ALTO Working Group                                                 Q. Wu
Internet-Draft                                                    Huawei
Intended status: Standards Track                                 Y. Yang
Expires: April 26, 2022                                  Yale University
                                                                  Y. Lee
                                                                 Samsung
                                                                D. Dhody
                                                                  Huawei
                                                          S. Randriamasy
                                                         Nokia Bell Labs
                                                            L. Contreras
                                                              Telefonica
                                                        October 23, 2021


                     ALTO Performance Cost Metrics
                 draft-ietf-alto-performance-metrics-19

Abstract

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

   This document addresses this issue by extending the specification to
   provide a variety of network performance metrics, including network
   delay, delay variation (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
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at http://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 April 26, 2022.

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
   to this document.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of
   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  . . . . . . . . . . . . . . . . .  10
     3.1.  Cost Metric: One-Way Delay (delay-ow) . . . . . . . . . .  11
       3.1.1.  Base Identifier . . . . . . . . . . . . . . . . . . .  11
       3.1.2.  Value Representation  . . . . . . . . . . . . . . . .  11
       3.1.3.  Intended Semantics and Use  . . . . . . . . . . . . .  11
       3.1.4.  Cost-Context Specification Considerations . . . . . .  12
     3.2.  Cost Metric: Round-trip Delay (delay-rt)  . . . . . . . .  13
       3.2.1.  Base Identifier . . . . . . . . . . . . . . . . . . .  13
       3.2.2.  Value Representation  . . . . . . . . . . . . . . . .  13
       3.2.3.  Intended Semantics and Use  . . . . . . . . . . . . .  13



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       3.2.4.  Cost-Context Specification Considerations . . . . . .  14
     3.3.  Cost Metric: Delay Variation (delay-variation)  . . . . .  15
       3.3.1.  Base Identifier . . . . . . . . . . . . . . . . . . .  15
       3.3.2.  Value Representation  . . . . . . . . . . . . . . . .  15
       3.3.3.  Intended Semantics and Use  . . . . . . . . . . . . .  15
       3.3.4.  Cost-Context Specification Considerations . . . . . .  16
     3.4.  Cost Metric: Hop Count (hopcount) . . . . . . . . . . . .  17
       3.4.1.  Base Identifier . . . . . . . . . . . . . . . . . . .  17
       3.4.2.  Value Representation  . . . . . . . . . . . . . . . .  17
       3.4.3.  Intended Semantics and Use  . . . . . . . . . . . . .  17
       3.4.4.  Cost-Context Specification Considerations . . . . . .  18
     3.5.  Cost Metric: Loss Rate (lossrate) . . . . . . . . . . . .  19
       3.5.1.  Base Identifier . . . . . . . . . . . . . . . . . . .  19
       3.5.2.  Value Representation  . . . . . . . . . . . . . . . .  19
       3.5.3.  Intended Semantics and Use  . . . . . . . . . . . . .  19
       3.5.4.  Cost-Context Specification Considerations . . . . . .  20
   4.  Bandwidth Performance Metrics . . . . . . . . . . . . . . . .  20
     4.1.  Cost Metric: TCP Throughput (tput)  . . . . . . . . . . .  21
       4.1.1.  Base Identifier . . . . . . . . . . . . . . . . . . .  21
       4.1.2.  Value Representation  . . . . . . . . . . . . . . . .  21
       4.1.3.  Intended Semantics and Use  . . . . . . . . . . . . .  21
       4.1.4.  Cost-Context Specification Considerations . . . . . .  22
     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  . . . . . . . . . . . . .  23
       4.2.4.  Cost-Context Specification Considerations . . . . . .  24
     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  . . . . . . . . . . . . .  25
       4.3.4.  Cost-Context Specification Considerations . . . . . .  26
   5.  Operational Considerations  . . . . . . . . . . . . . . . . .  26
     5.1.  Source Considerations . . . . . . . . . . . . . . . . . .  27
     5.2.  Metric Timestamp Consideration  . . . . . . . . . . . . .  27
     5.3.  Backward Compatibility Considerations . . . . . . . . . .  27
     5.4.  Computation Considerations  . . . . . . . . . . . . . . .  28
       5.4.1.  Configuration Parameters Considerations . . . . . . .  28
       5.4.2.  Aggregation Computation Considerations  . . . . . . .  28
   6.  Security Considerations . . . . . . . . . . . . . . . . . . .  28
   7.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  29
   8.  Acknowledgments . . . . . . . . . . . . . . . . . . . . . . .  31
   9.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  31
     9.1.  Normative References  . . . . . . . . . . . . . . . . . .  31
     9.2.  Informative References  . . . . . . . . . . . . . . . . .  32
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  33





