ALTO Performance Cost Metrics
draft-ietf-alto-performance-metrics-15

Document Type Active Internet-Draft (alto WG)
Authors Qin Wu  , Y. Yang  , Young Lee  , Dhruv Dhody  , Sabine Randriamasy  , Luis Contreras 
Last updated 2021-03-29 (latest revision 2021-02-04)
Replaces draft-wu-alto-te-metrics
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Stream WG state Submitted to IESG for Publication (wg milestone: Jul 2020 - Submit cost property... )
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Y. Yang  for 81 days
Dhruv Dhody  for 81 days
Sabine Randriamasy  for 81 days
Qin Wu  for 81 days
Luis Contreras  for 81 days
Young Lee  for 81 days
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ALTO Working Group                                                 Q. Wu
Internet-Draft                                                    Huawei
Intended status: Standards Track                                 Y. Yang
Expires: August 7, 2021                                  Yale University
                                                                  Y. Lee
                                                                 Samsung
                                                                D. Dhody
                                                                  Huawei
                                                          S. Randriamasy
                                                         Nokia Bell Labs
                                                            L. Contreras
                                                              Telefonica
                                                        February 3, 2021

                     ALTO Performance Cost Metrics
                 draft-ietf-alto-performance-metrics-15

Abstract

   Cost metric is a basic concept in Application-Layer Traffic
   Optimization (ALTO), and different applications may use different
   cost metrics.  Since the ALTO base protocol (RFC 7285) defines only a
   single cost metric (i.e., the generic "routingcost" metric), if an
   application wants to issue a cost map or an endpoint cost request to
   determine the resource provider that offers better delay performance,
   the base protocol does not define the cost metric to be used.

   This document addresses the issue by introducing network performance
   metrics, including network delay, jitter, packet loss rate, hop
   count, and bandwidth.

   There are multiple sources (e.g., estimation based on measurements or
   service-level agreement) to derive a performance metric.  This
   document introduces an additional "cost-context" field to the ALTO
   "cost-type" field to convey the source of a performance metric.

Requirements Language

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
   "OPTIONAL" in this document are to be interpreted as described in BCP
   14 [RFC2119][RFC8174] when, and only when, they appear in all
   capitals, as shown here.

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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
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   Internet-Drafts are draft documents valid for a maximum of six months
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   This Internet-Draft will expire on August 7, 2021.

Copyright Notice

   Copyright (c) 2021 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (http://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
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   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   4
   2.  Performance Metric Attributes . . . . . . . . . . . . . . . .   5
     2.1.  Performance Metric Context: cost-context  . . . . . . . .   6
     2.2.  Performance Metric Statistics . . . . . . . . . . . . . .   8
   3.  Packet Performance Metrics  . . . . . . . . . . . . . . . . .   9
     3.1.  Cost Metric: One-Way Delay (delay-ow) . . . . . . . . . .  10
       3.1.1.  Base Identifier . . . . . . . . . . . . . . . . . . .  10
       3.1.2.  Value Representation  . . . . . . . . . . . . . . . .  10
       3.1.3.  Intended Semantics and Use  . . . . . . . . . . . . .  10
       3.1.4.  Cost-Context Specification Considerations . . . . . .  12
     3.2.  Cost Metric: Round-trip Delay (delay-rt)  . . . . . . . .  12
       3.2.1.  Base Identifier . . . . . . . . . . . . . . . . . . .  12
       3.2.2.  Value Representation  . . . . . . . . . . . . . . . .  12
       3.2.3.  Intended Semantics and Use  . . . . . . . . . . . . .  12

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       3.2.4.  Cost-Context Specification Considerations . . . . . .  14
     3.3.  Cost Metric: Delay Variation (delay-variation)  . . . . .  14
       3.3.1.  Base Identifier . . . . . . . . . . . . . . . . . . .  14
       3.3.2.  Value Representation  . . . . . . . . . . . . . . . .  14
       3.3.3.  Intended Semantics and Use  . . . . . . . . . . . . .  14
       3.3.4.  Cost-Context Specification Considerations . . . . . .  16
     3.4.  Cost Metric: Hop Count (hopcount) . . . . . . . . . . . .  16
       3.4.1.  Base Identifier . . . . . . . . . . . . . . . . . . .  16
       3.4.2.  Value Representation  . . . . . . . . . . . . . . . .  16
       3.4.3.  Intended Semantics and Use  . . . . . . . . . . . . .  16
       3.4.4.  Cost-Context Specification Considerations . . . . . .  18
     3.5.  Cost Metric: Loss Rate (lossrate) . . . . . . . . . . . .  18
       3.5.1.  Base Identifier . . . . . . . . . . . . . . . . . . .  18
       3.5.2.  Value Representation  . . . . . . . . . . . . . . . .  18
       3.5.3.  Intended Semantics and Use  . . . . . . . . . . . . .  18
       3.5.4.  Cost-Context Specification Considerations . . . . . .  19
   4.  Bandwidth Performance Metrics . . . . . . . . . . . . . . . .  20
     4.1.  Cost Metric: TCP Throughput (tput)  . . . . . . . . . . .  20
       4.1.1.  Base Identifier . . . . . . . . . . . . . . . . . . .  20
       4.1.2.  Value Representation  . . . . . . . . . . . . . . . .  20
       4.1.3.  Intended Semantics and Use  . . . . . . . . . . . . .  20
       4.1.4.  Cost-Context Specification Considerations . . . . . .  21
     4.2.  Cost Metric: Residual Bandwidth (bw-residual) . . . . . .  22
       4.2.1.  Base Identifier . . . . . . . . . . . . . . . . . . .  22
       4.2.2.  Value Representation  . . . . . . . . . . . . . . . .  22
       4.2.3.  Intended Semantics and Use  . . . . . . . . . . . . .  22
       4.2.4.  Cost-Context Specification Considerations . . . . . .  23
     4.3.  Cost Metric: Maximum Reservable Bandwidth (bw-maxres) . .  24
       4.3.1.  Base Identifier . . . . . . . . . . . . . . . . . . .  24
       4.3.2.  Value Representation  . . . . . . . . . . . . . . . .  24
       4.3.3.  Intended Semantics and Use  . . . . . . . . . . . . .  24
       4.3.4.  Cost-Context Specification Considerations . . . . . .  25
   5.  Operational Considerations  . . . . . . . . . . . . . . . . .  26
     5.1.  Source Considerations . . . . . . . . . . . . . . . . . .  26
     5.2.  Metric Timestamp Consideration  . . . . . . . . . . . . .  27
     5.3.  Backward Compatibility Considerations . . . . . . . . . .  27
     5.4.  Computation Considerations  . . . . . . . . . . . . . . .  27
       5.4.1.  Configuration Parameters Considerations . . . . . . .  27
       5.4.2.  Availability Considerations . . . . . . . . . . . . .  28
   6.  Security Considerations . . . . . . . . . . . . . . . . . . .  28
   7.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  28
   8.  Acknowledgments . . . . . . . . . . . . . . . . . . . . . . .  29
   9.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  29
     9.1.  Normative References  . . . . . . . . . . . . . . . . . .  29
     9.2.  Informative References  . . . . . . . . . . . . . . . . .  30
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  32

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1.  Introduction

   Cost Metric is a basic concept in Application-Layer Traffic
   Optimization (ALTO).  It is used in both the ALTO cost map service
   and the ALTO endpoint cost service in the ALTO base protocol
   [RFC7285].

