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Versions: (draft-eardley-pcn-architecture) 00 01 02 03 04 05 06 07 08 09 10 11 RFC 5559

Congestion and Pre-Congestion                   Philip. Eardley (Editor)
Notification Working Group                                            BT
Internet-Draft                                             April 7, 2009
Intended status: Informational
Expires: October 9, 2009


             Pre-Congestion Notification (PCN) Architecture
                     draft-ietf-pcn-architecture-11

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   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents in effect on the date of
   publication of this document (http://trustee.ietf.org/license-info).
   Please review these documents carefully, as they describe your rights
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Abstract

   This document describes a general architecture for flow admission and
   termination based on pre-congestion information in order to protect
   the quality of service of established inelastic flows within a single
   Diffserv domain.

Status


Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  4
     1.1.   Overview of PCN . . . . . . . . . . . . . . . . . . . . .  4
     1.2.   Example use case for PCN  . . . . . . . . . . . . . . . .  4
     1.3.   Applicability of PCN  . . . . . . . . . . . . . . . . . .  8
     1.4.   Documents about PCN . . . . . . . . . . . . . . . . . . .  9
   2.  Terminology  . . . . . . . . . . . . . . . . . . . . . . . . . 10
   3.  High-level functional architecture . . . . . . . . . . . . . . 12
     3.1.   Flow admission  . . . . . . . . . . . . . . . . . . . . . 14
     3.2.   Flow termination  . . . . . . . . . . . . . . . . . . . . 15
     3.3.   Flow admission and/or flow termination when there are
            only two PCN encoding states  . . . . . . . . . . . . . . 16
     3.4.   Information transport . . . . . . . . . . . . . . . . . . 17
     3.5.   PCN-traffic . . . . . . . . . . . . . . . . . . . . . . . 17
     3.6.   Backwards compatibility . . . . . . . . . . . . . . . . . 18
   4.  Detailed Functional architecture . . . . . . . . . . . . . . . 19
     4.1.   PCN-interior-node functions . . . . . . . . . . . . . . . 20
     4.2.   PCN-ingress-node functions  . . . . . . . . . . . . . . . 21
     4.3.   PCN-egress-node functions . . . . . . . . . . . . . . . . 22
     4.4.   Admission control functions . . . . . . . . . . . . . . . 22
     4.5.   Flow termination functions  . . . . . . . . . . . . . . . 23
     4.6.   Addressing  . . . . . . . . . . . . . . . . . . . . . . . 24
     4.7.   Tunnelling  . . . . . . . . . . . . . . . . . . . . . . . 24
     4.8.   Fault handling  . . . . . . . . . . . . . . . . . . . . . 26
   5.  Operations and Management  . . . . . . . . . . . . . . . . . . 26
     5.1.   Configuration Operations and Management . . . . . . . . . 27
       5.1.1.  System options . . . . . . . . . . . . . . . . . . . . 27
       5.1.2.  Parameters . . . . . . . . . . . . . . . . . . . . . . 28
     5.2.   Performance & Provisioning Operations and Management  . . 30
     5.3.   Accounting Operations and Management  . . . . . . . . . . 31
     5.4.   Fault Operations and Management . . . . . . . . . . . . . 31



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     5.5.   Security Operations and Management  . . . . . . . . . . . 32
   6.  Applicability of PCN . . . . . . . . . . . . . . . . . . . . . 33
     6.1.   Benefits  . . . . . . . . . . . . . . . . . . . . . . . . 33
     6.2.   Deployment scenarios  . . . . . . . . . . . . . . . . . . 35
     6.3.   Assumptions and constraints on scope  . . . . . . . . . . 36
       6.3.1.  Assumption 1: Trust and support of PCN -
               controlled environment . . . . . . . . . . . . . . . . 37
       6.3.2.  Assumption 2: Real-time applications . . . . . . . . . 37
       6.3.3.  Assumption 3: Many flows and additional load . . . . . 38
       6.3.4.  Assumption 4: Emergency use out of scope . . . . . . . 38
     6.4.   Challenges  . . . . . . . . . . . . . . . . . . . . . . . 39
   7.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 41
   8.  Security considerations  . . . . . . . . . . . . . . . . . . . 41
   9.  Conclusions  . . . . . . . . . . . . . . . . . . . . . . . . . 42
   10. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 42
   11. Comments Solicited (to be removed by RFC Editor) . . . . . . . 43
   12. Changes (to be removed by RFC Editor)  . . . . . . . . . . . . 43
     12.1.  Changes from -10 to -11 . . . . . . . . . . . . . . . . . 43
     12.2.  Changes from -09 to -10 . . . . . . . . . . . . . . . . . 44
     12.3.  Changes from -08 to -09 . . . . . . . . . . . . . . . . . 44
     12.4.  Changes from -07 to -08 . . . . . . . . . . . . . . . . . 44
     12.5.  Changes from -06 to -07 . . . . . . . . . . . . . . . . . 45
     12.6.  Changes from -05 to -06 . . . . . . . . . . . . . . . . . 45
     12.7.  Changes from -04 to -05 . . . . . . . . . . . . . . . . . 46
     12.8.  Changes from -03 to -04 . . . . . . . . . . . . . . . . . 46
     12.9.  Changes from -02 to -03 . . . . . . . . . . . . . . . . . 47
     12.10. Changes from -01 to -02 . . . . . . . . . . . . . . . . . 48
     12.11. Changes from -00 to -01 . . . . . . . . . . . . . . . . . 49
   13. References . . . . . . . . . . . . . . . . . . . . . . . . . . 51
     13.1.  Normative References  . . . . . . . . . . . . . . . . . . 51
     13.2.  Informative References  . . . . . . . . . . . . . . . . . 51
   Appendix A.  Possible future work items  . . . . . . . . . . . . . 55
     A.1.   Probing . . . . . . . . . . . . . . . . . . . . . . . . . 57
       A.1.1.  Introduction . . . . . . . . . . . . . . . . . . . . . 57
       A.1.2.  Probing functions  . . . . . . . . . . . . . . . . . . 58
       A.1.3.  Discussion of rationale for probing, its downsides
               and open issues  . . . . . . . . . . . . . . . . . . . 58
   Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 61













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1.  Introduction

1.1.  Overview of PCN

   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 "pre-congestion
   notification").  The level of marking allows boundary nodes to make
   decisions about whether to admit or terminate.

   Within a PCN-domain, PCN-traffic is forwarded in a prioritised
   Diffserv traffic class.  Every link in the PCN-domain is configured
   with two rates (PCN-threshold-rate and PCN-excess-rate).  If the
   overall rate of PCN-traffic on a link exceeds a configured rate, then
   a PCN-interior-node marks PCN-packets appropriately.  The PCN-egress-
   nodes use this information to make admission control and flow
   termination decisions.  Flow admission control determines whether a
   new flow can be admitted without any impact, in normal circumstances,
   on the QoS of existing PCN-flows.  However, in abnormal
   circumstances, for instance a disaster affecting multiple nodes and
   causing traffic re-routes, then the QoS on existing PCN-flows may
   degrade even though care was exercised when admitting those flows.
   The flow termination mechanism removes sufficient traffic in order to
   protect the QoS of the remaining PCN-flows.  All PCN-boundary-nodes
   and PCN-interior-nodes are PCN-enabled and are trusted for correct
   PCN operation.  PCN-ingress-nodes police arriving packets to check
   that they are part of an admitted PCN-flow that keeps within its
   agreed flowspec, and hence they maintain per flow state.  PCN-
   interior-nodes meter all PCN traffic, and hence do not need to
   maintain any per flow state.  Decisions about flow admission and
   termination are made for a particular pair of PCN-boundary-nodes, and
   hence PCN-egress-nodes must be able to identify which PCN-ingress-
   node sent each PCN-packet.

1.2.  Example use case for PCN

   This section outlines an end-to-end QoS scenario that uses the PCN
   mechanisms within one domain.  The parts outside the PCN-domain are
   out of scope for PCN, but are included to help clarify how PCN could
   be used.  Note that the section is only an example - in particular



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   there are other possibilities (see later) for how the PCN-boundary-
   nodes perform admission control and flow termination.

   As a fundamental building block, each link of the PCN-domain operates
   a [PCN08-2] (Figure 1):

   o  Threshold meter and marker, which marks all PCN-packets if the PCN
      traffic rate is greater than a first configured rate, the PCN-
      threshold-rate.  The admission control mechanism limits the PCN-
      traffic on each link to *roughly* its PCN-threshold-rate.

   o  Excess traffic meter and marker, which marks a proportion of PCN-
      packets, such that the amount marked equals the traffic rate in
      excess of a second configured rate, the PCN-excess-rate.  The flow
      termination mechanism limits the PCN-traffic on each link to
      *roughly* its PCN-excess-rate.

   Overall the aim is to give an "early warning" of potential congestion
   before there is any significant build-up of PCN-packets in the queue
   on the link; we term this "pre-congestion notification" by analogy
   with ECN (Explicit Congestion Notification, [RFC3168]).  Note that
   the link only meters the bulk PCN-traffic (and not per flow).





























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                          ==   Metering &    ==
                          ==Marking behaviour==       ==PCN mechanisms==
                       ^
           Rate of     ^
      PCN-traffic on   |
     bottleneck link   |
                       |
                       |       Some pkts                  Terminate some
                       |  excess-traffic-marked           admitted flows
                       |           &                            &
                       |     Rest of pkts                Block new flows
                       |   threshold-marked
                       |
     PCN-excess-rate  -|------------------------------------------------
(=PCN-supportable-rate)|
                       |       All pkts                  Block new flows
                       |   threshold-marked
                       |
   PCN-threshold-rate -|------------------------------------------------
 (=PCN-admissible-rate)|
                       |        No pkts                  Admit new flows
                       |      PCN-marked
                       |

   Figure 1: Example of how the PCN admission control and flow
   termination mechanisms operate as the rate of PCN-traffic increases.

   The two forms of PCN-marking are indicated by setting of the ECN and
   DSCP (Differentiated Services Codepoint [RFC2474]) fields to known
   values, which are configured for the domain.  Thus the PCN-egress-
   nodes can monitor the PCN-markings in order to measure the severity
   of pre-congestion.  In addition, the PCN-ingress-nodes need to set
   the ECN and DSCP fields to that configured for an unmarked PCN-
   packet, and the PCN-egress-nodes need to revert to values appropriate
   outside the PCN-domain.

   For admission control, we assume end-to-end RSVP signalling (Resource
   Reservation Protocol) [RFC2205]) in this example.  The PCN-domain is
   a single RSVP hop.  The PCN-domain operates Diffserv, and we assume
   that PCN-traffic is scheduled with the expedited forwarding (EF) per-
   hop behaviour, [RFC3246].  Hence the overall solution is in line with
   the "IntServ over Diffserv" framework defined in [RFC2998], as shown
   in Figure 2.








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   ___    ___    _______________________________________    ____    ___
  |   |  |   |  | PCN-             PCN-            PCN- |  |    |  |   |
  |   |  |   |  |ingress         interior         egress|  |    |  |   |
  |   |  |   |  | -node           -nodes          -node |  |    |  |   |
  |   |  |   |  |-------+  +-------+  +-------+  +------|  |    |  |   |
  |   |  |   |  |       |  | PCN   |  | PCN   |  |      |  |    |  |   |
  |   |..|   |..|Ingress|..|meter &|..|meter &|..|Egress|..|    |..|   |
  |   |..|   |..|Policer|..|marker |..|marker |..|Meter |..|    |..|   |
  |   |  |   |  |-------+  +-------+  +-------+  +------|  |    |  |   |
  |   |  |   |  |  \                                 /  |  |    |  |   |
  |   |  |   |  |   \                               /   |  |    |  |   |
  |   |  |   |  |    \  PCN-feedback-information   /    |  |    |  |   |
  |   |  |   |  |     \  (for admission control)  /     |  |    |  |   |
  |   |  |   |  |      --<-----<----<----<-----<--      |  |    |  |   |
  |   |  |   |  |       PCN-feedback-information        |  |    |  |   |
  |   |  |   |  |        (for flow termination)         |  |    |  |   |
  |___|  |___|  |_______________________________________|  |____|  |___|

  Sx     Access               PCN-domain                   Access    Rx
  End    Network                                          Network   End
  Host                                                              Host
                  <---- signalling across PCN-domain--->
                (for admission control & flow termination)

  <-------------------end-to-end QoS signalling protocol--------------->


   Figure 2: Example of possible overall QoS architecture

   A source wanting to start a new QoS flow sends an RSVP PATH message.
   Normal hop-by-hop IntServ [RFC1633] is used outside the PCN-domain
   (we assume successfully).  The PATH message travels across the PCN-
   domain; the PCN-egress-node reads the PHOP object to discover the
   specific PCN-ingress-node for this flow.  The RESV message travels
   back from the receiver, and triggers the PCN-egress-node to check
   what fraction of the PCN-traffic, from the relevant PCN-ingress-node,
   is currently being threshold-marked.  It adds an object with this
   information onto the RESV message, and hence the PCN-ingress-node
   learns about the level of pre-congestion on the path.  If this level
   is below some threshold, then the PCN-ingress-node admits the new
   flow into the PCN-domain.  The RSVP message triggers the PCN-ingress-
   node to install two normal IntServ items: five-tuple information, so
   that it can subsequently identify data packets that are part of a
   previously admitted PCN-flow; and a traffic profile, so that it can
   police the flow to within its contract.  Similarly, the RSVP message
   triggers the PCN-egress-node to install five-tuple and PHOP
   information, so that it can identify packets as part of a flow from a
   specific PCN-ingress-node.



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   The flow termination mechanism may happen when some abnormal
   circumstances causes a link to become so pre-congested that it
   excess-traffic-marks (and perhaps also drops) PCN-packets.  In this
   example, when a PCN-egress-node observes such a packet it then, with
   some probability, terminates this PCN-flow; the probability is
   configured low enough to avoid over-termination and high enough to
   ensure rapid termination of enough flows.  It also informs the
   relevant PCN-ingress-node, so it can block any further traffic on the
   terminated flow.

1.3.  Applicability of PCN

   Compared with alternative QoS mechanisms, PCN has certain advantages
   and disadvantages that will make it appropriate in particular
   scenarios.  For example, compared with hop-by-hop IntServ [RFC1633],
   PCN only requires per flow state at the PCN-ingress-nodes.  Compared
   with the Diffserv architecture [RFC2475], an operator needs to be
   less accurate and/or conservative in its prediction of the traffic
   matrix.  The Diffserv architecture's traffic conditioning agreements
   are static and coarse; they are defined at subscription time, and
   they are used (for instance) to limit the total traffic at each
   ingress of the domain regardless of the egress for the traffic.  On
   the other hand, PCN firstly uses admission control based on
   measurements of the current conditions between the specific pair of
   PCN-boundary-nodes, and secondly, in case of a disaster, PCN protects
   the QoS of most flows by terminating a few selected ones.

