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Network Working Group                                    P. Eardley, Ed.Request for Comments: 5559                                            BTCategory: Informational                                        June 2009Pre-Congestion Notification (PCN) ArchitectureStatus of This Memo   This memo provides information for the Internet community.  It does   not specify an Internet standard of any kind.  Distribution of this   memo is unlimited.Copyright Notice   Copyright (c) 2009 IETF Trust and the persons identified as the   document authors.  All rights reserved.   This document is subject toBCP 78 and the IETF Trust's Legal   Provisions Relating to IETF Documents in effect on the date of   publication of this document (http://trustee.ietf.org/license-info).   Please review these documents carefully, as they describe your rights   and restrictions with respect to this document.   This document may contain material from IETF Documents or IETF   Contributions published or made publicly available before November   10, 2008.  The person(s) controlling the copyright in some of this   material may not have granted the IETF Trust the right to allow   modifications of such material outside the IETF Standards Process.   Without obtaining an adequate license from the person(s) controlling   the copyright in such materials, this document may not be modified   outside the IETF Standards Process, and derivative works of it may   not be created outside the IETF Standards Process, except to format   it for publication as an RFC or to translate it into languages other   than English.Abstract   This document describes a general architecture for flow admission and   termination based on pre-congestion information in order to protect   the quality of service of established, inelastic flows within a   single Diffserv domain.Eardley                      Informational                      [Page 1]

RFC 5559                    PCN Architecture                   June 2009Table of Contents1. Introduction ....................................................31.1. Overview of PCN ............................................31.2. Example Use Case for PCN ...................................41.3. Applicability of PCN .......................................71.4. Documents about PCN ........................................82. Terminology .....................................................93. High-Level Functional Architecture .............................113.1. Flow Admission ............................................133.2. Flow Termination ..........................................14      3.3. Flow Admission and/or Flow Termination When There Are Only           Two PCN Encoding States ...................................153.4. Information Transport .....................................163.5. PCN-Traffic ...............................................163.6. Backwards Compatibility ...................................174. Detailed Functional Architecture ...............................184.1. PCN-Interior-Node Functions ...............................194.2. PCN-Ingress-Node Functions ................................194.3. PCN-Egress-Node Functions .................................204.4. Admission Control Functions ...............................214.5. Flow Termination Functions ................................224.6. Addressing ................................................224.7. Tunnelling ................................................234.8. Fault Handling ............................................255. Operations and Management ......................................255.1. Fault Operations and Management ...........................255.2. Configuration Operations and Management ...................265.2.1. System Options .....................................275.2.2. Parameters .........................................285.3. Accounting Operations and Management ......................305.4. Performance and Provisioning Operations and Management ....305.5. Security Operations and Management ........................316. Applicability of PCN ...........................................326.1. Benefits ..................................................326.2. Deployment Scenarios ......................................336.3. Assumptions and Constraints on Scope ......................35           6.3.1. Assumption 1: Trust and Support of PCN -                  Controlled Environment .............................366.3.2. Assumption 2: Real-Time Applications ...............366.3.3. Assumption 3: Many Flows and Additional Load .......376.3.4. Assumption 4: Emergency Use Out of Scope ...........376.4. Challenges ................................................377. Security Considerations ........................................408. Conclusions ....................................................419. Acknowledgements ...............................................41Eardley                      Informational                      [Page 2]

RFC 5559                    PCN Architecture                   June 200910. References ....................................................4210.1. Normative References .....................................4210.2. Informative References ...................................42Appendix A.  Possible Future Work Items ...........................48A.1.  Probing .................................................50A.1.1.  Introduction ....................................50A.1.2.  Probing Functions ...............................50             A.1.3.  Discussion of Rationale for Probing, Its                     Downsides and Open Issues .......................511.  Introduction1.1.  Overview of PCN   The objective of Pre-Congestion Notification (PCN) is to protect the   quality of service (QoS) of inelastic flows within a Diffserv domain   in a simple, scalable, and robust fashion.  Two mechanisms are used:   admission control, to decide whether to admit or block a new flow   request, and (in abnormal circumstances) flow termination, to decide   whether to terminate some of the existing flows.  To achieve this,   the overall rate of PCN-traffic is metered on every link in the   domain, and PCN packets are appropriately marked when certain   configured rates are exceeded.  These configured rates are below the   rate of the link, thus providing notification to boundary nodes about   overloads before any congestion occurs (hence, "Pre-Congestion   Notification").  The level of marking allows boundary nodes to make   decisions about whether to admit or terminate.   Within a PCN-domain, PCN-traffic is forwarded in a prioritised   Diffserv traffic class.  Every link in the PCN-domain is configured   with two rates (PCN-threshold-rate and PCN-excess-rate).  If the   overall rate of PCN-traffic on a link exceeds a configured rate, then   a PCN-interior-node marks PCN-packets appropriately.  The PCN-egress-   nodes use this information to make admission control and flow   termination decisions.  Flow admission control determines whether a   new flow can be admitted without any impact, in normal circumstances,   on the QoS of existing PCN-flows.  However, in abnormal circumstances   (for instance, a disaster affecting multiple nodes and causing   traffic re-routes), the QoS on existing PCN-flows may degrade even   though care was exercised when admitting those flows.  The flow   termination mechanism removes sufficient traffic in order to protect   the QoS of the remaining PCN-flows.  All PCN-boundary-nodes and PCN-   interior-nodes are PCN-enabled and are trusted for correct PCN   operation.  PCN-ingress-nodes police arriving packets to check that   they are part of an admitted PCN-flow that keeps within its agreed   flowspec, and hence they maintain per-flow state.  PCN-interior-nodes   meter all PCN-traffic, and hence do not need to maintain any per-flowEardley                      Informational                      [Page 3]

RFC 5559                    PCN Architecture                   June 2009   state.  Decisions about flow admission and termination are made for a   particular pair of PCN-boundary-nodes, and hence PCN-egress-nodes   must be able to identify which PCN-ingress-node sent each PCN-packet.1.2.  Example Use Case for PCN   This section outlines an end-to-end QoS scenario that uses the PCN   mechanisms within one domain.  The parts outside the PCN-domain are   out of scope for PCN, but are included to help clarify how PCN could   be used.  Note that this section is only an example -- in particular,   there are other possibilities (seeSection 3) for how the PCN-   boundary-nodes perform admission control and flow termination.   As a fundamental building block, each link of the PCN-domain operates   the following.  Please refer to [Eardley09] and Figure 1.   o  A threshold meter and marker, which marks all PCN-packets if the      rate of PCN-traffic is greater than a first configured rate, the      PCN-threshold-rate.  The admission control mechanism limits the      PCN-traffic on each link to *roughly* its PCN-threshold-rate.   o  An excess-traffic meter and marker, which marks a proportion of      PCN-packets such that the amount marked equals the traffic rate in      excess of a second configured rate, the PCN-excess-rate.  The flow      termination mechanism limits the PCN-traffic on each link to      *roughly* its PCN-excess-rate.   Overall, the aim is to give an "early warning" of potential   congestion before there is any significant build-up of PCN-packets in   the queue on the link; we term this "Pre-Congestion Notification" by   analogy with ECN (Explicit Congestion Notification, [RFC3168]).  Note   that the link only meters the bulk PCN-traffic (and not per flow).Eardley                      Informational                      [Page 4]

RFC 5559                    PCN Architecture                   June 2009                          ==   Metering &    ==                          ==Marking behaviour==       ==PCN mechanisms==                       ^           Rate of     ^      PCN-traffic on   |     bottleneck link   |                       |                       |       Some pkts                  Terminate some                       |  excess-traffic-marked           admitted flows                       |           &                            &                       |     Rest of pkts                Block new flows                       |   threshold-marked                       |     PCN-excess-rate  -|------------------------------------------------(=PCN-supportable-rate)|                       |       All pkts                  Block new flows                       |   threshold-marked                       |   PCN-threshold-rate -|------------------------------------------------ (=PCN-admissible-rate)|                       |        No pkts                  Admit new flows                       |      PCN-marked                       |   Figure 1: Example of how the PCN admission control and flow   termination mechanisms operate as the rate of PCN-traffic increases.   The two forms of PCN-marking are indicated by setting the ECN and   DSCP (Differentiated Services Codepoint [RFC2474]) fields to known   values, which are configured for the domain.  Thus, the PCN-egress-   nodes can monitor the PCN-markings in order to measure the severity   of pre-congestion.  In addition, the PCN-ingress-nodes need to set   the ECN and DSCP fields to that configured for an unmarked PCN-   packet, and the PCN-egress-nodes need to revert to values appropriate   outside the PCN-domain.   For admission control, we assume end-to-end RSVP (Resource   Reservation Protocol) [RFC2205]) signalling in this example.  The   PCN-domain is a single RSVP hop.  The PCN-domain operates Diffserv,   and we assume that PCN-traffic is scheduled with the expedited   forwarding (EF) per-hop behaviour [RFC3246].  Hence, the overall   solution is in line with the "IntServ over Diffserv" framework   defined in [RFC2998], as shown in Figure 2.Eardley                      Informational                      [Page 5]

