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Versions: 00

Network Working Group                                          A. Charny
Internet-Draft                                       Cisco Systems, Inc.
Intended status: Informational                                J. Babiarz
Expires: May 14, 2008                                             Nortel
                                                                M. Menth
                                                 University of Wuerzburg
                                                                J. Zhang
                                            Cisco Systems, Inc & Cornell
                                                              University
                                                       November 11, 2007


                 Comparison of Proposed PCN Approaches
                   draft-charny-pcn-comparison-00.txt

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Copyright Notice

   Copyright (C) The IETF Trust (2007).

Abstract

   Several, sometimes conflicting proposals have been offered for the
   consideration of the PCN WG regarding PCN internal node and PCN edge



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   node behaviors.  Based on the WG charter, the WG needs to make a
   decision on which of the proposed PCN-interior-node and PCN-boundary-
   nodes behaviors to endorse.  The primary goal of this draft is
   twofold.  First, we attempt to summarize the functional differences
   between the proposed alternatives.  Second, we provide a brief
   summary of performance evaluation results.  Finally we propose a view
   on how a (parameterized) specification of the PCN-interior-node
   metering and marking function can be described to enable several of
   the proposed behaviors.  We argue that if this parameterized
   specification is used for specifying the PCN-interior-node behavior,
   then it can support a range of behaviors at the PCN-boundary-node.
   The decision on which of the PCN-boundary-node behaviors to choose
   can then be considered separately.  We also discuss complexities
   associated with choosing such uniform approach.

Requirements Language

   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 RFC 2119 [RFC2119].































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Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  4
     1.1.  Terminology  . . . . . . . . . . . . . . . . . . . . . . .  4
     1.2.  Introduction . . . . . . . . . . . . . . . . . . . . . . .  4
   2.  PCN-Interior-Node Metering and Marking Functions . . . . . . .  5
     2.1.  Metering and Marking Types . . . . . . . . . . . . . . . .  5
     2.2.  Metering and Re-Marking of Previously Marked Packets . . .  6
       2.2.1.  Admission Marking  . . . . . . . . . . . . . . . . . .  7
       2.2.2.  Termination Marking  . . . . . . . . . . . . . . . . .  7
     2.3.  Number of Metering Functions and Marking Codepoints  . . .  7
   3.  Dropping Policies  . . . . . . . . . . . . . . . . . . . . . .  7
   4.  PCN-Boundary-Node Behaviors  . . . . . . . . . . . . . . . . .  8
     4.1.  CL Boundary Behavior . . . . . . . . . . . . . . . . . . .  8
     4.2.  SM Boundary Behavior . . . . . . . . . . . . . . . . . . .  9
     4.3.  3SM Boundary Behavior  . . . . . . . . . . . . . . . . . .  9
     4.4.  Notes on Using Different Boundary Behaviors with
           Different Marking/Metering Behaviors . . . . . . . . . . . 10
   5.  Informationed Signaled Between the Boundary Nodes  . . . . . . 10
   6.  Dealing with ECMP  . . . . . . . . . . . . . . . . . . . . . . 11
   7.  Suitability for Probing for Admission Control  . . . . . . . . 11
   8.  Configuration Complexity and Configuratio Restrictions . . . . 12
   9.  Functional Comparison Summary  . . . . . . . . . . . . . . . . 13
   10. Other Comparison Criteria  . . . . . . . . . . . . . . . . . . 17
     10.1. Performance Comparison . . . . . . . . . . . . . . . . . . 17
   11. Unified Descriprion of Marking and Metering Functions  . . . . 19
   12. Difficulties with Allowing Multiple Marking Behaviors  . . . . 23
   13. Security Considerations  . . . . . . . . . . . . . . . . . . . 24
   14. IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 24
   15. Appendix . . . . . . . . . . . . . . . . . . . . . . . . . . . 24
     15.1. Formulation of the Simple Generalized Metering and
           Marking Algorithm  . . . . . . . . . . . . . . . . . . . . 24
     15.2. VQ Formulation of the Complex Generalized Metering and
           Marking Algorithm  . . . . . . . . . . . . . . . . . . . . 26
   16. References . . . . . . . . . . . . . . . . . . . . . . . . . . 29
     16.1. Normative References . . . . . . . . . . . . . . . . . . . 29
     16.2. Informative References . . . . . . . . . . . . . . . . . . 29
     16.3. References . . . . . . . . . . . . . . . . . . . . . . . . 31
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 31
   Intellectual Property and Copyright Statements . . . . . . . . . . 33











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1.  Introduction

1.1.  Terminology

   This draft uses the terminology defined in
   [I-D.ietf-pcn-architecture]

1.2.  Introduction

   A number of seemingly diverse approaches have been presented in the
   context of the work in the PCN WG, encompassing both the PCN-
   interior-node and PCN-boundary-node node behaviors.  The goal of this
   informational draft is to provide a functional comparison of several
   candidate approaches for PCN behaviors.  We refer the reader to
   [I-D.ietf-pcn-architecture] for an architectural overview of PCN.

   The first draft of this document concentrates on the CL approach
   proposed in [I-D.briscoe-tsvwg-cl-architecture] , the 3SM approach
   described in [I-D.babiarz-pcn-3sm] , and the Single-Marking (SM)
   approach proposed in [I-D.charny-pcn-single-marking].  The approach
   proposed in [I-D.westberg-pcn-load-control] is not covered in the
   initial version of this draft, pending clarifications on the open
   questions regarding the details of the algorithms described in that
   draft.

   At the time of writing of this draft, several performance studies
   have been undertaken and are on-going.  The studies performed in
   [I-D.zhang-pcn-performance-evaluation] and
   [I-D.charny-pcn-single-marking] provide a side-by-side comparison of
   performance of the CL and SM proposals.  A performance evaluation of
   the 3SM approach was reported in [I-D.babiarz-pcn-explicit-marking]
   and [TR437].  However, due to a number of differences in the
   experimental setups in different simulation studies, a side-by-side
   performance comparison between 3SM and the other two approaches is
   not fully possible at this point.  Therefore, we present only a short
   overview of some of the performance evaluation results where
   possible.

   We then argue that a unified (parameterized) formulation of the
   metering and marking behavior at the PCN-interior-nodes can be
   defined.  Thus defined unified PCN-interior-node behavior may support
   multiple PCN-boundary-node behaviors, and hence in principle can be
   used in a variety of environments.  We also discuss some of the
   tradeoffs and additional complexities associated with such unified
   PCN-interior-node behavior definition.

   Sections 2-8 provide an overview of the three proposals, followed by
   a summary of functional differences between the proposals in Section



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   9.  Section 10 briefly covers other comparison criteria not covered
   in this draft, including a brief summary of performance evaluation
   efforts as of the time of writing of this document.  Section 11
   provides a unified description of admission and termination functions
   of the PCN-interior-node covering all three proposals of CL, SM and
   3SM, followed by a brief discussion of the tradeoffs associated with
   such unified definition in Section 12.


2.  PCN-Interior-Node Metering and Marking Functions

   This section provides a high level functional comparison of the
   metering and marking functions at the PCN-interior-node.  For more
   detail, please see [I-D.briscoe-tsvwg-cl-phb],
   [I-D.charny-pcn-single-marking], and [I-D.babiarz-pcn-3sm].

