 for NAT, and TR-459.3 for multicast. 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 cases. Today, the MAG solution can be deployed in a virtualized network function (VNF) environment or a cloud-native (CN) environment. Nokia refers to the CN version of the MAG as the containerized MAG controller (cMAG-c). The cMAG-c supports wireline use cases, 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)
  • in-band control plane (IBCP), also referred to as 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

cMAG-c control plane overview

The Nokia cMAG-c 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 for basic broadband functions. Nokia innovation includes many more added features to enrich functionalities, efficiency, resiliency, and performance improvements.

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 BNG.

DBNG-UP 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, and it performs the following key functions:
  • traffic management, including NAT and multicast replication
  • policy enforcement, such as QoS and charging
  • subscriber traffic statistics
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 controlling the routing function on the UP.

cMAG-c interfaces

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

State control interface

The SCi is responsible for programming subscriber data forwarding rules on the UP. The cMAG-c 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 cMAG-c and from the cMAG-c to the RG
  • subscriber data forwarding rules – bidirectional rules to forward subscriber traffic between the access interface and the network interface

In-band control plane interface

The purpose of the IBCP is to tunnel the control packets of the RG that are received on the UP to the cMAG-c, from the cMAG-c 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 IBCP tunnels:
  • per-session IBCP tunnels
  • common IBCP tunnels
The cMAG-c programs the IBCP tunnels through the SCi interface.

Management interface

The cMAG-c uses the Mi to program the DBNG-UP for everything that is not related to subscriber forwarding rules. BBF TR-459 has no specifications for this interface.