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TSVWG                                                        B. Briscoe
Internet Draft                                               P. Eardley
draft-briscoe-tsvwg-cl-phb-01.txt                           D. Songhurst
Expires: September 2006                                              BT

                                                        F. Le Faucheur
                                                              A. Charny
                                                              V. Liatsos
                                                    Cisco Systems, Inc

                                                           J. Babiarz
                                                                K. Chan
                                                            S. Dudley
                                                               Nortel

                                                         6 March, 2006

                    Pre-Congestion Notification marking
                     draft-briscoe-tsvwg-cl-phb-01.txt


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Copyright Notice

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

Abstract

   Pre-Congestion Notification (PCN) builds on the concepts of RFC 3168,
   "The addition of Explicit Congestion Notification to IP". However,
   Pre-Congestion Notification aims at providing notification before any
   congestion actually occurs. Pre-Congestion Notification is applied to
   real-time flows (such as voice, video and multimedia streaming) in
   DiffServ networks. As described in [CL-ARCH], it enables "pre"
   congestion control through two procedures, flow admission control and
   flow pre-emption. The draft proposes algorithms that determine when a
   PCN-enabled router writes Admission Marking and Pre-emption Marking
   in a packet header, depending on the traffic level. The draft also
   proposes how to encode these markings. We present simulation results
   with PCN working in an edge-to-edge scenario using the marking
   algorithms described. Other marking algorithms will be investigated
   in the future.



Conventions used in this document

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in [RFC2119].

Table of Contents


   1. Overview....................................................4
      1.1. Introduction...........................................4
      1.2. Terminology............................................8
   2. Admission Marking algorithm..................................9
      2.1. Outline................................................9
      2.2. Virtual queue based algorithm for Admission Marking......9
      2.3. Admission control within a CL-region using Pre-Congestion
      Notification...............................................11
   3. Pre-emption Marking........................................12
      3.1. Outline...............................................12
      3.2. Token bucket based algorithm for Pre-emption Marking....12
      3.3. Flow pre-emption within a CL-region using Pre-Congestion
      Notification...............................................15
   4. Simulation results.........................................16
   5. Encoding the Admission Marked and Pre-emption Marked states..17


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   6. Acknowledgements...........................................19
   7. Comments solicited.........................................19
   8. Changes from earlier version of the draft...................19
   9. Appendix A: Explicit Congestion Notification................20
   10. Appendix B - Details of simulations........................22
      10.1. Network and signalling model..........................22
      10.2. Simulated Traffic types...............................23
         10.2.1. Voice CBR........................................24
         10.2.2. On-off traffic approximating voice with silence
         compression.............................................24
         10.2.3. High-rate on-off traffic.........................24
      10.3. Admission Control Simulations.........................24
         10.3.1. Summary of the key parameters for CAC............24
            10.3.1.1. Virtual Queue settings......................24
            10.3.1.2. Egress measurement parameters...............25
         10.3.2. Overview of the Admission Control Results.........25
         10.3.3. Sensitivity to Poisson Arrivals assumption........27
         10.3.4. Sensitivity to marking parameters................29
         10.3.5. Sensitivity to RTT...............................30
         10.3.6. Future Work for Admission Control Experiments.....31
      10.4. Flow Pre-emption Simulations..........................31
         10.4.1. Flow Pre-emption Model and key parameters.........31
         10.4.2. Summary of Flow Pre-emption Experiments...........33
         10.4.3. Future Work on Flow Pre-emption Experiments.......33
   11. Appendix C - Alternative ways of encoding the Admission Marked
   and Pre-emption Marked States..................................35
      11.1. Alternative 1........................................35
      11.2. Alternative 2........................................35
      11.3. Alternative 3........................................36
      11.4. Alternative 4........................................36
      11.5. Alternative 5........................................37
      11.6. Comparison of Alternatives............................37
         11.6.1. How compatible is the encoding scheme with RFC 3168
         ECN?....................................................38
         11.6.2. Does the encoding scheme allow an "ECN-nonce"?....40
         11.6.3. Does the encoding scheme require new DSCP(s)?.....41
         11.6.4. Impact on measurements...........................42
         11.6.5. Other issues.....................................42
   12. References................................................43
   Authors' Addresses............................................45
   Intellectual Property Statement................................46
   Disclaimer of Validity........................................47
   Copyright Statement...........................................47






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1. Overview

1.1. Introduction

   Pre-Congestion Notification builds on the concepts of RFC 3168, "The
   addition of Explicit Congestion Notification to IP". Pre-Congestion
   Notification is applied to real-time flows (such as voice, video and
   multimedia streaming) in DiffServ-enabled networks. The reader is
   referred to [CL-ARCH] for description of how PCN enables "pre"
   congestion control through two procedures, flow admission control and
   flow pre-emption. Flow admission control determines whether a new
   microflow is added into the network. Flow pre-emption reduces the
   current traffic load by terminating selected microflows.

   Note this draft concerns the admission control and pre-emption of
   *flows*, not of packets.

   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 RED (Random Early Detection)
   is expected to be used. RFC3168 states that "specifications for
   Diffserv PHBs [RFC2475] MAY provide more specifics" on the CE marking
   algorithm. This document can be seen as effectively providing such
   "specifics" for PHBs targeting real time services. We imagine future
   specifications for Diffserv PHBs MAY define their ECN marking
   algorithm by reference to this document. In particular we imagine a
   CL PHB definition would refer to EF [RFC3246] for its scheduling
   behaviour and to this draft for its ECN marking behaviour. However,
   currently this draft merely documents pre-congestion notification
   algorithms and encoding schemes that we believe are reasonably good,
   but not necessarily the best. On-going work will consider various
   alternatives and reach rough consensus on the best.

   This draft does not propose to change the name of the ECN field. The
   term PCN is solely used for the marking process. So we say pre-
   congestion marking is applied to the ECN field (not to the PCN
   field). We also keep the names of the ECN codepoints, except wherever
   new codepoint semantics are required. When we talk of PCN-routers, we
   mean routers arranged so that they will use PCN to mark packets
   carrying specific, configured DSCPs. PCN routers will still use
   default ECN semantics to mark packets carrying other DSCPs.

   A router enabled with Pre-Congestion Notification marks packets at a
   lower traffic level than an ECN-router, when there still isn't any
   significant build-up of real-time packets in the queue. So PCN-marked


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   packets act as an "early warning" that the amount of packets flowing
   is getting close to the engineered capacity and hence indicate to the
   admission control system that requests to admit new real-time flows
   should be rejected.

   In addition to admission control, another essential Quality of
   Service feature in deployed networks is the ability to cope with
   failures of nodes and links. In this situation the network's capacity
   is reduced and selected flows may need to be terminated (pre-empted)
   in order to preserve the quality of service of the remaining real-
   time flows. Therefore PCN-routers also include the ability to PCN-
   mark packets to alert that flow pre-emption may be needed.

   So a PCN-router needs to be configured with two reference rates:

   o configured-admission-rate

   o configured-pre-emption-rate

   Clearly flow pre-emption should happen at a higher traffic rate than
   admission control. Both these rates will be lower than the physical
   line rate.

   Note that admission control is the primary mechanism used to prevent
   congestion from occurring and flow pre-emption would rarely be
   invoked under normal conditions; it is a safety mechanism to prevent
   congestion from persisting after link failures, re-routes and other
   similar events.

   Together, admission control and flow pre-emption protect the
   forwarding service offered to admitted and non-pre-empted flows, as
   well as protecting service to the traffic classes using the remainder
   of the link capacity.

   Note well that a PCN-router does not achieve admission control or
   flow pre-emption on its own. Just like ECN, a PCN router requires a
   feedback system in order to control the load causing the congestion
   it is suffering. [CL-ARCH] describes a framework 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 (CL)
   service is a quality of service (QoS) closely approximating the QoS
   that the same flow would receive from a lightly loaded network
   element [RFC2211]. The edge-to-edge region (which we call the CL-
   region) is a controlled environment, in that all routers in the CL-
   region are enabled with Pre-Congestion Notification and packets can
   only enter / leave the CL-region through (enhanced) gateways. PCN-


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   marked packets are detected by an egress gateway and associated
   information is sent to the relevant ingress gateway to decide whether
   to admit a new flow, or even pre-empt an existing flow. [CL-ARCH]
   also describes a number of assumptions about the CL-region, such as
   that there are a large number of real-time flows between each pair of
   gateways; hence the CL-region is typically the backbone of an
   operator.



