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RFC 5559

 
 
 

Pre-Congestion Notification (PCN) Architecture

Part 2 of 3, p. 18 to 40
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4.  Detailed Functional Architecture

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

   1.  functions needed at PCN-interior-nodes

   2.  functions needed at PCN-ingress-nodes

   3.  functions needed at PCN-egress-nodes

   4.  other functions needed for flow admission control

   5.  other functions needed for flow termination control

   Note: Probing is covered in the Appendix.

   The section then discusses some other detailed topics:

   1.  addressing

   2.  tunnelling

   3.  fault handling

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4.1.  PCN-Interior-Node Functions

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

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

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

   o  PCN-mark - as triggered by indications from the PCN-meter
      functionality; if necessary, PCN-mark packets with the appropriate
      encoding.

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

   The functions are defined in [Eardley09] and the baseline encoding in
   [Moncaster09-1] (extended encodings are to be defined in other
   documents).

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

   Figure 4: Schematic of PCN-interior-node functionality.

4.2.  PCN-Ingress-Node Functions

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

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

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

   o  PCN-colour - set the DSCP and ECN fields appropriately for the
      PCN-domain, for example, as in [Moncaster09-1].

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

   The first two are policing functions, needed to make sure that PCN-
   packets admitted into the PCN-domain belong to a flow that has been
   admitted and to ensure that the flow keeps to the flowspec agreed
   (eg, doesn't exceed an agreed maximum rate and is inelastic traffic).
   Installing the filter spec will typically be done by the signalling
   protocol, as will re-installing the filter, for example, after a re-
   route that changes the PCN-ingress-node (see [Briscoe06] for an
   example using RSVP).  PCN-colouring allows the rest of the PCN-domain
   to recognise PCN-packets.

4.3.  PCN-Egress-Node Functions

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

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

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

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

   The metering functionality, of course, depends on whether it is
   targeted at admission control or flow termination.  Alternatives
   involve the PCN-egress-node "measuring", as an aggregate (ie, not per
   flow), all PCN-packets from a particular PCN-ingress-node, or
   "monitoring" the PCN-traffic and reacting to one (or several) PCN-

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   marked packets.  For PCN-colouring, [Moncaster09-1] specifies that
   the PCN-egress-node resets the ECN field to 00; other encodings may
   define different behaviour.

4.4.  Admission Control Functions

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

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

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

   There are various possibilities for how the functionality could be
   distributed (we assume the operator will 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.

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

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

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

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

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4.5.  Flow Termination Functions

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

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

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

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

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

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

   There are various possibilities for how the functionality could be
   distributed, similar to those discussed above in Section 4.4.

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

4.6.  Addressing

   PCN-nodes may need to know the address of other PCN-nodes.  Note that
   PCN-interior-nodes don't need to know the address of other PCN-nodes
   (except their next-hop neighbours for routing purposes).

   At a minimum, the PCN-egress-node needs to know the address of the
   PCN-ingress-node associated with a flow so that the PCN-ingress-node
   can be informed of the admission decision (and any flow termination
   decision) and enforce it through policing.  There are various

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   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, through the regular processing of an RSVP PATH message,
      as the PCN-ingress-node is the previous RSVP hop (PHOP)
      ([Lefaucheur06]).  Another option is that the PCN-ingress-node
      could signal its address to the PCN-egress-node.

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

4.7.  Tunnelling

   Tunnels may originate and/or terminate within a PCN-domain (eg, IP
   over IP, IP over MPLS).  It is important that the PCN-marking of any
   packet can potentially influence PCN's flow admission control and
   termination -- it shouldn't matter whether the packet happens to be
   tunnelled at the PCN-node that PCN-marks the packet, or indeed
   whether it's decapsulated or encapsulated by a subsequent PCN-node.
   This suggests that the "uniform conceptual model" described in
   [RFC2983] should be re-applied in the PCN context.  In line with both
   this and the approach of [RFC4303] and [Briscoe09], the following
   rule is applied if encapsulation is done within the PCN-domain:

   o  Any PCN-marking is copied into the outer header.

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

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

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

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

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

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

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

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

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

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

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

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

   Tunnelling constraints were a major factor in the choice of the
   baseline encoding.  As explained in [Moncaster09-1], with current
   tunnelling endpoints, only the 11 codepoint of the ECN field survives
   decapsulation, and hence the baseline encoding only uses the 11
   codepoint to indicate PCN-marking.  Extended encoding schemes need to

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   explain their interactions with (or assumptions about) tunnelling.  A
   lengthy discussion of all the issues associated with layered
   encapsulation of congestion notification (for ECN as well as PCN) is
   in [Briscoe09].