   Since different applications may use different cost metrics, the ALTO
   base protocol introduces an ALTO Cost Metric Registry (Section 14.2
   of [RFC7285]), as a systematic mechanism to allow different metrics
   to be specified.  For example, a delay-sensitive application may want
   to use latency related metrics, and a bandwidth-sensitive application
   may want to use bandwidth related metrics.  However, the ALTO base
   protocol has registered only a single cost metric, i.e., the generic
   "routingcost" metric (see Sec. 14.2 of [RFC7285]); no latency or
   bandwidth related metrics are defined.

   This document registers a set of new cost metrics specified in
   Table 1, to allow applications to determine "where" to connect based
   on network performance criteria such as delay and bandwidth related
   metrics.  This document follows the guideline defined in Section 14.2
   of the ALTO base protocol [RFC7285]) on registering ALTO cost
   metrics.  Hence it specifies the identifier, the intended semantics,
   and the security considerations of each one of the metrics defined in
   Table 1.

   +--------------------------+-------------+-------------------+
   | Metric                   | Definition  |  Origin Example   |
   +--------------------------+-------------+-------------------+
   | One-way Delay            | Section 3.1 | [RFC7679]         |
   | Round-trip Delay         | Section 3.2 | [RFC2681]         |
   | Delay Variation          | Section 3.3 | [RFC3393]         |
   | Hop Count                | Section 3.4 | [RFC7285]         |
   | Loss Rate                | Section 3.5 | [RFC7680]         |
   |                          |             |                   |
   | TCP Throughput           | Section 4.1 | [RFC6349]         |
   | Residual Bandwidth        | Section 4.2 | [RFC8570]         |
   | Max Reservable Bandwidth | Section 4.3 | [RFC5305]         |
   +------------+-----------------------------------------------+
      Table 1. Cost Metrics Defined in this Document.

   The purpose of this document is to ensure proper usage of the
   performance metrics defined in Table 1; it does not claim novelty of
   the metrics.  The "Origin Example" column of Table 1 gives an example
   RFC that has defined each metric.

   The performance metrics can be classified into two categories: those
   derived from the performance of individual packets (i.e., one-way

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   delay, round-trip delay, delay variation, hop count, and loss rate),
   and those related with bandwidth (TCP throughput, residual bandwidth,
   and maximum reservable bandwidth).  These two categories are defined
   in Sections 3 and 4 respectively.  Note that all metrics except round
   trip delay in Table 1 are unidirectional; hence, a client will need
   to query both directions if needed.

   An ALTO server may provide only a subset of the metrics described in
   this document.  For example, those that are subject to privacy
   concerns should not be provided to unauthorized ALTO clients.  Hence,
   all cost metrics defined in this document are optional and not all of
   them need to be exposed to a given application.  When an ALTO server
   supports a cost metric defined in this document, it should announce
   this metric in its information resource directory (IRD).

   [RFC7285] specifies that cost values should be assumed by default as
   JSONNumber.  When defining the value representation of each metric in
   Table 1, this document conforms to this specification, but specifies
   additional, generic constraints on valid JSONNumbers for each metric.
   For example, each metric in Table 1 will be specified as non-negative
   (>= 0); Hop Count is specified to be an integer.

   An ALTO server introducing these metrics should consider security
   issues.  As a generic security consideration on the reliability and
   trust in the exposed metric values, applications SHOULD rapidly give
   up using ALTO-based guidance if they detect that the exposed
   information does not preserve their performance level or even
   degrades it.  This document discusses security considerations in more
   detail in Section 6.

   Following the ALTO base protocol, this document uses JSON to specify
   the value type of each defined metric.  See [RFC8259] for JSON data
   type specification.

2.  Performance Metric Attributes

   When defining the metrics in Table 1, this document considers the
   guidelines specified in [RFC6390], which requires fine-grained
   specification of (i) Metric Name, (ii) Metric Description, (iii)
   Method of Measurement or Calculation, (iv) Units of Measurement, (v)
   Measurement Points, and (vi) Measurement Timing.  In particular, for
   each metric, this document defines (i) Metric Name, (ii) Metric
   Description, and (iv) Units of Measurement.  The Measurement Points
   are always specified by the specific ALTO services; for example,
   endpoint cost service is between the two endpoints.

   On the other hand, to be able to use coarse-grained information such
   as routing system information (e.g., [RFC8571]), which may not

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   provide fine-grained information such as (iii) Method of Measurement
   or Calculation and (vi) Measurement Timing, this document provides
   context information to indicate the source of information and hence
   available metric details.

2.1.  Performance Metric Context: cost-context

   The details of a performance metric depend on the source that defines
   the metric.  Specifically, this document defines four types of
   information sources: "nominal", and "sla" (service level agreement),
   "import", and "estimation".

   For a given type of source, precise interpretation of a performance
   metric value can depend on particular measurement and computation
   parameters.  For example, see Section 3.8 of [RFC7679] on items that
   a more complete measurement-based report should include.

   To make it possible to specify the source and the aforementioned
   parameters, this document introduces an optional "cost-context" field
   to the "cost-type" field defined by the ALTO base protocol
   (Section 10.7 of [RFC7285]) as the following:

       object {
         CostMetric   cost-metric;
         CostMode     cost-mode;
         [CostContext cost-context;]
         [JSONString  description;]
       } CostType;

       object {
         JSONString    cost-source;
         [JSONValue    parameters;]
       } CostContext;

   The "cost-source" field of the "cost-context" field MUST be one of
   four category values: "nominal", "sla", "import", and "estimation".
   "cost-context" will not be used as a key to distinguish among
   performance metrics.  Hence, an ALTO information resource MUST NOT
   announce multiple CostType with the same "cost-metric" and "cost-
   mode".  They must be placed into different information resources.

   The "nominal" category indicates that the metric value is statically
   configured by the underlying devices.  Not all metrics have
   reasonable "nominal" values.  For example, throughput can have a

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   nominal value, which indicates the configured transmission rate of
   the devices; latency typically does not have a nominal value.

   The "sla" category indicates that the metric value is derived from
   some commitment which this document refers to as service-level
   agreement (SLA).  Some operators also use terms such as "target" or
   "committed" values.  For an "sla" metric, it is RECOMMENDED that the
   "parameters" field provides a link to the SLA definition.

