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Content Delivery Network Interconnection (CDNI) Delegation Using the Automated Certificate Management Environment
RFC 9538

Document Type RFC - Proposed Standard (February 2024)
Authors Frédéric Fieau , Emile Stephan , Sanjay Mishra
Last updated 2024-02-22
RFC stream Internet Engineering Task Force (IETF)
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IESG Responsible AD Francesca Palombini
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RFC 9538

Internet Engineering Task Force (IETF)                     F. Fieau, Ed.
Request for Comments: 9538                                    E. Stephan
Category: Standards Track                                         Orange
ISSN: 2070-1721                                                S. Mishra
                                                           February 2024

  Content Delivery Network Interconnection (CDNI) Delegation Using the
              Automated Certificate Management Environment


   This document defines metadata to support delegating the delivery of
   HTTPS content between two or more interconnected Content Delivery
   Networks (CDNs).  Specifically, this document defines a Content
   Delivery Network Interconnection (CDNI) Metadata interface object to
   enable delegation of X.509 certificates leveraging delegation schemes
   defined in RFC 9115.  Per RFC 9115, delegating entities can remain in
   full control of the delegation and can revoke it at any time.  This
   avoids the need to share private cryptographic key material between
   the involved entities.

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

Copyright Notice

   Copyright (c) 2024 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
   ( 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 Revised BSD License text as described in Section 4.e of the
   Trust Legal Provisions and are provided without warranty as described
   in the Revised BSD License.

Table of Contents

   1.  Introduction
     1.1.  Terminology
   2.  Advertising Delegation Metadata for CDNI through FCI
   3.  ACME Delegation Metadata for CDNI
     3.1.  ACMEDelegationMethod Object
       3.1.1.  Examples
   4.  IANA Considerations
     4.1.  CDNI MI ACMEDelegationMethod Payload Type
   5.  Security Considerations
   6.  References
     6.1.  Normative References
     6.2.  Informative References
   Authors' Addresses

1.  Introduction

   Content delivery over HTTPS using two or more cooperating CDNs along
   the path requires credential management, specifically when DNS-based
   redirection is used.  In such cases, an upstream CDN (uCDN) needs to
   delegate its credentials to a downstream CDN (dCDN) for content

   [RFC9115] defines delegation methods that allow a uCDN on behalf of
   the content provider, the holder of the domain, to generate on-demand
   an X.509 certificate that binds the designated domain name with a key
   pair owned by the dCDN.  For further details, please refer to
   Sections 1 and of [RFC9115].

   This document defines CDNI Metadata to make use of HTTPS delegation
   between a uCDN and a dCDN based on the mechanism specified in
   [RFC9115].  Furthermore, it adds a delegation method to the "CDNI
   Payload Types" IANA registry.

   Section 2 presents delegation metadata for the Footprint &
   Capabilities Advertisement interface (FCI).  Section 3 addresses the
   metadata for handling HTTPS delegation with the Metadata interface.

1.1.  Terminology

   This document uses terminology from CDNI framework documents such as:
   CDNI framework document [RFC7336] and CDNI interface specifications
   documents: CDNI Metadata interface [RFC8006] and CDNI Footprint and
   Capabilities Advertisement interface [RFC8008].  It also uses
   terminology from Section 1.2 of [RFC8739] and Section 1.1 of
   [RFC9115], including Short-Term, Automatically Renewed (STAR), as
   applied to X.509 certificates.

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "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.

2.  Advertising Delegation Metadata for CDNI through FCI

   The Footprint & Capabilities Advertisement interface (FCI) defined in
   [RFC8008] allows a dCDN to send a FCI capability type object to a

   This document uses the CDNI Metadata capability object serialization
   from [RFC8008] for a CDN that supports delegation methods.

   The following is an example of the supported delegated methods
   capability object for a dCDN implementing the ACME delegation method.

     "capabilities": [
         "capability-type": "FCI.Metadata",
         "capability-value": {
           "metadata": [
             // list of supported delegation methods
         "footprints": [
           "Footprint objects"

3.  ACME Delegation Metadata for CDNI

   When a uCDN delegates the delivery of HTTPS traffic to a dCDN using
   DNS redirection [RFC7336], the dCDN must use a certificate bound to
   the origin's name to successfully authenticate to the end-user (see
   also Section of [RFC9115]).

   To that end, this section defines the AcmeDelegationMethod object,
   which describes metadata for using the ACME delegation interface

   The ACMEDelegationMethod applies to both ACME STAR delegation, which
   provides a delegation model based on short-term certificates with
   automatic renewal (Section 2.3.2 of [RFC9115]), and non-STAR
   delegation, which allows delegation between CDNs using long-term
   certificates (Section 2.3.3 of [RFC9115]).

