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Internet Engineering Task Force                                A. Charny
Internet-Draft                                                  J. Zhang
Intended status: Informational                             Cisco Systems
Expires: September 7, 2010                                G. Karagiannis
                                                               U. Twente
                                                                M. Menth
                                                 University of Wuerzburg
                                                          T. Taylor, Ed.
                                                     Huawei Technologies
                                                           March 6, 2010


    PCN Boundary Node Behaviour for the Single Marking (SM) Mode of
                               Operation
                  draft-ietf-pcn-sm-edge-behaviour-02

Abstract

   Precongestion notification (PCN) is a means for protecting quality of
   service for inelastic traffic admitted to a Diffserv domain.  The
   overall PCN architecture is described in RFC 5559.  This memo is one
   of a series describing possible boundary node behaviours for a PCN
   domain.  The behaviour described here is that for two-state
   measurement-based load control, known informally as Single Marking
   (SM).

Status of this Memo

   This Internet-Draft is submitted to IETF in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF), its areas, and its working groups.  Note that
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   The list of Internet-Draft Shadow Directories can be accessed at
   http://www.ietf.org/shadow.html.

   This Internet-Draft will expire on September 7, 2010.



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Copyright Notice

   Copyright (c) 2010 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
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   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 BSD License.


Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  4
     1.1.  Terminology  . . . . . . . . . . . . . . . . . . . . . . .  4
   2.  Assumed Core Network Behaviour for SM  . . . . . . . . . . . .  5
   3.  Node Behaviours  . . . . . . . . . . . . . . . . . . . . . . .  5
     3.1.  Overview . . . . . . . . . . . . . . . . . . . . . . . . .  5
     3.2.  Behaviour of the PCN-Egress-Node . . . . . . . . . . . . .  6
       3.2.1.  Reporting the PCN Data . . . . . . . . . . . . . . . .  6
     3.3.  Behaviour of the Ingress Node  . . . . . . . . . . . . . .  6
     3.4.  Behaviour at the Decision Point  . . . . . . . . . . . . .  7
       3.4.1.  Flow Admission . . . . . . . . . . . . . . . . . . . .  7
       3.4.2.  Flow Termination . . . . . . . . . . . . . . . . . . .  7
       3.4.3.  Decision Point Action For Missing Egress Node
               Reports  . . . . . . . . . . . . . . . . . . . . . . .  8
     3.5.  Summary of Timers  . . . . . . . . . . . . . . . . . . . .  9
   4.  Identifying Ingress-Egress-Aggregates and Their Edge Points  .  9
   5.  Specification of Diffserv Per-Domain Behaviour . . . . . . . .  9
     5.1.  Applicability  . . . . . . . . . . . . . . . . . . . . . .  9
     5.2.  Technical Specification  . . . . . . . . . . . . . . . . . 10
     5.3.  Attributes . . . . . . . . . . . . . . . . . . . . . . . . 10
     5.4.  Parameters . . . . . . . . . . . . . . . . . . . . . . . . 10
     5.5.  Assumptions  . . . . . . . . . . . . . . . . . . . . . . . 10
     5.6.  Example Uses . . . . . . . . . . . . . . . . . . . . . . . 10
     5.7.  Environmental Concerns . . . . . . . . . . . . . . . . . . 11
     5.8.  Security Considerations  . . . . . . . . . . . . . . . . . 11
   6.  Security Considerations  . . . . . . . . . . . . . . . . . . . 11
   7.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 11
   8.  Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 11
   9.  References . . . . . . . . . . . . . . . . . . . . . . . . . . 11
     9.1.  Normative References . . . . . . . . . . . . . . . . . . . 11
     9.2.  Informative References . . . . . . . . . . . . . . . . . . 12



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   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 12


















































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1.  Introduction

   The objective of Pre-Congestion Notification (PCN) is to protect the
   quality of service (QoS) of inelastic flows within a Diffserv domain,
   in a simple, scalable, and robust fashion.  Two mechanisms are used:
   admission control, to decide whether to admit or block a new flow
   request, and (in abnormal circumstances) flow termination to decide
   whether to terminate some of the existing flows.  To achieve this,
   the overall rate of PCN-traffic is metered on every link in the
   domain, and PCN-packets are appropriately marked when certain
   configured rates are exceeded.  These configured rates are below the
   rate of the link thus providing notification to boundary nodes about
   overloads before any congestion occurs (hence the "pre" of "pre-
   congestion notification").  The level of marking allows decisions to
   be made about whether to admit or terminate individual flows.  For
   more details see [RFC5559].

