Path Computation Based on Precision Availability Metrics
draft-contreras-pce-pam-06
This document is an Internet-Draft (I-D).
Anyone may submit an I-D to the IETF.
This I-D is not endorsed by the IETF and has no formal standing in the
IETF standards process.
| Document | Type | Active Internet-Draft (individual) | |
|---|---|---|---|
| Authors | Luis M. Contreras , Fernando Agraz , Salvatore Spadaro , Quan Xiong | ||
| Last updated | 2025-10-20 | ||
| 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-06
PCE L. M. Contreras
Internet-Draft Telefonica
Intended status: Informational F. Agraz
Expires: 23 April 2026 S. Spadaro
Universitat Politecnica de Catalunya
Q. Xiong
ZTE Corporation
20 October 2025
Path Computation Based on Precision Availability Metrics
draft-contreras-pce-pam-06
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 23 April 2026.
Copyright Notice
Copyright (c) 2025 IETF Trust and the persons identified as the
document authors. All rights reserved.
Contreras, et al. Expires 23 April 2026 [Page 1]
Internet-Draft PAM-based PCE October 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 . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 3
3. Rationale of the usage of PAM for path calculation . . . . . 4
3.1. Dynamic behavior of performance parameters . . . . . . . 4
3.2. Applicability . . . . . . . . . . . . . . . . . . . . . . 4
3.3. Usage of collected metrics . . . . . . . . . . . . . . . 5
3.4. Calculation or selection of the path . . . . . . . . . . 6
4. PRECISION METRIC Object . . . . . . . . . . . . . . . . . . . 6
4.1. Motivation for a new object to express precision
metrics . . . . . . . . . . . . . . . . . . . . . . . . . 7
4.2. Definition of the PRECISION METRIC Object . . . . . . . . 7
4.3. Summary of the PRECISION METRIC Object . . . . . . . . . 11
4.4. Examples on the usage of the PRECISION METRIC Object. . . 13
4.4.1. PRECISION METRIC coding examples . . . . . . . . . . 13
4.4.2. PRECISION METRIC operation examples . . . . . . . . . 15
4.5. Interaction between precision metric object and the metric
object . . . . . . . . . . . . . . . . . . . . . . . . . 16
5. PCEP message extensions . . . . . . . . . . . . . . . . . . . 16
5.1. The PCReq Message . . . . . . . . . . . . . . . . . . . . 16
5.2. The PCRep Message . . . . . . . . . . . . . . . . . . . . 17
5.3. The PCRpt Message . . . . . . . . . . . . . . . . . . . . 18
6. Security and operational considerations . . . . . . . . . . . 19
6.1. Security considerations . . . . . . . . . . . . . . . . . 19
6.2. Operational considerations . . . . . . . . . . . . . . . 19
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 19
8. References . . . . . . . . . . . . . . . . . . . . . . . . . 20
8.1. Normative References . . . . . . . . . . . . . . . . . . 20
8.2. Informative References . . . . . . . . . . . . . . . . . 20
Appendix A. Path Histogram Composition . . . . . . . . . . . . . 21
A.1. Additive Metrics . . . . . . . . . . . . . . . . . . . . 22
A.2. Multiplicative Metrics . . . . . . . . . . . . . . . . . 22
A.3. Maximization / Minimization Metrics (Bottleneck
Metrics) . . . . . . . . . . . . . . . . . . . . . . . . 22
Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . 22
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 23
Contreras, et al. Expires 23 April 2026 [Page 2]
Internet-Draft PAM-based PCE October 2025
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)
[RFC9544]. That is the case of services like Network Slice [RFC9543]
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.
Contreras, et al. Expires 23 April 2026 [Page 3]
Internet-Draft PAM-based PCE October 2025
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
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.
Contreras, et al. Expires 23 April 2026 [Page 4]
Internet-Draft PAM-based PCE October 2025
* 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].
3.3. Usage of collected metrics
The Traffic Engineering Database (TED) defined in [RFC4655] could be
considered as the component providing the precision metrics of
interest.
The TED stores information related to the network topology, including
nodes, links, link attributes (e.g., bandwidth, delay), and any
constraints relevant for traffic engineering. It is dynamically
updated with information received via routing protocols (e.g., OSPF-
TE, IS-IS-TE), ensuring the PCE has up-to-date knowledge. It is also
possible to define policies like administrative group (coloring), to
be used used in constraint-based path computation.
In order to support precision metrics, the TED could be extended to
support e.g. time-series storage and processing capabilities (e.g.,
to derive histograms from them, as described for instance in
Appendix A). The metrics could be gathered from in-band telemetry,
active probing mechanisms, or streaming telemetry via standardized
interfaces, as complementary information sources to the information
received from routing protocols.