4.8.  Fault Handling

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

   If a PCN-boundary-node fails, then we would like the regular QoS
   signalling protocol to be responsible for taking appropriate action.
   As an example, [Briscoe09] considers what happens if RSVP is the QoS
   signalling protocol.

5.  Operations and Management

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

5.1.  Fault Operations and Management

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

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

   Section 4 describes how the PCN architecture has been designed to
   ensure admitted flows continue gracefully after recovering
   automatically from link or node failures.  The need to record and
   monitor re-routing events affecting signalling is unchanged by the

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   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 5.2) to upgrade resources in danger of causing
   service degradation or to trigger manual tracing of intermittent
   incipient errors that would otherwise have gone unnoticed.

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

5.2.  Configuration Operations and Management

   Threshold-metering and -marking and excess-traffic-metering and
   -marking are standardised in [Eardley09].  However, more diversity in
   PCN-boundary-node behaviours is expected, in order to interface with
   diverse industry architectures.  It may be possible to have different
   PCN-boundary-node behaviours for different ingress-egress-aggregates
   within the same PCN-domain.

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

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   PCN configuration control variables fall into the following
   categories:

   o  system options (enabling or disabling behaviours)

   o  parameters (setting levels, addresses, etc.)

   One possibility is that all configurable variables sit within an SNMP
   (Simple Network Management Protocol) 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 (such as SNMPv3).

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

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

5.2.1.  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.

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

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

   o  Which of admission and flow termination are enabled.  If any PCN-
      interior-node is configured to generate a marking, all PCN-
      boundary-nodes must be able to interpret that marking (which

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      includes understanding, in a PCN-domain that uses only one type of
      PCN-marking, whether they are generated by PCN-interior-nodes'
      threshold meters or their excess-traffic meters).  Therefore, all
      PCN-boundary-nodes must be configured the same in this respect.

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

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

   PCN-egress-nodes will have further system options:

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

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

5.2.2.  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 the first deployment of PCN will be very
   similar to the traditional process for sizing an admission-controlled
   network, depending on: the operator's requirements for minimising
   flow blocking (grade of service), the expected PCN-traffic load on
   each link and its statistical characteristics (the traffic matrix),
   contingency for re-routing the PCN-traffic matrix in the event of
   single or multiple failures, and the expected load from other classes
   relative to link capacities [Menth09-1].  But, once a domain is in
   operation, a PCN design goal is to be able to determine growth in

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   these configured rates much more simply, by monitoring PCN-marking
   rates from actual rather than expected traffic (see Section 5.4 on
   Performance and Provisioning).

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

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

   The PCN-egress-node may allow configuration of:

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

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

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

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

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

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

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5.3.  Accounting Operations and Management

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

5.4.  Performance and Provisioning Operations and Management

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

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

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

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

5.5.  Security Operations and Management

   Security Operations and Management is about using secure operational
   practices as well as being able to track security breaches or near-
   misses at run-time.  PCN adds few specifics to the general good
   practice required in this field [RFC4778].  The correct functions of
   the system should be monitored (Section 5.4) in multiple independent
   ways and correlated to detect possible security breaches.  Persistent
   (pre-)congestion marking should raise an alarm (both on the node
   doing the marking and on the PCN-egress-node metering it).
   Similarly, persistently poor external QoS metrics (such as jitter or
   mean opinion score) should raise an alarm.  The following are
   examples of symptoms that may be the result of innocent faults,
   rather than attacks; however, until diagnosed, they should be logged
   and should trigger a security alarm:

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

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

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

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

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6.  Applicability of PCN

6.1.  Benefits

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

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

   o  Admission control is resilient: with PCN, QoS is decoupled from
      the routing system.  Hence, in general, admitted flows can survive
      capacity, routing, or topology changes without additional
      signalling.  The PCN-admissible-rate on each link can be chosen to
      be small enough that admitted traffic can still be carried after a
      re-routing in most failure cases [Menth09-1].  This is an
      important feature, as QoS violations in core networks due to link
      failures are more likely than QoS violations due to increased
      traffic volume [Iyer03].

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

   o  The information of these measurements is signalled to the PCN-
      egress-nodes by the PCN-marks in the packet headers, ie, "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.  It also adapts 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.

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   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 the malfunction of
      measurement-based admission control in the presence of admitted
      flows that send for a while with an atypically low rate and then
      increase their rates in a correlated way.

