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 DiffServ architecture model (RFC 2475, December 1998) divides traffic into a small number of classes, and allocates resources on a per-class basis. Because DiffServ has only a few classes of traffic, a packet's "class" can be marked directly in the packet.
In the DiffServ model, packets are classified and marked to receive a particular forwarding treatment (per-hop behavior or PHB) on nodes along their path. Sophisticated classification, marking, policing, and shaping operations need only be implemented at network boundaries or hosts, enabling greater scalability than other models of service differentiation.