RFC 5559                    PCN Architecture                   June 2009   ___    ___    _______________________________________    ____    ___  |   |  |   |  | PCN-             PCN-            PCN- |  |    |  |   |  |   |  |   |  |ingress         interior         egress|  |    |  |   |  |   |  |   |  | -node           -nodes          -node |  |    |  |   |  |   |  |   |  |-------+  +-------+  +-------+  +------|  |    |  |   |  |   |  |   |  |       |  | PCN   |  | PCN   |  |      |  |    |  |   |  |   |..|   |..|Ingress|..|meter &|..|meter &|..|Egress|..|    |..|   |  |   |..|   |..|Policer|..|marker |..|marker |..|Meter |..|    |..|   |  |   |  |   |  |-------+  +-------+  +-------+  +------|  |    |  |   |  |   |  |   |  |  \                                 /  |  |    |  |   |  |   |  |   |  |   \                               /   |  |    |  |   |  |   |  |   |  |    \  PCN-feedback-information   /    |  |    |  |   |  |   |  |   |  |     \  (for admission control)  /     |  |    |  |   |  |   |  |   |  |      --<-----<----<----<-----<--      |  |    |  |   |  |   |  |   |  |       PCN-feedback-information        |  |    |  |   |  |   |  |   |  |        (for flow termination)         |  |    |  |   |  |___|  |___|  |_______________________________________|  |____|  |___|  Sx     Access               PCN-domain                   Access    Rx  End    Network                                          Network   End  Host                                                              Host                  <---- signalling across PCN-domain--->                (for admission control & flow termination)  <-------------------end-to-end QoS signalling protocol--------------->   Figure 2: Example of possible overall QoS architecture.   A source wanting to start a new QoS flow sends an RSVP PATH message.   Normal hop-by-hop IntServ [RFC1633] is used outside the PCN-domain   (we assume successfully).  The PATH message travels across the PCN-   domain; the PCN-egress-node reads the PHOP (previous RSVP hop) object   to discover the specific PCN-ingress-node for this flow.  The RESV   message travels back from the receiver, and triggers the PCN-egress-   node to check what fraction of the PCN-traffic from the relevant PCN-   ingress-node is currently being threshold-marked.  It adds an object   with this information onto the RESV message, and hence the PCN-   ingress-node learns about the level of pre-congestion on the path.   If this level is below some threshold, then the PCN-ingress-node   admits the new flow into the PCN-domain.  The RSVP message triggers   the PCN-ingress-node to install two normal IntServ items: five-tuple   information, so that it can subsequently identify data packets that   are part of a previously admitted PCN-flow, and a traffic profile, so   that it can police the flow to within its reservation.  Similarly,   the RSVP message triggers the PCN-egress-node to install five-tuple   and PHOP information so that it can identify packets as part of a   flow from a specific PCN-ingress-node.Eardley                      Informational                      [Page 6]

RFC 5559                    PCN Architecture                   June 2009   The flow termination mechanism may happen when some abnormal   circumstance causes a link to become so pre-congested that it excess-   traffic-marks (and perhaps also drops) PCN-packets.  In this example,   when a PCN-egress-node observes such a packet, it then, with some   probability, terminates this PCN-flow; the probability is configured   low enough to avoid over termination and high enough to ensure rapid   termination of enough flows.  It also informs the relevant PCN-   ingress-node so that it can block any further traffic on the   terminated flow.1.3.  Applicability of PCN   Compared with alternative QoS mechanisms, PCN has certain advantages   and disadvantages that will make it appropriate in particular   scenarios.  For example, compared with hop-by-hop IntServ [RFC1633],   PCN only requires per-flow state at the PCN-ingress-nodes.  Compared   with the Diffserv architecture [RFC2475], an operator needs to be   less accurate and/or conservative in its prediction of the traffic   matrix.  The Diffserv architecture's traffic-conditioning agreements   are static and coarse; they are defined at subscription time and are   used (for instance) to limit the total traffic at each ingress of the   domain, regardless of the egress for the traffic.  On the other hand,   PCN firstly uses admission control based on measurements of the   current conditions between the specific pair of PCN-boundary-nodes,   and secondly, in case of a disaster, PCN protects the QoS of most   flows by terminating a few selected ones.   PCN's admission control is a measurement-based mechanism.  Hence, it   assumes that the present is a reasonable prediction of the future:   the network conditions are measured at the time of a new flow   request, but the actual network performance must be acceptable during   the call some time later.  Hence, PCN is unsuitable in several   circumstances:   o  If the source adapts its bit rate dependent on the level of pre-      congestion, because then the aggregate traffic might become      unstable.  The assumption in this document is that PCN-packets      come from real-time applications generating inelastic traffic,      such as the Controlled Load Service [RFC2211].   o  If a potential bottleneck link has capacity for only a few flows,      because then a new flow can move a link directly from no pre-      congestion to being so overloaded that it has to drop packets.      The assumption in this document is that this isn't a problem.   o  If there is the danger of a "flash crowd", in which many admission      requests arrive within the reaction time of PCN's admission      mechanism, because then they all might get admitted and soEardley                      Informational                      [Page 7]

RFC 5559                    PCN Architecture                   June 2009      overload the network.  The assumption in this document is that, if      it is necessary, then flash crowds are limited in some fashion      beyond the scope of this document, for instance by rate-limiting      QoS requests.   The applicability of PCN is discussed further inSection 6.1.4.  Documents about PCN   The purpose of this document is to describe a general architecture   for flow admission and termination based on (pre-)congestion   information in order to protect the quality of service of flows   within a Diffserv domain.  This document describes the PCN   architecture at a high level (Section 3) and in more detail   (Section 4).  It also defines some terminology, and provides   considerations about operations, management, and security.Section 6   considers the applicability of PCN in more detail, covering its   benefits, deployment scenarios, assumptions, and potential   challenges.  The Appendix covers some potential future work items.   Aspects of PCN are also documented elsewhere:   o  Metering and marking: [Eardley09] standardises threshold metering      and marking and excess-traffic metering and marking.  A PCN-packet      may be marked, depending on the metering results.   o  Encoding: the "baseline" encoding is described in [Moncaster09-1],      which standardises two PCN encoding states (PCN-marked and not      PCN-marked), whilst (experimental) extensions to the baseline      encoding can provide three encoding states (threshold-marked,      excess-traffic-marked, or not PCN-marked), for instance, see      [Moncaster09-2].  (There may be further encoding states as      suggested in [Westberg08].)Section 3.6 considers the backwards      compatibility of PCN encoding with ECN.   o  PCN-boundary-node behaviour: how the PCN-boundary-nodes convert      the PCN-markings into decisions about flow admission and flow      termination, as described in Informational documents such as      [Taylor09] and [Charny07-2].  The concept is that the standardised      metering and marking by PCN-nodes allows several possible PCN-      boundary-node behaviours.  A number of possibilities are outlined      in this document; detailed descriptions and comparisons are in      [Charny07-1] and [Menth09-2].   o  Signalling between PCN-boundary-nodes: signalling is needed to      transport PCN-feedback-information between the PCN-boundary-nodes      (in the example above, this is the fraction of traffic, between      the pair of PCN-boundary-nodes, that is PCN-marked).  The exactEardley                      Informational                      [Page 8]

