Congestion and Pre-Congestion                   Philip. Eardley (Editor)
Notification Working Group                                            BT
Internet-Draft                                             July 14, 2008
Intended status: Informational
Expires: January 15, 2009

                Pre-Congestion Notification Architecture

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Copyright Notice

   Copyright (C) The IETF Trust (2008).


   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 established
   inelastic flows within a single DiffServ domain.

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Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  4
   2.  Terminology  . . . . . . . . . . . . . . . . . . . . . . . . .  5
   3.  Benefits . . . . . . . . . . . . . . . . . . . . . . . . . . .  6
   4.  Deployment scenarios . . . . . . . . . . . . . . . . . . . . .  8
   5.  Assumptions and constraints on scope . . . . . . . . . . . . . 10
     5.1.  Assumption 1: Trust and support of PCN - controlled
           environment  . . . . . . . . . . . . . . . . . . . . . . . 11
     5.2.  Assumption 2: Real-time applications . . . . . . . . . . . 11
     5.3.  Assumption 3: Many flows and additional load . . . . . . . 12
     5.4.  Assumption 4: Emergency use out of scope . . . . . . . . . 12
   6.  High-level functional architecture . . . . . . . . . . . . . . 12
     6.1.  Flow admission . . . . . . . . . . . . . . . . . . . . . . 14
     6.2.  Flow termination . . . . . . . . . . . . . . . . . . . . . 15
     6.3.  Flow admission and flow termination when there are
           only two PCN encoding states . . . . . . . . . . . . . . . 16
     6.4.  Information transport  . . . . . . . . . . . . . . . . . . 16
     6.5.  PCN-traffic  . . . . . . . . . . . . . . . . . . . . . . . 17
     6.6.  Backwards compatibility  . . . . . . . . . . . . . . . . . 17
   7.  Detailed Functional architecture . . . . . . . . . . . . . . . 18
     7.1.  PCN-interior-node functions  . . . . . . . . . . . . . . . 19
     7.2.  PCN-ingress-node functions . . . . . . . . . . . . . . . . 19
     7.3.  PCN-egress-node functions  . . . . . . . . . . . . . . . . 20
     7.4.  Other admission control functions  . . . . . . . . . . . . 20
     7.5.  Other flow termination functions . . . . . . . . . . . . . 21
     7.6.  Addressing . . . . . . . . . . . . . . . . . . . . . . . . 22
     7.7.  Tunnelling . . . . . . . . . . . . . . . . . . . . . . . . 23
     7.8.  Fault handling . . . . . . . . . . . . . . . . . . . . . . 24
   8.  Design goals and challenges  . . . . . . . . . . . . . . . . . 24
   9.  Operations and Management  . . . . . . . . . . . . . . . . . . 27
     9.1.  Configuration OAM  . . . . . . . . . . . . . . . . . . . . 27
       9.1.1.  System options . . . . . . . . . . . . . . . . . . . . 28
       9.1.2.  Parameters . . . . . . . . . . . . . . . . . . . . . . 29
     9.2.  Performance & Provisioning OAM . . . . . . . . . . . . . . 31
     9.3.  Accounting OAM . . . . . . . . . . . . . . . . . . . . . . 32
     9.4.  Fault OAM  . . . . . . . . . . . . . . . . . . . . . . . . 32
     9.5.  Security OAM . . . . . . . . . . . . . . . . . . . . . . . 33
   10. IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 34
   11. Security considerations  . . . . . . . . . . . . . . . . . . . 34
   12. Conclusions  . . . . . . . . . . . . . . . . . . . . . . . . . 35
   13. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 35
   14. Comments Solicited . . . . . . . . . . . . . . . . . . . . . . 36
   15. Changes  . . . . . . . . . . . . . . . . . . . . . . . . . . . 36
     15.1. Changes from -03 to -04  . . . . . . . . . . . . . . . . . 36

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     15.2. Changes from -02 to -03  . . . . . . . . . . . . . . . . . 37
     15.3. Changes from -01 to -02  . . . . . . . . . . . . . . . . . 38
     15.4. Changes from -00 to -01  . . . . . . . . . . . . . . . . . 39
   16. Appendix A: Possible work items beyond the scope of the
       current PCN WG Charter . . . . . . . . . . . . . . . . . . . . 40
   17. Appendix B: Probing  . . . . . . . . . . . . . . . . . . . . . 42
     17.1. Introduction . . . . . . . . . . . . . . . . . . . . . . . 42
     17.2. Probing functions  . . . . . . . . . . . . . . . . . . . . 43
     17.3. Discussion of rationale for probing, its downsides and
           open issues  . . . . . . . . . . . . . . . . . . . . . . . 43
   18. Informative References . . . . . . . . . . . . . . . . . . . . 46
   Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 51
   Intellectual Property and Copyright Statements . . . . . . . . . . 52

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1.  Introduction

   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 [RFC2475].  This document defines an
   architecture for implementing two mechanisms to protect the quality
   of service of established inelastic flows within a single DiffServ
   domain, where all boundary and interior nodes are PCN-enabled and
   trust each other for correct PCN operation.  Flow admission control
   determines whether a new flow should be admitted, in order to protect
   the QoS of existing PCN-flows in normal circumstances.  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.  Therefore we also propose a mechanism for flow termination,
   which removes enough traffic in order to protect the QoS of the
   remaining PCN-flows.