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1.  Introduction

   Application-Layer Traffic Optimization (ALTO) provides a means for
   network applications to obtain network status information so that the
   applications can identify efficient application-layer traffic
   patterns using the networks.  The cost metric is a basic concept in
   realizing ALTO, and the concept 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 Section 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.



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   The performance metrics can be classified into two categories: those
   derived from the performance of individual packets (i.e., one-way
   delay, round-trip delay, delay variation, hop count, and loss rate),
   and those related to bandwidth (TCP throughput, residual bandwidth,
   and maximum reservable bandwidth).  These two categories are defined
   in Section 3 and Section 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) as defined in
   Section 9.2 of [RFC7285].

   [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
   guideline specified in [RFC6390], which requires that the fine-
   grained specification of a network performance metric include 6
   components: (i) Metric Name, (ii) Metric Description, (iii) Method of
   Measurement or Calculation, (iv) Units of Measurement, (v)
   Measurement Points, and (vi) Measurement Timing.  Requiring that an
   ALTO server provide precise, fine-grained values for all 6 components
   for each metric that it exposes may not be feasible or necessary for
   all ALTO use cases.  For example, the method of measurement or



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   calculation can be complex with substantial details that cannot be
   exposed to or are unnecessary for ALTO clients in many use cases.

   To address the issue and realize ALTO use cases, for metrics in
   Table 1, this document defines performance metric identifiers which
   can be used in the ALTO protocol with well-defined (i) Metric Name,
   (ii) Metric Description, (iv) Units of Measurement, and (v)
   Measurement Points, which are always specified by the specific ALTO
   services; for example, endpoint cost service is between the two
   endpoints.  Hence, the ALTO performance metric identifiers provide
   basic metric attributes.

   To allow the flexibility of allowing an ALTO server to provide fine-
   grained information such as Method of Measurement or Calculation,
   according to its policy and use cases, this document introduces
   context information so that the server can provide these additional
   details.

2.1.  Performance Metric Context: cost-context

   The core additional details of a performance metric specify "how" the
   metric is obtained.  This is referred to as the source of the metric.
   Specifically, this document defines three types of coarse-grained
   metric information sources: "nominal", and "sla" (service level
   agreement), 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:
















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

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



   "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 "cost-source" field of the "cost-context" field is defined as a
   string consisting of only US-ASCII alphanumeric characters
   (U+0030-U+0039, U+0041-U+005A, and U+0061-U+007A).  The cost-source
   is used in this document to indicate a string of this format.

   This document defines three values for "cost-source": "nominal",
   "sla", and "estimation".  The "cost-source" field of the "cost-
   context" field MUST be one registered in "ALTO Cost Source Registry"
   (Section 7).

   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
   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.  An illustration of potential information flows
   used for estimating these metrics is shown in Figure 1 below.



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   Section 5 will discuss in more detail the operational issues and how
   a network may address them.

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

   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.  A statistical operator is applied to the samples to obtain a
   value to be reported to the client.  Multiple statistical operators
   (e.g., min, median, max) are commonly being used.

   Hence, this document extends the general US-ASCII alphanumeric cost
   metric strings, formally specified as the CostMetric type defined in
   Section 10.6 of [RFC7285]; see above in the CostType definition, as
   follows:

      A cost metric string consists of a base metric identifier (or base
      identifier for short) string, followed by an optional statistical
      operator string, connected by the ASCII character colon (':',
      U+003A), if the statistical operator string exists.




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   Examples of cost metric strings then include "delay-ow", "delay-
   ow:min", "delay-ow:p99", where "delay-ow" is the base metric
   identifier string; "min" and "p99" are example statistical operator
   strings.

