Overview of Pre-Congestion Notification Encoding
RFC 6627

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Internet Engineering Task Force (IETF)                    G. Karagiannis
Request for Comments: 6627                          University of Twente
Category: Informational                                          K. Chan
ISSN: 2070-1721                                               Consultant
                                                            T. Moncaster
                                                 University of Cambridge
                                                                M. Menth
                                                 University of Tuebingen
                                                              P. Eardley
                                                              B. Briscoe
                                                                      BT
                                                               July 2012

            Overview of Pre-Congestion Notification Encoding

Abstract

   The objective of Pre-Congestion Notification (PCN) is to protect the
   quality of service (QoS) of inelastic flows within a Diffserv domain.
   On every link in the PCN-domain, the overall rate of PCN-traffic is
   metered, and PCN-packets are appropriately marked when certain
   configured rates are exceeded.  Egress nodes provide decision points
   with information about the PCN-marks of PCN-packets that allows them
   to take decisions about whether to admit or block a new flow request,
   and to terminate some already admitted flows during serious
   pre-congestion.

   The PCN working group explored a number of approaches for encoding
   this pre-congestion information into the IP header.  This document
   provides details of those approaches along with an explanation of the
   constraints that apply to any solution.

Status of This Memo

   This document is not an Internet Standards Track specification; it is
   published for informational purposes.

   This document is a product of the Internet Engineering Task Force
   (IETF).  It represents the consensus of the IETF community.  It has
   received public review and has been approved for publication by the
   Internet Engineering Steering Group (IESG).  Not all documents
   approved by the IESG are a candidate for any level of Internet
   Standard; see Section 2 of RFC 5741.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at
   http://www.rfc-editor.org/info/rfc6627.

Karagiannis, et al.           Informational                     [Page 1]
RFC 6627          Pre-Congestion Notification Encoding         July 2012

Copyright Notice

   Copyright (c) 2012 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (http://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Karagiannis, et al.           Informational                     [Page 2]
RFC 6627          Pre-Congestion Notification Encoding         July 2012

Table of Contents

   1. Introduction ....................................................4
   2. General PCN Encoding Requirements ...............................5
      2.1. Metering and Marking Algorithms ............................5
      2.2. Approaches for PCN-Based Admission Control and Flow
           Termination ................................................5
           2.2.1. Dual Marking (DM) ...................................5
           2.2.2. Single Marking (SM) .................................6
           2.2.3. Packet-Specific Dual Marking (PSDM) .................7
           2.2.4. Preferential Packet Dropping ........................8
   3. Encoding Constraints ............................................9
      3.1. Structure of the DS Field ..................................9
      3.2. Constraints from the DS Field ..............................9
           3.2.1. General Scarcity of DSCPs ...........................9
           3.2.2. Handling of the DSCP in Tunneling Rules ............10
           3.2.3. Restoration of Original DSCPs at the Egress Node ...10
      3.3. Constraints from the ECN Field ............................11
           3.3.1. Structure and Use of the ECN Field .................11
           3.3.2. Redefinition of the ECN Field ......................12
           3.3.3. Handling of the ECN Field in Tunneling Rules .......12
                  3.3.3.1. Limited-Functionality Option ..............12
                  3.3.3.2. Full-Functionality Option .................13