RFC 5559                    PCN Architecture                   June 2009      details vary for different PCN-boundary-node behaviours, and so      should be described in those documents.  It may require an      extension to the signalling protocol -- standardisation is out of      scope of the PCN WG.   o  The interface by which the PCN-boundary-nodes learn identification      information about the admitted flows: the exact requirements vary      for different PCN-boundary-node behaviours and for different      signalling protocols, and so should be described in those      documents.  They will be similar to those described in the example      above -- a PCN-ingress-node needs to be able to identify that a      packet is part of a previously admitted flow (typically from its      five-tuple) and each PCN-boundary-node needs to be able to      identify the other PCN-boundary-node for the flow.2.  Terminology   o  PCN-domain: a PCN-capable domain; a contiguous set of PCN-enabled      nodes that perform Diffserv scheduling [RFC2474]; the complete set      of PCN-nodes that in principle can, through PCN-marking packets,      influence decisions about flow admission and termination for the      PCN-domain; includes the PCN-egress-nodes, which measure these      PCN-marks, and the PCN-ingress-nodes.   o  PCN-boundary-node: a PCN-node that connects one PCN-domain to a      node either in another PCN-domain or in a non-PCN-domain.   o  PCN-interior-node: a node in a PCN-domain that is not a PCN-      boundary-node.   o  PCN-node: a PCN-boundary-node or a PCN-interior-node.   o  PCN-egress-node: a PCN-boundary-node in its role in handling      traffic as it leaves a PCN-domain.   o  PCN-ingress-node: a PCN-boundary-node in its role in handling      traffic as it enters a PCN-domain.   o  PCN-traffic, PCN-packets, PCN-BA: a PCN-domain carries traffic of      different Diffserv behaviour aggregates (BAs) [RFC2474].  The      PCN-BA uses the PCN mechanisms to carry PCN-traffic, and the      corresponding packets are PCN-packets.  The same network will      carry traffic of other Diffserv BAs.  The PCN-BA is distinguished      by a combination of the Diffserv codepoint (DSCP) and ECN fields.Eardley                      Informational                      [Page 9]

RFC 5559                    PCN Architecture                   June 2009   o  PCN-flow: the unit of PCN-traffic that the PCN-boundary-node      admits (or terminates); the unit could be a single microflow (as      defined in [RFC2474]) or some identifiable collection of      microflows.   o  Pre-congestion: a condition of a link within a PCN-domain such      that the PCN-node performs PCN-marking, in order to provide an      "early warning" of potential congestion before there is any      significant build-up of PCN-packets in the real queue.  (Hence, by      analogy with ECN, we call our mechanism Pre-Congestion      Notification.)   o  PCN-marking: the process of setting the header in a PCN-packet      based on defined rules, in reaction to pre-congestion; either      threshold-marking or excess-traffic-marking.  Such a packet is      then called PCN-marked.   o  Threshold-metering: a metering behaviour that, if the PCN-traffic      exceeds the PCN-threshold-rate, indicates that all PCN-traffic is      to be threshold-marked.   o  PCN-threshold-rate: the reference rate of a threshold-meter, which      is configured for each link in the PCN-domain and which is lower      than the PCN-excess-rate.   o  Threshold-marking: the setting of the header in a PCN-packet to a      specific encoding, based on indications from the threshold-meter.      Such a packet is then called threshold-marked.   o  Excess-traffic-metering: a metering behaviour that, if the PCN-      traffic exceeds the PCN-excess-rate, indicates that the amount of      PCN-traffic to be excess-traffic-marked is equal to the amount in      excess of the PCN-excess-rate.   o  PCN-excess-rate: the reference rate of an excess-traffic-meter,      which is a configured for each link in the PCN-domain and which is      higher than the PCN-threshold-rate.   o  Excess-traffic-marking: the setting of the header in a PCN-packet      to a specific encoding, based on indications from the excess-      traffic-meter.  Such a packet is then called excess-traffic-      marked.   o  PCN-colouring: the process of setting the header in a PCN-packet      by a PCN-boundary-node; performed by a PCN-ingress-node so that      PCN-nodes can easily identify PCN-packets; performed by a PCN-      egress-node so that the header is appropriate for nodes beyond the      PCN-domain.Eardley                      Informational                     [Page 10]

RFC 5559                    PCN Architecture                   June 2009   o  Ingress-egress-aggregate: The collection of PCN-packets from all      PCN-flows that travel in one direction between a specific pair of      PCN-boundary-nodes.   o  PCN-feedback-information: information signalled by a PCN-egress-      node to a PCN-ingress-node (or a central control node), which is      needed for the flow admission and flow termination mechanisms.   o  PCN-admissible-rate: the rate of PCN-traffic on a link up to which      PCN admission control should accept new PCN-flows.   o  PCN-supportable-rate: the rate of PCN-traffic on a link down to      which PCN flow termination should, if necessary, terminate already      admitted PCN-flows.3.  High-Level Functional Architecture   The high-level approach is to split functionality between:   o  PCN-interior-nodes "inside" the PCN-domain, which monitor their      own state of pre-congestion and mark PCN-packets as appropriate.      They are not flow-aware, nor are they aware of ingress-egress-      aggregates.  The functionality is also done by PCN-ingress-nodes      for their outgoing interfaces (ie, those "inside" the PCN-domain).   o  PCN-boundary-nodes at the edge of the PCN-domain, which control      admission of new PCN-flows and termination of existing PCN-flows,      based on information from PCN-interior-nodes.  This information is      in the form of the PCN-marked data packets (which are intercepted      by the PCN-egress-nodes) and is not in signalling messages.      Generally, PCN-ingress-nodes are flow-aware.   The aim of this split is to keep the bulk of the network simple,   scalable, and robust, whilst confining policy, application-level, and   security interactions to the edge of the PCN-domain.  For example,   the lack of flow awareness means that the PCN-interior-nodes don't   care about the flow information associated with PCN-packets, nor do   the PCN-boundary-nodes care about which PCN-interior-nodes its   ingress-egress-aggregates traverse.   In order to generate information about the current state of the PCN-   domain, each PCN-node PCN-marks packets if it is "pre-congested".   Exactly when a PCN-node decides if it is "pre-congested" (the   algorithm) and exactly how packets are "PCN-marked" (the encoding)   will be defined in separate Standards Track documents, but at a high   level it is as follows:Eardley                      Informational                     [Page 11]

RFC 5559                    PCN Architecture                   June 2009   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, not per flow.  There are      two algorithms: one for threshold-metering and one for excess-      traffic-metering.  The meters trigger PCN-marking as necessary.   o  the encoding(s): a PCN-node PCN-marks a PCN-packet by modifying a      combination of the DSCP and ECN fields.  In the "baseline"      encoding [Moncaster09-1], the ECN field is set to 11 and the DSCP      is not altered.  Extension encodings may be defined that, at most,      use a second DSCP (eg, as in [Moncaster09-2]) and/or set the ECN      field to values other than 11 (eg, as in [Menth08-2]).   In a PCN-domain, the operator may have two or three encoding states   available.  The baseline encoding provides two encoding states (not   PCN-marked and PCN-marked), whilst extended encodings can provide   three encoding states (not PCN-marked, threshold-marked, and excess-   traffic-marked).   An operator may choose to deploy either admission control or flow   termination or both.  Although designed to work together, they are   independent mechanisms, and the use of one does not require or   prevent the use of the other.  Three encoding states naturally allows   both flow admission and flow termination.  If there are only two   encoding states, then there are several options -- seeSection 3.3.   The PCN-boundary-nodes monitor the PCN-marked packets in order to   extract information about the current state of the PCN-domain.  Based   on this monitoring, a distributed decision is made about whether to   admit a prospective new flow or terminate existing flow(s).  Sections   4.4 and 4.5 mention various possibilities for how the functionality   could be distributed.   PCN-metering and PCN-marking need to be configured on all   (potentially pre-congested) links in the PCN-domain to ensure that   the PCN mechanisms protect all links.  The actual functionality can   be configured on the outgoing or incoming interfaces of PCN-nodes --   or one algorithm could be configured on the outgoing interface and   the other on the incoming interface.  The important point is that a   consistent choice is made across the PCN-domain to ensure that the   PCN mechanisms protect all links.  See [Eardley09] for further   discussion.   The objective of threshold-marking, as triggered by the threshold-   metering algorithm, is to threshold-mark all PCN-packets whenever the   bit rate of PCN-packets is greater than some configured rate, the   PCN-threshold-rate.  The objective of excess-traffic-metering, as   triggered by the excess-traffic-marking algorithm, is to excess-Eardley                      Informational                     [Page 12]

