Congestion and Pre-Congestion Notification (pcn)

Last modified: 2012-07-19


Transport Area Directors

Transport Area Advisor

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The Congestion and Pre-Congestion Notification (PCN) working group develops mechanisms to protect the quality-of-service of established inelastic flows within a DiffServ domain when congestion is imminent or existing. These mechanisms operate at the domain boundary, based on aggregated congestion and pre-congestion information from within the domain. The focus of the WG is on developing standards for the marking behavior of the interior nodes and the encoding and transport of the congestion information. To allow for future extensions to the mechanisms and their application to new deployment scenarios, they are logically separated into several components, namely, encoding and transport along forward path from marker to egress, metering of congestion information at the egress, and transport of congestion information back to the controlling ingress. Reaction mechanisms at the boundary consist of flow admission and flow termination. Although designed to work together, flow admission and flow termination are independent mechanisms, and the use of one does not require or prevent the use of the other. The WG may produce a small number of informational documents that describe how specific quality-of-service policies for a domain can be implemented using these two mechanisms.

The PCN WG will specify the following components to protect the quality-of-service of flows within a DiffServ domain:

(1) a general architecture for flow admission and termination based on aggregated (pre-)congestion information

(2) a specification of conditions under which interior nodes generate (pre-)congestion information

(3) encoding and transport of (pre-)congestion information between the interior and domain egress

(4) metering of (pre-)congestion information at the domain egress

(5) encoding and transport of (pre-)congestion information between the egress and the controlling domain ingress

(6) ingress node control mechanisms for flow admission or termination, based on aggregated (pre-) congestion information

The WG focuses on the marking behavior and encoding and transport mechanisms needed to realize the overall PCN architecture. Standards- track protocols and mechanisms are only developed where necessary for interoperability. For other components of the architecture, the WG may document examples or provide recommended solutions in informational documents. The architecture document will be comprehensive, and include security, manageability and operational considerations. All PCN mechanisms, including transport and encoding of (pre-congestion) information, are required to cleanly integrate with existing architectures and protocols such as DiffServ and ECN. If the PCN WG requires extensions or modifications to protocols that are products of other WGs, it may motivate their need and describe requirements in informational documents; design of such extensions and modifications will take place in the appropriate WGs.

The initial scope of the PCN WG is restricted by the following assumptions:

(A) these components are deployed in a single DiffServ domain, where all boundary and interior nodes are PCN-enabled and trust each other for correct PCN marking, encoding, transport and aggregation

(B) all flows handled by these mechanisms are inelastic and constrained to a known maximum rate through policing or shaping

(C) the number of flows across any potential aggregation bottleneck is sufficiently large for stateless, statistical mechanisms to be effective

(D) flows may have different precedence, but the applicability of the PCN mechanisms for emergency use (911, GETS, WPS, MLPP, etc.) is out of scope

After completion of the initial phase, the PCN WG may re-charter to develop solutions for scenarios where some of these restrictions are not in place. It may also re-charter to consider applying the PCN mechanisms to additional deployment scenarios (operation over concatenated DiffServ domains, PCN-aware application mechanisms, etc.). The WG may also consider to investigate additional response mechanisms that act on (pre-)congestion information. One example could be flow-rate adaptation (rather than flow admission/ termination) during times of congestion. The details of these work items are outside the scope of the initial phase; but the WG may consider their requirements to design components that are sufficiently general to support such extensions in the future.

Goals and Milestones

Done Submit Pre-Congestion Notification (PCN) Architecture to the IESG for consideration as an Informational RFC
Mar 2009 Submit Encoding and Transport of (Pre-) Congestion Information from within a DiffServ Domain to the Egress to the IESG for consideration as a Proposed Standard RFC
Mar 2009 Submit (Pre-) Congestion Detection within a DiffServ Domain to the IESG for consideration as a Proposed Standard RFC
Mar 2009 Submit Survey of Encoding and Transport Choices of (Pre-) Congestion Information within a DiffServ Domain to the IESG for consideration as an Informational RFC
Jul 2009 Submit Requirements for Signaling of (Pre-) Congestion Information from Egress to Ingress in a DiffServ Domain to the IESG for consideration as a Informational RFC


Request for Comments

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