   Boundary node behaviours specify a detailed set of algorithms and
   edge node behaviours used to implement the PCN mechanisms.  Since the
   algorithms depend on specific metering and marking behaviour at the
   interior nodes, it is also necessary to specify the assumptions made
   about interior node behaviour.  Finally, because PCN uses DSCP values
   to carry its markings, a specification of boundary node behaviour
   must include the per domain behaviour (PDB) template specified in
   [RFC3086], filled out with the appropriate content.  The present
   document accomplishes these tasks for the Single Marking (SM) mode of
   operation.

1.1.  Terminology

   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 RFC2119 [RFC2119].

   In addition to the terms defined in [RFC5559], this document uses the
   following terms:

   decision point
      The node that makes the decision about which flows to admit and to
      terminate.  In a given network deployment, this may be the ingress
      node or a centralized control node.  Regardless of the location of
      the decision point, the ingress node is the point where the
      decisions are enforced.

   PCN-admission-state
      The state ("admit" or "block") derived by the decision point for a
      given ingress-egress-aggregate based on PCN packet marking
      statistics.  The decision point decides to admit or block new



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      flows offered to the aggregate based on the current value of the
      PCN-admission-state.  For further details see Section 3.4.1.

   Congestion level estimate (CLE)
      A value derived from the measurement of PCN packets received at a
      PCN-egress-node for a given ingress-egress-aggregate, representing
      the ratio of excess-traffic-marked to total PCN traffic (measured
      in octets) over a short period.  For further details see
      Section 3.4.1.

   Admission decision threshold
      A fractional value to which decision point compares the CLE to
      determine the PCN-admission-state for a given ingress-egress-
      aggregate.  If the CLE is below the admission decision threshold
      the PCN-admission-state is set to "admit".  If the CLE is above
      the admission decision threshold the PCN-admission-state is set to
      "block".  For further details see Section 3.4.1.


2.  Assumed Core Network Behaviour for SM

   This section describes the assumed behaviour for nodes of the PCN-
   domain when acting in their role as PCN-interior-nodes.  The SM mode
   of operation assumes that:

   o  on each link the reference rate for the excess traffic meter is
      configured with a PCN-excess-rate to be equal to the PCN-
      admissible-rate for the link;

   o  PCN-interior-nodes perform excess-traffic-metering of packets
      according to the rules specified in [RFC5670].

   o  excess-traffic-marking of packets uses the PCN-Marked (PM)
      codepoint defined in [RFC5696];

   o  no link PCN-threshold-rate is configured, and PCN-interior nodes
      perform no threshold-metering.


3.  Node Behaviours

3.1.  Overview

   This section describes the behaviour of the PCN ingress and egress
   nodes and the decision point (which may be collocated with the
   ingress node).  The PCN egress node collects and reports the rates of
   total, threshold-marked, and excess-traffic-marked PCN traffic to the
   decision point.  For a detailed description, see Section 3.2.



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   The PCN ingress node reports the rate of PCN traffic admitted to a
   given ingress-egress aggregate when requested by the decision point.
   It also enforces flow admission and termination decisions.  For
   details, see Section 3.3.

   Finally, the decision point makes flow admission decisions and
   selects flows to terminate based on the information provided by the
   ingress and egress nodes for a given ingress-egress-aggregate.  For
   details, see Section 3.4.

3.2.  Behaviour of the PCN-Egress-Node

   The PCN-egress-node MUST meter received PCN traffic in order to
   derive periodically the following rates for each ingress-egress-
   aggregate passing through it:

   o  NM-rate: octets per second of PCN traffic in PCN-unmarked packets;

   o  ETM-rate: octets per second of PCN traffic in PCN-excess-marked
      packets.