RFC 5559                    PCN Architecture                   June 2009   traffic-mark PCN-packets at a rate equal to the difference between   the bit rate of PCN-packets and some configured rate, the PCN-excess-   rate.  Note that this description reflects the overall intent of the   algorithms rather than their instantaneous behaviour, since the rate   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   [Eardley09].   Admission and termination approaches are detailed and compared in   [Charny07-1] and [Menth09-2].  The discussion below is just a brief   summary.  Sections3.1 and3.2 assume there are three encoding states   available, whilstSection 3.3 assumes there are two encoding states   available.   From the perspective of the outside world, a PCN-domain essentially   looks like a Diffserv domain, but without the Diffserv architecture's   traffic-conditioning agreements.  PCN-traffic is either transported   across it transparently or policed at the PCN-ingress-node (ie,   dropped or carried at a lower QoS).  One difference is that PCN-   traffic has better QoS guarantees than normal Diffserv traffic   because the PCN mechanisms better protect the QoS of admitted flows.   Another difference may occur in the rare circumstance when there is a   failure: on the one hand, some PCN-flows may get terminated but, on   the other hand, other flows will get their QoS restored.  Non-PCN-   traffic is treated transparently, ie, the PCN-domain is a normal   Diffserv domain.3.1.  Flow Admission   The objective of PCN's flow admission control mechanism is to limit   the PCN-traffic on each link in the PCN-domain to *roughly* its PCN-   admissible-rate by admitting or blocking prospective new flows, in   order to protect the QoS of existing PCN-flows.  With three encoding   states available, the PCN-threshold-rate is configured by the   operator as equal to the PCN-admissible-rate on each link.  It is set   lower than the traffic rate at which the link becomes congested and   the node drops packets.   Exactly how the admission control decision is made will be defined   separately in Informational documents.  This document describes two   approaches (others might be possible):   o  The PCN-egress-node measures (possibly as a moving average) the      fraction of the PCN-traffic that is threshold-marked.  The      fraction is measured for a specific ingress-egress-aggregate.  If      the fraction is below a threshold value, then the new flow isEardley                      Informational                     [Page 13]

RFC 5559                    PCN Architecture                   June 2009      admitted; if the fraction is above the threshold value, then it is      blocked.  The fraction could be measured as an EWMA (exponentially      weighted moving average), which has sometimes been called the      "congestion level estimate".   o  The PCN-egress-node monitors PCN-traffic and if it receives one      (or several) threshold-marked packets, then the new flow is      blocked; otherwise, it is admitted.  One possibility may be to      react to the marking state of an initial flow-setup packet (eg,      RSVP PATH).  Another is that after one (or several) threshold-      marks, all flows are blocked until after a specific period of no      congestion.   Note that the admission control decision is made for a particular   pair of PCN-boundary-nodes.  So it is quite possible for a new flow   to be admitted between one pair of PCN-boundary-nodes, whilst at the   same time another admission request is blocked between a different   pair of PCN-boundary-nodes.3.2.  Flow Termination   The objective of PCN's flow termination mechanism is to limit the   PCN-traffic on each link to *roughly* its PCN-supportable-rate, by   terminating some existing PCN-flows, in order to protect the QoS of   the remaining PCN-flows.  With three encoding states available, the   PCN-excess-rate is configured by the operator as equal to the PCN-   supportable-rate on each link.  It may be set lower than the traffic   rate at which the link becomes congested and at which the node drops   packets.   Exactly how the flow termination decision is made will be defined   separately in Informational documents.  This document describes   several approaches (others might be possible):   o  In one approach, the PCN-egress-node measures the rate of PCN-      traffic that is not excess-traffic-marked, which is the amount of      PCN-traffic that can actually be supported, and communicates this      to the PCN-ingress-node.  Also, the PCN-ingress-node measures the      rate of PCN-traffic that is destined for this specific PCN-egress-      node.  The difference represents the excess amount that should be      terminated.   o  Another approach instead measures the rate of excess-traffic-      marked traffic and terminates this amount of traffic.  This      terminates less traffic than the previous approach, if some nodes      are dropping PCN-traffic.Eardley                      Informational                     [Page 14]

RFC 5559                    PCN Architecture                   June 2009   o  Another approach monitors PCN-packets and terminates some of the      PCN-flows that have an excess-traffic-marked packet.  (If all such      flows were terminated, far too much traffic would be terminated,      so a random selection needs to be made from those with an excess-      traffic-marked packet [Menth08-1].)   Since flow termination is designed for "abnormal" circumstances, it   is quite likely that some PCN-nodes are congested and, hence, that   packets are being dropped and/or significantly queued.  The flow   termination mechanism must accommodate this.   Note also that the termination control decision is made for a   particular pair of PCN-boundary-nodes.  So it is quite possible for   PCN-flows to be terminated between one pair of PCN-boundary-nodes,   whilst at the same time none are terminated between a different pair   of PCN-boundary-nodes.3.3.  Flow Admission and/or Flow Termination When There Are Only Two PCN      Encoding States   If a PCN-domain has only two encoding states available (PCN-marked   and not PCN-marked), ie, it is using the baseline encoding   [Moncaster09-1], then an operator has three options (others might be   possible):   o  admission control only: PCN-marking means threshold-marking, ie,      only the threshold-metering algorithm triggers PCN-marking.  Only      PCN admission control is available.   o  flow termination only: PCN-marking means excess-traffic-marking,      ie, only the excess-traffic-metering algorithm triggers PCN-      marking.  Only PCN termination control is available.   o  both admission control and flow termination: only the excess-      traffic-metering algorithm triggers PCN-marking; however, the      configured rate (PCN-excess-rate) is set equal to the PCN-      admissible-rate, as shown in Figure 3.  [Charny07-2] describes how      both admission control and flow termination can be triggered in      this case and also gives some pros and cons of this approach.  The      main downside is that admission control is less accurate.Eardley                      Informational                     [Page 15]

RFC 5559                    PCN Architecture                   June 2009                          ==   Metering &    ==                          ==Marking behaviour==       ==PCN mechanisms==                       ^           Rate of     ^      PCN-traffic on   |     bottleneck link   |                                  Terminate some                       |                                  admitted flows                       |                                         &                       |                                 Block new flows                       |                       |       Some pkts   U*PCN-excess-rate  -|  excess-traffic-marked        -----------------(=PCN-supportable-rate)|                       |                                 Block new flows                       |                       |     PCN-excess-rate  -|------------------------------------------------ (=PCN-admissible-rate)|                       |         No pkts                 Admit new flows                       |       PCN-marked                       |   Figure 3: Schematic of how the PCN admission control and flow   termination mechanisms operate as the rate of PCN-traffic increases,   for a PCN-domain with two encoding states and using the approach of   [Charny07-2].  Note: U is a global parameter for all links in the   PCN-domain.3.4.  Information Transport   The transport of pre-congestion information from a PCN-node to a PCN-   egress-node is through PCN-markings in data packet headers, ie, "in-   band"; no signalling protocol messaging is needed.  Signalling is   needed to transport PCN-feedback-information -- for example, to   convey the fraction of PCN-marked traffic from a PCN-egress-node to   the relevant PCN-ingress-node.  Exactly what information needs to be   transported will be described in future documents about possible   boundary mechanisms.  The signalling could be done by an extension of   RSVP or NSIS (Next Steps in Signalling), for instance; [Lefaucheur06]   describes the extensions needed for RSVP.3.5.  PCN-Traffic   The following are some high-level points about how PCN works:   o  There needs to be a way for a PCN-node to distinguish PCN-traffic      from other traffic.  This is through a combination of the DSCP      field and/or ECN field.Eardley                      Informational                     [Page 16]