   The "estimation" category indicates that the metric value is computed
   through an estimation process.  An ALTO server may compute
   "estimation" values by retrieving and/or aggregating information from
   routing protocols (e.g., [RFC8571]) and traffic measurement
   management tools (e.g., TWAMP [RFC5357]), with corresponding
   operational issues.  A potential architecture on estimating these
   metrics is shown in Figure 1 below.  Section 5 will discuss in more
   detail the operational issues and how a network may address them.

     +--------+   +--------+  +--------+
     | Client |   | Client |  | Client |
     +----^---+   +---^----+  +---^----+
          |           |           |
          +-----------|-----------+
                NBI   |ALTO protocol
                      |
                      |
                   +--+-----+  retrieval      +-----------+
                   |  ALTO  |<----------------| Routing   |
                   | Server |  and aggregation|           |
                   |        |<-------------+  | Protocols |
                   +--------+              |  +----------+
                                           |
                                           |  +-----------+
                                           |  |Management |
                                           ---|           |
                                              |  Tool     |
                                              +-----------+
   Figure 1. A framework to compute estimation to performance metrics

   A particular type of "estimation" is direct "import", which indicates
   that the metric value is imported directly from a specific existing
   protocol or system.  Specifying "import" as the source instead of the
   more generic "estimation" may allow better tracking of information
   flow.  For an "import" metric, it is RECOMMENDED that the
   "parameters" field provides details to the system from which raw data
   is imported.  In particular, one may notice that the set of end-to-
   end metrics defined in Table 1 has a large overlap with the set
   defined in [RFC8571], in the setting of IGP traffic engineering

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   performance metrics for each link (i.e., unidirectional link delay,
   min/max unidirectional link delay, unidirectional delay variation,
   unidirectional link loss, unidirectional residual bandwidth,
   unidirectional available bandwidth, unidirectional utilized
   bandwidth).  Hence, an ALTO server may use "import" to indicate that
   its end-to-end metrics are computed from link metrics imported from
   [RFC8571].

   There can be multiple choices in deciding the cost-source category.
   It is the operator of an ALTO server who chooses the category.  If a
   metric does not include a "cost-source" value, the application MUST
   assume that the value of "cost-source" is the most generic
   "estimation".

2.2.  Performance Metric Statistics

   The measurement of a performance metric often yields a set of samples
   from an observation distribution ([Prometheus]), instead of a single
   value.  This document considers that the samples are aggregated as a
   statistic when reported.  Hence, each performance metric's identifier
   should indicate the statistic (i.e., an aggregation operation), to
   become

     <metric-identifier> ::= <metric-base-identifier> [ '-' <stat> ]

   where <stat> MUST be one of the following:

   percentile, with letter 'p' followed by a number:

      gives the p percentile.  Specifically, consider the samples coming
      from a random variable X.  The metric returns x, relative to 100,
      such that the probability of X is less than or equal to x, i.e.,
      Prob(X <= x) = p/100.  The number p MUST be a non-negative JSON
      number in the range [0, 100] (i.e., greater than or equal to 0 and
      less than or equal to 100).  To avoid complex identifiers, the
      number MUST NOT include the minus or the exp component (Section 6
      of [RFC8259]).  For example, delay-ow-p75 gives the 75% percentile
      of observed one-way delay; delay-ow-p99.9 gives the 99.9%
      percentile of delay.  Note that some systems use quantile, which
      is in the range [0, 1].  This document uses percentile to make the
      identifier easier to read.

   min:

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      the minimal value of the observations.

   max:

      the maximal value of the observations.

   median:

      the mid point (i.e., p50) of the observations.

   mean:

      the arithmetic mean value of the observations.

   stddev:

      the standard deviation of the observations.

   stdvar:

      the standard variance of the observations.

   If a metric has no <stat> (i.e., <metric-identifier> does not include
   '-' <stat>), the metric MUST be considered as the 50 percentile
   (median).

3.  Packet Performance Metrics

   This section introduces ALTO network performance metrics on one way
   delay, round trip delay, delay variation, hop count, and packet loss
   rate.  They measure the "quality of experience" of the stream of
   packets sent from a resource provider to a resource consumer.  The
   measures of each individual packet (pkt) can include the delay from
   the time when the packet enters the network to the time when the
   packet leaves the network (pkt.delay); the number of network hops
   that the packet traverses (pkt.hopcount); and whether the packet is

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   dropped before reaching the destination (pkt.dropped).  The semantics
   of the performance metrics defined in this section is that they are
   statistics (percentiles) computed from these measures; for example,
   the x-percentile of the one-way delay is the x-percentile of the set
   of delays {pkt.delay} for the packets in the stream.

3.1.  Cost Metric: One-Way Delay (delay-ow)

3.1.1.  Base Identifier

   The base identifier for this performance metric is "delay-ow".

3.1.2.  Value Representation

   The metric value type is a single 'JSONNumber' type value conforming
   to the number specification of [RFC8259] Section 6.  The unit is
   expressed in milliseconds.  Hence, the number can be a floating point
   number to express delay that is smaller than milliseconds.  The
   number MUST be non-negative.

3.1.3.  Intended Semantics and Use

   Intended Semantics: To specify the spatial and temporal aggregated
   delay of a stream of packets from the specified source and the
   specified destination.  The spatial aggregation level is specified in
   the query context (e.g., PID to PID, or endpoint to endpoint).

   Use: This metric could be used as a cost metric constraint attribute
   or as a returned cost metric in the response.

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   Example 1: Delay value on source-destination endpoint pairs

   POST /endpointcost/lookup HTTP/1.1
   Host: alto.example.com
   Content-Length: TBA
   Content-Type: application/alto-endpointcostparams+json
   Accept:
     application/alto-endpointcost+json,application/alto-error+json

   {
     "cost-type": {"cost-mode" : "numerical",
                   "cost-metric" : "delay-ow"},
     "endpoints" : {
       "srcs": [ "ipv4:192.0.2.2" ],
       "dsts": [
         "ipv4:192.0.2.89",
         "ipv4:198.51.100.34",
         "ipv6:2001:db8::1234:5678"
       ]
     }
   }

   HTTP/1.1 200 OK
   Content-Length: TBA
   Content-Type: application/alto-endpointcost+json
   {
     "meta" :{
       "cost-type": {"cost-mode" : "numerical",
                     "cost-metric" : "delay-ow"
        }
      },
       "endpoint-cost-map" : {
         "ipv4:192.0.2.2": {
           "ipv4:192.0.2.89"    : 10,
           "ipv4:198.51.100.34" : 20,
           "ipv6:2001:db8::1234:5678"  : 30,
       }
     }
   }

   Comment: Since the "cost-type" does not include the "cost-source"
   field, the values are based on "estimation".  Since the identifier
   does not include the -<percentile> component, the values will
   represent median values.

