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TSVWG                                                        B. Briscoe
Internet Draft                                               P. Eardley
draft-briscoe-tsvwg-cl-architecture-03.txt                 D. Songhurst
Expires: December 2006                                               BT

                                                       F. Le Faucheur
                                                              A. Charny
                                                    Cisco Systems, Inc

                                                           J. Babiarz
                                                                K. Chan
                                                            S. Dudley
                                                               Nortel

                                                       G. Karagiannis
                                       University of Twente / Ericsson

                                                             A. Bader
                                                          L. Westberg
                                                             Ericsson

                                                        26 June, 2006


     An edge-to-edge Deployment Model for Pre-Congestion Notification:
                 Admission Control over a DiffServ Region
                draft-briscoe-tsvwg-cl-architecture-03.txt


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   The list of Internet-Draft Shadow Directories can be accessed at
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   This Internet-Draft will expire on September 6, 2006.

Copyright Notice

   Copyright (C) The Internet Society (2006).  All Rights Reserved.

Abstract

   This document describes a deployment model for pre-congestion
   notification (PCN). PCN-based flow admission control and if necessary
   flow pre-emption preserve the Controlled Load service to admitted
   flows. Routers in a large DiffServ-based region of the Internet use
   new pre-congestion notification marking to give early warning of
   their own congestion. Gateways around the edges of the region convert
   measurements of this packet granularity marking into admission
   control and pre-emption functions at flow granularity. Note that
   interior routers of the DiffServ-based region do not require flow
   state or signalling - they only have to do the bulk packet marking of
   PCN. Hence an end-to-end Controlled Load service can be achieved
   without any scalability impact on interior routers.



Authors' Note (TO BE DELETED BY THE RFC EDITOR UPON PUBLICATION)

   This document is posted as an Internet-Draft with the intention of
   eventually becoming an INFORMATIONAL RFC.



















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Table of Contents


   1. Introduction......................................... 5
      1.1. Summary......................................... 5
         1.1.1. Flow admission control........................ 7
         1.1.2. Flow pre-emption............................. 9
         1.1.3. Both admission control and pre-emption.......... 10
      1.2. Terminology.................................... 10
      1.3. Existing terminology............................. 12
      1.4. Standardisation requirements...................... 12
      1.5. Structure of rest of the document.................. 13
   2. Key aspects of the deployment model..................... 14
      2.1. Key goals...................................... 14
      2.2. Key assumptions................................. 15
      2.3. Key benefits ................................... 17
   3. Deployment model.................................... 19
      3.1. Admission control ............................... 19
         3.1.1. Pre-Congestion Notification for Admission Marking. 19
         3.1.2. Measurements to support admission control........ 19
         3.1.3. How edge-to-edge admission control supports end-to-end
         QoS signalling ................................... 20
         3.1.4. Use case................................... 20
      3.2. Flow pre-emption................................ 22
         3.2.1. Alerting an ingress gateway that flow pre-emption may be
         needed.......................................... 22
         3.2.2. Determining the right amount of CL traffic to drop 24
         3.2.3. Use case for flow pre-emption ................. 25
   4. Summary of Functionality.............................. 27
      4.1. Ingress gateways................................ 27
      4.2. Interior routers................................ 28
      4.3. Egress gateways................................. 28
      4.4. Failures....................................... 29
   5. Limitations and some potential solutions................. 31
      5.1. ECMP.......................................... 31
      5.2. Beat down effect................................ 33
      5.3. Bi-directional sessions .......................... 35
      5.4. Global fairness................................. 37
      5.5. Flash crowds ................................... 39
      5.6. Pre-empting too fast............................. 40
      5.7. Other potential extensions........................ 42
         5.7.1. Tunnelling................................. 42
         5.7.2. Multi-domain and multi-operator usage........... 43
         5.7.3. Preferential dropping of pre-emption marked packets43
         5.7.4. Adaptive bandwidth for the Controlled Load service 44
         5.7.5. Controlled Load service with end-to-end Pre-Congestion
         Notification..................................... 44


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         5.7.6. MPLS-TE ................................... 45
   6. Relationship to other QoS mechanisms.................... 46
      6.1. IntServ Controlled Load .......................... 46
      6.2. Integrated services operation over DiffServ.......... 46
      6.3. Differentiated Services .......................... 46
      6.4. ECN........................................... 47
      6.5. RTECN......................................... 47
      6.6. RMD........................................... 47
      6.7. RSVP Aggregation over MPLS-TE...................... 48
   7. Security Considerations............................... 49
   8. Acknowledgements.................................... 49
   9. Comments solicited................................... 49
   10. Changes from earlier versions of the draft.............. 50
   11. Appendices ........................................ 51
      11.1. Appendix A: Explicit Congestion Notification ........ 51
      11.2. Appendix B: What is distributed measurement-based admission
      control?........................................... 52
      11.3. Appendix C: Calculating the Exponentially weighted moving
      average (EWMA)...................................... 53
   12. References ........................................ 55
   Authors' Addresses..................................... 60
   Intellectual Property Statement .......................... 62
   Disclaimer of Validity.................................. 62
   Copyright Statement.................................... 62
























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1. Introduction

1.1. Summary

   This document describes a deployment model to achieve an end-to-end
   Controlled Load service by using (within a large region of the
   Internet) DiffServ and edge-to-edge distributed measurement-based
   admission control and flow pre-emption. Controlled load service is a
   quality of service (QoS) closely approximating the QoS that the same
   flow would receive from a lightly loaded network element [RFC2211].
   Controlled Load (CL) is useful for inelastic flows such as those for
   real-time media.

   In line with the "IntServ over DiffServ" framework defined in
   [RFC2998], the CL service is supported end-to-end and RSVP signalling
   [RFC2205] is used end-to-end, over an edge-to-edge DiffServ region.

 ___    ___    _______________________________________    ____    ___
|   |  |   |  |                                       |  |    |  |   |
|   |  |   |  |Ingress         Interior         Egress|  |    |  |   |
|   |  |   |  |gateway         routers         gateway|  |    |  |   |
|   |  |   |  |-------+  +-------+  +-------+  +------|  |    |  |   |
|   |  |   |  | PCN-  |  | PCN-  |  | PCN-  |  |      |  |    |  |   |
|   |..|   |..|marking|..|marking|..|marking|..| Meter|..|    |..|   |
|   |  |   |  |-------+  +-------+  +-------+  +------|  |    |  |   |
|   |  |   |  |  \                                 /  |  |    |  |   |
|   |  |   |  |   \                               /   |  |    |  |   |
|   |  |   |  |    \  Congestion-Level-Estimate  /    |  |    |  |   |
|   |  |   |  |     \  (for admission control)  /     |  |    |  |   |
|   |  |   |  |      --<-----<----<----<-----<--      |  |    |  |   |
|   |  |   |  |      Sustainable-Aggregate-Rate       |  |    |  |   |
|   |  |   |  |        (for flow pre-emption)         |  |    |  |   |
|___|  |___|  |_______________________________________|  |____|  |___|

Sx     Access               CL-region                   Access    Rx
End    Network                                          Network   End
Host                                                              Host
                <------ edge-to-edge signalling ----->
              (for admission control & flow pre-emption)

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

Figure 1: Overall QoS architecture (NB terminology explained later)


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   Figure 1 shows an example of an overall QoS architecture, where the
   two access networks are connected by a CL-region. Another possibility
   is that there are several CL-regions between the access networks -
   each would operate the Pre-Congestion Notification mechanisms
   separately.

   In Section 1.1.1 we summarise how admission of new CL microflows is
   controlled so as to deliver the required QoS. In abnormal
   circumstances, for instance a disaster affecting multiple interior
   routers, then the QoS on existing CL microflows may degrade even if
   care was exercised when admitting those microflows before those
   circumstances. Therefore we also propose a mechanism (summarised in
   Section 1.1.2) to pre-empt some of the existing microflows. Then
   remaining microflows retain their expected QoS, while improved QoS is
   quickly restored to lower priority traffic.

   As a fundamental building block to support these two mechanisms, we
   introduce "Pre-Congestion Notification". Pre-Congestion Notification
   (PCN) builds on the concepts of RFC 3168, "The addition of Explicit
   Congestion Notification to IP". The [PCN] document defines the
   respective algorithms that determine when a PCN-enabled router marks
   a packet with Admission Marking or Pre-emption Marking, depending on
   the traffic level.

   In order to support CL traffic we would expect PCN to supplement the
   existing Expedited Forwarding (EF). Within the controlled edge-to-
   edge region, a particular packet receives the Pre-Congestion
   Notification (PCN) behaviour if the packet's differentiated services
   codepoint (DSCP) is set to EF and also the ECN field indicates ECN
   Capable Transport. However, PCN is not only intended to supplement
   EF. PCN is specified (in [PCN]) as a building block which can
   supplement the scheduling behaviour of other PHBs.

   There are various possible ways to encode the markings into a packet,
   using the ECN field and perhaps other DSCPs, which are discussed in
   [PCN]. In this draft we use the abstract names Admission Marking and
   Pre-emption Marking.

   This framework assumes that the Pre-Congestion Notification behaviour
   is used in a controlled environment, i.e. within the controlled edge-
   to-edge region.






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1.1.1. Flow admission control

   This document describes a new admission control procedure for an
   edge-to-edge region, which uses new per-hop Pre-Congestion
   Notification 'admission marking' as a fundamental building block. In
   turn, an end-to-end CL service would use this as a building block
   within a broader QoS architecture.

   The per-hop, edge-to-edge and end-to-end aspects are now briefly
   introduced in turn.

   Appendix A provides a brief summary of Explicit Congestion
   Notification (ECN) [RFC3168]. It specifies that a router sets the ECN
   field to the Congestion Experienced (CE) value as a warning of
   incipient congestion. RFC3168 doesn't specify a particular algorithm
   for setting the CE codepoint, although Random Early Detection (RED)
   is expected to be used.

   Pre-Congestion Notification (PCN) builds on the concepts of ECN. PCN
   introduces a new algorithm that Admission Marks packets before there
   is any significant build-up of CL packets in the queue. Admission
   marked packets therefore act as an "early warning" when the amount of
   packets flowing is getting close to the engineered capacity. Hence it
   can be used with per-hop behaviours (PHBs) designed to operate with
   very low queue occupancy, such as Expedited Forwarding (EF). Note
   that our use of the ECN field operates across the CL-region, i.e.
   edge-to-edge, and not host-to-host as in [RFC3168].

   Turning next to the edge-to-edge aspect. All routers within a region
   of the Internet, which we call the CL-region, apply the PHB used for
   CL traffic and the Pre-Congestion Notification behaviour. Traffic
   must enter/leave the CL-region through ingress/egress gateways, which
   have special functionality. Typically the CL-region is the core or
   backbone of an operator. The CL service is achieved "edge-to-edge"
   across the CL-region, by using distributed measurement-based
   admission control: the decision whether to admit a new microflow
   depends on a measurement of the existing traffic between the same
   pair of ingress and egress gateways (i.e. the same pair as the
   prospective new microflow). (See Appendix B for further discussion on
   "What is distributed measurement-based admission control?")

   As CL packets travel across the CL-region, routers will admission
   mark packets (according to the Pre-Congestion Notification algorithm)
   as an "early warning" of potential congestion, i.e. before there is
   any significant build-up of CL packets in the queue. For traffic from
   each remote ingress gateway, the CL-region's egress gateway measures
   the fraction of CL traffic that is admission marked. The egress


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   gateway calculates the value on a per bit basis as a moving average
   (exponentially weighted is suggested), and which we term Congestion-
   Level-Estimate (CLE). Then it reports it to the CL-region's ingress
   gateway, piggy-backed on the signalling for a new flow. The ingress
   gateway only admits the new CL microflow if the Congestion-Level-
   Estimate is less than the value of the CLE-threshold. Hence
   previously accepted CL microflows will suffer minimal queuing delay,
   jitter and loss.

   In turn, the edge-to-edge architecture is a building block in
   delivering an end-to-end CL service. The approach is similar to that
   described in [RFC2998] for Integrated services operation over
   DiffServ networks. Like [RFC2998], an IntServ class (CL in our case)
   is achieved end-to-end, with a CL-region viewed as a single
   reservation hop in the total end-to-end path. Interior routers of the
   CL-region do not process flow signalling nor do they hold per flow
   state. We assume that the end-to-end signalling mechanism is RSVP
   (Section 2.2). However, the RSVP signalling may itself be originated
   or terminated by proxies still closer to the edge of the network,
   such as home hubs or the like, triggered in turn by application layer
   signalling. [RFC2998] and our approach are compared further in
   Section 6.2.

