Requests/Responses

Things accessible via CoAP provide interaction affordances that direct Consumers to send CoAP requests to a CoAP endpoint. The Thing processes these requests and sends appropriate CoAP responses.

The target resource is specified in the Thing Description by the href member of a form, using one of the CoAP URI schemes (see section [[[#url]]]). The request method (e.g., "GET", "PUT", "POST", or "DELETE") is specified using the cov:method member of a form.

{
    "@context": "https://www.w3.org/2022/wot/td/v1.1",
    ...
    "properties": {
        "status": {
            "type": "string",
            "readOnly": true,
            "forms": [{
                "cov:method": "GET",
                "href": "coap://[2001:DB8::1]/status",
                "contentType": "text/plain;charset=utf-8",
                "op": ["readproperty"]
            }]
        }
    }
}

The cov:method member may be omitted if the request method is the default for the operation (see section [[[#default-mappings]]]).

{
    "@context": "https://www.w3.org/2022/wot/td/v1.1",
    ...
    "properties": {
        "status": {
            "type": "string",
            "readOnly": true,
            "forms": [{
                "href": "coap://[2001:DB8::1]/status",
                "contentType": "text/plain;charset=utf-8"
            }]
        }
    }
}

The contentType and contentCoding members of a form specify the CoAP content format of the request and/or response. In CoAP, combinations of content type and content coding are encoded using standardized values listed in the IANA CoAP Content-Formats registry. Therefore, not every combination of content type and content coding can be used with CoAP. Furthermore, even parameters are included in the mapping: in [[[#example-request-defaults]]], the content type text/plain;charset=utf-8 would be mapped to Content-Format ID 0. Omitting the charset parameter, for instance, would break a literal mapping from content type to Content-Format in this case.

Section [[[#content-negotiation]]] introduces the additional vocabulary terms cov:contentFormat and cov:accept to deal with ambiguities of content types and content codings in the context of CoAP, and to give Consumers guidance for the content negotiation process.

Content Negotiation

Content negotiation in CoAP is used to negotiate the representation of CoAP resources that may have different representations available.

Content negotiation is accomplished through the use of CoAP Accept and Content-Format options. The CoAP Accept option is used by clients to request a particular content format, while the Content-Format option is used by clients and servers to indicate the content format of the representation in requests and responses, respectively.

Mapping the string-based contentType and contentCoding fields to a numeric Content-Format can be difficult for a Consumer for a number of reasons: First, a Consumer needs to be aware of all potential mappings between the string-based and the numerical representations. Furthermore, since there is no well-defined algorithm for converting a combination of contentType and contentCoding to a CoAP Content-Format, the process might be prone to error.

Therefore, this document introduces the vocabulary term cov:contentFormat for providing a hint to simplify the conversion for a Consumer. It can be used in all Thing Description classes that contain the contentType and contentCoding fields. If the field cov:contentFormat is present, it must match the string representation expressed by contentType and contentCoding. Corresponding with the default contentType value application/json, Consumers are supposed to assume a Content-Format value 50 if cov:contentFormat and application/json is not present.

Forms may contain a cov:accept field to explicitly require a Consumer to provide an Accept option. If a cov:accept member is present in a Form, then the Consumer must use the contained value in an Accept option when requesting the described resource.

In the case of the following example, a Consumer trying to invoke the start Action would include a Content-Format option with value 50 (application/json) and an Accept option with value 60 (application/cbor) in its request when choosing the first Form provided. Since both forms have the same href, use of Accept option allows the Consumer to choose the correct form.

                {
                    "@context": "https://www.w3.org/2022/wot/td/v1.1",
                    ...
                    "actions": {
                        "start": {
                            "input": { "type": "integer" },
                            "output": { "type": "string" },
                            "forms": [
                                {
                                    "href": "coap://[2001:DB8::1]/start",
                                    "cov:accept": 60,
                                    "response": {
                                        "contentType": "application/cbor",
                                        "cov:contentFormat": 60
                                    }
                                },
                                {
                                    "href": "coap://[2001:DB8::1]/start",
                                    "contentType": "application/cbor",
                                    "cov:contentFormat": 60,
                                    "cov:accept": 50,
                                    "response": {
                                        "contentType": "application/json",
                                        "cov:contentFormat": 50
                                    }
                                }
                            ]
                        }
                    }
                }
                

Caching

CoAP features two mechanisms to make efficient use of network resources and minimize the number of required transmissions. First, a CoAP client may store responses to requests in order to satisfy future requests without sending a new request. A stored response may be used if it is still valid, i.e., if the response's Max-Age value has not been exceeded. Second, a CoAP client may validate a stored response by sending a new request with the ETag from the stored response. This allows the server to quickly respond with a 2.03 (Valid) response if the stored response is still valid.

Consumers are generally expected to make use of both mechanisms to minimize the number of required CoAP transmissions.

Polling Resources

A CoAP client may need a current representation of a resource over a period of time. In this case, the client may poll the CoAP server periodically for new content, taking advantage of caching to keep traffic to a minimum.

It is important to note that a response's Max-Age is independent of how often a client polls. For example, if a response contains the current position of an aircraft, after less than a second it can no longer be assumed that the aircraft is still in the same position. This means the server would set Max-Age to 0 in the response, indicating the response cannot be reused for a future request.

