Content Delivery Network Interconnection (CDNI) Capacity Capability Advertisement Extensions

Internet Engineering Task Force (IETF)                           A. Ryan
Request for Comments: 9808                              Disney Streaming
Category: Standards Track                                   B. Rosenblum
ISSN: 2070-1721                                                   Vecima
                                                               N. Sopher
                                                                   Qwilt
                                                               July 2025


  Content Delivery Network Interconnection (CDNI) Capacity Capability
                        Advertisement Extensions

Abstract

   This specification defines a set of additional Capability Objects
   that provide information about current downstream CDN (dCDN)
   utilization and specified usage limits to the delegating upstream CDN
   (uCDN) in order to inform traffic delegation decisions.

   This document supplements the CDNI Capability Objects, defined in RFC
   8008 as part of the Footprint & Capabilities Advertisement Interface
   (FCI), with two additional Capability Objects: FCI.CapacityLimits and
   FCI.Telemetry.

Status of This Memo

   This is an Internet Standards Track document.

   This document is a product of the Internet Engineering Task Force
   (IETF).  It represents the consensus of the IETF community.  It has
   received public review and has been approved for publication by the
   Internet Engineering Steering Group (IESG).  Further information on
   Internet Standards is available in Section 2 of RFC 7841.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at
   https://www.rfc-editor.org/info/rfc9808.

Copyright Notice

   Copyright (c) 2025 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
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   in the Revised BSD License.

Table of Contents

   1.  Introduction
     1.1.  Terminology
     1.2.  Requirements Language
     1.3.  Objectives
   2.  CDNI Additional Capability Objects
     2.1.  Telemetry Capability Object
       2.1.1.  Telemetry Source Object
         2.1.1.1.  Telemetry Source Types
         2.1.1.2.  Telemetry Source Metric Object
       2.1.2.  Telemetry Capability Object Serialization
     2.2.  CapacityLimits Capability Object
       2.2.1.  CapacityLimit Object
         2.2.1.1.  CapacityLimit Types
         2.2.1.2.  CapacityLimitTelemetrySource Object
       2.2.2.  CapacityLimit Object Serialization
   3.  IANA Considerations
     3.1.  CDNI Payload Types
       3.1.1.  CDNI FCI.Telemetry Payload Type
       3.1.2.  CDNI FCI.CapacityLimits Payload Type
     3.2.  CDNI Telemetry Source Types Registry
       3.2.1.  CDNI Generic Telemetry Source Type
     3.3.  CDNI Capacity Limit Types Registry
   4.  Security Considerations
   5.  References
     5.1.  Normative References
     5.2.  Informative References
   Acknowledgements
   Authors' Addresses

1.  Introduction

   While delegating traffic from an upstream CDN (uCDN) to a downstream
   CDN (dCDN), it is important to ensure that an appropriate amount of
   traffic is delegated.  To achieve that, this specification defines a
   feedback mechanism to inform the delegator how much traffic may be
   delegated.  The traffic level information provided by that interface
   will be consumed by services, such as a request router, to inform
   that service's traffic delegation decisions.  The provided
   information is advisory and does not represent a guarantee,
   commitment, or reservation of capacity.

   This document defines and registers CDNI Payload Types (as defined in
   Section 7.1 of [RFC8006]).  These Payload Types are used for
   Capability Objects, which are added to those defined in Section 4 of
   [RFC8008].

1.1.  Terminology

   The following term is used throughout this document:

   CDN:  Content Delivery Network

   Additionally, this document reuses the terminology defined in
   [RFC6707].  Specifically, the following CDNI acronyms are used:

   uCDN:  upstream CDN

   dCDN:  downstream CDN

1.2.  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.

1.3.  Objectives

   To enable information exchange between a uCDN and a dCDN regarding
   acceptable levels of traffic delegation, the following process has
   been defined:

      In normal operation, a uCDN will communicate with a dCDN, via an
      interface, to collect and understand any limits that a dCDN has
      set forth for traffic delegation from a uCDN.  These limits will
      come in the form of metrics such as bits per second, requests per
      second, etc.  These limits can be thought of as Not to Exceed
      (NTE) limits.

      The dCDN should provide access to a Telemetry Source of near real-
      time metrics that the uCDN can use to track current usage.  The
      uCDN should compare its current usage to the limits the dCDN has
      put forth and adjust traffic delegation decisions accordingly to
      keep current usage under the specified limits.