RFC 5559                    PCN Architecture                   June 2009   o  It is not advised to have competing-non-PCN-traffic but, if there      is such traffic, there needs to be a mechanism to limit it.      "Competing-non-PCN-traffic" means traffic that shares a link with      PCN-traffic and competes for its forwarding bandwidth.  Hence,      more competing-non-PCN-traffic results in poorer QoS for PCN.      Further, the unpredictable amount of competing-non-PCN-traffic      makes the PCN mechanisms less accurate and so reduces PCN's      ability to protect the QoS of admitted PCN-flows.   o  Two examples of such competing-non-PCN-traffic are:      1.  traffic that is priority scheduled over PCN (perhaps a          particular application or an operator's control messages);      2.  traffic that is scheduled at the same priority as PCN (for          example, if the Voice-Admit codepoint is used for PCN-traffic          [Moncaster09-1] and there is non-PCN, voice-admit traffic in          the PCN-domain).   o  If there is such competing-non-PCN-traffic, then PCN's mechanisms      should take account of it, in order to improve the accuracy of the      decision about whether to admit (or terminate) a PCN-flow.  For      example, one mechanism is that such competing-non-PCN-traffic      contributes to the PCN-meters (ie, is metered by the threshold-      marking and excess-traffic-marking algorithms).   o  There will be other non-PCN-traffic that doesn't compete for the      same forwarding bandwidth as PCN-traffic, because it is forwarded      at lower priority.  Hence, it shouldn't contribute to the PCN-      meters.  Examples are best-effort and assured-forwarding traffic.      However, a PCN-node should dedicate some capacity to lower-      priority traffic so that it isn't starved.   o  This document assumes that the PCN mechanisms are applied to a      single behaviour aggregate in the PCN-domain.  However, it would      also be possible to apply them independently to more than one      behaviour aggregate, which are distinguished by DSCP.3.6.  Backwards Compatibility   PCN specifies semantics for the ECN field that differ from the   default semantics of [RFC3168].  A particular PCN encoding scheme   needs to describe how it meets the guidelines ofBCP 124 [RFC4774]   for specifying alternative semantics for the ECN field.  In summary,   the approach is to:   o  use a DSCP to allow PCN-nodes to distinguish PCN-traffic that uses      the alternative ECN semantics;Eardley                      Informational                     [Page 17]

RFC 5559                    PCN Architecture                   June 2009   o  define these semantics for use within a controlled region, the      PCN-domain;   o  take appropriate action if ECN-capable, non-PCN-traffic arrives at      a PCN-ingress-node with the DSCP used by PCN.   For the baseline encoding [Moncaster09-1], the "appropriate action"   is to block ECN-capable traffic that uses the same DSCP as PCN from   entering the PCN-domain directly.  "Blocking" means it is dropped or   downgraded to a lower-priority behaviour aggregate, or alternatively   such traffic may be tunnelled through the PCN-domain.  The reason   that "appropriate action" is needed is that the PCN-egress-node   clears the ECN field to 00.   Extended encoding schemes may need to take different "appropriate   action".4.  Detailed Functional Architecture   This section is intended to provide a systematic summary of the new   functional architecture in the PCN-domain.  First, it describes   functions needed at the three specific types of PCN-node; these are   data plane functions and are in addition to 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 handlingEardley                      Informational                     [Page 18]

RFC 5559                    PCN Architecture                   June 20094.1.  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     |   |         |  |                |      |    MarkedPacket =>|Classifier|==>| Dropper |==?===============>|Marker|==> PacketStream   |          |   |         |  |                |      |    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:Eardley                      Informational                     [Page 19]

RFC 5559                    PCN Architecture                   June 2009   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-Eardley                      Informational                     [Page 20]

RFC 5559                    PCN Architecture                   June 2009   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.Eardley                      Informational                     [Page 21]

RFC 5559                    PCN Architecture                   June 20094.5.  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 inSection 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 variousEardley                      Informational                     [Page 22]

RFC 5559                    PCN Architecture                   June 2009   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.Eardley                      Informational                     [Page 23]

RFC 5559                    PCN Architecture                   June 2009   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, seeSection 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 toEardley                      Informational                     [Page 24]

RFC 5559                    PCN Architecture                   June 2009   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 theEardley                      Informational                     [Page 25]

RFC 5559                    PCN Architecture                   June 2009   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.Eardley                      Informational                     [Page 26]

RFC 5559                    PCN Architecture                   June 2009   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 (whichEardley                      Informational                     [Page 27]

RFC 5559                    PCN Architecture                   June 2009      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 Sections3.1 and3.2).   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 inEardley                      Informational                     [Page 28]

RFC 5559                    PCN Architecture                   June 2009   these configured rates much more simply, by monitoring PCN-marking   rates from actual rather than expected traffic (seeSection 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.Eardley                      Informational                     [Page 29]

RFC 5559                    PCN Architecture                   June 20095.3.  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.Eardley                      Informational                     [Page 30]

RFC 5559                    PCN Architecture                   June 2009   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.Eardley                      Informational                     [Page 31]

RFC 5559                    PCN Architecture                   June 20096.  Applicability of PCN6.1.  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.Eardley                      Informational                     [Page 32]

RFC 5559                    PCN Architecture                   June 2009   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.  AsEardley                      Informational                     [Page 33]

RFC 5559                    PCN Architecture                   June 2009   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 alsoSection 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 inSection 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 networkEardley                      Informational                     [Page 34]

RFC 5559                    PCN Architecture                   June 2009   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 inSection 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.Eardley                      Informational                     [Page 35]

RFC 5559                    PCN Architecture                   June 2009   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 inSection 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 ofEardley                      Informational                     [Page 36]

RFC 5559                    PCN Architecture                   June 2009   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 ofEardley                      Informational                     [Page 37]

RFC 5559                    PCN Architecture                   June 2009   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].Eardley                      Informational                     [Page 38]

RFC 5559                    PCN Architecture                   June 2009   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 requestsEardley                      Informational                     [Page 39]

RFC 5559                    PCN Architecture                   June 2009      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.7.  Security Considerations   Security considerations essentially come from the Trust AssumptionSection 6.3.1, ie, that all PCN-nodes are PCN-enabled and are trusted   for truthful PCN-metering and PCN-marking.  PCN splits functionality   between PCN-interior-nodes and PCN-boundary-nodes, and the security   considerations are somewhat different for each, mainly because PCN-   boundary-nodes are flow-aware and PCN-interior-nodes are not.   o  Because PCN-boundary-nodes are flow-aware, they are trusted to use      that awareness correctly.  The degree of trust required depends on      the kinds of decisions they have to make and the kinds of      information they need to make them.  There is nothing specific to      PCN.   o  The PCN-ingress-nodes police packets to ensure a PCN-flow sticks      within its agreed limit, and to ensure that only PCN-flows that      have been admitted contribute PCN-traffic into the PCN-domain.      The policer must drop (or perhaps downgrade to a different DSCP)      any PCN-packets received that are outside this remit.  This is      similar to the existing IntServ behaviour.  Between them, the PCN-      boundary-nodes must encircle the PCN-domain; otherwise, PCN-      packets could enter the PCN-domain without being subject to      admission control, which would potentially destroy the QoS of      existing flows.   o  PCN-interior-nodes are not flow-aware.  This prevents some      security attacks where an attacker targets specific flows in the      data plane -- for instance, for DoS or eavesdropping.Eardley                      Informational                     [Page 40]

