CoAP Transport for CMPV2
draft-msahni-ace-cmpv2-coap-transport-00

ACE                                                        M. Sahni, Ed.
Internet-Draft                                          S. Tripathi, Ed.
Intended status: Standards Track                      Palo Alto Networks
Expires: January 14, 2021                                  July 13, 2020


                        CoAP Transport for CMPV2
                draft-msahni-ace-cmpv2-coap-transport-00

Abstract

   This document specifies how to use Constrained Application Protocol
   (CoAP) as a Transport Medium for the Certificate management protocol
   version 2 (CMPv2) and Lightweight CMP Profile
   [Lightweight-CMP-Profile] which is a subset of CMPv2 defined for
   Constrained devices.  The CMPv2 defines the interaction between
   various PKI entities for the purpose of certificate creation and
   management.  The CoAP is a HTTP like client-server protocol used by
   various constrained devices in the IoT and industrial scenarios.
   Constrained devices are devices that have low memory or CPU or power
   constraints and avoid the use of complex protocols like TCP to save
   resources.

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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   This Internet-Draft will expire on January 14, 2021.

Copyright Notice

   Copyright (c) 2020 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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   (https://trustee.ietf.org/license-info) in effect on the date of



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   publication of this document.  Please review these documents
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   described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
     1.1.  Terminology . . . . . . . . . . . . . . . . . . . . . . .   3
   2.  CoAP Transport For CMPv2  . . . . . . . . . . . . . . . . . .   3
     2.1.  Discovery of CMP Entities . . . . . . . . . . . . . . . .   3
     2.2.  CoAP URI Format . . . . . . . . . . . . . . . . . . . . .   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  . . . . . . . . . . . . . . . . . . . . .   4
   3.  Using CoAP over DTLS  . . . . . . . . . . . . . . . . . . . .   5
   4.  Proxy support . . . . . . . . . . . . . . . . . . . . . . . .   5
     4.1.  CoAP to HTTP Proxy  . . . . . . . . . . . . . . . . . . .   5
     4.2.  CoAPs to HTTPs Proxy  . . . . . . . . . . . . . . . . . .   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
     8.3.  URL References  . . . . . . . . . . . . . . . . . . . . .   7
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .   7

1.  Introduction

   The CMPv2 is used by the entities in PKI for the generation and
   management of the certificates.  One of the requirements of CMPv2
   [RFC4210] is to be usable over a variety of transport mechanisms.
   The CMP is designed to be independent of the transport protocol being
   used and has mechanisms to take care of transactions, error reporting
   and encryption of messages where ever required.  The CoAP defined in
   [RFC7252], [RFC7959] and [RFC8323] is a client-server protocol, like
   HTTP, that is designed to be used by constrained devices over
   constrained networks (low power lossy 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 as a transport medium for the CMPv2 and Lightweight CMP
   Profile [Lightweight-CMP-Profile].  This document, in general,
   follows the HTTP transport specifications for CMPv2 defined in



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   [RFC6712] and specifies the additional requirements for CoAP
   transport.  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
   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 being used for the CMPv2 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 CMPv2

   CMPv2 transaction consists of passing PKIMesssage [RFC4210] between
   the PKI End Entities (EEs), Registration Authorities (RAs), and
   Certification Authorities (CAs).  If the EEs are constrained devices
   then they will prefer, as a client, the use of CoAP instead of HTTP
   as a transport medium, while the RAs and CAs, in general, are not
   constrained and can support both CoAP and HTTP Client and Server
   implementation.  This section specifes how to use CoAP as transport
   mechanism for CMPv2 or Lightweight CMP Profile
   [Lightweight-CMP-Profile].

2.1.  Discovery of CMP Entities

   The information about the URIs of CA and RA that is required by EEs
   can be either configured out of band on EEs or the EEs can use the
   service discovery mechanism described in section 7 of [RFC7252] to
   find them.  The EE, RA SHOULD support service discovery as described
   in section 7 of [RFC7252].  An EE MUST verify the configured Root CA
   certificate against the Root CA certificate of the discovered entity
   to make sure it is talking to correct endpoint.

2.2.  CoAP URI Format

   The CoAP URI MUST follow the guidelines defined in section 3.6 of
   [RFC6712] for CMPv2 protocol.  Implementations supporting the
   Lightweight CMP Profile [Lightweight-CMP-Profile] MUST follow the
   guidelines specified for HTTP transport defined in section 7.1 of
   Lightweight CMP Profile [Lightweight-CMP-Profile].  The URI's for




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   CoAP resources should start with coap:// instead of http:// and
   coaps:// instead of https://

2.3.  CoAP Request Format

   The CMPv2 PKIMessage MUST be DER encoded and sent as the body of the
   CoAP POST request.  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.

