ALTO Performance Cost Metrics
draft-ietf-alto-performance-metrics-08

Abstract

Cost metric is a basic concept in Application-Layer Traffic Optimization (ALTO), and is used in basic services including both the cost map service and the endpoint cost service.

Different applications may use different cost metrics, but the ALTO base protocol documents only one single cost metric, i.e., the generic "routingcost" metric; see Sec. 14.2 of ALTO base specification [RFC7285]. Hence, if the resource consumer of an application prefers a resource provider that offers low-delay delivery to the resource consumer, the base protocol does not define the cost metric to be used.

ALTO cost metrics can be generic metrics and this document focuses on network performance metrics, including network delay, jitter, packet loss, hop count, and bandwidth. Additional cost metrics may be documented in other documents.

When using an ALTO performance metric, applications need additional information beyond the metric. In particular, its "cost-source", such as it being an estimation or an SLA, is key to define the meaning of a performance metric. Hence, each ALTO performance metric should include the "cost-source" of the metric. To report an estimated value of a performance metric, the ALTO server may derive and aggregate from routing protocols with different granularity and scope, such as BGP-LS, OSPF-TE and ISIS-TE, or from end-to-end traffic management tools. These metrics may then be exposed by an ALTO Server to allow applications to determine "where" to connect based on network performance criteria.

Requirements Language The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in RFC 2119 [RFC2119].

Status of This Memo

This Internet-Draft is submitted in full conformance with the provisions of BCP 78 and BCP 79.

Internet-Drafts are working documents of the Internet Engineering Task Force (IETF). Note that other groups may also distribute working documents as Internet-Drafts. The list of current Internet-Drafts is at https://datatracker.ietf.org/drafts/current/.

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This Internet-Draft will expire on May 7, 2020.

Copyright Notice

Copyright (c) 2019 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 (https://trustee.ietf.org/license-info) in effect on the date of publication of this document. Please review these documents carefully, as they describe your rights and restrictions with respect to this document. Code Components extracted from this document must include 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
• 2. Network Performance Cost Metrics
• 2.1. Cost Metric: One Way Delay (owdelay)
• 2.1.1. Intended Semantics
• 2.1.2. Use and Example
• 2.1.3. Measurement Considerations
• 2.2. Cost Metric: RoundTrip Time (rtt)
• 2.2.1. Intended Semantics
• 2.2.2. Use and Example
• 2.2.3. Measurement Considerations
• 2.3. Cost Metric: Packet Delay Variation (pdv)
• 2.3.1. Intended Semantics
• 2.3.2. Use and Example
• 2.3.3. Measurement Considerations
• 2.4. Cost Metric: Hop Count
• 2.4.1. Intended Semantics
• 2.4.2. Use and Example
• 2.4.3. Measurement Considerations
• 2.5. Cost Metric: Packet Loss
• 2.5.1. Intended Semantics
• 2.5.2. Use and Example
• 2.5.3. Measurement Considerations
• 2.6. Cost Metric: Throughput
• 2.6.1. Intended Semantics
• 2.6.2. Use and Example
• 2.6.3. Measurement Considerations
• 3. Traffic Engineering Performance Cost Metrics
• 3.1. Cost Metric: Link Maximum Reservable Bandwidth
• 3.1.1. Intended Semantics
• 3.1.2. Use and Example
• 3.1.3. Measurement Considerations
• 3.2. Cost Metric: Link Residue Bandwidth
• 3.2.1. Intended Semantics
• 3.2.2. Use and Example
• 3.2.3. Measurement Considerations
• 4. Operational Considerations
• 4.1. Source Considerations
• 4.2. Backward Compatibility Considerations
• 4.3. Computation Considerations
• 4.3.1. Configuration Parameters Considerations
• 4.3.2. Availability Considerations
• 5. Security Considerations
• 6. IANA Considerations
• 7. Acknowledgments
• 8. References
• 8.1. Normative References
• 8.2. Informative References
• Authors' Addresses

1. Introduction

Cost Metric is a basic concept in Application-Layer Traffic Optimization (ALTO). It is used in both the ALTO cost map service and the ALTO endpoint cost service, to allow applications to request network cost metrics.