   Figure 1 provides a high-level view of the combined CDNI and ACME
   delegation message flows to obtain a STAR certificate from the
   Certification Authority (CA) bound to the Content Provider's (CP)

.----.                .----.               .----.                 .----.
|dCDN|                |uCDN|               | CP |                 | CA |
'-+--'                '-+--'               '--+-'                 '--+-'
  |     GET metadata    |                     |                      |
  +--------[CDNI]------>|                     |                      |
  |   200 OK, metadata  |                     |                      |
  |  (inc. dele config) |                     |                      |
  |<-------[CDNI]-------+                     |                      |
  |                     |                     |                      |
  |    GET delegation   |                     |                      |
  +-----[ACME dele]---->|                     |                      |
  | 200 OK, delegation  |                     |                      |
  | (inc. CSR template) |                     |                      |
  |<----[ACME dele]-----+                     |                      |
  |                     |                     |                      |
  +----.                |                     |                      |
  |    |                |                     |                      |
  |  create key pair and|                     |                      |
  |  CSR w/ delegated   |                     |                      |
  |  name               |                     |                      |
  |    |                |                     |                      |
  |<---'                |                     |                      |
  |                     |                     |                      |
  |     POST Order1     |                     |                      |
  +-----[ACME dele]---->|                     |                      |
  |                     |   forward Order1    |                      |
  |                     +-----[ACME dele]---->|                      |
  |                     |                     |     POST Order2      |
  |                     |                     +-----[ACME STAR]----->|
  |                     |                     |                      |
  |                     |                     |<---authorizations--->|
  |                     |                     |                      |
  |<---wait issuance--->|<---wait issuance--->|<---wait issuance---->|
  |                                                                  |
  |              (unauthenticated) GET star-certificate              |
  |                          certificate #1                          |
  |                              ...                                 |

    Figure 1: Example Call Flow of STAR Delegation in CDNI Showing
                       Two Levels of Delegation

      |  Note: The delegation object defined in Section of
      |  [RFC9115] only allows DNS mappings to be specified using CNAME
      |  RRs.  A future document updating [RFC9115] could expand the
      |  delegation object to also include SVCB/HTTPS-based mappings
      |  [RFC9460].

   Section 3.1 defines the objects used for bootstrapping the ACME
   delegation method between a uCDN and a delegate dCDN.

3.1.  ACMEDelegationMethod Object

   The ACMEDelegationMethod object allows a uCDN to define both STAR and
   non-STAR delegations.  The dCDN, the consumer of the delegation, can
   determine the type of delegation by the presence (or absence) of the
   "lifetime" property.  That is, the presence of the "lifetime"
   property explicitly means a short-term delegation with lifetime of
   the certificate based on that property (and the optional "lifetime-
   adjust" attribute).  A non-STAR delegation will not have the
   "lifetime" property in the delegation.  See also the examples in
   Section 3.1.1.

   The ACMEDelegationMethod object is defined with the properties shown

   *  Property: acme-delegation

      -  Description: A URL pointing at an ACME delegation object,
         either STAR or non-STAR, associated with the dCDN account on
         the uCDN ACME server (see Section of [RFC9115] for the
         details).  The URL MUST use the https scheme.

      -  Type: String

      -  Mandatory-to-Specify: Yes

   *  Property: time-window

      -  Description: Validity period of the certificate.  According to
         Section 4.3.4 of [RFC8006], a TimeWindow object is defined by a
         window "start" time and a window "end" time.  In the case of a
         STAR method, the "start" and "end" properties of the window
         MUST be understood respectively as the start-date and end-date
         of the certificate validity.  In the case of a non-STAR method,
         the "start" and "end" properties of the window MUST be
         understood, respectively, as the notBefore and notAfter fields
         of the certificate.

      -  Type: TimeWindow

      -  Mandatory-to-Specify: Yes

   *  Property: lifetime

      -  Description: See lifetime in Section 3.1.1 of [RFC8739]

      -  Type: Integer

      -  Mandatory-to-Specify: Yes, only if a STAR delegation method is

   *  Property: lifetime-adjust

      -  Description: See lifetime-adjust in Section 3.1.1 of [RFC8739]

      -  Type: Integer

      -  Mandatory-to-Specify: No

3.1.1.  Examples

   The following example shows an ACMEDelegationMethod object for a
   STAR-based ACME delegation.