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

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

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

6.2.  Deployment Scenarios

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

   A PCN-domain may have three encoding states (or pedantically, an
   operator may choose to use up three encoding states for PCN): not
   PCN-marked, threshold-marked, and excess-traffic-marked.  This way,
   both PCN admission control and flow termination can be supported.  As

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   illustrated in Figure 1, admission control accepts new flows until
   the PCN-traffic rate on the bottleneck link rises above the PCN-
   threshold-rate, whilst, if necessary, the flow termination mechanism
   terminates flows down to the PCN-excess-rate on the bottleneck link.

   On the other hand, a PCN-domain may have two encoding states (as in
   [Moncaster09-1]) (or pedantically, an operator may choose to use up
   two encoding states for PCN): not PCN-marked and PCN-marked.  This
   way, there are three possibilities, as discussed in the following
   paragraphs (see also Section 3.3).

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

   Second, an operator could just rely on statically provisioned
   capacity per PCN-ingress-node (regardless of the PCN-egress-node of a
   flow) for admission control, as is typical in the hose model of the
   Diffserv architecture [Kumar01].  Such traffic-conditioning
   agreements can lead to focused overload: many flows happen to focus
   on a particular link and then all flows through the congested link
   fail catastrophically.  PCN's flow termination mechanism could then
   be used to counteract such a problem.

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

   Within the PCN-domain, there is some flexibility about how the
   decision-making functionality is distributed.  These possibilities
   are outlined in Section 4.4 and are also discussed elsewhere, such as
   in [Menth09-2].

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

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   and not be done at the PCN-ingress-node.  Note that, to aid
   readability, the rest of this document assumes that policing is done
   by the PCN-ingress-nodes.

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

   It is possible that a user wants its inelastic traffic to use the PCN
   mechanisms but also react to ECN markings outside the PCN-domain
   [Sarker08].  Two possible ways to do this are to tunnel all PCN-
   packets across the PCN-domain, so that the ECN marks are carried
   transparently across the PCN-domain, or to use an encoding like
   [Moncaster09-2].  Tunnelling is discussed further in Section 4.7.

   Some further possible deployment models are outlined in the Appendix.

6.3.  Assumptions and Constraints on Scope

   The scope of this document is restricted by the following
   assumptions:

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

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

   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 (Government
       Telecommunications Service), WPS (Wireless Priority Service),
       MLPP (Multilevel Precedence and Premption), etc.) is out of
       scope.

6.3.1.  Assumption 1: Trust and Support of PCN - Controlled Environment

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

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

   o  Similarly, a PCN-boundary-node has to trust that all the PCN-nodes
      mark PCN-traffic consistently.  A node not performing PCN-marking
      wouldn't be able to send an 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 Section 7.

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

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

6.3.2.  Assumption 2: Real-Time Applications

   It is assumed that any variation of source bit rate is independent of
   the level of pre-congestion.  We assume that PCN-packets come from
   real-time applications generating inelastic traffic, ie, sending
   packets at the rate the codec produces them, regardless of the
   availability of capacity [RFC4594].  Examples of such real-time
   applications include 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

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   applications where per-flow QoS is a known requirement.  In other
   words, we focus on PCN providing a benefit to inelastic traffic (PCN
   may or may not provide a benefit to other types of traffic).

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

6.3.3.  Assumption 3: Many Flows and Additional Load

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

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

6.3.4.  Assumption 4: Emergency Use Out of Scope

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

6.4.  Challenges

   Prior work on PCN and similar mechanisms has led to a number of
   considerations about PCN's design goals (things PCN should be good
   at) and some issues that have been hard to solve in a fully
   satisfactory manner.  Taken as a whole, PCN represents a list of

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   trade-offs (it is unlikely that they can all be 100% achieved) and
   perhaps a list of evaluation criteria to help an operator (or the
   IETF) decide between options.

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

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

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

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

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

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

   o  ECMP and Signalling: It is possible that, in a PCN-domain running
      ECMP, the signalling packets (eg, RSVP, NSIS) follow a different
      path than the data packets, which could matter if the signalling
      packets are used as probes.  Whether this is an issue depends on
      which fields the ECMP algorithm uses; if the ECMP algorithm is
      restricted to the source and destination IP addresses, then it
      will not be an issue.  ECMP and signalling interactions are a
      specific instance of a general issue for non-traditional routing
      combined with resource management along a path [Hancock02].

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

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

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

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

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

   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, if a big burst of admission requests

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      potentially occurs in a very short space of time (eg, prompted by
      a televote), they could all get admitted before enough PCN-marks
      are seen to block new flows.  In other words, any additional load
      offered within the reaction time of the mechanism must not move
      the PCN-domain directly from a no congestion state to overload.
      This "vulnerability period" may have an impact at the signalling
      level, for instance, QoS requests should be rate-limited to bound
      the number of requests able to arrive within the vulnerability
      period.

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



(page 40 continued on part 3)

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