ALTO Performance Cost Metrics

Internet-Draft	ALTO Performance Cost Metrics	October 2021
Wu, et al.	Expires 26 April 2022	[Page]

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.¶

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.¶

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 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:¶


    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. 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.¶

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.¶

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.¶

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"
    ]
  }
}

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 [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.¶

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 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.¶

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 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.¶

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 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"
      ]
    }
  }

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.¶

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"
    ]
  }
}

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.¶

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"
     ]
   }
  }

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.¶

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"
      ]
    }
  }

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.¶

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.¶

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.¶

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 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:¶

Identifier: The name of the desired ALTO cost source type.¶
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.¶
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         |
+------------+----------------------------------+----------------+

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>.
[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>.
[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>.