| core | B. Greevenbosch |
| Internet-Draft | Huawei Technologies |
| Intended status: Informational | June 2012 |
| Expires: December 01, 2012 |
CoAP Profile Description Format
draft-greevenbosch-core-profile-description-00
This document provides a profile description format, that can be used to express capabilities of a CoAP server.
Discussion and suggestions for improvement are requested, and should be sent to core@ietf.org.
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The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in [RFC2119].
The Constrained Application Protocol (CoAP) [I-D.ietf-core-coap] is a RESTful protocol for constrained nodes and networks.
Often, a client first learns about a resource through the link format [I-D.ietf-core-link-format]. The link format only provides basic information, for example the resource URL. However, it would be good if the client could get more extensive information on the resources when required. This document defines a profile description format, which can be used to signal several parameters about resources and their servers.
One of the main features of the CoAP protocol is the ability to include CoAP options. These options can be either elective or critical. A message with an unsupported critical option will be rejected, whereas unsupported elective options will be ignored.
Even though it is well defined how the server must respond to unsupported options, it is useful for the client to know which options are supported in advance. In this way, it can determine which options are viable to use in a transaction, and which features cannot be exploited. This specification allows signalling of the supported options by the resource.
Another important feature of a server is which media types it supports. This document allows signalling of that information too.
It is anticipated that more profile descriptions will become available in the future.
The profile description can be obtained through a link with interface type "core#p" (see also [I-D.shelby-core-interfaces]). The URI of the profile description should be equal to the URI of the associated resource, but with a "/p" suffix. For example, if the client wants to get profile description on a server witn URI "www.example.org", it can send a GET request for "coap://www.example.org/p".
If the profile of a particular resource is available, it should use the same naming convention. For example, the profile of the sensor "coap://www.example.org/s/tmp" (on the same server) can be acquired with a GET to: "coap://www.example.org/s/tmp/p".
The profile description is formatted as a JSON document. It consists of several profile fields, each of which has associated parameters.
"op":[op1,op2,...]
The Options "op" profile field contains a list of options that are supported by a resource. It has the format depicted in Figure 1, where op1, op2, ... are integers representing the option numbers of the supported options, as defined in [I-D.ietf-core-coap] and/or through IANA. The option numbers MUST appear in numerical order, starting with the lowest number.
When including the "op" profile field in the profile description of a resource, all option numbers of the CoAP options supported by that resource MUST be included. Options that are not supported by the resource MUST NOT be included in the "op" profile field.
If the "op" profile field is available, the receiving party SHALL assume a non-listed option is not supported by the associated resource.
"mt":[mt1,mt2,...]
The Media-Types "mt" profile field indicates which media-types are supported. It has the format depicted in Figure 2, where mt1, mt2, ... are integers representing the media-type numbers of the supported media-types, as defined in [I-D.ietf-core-coap] and/or through IANA. The media-type numbers MUST appear in numerical order, starting with the lowest number.
When including the "mt" profile field in the profile description of a resource, all media-types of the CoAP options supported by that resource MUST be included. Media-types that are not supported by the resource MUST NOT be included in the "mt" profile field.
If the "mt" profile field is available, the receiving party SHALL assume a non-listed media-type is not supported by the associated resource.
A CoAP server may cater for multiple resources. In addition, the document [I-D.shelby-core-interfaces] provides a batch mechanism that allows querying/manipulating multiple resources at once.
The "/p" URI suffix allows obtaining profile description on each of these three levels. For example, a GET to "http://www.example.org/p" would return profile description about the complete server, whereas a GET to "http://www.example.org/s/tmp/p" only returns profile description about the temperature sensor "tmp". Lastly, a GET to "http://www.example.org/s/p", where "/s" is a batch of sensors, would return common profile description for all these sensors.
The hierarchical nature of these three entities implies that profiles can only become more restrictive when going to a lower level. For example, a server may support the block option, whereas one of its resources does not. In that case, the profile of the server would indicate block option support, whereas the profile of the resource would not do so.
To allow addition of new profile fields in the future, unknown profile fields SHOULD be ignored by the client.
The following is an example of a camera sensor at "coap://www.example.org/s/cam", that supports the "Content-Type" (1), "Proxy-Uri" (3), "ETag" (4), "Uri-Host" (5), "Uri-Port" (7), "Uri-Path" (9), "Token" (11) and "Block2" (17) options.
The supported media types are "text/plan" (0), "application/link-format" (40) and "application/json" (50).
Req: GET /s/cam/p
Res: 2.05 Content (application/json)
{
"op":[1,3,4,5,7,9,11,17],
"mt":[0,40,50]
}
For general CoAP security considerations see [I-D.ietf-core-coap].
In an unprotected environment, an attacker can change the profile description. For example, the list of supported options may be changed. This could cause the client to make a wrong decision on which mechanisms to use. However, such a threat is normal in environments that lack secure authentication.
The author would like to thank Kepeng Li for his ideas and feedback.
| [RFC2119] | Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997. |
| [I-D.ietf-core-coap] | Shelby, Z., Hartke, K., Bormann, C. and B. Frank, "Constrained Application Protocol (CoAP)", draft-ietf-core-coap-09 (work in progress), March 2012. |
| [I-D.ietf-core-link-format] | Shelby, Z., "CoRE Link Format", draft-ietf-core-link-format-12 (work in progress), May 2012. |
| [I-D.shelby-core-interfaces] | Shelby, Z., "CoRE Link Format", draft-shelby-core-interfaces-02 (work in progress), March 2012. |