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Internet Engineering Task Force                                A. Charny
Internet-Draft                                                  J. Zhang
Intended status: Informational                             Cisco Systems
Expires: March 18, 2011                                   G. Karagiannis
                                                               U. Twente
                                                                M. Menth
                                                 University of Wuerzburg
                                                          T. Taylor, Ed.
                                                     Huawei Technologies
                                                      September 14, 2010


    PCN Boundary Node Behaviour for the Single Marking (SM) Mode of
                               Operation
                  draft-ietf-pcn-sm-edge-behaviour-04

Abstract

   Pre-congestion notification (PCN) is a means for protecting the
   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 a form of
   measurement-based load control using two PCN marking states, not-
   marked, and excess-traffic-marked.  This behaviour is known
   informally as the Single Marking (SM) PCN edge behaviour.

Status of this Memo

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

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
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   Internet-Drafts are draft documents valid for a maximum of six months
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   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on March 18, 2011.

Copyright Notice

   Copyright (c) 2010 IETF Trust and the persons identified as the
   document authors.  All rights reserved.



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   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.


Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
     1.1.  Terminology  . . . . . . . . . . . . . . . . . . . . . . .  3
   2.  Assumed Core Network Behaviour for SM  . . . . . . . . . . . .  5
   3.  Node Behaviours  . . . . . . . . . . . . . . . . . . . . . . .  6
     3.1.  Overview . . . . . . . . . . . . . . . . . . . . . . . . .  6
     3.2.  Behaviour of the PCN-Egress-Node . . . . . . . . . . . . .  6
       3.2.1.  Data Collection  . . . . . . . . . . . . . . . . . . .  6
       3.2.2.  Reporting the PCN Data . . . . . . . . . . . . . . . .  7
       3.2.3.  Optional Report Suppression  . . . . . . . . . . . . .  7
     3.3.  Behaviour at the Decision Point  . . . . . . . . . . . . .  8
       3.3.1.  Flow Admission . . . . . . . . . . . . . . . . . . . .  8
       3.3.2.  Flow Termination . . . . . . . . . . . . . . . . . . .  9
       3.3.3.  Decision Point Action For Missing
               PCN-Boundary-Node Reports  . . . . . . . . . . . . . . 10
     3.4.  Behaviour of the Ingress Node  . . . . . . . . . . . . . . 10
     3.5.  Summary of Timers  . . . . . . . . . . . . . . . . . . . . 10
   4.  Identifying Ingress and Egress Nodes For PCN     Traffic  . . . . 12
   5.  Specification of Diffserv Per-Domain Behaviour . . . . . . . . 12
     5.1.  Applicability  . . . . . . . . . . . . . . . . . . . . . . 12
     5.2.  Technical Specification  . . . . . . . . . . . . . . . . . 12
     5.3.  Attributes . . . . . . . . . . . . . . . . . . . . . . . . 13
     5.4.  Parameters . . . . . . . . . . . . . . . . . . . . . . . . 13
     5.5.  Assumptions  . . . . . . . . . . . . . . . . . . . . . . . 14
     5.6.  Example Uses . . . . . . . . . . . . . . . . . . . . . . . 14
     5.7.  Environmental Concerns . . . . . . . . . . . . . . . . . . 14
     5.8.  Security Considerations  . . . . . . . . . . . . . . . . . 14
   6.  Security Considerations  . . . . . . . . . . . . . . . . . . . 14
   7.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 15
   8.  Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 15
   9.  References . . . . . . . . . . . . . . . . . . . . . . . . . . 15
     9.1.  Normative References . . . . . . . . . . . . . . . . . . . 15
     9.2.  Informative References . . . . . . . . . . . . . . . . . . 15
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 16





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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 PCN-
   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 PCN-boundary-nodes
   about overloads before any congestion occurs (hence the "pre" part 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].

   PCN-boundary-node behaviours specify a detailed set of algorithms and
   procedures 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 PCN-interior-node behaviour.  Finally, because PCN uses DSCP
   values to carry its markings, a specification of PCN-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 PCN-
      ingress-node or a centralized control node.  Regardless of the
      location of the Decision Point, the PCN-ingress-node is the point
      where the decisions are enforced.