   As a fundamental building block to enable these two mechanisms, PCN-
   interior-nodes generate, encode and transport pre-congestion
   information towards the PCN-egress-nodes.  Two rates, a PCN-
   threshold-rate and a PCN-excess-rate, are associated with each link
   of the PCN-domain.  Each rate is used by a marking behaviour that
   determines how and when PCN-packets are marked, and how the markings
   are encoded in packet headers.  Overall the aim is to enable PCN-
   nodes to give an "early warning" of potential congestion before there
   is any significant build-up of PCN-packets in the queue.

   PCN-boundary-nodes convert measurements of these PCN-markings into
   decisions about flow admission and termination.  The admission
   control mechanism limits the PCN-traffic on each link to *roughly*
   its PCN-threshold-rate and the flow termination mechanism limits the
   PCN-traffic on each link to *roughly* its PCN-excess-rate.

   This document describes the PCN architecture and outlines some
   benefits, deployment scenarios, assumptions and terminology for PCN.
   The behaviour of PCN-interior-nodes is standardised in three
   documents, which are summarised in this
   document.[I-D.eardley-pcn-marking-behaviour] standardises the two
   marking behaviours of PCN-nodes: threshold marking and excess traffic
   marking.  Threshold marking marks all PCN-packets if the PCN traffic
   rate is greater than a first configured rate, "PCN-threshold-rate".
   Excess traffic marking marks a proportion of PCN-packets, such that
   the amount marked equals the traffic rate in excess of a second
   configured rate, "PCN-excess-rate".  PCN encoding uses a combination
   of the DSCP field and ECN field in the IP header to indicate that a
   packet is a PCN-packet and whether it is PCN-marked.

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   [I-D.moncaster-pcn-baseline-encoding] standardises two PCN encoding
   states (PCN-marked and not PCN-marked) whilst
   [I-D.moncaster-pcn-3-state-encoding] standardises an extended scheme
   with three encoding states (threshold-marked, excess-traffic-marked,
   not PCN-marked) but requires an extra DiffServ codepoint.  PCN
   therefore defines semantics for the ECN field different from the
   default semantics of [RFC3168]; PCN's encoding has been chosen to
   meet the guidelines of BCP124, [RFC4774].  The behaviour of PCN-
   boundary-nodes is described in Informational documents.  Several
   possibilities are outlined in this document; detailed descriptions
   and comparisons are in [I-D.charny-pcn-comparison] and [Menth08].

2.  Terminology

   o  PCN-domain: a PCN-capable domain; a contiguous set of PCN-enabled
      nodes that perform DiffServ scheduling; the complete set of PCN-
      nodes whose PCN-marking can in principle influence decisions about
      flow admission and termination for the PCN-domain, including the
      PCN-egress-nodes which measure these PCN-marks.

   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-

   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) [RFC2475].  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;
      note that a packet that shares the same DSCP as PCN-traffic but
      its ECN field is 00 (Not ECT) is not part of the PCN-BA.

   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 [RFC2475]) or some identifiable collection of

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

   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 marking behaviour that determines whether a packet
      should be PCN-marked with a first encoding, "threshold-marked".
      It's roughly the rate up to which PCN admission control should
      accept new flows.

   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 marking behaviour that determines whether a packet
      should be PCN-marked with a second encoding, "excess-traffic-
      marked".  It's roughly that rate down to which flow termination
      should, if necessary, terminate already admitted flows.

   o  Threshold-marking: a PCN-marking behaviour with the objective that
      all PCN-traffic is marked if the PCN-traffic exceeds the PCN-

   o  Excess-traffic-marking: a PCN-marking behaviour with the objective
      that the amount of PCN-traffic that is PCN-marked is equal to the
      amount that exceeds the PCN-excess-rate.

   o  Pre-congestion: a condition of a link within a PCN-domain in which
      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-feedback-information: information signalled by a PCN-egress-
      node to a PCN-ingress-node or central control node, which is
      needed for the flow admission and flow termination mechanisms.

3.  Benefits

   We believe that the key benefits of the PCN mechanisms described in
   this document 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

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      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: PCN's QoS is decoupled from the
      routing system; hence in general admitted flows can survive
      capacity, routing or topology changes without additional
      signalling, and they don't have to be told (or learn) about such
      changes.  The PCN-threshold-rate on each PCN-node can be chosen
      small enough that admitted traffic can still be carried after a
      rerouting in most failure cases [Menth].  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 [Iyer].

   o  The PCN-marking 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 "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
      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

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      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].  A requirement to fully protect traffic against a
      single SRLG failure requires low utilisation (~10%) of the link
      bandwidth on some links before failure [PCN-email-SRLG].

   o  The PCN-excess-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 that statically define the parameters of the traffic
      that will be accepted from a customer, and so the operator has to
      run the provisioning process 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.

