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Path Computation Based on Precision Availability Metrics
draft-contreras-pce-pam-00

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This is an older version of an Internet-Draft whose latest revision state is "Active".
Authors Luis M. Contreras , Fernando Agraz , Salvatore Spadaro
Last updated 2023-10-23
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draft-contreras-pce-pam-00
PCE                                                      L. M. Contreras
Internet-Draft                                                Telefonica
Intended status: Informational                                  F. Agraz
Expires: 25 April 2024                                        S. Spadaro
                                    Universitat Politecnica de Catalunya
                                                         23 October 2023

        Path Computation Based on Precision Availability Metrics
                       draft-contreras-pce-pam-00

Abstract

   The Path Computation Element (PCE) is able of determining paths
   according to constraints expressed in the form of metrics.  The value
   of the metric can be signaled as a bound or maximum, meaning that
   path metric must be less than or equal such value.  While this can be
   sufficient for certain services, some others can require the
   utilization of Precision Availability Metrics (PAM).  This document
   defines extensions to the METRIC object networking services with
   performance requirements expressed as Service Level Objectives (SLO)
   using PAM.

Status of This Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
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   Drafts is at https://datatracker.ietf.org/drafts/current/.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on 25 April 2024.

Copyright Notice

   Copyright (c) 2023 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents (https://trustee.ietf.org/
   license-info) in effect on the date of publication of this document.

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   Please review these documents carefully, as they describe your rights
   and restrictions with respect to this document.  Code Components
   extracted from this document must include Revised BSD License text as
   described in Section 4.e of the Trust Legal Provisions and are
   provided without warranty as described in the Revised BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   3
   3.  Rationale of the usage of PAM for path calculation  . . . . .   3
   4.  Extension to the METRIC Object  . . . . . . . . . . . . . . .   4
     4.1.  PAM Metric Type . . . . . . . . . . . . . . . . . . . . .   4
     4.2.  PAM metric field structure  . . . . . . . . . . . . . . .   4
     4.3.  Summary of the PAM METRIC Object  . . . . . . . . . . . .   8
     4.4.  Examples of usage of the extended METRIC Object.  . . . .   9
   5.  Related work  . . . . . . . . . . . . . . . . . . . . . . . .  11
   6.  Security and operational considerations . . . . . . . . . . .  11
   7.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  12
   8.  Informative References  . . . . . . . . . . . . . . . . . . .  12
   Acknowledgements  . . . . . . . . . . . . . . . . . . . . . . . .  13
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  13

1.  Introduction

   The Path Computation Element (PCE) [RFC4655] is able of determining
   paths according to constraints expressed in the form of metrics.  For
   that purpose, the METRIC object is defined in [RFC5440].  The value
   of the metric included in the METRIC object can be signaled as a
   bound or maximum, meaning that path metric must be less than or equal
   such value.  While this can be sufficient for certain services, some
   others can require the utilization of Precision Availability Metrics
   (PAM) [I-D.ietf-ippm-pam].  That is the case of services like RFC
   XXXX Network Slice [I-D.ietf-teas-ietf-network-slices] or
   deterministic [RFC5878] [RFC8655] services.  These networking
   services express their performance requirements expressed as Service
   Level Objectives (SLO) with target values for certain metrics.

   At the time of calculating a path by the PCE, the METRIC object
   [RFC5440] serves for the purposes of indicating either the metric
   that MUST be optimized by the path computation algorithm, or a bound
   on the path cost that MUST NOT be exceeded for the path to be
   considered as acceptable.  The value of the metric can refer to the
   instantaneous observed behavior of that parameter, without a notion
   of behavior along the preceding time.  This cannot be sufficient for
   certain networking services which require to experience stable
   behavior along the time according to their SLOs.

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   The precision availability metrics indicate whether or not a given
   service has been available according to expectations at all times,
   for whatever SLO considered as constraint.  Thus, at the time of
   computing a path for networking services with SLOs, it is convenient
   to express the applicable metric constraints according to the
   definition of precision availability metrics.  This permits the PCE
   to calculate paths showing a behavior compatible to the desired SLOs
   over a period.  This document defines extensions to the METRIC object
   services using PAM for that purpose.