   An important benefit compared with the IntServ over DiffServ model
   [RFC2998] arises from the fact that the load is controlled
   dynamically rather than with traffic conditioning agreements (TCAs).
   TCAs were originally introduced in the (informational) DiffServ
   architecture [RFC2475] as an alternative to reservation processing in
   the interior region in order to reduce the burden on interior
   routers. With TCAs, in practice service providers rely on
   subscription-time Service Level Agreements that statically define the
   parameters of the traffic that will be accepted from a customer. The
   problem arises because the TCA at the ingress must allow any
   destination address, if it is to remain scalable. But for longer
   topologies, the chances increase that traffic will focus on an
   interior resource, even though it is within contract at the ingress
   [Reid], e.g. all flows converge on the same egress gateway. Even
   though networks can be engineered to make such failures rare, when
   they occur all inelastic flows through the congested resource fail
   catastrophically.

   Distributed measurement-based admission control avoids reservation
   processing (whether per flow or aggregated) on interior routers but
   flows are still blocked dynamically in response to actual congestion
   on any interior router. Hence there is no need for accurate or
   conservative prediction of the traffic matrix.



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1.1.2. Flow pre-emption

   An essential QoS issue in core and backbone networks is being able to
   cope with failures of routers and links. The consequent re-routing
   can cause severe congestion on some links and hence degrade the QoS
   experienced by on-going microflows and other, lower priority traffic.
   Even when the network is engineered to sustain a single link failure,
   multiple link failures (e.g. due to a fibre cut, router failure or a
   natural disaster) can cause violation of capacity constraints and
   resulting QoS failures. Our solution uses rate-based flow pre-
   emption, so that sufficient of the previously admitted CL microflows
   are dropped to ensure that the remaining ones again receive QoS
   commensurate with the CL service and at least some QoS is quickly
   restored to other traffic classes.

   The solution involves four steps. First, triggering the ingress
   gateway to test whether pre-emption may be needed. A router enhanced
   with Pre-Congestion Notification may optionally include an algorithm
   that Pre-emption Marks packets. Reception of a packet with such a
   marking alerts the egress gateway that pre-emption may be needed,
   which in turn sends a Pre-emption Alert message to the ingress
   gateway. Secondly, calculating the right amount of traffic to drop.
   This involves the egress gateway measuring, and reporting to the
   ingress gateway, the current rate of CL traffic received from that
   particular ingress gateway. This is the CL rate which the network can
   actually support from that ingress gateway to that egress gateway,
   and we thus call it the Sustainable-Aggregate-Rate. The ingress
   gateway compares the Sustainable-Aggregate-Rate) with the rate that
   it is sending and hence determines how much traffic needs to be pre-
   empted. Thirdly, choosing which flows to shed in order to drop the
   traffic calculated in the second step. Information on the priority of
   flows may be held by the ingress gateway, or by some out of band
   policy decision point. How these systems co-ordinate to determine
   which flows to drop is outside the scope of this document, but
   between them they have all the information necessary to make the
   decision. Fourthly, tearing down reservations for the chosen flows.
   The ingress gateway triggers standard tear-down messages for the
   reservation protocol in use. In turn, this is expected to result in
   end-systems tearing down the corresponding sessions (e.g. voice
   calls) using the corresponding session control protocols.

   The focus of this document is on the first two steps, i.e.
   determining that pre-emption may be needed and estimating how much
   traffic needs to be pre-empted. We provide some hints about the
   latter two steps in Section 3.2.3, but don't try to provide full
   guidance as it greatly depends on the particular detailed operational
   situation.


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   The solution operates within a little over one round trip time - the
   time required for microflow packets that have experienced Pre-emption
   Marking to travel downstream through the CL-region and arrive at the
   egress gateway, plus some additional time for the egress gateway to
   measure the rate seen after it has been alerted that pre-emption may
   be needed, and the time for the egress gateway to report this
   information to the ingress gateway.

1.1.3. Both admission control and pre-emption

   This document describes both the admission control and pre-emption
   mechanisms, and we suggest that an operator uses both. However, we do
   not require this and some operators may want to implement only one.

   For example, an operator could use just admission control, solving
   heavy congestion (caused by re-routing) by 'just waiting' - as
   sessions end, existing microflows naturally depart from the system
   over time, and the admission control mechanism will prevent admission
   of new microflows that use the affected links. So the CL-region will
   naturally return to normal controlled load service, but with reduced
   capacity. The drawback of this approach would be that until flows
   naturally depart to relieve the congestion, all flows and lower
   priority services will be adversely affected. As another example, an
   operator could use just admission control, avoiding heavy congestion
   (caused by re-routing) by 'capacity planning' - by configuring
   admission control thresholds to lower levels than the network could
   accept in normal situations such that the load after failure is
   expected to stay below acceptable levels even with reduced network
   resources.

   On the other hand, an operator could just rely for admission control
   on the traffic conditioning agreements of the DiffServ architecture
   [RFC2475]. The pre-emption mechanism described in this document would
   be used to counteract the problem described at the end of Section
   1.1.1.



1.2. Terminology

   This terminology is copied from the pre-congestion notification
   marking draft [PCN]:

   o Pre-Congestion Notification (PCN): two new algorithms that
      determine when a PCN-enabled router Admission Marks and Pre-
      emption Marks a packet, depending on the traffic level.



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   o Admission Marking condition: the traffic level is such that the
      router Admission Marks packets. The router provides an "early
      warning" that the load is nearing the engineered admission control
      capacity, before there is any significant build-up of CL packets
      in the queue.

   o Pre-emption Marking condition: the traffic level is such that the
      router Pre-emption Marks packets. The router warns explicitly that
      pre-emption may be needed.

   o Configured-admission-rate: the reference rate used by the
      admission marking algorithm in a PCN-enabled router.

   o Configured-pre-emption-rate - the reference rate used by the pre-
      emption marking algorithm in a PCN-enabled router.



   The following terms are defined here:

   o Ingress gateway: router at an ingress to the CL-region. A CL-
      region may have several ingress gateways.

   o Egress gateway: router at an egress from the CL-region. A CL-
      region may have several egress gateways.

   o Interior router: a router which is part of the CL-region, but
      isn't an ingress or egress gateway.

   o CL-region: A region of the Internet in which all traffic
      enters/leaves through an ingress/egress gateway and all routers
      run Pre-Congestion Notification marking. A CL-region is a DiffServ
      region (a DiffServ region is either a single DiffServ domain or
      set of contiguous DiffServ domains), but note that the CL-region
      does not use the traffic conditioning agreements (TCAs) of the
      (informational) DiffServ architecture.

   o CL-region-aggregate: all the microflows between a specific pair of
      ingress and egress gateways. Note there is no field in the flow
      packet headers that uniquely identifies the aggregate.








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   o Congestion-Level-Estimate: the number of bits in CL packets that
      are admission marked (or pre-emption marked), divided by the
      number of bits in all CL packets. It is calculated as an
      exponentially weighted moving average. It is calculated by an
      egress gateway for the CL packets from a particular ingress
      gateway, i.e. there is a Congestion-Level-Estimate for each CL-
      region-aggregate.

   o Sustainable-Aggregate-Rate: the rate of traffic that the network
      can actually support for a specific CL-region-aggregate. So it is
      measured by an egress gateway for the CL packets from a particular
      ingress gateway.

   o Ingress-Aggregate-Rate: the rate of traffic that is being sent on
      a specific CL-region-aggregate. So it is measured by an ingress
      gateway for the CL packets sent towards a particular egress
      gateway.



1.3. Existing terminology

   This is a placeholder for useful terminology that is defined
   elsewhere.

1.4. Standardisation requirements

   The framework described in this document has two new standardisation
   requirements:

   o new Pre-Congestion Notification for Admission Marking and Pre-
      emption Marking are required, as detailed in [PCN].

   o the end-to-end signalling protocol needs to be modified to carry
      the Congestion-Level-Estimate report (for admission control) and
      the Sustainable-Aggregate-Rate (for flow pre-emption). With our
      assumption of RSVP (Section 2.2) as the end-to-end signalling
      protocol, it means that extensions to RSVP are required, as
      detailed in [RSVP-PCN], for example to carry the Congestion-Level-
      Estimate and Sustainable-Aggregate-Rate information from egress
      gateway to ingress gateway.

   o We are discussing what to standardise about the gateway's
      behaviour.




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   Other than these things, the arrangement uses existing IETF protocols
   throughout, although not in their usual architecture.

1.5. Structure of rest of the document

   Section 2 describes some key aspects of the deployment model: our
   goals, assumptions and the benefits we believe it has. Section 3
   describes the deployment model, whilst Section 4 summarises the
   required changes to the various routers in the CL-region. Section 5
   outlines some limitations of PCN that we've identified in this
   deployment model; it also discusses some potential solutions, and
   other possible extensions. Section 6 provides some comparison with
   existing QoS mechanisms.



































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2. Key aspects of the deployment model

   In this section we discuss the key aspects of the deployment model:

   o At a high level, our key goals, i.e. the functionality that we
      want to achieve

   o The assumptions that we're prepared to make

   o The consequent benefits they bring

2.1. Key goals

   The deployment model achieves an end-to-end controlled load (CL)
   service where a segment of the end-to-end path is an edge-to-edge
   Pre-Congestion Notification region. CL is a quality of service (QoS)
   closely approximating the QoS that the same flow would receive from a
   lightly loaded network element [RFC2211]. It is useful for inelastic
   flows such as those for real-time media.

   o The CL service should be achieved despite varying load levels of
      other sorts of traffic, which may or may not be rate adaptive
      (i.e. responsive to packet drops or ECN marks).

   o The CL service should be supported for a variety of possible CL
      sources: Constant Bit Rate (CBR), Variable Bit Rate (VBR) and
      voice with silence suppression. VBR is the most challenging to
      support.

   o After a localised failure in the interior of the CL-region causing
      heavy congestion, the CL service should recover gracefully by pre-
      empting (dropping) some of the admitted CL microflows, whilst
      preserving as many of them as possible with their full CL QoS.

   o It needs to be possible to complete flow pre-emption within 1-2
      seconds. Operators will have varying requirements but, at least
      for voice, it has been estimated that after a few seconds then
      many affected users will start to hang up, making the flow pre-
      emption mechanism redundant and possibly even counter-productive.
      Until flow pre-emption kicks in, other applications using CL (e.g.
      video) and lower priority traffic (e.g. Assured Forwarding (AF))
      could be receiving reduced service. Therefore an even faster flow
      pre-emption mechanism would be desirable (even if, in practice,
      operators have to add a deliberate pause to ride out a transient
      while the natural rate of call tear down or lower layer protection
      mechanisms kick in).



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   o The CL service should support emergency services ([EMERG-RQTS],
      [EMERG-TEL]) as well as the Assured Service which is the IP
      implementation of the existing ITU-T/NATO/DoD telephone system
      architecture known as Multi-Level Pre-emption and Precedence
      [ITU.MLPP.1990] [ANSI.MLPP.Spec][ANSI.MLPP.Supplement], or MLPP.
      In particular, this involves admitting new flows that are part of
      high priority sessions even when admission control would reject
      new routine flows. Similarly, when having to choose which flows to
      pre-empt, this involves taking into account the priorities and
      properties of the sessions that flows are part of.


2.2. Key assumptions

   The framework does not try to deliver the above functionality in all
   scenarios. We make the following assumptions about the type of
   scenario to be solved.

   o Edge-to-edge: all the routers in the CL-region are upgraded with
      Pre-Congestion Notification, and all the ingress and egress
      gateways are upgraded to perform the measurement-based admission
      control and flow pre-emption. Note that although the upgrades
      required are edge-to-edge, the CL service is provided end-to-end.

   o Additional load: we assume that any additional load offered within
      the reaction time of the admission control mechanism doesn't move
      the CL-region directly from no congestion to overload. So it
      assumes there will always be an intermediate stage where some CL
      packets are Admission Marked, but they are still delivered without
      significant QoS degradation. We believe this is valid for core and
      backbone networks with typical call arrival patterns (given the
      reaction time is little more than one round trip time across the
      CL-region), but is unlikely to be valid in access networks where
      the granularity of an individual call becomes significant.

   o Aggregation: we assume that in normal operations, there are many
      CL microflows within the CL-region, typically at least hundreds
      between any pair of ingress and egress gateways. The implication
      is that the solution is targeted at core and backbone networks and
      possibly parts of large access networks.

   o Trust: we assume that there is trust between all the routers in
      the CL-region. For example, this trust model is satisfied if one
      operator runs the whole of the CL-region. But we make no such
      assumptions about the end hosts, i.e. depending on the scenario
      they may be trusted or untrusted by the CL-region.



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   o Signalling: we assume that the end-to-end signalling protocol is
      RSVP. Section 3 describes how the CL-region fits into such an end-
      to-end QoS scenario, whilst [RSVP-PCN] describes the extensions to
      RSVP that are required.

   o Separation: we assume that all routers within the CL-region are
      upgraded with the CL mechanism, so the requirements of [Floyd] are
      met because the CL-region is an enclosed environment. Also, an
      operator separates CL-traffic in the CL-region from outside
      traffic by administrative configuration of the ring of gateways
      around the region. Within the CL-region we assume that the CL-
      traffic is separated from non-CL traffic.

   o Routing: we assume that all packets between a pair of ingress and
      egress gateways follow the same path, or that they follow
      different paths but that the load balancing scheme is tuned in the
      CL-region to distribute load such that the different paths always
      receive comparable relative load. This ensures that the
      Congestion-Level-Estimate used in the admission control procedure
      (and which is computed taking into account packets travelling on
      all the paths) approximately reflects the status of the actual
      path that will be followed by the new microflow's packets.



