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CoAP Transfer for the Certificate Management Protocol

The information below is for an old version of the document.
Document Type This is an older version of an Internet-Draft whose latest revision is Active
Authors Mohit Sahni , Saurabh Tripathi
Last updated 2022-03-23 (Latest revision 2021-11-08)
Replaces draft-msahni-ace-cmpv2-coap-transport
Stream Internet Engineering Task Force (IETF)
Stream WG state Submitted to IESG for Publication
Document shepherd Daniel Migault
Shepherd write-up Show Last changed 2021-11-08
IESG IESG state AD Evaluation::Revised I-D Needed
Consensus boilerplate Yes
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Responsible AD Paul Wouters
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ACE                                                        M. Sahni, Ed.
Internet-Draft                                          S. Tripathi, Ed.
Intended status: Standards Track                      Palo Alto Networks
Expires: May 12, 2022                                   November 8, 2021

         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).  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 May 12, 2022.

Copyright Notice

   Copyright (c) 2021 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
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   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified 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  . . . . . . . . . . . . . . . . .   3
     2.3.  CoAP Request Format . . . . . . . . . . . . . . . . . . .   4
     2.4.  CoAP Block-Wise Transfer Mode . . . . . . . . . . . . . .   4
     2.5.  Multicast CoAP  . . . . . . . . . . . . . . . . . . . . .   4
     2.6.  Announcement PKIMessage . . . . . . . . . . . . . . . . .   5
   3.  Using CoAP over DTLS  . . . . . . . . . . . . . . . . . . . .   5
   4.  Proxy Support . . . . . . . . . . . . . . . . . . . . . . . .   6
   5.  Security Considerations . . . . . . . . . . . . . . . . . . .   6
   6.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   6
   7.  Acknowledgments . . . . . . . . . . . . . . . . . . . . . . .   7
   8.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   7
     8.1.  Normative References  . . . . . . . . . . . . . . . . . .   7
     8.2.  Informative References  . . . . . . . . . . . . . . . . .   8
     8.3.  URIs  . . . . . . . . . . . . . . . . . . . . . . . . . .   9
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .   9

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
   messages.  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

   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 to support multiple PKI entities
   on the same endpoint.  A valid full operation path segment can look
   like this:


   Here operationalLabel may represent different CAs or Certificate
   profiles or supported End Entity types and profileLabel may represent
   different set of supported PKI operations on that particular path.

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 port 5683 MUST be assumed for the "coap://"
   scheme and port 5684 MUST be assumed for the "coaps://" scheme.
   Optionally, in the environments where a Local Registration Authority

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   (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 CoAP requests marked as
   Confirmable message ([RFC7252] section 2.1).  If the CoAP request is
   successful then the server MUST 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 chose to send a Piggybacked response
   ([RFC7252] section 5.2.1) to the client or it MAY send a Separate
   response ([RFC7252] section 5.2.2) in case it takes some time for CA
   RA to process the CMP transaction.

   When transferring CMP PKIMesssage over CoAP the media type
   "application/pkixcmp" MUST be used.

2.4.  CoAP Block-Wise Transfer Mode

   A CMP PKIMesssage consists of a header, body, protection, and
   extraCerts structures.  These structures may contain many optional
   and potentially large fields, a CMP message can be much larger than
   the Maximum Transmission Unit (MTU) of the outgoing interface of the
   device.  In order to avoid IP fragmentation of messages exchanged
   between EEs and RAs or CAs, the Block-Wise transfer [RFC7959] mode
   MUST be used for the CMP Transactions over CoAP.  If a CoAP-to-HTTP
   proxy is in the path between EEs and CA or EEs and RA then it MUST
   receive the entire body from the client before sending the HTTP
   request to the server.  This will avoid unnecessary errors in case
   the entire content of the PKIMesssage is not received and the proxy
   opens a connection with the server.