2.  Terminology

   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].

   In addition, the terms defined in [I-D.ietf-ippm-pam] are also used
   in this document.

3.  Rationale of the usage of PAM for path calculation

   [I-D.ietf-ippm-pam] introduced the concept of intervals for measuring
   the behavior of measurable performance parameters against some
   predefined thresholds.  Those intervals consider a given time window.
   Thus, it is possible to define a Violated Interval (VI) as the time
   interval during which at least one of the performance parameters
   presents degradation respect to a predefined optimal level threshold.
   Similarly, when the threshold is defined as critical, the degradation
   of the performance parameter in a time window generates a Severe
   Violated Interval (SVI).

   Taking into account the VIs and SVIs it is feasible to generate
   availability metrics showing some degree of historic behavior in the
   form of the following ratios:

   *  Violated Interval Ratio (VIR) defined as the ratio of the summed
      numbers of VIs and SVIs to the total number of time unit intervals
      along a predefined availability period.

   *  Severely Violated Interval Ratio (SVIR), defined as the ratio of
      SVIs to the total number of time unit intervals along a predefined
      availability period.

   At the time of provisioning a networking service which requires
   stable SLOs along the time, it is important to ensure that the
   selected path has shown such stable behavior in the past.  Despite
   that the past behavior is not a guarantee of future behavior, it can
   be presumed that those paths with better VIR and SVIR will better

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   satisfy the SLOs of the networking service.  Alternatively, PAM can
   be used by the path computation entity for fine-grained path
   computation.  Then PAM is a useful criteria for calculating and
   selecting paths.

4.  Extension to the METRIC Object

   The original METRIC object is defined in [RFC5440] as follows.

       0                   1                   2                   3
       0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |          Reserved             |    Flags  |C|B|       T       |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                          metric-value                         |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   The extension described in this document considers a new value for
   the metric type T and a new structure for the metric-value field.

4.1.  PAM Metric Type

   There can be multiple parameters that can be expressed as precision
   availability metrics such as latency, delay variation, loss,
   capacity/throughput, ordering, or fragmentation [I-D.ietf-ippm-pam].
   However all of them can be described in the same form.  In
   consequence, a generic PAM metric type is defined as follows:

       * T=TBD1: PAM metric

4.2.  PAM metric field structure

   In order to fully described the characteristics of the PAM metric,
   the following fields are defined.

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       0                   1                   2                   3
       0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |      Metric   |S|  Function   | Metric_Units  |   TI_Units    |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |    AvPeriod   |     Tiers     |          TI_Value             |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                   Violated Interval Ratio                     |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |               Severely Violated Interval Ratio                |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                                                               |
      ~                        Thresholds                             ~
      |                                                               |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   *  Metric field (8 bits): specifies the particular metric to take
      into consideration (e.g., latency, delay variation, etc).

   *  S flag (Statistical - 1 bit): determines if the metric follows a
      statistical distribution function.  When S=0, it means that the
      metric will be assessed against an optimal (for VI) and a critical
      (for SVI) thresholds.  When S=1, it means that the metric will be
      assessed against a multi-tiered SLO, presenting different
      thresholds per tier.  In case the SLO is defined in N tiers, each
      tier is associated with a threshold.  Following the example in
      [I-D.ietf-ippm-pam], a latency metric defined in this way could be
      expressed in the form of

        - not to exceed 30 ms for any packet;
        - to not exceed 25 ms for 99.999% of packets;
        - to not exceed 20 ms for 99% of packets.

   *  Function field (7 bits): in case S=1, this field determines the
      statistical function for the SLO.  The following functions are
      considered:

        - 0x0: this is a reserved value.
        - 0x1: histogram
        - 0x2: cumulative distribution function
        - 0x3 - 0x127: these are reserved for future use.
        When S=0, this field SHOULD be ignored.

   *  Metric_Units field (8 bits): specifies the unit of the metric.
      (Note: To be discussed if the description of the particular
      metrics should be part of this document).