   We are investigating ways of loosening the restrictions set by some
   of these assumptions, for instance:

   o Trust: to allow the CL-region to span multiple, non-trusting
      operators, using the technique of [Re-PCN] as mentioned in Section
      5.7.2.

   o Signalling: we believe that the solution could operate with
      another signalling protocol, such as the one produced by the NSIS
      working group. It could also work with application level
      signalling as suggested in [RT-ECN].

   o Additional load: we believe that the assumption is valid for core
      and backbone networks, with an appropriate margin between the
      configured-admission-rate and the capacity for CL traffic.
      However, in principle a burst of admission requests can occur in a
      short time. We expect this to be a rare event under normal
      conditions, but it could happen e.g. due to a 'flash crowd'. If it
      does, then more flows may be admitted than should be, triggering
      the pre-emption mechanism. There are various ways an operator
      might try to alleviate this issue, which are discussed in the
      'Flash crowds' section 5.5 later.


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   o Separation: the assumption that CL traffic is separated from non-
      CL traffic implies that the CL traffic has its own PHB, not shared
      with other traffic. We are looking at whether it could share
      Expedited Forwarding's PHB, but supplemented with Pre-Congestion
      Notification. If this is possible, other PHBs (like Assured
      Forwarding) could be supplemented with the same new behaviours.
      This is similar to how RFC3168 ECN was defined to supplement any
      PHB.

   o Routing: we are looking in greater detail at the solution in the
      presence of Equal Cost Multi-Path routing and at suitable
      enhancements. See also the 'ECMP' section 5.1 later.



2.3. Key benefits

   We believe that the mechanism described in this document has several
   advantages:

   o It achieves statistical guarantees of quality of service for
      microflows, delivering a very low delay, jitter and packet loss
      service suitable for applications like voice and video calls that
      generate real time inelastic traffic. This is because of its per
      microflow admission control scheme, combined with its dynamic on-
      path "early warning" of potential congestion. The guarantee is at
      least as strong as with IntServ Controlled Load (Section 6.1
      mentions why the guarantee may be somewhat better), but without
      the scalability problems of per-microflow IntServ.

   o It can support "Emergency" and military Multi-Level Pre-emption
      and Priority (MLPP) services, even in times of heavy congestion
      (perhaps caused by failure of a router within the CL-region), by
      pre-empting on-going "ordinary CL microflows". See also Section
      4.5.

   o It scales well, because there is no signal processing or per flow
      state held by the interior routers of the CL-region. Note that
      interior routers only hold state per outgoing interface - they do
      not hold state per CL-region-aggregate nor per flow.

   o It is resilient, again because no per flow state is held by the
      interior routers of the CL-region. Hence during an interior
      routing change caused by a router failure, no microflow state has
      to be relocated. The flow pre-emption mechanism further helps
      resilience because it rapidly reduces the load to one that the CL-
      region can support.


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   o It helps preserve, through the flow pre-emption mechanism, QoS to
      as many microflows as possible and to lower priority traffic in
      times of heavy congestion (e.g. caused by failure of an interior
      router). Otherwise long-lived microflows could cause loss on all
      CL microflows for a long time.

   o It avoids the potential catastrophic failure problem when the
      DiffServ architecture is used in large networks using statically
      provisioned capacity. This is achieved by controlling the load
      dynamically, based on edge-to-edge-path real-time measurement of
      Pre-Congestion Notification, as discussed in Section 1.1.1.

   o It requires minimal new standardisation, because it reuses
      existing QoS protocols and algorithms.

   o It can be deployed incrementally, region by region or network by
      network. Not all the regions or networks on the end-to-end path
      need to have it deployed. Two CL-regions can even be separated by
      a network that uses another QoS mechanism (e.g. MPLS-TE).

   o It provides a deployment path for use of ECN for real-time
      applications. Operators can gain experience of ECN before its
      applicability to end-systems is understood and end terminals are
      ECN capable.
























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3. Deployment model

3.1. Admission control

   In this section we describe the admission control mechanism. We
   discuss the three pieces of the solution and then give an example of
   how they fit together in a use case:

   o the new Pre-Congestion Notification for Admission Marking used by
      all routers in the CL-region

   o how the measurements made support our admission control mechanism

   o how the edge to edge mechanism fits into the end to end RSVP
      signalling



3.1.1. Pre-Congestion Notification for Admission Marking

   This is discussed in [PCN]. Here we only give a brief outline.

   To support our admission control mechanism, each router in the CL-
   region runs an algorithm to determine whether to Admission Mark the
   packet. The algorithm measures the aggregate CL traffic on the link
   and ensures that packets are admission marked before the actual queue
   builds up, but when it is in danger of doing so soon; the probability
   of admission marking increases with the danger. The algorithm's main
   parameter is the configured-admission-rate, which is set lower than
   the link speed, perhaps considerably so. Admission marked packets
   indicate that the CL traffic rate is reaching the configured-
   admission-rate and so act as an "early warning" that the engineered
   capacity is nearly reached. Therefore they indicate that requests to
   admit prospective new CL flows may need to be refused.



3.1.2. Measurements to support admission control

   To support our admission control mechanism the egress measures the
   Congestion-Level-Estimate for traffic from each remote ingress
   gateway, i.e. per CL-region-aggregate. The Congestion-Level-Estimate
   is the number of bits in CL packets that are admission marked or pre-
   emption marked, divided by the number of bits in all CL packets. It
   is calculated as an exponentially weighted moving average. It is
   calculated by an egress gateway separately for the CL packets from
   each particular ingress gateway.


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   Why are pre-emption marked packets included in the Congestion-Level-
   Estimate? Pre-emption marking over-writes admission marking, i.e. a
   packet cannot be both admission and pre-emption marked. So if pre-
   emption marked packets weren't counted we would have the anomaly that
   as the traffic rate grew above the configured-pre-emption-rate, the
   Congestion-Level-Estimate would fall. If a particular encoding scheme
   is chosen where a packet can be both admission and pre-emption marked
   (such as Alternative 4 in Appendix C of [PCN]), then this is not
   necessary.

   This Congestion-Level-Estimate provides an estimate of how near the
   links on the path inside the CL-region are getting to the configured-
   admission-rate. Note that the metering is done separately per ingress
   gateway, because there may be sufficient capacity on all the routers
   on the path between one ingress gateway and a particular egress, but
   not from a second ingress to that same egress gateway.

3.1.3. How edge-to-edge admission control supports end-to-end QoS
   signalling

   Consider a scenario that consists of two end hosts, each connected to
   their own access networks, which are linked by the CL-region. A
   source tries to set up a new CL microflow by sending an RSVP PATH
   message, and the receiving end host replies with an RSVP RESV
   message. Outside the CL-region some other method, for instance
   IntServ, is used to provide QoS. From the perspective of RSVP the CL-
   region is a single hop, so the RSVP PATH and RESV messages are
   processed by the ingress and egress gateways but are carried
   transparently across all the interior routers; hence, the ingress and
   egress gateways hold per microflow state, whilst no per microflow
   state is kept by the interior routers. So far this is as in IntServ
   over DiffServ [RFC2998]. However, in order to support our admission
   control mechanism, the egress gateway adds to the RESV message an
   opaque object which states the current Congestion-Level-Estimate for
   the relevant CL-region-aggregate. Details of the corresponding RSVP
   extensions are described in [RSVP-PCN].

3.1.4. Use case

   To see how the three pieces of the solution fit together, we imagine
   a scenario where some microflows are already in place between a given
   pair of ingress and egress gateways, but the traffic load is such
   that no packets from these flows are admission marked as they travel
   across the CL-region. A source wanting to start a new CL microflow
   sends an RSVP PATH message. The egress gateway adds an object to the
   RESV message with the Congestion-Level-Estimate, which is zero. The
   ingress gateway sees this and consequently admits the new flow. It


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   then forwards the RSVP RESV message upstream towards the source end
   host. Hence, assuming there's sufficient capacity in the access
   networks, the new microflow is admitted end-to-end.

   The source now sends CL packets, which arrive at the ingress gateway.
   The ingress uses a five-tuple filter to identify that the packets are
   part of a previously admitted CL microflow, and it also polices the
   microflow to ensure it remains within its traffic profile. (The
   ingress has learnt the required information from the RSVP messages.)
   When forwarding a packet belonging to an admitted microflow, the
   ingress sets the packet's DSCP and ECN fields to the appropriate
   values configured for the CL region. The CL packet now travels across
   the CL-region, getting admission marked if necessary.

   Next, we imagine the same scenario but at a later time when load is
   higher at one (or more) of the interior routers, which start to
   Admission Mark CL packets, because their load on the outgoing link is
   nearing the configured-admission-rate. The next time a source tries
   to set up a CL microflow, the ingress gateway learns (from the
   egress) the relevant Congestion-Level-Estimate. If it is greater than
   some CLE-threshold value then the ingress refuses the request,
   otherwise it is accepted. The ingress gateway could also take into
   account attributes of the RSVP reservation (such as for example the
   RSVP pre-emption priority of [RSVP-PREEMPTION] or the RSVP admission
   priority of [RSVP-EMERGENCY]) as well as information provided by a
   policy decision point in order to make a more sophisticated admission
   decision. This way, flow admission can help emergency/military calls
   by taking into account the corresponding priorities (as conveyed in
   RSVP policy elements) when deciding to admit or reject a new
   reservation. Use of RSVP for the support of emergency/military
   applications is discussed in further detail in [RFC4542] and [RSVP-
   EMERGENCY].

   It is also possible for an egress gateway to get a RSVP RESV message
   and not know what the Congestion-Level-Estimate is. For example, if
   there are no CL microflows at present between the relevant ingress
   and egress gateways. In this case the egress requests the ingress to
   send probe packets, from which it can initialise its meter. RSVP
   Extensions for such a request to send probe data can be found in
   [RSVP-PCN].









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3.2. Flow pre-emption

   In this section we describe the flow pre-emption mechanism. We
   discuss the two parts of the solution and then give an example of how
   they fit together in a use case:

   o How an ingress gateway is triggered to test whether flow pre-
      emption may be needed

   o How an ingress gateway determines the right amount of CL traffic
      to drop

   The mechanism is defined in [PCN] and [RSVP-PCN].

3.2.1. Alerting an ingress gateway that flow pre-emption may be needed

   Alerting an ingress gateway that flow pre-emption may be needed is a
   two stage process: a router in the CL-region alerts an egress gateway
   that flow pre-emption may be needed; in turn the egress gateway
   alerts the relevant ingress gateway. Every router in the CL-region
   has the ability to alert egress gateways, which may be done either
   explicitly or implicitly:

   o Explicit - the router per-hop behaviour is supplemented with a new
      Pre-emption Marking behaviour, which is outlined below. Reception
      of such a packet by the egress gateway alerts it that pre-emption
      may be needed.

   o Implicit - the router behaviour is unchanged from the Admission
      Marking behaviour described earlier. The egress gateway treats a
      Congestion-Level-Estimate of (almost) 100% as an implicit alert
      that pre-emption may be required. ('Almost' because the
      Congestion-Level-Estimate is a moving average, so can never reach
      exactly 100%.)

   To support explicit pre-emption alerting, each router in the CL-
   region runs an algorithm to determine whether to Pre-emption Mark the
   packet. The algorithm measures the aggregate CL traffic and ensures
   that packets are pre-emption marked before the actual queue builds
   up. The algorithm's main parameter is the configured-pre-emption-
   rate, which is set lower than the link speed (but higher than the
   configured-admission-rate). Thus pre-emption marked packets indicate
   that the CL traffic rate is reaching the configured-pre-emption-rate
   and so act as an "early warning" that the engineered capacity is
   nearly reached. Therefore they indicate that it may be advisable to
   pre-empt some of the existing CL flows in order to preserve the QoS
   of the others.


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   Note that the explicit mechanism only makes sense if all the routers
   in the CL-region have the functionality so that the egress gateways
   can rely on the explicit mechanism. Otherwise there is the danger
   that the traffic happens to focus on a router without it, and egress
   gateways then have also to watch for implicit pre-emption alerts.