   PCN's admission control is a measurement-based mechanism.  Hence it
   assumes that the present is a reasonable prediction of the future:
   the network conditions are measured at the time of a new flow
   request, but the actual network performance must be acceptable during
   the call some time later.  Hence PCN is unsuitable in several
   circumstances:

   o  If the source adapts its bit rate dependent on the level of pre-
      congestion, because then the aggregate traffic might become
      unstable.  The assumption in this document is that PCN-packets
      come from real time applications generating inelastic traffic,
      such as the Controlled Load Service, [RFC2211].

   o  If a potential bottleneck link has capacity for only a few flows,
      because then a new flow can move a link directly from no pre-
      congestion to being so overloaded that it has to drop packets.
      The assumption in this document is that this isn't a problem.

   o  If there is the danger of a "flash crowd" in which many admission
      requests arrive within the reaction time of PCN's admission
      mechanism, because then they all might get admitted and so



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      overload the network.  The assumption in this document is that, if
      it is necessary, then flash crowds are limited in some fashion
      beyond the scope of this document, for instance by rate limiting
      QoS requests.

   The applicability of PCN is discussed further in Section 6.

1.4.  Documents about PCN

   The purpose of this document is to describe a general architecture
   for flow admission and termination based on (pre-) congestion
   information in order to protect the quality of service of flows
   within a Diffserv domain.  This document describes the PCN
   architecture at a high level (Section 3) and in more detail (Section
   4).  It also defines some terminology, and considerations about
   operations and management, and security.  Section 6 considers the
   applicability of PCN in more detail, covering its benefits,
   deployment scenarios, assumptions and potential challenges.  The
   Appendix covers some potential future work items.

   Aspects of PCN are also documented elsewhere:

   o  Metering and marking: [PCN08-2] standardises threshold metering
      and marking, and excess traffic metering and marking.  A PCN-
      packet may be marked, depending on the metering results.

   o  Encoding: the "baseline" encoding is described in [PCN08-1], which
      standardises two PCN encoding states (PCN-marked and not PCN-
      marked), whilst (experimental) extensions to the baseline encoding
      can provide three encoding states (threshold-marked, excess-
      traffic-marked, not PCN-marked, or perhaps further encoding states
      as suggested in [Westberg08]).  Section 3.6 considers backwards
      compatability of PCN encoding with ECN.

   o  PCN-boundary-node behaviour: how the PCN-boundary-nodes convert
      the PCN-markings into decisions about flow admission and flow
      termination, as described in Informational documents.  The concept
      is that the standardised metering and marking by PCN-nodes allows
      several possible PCN-boundary-node behaviours.  A number of
      possibilities are outlined in this document; detailed descriptions
      and comparisons are in [Charny07-1] and [Menth08-3].

   o  Signalling between PCN-boundary-nodes: Signalling is needed to
      transport PCN-feedback-information between the PCN-boundary-nodes
      (in the example above, this is the fraction of traffic, between
      the pair of PCN-boundary-nodes, that is PCN-marked).  The exact
      details vary for different PCN-boundary-node behaviours, and so
      should be described in those documents.  It may require an



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      extension to the signalling protocol - standardisation is out of
      scope of the PCN WG.

   o  The interface by which the PCN-boundary-nodes learn identification
      information about the admitted flows: the exact requirements vary
      for different PCN-boundary-node behaviours and for different
      signalling protocols, and so should be described in those
      documents.  They will be similar to those described in the example
      above - a PCN-ingress-node needs to be able to identify that a
      packet is part of a previously admitted flow (typically from its
      five-tuple) and each PCN-boundary-node needs to be able to
      identify the other PCN-boundary-node for the flow.


2.  Terminology

   o  PCN-domain: a PCN-capable domain; a contiguous set of PCN-enabled
      nodes that perform Diffserv scheduling [RFC2474]; the complete set
      of PCN-nodes that in principle can, through PCN-marking packets,
      influence decisions about flow admission and termination for the
      PCN-domain; the PCN-domain includes the PCN-egress-nodes, which
      measure these PCN-marks, and the PCN-ingress-nodes.

   o  PCN-boundary-node: a PCN-node that connects one PCN-domain to a
      node either in another PCN-domain or in a non PCN-domain.

   o  PCN-interior-node: a node in a PCN-domain that is not a PCN-
      boundary-node.

   o  PCN-node: a PCN-boundary-node or a PCN-interior-node

   o  PCN-egress-node: a PCN-boundary-node in its role in handling
      traffic as it leaves a PCN-domain.

   o  PCN-ingress-node: a PCN-boundary-node in its role in handling
      traffic as it enters a PCN-domain.

   o  PCN-traffic, PCN-packets, PCN-BA: a PCN-domain carries traffic of
      different Diffserv behaviour aggregates (BAs) [RFC2474].  The
      PCN-BA uses the PCN mechanisms to carry PCN-traffic and the
      corresponding packets are PCN-packets.  The same network will
      carry traffic of other Diffserv BAs.  The PCN-BA is distinguished
      by a combination of the Diffserv codepoint (DSCP) and ECN fields.

   o  PCN-flow: the unit of PCN-traffic that the PCN-boundary-node
      admits (or terminates); the unit could be a single microflow (as
      defined in [RFC2474]) or some identifiable collection of
      microflows.



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   o  Pre-congestion: a condition of a link within a PCN-domain such
      that the PCN-node performs PCN-marking, in order to provide an
      "early warning" of potential congestion before there is any
      significant build-up of PCN-packets in the real queue.  (Hence, by
      analogy with ECN we call our mechanism Pre-Congestion
      Notification.)

   o  PCN-marking: the process of setting the header in a PCN-packet
      based on defined rules, in reaction to pre-congestion; either
      threshold-marking or excess-traffic-marking.

   o  PCN-threshold-rate: a reference rate configured for each link in
      the PCN-domain, which is lower than the PCN-excess-rate.  It is
      used by a metering behaviour that determines whether a packet
      should be PCN-marked with a first encoding, "threshold-marked".

   o  Threshold-metering: a metering behaviour that, if the PCN-traffic
      exceeds the PCN-threshold-rate, indicates that all PCN-traffic is
      to be threshold-marked.

   o  Threshold-marking: the setting of the header in a PCN-packet to a
      specific encoding, based on indications from the threshold-meter.

   o  PCN-excess-rate: a reference rate configured for each link in the
      PCN-domain, which is higher than the PCN-threshold-rate.  It is
      used by a metering behaviour that determines whether a packet
      should be PCN-marked with a second encoding, "excess-traffic-
      marked".

   o  Excess-traffic-metering: a metering behaviour that, if the PCN-
      traffic exceeds the PCN-excess-rate, indicates that the amount of
      PCN-traffic to be PCN-marked is equal to the amount in excess of
      the PCN-excess-rate.

   o  Excess-traffic-marking: the setting of the header in a PCN-packet
      to a specific encoding, based on indications from the excess-
      traffic-meter.

   o  PCN-colouring: the process of setting the header in a PCN-packet
      by a PCN-boundary-node; performed by a PCN-ingress-node so that
      PCN-nodes can easily identify PCN-packets; performed by a PCN-
      egress-node so that the header is appropriate for nodes beyond the
      PCN-domain.

   o  Ingress-egress-aggregate: The collection of PCN-packets from all
      PCN-flows that travel in one direction between a specific pair of
      PCN-boundary-nodes.




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   o  PCN-feedback-information: information signalled by a PCN-egress-
      node to a PCN-ingress-node (or a central control node), which is
      needed for the flow admission and flow termination mechanisms.

   o  PCN-admissible-rate: the rate of PCN-traffic on a link up to which
      PCN admission control should accept new PCN-flows.

   o  PCN-supportable-rate: the rate of PCN-traffic on a link down to
      which PCN flow termination should, if necessary, terminate already
      admitted PCN-flows.


3.  High-level functional architecture

   The high-level approach is to split functionality between:

   o  PCN-interior-nodes 'inside' the PCN-domain, which monitor their
      own state of pre-congestion and mark PCN-packets as appropriate.
      They are not flow-aware, nor aware of ingress-egress-aggregates.
      The functionality is also done by PCN-ingress-nodes for their
      outgoing interfaces (ie those 'inside' the PCN-domain).

   o  PCN-boundary-nodes at the edge of the PCN-domain, which control
      admission of new PCN-flows and termination of existing PCN-flows,
      based on information from PCN-interior-nodes.  This information is
      in the form of the PCN-marked data packets (which are intercepted
      by the PCN-egress-nodes) and not signalling messages.  Generally
      PCN-ingress-nodes are flow-aware.

   The aim of this split is to keep the bulk of the network simple,
   scalable and robust, whilst confining policy, application-level and
   security interactions to the edge of the PCN-domain.  For example the
   lack of flow awareness means that the PCN-interior-nodes don't care
   about the flow information associated with PCN-packets, nor do the
   PCN-boundary-nodes care about which PCN-interior-nodes its ingress-
   egress-aggregates traverse.

   In order to generate information about the current state of the PCN-
   domain, each PCN-node PCN-marks packets if it is "pre-congested".
   Exactly when a PCN-node decides if it is "pre-congested" (the
   algorithm) and exactly how packets are "PCN-marked" (the encoding)
   will be defined in separate standards-track documents, but at a high
   level it is as follows:

   o  the algorithms: a PCN-node meters the amount of PCN-traffic on
      each one of its outgoing (or incoming) links.  The measurement is
      made as an aggregate of all PCN-packets, and not per flow.  There
      are two algorithms, one for threshold-metering and one for excess-



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      traffic-metering.  The meters trigger PCN-marking as necessary.

   o  the encoding(s): a PCN-node PCN-marks a PCN-packet by modifying a
      combination of the DSCP and ECN fields.  In the "baseline"
      encoding [PCN08-1], the ECN field is set to 11 and the DSCP is not
      altered.  Extension encodings may be defined that, at most, use a
      second DSCP (eg as in [Moncaster08]) and/or set the ECN field to
      values other than 11 (eg as in [Menth08-2]).

   In a PCN-domain the operator may have two or three encoding states
   available.  The baseline encoding provides two encoding states (not
   PCN-marked, PCN-marked), whilst extended encodings can provide three
   encoding states (not PCN-marked, threshold-marked, excess-traffic-
   marked).

   An operator may choose to deploy either admission control or flow
   termination or both.  Although designed to work together, they are
   independent mechanisms, and the use of one does not require or
   prevent the use of the other.  Three encoding states naturally allows
   both flow admission and flow termination.  If there are only two
   encoding states, then there are several options - see Section 3.3.

   The PCN-boundary-nodes monitor the PCN-marked packets in order to
   extract information about the current state of the PCN-domain.  Based
   on this monitoring, a distributed decision is made about whether to
   admit a prospective new flow or whether to terminate existing
   flow(s).  Sections 4.4 and 4.5 mention various possibilities for how
   the functionality could be distributed.

   PCN-metering and PCN-marking needs to be configured on all
   (potentially pre-congested) links in the PCN-domain to ensure that
   the PCN mechanisms protect all links.  The actual functionality can
   be configured on the outgoing or incoming interfaces of PCN-nodes -
   or one algorithm could be configured on the outgoing interface and
   the other on the incoming interface.  The important point is that a
   consistent choice is made across the PCN-domain to ensure that the
   PCN mechanisms protect all links.  See [PCN08-2] for further
   discussion.

   The objective of threshold-marking, as triggerd by the threshold-
   metering algorithm, is to threshold-mark all PCN-packets whenever the
   rate of PCN-packets is greater than some configured rate, the PCN-
   threshold-rate.  The objective of excess-traffic-metering, as
   triggered by the excess-traffic-marking algorithm, is to excess-
   traffic-mark PCN-packets at a rate equal to the difference between
   the bit rate of PCN-packets and some configured rate, the PCN-excess-
   rate.  Note that this description reflects the overall intent of the
   algorithms rather than their instantaneous behaviour, since the rate



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   measured at a particular moment depends on the detailed algorithm,
   its implementation, and the traffic's variance as well as its rate
   (eg marking may well continue after a recent overload even after the
   instantaneous rate has dropped).  The algorithms are specified in
   [PCN08-2].

   Admission and termination approaches are detailed and compared in
   [Charny07-1] and [Menth08-3].  The discussion below is just a brief
   summary.  Sections 3.1 and 3.2 assume there are three encoding states
   available, whilst Section 3.3 assumes there are two encoding states
   available.

   From the perspective of the outside world, a PCN-domain essentially
   looks like a Diffserv domain, but without the Diffserv architecture's
   traffic conditioning agreements.  PCN-traffic is either transported
   across it transparently or policed at the PCN-ingress-node (ie
   dropped or carried at a lower QoS).  One difference is that PCN-
   traffic has better QoS guarantees than normal Diffserv traffic,
   because the PCN mechanisms better protect the QoS of admitted flows.
   Another difference may occur in the rare circumstance when there is a
   failure: on the one hand some PCN-flows may get terminated, but on
   the other hand other flows will get their QoS restored.  Non PCN-
   traffic is treated transparently, ie the PCN-domain is a normal
   Diffserv domain.

3.1.  Flow admission

   The objective of PCN's flow admission control mechanism is to limit
   the PCN-traffic on each link in the PCN-domain to *roughly* its PCN-
   admissible-rate, by admitting or blocking prospective new flows, in
   order to protect the QoS of existing PCN-flows.  With three encoding
   states available, the PCN-threshold-rate is configured by the
   operator as equal to the PCN-admissible-rate on each link.  It is set
   lower than the traffic rate at which the link becomes congested and
   the node drops packets.

   Exactly how the admission control decision is made will be defined
   separately in informational documents.  This document describes two
   approaches (others might be possible):

   o  the PCN-egress-node measures (possibly as a moving average) the
      fraction of the PCN-traffic that is threshold-marked.  The
      fraction is measured for a specific ingress-egress-aggregate.  If
      the fraction is below a threshold value then the new flow is
      admitted, and if the fraction is above the threshold value then it
      is blocked.  The fraction could be measured as an EWMA
      (exponentially weighted moving average), which has sometimes been
      called the "congestion level estimate".



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   o  the PCN-egress-node monitors PCN-traffic and if it receives one
      (or several) threshold-marked packets, then the new flow is
      blocked, otherwise it is admitted.  One possibility may be to
      react to the marking state of an initial flow set-up packet (eg
      RSVP PATH).  Another is that after one (or several) threshold-
      marks then all flows are blocked until after a specific period of
      no congestion.