   We also would like to use PCN-routers in other frameworks, such as:

   o Where the CL-region spans networks run by different operators.

   o End-host to end-host, i.e. a similar architecture to that
      described in [RTECN]

   o a similar architecture to that described in [RMD]

   These scenarios are for further study as some of the assumptions made
   about the CL-region in [CL-ARCH] no longer hold. We plan later drafts
   to describe if and how PCN can work in these frameworks.



   This document describes Pre-Congestion Notification:

   o (Section 2) The algorithm that determines when a packet is marked
      so as to warn the admission control mechanism that admission
      control may be needed

   o (Section 3) The algorithm that determines when a packet is marked
      so as to warn the pre-emption mechanism that pre-emption may be
      needed

   o (Section 4 & Appendix B) Simulation results that demonstrate the
      effectiveness of stateless admission control and flow pre-emption.
      The results were obtained using the algorithms of Sections 2 and
      3. The pdf version of this document includes graphs of simulation
      results that aren't in the text version. It can be found at
      http://www.cs.ucl.ac.uk/staff/B.Briscoe/projects/ipe2eqos/gqs/pape
      rs/draft-briscoe-tsvwg-cl-phb-01.pdf

   o (Section 5 & Appendix C) How to encode the markings, i.e. what
      change to make to which bits of a packet so as to convey the
      admission marking and pre-emption marking to the admission control
      and pre-emption mechanisms on the egress gateway


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   Sections 2 and 3 describe the algorithms a PCN-enabled router uses to
   decide whether it needs to set a packet into the Admission Marked or
   Pre-emption Marked state. The algorithms are driven by the amount of
   traffic in the specified real-time service class. Note that the
   measurement is made on an aggregate basis, i.e. it doesn't
   distinguish between real-time microflows. We present example
   implementations but the same effect may be implemented in different
   ways. Indeed, both the admission control and pre-emption algorithms
   could have been implemented as variants of token buckets, but the
   former is implemented as a virtual queue, to present an alternative
   (yet still fairly similar) implementation.

                          +------------+
                          |   Result   |
                          |            V
                      +-------+    +--------+
                      | Bulk  |    |  PCN   |
       Packets    ===>| Meter |===>| Marker |===> Marked Packets
                      |       |    |        |
                      +-------+    +--------+

   Figure 1: Block Diagram of Meter and Marker Function

   In Sections 2 and 3 we also hint at how Pre-Congestion Notification
   can be used within the CL-region, in order to achieve measurement-
   based admission control and flow pre-emption "edge-to-edge" across
   the CL-region. Details are in [CL-ARCH].

   Section 4 reports some simulation results obtained using these
   algorithms in the CL-region framework. Note that the aim of our
   simulations is to demonstrate to the IETF community that these
   measurement-based admission control and flow pre-emption mechanisms
   work successfully. It isn't to show that the particular marking
   algorithms simulated are the optimum ones; although we believe they
   are a reasonably good choice, on-going work will compare them with
   various alternatives.

   Section 5 presents one possibility for how to encode the markings.
   Although we believe it is a reasonable choice, there are other
   possibilities, some of which are listed and discussed in Appendix C.
   We seek advice and debate as to what scheme should be standardised.
   Note that the choice of how to encode the markings is non-trivial



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   because we have five things we potentially want to encode, and only
   have four states in the two bits of the ECN field:

   o Admission Marking - the traffic level is such that the router
      Admission Marks the packet

   o Pre-emption Marking - the traffic level is such that the router
      Pre-emption Marks the packet

   o ECT(0) - the first ECT codepoint, for backwards compatibility with
      the ECN nonce

   o ECT(1) - the other ECT codepoint, for backwards compatibility with
      the ECN nonce

   o Not ECT - to indicate to a router that the traffic is not PCN-
      capable.



1.2. Terminology

   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.

   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.








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2. Admission Marking algorithm

2.1. Outline

   A PCN-enabled router monitors the aggregate traffic in the specified
   real-time service class. Based on this measurement, the probability
   that the router sets a packet into the Admission Marked state is
   determined by the algorithm detailed below, configured to use the
   configured-admission-rate. The algorithm ensures that packets are set
   into the Admission Marked state before the actual queue builds up,
   but when it is in danger of doing so soon; the probability increases
   with the danger. Hence such packets act as an "early warning" that
   the engineered capacity is nearly reached, and that no more real-time
   flows should be admitted.

2.2. Virtual queue based algorithm for Admission Marking

   In order to make the description more specific we assume a virtual
   queue is used; other implementations are possible. By a virtual queue
   we mean a *conceptual* queue - it doesn't store packets, it is just
   an integer. The integer represents the dynamically changing length of
   a queue that would exist if the real-time packets were drained at the
   configured-admission-rate instead of the real scheduling rate for the
   relevant PHB. Note that there is a virtual queue for each outgoing
   link and it operates in bulk and not per microflow, i.e. the same
   virtual queue is used for all the real-time packets on that link. The
   virtual queue could be implemented, for example, with a variation of
   a leaky bucket.

   The virtual queue is:

   o Emptied at the configured-admission-rate, which is slower (perhaps
      considerably slower) than the link speed and the relevant PHB
      scheduling rate. This provides a safety margin to minimise the
      chances of unnecessarily triggering the pre-emption mechanism, for
      instance.

   o Filled when a packet arrives carrying a DSCP that has been
      configured for PCN (even if the packet is already admission or
      pre-emption marked). The amount added is the same as the number of
      octets in the packet.

   The procedure is visualised in Figure 2:





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            _________________      _________________      ____________
PCN        |increment length |    | calculate       |    |decide      |
packet --> |of virtual queue | -> |probability of   | -> |whether to  |
arrives    | by size of      |    |admission marking|    |admission   |
           |   packet        |    | packet          |    |mark packet |
            -----------------      -----------------      ------------
Figure 2: Router action to support admission marking


   The router computes the probability that the packet should be set
   into the Admission Marked state according to the size of the virtual
   queue, using the following RED-like algorithm:

   Size of virtual queue < min-marking-threshold, probability = 0;

   min-marking-threshold < Size of virtual queue < max-marking-
   threshold,

     probability = (Size of virtual queue - min-marking-threshold) /
     (max-marking-threshold - min-marking-threshold);

   Size of virtual queue > max-marking-threshold, probability = 1

Probability   ^
of setting    |
packet into   |
Admission   1_|                   _______________
Marked        |                  /
state         |                 /
              |                /
              |               /
              |              /
              |             /
              |            /
            0_|___________/
              |
               -----------|-------|-------------->
                         min-    max-          Size of virtual queue
                     marking-    marking-
                    threshold    threshold

Figure 3: Probability of router setting a packet into the Admission
Marked state


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   So if the CL traffic is sustained at a level greater than the
   configured-admission-rate then all packets are eventually admission
   marked. However, a short burst of traffic at greater than the
   configured-admission-rate (measured over the burst) may not trigger
   any admission marking if the burst is sufficiently short that the
   virtual queue doesn't grow beyond the min-marking-threshold.

   A packet that is already pre-emption marked is never re-marked to the
   admission marked state. The decision whether to set a particular
   packet into the Admission Marked state is made on a per-packet basis
   i.e. independently of the decision for the previous packet.

2.3. Admission control within a CL-region using Pre-Congestion
   Notification

   As an example of how the Admission Marking algorithm enables
   admission control, we briefly consider the edge-to-edge framework
   described in [CL-ARCH]. As real-time packets enter a CL-region, they
   are re-marked to enable PCN marking using the CL DSCP and the
   appropriate ECT field. As these CL-packets travel across the edge-to-
   edge CL-region, nodes may set the packets into the Admission Marked
   state, as determined by the algorithm described above. The egress
   gateway of the region measures the fraction of the real-time traffic
   that is in the Admission Marked state, with a separate measurement
   made for traffic from each ingress gateway. It calculates the
   fraction as an exponentially weighted moving average (which we term
   Congestion-Level-Estimate, or CLE). When signalling for a new flow
   arrives at the egress gateway, it reports the CLE to the CL-region's
   ingress gateway piggy-backed on the signalling. The ingress gateway
   only admits the new real-time microflow if the CLE is less than the
   CLE-threshold. Hence previously accepted microflows are protected and
   so suffer minimal queuing delay, jitter and loss.

