   When one or more packets in a CL-region-aggregate alert the egress
   gateway of the need for flow pre-emption, whether explicitly or
   implicitly, the egress puts that CL-region-aggregate into the Pre-
   emption Alert state. For each CL-region-aggregate in alert state it
   measures the rate of traffic at the egress gateway (i.e. the traffic
   rate of the appropriate CL-region-aggregate) and reports this to the
   relevant ingress gateway. The steps are:

   o Determine the relevant ingress gateway - for the explicit case the
      egress gateway examines the pre-emption marked packet and uses the
      state installed at the time of admission to determine which
      ingress gateway the packet came from. For the implicit case the
      egress gateway has already determined this information, because
      the Congestion-Level-Estimate is calculated per ingress gateway.

   o Measure the traffic rate of CL packets - as soon as the egress
      gateway is alerted (whether explicitly or implicitly) it measures
      the rate of CL traffic from this ingress gateway (i.e. for this
      CL-region-aggregate). Note that pre-emption marked packets are
      excluded from that measurement. It should make its measurement
      quickly and accurately, but exactly how is up to the
      implementation.

   o Alert the ingress gateway - the egress gateway then immediately
      alerts the relevant ingress gateway about the fact that flow pre-
      emption may be required. This Alert message also includes the
      measured Sustainable-Aggregate-Rate, i.e. the rate of CL-traffic
      received from this ingress gateway. The Alert message is sent
      using reliable delivery. Procedures for the support of such an
      Alert using RSVP are defined in [RSVP-PCN].

             --------------       _       _          -----------------
CL packet   |Update        |     / Is it a \   Y    | Measure CL rate |
arrives --->|Congestion-   |--->/pre-emption\-----> | from ingress and|
            |Level-Estimate|    \  marked   /       | alert ingress   |
             --------------      \ packet? /         -----------------
                                  \_     _/

Figure 2: Egress gateway action for explicit Pre-emption Alert



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                                   _     _
             --------------       /       \          -----------------
CL packet   |Update        |     /  Is     \   Y    | Measure CL rate |
arrives --->|Congestion-   |--->/  C.L.E.   \-----> | from ingress and|
            |Level-Estimate|    \ (nearly)  /       | alert ingress   |
             --------------      \ 100%?   /         -----------------
                                  \_     _/

Figure 3: Egress gateway action for implicit Pre-emption Alert


3.2.2. Determining the right amount of CL traffic to drop

   The method relies on the insight that the amount of CL traffic that
   can be supported between a particular pair of ingress and egress
   gateways, is the amount of CL traffic that is actually getting across
   the CL-region to the egress gateway without being Pre-emption Marked.
   Hence we term it the Sustainable-Aggregate-Rate.

   So when the ingress gateway gets the Alert message from an egress
   gateway, it compares:

   o The traffic rate that it is sending to this particular egress
      gateway (which we term Ingress-Aggregate-Rate)

   o The traffic rate that the egress gateway reports (in the Alert
      message) that it is receiving from this ingress gateway (which is
      the Sustainable-Aggregate-Rate)

   If the difference is significant, then the ingress gateway pre-empts
   some microflows. It only pre-empts if:

        Ingress-Aggregate-Rate > Sustainable-Aggregate-Rate + error

   The "error" term is partly to allow for inaccuracies in the
   measurements of the rates. It is also needed because the Ingress-
   Aggregate-Rate is measured at a slightly later moment than the
   Sustainable-Aggregate-Rate, and it is quite possible that the
   Ingress-Aggregate-Rate has increased in the interim due to natural
   variation of the bit rate of the CL sources. So the "error" term
   allows for some variation in the ingress rate without triggering pre-
   emption.

   The ingress gateway should pre-empt enough microflows to ensure that:



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        New Ingress-Aggregate-Rate < Sustainable-Aggregate-Rate - error

   The "error" term here is used for similar reasons but in the other
   direction, to ensure slightly more load is shed than seems necessary,
   in case the two measurements were taken during a short-term fall in
   load.

   When the routers in the CL-region are using explicit pre-emption
   alerting, the ingress gateway would normally pre-empt microflows
   whenever it gets an alert (it always would if it were possible to set
   "error" equal to zero). For the implicit case however this is not so.
   It receives an Alert message when the Congestion-Level-Estimate
   reaches (almost) 100%, which is roughly when traffic exceeds the
   configured-admission-rate. However, it is only when packets are
   indeed dropped en route that the Sustainable-Aggregate-Rate becomes
   less than the Ingress-Aggregate-Rate so only then will pre-emption
   actually occur on the ingress gateway.

   Hence with the implicit scheme, pre-emption can only be triggered
   once the system starts dropping packets and thus the QoS of flows
   starts being significantly degraded. This is in contrast with the
   explicit scheme which allows flow pre-emption to be triggered before
   any packet drop, simply when the traffic reaches the configured-pre-
   emption-rate. Therefore we believe that the explicit mechanism is
   superior. However it does require new functionality on all the
   routers (although this is little more than a bulk token bucket - see
   [PCN] for details).



3.2.3. Use case for flow pre-emption

   To see how the pieces of the solution fit together in a use case, we
   imagine a scenario where many microflows have already been admitted.
   We confine our description to the explicit pre-emption mechanism. Now
   an interior router in the CL-region fails. The network layer routing
   protocol re-routes round the problem, but as a consequence traffic on
   other links increases. In fact let's assume the traffic on one link
   now exceeds its configured-pre-emption-rate and so the router pre-
   emption marks CL packets. When the egress sees the first one of the
   pre-emption marked packets it immediately determines which microflow
   this packet is part of (by using a five-tuple filter and comparing it
   with state installed at admission) and hence which ingress gateway
   the packet came from. It sets up a meter to measure the traffic rate
   from this ingress gateway, and as soon as possible sends a message to
   the ingress gateway. This message alerts the ingress gateway that
   pre-emption may be needed and contains the traffic rate measured by


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   the egress gateway. Then the ingress gateway determines the traffic
   rate that it is sending towards this egress gateway and hence it can
   calculate the amount of traffic that needs to be pre-empted.

   The ingress gateway could now just shed random microflows, but it is
   better if the least important ones are dropped. The ingress gateway
   could use information stored locally in each reservation's state
   (such as for example the RSVP pre-emption priority of [RSVP-
   PREEMPTION] or the RSVP admission priority of [RSVP-EMERGENCY]) as
   well as information provided by a policy decision point in order to
   decide which of the flows to shed (or perhaps which ones not to
   shed). This way, flow pre-emption can also helps emergency/military
   calls by taking into account the corresponding priorities (as
   conveyed in RSVP policy elements) when selecting calls to be pre-
   empted, which is likely to be particularly important in a disaster
   scenario. Use of RSVP for support of emergency/military applications
   is discussed in further details in [RFC4542] and [RSVP-EMERGENCY].

   The ingress gateway then initiates RSVP signalling to instruct the
   relevant destinations that their reservation has been terminated, and
   to tell (RSVP) nodes along the path to tear down associated RSVP
   state. To guard against recalcitrant sources, normal IntServ policing
   may be used to block any future traffic from the dropped flows from
   entering the CL-region. Note that - with the explicit Pre-emption
   Alert mechanism - since the configured-pre-emption-rate may be
   significantly less than the physical line capacity, flow pre-emption
   may be triggered before any congestion has actually occurred and
   before any packet is dropped.

   We extend the scenario further by imagining that (due to a disaster
   of some kind) further routers in the CL-region fail during the time
   taken by the pre-emption process described above. This is handled
   naturally, as packets will continue to be pre-emption marked and so
   the pre-emption process will happen for a second time.















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4. Summary of Functionality

   This section is intended to provide a systematic summary of the new
   functionality required by the routers in the CL-region.

   A network operator upgrades normal IP routers by:

   o Adding functionality related to admission control and flow pre-
      emption to all its ingress and egress gateways

   o Adding Pre-Congestion Notification for Admission Marking and Pre-
      emption Marking to all the routers in the CL-region.

   We consider the detailed actions required for each of the types of
   router in turn.

4.1. Ingress gateways

   Ingress gateways perform the following tasks:

   o Classify incoming packets - decide whether they are CL or non-CL
      packets. This is done using an IntServ filter spec (source and
      destination addresses and port numbers), whose details have been
      gathered from the RSVP messaging.

   o Police - check that the microflow conforms with what has been
      agreed (i.e. it keeps to its agreed data rate). If necessary,
      packets which do not correspond to any reservations, packets which
      are in excess of the rate agreed for their reservation, and
      packets for a reservation that has earlier been pre-empted may be
      policed. Policing may be achieved via dropping or via re-marking
      of the packet's DSCP to a value different from the CL behaviour
      aggregate.

   o ECN colouring packets - for CL microflows, set the ECN field of
      packets appropriately (see [PCN] for some discussion of encoding).

   o Perform 'interior router' functions (see next sub-section).

   o Admission Control - on new session establishment, consider the
      Congestion-Level-Estimate received from the corresponding egress
      gateway and most likely based on a simple configured CLE-threshold
      decide if a new call is to be admitted or rejected (taking into
      account local policy information as well as optionally information
      provided by a policy decision point).




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   o Probe - if requested by the egress gateway to do so, the ingress
      gateway generates probe traffic so that the egress gateway can
      compute the Congestion-Level-Estimate from this ingress gateway.
      Probe packets may be simple data addressed to the egress gateway
      and require no protocol standardisation, although there will be
      best practice for their number, size and rate.

   o Measure - when it receives a Pre-emption Alert message from an
      egress gateway, it determines the rate at which it is sending
      packets to that egress gateway

   o Pre-empt - calculate how much CL traffic needs to be pre-empted;
      decide which microflows should be dropped, perhaps in consultation
      with a Policy Decision Point; and do the necessary signalling to
      drop them.

4.2. Interior routers

   Interior routers do the following tasks:

   o Classify packets - examine the DSCP and ECN field to see if it's a
      CL packet

   o Non-CL packets are handled as usual, with respect to dropping them
      or setting their CE codepoint.

   o Pre-Congestion Notification - CL packets are Admission Marked and
      Pre-emption Marked according to the algorithm detailed in [PCN]
      and outlined in Section 3.



4.3. Egress gateways

   Egress gateways do the following tasks:

   o Classify packets - determine which ingress gateway a CL packet has
      come from. This is the previous RSVP hop, hence the necessary
      details are obtained just as with IntServ from the state
      associated with the packet five-tuple, which has been built using
      information from the RSVP messages.








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   o Meter - for CL packets, calculate the fraction of the total number
      of bits which are in Admission marked packets or in Pre-emption
      Marked packets. The calculation is done as an exponentially
      weighted moving average (see Appendix C). A separate calculation
      is made for CL packets from each ingress gateway. The meter works
      on an aggregate basis and not per microflow.

   o Signal the Congestion-Level-Estimate - this is piggy-backed on the
      reservation reply. An egress gateway's interface is configured to
      know it is an egress gateway, so it always appends this to the
      RESV message. If the Congestion-Level-Estimate is unknown or is
      too stale, then the egress gateway can request the ingress gateway
      to send probes.

   o Packet colouring - for CL packets, set the DSCP and the ECN field
      to whatever has been agreed as appropriate for the next domain. By
      default the ECN field is set to the Not-ECT codepoint. See also
      the discussion in the Tunnelling section later.

   o Measure the rate - measure the rate of CL traffic from a
      particular ingress gateway, excluding packets that are Pre-emption
      Marked (i.e. the Sustainable-Aggregate-Rate for the CL-region-
      aggregate), when alerted (either explicitly or implicitly) that
      pre-emption may be required. The measured rate is reported back to
      the appropriate ingress gateway [RSVP-PCN].

4.4. Failures

   If an interior router fails, then the regular IP routing protocol
   will re-route round it. If the new route can carry all the admitted
   traffic, flows will gracefully continue. If instead this causes early
   warning of pre-congestion on the new route, then admission control
   based on pre-congestion notification will ensure new flows will not
   be admitted until enough existing flows have departed. Finally re-
   routing may result in heavy congestion, when the flow pre-emption
   mechanism will kick in.

   If a gateway fails then we would like regular RSVP procedures
   [RFC2205] to take care of things. With the local repair mechanism of
   [RFC2205], when a route changes the next RSVP PATH refresh message
   will establish path state along the new route, and thus attempt to
   re-establish reservations through the new ingress gateway.
   Essentially the same procedure is used as described earlier in this
   document, with the re-routed session treated as a new session
   request.




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   In more detail, consider what happens if an ingress gateway of the
   CL-region fails. Then RSVP routers upstream of it do IP re-routing to
   a new ingress gateway. The next time the upstream RSVP router sends a
   PATH refresh message it reaches the new ingress gateway which
   therefore installs the associated RSVP state. The next RSVP RESV
   refresh will pick up the Congestion-Level-Estimate from the egress
   gateway, and the ingress compares this with its threshold to decide
   whether to admit the new session. This could result in some of the
   flows being rejected, but those accepted will receive the full QoS.