2.1.  Metering and Marking Types

   Metering functions are defined in different proposals via the notions
   of Token Bucket (TB) or Virtual Queue (VQ).  These two formulations
   are equivalent in the sense that each one can be implemented via the
   other with appropriate settings.  They may count packet or bytes.
   The marking functions differ with respect to how the queue length of
   the Q or the fill state of the TB is used which has a direct
   influence on the marking result.  The following are the marking
   behaviours used in [I-D.briscoe-tsvwg-cl-phb],
   [I-D.charny-pcn-single-marking] and [I-D.babiarz-pcn-3sm].

   o  Excess-rate-marking marks packets which exceed the configured
      rate.  In the TB formulation, the packets are marked when the
      arriving packet does not find enough tokens in the token bucket
      (and the marked packet does not consume tokens from the TB).  In
      the VQ formulation, the packets are marked when the arriving
      packet would exceed the configured maximum size of the VQ (and the
      marked packet is not added to the Q).  These two formulations are
      equivalent with the same rates of the TB and VQ, and the depth of
      the TB numerically equal to the maximum size of the Q. This is the
      marking used for termination in CL, and for both admission and
      termination in SM.

   o  Excess-rate-marking-with-marking-frequency-reduction is similar to
      the Excess-rate-marking in the sense that it also marks packets
      which do not find tokens in the TB (in the TB formulation), or
      would exceed the maximum size of the VQ (in the VQ formulation).
      However, to reduce the number of marked packets, whenever a packet
      is marked a certain amount of tokens are added to the TB (in the
      TB formulation) or the same number of bytes is removed from the
      queue of the VQ (in the VQ formulation).  This is the marking used



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      for termination in 3SM.

   o  Ramp-marking is defined as follows.  In the VQ formulation, two VQ
      thresholds are defined (below the maximum VQ size).  Packets are
      marked with a certain probability depending on the VQ size at the
      time of the packet arrival.  This probability is 0 for VQ queue
      length from 0 to a lower VQ threshold, it rises linearly from 0 to
      1 between the lower and a upper VQ threshold, and it is 1 above
      the upper VQ threshold.  As a result, no packets are marked when
      the queue length is below the lower VQ threshold, a few packets
      are marked when the queue length is between the lower and the
      upper VQ thresholds, and all packets are marked when the queue
      length is above the upper VQ threshold.  In the equivalent TB
      formulation, two additional TB fill thresholds (called the lower
      and upper TB thresholds, both not exceeding the TB depth) are
      defined .  The packers are marked with the probability 1 for a TB
      fill state from 0 to a lower TB threshold.  The probability
      decreases linearly from 1 to 0 between the lower and upper TB
      thresholds, and it is 0 above the upper TB threshold.  As a
      result, all packets are marked when the fill state is below the
      lower threshold, a few packets are marked when the fill state is
      between the lower and the upper threshold, and no packets are
      marked above the upper threshold.  This is the marking described
      in [I-D.briscoe-tsvwg-cl-phb] (in the VQ formulation) where it is
      used for admission.

   o  Threshold-marking marks packets that make the queue length of the
      VQ exceed a certain threshold which is lower than the queue size.
      As a result, all packets are marked when the metered traffic
      exceeds the VQ rate.  In the equivalent TB formulation, Threshold-
      marking marks packets that make the number of tokens in the TB
      fall below a certain threshold which is larger than zero.  As a
      result, all packets are marked when the metered traffic exceeds
      the TB rate.  The VQ and TB formulations are equivalent with the
      VQ of rate R, maximum size S and VQ threshold T, and TB with rate
      R, depth B and threshold T, and TB threshold B-T.  Threshold-
      marking s a special case of Ramp marking when the lower and the
      upper (TB or Q) thresholds are identical.  It is used for
      admission in CL and 3SM.

2.2.  Metering and Re-Marking of Previously Marked Packets

   When packets travel over several links within a PCN domain, they are
   possibly marked.  This section clarifies the question concerning
   packets of which markings should be taken into account for the
   metering process on subsequent links and if so which markings are re-
   marked.




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2.2.1.  Admission Marking

   3SM and CL consider packets of all markings for metering, but TM-
   marked packets must not be re-marked to AM.  SM requires that
   previously marked packets are excluded from metering, as not doing so
   would result in underestimation of sustainable admission rate in the
   multiple bottleneck scenarios, and consequently will result in the
   under-estimation of the sustainable termination rate at the PCN-
   ingress-node, in turn causing over-termination in the multiple
   bottlenecks scenarios.

2.2.2.  Termination Marking

   CL needs to exclude all previously termination-marked packets from
   metering in order to prevent underestimation of sustainable
   termination rate.  If previously termination-marked packets are not
   excluded from metering, substantial over-termination in the multiple
   bottleneck scenarios might occur. 3SM can accommodate previously
   termination- marked packets being included for termination metering
   (although the exact impact of doing so needs to be further
   evaluated).  Both in CL and 3SM, AM-marked packets may be remarked to
   TM.  Note that SM does not employ termination marking at the PCN-
   ingress-node, an hence does not have any termination-marked packets
   at all.

2.3.  Number of Metering Functions and Marking Codepoints

   Both CL and 3SM require 2 different metering functions - one (pcn-
   lower-threshold) for admission and one (pcn-upper-threshold) for
   termination.  Three marking codepoints are needed for both: unmarked,
   admission-marked (first PCN encoding) for admission, and termination-
   marked (second PCN encoding) to terminate flows.

   In contrast, SM requires a single metering threshold and two
   different marking codepoints: marked and unmarked.  (Note: There is a
   choice to be made whether the "marked" codepoint should use the first
   or the second PCN encoding state, if both are defined.)


3.  Dropping Policies

   A subtle difference in existing proposals for the PCN-interior-node
   behavior is related to how already marked packets need to be treated
   in the presence of loss.  It turns out that all three approaches
   assume different drop preferences.

   CL needs preferential dropping of termination-marked packets.  If
   such preferential dropping is not implemented, then possible over-



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   termination may occur.  It can be argued that the admission function
   of CL is not sensitive to whether or not admission-marked packets are
   preferentially dropped or not.

   SM relies on preferential drop of marked packets.  While admission
   function of this approach appears to be insensitive to the drop
   preference just as CL admission function, the termination function of
   SM will result in over-termination if preferential dropping of
   already marked packets is not implemented.  While, insensitivity of
   CL admission function to marked packet drop remains to be studied,
   especially in the presence of large differences in packet sizes.

   In contrast, the proposal in 3SM can benefit from preferential
   dropping of unmarked flow-termination packets, but it can function
   without at the expense of longer termination time.  It can be argued
   that the admission function of 3SM is not sensitive to whether or not
   admission-marked packets are preferentially dropped or not.


4.  PCN-Boundary-Node Behaviors

4.1.  CL Boundary Behavior

   In the CL approach, the PCN-egress-node measures the rate of
   admission-marked, termination-marked, and unmarked PCN-traffic per
   ingress-egress-aggregate.  In addition, the PCN-ingress-node measures
   the rate of sent PCN-traffic per ingress-egress-aggregate.  To
   support admission control, the PCN-egress-node calculates the
   Congestion Level Estimate (CLE) defined as a fraction of (admission-
   or termination-) marked traffic and the overall traffic, and signals
   the CLE to the PCN-ingress-node.  The PCN-ingress-node accepts or
   rejects flows based on whether this CLE value for a particular
   ingress-egress aggregate exceeds a pre-defined threshold.

   To support flow termination, the PCN-egress-node calculates (on a
   per-ingress-egress basis) the Sustainable (Termination) Rate, defined
   as the combined rate of unmarked and admission-marked packets, and
   signals the Sustainable Rate to the PCN-ingress-node.  The PCN-
   ingress-node measures the ingress sending rate (again on a per-
   ingress-egress basis) and calculates the difference to the
   sustainable termination rate.  It chooses an appropriate set of flows
   whose combined rate corresponds to that difference and terminates
   these flows.  (Note: this choice is done without any knowledge of
   which flows had termination-marked packets.)