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3. Pre-emption Marking

3.1. Outline

   A PCN-enabled router monitors the aggregate traffic in the specified
   real-time service class. Based on this measurement, when the rate of
   real-time traffic exceeds the configured-pre-emption-rate for some
   time, the router will set packets into the Pre-emption Marked state
   as determined by the algorithm detailed below. The configured-pre-
   emption-rate is less than the link speed and less than the relevant
   PHB scheduling rate, so that Pre-emption Marked packets act as an
   explicit alert that the engineered capacity is nearly reached, and
   that some real-time flows may need to be pre-empted. This minimises
   the chances of a router randomly dropping packets, and hence the
   Quality of Service of the remaining flows is fully preserved. Also,
   service is preserved to traffic in other service classes using the
   remaining capacity.

   Pre-emption Marking of packets is similar in motivations to ECN-
   marking of packets in [RFC3168]. With [RFC3168] feedback of an ECN-
   marked packet causes the TCP source to halve its effective rate,
   whereas in our mechanism feedback of pre-emption marking enables an
   upstream node to terminate real-time flow(s). Pre-emption is
   therefore more aggressive against selected flows, but the gain is
   that it enables the full QoS of the remaining flows to be preserved.
   Note that in [RFC3168] ECN-marking a given packet is intended to
   result in rate adjustment of the flow to which the packet belongs;
   while in this draft and [CL-ARCH], Pre-emption marking a packet
   simply provides an indication that pre-emption may be needed and the
   pre-emption algorithm will then select flows to be pre-empted
   independently of which flow the marked packet belonged to.



3.2. Token bucket based algorithm for Pre-emption Marking

   In order to make the description more specific we assume a token
   bucket is used; other implementations are possible.

   All PCN routers maintain a token bucket per outgoing link:

   o Tokens are added at the configured-pre-emption-rate, which is
      slower than the link speed (and the relevant PHB scheduling rate).






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   o Tokens are removed when a real-time packet arrives; the amount
      removed is the same as the number of octets in the packet.
      However, if the real-time packet has already been Pre-emption
      marked, then tokens are not removed. Also, if there are
      insufficient tokens (because removing them would cause a negative
      number of tokens in the token bucket), then tokens are not removed
      and the packet is set into the Pre-emption Marked state. This
      procedure is visualised in Figure 4.



                _   _
               /     \
              /packet \           ----------------
RT packet    /  in     \     Y   |Don't remove    |
arrives --->/Pre-emption\ -----> |any tokens from |
            \ Marked    /        |token bucket    |
             \ state?  /          ----------------
              \       /                  ^
               \_   _/                   |
                  |                      |
                N |               ---------------
                  |              | Set pkt into  |
                  |              | Pre-emption   |
                  |              | Marked state  |
                  |                --------------
                  v                      ^
                _   _                    |
               /     \                   |
              / are   \                  |
             / there   \                N|
            /sufficient \----------------+
            \ tokens in /               Y|        -------------------
             \ token   /                 |       |  Remove tokens    |
              \bucket?/                  +-----> | (= octets in pkt) |
               \_   _/                           | from token bucket |
                                                  ------------------

Figure 4: Router action to support explicit pre-emption alerting






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   The router computes the probability that an 'unmarked' packet should
   be set into the Pre-emption Marked state according to the amount of
   tokens in the token bucket:

   Size of packet <= tokens in token bucket, probability = 0;

   Size of packet >  tokens in token bucket, probability = 1.

   'Unmarked' here means 'not in the Pre-emption Marked state'.



Probability
of setting      ^
unmarked-packet |
into            |
Pre-emption   1_|___________
Marked          |           |
state           |           |
                |           |
                |           |
                |           |
                |           |
                |           |
                |           |
                |           |
              0_|           |__________________
                |
                 -----------|------------------>
                           size of          Amount of tokens
                           packet           in token bucket


Figure 5: Probability of router setting a packet into Pre-emption
Marked state


   So if the CL traffic is sustained at a level greater than the
   configured-pre-emption-rate then 'unmarked' packet arrivals in excess
   of this rate (but not those below it) are pre-emption marked.
   However, a short burst of traffic at greater than the configured-pre-
   emption-rate (measured over the burst) may not trigger any pre-




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   emption marking if the burst is sufficiently short that the token
   bucket doesn't run out of tokens.



3.3. Flow pre-emption within a CL-region using Pre-Congestion
   Notification

   As an example of how the Pre-emption Marking algorithm enables flow
   pre-emption, we briefly consider the edge-to-edge framework described
   in [CL-ARCH]. As real-time packets travel across the edge-to-edge CL-
   region, nodes may set the packets into the Pre-emption Marked state,
   as determined by the algorithm described above.

   When the egress gateway of the region detects a Pre-emption Marked
   packet, it measures the rate of real-time traffic *excluding* any
   packets that are set into the Pre-emption Marked state. Hence it
   measures the amount of traffic that the network can actually support
   safely (which we term Sustainable-Aggregate-Rate). The measurement is
   made for traffic from a particular ingress gateway, and then reported
   to that ingress gateway. When it receives this message, the ingress
   gateway measures the aggregate-rate of real-time traffic that is
   being sent towards the particular egress gateway. If this measured
   aggregate-rate exceeds the Sustainable-Aggregate-Rate, then the
   ingress gateway pre-empts sufficient number of real-time flow(s) to
   bring down the aggregate-rate to (approximately) the Sustainable-
   Aggregate-Rate.

   Different implementations of the rate measurement (and the timescale
   of this measurement) at the egress and ingress nodes are possible.



















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4. Simulation results

   We have performed an initial set of simulations of admission control
   and flow pre-emption mechanisms described in this document and
   consistent with [CL-ARCH].

   We investigated the performance of the admission control and flow
   pre-emption mechanisms with traffic modelling CBR voice, on-off
   traffic approximating voice with silence compression, and more
   aggressive on-off traffic with larger packet sizes and peak and mean
   rates approximating that of video traffic.

   In summary, both the admission control and flow pre-emption
   mechanisms worked well for all of these traffic types under the
   assumptions of [CL-ARCH] (in particular under the assumption that
   there are many micro-flows between any pair of ingress / egress
   gateways, which, in turn, translates in the assumption that
   relatively high speed links are used). Details of the simulation
   study are given in Appendix B. In the pdf version of this document
   Appendix B also include graphs of simulation results. It can be found
   at
   http://www.cs.ucl.ac.uk/staff/B.Briscoe/projects/ipe2eqos/gqs/papers/
   draft-briscoe-tsvwg-cl-phb-01.pdf

   So far the simulations have been run with a sensible estimate of
   suitable parameters. While a limited amount of work has been done to
   evaluate sensitivity of the results to the simulation parameters (see
   Appendix B), investigating further the sensitivity to these
   parameters is the next step.

   Due to time constraints, we were able to simulate a single
   "congestion point" only, i.e. there was a single node where pre-
   congestion notification for admission control and/or pre-emption was
   triggered. Furthermore, admission control and flow pre-emption
   simulations were performed independently.  A study of the interaction
   of admission control and flow pre-emption is also a subject of future
   work.












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5. Encoding the Admission Marked and Pre-emption Marked states

   In this Section we describe one proposal for how to encode the
   Admission Marking and Pre-emption Marking states in a packet, i.e.
   what change to make to which bits of a packet.