   An issue with this is that we have to wait until a PATH and RESV
   refresh messages are sent - which may not be very often - the default
   value is 30 seconds. [RFC2205] discusses how to speed up the local
   repair mechanism. First, the RSVP module is notified by the local
   routing protocol module of a route change to particular destinations,
   which triggers it to rapidly send out PATH refresh messages. Further,
   when a PATH refresh arrives with a previous hop address different
   from the one stored, then RESV refreshes are immediately sent to that
   previous hop. Where RSVP is operating hop-by-hop, i.e. on every
   router, then triggering the PATH refresh is easy as the router can
   simply monitor its local link. Thus, this fast local repair mechanism
   can be  used to deal with failures upstream of the ingress gateway,
   with failures of the ingress gateway and with failures downstream of
   the egress gateway.

   But where RSVP is not operating hop-by-hop (as is the case within the
   CL-region), it is not so easy to trigger the PATH refresh.

   Unfortunately, this problem applies if an egress gateway fails, since
   it's very likely that an egress gateway is several IP hops from the
   ingress gateway. (If the ingress is several IP hops from its previous
   RSVP node, then there is the same issue.) The options appear to be:

   o the ingress gateway has a link state database for the CL-region,
      so it can detect that an egress gateway has failed or became
      unreachable

   o there is an inter-gateway protocol, so the ingress can
      continuously check that the egress gateways are still alive

   o (default) do nothing and wait for the regular PATH/RESV refreshes
      (and, if needed, the pre-emption mechanism) to sort things out.







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5. Limitations and some potential solutions

   In this section we describe various limitations of the deployment
   model, and some suggestions about potential ways of alleviating them.
   The limitations fall into three broad categories:

   o ECMP (Section 5.1): the assumption about routing (Section 2.2) is
      that all packets between a pair of ingress and egress gateways
      follow the same path; ECMP breaks this assumption

   o The lack of global coordination (Sections 5.2, 5.3 and 5.4): a
      decision about admission control or flow pre-emption is made for
      one aggregate independently of other aggregates

   o Timing and accuracy of measurements (Sections 5.5 and 5.6): the
      assumption (Section 2.2) that additional load, offered within the
      reaction time of the measurement-based admission control
      mechanism, doesn't move the system directly from no congestion to
      overload (dropping packets). A 'flash crowd' may break this
      assumption (Section 5.5). There are a variety of more general
      issues associated with marking measurements, which may mean it's a
      good idea to do pre-emption 'slower' (Section 5.6).

   Each section describes a limitation and some possible solutions to
   alleviate the limitation. These are intended as options for an
   operator to consider, based on their particular requirements.

   We would welcome feedback, for example suggestions as to which
   potential solutions are worth working out in more detail, and ideas
   on new potential solutions.

   Finally Section 5.7 considers some other potential extensions.



5.1. ECMP

   If the CL-region uses Equal Cost Multipath Routing (ECMP), then
   traffic between a particular pair of ingress and egress gateways may
   follow several different paths.

   Why? An ECMP-enabled router runs an algorithm to choose between
   potential outgoing links, based on a hash of fields such as the
   packet's source and destination addresses - exactly what depends on
   the proprietary algorithm. Packets are addressed to the CL flow's
   end-point, and therefore different flows may follow different paths



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   through the CL-region. (All packets of an individual flow follow the
   same ECMP path.)

   The problem is that if one of the paths is congested such that
   packets are being admission marked, then the Congestion-Level-
   Estimate measured by the egress gateway will be diluted by unmarked
   packets from other non-congested paths. Similarly, the measurement of
   the Sustainable-Aggregate-Rate will also be diluted.

   Possible solution approaches are:

   o tunnel: traffic is tunnelled across the CL-region. Then the
      destination address (and so on) seen by the ECMP algorithm is that
      of the egress gateway, so all flows follow the same path.
      Effectively ECMP is turned off. As a compromise, to try to retain
      some of the benefits of ECMP, there could be several tunnels, each
      following a different ECMP path, with flows randomly assigned to
      different tunnels.

   o assume worst case: the operator sets the configured-admission-rate
      (and configured-pre-emption-rate) to below the optimum level to
      compensate for the fact that the effect on the Congestion-Level-
      Estimate (and Sustainable-Aggregate-Rate) of the congestion
      experienced over one of the paths may be diluted by traffic
      received over non-congested paths. Hence lower thresholds need to
      be used to ensure early admission control rejection and pre-
      emption over the congested path. This approach will waste capacity
      (e.g. flows following a non-congested ECMP path are not admitted
      or are pre-empted), and there is still the danger that for some
      traffic mixes the operator hasn't been cautious enough.

   o for admission control, probe to obtain a flow-specific congestion-
      level-estimate. Earlier this document suggests continuously
      monitoring the congestion-level-estimate. Instead, probe packets
      could be sent for each prospective new flow. The probe packets
      have the same IP address etc as the data packets would have, and
      hence follow the same ECMP path. However, probing is an extra
      overhead, depending on how many probe packets need to be sent to
      get a sufficiently accurate congestion-level-estimate.

   o for flow pre-emption, only select flows for pre-emption from
      amongst those that have actually received a Pre-emption Marked
      packet. Because these flows must have followed an ECMP path that
      goes through an overloaded router. However, it needs some extra
      work by the egress gateway, to record this information and report
      it to the ingress gateway.



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   o for flow pre-emption, a variant of this idea involves introducing
      a new marking behaviour, 'Router Marking'. A router that is pre-
      emption marking packets on an outgoing link, also 'Router Marks'
      all other packets. When selecting flows for pre-emption, the
      selection is made from amongst those that have actually received a
      Router Marked or Pre-emption Marked packet. Hence compared with
      the previous bullet, it may extend the range of flows from which
      the pre-emption selection is made (i.e. it includes those which,
      by chance, haven't had any pre-emption marked packets). However,
      it also requires that the 'Router Marking' state is somehow
      encoded into a packet, i.e. it makes harder the encoding challenge
      discussed in Appendix C of [PCN]. The extra work required by the
      egress gateway would also be somewhat higher than for the previous
      bullet.



5.2. Beat down effect

   This limitation concerns the pre-emption mechanism in the case where
   more than one router is pre-emption marking packets. The result
   (explained in the next paragraph) is that the measurement of
   sustainable-aggregate-rate is lower than its true value, so more
   traffic is pre-empted than necessary.

   Imagine the scenario:

                 +-------+     +-------+     +-------+
   IAR-b=3 >@@@@@| CPR=2 |@@@@@| CPR>2 |@@@@@| CPR=1 |@@> SAR-b=1
   IAR-a=1 >#####|  R1   |#####|  R2   |     |  R3   |
                 +-------+     +-------+     +-------+
                                  #
                                  #
                                  #
                                  v SAR-a=0.5

   Figure 4: Scenario to illustrate 'beat down effect' limitation

   Aggregate-a (ingress-aggregate-rate, IAR, 1 unit) takes a 'short'
   route through two routers, one of which (R1) is above its configured-
   pre-emption-rate (CPR, 2 units). Aggregate-b takes a 'long' route,
   going through a second congested router (R3, with a CPR of 1 unit).

   R1's input traffic is 4 units, twice its configured-pre-emption-rate,
   so 50% of packets are pre-emption marked. Hence the measured
   sustainable-aggregate-rate (SAR) for aggregate-a is 0.5, and half of
   its traffic will be pre-empted.


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   R3's input of non-pre-emption-marked traffic is 1.5 units, and
   therefore it has to do further marking.

   But this means that aggregate-a has taken a bigger hit than it needed
   to; the router R1 could have let through all of aggregate-a's traffic
   unmarked if it had known that the second router R2 was going to "beat
   down" aggregate-b's traffic further.

   Generalising, the result is that in a scenario where more than one
   router is pre-emption marking packets, only the final router is sure
   to be fully loaded after flow pre-emption. The fundamental reason is
   that a router makes a local decision about which packets to pre-
   emption mark, i.e. independently of how other routers are pre-emption
   marking. A very similar effect has been noted in XCP [Low].

   Potential solutions:

   o a full solution would involve routers learning about other routers
      that are pre-emption marking, and being able to differentially
      mark flows (e.g. in the example above, aggregate-a's packets
      wouldn't be marked by R1). This seems hard and complex.

   o do nothing about this limitation. It causes over-pre-emption,
      which is safe. At the moment this is our suggested option.

   o do pre-emption 'slowly'. The description earlier in this document
      assumes that after the measurements of ingress-aggregate-rate and
      sustainable-aggregate-rate, then sufficient flows are pre-empted
      in 'one shot' to eliminate the excess traffic. An alternative is
      to spread pre-emption over several rounds: initially, only pre-
      empt enough to eliminate some of the excess traffic, then re-
      measure the sustainable-aggregate-rate, and then pre-empt some
      more, etc. In the scenario above, the re-measurement would be
      lower than expected, due to the beat down effect, and hence in the
      second round of pre-emption less of aggregate-a's traffic would be
      pre-empted (perhaps none). Overall, therefore the impact of the
      'beat down' effect would be lessened, i.e. there would be a
      smaller degree of over pre-emption. The downside is that the
      overall pre-emption is slower, and therefore routers will be
      congested longer.









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5.3. Bi-directional sessions

   The document earlier describes how to decide whether or not to admit
   (or pre-empt) a particular flow. However, from a user/application
   perspective, the session is the relevant unit of granularity. A
   session can consist of several flows which may not all be part of the
   same aggregate. The most obvious example is a bi-directional session,
   where the two flows should ideally be admitted or pre-empted as a
   pair - for instance a voice call only makes sense if A can send to B
   as well as B to A! But the admission and pre-emption mechanisms
   described earlier in this document operate on a per-aggregate basis,
   independently of what's happening with other aggregates. For
   admission control the problem isn't serious: e.g. the SIP server for
   the voice call can easily detect that the A-to-B flow has been
   admitted but the B-to-A flow blocked, and inform the user perhaps via
   a busy tone. For flow pre-emption, the problem is similar but more
   serious. If both the aggregate-1-to-2 (i.e. from gateway 1 to gateway
   2) and the aggregate-2-to-1 have to pre-empt flows, then it would be
   good if either all of the flows of a particular session were pre-
   empted or none of them. Therefore if the two aggregates pre-empt
   flows independently of each other, more sessions will end up being
   torn down than is really necessary. For instance, pre-empting one
   direction of a voice call will result in the SIP server tearing down
   the other direction anyway.





   Potential solutions:

   o if it's known that all session are bi-directional, simply pre-
      empting roughly half as many flows as suggested by the
      measurements of {ingress-aggregate-rate - sustainable-aggregate-
      rate}. But this makes a big assumption about the nature of
      sessions, and also that the aggregate-1-to-2 and aggregate-2-to-1
      are equally overloaded.

   o ignore the limitation. The penalty will be quite small if most
      sessions consist of one flow or of flows part of the same
      aggregate.








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   o introduce a gateway controller. It would receive reports for all
      aggregates where the ingress-aggregate-rate exceeds the
      sustainable-aggregate-rate. It then would make a global decision
      about which flows to pre-empt. However it requires quite some
      complexity, for example the controller needs to understand which
      flows map to which sessions. This may be an option in some
      scenarios, for example where gateways aren't handling too many
      flows (but note that this breaks the aggregation assumption of
      Section 2.2). A variant of this idea would be to introduce a
      gateway controller per pair of gateways, in order to handle bi-
      directional sessions but not try to deal with more complex
      sessions that include flows from an arbitrary number of
      aggregates.

   o do pre-emption 'slowly'. As in the "beat down" solution 4, this
      would reduce the impact of this limitation. The downside is that
      the overall pre-emption is slower, and therefore router(s) will be
      congested longer.

   o each ingress gateway 'loosely coordinates' with other gateways its
      decision about which specific flows to pre-empt. Each gateway
      numbers flows in the order they arrive (note that this number has
      no meaning outside the gateway), and when pre-empting flows, the
      most recent (or most recent low priority flow) is selected for
      pre-emption; the gateway then works backwards selecting as many
      flows as needed. Gateways will therefore tend to pre-empt flows
      that are part of the same session (as they were admitted at the
      same time). Of course this isn't guaranteed for several reasons,
      for instance gateway A's most recent bi-directional sessions may
      be with gateway C, whereas gateway B's are with gateway A (so
      gateway A will pre-empt A-to-C flows and gateway B will pre-empt
      B-to-A flows). Rather than pre-empting the most recent (low
      priority) flow, an alternative algorithm (for further study) may
      be to select flows based on a hash of particular fields in the
      packet, such that both gateways produce the same hash for flows of
      the same bi-directional session. We believe that this approach
      should be investigated further.












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5.4. Global fairness

   The limitation here is that 'high priority' traffic may be pre-empted
   (or not admitted) when a global decision would instead pre-empt (or
   not admit) 'lower priority' traffic on a different aggregate.

   Imagine the following scenario (extreme to illustrate the point
   clearly). Aggregate_a is all Assured Services (MLPP) traffic, whilst
   aggregate_b is all ordinary traffic (i.e. comparatively low
   priority). Together the two aggregates cause a router to be at twice
   its configured-pre-emption-rate. Ideally we'd like all of aggregate_b
   to be pre-empted, as then all of aggregate_a could be carried.
   However, the approach described earlier in this document leads to
   half of each aggregate being pre-empted.

