Related Excerpts from Fielding's Dissertation
"A layered system is organized hierarchically, each layer providing services to the layer above it and using services of the layer below it. Although layered system is considered a "pure" style, its use within network-based systems is limited to its combination with the client-server style to provide layered-client-server.
...Layered-client-server adds proxy and gateway components to the client-server style. A proxy acts as a shared server for one or more client components, taking requests and forwarding them, with possible translation, to server components. A gateway component appears to be a normal server to clients or proxies that request its services, but is in fact forwarding those requests, with possible translation, to its "inner-layer" servers. These additional mediator components can be added in multiple layers to add features like load balancing and security checking to the system.
...the layered system style allows an architecture to be composed of hierarchical layers by constraining component behavior such that each component cannot "see" beyond the immediate layer with which they are interacting. By restricting knowledge of the system to a single layer, we place a bound on the overall system complexity and promote substrate independence.
Layered systems reduce coupling across multiple layers by hiding the inner layers from all except the adjacent outer layer, thus improving evolvability and reusability. Examples include the processing of layered communication protocols, such as the TCP/IP and OSI protocol stacks, and hardware interface libraries. The primary disadvantage of layered systems is that they add overhead and latency to the processing of data, reducing user-perceived performance.
...LCS is also a solution to managing identity in a large scale distributed system, where complete knowledge of all servers would be prohibitively expensive. Instead, servers are organized in layers such that rarely used services are handled by intermediaries rather than directly by each client.
...Layers can be used to encapsulate legacy services and to protect new services from legacy clients, simplifying components by moving infrequently used functionality to a shared intermediary. Intermediaries can also be used to improve system scalability by enabling load balancing of services across multiple networks and processors."