Internet DRAFT - draft-msahni-tbd-cmpv2-coap-transport
draft-msahni-tbd-cmpv2-coap-transport
TBD M. Sahni, Ed.
Internet-Draft Palo Alto Networks
Intended status: Standards Track June 4, 2020
Expires: December 6, 2020
CoAP Transport for CMPV2
draft-msahni-tbd-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 an 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
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provisions of BCP 78 and BCP 79.
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This Internet-Draft will expire on December 6, 2020.
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
Provisions Relating to IETF Documents
(https://trustee.ietf.org/license-info) in effect on the date of
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
1.1. Terminology . . . . . . . . . . . . . . . . . . . . . . . 3
2. CoAP Transport For CMPv2 . . . . . . . . . . . . . . . . . . 3
2.1. CoAP URI Format . . . . . . . . . . . . . . . . . . . . . 3
2.2. CoAP Request Format . . . . . . . . . . . . . . . . . . . 3
2.3. CoAP Content-Format . . . . . . . . . . . . . . . . . . . 3
2.4. Announcement PKIMessage . . . . . . . . . . . . . . . . . 4
2.5. CoAP Block Wise Transfer Mode . . . . . . . . . . . . . . 4
2.6. Multicast CoAP . . . . . . . . . . . . . . . . . . . . . 4
3. Using CoAP over DTLS . . . . . . . . . . . . . . . . . . . . 4
4. Proxy support . . . . . . . . . . . . . . . . . . . . . . . . 4
4.1. CoAP to HTTP Proxy . . . . . . . . . . . . . . . . . . . 5
4.2. CoAPs to HTTPs Proxy . . . . . . . . . . . . . . . . . . 5
5. Security Considerations . . . . . . . . . . . . . . . . . . . 5
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 5
7. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 5
8. References . . . . . . . . . . . . . . . . . . . . . . . . . 5
8.1. Normative References . . . . . . . . . . . . . . . . . . 6
8.2. Informative References . . . . . . . . . . . . . . . . . 6
8.3. URL References . . . . . . . . . . . . . . . . . . . . . 6
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 7
1. Introduction
The CMPv2 is used by the entities in PKI for the generation and
management of the certificate. One of the requirements of CMPv2
[RFC4210] is to be usable over a variety of protocols. The CoAP
[RFC7252] and [RFC7959] is a client-server protocol like HTTP that is
designed to be used by constrained devices over constrained networks
(low power lossy networks). 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 [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 RAs and CAs or between CAs, the scope of
this document is for communication between EEs and RAs or EEs and
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CAs.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 over the 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 describes how to use CoAP as transport
for CMPv2 or Lightweight CMP Profile [Lightweight-CMP-Profile].
2.1. 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
CoAP resources should start of coap:// instead of http:// and
coaps:// instead of https://
2.2. 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.
2.3. CoAP Content-Format
When transferring CMPv2 PKIMesssage over CoAP the media type
application/pkixcmp MUST be used.
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2.4. 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.
2.5. CoAP Block Wise Transfer Mode
Since the CMPv2 PKIMesssage consists of a header body and optional
fields, when using CoAP as transport for the CMPv2 protocol the Block
Wise transfer [RFC7959] mode MUST be used for the CMPv2 Transaction.
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.
2.6. 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.
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 for setting up DLTS connection. 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.
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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 CMPv2
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) SHOULD not be used as it can be insecure for the
client to trust the Man in the Middle (MiTM) certificate issued by
the proxy to share client and server shared secret with the proxy.
If a server requires Mutual TLS [MTLS] then a proxy will not work.
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].
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 TCP.
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
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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>.
[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>.
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
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[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>.
[MTLS] Rescorla, E., "Mutual TLS", August 2018,
<https://tools.ietf.org/html/rfc8446#section-4.3.2>.
Author's Address
Mohit Sahni (editor)
Palo Alto Networks
3000 Tannery Way
Santa Clara, CA 95054
US
EMail: msahni@paloaltonetworks.com
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