                       IAR_b=1
                         v
                         v
                     +-------+
   IAR_a=1 ---->-----| CPR=1 |-----> SAR_a=0.5
                     |       |
                     +-------+
                         v
                         v
                       SAR_a=0.5

   Figure 5: Scenario to illustrate 'global fairness' limitation


   Similarly, for admission control - Section 4.1 describes how if the
   Congestion-Level-Estimate is greater than the CLE-threshold all new
   sessions are refused. But it is unsatisfactory to block emergency
   calls, for instance.
















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   Potential solutions:

   o in the admission control case, it is recommended that an
      'emergency / Assured Services' call is admitted immediately even
      if the CLE-threshold is exceeded. Usually the network can actually
      handle the additional microflow, because there is a safety margin
      between the configured-admission-rate and the configured-pre-
      emption-rate. Normal call termination behaviour will soon bring
      the traffic level down below the configured-admission-rate.
      However, in exceptional circumstances the 'emergency / higher
      precedence' call may cause the traffic level to exceed the
      configured-pre-emption-rate; then the usual pre-emption mechanism
      will pre-empt enough (non 'emergency / higher precedence')
      microflows to bring the total traffic back under the configured-
      pre-emption-rate.

   o all egress gateways report to a global coordinator that makes
      decisions about what flows to pre-empt. However this solution adds
      complexity and probably isn't scalable, but it may be an option in
      some scenarios, for example where gateways aren't handling too
      many flows (but note that this breaks the aggregation assumption
      of Section 2.2).

   o introduce a heuristic rule: before pre-empting a 'high priority'
      flow the egress gateway should wait to see if sufficient (lower
      priority) traffic is pre-empted on other aggregates. This is a
      reasonable option.

   o enhance the functionality of all the interior routers, so they can
      detect the priority of a packet, and then differentially mark
      them. As well as adding complexity, in general this would be an
      unacceptable security risk for MLPP traffic, since only controlled
      nodes (like gateways) should know which packets are high priority,
      as this information can be abused by an attacker.

   o do nothing, i.e. accept the limitation. Whilst it's unlikely that
      high priority calls will be quite so unbalanced as in the scenario
      above, just accepting this limitation may be risky. The sorts of
      situations that cause routers to start pre-emption marking are
      also likely to cause a surge of emergency / MLPP calls.









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5.5. Flash crowds

   This limitation concerns admission control and arises because there
   is a time lag between the admission control decision (which depends
   on the Congestion-Level-Estimate during RSVP signalling during call
   set-up) and when the data is actually sent (after the called party
   has answered). In PSTN terms this is the time the phone rings.
   Normally the time lag doesn't matter much because (1) in the CL-
   region there are many flows and they terminate and are answered at
   roughly the same rate, and (2) the network can still operate safely
   when the traffic level is some margin above the configured-admission-
   rate.

   A 'flash crowd' occurs when something causes many calls to be
   initiated in a short period of time - for instance a 'tele-vote'. So
   there is a danger that a 'flash' of calls is accepted, but when the
   calls are answered and data flows the traffic overloads the network.
   Therefore potentially the 'additional load' assumption of Section 2.2
   doesn't hold.

   Potential solutions:

   o The simplest option is to do nothing; an operator relies on the
      pre-emption mechanism if there is a problem. This doesn't seem a
      good choice, as 'flash crowds' are reasonably common on the PSTN,
      unless the operator can ensure that nearly all 'flash crowd'
      events are blocked in the access network and so do not impact on
      the CL-region.

   o A second option is to send 'dummy data' as soon as the call is
      admitted, thus effectively reserving the bandwidth whilst waiting
      for the called party to answer. Reserving bandwidth in advance
      means that the network cannot admit as many calls. For example,
      suppose sessions last 100 seconds and ringing for 10 seconds, the
      cost is a 10% loss of capacity. It may be possible to offset this
      somewhat by increasing the configured-admission-rate in the
      routers, but it would need further investigation. A concern with
      this 'dummy data' option is that it may allow an attacker to
      initiate many calls that are never answered (by a cooperating
      attacker), so eventually the network would only be carrying 'dummy
      data'. The attack exploits that charging only starts when the call
      is answered and not when it is dialled. It may be possible to
      alleviate the attack at the session layer - for example, when the
      ingress gateway gets an RSVP PATH message it checks that the
      source has been well-behaved recently; and limiting the maximum
      time that ringing can last. We believe that if this attack can be
      dealt with then this is a good option.


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   o A third option is that the egress gateway limits the rate at which
      it sends out the Congestion-Level-Estimate, or limits the rate at
      which calls are accepted by replying with a Congestion-Level-
      Estimate of 100% (this is the equivalent of 'call gapping' in the
      PSTN). There is a trade-off, which would need to be investigated
      further, between the degree of protection and possible adverse
      side-effects like slowing down call set-up.

   o A final option is to re-perform admission control before the call
      is answered. The ingress gateway monitors Congestion-Level-
      Estimate updates received from each egress. If it notices that a
      Congestion-Level-Estimate has risen above the CLE-threshold, then
      it terminates all unanswered calls through that egress (e.g. by
      instructing the session protocol to stop the 'ringing tone'). For
      extra safety the Congestion-Level-Estimate could be re-checked
      when the call is answered. A potential drawback for an operator
      that wants to emulate the PSTN is that the PSTN network never
      drops a 'ringing' PSTN call.





5.6. Pre-empting too fast

   As a general idea it seems good to pre-empt excess flows rapidly, so
   that the full QoS is restored to the remaining CL users as soon as
   possible, and partial service is restored to lower priority traffic
   classes on shared links. Therefore the pre-emption mechanism
   described earlier in this document works in 'one shot', i.e. one
   measurement is made of the sustainable-aggregate-rate and the
   ingress-aggregate-rate, and the excess is pre-empted immediately.
   However, there are some reasons why an operator may potentially want
   to pre-empt 'more slowly':

   o To allow time to modify the ingress gateway's policer, as the
      ingress wants to be able to drop any packets that arrive from a
      pre-empted flow. There will be a limit on how many new filters an
      ingress gateway can install in a certain time period. Otherwise
      the source may cheat and ignore the instruction to drop its flow.

   o The operator may decide to slow down pre-emption in order to
      ameliorate the 'beat down' and/or 'bi-directional sessions'
      limitations (see above)





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   o To help combat inaccuracies in measurements of the sustainable-
      aggregate-rate and ingress-aggregate-rate. For a CL-region where
      it's assumed there are many flows in an aggregate these
      measurements can be obtained in a short period of time, but where
      there are fewer flows it will take longer.

   o To help combat over pre-emption because, during the time it takes
      to pre-empt flows, others may be ending anyway (either the call
      has naturally ended, or the user hangs up due to poor QoS).
      Slowing pre-emption may seem counter-intuitive here, as it makes
      it more likely that calls will terminate anyway - however it also
      gives time to adjust the amount pre-empted to take account of
      this.

   o Earlier in this document we said that an egress starts measuring
      the sustainable-aggregate-rate immediately it sees a single pre-
      emption marked packet. However, when a link or router fails the
      network's underlying recovery mechanism will kick in (e.g.
      switching to a back up path), which may result in the network
      again being able to support all the traffic.





   Potential solutions

   o To combat the final issue, the egress could measure the
      sustainable-aggregate-rate over a longer time period than the
      network recovery time (say 100ms vs. 50ms). If it detects no pre-
      emption marked packets towards the end of its measurement period
      (say in the last 30 ms) then it doesn't send a pre-emption alert
      message to the ingress.















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   o We suggest that optionally (the choice of the operator) pre-
      emption is slowed by pre-empting traffic in several rounds rather
      than in one shot. One possible algorithm is to pre-empt most of
      the traffic in the first round and the rest in the second round;
      the amount pre-empted in the second round is influenced by both
      the first and second round measurements:     * Round 1: pre-empt h
      * S_1                                 where 0.5 <= h <= 1
      where S_1 is the amount the normal mechanism calculates that it
      should shed, i.e. {ingress-aggregate-rate - sustainable-aggregate-
      rate}        * Round 2: pre-empt Predicted-S_2 - h * (Predicted-
      S_2 - Measured-S_2)
      where Predicted-S_2 = (1-h)*S_1                              Note
      that the second measurement should be made when sufficient time
      has elapsed for the first round of pre-emption to have happened.
      One idea to achieve this is for the egress gateway to continuously
      measure and report its sustainable-aggregate-rate, in (say) 100ms
      windows. Therefore the ingress gateway knows when the egress
      gateway made its measurement (assuming the round trip time is
      known). Therefore the ingress gateway knows when measurements
      should reflect that it has pre-empted flows.





5.7. Other potential extensions

   In this section we discuss some other potential extensions not
   already covered above.

5.7.1. Tunnelling

   It is possible to tunnel all CL packets across the CL-region.
   Although there is a cost of tunnelling (additional header on each
   packet, additional processing at tunnel ingress and egress), there
   are three reasons it may be interesting.

   ECMP:

   Tunnelling is one of the possible solutions given earlier in Section
   5.1 on Equal Cost Multipath Routing (ECMP).

   Ingress gateway determination:

   If packets are tunnelled from ingress gateway to egress gateway, the
   egress gateway can very easily determine in the data path which
   ingress gateway a packet comes from (by simply looking at the source


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   address of the tunnel header). This can facilitate operations such as
   computing the Congestion-Level-Estimate on a per ingress gateway
   basis.

   End-to-end ECN:

   The ECN field is used for PCN marking (see [PCN] for details), and so
   it needs to be re-set by the egress gateway to whatever has been
   agreed as appropriate for the next domain. Therefore if a packet
   arrives at the ingress gateway with its ECN field already set (i.e.
   not '00'), it may leave the egress gateway with a different value.
   Hence the end-to-end meaning of the ECN field is lost.

   It is open to debate whether end-to-end congestion control is ever
   necessary within an end-to-end reservation. But if a genuine need is
   identified for end-to-end ECN semantics within a reservation, then
   one solution is to tunnel CL packets across the CL-region. When the
   egress gateway decapsulates them the original ECN field is recovered.



5.7.2. Multi-domain and multi-operator usage

   This potential extension would eliminate the trust assumption
   (Section 2.2), so that the CL-region could consist of multiple
   domains run by different operators that did not trust each other.
   Then only the ingress and egress gateways of the CL-region would take
   part in the admission control procedure, i.e. at the ingress to the
   first domain and the egress from the final domain. The border routers
   between operators within the CL-region would only have to do bulk
   accounting - they wouldn't do per microflow metering and policing,
   and they wouldn't take part in signal processing or hold per flow
   state [Briscoe]. [Re-feedback] explains how a downstream domain can
   police that its upstream domain does not 'cheat' by admitting traffic
   when the downstream path is over-congested. [Re-PCN] proposes how to
   achieve this with the help of another recently proposed extension to
   ECN, involving re-echoing ECN feedback [Re-ECN].

5.7.3. Preferential dropping of pre-emption marked packets

   When the rate of real-time traffic in the specified class exceeds the
   maximum configured rate, then a router has to drop some packet(s)
   instead of forwarding them on the out-going link. Now when the egress
   gateway measures the Sustainable-Aggregate-Rate, neither dropped
   packets nor pre-emption marked packets contribute to it. Dropping
   non-pre-emption-marked packets therefore reduces the measured



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   Sustainable-Aggregate-Rate below its true value. Thus a router should
   preferentially drop pre-emption marked packets.

   Note that it is important that the operator doesn't set the
   configured-pre-emption-rate equal to the rate at which packets start
   being dropped (for the specified real-time service class). Otherwise
   the egress gateway may never see a pre-emption marked packet and so
   won't be triggered into the Pre-emption Alert state.

   This optimisation is optional. When considering whether to use it an
   operator will consider issues such as whether the over-pre-emption is
   serious, and whether the particular routers can easily do this sort
   of selective drop.



5.7.4. Adaptive bandwidth for the Controlled Load service

   The admission control mechanism described in this document assumes
   that each router has a fixed bandwidth allocated to CL flows. A
   possible extension is that the bandwidth is flexible, depending on
   the level of non-CL traffic. If a large share of the current load on
   a path is CL, then more CL traffic can be admitted. And if the
   greater share of the load is non-CL, then the admission threshold can
   be proportionately lower. The approach re-arranges sharing between
   classes to aim for economic efficiency, whatever the traffic load
   matrix. It also deals with unforeseen changes to capacity during
   failures better than configuring fixed engineered rates. Adaptive
   bandwidth allocation can be achieved by changing the admission
   marking behaviour, so that the probability of admission marking a
   packet would now depend on the number of queued non-CL packets as
   well as the size of the virtual queue. The adaptive bandwidth
   approach would be supplemented by placing limits on the adaptation to
   prevent starvation of the CL by other traffic classes and of other
   classes by CL traffic. [Songhurst] has more details of the adaptive
   bandwidth approach.

