Skip to main content

Path Computation Based on Precision Availability Metrics
draft-contreras-pce-pam-02

Document Type Active Internet-Draft (individual)
Authors Luis M. Contreras , Fernando Agraz , Salvatore Spadaro
Last updated 2024-02-13
RFC stream (None)
Intended RFC status (None)
Formats
Stream Stream state (No stream defined)
Consensus boilerplate Unknown
RFC Editor Note (None)
IESG IESG state I-D Exists
Telechat date (None)
Responsible AD (None)
Send notices to (None)
draft-contreras-pce-pam-02
PCE                                                      L. M. Contreras
Internet-Draft                                                Telefonica
Intended status: Informational                                  F. Agraz
Expires: 16 August 2024                                       S. Spadaro
                                    Universitat Politecnica de Catalunya
                                                        13 February 2024

        Path Computation Based on Precision Availability Metrics
                       draft-contreras-pce-pam-02

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 a new object, namely the PRECISION METRIC object, to be used
   for path calculation or selection for 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
   working documents as Internet-Drafts.  The list of current Internet-
   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 16 August 2024.

Copyright Notice

   Copyright (c) 2024 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

Contreras, et al.        Expires 16 August 2024                 [Page 1]
Internet-Draft                PAM-based PCE                February 2024

   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.
   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.  Definition of the PRECISION METRIC Object . . . . . . . . . .   4
     4.1.  Summary of the PRECISION METRIC Object  . . . . . . . . .   8
     4.2.  Examples on the usage of the PRECISION METRIC Object. . .   9
       4.2.1.  PRECISION METRIC coding examples  . . . . . . . . . .   9
       4.2.2.  Example on the usage of PRECISION METRIC Object . . .  11
   5.  Related work  . . . . . . . . . . . . . . . . . . . . . . . .  12
   6.  Security and operational considerations . . . . . . . . . . .  12
   7.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  12
   8.  Informative References  . . . . . . . . . . . . . . . . . . .  12
   Acknowledgements  . . . . . . . . . . . . . . . . . . . . . . . .  14
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  14

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 Network Slice
   [I-D.ietf-teas-ietf-network-slices] or deterministic [RFC5878]
   [RFC8655] services.  These networking services express their
   performance requirements by means of 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 refers to the

Contreras, et al.        Expires 16 August 2024                 [Page 2]
Internet-Draft                PAM-based PCE                February 2024

   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.

   The precision availability metrics indicate whether or not a given
   service has been available according to expectations along the time,
   for whatever SLO considered as constraint.  Thus, at the time of
   computing a path for networking services described by means of 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 new object,
   namely the PRECISION METRIC object, 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.

Contreras, et al.        Expires 16 August 2024                 [Page 3]
Internet-Draft                PAM-based PCE                February 2024

   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
   the fact that the past behavior is not a guarantee of future
   behavior, it can be presumed that those paths with lower VIR and SVIR
   will better 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.  Definition of the PRECISION METRIC Object

   The PRECISION METRIC object is defined according to 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
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |    Flags  |C|S|     Type      | Stat Function |     Tiers     |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |    AvPeriod   |    TI_Units   |          TI_Value             |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                   Violated Interval Ratio                     |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |               Severely Violated Interval Ratio                |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                                                               |
      ~                        Thresholds                             ~
      |                                                               |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   The following fields are defined.

   *  From the Flags field, two flags are defined in this document.

      o 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.

Contreras, et al.        Expires 16 August 2024                 [Page 4]
Internet-Draft                PAM-based PCE                February 2024

      o C (Computed Metric - 1 bit), with similar meaning and
      implications to the C flag defined on the METRIC object in
      [RFC5440].  That is, when C=1 in a PCReq message it indicates that
      the PCE MUST provide the computed path precision metric value in
      the PCRep message.

      o Unassigned flags MUST be set to zero on transmission and MUST be
      ignored on receipt.

   *  Type (8 bits): specifies the metric type.  The valid metric type
      values are those allocated by IANA for the original METRIC object
      T field.

      (Note.  To check with PCE WG if this is the correct approach, or
      if alternatively it is convenient to allocate specific values for
      the PRECISION METRIC object).

   *  Stat Function field (0 bits): in case S=1, this field determines
      the statistical function for describing the SLO.  The following
      functions are considered:

        - 0x0: this is a reserved value.
        - 0x1: histogram
        - 0x2: cumulative distribution function
        - 0x3 - 0x255: these are reserved for future use.
        When S=0, this field SHOULD be ignored.

   *  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.

   *  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.

   *  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:

Contreras, et al.        Expires 16 August 2024                 [Page 5]
Internet-Draft                PAM-based PCE                February 2024

        - 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.

   A PRECISION METRIC Object with values 0x0 or 0x1 SHOULD be discarded.
   A PRECISION 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 PRECISION 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.