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4.2.  SM Boundary Behavior

   In the SM approach, the PCN-egress-node measures the rate of marked
   and unmarked traffic per ingress-egress-aggregate.  In addition, the
   PCN-ingress-node measures the rate of sent PCN-traffic per ingress-
   egress-aggregate.

   To support admission control, the PCN-egress-node calculates the
   congestion level estimate (CLE) as the fraction of marked traffic and
   the overall traffic, and signals the CLE to the PCN-ingress-node.
   The PCN-ingress-node accepts or rejects flows based on whether this
   CLE value exceeds a pre-defined threshold.

   Although the boundary functions necessary to support admission
   control are similar for CL and SM, an important difference between
   the two algorithms stems from the fact that CL is using threshold (or
   ramp) marking, while SM uses excess-rate-marking.  As a result, the
   meaningful value of CLE is also different.  For example, in case of
   small overloads, SM will have only a small fraction of packets marked
   (and hence the appropriate CLE value needed to detect the overload
   without over admission is small), while a small (but consistent)
   overload with CL results in the majority of packets being marked,
   resulting in CL being quite robust for a range of CLE values.

   To support flow termination, the PCN-egress-node signals the rate of
   unmarked packets as the so-called Sustainable (Admission) Rate to the
   PCN-ingress-node.  The PCN-ingress-node multiplies it by a system-
   wide constant to get the Sustainable Termination Rate.  The rest of
   the termination is done like in the CL approach.  The PCN-ingress-
   node measures the ingress rate and calculates the difference to the
   sustainable termination rate.  It chooses an appropriate set of flows
   whose overall rate corresponds to that difference and terminates
   theses flows.  (NOTE: This choice is done without any knowledge of
   which flows had marked packets.)

4.3.  3SM Boundary Behavior

   To support flow admission, a PCN-egress-node analyzes the packet
   markings per ingress-egress-aggregate.  Depending on the markings it
   sends from time to time "admission-stop" or "admission-continue"
   messages to the corresponding PCN-ingress-nodes to control their
   admission of new flows.  The PCN-ingress-node admits new flows when
   the last control message was "admission-continue" and it rejects them
   when it was "admission-stop". 3SM leaves deliberately open the way
   how the PCN-egress-node decides when to send a specific control
   message.  A very simple option for the PCN-egress-node is to send an
   "admission-stop" message when a single packet with admission- or
   termination-marking is observed and to send an "admission-continue"



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   message some time after the last marked packet has been observed.
   This is the method used for performance evaluation of 3SM reported in
   [I-D.babiarz-pcn-explicit-marking].  A more sophisticated option is
   to calculate a CLE based on an exponentially weighted moving average
   (EWMA) counting marked messages as 1 and umarked messages as 0.  When
   the CLE exceeds an upper threshold, an "admission-stop" message is
   sent, and when the CLE falls below a lower threshold, an "admission-
   continue" message is sent.  Other implementations are possible since
   the decision logic is local to the PCN-egress-node.

   To support flow termination, the PCN-egress-node in the 3SM approach
   again monitors the packet markings and signals the flow ID of
   termination-marked packets to the PCN-ingress-node whereby several
   flow IDs may be sent in a single message.  The PCN-ingress-node
   terminates these flows.  Note that neither the PCN-ingress-node nor
   the PCN-egress-node are required to perform rate measurements in 3SM.

4.4.  Notes on Using Different Boundary Behaviors with Different
      Marking/Metering Behaviors

   The previous three Subsections describe the PCN-boundary-node
   behaviors in conjunction with specific proposals where these
   behaviors were defined.  It should be noted, however, that various
   features of specific boundary behaviors described in the previous
   sections may be used with different marking/metering strategies.  For
   example, as indicated in Section 4.3, the boundary behavior that CL
   uses for admission can also be used with 3SM.  Likewise, the
   Termination function of CL may choose to only terminate those flows
   whose packets are TM-marked - see Section 6 for discussion on how
   this additional functionality can be used to address ECMP issues.

   In general, new boundary behaviors may be designed to work with
   proposed metering and marking mechanism.  Nevertheless, in the
   remaining portion of this document we will assume specific boundary
   mechanisms as described in sections 4.1 - 4.3 unless stated
   otherwise.


5.  Informationed Signaled Between the Boundary Nodes

   In CL, the PCN-egress-node must send to the PCN-ingress-node two
   values: a CLE for Admission and Sustainable (Termination) rate for
   Termination

   In SM, the PCN-egress-node sends to the PCN-ingress-node the two
   values as in CL: the CLE and Sustainable (Admission) Rate.  (Note
   that while the format of the signaling information is the same for CL
   and SM, the meaning of Sustainable Rate is different in the two



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   cases).

   In 3SM, the PCN-egress-node signals to the PCN-ingress-node using
   control messages for the Admission process (e.g. admission-stop or
   admission-continue messages).  For Termination, the PCN-egress-node
   signals to the ingress the flow ID of each flow to be terminated.
   Note that several IDs can be communicated in a single signalling
   (i.e.  RSVP) message.


6.  Dealing with ECMP

   For admission control, neither of the three algorithms considered in
   this draft address ECMP issue in the absence of probing of some sort.
   The probing discussion is deferred to the next section.

   Without probing, in the case when congestion state of different paths
   in the network differ, flows may be admitted on a congested path
   while the other path can remain uncongested, or, conversely,
   admission may stop on all paths, when only one path becomes pre-
   congested.

   For termination, 3SM will correctly identify for termination only
   those flows which pass congested paths even in the presence of ECMP.
   In contrast, both CL and SM may erroneously terminate flows that do
   not traverse congested paths.  For CL, an option is available to
   choose for termination only those flows that are termination-marked.
   However, doing so then requires that the egress needs to additionally
   signal to the ingress which flows have been marked, on top of the
   other signaling information described in Section 5.

   Single-marking does not explicitly mark traffic by termination-
   marking and so the above option to identify flows of congested path
   does not work.  SM does have an option to terminate only those flows
   that are marked for admission.  However, this may result in erroneous
   termination of flows on paths where traffic is above the Admission
   threshold but below the level that should cause Termination.  Just as
   in the case of CL, doing so also involves the necessity to signal
   additional information (flow IDs) between the PCN-egress-node and
   PCN-ingress node.


7.  Suitability for Probing for Admission Control

   Although probing is currently out-of-scope of the PCN WG charter, it
   may be useful in a number of situations (in particular, dealing with
   ECMP for admission, or addressing flash crowd situations in the
   presence of many small ingress-egress aggregates).  We do not attempt



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   here to address the issue of whether or not and exactly how well
   probing might work, nor do we discuss any protocol issues and
   complexities associated with probing.  Instead we touch upon only one
   aspect of it - how well the PCN admission marking of by the three
   proposals in question might work with probing.

   Threshold-marking employed by both CL and 3SM results in all packets
   being marked once the traffic rate exceeds PCN-lower-threshold (i.e
   the admissible rate threshold).  Therefore, a single probe packet is
   guaranteed to be marked if the PCN-traffic rate exceeds the PCN-
   lower-threshold threshold.  Therefore, the admission decision can be
   reliably made based on a single probe.  One possibility may be to use
   signaling (e.g.  RSVP) messages as probes in this case.

   In SM, admission metering and marking is based on Excess-rate-
   marking.  In this case, only a fraction of traffic is marked when
   traffic exceeds the configured (admissible rate) threshold.
   Therefore, when the PCN-traffic rate exceeds this threshold, a single
   probe will only be marked with a certain probability, and so a series
   of probes need to be sent to detect congestion with high probability.