   NM-rate
      The rate of not-marked PCN-traffic received at a PCN-egress-node
      for a given ingress-egress-aggregate in octets per second.  For
      further details see Section 3.2.1.



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   ETM-rate
      The rate of excess-traffic-marked PCN traffic received at a PCN-
      egress-node for a given ingress-egress-aggregate in octets per
      second.  For further details see Section 3.2.1.

   PCN-sent-rate
      The rate of PCN-traffic received at a PCN-ingress-node and
      destined for a given ingress-egress-aggregate in octets per
      second.  For further details see Section 3.4.

   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.  The CLE is used to derive the
      PCN-admission-state (Section 3.3.1) and also by the report
      suppression procedure (Section 3.2.3) if report suppression is
      activated.

   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
      flows offered to the aggregate based on the current value of the
      PCN-admission-state.  For further details see Section 3.3.1.

   Sustainable aggregate rate (SAR)
      The estimated maximum rate of PCN-traffic that can be admitted to
      a given ingress-egress-aggregate at a given moment without risking
      degradation of quality of service for the admitted flows.  The
      intention is that if the PCN-sent-rate of every ingress-egress-
      aggregate passing through a given link is limited to its
      sustainable aggregate rate, the total rate of PCN-traffic flowing
      through the link will be limited to the PCN-supportable-rate for
      that link.  An estimate of the sustainable aggregate rate for a
      given ingress-egress-aggregate is derived as part of the flow
      termination procedure, and is used to determine how much PCN-
      traffic must be terminated.  For further details see
      Section 3.3.2.

   CLE-reporting-threshold
      A configurable value against which the CLE is compared as part of
      the report suppression procedure.  For further details, see
      Section 3.2.3.







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   CLE-limit
      A configurable value against which the CLE is compared in order to
      derive the PCN-admission-state for a given ingress-egress-
      aggregate.  For further details see Section 3.3.1.

   T-meas
      An interval, the value of which is configurable, defining the
      measurement period at the PCN-egress-node during which statistics
      relating to PCN-traffic marking are collected.  At the end of the
      interval the values NM-rate and ETM-rate as defined above are
      calculated and a report is sent to the Decision Point, subject to
      the operation of the report suppression feature.  For further
      details see Section 3.2.

   T-maxsuppress
      An interval, the value of which is configurable, after which the
      PCN-egress-node must send a report to the Decision Point for a
      given ingress-egress-aggregate regardless of the most recent
      values of the CLE.  This is used as a keep-alive mechanism for
      signalling between the PCN-egress-node and the Decision Point when
      report suppression is activated.  For further details, see
      Section 3.2.3.

   T-fail
      An interval, the value of which is configurable, after which the
      Decision Point concludes that communication from a given PCN-
      egress-node has failed if it has received no reports from the PCN-
      egress-node during that interval.  For further details see
      Section 3.3.3.


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  PCN-interior-nodes perform excess-traffic-marking 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  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;





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   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-node, PCN-
   egress-node and the Decision Point (which may be collocated with the
   PCN-ingress-node).

   The PCN-egress-node collects and reports the rates of not-marked and
   excess-traffic-marked PCN-traffic to the Decision Point.  For a
   detailed description, see Section 3.2.

   The PCN-ingress-node enforces flow admission and termination
   decisions.  It also reports the rate of PCN-traffic sent to a given
   ingress-egress-aggregate when requested by the Decision Point.  For
   details, see Section 3.4.

   Finally, the Decision Point makes flow admission decisions and
   selects flows to terminate based on the information provided by the
   PCN-ingress-node and PCN-egress-node for a given ingress-egress-
   aggregate.  For details, see Section 3.3.

3.2.  Behaviour of the PCN-Egress-Node

3.2.1.  Data Collection

   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-packets that are
      not PCN-Marked;

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

   It is RECOMMENDED that the measurement interval, T-meas, between
   successive calculations 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.  The starting
   and ending times of the measurement intervals for different ingress-



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   egress-aggregates MAY be the same or MAY be different.