4.  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

   From the perspective of the outside world, a PCN-domain essentially
   looks like a DiffServ domain.  PCN-traffic is either transported
   across it transparently or policed at the PCN-ingress-node (ie
   dropped or carried at a lower QoS).  A couple of differences are
   that: PCN-traffic has better QoS guarantees than normal DiffServ
   traffic (because PCN's mechanisms better protect the QoS of admitted
   flows); and in rare circumstances (failures), 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.

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   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.

   For example, an operator could use just 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 PCN-traffic naturally departs to relieve
   the congestion, all PCN-flows as well as lower priority services will
   be adversely affected.

   Another example is that 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.

   The possibility of deploying just one of PCN's flow admission and
   termination mechanisms is certainly an option when only two PCN
   encoding states are available (PCN-marked and not PCN-marked), as in
   [I-D.moncaster-pcn-baseline-encoding].  Another option in this
   circumstance is to trigger both admission control and flow
   termination from the single type of PCN-marking; the main downside is
   that admission control is less accurate.

   Within the PCN-domain there is some flexibility about where the
   decision making functionality is located.  For admission control, the
   most natural place is the PCN-ingress-node.  For flow termination,
   whether the PCN-ingress-node or PCN-egress-node is more natural
   depends on the mechanism used to convert packet markings into a flow
   termination decision.  These possibilities are outlined more later
   and also discussed elsewhere, such as in [Menth08].  Another
   possibility is that the decision making functionality is at some
   central control node.  This is briefly discussed in Appendix A and
   described in [I-D.tsou-pcn-racf-applic].

   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-

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   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-

   PCN admission control has to fit with the overall approach to
   admission control.  For instance [I-D.briscoe-tsvwg-cl-architecture]
   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 is used instead of RSVP.

   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
   [I-D.sarker-pcn-ecn-pcn-usecases].  Two ways to do this are to tunnel
   all PCN-packets across the PCN-domain, so that the ECN marks is
   carried transparently across the PCN-domain, or to use the three
   state PCN encoding [I-D.moncaster-pcn-3-state-encoding].  This is
   discussed further in Section Section 7.

   Some possible deployment models that are outside the current PCN WG
   Charter are outlined in Appendix A.

5.  Assumptions and constraints on scope

   The scope of PCN is, at least initially (see Appendix A), restricted
   by the following assumptions:

   1.  these components are deployed in a single DiffServ domain, within
       which all PCN-nodes are PCN-enabled and trust each other 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

   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.

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   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.

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

   We assume that the PCN-domain is a controlled environment, ie all the
   nodes in a PCN-domain run PCN and trust each other.  There are
   several reasons for proposing 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 doing 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

   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 but they trust each other to a sufficient
   level, in their handling of PCN-traffic.

   Note: All PCN-nodes need to be trustworthy.  However if it's known
   that an interface cannot become pre-congested then it's 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

5.2.  Assumption 2: Real-time applications

   We assume 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 it sends packets
   at the rate the codec produces them, regardless of the 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).  For instance, the impact of this assumption

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   would be to guide simulations work.

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

5.3.  Assumption 3: Many flows and additional load

   We assume 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 OK 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 things 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

   We do not make explicit assumptions on 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.

5.4.  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 for consideration by the PCN WG.

6.  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 if 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).

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   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 the PCN-packets that they
   carry, nor do the PCN-boundary-nodes care about which PCN-interior-
   nodes its flows traverse.  The objective is to standardise PCN-
   marking behaviour, but potentially produce more than one
   (informational) RFC describing how PCN-boundary-nodes react to PCN-

   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)
   are 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-marking and one for excess-

   o  the encoding(s): a PCN-node PCN-marks a PCN-packet by setting the
      ECN field to 11 and potentially altering the DSCP.

   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 decision is made about whether to admit a
   prospective new flow or whether to terminate existing flow(s).

   PCN-marking needs to be configured on all 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 thing is that a consistent choice is made across the PCN-
   domain to ensure that the PCN mechanisms protect all links.  See
   [I-D.eardley-pcn-marking-behaviour] for further discussion.

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   The objective of the threshold-marking 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 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 algorithm rather
   than its instantaneous behaviour, since the rate 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

   In a PCN-domain the operator may have two or three encoding states
   available.  In both cases the ECN field is set to 11 to indicate PCN-
   marking.  In the former case, one DSCP is used.  In the latter case a
   second DSCP is used, which allows distinct threshold-marks and
   excess-traffic-marks.  The encoding is specified in
   [I-D.moncaster-pcn-baseline-encoding] and

   All the various admission and termination approaches are detailed and
   compared in [I-D.charny-pcn-comparison] and [Menth08].  The
   discussion below is just a brief summary.  It initially assumes there
   are three encoding states available.

6.1.  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-
   threshold-rate, by admitting or blocking prospective new flows, in
   order to protect the QoS of existing PCN-flows.  The PCN-threshold-
   rate is a parameter that can be configured by the operator and will
   be 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.  At a high level two
   approaches are proposed:

   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.  In [I-D.eardley-pcn-architecture] the fraction is
      measured as an EWMA (exponentially weighted moving average) and

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      termed the "congestion level estimate".

   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 is 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.

