CoAP Transport for CMPV2
draft-ietf-ace-cmpv2-coap-transport-01

Document Type Active Internet-Draft (ace WG)
Authors Mohit Sahni  , Saurabh Tripathi 
Last updated 2021-04-22
Replaces draft-msahni-ace-cmpv2-coap-transport
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ACE                                                        M. Sahni, Ed.
Internet-Draft                                          S. Tripathi, Ed.
Intended status: Standards Track                      Palo Alto Networks
Expires: October 25, 2021                                 April 23, 2021

                        CoAP Transport for CMPV2
                 draft-ietf-ace-cmpv2-coap-transport-01

Abstract

   This document specifies the use of Constrained Application Protocol
   (CoAP) as a transport medium 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
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   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on October 25, 2021.

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
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   include Simplified BSD License text as described in Section 4.e of

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   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
     1.1.  Terminology . . . . . . . . . . . . . . . . . . . . . . .   3
   2.  CoAP Transport 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 Content-Format . . . . . . . . . . . . . . . . . . .   4
     2.5.  Announcement PKIMessage . . . . . . . . . . . . . . . . .   4
     2.6.  CoAP Block-Wise Transfer Mode . . . . . . . . . . . . . .   4
     2.7.  Multicast CoAP  . . . . . . . . . . . . . . . . . . . . .   5
   3.  Using CoAP over DTLS  . . . . . . . . . . . . . . . . . . . .   5
   4.  Proxy support . . . . . . . . . . . . . . . . . . . . . . . .   5
   5.  Security Considerations . . . . . . . . . . . . . . . . . . .   5
   6.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   6
   7.  Acknowledgments . . . . . . . . . . . . . . . . . . . . . . .   6
   8.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   6
     8.1.  Normative References  . . . . . . . . . . . . . . . . . .   6
     8.2.  Informative References  . . . . . . . . . . . . . . . . .   7
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .   8

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 encryption 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
   Websockets.  This document specifies the use of CoAP over UDP as a
   transport medium for the CMP version 2 [RFC4210], CMP version 3
   [Certificate-Management-Protocol-Updates] and Lightweight CMP Profile
   [Lightweight-CMP-Profile].  This document, in general, follows the
   HTTP transport specifications for CMP defined in [RFC6712] and
   specifies the additional requirements for using CoAP as a transport
   medium.  This document also provides guidance on how to use a "CoAP-
   to-HTTP" proxy for a better adaptation of CoAP transport without
   significant changes to the existing PKI entities.  Although CoAP
   transport can be used for communication between Registration
   Authority (RA) and Certification Authority (CA) or between CAs, the

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   scope of this document is for communication between End Entity (EE)
   and RA or EE and CA.  This document is applicable only when the CoAP
   transport is used for the CMP transactions.

1.1.  Terminology

   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.

2.  CoAP Transport 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 transport medium, while 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
   transport medium 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 path specific to a CA, certificate profile or
   PKI management operations.  A valid full operation path segment can
   look like this:

    coap://www.example.com/.well-known/cmp
    coap://www.example.com/.well-known/cmp/operationalLabel
    coap://www.example.com/.well-known/cmp/profileLabel
    coap://www.example.com/.well-known/cmp/profileLabel/operationalLabel

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. "example.com:5683".  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's CMP endpoint.  The CoAP CMP endpoints supporting service
   discovery MUST also support resource discovery in the CoRE Link
   Format as described in [RFC6690].

2.3.  CoAP Request Format

   The CMP PKIMessages MUST be DER encoded and sent as the body of the
   CoAP POST request.  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.

2.4.  CoAP Content-Format

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

2.5.  Announcement PKIMessage

   When using the CoAP protocol, a PKI EE SHOULD 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 [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.

2.6.  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.

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2.7.  Multicast CoAP

   CMP PKIMessages sent over CoAP transport MUST NOT use a Multicast
   destination address.

3.  Using CoAP over DTLS

   Although CMP protocol does not depend upon the underlying transport
   for protecting the messages but in cases when an end to end secrecy
   is desired for the CoAP transport, CoAP over DTLS [RFC6347] as a
   transport medium SHOULD be used.  Section 9.1 of [RFC7252] defines
   how to use DTLS [RFC6347] for securing the CoAP.  Once a DTLS
   [RFC6347] connection is established it SHOULD be used for as long as
   possible to avoid the frequent overhead of setting up a DTLS
   [RFC6347] connection for constrained devices.

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
   transport, 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 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
   protocol are mentioned in the [RFC7252].

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   In order to protect themselves against DDoS attacks, the
   implementations SHOULD avoid sending or receiving very 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" and a new
   entry in Well-Known URIs for URI Suffix "cmp".

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,
              <https://www.rfc-editor.org/info/rfc2119>.

   [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,
              <https://www.rfc-editor.org/info/rfc4210>.

   [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,
              <https://www.rfc-editor.org/info/rfc5280>.

   [RFC6690]  Shelby, Z., "Constrained RESTful Environments (CoRE) Link
              Format", RFC 6690, DOI 10.17487/RFC6690, August 2012,
              <https://www.rfc-editor.org/info/rfc6690>.

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   [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,
              <https://www.rfc-editor.org/info/rfc6712>.

   [RFC7252]  Shelby, Z., Hartke, K., and C. Bormann, "The Constrained
              Application Protocol (CoAP)", RFC 7252,
              DOI 10.17487/RFC7252, June 2014,
              <https://www.rfc-editor.org/info/rfc7252>.

   [RFC7959]  Bormann, C. and Z. Shelby, Ed., "Block-Wise Transfers in
              the Constrained Application Protocol (CoAP)", RFC 7959,
              DOI 10.17487/RFC7959, August 2016,
              <https://www.rfc-editor.org/info/rfc7959>.

   [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>.

   [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,
              <https://www.rfc-editor.org/info/rfc8323>.

   [RFC8615]  Nottingham, M., "Well-Known Uniform Resource Identifiers
              (URIs)", RFC 8615, DOI 10.17487/RFC8615, May 2019,
              <https://www.rfc-editor.org/info/rfc8615>.

8.2.  Informative References

   [Certificate-Management-Protocol-Updates]
              Brockhaus, H. and D. von Oheimb, "Certificate Management
              Protocol (CMP) Updates", Work in Progress, draft-
              brockhaus-lamps-cmp-updates-03, 2021,
              <https://tools.ietf.org/html/draft-brockhaus-lamps-cmp-
              updates-03>.

   [Lightweight-CMP-Profile]
              Brockhaus, H., Fries, S., and D. von Oheimb, "Lightweight
              CMP Profile", Work in Progress, draft-ietf-lamps-
              lightweight-cmp-profile-05, 2021,
              <https://tools.ietf.org/html/draft-ietf-lamps-lightweight-
              cmp-profile-05>.

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   [RFC6347]  Rescorla, E. and N. Modadugu, "Datagram Transport Layer
              Security Version 1.2", RFC 6347, DOI 10.17487/RFC6347,
              January 2012, <https://www.rfc-editor.org/info/rfc6347>.

   [RFC8446]  Rescorla, E., "The Transport Layer Security (TLS) Protocol
              Version 1.3", RFC 8446, DOI 10.17487/RFC8446, August 2018,
              <https://www.rfc-editor.org/info/rfc8446>.

Authors' Addresses

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

   EMail: msahni@paloaltonetworks.com

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

   EMail: stripathi@paloaltonetworks.com

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