3.2.2.  Reporting the PCN Data

   If the report suppression option described in the next sub-section is
   not activated, the PCN-egress-node MUST report the latest values of
   NM-rate and ETM-rate to the Decision Point each time that it
   calculates them.

3.2.3.  Optional Report Suppression

   Report suppression MUST be provided as a configurable option, along
   with two configurable parameters, the CLE-reporting-threshold and the
   maximum report suppression interval T-maxsuppress.  The default value
   of the CLE-reporting-threshold is zero.  T-maxsuppress is discussed
   further at the end of this sub-section, but functions as a keep-alive
   mechanism for signalling between the PCN-egress-node and the Decision
   Point.

   If the report suppression option is enabled, the PCN-egress-node MUST
   apply the following procedure to decide whether to send a report to
   the Decision Point, rather than sending a report automatically at the
   end of each measurement interval.

   1.  As well as the quantities NM-rate and ETM-rate, the PCN-egress-
       node MUST calculate the congestion level estimate (CLE) for each
       measurement interval.  The CLE is equal to the ratio:

          ETM-rate / (NM-rate + ETM-rate)

       if any PCN-traffic was observed, or zero otherwise.

   2.  If the calculated CLE for the latest measurement interval or for
       the immediately previous interval is greater than the CLE-
       reporting-threshold, then the PCN-egress-node MUST send a report
       to the Decision Point.  The contents of the report are described
       below.

   3.  If an interval T-maxsuppress has elapsed since the last report
       was sent to the Decision Point, then the PCN-egress-node MUST
       send a report to the Decision Point regardless of the CLE value.

   4.  If neither of the preceding conditions holds, the PCN-egress-node
       MUST NOT send a report for the latest measurement interval.

   Each report sent to the Decision Point when report suppression has
   been activated MUST contain the values of NM-rate, ETM-rate, and CLE
   that were calculated for the most recent measurement interval.



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   The above procedure ensures that at least one report is sent per
   interval (T-maxsuppress + T-meas).  This provides some protection
   against loss of egress reports and also demonstrates to the Decision
   Point that both the PCN-egress-node and the communication path
   between the two nodes are in operation.  However, depending on the
   transport used for reporting, the operator may choose to set
   T-maxsuppress to an effectively infinite value.  For example, the
   transport may include its own keep-alive signalling at a frequency
   such that PCN keep-alive is redundant.

3.3.  Behaviour at the Decision Point

   Operators may choose to use PCN procedures just for flow admission,
   or just for flow termination, or for both.  The Decision Point MUST
   implement both mechanisms, but configurable options MUST be provided
   to activate or deactivate PCN-based flow admission and flow
   termination independently of each other at a given Decision Point.

   If PCN-based flow termination is enabled but PCN-based flow admission
   is not, flow termination operates as specified in this document.
   Logically, some other system of flow admission control must be in
   operation, but the description of such a system is out of scope of
   this document and depends on local arrangements.

3.3.1.  Flow Admission

   The Decision Point determines the PCN-admission-state for a given
   ingress-egress-aggregate each time it receives a report from the
   egress node.  It makes this determination on the basis of the
   congestion level estimate (CLE).  If the CLE is provided in the
   egress node report, the Decision Point SHOULD use the reported value.
   If the CLE was not provided in the report, the Decision Point MUST
   calculate it based on the other values provided in the report, using
   the formula:

      CLE = ETM-rate / (NM-rate + ETM-rate)

   if any PCN-traffic was observed, or CLE = 0 if all the rates are
   zero.

   The Decision Point MUST compare the reported or calculated CLE to a
   configurable value, the CLE-limit.  If the CLE is less than the CLE-
   limit, the PCN-admission-state for that aggregate MUST be set to
   "admit"; otherwise it MUST be set to "block".

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



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   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, but such policy intervention
   risks defeating the purpose of using PCN.