6.2.  Flow termination

   The objective of PCN's flow termination mechanism is to limit the
   PCN-traffic on each link to *roughly* its PCN-excess-rate, by
   terminating some existing PCN-flows, in order to protect the QoS of
   the remaining PCN-flows.  The PCN-excess-rate is a parameter that can
   be configured by the operator and 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.  At a high level several
   approaches are proposed:

   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.  Also the PCN-ingress-
      node measures the rate of PCN-traffic that is destined for this
      specific PCN-egress-node, and hence it can calculate 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 more traffic than the previous bullet if some nodes are
      dropping PCN-traffic.

   o  Another approach monitors PCN-packets and terminates any PCN-flow
      with an excess-traffic-marked packet.  Compared with the
      approaches above, PCN-marking needs to be done at a reduced rate
      (every "s" bytes of excess traffic) otherwise far too much traffic
      would be terminated.

   Since flow termination is designed for "abnormal" circumstances, it
   is quite likely that some PCN-nodes are congested and hence packets

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   are being dropped and/or significantly queued.  The flow termination
   mechanism must bear this in mind.

   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.

6.3.  Flow admission and 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's using the baseline encoding
   [I-D.moncaster-pcn-baseline-encoding], then an operator has three

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

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

   o  both admission control and flow termination: only the excess-
      traffic-marking algorithm writes PCN-marks, however the configured
      rate (PCN-excess-rate) is set at the rate the admission control
      mechanism needs to limit PCN-traffic to.
      [I-D.charny-pcn-single-marking] 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.

6.4.  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 between the PCN-
   boundary-nodes, 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
   the future PCN WG document(s) about the boundary mechanisms.  The
   signalling could be done by an extension of RSVP or NSIS, for
   instance; protocol work will be done by the relevant WG, but for
   example [I-D.lefaucheur-rsvp-ecn] describes the extensions needed for

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6.5.  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.

   o  The PCN mechanisms may be applied to more than one behaviour
      aggregate which are distinguished by DSCP.  However the current
      PCN encodings, [I-D.moncaster-pcn-baseline-encoding] and
      [I-D.moncaster-pcn-3-state-encoding], only allow one PCN-BA.

   o  There may be traffic that is more important than PCN, perhaps a
      particular application or an operator's control messages.  A PCN-
      node may dedicate capacity to such traffic or priority schedule it
      over PCN.  In the latter case its traffic needs to contribute to
      the PCN meters (ie be metered by the threshold-marking and excess-
      traffic-marking algorithms).

   o  There may be other traffic that uses the same DSCP as PCN-traffic
      but with the ECN field is 00 (Not ECT), and so not subject to PCN-
      marking, nor PCN's admission control and flow termination
      mechanisms..  To quote [I-D.moncaster-pcn-baseline-encoding]: "To
      conserve DSCPs, DiffServ Codepoints SHOULD be chosen that are
      already defined for use with admission controlled traffic, such as
      the Voice-Admit codepoint defined in [voice-admit]."  Since
      scheduling behaviour is coupled with the DSCP only, therefore the
      same scheduling and buffer management rules are applied to non-
      PCN-traffic and PCN-traffic using the same PCN-enabled DSCP.
      There may be no "non-PCN-traffic", but if there is it needs to
      contribute to the PCN meters.

   o  There will be traffic less important than PCN.  For instance best
      effort or assured forwarding traffic.  It will be scheduled at
      lower priority than PCN, and use a separate queue or queues.
      However, a PCN-node should dedicate some capacity to lower
      priority traffic so that it isn't starved.  Such traffic doesn't
      contribute to the PCN meters.

6.6.  Backwards compatibility

   PCN specifies semantics for the ECN field that differ from the
   default semantics of [RFC3168].  BCP124 [RFC4774] gives guidelines
   for specifying alternative semantics for the ECN field.  These are
   discussed in the baseline encoding
   [I-D.moncaster-pcn-baseline-encoding] and extended encoding
   [I-D.moncaster-pcn-3-state-encoding] documents.  In summary, PCN

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   meets these guidelines by:

   o  using a DSCP (or two DSCPs in the extended encoding) to allow PCN-
      nodes to distinguish PCN-traffic that uses the alternative ECN

   o  defining these semantics for use within a controlled region, the

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

   The 'appropriate action' can differ in the case of baseline encoding
   and extended encoding.  In the former, ECN-capable traffic that uses
   the same DSCP as PCN is blocked 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 blocking is needed
   is that the PCN-egress-node clears the ECN field to 00.  The extended
   encoding adds support for end-to-end ECN, since the value of the ECN
   field is preserved across the PCN-domain.  However, PCN-packets that
   get PCN-marked emerge from the PCN-domain with the ECN field set to
   11 (CE).  It may make sense to expose such marks to a rate adaptive
   endpoint.  However, it could violate [RFC4774] if the endpoint
   doesn't understand ECN, and therefore the PCN-domain first needs to
   ensure that the end-to-end transport is ECN capable (probably through

7.  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

   1.  functions needed at PCN-interior-nodes

   2.  functions needed at PCN-ingress-nodes

   3.  functions needed at PCN-egress-nodes

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   4.  other functions needed for flow admission control

   5.  other functions needed for flow termination control

   Note: Probing is covered in Appendix B.