   Note that the admission control decision is made for a particular
   pair of PCN-boundary-nodes.  So it is quite possible for a new flow
   to be admitted between one pair of PCN-boundary-nodes, whilst at the
   same time another admission request is blocked between a different
   pair of PCN-boundary-nodes.

3.2.  Flow termination

   The objective of PCN's flow termination mechanism is to limit the
   PCN-traffic on each link to *roughly* its PCN-supportable-rate, by
   terminating some existing PCN-flows, in order to protect the QoS of
   the remaining PCN-flows.  With three encoding states available, the
   PCN-excess-rate is configured by the operator as equal to the PCN-
   supportable-rate on each link.  It may be set lower than the traffic
   rate at which the link becomes congested and the node drops packets.

   Exactly how the flow termination decision is made will be defined
   separately in informational documents.  This document describes
   several approaches (others might be possible):

   o  In one approach the PCN-egress-node measures the rate of PCN-
      traffic that is not excess-traffic-marked, which is the amount of
      PCN-traffic that can actually be supported, and communicates this
      to the PCN-ingress-node.  Also the PCN-ingress-node measures the
      rate of PCN-traffic that is destined for this specific PCN-egress-
      node.  The difference represents the excess amount that should be
      terminated.

   o  Another approach instead measures the rate of excess-traffic-
      marked traffic and terminates this amount of traffic.  This
      terminates less traffic than the previous bullet if some nodes are
      dropping PCN-traffic.

   o  Another approach monitors PCN-packets and terminates some of the
      PCN-flows that have an excess-traffic-marked packet.  (If all such
      flows were terminated, far too much traffic would be terminated,
      so a random selection needs to be made from those with an excess-
      traffic-marked packet, [Menth08-1].)

   Since flow termination is designed for "abnormal" circumstances, it



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   is quite likely that some PCN-nodes are congested and hence packets
   are being dropped and/or significantly queued.  The flow termination
   mechanism must accommodate this.

   Note also that the termination control decision is made for a
   particular pair of PCN-boundary-nodes.  So it is quite possible for
   PCN-flows to be terminated between one pair of PCN-boundary-nodes,
   whilst at the same time none are terminated between a different pair
   of PCN-boundary-nodes.

3.3.  Flow admission and/or flow termination when there are only two PCN
      encoding states

   If a PCN-domain has only two encoding states available (PCN-marked
   and not PCN-marked), ie it is using the baseline encoding [PCN08-1],
   then an operator has three options (others might be possible):

   o  admission control only: PCN-marking means threshold-marking, ie
      only the threshold-metering algorithm triggers PCN-marking.  Only
      PCN admission control is available.

   o  flow termination only: PCN-marking means excess-traffic-marking,
      ie only the excess-traffic-metering algorithm triggers PCN-
      marking.  Only PCN termination control is available.

   o  both admission control and flow termination: only the excess-
      traffic-metering algorithm triggers PCN-marking, however the
      configured rate (PCN-excess-rate) is set equal to the PCN-
      admissible-rate, as shown in Figure 3.  [Charny07-2] describes how
      both admission control and flow termination can be triggered in
      this case and also gives some of the pros and cons of this
      approach.  The main downside is that admission control is less
      accurate.


















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                          ==   Metering &    ==
                          ==Marking behaviour==       ==PCN mechanisms==
                       ^
           Rate of     ^
      PCN-traffic on   |
     bottleneck link   |                                  Terminate some
                       |                                  admitted flows
                       |                                         &
                       |                                 Block new flows
                       |
                       |       Some pkts
   U*PCN-excess-rate  -|  excess-traffic-marked        -----------------
(=PCN-supportable-rate)|
                       |                                 Block new flows
                       |
                       |
     PCN-excess-rate  -|------------------------------------------------
 (=PCN-admissible-rate)|
                       |         No pkts                 Admit new flows
                       |       PCN-marked
                       |

   Figure 3: Schematic of how the PCN admission control and flow
   termination mechanisms operate as the rate of PCN-traffic increases,
   for a PCN-domain with two encoding states and using the approach of
   [Charny07-2].  Note: U is a global parameter for all links in the
   PCN-domain.

3.4.  Information transport

   The transport of pre-congestion information from a PCN-node to a PCN-
   egress-node is through PCN-markings in data packet headers, ie "in-
   band": no signalling protocol messaging is needed.  Signalling is
   needed to transport PCN-feedback-information, for example to convey
   the fraction of PCN-marked traffic from a PCN-egress-node to the
   relevant PCN-ingress-node.  Exactly what information needs to be
   transported will be described in future documents about possible
   boundary mechanisms.  The signalling could be done by an extension of
   RSVP or NSIS, for instance; [Lefaucheur06] describes the extensions
   needed for RSVP.

3.5.  PCN-traffic

   The following are some high-level points about how PCN works:

   o  There needs to be a way for a PCN-node to distinguish PCN-traffic
      from other traffic.  This is through a combination of the DSCP
      field and/or ECN field.



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   o  It is not advised to have non PCN-traffic that competes for the
      same capacity as PCN-traffic but, if there is such traffic, there
      needs to be a mechanism to limit it.  "Capacity" means the
      forwarding bandwidth on a link; "competes" means that non PCN-
      packets will delay PCN-packets in the queue for the link.  Hence
      more non PCN-traffic results in poorer QoS for PCN.  Further, the
      unpredictable amount of non PCN-traffic makes the PCN mechanisms
      less accurate and so reduces PCN's ability to protect the QoS of
      admitted PCN-flows

   o  Two examples of such non PCN-traffic (ie that competes for the
      same capacity as PCN-traffic) are:

   1.  traffic that is priority scheduled over PCN (perhaps a particular
       application or an operator's control messages).

   2.  traffic that is scheduled at the same priority as PCN (for
       example if the Voice-Admit codepoint is used for PCN-traffic
       [PCN08-1] and there is non-PCN voice-admit traffic in the PCN-
       domain).

   o  If there is such non PCN-traffic (ie that competes for the same
      capacity as PCN-traffic), then PCN's mechanisms should take
      account of it, in order to improve the accuracy of the decision
      about whether to admit (or terminate) a PCN-flow.  For example,
      one mechanism is that such non PCN-traffic contributes to the PCN
      meters (ie is metered by the threshold-marking and excess-traffic-
      marking algorithms).

   o  There will be non PCN-traffic that doesn't compete for the same
      capacity as PCN-traffic, because it is forwarded at lower
      priority.  Hence it shouldn't contribute to the PCN meters.
      Examples are best effort and assured forwarding traffic.  However,
      a PCN-node should dedicate some capacity to lower priority traffic
      so that it isn't starved.

   o  The document assumes that the PCN mechanisms are applied to a
      single behaviour aggregate in the PCN-domain.  However, it would
      also be possible to apply them independently to more than one
      behaviour aggregate, which are distinguished by DSCP.

3.6.  Backwards compatibility

   PCN specifies semantics for the ECN field that differ from the
   default semantics of [RFC3168].  A particular PCN encoding scheme
   needs to describe how it meets the guidelines of BCP 124 [RFC4774]
   for specifying alternative semantics for the ECN field.  In summary
   the approach is to:



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   o  use a DSCP to allow PCN-nodes to distinguish PCN-traffic that uses
      the alternative ECN semantics;

   o  define these semantics for use within a controlled region, the
      PCN-domain;

   o  take appropriate action if ECN capable, non-PCN traffic arrives at
      a PCN-ingress-node with the DSCP used by PCN.

   For the baseline encoding [PCN08-1], the 'appropriate action' is to
   block ECN-capable traffic that uses the same DSCP as PCN from
   entering the PCN-domain directly.  Blocking means it is dropped or
   downgraded to a lower priority behaviour aggregate, or alternatively
   such traffic may be tunnelled through the PCN-domain.  The reason
   that 'appropriate action' is needed is that the PCN-egress-node
   clears the ECN field to 00.

   Extended encoding schemes may need to take different 'appropriate
   action'.


4.  Detailed Functional architecture

   This section is intended to provide a systematic summary of the new
   functional architecture in the PCN-domain.  First it describes
   functions needed at the three specific types of PCN-node; these are
   data plane functions and are in addition to their normal router
   functions.  Then it describes further functionality needed for both
   flow admission control and flow termination; these are signalling and
   decision-making functions, and there are various possibilities for
   where the functions are physically located.  The section is split
   into:

   1.  functions needed at PCN-interior-nodes

   2.  functions needed at PCN-ingress-nodes

   3.  functions needed at PCN-egress-nodes

   4.  other functions needed for flow admission control

   5.  other functions needed for flow termination control

   Note: Probing is covered in the Appendix.

   The section then discusses some other detailed topics:





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   1.  addressing

   2.  tunnelling

   3.  fault handling

4.1.  PCN-interior-node functions

   Each link of the PCN-domain is configured with the following
   functionality:

   o  Behaviour aggregate classification - determine whether an incoming
      packet is a PCN-packet or not.

   o  PCN-meter - measure the 'amount of PCN-traffic'.  The measurement
      is made on the overall PCN-traffic, and not per flow.  Algorithms
      determine whether to indicate to the PCN-marking functionality
      that packets should be PCN-marked.

   o  PCN-mark - as triggered by indications from the PCN-meter
      functionality, if necessary PCN-mark packets wth the appropiate
      encoding.

   o  Drop - if the queue overflows then naturally packets are dropped.
      In addition, the link may be configured with a maximum rate for
      PCN-traffic (below the physical link rate), above which PCN-
      packets are dropped.

   The functions are defined in [PCN08-2] and the baseline encoding in
   [PCN08-1] (extended encodings are to be defined in other documents).





















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                                       +---------+   Result
                                    +->|Threshold|-------+
                                    |  |  Meter  |       |
                                    |  +---------+       V
         +----------+   +- - - - -+  |                +------+
         |   BA     |   |         |  |                |      |    Marked
Packet =>|Classifier|==>| Dropper |==?===============>|Marker|==> Packet
Stream   |          |   |         |  |                |      |    Stream
         +----------+   +- - - - -+  |                +------+
                                    |  +---------+       ^
                                    |  | Excess  |       |
                                    +->| Traffic |-------+
                                       |  Meter  |   Result
                                       +---------+

   Figure 4: Schematic of PCN-interior-node functionality

4.2.  PCN-ingress-node functions

   Each ingress link of the PCN-domain is configured with the following
   functionality:

   o  Packet classification - determine whether an incoming packet is
      part of a previously admitted flow, by using a filter spec (eg
      DSCP, source and destination addresses, port numbers, and
      protocol).

   o  Police - police, by dropping, any packets received with a DSCP
      indicating PCN transport that do not belong to an admitted flow.
      (A prospective PCN-flow that is rejected could be blocked or
      admitted into a lower priority behaviour aggregate.)  Similarly,
      police packets that are part of a previously admitted flow, to
      check that the flow keeps to the agreed rate or flowspec (eg
      [RFC1633] for a microflow and its NSIS equivalent).

   o  PCN-colour - set the DSCP and ECN fields appropriately for the
      PCN-domain, for example as in [PCN08-1].

   o  Meter - some approaches to flow termination require the PCN-
      ingress-node to measure the (aggregate) rate of PCN-traffic
      towards a particular PCN-egress-node.

   The first two are policing functions, needed to make sure that PCN-
   packets admitted into the PCN-domain belong to a flow that has been
   admitted and to ensure that the flow keeps to the flowspec agreed (eg
   doesn't exceed an agreed maximum rate and is inelastic traffic).
   Installing the filter spec will typically be done by the signalling
   protocol, as will re-installing the filter, for example after a re-



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   route that changes the PCN-ingress-node (see [Briscoe06] for an
   example using RSVP).  PCN-colouring allows the rest of the PCN-domain
   to recognise PCN-packets.

4.3.  PCN-egress-node functions

   Each egress link of the PCN-domain is configured with the following
   functionality:

   o  Packet classify - determine which PCN-ingress-node a PCN-packet
      has come from.

   o  Meter - "measure PCN-traffic" or "monitor PCN-marks".

   o  PCN-colour - for PCN-packets, set the DSCP and ECN fields to the
      appropriate values for use outside the PCN-domain.

   The metering functionality of course depends on whether it is
   targeted at admission control or flow termination.  Alternatives
   involve the PCN-egress-node "measuring" as an aggregate (ie not per
   flow) all PCN-packets from a particular PCN-ingress-node, or
   "monitoring" the PCN-traffic and reacting to one (or several) PCN-
   marked packets.  For PCN-colouring, [PCN08-1] specifies that the PCN-
   egress-node re-sets the ECN field to 00; other encodings may define
   different behaviour.

4.4.  Admission control functions

   As well as the functions covered above, other specific admission
   control functions need to be performed (others might be possible):

   o  Make decision about admission - based on the output of the PCN-
      egress-node's meter function.  In the case where it "measures PCN-
      traffic", the measured traffic on the ingress-egress-aggregate is
      compared with some reference level.  In the case where it
      "monitors PCN-marks", then the decision is based on whether one
      (or several) packets is (are) PCN-marked or not (eg the RSVP PATH
      message).  In either case, the admission decision also takes
      account of policy and application layer requirements [RFC2753].

   o  Communicate decision about admission - signal the decision to the
      node making the admission control request (which may be outside
      the PCN-domain), and to the policer (PCN-ingress-node function)
      for enforcement of the decision.

   There are various possibilities for how the functionality could be
   distributed (we assume the operator would configure which is used):




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   o  The decision is made at the PCN-egress-node and the decision
      (admit or block) is signalled to the PCN-ingress-node.

   o  The decision is recommended by the PCN-egress-node (admit or
      block) but the decision is definitively made by the PCN-ingress-
      node.  The rationale is that the PCN-egress-node naturally has the
      necessary information about the amount of PCN-marks on the
      ingress-egress-aggregate, but the PCN-ingress-node is the policy
      enforcement point [RFC2753], which polices incoming traffic to
      ensure it is part of an admitted PCN-flow.

   o  The decision is made at the PCN-ingress-node, which requires that
      the PCN-egress-node signals PCN-feedback-information to the PCN-
      ingress-node.  For example, it could signal the current fraction
      of PCN-traffic that is PCN-marked.

   o  The decision is made at a centralised node (see Appendix).

   Note: Admission control functionality is not performed by normal PCN-
   interior-nodes.