     "generic-metadata-type": "MI.ACMEDelegationMethod",
     "generic-metadata-value": {
       "acme-delegation": "https://acme.ucdn.example/delegation/ogfr",
       "time-window": {
         "start": 1665417434,
         "end": 1665676634
       "lifetime": 345600,
       "lifetime-adjust": 259200

   The example below shows an ACMEDelegationMethod object for a non-STAR
   ACME delegation.  The delegation object is defined as per Section 4.3
   of [RFC8006].

     "generic-metadata-type": "MI.ACMEDelegationMethod",
     "generic-metadata-value": {
       "acme-delegation": "https://acme.ucdn.example/delegation/wSi5",
       "time-window": {
         "start": 1570982234,
         "end": 1665417434

4.  IANA Considerations

   Per this document, the following type has been registered in the
   "CDNI Payload Types" registry:

                  | Payload Type            | Reference |
                  | MI.ACMEDelegationMethod | RFC 9538  |

                                  Table 1

4.1.  CDNI MI ACMEDelegationMethod Payload Type

   Purpose:  The purpose of this Payload Type is to distinguish
      AcmeDelegationMethod MI objects (and any associated capability

   Interface:  MI/FCI

   Encoding:  See Section 3.1

5.  Security Considerations

   The metadata object defined in this document does not introduce any
   new security or privacy concerns over those already discussed in
   [RFC9115], [RFC8006], and [RFC8008].

   The reader is expected to understand the ACME delegation trust model
   (Section 7.1 of [RFC9115]) and security goal (Section 7.2 of
   [RFC9115]).  In particular, the reader is expected to understand that
   it is critical to protect the user account associated with the
   delegation; this account authorizes all the security-relevant
   operations between a dCDN and a uCDN over the ACME channel.  The
   dCDN's ACME account is also relevant to the privacy of the entire
   scheme; for example, the acme-delegation resource in the Metadata
   object is only accessible to the holder of the account key, who is
   allowed to fetch its content exclusively via POST-as-GET
   (Section of [RFC9115]).

   In addition, the Metadata interface authentication and
   confidentiality requirements defined in Section 8 of [RFC8006] MUST
   be followed.

   Implementers MUST adhere to the security considerations defined in
   Section 7 of [RFC8008], "Content Delivery Network Interconnection
   (CDNI) Request Routing: Footprint and Capabilities Semantics".

   When TLS is used to achieve the above security objectives, the
   general TLS usage guidance in [RFC9325] MUST be followed.

6.  References

6.1.  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,

   [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,

   [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,

   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
              2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
              May 2017, <>.

   [RFC8739]  Sheffer, Y., Lopez, D., Gonzalez de Dios, O., Pastor
              Perales, A., and T. Fossati, "Support for Short-Term,
              Automatically Renewed (STAR) Certificates in the Automated
              Certificate Management Environment (ACME)", RFC 8739,
              DOI 10.17487/RFC8739, March 2020,

   [RFC9115]  Sheffer, Y., López, D., Pastor Perales, A., and T.
              Fossati, "An Automatic Certificate Management Environment
              (ACME) Profile for Generating Delegated Certificates",
              RFC 9115, DOI 10.17487/RFC9115, September 2021,

   [RFC9325]  Sheffer, Y., Saint-Andre, P., and T. Fossati,
              "Recommendations for Secure Use of Transport Layer
              Security (TLS) and Datagram Transport Layer Security
              (DTLS)", BCP 195, RFC 9325, DOI 10.17487/RFC9325, November
              2022, <>.

6.2.  Informative References

   [RFC7336]  Peterson, L., Davie, B., and R. van Brandenburg, Ed.,
              "Framework for Content Distribution Network
              Interconnection (CDNI)", RFC 7336, DOI 10.17487/RFC7336,
              August 2014, <>.

   [RFC9460]  Schwartz, B., Bishop, M., and E. Nygren, "Service Binding
              and Parameter Specification via the DNS (SVCB and HTTPS
              Resource Records)", RFC 9460, DOI 10.17487/RFC9460,
              November 2023, <>.


   We would like to thank authors of the [RFC9115], Antonio Augustín
   Pastor Perales, Diego López, Thomas Fossati, and Yaron Sheffer.
   Additionally, our gratitude to Thomas Fossati who participated in the
   drafting, reviewing, and giving his feedback in finalizing this
   document.  We also thank CDNI co-chair Kevin Ma for his continual
   review and feedback during the development of this document.

Authors' Addresses

   Frédéric Fieau (editor)
   40-48, avenue de la République
   92320 Châtillon

   Emile Stephan
   2, avenue Pierre Marzin
   22300 Lannion

   Sanjay Mishra
   13100 Columbia Pike
   Silver Spring, MD 20904
   United States of America