Precongestion notification: new QoS support for differentiated services IP networks

Admission control is a well-known technique to explicitly admit or block new flows in order to keep a network's traffic load at a moderate level and to guarantee QoS for admitted flows. Flow termination is a new flow control function that terminates some admitted flows when the network capacity does not suffice, such as in the case of unexpected failures. Admission control and flow termination are useful to protect QoS for inelastic flows that require a minimum bit rate. Examples are real-time applications like voice and video. Precongestion notification (PCN) provides feedback about load conditions on the path to boundary nodes. They use this information to implement lightweight admission control and flow termination without per-flow state on interior nodes of a domain. These mechanisms are significantly simpler than explicit reservation schemes. In the context of DiffServ, IP, MPLS, and Ethernet networks, we explain the concept of PCN-based admission control and flow termination, present recent standards of the IETF, and discuss benefits and limitations.