4.5.  Flow termination functions

   As well as the functions covered above, other specific termination
   control functions need to be performed (others might be possible):

   o  PCN-meter at PCN-egress-node - similarly to flow admission, there
      are two types of possibilities: to "measure PCN-traffic" on the
      ingress-egress-aggregate, and to "monitor PCN-marks" and react to
      one (or several) PCN-marks.

   o  (if required) PCN-meter at PCN-ingress-node - make "measurements
      of PCN-traffic" being sent towards a particular PCN-egress-node;
      again, this is done for the ingress-egress-aggregate and not per
      flow.

   o  (if required) Communicate PCN-feedback-information to the node
      that makes the flow termination decision.  For example, as in
      [Briscoe06], communicate the PCN-egress-node's measurements to the
      PCN-ingress-node.

   o  Make decision about flow termination - use the information from
      the PCN-meter(s) to decide which PCN-flow or PCN-flows to
      terminate.  The decision takes account of policy and application
      layer requirements [RFC2753].

   o  Communicate decision about flow termination - signal the decision
      to the node that is able to terminate the flow (which may be



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      outside the PCN-domain), and to the policer (PCN-ingress-node
      function) for enforcement of the decision.

   There are various possibilities for how the functionality could be
   distributed, similar to those discussed above in the Admission
   control section.

   Note: Flow termination functionality is not performed by normal PCN-
   interior-nodes.

4.6.  Addressing

   PCN-nodes may need to know the address of other PCN-nodes.  Note: in
   all cases PCN-interior-nodes don't need to know the address of any
   other PCN-nodes (except as normal their next hop neighbours, for
   routing purposes).

   The PCN-egress-node needs to know the address of the PCN-ingress-node
   associated with a flow, at a minimum so that the PCN-ingress-node can
   be informed to enforce the admission decision (and any flow
   termination decision) through policing.  There are various
   possibilities for how the PCN-egress-node can do this, ie associate
   the received packet to the correct ingress-egress-aggregate.  It is
   not the intention of this document to mandate a particular mechanism.

   o  The addressing information can be gathered from signalling.  For
      example, regular processing of an RSVP PATH message, as the PCN-
      ingress-node is the previous RSVP hop (PHOP) ([Lefaucheur06]).  Or
      the PCN-ingress-node could signal its address to the PCN-egress-
      node.

   o  Always tunnel PCN-traffic across the PCN-domain.  Then the PCN-
      ingress-node's address is simply the source address of the outer
      packet header.  The PCN-ingress-node needs to learn the address of
      the PCN-egress-node, either by manual configuration or by one of
      the automated tunnel endpoint discovery mechanisms (such as
      signalling or probing over the data route, interrogating routing
      or using a centralised broker).

4.7.  Tunnelling

   Tunnels may originate and/or terminate within a PCN-domain (eg IP
   over IP, IP over MPLS).  It is important that the PCN-marking of any
   packet can potentially influence PCN's flow admission control and
   termination - it shouldn't matter whether the packet happens to be
   tunnelled at the PCN-node that PCN-marks the packet, or indeed
   whether it's decapsulated or encapsulated by a subsequent PCN-node.
   This suggests that the "uniform conceptual model" described in



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   [RFC2983] should be re-applied in the PCN context.  In line with this
   and the approach of [RFC4303] and [Briscoe08-2], the following rule
   is applied if encapsulation is done within the PCN-domain:

   o  any PCN-marking is copied into the outer header

   Note: A tunnel will not provide this behaviour if it complies with
   [RFC3168] tunnelling in either mode, but it will if it complies with
   [RFC4301] IPSec tunnelling.

   Similarly, in line with the "uniform conceptual model" of [RFC2983],
   the "full-functionality option" of [RFC3168], and [RFC4301], the
   following rule is applied if decapsulation is done within the PCN-
   domain:

   o  if the outer header's marking state is more severe then it is
      copied onto the inner header.

   Note: the order of increasing severity is: not PCN-marked; threshold-
   marked; excess-traffic-marked.

   An operator may wish to tunnel PCN-traffic from PCN-ingress-nodes to
   PCN-egress-nodes.  The PCN-marks shouldn't be visible outside the
   PCN-domain, which can be achieved by the PCN-egress-node doing the
   PCN-colouring function (Section 4.3) after all the other (PCN and
   tunnelling) functions.  The potential reasons for doing such
   tunnelling are: the PCN-egress-node then automatically knows the
   address of the relevant PCN-ingress-node for a flow; even if ECMP is
   running, all PCN-packets on a particular ingress-egress-aggregate
   follow the same path.  (ECMP: Equal Cost Multi-Path, Section 6.4.)
   But it also has drawbacks, for example the additional overhead in
   terms of bandwidth and processing, and the cost of setting up a mesh
   of tunnels between PCN-boundary-nodes (there is an N^2 scaling
   issue).

   Potential issues arise for a "partially PCN-capable tunnel", ie where
   only one tunnel endpoint is in the PCN domain:

   1.  The tunnel originates outside a PCN-domain and ends inside it.
       If the packet arrives at the tunnel ingress with the same
       encoding as used within the PCN-domain to indicate PCN-marking,
       then this could lead the PCN-egress-node to falsely measure pre-
       congestion.

   2.  The tunnel originates inside a PCN-domain and ends outside it.
       If the packet arrives at the tunnel ingress already PCN-marked,
       then it will still have the same encoding when it's decapsulated
       which could potentially confuse nodes beyond the tunnel egress.



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   In line with the solution for partially capable Diffserv tunnels in
   [RFC2983], the following rules are applied:

   o  For case (1), the tunnel egress node clears any PCN-marking on the
      inner header.  This rule is applied before the 'copy on
      decapsulation' rule above.

   o  For case (2), the tunnel ingress node clears any PCN-marking on
      the inner header.  This rule is applied after the 'copy on
      encapsulation' rule above.

   Note that the above implies that one has to know, or determine, the
   characteristics of the other end of the tunnel as part of
   establishing it.

   Tunnelling constraints were a major factor in the choice of the
   baseline encoding.  As explained in [PCN08-1], with current
   tunnelling endpoints only the 11 codepoint of the ECN field survives
   decapsulation, and hence the baseline encoding only uses the 11
   codepoint to indicate PCN-marking.  Extended encoding schemes need to
   explain their interactions with (or assumptions about) tunnelling.  A
   lengthy discussion of all the issues associated with layered
   encapsulation of congestion notification (for ECN as well as PCN) is
   in [Briscoe08-2].

4.8.  Fault handling

   If a PCN-interior-node (or one of its links) fails, then lower layer
   protection mechanisms or the regular IP routing protocol will
   eventually re-route around it.  If the new route can carry all the
   admitted traffic, flows will gracefully continue.  If instead this
   causes early warning of pre-congestion on the new route, then
   admission control based on pre-congestion notification will ensure
   new flows will not be admitted until enough existing flows have
   departed.  Re-routing may result in heavy (pre-)congestion, when the
   flow termination mechanism will kick in.

   If a PCN-boundary-node fails then we would like the regular QoS
   signalling protocol to be responsible for taking appropriate action.
   As an example [Briscoe08-2] considers what happens if RSVP is the QoS
   signalling protocol.


5.  Operations and Management

   This Section considers operations and management issues, under the
   FCAPS headings: the Operations and Management of Faults,
   Configuration, Accounting, Performance and Security.  Provisioning is



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   discussed with performance.

5.1.  Configuration Operations and Management

   Threshold-metering and -marking and excess-traffic-metering and
   -marking are standardised in [PCN08-2].  However, more diversity in
   PCN-boundary-node behaviours is expected, in order to interface with
   diverse industry architectures.  It may be possible to have different
   PCN-boundary-node behaviours for different ingress-egress-aggregates
   within the same PCN-domain.

   PCN metering behaviour is enabled on either the egress or the ingress
   interfaces of PCN-nodes.  A consistent choice must be made across the
   PCN-domain to ensure that the PCN mechanisms protect all links.

   PCN configuration control variables fall into the following
   categories:

   o  system options (enabling or disabling behaviours)

   o  parameters (setting levels, addresses etc)

   One possibility is that all configurable variables sit within an SNMP
   management framework [RFC3411], being structured within a defined
   management information base (MIB) on each node, and being remotely
   readable and settable via a suitably secure management protocol
   (SNMPv3).

   Some configuration options and parameters have to be set once to
   'globally' control the whole PCN-domain.  Where possible, these are
   identified below.  This may affect operational complexity and the
   chances of interoperability problems between equipment from different
   vendors.

   It may be possible for an operator to configure some PCN-interior-
   nodes so that they don't run the PCN mechanisms, if it knows that
   these links will never become (pre-)congested.

5.1.1.  System options

   On PCN-interior-nodes there will be very few system options:

   o  Whether two PCN-markings (threshold-marked and excess-traffic-
      marked) are enabled or only one.  Typically all nodes throughout a
      PCN-domain will be configured the same in this respect.  However,
      exceptions could be made.  For example, if most PCN-nodes used
      both markings, but some legacy hardware was incapable of running
      two algorithms, an operator might be willing to configure these



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      legacy nodes solely for excess-traffic-marking to enable flow
      termination as a back-stop.  It would be sensible to place such
      nodes where they could be provisioned with a greater leeway over
      expected traffic levels.

   o  In the case where only one PCN-marking is enabled, all nodes must
      be configured to generate PCN-marks from the same meter (ie either
      the threshold meter or the excess traffic meter).

   PCN-boundary-nodes (ingress and egress) will have more system
   options:

   o  Which of admission and flow termination are enabled.  If any PCN-
      interior-node is configured to generate a marking, all PCN-
      boundary-nodes must be able to interpret that marking (which
      includes understanding, in a PCN-domain that uses only one type of
      PCN-marking, whether they are generated by PCN-interior-nodes'
      threshold meters or the excess traffic meters).  Therefore all
      PCN-boundary-nodes must be configured the same in this respect.

   o  Where flow admission and termination decisions are made: at PCN-
      ingress-nodes or at PCN-egress-nodes (or at a centralised node,
      see Appendix).  Theoretically, this configuration choice could be
      negotiated for each pair of PCN-boundary-nodes, but we cannot
      imagine why such complexity would be required, except perhaps in
      future inter-domain scenarios.

   o  How PCN-markings are translated into admission control and flow
      termination decisions (see Section 3.1 and Section 3.2).

   PCN-egress-nodes will have further system options:

   o  How the mapping should be established between each packet and its
      aggregate, eg by MPLS label, by IP packet filter spec; and how to
      take account of ECMP.

   o  If an equipment vendor provides a choice, there may be options to
      select which smoothing algorithm to use for measurements.

5.1.2.  Parameters

   Like any Diffserv domain, every node within a PCN-domain will need to
   be configured with the DSCP(s) used to identify PCN-packets.  On each
   interior link the main configuration parameters are the PCN-
   threshold-rate and PCN-excess-rate.  A larger PCN-threshold-rate
   enables more PCN-traffic to be admitted on a link, hence improving
   capacity utilisation.  A PCN-excess-rate set further above the PCN-
   threshold-rate allows greater increases in traffic (whether due to



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   natural fluctuations or some unexpected event) before any flows are
   terminated, ie minimises the chances of unnecessarily triggering the
   termination mechanism.  For instance, an operator may want to design
   their network so that it can cope with a failure of any single PCN-
   node without terminating any flows.

   Setting these rates on first deployment of PCN will be very similar
   to the traditional process for sizing an admission controlled
   network, depending on: the operator's requirements for minimising
   flow blocking (grade of service), the expected PCN traffic load on
   each link and its statistical characteristics (the traffic matrix),
   contingency for re-routing the PCN traffic matrix in the event of
   single or multiple failures, and the expected load from other classes
   relative to link capacities [Menth07].  But once a domain is in
   operation, a PCN design goal is to be able to determine growth in
   these configured rates much more simply, by monitoring PCN-marking
   rates from actual rather than expected traffic (see Section 5.2 on
   Performance & Provisioning).

   Operators may also wish to configure a rate greater than the PCN-
   excess-rate that is the absolute maximum rate that a link allows for
   PCN-traffic.  This may simply be the physical link rate, but some
   operators may wish to configure a logical limit to prevent starvation
   of other traffic classes during any brief period after PCN-traffic
   exceeds the PCN-excess-rate but before flow termination brings it
   back below this rate.

   Threshold-metering requires a threshold token bucket depth to be
   configured, excess-traffic-metering needs a value for the MTU
   (maximum size of a PCN-packet on the link) and both require setting a
   maximum size of their token buckets.  It will be preferable for there
   to be rules to set defaults for these parameters, but then allow
   operators to change them, for instance if average traffic
   characteristics change over time.

   The PCN-egress-node may allow configuration of the following:

   o  how it smooths metering of PCN-markings (eg EWMA parameters)

   Whichever node makes admission and flow termination decisions will
   contain algorithms for converting PCN-marking levels into admission
   or flow termination decisions.  These will also require configurable
   parameters, for instance:

   o  an admission control algorithm that is based on the fraction of
      marked packets will at least require a marking threshold setting
      above which it denies admission to new flows;




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   o  flow termination algorithms will probably require a parameter to
      delay termination of any flows until it is more certain that an
      anomalous event is not transient;

   o  a parameter to control the trade-off between how quickly excess
      flows are terminated, and over-termination.

   One particular approach, [Charny07-2] would require a global
   parameter to be defined on all PCN-nodes, but only needs one PCN
   marking rate to be configured on each link.  The global parameter is
   a scaling factor between admission and termination (the PCN-traffic
   rate on a link up to which flows are admitted vs the rate above which
   flows are terminated).  [Charny07-2] discusses in full the impact of
   this particular approach on the operation of PCN.

5.2.  Performance & Provisioning Operations and Management

   Monitoring of performance factors measurable from *outside* the PCN
   domain will be no different with PCN than with any other packet-based
   flow admission control system, both at the flow level (blocking
   probability, etc) and the packet level (jitter [RFC3393], [Y.1541],
   loss rate [RFC4656], mean opinion score [P.800], etc).  The
   difference is that PCN is intentionally designed to indicate
   *internally* which exact resource(s) are the cause of performance
   problems and by how much.

   Even better, PCN indicates which resources will probably cause
   problems if they are not upgraded soon.  This can be achieved by the
   management system monitoring the total amount (in bytes) of PCN-
   marking generated by each queue over a period.  Given possible long
   provisioning lead times, pre-congestion volume is the best metric to
   reveal whether sufficient persistent demand has occurred to warrant
   an upgrade.  Because, even before utilisation becomes problematic,
   the statistical variability of traffic will cause occasional bursts
   of pre-congestion.  This 'early warning system' decouples the process
   of adding customers from the provisioning process.  This should cut
   the time to add a customer when compared against admission control
   provided over native Diffserv [RFC2998], because it saves having to
   verify the capacity planning process before adding each customer.