      In summary, the dCDN will inform the uCDN of the amount of traffic
      that may be delegated.  Additionally, it will provide a Telemetry
      Source aligned with this limit, allowing the uCDN to monitor its
      current usage against the advertised value.  Having a limit and a
      corresponding Telemetry Source creates an unambiguous definition
      understood by both parties.

   Limits that are communicated from the dCDN to the uCDN should be
   considered valid based on the Time to Live (TTL) provided by a
   mechanism of the underlying transport, e.g., an HTTP Cache-Control
   header.  The intention is that the limits would have a long-lived TTL
   and would represent a reasonable peak utilization limit that the uCDN
   should target.  If the underlying transport does not provide a
   mechanism for the dCDN to communicate the TTL of the limits, the TTL
   should be communicated through an out-of-band mechanism agreed upon
   between the dCDN and uCDN.

2.  CDNI Additional Capability Objects

   Section 5 of [RFC8008] describes the FCI Capability Advertisement
   Object, which contains a CDNI Capability Object as well as the
   capability-type (a CDNI Payload Type).  The section also defines the
   Capability Objects per such type.  Below, we define two additional
   Capability Objects.

      |  Note: In the following sections, the term "mandatory-to-
      |  specify" is used to convey which properties MUST be included
      |  when serializing a given capability object.  When mandatory-to-
      |  specify is defined as a "Yes" for an individual property, it
      |  means that if the object containing that property is included
      |  in an FCI message, then the mandatory-to-specify property MUST
      |  be included.

2.1.  Telemetry Capability Object

   The Telemetry Capability Object advertises a list of Telemetry
   Sources made available to the uCDN by the dCDN.  In this document,
   telemetry data is being defined as near real-time aggregated metrics
   of dCDN utilization, such as bits per second egress, and is specific
   to the uCDN and dCDN traffic delegation relationship.

   Telemetry data is uniquely defined by a source ID, a metric name, and
   the footprints that are associated with an FCI Capabilities
   advertisement.  When defining a CapacityLimit, the meaning of a limit
   might be ambiguous if the uCDN and dCDN are observing telemetry via
   different data sources.  A dCDN-provided Telemetry Source that both
   parties reference serves as a non-ambiguous metric for use when
   comparing current usage to a limit.

   Telemetry data is important for making informed traffic delegation
   decisions.  Additionally, it is essential in providing visibility of
   traffic that has been delegated.  In situations where there are
   multiple CDN delegations, a uCDN will need to aggregate the usage
   information from any dCDNs to which it delegated when asked to
   provide usage information, otherwise the traffic may seem unaccounted
   for.

   Example: A Content Provider delegates traffic directly to a uCDN, and
   that uCDN delegates that traffic to a dCDN.  When the Content
   Provider polls the uCDN telemetry interface, any of the traffic the
   uCDN delegated to the dCDN would become invisible to the Content
   Provider, unless the uCDN aggregates the dCDN telemetry with its own
   metrics.

   Property:  sources

      Description:  Telemetry Sources made available to the uCDN.

      Type:  A JSON array of Telemetry Source objects (see
         Section 2.1.1).

      Mandatory-to-Specify:  Yes

2.1.1.  Telemetry Source Object

   The Telemetry Source Object is made of an associated type, a list of
   exposed metrics, and type-specific configuration data.

   Property:  id

      Description:  An identifier of a telemetry source.  The ID string
         assigned to this Telemetry Source MUST be unique across all
         Telemetry Source objects in the advertisement containing this
         Telemetry Source Object.  The ID string MUST remain consistent
         for the same source reference across advertisements.

      Type:  String

      Mandatory-to-Specify:  Yes

   Property:  type

      Description:  A valid Telemetry Source Type (see Section 2.1.1.1).

      Type:  String

      Mandatory-to-Specify:  Yes

   Property:  metrics

      Description:  The metrics exposed by this source.

      Type:  A JSON array of Telemetry Source Metric Objects (see
         Section 2.1.1.2).

      Mandatory-to-Specify:  Yes

   Property:  configuration

      Description:  A source-specific representation of the Telemetry
         Source configuration.  For the generic source type, this
         configuration format is defined as out-of-band.  For other
         types, the configuration format will be specified in a yet-to-
         be-defined telemetry interface specification.  The goal of this
         element is to allow for forward compatibility with a formal
         telemetry interface.

      Type:  A JSON object, the structure of which is specific to the
         Telemetry Source and outside the scope of this document.

