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CoAP Transfer for the Certificate Management Protocol

Document Type Active Internet-Draft (ace WG)
Authors Mohit Sahni , Saurabh Tripathi
Last updated 2023-02-03 (Latest revision 2023-01-27)
Replaces draft-msahni-ace-cmpv2-coap-transport
RFC stream Internet Engineering Task Force (IETF)
Intended RFC status Proposed Standard
ARTART Last Call Review Incomplete, due 2022-10-27
OPSDIR Last Call Review Incomplete, due 2022-10-27
Additional resources Mailing list discussion
Stream WG state Submitted to IESG for Publication
Document shepherd Daniel Migault
Shepherd write-up Show Last changed 2022-06-15
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Consensus boilerplate Yes
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Responsible AD Paul Wouters
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IANA IANA review state Version Changed - Review Needed
IANA expert review state Need IANA Expert(s)
ACE                                                        M. Sahni, Ed.
Internet-Draft                                          S. Tripathi, Ed.
Intended status: Standards Track                      Palo Alto Networks
Expires: 31 July 2023                                    27 January 2023

         CoAP Transfer for the Certificate Management Protocol


   This document specifies the use of Constrained Application Protocol
   (CoAP) as a transfer mechanism for the Certificate Management
   Protocol (CMP).  CMP defines the interaction between various PKI
   entities for the purpose of certificate creation and management.
   CoAP is an HTTP-like client-server protocol used by various
   constrained devices in the IoT space.

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

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on 31 July 2023.

Copyright Notice

   Copyright (c) 2023 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 (
   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 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.

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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
     1.1.  Terminology . . . . . . . . . . . . . . . . . . . . . . .   3
   2.  CoAP Transfer Mechanism for CMP . . . . . . . . . . . . . . .   3
     2.1.  CoAP URI Format . . . . . . . . . . . . . . . . . . . . .   3
     2.2.  Discovery of CMP RA/CA  . . . . . . . . . . . . . . . . .   4
     2.3.  CoAP Request Format . . . . . . . . . . . . . . . . . . .   4
     2.4.  CoAP Block-Wise Transfer Mode . . . . . . . . . . . . . .   4
     2.5.  Multicast CoAP  . . . . . . . . . . . . . . . . . . . . .   5
     2.6.  Announcement PKIMessage . . . . . . . . . . . . . . . . .   5
   3.  Proxy Support . . . . . . . . . . . . . . . . . . . . . . . .   6
   4.  Security Considerations . . . . . . . . . . . . . . . . . . .   6
   5.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   8
   6.  Acknowledgments . . . . . . . . . . . . . . . . . . . . . . .   8
   7.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   9
     7.1.  Normative References  . . . . . . . . . . . . . . . . . .   9
     7.2.  Informative References  . . . . . . . . . . . . . . . . .  10
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  10

1.  Introduction

   The Certificate Management Protocol (CMP) [RFC4210] is used by the
   PKI entities for the generation and management of certificates.  One
   of the requirements of Certificate Management Protocol is to be
   independent of the transport protocol in use.  CMP has mechanisms to
   take care of required transactions, error reporting and protection of

   The Constrained Application Protocol (CoAP) defined in [RFC7252] ,
   [RFC7959] and [RFC8323] is a client-server protocol like HTTP.  It is
   designed to be used by constrained devices over constrained networks.
   The recommended transport for CoAP is UDP, however [RFC8323]
   specifies the support of CoAP over TCP, TLS and Websockets.

   This document specifies the use of CoAP over UDP as a transport
   medium for the CMP version 2 [RFC4210] , CMP version 3
   [I-D.ietf-lamps-cmp-updates] designated as CMP in this document and
   Lightweight CMP Profile [I-D.ietf-lamps-lightweight-cmp-profile] .
   This document, in general, follows the HTTP transfer for CMP
   specifications defined in [RFC6712] and specifies the requirements
   for using CoAP as a transfer mechanism for the CMP.

   This document also provides guidance on how to use a "CoAP-to-HTTP"
   proxy to ease adoption of CoAP transfer mechanism by enabling the
   interconnection with existing PKI entities already providing CMP over

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1.1.  Terminology

   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.  CoAP Transfer Mechanism for CMP

   A CMP transaction consists of exchanging PKIMessages [RFC4210]
   between PKI End Entities (EEs), Registration Authorities (RAs), and
   Certification Authorities (CAs).  If the EEs are constrained devices
   then they may prefer, as a CMP client, the use of CoAP instead of
   HTTP as the transfer mechanism.  The RAs and CAs, in general, are not
   constrained and can support both CoAP and HTTP Client and Server
   implementations.  This section specifies how to use CoAP as the
   transfer mechanism for the Certificate Management Protocol.

