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Internet Engineering Task Force                                 M. Menth
Internet-Draft                                              F. Lehrieder
Expires: January 8, 2009                         University of Wuerzburg
                                                            July 7, 2008


              Marking Converter for Excess-Marked Traffic
                  draft-menth-pcn-marking-converter-00

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Copyright Notice

   Copyright (C) The IETF Trust (2008).














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Abstract

   This document proposes an algorithm that converts packet markings of
   a stream that was excess-marked based on a lower-rate into packet
   markings that correspond to a stream that was excess-marked based on
   a higher-rate.  It may be applied in the PCN context to convert
   marked admissible-rate-overload into marked supportable-rate-
   overload.  This allows to perform marked flow termination when
   packets are excess-marked based on the admissible rate only.


Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
   2.  Terminology  . . . . . . . . . . . . . . . . . . . . . . . . .  4
   3.  Algorithm for Conversion of AS-Markings into ET-Markings . . .  5
   4.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . .  7
   5.  Security Considerations  . . . . . . . . . . . . . . . . . . .  8
   6.  Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . .  9
   7.  References . . . . . . . . . . . . . . . . . . . . . . . . . . 10
     7.1.  Normative References . . . . . . . . . . . . . . . . . . . 10
     7.2.  Informative References . . . . . . . . . . . . . . . . . . 10
     7.3.  Other References . . . . . . . . . . . . . . . . . . . . . 10
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 11
   Intellectual Property and Copyright Statements . . . . . . . . . . 12


























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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 [I-D.ietf-pcn-architecture].  This is achieved by marking
   packets on interior nodes according to some metering function
   implemented at each node.  Links are associated with an admissible
   and a supportable rate threshold (AR, SR).

   o  When the PCN traffic rate on a link exceeds the AR of that link,
      the link is AR-pre-congested and the PCN rate above AR is AR-
      overload.

   o  When the PCN traffic rate on a link exceeds the SR of that link,
      the link is SR-pre-congested and the PCN rate above SR is SR-
      overload.

   Excess marking is a mechanism marking packets exceeding a certain
   reference rate.  If applied with AR or SR as reference rate on a link
   of the PCN domain, excess marking marks the AR- or SR-overload.  We
   call the marks based on AR admission-stop (AS) marking and the marks
   based on SR excess-traffic (ET) marking.  Admission control requires
   AS-marking while flow termination requires ET-marking.  Having two
   different markers is desirable to perform admission control and flow
   termination based on direct feedback from the network, but it
   increases hardware and encoding complexity.

   The single-marking draft [I-D.charny-pcn-single-marking] proposes one
   method to perform measured rate termination based on AR-overload.  It
   requires that SR=u*AR on all links within the PCN domain.

   In this document we present a conversion algorithm that converts AS-
   markings of a packet stream into ET-markings.  Admission control can
   be performed based on the original AS-markings and flow termination
   can be performed based on the converted ET-markings.  To that end,
   any flow termination method working with SR-overload can be applied
   ([Menth08-PCN-Comparison], Section 7).

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in RFC 2119 [RFC2119].









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2.  Terminology

   Most of the terminology used in this document is defined in
   [I-D.ietf-pcn-architecture].  The following additional terms are
   defined.

   o  Admissible rate (AR) - PCN lower rate

   o  Supportable rate (SR) - PCN upper rate

   o  AR-overload - PCN traffic rate above AR

   o  SR-overload - PCN traffic rate above SR

   o  Excess marking - metering and marking mechanism marking all
      packets exceeding a reference rate (excess rate marking in
      [I-D.eardley-pcn-marking-behaviour])

   o  Admission-stop (AS) marking - marking based on AR as reference
      rate

   o  Excess-traffic (ET) marking - marking based on SR as reference
      rate




























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3.  Algorithm for Conversion of AS-Markings into ET-Markings

   The conversion algorithm is applied by the egress node on an ingress-
   egress aggregate basis.  It is called for each packet arrival and
   either converts an existing AS-mark into an ET-mark or clears it.
   The algorithm is based on a token bucket (TB) with size S, fill state
   F, and threshold T. It differs from conventional TB implementations
   as it does not have a constant fill rate R. Its operation is
   explained in Algorithm 1.

