User-User Protocol

Description

The User-User Protocol (UUP) is a concept defined in 3GPP TS 21.905 and referenced in TS 22.060, adopted from ITU-T Recommendation I.112. It is described as 'a protocol that is adopted between two or more users in order to ensure communication between them.' This definition places the UUP at the highest level of the protocol stack, dealing with the rules that govern how users (which can be human users or application processes) exchange information end-to-end.

In the OSI reference model, the UUP corresponds to the application layer or, more precisely, to the protocols that operate directly between end systems. Unlike lower-layer protocols that are concerned with the transport of data across the network, the UUP is concerned with the semantics and syntax of the information exchanged. For example, in a voice call, the UUP would include the rules for setting up, managing, and tearing down the call, as well as the encoding of voice signals. In a data application, the UUP would define the message formats and procedures for a specific service.

The UUP is distinct from the User-Network Interface (UNI) and the Network-Network Interface (NNI). While the UNI and NNI define how users and networks interact with each other, the UUP defines how users interact with each other through the network. The network provides the transport, but the UUP ensures that the users can understand each other. This separation of concerns is a fundamental principle in layered network architectures.

In 3GPP networks, examples of UUPs include the protocols used for end-to-end services such as IP Multimedia Subsystem (IMS) session control (SIP), which operates between user equipment, and the Real-time Transport Protocol (RTP) for media transport. These protocols are not defined by 3GPP alone but are often adopted from other standards bodies such as the IETF. 3GPP specifies how these protocols are used in the context of mobile networks, including the necessary adaptations for mobility and quality of service.

The UUP concept is also relevant in the context of the Open Service Access (OSA) defined in TS 22.127 and TS 29.198. OSA provides a framework for applications to access network capabilities, and the UUP can be seen as the protocol between the application and the user. In this context, the UUP ensures that the application can communicate with the user regardless of the underlying network technology.

The UUP has evolved with the shift from circuit-switched to packet-switched networks. In circuit-switched networks, the UUP was often tightly coupled with the network signalling, such as in the ISDN User Part (ISUP) for telephony. In packet-switched networks, the UUP is more decoupled, with protocols like SIP and HTTP operating over IP. This decoupling has enabled greater flexibility and innovation in service creation.

Purpose & Motivation

The User-User Protocol exists to ensure that two or more users can communicate effectively, regardless of the network they are using. It provides a common set of rules for interaction, enabling interoperability between different applications and devices. Without a UUP, each application would need to define its own communication rules, leading to fragmentation and incompatibility.

The concept is rooted in the OSI model, which separates the functions of a communication system into layers. The UUP operates at the highest layer, focusing on the meaning and format of the information exchanged. This separation allows the network to evolve independently of the applications, and vice versa. For example, a new UUP can be deployed without changing the underlying transport network.

In the context of 3GPP, the UUP is important for enabling end-to-end services that span multiple networks and devices. It allows operators to offer services that are not tied to a specific access technology, such as IMS-based voice and messaging. The UUP also facilitates the creation of new services by providing a standardised way for applications to communicate with users.