Different applications may use different cost metrics. Hence, the ALTO base protocol [RFC7285] 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 more delay-sensitive application may want to use latency related metrics, and a more bandwidth-sensitive application may want to use bandwidth related metrics. The ALTO base protocol [RFC7285], however, has registered only one single cost metric, i.e., the generic "routingcost" metric; no latency or bandwidth related metrics are defined.

This document registers a set of new cost metrics specified in Table 1, to support the aforementioned applications, to allow them to determine "where" to connect based on network performance criteria. This document follows the guideline (Section 14.2 of [RFC7285]) of the ALTO base protocol 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               |
+--------------------------+-------------+-----------------------+
| One Way Delay            | Section 2.1 | [RFC2679] Section 3.6 |
| Round Trip Delay         | Section 2.2 | [RFC2681] Section 2.6 |
| Packet Delay Variation   | Section 2.3 | [RFC3393] Section 2.6 |
| Hop Count                | Section 2.4 | [RFC7285]             |
| Packet Loss              | Section 2.5 | [RFC7680] Section 2.6 |
| Throughput               | Section 2.6 | [RFC6349] Section 3.3 |
| Max Reservable Bandwidth | Section 3.1 | [RFC5305] Section 3.5 |
| Residue Bandwidth        | Section 3.2 | [RFC7810] Section 4.5 |
+------------+---------------------------------------------------+
   Table 1. Cost Metrics Defined in this Document
        

The purpose of this document is to ensure proper usage of the metrics by ALTO clients. It does not claim novelty of the metrics. Some of these metrics are already specified by standards such as IPPM; some are ISP dependent such as those registered in ISIS or OSPF-TE. This document will refer to the relevant specifications.

An ALTO server may provide only a subset of the cost metrics described in this document. Hence, all cost metrics defined in this document are optional and not all them need to be exposed to applications. For example, those that are subject to privacy concerns should not be provided to unauthorized ALTO clients.

When an ALTO server supports a cost metric defined in this document, it MUST announce this metric in its information resource directory (IRD).

To make clear how the value of an ALTO performance metric is derived, this document defines an optional field named "cost-source" to extend the ALTO "cost-type". The "cost-source" indicates how the metric is derived, and currently it can be either "estimation" or "sla". If a "cost-type" does not include the optional "cost-source" field, the application MUST assume that the value of "cost-source" is "estimation".

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

An ALTO server may compute "estimation" values by retrieving and/or aggregating information from routing protocols or other traffic measurement management tools, with corresponding operational issues. A potential architecture on estimating these metrics is shown in Figure 1 below. In Section 4, we discuss in more detail the operations issues and how to address them.

An ALTO server introducing these metrics should also 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 feel the exposed information does not preserve their performance level or even degrades it. We discuss security considerations in more details in Section 5.

Following the ALTO base protocol, this document uses JSON to specify the value type of each defined metric. See [RFC4627] for JSON data type specification.

2. Network Performance Cost Metrics

This section introduces generic ALTO network performance metrics such as one way delay, round trip delay, hop count, packet loss, and throughput.

2.1. Cost Metric: One Way Delay (owdelay)

Metric name:

One Way Delay

Metric Identifier:

owdelay

2.1.1. Intended Semantics

Metric Description: To specify spatial and temporal aggregated delay of a stream of packets exchanged between the specified source and destination or the time that the packet spends to travel from source to destination. The spatial aggregation level is specified in the query context (e.g., PID to PID, or endpoint to endpoint).

Metric 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 in this document.