   Alternatively, before triggering an upgrade, the long term pre-
   congestion volume on each link can be used to balance traffic load
   across the PCN-domain by adjusting the link weights of the routing
   system.  When an upgrade to a link's configured PCN-rates is
   required, it may also be necessary to upgrade the physical capacity
   available to other classes.  But usually there will be sufficient
   physical capacity for the upgrade to go ahead as a simple
   configuration change.  Alternatively, [Songhurst06] describes an



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   adaptive rather than preconfigured system, where the configured PCN-
   threshold-rate is replaced with a high and low water mark and the
   marking algorithm automatically optimises how physical capacity is
   shared using the relative loads from PCN and other traffic classes.

   All the above processes require just three extra counters associated
   with each PCN queue: threshold-markings, excess-traffic-markings and
   drop.  Every time a PCN packet is marked or dropped its size in bytes
   should be added to the appropriate counter.  Then the management
   system can read the counters at any time and subtract a previous
   reading to establish the incremental volume of each type of
   (pre-)congestion.  Readings should be taken frequently, so that
   anomalous events (eg re-routes) can be distinguished from regular
   fluctuating demand if required.

5.3.  Accounting Operations and Management

   Accounting is only done at trust boundaries so it is out of scope of
   this document, which is confined to intra-domain issues.  Use of PCN
   internal to a domain makes no difference to the flow signalling
   events crossing trust boundaries outside the PCN-domain, which are
   typically used for accounting.

5.4.  Fault Operations and Management

   Fault Operations and Management is about preventing faults, telling
   the management system (or manual operator) that the system has
   recovered (or not) from a failure, and about maintaining information
   to aid fault diagnosis.

   Admission blocking and particularly flow termination mechanisms
   should rarely be needed in practice.  It would be unfortunate if they
   didn't work after an option had been accidentally disabled.
   Therefore it will be necessary to regularly test that the live system
   works as intended (devising a meaningful test is left as an exercise
   for the operator).

   Section 4 describes how the PCN architecture has been designed to
   ensure admitted flows continue gracefully after recovering
   automatically from link or node failures.  The need to record and
   monitor re-routing events affecting signalling is unchanged by the
   addition of PCN to a Diffserv domain.  Similarly, re-routing events
   within the PCN-domain will be recorded and monitored just as they
   would be without PCN.

   PCN-marking does make it possible to record 'near-misses'.  For
   instance, at the PCN-egress-node a 'reporting threshold' could be set
   to monitor how often - and for how long - the system comes close to



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   triggering flow blocking without actually doing so.  Similarly,
   bursts of flow termination marking could be recorded even if they are
   not sufficiently sustained to trigger flow termination.  Such
   statistics could be correlated with per-queue counts of marking
   volume (Section 5.2) to upgrade resources in danger of causing
   service degradation, or to trigger manual tracing of intermittent
   incipient errors that would otherwise have gone unnoticed.

   Finally, of course, many faults are caused by failings in the
   management process ('human error'): a wrongly configured address in a
   node, a wrong address given in a signalling protocol, a wrongly
   configured parameter in a queueing algorithm, a node set into a
   different mode from other nodes, and so on.  Generally, a clean
   design with few configurable options ensures this class of faults can
   be traced more easily and prevented more often.  Sound management
   practice at run-time also helps.  For instance: a management system
   should be used that constrains configuration changes within system
   rules (eg preventing an option setting inconsistent with other
   nodes); configuration options should also be recorded in an offline
   database; and regular automatic consistency checks between live
   systems and the database should be performed.  PCN adds nothing
   specific to this class of problems.

5.5.  Security Operations and Management

   Security Operations and Management is about using secure operational
   practices as well as being able to track security breaches or near-
   misses at run-time.  PCN adds few specifics to the general good
   practice required in this field [RFC4778], other than those below.
   The correct functions of the system should be monitored (Section 5.2)
   in multiple independent ways and correlated to detect possible
   security breaches.  Persistent (pre-)congestion marking should raise
   an alarm (both on the node doing the marking and on the PCN-egress-
   node metering it).  Similarly, persistently poor external QoS metrics
   (such as jitter or mean opinion score) should raise an alarm.  The
   following are examples of symptoms that may be the result of innocent
   faults, rather than attacks, but until diagnosed they should be
   logged and trigger a security alarm:

   o  Anomalous patterns of non-conforming incoming signals and packets
      rejected at the PCN-ingress-nodes (eg packets already marked PCN-
      capable, or traffic persistently starving token bucket policers).

   o  PCN-capable packets arriving at a PCN-egress-node with no
      associated state for mapping them to a valid ingress-egress-
      aggregate.





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   o  A PCN-ingress-node receiving feedback signals about the pre-
      congestion level on a non-existent aggregate, or that are
      inconsistent with other signals (eg unexpected sequence numbers,
      inconsistent addressing, conflicting reports of the pre-congestion
      level, etc).

   o  Pre-congestion marking arriving at a PCN-egress-node with
      (pre-)congestion markings focused on particular flows, rather than
      randomly distributed throughout the aggregate.


6.  Applicability of PCN

6.1.  Benefits

   The key benefits of the PCN mechanisms are that they are simple,
   scalable, and robust because:

   o  Per flow state is only required at the PCN-ingress-nodes
      ("stateless core").  This is required for policing purposes (to
      prevent non-admitted PCN traffic from entering the PCN-domain) and
      so on.  It is not generally required that other network entities
      are aware of individual flows (although they may be in particular
      deployment scenarios).

   o  Admission control is resilient: with PCN QoS is decoupled from the
      routing system.  Hence in general admitted flows can survive
      capacity, routing or topology changes without additional
      signalling.  The PCN-admissible-rate on each link can be chosen
      small enough that admitted traffic can still be carried after a
      rerouting in most failure cases [Menth07].  This is an important
      feature as QoS violations in core networks due to link failures
      are more likely than QoS violations due to increased traffic
      volume [Iyer03].

   o  The PCN-metering behaviours only operate on the overall PCN-
      traffic on the link, not per flow.

   o  The information of these measurements is signalled to the PCN-
      egress-nodes by the PCN-marks in the packet headers, ie [Style]
      "in-band".  No additional signalling protocol is required for
      transporting the PCN-marks.  Therefore no secure binding is
      required between data packets and separate congestion messages.

   o  The PCN-egress-nodes make separate measurements, operating on the
      aggregate PCN-traffic from each PCN-ingress-node, ie not per flow.
      Similarly, signalling by the PCN-egress-node of PCN-feedback-
      information (which is used for flow admission and termination



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      decisions) is at the granularity of the ingress-egress-aggregate.
      An alternative approach is that the PCN-egress-nodes monitor the
      PCN-traffic and signal PCN-feedback-information (which is used for
      flow admission and termination decisions) at the granularity of
      one (or a few) PCN-marks.

   o  The admitted PCN-load is controlled dynamically.  Therefore it
      adapts as the traffic matrix changes, and also if the network
      topology changes (eg after a link failure).  Hence an operator can
      be less conservative when deploying network capacity, and less
      accurate in their prediction of the PCN-traffic matrix.

   o  The termination mechanism complements admission control.  It
      allows the network to recover from sudden unexpected surges of
      PCN-traffic on some links, thus restoring QoS to the remaining
      flows.  Such scenarios are expected to be rare but not impossible.
      They can be caused by large network failures that redirect lots of
      admitted PCN-traffic to other links, or by malfunction of the
      measurement-based admission control in the presence of admitted
      flows that send for a while with an atypically low rate and then
      increase their rates in a correlated way.

   o  Flow termination can also enable an operator to be less
      conservative when deploying network capacity.  It is an
      alternative to running links at low utilisation in order to
      protect against link or node failures.  This is especially the
      case with SRLGs (shared risk link groups, which are links that
      share a resource, such as a fibre, whose failure affects all those
      links [RFC4216]).  Fully protecting traffic against a single SRLG
      failure requires low utilisation (~10%) of the link bandwidth on
      some links before failure [Charny08].

   o  The PCN-supportable-rate may be set below the maximum rate that
      PCN-traffic can be transmitted on a link, in order to trigger
      termination of some PCN-flows before loss (or excessive delay) of
      PCN-packets occurs, or to keep the maximum PCN-load on a link
      below a level configured by the operator.

   o  Provisioning of the network is decoupled from the process of
      adding new customers.  By contrast, with the Diffserv architecture
      [RFC2475] operators rely on subscription-time Service Level
      Agreements, which statically define the parameters of the traffic
      that will be accepted from a customer, and so the operator has to
      verify provision is sufficient each time a new customer is added
      to check that the Service Level Agreement can be fulfilled.  A
      PCN-domain doesn't need such traffic conditioning.





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6.2.  Deployment scenarios

   Operators of networks will want to use the PCN mechanisms in various
   arrangements, for instance depending on how they are performing
   admission control outside the PCN-domain (users after all are
   concerned about QoS end-to-end), what their particular goals and
   assumptions are, how many PCN encoding states are available, and so
   on.

   A PCN-domain may have three encoding states (or pedantically, an
   operator may choose to use up three encoding states for PCN): not
   PCN-marked, threshold-marked, excess-traffic-marked.  Then both PCN
   admission control and flow termination can be supported.  As
   illustrated in Figure 1, admission control accepts new flows until
   the PCN-traffic rate on the bottleneck link rises above the PCN-
   threshold-rate, whilst if necessary the flow termination mechanism
   terminates flows down to the PCN-excess-rate on the bottleneck link.

   On the other hand, a PCN-domain may have two encoding states (as in
   [PCN08-1]) (or pedantically, an operator may choose to use up two
   encoding states for PCN): not PCN-marked, PCN-marked.  Then there are
   three possibilities, as discussed in the following paragraphs (see
   also Section 3.3).

   First, an operator could just use PCN's admission control, solving
   heavy congestion (caused by re-routing) by 'just waiting' - as
   sessions end, PCN-traffic naturally reduces, and meanwhile the
   admission control mechanism will prevent admission of new flows that
   use the affected links.  So the PCN-domain will naturally return to
   normal operation, but with reduced capacity.  The drawback of this
   approach would be that, until sufficient sessions have ended to
   relieve the congestion, all PCN-flows as well as lower priority
   services will be adversely affected.

   Second, an operator could just rely for admission control on
   statically provisioned capacity per PCN-ingress-node (regardless of
   the PCN-egress-node of a flow), as is typical in the hose model of
   the Diffserv architecture [RFC2475].  Such traffic conditioning
   agreements can lead to focused overload: many flows happen to focus
   on a particular link and then all flows through the congested link
   fail catastrophically.  PCN's flow termination mechanism could then
   be used to counteract such a problem.

   Third, both admission control and flow termination can be triggered
   from the single type of PCN-marking; the main downside is that
   admission control is less accurate [Charny07-2].  This possibility is
   illustrated in Figure 3.




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   Within the PCN-domain there is some flexibility about how the
   decision making functionality is distributed.  These possibilities
   are outlined in Section 4.4 and also discussed elsewhere, such as in
   [Menth08-3].

   The flow admission and termination decisions need to be enforced
   through per flow policing by the PCN-ingress-nodes.  If there are
   several PCN-domains on the end-to-end path, then each needs to police
   at its PCN-ingress-nodes.  One exception is if the operator runs both
   the access network (not a PCN-domain) and the core network (a PCN-
   domain); per flow policing could be devolved to the access network
   and not done at the PCN-ingress-node.  Note: to aid readability, the
   rest of this draft assumes that policing is done by the PCN-ingress-
   nodes.

   PCN admission control has to fit with the overall approach to
   admission control.  For instance [Briscoe06] describes the case where
   RSVP signalling runs end-to-end.  The PCN-domain is a single RSVP
   hop, ie only the PCN-boundary-nodes process RSVP messages, with RSVP
   messages processed on each hop outside the PCN-domain, as in IntServ
   over Diffserv [RFC2998].  It would also be possible for the RSVP
   signalling to be originated and/or terminated by proxies, with
   application-layer signalling between the end user and the proxy (eg
   SIP signalling with a home hub).  A similar example would use NSIS
   signalling instead of RSVP.  (NSIS: Next Steps in Signalling,
   [RFC3726].)

   It is possible that a user wants its inelastic traffic to use the PCN
   mechanisms but also react to ECN marking outside the PCN-domain
   [Sarker08].  Two possible ways to do this are to tunnel all PCN-
   packets across the PCN-domain, so that the ECN marks are carried
   transparently across the PCN-domain, or to use an encoding like
   [Moncaster08].  Tunnelling is discussed further in Section 4.7.

   Some further possible deployment models are outlined in the Appendix.

6.3.  Assumptions and constraints on scope

   The scope is restricted by the following assumptions:

   1.  these components are deployed in a single Diffserv domain, within
       which all PCN-nodes are PCN-enabled and are trusted for truthful
       PCN-marking and transport

   2.  all flows handled by these mechanisms are inelastic and
       constrained to a known peak rate through policing or shaping





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   3.  the number of PCN-flows across any potential bottleneck link is
       sufficiently large that stateless, statistical mechanisms can be
       effective.  To put it another way, the aggregate bit rate of PCN-
       traffic across any potential bottleneck link needs to be
       sufficiently large relative to the maximum additional bit rate
       added by one flow.  This is the basic assumption of measurement-
       based admission control.

   4.  PCN-flows may have different precedence, but the applicability of
       the PCN mechanisms for emergency use (911, GETS, WPS, MLPP, etc.)
       is out of scope.

6.3.1.  Assumption 1: Trust and support of PCN - controlled environment

   It is assumed that the PCN-domain is a controlled environment, ie all
   the nodes in a PCN-domain run PCN and are trusted.  There are several
   reasons this assumption:

   o  The PCN-domain has to be encircled by a ring of PCN-boundary-
      nodes, otherwise traffic could enter a PCN-BA without being
      subject to admission control, which would potentially degrade the
      QoS of existing PCN-flows.

   o  Similarly, a PCN-boundary-node has to trust that all the PCN-nodes
      mark PCN-traffic consistently.  A node not performing PCN-marking
      wouldn't be able to alert when it suffered pre-congestion, which
      potentially would lead to too many PCN-flows being admitted (or
      too few being terminated).  Worse, a rogue node could perform
      various attacks, as discussed in the Security Considerations
      section.

   One way of assuring the above two points is that the entire PCN-
   domain is run by a single operator.  Another possibility is that
   there are several operators that trust each other in their handling
   of PCN-traffic.

   Note: All PCN-nodes need to be trustworthy.  However if it is known
   that an interface cannot become pre-congested then it is not strictly
   necessary for it to be capable of PCN-marking.  But this must be
   known even in unusual circumstances, eg after the failure of some
   links.