   The section then discusses some other detailed topics:

   1.  addressing

   2.  tunnelling

   3.  fault handling

7.1.  PCN-interior-node functions

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

   o  Packet classify - decide whether an incoming packet is a PCN-
      packet or not.

   o  Packet condition - if the level if traffic is sufficiently high to
      overload the PCN_BA, ie cause real congestion, then drop or
      downgrade PCN-packets.

   o  Meter - measure the 'amount of PCN-traffic'.  The measurement is
      made as an aggregate of all PCN-packets, and not per flow.

   o  Mark - algorithms determine whether to PCN-mark PCN-packets and
      what packet encoding is used.

   The functions are specified in [I-D.eardley-pcn-marking-behaviour]
   and the encodings in [I-D.moncaster-pcn-baseline-encoding] and

7.2.  PCN-ingress-node functions

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

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

   o  Police - police, by dropping or downgrading, any packets received
      with a DSCP demanding PCN transport that do not belong to an
      admitted flow.  Similarly, police packets that are part of a

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      previously admitted flow, to check that the flow keeps to the
      agreed rate or flowspec (eg RFC1633 [RFC1633] for a microflow and
      its NSIS equivalent).

   o  Packet colour - set the DSCP and ECN fields appropriately, see
      [I-D.moncaster-pcn-baseline-encoding] or
      [I-D.moncaster-pcn-3-state-encoding] as appropriate for the PCN-

   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's been
   admitted and to ensure that the flow keeps to the flowspec agreed (eg
   doesn't go at a faster 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-route that
   changes the PCN-ingress-node (see [I-D.briscoe-tsvwg-cl-architecture]
   for an example using RSVP).  Packet colouring allows the rest of the
   PCN-domain to recognise PCN-packets.

7.3.  PCN-egress-node functions

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

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

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

   o  Packet 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.  Alternative
   proposals 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.

7.4.  Other admission control functions

   As well as the functions covered above, other specific admission
   control functions can be performed at a PCN-boundary-node (PCN-
   ingress-node or PCN-egress-node) or at a centralised node, but not at

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   normal PCN-interior-nodes.  The functions are:

   o  Make decision about admission - based on the output of the PCN-
      egress-node's PCN 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.  In either case,
      the admission decision also takes account of policy and
      application layer requirements.

   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 can be
   distributed (we assume the operator would configure which is used):

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

   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 A).

7.5.  Other flow termination functions

   Specific termination control functions can be performed at a PCN-
   boundary-node (PCN-ingress-node or PCN-egress-node) or at a
   centralised node, but not at normal PCN-interior-nodes.  There are
   various possibilities for how the functionality can be distributed,
   similar to those discussed above in the Admission control section;
   the flow termination decision could be made at the PCN-ingress-node,
   the PCN-egress-node or at some centralised node.  The functions are:

   o  PCN-meter at PCN-egress-node - similarly to flow admission, there
      are two types of proposals: 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

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   o  (if required) Communicate PCN-feedback-information to the node
      that makes the flow termination decision.  For example, as in
      [I-D.briscoe-tsvwg-cl-architecture], 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.

   o  Communicate decision about flow termination - signal the decision
      to the node that is able to terminate the flow (which may be
      outside the PCN-domain), and to the policer (PCN-ingress-node
      function) for enforcement of the decision.

7.6.  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)
      ([I-D.lefaucheur-rsvp-ecn]).  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).

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7.7.  Tunnelling

   Tunnels may originate and/or terminate within a PCN-domain.  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 [RFC2983] should be re-
   applied in the PCN context.  In line with this and the approach of
   [RFC4303] and [I-D.briscoe-tsvwg-ecn-tunnel], the following rule is
   applied if encapsulation is done within the PCN-domain:

   o  any PCN-marking is copied into the outer header

   Similarly, in line with the "uniform conceptual model" of [RFC2983]
   and the "full-functionality option" of [RFC3168], 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

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

   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
   packet colouring function (Section 7.3) 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.  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 starts outside a PCN-domain and finishes 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-

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   2.  The tunnel starts inside a PCN-domain and finishes 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.

   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 figure out, the
   characteristics of the other end of the tunnel as part of setting it

   Tunnelling constraints were a major factor in the choice of encoding,
   as explained in [I-D.moncaster-pcn-baseline-encoding] and
   [I-D.moncaster-pcn-3-state-encoding].  A lengthy discussion of all
   the issues associated with layered encapsulation of congestion
   notification (for ECN as well as PCN) is in

7.8.  Fault handling

   If a PCN-interior-node fails (or one of its links), then lower layer
   protection mechanisms or the regular IP routing protocol will
   eventually re-route round 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 take care of things.  As an example
   [I-D.briscoe-tsvwg-cl-architecture] considers what happens if RSVP is
   the QoS signalling protocol.

