PCE L. M. Contreras
Internet-Draft Telefonica
Intended status: Informational F. Agraz
Expires: 4 September 2025 S. Spadaro
Universitat Politecnica de Catalunya
Q. Xiong
ZTE Corporation
3 March 2025
Path Computation Based on Precision Availability Metrics
draft-contreras-pce-pam-04
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 4 September 2025.
Copyright Notice
Copyright (c) 2025 IETF Trust and the persons identified as the
document authors. All rights reserved.
Contreras, et al. Expires 4 September 2025 [Page 1]
Internet-Draft PAM-based PCE March 2025
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
3.1. Dynamic behavior of performance parameters . . . . . . . 3
3.2. Applicability . . . . . . . . . . . . . . . . . . . . . . 4
4. PRECISION METRIC Object . . . . . . . . . . . . . . . . . . . 5
4.1. Motivation for a new object to express precision
metrics . . . . . . . . . . . . . . . . . . . . . . . . . 5
4.2. Definition of the PRECISION METRIC Object . . . . . . . . 5
4.3. Summary of the PRECISION METRIC Object . . . . . . . . . 9
4.4. Examples on the usage of the PRECISION METRIC Object. . . 11
4.4.1. PRECISION METRIC coding examples . . . . . . . . . . 11
4.4.2. PRECISION METRIC operation examples . . . . . . . . . 12
5. PCEP message extensions . . . . . . . . . . . . . . . . . . . 13
5.1. The PCReq Message . . . . . . . . . . . . . . . . . . . . 13
5.2. The PCRep Message . . . . . . . . . . . . . . . . . . . . 14
5.3. The PCRpt Message . . . . . . . . . . . . . . . . . . . . 15
6. Related work . . . . . . . . . . . . . . . . . . . . . . . . 16
7. Security and operational considerations . . . . . . . . . . . 16
8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 16
9. References . . . . . . . . . . . . . . . . . . . . . . . . . 16
9.1. Normative References . . . . . . . . . . . . . . . . . . 16
9.2. Informative References . . . . . . . . . . . . . . . . . 17
Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . 18
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 19
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.
Contreras, et al. Expires 4 September 2025 [Page 2]
Internet-Draft PAM-based PCE March 2025
While this can be sufficient for certain services, some others can
require the utilization of Precision Availability Metrics (PAM)
[RFC9544]. That is the case of services like Network Slice
[I-D.ietf-teas-ietf-network-slices] or deterministic [RFC8578]
[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
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 [RFC9544] are also used in this
document.
3. Rationale of the usage of PAM for path calculation
3.1. Dynamic behavior of performance parameters
[RFC9544] 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
Contreras, et al. Expires 4 September 2025 [Page 3]
Internet-Draft PAM-based PCE March 2025
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
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.
3.2. Applicability
Three situations of applicability of precision metrics can be
identified:
* The provision of a path according to the desired behavior along
the time. In this scenario different segments of a potential path
could be monitored before the path is created. The path
calculation can take into consideration the measured
characteristics of the segments forming that path for decision.
* The selection of a path according to its long-run characteristics.
In this scenario, an existing path being monitored along the time
can be selected if its behavior is compliant with the long-run
behavior expected by the customer.
* The triggering of corrective actions for a selected path. It
could be the case that a selected path suffers degradation. The
precision metrics can assist on the identification of such
potential problems, e.g, raising incidents or anomalies to
operational groups, as described in
[I-D.ietf-nmop-network-incident-yang].
Contreras, et al. Expires 4 September 2025 [Page 4]
Internet-Draft PAM-based PCE March 2025
4. PRECISION METRIC Object
4.1. Motivation for a new object to express precision metrics
The existing METRIC object [RFC5440] is used to specify either the
metric that the path computation algorithm MUST optimize or a
constraint on the path cost (i.e., an upper bound) that MUST NOT be
exceeded for the path to be deemed acceptable. A number of metric
types to be used in the METRIC object have been already defined in
IANA registries [IANA_METRIC_Object].