RFC 5559                    PCN Architecture                   June 2009   o  The PCN-boundary-nodes rely on correct PCN-marking by the PCN-      interior-nodes.  For instance, a rogue PCN-interior-node could      PCN-mark all packets so that no flows were admitted.  Another      possibility is that it doesn't PCN-mark any packets, even when it      is pre-congested.  More subtly, the rogue PCN-interior-node could      perform these attacks selectively on particular flows, or it could      PCN-mark the correct fraction overall but carefully choose which      flows it marked.   o  The PCN-boundary-nodes should be able to deal with DoS attacks and      state exhaustion attacks based on fast changes in per-flow      signalling.   o  The signalling between the PCN-boundary-nodes must be protected      from attacks.  For example, the recipient needs to validate that      the message is indeed from the node that claims to have sent it.      Possible measures include digest authentication and protection      against replay and man-in-the-middle attacks.  For the RSVP      protocol specifically, hop-by-hop authentication is in [RFC2747],      and [Behringer09] may also be useful.   Operational security advice is given inSection 5.5.8.  Conclusions   This document describes a general architecture for flow admission and   termination based on pre-congestion information, in order to protect   the quality of service of established, inelastic flows within a   single Diffserv domain.  The main topic is the functional   architecture.  This document also mentions other topics like the   assumptions and open issues associated with the PCN architecture.9.  Acknowledgements   This document is a revised version of an earlier individual working   draft authored by: P. Eardley, J. Babiarz, K. Chan, A. Charny, R.   Geib, G. Karagiannis, M. Menth, and T. Tsou.  They are therefore   contributors to this document.   Thanks to those who have made comments on this document: Lachlan   Andrew, Joe Babiarz, Fred Baker, David Black, Steven Blake, Ron   Bonica, Scott Bradner, Bob Briscoe, Ross Callon, Jason Canon, Ken   Carlberg, Anna Charny, Joachim Charzinski, Andras Csaszar, Francis   Dupont, Lars Eggert, Pasi Eronen, Adrian Farrel, Ruediger Geib, Wei   Gengyu, Robert Hancock, Fortune Huang, Christian Hublet, Cullen   Jennings, Ingemar Johansson, Georgios Karagiannis, Hein Mekkes,   Michael Menth, Toby Moncaster, Dimitri Papadimitriou, Dan Romascanu,   Daisuke Satoh, Ben Strulo, Tom Taylor, Hannes Tschofenig, Tina Tsou,Eardley                      Informational                     [Page 41]

RFC 5559                    PCN Architecture                   June 2009   David Ward, Lars Westberg, Magnus Westerlund, and Delei Yu.  Thanks   to Bob Briscoe who extensively revised the Operations and Management   section.   This document is the result of discussions in the PCN WG and   forerunner activity in the TSVWG.  A number of previous drafts were   presented to TSVWG; their authors were: B. Briscoe, P. Eardley, D.   Songhurst, F. Le Faucheur, A. Charny, J. Babiarz, K. Chan, S. Dudley,   G. Karagiannis, A. Bader, L. Westberg, J. Zhang, V. Liatsos, X-G.   Liu, and A. Bhargava.   The admission control mechanism evolved from the work led by Martin   Karsten on the Guaranteed Stream Provider developed in the M3I   project [Karsten02] [M3I], which in turn was based on the theoretical   work of Gibbens and Kelly [Gibbens99].10.  References10.1.  Normative References   [RFC2474]        Nichols, K., Blake, S., Baker, F., and D. Black,                    "Definition of the Differentiated Services Field (DS                    Field) in the IPv4 and IPv6 Headers",RFC 2474,                    December 1998.   [RFC3246]        Davie, B., Charny, A., Bennet, J., Benson, K., Le                    Boudec, J., Courtney, W., Davari, S., Firoiu, V.,                    and D. Stiliadis, "An Expedited Forwarding PHB (Per-                    Hop Behavior)",RFC 3246, March 2002.10.2.  Informative References   [RFC1633]        Braden, B., Clark, D., and S. Shenker, "Integrated                    Services in the Internet Architecture: an Overview",RFC 1633, June 1994.   [RFC2205]        Braden, B., Zhang, L., Berson, S., Herzog, S., and                    S. Jamin, "Resource ReSerVation Protocol (RSVP) --                    Version 1 Functional Specification",RFC 2205,                    September 1997.   [RFC2211]        Wroclawski, J., "Specification of the Controlled-                    Load Network Element Service",RFC 2211,                    September 1997.   [RFC2475]        Blake, S., Black, D., Carlson, M., Davies, E., Wang,                    Z., and W. Weiss, "An Architecture for                    Differentiated Services",RFC 2475, December 1998.Eardley                      Informational                     [Page 42]

RFC 5559                    PCN Architecture                   June 2009   [RFC2747]        Baker, F., Lindell, B., and M. Talwar, "RSVP                    Cryptographic Authentication",RFC 2747,                    January 2000.   [RFC2753]        Yavatkar, R., Pendarakis, D., and R. Guerin, "A                    Framework for Policy-based Admission Control",RFC 2753, January 2000.   [RFC2983]        Black, D., "Differentiated Services and Tunnels",RFC 2983, October 2000.   [RFC2998]        Bernet, Y., Ford, P., Yavatkar, R., Baker, F.,                    Zhang, L., Speer, M., Braden, R., Davie, B.,                    Wroclawski, J., and E. Felstaine, "A Framework for                    Integrated Services Operation over Diffserv                    Networks",RFC 2998, November 2000.   [RFC3168]        Ramakrishnan, K., Floyd, S., and D. Black, "The                    Addition of Explicit Congestion Notification (ECN)                    to IP",RFC 3168, September 2001.   [RFC3270]        Le Faucheur, F., Wu, L., Davie, B., Davari, S.,                    Vaananen, P., Krishnan, R., Cheval, P., and J.                    Heinanen, "Multi-Protocol Label Switching (MPLS)                    Support of Differentiated Services",RFC 3270,                    May 2002.   [RFC3393]        Demichelis, C. and P. Chimento, "IP Packet Delay                    Variation Metric for IP Performance Metrics (IPPM)",RFC 3393, November 2002.   [RFC3411]        Harrington, D., Presuhn, R., and B. Wijnen, "An                    Architecture for Describing Simple Network                    Management Protocol (SNMP) Management Frameworks",                    STD 62,RFC 3411, December 2002.   [RFC3726]        Brunner, M., "Requirements for Signaling Protocols",RFC 3726, April 2004.   [RFC4216]        Zhang, R. and J. Vasseur, "MPLS Inter-Autonomous                    System (AS) Traffic Engineering (TE) Requirements",RFC 4216, November 2005.   [RFC4301]        Kent, S. and K. Seo, "Security Architecture for the                    Internet Protocol",RFC 4301, December 2005.   [RFC4303]        Kent, S., "IP Encapsulating Security Payload (ESP)",RFC 4303, December 2005.Eardley                      Informational                     [Page 43]

RFC 5559                    PCN Architecture                   June 2009   [RFC4594]        Babiarz, J., Chan, K., and F. Baker, "Configuration                    Guidelines for DiffServ Service Classes",RFC 4594,                    August 2006.   [RFC4656]        Shalunov, S., Teitelbaum, B., Karp, A., Boote, J.,                    and M. Zekauskas, "A One-way Active Measurement                    Protocol (OWAMP)",RFC 4656, September 2006.   [RFC4774]        Floyd, S., "Specifying Alternate Semantics for the                    Explicit Congestion Notification (ECN) Field",BCP 124,RFC 4774, November 2006.   [RFC4778]        Kaeo, M., "Operational Security Current Practices in                    Internet Service Provider Environments",RFC 4778,                    January 2007.   [RFC5129]        Davie, B., Briscoe, B., and J. Tay, "Explicit                    Congestion Marking in MPLS",RFC 5129, January 2008.   [RFC5462]        Andersson, L. and R. Asati, "Multiprotocol Label                    Switching (MPLS) Label Stack Entry: "EXP" Field                    Renamed to "Traffic Class" Field",RFC 5462,                    February 2009.   [P.800]          "Methods for subjective determination of                    transmission quality", ITU-T Recommendation P.800,                    August 1996.   [Y.1541]         "Network Performance Objectives for IP-based                    Services", ITU-T Recommendation Y.1541,                    February 2006.   [Babiarz06]      Babiarz, J., Chan, K., Karagiannis, G., and P.                    Eardley, "SIP Controlled Admission and Preemption",                    Work in Progress, October 2006.   [Behringer09]    Behringer, M. and F. Le Faucheur, "Applicability of                    Keying Methods for RSVP Security", Work in Progress,                    March 2009.   [Briscoe06]      Briscoe, B., Eardley, P., Songhurst, D., Le                    Faucheur, F., Charny, A., Babiarz, J., Chan, K.,                    Dudley, S., Karagiannis, G., Bader, A., and L.                    Westberg, "An edge-to-edge Deployment Model for Pre-                    Congestion Notification: Admission Control over a                    Diffserv Region", Work in Progress, October 2006.Eardley                      Informational                     [Page 44]