   Thus, Threshold-marking of CL and 3SM allows faster and simpler
   admission decisions than Excess-rate-marking of SM.

   In addition it should be noted that the necessity to send multiple
   probes for SM adds a potential problem with using RSVP messages as
   probes.  Extensions necessary to do so have not been considered at
   this time.


8.  Configuration Complexity and Configuratio Restrictions

   The approach in SM requires a global configuration parameter at the
   edges reflecting the assumed ratio between the (implicit) termination
   threshold and (explicit) admission threshold, which is assumed to be
   global on all links in the PCN domain.  This necessitates the use of
   a global parameter that needs to be configured to the same value on
   all PCN-boundary-nodes in the network.  This clearly leads to an
   additional configuration complexity of SM compared to CL and 3SM.

   An additional issue caused by the assumption of the global ratio
   between (implicit) termination threshold and (explicit) admission
   threshold of SM is related to the flexibility of resiliency planning.
   In this context, a natural approach is to use PCN-lower-threshold to
   represent the expected utilisation on different links for expected
   traffic matrix under normal, non-failure, conditions, and to view
   PCN-upper-threshold to represent expected worst case utilization
   under any of the "planned" failures.



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   As discussed in [Menth] and [I-D.charny-pcn-single-marking] , the
   global ratio between the termination and admission utilisation levels
   assumed in SM limits the flexibility of traffic engineering for
   resiliency to a certain extent.  Specifically, While all three
   approaches can be configured to ensure that the planned matrix can be
   protected against all the planned failure conditions, the nature of
   the guarantee for the admitted traffic is different.  Both CL and 3SM
   can be configured to protect all admitted traffic (but would not
   admit more than the planned traffic matrix), while SM can be
   configured to admit more traffic than planned, but will not guarantee
   protection against planned failures for traffic exceeding planned
   utilization for the planned traffic matrix.

   It should be noted that in principle, all three algorithms can be
   configured to protect all admitted traffic (whether or not this
   admitted traffic exceeds the planned traffic matrix or not).
   However, as argued in [Menth], in this case SM can generally admit
   less traffic than CL or 3SM.

   We refer the reader to [Menth] and [I-D.charny-pcn-single-marking]
   for a more detailed discussion on this issue.


9.  Functional Comparison Summary

   The following Table summarizes the differences between the three
   approaches discussed so far.

   (preamble)
    -------------------------------------------------------------------|
   |Comparison    |      SM       |       3SM        |      CL         |
   |criteria      |               |                  |                 |
   |--------------------------------------------------------------------
   |# of encoding |       2       |        3         |       3         |
   |encoding      |               |                  |                 |
   |states        |               |                  |                 |
   |-------------------------------------------------------------------|
   |# of metering |               |                  |                 |
   |mechanisms    |       1       |        2         |       2         |
   |in forwarding |               |                  |                 |
   |path of       |               |                  |                 |
   |interior nodes|               |                  |                 |
   |-------------------------------------------------------------------|
   |Type of       |   excess-rate |     threshold    |  threshold or   |
   |metering  &   |     marking   |     marking      |  ramp marking   |
   |marking for   |               |                  |                 |
   |admission     |               |                  |                 |
   |-------------------------------------------------------------------|



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   |Type of       |     N/A       | excess-rate with |  excess-rate    |
   |metering and  |               | marking frequency|     marking     |
   |marking for   |               |     reduction    |                 |
   |termination   |               |                  |                 |
   |-------------------------------------------------------------------|
   |Metering and  | do not meter  |  meter all pkts  | do not meter    |
   |remarking of  | AM-marked     |  for admission   | TM-marked pkts  |
   |previously    | packets       |  and termination;| for termination |
   |marked        |               |  do not re-mark  | meter all pkts;|
   |packets       |               |  TM-marked pkts  | for admission,  |
   |              |               |                  | do not re-mark  |
   |              |               |                  | TM-marked pkts  |
   |-------------------------------------------------------------------|
   |Packet Drop   |preferentially | no sensitivity   | no sensitivity  |
   |preference    | drop AM-marked| to moderate drop | to moderate drop|
   |              | packets; over-| of AM-marked pkts| of AM-marked    |
   |              |termination    | preferentially   | packets;        |
   |              | otherwise     | drop non-TM-     | preferentially  |
   |              |               | marked packets-  | drop TM-marked  |
   |              |               | over-termination | packets -       |
   |              |               | otherwise, espe- |over-termination |
   |              |               | cially under     | otherwise       |
   |              |               | high loss        |                 |
   |------------------------------------------------ ------------------|
   |Egress        | measure rates | observe packet   | measure rates of|
   |Function      | of marked and | markings, send   | AM/TM marked and|
   |              | unmarked pkts,| control messages | unmarked packets|
   |              | compute CLE,  | to control admis-| compute CLE,send|
   |              | send both to  | sion at ingress; | CLE and the rate|
   |              | ingress       | send IDs of TM-  | of unmarked pkts|
   |              |               | marked packets to| to ingress      |
   |              |               | ingress          |                 |
   |-------------------------------------------------------------------|
   |Ingress       |compute  sus-  | terminate those  | measure sending |
   |Termination   |tainable rate; | flows whose ids  | rate; compute   |
   |function      |measure sending| were signalled   | rate to termi-  |
   |              |rate; compute  | by the egress    | nate, select    |
   |              |rate to        |                  | flows to        |
   |              |terminate, se- |                  | terminate       |
   |              |lect flows to  |                  |                 |
   |              |terminate      |                  |                 |
   |-------------------------------------------------------------------|
   |Ingress       |stop admitting | stop admitting   |stop admitting   |
   |Admission     |when CLE       | when notified by |when CLE exceeds |
   |Function      |exceeds confi- | egress; resume   |configured value;|
   |              |gured value;   | after a timeout  |restart admitting|
   |              |restart admit- | or when notified |when CLE reduces |
   |              |ting when CLE  | by ingress       |below configured |



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   |              |falls below    |                  |value            |
   |              |configured     |                  |                 |
   |              |value          |                  |                 |
   |-------------------------------------------------------------------|
   |rate          | required      |    optional      |   required      |
   |measurement   | 1 at ingress  |   1 at egress    |   1 at ingress  |
   |at boundary   | 2 at egress   |                  |   2 at egress   |
   |nodes         |               |                  |                 |
   |-------------------------------------------------------------------|
   |Information   |CLE, sustaina- |control messages  |CLE, sustainable |
   |signalled     |ble(admission) |to stop & possibly|(termination)    |
   |from egress   |rate           |restart admission;|rate             |
   |to ingress    |               |ids of flows with |                 |
   |              |               | TM-marked packets|                 |
   |-------------------------------------------------------------------|
   |ECMP support  |no;   only     |     yes          | no;  but full   |
   |for           |partial support|                  | support with    |
   |Termination   |with additional|                  | additional      |
   |              | complexity at |                  | complexity at   |
   |              | the edge +    |                  | the edge +      |
   |              | signaling flow|                  | plus signalling |
   |              | flow IDs from |                  | flow IDs from   |
   |              | egress to     |                  | egress to       |
   |              | ingress       |                  | ingress         |
   |-------------------------------------------------------------------|
   |ECMP support  |no w/out probes| no w/out probing | no w/out probing|
   |for Admission |yes with probes| yes with probing | yes with probing|
   |              |but needs many |(needs one probe, |(needs one probe,|
   |              |probes; use of |can use RSVP as   | can use RSVP    |
   |              |RSVP as probes |probe)            | as probe)       |
   |              |not understood |                  |                 |
   |-------------------------------------------------------------------|
   |Network-wide  |      yes      |        no        |      no         |
   |parameter     |  (one global  |                  |                 |
   |configuration |   parameter   |                  |                 |
   |coordination  |  at the edges)|                  |                 |
   |-------------------------------------------------------------------|
   | Support for  |      yes      |      yes         |      yes        |
   | resiliency   | but weaker    |                  |                 |
   | planning     | guarantees    |                  |                 |
   |              | under planned |                  |                 |
   |              | failures for  |                  |                 |
   |              | traffic excee-|                  |                 |
   |              | ding planned  |                  |                 |
   |              |traffic matrix;|                  |                 |
   |              |if all admitted|                  |                 |
   |              |traffic is to  |                  |                 |
   |              |be protected,  |                  |                 |