2.4.  CoAP Content-Format

   When transferring CMPv2 PKIMesssage over CoAP the media type
   application/pkixcmp MUST be used.

2.5.  Announcement PKIMessage

   When using the CoAP protocol, a PKI entity SHOULD poll for the
   possible changes via PKI Information request using General Message
   defined in a PKIMessage for various type of changes like CA key
   update or to get current CRL to check revocation or using Support
   messages defined in section 5.4 of Lightweight CMP Profile
   [Lightweight-CMP-Profile].  This will make use of a CoAP to HTTP
   proxy transparent to the client.

2.6.  CoAP Block Wise Transfer Mode

   Since the CMPv2 PKIMesssage consists of a header body and optional
   fields a CMPv2 message can be much larger than the MTU of the
   outgoing interface of the device.  In order to avoid IP fragmentation
   of messages that are exchanged between EEs and RAs or CAs, the Block
   Wise transfer [RFC7959] mode MUST be used for the CMPv2 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 Proxy opens a connection with the server.

2.7.  Multicast CoAP

   CMPv2 PKIMessage request messages sent from EEs to RAs or from EEs to
   CAs over CoAP transport MUST not use a Multicast destination address.








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3.  Using CoAP over DTLS

   When the end to end secrecy is desired for 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.
   For CMPv2 and Lightweight CMP Profile [Lightweight-CMP-Profile] the
   clients should follow specifications defined in section 7.1 and
   section 7.2 of Lightweight CMP Profile [Lightweight-CMP-Profile] for
   setting up DTLS [RFC6347] connection either using certificates or
   shared secret.  Once a DTLS [RFC6347] connection is established it
   SHOULD be used for as long as possible to avoid the frequent overhead
   of using DTLS [RFC6347] connection for constrained devices.

4.  Proxy support

   The use of a CoAP to HTTP proxy is recommended to avoid significant
   changes in the implementation of the CAs and RAs.  However, if a
   proxy is in place then Announcements Messages cannot be passed to EEs
   efficiently.  In case a CoAP to HTTP proxy is used for CMP
   transactions, it SHOULD support service discovery mentioned in
   section 2.1

4.1.  CoAP to HTTP Proxy

   If a CoAP to HTTP proxy is used then it MUST be positioned between
   EEs and RAs or between EEs and CAs when RA is not part of CMP
   transactions.  The use of a CoAP to HTTP proxy between CAs and RAs is
   not recommended.  The implementation of a CoAP to HTTP proxy is
   specified in Section 10 of [RFC7252].  The CoAP to HTTP proxy will
   also protect the CAs and RAs from UDP based Denial of Service
   attacks.

4.2.  CoAPs to HTTPs Proxy

   A CoAPs to HTTPS proxy (DTLS [RFC6347] transport to TLS [RFC8446]
   transport proxy) can be used instead of the CoAP to HTTP proxy if the
   server support HTTPS protocol, however client SHOULD be configured to
   trust the CA certificate used by proxy to sign the Man in the Middle
   (MITM) certificate for certificate chain validation [RFC5280].

5.  Security Considerations

   The CMPv2 protocol itself does not require secure transport and
   depends upon various mechanisms in the protocol itself to make sure
   that the transactions are secure.  However, the CoAP protocol which
   uses UDP as layer 4 transport 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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   Using a CoAP to HTTP proxy mitigates some of the risks as the
   requests from the EE's can terminate inside the trusted network and
   will not require the server to listen on a UDP port making it safe
   from UDP based address spoofing, Denial of Service, and amplification
   attacks due to the characteristics of UDP.

6.  IANA Considerations

   This document requires a new entry to the CoAP Content-Formats
   Registry code for the content-type application/pkixcmp

7.  Acknowledgments

   The author 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>.

   [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>.



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   [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>.

8.2.  Informative References

   [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>.

8.3.  URL References

   [Lightweight-CMP-Profile]
              Brockhaus, H., Fries, S., and D. von Oheimb, "Lightweight
              CMP Profile", 2020, <https://tools.ietf.org/html/draft-
              brockhaus-lamps-lightweight-cmp-profile-03>.

Authors' Addresses

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

   EMail: msahni@paloaltonetworks.com










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

   EMail: stripathi@paloaltonetworks.com












































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