5.7.5. Controlled Load service with end-to-end Pre-Congestion
   Notification

   It may be possible to extend the framework to parts of the network
   where there are only a low number of CL microflows, i.e. the
   aggregation assumption (Section 2.2) doesn't hold. In the extreme it
   may be possible to operate the framework end-to-end, i.e. between end
   hosts. One potential method is to send probe packets to test whether
   the network can support a prospective new CL microflow. The probe
   packets would be sent at the same traffic rate as expected for the


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   actual microflow, but in order not to disturb existing CL traffic a
   router would always schedule probe packets behind CL ones (compare
   [Breslau00]); this implies they have a new DSCP. Otherwise the
   routers would treat probe packets identically to CL packets. In order
   to perform admission control quickly, in parts of the network where
   there are only a few CL microflows, the Pre-Congestion marking
   behaviour for probe packets would switch from admission marking no
   packets to admission marking them all for only a minimal increase in
   load.

5.7.6. MPLS-TE

   [ECN-MPLS] discusses how to extend the deployment model to MPLS, i.e.
   for admission control of microflows into a set of MPLS-TE aggregates
   (Multi-protocol label switching traffic engineering). It would
   require that the MPLS header could include the ECN field, which is
   not precluded by RFC3270. See [ECN-MPLS].
































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6. Relationship to other QoS mechanisms

6.1. IntServ Controlled Load

   The CL mechanism delivers QoS similar to Integrated Services
   controlled load, but rather better. The reason the QoS is better is
   that the CL mechanism keeps the real queues empty, by driving
   admission control from a bulk virtual queue on each interface. The
   virtual queue [AVQ, vq] can detect a rise in load before the real
   queue builds. It is also more robust to route changes.

6.2. Integrated services operation over DiffServ

   Our approach to end-to-end QoS is similar to that described in
   [RFC2998] for Integrated services operation over DiffServ networks.
   Like [RFC2998], an IntServ class (CL in our case) is achieved end-to-
   end, with a CL-region viewed as a single reservation hop in the total
   end-to-end path. Interior routers of the CL-region do not process
   flow signalling nor do they hold per flow state. Unlike [RFC2998] we
   do not require the end-to-end signalling mechanism to be RSVP,
   although it can be.

   Bearing in mind these differences, we can describe our architecture
   in the terms of the options in [RFC2998]. The DiffServ network region
   is RSVP-aware, but awareness is confined to (what [RFC2998] calls)
   the "border routers" of the DiffServ region. We use explicit
   admission control into this region, with static provisioning within
   it. The ingress "border router" does per microflow policing and sets
   the DSCP and ECN fields to indicate the packets are CL ones (i.e. we
   use router marking rather than host marking).

6.3. Differentiated Services

   The DiffServ architecture does not specify any way for devices
   outside the domain to dynamically reserve resources or receive
   indications of network resource availability.  In practice, service
   providers rely on subscription-time Service Level Agreements (SLAs)
   that statically define the parameters of the traffic that will be
   accepted from a customer. The CL mechanism allows dynamic reservation
   of resources through the DiffServ domain and, with the potential
   extension mentioned in Section 5.7.2, it can span multiple domains
   without active policing mechanisms at the borders (unlike DiffServ).
   Therefore we do not use the traffic conditioning agreements (TCAs) of
   the (informational) DiffServ architecture [RFC2475].

   [Johnson] compares admission control with a 'generously dimensioned'
   DiffServ network as ways to achieve QoS. The former is recommended.


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6.4. ECN

   The marking behaviour described in this document complies with the
   ECN aspects of the IP wire protocol RFC3168, but provides its own
   edge-to-edge feedback instead of the TCP aspects of RFC3168. All
   routers within the CL-region are upgraded with the admission marking
   and pre-emption marking of Pre-Congestion Notification, so the
   requirements of [Floyd] are met because the CL-region is an enclosed
   environment. The operator prevents traffic arriving at a router that
   doesn't understand CL by administrative configuration of the ring of
   gateways around the CL-region.

6.5. RTECN

   Real-time ECN (RTECN) [RTECN, RTECN-usage] has a similar aim to this
   document (to achieve a low delay, jitter and loss service suitable
   for RT traffic) and a similar approach (per microflow admission
   control combined with an "early warning" of potential congestion
   through setting the CE codepoint). But it explores a different
   architecture without the aggregation assumption: host-to-host rather
   than edge-to-edge. We plan to document such a host-to-host framework
   in a parallel draft to this one, and to describe if and how [PCN] can
   work in this framework.



6.6. RMD

   Resource Management in DiffServ (RMD) [RMD] is similar to this work,
   in that it pushes complex classification, traffic conditioning and
   admission control functions to the edge of a DiffServ domain and
   simplifies the operation of the interior routers. One of the RMD
   modes ("Congestion notification function based on probing") uses
   measurement-based admission control in a similar way to this
   document. The main difference is that in RMD probing plays a
   significant role in the admission control process. Other differences
   are that the admission control decision is taken on the egress
   gateway (rather than the ingress); 'admission marking' is encoded in
   a packet as a new DSCP (rather than in the ECN field), and that the
   NSIS protocols are used for signalling (rather than RSVP).

   RMD also includes the concept of Severe Congestion handling. The pre-
   emption mechanism described in the CL architecture has similar
   objectives but relies on different mechanisms. The main difference is
   that the interior routers measure the data rate that causes an
   overload and mark packets according to this rate.



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6.7. RSVP Aggregation over MPLS-TE

   Multi-protocol label switching traffic engineering (MPLS-TE) allows
   scalable reservation of resources in the core for an aggregate of
   many microflows. To achieve end-to-end reservations, admission
   control and policing of microflows into the aggregate can be achieved
   using techniques such as RSVP Aggregation over MPLS TE Tunnels as per
   [AGGRE-TE]. However, in the case of inter-provider environments,
   these techniques require that admission control and policing be
   repeated at each trust boundary or that MPLS TE tunnels span multiple
   domains.






































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7. Security Considerations

   To protect against denial of service attacks, the ingress gateway of
   the CL-region needs to police all CL packets and drop packets in
   excess of the reservation. This is similar to operations with
   existing IntServ behaviour.

   For pre-emption, it is considered acceptable from a security
   perspective that the ingress gateway can treat "emergency/military"
   CL flows preferentially compared with "ordinary" CL flows. However,
   in the rest of the CL-region they are not distinguished (nonetheless,
   our proposed technique does not preclude the use of different DSCPs
   at the packet level as well as different priorities at the flow
   level.). Keeping emergency traffic indistinguishable at the packet
   level minimises the opportunity for new security attacks. For
   example, if instead a mechanism used different DSCPs for
   "emergency/military" and "ordinary" packets, then an attacker could
   specifically target the former in the data plane (perhaps for DoS or
   for eavesdropping).

   Further security aspects to be considered later.



8. Acknowledgements

   The admission control mechanism evolved from the work led by Martin
   Karsten on the Guaranteed Stream Provider developed in the M3I
   project [GSPa, GSP-TR], which in turn was based on the theoretical
   work of Gibbens and Kelly [DCAC]. Kennedy Cheng, Gabriele Corliano,
   Carla Di Cairano-Gilfedder, Kashaf Khan, Peter Hovell, Arnaud Jacquet
   and June Tay (BT) helped develop and evaluate this approach.

   Many thanks to those who have commented on this work at Transport
   Area Working Group meetings and on the mailing list, including: Ken
   Carlberg, Ruediger Geib, Lars Westberg, David Black, Robert Hancock,
   Cornelia Kappler.

9. Comments solicited

   Comments and questions are encouraged and very welcome. They can be
   sent to the Transport Area Working Group's mailing list,
   tsvwg@ietf.org, and/or to the authors.






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10. Changes from earlier versions of the draft

   The main changes are:

   From -00 to -01

   The whole of the Pre-emption mechanism is added.

   There are several modifications to the admission control mechanism.



   From -01 to -02

   The pre-congestion notification algorithms for admission marking and
   pre-emption marking are now described in [PCN].

   There are new sub-sections in Section 4 on Failures, Admission of
   'emergency / higher precedence' session, and Tunnelling; and a new
   sub-section in Section 5 on Mechanisms to deal with 'Flash crowds'.



   From -02 to -03

   Section 5 has been updated and expanded. It is now about the
   'limitations' of the PCN mechanism, as described in the earlier
   sections, plus discussion of 'possible solutions' to those
   limitations.

   The measurement of the Congestion-Level-Estimate now includes pre-
   emption marked packets as well as admission marked ones. Section
   3.1.2 explains.
















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11. Appendices

11.1. Appendix A: Explicit Congestion Notification

   This Appendix provides a brief summary of Explicit Congestion
   Notification (ECN).

   [RFC3168] specifies the incorporation of ECN to TCP and IP, including
   ECN's use of two bits in the IP header. It specifies a method for
   indicating incipient congestion to end-hosts (e.g. as in RED, Random
   Early Detection), where the notification is through ECN marking
   packets rather than dropping them.

   ECN uses two bits in the IP header of both IPv4 and IPv6 packets:

            0     1     2     3     4     5     6     7
         +-----+-----+-----+-----+-----+-----+-----+-----+
         |          DS FIELD, DSCP           | ECN FIELD |
         +-----+-----+-----+-----+-----+-----+-----+-----+

           DSCP: differentiated services codepoint
           ECN:  Explicit Congestion Notification

   Figure A.1: The Differentiated Services and ECN Fields in IP.

   The two bits of the ECN field have four ECN codepoints, '00' to '11':
         +-----+-----+
         | ECN FIELD |
         +-----+-----+
           ECT   CE
            0     0         Not-ECT
            0     1         ECT(1)
            1     0         ECT(0)
            1     1         CE

   Figure A.2: The ECN Field in IP.

   The not-ECT codepoint '00' indicates a packet that is not using ECN.

   The CE codepoint '11' is set by a router to indicate congestion to
   the end hosts. The term 'CE packet' denotes a packet that has the CE
   codepoint set.

   The ECN-Capable Transport (ECT) codepoints '10' and '01' (ECT(0) and
   ECT(1) respectively) are set by the data sender to indicate that the
   end-points of the transport protocol are ECN-capable. Routers treat
   the ECT(0) and ECT(1) codepoints as equivalent. Senders are free to


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   use either the ECT(0) or the ECT(1) codepoint to indicate ECT, on a
   packet-by-packet basis. The use of both the two codepoints for ECT is
   motivated primarily by the desire to allow mechanisms for the data
   sender to verify that network elements are not erasing the CE
   codepoint, and that data receivers are properly reporting to the
   sender the receipt of packets with the CE codepoint set.

   ECN requires support from the transport protocol, in addition to the
   functionality given by the ECN field in the IP packet header.
   [RFC3168] addresses the addition of ECN Capability to TCP, specifying
   three new pieces of functionality: negotiation between the endpoints
   during connection setup to determine if they are both ECN-capable; an
   ECN-Echo (ECE) flag in the TCP header so that the data receiver can
   inform the data sender when a CE packet has been received; and a
   Congestion Window Reduced (CWR) flag in the TCP header so that the
   data sender can inform the data receiver that the congestion window
   has been reduced.

   The transport layer (e.g.. TCP) must respond, in terms of congestion
   control, to a *single* CE packet as it would to a packet drop.

   The advantage of setting the CE codepoint as an indication of
   congestion, instead of relying on packet drops, is that it allows the
   receiver(s) to receive the packet, thus avoiding the potential for
   excessive delays due to retransmissions after packet losses.



11.2. Appendix B: What is distributed measurement-based admission
   control?

   This Appendix briefly explains what distributed measurement-based
   admission control is [Breslau99].

   Traditional admission control algorithms for 'hard' real-time
   services (those providing a firm delay bound for example) guarantee
   QoS by using 'worst case analysis'. Each time a flow is admitted its
   traffic parameters are examined and the network re-calculates the
   remaining resources. When the network gets a new request it therefore
   knows for certain whether the prospective flow, with its particular
   parameters, should be admitted. However, parameter-based admission
   control algorithms result in under-utilisation when the traffic is
   bursty. Therefore 'soft' real time services - like Controlled Load -
   can use a more relaxed admission control algorithm.

   This insight suggests measurement-based admission control (MBAC). The
   aim of MBAC is to provide a statistical service guarantee. The


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   classic scenario for MBAC is where each router participates in hop-
   by-hop admission control, characterising existing traffic locally
   through measurements (instead of keeping an accurate track of traffic
   as it is admitted), in order to determine the current value of some
   parameter e.g. load. Note that for scalability the measurement is of
   the aggregate of the flows in the local system. The measured
   parameter(s) is then compared to the requirements of the prospective
   flow to see whether it should be admitted.

   MBAC may also be performed centrally for a network, it which case it
   uses centralised measurements by a bandwidth broker.