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3.1.4.  Cost-Context Specification Considerations

   "nominal": Typically network one-way delay does not have a nominal
   value.

   "sla": Many networks provide delay in their application-level service
   level agreements.  It is RECOMMENDED that the "parameters" field of
   an "sla" one-way delay metric provides a link (i.e., a field named
   "link") to the SLA definition, if available.

   "import": There can be multiple sources to import one-way delay.  For
   example, if the import is from [RFC8571] (by using unidirectional
   link delay, min/max unidirectional link delay), it is RECOMMENDED
   that "parameters" provides "protocol" as a field and "RFC8571" as the
   value.  During import, the server should be cognizant of potential
   issues when computing an end-to-end summary statistic from link
   statistics.  Another example of an import source is the IPPM
   framework.  For IPPM, it is recommended that "parameters" provides
   "protocol" as a field and "ippm" as the value; see Section 4 of [I-
   D.ietf-ippm-initial-registry] for additional fields which can be
   specified for "ippm" in "parameters".

   "estimation": The exact estimation method is out of the scope of this
   document.  It is RECOMMENDED that the "parameters" field of an
   "estimation" one-way delay metric provides a link ("link") to a
   description of the "estimation" method.

3.2.  Cost Metric: Round-trip Delay (delay-rt)

3.2.1.  Base Identifier

   The base identifier for this performance metric is "delay-rt".

3.2.2.  Value Representation

   The metric value type is a single 'JSONNumber' type value conforming
   to the number specification of [RFC8259] Section 6.  The number MUST
   be non-negative.  The unit is expressed in milliseconds.

3.2.3.  Intended Semantics and Use

   Intended Semantics: To specify spatial and temporal aggregated round-
   trip delay between the specified source and specified destination.
   The spatial aggregation level is specified in the query context
   (e.g., PID to PID, or endpoint to endpoint).

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   Note that it is possible for a client to query two one-way delays and
   then compute the round-trip delay.  The server should be cognizant of
   the consistency of values.

   Use: This metric could be used either as a cost metric constraint
   attribute or as a returned cost metric in the response.

   Example 2: Round-trip Delay of source-destination endpoint pairs

   POST /endpointcost/lookup HTTP/1.1
   Host: alto.example.com
   Content-Length: TBA
   Content-Type: application/alto-endpointcostparams+json
   Accept:
     application/alto-endpointcost+json,application/alto-error+json

   {
    "cost-type": {"cost-mode" : "numerical",
                  "cost-metric" : "delay-rt"},
     "endpoints" : {
        "srcs": [ "ipv4:192.0.2.2" ],
        "dsts": [
          "ipv4:192.0.2.89",
          "ipv4:198.51.100.34",
          "ipv6:2001:db8::1234:5678"
        ]
      }
   }

    HTTP/1.1 200 OK
    Content-Length: TBA
    Content-Type: application/alto-endpointcost+json
    {
      "meta" :{
        "cost-type": {"cost-mode" : "numerical",
                      "cost-metric" : "delay-rt"
         }
       },
        "endpoint-cost-map" : {
          "ipv4:192.0.2.2": {
            "ipv4:192.0.2.89"    : 4,
            "ipv4:198.51.100.34" : 3,
            "ipv6:2001:db8::1234:5678" : 2,
        }
      }
    }

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3.2.4.  Cost-Context Specification Considerations

   "nominal": Typically network round-trip delay does not have a nominal
   value.

   "sla": It is RECOMMENDED that the "parameters" field of an "sla"
   round-trip delay metric provides a link ("link") to the SLA
   definition.

   "import": There can be multiple sources to import round-trip delay.
   If the import is from [RFC8571] (by using unidirectional link delay,
   min/max unidirectional link delay), it is RECOMMENDED that
   "parameters" provides "protocol" as a field and "RFC8571" as the
   value; see Section 3.1.4 for discussions on summing up link metrics
   to obtain end-to-end metrics.  If the import is from the IPPM
   framework, it is recommended that "parameters" provides "protocol" as
   a field and "ippm" as the value; see Section 4 of [I-D.ietf-ippm-
   initial-registry] for additional fields which can be specified for
   "ippm" in "parameters".

   "estimation": The exact estimation method is out of the scope of this
   document.  It is RECOMMENDED that the "parameters" field of an
   "estimation" round-trip delay metric provides a link ("link") to a
   description of the "estimation" method.

3.3.  Cost Metric: Delay Variation (delay-variation)

3.3.1.  Base Identifier

   The base identifier for this performance metric is "delay-variation".

3.3.2.  Value Representation

   The metric value type is a single 'JSONNumber' type value conforming
   to the number specification of [RFC8259] Section 6.  The number MUST
   be non-negative.  The unit is expressed in milliseconds.

3.3.3.  Intended Semantics and Use

   Intended Semantics: To specify spatial and temporal aggregated delay
   variation (also called delay jitter)) with respect to the minimum
   delay observed on the stream over the specified source and
   destination.  The spatial aggregation level is specified in the query
   context (e.g., PID to PID, or endpoint to endpoint).

   Note that in statistics, variations are typically evaluated by the
   distance from samples relative to the mean.  In networking context,

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   it is more commonly defined from samples relative to the min.  This
   definition follows the networking convention.

   Use: This metric could be used either as a cost metric constraint
   attribute or as a returned cost metric in the response.

   Example 3: Delay variation value on source-destination endpoint pairs

   POST /endpointcost/lookup HTTP/1.1
   Host: alto.example.com
   Content-Length: TBA
   Content-Type: application/alto-endpointcostparams+json
   Accept:
      application/alto-endpointcost+json,application/alto-error+json

   {
     "cost-type": {"cost-mode" : "numerical",
      "cost-metric" : "delay-variation"},
     "endpoints" : {
       "srcs": [ "ipv4:192.0.2.2" ],
       "dsts": [
         "ipv4:192.0.2.89",
         "ipv4:198.51.100.34",
         "ipv6:2001:db8::1234:5678"
       ]
     }
   }
   HTTP/1.1 200 OK
    Content-Length: TBA
    Content-Type: application/alto-endpointcost+json
   {
     "meta": {
              "cost type": {
              "cost-mode": "numerical",
              "cost-metric":"delay-variation"
       }
      },
     "endpoint-cost-map": {
              "ipv4:192.0.2.2": {
              "ipv4:192.0.2.89"    : 0
              "ipv4:198.51.100.34" : 1
              "ipv6:2001:db8::1234:5678" : 5
            }
         }
      }

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3.3.4.  Cost-Context Specification Considerations

   "nominal": Typically network delay variation does not have a nominal
   value.

   "sla": It is RECOMMENDED that the "parameters" field of an "sla"
   delay variation metric provides a link ("link") to the SLA
   definition.