   The statistical operator string MUST be one of the following:

   cur:

      the instantaneous observation value of the metric from the most
      recent sample (i.e., the current value).



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

      gives the percentile specified by the number following the letter
      'p'.  The number MUST be a non-negative JSON integer in the range
      [0, 100] (i.e., greater than or equal to 0 and less than or equal
      to 100), followed by an optional decimal part, if a higher
      precision is needed.  The decimal part should start with the '.'
      separator (U+002E), and followed by a sequence of one or more
      ASCII numbers between '0' and '9'.  The total length of the cost
      metric string MUST not exceed 32, as required by [RFC7285].
      Assume this number is y and consider the samples coming from a
      random variable X.  Then the metric returns x, such that the
      probability of X is less than or equal to x, i.e., Prob(X <= x), =
      y/100.  For example, delay-ow:p99 gives the 99% percentile of
      observed one-way delay; delay-ow:p99.9 gives the 99.9% percentile.
      Note that some systems use quantile, which is in the range [0, 1].
      When there is a more common form for a given percentile, it is
      RECOMMENDED that the common form being used; that is, instead of
      p0, use min; instead of p50, use median; instead of p100, use max.



   min:

      the minimal value of the observations.



   max:

      the maximal value of the observations.






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   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 cost metric string does not have the optional statical operator
   string, the statistical operator SHOULD be interpreted as the default
   statical operator in the definition of the base metric.  If the
   definition of the base metric does not provide a definition for the
   default statistical operator, the metric MUST be considered as the
   median value.

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
   dropped before reaching the destination (pkt.dropped).  The semantics
   of the performance metrics defined in this section are 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.





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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., provider-defined identifier (PID) to PID, or
   endpoint to endpoint, where PID is defined in Section 5.1 of
   [RFC7285].

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

   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"
       ]
     }
   }





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   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
       }
     }
   }

   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.

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 includes a link (i.e., a field named
   "link") providing an URI to the specification of SLA details, if
   available.  This specification can be either free text for possible
   presentation to the user, or a formal specification.  The format of
   the specification is out of the scope of this document.

   "estimation": The exact estimation method is out of the scope of this
   document.  There can be multiple sources to estimate one-way delay.
   For example, the server may use [RFC8571] (by using unidirectional
   link delay, min/max unidirectional link delay) to estimate the path
   delay.  During estimation, the server should be cognizant of
   potential issues when computing an end-to-end summary statistic from
   link statistics.  Another example of a source to estimate the delay
   is the IPPM framework [RFC2330].  It is RECOMMENDED that the
   "parameters" field of an "estimation" one-way delay metric includes a
   link (a field named "link") providing an URI to a description of the
   "estimation" method.  This description can be either free text for
   possible presentation to the user, or a formal specification; see




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   [IANA-IPPM] for the specification on fields which should be included.
   The format of the description is out of the scope of this document.

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).

   Note that it is possible for a client to query two one-way delays
   (delay-ow) 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.























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   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"
        ]
      }
   }

    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
        }
      }
    }


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 includes a link (a field named "link")
   providing an URI to the specification of SLA details, if available.
   This specification can be either free text for possible presentation



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   to the user, or a formal specification.  The format of the
   specification is out of the scope of this document.

   "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 includes a link (a field named
   "link") providing an URI to a description of the "estimation" method;
   see Section 3.1.4 for related discussions on the link.

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 one-way delay from 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,
   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.














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   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"
       ]
     }
   }
   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
            }
         }
      }

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 includes a link (a field named "link")
   providing an URI to the specification of SLA details, if available.
   This specification can be either free text for possible presentation




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   to the user, or a formal specification.  The format of the
   specification is out of the scope of this document.

   "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.  See Section 3.1.4 for
   related discussions.

3.4.  Cost Metric: Hop Count (hopcount)

   The hopcount metric 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
   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 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"
         ]
       }
     }

   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
             }
       }
    }

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.

   "estimation": The exact estimation method is out of the scope of this
   document.  An example of estimating hopcounts is by importing from
   IGP routing protocols.  It is RECOMMENDED that the "parameters" field



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   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.