3.3.2.  Flow Termination

   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 (PCN-sent-rate) at which the
   PCN-ingress-node is receiving PCN-traffic that is destined for the
   given ingress-egress-aggregate.

      If the Decision Point is collocated with the PCN-ingress-node, the
      request and response are internal operations.

   The Decision Point MUST then wait, for both the requested rate from
   the PCN-ingress-node and the next report from the PCN-egress-node for
   the ingress-egress-aggregate concerned.  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:

          PCN-sent-rate - SAR,

       where PCN-sent-rate is the value provided by the ingress node.

   If the difference calculated in the second step is positive, the
   Decision Point SHOULD select PCN-flows to terminate, until it
   determines that the PCN-traffic admission rate will no longer be
   greater than the estimated sustainable aggregate rate.  If the
   Decision Point knows the bandwidth required by individual flows
   (e.g., from resource signalling used to establish the flows), it MAY



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   choose to complete its selection of flows to terminate in a single
   round of decisions.

   Alternatively, the Decision Point MAY spread flow termination 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 [IEEE-Satoh] and sections 4.2 and 4.3 of [Menth08-sub-9]).

3.3.3.  Decision Point Action For Missing  PCN-Boundary-Node Reports

   If the Decision Point fails to receive any report from a given PCN-
   egress-node for a configurable interval T-fail, it SHOULD raise an
   alarm to management.  A Decision Point collocated with a PCN-ingress-
   node SHOULD cease to admit flows to the ingress-egress-aggregate
   passing from the PCN-ingress-node to the given PCN-egress-node, until
   it again receives a report from that node.  A centralized Decision
   Point MAY cease to admit PCN-flows to all ingress-egress-aggregates
   destined to the PCN-egress-node concerned, until it again receives a
   report from that node.

   If a centralized Decision Point fails to receive a reply within a
   reasonable period of time to a request for a PCN-sent-rate value sent
   to a given PCN-ingress-node, it SHOULD raise an alarm to management.

3.4.  Behaviour of the Ingress Node

   The PCN-ingress-node MUST provide the estimated current rate of PCN-
   traffic received at that node and destined for a given ingress-
   egress-aggregate in octets per second (the PCN-sent-rate) when the
   Decision Point requests it.  The way this rate estimate is derived is
   a matter of implementation.

      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.

3.5.  Summary of Timers

   Table 1 summarizes the timers implied by the preceding procedures.
   The three limits T-meas, T-maxsuppress, and T-fail apply to the three
   timers t-meas, t-maxsuppress, and t-fail respectively. t-meas and
   t-maxsuppress are reset upon expiry.  T-fail is reset by management
   action or by receipt of a report from the PCN-egress-node concerned.




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   +-------------+---------+-------------+--------------+--------------+
   | Timer       | Locatio | Incidence   | Limit        | Action on    |
   |             | n       |             |              | Expiry       |
   +-------------+---------+-------------+--------------+--------------+
   | t-meas      | Egress  | One per     | T-meas       | Calculate    |
   |             | node    | node        |              | and possibly |
   |             |         |             |              | report       |
   |             |         |             |              | NM-rate,     |
   |             |         |             |              | ETM-rate and |
   |             |         |             |              | conditionall |
   |             |         |             |              | yCLE for eac |
   |             |         |             |              | hIEA.        |
   | -           | -       | -           | -            | -            |
   | t-maxsuppre | Egress  | One per IEA | T-maxsuppres | Send a       |
   | ss          | node    | if report   | s            | report for   |
   |             |         | suppression |              | that IEA at  |
   |             |         | is enabled. |              | the next     |
   |             |         |             |              | expiry of    |
   |             |         |             |              | t-meas.      |
   | -           | -       | -           | -            | -            |
   | t-fail      | Decisio | One per     | T-fail       | Assume       |
   |             | npoint  | egress node |              | failure and  |
   |             |         |             |              | cease to     |
   |             |         |             |              | admit flows  |
   |             |         |             |              | passing      |
   |             |         |             |              | through that |
   |             |         |             |              | egress node. |
   +-------------+---------+-------------+--------------+--------------+

                      IEA = ingress-egress-aggregate

          Table 1: Timers Used For the SM Boundary Node Behaviour

   The value of T-meas SHOULD be configurable, and is RECOMMENDED to be
   of the order of 100 to 500 ms.

   t-maxsuppress is active only when report suppression is enabled.  The
   value of T-maxsuppress SHOULD be configurable.  The appropriate value
   depends on the transport used to carry the egress node reports.  For
   unreliable transport, T-maxsuppress is RECOMMENDED to be of the order
   of one second.