   "import": There can be multiple sources to import delay variation.
   If the import is from [RFC8571] (by using unidirectional delay
   variation), it is RECOMMENDED that "parameters" provides "protocol"
   as a field and "RFC8571" as the value; see Section 3.1.4 for
   discussions on summing up link metrics to obtain end-to-end metrics.
   If the import is from the IPPM framework, it is recommended that
   "parameters" provides "protocol" as a field and "ippm" as the value;
   see Section 4 of [I-D.ietf-ippm-initial-registry] for additional
   fields which can be specified for "ippm" in "parameters".

   "estimation": The exact estimation method is out of the scope of this
   document.  It is RECOMMENDED that the "parameters" field of an
   "estimation" delay variation metric provides a link ("link") to a
   description of the "estimation" method.

3.4.  Cost Metric: Hop Count (hopcount)

   The metric hopcount is mentioned in [RFC7285] Section 9.2.3 as an
   example.  This section further clarifies its properties.

3.4.1.  Base Identifier

   The base identifier for this performance metric is "hopcount".

3.4.2.  Value Representation

   The metric value type is a single 'JSONNumber' type value conforming
   to the number specification of [RFC8259] Section 6.  The number MUST
   be a non-negative integer (greater than or equal to 0).  The value
   represents the number of hops.

3.4.3.  Intended Semantics and Use

   Intended Semantics: To specify the number of hops in the path from
   the specified source to the specified destination.  The hop count is
   a basic measurement of distance in a network and can be exposed as
   the number of router hops computed from the routing protocols
   originating this information.  The spatial aggregation level is

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   specified in the query context (e.g., PID to PID, or endpoint to
   endpoint).

   Use: This metric could be used as a cost metric constraint attribute
   or as a returned cost metric in the response.

   Example 4: hopcount value on source-destination endpoint pairs

   POST /endpointcost/lookup HTTP/1.1
   Host: alto.example.com
   Content-Length: TBA
   Content-Type: application/alto-endpointcostparams+json
   Accept:
     application/alto-endpointcost+json,application/alto-error+json

     {
       "cost-type": {"cost-mode" : "numerical",
        "cost-metric" : "hopcount"},
       "endpoints" : {
         "srcs": [ "ipv4:192.0.2.2" ],
         "dsts": [
           "ipv4:192.0.2.89",
           "ipv4:198.51.100.34",
           "ipv6:2001:db8::1234:5678"
         ]
       }
     }

   HTTP/1.1 200 OK
   Content-Length: TBA
   Content-Type: application/alto-endpointcost+json
   {
       "meta": {
                  "cost type": {
                "cost-mode": "numerical",
                "cost-metric":"hopcount"}
          }
       },
      "endpoint-cost-map": {
              "ipv4:192.0.2.2": {
              "ipv4:192.0.2.89"   : 5,
              "ipv4:198.51.100.34": 3,
              "ipv6:2001:db8::1234:5678" : 2,
             }
       }
    }

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3.4.4.  Cost-Context Specification Considerations

   "nominal": Typically hop count does not have a nominal value.

   "sla": Typically hop count does not have an SLA value.

   "import": There can be multiple sources to import hop count, such as
   from IGP routing protocols.

   "estimation": The exact estimation method is out of the scope of this
   document.  It is RECOMMENDED that the "parameters" field of an
   "estimation" hop count metric provides a link ("link") to a
   description of the "estimation" method.

3.5.  Cost Metric: Loss Rate (lossrate)

3.5.1.  Base Identifier

   The base identifier for this performance metric is "lossrate".

3.5.2.  Value Representation

   The metric value type is a single 'JSONNumber' type value conforming
   to the number specification of [RFC8259] Section 6.  The number MUST
   be non-negative.  The value represents the percentage of packet
   losses.

3.5.3.  Intended Semantics and Use

   Intended Semantics: To specify spatial and temporal aggregated packet
   loss rate from the specified source and the specified destination.
   The spatial aggregation level is specified in the query context
   (e.g., PID to PID, or endpoint to endpoint).

   Use: This metric could be used as a cost metric constraint attribute
   or as a returned cost metric in the response.

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   Example 5: Loss rate value on source-destination endpoint pairs

   POST /endpointcost/lookup HTTP/1.1
   Host: alto.example.com
   Content-Length: TBA
   Content-Type: application/alto-endpointcostparams+json
   Accept:
     application/alto-endpointcost+json,application/alto-error+json

     {
       "cost-type": {"cost-mode" : "numerical",
                     "cost-metric" : "lossrate"
       },
       "endpoints" : {
         "srcs": [ "ipv4:192.0.2.2" ],
         "dsts": [
           "ipv4:192.0.2.89",
           "ipv4:198.51.100.34",
           "ipv6:2001:db8::1234:5678"
         ]
       }
     }

   HTTP/1.1 200 OK
   Content-Length: TBA
   Content-Type: application/alto-endpointcost+json
   {
       "meta": {
         "cost-type": {
           "cost-mode": "numerical",
           "cost-metric":"lossrate"
         }
       },
      "endpoint-cost-map": {
         "ipv4:192.0.2.2": {
           "ipv4:192.0.2.89"   : 0,
           "ipv4:198.51.100.34": 0,
           "ipv6:2001:db8::1234:5678" : 0,
         }
       }
    }

3.5.4.  Cost-Context Specification Considerations

   "nominal": Typically packet loss rate does not have a nominal value,
   although some networks may specify zero losses.

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   "sla": It is RECOMMENDED that the "parameters" field of an "sla"
   packet loss rate provides a link ("link") to the SLA definition.

   "import": There can be multiple sources to import packet loss rate.
   If the import is from [RFC8571] (by using unidirectional link loss),
   it is RECOMMENDED that "parameters" provides "protocol" as a field
   and "RFC8571" as the value; see Section 3.1.4 for discussions on
   summing up link metrics to obtain end-to-end metrics.  If the import
   is from the IPPM framework, it is recommended that "parameters"
   provides "protocol" as a field and "ippm" as the value; see Section 4
   of [I-D.ietf-ippm-initial-registry] for additional fields which can
   be specified for "ippm" in "parameters".

   "estimation": The exact estimation method is out of the scope of this
   document.  It is RECOMMENDED that the "parameters" field of an
   "estimation" packet loss rate metric provides a link ("link") to a
   description of the "estimation" method.

4.  Bandwidth Performance Metrics

   This section introduces three bandwidth related metrics.  Given a
   specified source to a specified destination, these metrics reflect
   the volume of traffic that the network can carry from the source to
   the destination.

4.1.  Cost Metric: TCP Throughput (tput)

4.1.1.  Base Identifier

   The base identifier for this performance metric is "tput".