   It is RECOMMENDED that the interval Tcalc between calculation of
   these quantities be in the range of 100 to 500 ms to provide a
   reasonable tradeoff between signalling demands on the network and the
   time taken to react to impending congestion.

   The PCN-traffic SHOULD be metered continuously and the intervals
   themselves SHOULD be of equal length, to minimize the statistical
   variance introduced by the measurement process itself.

3.2.1.  Reporting the PCN Data

   At the end of each interval, the PCN-egress-node SHOULD report the
   latest calculated rates to the decision point.  To reduce the volume
   of signalling, the egress node MAY choose not to send a report if no
   PCN traffic was received either during the present interval or during
   the previous one.  The egress node MUST send a report at least once
   per configurable interval Tmax (of the order of a second) to
   demonstrate liveness, even if all of the rates have value zero.

3.3.  Behaviour of the Ingress Node

   The PCN-ingress-node MUST provide the estimated current rate of
   admitted PCN traffic (octets per second) for a specific ingress-
   egress-aggregate when the decision point requests it.  The way this
   rate estimate is derived is a matter of implementation.





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      For example, the rate that the PCN-ingress-node supplies MAY be
      based on a quick sample taken at the time the information is
      required.  It is RECOMMENDED that such a sample be based on
      observation of at least 30 PCN packets to achieve reasonable
      statistical reliability.

3.4.  Behaviour at the Decision Point

3.4.1.  Flow Admission

   When the decision point (e.g., the PCN-ingress-node) receives a
   report from the egress node for a given ingress-egress-aggregate that
   contains non-zero rates, it MUST calculate a congestion level
   estimate (CLE) for the interval, where

      CLE = ETM-Rate/(NM-Rate + ETM-Rate).

   The decision point MUST compare the CLE to an admission decision
   threshold.  If the CLE is less than the threshold, the PCN-admission-
   state for that aggregate MUST be set to "admit"; otherwise it MUST be
   set to "block".

      It is RECOMMENDED that the admission decision threshold for SM be
      set fairly low, in the order of 0.05.  The admission decision
      threshold MAY vary for different flows based on policy.

   If the PCN-admission-state for a given ingress-egress-aggregate is
   "admit", the decision point SHOULD allow new flows to be admitted to
   that aggregate.  If the PCN-admission-state for a given ingress-
   egress-aggregate is "block", the decision point SHOULD NOT allow new
   flows to be admitted to that aggregate.  These actions MAY be
   modified by policy in specific cases.

3.4.2.  Flow Termination

   Not all operators will wish to deploy flow termination.  Hence
   deactivation of flow termination at the decision node MUST be a
   configurable option.

   When the report from the egress node that the PCN-admission-state
   computed on the basis of the CLE is "block" for the given ingress-
   egress-aggregate, the decision point MUST request the PCN-ingress-
   node to provide an estimate of the rate (Admit-Rate) at which PCN-
   traffic is being admitted to the aggregate.

      If the decision point is collocated with the ingress node, the
      request and response are internal operations.




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   The decision point MUST then wait, both for the requested rate from
   the ingress node and for the next report from the egress node.  If
   this next egress node report also includes a non-zero value for the
   ETM-Rate, the decision point MUST determine an amount of flow to
   terminate in the following steps:

   1.  The sustainable aggregate rate (SAR) for the given ingress-
       egress-aggregate is estimated by the product:

          SAR = U * NM-Rate

       for the latest reported interval, where U is a configurable
       factor less than one which is the same for all ingress-egress-
       aggregates.

   2.  The amount of traffic that must be terminated is the difference:

          Admit-Rate - SAR,

       where Admit-Rate is the value provided by the ingress node.

   If the difference calculated in the second step is positive, the
   decision point MUST select flows to terminate using its knowledge of
   the bandwidth required by individual flows gained, e.g., from
   resource signalling, until it determines that the PCN traffic
   admission rate will no longer be greater than the estimated
   sustainable aggregate rate.