2.1.2. Use and Example

This metric could be used as a cost metric constraint attribute used either together with cost metric attribute 'routingcost' or on its own 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" : "owdelay"},
  "endpoints" : {
    "srcs": [ "ipv4:192.0.2.2" ],
    "dsts": [
      "ipv4:192.0.2.89",
      "ipv4:198.51.100.34",
      "ipv6:2000::1:2345:6789:abcd"
    ]
  }
}

HTTP/1.1 200 OK
Content-Length: TBA
Content-Type: application/alto-endpointcost+json
{
  "meta" :{
    "cost-type": {"cost-mode" : "numerical",
                  "cost-metric" : "owdelay"
     }
   },
    "endpoint-cost-map" : {
      "ipv4:192.0.2.2": {
        "ipv4:192.0.2.89"    : 10,
        "ipv4:198.51.100.34" : 20,
        "ipv6:2000::1:2345:6789:abcd"  : 30,
    }
  }
}

Comment: Since the "cost-type" does not include the "cost-source" field, the values are based on "estimation".

2.1.3. Measurement Considerations

Method of Measurement or Calculation:

See section 8.3 of [I-D.ietf-ippm-initial-registry] for potential measurement method.

Measurement Point(s) with Potential Measurement Domain:

See Section 4.1, Data sources for potential data sources.

Measurement Timing:

See section 8.3.5 of [I-D.ietf-ippm-initial-registry] for potential measurement timing considerations.

2.2. Cost Metric: RoundTrip Time (rtt)

Metric name:

Round Trip Time

Metric Identifier:

rtt

2.2.1. Intended Semantics

Metric Description: To specify spatial and temporal aggregated round trip delay between the specified source and destination or the time that the packet spends to travel from source to destination and then from destination to source. The spatial aggregation level is specified in the query context (e.g., PID to PID, or endpoint to endpoint).

Metric 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 in this document.

2.2.2. Use and Example

This metric could be used as a cost metric constraint attribute used either together with cost metric attribute 'routingcost' or on its own or as a returned cost metric in the response.

 
Example 2: Roundtrip 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" : "rtt"},
  "endpoints" : {
     "srcs": [ "ipv4:192.0.2.2" ],
     "dsts": [
       "ipv4:192.0.2.89",
       "ipv4:198.51.100.34",
       "ipv6:2000::1:2345:6789:abcd"
     ]
   }
}

 HTTP/1.1 200 OK
 Content-Length: TBA
 Content-Type: application/alto-endpointcost+json
 {
   "meta" :{
     "cost-type": {"cost-mode" : "numerical",
                   "cost-metric" : "rtt"
      }
    },
     "endpoint-cost-map" : {
       "ipv4:192.0.2.2": {
         "ipv4:192.0.2.89"    : 4,
         "ipv4:198.51.100.34" : 3,
         "ipv6:2000::1:2345:6789:abcd"  : 2,
     }
   }
 }

2.2.3. Measurement Considerations

Method of Measurement or Calculation:

See section 4.3 of [I-D.ietf-ippm-initial-registry] for potential measurement method.

Measurement Point(s) with Potential Measurement Domain:

See section 4.1, Data sources.

Measurement Timing:

See section 4.3.5 of [I-D.ietf-ippm-initial-registry] for Measurement Timing.

2.3. Cost Metric: Packet Delay Variation (pdv)

Metric name:

Packet Delay Variation

Metric Identifier:

pdv

2.3.1. Intended Semantics

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

Metric 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 in this document.

2.3.2. Use and Example

This metric could be used as a cost metric constraint attribute used either together with cost metric attribute 'routingcost' or on its own or as a returned cost metric in the response.

Example 3: PDV 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" : "pdv"},
  "endpoints" : {
    "srcs": [ "ipv4:192.0.2.2" ],
    "dsts": [
      "ipv4:192.0.2.89",
      "ipv4:198.51.100.34",
      "ipv6:2000::1:2345:6789:abcd"
    ]
  }
}
HTTP/1.1 200 OK
 Content-Length: TBA
 Content-Type: application/alto-endpointcost+json
{
  "meta": {
           "cost type": {
           "cost-mode": "numerical",
           "cost-metric":"delayjitter"
    }
   },
  "endpoint-cost-map": {
           "ipv4:192.0.2.2": {
           "ipv4:192.0.2.89"    : 0
           "ipv4:198.51.100.34" : 1
           "ipv6:2000::1:2345:6789:abcd"  : 5
         }
      }
   }

2.3.3. Measurement Considerations

Method of Measurement or Calculation:

See Section 5.3 of [I-D.ietf-ippm-initial-registry] for potential measurement method.

