IntServ

Integrated Services (IntServ) is a network architecture designed to provide quality of service for individual data streams by reserving resources along the path between communicating hosts. It contrasts with Differentiated Services (DiffServ), which provides scalable QoS by classifying traffic rather than reserving per flow.

IntServ relies on signaling to perform end-to-end resource reservation. Routers along the path maintain per-flow state and participate in admission control. Endpoints specify their traffic and QoS requirements using a traffic specification (Tspec) and a resource specification (Rspec) within signaling messages. A PATH message traverses from sender to receiver to establish the flow state, while a RESV message travels back to reserve the necessary resources along the path. If adequate resources cannot be reserved, the reservation may fail and the flow is not admitted.

There are two principal service models. Guaranteed Service aims to provide predictable bounds on delay and loss for a flow, suitable for latency-sensitive applications. Controlled Load offers QoS features such as reduced loss and lower delay under moderate congestion, but does not guarantee strict bounds. Implementations require per-flow state in each router along the path and careful admission control to prevent overcommitment.

IntServ gained substantial attention in the 1990s as a principled approach to QoS but faced scalability challenges due to the overhead of maintaining per-flow state in routers. Consequently, it has seen limited deployment in the public Internet, with DiffServ becoming the dominant QoS framework. IntServ remains relevant in controlled networks, research testbeds, and certain enterprise deployments.

See also: RSVP, QoS, DiffServ, MPLS.