If a client polls too frequently, a server may send a 4.29 (Too Many Requests) response [[RFC8516]]. This error response is cacheable, with the Max-Age indicating after which time the client is allowed to try again.

In a form, the cov:minPollingInterval member can be used to provide a hint of what minimum polling interval the server is willing to accept. (This doesn't mean that the client won't get a 4.29 response if it complies; just that the client will most likely get a 4.29 response if it doesn't.)

{
    "@context": "https://www.w3.org/2022/wot/td/v1.1",
    ...
    "properties": {
        "status": {
            "type": "string",
            "readOnly": true,
            "forms": [{
                "href": "coap://[2001:DB8::1]/status",
                "contentType": "text/plain;charset=utf-8",
                "cov:minPollingInterval": 60
            }]
        }
    }
}
        

Observing Resources

As a significant improvement over polling, CoAP provides a mechanism for clients to "observe" resources [[RFC7641]], i.e., to request notifications from the server whenever the resource changes. This is especially useful in the Web of Things where many Things are expected to have resources that change over time, such as sensor readings.

A resource can be observed by sending a GET request with the Observe option set. The server will then respond with the current representation of the resource, as well as send notifications to the client. A server is not required to support the Observe option for all resources; in this case, a resource can still be observed by polling it at regular intervals.

Support for observing resources can be indicating by including a subprotocol value of cov:observe in a form.

{
    "@context": "https://www.w3.org/2022/wot/td/v1.1",
    ...
    "properties": {
        "status": {
            "type": "string",
            "readOnly": true,
            "forms": [{
                "cov:method": "GET",
                "href": "coap://[2001:DB8::1]/status",
                "contentType": "text/plain;charset=utf-8",
                "subprotocol": "cov:observe",
                "op": ["observeproperty"]
            }]
        }
    }
}

A consumer can always fall back to polling a resource if observing a resource is permanently or temporarily not possible. In fact, a typical CoAP client implementation is to poll for as long as the client is interested in having a current representation, setting the Observe option each time. As long as the client is receiving notifications, it doesn't need to poll; as soon as the notifications are missing, the client polls again with the Observe option set.

Block-wise Transfers

CoAP supports block-wise transfers to allow large resource representations to be transferred between clients and servers. This feature enables clients and servers to request or provide resource representations in smaller blocks, which can be useful when constrained network conditions make it undesirable to transfer large amounts of data at once.

At the time of writing, CoAP supports two mechanisms for block-wise transfers: Block-Wise Transfers [[RFC7959]] and Block-Wise Transfer Options Supporting Robust Transmission [[RFC9177]]. In a form, support for these can be indicated by a cov:blockwise or cov:quickblockwise member, respectively.

{
    "@context": "https://www.w3.org/2022/wot/td/v1.1",
    ...
    "properties": {
        "status": {
            "type": "string",
            "readOnly": true,
            "forms": [{
                "href": "coap://[2001:DB8::1]/status",
                "contentType": "text/plain;charset=utf-8",
                "cov:blockwise": { }
            }]
        }
    }
}

Additionally, a cov:blockwise or cov:quickblockwise member may indicate relevant parameters, such as the largest block size that may be used in a Block2 Option.

{
    "@context": "https://www.w3.org/2022/wot/td/v1.1",
    ...
    "properties": {
        "status": {
            "type": "string",
            "readOnly": true,
            "forms": [{
                "href": "coap://[2001:DB8::1]/status",
                "contentType": "text/plain;charset=utf-8",
                "cov:blockwise": {
                    "cov:block2Size": 64
                }
            }]
        }
    }
}

Conditional Requests

With conditional requests, a CoAP client can include conditions in a request that specify how the server should process the request. If the server cannot satisfy the conditions, it returns a 4.12 (Precondition Failed) response.

At the time of writing, CoAP supports three types of conditional requests:

• If-Match option with an ETag: "only process the request if the target resource exists and has a representation with this ETag"
• If-Match option without an ETag: "only process the request if the target resource exists"
• If-None-Match option without an ETag: "only process the request if the target resource does not exist"

Conditional requests are typically used in the context of larger, multi-step workflows. For example, a CoAP client might want to make sure that a resource has not been modified before it performs some action on it. Therefore, no vocabulary is provided for driving conditional requests from the Thing Description. Instead, consumers are expected to be familiar with the specifics of the workflows in which they are participating and to use conditional requests accordingly.

Hop Limit

The CoAP Hop-Limit option [[RFC8768]] limits the number of hops a CoAP message can take before it is considered undeliverable. This prevents infinite message loops in CoAP networks.

In a form, the cov:hopLimit member can be used to set the desired hop limit for a particular CoAP request.

{
    "@context": "https://www.w3.org/2022/wot/td/v1.1",
    ...
    "properties": {
        "status": {
            "type": "string",
            "readOnly": true,
            "forms": [{
                "href": "coap://[2001:DB8::1]/status",
                "contentType": "text/plain;charset=utf-8",
                "cov:hopLimit": 5
            }]
        }
    }
}

Form terms

Response terms

The following vocabulary terms are usable in the ExpectedResponse and AdditionalExpectedResponse classes.

BlockWiseTransferParameters

Default mappings

Possible mappings

TODO: This section should describe other mappings that can be used by TD designers. It is meant to be informative but it provides guidelines for implementers.