2.8.10 Congestion and Pre-Congestion Notification (pcn)

NOTE: This charter is a snapshot of the 76th IETF Meeting in Hiroshima, Japan. It may now be out-of-date.

Last Modified: 2009-03-27


Scott Bradner <sob@harvard.edu>
Steven Blake <slblake@petri-meat.com>

Transport Area Director(s):

Magnus Westerlund <magnus.westerlund@ericsson.com>
Lars Eggert <lars.eggert@nokia.com>

Transport Area Advisor:

Lars Eggert <lars.eggert@nokia.com>

Mailing Lists:

General Discussion: pcn@ietf.org
To Subscribe: pcn-request@ietf.org
In Body: (un)subscribe
Archive: http://www.ietf.org/mail-archive/web/pcn/index.html

Description of Working Group:

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

(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
Nov 2009  Submit Encoding and Transport of (Pre-) Congestion Information from the Domain Egress to the Ingress to the IESG for consideration as a Proposed Standard RFC
Nov 2009  Submit Suggested Flow Admission and Termination Boundary Mechanisms to the IESG for consideration as an Informational RFC


  • draft-ietf-pcn-encoding-comparison-01.txt
  • draft-ietf-pcn-cl-edge-behaviour-01.txt
  • draft-ietf-pcn-sm-edge-behaviour-01.txt

    Request For Comments:

    RFC5559 I Pre-Congestion Notification (PCN) Architecture
    RFC5670 PS Metering and Marking Behaviour of PCN-Nodes
    RFC5696 PS Baseline Encoding and Transport of Pre-Congestion Information

    Meeting Minutes


    Encoding Comparison Status
    Signaling Requirements
    Controlled Load and Single Marking Edge Behaviors
    Piggybacking Edge Behavior
    HOSE Edge Behavior