      Mandatory-to-Specify:  No

2.1.1.1.  Telemetry Source Types

   At the time of this writing, the "CDNI Telemetry Source Types"
   registry is limited to a single type: generic (see Table 3 in
   Section 3.2.1).

2.1.1.2.  Telemetry Source Metric Object

   The Telemetry Source Metric Object describes the metric to be
   exposed.

   Property:  name

      Description:  An identifier for this metric.  This name MUST be
         unique among metric objects within the containing Telemetry
         Source.  The name MUST remain consistent for the same source
         reference across advertisements.

      Type:  String

      Mandatory-to-Specify:  Yes

   Property:  time-granularity

      Description:  The time, in seconds, representing the metric data.
         For example, a value representing the last 5 minutes would have
         a time-granularity of 300.

      Type:  Unsigned Integer

      Mandatory-to-Specify:  No

   Property:  data-percentile

      Description:  The percentile calculation the data represents,
         i.e., 50 percentile would equate to the median over the time-
         granularity.  Lack of a data-percentile indicates that the data
         MUST be the mean over the time representation.

      Type:  Unsigned Integer

      Mandatory-to-Specify:  No

   Property:  latency

      Description:  Time in seconds that the data is behind real-time.
         This is important to specify to help the uCDN understand how
         long it might take to reflect traffic adjustments in the
         metrics.

      Type:  Unsigned Integer

      Mandatory-to-Specify:  No

2.1.2.  Telemetry Capability Object Serialization

   The following shows an example of a Telemetry Capability Object,
   including two metrics for a source, that is scoped to a footprint.

   {
     "capabilities": [
       {
         "capability-type": "FCI.Telemetry",
         "capability-value": {
           "sources": [
             {
               "id": "capacity_metrics_region1",
               "type": "generic",
               "metrics": [
                 {
                   "name": "egress_5m",
                   "time-granularity": 300,
                   "data-percentile": 50,
                   "latency": 1500
                 },
                 {
                   "name": "requests_5m",
                    ...
                 }
               ]
             }
           ]
         },
         "footprints": [
           <footprint objects>
         ]
       }
     ]
   }

2.2.  CapacityLimits Capability Object

   The CapacityLimits Capability Object enables the dCDN to specify
   traffic delegation limits to a uCDN within an FCI Capabilities
   advertisement.  The limits specified by the dCDN will inform the uCDN
   on how much traffic may be delegated to the dCDN.  The limits
   specified by the dCDN should be considered NTE limits.  The limits
   should be based on near real-time telemetry data that the dCDN
   provides to the uCDN.  In other words, for each limit that is
   advertised, there should also exist a Telemetry Source that provides
   current utilization data against the particular advertised limit.

   Property:  limits

      Description:  A collection of CapacityLimit Objects.

      Type:  A JSON array of CapacityLimit Objects (see Section 2.2.1).

      Mandatory-to-Specify:  Yes

2.2.1.  CapacityLimit Object

   A CapacityLimit Object is used to represent traffic limits for
   delegation from the uCDN towards the dCDN.  The limit object is
   scoped to the footprint associated with the FCI Capabilities
   advertisement encompassing this object.  Limits MUST be considered
   using a logical "AND": A uCDN will need to ensure that all limits are
   considered rather than choosing only the most specific.

   Property:  limit-type

      Description:  The units of maximum-hard and maximum-soft.

      Type:  String.  One of the values listed in Section 2.2.1.1.

      Mandatory-to-Specify:  Yes

   Property:  id

      Description:  Specifies an identifier associated with a limit.
         This MAY be used as a relational identifier to a specific
         CapacityLimit Object.  If specified, this identifier MUST be
         unique among specified identifiers associated with any other
         CapacityLimit Objects in the advertisement containing this
         CapacityLimit Object.

      Type:  String

      Mandatory-to-Specify:  No

   Property:  maximum-hard

      Description:  The maximum unit of capacity that is available for
         use.

      Type:  Unsigned Integer

      Mandatory-to-Specify:  Yes

   Property:  maximum-soft

      Description:  A soft limit at which a uCDN SHOULD reduce traffic
         before hitting the hard limit.  This value MUST be less than
         the value of maximum-hard.  If this value is not specified, it
         is equal to the value of maximum-hard.

      Type:  Unsigned Integer

      Mandatory-to-Specify:  No

   Property:  current

      Description:  Specifies the current usage value of the limit.  It
         is NOT RECOMMENDED to specify the current usage value inline
         with the FCI.CapacityLimits advertisements as it will reduce
         the ability to cache the response, but this mechanism exists
         for simple use cases where an external Telemetry Source cannot
         be feasibly implemented.  The intended method for providing
         telemetry data is to reference a Telemetry Source Object (see
         Section 2.1.1) to poll for the current usage.