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   |              |SM can admit   |                  |                 |
   |              |less traffic   |                  |                 |
   |              |than CL or 3SM |                  |                 |
   |-------------------------------------------------------------------|


   (Table 8.1.  Functional Comparison of CL, SM and 3SM)

   Finally, a few words are due on relative complexities of the three
   schemes.  It should be clear from the above table that relative
   complexity has a number of dimensions.  Specifically:

   o  Complexity of the forwarding path.  From that standpoint SM
      appears to be the simplest, as it requires only a single metering
      mechanism, CL appears to come next, closely followed by 3SM.

   o  Complexity of conditional metering (i.e checking whether a
      particularly marked packet needs to be metered ).  From that
      standpoint, all three approaches appear to be comparable.  (Note
      that while SM admission function is more complex from this point
      of view than admission functions of CL and 3SM, the additional
      complexity of not metering AM-marked packets has to be balanced
      against similar complexity of the termination functions of CL and
      3SM).

   o  Complexity of implementing preferential packet loss.  From that
      standpoint all three approaches appear comparable, when both
      admission and termination functions are considered.

   o  Complexity of boundary behaviors.  From that standpoint 3SM
      appears to be the least complex, CL coming next and SM following
      closely.

   o  Complexity of support for ECMP.  From that standpoint 3SM requires
      no additional functionality, while CL and SM require extra
      complexity at the boundary nodes and extra signaling information
      exchange between the egress and the ingress (and in addition SM
      has only partial support - see section 6 and table 8.1 for its
      limitations).

   o  Global configuration complexity.  From that standpoint Cl and 3SM
      come first, with SM being the more complex as the only one
      requiring global parameter setting.

   These comparisons are very crude and are only intended to roughly
   summarize the detailed differences described in Table 8.1.





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10.  Other Comparison Criteria

   This section discusses a number of other comparison criteria that
   have not been studied in detail or are currently under consideration

   1.  Co-existence with RFC3168 (work in progress);

   2.  Multicast support (TBD, NOTE: this is out-of-scope for the
       current charter)

   3.  Future extensions to multiple domains
       ([I-D.briscoe-tsvwg-re-ecn-border-cheat] for CL, not clarity on
       other approaches; NOTE: this is out-of-scope of the current
       charter)

   4.  Extensibility to host-initiated PCN (3SM designed to support
       host-initiated PCN but performance analysis is ongoing; NOTE:
       this is out-of-scope for the current charter.

   5.  Extensibility to rate-adaptive traffic (TBD; NOTE: this is out-
       of-scope of the current charter)

   6.  Support of multiple precedence levels (TBD; NOTE: this is out-of-
       scope of the current charter)

   7.  Performance comparison (ongoing, see next Subsection).

10.1.  Performance Comparison

   As mentioned in the Introduction, at the time of writing this
   document several performance studies have been reported in
   [I-D.zhang-pcn-performance-evaluation],
   [I-D.charny-pcn-single-marking][I-D.babiarz-pcn-explicit-marking],
   and [TR437].  While the first two studies have attempted a careful
   side-by-side comparisons of CL and SM, the set of experiments
   reported is less extensive, and a number of differences existing in
   the simulation models and experiment setup make a side-by-side
   apples-to-apples comparison of the 3 schemes difficult.  In this we
   will attempt to summarize performance evaluation criteria that could
   be used for comparison of the different approaches, as well as
   provide high level conclusions on some of them, where available
   studies allow those conclusions.

   We refer the reader to [I-D.zhang-pcn-performance-evaluation],
   [I-D.charny-pcn-single-marking], [I-D.babiarz-pcn-explicit-marking],
   and [TR437] for details that could not be captured within the format
   of the table below.




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   DISCLAIMER: statements in the table below should be understood only
   in terms relative to the three considered algorithms and with respect
   to specific experiments performed; they and are intended for crude
   qualitative comparison only based on (ongoing) simulation studies as
   of the time of writing of this document .  Quantification of these
   statements can be found in the corresponding performance studies
   referenced earlier in this section.

   (preamble)
    ------------------------------------------------------------------
   |Comparison   |      SM      |       3SM        |        CL       |
   |Criteria     |              |                  |                 |
   |-----------------------------------------------------------------|
   |Sensitivity  | poor perfor- | good performance | poor perfor-    |
   |to low       | mance at low | at low aggrega-  | mance at low    |
   |bottleneck   | bottleneck   | tion reported    | bottleneck      |
   |aggregation  | aggregation  | (evaluation      | aggregation     |
   |             |              |  ongoing)        |                 |
   |-----------------------------------------------------------------|
   |Sensitivity  | performance  | good in reported | admission       |
   |to low       |degradation   | experiments;     | insensitive;    |
   |ingress-     |for both      | traffic and      | termination     |
   |egress       |termination   | topology         | sensitive for   |
   |aggregation  |and admission | sensitivity study| some traffic    |
   |             |under some    | needed           | types           |
   |             |traffic types |                  |                 |
   |-----------------------------------------------------------------|
   |Sensitivity  | a range of   |evaluation ongoing|  relatively     |
   |to marking & | params exist |slow-down param. S|  insensitive to |
   |measurement  | with consis- |has the biggest   |  marking and    |
   |parameters   | tent perfor- |impact on flow    |  measurement    |
   |             | mance across |termination speed |  params across  |
   |             | a range of   |(its choice de-   |  a range of     |
   |             | traffic types|pends on topology |  traffic types  |
   |             | & topologies |and traffic rate) |  and topologies |
   |-----------------------------------------------------------------|
   |Accuracy of  |good for large|      good        |    good         |
   |admission    |ingress-egress|(but sensitivity  |                 |
   |across traf- |aggregation   |to parameters TBD)|                 |
   |fic types and|              |                  |                 |
   |topologies   |              |                  |                 |
   |-----------------------------------------------------------------|
   |Accuracy of  |over-         |        good      |     good        |
   |termination  |termination   | (but sensitivity |                 |
   |(single      |compared to CL|  to params TBD)  |                 |
   | bottleneck) |if termination|                  |                 |
   |             |trigger is not|                  |                 |
   |             |smoothed;     |                  |                 |