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   *  TI_Units (Time Interval Units - 8 bits): specifies the units for
      the definition of the time window of the interval.  The following
      units are considered:

        - 0x0: this is a reserved value.
        - 0x1: microsecond
        - 0x2: millisecond
        - 0x3: second
        - 0x4: minute
        - 0x5: hour
        - 0x6: day
        - 0x7: week
        - 0x8: month
        - 0x9: year
        - 0x10 - 0x255: these are reserved for future use.

   *  AvPeriod (Availability Period - 8 bits): specifies the total
      number of of time unit intervals to be considered for the
      calculation of VIR and SVIR shown by the path.

   *  Tiers (8 bits): determines the number of tiers in which the
      statistical distribution of the SLO is defined.  The following
      values are considered:

        - 0x0-0x1: these are invalid values.
        - 0x2: two tiers, valid for the case S=0.
        - 0x3- 0x255: multiple tiers, valid for the case S=1.

   A PAM METRIC Object with values 0x0 or 0x1 SHOULD be discarded.  A
   PAM METRIC Object with S=0 and Tiers field different than 0x2 SHOULD
   be discarded.  This value implies that the Threshold field will be
   composed by an Optimal Threshold (for VI) and a Critical Threshold
   (for SVI).  Finally, a PAM METRIC Object with S=1 and Tiers field
   lower than 0x3 SHOULD be discarded.  When a generic value of N is
   provided in this field, it implies that the Threshold field will be
   compose by N-1 thresholds (for VI per tier) and a Critical Threshold
   (for SVI corresponding to the highest tier).

   *  TI_Value (Time Interval Value - 8 bits): specifies the numerical
      value for the definition of the time window of the interval.

   *  Violated Interval Ratio (32 bits): specifies the expected VIR for
      the path, encoded in 32 bits in IEEE floating point format
      [IEEE.754.2019].  The VIR of the path calculated by the PCE SHOULD
      be lower or equal than this value.  The way in which the PCE
      calculates the VIR is out of scope of this document.

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   *  Severely Violated Interval Ratio (32 bits): specifies the expected
      SVIR for the path, encoded in 32 bits in IEEE floating point
      format [IEEE.754.2019].  The SVIR of the path calculated by the
      PCE SHOULD be lower or equal than this value.  The way in which
      the PCE calculates the SVIR is out of scope of this document.

   Regarding the Thresholds field, this will be variable in size
   depending on the statistical nature of the metric.  When the metric
   is defined only according to an optimal and critical thresholds (S=0
   case), then only those thresholds are included in the field.
   However, when the SLO is defined by means of a multi-tiered
   statistical distribution (S=1 case), then one threshold field is
   included per tier.  In summary, this would be the different possible
   situations for the Threshold field:

   *  S=0, meaning that only an optimal and critical thresholds are
      considered.  In this case, the Thresholds field follows the
      following structure:

       0                   1                   2                   3
       0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                     Optimal Threshold                         |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                    Critical  Threshold                        |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Both Optimal Threshold and Critical Threshold fields are encoded in
   32 bits in IEEE floating point format [IEEE.754.2019].

   *  S=1, meaning that only an optimal and critical thresholds are
      considered.  In this case, the Thresholds field follows the
      following structure:

       0                   1                   2                   3
       0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                   Threshold for Tier 1                        |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      ~                             ...                               ~
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                  Threshold for Tier N-1                       |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                    Critical  Threshold                        |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   All the Threshold fields are encoded in 32 bits in IEEE floating
   point format [IEEE.754.2019].

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   The way in which the PCE calculates the different thresholds is out
   of scope of this document.

4.3.  Summary of the PAM METRIC Object

   The METRIC Object is extended to take into consideration PAMs.  The
   first 32 bits of the original METRIC object in [RFC5440] are used in
   a similar way as before.  However, the original "metric-value" of the
   METRIC object is extended to accommodate the expression of
   constraints following the PAM proposition in [I-D.ietf-ippm-pam].

   According to the definition before, and depending on the statistical
   description of the SLO, two different messages can be found.

   When S=0 the SLO or metric is defined against an optimal and a
   critical thresholds.  In consequence, the message format is as
   follows:

       0                   1                   2                   3
       0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |          Reserved             |    Flags  |C|B|  T=TBD1 (PAM) |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |      Metric   |0|  Function   | Metric_Units  |   TI_Units    |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |    AvPeriod   |      0x2      |          TI_Value             |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                   Violated Interval Ratio                     |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |               Severely Violated Interval Ratio                |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                     Optimal Threshold                         |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                    Critical  Threshold                        |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   In this case, the message has a fixed size of 28 bytes.

