ACE M. Sahni, Ed.
Internet-Draft S. Tripathi, Ed.
Intended status: Standards Track Palo Alto Networks
Expires: April 4, 2022 October 1, 2021
CoAP Transfer for the Certificate Management Protocol
draft-ietf-ace-cmpv2-coap-transport-03
Abstract
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
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provisions of BCP 78 and BCP 79.
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This Internet-Draft will expire on April 4, 2022.
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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
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 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
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
HTTP.
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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 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:
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
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. "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. 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
address.
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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
transfering CMP transactions over CoAP.
Type name: application
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Subtype name: pkixcmp
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>.
[RFC6690] Shelby, Z., "Constrained RESTful Environments (CoRE) Link
Format", RFC 6690, DOI 10.17487/RFC6690, August 2012,
<https://www.rfc-editor.org/info/rfc6690>.
[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>.
[RFC7641] Hartke, K., "Observing Resources in the Constrained
Application Protocol (CoAP)", RFC 7641,
DOI 10.17487/RFC7641, September 2015,
<https://www.rfc-editor.org/info/rfc7641>.
[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>.
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[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>.
[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
[I-D.ietf-lamps-cmp-updates]
Brockhaus, H. and D. von Oheimb, "Certificate Management
Protocol (CMP) Updates", draft-ietf-lamps-cmp-updates-12
(work in progress), July 2021.
[I-D.ietf-lamps-lightweight-cmp-profile]
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.
[I-D.ietf-tls-dtls13]
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
2021.
[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>.
[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>.
[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
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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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