4.1.2.  Value Representation

   The metric value type is a single 'JSONNumber' type value conforming
   to the number specification of [RFC8259] Section 6.  The number MUST
   be non-negative.  The unit is bytes per second.

4.1.3.  Intended Semantics and Use

   Intended Semantics: To give the throughput of a TCP flow from the
   specified source to the specified destination.  The spatial
   aggregation level is specified in the query context (e.g., PID to
   PID, or endpoint to endpoint).

   Use: This metric could be used as a cost metric constraint attribute
   or as a returned cost metric in the response.

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   Example 5: TCP throughput value on source-destination endpoint pairs

   POST /endpointcost/lookup HTTP/1.1
   Host: alto.example.com
   Content-Length: TBA
   Content-Type: application/alto-endpointcostparams+json
   Accept:
     application/alto-endpointcost+json,application/alto-error+json

   {
     "cost-type": {"cost-mode" : "numerical",
                   "cost-metric" : "tput"},
     "endpoints" : {
       "srcs": [ "ipv4:192.0.2.2" ],
       "dsts": [
          "ipv4:192.0.2.89",
          "ipv4:198.51.100.34",
          "ipv6:2001:db8::1234:5678"
       ]
     }
   }

   HTTP/1.1 200 OK
   Content-Length: TBA
   Content-Type: application/alto-endpointcost+json
   {
     "meta": {
        "cost type": {
           "cost-mode": "numerical",
           "cost-metric":"tput"
       }
     }
     "endpoint-cost-map": {
       "ipv4:192.0.2.2": {
         "ipv4:192.0.2.89"   : 256000,
         "ipv4:198.51.100.34": 128000,
         "ipv6:2001:db8::1234:5678" : 428000,
     }
   }

4.1.4.  Cost-Context Specification Considerations

   "nominal": Typically TCP throughput does not have a nominal value.

   "sla": Typically TCP throughput does not have an SLA value.

   "import": Typically there is not a routing protocol through which one
   can import TCP throughput.  If the import is from the IPPM framework,

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   it is recommended that "parameters" provides "protocol" as a field
   and "ippm" as the value; see Section 4 of [I-D.ietf-ippm-initial-
   registry] for additional fields which can be specified for "ippm" in
   "parameters".

   "estimation": The exact estimation method is out of the scope of this
   document.  See [Prophet] for a method to estimate TCP throughput.  It
   is RECOMMENDED that the "parameters" field of an "estimation" TCP
   throughput metric provides a link ("link") to a description of the
   "estimation" method.

4.2.  Cost Metric: Residual Bandwidth (bw-residual)

4.2.1.  Base Identifier

   The base identifier for this performance metric is "bw-residual".

4.2.2.  Value Representation

   The metric value type is a single 'JSONNumber' type value that is
   non-negative.  The unit of measurement is bytes per second.

4.2.3.  Intended Semantics and Use

   Intended Semantics: To specify spatial and temporal residual
   bandwidth from the specified source and the specified destination.
   The value is calculated by subtracting tunnel reservations from
   Maximum Bandwidth (motivated from [RFC8570], Section 4.5).  The
   spatial aggregation unit is specified in the query context (e.g., PID
   to PID, or endpoint to endpoint).

   Use: This metric could be used either as a cost metric constraint
   attribute or as a returned cost metric in the response.

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   Example 7: bw-residual value on source-destination endpoint pairs

   POST/ endpointcost/lookup HTTP/1.1
   Host: alto.example.com
   Content-Length: TBA
   Content-Type: application/alto-endpointcostparams+json
   Accept:
     application/alto-endpointcost+json,application/alto-error+json

     {
      "cost-type": { "cost-mode":   "numerical",
                     "cost-metric": "bw-residual"},
      "endpoints":  {
        "srcs": [ "ipv4 : 192.0.2.2" ],
        "dsts": [
          "ipv4:192.0.2.89",
          "ipv4:198.51.100.34",
          "ipv6:2001:db8::1234:5678"
        ]
      }
     }

   HTTP/1.1 200 OK
   Content-Length: TBA
   Content-Type: application/alto-endpointcost+json
   {
     "meta": {
       "cost-type" {
         "cost-mode": "numerical",
         "cost-metric": "bw-residual"
       }
     },
     "endpoint-cost-map" {
       "ipv4:192.0.2.2" {
         "ipv4:192.0.2.89" :    0,
         "ipv4:198.51.100.34": 2000,
         "ipv6:2001:db8::1234:5678" : 5000,
       }
     }
   }

4.2.4.  Cost-Context Specification Considerations

   "nominal": Typically residual bandwidth does not have a nominal
   value.

   "sla": Typically residual bandwidth does not have an "sla" value.

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   "import": There can be multiple sources to import residual bandwidth.
   If the import is from [RFC8571] (by using unidirectional residual
   bandwidth), it is RECOMMENDED that "parameters" provides "protocol"
   as a field and "RFC8571" as the value.  The server should be
   cognizant of issues when computing end-to-end summary statistics from
   link statistics.  For example, the min of the end-to-end path
   residual bandwidth is the min of all links on the path.

   "estimation": The exact estimation method is out of the scope of this
   document.  It is RECOMMENDED that the "parameters" field of an
   "estimation" residual bandwidth metric provides a link ("link") to a
   description of the "estimation" method.

4.3.  Cost Metric: Maximum Reservable Bandwidth (bw-maxres)

4.3.1.  Base Identifier

   The base identifier for this performance metric is "bw-maxres".

4.3.2.  Value Representation

   The metric value type is a single 'JSONNumber' type value that is
   non-negative.  The unit of measurement is bytes per second.

4.3.3.  Intended Semantics and Use

   Intended Semantics: To specify spatial and temporal maximum
   reservable bandwidth from the specified source to the specified
   destination.  The value corresponds to the maximum bandwidth that can
   be reserved (motivated from [RFC3630] Section 2.5.7).  The spatial
   aggregation unit is specified in the query context (e.g., PID to PID,
   or endpoint to endpoint).

   Use: This metric could be used either as a cost metric constraint
   attribute or as a returned cost metric in the response.

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     Example 6: bw-maxres value on source-destination endpoint pairs

   POST/ endpointcost/lookup HTTP/1.1
   Host: alto.example.com
   Content-Length: TBA
   Content-Type: application/alto-endpointcostparams+json
   Accept:
     application/alto-endpointcost+json,application/alto-error+json

     {
       "cost-type" { "cost-mode":   "numerical",
                     "cost-metric": "bw-maxres"},
       "endpoints":  {
         "srcs": [ "ipv4 : 192.0.2.2" ],
         "dsts": [
           "ipv4:192.0.2.89",
           "ipv4:198.51.100.34",
           "ipv6:2001:db8::1234:5678"
         ]
       }
     }

   HTTP/1.1 200 OK
   Content-Length: TBA
   Content-Type: application/alto-endpointcost+json
   {
     "meta": {
       "cost-type": {
         "cost-mode":   "numerical",
         "cost-metric": "bw-maxres"
       }
     },
     "endpoint-cost-map": {
       "ipv4:192.0.2.2" {
         "ipv4:192.0.2.89" :    0,
         "ipv4:198.51.100.34": 2000,
         "ipv6:2001:db8::1234:5678" :  5000,
       }
     }
   }

4.3.4.  Cost-Context Specification Considerations

   "nominal": Typically maximum reservable bandwidth does not have a
   nominal value.