Assuming that capability, the PCE queries the TED for compliance with
precision constraints.
Contreras, et al. Expires 23 April 2026 [Page 5]
Internet-Draft PAM-based PCE October 2025
- Topology
Path computation request - Single value metrics
based on PAM metrics - Precision metrics
+-------------+ +-------------+ +-------------+
| Path | | Path | | Traffic |
| Computation |<------->| Computation |<------->| Engineering |
| Client | | Element | | Database |
+-------------+ +-------------+ +-------------+
^
|
v
+-------------+
| Data |
| Sources |
+-------------+
- Link state info
- Active Probes
- Streaming telemetry
- In-band OAM
- etc
Figure 1: Usage of precision metrics stored in TED
The implementation of the TED and its support to the collection,
processing and generation of the precision metrics is out of scope of
this document.
3.4. Calculation or selection of the path
For a given metric, i.e. metric X, it is defined a frequency of
values per bin for such a metric (e.g., if the metric refers to
latency, a way of expressing it could be to consider the latency
below 20 ms the 90% of the time, and below 25 ms the 99% of the
time). Thus, the calculation or selection of a path for such a
metric X will consist on the comparison of the frequency of the
metric values per bin, so that the intended path behaves equal or
better than the intended path. For that purpose, the statistical
behavior of the path is characterized e.g. as described in
Appendix A.
4. PRECISION METRIC Object
Contreras, et al. Expires 23 April 2026 [Page 6]
Internet-Draft PAM-based PCE October 2025
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 precision 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. Then 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 existing metric in
the METRIC object, and can be implemented simultaneously or
separately.
The former limitations make preferable 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 23 April 2026 [Page 7]
Internet-Draft PAM-based PCE October 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 ~
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 2: PRECISION METRIC object
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 23 April 2026 [Page 8]
Internet-Draft PAM-based PCE October 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 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 23 April 2026 [Page 9]
Internet-Draft PAM-based PCE October 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 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 3: Structure of Thresholds field for S=0
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 23 April 2026 [Page 10]
Internet-Draft PAM-based PCE October 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 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 4: Structure of Thresholds field for S=1
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 23 April 2026 [Page 11]
Internet-Draft PAM-based PCE October 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 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 5: Complete structure of PRECISION METRIC object for S=0
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 23 April 2026 [Page 12]
Internet-Draft PAM-based PCE October 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 | 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 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 6: Complete structure of PRECISION METRIC object for S=1
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 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 described as:
Contreras, et al. Expires 23 April 2026 [Page 13]
Internet-Draft PAM-based PCE October 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 = 12 | n/a | 0x2 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| AvPeriod= 24 | TI_Units= sec | TI_Value= 3600 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Violated Interval Ratio= 5 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Severely Violated Interval Ratio= 0.2 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Optimal Threshold Tier Boundary= 99.9 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Optimal Threshold= 20 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Critical Threshold= 25 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 7: Example of usage of PRECISION METRIC object with a SLO
defined by two tiers
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 23 April 2026 [Page 14]
Internet-Draft PAM-based PCE October 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 | ST= Histogram | 0x3 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| AvPeriod= 24 | TI_Units= sec | TI_Value= 3600 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Violated Interval Ratio= 5 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Severely Violated Interval Ratio= 0.2 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Tier 1 Boundary= 99.9 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Threshold for Tier 1= 20 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Tier 2 Boundary= 99.999 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Threshold for Tier 1= 25 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Critical Threshold= 30 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 8: Example of usage of PRECISION METRIC object with a SLO
defined by a statistical distribution
Once the PCE processes these PRECISION METRIC 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
Contreras, et al. Expires 23 April 2026 [Page 15]
Internet-Draft PAM-based PCE October 2025
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.
4.5. Interaction between precision metric object and the metric object
The precision metric type values in the PRECISION METRIC object are
intended to reuse the valid metric type values which are allocated by
IANA for the original METRIC object T field. The precision metric
object and metric object may be both carried as the constraints for
path computing, but they must be set to different type values for
valid path requests.
It could be the case that a path request could contain both a METRIC
and a PRECISION METRIC objects for the same type of metric, for
instance, delay. This behavior can be considered anomalous and, as
such, the PCE will send back a PCErr message , for example, a Error-
Type "Invalid Operation" and Error-Value "Unsupported Metric Object
and PAM metric Object with same type".