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   |             |otherwise     |                  |                 |
   |             |slower reac-  |                  |                 |
   |             |tion than CL  |                  |                 |
   |-----------------------------------------------------------------|
   |Accuracy of  | more over-   |   not reported   |      good       |
   |termination  | termination  |                  |                 |
   |wet bottle-  | than CL      |                  |                 |
   |neck utile.   |              |                  |                 |
   |(multi-bot-  |              |                  |                 |
   |neck case)   |              |                  |                 |
   |-----------------------------------------------------------------|
   |Reaction time| slower than  | slower than CL,  |      fast       |
   |time to      | CL with      | comparison with  |                 |
   |termination  | smoothing of | SM TBD; parameter|                 |
   |             | termination  | sensitivity TBD  |                 |
   |             | trigger, fast|                  |                 |
   |             | otherwise    |                  |                 |
   |             | to 3SM       |                  |                 |
   |-----------------------------------------------------------------|
   |Sensitivity  |not reported; |preferentially    | not reported ;  |
   |to large and |flow selection|selects large     | flow selection  |
   |small flows  |at ingress    |flows; slow down  | at ingress more |
   |(termination)|more compli-  |parameter hard to | complicated (or |
   |             |cated (or less|select for mix of | less accurate)  |
   |             |accurate with |traffic rates     | with widely dif-|
   |             |widely differ- |(over-termination | ferrent rates;  |
   |             |rent rates;   |or slower react-  | (study ongoing) |
   |             |study ongoing |tion if slowdown  |                 |
   |             |              |parameter does    |                 |
   |             |              |not reflect       |                 |
   |             |              |average rate;     |                 |
   |             |              |evaluation ongoing|                 |
   |-----------------------------------------------------------------|
   |Beat-down    | more beatdown| not reported     | beat-down of    |
   |effect       | of long-haul |                  | flows traversing|
   |multi-bottle-| aggregates   |                  | more bottlenecks|
   |neck case)   | than CL      |                  |                 |
    ------------------------------------------------------------------
   (Table 9.1.  Summary of (up-to-date) performance comparison. )


11.  Unified Descriprion of Marking and Metering Functions

   In this section we address the question of how the metering and
   marking functions of the three approaches can be described in a
   unified way so that different marking behaviors considered in this
   draft can be obtained by different parametrization choices.  A
   potential benefit of such unified description is that a single PCN-



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   internal-node behavior could support a wider range of different PCN-
   boundary-node behaviors, and hence, potentially, can be of use under
   different deployment scenarios.  We discuss the difficulties
   associated with such unified description in the following section,
   where we also raise the question of whether the benefits of such
   unified behavior outweigh a number of drawbacks discussed in that
   section.

   In Figure 10.1, we show a relatively compact version of such unified
   algorithm using the notion of Token Bucket (TB) .  This pseudocode
   does not support ramp-marking, as the current performance evaluation
   studies indicate little additional benefit of ramp-marking compared
   to threshold-marking.  However, in the Appendix we present a (more
   complex) version of the marking algorithm that does support ramp-
   marking as well.

   We note that the algorithm below (as well as a more complex complete
   version in the Appendix) can be further optimized.  We make no
   optimization attempt in the interest of clarity.

   The TB has a rate of TB.rate and a depth of TB.size.  It has one
   marking thresholds TB.threshold to support threshold marking.  If
   TB.threshold is configured to be greater than zero, then packets are
   marked if the TB fill state is below TB.threshold after their arrival
   and removal of tokens from the queue; otherwise packets are not
   marked.  The "classic" token bucket used by SM and the termination
   function of CL is obtained by setting TB.theshold = 0.

   In addition, the slowdown factor TB.slowdown is used to implement
   marking frequency reduction: TB.slowdown tokens are added to the
   token bucket when a packet is marked.  The metering is applied only
   to packets whose marking is part of a specific subset that we call
   TB.meteredMarking.  The TB.markingType indicates the type of
   codepoint that is used for marking.  In addition, the TB has a
   variable TB.fill that records the number of tokens in the bucket and
   the variable TB.lastUpdate records the last update instant of the
   bucket.  The global variable "now" indicates the current time.
   Packets have size packet.size (in bytes) and marking packet.mark.
   The algorithm is followed by a table describing the parameterization
   necessary to implement Admission and Termination Functions for
   different proposals.










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   (preamble)
Parameters:
TB.rate: token rate of TB in bytes/s
TB.size: depth of TB in tokens (bytes)
TB.threshold: marking threshold of TB in bytes
TB.slowdown: slowdown factor for marking frequency reduction of TB in bytes
TB.markingType: PCN-first-encoding ("admission")  or
                PCN-second-encoding ("termination").
TB.meteredMarkings: set of packet markings that are eligible for metering by TB,
                    it is a subset of ("unmarked", "admission", "termination").
                    NOTE: this set depends on whether it is admission or
                    termination that the function below is used for.
NOTE: settings of these parameters for different approaches are shown
in Tables 10.2 and 10.3

Input: packet
    // take passed time since last update into account
    TB.fill = min(TB.size, TB.fill+(now-TB.lastUpdate) * TB.rate);
    TB.lastUpdate = now;

    // meter and mark
    If (packet.mark in TB.meteredMarkings)
        if (TB.fill < packet.size)
            // re-marking of TM-marked packets to AM not allowed
            if (!(packet.mark == "termination" and TB.markingType == "admission"))
                packet.mark = TB.markingType;
            endif
        else
            TB.fill = TB.fill - packet.size;
            if (TB.fill < TB.threshold)
                // re-marking of TM-marked packets to AM not allowed
                if (!(packet.mark == "termination" and TB.markingType == "admission"))
                    packet.mark = TB.markingType;
                endif
            endif
        endif
    endif

    // marking frequency reduction
    if (packet.mark == "termination")
        TB.fill = min(TB.size, TB.fill + TB.slowdown);
    endif

Output: void

   (Figure 10.1.  Simple generalized metering and marking algorithm
   based on TB-formulation)




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   (preamble)
|----------------------------------------------------------------------|
|                |  CL Admission   |      SM         | 3SM Admission   |
|----------------|-----------------|-----------------|-----------------|
| TB.rate        |      PCN-       |      PCN-       |      PCN-       |
|                | lower-threshold | lower-threshold | lower-threshold |
|----------------|-----------------|-----------------|-----------------|
| TB.size        |   configured    |   configured    |   configured    |
|----------------|-----------------|-----------------|-----------------|
| TB.threshold   |   configured    |       0         |   configured    |
|----------------|-----------------|-----------------|-----------------|
| TB.slowdown    |       0         |       0         |       0         |
|----------------|-----------------|-----------------|-----------------|
| TB.metered-    |  "unmarked"     |  "unmarked"     |  "unmarked"     |
| Markings       |  "admission"    |                 |  "admission"    |
|                |  "termination"  |                 |  "termination"  |
|----------------|-----------------|-----------------|-----------------|
| TB.markingType |  "admission"    |  "admission"    |  "admission"    |
 ----------------------------------------------------------------------|
   (Figure 10.2 Admission Settings for the Three Algorithms)

   (preamble)
 ----------------------------------------------------------------------|
|                | CL Termination  |      SM         | 3SM Termination |
|----------------|-----------------|-----------------|-----------------|
| TB.rate        |  PCN-upper-     |     N/A         | PCN-upper-      |
|                |  threshold      |                 | threshold       |
|----------------|-----------------|-----------------|-----------------|
| TB.size        |   configured    |     N/A         |   configured    |
|----------------|-----------------|-----------------|-----------------|
| TB.threshold   |       0         |     N/A         |       0         |
|----------------|-----------------|-----------------|-----------------|
| TB.slowdown    |       0         |     N/A         |   configured    |
|----------------|-----------------|-----------------|-----------------|
| TB.metered-    |  "unmarked"     |     N/A         |  "unmarked"     |
| Markings       |  "admission"    |                 |  "admission"    |
|                |                 |                 |  "termination"  |
|----------------|-----------------|-----------------|-----------------|
| TB.markingType |  "termination"  |     N/A         |  "termination"  |
 ----------------------------------------------------------------------|
   (Figure 10.3 Termination Settings for the Three Algorithms)

   Figures 10.2 and 10.3 provide parameter settings corresponding to
   different metering and marking functions.