      Flow termination MAY be spread out over multiple rounds to avoid
      over-termination.  If this is done, it is RECOMMENDED that enough
      time elapse between successive rounds of termination to allow the
      effects of previous rounds to be reflected in the measurements
      upon which the termination decisions are based (see
      [I-D.satoh-pcn-performance-termination] and sections 4.2 and 4.3
      of [Menth08-sub-9]).

3.4.3.  Decision Point Action For Missing Egress Node Reports

   As mentioned in Section 3.2.1, the egress node MAY choose not to send
   reports for a configurable interval Tmax while it does not receive
   any PCN traffic for a given ingress-egress-aggregate.  However, if
   the decision point fails to receive reports for a given ingress-
   egress-aggregate for a configurable interval Tfail (of the order of 2
   * Tmax or less), it SHOULD cease to admit flows to that aggregate and
   raise an alarm to management.  This provides some protection against
   the case where congestion is preventing the transfer of reports from
   the egress node to the decision point.




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3.5.  Summary of Timers

   This section has referred to three timers:

   o  Tcalc: a timer which SHOULD be configurable, specifying the
      frequency with which the PCN-egress-node calculates NM-Rate, ThM-
      Rate, and ETM-Rate and reports them to the decision point.  This
      timer is RECOMMENDED to be of the order of 100 to 500 ms.

   o  Tmax: a configurable timer, specifying the maximum amount of time
      between successive reports from the PCN-egress-node for a given
      ingress-egress-aggregate.  This is RECOMMENDED to be of the order
      of one second.

   o  Tfail: a configurable timer, specifying the maximum amount of time
      between successive reports for a given ingress-egress-aggregate
      received at the decision point, after which the latter SHOULD
      cease to admit flows to the aggregate concerned and raise an alarm
      to management.  This is RECOMMENDED to be of the order of 2 * Tmax
      or less.


4.  Identifying Ingress-Egress-Aggregates and Their Edge Points

   The operation of PCN depends on the ability of the ingress and egress
   nodes to identify the aggregate to which each flow belongs.  The
   egress node also needs to associate an aggregate with the address of
   the ingress node for receiving reports, if the ingress node is the
   decision point.

   The means by which this is done depends on the packet routing
   technology in use in the network.  In general, classification of
   individual packets at the ingress node (for enforcement and metering
   of admission rates) and at the egress node must use the content of
   the outer packet header.  The process may well require configuration
   of routing information in the ingress and egress nodes.


5.  Specification of Diffserv Per-Domain Behaviour

   This section provides the specification required by [RFC3086] for a
   per-domain behaviour.

5.1.  Applicability

   This section draws heavily upon points made in the PCN architecture
   document, [RFC5559].




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   The PCN SM boundary node behaviour specified in this document is
   applicable to inelastic traffic (particularly video and voice) where
   quality of service for admitted flows is protected primarily by
   admission control at the ingress to the domain.  In exceptional
   circumstances (e.g. due to network failures) already-admitted flows
   may be terminated to protect the quality of service of the remainder.
   The SM boundary node behaviour is more likely to terminate too many
   flows under such circumstances than some alternative PCN boundary
   node behaviours.

   Single-Marking requires no extension to the baseline PCN encoding
   described in [RFC5696], thus reducing the work expected to be
   performed in the data path of the high-speed routing equipment, and
   saving valuable real estate in the packet header.

5.2.  Technical Specification

   The technical specification of the PCN SM per domain behaviour is
   provided by the contents of [RFC5559], [RFC5696], [RFC5670], and the
   present document.

5.3.  Attributes

   The purpose of this per-domain behaviour is to achieve low loss and
   jitter for the target class of traffic.  Recovery from overloads by
   flow termination should happen within 1-3 seconds.

5.4.  Parameters

   The SM per-domain behaviour specifies three timers, two at the PCN-
   egress-node and one at the PCN-ingress-node; see Section 3.5.
   Reference rates must be specified at each interior router for the
   PCN-excess-rate on each link; see Section 2.  An admission decision
   threshold must be specified at each PCN-ingress-node; see
   Section 3.4.1.  A fraction U must be specified at each PCN-ingress-
   node, with a common value over the whole domain; see Section 3.4.2.