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:
Contreras, et al. Expires 23 April 2026 [Page 16]
Internet-Draft PAM-based PCE October 2025
<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>]
Figure 9: PCReq Message
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 23 April 2026 [Page 17]
Internet-Draft PAM-based PCE October 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>]
Figure 10: PCRep Message
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 23 April 2026 [Page 18]
Internet-Draft PAM-based PCE October 2025
6. Security and operational considerations
6.1. Security considerations
Same security and operational considerations as described in
[RFC5440] apply also in this document.
When a path request could contain both a METRIC and a PRECISION
METRIC objects for the same type of metric, the PCE will send back a
PCErr message. In order to avoid denial of service attacks, new
similar requests could be silently ignored during periods or time, or
even requests from the same PCC could be filtered to prevent PCE
affection.
Other security considerations will be addressed in future versions of
the document.
6.2. Operational considerations
The work with precision metrics can impose stringent requirements in
terms of collection, processing and assessment of metrics of
interest. Such capabilities are expected to be supported by external
systems, such as the TED, with the role of the PCE being limited to
the work with processed information (e.g., histograms) so to assess
that the precision metric used as constraint is compliant with the
expectation of the PCC. Such external supportive systems are out of
scope of this 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
Furthermore, the following Error-Type and Error-value of the PCEP
Error Object are specified
Value Description Reference
------ ------------------------------- -------------
TBD2 Error-Type "Invalid Operation" This document
TBD3 Error Value "Unsupported Metric This document
Object and PAM metric Object with
same type"
Contreras, et al. Expires 23 April 2026 [Page 19]
Internet-Draft PAM-based PCE October 2025
Additional IANA considerations required by this extension will be
documented in future document versions (for instance, in respect to
precision metric types).
8. References
8.1. 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>.
[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>.
8.2. 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-06, 12 October 2025,
<https://datatracker.ietf.org/doc/html/draft-ietf-nmop-
network-incident-yang-06>.
[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>.
Contreras, et al. Expires 23 April 2026 [Page 20]
Internet-Draft PAM-based PCE October 2025
[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>.
[RFC9543] Farrel, A., Ed., Drake, J., Ed., Rokui, R., Homma, S.,
Makhijani, K., Contreras, L., and J. Tantsura, "A
Framework for Network Slices in Networks Built from IETF
Technologies", RFC 9543, DOI 10.17487/RFC9543, March 2024,
<https://www.rfc-editor.org/info/rfc9543>.
[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>.
Appendix A. Path Histogram Composition
In order to obtain the statistical distribution of a metric over a
complete path from the corresponding distributions of its constituent
segments (e.g., hops) it is necessary to consider the class of the
metric under evaluation, i.e., if the metric is additive,
multiplicative, or maximal/minimal.
Contreras, et al. Expires 23 April 2026 [Page 21]
Internet-Draft PAM-based PCE October 2025
A.1. Additive Metrics
Additive metrics are those that sum along the path, such as delay or
IGP cost [RFC4655], [RFC8233]. To generate a path histogram from
segment histograms, the total path value can be obtained by summing
the individual segment values along a period, and then forming the
histogram.
Alternatively, considering that a histogram is divided into discrete
bins representing value ranges, it is possible to perform a bin-by-
bin summation. The histogram for the path is then obtained by
summing the bin values across the segments.
A.2. Multiplicative Metrics
Multiplicative metrics, for example link availability or success
probability [RFC8233], combine along a path by multiplying segment
(e.g., per hop) values. The path histogram can be obtained by
combining the segment values and computing the product for each
combination.
Alternatively, logarithmic transformation can be applied to convert
multiplicative aggregation into additive form, enabling reuse of
additive histogram composition techniques. In this method, the
values of each histogram bin are transformed by taking the logarithm,
effectively converting multiplication into addition. The histograms
can then be combined by summing the log-transformed bin values across
segments, using the values of each bin per segment to calculate the
resulting distribution. After aggregating the histograms in the log
domain, the path histogram can be transformed back to the original
metric domain by applying the exponential function, yielding the
final probabilities for the multiplicative path values.
A.3. Maximization / Minimization Metrics (Bottleneck Metrics)
Bottleneck metrics are defined by taking the maximum or minimum value
along the path, such as bandwidth, MTU, etc [RFC4655]. To construct
a path histogram, the values of each segment are considered to build
the cumulative distribution function (CDF) of the path.
Acknowledgements
The authors thank Dhruv Dhody, Ruediger Geib and Greg Mirsky for the
comments received that helped to improve the document.
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
Contreras, et al. Expires 23 April 2026 [Page 22]
Internet-Draft PAM-based PCE October 2025
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
Quan Xiong
ZTE Corporation
China
Email: xiong.quan@zte.com.cn
Contreras, et al. Expires 23 April 2026 [Page 23]