   *  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 precision 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 Thresholds field:

Contreras, et al.        Expires 16 August 2024                 [Page 6]
Internet-Draft                PAM-based PCE                February 2024

   *  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 Tier Boundary                  |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                     Optimal Threshold                         |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                    Critical  Threshold                        |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   The Optimal Threshold Tier Boundary, the Optimal Threshold and the
   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
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                      Tier 1 Boundary                          |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                   Threshold for Tier 1                        |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      ~                             ...                               ~
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                     Tier N-1 Boundary                         |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                   Threshold for Tier N-1                      |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                    Critical  Threshold                        |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

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

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

Contreras, et al.        Expires 16 August 2024                 [Page 7]
Internet-Draft                PAM-based PCE                February 2024

4.1.  Summary of the PRECISION METRIC Object

   The PRECISION METRIC Object is extended to take into consideration
   PAMs.  The PRECISION METRIC object is defined 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
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |    Flags  |C|0|     Type      | Stat Function |      0x2      |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |    AvPeriod   |    TI_Units   |          TI_Value             |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                   Violated Interval Ratio                     |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |               Severely Violated Interval Ratio                |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |               Optimal Threshold Tier Boundary                 |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                     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:

Contreras, et al.        Expires 16 August 2024                 [Page 8]
Internet-Draft                PAM-based PCE                February 2024

       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
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |    Flags  |C|1|     Type      | Stat Function |      0xN      |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |    AvPeriod   |    TI_Units   |          TI_Value             |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                   Violated Interval Ratio                     |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |               Severely Violated Interval Ratio                |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                      Tier 1 Boundary                          |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                   Threshold for Tier 1                        |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      ~                             ...                               ~
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                     Tier N-1 Boundary                         |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                  Threshold for Tier N-1                       |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                    Critical  Threshold                        |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   In this case, the message has a variable size determined by (4+(2N-
   1))*4 bytes, being N the number of tiers of the SLO statistical
   distribution.

4.2.  Examples on the usage of the PRECISION METRIC Object.

4.2.1.  PRECISION METRIC coding examples

   The following are examples of usage of the PRECISION METRIC Object.
   Path Delay metric type is used as precision metric in these examples.

   The first example assumes a a networking service characterized by a
   SLO defined by means of 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:

Contreras, et al.        Expires 16 August 2024                 [Page 9]
Internet-Draft                PAM-based PCE                February 2024

       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
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |    Flags  |C|0|  Type = 12    | Stat Function |      0x2      |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |       24      |      sec      |            3600               |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                              5                                |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                             0.2                               |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                            99.9                               |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                             20                                |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                             25                                |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   The second example takes the example of statistical distribution in
   [I-D.ietf-ippm-pam], where the path delay 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:

Contreras, et al.        Expires 16 August 2024                [Page 10]
Internet-Draft                PAM-based PCE                February 2024

       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
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |    Flags  |C|1|  Type = 12    |   Histogram   |      0x3      |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |       24      |      sec      |            3600               |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                             5                                 |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                            0.2                                |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                            99                                 |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                            20                                 |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                          99.999                               |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                            25                                 |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                            30                                 |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Once the PCE processes these PRECISION 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.

4.2.2.  Example on the usage of PRECISION METRIC Object

   The example considers a PCC sending a path computation request to the
   PCE, including a PRECISION METRIC object detailing path delay
   described in terms of SLO, and a METRIC object indicating that the
   path loss must not exceed the value of M.  The two objects are
   inserted in the PCReq message as follows:

   o First PRECISION METRIC object coded as in the previous examples,
   depending on the applicable SLO.

   o Second METRIC object with B=1, T=14, metric-value=M

   In case the PRECISION METRIC contains flag C = 1, as per [RFC5440],
   in case there is a path satisfying the set of constraints and there
   is no policy that prevents the return of the computed metric, then
   the PCE inserts in its response one PRECISION METRIC object with T=12
   and the corresponding SLO description for that path (i.e., all the
   fields contained in the definition of the PRECISION METRIC Object).
   Additionally, the PCE MAY insert a second METRIC object with B=1,
   T=14, metric-value=computed path loss.

Contreras, et al.        Expires 16 August 2024                [Page 11]
Internet-Draft                PAM-based PCE                February 2024

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.

7.  IANA Considerations

   This document defines a new object class for the PCEP.  IANA is
   requested to allocate the following codepoint in the PCEP "Objects"
   registry.

       Value     Description                        Reference
       ------    -------------------------------    -------------
       TBD1      PRECISION METRIC object            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-09,
              1 December 2023, <https://datatracker.ietf.org/doc/html/
              draft-ietf-ippm-pam-09>.

   [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

Contreras, et al.        Expires 16 August 2024                [Page 12]
Internet-Draft                PAM-based PCE                February 2024

              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>.

   [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>.

Contreras, et al.        Expires 16 August 2024                [Page 13]
Internet-Draft                PAM-based PCE                February 2024

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

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

   Salvatore Spadaro
   Universitat Politecnica de Catalunya
   08034 Barcelona
   Spain
   Email: salvatore.spadaro@upc.edu

Contreras, et al.        Expires 16 August 2024                [Page 14]