 Algorithm 1:

 Input: token bucket parameters S, F, and T, packet size B and marking M

 1: if (M == unmarked) then
 2:    F = min(S, F + (u - 1) * B);
 3: else if (F >= T) then
 4:    F = F - B;
 5:    M = unmarked;
 6: else
 7:    M = ET;
 8: end if

   The number of tokens in the bucket F indicates how many AS-marked
   bytes can be re-marked to unmarked.  Initially, the token bucket
   should be filled.  For each non-AS-marked byte, the fill state F is
   incremented by u-1 tokens (cf. line 1-2).  When a packet arrives AS-
   marked and if the fill state F is larger than a certain threshold T,
   the packet is re-marked to unmarked and the fill state of the TB is
   reduced by the packet size B. Otherwise, the packet remains marked
   which is then interpreted as ET-marking (cf. line 3-8).

   The threshold T is used to achieve packet-size independent marking
   conversion and should be set to the maximum transfer unit.  A
   sufficiently large TB size S is needed to tolerate short-term
   variations of packet markings, i.e. a burst of S AS-marked bytes
   should not be ET-marked.  However, this tolerance also delays initial
   re-marking.  Further experimentation and performance evaluation of
   this approach is required.

   First simulations give a proof of concept.  The conversion algorithm
   works well if the rate of the controlled ingress-egress-aggregate is
   large enough and if it is a rather large fraction (>10%) of the
   traffic rate on the bottleneck link.  If this is not the case,
   packets with AS-markings occur almost random which leads to a
   geometrically distributed distance between packet markings within an
   ingress-egress-aggregate such that very large bursts of AS-marked
   packets can occur even when the PCN rate is between AR and SR on the



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   bottleneck link.  This leads to wrong ET-markings.  As a result,
   there is some chance for overtermination
   ([I-D.menth-pcn-performance]) when marked flow termination for
   ingress-egress-aggregates is used.  This, however, is not a property
   of the conversion algorithm, it's rather a property of the single
   marking approach and also measured rate termination suffers from this
   phenomenon.  Further evaluation is required to configure the
   conversion algorithm appropriately and to validate flow termination
   mechanisms in combination with this converter.










































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4.  IANA Considerations

   TBD
















































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5.  Security Considerations

   TBD
















































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6.  Conclusion

   This document describes an algorithm that converts marked AR-overload
   into marked SR-overload.  It makes flow termination mechanisms
   requiring SR-overload applicable in networks that mark AR-overload
   only.  This algorithm does not solve the problem that flow
   termination based on AR-overload does not work well for multipath
   routing ([Menth08-PCN-Comparison], Section 8.3).











































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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.

7.2.  Informative References

   [I-D.charny-pcn-single-marking]
              Charny, A., Zhang, X., Faucheur, F., and V. Liatsos, "Pre-
              Congestion Notification Using Single Marking for Admission
              and  Termination", draft-charny-pcn-single-marking-03
              (work in progress), November 2007.

   [I-D.eardley-pcn-marking-behaviour]
              Eardley, P., "", draft-eardley-pcn-marking-behaviour-01
              (work in progress), I-D Status active, June 2008.

   [I-D.ietf-pcn-architecture]
              Eardley, P., "Pre-Congestion Notification Architecture",
              draft-ietf-pcn-architecture-01 (work in progress),
              October 2007.

   [I-D.menth-pcn-performance]
              Menth, M. and F. Lehrieder, "Performance Evaluation of
              PCN-Based Algorithms", draft-menth-pcn-performance-02
              (work in progress), February 2008.

7.3.  Other References

   [Menth08-PCN-Comparison]
              Menth, M., Lehrieder, F., Briscoe, B., Eardley, P.,
              Moncaster, T., Babiarz, J., Chan, K., Charny, A.,
              Karagiannis, G., and X. Zhang, "PCN-Based Admission
              Control and Flow Termination", 2008, <http://
              www3.informatik.uni-wuerzburg.de/staff/menth/Publications/
              Menth08-PCN-Comparison.pdf>.













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Authors' Addresses

   Michael Menth
   University of Wuerzburg
   Am Hubland
   Wuerzburg  D-97074
   Germany

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


   Frank Lehrieder
   University of Wuerzburg
   Am Hubland
   Wuerzburg  D-97074
   Germany

   Phone: +49-931-888-6651
   Email: lehrieder@informatik.uni-wuerzburg.de































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Full Copyright Statement

   Copyright (C) The IETF Trust (2008).

   This document is subject to the rights, licenses and restrictions
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Acknowledgment

   Funding for the RFC Editor function is provided by the IETF
   Administrative Support Activity (IASA).





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