2.1.  CoAP URI Format

   The CoAP URI format is described in section 6 of [RFC7252].  The CoAP
   endpoints MUST support use of the path prefix "/.well-known/" as
   defined in [RFC8615] and the registered name "cmp" to help with
   endpoint discovery and interoperability.  Optional path segments MAY
   be added after the registered application name (i.e. after "/.well-
   known/cmp") to provide distinction.  The path segment 'p' followed by
   an arbitraryLabel <name> could for example support the
   differentiation of specific CAs or certificate profiles.  Further
   path segments, e.g., as specified in the Lightweight CMP Profile [I-
   D.ietf-lamps-lightweight-cmp-profile], could indicate PKI management
   operations using an operationLabel <operation>.  A valid full CMP URI
   can look like this:


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2.2.  Discovery of CMP RA/CA

   The EEs can be configured with enough information to form the CMP
   server URI.  The minimum information that can be configured is the
   scheme i.e. "coap:" or "coaps:" and the authority portion of the URI,
   e.g. "".  If the port number is not specified in the
   authority, then the default ports numbers MUST be assumed for the
   "coap:" and the "coaps:" scheme URIs.  The default port for coap:
   scheme URIs is 5683 and the default port for coaps: scheme URIs is
   5684 [RFC7252] .

   Optionally, in the environments where a Local Registration Authority
   (LRA) or a Local CA is deployed, EEs can also use the CoAP service
   discovery mechanism [RFC7252] to discover the URI of the Local RA or
   CA.  The CoAP CMP endpoints supporting service discovery MUST also
   support resource discovery in the CoRE Link Format as described in
   [RFC6690] . The Link MUST include the 'ct' attribute defined in
   section 7.2.1 of [RFC7252] with the value of "application/pkixcmp" as
   defined in the CoAP Content-Formats IANA registry.

2.3.  CoAP Request Format

   The CMP PKIMessages MUST be DER encoded and sent as the body of the
   CoAP POST request.  A CMP client SHOULD send each CoAP requests
   marked as a Confirmable message Section 2.1 of [RFC7252].  If the
   CoAP request is successful then the server SHOULD return a "2.05
   Content" response code.  If the CoAP request is not successful then
   an appropriate CoAP Client Error 4.xx or a Server Error 5.xx response
   code MUST be returned.  A CMP RA or CA may choose to send a
   Piggybacked response Section 5.2.1 of [RFC7252], to the client or it
   MAY send a Separate response Section 5.2.2 of [RFC7252], in case it
   takes some time for CA RA to process the CMP transaction.

   When transferring CMP PKIMesssage over CoAP the content-format
   "application/pkixcmp" MUST be used.

2.4.  CoAP Block-Wise Transfer Mode

   A CMP PKIMesssage consists of a header, body, protection, and
   extraCerts structures which may contain many optional and potentially
   large fields.  Thus a CMP message can be much larger than the Maximum
   Transmission Unit (MTU) of the outgoing interface of the device.  The
   EEs and RAs or CAs, MUST use the Block-Wise transfer mode [RFC7959]
   to transfer such large messages instead of relying on IP

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   If a CoAP-to-HTTP proxy is in the path between EEs and CA or EEs and
   RA then, if the server supports, it MUST use the chunked transfer
   encoding [RFC9112] to send data over the HTTP transport.  The proxy
   MUST try to reduce the number of packets sent by using an optimal
   chunk length for the HTTP transport.

2.5.  Multicast CoAP

   CMP PKIMessages sent over CoAP MUST NOT use a Multicast destination

2.6.  Announcement PKIMessage

   A CMP server may publish announcements, that can be event triggered
   or periodic, for the other PKI entities.  Here is the list of CMP
   announcement messages prefixed by their respective ASN.1 identifier
   (section 5.1.2 [RFC4210])

         [15] CA Key Update Announcement
         [16] Certificate Announcement
         [17] Revocation Announcement
         [18] CRL Announcement

   An EE MAY use CoAP Observe option [RFC7641] to register itself to get
   any announcement messages from the RA or CA.  The EE can send a GET
   request to the server's URI suffixed by "/ann".  For example a path
   to register for announcement messages may look like this:


   If the server supports CMP Announcements messages, then it SHOULD
   send appropriate 2.xx success response code, otherwise a 4.xx or 5.xx
   error response code.  If for some reason server cannot add the client
   to its list of observers for the announcements, it can omit the
   Observe option [RFC7641] in the response to the client.  A client on
   receiving a 2.xx success response without the Observe option
   [RFC7641] can try after some time to register again for announcements
   from the CMP server.  Since server can remove the EE from the list of
   observers for announcement messages, an EE SHOULD periodically re-
   register itself for announcement messages.