   "sla": Typically maximum reservable bandwidth does not have an "sla"
   value.

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   "import": There can be multiple sources to import maximum reservable
   bandwidth.  For example, Maximum reservable bandwidth is defined by
   IS-IS/OSPF TE, and measures the reservable bandwidth between two
   directly connected IS-IS neighbors or OSPF neighbors; see Section 3.5
   of [RFC5305].  If the import is from [RFC8571] (by using
   unidirectional maximum reservable bandwidth), it is RECOMMENDED that
   "parameters" provides "protocol" as a field and "RFC8571" as the
   value.

   "estimation": The exact estimation method is out of the scope of this
   document.  It is RECOMMENDED that the "parameters" field of an
   "estimation" maximum reservable bandwidth metric provides a link
   ("link") to a description of the "estimation" method.

5.  Operational Considerations

   The exact measurement infrastructure, measurement condition, and
   computation algorithms can vary from different networks, and are
   outside the scope of this document.  Both the ALTO server and the
   ALTO clients, however, need to be cognizant of the operational issues
   discussed below.

   Also, the performance metrics specified in this document are similar,
   in that they may use similar data sources and have similar issues in
   their calculation.  Hence, we specify common issues unless one metric
   has its unique challenges.

5.1.  Source Considerations

   The addition of the "cost-source" field is to solve a key issue: An
   ALTO server needs data sources to compute the cost metrics described
   in this document, and an ALTO client needs to know the data sources
   to better interpret the values.

   To avoid too fine-grained information, this document introduces
   "cost-source" to indicate only the high-level type of data sources:
   "estimation" or "sla", where "estimation" is a type of measurement
   data source, and "sla" is a type that is more based on policy.

   For estimation, for example, the ALTO server may use log servers or
   the OAM system as its data source as recommended by [RFC7971].  In
   particular, the cost metrics defined in this document can be computed
   using routing systems as the data sources.

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5.2.  Metric Timestamp Consideration

   Despite the introduction of the additional cost-context information,
   the metrics do not have a field to indicate the timestamps of the
   data used to compute the metrics.  To indicate this attribute, the
   ALTO server SHOULD return HTTP "Last-Modified", to indicate the
   freshness of the data used to compute the performance metrics.

   If the ALTO client obtains updates through an incremental update
   mechanism [RFC8895]), the client SHOULD assume that the metric is
   computed using a snapshot at the time that is approximated by the
   receiving time.

5.3.  Backward Compatibility Considerations

   One potential issue introduced by the optional "cost-source" field is
   backward compatibility.  Consider that an IRD which defines two cost-
   types with the same "cost-mode" and "cost-metric", but one with
   "cost-source" being "estimation" and the other being "sla".  Then an
   ALTO client that is not aware of the extension will not be able to
   distinguish between these two types.  A similar issue can arise even
   with a single cost-type, whose "cost-source" is "sla": an ALTO client
   that is not aware of this extension will ignore this field and
   consider the metric estimation.

   To address this issue, the only defined "routingcost" metric can be
   only "estimation".

5.4.  Computation Considerations

   The metric values exposed by an ALTO server may result from
   additional processing on measurements from data sources to compute
   exposed metrics.  This may involve data processing tasks such as
   aggregating the results across multiple systems, removing outliers,
   and creating additional statistics.  There are two challenges on the
   computation of ALTO performance metrics.

5.4.1.  Configuration Parameters Considerations

   Performance metrics often depend on configuration parameters.  For
   example, the value of packet loss rate depends on the measurement
   interval and varies over time.  To handle this issue, an ALTO server
   may collect data on time periods covering the previous and current
   time or only collect data on present time.  The ALTO server may
   further aggregate these data to provide an abstract and unified view
   that can be more useful to applications.  To make the ALTO client
   better understand how to use these performance data, the ALTO server

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   may provide the client with the validity period of the exposed metric
   values.

5.4.2.  Availability Considerations

   Applications value information relating to bandwidth availability,
   whereas bandwidth related metrics can often be measured only at the
   link level.  This document specifies a set of link-level bandwidth
   related values that may be exposed as such by an ALTO server.  The
   server may also expose other metrics derived from their aggregation,
   using different levels of endpoint granularity, e.g., link endpoints
   or session endpoints.  The metric specifications may also expose the
   utilized aggregation laws.

6.  Security Considerations

   The properties defined in this document present no security
   considerations beyond those in Section 15 of the base ALTO
   specification [RFC7285].

   However, concerns addressed in Sections "15.1 Authenticity and
   Integrity of ALTO Information", "15.2 Potential Undesirable Guidance
   from Authenticated ALTO Information", and "15.3 Confidentiality of
   ALTO Information" remain of utmost importance.  Indeed, TE
   performance is a highly sensitive ISP information; therefore, sharing
   TE metric values in numerical mode requires full mutual confidence
   between the entities managing the ALTO server and the ALTO client.
   ALTO servers will most likely distribute numerical TE performance to
   ALTO clients under strict and formal mutual trust agreements.  On the
   other hand, ALTO clients must be cognizant on the risks attached to
   such information that they would have acquired outside formal
   conditions of mutual trust.

7.  IANA Considerations

   IANA has created and now maintains the "ALTO Cost Metric Registry",
   listed in Section 14.2, Table 3 of [RFC7285].  This registry is
   located at <http://www.iana.org/assignments/alto-protocol/alto-
   protocol.xhtml#cost-metrics>.  This document requests to add the
   following entries to "ALTO Cost Metric Registry".

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   +-----------------+--------------------+
   | Identifier      | Intended Semantics |
   +-----------------+--------------------+
   | delay-ow        | See Section 3.1    |
   | delay-rt        | See Section 3.2    |
   | delay-variation | See Section 3.3    |
   | hopcount        | See Section 3.4    |
   | lossrate        | See Section 3.5    |
   | tput            | See Section 4.1    |
   | bw-residual     | See Section 4.2    |
   | bw-maxres       | See Section 4.3    |
   +-----------------+--------------------+

   This document requests the creation of the "ALTO Cost Source
   Registry" with the following currently defined values:

   +------------+-----------------------------+
   | Identifier | Intended Semantics          |
   +------------+-----------------------------+
   | nominal    | Values in nominal cases     |
   | sla        | Values reflecting service   |
   |            | level agreement             |
   | import     | Values from a given protocol|
   | estimation | Values by estimation        |
   +------------+-----------------------------+

8.  Acknowledgments

   The authors of this document would also like to thank Brian Trammell,
   Haizhou Du, Kai Gao, Lili Liu, Geng Li, Danny Alex Lachos Perez for
   the reviews and comments.