RFC 5559                    PCN Architecture                   June 2009   [Briscoe08]      Briscoe, B., "Emulating Border Flow Policing using                    Re-PCN on Bulk Data", Work in Progress,                    September 2008.   [Briscoe09]      Briscoe, B., "Tunnelling of Explicit Congestion                    Notification", Work in Progress, March 2009.   [Bryant08]       Bryant, S., Davie, B., Martini, L., and E.  Rosen,                    "Pseudowire Congestion Control Framework", Work                    in Progress, May 2008.   [Charny07-1]     Charny, A., Babiarz, J., Menth, M., and X. Zhang,                    "Comparison of Proposed PCN Approaches", Work                    in Progress, November 2007.   [Charny07-2]     Charny, A., Zhang, X., Le Faucheur, F., and V.                    Liatsos, "Pre-Congestion Notification Using Single                    Marking for Admission and Termination", Work                    in Progress, November 2007.   [Charny07-3]     Charny, A., "Email to PCN WG mailing list",                    November 2007, <http://www1.ietf.org/mail-archive/web/pcn/current/msg00871.html>.   [Charny08]       Charny, A., "Email to PCN WG mailing list",                    March 2008, <http://www1.ietf.org/mail-archive/web/pcn/current/msg01359.html>.   [Eardley07]      Eardley, P., "Email to PCN WG mailing list",                    October 2007, <http://www1.ietf.org/mail-archive/web/pcn/current/msg00831.html>.   [Eardley09]      Eardley, P., "Metering and marking behaviour of PCN-                    nodes", Work in Progress, May 2009.   [Gibbens99]      Gibbens, R. and F. Kelly, "Distributed connection                    acceptance control for a connectionless network",                    Proceedings International Teletraffic Congress                    (ITC16), Edinburgh, pp. 941-952, 1999.   [Hancock02]      Hancock, R. and E. Hepworth, "Slide 14 of 'NSIS: An                    Outline Framework for QoS Signalling'", May 2002, <h                    ttp://www-nrc.nokia.com/sua/nsis/interim/                    nsis-framework-outline.ppt>.Eardley                      Informational                     [Page 45]

RFC 5559                    PCN Architecture                   June 2009   [Iyer03]         Iyer, S., Bhattacharyya, S., Taft, N., and C. Diot,                    "An approach to alleviate link overload as observed                    on an IP backbone", IEEE INFOCOM, 2003,                    <http://www.ieee-infocom.org/2003/papers/10_04.pdf>.   [Karsten02]      Karsten, M. and J. Schmitt, "Admission Control Based                    on Packet Marking and Feedback Signalling --                    Mechanisms, Implementation and Experiments", TU-                    Darmstadt Technical Report TR-KOM-2002-03, May 2002,                    <http://www.kom.e-technik.tu-darmstadt.de/publications/abstracts/KS02-5.html>.   [Kumar01]        Kumar, A., Rastogi, R., Silberschatz, A., and B.                    Yener, "Algorithms for Provisioning Virtual Private                    Networks in the Hose Model", Proceedings ACM SIGCOMM                    (ITC16), , 2001.   [Lefaucheur06]   Le Faucheur, F., Charny, A., Briscoe, B., Eardley,                    P., Babiarz, J., and K. Chan, "RSVP Extensions for                    Admission Control over Diffserv using Pre-congestion                    Notification (PCN)", Work in Progress, June 2006.   [M3I]            "M3I - Market Managed Multiservice Internet",                    <http://www.m3iproject.org/>.   [Menth08-1]      Menth, M., Lehrieder, F., Eardley, P., Charny, A.,                    and J. Babiarz, "Edge-Assisted Marked Flow                    Termination", Work in Progress, February 2008.   [Menth08-2]      Menth, M., Babiarz, J., Moncaster, T., and B.                    Briscoe, "PCN Encoding for Packet-Specific Dual                    Marking (PSDM)", Work in Progress, July 2008.   [Menth09-1]      Menth, M. and M. Hartmann, "Threshold Configuration                    and Routing Optimization for PCN-Based Resilient                    Admission Control", Computer Networks, 2009,                    <http://dx.doi.org/10.1016/j.comnet.2009.01.013>.   [Menth09-2]      Menth, M., Lehrieder, F., Briscoe, B., Eardley, P.,                    Moncaster, T., Babiarz, J., Chan, K., Charny, A.,                    Karagiannis, G., Zhang, X., Taylor, T., Satoh, D.,                    and R. Geib, "A Survey of PCN-Based Admission                    Control and Flow Termination", IEEE                    Communications Surveys and Tutorials, <http://www3.informatik.uni-wuerzburg.de/staff/menth/Publications/papers/Menth08-PCN-Overview.pdf>>.Eardley                      Informational                     [Page 46]

RFC 5559                    PCN Architecture                   June 2009   [Moncaster09-1]  Moncaster, T., Briscoe, B., and M. Menth, "Baseline                    Encoding and Transport of Pre-Congestion                    Information", Work in Progress, May 2009.   [Moncaster09-2]  Moncaster, T., Briscoe, B., and M. Menth, "A PCN                    encoding using 2 DSCPs to provide 3 or more states",                    Work in Progress, April 2009.   [Sarker08]       Sarker, Z. and I. Johansson, "Usecases and Benefits                    of end to end ECN support in PCN Domains", Work                    in Progress, November 2008.   [Songhurst06]    Songhurst, DJ., Eardley, P., Briscoe, B., Di Cairano                    Gilfedder, C., and J. Tay, "Guaranteed QoS Synthesis                    for Admission Control with Shared Capacity", BT                    Technical Report TR-CXR9-2006-001, Feburary 2006,                    <http://www.cs.ucl.ac.uk/staff/B.Briscoe/projects/ipe2eqos/gqs/papers/GQS_shared_tr.pdf>.   [Taylor09]       Charny, A., Huang, F., Menth, M., and T. Taylor,                    "PCN Boundary Node Behaviour for the Controlled Load                    (CL) Mode of Operation", Work in Progress,                    March 2009.   [Tsou08]         Tsou, T., Huang, F., and T. Taylor, "Applicability                    Statement for the Use of Pre-Congestion Notification                    in a Resource-Controlled Network", Work in Progress,                    November 2008.   [Westberg08]     Westberg, L., Bhargava, A., Bader, A., Karagiannis,                    G., and H. Mekkes, "LC-PCN: The Load Control PCN                    Solution", Work in Progress, November 2008.Eardley                      Informational                     [Page 47]

RFC 5559                    PCN Architecture                   June 2009Appendix A.  Possible Future Work Items   This section mentions some topics that are outside the PCN WG's   current charter but that 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, or an operator-specific   usage that is not standardised.   Note: It should be crystal clear that this section discusses   possibilities only.   The first set of possibilities relate to the restrictions described   inSection 6.3:   o  A single PCN-domain encompasses several autonomous systems that do      not trust each other.  A possible solution is a mechanism like re-      PCN [Briscoe08].   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 of the good QoS on the      virtual link (the tunnel).  Another example is that PCN is      deployed on the general Internet (ie, widely but not universally      deployed).   o  Applying the PCN mechanisms to other types of traffic, ie, beyond      inelastic traffic -- for instance, applying the PCN mechanisms to      traffic scheduled with the Assured Forwarding per-hop behaviour.      One example could be flow-rate adaptation by elastic applications      that adapt according to the pre-congestion information.   o  The aggregation assumption doesn't hold, because the link capacity      is too low.  Measurement-based admission control is less accurate,      with a greater risk of over-admission for instance.   o  The applicability of PCN mechanisms for emergency use (911, GETS,      WPS, MLPP, etc.).   Other possibilities include:   o  Probing.  This is discussed inAppendix A.1 below.   o  The PCN-domain extends to the end users.  This scenario is      described in [Babiarz06].  The end users need to be trusted to do      their own policing.  If there is sufficient traffic, then the      aggregation assumption may hold.  A variant is that the PCN-domain      extends out as far as the LAN edge switch.Eardley                      Informational                     [Page 48]

