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Internet Engineering Task Force                                 M. Menth
Internet-Draft                                   University of Wuerzburg
Expires: January 8, 2009                                      J. Babiarz
                                                            T. Moncaster
                                                            July 7, 2008

                 End-to-end Extension for PCN Encoding

Status of this Memo

   By submitting this Internet-Draft, each author represents that any
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Copyright Notice

   Copyright (C) The IETF Trust (2008).

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   This document proposes an encoding of PCN marks based on the ECN
   field of the Voice-Admit DSCP.  It has global meaning and ECN
   semantics are not applied to that field.  The PCN codepoints are the
   same as those for packet-specific dual marking (PSDM) within a single
   PCN domain, but the general concept can also be applied to other
   encodings requiring only a single PCN-enabled DSCP.

Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
   2.  Terminology  . . . . . . . . . . . . . . . . . . . . . . . . .  4
   3.  Encoding Support for End-to-End PCN  . . . . . . . . . . . . .  5
   4.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . .  6
   5.  Security Considerations  . . . . . . . . . . . . . . . . . . .  7
   6.  Conclusions  . . . . . . . . . . . . . . . . . . . . . . . . .  8
   7.  References . . . . . . . . . . . . . . . . . . . . . . . . . .  9
     7.1.  Normative References . . . . . . . . . . . . . . . . . . .  9
     7.2.  Informative References . . . . . . . . . . . . . . . . . .  9
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 10
   Intellectual Property and Copyright Statements . . . . . . . . . . 11

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1.  Introduction

   Pre-congestion notification provides information to support admission
   control and flow termination at the boundary nodes of a Diffserv
   region in order to protect the quality of service (QoS) of inelastic
   flows [PCN-arch].  This is achieved by marking packets on interior
   nodes according to some metering function implemented at each node.
   Excess traffic marking marks PCN packets that exceed a certain
   reference rate on a link while exhaustive marking marks all PCN
   packets on a link when the PCN traffic rate exceeds a reference rate
   [PCN-marking-behaviour].  These marks are monitored by the egress
   nodes of the PCN domain.  This idea that the use of PCN marks is
   limited to a single domain is called edge-to-edge PCN.

   Packet-specific dual marking (PSDM) [PCN-pcn-encoding-PSDM] proposes
   an encoding of PCN marks that reuse the ECN field of PCN-enabled DSCP
   to record the marks in IP packets.  However, these codepoints can be
   used only within PCN domains and packets with such PCN codepoints
   must not leave the PCN domain.

   With end-to-end PCN the entities monitoring and evaluating the PCN
   marks reside in the end systems such that the notion of PCN domains
   no longer exists [Menth08-PCN-Comparison].  Here, an encoding is
   required that has a global scope.  In this document, we propose to
   use the ECN field of the Voice-Admit DSCP for that use.  As a
   consequence, ECN cannot be used with Voice-Admit anymore.  Note that
   Voice-Admit DSCP with this modified meaning can be used both for
   edge-to-edge and for end-to-end PCN.

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   document are to be interpreted as described in [RFC2119].

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2.  Terminology

   The terminology used in this document is defined either in [PCN-arch]
   or in [PCN-pcn-encoding-PSDM].

   o  Edge-to-edge PCN - Admission for a PCN domain is requested
      explicitly for flows.  When their packets enter the PCN domain,
      the ingress node sets a PCN-enabled DSCP and a PCN-capable
      codepoint (ECN field).  The packets are metered and possibly re-
      marked by the traversed node in the PCN domain.  The markings are
      evaluated by the egress node of that domain, and the DSCP and ECN
      field of the packet is reset before packets leave the PCN domain.

   o  End-to-end PCN - The source node or a proxy thereof issues PCN
      packets with a PCN-enabled DSCP and PCN-capable codepoint (ECN
      field).  Packets are metered and possibly marked by PCN nodes on
      their way.  They pass ingress and egress nodes without special
      treatment of their DSCP or ECN field.  The destination node or a
      proxy thereof evaluate the PCN markings.