   The encoding scheme uses the two ECN (Explicit Congestion
   Notification) bits in the IP header. The four ECN codepoints are used
   as follows:

         +-----+-----+
         | ECN FIELD |
         +-----+-----+
         bit 6  bit 7
            0     0         Admission Marking
            0     1         ECT(1)
            1     0         ECT(0)
            1     1         Pre-emption Marking
          Other DSCPs       Non-PCN-Capable

   Figure 6: Pre-Congestion Notification's use of the ECN Field in IP



   To explain this, we assume that Pre-Congestion Notification is being
   used in the architecture described in [CL-ARCH]. It is therefore a
   controlled environment, with all routers in the CL-region upgraded
   with the PCN capability. Within the CL-region, this encoding meets
   the requirements of [Floyd] because a router knows a packet is PCN-
   capable if

   o Its differentiated services codepoint (DSCP) is one configured for
      PCN marking.

   When an ingress gateway gets a packet that it has agreed to treat as
   part of a PCN-capable microflow, then it sets the ECN field to either
   ECT(0) or ECT(1) as it chooses, and if necessary it sets the DSCP to
   a PCN-capable Diffserv codepoint. Packets with this DSCP indicate a
   PCN-capable transport if any of the four ECN codepoints are set.

   When a router gets a PCN-capable packet, then (if necessary) it re-
   sets the ECN field to '00' to indicate Admission Marking and to '11'
   to indicate Pre-emption Marking. Packets with Admission Marking may
   be re-marked to Pre-emption Marking, but not vice-versa.

   Other frameworks would be very similar. For example, in a framework
   where Pre-Congestion Notification operates from one end-host to


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   another, then the sending end-host would set the ECN field to either
   ECT(0) or ECT(1).



   One advantage of this encoding scheme is that it allows the use of
   the ECN nonce, thus providing similar protection against a cheater as
   [RFC3540]. However, if PCN marking is desired on traffic with a pre-
   existing scheduling behaviour (such as EF) a drawback is that a new
   DSCP will be required to distinguish PCN-capable traffic from traffic
   that isn't PCN-capable, so that a router can identify which traffic
   it should PCN mark.

   Note that although we believe the encoding scheme is reasonable, it
   is not our final proposal. Alternatives are listed and discussed in
   Appendix C. We welcome advice and comments as to the most appropriate
   scheme.
































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6. Acknowledgements

   This work has evolved from several previous independent efforts:

   o Guaranteed QoS Synthesis [Hovell], which evolved from 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]

   o RTECN (Real-Time Explicit Congestion Notification) [RTECN]

   o RMD (Resource Management in DiffServ) [RMD] and [Westberg]



7. 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.



8. Changes from earlier version of the draft

   The main changes are:

   From -00 to -01

   The description of how to use pre-congestion notification marking in
   a CL-region is now described in [CL-arch].

   Only one admission marking algorithm is now described.

   A pre-emption marking scheme has been added.

   Various options for encoding the marking are described and discussed
   in Appendix C.

   Simulation results are described in Appendix B and summarised in
   Section 4.








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9. 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-nodes (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 nodes. 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
   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


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   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.


























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10. Appendix B - Details of simulations

   This section provides some details on the simulation study reference
   in Section 4.

   Note that the pdf version of this document includes graphs of
   simulation results that aren't in the text version. It can be found
   at
   http://www.cs.ucl.ac.uk/staff/B.Briscoe/projects/ipe2eqos/gqs/papers/
   draft-briscoe-tsvwg-cl-phb-01.pdf

10.1. Network and signalling model

   In most simulations, the network is modelled as a single link between
   an ingress and an egress node, all flows sharing the same link.
   Figure B.1 shows the modelled network. A is the ingress node and B is
   the egress node.



         A --- B

Figure B.1: Simulated Single Link Network.



                           A

                            \

                          B  - D - F

                              /

                           C

   Figure B.2: Simulated Multi Link Network.

   A subset of simulations uses a network structured similarly to the
   network shown on figure B.2. A set of ingresses (A,B,C) connected to
   an interior node in the network (D) with links of different
   propagation delay. This node in turn is connected to the egress (F).
   In this topology, different sets of flows between each ingress and
   the egress converge on the single link, where pre-congestion
   notification algorithm is enabled. In our simulations, the network
   has 100 ingress nodes, each connected to the interior node with a


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   different propagation delay (1ms to 100ms). The point of congestion
   is taken to be the link (D-F) connecting the interior node to the
   egress node. This link is modelled with a 10ms propagation delay.
   Therefore the range of RTTs is from 22ms to 220ms.

   The simple network topology was due to a lack of time for the
   simulations.

   Our simulations concentrated primarily on the range of capacities of
   'bottleneck' links with sufficient aggregation - above 10 Mbps for
   voice and 622 Mbps for "video", up to 1 Gbps. But we also
   investigated slower 'bottleneck' links down to 512 kbps.

   In the simulation model, a call request arrives at the ingress and
   immediately sends a message to the egress. The message arrives at the
   egress after the propagation time plus link processing time (but no
   queuing delay). When the egress receives this message, it immediately
   responds to the ingress with the current Congestion-Level-Estimate.
   If the Congestion-Level-Estimate is below the specified CLE-
   threshold, the call is admitted, otherwise it is rejected.

   The life of a call outside the domain described above is not
   modelled. Propagation delay from source to the ingress and from
   destination to the egress is assumed negligible and is not modelled.



10.2. Simulated Traffic types

   Three types of traffic were simulated (CBR voice, on-off traffic
   approximating voice with silence compression, and on-off traffic with
   higher peak and mean rates (we termed the latter "video" as the
   chosen peak and mean rate was similar to that of an mpeg video
   stream, although no attempt was made to match any other parameters of
   this traffic to those of a video stream).  The distribution of flow
   duration was chosen to be exponentially distributed with mean 2min,
   regardless of the traffic type. In most of the experiments flows
   arrived according to a Poisson distribution with mean arrival rate
   chosen to achieve a desired amount of overload over the configured-
   pre-emption-rate or configured-admission-limit in each experiment.
   Overloads in the range 2x to 5x have been investigated.

   In addition, some experiments investigated a batch Poisson model.
   Here the batch represented a set of calls arriving at almost the same
   time. The batch arrival process was Poisson, and the batch size was
   geometrically distributed with a mean of up to 5 calls per batch.



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   For on-off traffic, on and off periods were exponentially distributed
   with the specified mean.

   Traffic parameters for each flow are summarized below:

10.2.1. Voice CBR

   * Average rate 64 Kbps,

   * Packet length 160 bytes

   * packet inter-arrival time 20ms

10.2.2. On-off traffic approximating voice with silence compression

   * Packet length 160 bytes

   * Long-term average rate 21.76 Kbps

   * On Period mean duration 340ms; during the on period traffic is sent
   with the CBR voice parameters described above

   * Off Period mean duration 660ms; no traffic is sent during the off
   period.

10.2.3. High-rate on-off traffic

   * Long term average rate 4 Mbps

   * On Period mean duration 340ms; during the on-period the packets are
   sent at 12 Mbps (1500 byte packets, packet inter-arrival: 1ms)

   * Off Period mean duration 660ms



10.3. Admission Control Simulations

10.3.1. Summary of the key parameters for CAC

10.3.1.1. Virtual Queue settings

   Most of the simulations were run with the following Virtual Queue
   thresholds:

   * min-marking-threshold: 5ms at link speed,



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   *  max-marking-threshold: 15ms at link speed,

   *  virtual-queue-upper-limit: 20ms at link speed.

   The virtual-queue-upper-limit puts an upper bound on how much the
   virtual queue can grow.

   Note that the virtual queue is drained at a configured rate smaller
   than the link speed. Most of the simulations were set with the
   configured-admission-rate of the virtual queue at half the link
   speed.

   Note that as long as there is no packet loss, the admission control
   scheme successfully keeps the load of admitted flows at the desired
   level regardless of the actual setting of the configured-admission-
   limit.  However, it is not clear if this remains true when the
   configured-admission-rate is close to the link speed/actual queue
   service rate.  Further work is necessary to quantify the performance
   of the scheme with smaller service rate/virtual queue rate ratio,
   where packet loss may be an issue.



10.3.1.2. Egress measurement parameters.

   In our simulations, the CLE-threshold was chosen as 0.5. The CLE is
   computed as an exponential weighted moving average (EWMA) with a
   weight of 0.01. The CLE is computed on a per-packet basis.