   When S=1 the SLO or metric is defined following an statistical
   distribution with N tiers, representing a total of N-1 optimal
   thresholds plus a critical one.  In consequence, the message format
   is as follows:

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       0                   1                   2                   3
       0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |          Reserved             |    Flags  |C|B|  T=TBD1 (PAM) |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |      Metric   |1|  Function   | Metric_Units  |   TI_Units    |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |    AvPeriod   |      0xN      |          TI_Value             |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                   Violated Interval Ratio                     |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |               Severely Violated Interval Ratio                |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                   Threshold for Tier 1                        |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      ~                             ...                               ~
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                  Threshold for Tier N-1                       |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                    Critical  Threshold                        |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   In this case, the message has a variable size determined by 20 + (N
   *4) bytes, being N the number of tiers of the SLO statistical
   distribution.

4.4.  Examples of usage of the extended METRIC Object.

   The following are examples of usage of the extended METRIC Object
   considering PAM.  Latency is used as PAM metric in these examples.

   The first example assumes a a networking service characterized by two
   tiers with optimal threshold of 20 ms for 99,9% of the packet latency
   samples, and critical threshold of 25 ms.  The availability
   expectation for this service is to show a VIR of 5% and a SVIR of
   0,2%. The availability period considered is one day, while the time
   interval is considered 1 hour.  In these conditions, the extended
   METRIC Object can be descrined as:

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        0                   1                   2                   3
        0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |          Reserved             |    Flags  |C|B|  T=TBD1 (PAM) |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |    Latency    |0|  Function   |       ms      |      sec      |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |      24       |      0x2      |             3600              |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                              5                                |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                             0.2                               |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                             20                                |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                             25                                |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   The second example takes the example of statistical distribution in
   [I-D.ietf-ippm-pam], where the latency metric is statistically
   defined in the form of:

   - not to exceed 30 ms for any packet;
   - to not exceed 25 ms for 99.999% of packets;
       - to not exceed 20 ms for 99% of packets

   Assuming similar VIR, SVIR, availability period and time interval
   duration.  In these conditions, he extended METRIC Object can be
   descrined as:

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        0                   1                   2                   3
        0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |          Reserved             |    Flags  |C|B|  T=TBD1 (PAM) |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |    Latency    |1|    Hist     |       ms      |      sec      |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |      24       |      0x3      |             3600              |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                             5                                 |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                            0,2                                |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                            20                                 |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                            25                                 |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                            30                                 |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Once the PCE processes these extended METRICT Objects, the PCE will
   calculate the VIR and SVIR of the different path alternatives and
   check them against the requested VIR and SVIR.  How the PCE calculate
   the VIR and SVIR is out of scope of this document.

5.  Related work

   In the case of deterministic networking, other documents like
   [I-D.xiong-pce-detnet-bounded-latency] and
   [I-D.zhang-pce-enhanced-detnet] propose extensions to PCE adapted to
   deterministic service capabilities.  As part of those capabilities
   specific metrics are considered.  Such metrics could be considered as
   SLOs that can be handled as PAM.  This document presents a generic
   form of using precision availability metrics in PCEP messages, and
   then permitting its applicability to broader networking scenarios.
   Thus, this extension could be used instead of ad-hoc extensions in
   [I-D.xiong-pce-detnet-bounded-latency] and
   [I-D.zhang-pce-enhanced-detnet].

6.  Security and operational considerations

   Same security and operational considerations as described in
   [RFC5440] apply also in this document.

   Other security considerations will be addressed in future versions of
   the document.

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7.  IANA Considerations

   This document defines a new metric type for the PCEP.  IANA is
   requested to allocate the following codepoint in the PCEP "METRIC
   Object T Field" registry.

       Value     Description                        Reference
       ------    -------------------------------    -------------
       TBD1      PAM metric                         This document

   Additional IANA considerations required by this extension will be
   documented in future document versions.

