ITU-T Q.3311 PDF

This Recommendation defines enhancements to resource and admission control protocols [ITU-T Q.3320] to make use of pre-congestion notification (PCN) which is defined in [IETF RFC 5559] and is a new approach to guarantee quality of service within Diffserv-controlled domains.

The basic concept of pre-congestion notification (PCN) is to measure the loading state of the network based on the experience of flow aggregates as they pass through the network. Aggregates are defined as the set of packets passing through given {ingress point, egress point} pairs. Based on the aggregate results as measured at the egress points, admission policies may be updated for flows offered to these aggregates at the ingress points to the PCN-controlled domain. The observed results can lead to one of three conclusions at a given point of time:

a) further flows may be admitted to the aggregate;

b) no further flows may be admitted to the aggregate; or

c) some of the flows already admitted to the aggregate must immediately be terminated to protect quality of service for further incoming flows.

PCN distinguishes and assigns roles to ingress nodes, interior nodes, and egress nodes relative to a given PCN domain.

Ingress nodes mark admitted packets to indicate that they should be PCN-metered.

Interior nodes check the next-hop link traffic status for each PCN-marked packet before routing it. Packets are either unmarked, threshold-marked or excess traffic marked where the use of threshold marking depends on the encoding and marking schemes deployed in the network. (For definitions of the marking terminology, see clause 3.)

The egress nodes relate the packets they receive to the aggregate flows they receive from individual ingress nodes and generate traffic marking statistics at regular intervals. In principle, the egress node reports these statistics to the decision point each time they are computed, although reports may be filtered in practice to reduce the amount of messaging to be handled.

The architecture on which the IETF works has focused on has the assumption that egress nodes report directly to ingress nodes to affect termination and admission decisions, but also allows for reporting to a centralized decision point. This Recommendation defines the protocol modifications required for resource and admission control protocols to mutually enhance the operation of resource and admission control function (RACF) and PCN when both are present.