RFC 5559                    PCN Architecture                   June 2009   o  Indicating pre-congestion through signalling messages rather than      in-band (in the form of PCN-marked packets).   o  The decision-making functionality is at a centralised node rather      than at the PCN-boundary-nodes.  This requires that the PCN-      egress-node signals PCN-feedback-information to the centralised      node, and that the centralised node signals to the PCN-ingress-      node the decision about admission (or termination).  Such      possibility may need the centralised node and the PCN-boundary-      nodes to be configured with each other's addresses.  The      centralised case is described further in [Tsou08].   o  Signalling extensions for specific protocols (eg, RSVP and 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 for coping      with failure of a PCN-boundary-node ([Briscoe06] considers what      happens if RSVP is the QoS signalling protocol) and for      establishing a tunnel across the PCN-domain if it is necessary to      carry ECN marks transparently.   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 [Bryant08].   o  PCN for MPLS.  [RFC3270] defines how to support the Diffserv      architecture in MPLS (Multiprotocol Label Switching) networks.      [RFC5129] describes how to add PCN for admission control of      microflows into a set of MPLS aggregates.  PCN-marking is done in      MPLS's EXP field (which [RFC5462] re-names the Class of Service      (CoS) field).   o  PCN for Ethernet.  Similarly, it may be possible to extend PCN      into Ethernet networks, where PCN-marking is done in the Ethernet      header.  Note: Specific consideration of this extension is outside      of the IETF's remit.Eardley                      Informational                     [Page 49]

RFC 5559                    PCN Architecture                   June 2009A.1.  ProbingA.1.1.  Introduction   Probing is a potential mechanism to assist admission control.   PCN's admission control, as described so far, is essentially a   reactive mechanism where the PCN-egress-node monitors the pre-   congestion level for traffic from each PCN-ingress-node; if the level   rises, then it blocks new flows on that ingress-egress-aggregate.   However, it's possible that an ingress-egress-aggregate carries no   traffic, and so the PCN-egress-node can't make an admission decision   using the usual method described earlier.   One approach is to be "optimistic" and simply admit the new flow.   However, it's possible to envisage a scenario where the traffic   levels on other ingress-egress-aggregates are already so high that   they're blocking new PCN-flows, and admitting a new flow onto this   "empty" ingress-egress-aggregate adds extra traffic onto a link that   is already pre-congested.  This 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, a   sufficient 'safety margin' above the PCN-threshold-rate.   An alternative approach is to make PCN a more proactive mechanism.   The PCN-ingress-node explicitly determines, before admitting the   prospective new flow, whether the ingress-egress-aggregate can   support it.  This can be seen as a "pessimistic" approach, in   contrast to the "optimism" of the approach above.  It involves   probing: a PCN-ingress-node generates and sends probe packets in   order to test the pre-congestion level that the flow would   experience.   One possibility is that a probe packet is just a dummy data packet,   generated by the PCN-ingress-node and addressed to the PCN-egress-   node.A.1.2.  Probing Functions   The probing functions are:   o  Make the decision that probing is needed.  As described above,      this is when the ingress-egress-aggregate (or the ECMP path -- seeSection 6.4) carries no PCN-traffic.  An alternative is to always      probe, ie, probe before admitting any PCN-flow.Eardley                      Informational                     [Page 50]

RFC 5559                    PCN Architecture                   June 2009   o  (if required) Communicate the request that probing is needed; the      PCN-egress-node signals to the PCN-ingress-node that probing is      needed.   o  (if required) Generate probe traffic; the PCN-ingress-node      generates the probe traffic.  The appropriate number (or rate) of      probe packets will depend on the PCN-metering algorithm; for      example, an excess-traffic-metering algorithm triggers fewer PCN-      marks than a threshold-metering algorithm, and so will need more      probe packets.   o  Forward probe packets; as far as PCN-interior-nodes are concerned,      probe packets are handled the same as (ordinary data) PCN-packets      in terms of routing, scheduling, and PCN-marking.   o  Consume probe packets; the PCN-egress-node consumes probe packets      to ensure that they don't travel beyond the PCN-domain.A.1.3.  Discussion of Rationale for Probing, Its Downsides and Open        Issues   It is an unresolved question whether probing is really needed, but   two viewpoints have been put forward as to why it is useful.  The   first is perhaps the most obvious: there is no PCN-traffic on the   ingress-egress-aggregate.  The second assumes that multipath routing   (eg, ECMP) is running in the PCN-domain.  We now consider each in   turn.   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, [Eardley07] suggests that, during the future   busy hour of a national network with about 100 PCN-boundary-nodes,   there are likely to be significant numbers of aggregates with very   few flows under nearly all circumstances.   The latter bullet could occur if new flows start on many of the empty   ingress-egress-aggregates, which together overload a link in the PCN-   domain.  To be a problem, this would probably have to happen in a   short time period (flash crowd) because, after the reaction time of   the system, other (non-empty) ingress-egress-aggregates that pass   through the link will measure pre-congestion and so block new flows.   Also, flows naturally end anyway.Eardley                      Informational                     [Page 51]

RFC 5559                    PCN Architecture                   June 2009   The downsides of probing for this viewpoint are:   o  Probing adds delay to the admission control process.   o  Sufficient probing traffic has to be generated to test the pre-      congestion level of the ingress-egress-aggregate.  But the probing      traffic itself may cause pre-congestion, causing other PCN-flows      to be blocked or even terminated -- and, in the flash crowd      scenario, there will be probing on many ingress-egress-aggregates.   The second viewpoint applies in the case where there is multipath   routing (eg, 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:        *  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 of           blocking a flow that should be admitted.        Note: [Charny07-3] suggests unbalanced traffic is quite        possible, even with quite a large number of flows on a PCN-link        (eg, 1000), when Assumption 3 (aggregation) is likely to be        satisfied.   (2)  If admission control is based on measurements of pre-congestion        on specific ECMP paths, then the viewpoint that probing is        useful assumes:        *  There is no PCN-traffic on the ECMP path on which to base an           admission decision.        *  Simply admitting the new flow has a significant risk of           leading to overload.        *  The PCN-egress-node can match a packet to an ECMP path.        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 ECMP paths between each pair of PCN-boundary-nodes,        it's presumably more likely that an ECMP path is "empty" than an        ingress-egress-aggregate is.  To constrain the number of ECMP        paths, a few tunnels could be set up between each pair of PCN-Eardley                      Informational                     [Page 52]

RFC 5559                    PCN Architecture                   June 2009        boundary-nodes.  Tunnelling also solves the issue in the point        immediately above (which is otherwise hard to solve because an        ECMP routing decision is made independently on each node).   The downsides of probing for this viewpoint are:   o  Probing adds delay to the admission control process.   o  Sufficient probing traffic has to be generated to test the pre-      congestion level of the ECMP path.  But there's the risk that the      probing traffic itself may cause pre-congestion, causing other      PCN-flows to be blocked or even terminated.   o  The PCN-egress-node needs to consume the probe packets to ensure      they don't travel beyond the PCN-domain, since they might confuse      the destination end node.  This is non-trivial, since probe      packets are addressed to the destination end node in order to test      the relevant ECMP path (ie, they are not addressed to the PCN-      egress-node, unlike the first viewpoint above).   The open issues associated with these viewpoints include:   o  What rate and pattern of probe packets does the PCN-ingress-node      need to generate so that there's enough traffic to make the      admission decision?   o  What difficulty does the delay (whilst probing is done), and      possible packet drops, cause applications?   o  Can the delay be alleviated by automatically and periodically      probing on the ingress-egress-aggregate?  Or does this add too      much overhead?   o  Are there other ways of dealing with the flash crowd scenario?      For instance, by limiting the rate at which new flows are      admitted, or perhaps by a PCN-egress-node blocking new flows on      its empty ingress-egress-aggregates when its non-empty ones are      pre-congested.   o  (Second viewpoint only) How does the PCN-egress-node disambiguate      probe packets from data packets (so it can consume the former)?      The PCN-egress-node must match the characteristic setting of      particular bits in the probe packet's header or body, but these      bits must not be used by any PCN-interior-node's ECMP algorithm.      In the general case, this isn't possible, but it should be      possible for a typical ECMP algorithm (which examines the source      and destination IP addresses and port numbers, the protocol ID,      and the DSCP).Eardley                      Informational                     [Page 53]

RFC 5559                    PCN Architecture                   June 2009Author's Address   Philip Eardley (editor)   BT   B54/77, Sirius House Adastral Park Martlesham Heath   Ipswich, Suffolk  IP5 3RE   United Kingdom   EMail: philip.eardley@bt.comEardley                      Informational                     [Page 54]