   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"
         ]
       }
     }




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   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
         }
       }
    }

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.

   "sla": It is RECOMMENDED that the "parameters" field of an "sla"
   packet loss rate includes a link (a field named "link") providing an
   URI to the specification of SLA details, if available.  This
   specification can be either free text for possible presentation to
   the user, or a formal specification.  The format of the specification
   is out of the scope of this document.

   "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.  See Section 3.1.4 on on
   related discussions such as summing up link metrics to obtain end-to-
   end metrics.

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.








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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 congestion-
   control conforming flow from the specified source to the specified
   destination; see [RFC3649, Section 5.1 of RFC8312] on how TCP
   throughput is estimated.  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.

   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"
       ]
     }
   }








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   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
     }
   }

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.

   "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 (a field named "link") to a
   description of the "estimation" method.  Note that as TCP congestion
   control algorithms evolve (e.g., TCP Cubic Congestion Control
   [RFC8312]), it helps to specify as much details as possible on the
   the congestion control algorithm used.  This description can be
   either free text for possible presentation to the user, or a formal
   specification.  The semantics are out of the scope of this document.

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.







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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).

   The default statical operator for residual bandwidth is the current
   instantaneous sample; that is, the default is assumed to be "cur".

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

   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"
        ]
      }
     }
















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   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
       }
     }
   }

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.

   "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.  See Section 3.1.4 on related
   discussions.  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.

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.







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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).

   The default statical operator for maximum reservable bandwidth is the
   current instantaneous sample; that is, the default is assumed to be
   "cur".

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

     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"
         ]
       }
     }















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   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
       }
     }
   }

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.

   "estimation": The exact estimation method is out of the scope of this
   document.  There can be multiple sources to estimate 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].  An estimation can also be computed from
   [RFC8571] (by using unidirectional maximum reservable bandwidth).  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.  This description can be either free text
   for possible presentation to the user, or a formal specification.
   The semantics are out of the scope of this document.

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.





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   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, this document specifies 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.

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.





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   To address the backward-compatibility issue, if a "cost-metric" is
   "routingcost" and the metric contains a "cost-context" field, then it
   MUST be "estimation"; if it is not, the client SHOULD reject the
   information as invalid.

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, and
   exposing such configuration parameters can help an ALTO client to
   better understand the exposed metrics.  In particular, an ALTO server
   may be configured to compute a TE metric (e.g., packet loss rate) in
   fixed intervals, say every T seconds.  To expose this information,
   the ALTO server may provide the client with two pieces of additional
   information: (1) when the metrics are last computed, and (2) when the
   metrics will be updated (i.e., the validity period of the exposed
   metric values).  The ALTO server can expose these two pieces of
   information by using the HTTP response headers Last-Modified and
   Expires.

5.4.2.  Aggregation Computation Considerations

   An ALTO server may not be able to measure the performance metrics to
   be exposed.  The basic issue is that the "source" information can
   often be link level.  For example, routing protocols often measure
   and report only per link loss, not end-to-end loss; similarly,
   routing protocols report link level available bandwidth, not end-to-
   end available bandwidth.  The ALTO server then needs to aggregate
   these data to provide an abstract and unified view that can be more
   useful to applications.  The server should consider that different
   metrics may use different aggregation computation.  For example, the
   end-to-end latency of a path is the sum of the latency of the links
   on the path; the end-to-end available bandwidth of a path is the
   minimum of the available bandwidth of the links on the path.

6.  Security Considerations

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



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   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 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.

   To mitigate confidentiality risks during information transport of TE
   performance metrics, the operator should address the risk of ALTO
   information being leaked to malicious Clients or third parties,
   through attacks such as the man-in-the-middle (MITM) attacks.  As
   specified in "Protection Strategies" (Section 15.3.2 of [RFC7285]),
   the ALTO Server should authenticate ALTO Clients when transmitting an
   ALTO information resource containing sensitive TE performance
   metrics.  "Authentication and Encryption" (Section 8.3.5 of
   [RFC7285]) specifies that "ALTO Server implementations as well as
   ALTO Client implementations MUST support the "https" URI scheme of
   [RFC2818] and Transport Layer Security (TLS) of [RFC8446]".