   It should be noted, that when the algorithm described by the
   pseudocode in Figure 10.1 is used to perform threshold-marking, it
   has a slightly different behavior than the algorithm described in CL



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   and 3SM.  The packet marking algorithm of 3SM bases its marking
   decision on TB.fill before tokens are removed while our algorithm
   makes the decision afterwards.  In addition, the admission marking
   algorithms of both 3SM and CL set TB.fill=0 when TB.fill<packet.size
   while our algorithm leaves TB.fill unchanged in this case.  This is
   done in the interest of simplification, as we believe that the impact
   of this change on performance will be minimal in practice.  The
   pseudocode we provide in the Appendix is a complete (and more
   complex) version of the unified algorithm to address this issue.  As
   mentioned earlier, this complete algorithm also supports the ramp
   marking.  In our judgement, however, a simpler version of algorithm
   10.1 would suffice in practice.

   We also present an equivalent VQ formulation of the same algorithm in
   the Appendix.


12.  Difficulties with Allowing Multiple Marking Behaviors

   There are a number of difficulties associated with allowing multiple
   edge behaviors in the PCN framework.  Below is a list of some of
   these difficulties.

   o  Additional implementation complexity in the core devices needed to
      support multiple options.

   o  Additional configuration complexity needed to support these
      different options (especially important in the case when different
      PCN domains configured for different options merge)

   o  Differences in packet dropping preferences represent additional
      complexity if different policies need to be used with different
      options (although the exact impact of not implementing any
      dropping preferences at all for different algorithms is under
      study).

   o  Difference in the PCN information signalled between the egress and
      ingress require definition and implementation of different
      signalling options

   o  Additional complexity of standardization process

   o  The unified specification is limited to the three approaches
      considered in this draft and does not consider any other possible
      approaches including that suggested
      in[I-D.westberg-pcn-load-control]

   A question then arises whether the benefits of specifying a more



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   flexible unified core behavior outweigh the above drawbacks.  We do
   not attempt to answer this question in this draft, but rather pose it
   for a general discussion of the WG.

   Finally, it should be noted that not all of the above difficulties
   pertain to different combination of the approaches in the same
   degree.  For example, the differences between SM and CL with respect
   to the PCN-boundary-node functions and the information signalled
   between the PCN-egress-node and PCN-ingress-node are relatively small
   compared to the substantial differences between the boundary behavior
   and information transport of 3SM compared to both CL and SM.
   Therefore, as described in [I-D.charny-pcn-single-marking], SM could
   be defined as a "stepping stone" to CL with relatively small
   additional complexity in such a way that the information transport is
   identical, and the boundary behavior is almost identical (and can be
   switched between the two schemes by a toggle of a configuration
   parameter).  Thus, transition from SM to CL essentially amounts to
   upgrading the core nodes only.  In contrast, transition from SM or CL
   to 3SM (or vice a versa) requires not only a change in the core
   behavior, but a substantial change in the boundary behavior and
   information transport.


13.  Security Considerations

   TBD


14.  IANA Considerations

   TBD


15.  Appendix

15.1.  Formulation of the Simple Generalized Metering and Marking
       Algorithm

   The simple generalized metering and marking algorithm can also be
   described based on a virtual queue (VQ).  The transformation is
   straightforward and the result is given in the algorithm shown in
   Figure 14.1.









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   (preamble)
Parameters:
VQ.rate: service rate of VQ in bytes/s
VQ.size: queue size of VQ in bytes
VQ.threshold: marking threshold of VQ in bytes
VQ.slowdown: slowdown factor for marking frequency reduction of VQ in bytes
VQ.markingType: PCN-first-encoding ("admission")  or  PCN-second-encoding ("termination").
VQ.meteredMarkings: set of packet markings that are eligible for metering by VQ,
                    it is a subset of ("unmarked", "admission", "termination").

VQ.length: number of bytes currently in the queue of the VQ

Input: packet
    // take passed time since last update into account
    VQ.length = max(0, VQ.length-(now-VQ.lastUpdate) * VQ.rate);
    VQ.lastUpdate = now;

    // meter and mark
    If (packet.mark in VQ.meteredMarkings)
        if (VQ.length+packet.size > VQ.size)
            if (!(packet.mark == "termination" and VQ.markingType == "admission"))
                packet.mark = VQ.markingType;
            endif
        else
            VQ.length = VQ.length + packet.size;
            if (VQ.length > VQ.threshold)
                // re-marking of TM-marked packets to AM not allowed
                if (!(packet.mark == "termination" and VQ.markingType == "admission"))
                    packet.mark = VQ.markingType;
                endif
            endif
        endif
    endif

    // marking frequency reduction
    if (packet.mark == "termination")
        VQ.length = max(0, VQ.length - VQ.slowdown);
    endif

Output: void

   (Figure 14.1)

   The tables in Figs 14.3 and 14.4 at the end of this Section give the
   corresponding parameter setting for the three proposals.

   NOTE: There are two further optimization options that are possible
   for this (and the TB-based) metering and marking algorithm:



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   1.  If TM-packets are not metered for admission-marking, the inner
       if-clause to avoid the re-marking of TM-marked packets to AM can
       be removed.  Current performance results suggest that this could
       be done, but for the sake of extensibility towards future rate
       adaptation it is better to keep open the option of metering all
       packets also for admission marking.

   2.  The marking frequency reduction may be applied when packets are
       re-marked to TM.  This is at the expense of increased unfairness
       and over termination in the presence of several simultaneously
       overloaded links.  The advantage is an improved runtime of the
       algorithm and a possibly simpler implementation.

15.2.  VQ Formulation of the Complex Generalized Metering and Marking
       Algorithm

   The simple generalized metering and marking algorithm in 14.1 has the
   following shortcomings:

   1.  It does not support ramp marking which may be desirable for CL;

   2.  If the packet does not fit into the queue, the queue cannot be
       filled up to its size.  This is a property for admission marking
       in CL and 3SM that is not implemented by a simplified algorithm
       (although the impact of this change appears to be minimal);

   3.  If the packet fits into the queue, the marking decision is always
       based on the queue size including the size of the new packet.
       This leads to a nicer formulation of threshold or ramp marking
       (however, the impact of this change seems minimal) If the
       generalized algorithm takes also these 3 issues into account, it
       becomes significantly more complex.

   To improve shortcoming 1, the algorithm has now two marking
   thresholds to support ramp and threshold marking instead of a single
   one: VQ.lowerThreshold and VQ.upperThreshold.  Packets are not marked
   if the queue length is below VQ.lowerThreshold, the marking
   probability linearly increases from 0 to 1 between VQ.lowerThreshold
   and VQ.upperThreshold, and packets are definitely marked if the queue
   length is VQ.upperThreshold and above.  If both thresholds are set to
   the same positive value, threshold marking is performed: packets are
   not marked if the queue length is below or equal to that threshold,
   otherwise they are marked.

   To improve shortcoming 2 and 3, the Boolean variable
   VQ.alwaysUpdateState is introduced.  If it is set to true, the queue
   length should always be increased by the packet size; this is the
   desired behaviour for admission marking when all packets are expected



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   to be marked when the admissible rate is exceeded by the PCN rate.
   If it is set to false, the queue length is only increased if the
   packet is not marked; this is the desired behaviour for excess-rate-
   marking.