10.3.2. Overview of the Admission Control Results

   We found that on links of capacity from 10Mbps to OC3, congestion
   control for CBR voice and ON_OFF voice traffic work reliably with the
   range of parameters we simulated, both with Poisson and Batch call
   arrivals.  As the performance of the algorithm was quite good at
   these speeds, and generally becomes the better the higher the degree
   of aggregation of traffic, we chose to not investigate higher link
   speeds for CBR and on-off voice, within the time constraints of this
   effort.

   For higher-rate on-off "video" traffic, due to time limitations we
   simulated 1Gbps and OC12 (622 Mbps) links and Poisson arrivals only.
   Note that due to the high mean and peak rates of this traffic model,
   slower links are unlikely to yield sufficient level of aggregation of
   this type of traffic to satisfy the flow aggregation assumptions of


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   [CL-ARCH]. Our simulations indicated that this model also behaved
   quite well, although the deviation from the configured-admission-rate
   is slightly higher in this case than for the less bursty traffic
   models.

   For these link speeds and traffic models, we investigated the demand
   overload of 2x-5x.

   Table B.1 below summarizes the worst case difference between the
   admitted load vs. configured-admission-rate. The worst case
   difference was taken over all experiments with the corresponding
   range of link speeds and demand overloads. In general, the higher the
   demand, the more challenging it is for the admission control
   algorithm due to a larger number of near-simultaneous arrivals at
   higher overloads, and as a result the worst case results in Table B.1
   correspond to the 5x demand overload experiments.

------------------------------------------------------------------
|               |         |           | diff between  |          |
| Link type     | traffic | call      | mean admitted | standard |
|               | type    | arrival   | load &        | deviation|
|               |         | process   | conf-adm-rate |          |
------------------------------------------------------------------
|T3,100Mbps,OC3 | CBR     | POISSON   |    0.5%       |   0.5%   |
------------------------------------------------------------------
|
|T3,100Mbps,OC3 |ON-OFF V | POISSON   |    2.5%       |   2.5%   |
------------------------------------------------------------------
|T3,100Mbps,OC3 | CBR     |  BATCH    |    1.0%       |   1.0%   |
------------------------------------------------------------------
|T3,100Mbps,OC3 |ON-OFF V |  BATCH    |    3.0%       |   3.0%   |
------------------------------------------------------------------
|  1Gbps        | "Video" |  POISSON  |    2.0%       |   8.0%   |
------------------------------------------------------------------
|  OC12        |"Video   |  POISSON  |    0.0%       |  10.0%    |
------------------------------------------------------------------
Table B.1. Summary of the admission control results for links above T3
speeds
Note: T1 = 1.5Mbps, T3 = 45Mbps, OC3 = 155Mbps, OC12 = 622Mbps

   Sample simulation graphs for the experiments summarized in Table 6.1
   can be viewed in the PDF version of this draft. It can be found at




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   http://www.cs.ucl.ac.uk/staff/B.Briscoe/projects/ipe2eqos/gqs/papers/
   draft-briscoe-tsvwg-cl-phb-01.pdf

   On slower links, accuracy of admission control algorithm was lower
   with Poisson arrivals, and was especially challenging with burstier
   Batch arrivals. This is described in section 6.3.3 below.

   In general, we find that the admission control algorithm perform the
   better the larger degree of aggregation of traffic on the link. The
   algorithm performs well in the range of link speeds we expect to see
   in a CL region.



10.3.3. Sensitivity to Poisson Arrivals assumption

   We investigated whether making the call arrival process burstier than
   Poisson has an effect on the performance of the admission control
   algorithm. To that end we investigated the comparative performance of
   the algorithm with Poisson and Batch call arrival processes,
   described in section 10.2. The mean call arrival rate was the same
   for both processes, with the demand overloads ranging from 2x to 5x.

   We found that the admission control algorithm works reliably for both
   CBR and VBR at links of 1Mbps and above for up to 5x overloads for
   both Poisson and Batch call arrivals. We also found that the
   admission control algorithm only works reasonably well at links of 1
   Mb/s if we assume CBR traffic and Poisson arrival. At T1 speeds and
   below, Batch arrivals resulted in over-admission, the degree of which
   increased on slower links.

   Table B.2 below summarizes the difference between the admitted load
   and the configured-admission-rate for CBR Voice in the case of
   Poisson and Batch arrivals. Table B.3 provides a similar summary for
   on-off traffic simulating voice with silence compression. The results
   in the tables correspond to the worst case across all overload
   factors (and when multiple links speeds are listed, across all those
   link speeds).










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-------------------------------------------------------------
|              |             | diff between  |              |
| Link type    |  arrival    | mean admitted | standard     |
|              |  model      | load &        | deviation    |
|              |             | conf-adm-rate |              |
------------------------------------------------------------
| 1Mbps, T1    |    BATCH    |      30.0%    |      30.0%   |
-------------------------------------------------------------
|  10 Mbps     |    BATCH    |       5.0%    |       8.0%   |
-------------------------------------------------------------
|T3,100Mbps,OC3|    BATCH    |       1.0%    |       1.0%   |
-------------------------------------------------------------
|  1Mbps, T1   |  POISSON    |       5.0%    |      10.0%   |
-------------------------------------------------------------
| 10 Mbps      |  POISSON    |       1.0%    |       2.0%   |
-------------------------------------------------------------
|T3,100Mbps,OC3|  POISSON    |       0.5%    |       0.5%   |
-------------------------------------------------------------
Table B.2. Comparison of Poisson and Batch call arrival models for CBR
voice.   Note: T1 = 1.5Mbps, T3 = 45Mbps, OC3 = 155Mbps, OC12 = 622Mbps
------------------------------------------------------------
|              |             | diff between  |              |
| Link type    |  arrival    | mean admitted | standard     |
|              |  model      | load &        | deviation    |
|              |             | conf-adm-rate |              |
------------------------------------------------------------
| 1Mbps, T1    |    BATCH    |      40.0%    |      30.0%   |
-------------------------------------------------------------
|  10 Mbps     |    BATCH    |       8.0%    |       6.0%   |
-------------------------------------------------------------
|T3,100Mbps,OC3|   BATCH     |       3.0%    |       3.0%   |
-------------------------------------------------------------
|  1Mbps, T1   |  POISSON    |      15.0%    |      20.0%   |
-------------------------------------------------------------
| 10 Mbps      |  POISSON    |       7.0%    |       6.0%   |
-------------------------------------------------------------
|T3,100Mbps,OC3|  POISSON    |       2.5%    |       2.5%   |
-------------------------------------------------------------
Table B.3. Comparison of Poisson and Batch call arrival models for on-
off voice with silence compression.
Note: T1 = 1.5Mbps, T3 = 45Mbps, OC3 = 155Mbps, OC12 = 622Mbps


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10.3.4. Sensitivity to marking parameters

   The behaviour of the congestion control algorithm in all simulation
   experiments did not substantially differ depending on whether the
   marking was "ramp", i.e. whether a separate min-marking-threshold and
   max-marking-threshold were used, with linear marking probability
   between these thresholds, or whether the marking was "step" with the
   min-marking-threshold and max-marking-threshold collapsed at the max-
   marking-threshold value, and marking all packets with probability 1
   above this collapsed threshold.

   However, the difference between "ramp" and "step" may be more visible
   in the multiple congestion point case (recall that only a single
   congestion point experiments were performed so far).

   Another possible reason for this apparent lack of difference between
   "ramp" and "step" may relate to the choice of the egress measurement
   parameters and a relatively high CLE threshold of 50%. Choosing a
   lower CLE-acceptance threshold and a faster measurement timescale may
   result in a better sensitivity to lower levels of marked traffic.
   Investigating the interaction between settings of the marking
   thresholds, the CLE-threshold, and the measurement parameters at the
   egress is an area of future investigation.