There are several reasons for proposing the definition of a new
object for dealing with precission metrics instead of forcing the
augmentation of the existing METRIC object:
* The notion of precision metric refers to the fact on how the
metric is described, that is, in terms of tiers constituting a
statistical distribution of the metric of interest. This implies
that the metric type does not change if the metric is expressed as
precision metric or not. Them extending the type in the METRIC
object for including the notion of precision can create
inconsistencies.
* Not all the existing metric types currently defined for the METRIC
object could (easily) adhere to the notion of precision metrics
(e.g., number of layers on a path).
* The current structure of the METRIC object is not sufficiently
flexible for permitting a clean description of the precision
metrics.
* The precision metrics can be independent of the exsiting metric in
the METRIC object, and can be implemented simultaneously or
separately.
The former limitations make preferble the introduction of a new
object facilitating a lean design for the way of expressing precision
metrics at the time of performing path calculations with the PCE.
4.2. Definition of the PRECISION METRIC Object
The PRECISION METRIC object is defined according to the following
structure.
Contreras, et al. Expires 4 September 2025 [Page 5]
Internet-Draft PAM-based PCE March 2025
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
[RFC9544], 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.
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.
Contreras, et al. Expires 4 September 2025 [Page 6]
Internet-Draft PAM-based PCE March 2025
(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 (8 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:
- 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
Contreras, et al. Expires 4 September 2025 [Page 7]
Internet-Draft PAM-based PCE March 2025
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 - 16 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:
* 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].
Contreras, et al. Expires 4 September 2025 [Page 8]
Internet-Draft PAM-based PCE March 2025
* 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.
4.3. 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 [RFC9544].
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:
Contreras, et al. Expires 4 September 2025 [Page 9]
Internet-Draft PAM-based PCE March 2025
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:
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 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Contreras, et al. Expires 4 September 2025 [Page 10]
Internet-Draft PAM-based PCE March 2025
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.
4.4. Examples on the usage of the PRECISION METRIC Object.
4.4.1. 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:
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
[RFC9544], 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 4 September 2025 [Page 11]
Internet-Draft PAM-based PCE March 2025
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.
4.4.2. PRECISION METRIC operation examples
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 4 September 2025 [Page 12]
Internet-Draft PAM-based PCE March 2025
5. PCEP message extensions
Message formats in this document are expressed using Routing Backus-
Naur Form (RBNF). This is an initial attempt for defining the
proposed extensions to PCEP messages on top of [RFC8233] definitions.
Note. Further revision of these formats is needed. Consider them as
an initial exercise by now.
5.1. The PCReq Message
The extension to the PCReq message would be as follows:
* New optional PRECISION METRIC object
* New metric types using the optional PRECISION METRIC object
The format of the PCReq message (with [RFC5541], [RFC8231] and
[RFC8233] as a base) is updated as follows:
<PCReq Message> ::= <Common Header>
[<svec-list>]
<request-list>
where:
<svec-list> ::= <SVEC>
[<OF>]
[<metric-list>]
[<precision-metric-list>]
[<svec-list>]
<request-list> ::= <request> [<request-list>]
<request> ::= <RP>
<END-POINTS>
[<LSP>]
[<LSPA>]
[<BANDWIDTH>]
[<bu-list>]
[<metric-list>]
[<precision-metric-list>]
[<OF>]
[<RRO>[<BANDWIDTH>]]
[<IRO>]
[<LOAD-BALANCING>]
and where:
<precision-metric-list> ::= <PRECISION-METRIC>[<precision-metric-list>]
Contreras, et al. Expires 4 September 2025 [Page 13]
Internet-Draft PAM-based PCE March 2025
5.2. The PCRep Message
The extension to the PCReq message would be as follows:
* New optional PRECISION METRIC object (during unsuccessful path
computation based on precision metrics, to indicate the precision
metrics requested which are reason for failure)
* New metric types using the optional PRECISION METRIC object
The format of the PCRep message (with [RFC5541], [RFC8231] and
[RFC8233] as a base) is updated as follows:
Contreras, et al. Expires 4 September 2025 [Page 14]
Internet-Draft PAM-based PCE March 2025
<PCRep Message> ::= <Common Header>
[<svec-list>]
<response-list>
where:
<svec-list> ::= <SVEC>
[<OF>]
[<metric-list>]
[<precision-metric-list>]
[<svec-list>]
<response-list> ::= <response> [<response-list>]
<response> ::= <RP>
[<LSP>]
[<NO-PATH>]
[<attribute-list>]
[<path-list>]
<path-list> ::= <path> [<path-list>]
<path> ::= <ERO>
<attribute-list>
and where:
<attribute-list> ::= [<OF>]
[<LSPA>]
[<BANDWIDTH>]
[<bu-list>]
[<metric-list>]
[<precision-metric-list>]
[<IRO>]
<precision-metric-list> ::= <PRECISION-METRIC>[<precision-metric-list>]
5.3. The PCRpt Message
The PCRpt message can use the updated attribute-list (as extended in
previous section) for the purpose of including the PRECISION-METRIC
object.