5.5.  Assumptions

   Assumed that a specific portion of link capacity has been reserved
   for PCN traffic.

5.6.  Example Uses

   The PCN SM behaviour may be used to carry real-time traffic,
   particularly voice and video.





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5.7.  Environmental Concerns

   The PCN SM per-domain behaviour may interfere with the use of end-to-
   end ECN due to reuse of ECN bits for PCN marking.  See Appendix B of
   [RFC5696] for details.

5.8.  Security Considerations

   Please see the security considerations in Section 6 as well as those
   in [RFC2474] and [RFC2475].


6.  Security Considerations

   [RFC5559] provides a general description of the security
   considerations for PCN.  This memo introduces no new considerations.


7.  IANA Considerations

   This memo includes no request to IANA.


8.  Acknowledgements

   The authors thank Ruediger Geib for his useful comments.


9.  References

9.1.  Normative References

   [RFC2474]  Nichols, K., Blake, S., Baker, F., and D. Black,
              "Definition of the Differentiated Services Field (DS
              Field) in the IPv4 and IPv6 Headers", RFC 2474,
              December 1998.

   [RFC2475]  Blake, S., Black, D., Carlson, M., Davies, E., Wang, Z.,
              and W. Weiss, "An Architecture for Differentiated
              Services", RFC 2475, December 1998.

   [RFC5559]  Eardley, P., "Pre-Congestion Notification (PCN)
              Architecture", RFC 5559, June 2009.

   [RFC5670]  Eardley, P., "Metering and Marking Behaviour of PCN-
              Nodes", RFC 5670, November 2009.

   [RFC5696]  Moncaster, T., Briscoe, B., and M. Menth, "Baseline



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              Encoding and Transport of Pre-Congestion Information",
              RFC 5696, November 2009.

9.2.  Informative References

   [I-D.babiarz-pcn-explicit-marking]
              Liu, X. and J. Babiarz, "Simulations Results for 3sM
              (expired Internet Draft)", July 2007.

   [I-D.satoh-pcn-performance-termination]
              Satoh, D., Ueno, H., and M. Menth, "Performance Evaluation
              of Termination in CL-Algorithm (Work in progress)",
              July 2009.

   [I-D.zhang-pcn-performance-evaluation]
              Zhang, X., "Performance Evaluation of CL-PHB Admission and
              Termination Algorithms (expired Internet Draft)",
              July 2007.

   [ID.briscoe-CL]
              Briscoe, B., "An edge-to-edge Deployment Model for Pre-
              Congestion Notification:  Admission Control over a
              DiffServ Region (expired Internet Draft)", 2006.

   [Menth08-sub-9]
              Menth, M. and F. Lehrieder, "PCN-Based Measured Rate
              Termination", July 2009, <http://
              www3.informatik.uni-wuerzburg.de/~menth/Publications/
              papers/Menth08-Sub-9.pdf>.

   [Menth08f]
              Menth, M. and F. Lehrieder, "Performance Evaluation of
              PCN-Based Admission Control", in Proceedings of the 16th
              International Workshop on Quality of Service (IWQoS)",
              June 2008, <http://www3.informatik.uni-wuerzburg.de/
              ~menth/Publications/papers/Menth08f.pdf>.

   [RFC3086]  Nichols, K. and B. Carpenter, "Definition of
              Differentiated Services Per Domain Behaviors and Rules for
              their Specification", RFC 3086, April 2001.











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Authors' Addresses

   Anna Charny
   Cisco Systems
   300 Apollo Drive
   Chelmsford, MA  01824
   USA

   Email: acharny@cisco.com


   Xinyan (Joy) Zhang
   Cisco Systems
   300 Apollo Drive
   Chelmsford, MA  01824
   USA


   Georgios Karagiannis
   U. Twente


   Phone:
   Email: karagian@cs.utwente.nl


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

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


   Tom Taylor (editor)
   Huawei Technologies
   1852 Lorraine Ave
   Ottawa, Ontario  K1H 6Z8
   Canada

   Phone: +1 613 680 2675
   Email: tom111.taylor@bell.net







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