Measurement Point(s) with Potential Measurement Domain:

See Section 4.1, Data sources for potential data sources.

Measurement Timing:

See Section 5.3.5 of [I-D.ietf-ippm-initial-registry] for Measurement Timing.

2.4. Cost Metric: Hop Count

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

Metric name:

Hop count

Metric Identifier:

hopcount

2.4.1. Intended Semantics

Metric Description:

To specify the number of hops in the path between the source endpoint and the destination endpoint. The hop count is a basic measurement of distance in a network and can be exposed as Router Hops, in direct relation to the routing protocols originating this information.

Metric Representation:

The metric value type is a single 'JSONNumber' type value conforming to the number specification [RFC8259], Section 6. The number MUST be an integer and non-negative. The value represents the number of hops.

2.4.2. Use and Example

This metric could be used as a cost metric constraint attribute used either together with cost metric attribute 'routingcost' or on its own or as a returned cost metric in the response.

Example 4: hopcount value on source-destination endpoint pairs

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

  {
    "cost-type": {"cost-mode" : "numerical",
     "cost-metric" : "hopcount"},
    "endpoints" : {
      "srcs": [ "ipv4:192.0.2.2" ],
      "dsts": [
        "ipv4:192.0.2.89",
        "ipv4:198.51.100.34",
        "ipv6:2000::1:2345:6789:abcd"
      ]
    }
  }

HTTP/1.1 200 OK
Content-Length: TBA
Content-Type: application/alto-endpointcost+json
{
    "meta": {
               "cost type": {
             "cost-mode": "numerical",
             "cost-metric":"hopcount"}
       }
    },
   "endpoint-cost-map": {
           "ipv4:192.0.2.2": {
           "ipv4:192.0.2.89"   : 5,
           "ipv4:198.51.100.34": 3,
           "ipv6:2000::1:2345:6789:abcd" : 2,
                             }
             }
 }

2.4.3. Measurement Considerations

Method of Measurement or Calculation:

The hop count can be calculated based on the number of routers from the source endpoint through which data must pass to reach the destination endpoint.

Measurement Point(s) with Potential Measurement Domain:

The hop count can be measured at the source endpoint by traceroute.

Measurement Timing:

Upon need, the traceroute can use UDP probe message or other implementations that use ICMP and TCP to discover the hop counts along the path from source endpoint to destination endpoint.

2.5. Cost Metric: Packet Loss

Metric name:

Packet loss

Metric Identifier:

pktloss

2.5.1. Intended Semantics

Metric Description:

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

Metric 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 loss.

2.5.2. Use and Example

This metric could be used as a cost metric constraint attribute used either together with cost metric attribute 'routingcost' or on its own or as a returned cost metric in the response.

Example 5: pktloss 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" : "pktloss"},
    "endpoints" : {
      "srcs": [ "ipv4:192.0.2.2" ],
      "dsts": [
        "ipv4:192.0.2.89",
        "ipv4:198.51.100.34",
        "ipv6:2000::1:2345:6789:abcd"
      ]
    }
  }

HTTP/1.1 200 OK
Content-Length: TBA
Content-Type: application/alto-endpointcost+json
{
    "meta": {
               "cost type": {
             "cost-mode": "numerical",
             "cost-metric":"pktloss"}
       }
    },
   "endpoint-cost-map": {
           "ipv4:192.0.2.2": {
           "ipv4:192.0.2.89"   : 0,
           "ipv4:198.51.100.34": 0,
           "ipv6:2000::1:2345:6789:abcd" : 0,
                             }
             }
 }

2.5.3. Measurement Considerations

Method of Measurement or Calculation:

See Section 2.6 of [RFC7680] for Measurement Method.

Measurement Point(s) with Potential Measurement Domain:

See Section 4.1 this document, Data sources.