2.5.  Multicast CoAP

   CMP PKIMessages sent over CoAP MUST NOT use a Multicast destination

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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

   As there are no request messages specified for these announcement
   messages, an EE MAY use CoAP Observe option [RFC7641] in the Get
   request to the CMP server's URI followed by "/ann" to register itself
   for any Announcements messages.  If the server supports CMP
   Announcements messages, then it can respond with response code 2.03
   "Valid", otherwise with response code 4.04 "Not Found".  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 2.03
   response to the client.  A client on receiving 2.03 response without
   Observe option [RFC7641] can try after some time to register again
   for announcements from the CMP server.

   Alternatively an EE MAY poll for the potential changes via "PKI
   Information" request using "PKI General Message" defined in the
   PKIMessage [RFC4210] for various type of changes like CA key update
   or to get current CRL [RFC5280] to check revocation or using Support
   messages defined in section 5.4 of Lightweight CMP Profile
   [I-D.ietf-lamps-lightweight-cmp-profile] . This will help constrained
   devices that are acting as EEs conserve resources by eliminating the
   need to create an endpoint for receiving notifications from RA or CA.
   It will also simplify the implementation of CoAP-to-HTTP proxy.

3.  Using CoAP over DTLS

   Although CMP protocol does not depend upon the underlying transfer
   mechanism for protecting the messages but in cases when an end to end
   secrecy is desired for the CoAP, CoAP over DTLS [I-D.ietf-tls-dtls13]
   SHOULD be used.  Section 9.1 of [RFC7252] defines how to use DTLS
   [I-D.ietf-tls-dtls13] for securing the CoAP.  Once a DTLS
   [I-D.ietf-tls-dtls13] connection is established it SHOULD be used for
   as long as possible to avoid the frequent overhead of setting up a
   DTLS [I-D.ietf-tls-dtls13] connection for constrained devices.

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4.  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 be either located closer to the EEs or closer to the RA or CA.
   In case the proxy is deployed closer to the EEs then it may also
   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 follows:

   o  Use Uri-Path option to identify a server.
   o  Use separate hostnames for each of the configured servers and then
      use the Uri-Host option for routing the CoAP requests.
   o  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.

5.  Security Considerations

   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 do not carry over to the CMP layer.  However
   the CoAP is vulnerable to many issues due to the connectionless
   characteristics of UDP itself.  The Security considerations for CoAP
   are mentioned in the [RFC7252].

   In order to to reduce the risks imposed by DoS attacks, the
   implementations SHOULD minimize fragmentation of messages, i.e. avoid
   small packets containing partial CMP PKIMessage data.

   A CoAP-to-HTTP proxy can also protect the PKI entities from various
   attacks by enforcing basic checks and validating messages before
   sending them to PKI entities.  Proxy can be deployed at the edge of
   End Entities" network or in front of an RA and CA to protect them.

6.  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.

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   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 references the cmp, a temporary entry, in the Well-
   Known URIs [1] IANA registry.  This document is expected to be
   published together with [I-D.ietf-lamps-cmp-updates] that makes the
   cmp registry entry permanent.  Please add a reference of this
   document to the Well-Known URIs [2] IANA registry for that entry

7.  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.

8.  References

8.1.  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,

   [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,

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

   [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,

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

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

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

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

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

8.2.  Informative References

              Brockhaus, H. and D. von Oheimb, "Certificate Management
              Protocol (CMP) Updates", draft-ietf-lamps-cmp-updates-12
              (work in progress), July 2021.

              Brockhaus, H., Fries, S., and D. von Oheimb, "Lightweight
              Certificate Management Protocol (CMP) Profile", draft-
              ietf-lamps-lightweight-cmp-profile-06 (work in progress),
              July 2021.

              Rescorla, E., Tschofenig, H., and N. Modadugu, "The
              Datagram Transport Layer Security (DTLS) Protocol Version
              1.3", draft-ietf-tls-dtls13-43 (work in progress), April

   [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,

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   [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,

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

8.3.  URIs



Authors' Addresses

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


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


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