9.  References

9.1.  Normative References

   [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>.

   [RFC6390]  Clark, A. and B. Claise, "Guidelines for Considering New
              Performance Metric Development", BCP 170, RFC 6390, DOI
              10.17487/RFC6390, October 2011, <https://www.rfc-
              editor.org/info/rfc6390>.

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   [RFC7285]  Alimi, R., Ed., Penno, R., Ed., Yang, Y., Ed., Kiesel, S.,
              Previdi, S., Roome, W., Shalunov, S., and R. Woundy,
              "Application-Layer Traffic Optimization (ALTO) Protocol",
              RFC 7285, DOI 10.17487/RFC7285, September 2014,
              <https://www.rfc-editor.org/info/rfc7285>.

   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
              2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
              May 2017, <https://www.rfc-editor.org/info/rfc8174>.

   [RFC8259]  Bray, T., Ed., "The JavaScript Object Notation (JSON) Data
              Interchange Format", STD 90, RFC 8259, DOI 10.17487/
              RFC8259, December 2017, <https://www.rfc-editor.org/info/
              rfc8259>.

   [RFC8895]  Roome, W. and Y. Yang, "Application-Layer Traffic
              Optimization (ALTO) Incremental Updates Using Server-Sent
              Events (SSE)", RFC 8895, DOI 10.17487/RFC8895, November
              2020, <https://www.rfc-editor.org/info/rfc8895>.

9.2.  Informative References

   [I-D.ietf-ippm-initial-registry]
              Morton, A., Bagnulo, M., Eardley, P., and K. D'Souza,
              "Initial Performance Metrics Registry Entries", draft-
              ietf-ippm-initial-registry-16 (work in progress), March
              2020.

   [Prometheus]
              Volz, J. and B. Rabenstein, "Prometheus: A Next-Generation
              Monitoring System", 2015.

   [Prophet]  Gao, K., Zhang, J., and YR. Yang, "Prophet: Fast, Accurate
              Throughput Prediction with Reactive Flows", ACM/IEEE
              Transactions on Networking July, 2020.

   [RFC2681]  Almes, G., Kalidindi, S., and M. Zekauskas, "A Round-trip
              Delay Metric for IPPM", RFC 2681, DOI 10.17487/RFC2681,
              September 1999, <https://www.rfc-editor.org/info/rfc2681>.

   [RFC3393]  Demichelis, C. and P. Chimento, "IP Packet Delay Variation
              Metric for IP Performance Metrics (IPPM)", RFC 3393, DOI
              10.17487/RFC3393, November 2002, <https://www.rfc-
              editor.org/info/rfc3393>.

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   [RFC3630]  Katz, D., Kompella, K., and D. Yeung, "Traffic Engineering
              (TE) Extensions to OSPF Version 2", RFC 3630, DOI
              10.17487/RFC3630, September 2003, <https://www.rfc-
              editor.org/info/rfc3630>.

   [RFC5305]  Li, T. and H. Smit, "IS-IS Extensions for Traffic
              Engineering", RFC 5305, DOI 10.17487/RFC5305, October
              2008, <https://www.rfc-editor.org/info/rfc5305>.

   [RFC5357]  Hedayat, K., Krzanowski, R., Morton, A., Yum, K., and J.
              Babiarz, "A Two-Way Active Measurement Protocol (TWAMP)",
              RFC 5357, DOI 10.17487/RFC5357, October 2008,
              <https://www.rfc-editor.org/info/rfc5357>.

   [RFC6349]  Constantine, B., Forget, G., Geib, R., and R. Schrage,
              "Framework for TCP Throughput Testing", RFC 6349, DOI
              10.17487/RFC6349, August 2011, <https://www.rfc-
              editor.org/info/rfc6349>.

   [RFC7679]  Almes, G., Kalidindi, S., Zekauskas, M., and A. Morton,
              Ed., "A One-Way Delay Metric for IP Performance Metrics
              (IPPM)", STD 81, RFC 7679, DOI 10.17487/RFC7679, January
              2016, <https://www.rfc-editor.org/info/rfc7679>.

   [RFC7680]  Almes, G., Kalidindi, S., Zekauskas, M., and A. Morton,
              Ed., "A One-Way Loss Metric for IP Performance Metrics
              (IPPM)", STD 82, RFC 7680, DOI 10.17487/RFC7680, January
              2016, <https://www.rfc-editor.org/info/rfc7680>.

   [RFC7971]  Stiemerling, M., Kiesel, S., Scharf, M., Seidel, H., and
              S. Previdi, "Application-Layer Traffic Optimization (ALTO)
              Deployment Considerations", RFC 7971, DOI 10.17487/
              RFC7971, October 2016, <https://www.rfc-editor.org/info/
              rfc7971>.

   [RFC8570]  Ginsberg, L., Ed., Previdi, S., Ed., Giacalone, S., Ward,
              D., Drake, J., and Q. Wu, "IS-IS Traffic Engineering (TE)
              Metric Extensions", RFC 8570, DOI 10.17487/RFC8570, March
              2019, <https://www.rfc-editor.org/info/rfc8570>.

   [RFC8571]  Ginsberg, L., Ed., Previdi, S., Wu, Q., Tantsura, J., and
              C. Filsfils, "BGP - Link State (BGP-LS) Advertisement of
              IGP Traffic Engineering Performance Metric Extensions",
              RFC 8571, DOI 10.17487/RFC8571, March 2019,
              <https://www.rfc-editor.org/info/rfc8571>.

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Authors' Addresses

   Qin Wu
   Huawei
   101 Software Avenue, Yuhua District
   Nanjing, Jiangsu  210012
   CHINA

   Email: bill.wu@huawei.com

   Y. Richard Yang
   Yale University
   51 Prospect St
   New Haven, CT  06520
   USA

   Email: yry@cs.yale.edu

   Young Lee
   Samsung
   1700 Alma Drive, Suite 500
   Plano, TX  75075
   USA

   Email: young.lee@gmail.com

   Dhruv Dhody
   Huawei
   Leela Palace
   Bangalore, Karnataka  560008
   INDIA

   Email: dhruv.ietf@gmail.com

   Sabine Randriamasy
   Nokia Bell Labs
   Route de Villejust
   Nozay  91460
   FRANCE

   Email: sabine.randriamasy@nokia-bell-labs.com

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   Luis Miguel Contreras Murillo
   Telefonica
   Madrid
   SPAIN

   Email: luismiguel.contrerasmurillo@telefonica.com

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