   In contrast, the limited number of simulation experiments we
   performed indicate that the choice of the absolute value of the min-
   marking-threshold, the max-marking-threshold and the virtual-queue-
   upper-limit can have an effect on the algorithm performance.
   Specifically, choosing the min-marking-threshold and the max-marking-
   threshold too small may cause substantial underutilization,
   especially on the slow links. However, at larger values of the min-
   marking-threshold and the max-marking-threshold, preliminary
   experiments suggest the algorithm's performance is insensitive to
   their values. The choice of the virtual-queue-upper-limit affects the
   amount of over-admission (above the configured-admission-rate
   threshold) in some cases, although this effect is not consistent
   throughout the experiments.

   The Table B.4 below gives a summary of the difference between the
   admitted load and the configured-admission-rate as a function of the
   virtual queue parameters, for the 4 Mbps on-off traffic model.  The
   results in the table represent the worst case result among the
   experiments with different degree of demand overloads in the range of
   2x-5x. Typically, higher deviation of admitted load from the




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   configured-admission-rate occurs for the higher degree of demand
   overload.



-------------------------------------------------------------
|            |               | diff between  |              |
| Link type  |min-threshold, | mean admitted | standard     |
|            |max-threshold, | load &        | deviation    |
|            |upper-limit(ms)| conf-adm-rate |              |
------------------------------------------------------------
|  1Gbps     |5, 15, 20      |       6.0%    |       8.0%   |
-------------------------------------------------------------
|  1Gbps     |1, 5, 10       |       2.0%    |       7.0%   |
-------------------------------------------------------------
|  1Gbps     |5, 15, 45      |       2.0%    |       8.0%   |
-------------------------------------------------------------
|  OC12      |5, 15, 20      |       5.0%    |      11.0%   |
-------------------------------------------------------------
|  OC12      |1, 5, 10       |       2.0%    |      13.0%   |
-------------------------------------------------------------
|  OC12      |5, 15, 45      |       0.0%    |      10.0%   |
-------------------------------------------------------------
Table B.4. Sensitivity of 4 Mbps on-off "video" traffic to the virtual
queue settings.
Note: T1 = 1.5Mbps, T3 = 45Mbps, OC3 = 155Mbps, OC12 = 622Mbps

   Impact of the virtual queue parameter setting is a subject of further
   study.



10.3.5. Sensitivity to RTT

   We performed a limited amount of sensitivity of the admission control
   algorithm used to the range of round trip propagation time (which is
   the dominant component of the control delay in the typical
   environment using pre-congestion notification).

   Specifically, we studied the case when different groups of flows
   sharing a single bottleneck link in the network have a range of
   roundtrip delays between 22 and 220 ms, as shown in Figure B.2.




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   The results were good for all types of traffic tested, implying that
   the admission control algorithm is not sensitive to the either the
   absolute value of the round-trip propagation time or relative value
   of the round-trip propagation time, at least in the range of values
   tested. We expect this to remain true for a wider range of round-trip
   propagation times.



10.3.6. Future Work for Admission Control Experiments

   Areas of future investigation include extending the study of
   sensitivity to multiple congestion points and topologies, further
   investigation of sensitivity to factors such as marking parameters,
   implementation details and time scale of egress measurements, the
   CLE-threshold. Also variations on the marking algorithm will be
   studied.

   Another area of investigation is to understand the sensitivity to the
   ratio of configured-admission-rate to the actual queue service
   rate/link speed, and specifically study how close the configured-
   admission-rate can be to the actual queue draining rate. A related
   investigation is to understand the effect of packet loss on the
   admission control mechanisms. Packet loss can occur if the
   configured-admission-rate is sufficiently close to the actual queue
   rate.

   More realistic Video modelling and the mix of video and voice traffic
   in the same queue is also an area of further study.

10.4. Flow Pre-emption Simulations

10.4.1. Flow Pre-emption Model and key parameters

   The same single-congestion-point network model as described in
   section 10.1 for admission control is used for flow pre-emption. Flow
   arrival and traffic models are also the same as for CAC admission
   control simulations.

   In all flow pre-emption simulations, flows arrive at the ingress
   according to a Poisson distribution, with the mean load of
   "unrestricted" arrivals exceeding the pre-emption threshold by a
   factor of 2 to 5. However, as explained below, the pre-emption
   simulation involve a very sudden surge of traffic to simulate a
   network failure scenario.




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   In the simulation, the router implementing PCN Pre-emption Marking
   operates as described in section 3, marking packets which find no
   token in the token bucket. When an egress gateway receives a marked
   packet from the ingress, it will start measuring its Sustainable-
   Aggregate-Rate for this ingress, if it is not already in the pre-
   emption mode.

   If a marked packet arrives while the egress is already in the pre-
   emption mode, the packet is ignored.

   The measurement is interval based, with 100ms measurement interval
   chosen in all simulations.

   At the end of the measurement interval, the egress sends the measured
   Sustainable-Aggregate-Rate to the ingress, and leaves the pre-emption
   mode.

   When the ingress receives the sustainable rate from the egress, it
   starts its own interval immediately (unless it is already in a
   measurement interval), and measures its sending rate to that egress.
   Then at the end of that measurement interval, it pre-empts the
   necessary amount of traffic. The ingress then leaves the pre-emption
   mode until the next time it receives the sustainable rate estimate
   from the egress.

   Due to time limitations, in all our simulations the ingress used the
   same length of the measurement interval as the egress. Investigation
   of the impact of different measurement intervals is an important area
   of future work.

   To avoid excessive pre-emption due to the rate measurement errors, we
   used two error factors, Error1 and Error2 to trigger decisions on
   when to pre-empt and how much to pre-empt at the ingress. To that
   end, the ingress did not trigger pre-emption unless the sending rate
   it measured was greater than SAR + Error1 (SAR=Sustainable Aggregate
   Rate). Similarly, the ingress pre-empted enough flows to reduce its
   sending rate to SAR - Error2. Both Error1 and Error2 in all
   simulations were in the range of 2-5%.

   The configured-pre-emption-rate was set to 50% of link speed. Token
   bucket depth was set to 64 packets for CBR and 128 packets for on-off
   traffic.

   We only tested on the network shown in Figure B.1 and we experimented
   with different propagation delay values: 10ms, 50ms and 100ms.




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   Due to time limitation, only links above T3 rate were simulated in
   Pre-emption experiments.

   In all pre-emption experiments, we simulated the base load of traffic
   below pre-emption threshold. At some point during the experiment, the
   load was suddenly increased to simulate sudden overload such that
   might occur after a link failure causes rerouting of some traffic to
   a previously un-congested link. In order to model the fact that a
   link failure may cause flows rerouting to a particular link over a
   period of time, we simulated a "one-wave" traffic surge, where the
   extra flows arrived near simultaneously, and a "three-wave" traffic
   surge, where there are two surges of traffic arriving close together
   (within one measurement interval), followed by a third surge at a
   later time.

10.4.2. Summary of Flow Pre-emption Experiments.

   Our initial simulations demonstrated that in general performance of
   the flow pre-emption mechanism was good, and the appropriate amount
   of traffic was pre-empted in all simulated cases, as long as the
   depth of the pre-emption token bucket was set appropriately (64
   packets for CBR, 128 or higher for on-off traffic). The pre-emption
   always occurred very fast (in particular, in the simulation graphs
   shown in the pdf version of this document with time granularity of 1
   second, pre-emption looks instantaneous).

   Perhaps the most useful result of the simulation experiments we were
   able to run so far was the importance of choosing the token bucket
   depth deep enough to accommodate the expected burstiness on CL
   traffic. If the token bucket depth is too small, instantaneous bursts
   may cause false pre-emption events. Note that if traffic load is
   stable or decreasing, then marking some packets erroneously during a
   an unexpected short burst does not cause any false pre-emption,
   because the rate measurement of the sustained rate is not affected by
   a small amount of pre-emption-marked packets.  However, if the
   traffic load is increasing (while still remaining below pre-emption
   level on the average), a packet marked for pre-emption because it
   found no tokens in the too-shallow token bucket, may cause a false
   pre-emption event.

10.4.3. Future Work on Flow Pre-emption Experiments

   Further work is required to study potential ways of reducing
   sensitivity of the algorithm to the token bucket depth. Potential
   approaches may be to smooth out pre-emption signal by requiring a
   certain amount of pre-emption-marked packets to arrive to the egress
   before measurement of the sustainable rate is triggered. An obvious


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   trade-off to be quantified is the corresponding increase in the
   reaction time to receiving a pre-emption-marked packet.