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".

   +-----------------+--------------------+
   | 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".  This registry serves two purposes.  First, it ensures



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   uniqueness of identifiers referring to ALTO cost source types.
   Second, it provides references to particular semantics of allocated
   cost source types to be applied by both ALTO servers and applications
   utilizing ALTO clients.

   A new ALTO cost source can be added after IETF Review [RFC8126], to
   ensure that proper documentation regarding the new ALTO cost source
   and its security considerations have been provided.  The RFC(s)
   documenting the new cost source should be detailed enough to provide
   guidance to both ALTO service providers and applications utilizing
   ALTO clients as to how values of the registered ALTO cost source
   should be interpreted.  Updates and deletions of ALTO cost source
   follow the same procedure.

   Registered ALTO address type identifiers MUST conform to the
   syntactical requirements specified in Section 2.1.  Identifiers are
   to be recorded and displayed as strings.

   Requests to add a new value to the registry MUST include the
   following information:

   o  Identifier: The name of the desired ALTO cost source type.

   o  Intended Semantics: ALTO cost source type carry with them
      semantics to guide their usage by ALTO clients.  Hence, a document
      defining a new type should provide guidance to both ALTO service
      providers and applications utilizing ALTO clients as to how values
      of the registered ALTO endpoint property should be interpreted.

   o  Security Considerations: ALTO cost source types expose information
      to ALTO clients.  ALTO service providers should be made aware of
      the security ramifications related to the exposure of a cost
      source type.

   This specification requests registration of the identifiers -
   "nominal", "sla", and "estimation" listed in the table below.
   Semantics for the these are documented in Section 2.1, and security
   considerations are documented in Section 6.

   +------------+----------------------------------+----------------+
   | Identifier | Intended Semantics               | Security       |
   |            |                                  | Considerations |
   +------------+----------------------------------+----------------+
   | nominal    | Values in nominal cases; Sec. 2.1| Sec. 6         |
   | sla        | Values reflecting service        | Sec. 6         |
   |            | level agreement; Sec. 2.1        |                |
   | estimation | Values by estimation; Sec. 2.1   | Sec. 6         |
   +------------+----------------------------------+----------------+



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8.  Acknowledgments

   The authors of this document would also like to thank Martin Duke for
   the highly informative, thorough AD reviews and comments.  We thank
   Christian Amsuess, Elwyn Davies, Haizhou Du, Kai Gao, Geng Li, Lili
   Liu, Danny Alex Lachos Perez, and Brian Trammell for the reviews and
   comments.

9.  References

9.1.  Normative References

   [IANA-IPPM]
              IANA, , "Performance Metrics Registry,
              https://www.iana.org/assignments/performance-metrics/
              performance-metrics.xhtml".

   [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>.

   [RFC2330]  Paxson, V., Almes, G., Mahdavi, J., and M. Mathis,
              "Framework for IP Performance Metrics", RFC 2330, DOI
              10.17487/RFC2330, May 1998, <https://www.rfc-
              editor.org/info/rfc2330>.

   [RFC2818]  Rescorla, E., "HTTP Over TLS", RFC 2818, DOI 10.17487/
              RFC2818, May 2000, <https://www.rfc-editor.org/info/
              rfc2818>.

   [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>.

   [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>.

   [RFC8126]  Cotton, M., Leiba, B., and T. Narten, "Guidelines for
              Writing an IANA Considerations Section in RFCs", BCP 26,
              RFC 8126, DOI 10.17487/RFC8126, June 2017,
              <https://www.rfc-editor.org/info/rfc8126>.





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   [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

   [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>.

   [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>.





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   [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>.

   [RFC8446]  Rescorla, E., "The Transport Layer Security (TLS) Protocol
              Version 1.3", RFC 8446, DOI 10.17487/RFC8446, August 2018,
              <https://www.rfc-editor.org/info/rfc8446>.

   [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>.

Authors' Addresses

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

   Email: bill.wu@huawei.com






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   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


   Luis Miguel Contreras Murillo
   Telefonica
   Madrid
   SPAIN

   Email: luismiguel.contrerasmurillo@telefonica.com









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