6.3.2.  Assumption 2: Real-time applications

   It is assumed that any variation of source bit rate is independent of
   the level of pre-congestion.  We assume that PCN-packets come from
   real time applications generating inelastic traffic, ie sending
   packets at the rate the codec produces them, regardless of the



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   availability of capacity [RFC4594].  For example, voice and video
   requiring low delay, jitter and packet loss, the Controlled Load
   Service, [RFC2211], and the Telephony service class, [RFC4594].  This
   assumption is to help focus the effort where it looks like PCN would
   be most useful, ie the sorts of applications where per flow QoS is a
   known requirement.  In other words we focus on PCN providing a
   benefit to inelastic traffic (PCN may or may not provide a benefit to
   other types of traffic).

   As a consequence, it is assumed that PCN-metering and PCN-marking is
   being applied to traffic scheduled with the expedited forwarding per-
   hop behaviour, [RFC3246], or a per-hop behaviour with similar
   characteristics.

6.3.3.  Assumption 3: Many flows and additional load

   It is assumed that there are many PCN-flows on any bottleneck link in
   the PCN-domain (or, to put it another way, the aggregate bit rate of
   PCN-traffic across any potential bottleneck link is sufficiently
   large relative to the maximum additional bit rate added by one PCN-
   flow).  Measurement-based admission control assumes that the present
   is a reasonable prediction of the future: the network conditions are
   measured at the time of a new flow request, however the actual
   network performance must be acceptable during the call some time
   later.  One issue is that if there are only a few variable rate
   flows, then the aggregate traffic level may vary a lot, perhaps
   enough to cause some packets to get dropped.  If there are many flows
   then the aggregate traffic level should be statistically smoothed.
   How many flows is enough depends on a number of factors such as the
   variation in each flow's rate, the total rate of PCN-traffic, and the
   size of the "safety margin" between the traffic level at which we
   start admission-marking and at which packets are dropped or
   significantly delayed.

   No explicit assumptions are made about how many PCN-flows are in each
   ingress-egress-aggregate.  Performance evaluation work may clarify
   whether it is necessary to make any additional assumption on
   aggregation at the ingress-egress-aggregate level.

6.3.4.  Assumption 4: Emergency use out of scope

   PCN-flows may have different precedence, but the applicability of the
   PCN mechanisms for emergency use (911, GETS, WPS, MLPP, etc) is out
   of scope of this document.







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6.4.  Challenges

   Prior work on PCN and similar mechanisms has thrown up a number of
   considerations about PCN's design goals (things PCN should be good
   at) and some issues that have been hard to solve in a fully
   satisfactory manner.  Taken as a whole it represents a list of trade-
   offs (it is unlikely that they can all be 100% achieved) and perhaps
   as evaluation criteria to help an operator (or the IETF) decide
   between options.

   The following are open issues.  They are mainly taken from
   [Briscoe06], which also describes some possible solutions.  Note that
   some may be considered unimportant in general or in specific
   deployment scenarios or by some operators.

   NOTE: Potential solutions are out of scope for this document.

   o  ECMP (Equal Cost Multi-Path) Routing: The level of pre-congestion
      is measured on a specific ingress-egress-aggregate.  However, if
      the PCN-domain runs ECMP, then traffic on this ingress-egress-
      aggregate may follow several different paths - some of the paths
      could be pre-congested whilst others are not.  There are three
      potential problems:

      1.  over-admission: a new flow is admitted (because the pre-
          congestion level measured by the PCN-egress-node is
          sufficiently diluted by unmarked packets from non-congested
          paths that a new flow is admitted), but its packets travel
          through a pre-congested PCN-node.

      2.  under-admission: a new flow is blocked (because the pre-
          congestion level measured by the PCN-egress-node is
          sufficiently increased by PCN-marked packets from pre-
          congested paths that a new flow is blocked), but its packets
          travel along an uncongested path.

      3.  ineffective termination: a flow is terminated, but its path
          doesn't travel through the (pre-)congested router(s).  Since
          flow termination is a 'last resort', which protects the
          network should over-admission occur, this problem is probably
          more important to solve than the other two.

   o  ECMP and signalling: It is possible that, in a PCN-domain running
      ECMP, the signalling packets (eg RSVP, NSIS) follow a different
      path than the data packets, which could matter if the signalling
      packets are used as probes.  Whether this is an issue depends on
      which fields the ECMP algorithm uses; if the ECMP algorithm is
      restricted to the source and destination IP addresses, then it



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      will not be an issue.  ECMP and signalling interactions are a
      specific instance of a general issue for non-traditional routing
      combined with resource management along a path [Hancock02].

   o  Tunnelling: There are scenarios where tunnelling makes it
      difficult to determine the path in the PCN-domain.  The problem,
      its impact, and the potential solutions are similar to those for
      ECMP.

   o  Scenarios with only one tunnel endpoint in the PCN domain may make
      it harder for the PCN-egress-node to gather from the signalling
      messages (eg RSVP, NSIS) the identity of the PCN-ingress-node.

   o  Bi-Directional Sessions: Many applications have bi-directional
      sessions - hence there are two microflows that should be admitted
      (or terminated) as a pair - for instance a bi-directional voice
      call only makes sense if microflows in both directions are
      admitted.  However, the PCN mechanisms concern admission and
      termination of a single flow, and coordination of the decision for
      both flows is a matter for the signalling protocol and out of
      scope of PCN.  One possible example would use SIP pre-conditions.
      However, there are others.

   o  Global Coordination: PCN makes its admission decision based on
      PCN-markings on a particular ingress-egress-aggregate.  Decisions
      about flows through a different ingress-egress-aggregate are made
      independently.  However, one can imagine network topologies and
      traffic matrices where, from a global perspective, it would be
      better to make a coordinated decision across all the ingress-
      egress-aggregates for the whole PCN-domain.  For example, to block
      (or even terminate) flows on one ingress-egress-aggregate so that
      more important flows through a different ingress-egress-aggregate
      could be admitted.  The problem may well be relatively
      insignificant.

   o  Aggregate Traffic Characteristics: Even when the number of flows
      is stable, the traffic level through the PCN-domain will vary
      because the sources vary their traffic rates.  PCN works best when
      there is not too much variability in the total traffic level at a
      PCN-node's interface (ie in the aggregate traffic from all
      sources).  Too much variation means that a node may (at one
      moment) not be doing any PCN-marking and then (at another moment)
      drop packets because it is overloaded.  This makes it hard to tune
      the admission control scheme to stop admitting new flows at the
      right time.  Therefore the problem is more likely with fewer,
      burstier flows.





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   o  Flash crowds and Speed of Reaction: PCN is a measurement-based
      mechanism and so there is an inherent delay between packet marking
      by PCN-interior-nodes and any admission control reaction at PCN-
      boundary-nodes.  For example, potentially if a big burst of
      admission requests occurs in a very short space of time (eg
      prompted by a televote), they could all get admitted before enough
      PCN-marks are seen to block new flows.  In other words, any
      additional load offered within the reaction time of the mechanism
      must not move the PCN-domain directly from a no congestion state
      to overload.  This 'vulnerability period' may have an impact at
      the signalling level, for instance QoS requests should be rate
      limited to bound the number of requests able to arrive within the
      vulnerability period.

   o  Silent at start: after a successful admission request the source
      may wait some time before sending data (eg waiting for the called
      party to answer).  Then the risk is that, in some circumstances,
      PCN's measurements underestimate what the pre-congestion level
      will be when the source does start sending data.


7.  IANA Considerations

   This memo includes no request to IANA.


8.  Security considerations

   Security considerations essentially come from the Trust Assumption
   (Section 6.3.1), ie that all PCN-nodes are PCN-enabled and are
   trusted for truthful PCN-metering and PCN-marking.  PCN splits
   functionality between PCN-interior-nodes and PCN-boundary-nodes, and
   the security considerations are somewhat different for each, mainly
   because PCN-boundary-nodes are flow-aware and PCN-interior-nodes are
   not.

   o  Because the PCN-boundary-nodes are flow-aware, they are trusted to
      use that awareness correctly.  The degree of trust required
      depends on the kinds of decisions they have to make and the kinds
      of information they need to make them.  There is nothing specific
      to PCN.

   o  The PCN-ingress-nodes police packets to ensure a PCN-flow sticks
      within its agreed limit, and to ensure that only PCN-flows that
      have been admitted contribute PCN-traffic into the PCN-domain.
      The policer must drop (or perhaps downgrade to a different DSCP)
      any PCN-packets received that are outside this remit.  This is
      similar to the existing IntServ behaviour.  Between them the PCN-



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      boundary-nodes must encircle the PCN-domain, otherwise PCN-packets
      could enter the PCN-domain without being subject to admission
      control, which would potentially destroy the QoS of existing
      flows.

   o  PCN-interior-nodes are not flow-aware.  This prevents some
      security attacks where an attacker targets specific flows in the
      data plane - for instance for DoS or eavesdropping.

   o  The PCN-boundary-nodes rely on correct PCN-marking by the PCN-
      interior-nodes.  For instance a rogue PCN-interior-node could PCN-
      mark all packets so that no flows were admitted.  Another
      possibility is that it doesn't PCN-mark any packets, even when it
      is pre-congested.  More subtly, the rogue PCN-interior-node could
      perform these attacks selectively on particular flows, or it could
      PCN-mark the correct fraction overall, but carefully choose which
      flows it marked.

   o  The PCN-boundary-nodes should be able to deal with DoS attacks and
      state exhaustion attacks based on fast changes in per flow
      signalling.

   o  The signalling between the PCN-boundary-nodes must be protected
      from attacks.  For example the recipient needs to validate that
      the message is indeed from the node that claims to have sent it.
      Possible measures include digest authentication and protection
      against replay and man-in-the-middle attacks.  For the specific
      protocol RSVP, hop-by-hop authentication is in [RFC2747], and
      [Behringer07] may also be useful.

   Operational security advice is given in Section 5.5.


9.  Conclusions

   The document describes a general architecture for flow admission and
   termination based on pre-congestion information in order to protect
   the quality of service of established inelastic flows within a single
   Diffserv domain.  The main topic is the functional architecture.  It
   also mentions other topics like the assumptions and open issues.


10.  Acknowledgements

   This document is a revised version of an earlier individual draft
   authored by: P. Eardley, J. Babiarz, K. Chan, A. Charny, R. Geib, G.
   Karagiannis, M. Menth, T. Tsou.  They are therefore contributors to
   this document.



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   Thanks to those who have made comments on this document: Lachlan
   Andrew, Joe Babiarz, Fred Baker, David Black, Steven Blake, Ron
   Bonica, Scott Bradner, Bob Briscoe, Ross Callon, Jason Canon, Ken
   Carlberg, Anna Charny, Joachim Charzinski, Andras Csaszar, Francis
   Dupont, Lars Eggert, Pasi Eronen, Adrian Farrel, Ruediger Geib, Wei
   Gengyu, Robert Hancock, Fortune Huang, Christian Hublet, Cullen
   Jennings, Ingemar Johansson, Georgios Karagiannis, Hein Mekkes,
   Michael Menth, Toby Moncaster, Dimitri Papadimitriou, Dan Romascanu,
   Daisuke Satoh, Ben Strulo, Tom Taylor, Hannes Tschofenig, Tina Tsou,
   David Ward, Lars Westberg, Magnus Westerlund, Delei Yu.  Thanks to
   Bob Briscoe who extensively revised the Operations and Management
   section.

   This document is the result of discussions in the PCN WG and
   forerunner activity in the TSVWG.  A number of previous drafts were
   presented to TSVWG; their authors were: B, Briscoe, P. Eardley, D.
   Songhurst, F. Le Faucheur, A. Charny, J. Babiarz, K. Chan, S. Dudley,
   G. Karagiannis, A. Bader, L. Westberg, J. Zhang, V. Liatsos, X-G.
   Liu, A. Bhargava.


11.  Comments Solicited (to be removed by RFC Editor)

   Comments and questions are encouraged and very welcome.  They can be
   addressed to the IETF PCN working group mailing list <pcn@ietf.org>.


12.  Changes (to be removed by RFC Editor)

12.1.  Changes from -10 to -11

   Changes to deal with IESG comments from routing area review:

   o  Small clarifications to Introduction

   o  the term "marking" now only used to refer only to setting the
      codepoint (not as a shorthand for 'metering and setting the
      codepoint')

   o  Added Figure 4 (Schematic of PCN-interior-node functionality)
      (from [PCN08-2]

   o  Appendix A brought back into the main body.

   o  Other minor clarifications






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12.2.  Changes from -09 to -10

   Changes to deal with IESG comments:

   o  New introduction to provide gentler introduction for the PCN
      novice: quick summary of PCN's applicability; quick example of how
      it all hangs together in one end-to-end qos scenario; quick
      summary of PCN "documentation"

   o  OAM changed to Operations and Management

   o  Processed some of the minor suggestions in the Gen-ART Review by
      Francis Dupont

   o  Two wording tweaks in Sections 3.2 & 3.4 (as agreed on mailing
      list)

   o  Updated boilerplate. this draft may include material pre- Nov 10
      2008 blah.

12.3.  Changes from -08 to -09

   Small changes to deal with WG Chair comments:

   o  tweak language in various places to make it more RFC-like and less
      that of a scholarly work, for instance from "we propose" to "this
      document describes"

   o  tweak language in various places to make it a stand alone
      architecture document rather than a discussion of the PCN WG.  Now
      only mentions WG at start of Annex.

   o  References: IDs are no longer referenced to by the draft name

   o  References: removed some of less important references to IDs

12.4.  Changes from -07 to -08

   Small changes from second WG last call:

   o  Section 2: added definition for PCN-admissible-rate and PCN-
      supportable-rate.  Small changes to use these terms as follows:
      Section 3, bullets 2 & 9; S6.1 para 1; S6.2 para1; S6.3 bullet 3;
      added to Figs 1 & 2.

   o  added the phrase "(others might be possible") before the list of
      approaches in Section 6.3, 7.4 & 7.5.




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   o  added references to RFC2753 (A framework for policy-based
      admission control) in S7.4 & S7.5.

   o  throughout, updated references now that marking behaviour &
      baseline encoding are WG drafts.

   o  a few typos corrected

12.5.  Changes from -06 to -07

   References re-formatted to pass ID nits.  No other changes.