      Type:  Unsigned Integer

      Mandatory-to-Specify:  No

   Property:  telemetry-source

      Description:  The mapping of each particular limit to a specific
         metric with relevant real-time data provided by a Telemetry
         Source.

      Type:  CapacityLimitTelemetrySource object (see Section 2.2.1.2).

      Mandatory-to-Specify:  No

2.2.1.1.  CapacityLimit Types

   Below are listed the valid limit-type entries registered in the "CDNI
   Capacity Limit Types" registry.  The values specified here represent
   the types that were identified as being the most relevant metrics for
   the purposes of traffic delegation between CDNs.

               +=====================+=====================+
               | Capacity Limit Type | Units               |
               +=====================+=====================+
               | egress              | Bits per second     |
               +---------------------+---------------------+
               | requests            | Requests per second |
               +---------------------+---------------------+
               | storage-size        | Total bytes         |
               +---------------------+---------------------+
               | storage-objects     | Count               |
               +---------------------+---------------------+
               | sessions            | Count               |
               +---------------------+---------------------+
               | cache-size          | Total bytes         |
               +---------------------+---------------------+

                                  Table 1

2.2.1.2.  CapacityLimitTelemetrySource Object

   The CapacityLimitTelemetrySource Object refers to a specific metric
   within a Telemetry Source.

   Property:  id

      Description:  Reference to the "id" of a Telemetry Source defined
         by a Telemetry Capability Object as defined in Section 2.1.

      Type:  String

      Mandatory-to-Specify:  Yes

   Property:  metric

      Description:  Reference to the "name" property of a metric defined
         within a Telemetry Source of a Telemetry Capability object.

      Type:  String

      Mandatory-to-Specify:  Yes

2.2.2.  CapacityLimit Object Serialization

   The following shows an example of an FCI.CapacityLimits object.

   {
     "capabilities": [
       {
         "capability-type":"FCI.CapacityLimits",
         "capability-value":{
           "limits":[
             {
               "id":"capacity_limit_region1",
               "limit-type":"egress",
               "maximum-hard":50000000000,
               "maximum-soft":25000000000,
               "telemetry-source":{
                 "id":"capacity_metrics_region1",
                 "metric":"egress_5m"
               }
             }
           ]
         },
         "footprints":[
           "<footprint objects>"
         ]
       }
     ]
   }

3.  IANA Considerations

3.1.  CDNI Payload Types

   Per this document, IANA has registered two additional payload types
   in the "CDNI Payload Types" registry within the "Content Delivery
   Network Interconnection (CDNI) Parameters" registry group:

                    +====================+===========+
                    | Payload Type       | Reference |
                    +====================+===========+
                    | FCI.Telemetry      | RFC 9808  |
                    +--------------------+-----------+
                    | FCI.CapacityLimits | RFC 9808  |
                    +--------------------+-----------+

                                 Table 2

3.1.1.  CDNI FCI.Telemetry Payload Type

   Purpose:  The purpose of this Payload Type is to list the supported
      Telemetry Sources and the metrics made available by each source.

   Interface:  FCI

   Encoding:  See Section 2.1.

3.1.2.  CDNI FCI.CapacityLimits Payload Type

   Purpose:  The purpose of this Payload Type is to define Capacity
      Limits based on utilization metrics corresponding to Telemetry
      Sources provided by the dCDN.

   Interface:  FCI

   Encoding:  See Section 2.2.

3.2.  CDNI Telemetry Source Types Registry

   IANA has added the following new registry within the "Content
   Delivery Network Interconnection (CDNI) Parameters" registry group at
   <https://www.iana.org/assignments/cdni-parameters>:

   Registry Name:  CDNI Telemetry Source Types

   Registry Description:  The "CDNI Telemetry Source Types" registry
      defines the valid values for the "type" property of the Telemetry
      Source object defined in Section 2.1.1.

   Registration Procedure:  The registry follows the Specification
      Required policy as defined in [RFC8126].  The designated expert
      should consider the following guidelines when evaluating
      registration requests:

      *  The new type definition does not duplicate existing types.

      *  The review should verify that the Telemetry Source is
         applicable to the CDNI use cases and that the description is
         clear and unambiguous.

      *  The registration is applicable for general use and is not
         proprietary.

