In general, methods and apparatus for implementing a Quality of Service (QoS) model are disclosed. A Quality of Service (QoS) contract with an initiating network device may be satisfied. A request may be received from the initiating network device in a first time less than or equal to an ordinal number times an arrival interval. The ordinal number signifies a position of the request among a group of requests. The request that has been serviced may be returned to the initiator in a second time less than or equal to a constant term plus the ordinal number times a service interval.