Contreras, et al. Expires 4 September 2025 [Page 15]
Internet-Draft PAM-based PCE March 2025
6. 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].
Note. To align with [RFC8655] which metrics from DetNet services can
be expressed as PAM and what other have strict behavior.
7. 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.
8. 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 (for instance, in respect to
precision metric types).
9. References
9.1. Normative References
[RFC5541] Le Roux, JL., Vasseur, JP., and Y. Lee, "Encoding of
Objective Functions in the Path Computation Element
Communication Protocol (PCEP)", RFC 5541,
DOI 10.17487/RFC5541, June 2009,
<https://www.rfc-editor.org/info/rfc5541>.
Contreras, et al. Expires 4 September 2025 [Page 16]
Internet-Draft PAM-based PCE March 2025
[RFC8231] Crabbe, E., Minei, I., Medved, J., and R. Varga, "Path
Computation Element Communication Protocol (PCEP)
Extensions for Stateful PCE", RFC 8231,
DOI 10.17487/RFC8231, September 2017,
<https://www.rfc-editor.org/info/rfc8231>.
[RFC8233] Dhody, D., Wu, Q., Manral, V., Ali, Z., and K. Kumaki,
"Extensions to the Path Computation Element Communication
Protocol (PCEP) to Compute Service-Aware Label Switched
Paths (LSPs)", RFC 8233, DOI 10.17487/RFC8233, September
2017, <https://www.rfc-editor.org/info/rfc8233>.
9.2. Informative References
[I-D.ietf-nmop-network-incident-yang]
Hu, T., Contreras, L. M., Wu, Q., Davis, N., and C. Feng,
"A YANG Data Model for Network Incident Management", Work
in Progress, Internet-Draft, draft-ietf-nmop-network-
incident-yang-03, 19 February 2025,
<https://datatracker.ietf.org/doc/html/draft-ietf-nmop-
network-incident-yang-03>.
[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
Bounded Latency", Work in Progress, Internet-Draft, draft-
xiong-pce-detnet-bounded-latency-05, 21 October 2024,
<https://datatracker.ietf.org/doc/html/draft-xiong-pce-
detnet-bounded-latency-05>.
[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-05, 29 June 2024,
<https://datatracker.ietf.org/doc/html/draft-zhang-pce-
enhanced-detnet-05>.
Contreras, et al. Expires 4 September 2025 [Page 17]
Internet-Draft PAM-based PCE March 2025
[IANA_METRIC_Object]
"METRIC Object T Field", n.d.,
<https://www.iana.org/assignments/pcep/pcep.xhtml#metric-
object-ni-field>.
[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>.
[RFC8578] Grossman, E., Ed., "Deterministic Networking Use Cases",
RFC 8578, DOI 10.17487/RFC8578, May 2019,
<https://www.rfc-editor.org/info/rfc8578>.
[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>.
[RFC9544] Mirsky, G., Halpern, J., Min, X., Clemm, A., Strassner,
J., and J. François, "Precision Availability Metrics
(PAMs) for Services Governed by Service Level Objectives
(SLOs)", RFC 9544, DOI 10.17487/RFC9544, March 2024,
<https://www.rfc-editor.org/info/rfc9544>.
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).
Contreras, et al. Expires 4 September 2025 [Page 18]
Internet-Draft PAM-based PCE March 2025
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
Quan Xiong
ZTE Corporation
China
Email: xiong.quan@zte.com.cn
Contreras, et al. Expires 4 September 2025 [Page 19]