   (preamble)
Additional parameters:
VQ.lowerThreshold: lower marking threshold of VQ in bytes
VQ.upperThreshold: upper marking threshold of VQ in bytes
VQ.alwaysUpdateState: VQ length state always increased or not

Input: packet

    // take passed time since last update into account
    VQ.length = max(0, VQ.length-(now-VQ.lastUpdate)*VQ.rate);
    VQ.lastUpdate = now;

    // meter and mark
    if (packet.mark in VQ.meteredMarkings)
        if (VQ.alwaysUpdateState == true)
            // threshold or ramp-marking
            if (VQ.length > VQ.upperThreshold)
                // re-marking of TM-marked packets to AM not allowed
                if (!(packet.mark == "termination" and VQ.markingType == "admission"))
                    packet.mark = VQ.markingType;
                endif
            elseif (VQ.length > VQ.lowerThreshold)
                choose random number u (0 < u < 1);
                if (u < (VQ.length-VQ.lowerThreshold)/(VQ.upperThreshold-VQ.lowerThreshold))
                    // re-marking of TM-marked packets to AM not allowed
                    if (!(packet.mark == "termination" and VQ.markingType == "admission"))
                        packet.mark = VQ.markingType;
                    endif
                endif
            endif
            VQ.length = min(VQ.size, VQ.length+packet.size);
        else
            // excess-rate-marking
            if (VQ.length + packet.size > VQ.size)
                // re-marking of TM-marked packets to AM not allowed
                if (!(packet.mark == "termination" and VQ.markingType == "admission"))
                    packet.mark = VQ.markingType;
                endif
            else
                VQ.length = VQ.length + packet.size;
            endif
        endif
    endif



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    // marking frequency reduction
    if (packet.mark == "termination")
        VQ.length = max(0, VQ.length - VQ.slowdown);
    endif

Output: void

   (Figure 14.2)

   (preamble)
|----------------------------------------------------------------------|
|                |  CL Admission   |       SM        | 3SM Admission   |
|----------------|-----------------|-----------------|-----------------|
|    VQ.rate     |      PCN-       |      PCN-       |      PCN-       |
|                | lower-threshold | lower-threshold | lower-threshold |
|----------------|-----------------|-----------------|-----------------|
|    VQ.size     |   configured    |   configured    |   configured    |
|----------------|-----------------|-----------------|-----------------|
|   VQ.always-   |     true        |      false      |      true       |
|  UpdateState   |                 |                 |                 |
|----------------|-----------------|-----------------|-----------------|
|      VQ.       |   configured    |       0         |   configured    |
| lowerThreshold |                 |                 |                 |
|----------------|-----------------|-----------------|-----------------|
|      VQ.       |   configured    |       0         |   configured    |
| upperThreshold |                 |                 |                 |
|----------------|-----------------|-----------------|-----------------|
|  VQ.slowdown   |       0         |       0         |       0         |
|----------------|-----------------|-----------------|-----------------|
|      VQ.       |  "unmarked"     |  "unmarked"     |  "unmarked"     |
| meteredMarkings|  "admission"    |                 |  "admission"    |
|                |  "termination"  |                 |  "termination"  |
|----------------|-----------------|-----------------|-----------------|
| VQ.markingType |  "admission"    |  "admission"    |  "admission"    |
 ----------------------------------------------------------------------|
   (Figure 14.3.  Admission settings for the three algorithms (VQ
   formulation))














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   (preamble)
 -----------------------------------------------------------------------
|                | CL Termination  |       SM        | 3SM Termination |
|----------------|-----------------|-----------------|-----------------|
|    VQ.rate     |     PCN-        |      N/A        |      PCN-       |
|                | lower-threshold |                 | upper-thrsehold |
|----------------|-----------------|-----------------|-----------------|
|    VQ.size     |   configured    |      N/A        |   configured    |
|----------------|-----------------|-----------------|-----------------|
|   VQ.always-   |     false       |      N/A        |      false      |
|  UpdateState   |                 |                 |                 |
|----------------|-----------------|-----------------|-----------------|
|      VQ.       |       0         |      N/A        |       0         |
| lowerThreshold |                 |                 |                 |
|----------------|-----------------|-----------------|-----------------|
|      VQ.       |       0         |      N/A        |       0         |
| upperThreshold |                 |                 |                 |
|----------------|-----------------|-----------------|-----------------|
|  VQ.slowdown   |       0         |      N/A        |   configured    |
|----------------|-----------------|-----------------|-----------------|
|      VQ.       |  "unmarked"     |      N/A        |  "unmarked"     |
| meteredMarkings|  "admission"    |                 |  "admission"    |
|                |                 |                 |  "termination"  |
|--------------- |-----------------|-----------------|-----------------|
| VQ.markingType |  "termination"  |      N/A        | "termination"   |
 ----------------------------------------------------------------------

   (Figure 14.4.  Termination settings for the three algorithms (VQ
   formulation))


16.  References

16.1.  Normative References

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

16.2.  Informative References

   [I-D.babiarz-pcn-3sm]
              Babiarz, J., "Three State PCN Marking",
              draft-babiarz-pcn-3sm-00 (work in progress), July 2007.

   [I-D.babiarz-pcn-explicit-marking]
              Liu, X. and J. Babiarz, "Simulations Results for 3sM",
              draft-babiarz-pcn-explicit-marking-01 (work in progress),
              July 2007.



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

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

   [I-D.briscoe-tsvwg-re-ecn-border-cheat]
              Briscoe, B., "Emulating Border Flow Policing using Re-ECN
              on Bulk Data", draft-briscoe-tsvwg-re-ecn-border-cheat-01
              (work in progress), June 2006.

   [I-D.briscoe-tsvwg-re-ecn-tcp]
              Briscoe, B., "Re-ECN: Adding Accountability for Causing
              Congestion to TCP/IP", draft-briscoe-tsvwg-re-ecn-tcp-04
              (work in progress), July 2007.

   [I-D.charny-pcn-single-marking]
              Charny, A., "Pre-Congestion Notification Using Single
              Marking for Admission and  Termination",
              draft-charny-pcn-single-marking-02 (work in progress),
              July 2007.

   [I-D.davie-ecn-mpls]
              Davie, B., "Explicit Congestion Marking in MPLS",
              draft-davie-ecn-mpls-01 (work in progress), October 2006.

   [I-D.ietf-pcn-architecture]
              Eardley, P., "Pre-Congestion Notification Architecture",
              draft-ietf-pcn-architecture-01 (work in progress),
              October 2007.

   [I-D.lefaucheur-emergency-rsvp]
              Faucheur, F., "RSVP Extensions for Emergency Services",
              draft-lefaucheur-emergency-rsvp-02 (work in progress),
              June 2006.

   [I-D.westberg-pcn-load-control]
              Westberg, L., "LC-PCN: The Load Control PCN Solution",
              draft-westberg-pcn-load-control-01 (work in progress),
              September 2007.

   [I-D.zhang-pcn-performance-evaluation]
              Zhang, X., "Performance Evaluation of CL-PHB Admission and



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              Termination Algorithms",
              draft-zhang-pcn-performance-evaluation-02 (work in
              progress), July 2007.

16.3.  References

   [Menth]    "PCN-Based Resilient Network Admission Control: The Impact
              of a Single Bit", 2007.

   [TR437]    "Comparison of Marking Algorithms for PCN-Based Admission
              Control, Technical Report No. 437, University of
              Wuerzburg", October 2007.


Authors' Addresses

   Anna Charny
   Cisco Systems, Inc.
   1414 Mass. Ave.
   Boxborough, MA  01719
   USA

   Email: acharny@cisco.com


   Joseph Babiarz
   Nortel
   3500 Carling Avenue
   Ottawa, Ontario  K2H 8E9
   Canada

   Email: babiarz@nortel.com


   Michael Menth
   University of Wuerzburg
   Informatik III Am Hubland
   Wuerzburg,   97074
   Germany

   Email: menth@menth@informatik.uni-wuerzburg.de










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   Joy Zhang
   Cisco Systems, Inc & Cornell University
   1414 Mass. Ave.
   Boxborough, MA  01719
   USA

   Email: acharny@cisco.com












































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