   We use distributed MBAC. "Distributed" means that the measurement is
   accumulated for the 'whole-path' using in-band signalling. In our
   case, this means that the measurement of existing traffic is for the
   same pair of ingress and egress gateways as the prospective
   microflow.

   In fact our mechanism can be said to be distributed in three ways:
   all routers on the ingress-egress path affect the Congestion-Level-
   Estimate; the admission control decision is made just once on behalf
   of all the routers on the path across the CL-region; and the ingress
   and egress gateways cooperate to perform MBAC.



11.3. Appendix C: Calculating the Exponentially weighted moving average
   (EWMA)

   At the egress gateway, for every CL packet arrival:

   [EWMA-total-bits]n+1  =  (w * bits-in-packet)  +  ((1-w) * [EWMA-
   total-bits]n )

   [EWMA-M-bits]n+1  =  (B * w * bits-in-packet)  +  ((1-w) * [EWMA-M-
   bits]n )

   Then, per new flow arrival:

   [Congestion-Level-Estimate]n+1  =  [EWMA-M-bits]n+1  /  [EWMA-total-
   bits]n+1



   where




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   EWMA-total-bits is the total number of bits in CL packets, calculated
   as an exponentially weighted moving average (EWMA)

   EWMA-M-bits is the total number of bits in CL packets that are
   Admission Marked or Pre-emption Marked, again calculated as an EWMA.

   B is either 0 or 1:

     B = 0 if the CL packet is not admission marked

     B = 1 if the CL packet is admission marked

   w is the exponential weighting factor.


   Varying the value of the weight trades off between the smoothness and
   responsiveness of the Congestion-Level-Estimate. However, in general
   both can be achieved, given our original assumption of many CL
   microflows and remembering that the EWMA is calculated on the basis
   of aggregate traffic between the ingress and egress gateways.
   There will be a threshold inter-arrival time between packets of the
   same aggregate below which the egress will consider the estimate of
   the Congestion-Level-Estimate as too stale, and it will then trigger
   generation of probes by the ingress.

   The first two per-packet algorithms can be simplified, if their only
   use will be where the result of one is divided by the result of the
   other in the third, per-flow algorithm.

   [EWMA-total-bits]'n+1  =  bits-in-packet  +  (w' * [EWMA- total-
   bits]n )

   [EWMA-AM-bits]'n+1  =  (B * bits-in-packet)  +  (w' * [EWMA-AM-bits]n
   )

   where w' = (1-w)/w.

   If w' is arranged to be a power of 2, these per packet algorithms can
   be implemented solely with a shift and an add.






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12. References

   A later version will distinguish normative and informative
   references.

   [AGGRE-TE]    Francois Le Faucheur, Michael Dibiasio, Bruce Davie,
                 Michael Davenport, Chris Christou, Jerry Ash, Bur
                 Goode, 'Aggregation of RSVP Reservations over MPLS
                 TE/DS-TE Tunnels', draft-ietf-tsvwg-rsvp-dste-03 (work
                 in progress), June 2006

   [ANSI.MLPP.Spec] American National Standards Institute,
                 "Telecommunications- Integrated Services Digital
                 Network (ISDN) - Multi-Level Precedence and Pre-
                 emption (MLPP) Service Capability", ANSI T1.619-1992
                 (R1999), 1992.

   [ANSI.MLPP.Supplement] American National Standards Institute, "MLPP
                 Service Domain Cause Value Changes", ANSI ANSI
                 T1.619a-1994 (R1999), 1990.

   [AVQ]         S. Kunniyur and R. Srikant "Analysis and Design of an
                 Adaptive Virtual Queue (AVQ) Algorithm for Active
                 Queue Management", In: Proc. ACM SIGCOMM'01, Computer
                 Communication Review 31 (4) (October, 2001).

   [Breslau99]   L. Breslau, S. Jamin, S. Shenker "Measurement-based
                 admission control: what is the research agenda?", In:
                 Proc. Int'l Workshop on Quality of Service 1999.

   [Breslau00]   L. Breslau, E. Knightly, S. Shenker, I. Stoica, H.
                 Zhang "Endpoint Admission Control: Architectural
                 Issues and Performance", In: ACM SIGCOMM 2000

   [Briscoe]     Bob Briscoe and Steve Rudkin, "Commercial Models for
                 IP Quality of Service Interconnect", BT Technology
                 Journal, Vol 23 No 2, April 2005.

   [DCAC]        Richard J. Gibbens and Frank P. Kelly "Distributed
                 connection acceptance control for a connectionless
                 network", In: Proc. International Teletraffic Congress
                 (ITC16), Edinburgh, pp. 941—952 (1999).

   [ECN-MPLS]    Bruce Davie, Bob Briscoe, June Tay, "Explicit
                 Congestion Marking in MPLS",                    draft-
                 davie-ecn-mpls-00.txt (work in progress), June 2006



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   [EMERG-RQTS]  Carlberg, K. and R. Atkinson, "General Requirements
                 for Emergency Telecommunication Service (ETS)", RFC
                 3689, February 2004.

   [EMERG-TEL]   Carlberg, K. and R. Atkinson, "IP Telephony
                 Requirements for Emergency Telecommunication Service
                 (ETS)", RFC 3690, February 2004.

   [Floyd]       S. Floyd, 'Specifying Alternate Semantics for the
                 Explicit Congestion Notification (ECN) Field', draft-
                 floyd-ecn-alternates-02.txt (work in progress), August
                 2005

   [GSPa]        Karsten (Ed.), Martin "GSP/ECN Technology &
                 Experiments", Deliverable: 15.3 PtIII, M3I Eu Vth
                 Framework Project IST-1999-11429, URL:
                 http://www.m3i.org/ (February, 2002) (superseded by
                 [GSP-TR])

   [GSP-TR]      Martin Karsten and Jens Schmitt, "Admission Control
                 Based on Packet Marking and Feedback Signalling ­--
                 Mechanisms, Implementation and Experiments", TU-
                 Darmstadt Technical Report TR-KOM-2002-03, URL:
                 http://www.kom.e-technik.tu-
                 darmstadt.de/publications/abstracts/KS02-5.html (May,
                 2002)

   [ITU.MLPP.1990] International Telecommunications Union, "Multilevel
                 Precedence and Pre-emption Service (MLPP)", ITU-T
                 Recommendation I.255.3, 1990.

   [Johnson]     DM Johnson, 'QoS control versus generous
                 dimensioning', BT Technology Journal, Vol 23 No 2,
                 April 2005

   [Low]         S. Low, L. Andrew, B. Wydrowski, 'Understanding XCP:
                 equilibrium and fairness', IEEE InfoCom 2005

   [PCN]         B. Briscoe, P. Eardley, D. Songhurst, F. Le Faucheur,
                 A. Charny, V. Liatsos, S. Dudley, J. Babiarz, K. Chan,
                 G. Karagiannis, A. Bader, L. Westberg. 'Pre-Congestion
                 Notification marking', draft-briscoe-tsvwg-cl-phb-02
                 (work in progress), June 2006.






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   [Re-ECN]      Bob Briscoe, Arnaud Jacquet, Alessandro Salvatori,
                 'Re-ECN: Adding Accountability for Causing Congestion
                 to TCP/IP', draft-briscoe-tsvwg-re-ecn-tcp-01 (work in
                 progress), March 2006.

   [Re-feedback] Bob Briscoe, Arnaud Jacquet, Carla Di Cairano-
                 Gilfedder, Andrea Soppera, 'Re-feedback for Policing
                 Congestion Response in an Inter-network', ACM SIGCOMM
                 2005, August 2005.

   [Re-PCN]      B. Briscoe, 'Emulating Border Flow Policing using Re-
                 ECN on Bulk Data', draft-briscoe-tsvwg-re-ecn-border-
                 cheat-00 (work in progress), February 2006.

   [Reid]        ABD Reid, 'Economics and scalability of QoS
                 solutions', BT Technology Journal, Vol 23 No 2, April
                 2005

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

   [RFC2309]     Braden, B., et al., "Recommendations on Queue
                 Management and Congestion Avoidance in the Internet",
                 RFC 2309, April 1998.

   [RFC2474]     Nichols, K., Blake, S., Baker, F. and D. Black,
                 "Definition of the Differentiated Services Field (DS
                 Field) in the IPv4 and IPv6 Headers", RFC 2474,
                 December 1998

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

   [RFC2597]     Heinanen, J., Baker, F., Weiss, W. and J. Wrocklawski,
                 "Assured Forwarding PHB Group", RFC 2597, June 1999.

   [RFC2998]     Bernet, Y., Yavatkar, R., Ford, P., 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.



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

   [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.

   [RFC4542]     F. Baker & J. Polk, "Implementing an Emergency
                 Telecommunications Service for Real Time Services in
                 the Internet Protocol Suite", RFC 4542, May 2006.

   [RMD]         Attila Bader, Lars Westberg, Georgios Karagiannis,
                 Cornelia Kappler, Tom Phelan, 'RMD-QOSM - The Resource
                 Management in DiffServ QoS model', draft-ietf-nsis-
                 rmd-03 Work in Progress, June 2005.

   [RSVP-PCN]    Francois Le Faucheur, Anna Charny, Bob Briscoe, Philip
                 Eardley, Joe Barbiaz, Kwok-Ho Chan, 'RSVP Extensions
                 for Admission Control over DiffServ using Pre-
                 Congestion Notification (PCN)', draft-lefaucheur-rsvp-
                 ecn-01 (work in progress), June 2006.

   [RSVP-PREEMPTION] Herzog, S., "Signaled Preemption Priority Policy
                 Element", RFC 3181, October 2001.

   [RSVP-EMERGENCY] Le Faucheur et al., RSVP Extensions for Emergency
                 Services, draft-lefaucheur-emergency-rsvp-02.txt

   [RTECN]       Babiarz, J., Chan, K. and V. Firoiu, 'Congestion
                 Notification Process for Real-Time Traffic', draft-
                 babiarz-tsvwg-rtecn-04 Work in Progress, July 2005.

   [RTECN-usage] Alexander, C., Ed., Babiarz, J. and J. Matthews,
                 'Admission Control Use Case for Real-time ECN', draft-
                 alexander-rtecn-admission-control-use-case-00, Work in
                 Progress, February 2005.

   [Songhurst]   David J. Songhurst, Philip Eardley, Bob Briscoe, Carla
                 Di Cairano Gilfedder and June Tay, 'Guaranteed QoS
                 Synthesis for Admission Control with Shared Capacity',
                 BT Technical Report TR-CXR9-2006-001, Feb 2006,
                 http://www.cs.ucl.ac.uk/staff/B.Briscoe/projects/ipe2e
                 qos/gqs/papers/GQS_shared_tr.pdf


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   [vq]          Costas Courcoubetis and Richard Weber "Buffer Overflow
                 Asymptotics for a Switch Handling Many Traffic
                 Sources" In: Journal Applied Probability 33 pp. 886--
                 903 (1996).












































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Authors' Addresses

   Bob Briscoe
   BT Research
   B54/77, Sirius House
   Adastral Park
   Martlesham Heath
   Ipswich, Suffolk
   IP5 3RE
   United Kingdom
   Email: bob.briscoe@bt.com

   Dave Songhurst
   BT Research
   B54/69, Sirius House
   Adastral Park
   Martlesham Heath
   Ipswich, Suffolk
   IP5 3RE
   United Kingdom
   Email: dsonghurst@jungle.bt.co.uk

   Philip Eardley
   BT Research
   B54/77, Sirius House
   Adastral Park
   Martlesham Heath
   Ipswich, Suffolk
   IP5 3RE
   United Kingdom
   Email: philip.eardley@bt.com

   Francois Le Faucheur
   Cisco Systems, Inc.
   Village d'Entreprise Green Side - Batiment T3
   400, Avenue de Roumanille
   06410 Biot Sophia-Antipolis
   France
   Email: flefauch@cisco.com

   Anna Charny
   Cisco Systems
   300 Apollo Drive
   Chelmsford, MA 01824
   USA
   Email: acharny@cisco.com



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   Kwok Ho Chan
   Nortel Networks
   600 Technology Park Drive
   Billerica, MA  01821
   USA
   Email: khchan@nortel.com


   Jozef Z. Babiarz
   Nortel Networks
   3500 Carling Avenue
   Ottawa, Ont  K2H 8E9
   Canada
   Email: babiarz@nortel.com

   Stephen Dudley
   Nortel Networks
   4001 E. Chapel Hill Nelson Highway
   P.O. Box 13010, ms 570-01-0V8
   Research Triangle Park, NC 27709
   USA
   Email: smdudley@nortel.com

   Georgios Karagiannis
   University of Twente
   P.O. BOX 217
   7500 AE Enschede,
   The Netherlands
   EMail: g.karagiannis@ewi.utwente.nl

   Attila Báder
   attila.bader@ericsson.com

   Lars Westberg
   Ericsson AB
   SE-164 80 Stockholm
   Sweden
   EMail: Lars.Westberg@ericsson.com











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