8.  Design goals and challenges

   Prior work on PCN and similar mechanisms has thrown up a number of

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   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's 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 key design goals for PCN (based on

   o  The PCN-enabled packet forwarding network should be simple,
      scalable and robust

   o  Compatibility with other traffic (ie a proposed solution should
      work well when non-PCN traffic is also present in the network)

   o  Support of different types of real-time traffic (eg should work
      well with CBR and VBR voice and video sources treated together)

   o  Reaction time of the mechanisms should be commensurate with the
      desired application-level requirements (eg a termination mechanism
      needs to terminate flows before significant QoS issues are
      experienced by real-time traffic, and before most users hang up).

   o  Compatibility with different precedence levels of real-time
      applications (eg preferential treatment of higher precedence calls
      over lower precedence calls, [ITU-MLPP]).

   The following are open issues.  They are mainly taken from
   [I-D.briscoe-tsvwg-cl-architecture] 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

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      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: flows are terminated, however their
          path doesn't travel through the (pre-)congested router(s).
          Since flow termination is a 'last resort' that 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
      won't be.  ECMP and signalling interactions are a specific
      instance of a general issue for non-traditional routing combined
      with resource management along a path [Hancock].

   o  Tunnelling: There are scenarios where tunnelling makes it hard 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 flows that should be admitted (or
      terminated) as a pair - for instance a bi-directional voice call
      only makes sense if flows in both directions are admitted.
      However, PCN's 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; 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

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      could be admitted.  The problem may well be second order.

   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's 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's 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.

   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
      mustn't move the PCN-domain directly from no congestion to
      overload.  This 'vulnerability period' may 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.

9.  Operations and Management

   This Section considers operations and management issues, under the
   FCAPS headings: OAM of Faults, Configuration, Accounting, Performance
   and Security.  Provisioning is discussed with performance.

9.1.  Configuration OAM

   This architecture document predates the detailed standards actions of
   the PCN WG.  Here we assume that only inter-operable PCN-marking
   behaviours will be standardised, otherwise we would have to consider
   how to avoid interactions between non inter-operable marking
   behaviours.  However, more diversity in PCN-boundary-node behaviours

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   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 functionality is configured 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

   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

   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 kit from different

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

9.1.1.  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
      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.

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   o  what marking algorithm to use, if an equipment vendor provides a

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

   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 handle that marking.  Therefore all
      PCN-boundary-nodes must be configured the same in this respect.

   o  Where flow admission and termination decisions are made: at the
      PCN-ingress-node, PCN-egress-node or at a centralised node (see
      Section 7).  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.

   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 filterspec; 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.

9.1.2.  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
   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

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   single or multiple failures and the expected load from other classes
   relative to link capacities [Menth].  But once a domain is up and
   running, 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 9.2 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.

   Specific marking algorithms will also depend on further configuration
   parameters.  For instance, threshold-marking will require a threshold
   queue depth and excess-traffic-marking may require a scaling
   parameter.  It will be preferable for each marking algorithm to have
   rules to set defaults for these parameters relative to the reference
   marking rate, 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  any admission control algorithm will at least require a marking
      threshold setting above which it denies admission to new flows;

   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 proposal, [I-D.charny-pcn-single-marking] 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, for
   example the amount by which the PCN-excess-rate is implicitly assumed
   to be above the PCN-threshold-rate.  [I-D.charny-pcn-single-marking]
   discusses in full the impact of this particular proposal on the

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   operation of PCN.

9.2.  Performance & Provisioning OAM

   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 mounted up 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
   re-run 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, [Songhurst] has proposed an
   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 separated from regular

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   fluctuating demand if required.

9.3.  Accounting OAM

   Accounting is only done at trust boundaries so it is out of scope of
   the initial Charter of the PCN WG 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.

9.4.  Fault OAM

   Fault OAM 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 7 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
   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 9.2) 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

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   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.  PCN adds nothing specific to this class of
   problems.  By the time standards are in place, we expect that the PCN
   WG will have ruthlessly removed gratuitous configuration choices.
   However, at the time of writing, the WG is yet to choose between
   multiple competing proposals, so the range of possible options in
   Section 9.1 does seem rather wide compared to the original near-zero
   configuration intent of the architecture.

9.5.  Security OAM

   Security OAM 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 9.2) 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
   MOS 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-

   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 an PCN-egress-node with
      (pre-)congestion markings focused on particular flows, rather than
      randomly distributed throughout the aggregate.

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10.  IANA Considerations

   This memo includes no request to IANA.

11.  Security considerations

   Security considerations essentially come from the Trust Assumption
   (Section 5.1), ie that all PCN-nodes are PCN-enabled and trust each
   other for truthful PCN-marking and transport.  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

   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.  For example when the PCN-
      boundary-node needs to know the contents of the sessions for
      making the admission and termination decisions, or when the
      contents are highly classified, then the security requirements for
      the PCN-boundary-nodes involved will also need to be high.

   o  the PCN-ingress-nodes police packets to ensure a PCN-flow sticks
      within its agreed limit, and to ensure that only PCN-flows which
      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-
      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

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

   o  PCN-marking by the PCN-interior-nodes along the packet forwarding
      path needs to be trusted, because the PCN-boundary-nodes rely on
      this information.  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's 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.