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   Alternatively, an EE MAY poll for the potential changes via "PKI
   Information" request using "PKI General Message" choice for the
   PKIBody of the PKIMessage [RFC4210].  The "PKI Information" request
   allows the EE to receive information about changes like the CA key
   update and CRL[RFC5280] updates.  If supported, EEs may also use
   "Support Messages" defined in section 4.3 of the Lightweight CMP
   Profile [I-D.ietf-lamps-lightweight-cmp-profile] to get information
   about the changes.

   These mechanisms will help constrained devices, that are acting as
   EEs, to conserve resources by eliminating the need to create an
   endpoint for receiving notifications from RA or CA.  It will also
   simplify the implementation of a CoAP-to-HTTP proxy.

3.  Proxy Support

   This section provides guidance on using a CoAP-to-HTTP proxy between
   EEs and RAs or CAs in order to avoid changes to the existing PKI
   implementation.  Since the CMP payload is same over CoAP and HTTP
   transfer mechanisms, a CoAP-to-HTTP cross-protocol proxy can be
   implemented based on section 10 of [RFC7252].

   The CoAP-to-HTTP proxy can either be located closer to the EEs or
   closer to the RA or CA.  The proxy MAY support service discovery and
   resource discovery as described in section 2.2.  The CoAP-to-HTTP
   proxy MUST function as a reverse proxy, only permitting connections
   to a limited set of pre-configured servers.  It is out of scope of
   this document on how a reverse proxy can route CoAP client requests
   to one of the configured servers.  Some recommended mechanisms are as

   *  Use the Uri-Path option to identify a server.

   *  Use separate hostnames for each of the configured servers and then
      use the Uri-Host option for routing the CoAP requests.

   *  Use separate hostnames for each of the configured servers and then
      use Server Name Indication [RFC8446] in case of "coaps://" scheme
      for routing CoAP requests.

4.  Security Considerations

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   *  The CMP protocol depends upon various mechanisms in the protocol
      itself for making the transactions secure therefore, security
      issues of CoAP due to using UDP without cryptographic protections
      for message confidentiality and integrity, do not carry over to
      the CMP layer.  The Security considerations for CoAP are mentioned
      in the [RFC7252].
   *  Although the CMP protocol does not depend upon the underlying
      transfer mechanism for protecting the messages but in cases when
      confidentiality protection is desired, CoAP over DTLS [RFC9147]
      MAY be used providing a hop-by-hop security.  The use of DTLS can
      provide confidentiality protection of the CMP-level metadata,
      however it cannot obscure the fact that CMP is being used in the
      underlying layer.
   *  Section 9.1 of [RFC7252] defines how to use DTLS [RFC9147] for
      securing the CoAP.  Once a DTLS [RFC9147] association is
      established it SHOULD be used for as long as possible to avoid the
      frequent overhead of setting up a DTLS [RFC9147] association for
      constrained devices.
   *  An EE may miss some of the Announcement messages when using CoAP
      Observe option [RFC7641] since Observe option is a "best-effort"
      approach and server can lose state about subscribers for
      announcement messages.  The EEs may use alternate method described
      in section 2.6 to get time critical changes like CRL updates.
   *  In order to to reduce the risks imposed by DoS attacks, the
      implementations SHOULD optimally use the available datagram size
      i.e. avoid small datagrams containing partial CMP PKIMessage data.
   *  A CoAP-to-HTTP proxy can also protect the PKI entities by handling
      UDP and CoAP messages.  Proxy can mitigate attacks like denial of
      service attacks, replay attacks and resource-exhaustion attacks by
      enforcing basic checks like validating that the ASN.1 syntax is
      compliant to CMP messages and validating the PKIMessage protection
      before sending them to PKI entities.
   *  Since the Proxy may have access to the CMP-Level metadata and
      control over the flow of CMP messages therefore proper role based
      access control should be in place.  Proxy can be deployed at the
      edge of the "End Entities" network or in front of an RA and CA to
      protect them.  The proxy however may itself be vulnerable to
      resource-exhaustion attacks as it's required to buffer the CMP
      messages received over CoAP transport before sending it to the
      HTTP endpoint.  This can be mitigated by using short timers for
      discarding the buffered messages and rate limiting clients based
      on the resource usage.

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5.  IANA Considerations

   This document requires a new entry to the CoAP Content-Formats
   Registry code for the content-type "application/pkixcmp" for
   transferring CMP transactions over CoAP from the identifier range
   256-9999 reserved for IETF specifications.

   Type name: application

   Subtype name: pkixcmp

   Encoding: Content may contain arbitrary octet values.  The octet
   values are the ASN.1 DER encoding of a PKI message, as defined in the
   [RFC4210] specifications.

