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Internet Engineering Task Force                                A. Charny
Internet-Draft                                                  J. Zhang
Intended status: Informational                             Cisco Systems
Expires: April 30, 2010                                   G. Karagiannis
                                                               U. Twente
                                                                M. Menth
                                                 University of Wuerzburg
                                                          T. Taylor, Ed.
                                                     Huawei Technologies
                                                        October 27, 2009


    PCN Boundary Node Behaviour for the Single Marking (SM) Mode of
                               Operation
                  draft-ietf-pcn-sm-edge-behaviour-01

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   Please review these documents carefully, as they describe your rights



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   and restrictions with respect to this document.

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).


Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
     1.1.  Terminology  . . . . . . . . . . . . . . . . . . . . . . .  3
   2.  Assumed Core Network Behaviour for SM  . . . . . . . . . . . .  4
   3.  Node Behaviours  . . . . . . . . . . . . . . . . . . . . . . .  5
     3.1.  Overview . . . . . . . . . . . . . . . . . . . . . . . . .  5
     3.2.  Behaviour of the PCN-Egress-Node . . . . . . . . . . . . .  5
       3.2.1.  PCN-Egress-Node Role In Flow Admission . . . . . . . .  6
       3.2.2.  PCN-Egress-Node Role In Flow Termination . . . . . . .  6
       3.2.3.  Reporting the PCN Data . . . . . . . . . . . . . . . .  7
     3.3.  Behaviour at the Decision Point  . . . . . . . . . . . . .  7
       3.3.1.  Flow Admission . . . . . . . . . . . . . . . . . . . .  7
       3.3.2.  Flow Termination . . . . . . . . . . . . . . . . . . .  7
     3.4.  Behaviour of the Ingress Node  . . . . . . . . . . . . . .  8
   4.  Identifying Ingress-Egress-Aggregates and Their Edge Points  .  8
   5.  Specification of Diffserv Per-Domain Behaviour . . . . . . . .  8
     5.1.  Applicability  . . . . . . . . . . . . . . . . . . . . . .  8
     5.2.  Technical Specification  . . . . . . . . . . . . . . . . .  9
     5.3.  Attributes . . . . . . . . . . . . . . . . . . . . . . . .  9
     5.4.  Parameters . . . . . . . . . . . . . . . . . . . . . . . .  9
     5.5.  Assumptions  . . . . . . . . . . . . . . . . . . . . . . .  9
     5.6.  Example Uses . . . . . . . . . . . . . . . . . . . . . . .  9
     5.7.  Environmental Concerns . . . . . . . . . . . . . . . . . .  9
     5.8.  Security Considerations  . . . . . . . . . . . . . . . . . 10
   6.  Security Considerations  . . . . . . . . . . . . . . . . . . . 10
   7.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 10
   8.  Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 10
   9.  References . . . . . . . . . . . . . . . . . . . . . . . . . . 10
     9.1.  Normative References . . . . . . . . . . . . . . . . . . . 10
     9.2.  Informative References . . . . . . . . . . . . . . . . . . 11
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 11






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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 boundary
   nodes to make decisions about whether to admit or terminate.  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.

   Some aspects of this specification are necessary for
   interoperability, while others are simply suggestions.  This document
   attempts to make the distinction as it proceeds.

1.1.  Terminology

   RFC 2119 requirements language does not seem appropriate for an
   Informational document.  This document uses three levels of
   requirement:

   o  "must" applies to requirements that affect the integrity of
      operation of the complete system;

   o  "recommended" applies to procedures that appear to give superior
      results at time of writing, but may be replaced by other
      procedures directed to the same objective without affecting the
      integrity of operation of the complete system;

   o  "suggested" applies to procedures that are not seen as superior at
      time of writing, but appear to be valid approaches for meeting a



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      particular objective.

   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.  Of course, 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 PCN-egress-node for a
      given ingress-egress-aggregate based on PCN packet marking
      statistics.  The PCN-ingress-node admits or blocks new flows
      offered to the aggregate based on the current value of the PCN-
      admission-state.  For further details see Section 3.2.1 and
      Section 3.3.1.

   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 marked to total PCN traffic (measured in octets) over
      a short period.  This specification suggests that the CLE be
      calculated as an exponentially weighted moving average of the
      ratios observed in successive fixed-length measurement intervals,
      but the exact algorithms used are not critical to
      interoperability.  For further details see Section 3.2.1.

   Admission decision threshold
      A fractional value to which the CLE is compared to determine the
      PCN-admission-state.  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 3.2.1.


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  each link has been configured with a PCN-excess-rate having a
      value equal to the PCN-admissible-rate for the link;





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

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

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


3.  Node Behaviours

3.1.  Overview

   The Single Marking (SM) mode of operation supports flow admission
   based on the ratio of PCN-marked to total PCN-traffic observed by the
   PCN-egress-node (the congestion level estimate, see Section 1.1) for
   each ingress-egress-aggregate.  The PCN-egress-node reports the
   latest value of the PCN-admission-state to the decision point at
   regular intervals.  The decision point decides to admit or block new
   PCN flows offered to a given ingress-egress-aggregate based on the
   PCN-admission-state.

   The decision to terminate flows requires measurement data from both
   the PCN-ingress-node and the PCN-egress-node.  Hence while the PCN-
   admission-state is "block" for a given ingress-egress-aggregate, the
   PCN-egress-node reports an estimate of the edge-to-edge supportable
   PCN traffic rate along with the PCN-admission-state.  If the admitted
   traffic rate measured at the PCN-ingress-node exceeds the reported
   estimate of the edge-to-edge supportable PCN traffic rate, the
   decision point selects flows for termination to reduce this
   difference to zero.  The PCN-ingress-node ceases to admit the
   selected flows.