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

   o  the signalling between the PCN-boundary-nodes (and possibly a
      central control node) 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
      [I-D.behringer-tsvwg-rsvp-security-groupkeying] may also be
      useful; for a generic signalling protocol the PCN WG document on
      "Requirements for signalling" will describe the requirements in
      more detail.

   Operational security advice is given in Section 9.5.

12.  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.

13.  Acknowledgements

   This document is a revised version of [I-D.eardley-pcn-architecture].
   Its authors were: P. Eardley, J. Babiarz, K. Chan, A. Charny, R.
   Geib, G. Karagiannis, M. Menth, T. Tsou.  They are therefore
   contributors to this document.

   Thanks to those who've made comments on
   [I-D.eardley-pcn-architecture] and on earlier versions of this draft:
   Lachlan Andrew, Joe Babiarz, Fred Baker, David Black, Steven Blake,
   Bob Briscoe, Jason Canon, Ken Carlberg, Anna Charny, Joachim
   Charzinski, Andras Csaszar, Lars Eggert, Ruediger Geib, Wei Gengyu,
   Robert Hancock, Ingemar Johansson, Georgios Karagiannis, Michael
   Menth, Toby Moncaster, Ben Strulo, Tom Taylor, Hannes Tschofenig,
   Tina Tsou, Lars Westberg, Magnus Westerlund, Delei Yu.  Thanks to Bob
   Briscoe who extensively revised the Operations and Management

   This document is the result of discussions in the PCN WG and
   forerunner activity in the TSVWG.  A number of previous drafts were

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   presented to TSVWG: [I-D.chan-pcn-problem-statement],
   [I-D.briscoe-tsvwg-cl-architecture], [I-D.briscoe-tsvwg-cl-phb],
   [I-D.charny-pcn-single-marking], [I-D.babiarz-pcn-sip-cap],
   [I-D.lefaucheur-rsvp-ecn], [I-D.westberg-pcn-load-control].  The
   authors of them 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.

14.  Comments Solicited

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

15.  Changes

15.1.  Changes from -03 to -04

   o  Minor changes throughout to reflect the consenus call about PCN-
      marking (as reflected in [I-D.eardley-pcn-marking-behaviour]).

   o  Minor changes throughout to reflect the current decisions about
      encoding (as reflected in [I-D.moncaster-pcn-baseline-encoding]and

   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-

   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.

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   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.

15.2.  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).

   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

   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).

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   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

   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.

15.3.  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.

   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

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15.4.  Changes from -00 to -01

   In addition to clarifications and nit squashing, the main changes

   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
      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

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   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

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

   o  S10: Conclusions: a small conclusions section added

16.  Appendix A: Possible work items beyond the scope of the current PCN
     WG Charter

   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's not standardised.

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

   The first set of possibilities relate to the restrictions on scope
   imposed by the PCN WG Charter (see Section 3):

   o  a single PCN-domain encompasses several autonomous systems that
      don't trust each other (perhaps by using a mechanism like re-ECN,

   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
      mechanisms still work properly because the of the good QoS on the
      virtual link (the tunnel).  Another example is that PCN is

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      deployed on the general Internet (ie widely but not universally

   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 adapts 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 then risky.

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

   Other possibilities include:

   o  The PCN-domain extends to the end users.  The scenario is
      described in [I-D.babiarz-pcn-sip-cap].  The end users need to be
      trusted to do their own policing.  This scenario is in the scope
      of the PCN WG charter if there is sufficient traffic for the
      aggregation assumption to 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 about the decision about admission (or termination).  It may
      also need the centralised node and the PCN-boundary-nodes to know
      each other's addresses.  It would be possible for the centralised
      node to be one of the PCN-boundary-nodes, when clearly the
      signalling would sometimes be replaced by a message internal to
      the node.

   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 ([I-D.briscoe-tsvwg-cl-architecture] 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.  Note: There is a PCN WG Milestone on
      "Requirements for signalling", which is potential input for the

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      appropriate WGs.

   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

   o  PCN for MPLS: [RFC3270] defines how to support the DiffServ
      architecture in MPLS networks.  [RFC5129] describes how to add PCN
      for admission control of microflows into a set of MPLS aggregates
      (Multi-protocol label switching).  PCN-marking is done in MPLS's
      EXP field (which [I-D.andersson-mpls-expbits-def] proposes to re-
      name to the Class of Service (CoS) bits).

   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.


17.  Appendix B: Probing

17.1.  Introduction

   Probing is an optional 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 the link that's
   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.

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

   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.  Another possibility is that a probe packet is a signalling
   packet that is anyway travelling from the PCN-ingress-node to the
   PCN-egress-node (eg an RSVP PATH message travelling from source to

17.2.  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 8)
      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

   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-marking algorithm; for
      example an excess-traffic-marking algorithm generates fewer PCN-
      marks than a threshold-marking 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.