12.6.  Changes from -05 to -06

   Minor clarifications throughout, the least insignificant are as
   follows:

   o  Section 1: added to the list of encoding states in an 'extended'
      scheme: "or perhaps further encoding states as suggested in
      draft-westberg-pcn-load-control"

   o  Section 2: added definition for PCN-colouring (to clarify that the
      term is used consistently differently from 'PCN-marking')

   o  Section 6.1 and 6.2: added "(others might be possible)" before the
      list of high level approaches for making flow admission
      (termination) decisions.

   o  Section 6.2: corrected a significant typo in 2nd bullet (more ->
      less)

   o  Section 6.3: corrected a couple of significant typos in Figure 2

   o  Section 6.5 (PCN-traffic) re-written for clarity.  Non PCN-traffic
      contributing to PCN meters is now given as an example (there may
      be cases where don't need to meter it).

   o  Section 7.7: added to the text about encapsulation being done
      within the PCN-domain: "Note: A tunnel will not provide this
      behaviour if it complies with [RFC3168] tunnelling in either mode,
      but it will if it complies with [RFC4301] IPSec tunnelling."

   o  Section 7.7: added mention of [RFC4301] to the text about
      decapsulation being done within the PCN-domain.

   o  Section 8: deleted the text about design goals, since this is
      already covered adequately earlier eg in S3.




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   o  Section 11: replaced the last sentence of bullet 1 by "There is
      nothing specific to PCN."

   o  Appendix: added to open issues: possibility of automatically and
      periodically probing.

   o  References: Split out Normative references (RFC2474 & RFC3246).

12.7.  Changes from -04 to -05

   Minor nits removed as follows:

   o  Further minor changes to reflect that baseline encoding is
      consensus, standards track document, whilst there can be
      (experimental track) encoding extensions

   o  Traffic conditioning updated to reflect discussions in Dublin,
      mainly that PCN-interior-nodes don't police PCN-traffic (so
      deleted bullet in S7.1) and that it is not advised to have non
      PCN-traffic that shares the same capacity (on a link) as PCN-
      traffic (so added bullet in S6.5)

   o  Probing moved into Appendix A and deleted the 'third viewpoint'
      (admission control based on the marking of a single packet like an
      RSVP PATH message) - since this isn't really probing, and in any
      case is already mentioned in S6.1.

   o  Minor changes to S9 Operations and management - mainly to reflect
      that consensus on marking behaviour has simplified things so eg
      there are fewer parameters to configure.

   o  A few terminology-related errors expunged, and two pictures added
      to help.

   o  Re-phrased the claim about the natural decision point in S7.4

   o  Clarified that extended encoding schemes need to explain their
      interactions with (or assumptions about) tunnelling (S7.7) and how
      they meet the guidelines of BCP124 (S6.6)

   o  Corrected the third bullet in S6.2 (to reflect consensus about
      PCN-marking)

12.8.  Changes from -03 to -04

   o  Minor changes throughout to reflect the consensus call about PCN-
      marking (as reflected in [PCN08-2]).




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   o  Minor changes throughout to reflect the current decisions about
      encoding (as reflected in [PCN08-1] and [Moncaster08]).

   o  Introduction: re-structured to create new sections on Benefits,
      Deployment scenarios and Assumptions.

   o  Introduction: Added pointers to other PCN documents.

   o  Terminology: changed PCN-lower-rate to PCN-threshold-rate and PCN-
      upper-rate to PCN-excess-rate; excess-rate-marking to excess-
      traffic-marking.

   o  Benefits: added bullet about SRLGs.

   o  Deployment scenarios: new section combining material from various
      places within the document.

   o  S6 (high level functional architecture): re-structured and edited
      to improve clarity, and reflect the latest PCN-marking and
      encoding drafts.

   o  S6.4: added claim that the most natural place to make an admission
      decision is a PCN-egress-node.

   o  S6.5: updated the bullet about non-PCN-traffic that uses the same
      DSCP as PCN-traffic.

   o  S6.6: added a section about backwards compatibility with respect
      to [RFC4774].

   o  Appendix A: added bullet about end-to-end PCN.

   o  Probing: moved to Appendix B.

   o  Other minor clarifications, typos etc.

12.9.  Changes from -02 to -03

   o  Abstract: Clarified by removing the term 'aggregated'.  Follow-up
      clarifications later in draft: S1: expanded PCN-egress-nodes
      bullet to mention case where the PCN-feedback-information is about
      one (or a few) PCN-marks, rather than aggregated information; S3
      clarified PCN-meter; S5 minor changes; conclusion.

   o  S1: added a paragraph about how the PCN-domain looks to the
      outside world (essentially it looks like a Diffserv domain).





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   o  S2: tweaked the PCN-traffic terminology bullet: changed PCN
      traffic classes to PCN behaviour aggregates, to be more in line
      with traditional Diffserv jargon (-> follow-up changes later in
      draft); included a definition of PCN-flows (and corrected a couple
      of 'PCN microflows' to 'PCN-flows' later in draft)

   o  S3.5: added possibility of downgrading to best effort, where PCN-
      packets arrive at PCN-ingress-node already ECN marked (CE or ECN
      nonce)

   o  S4: added note about whether talk about PCN operating on an
      interface or on a link.  In S8.1 (OAM) mentioned that PCN
      functionality needs to be configured consistently on either the
      ingress or the egress interface of PCN-nodes in a PCN-domain.

   o  S5.2: clarified that signalling protocol installs flow filter spec
      at PCN-ingress-node (& updates after possible re-route)

   o  S5.6: addressing: clarified

   o  S5.7: added tunnelling issue of N^2 scaling if you set up a mesh
      of tunnels between PCN-boundary-nodes

   o  S7.3: Clarified the "third viewpoint" of probing (always probe).

   o  S8.1: clarified that SNMP is only an example; added note that an
      operator may be able to not run PCN on some PCN-interior-nodes, if
      it knows that these links will never become (pre-)congested; added
      note that it may be possible to have different PCN-boundary-node
      behaviours for different ingress-egress-aggregates within the same
      PCN-domain.

   o  Appendix: Created an Appendix about "Possible work items beyond
      the scope of the current PCN WG Charter".  Material moved from
      near start of S3 and elsewhere throughout draft.  Moved text about
      centralised decision node to Appendix.

   o  Other minor clarifications.

12.10.  Changes from -01 to -02

   o  S1: Benefits: provisioning bullet extended to stress that PCN does
      not use RFC2475-style traffic conditioning.

   o  S1: Deployment models: mentioned, as variant of PCN-domain
      extending to end nodes, that may extend to LAN edge switch.





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   o  S3.1: Trust Assumption: added note about not needing PCN-marking
      capability if known that an interface cannot become pre-congested.

   o  S4: now divided into sub-sections

   o  S4.1: Admission control: added second proposed method for how to
      decide to block new flows (PCN-egress-node receives one (or
      several) PCN-marked packets).

   o  S5: Probing sub-section removed.  Material now in new S7.

   o  S5.6: Addressing: clarified how PCN-ingress-node can discover
      address of PCN-egress-node

   o  S5.6: Addressing: centralised node case, added that PCN-ingress-
      node may need to know address of PCN-egress-node

   o  S5.8: Tunnelling: added case of "partially PCN-capable tunnel" and
      degraded bullet on this in S6 (Open Issues)

   o  S7: Probing: new section.  Much more comprehensive than old S5.5.

   o  S8: Operations and Management: substantially revised.

   o  other minor changes not affecting semantics

12.11.  Changes from -00 to -01

   In addition to clarifications and nit squashing, the main changes
   are:

   o  S1: Benefits: added one about provisioning (and contrast with
      Diffserv SLAs)

   o  S1: Benefits: clarified that the objective is also to stop PCN-
      packets being significantly delayed (previously only mentioned not
      dropping packets)

   o  S1: Deployment models: added one where policing is done at ingress
      of access network and not at ingress of PCN-domain (assume trust
      between networks)

   o  S1: Deployment models: corrected MPLS-TE to MPLS

   o  S2: Terminology: adjusted definition of PCN-domain

   o  S3.5: Other assumptions: corrected, so that two assumptions (PCN-
      nodes not performing ECN and PCN-ingress-node discarding arriving



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      CE packet) only apply if the PCN WG decides to encode PCN-marking
      in the ECN-field.

   o  S4 & S5: changed PCN-marking algorithm to marking behaviour

   o  S4: clarified that PCN-interior-node functionality applies for
      each outgoing interface, and added clarification: "The
      functionality is also done by PCN-ingress-nodes for their outgoing
      interfaces (ie those 'inside' the PCN-domain)."

   o  S4 (near end): altered to say that a PCN-node "should" dedicate
      some capacity to lower priority traffic so that it isn't starved
      (was "may")

   o  S5: clarified to say that PCN functionality is done on an
      'interface' (rather than on a 'link')

   o  S5.2: deleted erroneous mention of service level agreement

   o  S5.5: Probing: re-written, especially to distinguish probing to
      test the ingress-egress-aggregate from probing to test a
      particular ECMP path.

   o  S5.7: Addressing: added mention of probing; added that in the case
      where traffic is always tunnelled across the PCN-domain, add a
      note that he PCN-ingress-node needs to know the address of the
      PCN-egress-node.

   o  S5.8: Tunnelling: re-written, especially to provide a clearer
      description of copying on tunnel entry/exit, by adding explanation
      (keeping tunnel encaps/decaps and PCN-marking orthogonal),
      deleting one bullet ("if the inner header's marking state is more
      sever then it is preserved" - shouldn't happen), and better
      referencing of other IETF documents.

   o  S6: Open issues: stressed that "NOTE: Potential solutions are out
      of scope for this document" and edited a couple of sentences that
      were close to solution space.

   o  S6: Open issues: added one about scenarios with only one tunnel
      endpoint in the PCN domain .

   o  S6: Open issues: ECMP: added under-admission as another potential
      risk

   o  S6: Open issues: added one about "Silent at start"





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   o  S10: Conclusions: a small conclusions section added


13.  References

13.1.  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.

   [RFC3246]  Davie, B., Charny, A., Bennet, J., Benson, K., Le Boudec,
              J., Courtney, W., Davari, S., Firoiu, V., and D.
              Stiliadis, "An Expedited Forwarding PHB (Per-Hop
              Behavior)", RFC 3246, March 2002.

13.2.  Informative References

   [RFC1633]  Braden, B., Clark, D., and S. Shenker, "Integrated
              Services in the Internet Architecture: an Overview",
              RFC 1633, June 1994.

   [RFC2205]  Braden, B., Zhang, L., Berson, S., Herzog, S., and S.
              Jamin, "Resource ReSerVation Protocol (RSVP) -- Version 1
              Functional Specification", RFC 2205, September 1997.

   [RFC2211]  Wroclawski, J., "Specification of the Controlled-Load
              Network Element Service", RFC 2211, September 1997.

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

   [RFC2747]  Baker, F., Lindell, B., and M. Talwar, "RSVP Cryptographic
              Authentication", RFC 2747, January 2000.

   [RFC2753]  Yavatkar, R., Pendarakis, D., and R. Guerin, "A Framework
              for Policy-based Admission Control", RFC 2753,
              January 2000.

   [RFC2983]  Black, D., "Differentiated Services and Tunnels",
              RFC 2983, October 2000.

   [RFC2998]  Bernet, Y., Ford, P., Yavatkar, R., Baker, F., Zhang, L.,
              Speer, M., Braden, R., Davie, B., Wroclawski, J., and E.
              Felstaine, "A Framework for Integrated Services Operation
              over Diffserv Networks", RFC 2998, November 2000.



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

   [RFC3270]  Le Faucheur, F., Wu, L., Davie, B., Davari, S., Vaananen,
              P., Krishnan, R., Cheval, P., and J. Heinanen, "Multi-
              Protocol Label Switching (MPLS) Support of Differentiated
              Services", RFC 3270, May 2002.

   [RFC3393]  Demichelis, C. and P. Chimento, "IP Packet Delay Variation
              Metric for IP Performance Metrics (IPPM)", RFC 3393,
              November 2002.

   [RFC3411]  Harrington, D., Presuhn, R., and B. Wijnen, "An
              Architecture for Describing Simple Network Management
              Protocol (SNMP) Management Frameworks", STD 62, RFC 3411,
              December 2002.

   [RFC3726]  Brunner, M., "Requirements for Signaling Protocols",
              RFC 3726, April 2004.

   [RFC4216]  Zhang, R. and J. Vasseur, "MPLS Inter-Autonomous System
              (AS) Traffic Engineering (TE) Requirements", RFC 4216,
              November 2005.

   [RFC4301]  Kent, S. and K. Seo, "Security Architecture for the
              Internet Protocol", RFC 4301, December 2005.

   [RFC4303]  Kent, S., "IP Encapsulating Security Payload (ESP)",
              RFC 4303, December 2005.

   [RFC4594]  Babiarz, J., Chan, K., and F. Baker, "Configuration
              Guidelines for DiffServ Service Classes", RFC 4594,
              August 2006.

   [RFC4656]  Shalunov, S., Teitelbaum, B., Karp, A., Boote, J., and M.
              Zekauskas, "A One-way Active Measurement Protocol
              (OWAMP)", RFC 4656, September 2006.

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

   [RFC4778]  Kaeo, M., "Operational Security Current Practices in
              Internet Service Provider Environments", RFC 4778,
              January 2007.

   [RFC5129]  Davie, B., Briscoe, B., and J. Tay, "Explicit Congestion



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              Marking in MPLS", RFC 5129, January 2008.

   [RFC5462]  Andersson, L. and R. Asati, "Multiprotocol Label Switching
              (MPLS) Label Stack Entry: "EXP" Field Renamed to "Traffic
              Class" Field", RFC 5462, February 2009.

   [P.800]    "Methods for subjective determination of transmission
              quality", ITU-T Recommendation P.800, August 1996.

   [Y.1541]   "Network Performance Objectives for IP-based Services",
              ITU-T Recommendation Y.1541, February 2006.

   [PCN08-1]  "Baseline Encoding and Transport of Pre-Congestion
              Information (work in progress)", Oct 2008.

   [PCN08-2]  "Metering and marking behaviour of PCN-nodes (work in
              progress)", Oct 2008.

   [PWE3-08]  "Pseudowire Congestion Control Framework (work in
              progress)", May 2008.

   [Babiarz06]
              "SIP Controlled Admission and Preemption (work in
              progress)", Oct 2006.

   [Behringer07]
              "Applicability of Keying Methods for RSVP Security (work
              in progress)", Nov 2007.

   [Briscoe06]
              "An edge-to-edge Deployment Model for Pre-Congestion
              Notification: Admission Control over a Diffserv Region
              (work in progress)", October 2006.

   [Briscoe08-1]
              "Emulating Border Flow Policing using Re-PCN on Bulk Data
              (work in progress)", Sept 2008.

   [Briscoe08-2]
              "Tunnelling of Congestion Notification (work in
              progress)", July 2008.

   [Charny07-1]
              "Comparison of Proposed PCN Approaches (work in
              progress)", November 2007.