      *  The "configuration" property has a fully specified object
         definition with a description of each defined property.

   The following value has been registered:

    +=============+=======================================+===========+
    | Source Type | Description                           | Reference |
    +=============+=======================================+===========+
    | generic     | An object that allows for             | RFC 9808  |
    |             | advertisement of generic data sources |           |
    +-------------+---------------------------------------+-----------+

                                  Table 3

3.2.1.  CDNI Generic Telemetry Source Type

   Purpose:  The purpose of this Telemetry Source Type is to provide a
      source-agnostic telemetry type that may be used for generic
      Telemetry Source advertisement.

   Usage:  See Section 2.1.1.

3.3.  CDNI Capacity Limit Types Registry

   IANA has added the following new registry within the "Content
   Delivery Network Interconnection (CDNI) Parameters" registry group at
   <https://www.iana.org/assignments/cdni-parameters>:

   Registry Name:  CDNI Capacity Limit Types

   Registry Description:  The "CDNI Capacity Limit Types" registry
      defines the valid values of the "limit-type" property of a
      CapacityLimit Object defined in Section 2.2.1.

   Registration Procedure:  The registry follows the Specification
      Required policy as defined in [RFC8126].  The designated expert
      should consider the following guidelines when evaluating
      registration requests:

      *  The new capacity limit-type does not duplicate existing
         entries.

      *  The submission has a defined purpose.  The newly defined
         capacity limit-type should be clearly justified in the context
         of one or more CDNI use cases.

      *  The description of the capacity limit-type is well-documented
         and unambiguous.

   The following values have been registered:

         +=====================+=====================+===========+
         | Capacity Limit Type | Units               | Reference |
         +=====================+=====================+===========+
         | egress              | Bits per second     | RFC 9808  |
         +---------------------+---------------------+-----------+
         | requests            | Requests per second | RFC 9808  |
         +---------------------+---------------------+-----------+
         | storage-size        | Total bytes         | RFC 9808  |
         +---------------------+---------------------+-----------+
         | storage-objects     | Count               | RFC 9808  |
         +---------------------+---------------------+-----------+
         | sessions            | Count               | RFC 9808  |
         +---------------------+---------------------+-----------+
         | cache-size          | Total bytes         | RFC 9808  |
         +---------------------+---------------------+-----------+

                                  Table 4

   Usage:  See Section 2.2.1.1.

4.  Security Considerations

   This specification is in accordance with the CDNI Request Routing:
   Footprint and Capabilities Semantics.  As such, it is subject to the
   security and privacy considerations as defined in Section 7 of
   [RFC8008].

5.  References

5.1.  Normative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,
              <https://www.rfc-editor.org/info/rfc2119>.

   [RFC8008]  Seedorf, J., Peterson, J., Previdi, S., van Brandenburg,
              R., and K. Ma, "Content Delivery Network Interconnection
              (CDNI) Request Routing: Footprint and Capabilities
              Semantics", RFC 8008, DOI 10.17487/RFC8008, December 2016,
              <https://www.rfc-editor.org/info/rfc8008>.

   [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>.

5.2.  Informative References

   [RFC6707]  Niven-Jenkins, B., Le Faucheur, F., and N. Bitar, "Content
              Distribution Network Interconnection (CDNI) Problem
              Statement", RFC 6707, DOI 10.17487/RFC6707, September
              2012, <https://www.rfc-editor.org/info/rfc6707>.

   [RFC8006]  Niven-Jenkins, B., Murray, R., Caulfield, M., and K. Ma,
              "Content Delivery Network Interconnection (CDNI)
              Metadata", RFC 8006, DOI 10.17487/RFC8006, December 2016,
              <https://www.rfc-editor.org/info/rfc8006>.

   [SVTA]     "Streaming Video Technology Alliance Home Page",
              <https://www.svta.org>.

Acknowledgements

   The authors would like to express their gratitude to the members of
   the Streaming Video Technology Alliance [SVTA] Open Caching Working
   Group for their guidance, contribution, and review.

Authors' Addresses

   Andrew Ryan
   Disney Streaming
   1211 Avenue of the Americas
   New York, NY 10036
   United States of America
   Email: andrew@andrewnryan.com


   Ben Rosenblum
   Vecima
   4375 River Green Pkwy #100
   Duluth, GA 30096
   United States of America
   Email: ben@rosenblum.dev


   Nir B. Sopher
   Qwilt
   6, Ha'harash
   Hod HaSharon 4524079
   Israel
   Email: nir@apache.org