This document describes a Diameter application that performs Authentication, Authorization, and Accounting for Quality of Service (QoS) reservations. This protocol is used by elements along the path of a given application flow to authenticate a reservation request, ensure that the reservation is authorized, and to account for resources consumed during the lifetime of the application flow. Clients that implement the Diameter QoS application contact an authorizing entity/application server that is located somewhere in the network, allowing for a wide variety of flexible deployment models.