Measurement Timing:

See Section 2 and Section 3 of [RFC7680] for Measurement Timing.

2.6. Cost Metric: Throughput

Metric name:

Throughput

Metric Identifier:

throughput

2.6.1. Intended Semantics

Metric Description:

To specify spatial and temporal throughput over the specified source and destination. The spatial aggregation level is specified in the query context (e.g., PID to PID, or endpoint to endpoint).

Metric 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 Mbps.

2.6.2. Use and Example

This metric could be used as a cost metric constraint attribute used either together with cost metric attribute 'routingcost' or on its own or as a returned cost metric in the response.

Example 5: throughtput 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" : "throughput"},
  "endpoints" : {
    "srcs": [ "ipv4:192.0.2.2" ],
    "dsts": [
       "ipv4:192.0.2.89",
       "ipv4:198.51.100.34",
       "ipv6:2000::1:2345:6789:abcd"
    ]
  }
}

HTTP/1.1 200 OK
Content-Length: TBA
Content-Type: application/alto-endpointcost+json
{
  "meta": {
     "cost type": {
        "cost-mode": "numerical",
        "cost-metric":"throughput"
    }
  }
  "endpoint-cost-map": {
    "ipv4:192.0.2.2": {
      "ipv4:192.0.2.89"   : 25.6,
      "ipv4:198.51.100.34": 12.8,
      "ipv6:2000::1:2345:6789:abcd" : 42.8,
  }
}

2.6.3. Measurement Considerations

Method of Measurement or Calculation:

See Section 3.3 of [RFC6349] for Measurement Method.

Measurement Point(s) with Potential Measurement Domain:

See Section 4.1 of this document.

Measurement Timing:

Similar to RTT. See Section 4.3.5 of [I-D.ietf-ippm-initial-registry] for Measurement Timing.

3. Traffic Engineering Performance Cost Metrics

This section introduces ALTO network performance metrics that may be aggregated from network metrics measured on links and specified in other documents. In particular, the bandwidth related metrics specified in this section are only available through link level measurements. For some of these metrics, the ALTO Server may further expose aggregated values while specifying the aggregation laws.

3.1. Cost Metric: Link Maximum Reservable Bandwidth

Metric name:

Maximum Reservable Bandwidth

Metric Identifier:

maxresbw

3.1.1. Intended Semantics

Metric Description:

To specify spatial and temporal maximum reservable bandwidth over the specified source and destination. The value is corresponding to the maximum bandwidth that can be reserved (motivated from RFC 3630 Sec. 2.5.7.). The spatial aggregation unit is specified in the query context (e.g., PID to PID, or endpoint to endpoint).

Metric Representation:

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

3.1.2. Use and Example

This metric could be used as a cost metric constraint attribute used either together with cost metric attribute 'routingcost' or on its own or as a returned cost metric in the response.

  Example 6: maxresbw 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":  "maxresbw"},
    "endpoints":  {
      "srcs": [ "ipv4 : 192.0.2.2" ],
      "dsts": [
        "ipv4:192.0.2.89",
        "ipv4:198.51.100.34",
        "ipv6:2000::1:2345:6789:abcd"
      ]
    }
  }

HTTP/1.1 200 OK
Content-Length: TBA
Content-Type: application/alto-endpointcost+json
{
    "meta": {
           "cost-type": {
           "cost-mode": "numerical",
           "cost-metric": "maxresbw"
           }
    },
  " endpoint-cost-map": {
          "ipv4:192.0.2.2" {
          "ipv4:192.0.2.89" :    0,
          "ipv4:198.51.100.34": 2000,
          "ipv6:2000::1:2345:6789:abcd":  5000,
                            }
           }
}

3.1.3. Measurement Considerations

Method of Measurement or Calculation:

Maximum Reservable Bandwidth is the bandwidth measured between two directly connected IS-IS neighbors or OSPF neighbors. See Section 3.5 of [RFC5305] for Measurement Method.

Measurement Point(s) with Potential Measurement Domain:

See Section 4.1 this document for discussions.