   The value of T-fail MUST be configurable.  When unreliable transport
   is used, the value of T-fail is RECOMMENDED to be of the order of 3 *
   T-maxsuppress if report suppression is activated, and of the order of
   3 * T-meas if report suppression is not activated.  When reliable
   transport is used, the operator may choose to provide similar values
   for T-fail or may choose to disable report timing by setting an



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   effectively infinite value for T-fail.


4.  Identifying Ingress and Egress Nodes For PCN        Traffic

   The operation of PCN depends on the ability of the PCN-ingress-node
   to identify the ingress-egress-aggregate to which each new PCN-flow
   belongs and the ability of the egress node to identify the ingress-
   egress-aggregate to which each received PCN-packet belongs.  If the
   Decision Point is collocated with the PCN-ingress-node, the PCN-
   egress-node also needs to associate each ingress-egress-aggregate
   with the address of the PCN-ingress-node to which it must send its
   reports.

   The means by which this is done depends on the packet routing
   technology in use in the network.The procedure to provide the
   required information is out of the scope of this document.


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].

   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



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   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.  The design requirement for
   PCN was that recovery from overloads by flow termination should
   happen within 1-3 seconds.  PCN probably does better than this.

5.4.  Parameters

   In the list that follows, note that most PCN-ingress-nodes are also
   PCN-egress-nodes, and vice versa.  Furthermore, the PCN-ingress-nodes
   may be collocated with Decision Points.

   Parameters at the PCN-ingress-node:
   ----------------------------------

   o  Filters for distinguishing PCN from non-PCN inbound traffic.

   o  The marking to be applied to PCN-traffic.

   o  Reference rates on each inward link for the PCN-excess-rate; see
      Section 2.

   o  The information needed to distinguish PCN-traffic belonging to a
      given ingress-egress-aggregate.

   Parameters at the PCN-egress-node:
   ---------------------------------

   o  The measurement interval T-meas.

   o  Whether report suppression is enabled and, if so, the values of
      the CLE-reporting-threshold and T-maxsuppress.

   o  The information needed to distinguish PCN-traffic belonging to a
      given ingress-egress-aggregate.

   o  The marking rules for re-marking PCN traffic leaving the PCN
      domain.

   Parameters at each interior node:
   --------------------------------

   o  A reference rate on each link for the PCN-excess-rate; see
      Section 2.




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   o  The markings to be applied to PCN-traffic, including the
      identification of PCN-packets and the encoding to indicate excess-
      traffic-marking.

   Parameters at the Decision Point:
   --------------------------------

   o  Activation/deactivation of PCN-based flow admission.

   o  Activation/deactivation of PCN-based flow termination.

   o  The value of CLE-limit.

   o  The fraction U used to derive the supportable aggregate rate from
      the NM-rate;

   o  The maximum interval Tfail between reports from a given egress
      node, for detecting failure of communications with that node.

   o  The information needed to map between each ingress-egress-
      aggregate and the corresponding PCN-ingress-node and PCN-egress-
      node.

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.

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



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   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.  The
   acknowledgements in the CL boundary node behaviour draft really apply
   here, too.


9.  References

9.1.  Normative References

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

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

9.2.  Informative References

   [IEEE-Satoh]
              Satoh, D. and H. Ueno, ""Cause and Countermeasure of
              Overtermination for PCN-Based Flow Termination",
              Proceedings of IEEE Symposium on Computers and
              Communications (ISCC '10), pp. 155-161, Riccione, Italy",



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              June 2010.

   [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>.

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


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




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   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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