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.

   This specification recommends that the interval between calculation
   of these quantities be in the range of 100 to 500ms to provide a
   reasonable tradeoff between signalling demands on the network and the



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   time taken to react to impending congestion.

   This specification suggests that PCN-traffic be metered continuously,
   that the counts of the number of octets of PCN traffic needed to
   calculate the above rates be accumulated continuously throughout the
   interval, and that the intervals themselves be of equal length, to
   minimize the statistical variance introduced by the measurement
   process itself.

3.2.1.  PCN-Egress-Node Role In Flow Admission

   Each time the egress node has calculated the rates listed above, the
   egress node must calculate a ratio R of marked to total traffic.  If
   both of the rates are zero for the interval, the ratio R must be set
   to zero.  Otherwise, the egress node must calculate the ratio as:

      R = ETM-rate / (NM-rate + ETM-rate).

   The egress node must then use this ratio to update a congestion level
   estimate (CLE, see Section 1.1).

   Exponential smoothing is suggested for this purpose, so that

      updated CLE = w*R + (1-w)*previous CLE.

   The value of w depends on the length of the measurement interval: for
   the equivalent system memory, a shorter interval calls for a smaller
   smoothing constant.  Simulation results
   ([I-D.babiarz-pcn-explicit-marking],
   [I-D.zhang-pcn-performance-evaluation]) show that the effectiveness
   of PCN is not sensitive to the specific value of w used.

   The egress node now compares the updated CLE against a decision
   threshold.  If the CLE is less than the threshold, the PCN-admission-
   state for the ingress-egress-aggregate is determined to be "admit",
   otherwise it is determined to be "block".

      Simulation results ([I-D.zhang-pcn-performance-evaluation] and
      [Menth08f]) show that the process is also not sensitive to the
      value of the decision threshold.

3.2.2.  PCN-Egress-Node Role In Flow Termination

   When the PCN-egress-node determines that the PCN-admission-state
   computed on the basis of the updated CLE is "block", it must compute
   an estimate of the edge-to-edge supportable rate of PCN traffic for
   the ingress-egress-aggregate concerned.  This specification
   recommends that the estimate be calculated by multiplying the latest



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   calculated NM-rate by a configured factor U, which is the same for
   all flows and all egress nodes in the domain.

3.2.3.  Reporting the PCN Data

   The PCN-egress-node must report the latest value of the PCN-
   admission-state to the decision point each time it calculates it.  If
   the PCN-admission-state is "block", the egress node must also report
   the estimate of the supportable edge-to-edge rate of PCN traffic
   calculated in the previous section.

3.3.  Behaviour at the Decision Point

3.3.1.  Flow Admission

   When the decision point (e.g., the PCN-ingress-node) receives a
   report indicating that the PCN-admission-state for a given ingress-
   egress-aggregate is "admit", it admits new flows to that aggregate.
   When the decision point receives a report indicating that the PCN-
   admission-state for a given ingress-egress-aggregate is "block", it
   ceases to admit new flows to that aggregate.  These actions may be
   modified by policy in specific cases.

3.3.2.  Flow Termination

   When the report from the egress node includes an estimate of the
   edge-to-edge supportable PCN traffic rate for the given ingress-
   egress-aggregate, the decision point must fetch the rate at which
   PCN-traffic has been admitted to the aggregate from the PCN-ingress-
   node.  If the rate of admitted traffic is greater than the estimate
   of the edge-to-edge supportable PCN traffic rate for the given
   ingress-egress-aggregate, 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 edge-to-edge supportable PCN traffic rate provided by
   the egress node.

      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].)

   If the egress node has supplied a list of flow identifiers
   (Section 3.2.2), the decision point first looks to terminate flows



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   from that list.  Flow selection may be guided by policy in specific
   cases.

3.4.  Behaviour of the Ingress Node

   In a specific deployment, the PCN-ingress-node may be the decision
   point.  If so, it carries out the procedures described in the
   previous section.

   Aside from those procedures, the PCN-ingress-node has the
   responsibility to provide the rate of admitted PCN traffic (octets
   per second) on a specific ingress-egress-aggregate when the decision
   point must determine how much flow to terminate in that aggregate.
   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.


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.  Some cases
   will be particularly challenging, as when a packet is carried by an
   MPLS tunnel through the ingress node to some node short of the egress
   node, and then turns into an ordinary IP packet.


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 [ID.PCN-baseline], 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], [ID.PCN-baseline],
   [ID.PCN-marking], and the present document.

5.3.  Attributes

   TBD -- basically low loss, low jitter.  Low delay would be nice but
   has to be quantified

5.4.  Parameters

   TBD.  Don't think RFC 3068 is looking for the list of configurable
   parameters given in the architecture document.

5.5.  Assumptions

   Assumed that a specific portion of link capacity has been reserved
   for PCN traffic.  Assumed that recovery from overloads by flow
   termination should happen within 1-3 seconds.

5.6.  Example Uses

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

5.7.  Environmental Concerns

   In some markets, traffic preemption is considered to be
   impermissible.  In such environments, flow termination would not be
   enabled.




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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

   TBD -- no time to rewrite.  Ruediger for sure.


9.  References

9.1.  Normative References

   [ID.PCN-baseline]
              Moncaster, T., Briscoe, B., and M. Menth, "Baseline
              Encoding and Transport of Pre-Congestion Information (Work
              in progress)", May 2009.

   [ID.PCN-marking]
              Eardley, P., "Metering and marking behaviour of PCN-nodes
              (Work in progress)", June 2009.

   [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.





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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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