   Further quantification of the sensitivity to traffic burstiness and
   rate measurement implementation and time scales is an important area
   for future work.

   More realistic Video modelling and the mix of video and voice traffic
   in the same queue is also an area of further study.

   Another area of further investigation is the interaction of flow pre-
   emption and admission control, and specifically understanding of how
   close the admission and pre-emption rates can be on one link. A
   related topic is the interaction of flow pre-emption and admission
   control triggered by different links for the same ingress-egress
   pair.

   The exact algorithm for selecting which flows to pre-empt in the case
   of variable rate flows and mixture of traffic profile is subject of
   further study.

   Representative graphs for pre-emption experiments are presented in
   the PDF version of this draft. It can be found at
   http://www.cs.ucl.ac.uk/staff/B.Briscoe/projects/ipe2eqos/gqs/papers/
   draft-briscoe-tsvwg-cl-phb-01.pdf
























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11. Appendix C - Alternative ways of encoding the Admission Marked and
   Pre-emption Marked States

   In this Appendix we list and discuss alternative ways of encoding the
   Admission Marked and Pre-emption Marked states. We ignore minor
   variants such as swapping the encoding for the Admission Marked and
   Pre-emption Marked states.



11.1. Alternative 1

   The first alternative is the one given in Section 5 above.

         +-----+-----+
         | ECN FIELD |
         +-----+-----+
         bit 6  bit 7
            0     0         Admission Marking
            0     1         ECT(1)
            1     0         ECT(0)
            1     1         Pre-emption Marking

         Other DSCPs        Not ECN capable

   Figure C.1: Encoding scheme Alternative 1



11.2. Alternative 2

   In the second alternative, both Admission Marking and Pre-emption
   Marking are encoded as '11', depending on the original ECT marking:

   o Setting the ECN field of an ECT(1) packet to '11' indicates
      Admission Marking

   o Setting the ECN field of an ECT(0) packet to '11' indicates Pre-
      emption Marking









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         +-----+-----+
         | ECN FIELD |
         +-----+-----+
         bit 6  bit 7
            0     0         Not-ECT
            0     1         ECT(1/A)  re-mark ECT(1) to '11' to encode
                                      Admission Marking
            1     0         ECT(0/P)  re-mark ECT(0) to '11' to encode
                                      Pre-emption Marking
            1     1         Admission Marking or Pre-emption Marking

   Figure C.2: Encoding scheme Alternative 2



11.3. Alternative 3

   The third alternative is a combination of the previous two schemes.

         +-----+-----+
         | ECN FIELD |
         +-----+-----+
         bit 6  bit 7
            0     0         Admission Marking
            0     1         ECT(1/A)  re-mark ECT(1) to '00' to encode
                                      Admission Marking
            1     0         ECT(0/P)  re-mark ECT(0) to '11' to encode
                                      Pre-emption Marking
            1     1         Pre-emption Marking

         Other DSCPs        Not ECN capable

   Figure C.3: Encoding scheme Alternative 3



11.4. Alternative 4

   In the fourth alternative a packet is re-marked with a new DSCP to
   indicate Pre-emption Marking.








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         +-----+-----+
         | ECN FIELD |
         +-----+-----+
         bit 6  bit 7
            0     0         Not ECN capable
            0     1         ECT(1)
            1     0         ECT(0)
            1     1         Admission Marking

            New DSCP        Pre-emption Marking

   Figure C.4: Encoding scheme Alternative 4



11.5. Alternative 5

   The fifth alternative doesn't include the ECN nonce.

         +-----+-----+
         | ECN FIELD |
         +-----+-----+
         bit 6  bit 7
            0     0         Not ECN capable
            0     1         PCN capable
            1     0         Admission Marking
            1     1         Pre-emption Marking

   Figure C.5: Encoding scheme Alternative 5



11.6. Comparison of Alternatives

   In this section we compare the encoding alternatives against various
   criteria. No scheme is perfect. We would like feedback and advice
   from the IETF community as to which is most suitable. The choice of
   how to encode the markings is non-trivial because we have five things
   we want to encode, and only have four states available in the two
   bits of the ECN field:

   o Admission Marking - the traffic level is such that the router
      Admission Marks the packet

   o Pre-emption Marking - the traffic level is such that the router
      Pre-emption Marks the packet



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   o ECT(0) - the first ECT codepoint, for backwards compatibility with
      the ECN nonce

   o ECT(1) - the other ECT codepoint, for backwards compatibility with
      the ECN nonce

   o Not ECN - to indicate to a router that the traffic is not ECN-
      capable, and indeed not PCN-capable.



   Some of the issues won't be relevant in particular scenarios. For
   example, with the CL-region framework[CL-ARCH], the edge-to-edge
   region is a controlled environment so an ECN (RFC3168) packet should
   never encounter a PCN-enabled router.

   Occasionally we use the terminology of the CL-region framework. This
   is merely to make the language more specific.



11.6.1. How compatible is the encoding scheme with RFC 3168 ECN?

   All the encoding schemes for Pre-Congestion Notification use the ECN
   field, so there will be interactions between PCN and ECN. Three
   aspects are:

   o What happens if an ECN (RFC3168) packet encounters a PCN-enabled
      router?

   o What happens if a PCN-capable packet encounters an ECN-enabled
      router?

   o What happens if a flow that has been admitted, using the PCN-based
      admission control mechanism, wants to use ECN (i.e. from end-point
      to end-point as in RFC3168)?

   The first two bullets are about an "unusual" situation, perhaps where
   re-routing means that a PCN-enabled packet gets routed onto an ECN
   router - or perhaps where one of the CL-regions ingress gateways is
   misconfigured so that it allows in ECN packets into the CL traffic
   class. The third bullet is when the end-point wants its flow, which
   has been reserved using PCN-based admission control, to also use ECN-
   congestion control. There has been some discussion (and disagreement)
   about whether this is a realistic requirement [Floyd] [tsvwg-ml].




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   o What happens if an ECN (RFC3168) packet encounters a PCN-enabled
      router?

   The main issue here is if traffic at the PCN-router is above the
   admission or pre-emption threshold, and what then happens when the
   ECN packet reaches the RFC3168 ECN end-point.

   Alternative 2 and 4 are very safe. If the PCN-router Admission Marks
   a packet ('11'), the ECN end-point interprets this as the CE
   codepoint. The admission threshold is lower (perhaps much lower) than
   an ECN threshold would be.

   Alternative 3 is also safe. If the PCN-router Pre-emption Marks a
   packet ('11'), the ECN end-point interprets this as the CE codepoint.
   The pre-emption threshold is likely to be lower than an ECN threshold
   would be, and is definitely lower than the traffic level at which
   packets would start to be dropped.

   Alternative 5 is probably OK. However if the level of RFC3168 traffic
   is above the PCN router's configured-admission-rate but below its
   configured-pre-emption-rate, then packets are admission marked (to
   '10') but not pre-emption marked (to '11'). Therefore the ECN traffic
   would tend to block new PCN flows, but not reduce its own rate. This
   would be safer with the encodings for admission marking and pre-
   emption marking swapped.

   With Alternatives 1 and 3, if traffic is above the admission
   threshold then packets will be re-marked to '00'. A subsequent ECN
   router will therefore think the packet isn't ECN-capable.

   With Alternative 5 packets are admission marked to '10', which could
   confuse an ECN RFC3168 end-point using the ECN nonce.



   o What happens if a PCN-capable packet encounters an ECN-enabled
      router?

   The main issue is if the ECN-router is becoming congested, so it
   changes the ECN field to '11', to indicate Congestion Experienced
   (CE).

   With Alternatives 1, 3 and 5 '11' will be interpreted as Pre-emption
   Marking, so the pre-emption mechanism will be triggered.





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   With Alternative 2 either the pre-emption or admission mechanism
   would be triggered (depending whether it was originally a '10' or
   '01' packet).