   [Charny07-2]
              "Pre-Congestion Notification Using Single Marking for



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              Admission and Termination (work in progress)",
              November 2007.

   [Charny07-3]
              "Email to PCN WG mailing list", November 2007, <http://
              www1.ietf.org/mail-archive/web/pcn/current/msg00871.html>.

   [Charny08]
              "Email to PCN WG mailing list", March 2008, <http://
              www1.ietf.org/mail-archive/web/pcn/current/msg01359.html>.

   [Eardley07]
              "Email to PCN WG mailing list", October 2007, <http://
              www1.ietf.org/mail-archive/web/pcn/current/msg00831.html>.

   [Hancock02]
              "Slide 14 of 'NSIS: An Outline Framework for QoS
              Signalling'", May 2002, <http://www-nrc.nokia.com/sua/
              nsis/interim/nsis-framework-outline.ppt>.

   [Iyer03]   "An approach to alleviate link overload as observed on an
              IP backbone", IEEE INFOCOM , 2003,
              <http://www.ieee-infocom.org/2003/papers/10_04.pdf>.

   [Lefaucheur06]
              "RSVP Extensions for Admission Control over Diffserv using
              Pre-congestion Notification (PCN) (work in progress)",
              June 2006.

   [Menth07]  "PCN-Based Resilient Network Admission Control: The Impact
              of a Single Bit"", Technical Report , 2007, <http://
              www3.informatik.uni-wuerzburg.de/staff/menth/Publications/
              Menth07-PCN-Config.pdf>.

   [Menth08-1]
              "Edge-Assisted Marked Flow Termination (work in
              progress)", February 2008.

   [Menth08-2]
              "PCN Encoding for Packet-Specific Dual Marking (PSDM)
              (work in progress)", July 2008.

   [Menth08-3]
              "PCN-Based Admission Control and Flow Termination", 2008,
              <http://www3.informatik.uni-wuerzburg.de/staff/menth/
              Publications/Menth08-PCN-Comparison.pdf>.

   [Moncaster08]



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              "A three state extended PCN encoding scheme (work in
              progress)", June 2008.

   [Sarker08]
              "Usecases and Benefits of end to end ECN support in PCN
              Domains (work in progress)", November 2008.

   [Songhurst06]
              "Guaranteed QoS Synthesis for Admission Control with
              Shared Capacity", BT Technical Report TR-CXR9-2006-001,
              Feburary 2006, <http://www.cs.ucl.ac.uk/staff/B.Briscoe/
              projects/ipe2eqos/gqs/papers/GQS_shared_tr.pdf>.

   [Style]    "Guardian Style", Note:  This document uses the
              abbreviations 'ie' and 'eg' (not 'i.e.' and 'e.g.'), as in
              many style guides, eg, 2007,
              <http://www.guardian.co.uk/styleguide/>.

   [Tsou08]   "Applicability Statement for the Use of Pre-Congestion
              Notification in a Resource-Controlled Network (work in
              progress)", November 2008.

   [Westberg08]
              "LC-PCN: The Load Control PCN Solution (work in
              progress)", November 2008.


Appendix A.  Possible future work items

   This section mentions some topics that are outside the PCN WG's
   current charter, but which have been mentioned as areas of interest.
   They might be work items for: the PCN WG after a future re-
   chartering; some other IETF WG; another standards body; an operator-
   specific usage that is not standardised.

   NOTE: it should be crystal clear that this section discusses
   possibilities only.

   The first set of possibilities relate to the restrictions described
   in Section 6.3:

   o  a single PCN-domain encompasses several autonomous systems that do
      not trust each other, perhaps by using a mechanism like re-PCN,
      [Briscoe08-1].

   o  not all the nodes run PCN.  For example, the PCN-domain is a
      multi-site enterprise network.  The sites are connected by a VPN
      tunnel; although PCN doesn't operate inside the tunnel, the PCN



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      mechanisms still work properly because of the good QoS on the
      virtual link (the tunnel).  Another example is that PCN is
      deployed on the general Internet (ie widely but not universally
      deployed).

   o  applying the PCN mechanisms to other types of traffic, ie beyond
      inelastic traffic.  For instance, applying the PCN mechanisms to
      traffic scheduled with the Assured Forwarding per-hop behaviour.
      One example could be flow-rate adaptation by elastic applications
      that adapt according to the pre-congestion information.

   o  the aggregation assumption doesn't hold, because the link capacity
      is too low.  Measurement-based admission control is less accurate,
      with a greater risk of over-admission for instance.

   o  the applicability of PCN mechanisms for emergency use (911, GETS,
      WPS, MLPP, etc.)

   Other possibilities include:

   o  Probing.  This is discussed in Section A.1 below.

   o  The PCN-domain extends to the end users.  The scenario is
      described in [Babiarz06].  The end users need to be trusted to do
      their own policing.  If there is sufficient traffic, then the
      aggregation assumption may hold.  A variant is that the PCN-domain
      extends out as far as the LAN edge switch.

   o  indicating pre-congestion through signalling messages rather than
      in-band (in the form of PCN-marked packets)

   o  the decision-making functionality is at a centralised node rather
      than at the PCN-boundary-nodes.  This requires that the PCN-
      egress-node signals PCN-feedback-information to the centralised
      node, and that the centralised node signals to the PCN-ingress-
      node the decision about admission (or termination).  It may need
      the centralised node and the PCN-boundary-nodes to be configured
      with each other's addresses.  The centralised case is described
      further in [Tsou08].

   o  Signalling extensions for specific protocols (eg RSVP, NSIS).  For
      example: the details of how the signalling protocol installs the
      flowspec at the PCN-ingress-node for an admitted PCN-flow; and how
      the signalling protocol carries the PCN-feedback-information.
      Perhaps also for other functions such as: coping with failure of a
      PCN-boundary-node ([Briscoe06] considers what happens if RSVP is
      the QoS signalling protocol); establishing a tunnel across the
      PCN-domain if it is necessary to carry ECN marks transparently.



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   o  Policing by the PCN-ingress-node may not be needed if the PCN-
      domain can trust that the upstream network has already policed the
      traffic on its behalf.

   o  PCN for Pseudowire: PCN may be used as a congestion avoidance
      mechanism for edge to edge pseudowire emulations [PWE3-08].

   o  PCN for MPLS: [RFC3270] defines how to support the Diffserv
      architecture in MPLS networks (Multi-protocol label switching).
      [RFC5129] describes how to add PCN for admission control of
      microflows into a set of MPLS aggregates.  PCN-marking is done in
      MPLS's EXP field (which [RFC5462] re-names the Class of Service
      (CoS) field).

   o  PCN for Ethernet: Similarly, it may be possible to extend PCN into
      Ethernet networks, where PCN-marking is done in the Ethernet
      header.  NOTE: Specific consideration of this extension is outside
      the IETF's remit.

A.1.  Probing

A.1.1.  Introduction

   Probing is a potential mechanism to assist admission control.

   PCN's admission control, as described so far, is essentially a
   reactive mechanism where the PCN-egress-node monitors the pre-
   congestion level for traffic from each PCN-ingress-node; if the level
   rises then it blocks new flows on that ingress-egress-aggregate.
   However, it's possible that an ingress-egress-aggregate carries no
   traffic, and so the PCN-egress-node can't make an admission decision
   using the usual method described earlier.

   One approach is to be "optimistic" and simply admit the new flow.
   However it's possible to envisage a scenario where the traffic levels
   on other ingress-egress-aggregates are already so high that they're
   blocking new PCN-flows, and admitting a new flow onto this 'empty'
   ingress-egress-aggregate adds extra traffic onto a link that is
   already pre-congested - which may 'tip the balance' so that PCN's
   flow termination mechanism is activated or some packets are dropped.
   This risk could be lessened by configuring on each link sufficient
   'safety margin' above the PCN-threshold-rate.

   An alternative approach is to make PCN a more proactive mechanism.
   The PCN-ingress-node explicitly determines, before admitting the
   prospective new flow, whether the ingress-egress-aggregate can
   support it.  This can be seen as a "pessimistic" approach, in
   contrast to the "optimism" of the approach above.  It involves



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   probing: a PCN-ingress-node generates and sends probe packets in
   order to test the pre-congestion level that the flow would
   experience.

   One possibility is that a probe packet is just a dummy data packet,
   generated by the PCN-ingress-node and addressed to the PCN-egress-
   node.

A.1.2.  Probing functions

   The probing functions are:

   o  Make decision that probing is needed.  As described above, this is
      when the ingress-egress-aggregate (or the ECMP path - Section 6.4)
      carries no PCN-traffic.  An alternative is always to probe, ie
      probe before admitting every PCN-flow.

   o  (if required) Communicate the request that probing is needed - the
      PCN-egress-node signals to the PCN-ingress-node that probing is
      needed

   o  (if required) Generate probe traffic - the PCN-ingress-node
      generates the probe traffic.  The appropriate number (or rate) of
      probe packets will depend on the PCN-metering algorithm; for
      example an excess-traffic-metering algorithm triggers fewer PCN-
      marks than a threshold-metering algorithm, and so will need more
      probe packets.

   o  Forward probe packets - as far as PCN-interior-nodes are
      concerned, probe packets are handled the same as (ordinary data)
      PCN-packets, in terms of routing, scheduling and PCN-marking.

   o  Consume probe packets - the PCN-egress-node consumes probe packets
      to ensure that they don't travel beyond the PCN-domain.

A.1.3.  Discussion of rationale for probing, its downsides and open
        issues

   It is an unresolved question whether probing is really needed, but
   two viewpoints have been put forward as to why it is useful.  The
   first is perhaps the most obvious: there is no PCN-traffic on the
   ingress-egress-aggregate.  The second assumes that multipath routing
   ECMP is running in the PCN-domain.  We now consider each in turn.

   The first viewpoint assumes the following:






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   o  There is no PCN-traffic on the ingress-egress-aggregate (so a
      normal admission decision cannot be made).

   o  Simply admitting the new flow has a significant risk of leading to
      overload: packets dropped or flows terminated.

   On the former bullet, [Eardley07] suggests that, during the future
   busy hour of a national network with about 100 PCN-boundary-nodes,
   there are likely to be significant numbers of aggregates with very
   few flows under nearly all circumstances.

   The latter bullet could occur if new flows start on many of the empty
   ingress-egress-aggregates, which together overload a link in the PCN-
   domain.  To be a problem this would probably have to happen in a
   short time period (flash crowd) because, after the reaction time of
   the system, other (non-empty) ingress-egress-aggregates that pass
   through the link will measure pre-congestion and so block new flows.
   Also, flows naturally end anyway.

   The downsides of probing for this viewpoint are:

   o  Probing adds delay to the admission control process.

   o  Sufficient probing traffic has to be generated to test the pre-
      congestion level of the ingress-egress-aggregate.  But the probing
      traffic itself may cause pre-congestion, causing other PCN-flows
      to be blocked or even terminated - and in the flash crowd scenario
      there will be probing on many ingress-egress-aggregates.

   The second viewpoint applies in the case where there is multipath
   routing (ECMP) in the PCN-domain.  Note that ECMP is often used on
   core networks.  There are two possibilities:

   (1) If admission control is based on measurements of the ingress-
   egress-aggregate, then the viewpoint that probing is useful assumes:

   o  there's a significant chance that the traffic is unevenly balanced
      across the ECMP paths, and hence there's a significant risk of
      admitting a flow that should be blocked (because it follows an
      ECMP path that is pre-congested) or blocking a flow that should be
      admitted.

   o  Note: [Charny07-3] suggests unbalanced traffic is quite possible,
      even with quite a large number of flows on a PCN-link (eg 1000)
      when Assumption 3 (aggregation) is likely to be satisfied.

   (2) If admission control is based on measurements of pre-congestion
   on specific ECMP paths, then the viewpoint that probing is useful



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   assumes:

   o  There is no PCN-traffic on the ECMP path on which to base an
      admission decision.

   o  Simply admitting the new flow has a significant risk of leading to
      overload.

   o  The PCN-egress-node can match a packet to an ECMP path.

   o  Note: This is similar to the first viewpoint and so similarly
      could occur in a flash crowd if a new flow starts more-or-less
      simultaneously on many of the empty ECMP paths.  Because there are
      several (sometimes many) ECMP paths between each pair of PCN-
      boundary-nodes, it's presumably more likely that an ECMP path is
      'empty' than an ingress-egress-aggregate is.  To constrain the
      number of ECMP paths, a few tunnels could be set-up between each
      pair of PCN-boundary-nodes.  Tunnelling also solves the issue in
      the bullet immediately above (which is otherwise hard because an
      ECMP routing decision is made independently on each node).

   The downsides of probing for this viewpoint are:

   o  Probing adds delay to the admission control process.

   o  Sufficient probing traffic has to be generated to test the pre-
      congestion level of the ECMP path.  But there's the risk that the
      probing traffic itself may cause pre-congestion, causing other
      PCN-flows to be blocked or even terminated.

   o  The PCN-egress-node needs to consume the probe packets to ensure
      they don't travel beyond the PCN-domain, since they might confuse
      the destination end node.  This is non-trivial, since probe
      packets are addressed to the destination end node, in order to
      test the relevant ECMP path (ie they are not addressed to the PCN-
      egress-node, unlike the first viewpoint above).

   The open issues associated with this viewpoint include:

   o  What rate and pattern of probe packets does the PCN-ingress-node
      need to generate, so that there's enough traffic to make the
      admission decision?

   o  What difficulty does the delay (whilst probing is done), and
      possible packet drops, cause applications?

   o  Can the delay be alleviated by automatically and periodically
      probing on the ingress-egress-aggregate?  Or does this add too



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      much overhead?

   o  Are there other ways of dealing with the flash crowd scenario?
      For instance, by limiting the rate at which new flows are
      admitted; or perhaps by a PCN-egress-node blocking new flows on
      its empty ingress-egress-aggregates when its non-empty ones are
      pre-congested.

   o  (Second viewpoint only) How does the PCN-egress-node disambiguate
      probe packets from data packets (so it can consume the former)?
      The PCN-egress-node must match the characteristic setting of
      particular bits in the probe packet's header or body - but these
      bits must not be used by any PCN-interior-node's ECMP algorithm.
      In the general case this isn't possible, but it should be possible
      for a typical ECMP algorithm (which examines: the source and
      destination IP addresses and port numbers, the protocol ID, and
      the DSCP).


Author's Address

   Philip Eardley
   BT
   B54/77, Sirius House Adastral Park Martlesham Heath
   Ipswich, Suffolk  IP5 3RE
   United Kingdom

   Email: philip.eardley@bt.com























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