Measurement Timing:

See Section 3.5 of [RFC5305] and Section 5 of [RFC7810] for Measurement Timing.

3.2. Cost Metric: Link Residue Bandwidth

Metric name:

Residue Bandwidth

Metric Identifier:

residuebw

3.2.1. Intended Semantics

Metric Description:

To specify spatial and temporal residual bandwidth over the specified source and destination. The value is calculated by subtracting tunnel reservations from Maximum Bandwidth (motivated from [RFC7810], Section 4.5.). The spatial aggregation unit is specified in the query context (e.g., PID to PID, or endpoint to endpoint).

Metric Representation:

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

3.2.2. Use and Example

This metric could be used as a cost metric constraint attribute used either together with cost metric attribute 'routingcost' or on its own or as a returned cost metric in the response.

Example 7: residuebw 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":  "residuebw"},
   "endpoints":  {
     "srcs": [ "ipv4 : 192.0.2.2" ],
     "dsts": [
       "ipv4:192.0.2.89",
       "ipv4:198.51.100.34",
       "ipv6:2000::1:2345:6789:abcd"
     ]
   }
  }

HTTP/1.1 200 OK
Content-Length: TBA
Content-Type: application/alto-endpointcost+json
{
   "meta": {
          "cost-type" {
          "cost-mode": "numerical",
          "cost-metric": "residuebw"
        }
    },
"endpoint-cost-map" {
         "ipv4:192.0.2.2" {
         "ipv4:192.0.2.89" :    0,
         "ipv4:198.51.100.34": 2000,
         "ipv6:2000::1:2345:6789:abcd":  5000,
                       }
        }
}

3.2.3. Measurement Considerations

Method of Measurement or Calculation:

Residue Bandwidth is the Unidirectional Residue bandwidth measured between two directly connected IS-IS neighbors or OSPF neighbors. See Section 4.5 of [RFC7810] for Measurement Method.

Measurement Point(s) with Potential Measurement Domain:

See Section 4.1 of this document.

Measurement Timing:

See Section 5 of [RFC7810] for Measurement Timing.

4. Operational Considerations

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

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

4.1. Source Considerations

The additiona 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 [RFC7971]. In particular, the cost metrics defined in this document can be computed using routing systems as the data sources. Mechanisms defined in [RFC2681], [RFC3393], [RFC7679], [RFC7680], [RFC3630], [RFC3784], [RFC7471], [RFC7810], [RFC7752] and [I-D.ietf-idr-te-pm-bgp] that allow an ALTO Server to retrieve and derive the necessary information to compute the metrics that we describe in this document.

4.2. 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 which has "cost-source" being "sla", but the backward client will ignore this field and consider the metric estimation. [RFC7285]

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

4.3. 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.

4.3.1. Configuration Parameters Considerations

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

4.3.2. Availability Considerations

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

5. Security Considerations

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

However concerns addressed in Sections "15.1 Authenticity and Integrity of ALTO Information", "15.2 Potential Undesirable Guidance from Authenticated ALTO Information" and "15.3 Confidentiality of ALTO Information" remain of utmost importance. Indeed, TE performance is a highly sensitive ISP information, therefore, sharing TE metric values in numerical mode requires full mutual confidence between the entities managing the ALTO Server and Client. Numerical TE performance information will most likely be distributed by ALTO Servers to 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.

6. 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 |
+------------+--------------------+
| owdelay    | See Section 2.1    |
| rtt        | See Section 2.2    |
| pdv        | See Section 2.3    |
| hopcount   | See Section 2.4    |
| pktloss    | See Section 2.5    |
| throughput | See Section 2.6    |
| maxresbw   | See Section 3.1    |
| residuebw  | See Section 3.2    |
+------------+--------------------+

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

+------------+------------------------+
| Identifier | Intended Semantics     |
+------------+------------------------+
| estimation | Values by estimation   |
| sla        | Values reflect service |
|            | level agreement        |
+------------+------------------------+

7. Acknowledgments

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

8. References

8.1. Normative References

8.2. Informative References

Authors' Addresses