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3.  Encoding Support for End-to-End PCN

   In this section describes the changes to [PCN-pcn-encoding-PSDM] to
   support end-to-end PCN.

   In [PCN-pcn-encoding-PSDM] the Voice-Admit DSCP is proposed as the
   preferred PCN-enabled DSCP to implement the PSDM encoding within
   single domains.  PCN codepoints are interpreted as such only within
   PCN domains while they are interpreted as ECN codepoints outside PCN
   domains.  Being conform with [RFC4774] requires special treatment of
   PCN packets by ingress and egress nodes of a PCN domain.

   We propose that the default ECN semantics [RFC3168] do not apply for
   the Voice-Admit DSCP [voice-admit] , but alternate PCN semantics are
   applied which are end-to-end PCN.  This requires an update to

   Then, the ECN field can be used for PCN purposes in the general
   Internet.  One of the benefits is that end-to-end PCN can be
   supported by such a codepoint.

   This extension of the scope of Voice-Admit DSCP for PCN has no impact
   on the treatment of the PCN encoding for edge-to-edge PCN wich is
   described in [PCN-pcn-encoding-PSDM]. o PCN packets have a PCN-
   capable codepoint only within a PCN domain. o Ingress and egress
   nodes need to perform special treatment of PCN packets of edge-to-
   edge controlled PCN flows.

   In contrast, packets from end-to-end PCN controlled flows are not
   dropped by the ingress node of a PCN domain and not re-marked by the
   egress node of a PCN domain.  Special treatment of ECN-capable PCN
   flows is not needed because PCN flows are not ECN capable.

   PCN ingress and egress nodes can differentiate packets from end-to-
   end and edge-to-edge PCN flows because edge-to-edge PCN flows
   explicitly request admission for the PCN domain such that PCN ingress
   and egress nodes keep per-flow states and know whether an incoming
   packet is edge-to-edge or end-to-end PCN controlled.

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4.  IANA Considerations

   This document makes no request to IANA.  It does however request a
   change to [voice-admit], requesting that a statement to be added that
   the default behaviour for the ECN semantics [RFC3168] does not apply
   for the Voice-Admit [voice-admit] DSCP.

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5.  Security Considerations


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6.  Conclusions

   The proposed encoding allows that PCN codepoint can be used in the
   general Internet.  This opens the possibility for end-to-end PCN.

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7.  References

7.1.  Normative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.

   [RFC3168]  Ramakrishnan, K., Floyd, S., and D. Black, "The Addition
              of Explicit Congestion Notification (ECN) to IP",
              RFC 3168, September 2001.

   [RFC4774]  Floyd, S., "Specifying Alternate Semantics for the
              Explicit Congestion Notification (ECN) Field", BCP 124,
              RFC 4774, November 2006.

7.2.  Informative References

              Eardley, P., "Pre-Congestion Notification Architecture",
              draft-ietf-pcn-architecture-03 (work in progress),
              February 2008.

              Eardley, P., "Marking behaviour of PCN-nodes",
              draft-eardley-pcn-marking-behaviour-01 (work in progress),
              June 2008.

              Menth, M., Babiarz, J., Moncaster, T., and B. Briscoe,
              "PCN Encoding for Packet-Specific Dual Marking (PSDM)",
              July 2008.

              Baker, F., Polk, J., and M. Dolly, "DSCPs for Capacity-
              Admitted Traffic",
              draft-ietf-tsvwg-admitted-realtime-dscp-04 (work in
              progress), February 2008.

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Authors' Addresses

   Michael Menth
   University of Wuerzburg
   Am Hubland
   Wuerzburg  D-97074

   Phone: +49-931-888-6644
   Email: menth@informatik.uni-wuerzburg.de

   Jozef Z. Babiarz
   3500 Carling Avenue
   Ottawa, Ont.  K2H 8E9

   Phone: +1-613-763-6098
   Email: babiarz@nortel.com

   Toby Moncaster
   B54/70, Adastral Park
   Ipswich  IP5 3RE

   Phone: +44 1473 648734
   Email: toby.moncaster@bt.com

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