8.  Informative References

   [I-D.ietf-ippm-pam]
              Mirsky, G., Halpern, J. M., Min, X., Clemm, A., Strassner,
              J., and J. Fran├žois, "Precision Availability Metrics for
              Services Governed by Service Level Objectives (SLOs)",
              Work in Progress, Internet-Draft, draft-ietf-ippm-pam-08,
              18 October 2023, <https://datatracker.ietf.org/doc/html/
              draft-ietf-ippm-pam-08>.

   [I-D.ietf-teas-ietf-network-slices]
              Farrel, A., Drake, J., Rokui, R., Homma, S., Makhijani,
              K., Contreras, L. M., and J. Tantsura, "A Framework for
              Network Slices in Networks Built from IETF Technologies",
              Work in Progress, Internet-Draft, draft-ietf-teas-ietf-
              network-slices-25, 14 September 2023,
              <https://datatracker.ietf.org/doc/html/draft-ietf-teas-
              ietf-network-slices-25>.

   [I-D.xiong-pce-detnet-bounded-latency]
              Xiong, Q., Liu, P., and R. Gandhi, "PCEP Extension for
              DetNet Bounded Latency", Work in Progress, Internet-Draft,
              draft-xiong-pce-detnet-bounded-latency-03, 8 June 2023,
              <https://datatracker.ietf.org/doc/html/draft-xiong-pce-
              detnet-bounded-latency-03>.

   [I-D.zhang-pce-enhanced-detnet]
              Zhang, L., Geng, X., and T. Zhou, "PCEP for Enhanced
              DetNet", Work in Progress, Internet-Draft, draft-zhang-
              pce-enhanced-detnet-03, 9 July 2023,
              <https://datatracker.ietf.org/doc/html/draft-zhang-pce-
              enhanced-detnet-03>.

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   [IEEE.754.2019]
              "754-2019 - IEEE Standard for Floating-Point Arithmetic",
              22 July 2019,
              <https://ieeexplore.ieee.org/document/8766229>.

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,
              <https://www.rfc-editor.org/info/rfc2119>.

   [RFC4655]  Farrel, A., Vasseur, J.-P., and J. Ash, "A Path
              Computation Element (PCE)-Based Architecture", RFC 4655,
              DOI 10.17487/RFC4655, August 2006,
              <https://www.rfc-editor.org/info/rfc4655>.

   [RFC5440]  Vasseur, JP., Ed. and JL. Le Roux, Ed., "Path Computation
              Element (PCE) Communication Protocol (PCEP)", RFC 5440,
              DOI 10.17487/RFC5440, March 2009,
              <https://www.rfc-editor.org/info/rfc5440>.

   [RFC5878]  Brown, M. and R. Housley, "Transport Layer Security (TLS)
              Authorization Extensions", RFC 5878, DOI 10.17487/RFC5878,
              May 2010, <https://www.rfc-editor.org/info/rfc5878>.

   [RFC8655]  Finn, N., Thubert, P., Varga, B., and J. Farkas,
              "Deterministic Networking Architecture", RFC 8655,
              DOI 10.17487/RFC8655, October 2019,
              <https://www.rfc-editor.org/info/rfc8655>.

Acknowledgements

   This work has been partially funded by the European Commission
   Horizon Europe SNS JU PREDICT-6G project (GA 101095890), and the
   Spanish Ministry of Economic Affairs and Digital Transformation and
   the European Union NextGenerationEU UNICO 5G I+D "Towards a smart and
   efficient telecom infrastructure meeting current and future industry
   needs" (TIMING) project (TSI-063000-2021-145, -148, -149).

Authors' Addresses

   Luis M. Contreras
   Telefonica
   Ronda de la Comunicacion, s/n
   28050 Madrid
   Spain
   Email: luismiguel.contrerasmurillo@telefonica.com
   URI:   http://lmcontreras.com

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   Fernando Agraz
   Universitat Politecnica de Catalunya
   08034 Barcelona
   Spain
   Email: fernando.agraz@upc.edu

   Salvatore Spadaro
   Universitat Politecnica de Catalunya
   08860 Castelldefels
   Spain
   Email: salvatore.spadaro@upc.edu

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