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). The reservation components consist of a traffic receiver (RSVP sender), an RSVP receiver, and a number of RSVP-enabled routers. An RSVP-enabled router is equipped with two local decision modules:
1. Admission control, which determines the sufficiency of required resources to support the reservation; and
2. Policy control, which determines whether the user had administrative permission to make that reservation.
Call admission control (CAC) determines the admissibility of real-time traffic to the network when there are not sufficient resources to accommodate such traffic, which cannot be simply dropped as could data traffic. A voice/video call that is being attempted when there are not enough bandwidth and delay guarantees would need to be attempted at a later time or possibly re-routed through PSTN or ISDN. Extensions are defined for policy control as well.
If the reservation passes the two fore-mentioned controls, RSVP sets packet classification and packet scheduling parameters to achieve the desired QoS. RSVP runs directly over IP, with two main message types to note: PATH and RESV, which respectively describe flow parameters and corresponding needed resource guarantees (Vegesna