Cisco IOS Software supports two fundamental Quality of Service architectures: Differentiated Services (DiffServ) and Integrated Services (IntServ). In the DiffServ model a packet's "class" can be marked directly in the packet, which contrasts with the IntServ model where a signaling protocol is required to tell the routers which flows of packets requires special QoS treatment. DiffServ achieves better QoS scalability, while IntServ provides a tighter QoS mechanism for real-time traffic. These approaches can be complimentary and are not mutually exclusive.
The IntServ architecture model (RFC 1633, June 1994) was motivated by the needs of real-time applications such as remote video, multimedia conferencing, visualization, and virtual reality. It provides a way to deliver the end-to-end Quality of Service (QoS) that real-time applications require by explicitly managing network resources to provide QoS to specific user packet streams (flows). It uses "resource reservation" and "admission control" mechanisms as key building blocks to establish and maintain QoS.
IntServ uses Resource Reservation Protocol (RSVP) to explicitly signal the QoS needs of an application's traffic along the devices in the end-to-end path through the network. If every network device along the path can reserve the necessary bandwidth, the originating application can begin transmitting.
Besides end-to-end signaling, IntServ requires several functions on routers and switches along the path: