Depending on scale and services involved, there is a choice between ignoring QoS and just over dimensioning resources and implementing QoS at one or more layers, using one of two architectures, through protocols of different robustness and complexity.
Second, layer 3 QoS may be considered for two classes of IP networks: 1) IP WANs, where virtual channels between sites are realized using Frame Relay or ATM; and 2) IP VPNs, where Multi-Protocol Label Switching (MPLS), also known as layer 3 switching, is used for defining layer 3 flows though coloring corresponding IP packets
Resource ReSerVation Protocol (RSVP) is the end-to-end signaling protocol used in the Intserv architecture, which requires guaranteed per-flow QoS over the IP network. RSVP incurs a scalability issue since the amount of state information that needs to be tracked about individual flows would become intractable as the number of flows increase in a realistic scenario.
In an RSVP implementation, receiving nodes send bandwidth reservation messages to each router en-route back to the service source. This technique allows RSVP-enabled routers to take note of receiving nodes reserving resources for the same flows, inferring a multicast service and aggregating traffic for bandwidth efficiency (Oram 2002). ...