   Reference: This document and [RFC4210]

   This document also requires a new path segment "ann" in the CMP
   protocol registry for the EEs to register themselves for the
   announcement messages.

   Path Segment: ann

   Description: The path to send a Get request with CoAP Observer Option
   to register for CMP announcement messages.

   Reference: This document.

   This document references the cmp, in the Well-Known URIs
   uris.xhtml) IANA registry.  This document is expected to be published
   together with [I-D.ietf-lamps-cmp-updates].  Please add a reference
   of this document to the Well-Known URIs
   uris.xhtml) IANA registry for that entry

   This document also refers the path segment "p" in the Certificate
   Management Protocol (CMP) (
   cmp.xhtml) IANA registry.  Please add a reference of this document to
   the Certificate Management Protocol (CMP)
   ( for that path

6.  Acknowledgments

   The authors would like to thank Hendrik Brockhaus, David von Oheimb,
   and Andreas Kretschmer for their guidance in writing the content of
   this document and providing valuable feedback.

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7.  References

7.1.  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,

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

   [RFC6712]  Kause, T. and M. Peylo, "Internet X.509 Public Key
              Infrastructure -- HTTP Transfer for the Certificate
              Management Protocol (CMP)", RFC 6712,
              DOI 10.17487/RFC6712, September 2012,

   [RFC4210]  Adams, C., Farrell, S., Kause, T., and T. Mononen,
              "Internet X.509 Public Key Infrastructure Certificate
              Management Protocol (CMP)", RFC 4210,
              DOI 10.17487/RFC4210, September 2005,

   [RFC7252]  Shelby, Z., Hartke, K., and C. Bormann, "The Constrained
              Application Protocol (CoAP)", RFC 7252,
              DOI 10.17487/RFC7252, June 2014,

   [RFC7959]  Bormann, C. and Z. Shelby, Ed., "Block-Wise Transfers in
              the Constrained Application Protocol (CoAP)", RFC 7959,
              DOI 10.17487/RFC7959, August 2016,

   [RFC8615]  Nottingham, M., "Well-Known Uniform Resource Identifiers
              (URIs)", RFC 8615, DOI 10.17487/RFC8615, May 2019,

   [RFC6690]  Shelby, Z., "Constrained RESTful Environments (CoRE) Link
              Format", RFC 6690, DOI 10.17487/RFC6690, August 2012,

   [RFC7641]  Hartke, K., "Observing Resources in the Constrained
              Application Protocol (CoAP)", RFC 7641,
              DOI 10.17487/RFC7641, September 2015,

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   [RFC9147]  Rescorla, E., Tschofenig, H., and N. Modadugu, "The
              Datagram Transport Layer Security (DTLS) Protocol Version
              1.3", RFC 9147, DOI 10.17487/RFC9147, April 2022,

   [RFC9112]  Fielding, R., Ed., Nottingham, M., Ed., and J. Reschke,
              Ed., "HTTP/1.1", STD 99, RFC 9112, DOI 10.17487/RFC9112,
              June 2022, <>.

7.2.  Informative References

   [RFC5280]  Cooper, D., Santesson, S., Farrell, S., Boeyen, S.,
              Housley, R., and W. Polk, "Internet X.509 Public Key
              Infrastructure Certificate and Certificate Revocation List
              (CRL) Profile", RFC 5280, DOI 10.17487/RFC5280, May 2008,

   [RFC8446]  Rescorla, E., "The Transport Layer Security (TLS) Protocol
              Version 1.3", RFC 8446, DOI 10.17487/RFC8446, August 2018,

   [RFC8323]  Bormann, C., Lemay, S., Tschofenig, H., Hartke, K.,
              Silverajan, B., and B. Raymor, Ed., "CoAP (Constrained
              Application Protocol) over TCP, TLS, and WebSockets",
              RFC 8323, DOI 10.17487/RFC8323, February 2018,

              Brockhaus, H., von Oheimb, D., and J. Gray, "Certificate
              Management Protocol (CMP) Updates", Work in Progress,
              Internet-Draft, draft-ietf-lamps-cmp-updates-23, 29 June
              2022, <

              Brockhaus, H., von Oheimb, D., and S. Fries, "Lightweight
              Certificate Management Protocol (CMP) Profile", Work in
              Progress, Internet-Draft, draft-ietf-lamps-lightweight-
              cmp-profile-13, 8 July 2022,

Authors' Addresses

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   Mohit Sahni (editor)
   Palo Alto Networks
   3000 Tannery Way
   Santa Clara, CA 95054
   United States of America

   Saurabh Tripathi (editor)
   Palo Alto Networks
   3000 Tannery Way
   Santa Clara, CA 95054
   United States of America

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