17.3.  Discussion of rationale for probing, its downsides and open

   It is an unresolved question whether probing is really needed, but
   three viewpoints have been put forward as to why it is useful.  The

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   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.  The third viewpoint is that
   admission control is always done by probing.  We now consider each in

   The first viewpoint assumes the following:

   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, [PCN-email-traffic-empty-aggregates] 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 a new flow starts on many of the
   empty ingress-egress-aggregates and causes overload on 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,
   and 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 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) cause
      applications, eg packets might be dropped?

   o  Are there other ways of dealing with the flash crowd scenario?
      For instance limit the rate at which new flows are admitted; or

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      perhaps for a PCN-egress-node to block new flows on its empty
      ingress-egress-aggregates when its non-empty ones are pre-

   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

   o  Note: [PCN-email-ECMP] 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

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

   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

   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.  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 third bullet
      (which is otherwise hard because an ECMP routing decision is made
      independently on each node).

   The downsides of probing for this viewpoint are:

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   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 (eg they might confuse the
      destination end node).  Hence somehow the PCN-egress-node has to
      be able to disambiguate a probe packet from a data packet, via the
      characteristic setting of particular bit(s) in the packet's header
      or body - but these bit(s) mustn't be used by any PCN-interior-
      node's ECMP algorithm.  In the general case this isn't possible,
      but it should be OK for a typical ECMP algorithm which examines:
      the source and destination IP addresses and port numbers, the
      protocol ID and the DSCP.

   The third viewpoint assumes the following:

   o  Every admission control decision involves probing, using the
      signalling set-up message as the probe packet (eg RSVP PATH).

   o  The PCN-marking behaviour is such that every packet is PCN-marked
      if the flow should be blocked, hence only a single probing packet
      is needed.

   This viewpoint [I-D.draft-babiarz-pcn-3sm] has in particular been
   suggested for the scenario where the PCN-domain reaches out towards
   the end terminals (note that it's assumed the trust and aggregation
   assumptions still hold), although it has also been suggested for
   other scenarios.

18.  Informative References

              Briscoe, B., "An edge-to-edge Deployment Model for Pre-
              Congestion Notification: Admission  Control over a
              DiffServ Region", draft-briscoe-tsvwg-cl-architecture-04
              (work in progress), October 2006.

              Briscoe, B., "Pre-Congestion Notification marking",
              draft-briscoe-tsvwg-cl-phb-03 (work in progress),
              October 2006.


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              Babiarz, J., "SIP Controlled Admission and Preemption",
              draft-babiarz-pcn-sip-cap-00 (work in progress),
              October 2006.

              Faucheur, F., "RSVP Extensions for Admission Control over
              Diffserv using Pre-congestion  Notification (PCN)",
              draft-lefaucheur-rsvp-ecn-01 (work in progress),
              June 2006.

              Chan, K., "Pre-Congestion Notification Problem Statement",
              draft-chan-pcn-problem-statement-01 (work in progress),
              October 2006.

              "Pseudowire Congestion Control Framework", May 2008, <http

              "Layered Encapsulation of Congestion Notification",
              July 2008, <

              "Pre-Congestion Notification Using Single Marking for
              Admission and Termination", November 2007, <http://

              "Pre-Congestion Notification Architecture", June 2007, <ht

              "LC-PCN: The Load Control PCN Solution", February 2008, <h

              "Applicability of Keying Methods for RSVP Security",
              November 2007, <

              "Emulating Border Flow Policing using Re-ECN on Bulk

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              Data", February 2008, <

              "Three State PCN Marking", November 2007, <http://

   [RFC5129]  "Explicit Congestion Marking in MPLS", RFC 5129,
              January 2008.

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

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

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

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

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

   [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.

   [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.

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

   [RFC2983]  Black, D., "Differentiated Services and Tunnels",

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              RFC 2983, October 2000.

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

   [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.

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

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

   [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.

              "Multilevel Precedence and Pre-emption Service (MLPP)",
              ITU-T Recommendation I.255.3, 1990.

   [Iyer]     "An approach to alleviate link overload as observed on an
              IP backbone", IEEE INFOCOM , 2003,

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

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

              "Guaranteed QoS Synthesis for Admission Control with
              Shared Capacity", BT Technical Report TR-CXR9-2006-001,
              Feburary 2006, <

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   [Menth]    "PCN-Based Resilient Network Admission Control: The Impact
              of a Single Bit"", Technical Report , 2007, <http://

              "Email to PCN WG mailing list", November 2007, <http://

              "Email to PCN WG mailing list", October 2007, <http://

              "Email to PCN WG mailing list", March 2008, <http://

              "Marking behaviour of PCN-nodes", June 2008, <http://

              "Baseline Encoding and Transport of Pre-Congestion
              Information", July 2008, <

              "A three state extended PCN encoding scheme", June 2008, <

              "Pre-Congestion Notification Using Single Marking for
              Admission and Termination", November 2007, <http://

              "Applicability Statement for the Use of Pre-Congestion
              Notification in a Resource-Controlled Network",
              February 2008, <


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              "Usecases and Benefits of end to end ECN support in PCN
              Domains", May 2008, <

              "MPLS EXP-bits definition", March 2008, <http://

   [Menth08]  "PCN-Based Admission Control and Flow Termination", 2008,

   [Hancock]  "Slide 14 of 'NSIS: An Outline Framework for QoS
              Signalling'", May 2002, <

Author's Address

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


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