| Internet-Draft | CoAP Transfer for the CMP | May 2023 |
| Sahni & Tripathi | Expires 16 November 2023 | [Page] |
This document specifies the use of Constrained Application Protocol (CoAP) as a transfer mechanism 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.¶
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 https://datatracker.ietf.org/drafts/current/.¶
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 16 November 2023.¶
Copyright (c) 2023 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 publication of this document. Please review these documents carefully, as they describe your rights and restrictions with respect to this document. Code Components extracted from this document must include Revised BSD License text as described in Section 4.e of the Trust Legal Provisions and are provided without warranty as described in the Revised BSD License.¶
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.¶
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.¶
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.¶
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. The path segment 'p' followed by an arbitraryLabel <name> could for example support the differentiation of specific CAs or certificate profiles. Further path segments, e.g., as specified in the Lightweight CMP Profile [I-D.ietf-lamps-lightweight-cmp-profile], could indicate PKI management operations using an operationLabel <operation>. A valid full CMP URI can look like this:¶
coap://www.example.com/.well-known/cmp
coap://www.example.com/.well-known/cmp/<operation>
coap://www.example.com/.well-known/cmp/p/<profileLabel>
coap://www.example.com/.well-known/cmp/p/<profileLabel>/<operation>
¶
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 the default ports numbers MUST be assumed for the "coap:" and the "coaps:" scheme URIs. The default port for coap: scheme URIs is 5683 and the default port for coaps: scheme URIs is 5684 [RFC7252].¶
Optionally, in the environments where a Local Registration Authority (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.¶
The CMP PKIMessages MUST be DER encoded and sent as the body of the CoAP POST request. A CMP client MUST send each CoAP requests marked as a Confirmable message [RFC7252]. If the CoAP request is successful then the CMP RA or CA MUST return a Success 2.xx response code otherwise CMP RA or CA MUST return an appropriate Client Error 4.xx or Server Error 5.xx response code. A CMP RA or CA may choose to send a Piggybacked response [RFC7252] to the client or it MAY send a Separate response [RFC7252] in case it takes some time for CA or RA to process the CMP transaction.¶
When transferring CMP PKIMesssage over CoAP the content-format "application/pkixcmp" MUST be used.¶
A CMP PKIMesssage consists of a header, body, protection, and extraCerts structures which may contain many optional and potentially large fields. Thus, a CMP message can be much larger than the Maximum Transmission Unit (MTU) of the outgoing interface of the device. The EEs and RAs or CAs, MUST use the Block-Wise transfer mode [RFC7959] to transfer such large messages instead of relying on IP fragmentation.¶
If a CoAP-to-HTTP proxy is in the path between EEs and CA or EEs and RA then, if the server supports, it MUST use the chunked transfer encoding [RFC9112] to send data over the HTTP transport. The proxy MUST try to reduce the number of packets sent by using an optimal chunk length for the HTTP transport.¶
CMP PKIMessages sent over CoAP MUST NOT use a Multicast destination address.¶
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
¶
An EE MAY use CoAP Observe option [RFC7641] to register itself to get any announcement messages from the RA or CA. The EE can send a GET request to the server's URI suffixed by "/ann". For example a path to register for announcement messages may look like this:¶
coap://www.example.com/.well-known/cmp/ann
coap://www.example.com/.well-known/cmp/p/<profileLabel>/ann
¶
If the server supports CMP Announcements messages, then it MUST send appropriate Success 2.xx response code, otherwise it MUST send an appropriate Client Error 4.xx or Server Error 5.xx response code. If for some reason the server cannot add the client to its list of observers for the announcements, it can omit the Observe option [RFC7641] in the response to the client. A client on receiving a 2.xx success response without the Observe option [RFC7641] MAY try after some time to register again for announcements from the CMP server. Since server can remove the EE from the list of observers for announcement messages, an EE SHOULD periodically re-register itself for announcement messages.¶
Alternatively, an EE MAY periodically poll for the current status of the CA via the "PKI Information Request" message, see section 6.5 of [RFC4210]. If supported, EEs MAY also use "Support Messages" defined in section 4.3 of Lightweight CMP Profile [I-D.ietf-lamps-lightweight-cmp-profile] to get information about the CA status. These mechanisms will help constrained devices, that are acting as EEs, to conserve resources by eliminating the need to create an endpoint for receiving notifications from RA or CA. It will also simplify the implementation of a CoAP-to-HTTP proxy.¶
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 CMP payload is the 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 either be located closer to the EEs or closer to the RA or CA. The proxy MAY 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 to specify how a reverse proxy can route CoAP client requests to one of the configured servers. Some recommended mechanisms are as follows:¶
This document adds a new entry to the CoAP Content-Formats IANA Registry for the code of content-type "application/pkixcmp", for transferring CMP transactions over CoAP, from the identifier range 256-9999 reserved for IETF specifications.¶
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 also adds a new path segment "ann" to the CMP Well-Known URI Path Segments IANA registry for the EEs to register themselves for the announcement messages.¶
Path Segment: ann¶
Description: The path to send a GET request with CoAP Observer Option to register for CMP announcement messages.¶
Reference: This document.¶
This document references the cmp, in the Well-Known URIs IANA registry. Please add a reference of this document to the Well-Known URIs IANA registry for that entry.¶
This document also refers the path segment "p" in the CMP Well-Known URI Path Segments IANA registry. Please add a reference of this document to the CMP Well-Known URI Path Segments for that path segment.¶
[Note RFC Editor]: This document should be published together or after the CMP version 3 [I-D.ietf-lamps-cmp-updates] as it references IANA entries created by that Internet draft.¶
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.¶