   With Alternative 4 the admission control mechanism will be triggered.

   Interpretation of '11' as pre-emption marking is probably safer than
   interpreting it as admission marking, because it then pre-empts flows
   going through a congested ECN router. However, it isn't clear-cut
   what 'safe' means in this context.



   o What happens if a flow that has been admitted, using the PCN-based
      admission control mechanism, wants to use ECN (i.e. from end-point
      to end-point as in RFC3168)?

   For instance with the CL-region framework, it isn't clear what the
   ingress gateway should do if it gets a packet with the CE codepoint,
   '11'. All the PCN encoding schemes have the same issue. Some options:

   - the ingress gateway could re-set a '11' packet to one of the ECT
      codepoints. However, as far as the ECN-end-point is concerned, the
      CE information is lost.

   - The ingress gateway could pre-empt the flow. This is safer, but
      perhaps harsh as the flow would now be handled by the non-PCN-
      capable class within the CL-region, and by the non-ECN-capable
      class after that.

   - Tunnelling between the ingress and egress gateways, e.g. all PCN-
      capable traffic could be tunnelled. This preserves both the ECN
      and PCN functionality, but at the cost of the tunnelling.



11.6.2. Does the encoding scheme allow an "ECN-nonce"?

   The Explicit Congestion Notification (ECN)-nonce is an optional
   addition to ECN that protects against accidental or malicious
   concealment of marked packets from the TCP sender. It uses the two
   ECN-Capable Transport (ECT) codepoints in the ECN field of the IP
   header. It improves the robustness of congestion control by enabling
   co-operative senders to prevent receivers from exploiting ECN to gain
   an unfair share of network bandwidth.




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   Pre-Congestion Notification is targeted at real-time traffic, which
   we'd expect to use UDP or DCCP rather than TCP. However, we imagine
   an "ECN-nonce" could be defined for DCCP and perhaps UDP with similar
   functionality to the ECN-nonce.

   Analysing the encoding schemes in the context of an ECN-nonce:

   o Alternatives 2 and 4 would allow an ECN-nonce

   o Alternatives 1 and 3 would party allow an ECN-nonce - in terms of
      the edge-to-edge framework, an egress gateway would be able to
      detect a cheating ingress gateway, but it wouldn't detect an
      interior router re-marking the ECN field from '11' to '00'.

   o Alternative 5 wouldn't allow an ECN-nonce

   An alternative scheme intended to prevent cheating when using ECN for
   admission control is proposed in [Re-PCN]. This scheme claims to
   provide protection against a much wider range of cheating strategies
   than the ECN-Nonce, including against cheating ingress nodes or
   senders. Whereas the ECN-nonce requires the sender to be trusted.
   This scheme uses a bit outside the ECN field, so Alternative 5
   combined with that scheme could solve the problem of fitting five
   states into four codepoints.

11.6.3. Does the encoding scheme require new DSCP(s)?

   o Alternatives 2 and 5 do not.

   o Alternative 1 does not allow indication of a non-PCN-capable
      transport within the same DSCP as used by PCN-capable transports.
      Therefore, if the PCN-routers are used with a pre-existing
      scheduling behaviour (such as EF) an extra DSCP would have to be
      used to indicate the combination of PCN marking with EF
      scheduling.

   o Alternative 4 needs a new DSCP so a PCN-router can Pre-emption
      Mark a packet.

   In Section 5 we suggested that the Expedited Forwarding DSCP might be
   used to indicate to a PCN-router that a packet is part of a PCN-
   capable flow. However PCN could be used similarly to add admission
   control and flow pre-emption to other DSCP classes. With Alternative
   4 a new DSCP would be needed for each PCN-enabled class.

   It's not clear to what extent the requirement for extra DSCP(s)
   matters. DSCPs are plentiful in an IP network, but scarce in an MPLS


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   network where the DSCP/ECN byte is mapped to the three MPLS header
   EXP bits [MPLS/EXP]. However, note that there is at least no need to
   encode the ECN-nonce in the MPLS EXP field, as it is sufficient to
   encode the ECN-nonce in the underlying IP header.



11.6.4. Impact on measurements

   With some of the Alternatives, the measurements by the egress gateway
   for instance, have to be modified:

   With Alternative 2 and 3, it has to measure the rate of ECT(1/A) in
   order to deduce the total number of bits in admission marked packets.

   With Alternative 2, the egress moves into the pre-emption alert state
   if the rate of ECT(0/P) is significantly less than 50%. This is
   slower than the other Alternatives which are triggered by a single
   pre-emption marked packet. It also makes it more likely that the
   egress moves into the pre-emption alert state when the traffic level
   actually doesn't justify this.

   With Alternative 4 the egress has to monitor the new DSCP in order to
   measure pre-emption marked packets.

11.6.5. Other issues

   With Alternatives 2 and 3, Admission Marking means re-marking the ECN
   field of a '01' packet and Pre-emption Marking means re-marking a
   '10' packet. Therefore extra work is required compared with the other
   Alternatives; exactly what the work is depends on the details of the
   framework using PCN.

   With Alternatives 1 and 5 Pre-emption Marking overwrites Admission
   Marking.

   With Alternative 4 Pre-emption Marking is indicated by a new DSCP.
   Some ECMP (Equal Cost Multipath Routing) algorithms use the DSCP
   field as one of the input fields used to calculate which link to
   forward a packet on. Therefore, with a network running ECMP there is
   a danger that a Pre-emption Marked packet might be forwarded on a
   different path to other PCN-capable packets. The extent that this
   matters is for further study. It is not an issue for the other
   encoding Alternatives.





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12. References

   A later version will distinguish normative and informative
   references.

   [CL-arch]     B. Briscoe, P. Eardley, D. Songhurst, F. Le Faucheur,
                 A.   Charny, S. Dudley, J. Babiarz, K. Chan. A
                 Framework for Admission Control over DiffServ using
                 Pre-Congestion Notification, draft-briscoe-tsvwg-cl-
                 architecture-02.txt", (work in progress), March 2006

   [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).

   [Floyd]       S. Floyd, 'Specifying Alternate Semantics for the
                 Explicit Congestion Notification (ECN) Field', draft-
                 floyd-ecn-alternates-00.txt (work in progress), April
                 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)

   [Hovell]      P. Hovell, R. Briscoe, G. Corliano, "Guaranteed QoS
                 Synthesis - an example of a scalable core IP quality
                 of service solution", BT Technology Journal, Vol 23 No
                 2, April 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

   [RFC2119]     Bradner, S., "Key words for use in RFCs to Indicate
                 Requirement Levels", BCP 14, RFC 2119, March 1997.



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

   [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, "An Architecture 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.

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

   [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.

   [RFC3540]     N. Spring, D. Wetherall, D. Ely, 'Robust Explicit
                 Congestion Notification (ECN) Signaling with Nonces',
                 RFC 3540, June 2003.

   [RMD]         A Bader, L Westberg, G Karagiannis, C Kappler, T
                 Phelan, 'RMD-QOSM - The Resource Management in
                 DiffServ QoS model', draft-ietf-nsis-rmd-06 Work in
                 Progress, February 2006

   [RTECN]       Babiarz, J., Chan, K. and V. Firoiu, 'Congestion
                 Notification Process for Real-Time Traffic', draft-
                 babiarz-tsvwg-rtecn-05 Work in Progress, October 2005.

   [tsvwg-ml]    Discussion on the TSVWG mailing list, Nov/Dec 2005.

   [Westberg]    L. Westberg, Z. R. Turanyi, D. Partain, A. Bader, G.
                 Karagiannis, "Load Control of Real-Time Traffic",
                 draft-westberg-loadcntr-04.txt (Work in progress), Dec
                 2005






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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

   Vassilis Liatsos
   Cisco Systems, Inc.
   1414 Massachusetts Avenue
   Boxborough,
   MA 01719,
   USA
   Email: vliatsos@ciscoyahoo.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


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   Anna Charny
   Cisco Systems, Inc.
   14164 Massachusetts Ave
   Boxborough,
   MA 01719
   USA
   Email: acharny@cisco.com

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

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



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Briscoe               Expires 6 September 2006               [Page 47]


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