CoAP

Overview

The Constrained Application Protocol (CoAP) is a specialized web transfer protocol for use with constrained nodes and constrained (e.g., low-power, lossy) networks. It provides a convenient API for RESTful Web services that support CoAP’s features. For more information about the protocol itself, see IETF RFC7252 The Constrained Application Protocol.

Zephyr provides a CoAP library which supports client and server roles. The library is configurable as per user needs. The Zephyr CoAP library is implemented using plain buffers. Users of the API create sockets for communication and pass the buffer to the library for parsing and other purposes. The library itself doesn’t create any sockets for users.

On top of CoAP, Zephyr has support for LWM2M “Lightweight Machine 2 Machine” protocol, a simple, low-cost remote management and service enablement mechanism. See Lightweight M2M (LWM2M) for more information.

Supported RFCs:

Supported RFCs:

Note

Not all parts of these RFCs are supported. Features are supported based on Zephyr requirements.

Sample Usage

CoAP Server

To create a CoAP server, resources for the server need to be defined. The .well-known/core resource should be added before all other resources that should be included in the responses of the .well-known/core resource.

static struct coap_resource resources[] = {
    { .get = well_known_core_get,
      .path = COAP_WELL_KNOWN_CORE_PATH,
    },
    { .get  = sample_get,
      .post = sample_post,
      .del  = sample_del,
      .put  = sample_put,
      .path = sample_path
    },
    { },
};

An application reads data from the socket and passes the buffer to the CoAP library to parse the message. If the CoAP message is proper, the library uses the buffer along with resources defined above to call the correct callback function to handle the CoAP request from the client. It’s the callback function’s responsibility to either reply or act according to CoAP request.

coap_packet_parse(&request, data, data_len, options, opt_num);
...
coap_handle_request(&request, resources, options, opt_num,
                    client_addr, client_addr_len);

If CONFIG_COAP_URI_WILDCARD enabled, server may accept multiple resources using MQTT-like wildcard style:

  • the plus symbol represents a single-level wild card in the path;

  • the hash symbol represents the multi-level wild card in the path.

static const char * const led_set[] = { "led","+","set", NULL };
static const char * const btn_get[] = { "button","#", NULL };
static const char * const no_wc[] = { "test","+1", NULL };

It accepts /led/0/set, led/1234/set, led/any/set, /button/door/1, /test/+1, but returns -ENOENT for /led/1, /test/21, /test/1.

This option is enabled by default, disable it to avoid unexpected behaviour with resource path like ‘/some_resource/+/#’.

CoAP Client

If the CoAP client knows about resources in the CoAP server, the client can start prepare CoAP requests and wait for responses. If the client doesn’t know about resources in the CoAP server, it can request resources through the .well-known/core CoAP message.

/* Initialize the CoAP message */
char *path = "test";
struct coap_packet request;
uint8_t data[100];
uint8_t payload[20];

coap_packet_init(&request, data, sizeof(data),
                 1, COAP_TYPE_CON, 8, coap_next_token(),
                 COAP_METHOD_GET, coap_next_id());

/* Append options */
coap_packet_append_option(&request, COAP_OPTION_URI_PATH,
                          path, strlen(path));

/* Append Payload marker if you are going to add payload */
coap_packet_append_payload_marker(&request);

/* Append payload */
coap_packet_append_payload(&request, (uint8_t *)payload,
                           sizeof(payload) - 1);

/* send over sockets */

Testing

There are various ways to test Zephyr CoAP library.

libcoap

libcoap implements a lightweight application-protocol for devices that are resource constrained, such as by computing power, RF range, memory, bandwidth, or network packet sizes. Sources can be found here libcoap. libcoap has a script (examples/etsi_coaptest.sh) to test coap-server functionality in Zephyr.

See the net-tools project for more details

The CoAP Server sample can be built and executed on QEMU as described in Networking with QEMU.

Use this command on the host to run the libcoap implementation of the ETSI test cases:

sudo ./libcoap/examples/etsi_coaptest.sh -i tap0 2001:db8::1

TTCN3

Eclipse has TTCN3 based tests to run against CoAP implementations.

Install eclipse-titan and set symbolic links for titan tools

sudo apt-get install eclipse-titan

cd /usr/share/titan

sudo ln -s /usr/bin bin
sudo ln /usr/bin/titanver bin
sudo ln -s /usr/bin/mctr_cli bin
sudo ln -s /usr/include/titan include
sudo ln -s /usr/lib/titan lib

export TTCN3_DIR=/usr/share/titan

git clone https://github.com/eclipse/titan.misc.git

cd titan.misc

Follow the instruction to setup CoAP test suite from here:

After the build is complete, the CoAP Server sample can be built and executed on QEMU as described in Networking with QEMU.

Change the client (test suite) and server (Zephyr coap-server sample) addresses in coap.cfg file as per your setup.

Execute the test cases with following command.

ttcn3_start coaptests coap.cfg

Sample output of ttcn3 tests looks like this.

Verdict statistics: 0 none (0.00 %), 10 pass (100.00 %), 0 inconc (0.00 %), 0 fail (0.00 %), 0 error (0.00 %).
Test execution summary: 10 test cases were executed. Overall verdict: pass

API Reference

group coap

COAP library.

Defines

COAP_REQUEST_MASK
coap_make_response_code(class, det)
COAP_CODE_EMPTY
GET_BLOCK_NUM(v)
GET_BLOCK_SIZE(v)
GET_MORE(v)
COAP_DEFAULT_MAX_RETRANSMIT
COAP_DEFAULT_ACK_RANDOM_FACTOR
COAP_WELL_KNOWN_CORE_PATH

This resource should be added before all other resources that should be included in the responses of the .well-known/core resource.

Typedefs

typedef int (*coap_method_t)(struct coap_resource *resource, struct coap_packet *request, struct sockaddr *addr, socklen_t addr_len)

Type of the callback being called when a resource’s method is invoked by the remote entity.

typedef void (*coap_notify_t)(struct coap_resource *resource, struct coap_observer *observer)

Type of the callback being called when a resource’s has observers to be informed when an update happens.

typedef int (*coap_reply_t)(const struct coap_packet *response, struct coap_reply *reply, const struct sockaddr *from)

Helper function to be called when a response matches the a pending request.

Enums

enum coap_option_num

Set of CoAP packet options we are aware of.

Users may add options other than these to their packets, provided they know how to format them correctly. The only restriction is that all options must be added to a packet in numeric order.

Refer to RFC 7252, section 12.2 for more information.

Values:

enumerator COAP_OPTION_IF_MATCH
enumerator COAP_OPTION_URI_HOST
enumerator COAP_OPTION_ETAG
enumerator COAP_OPTION_IF_NONE_MATCH
enumerator COAP_OPTION_OBSERVE
enumerator COAP_OPTION_URI_PORT
enumerator COAP_OPTION_LOCATION_PATH
enumerator COAP_OPTION_URI_PATH
enumerator COAP_OPTION_CONTENT_FORMAT
enumerator COAP_OPTION_MAX_AGE
enumerator COAP_OPTION_URI_QUERY
enumerator COAP_OPTION_ACCEPT
enumerator COAP_OPTION_LOCATION_QUERY
enumerator COAP_OPTION_BLOCK2
enumerator COAP_OPTION_BLOCK1
enumerator COAP_OPTION_SIZE2
enumerator COAP_OPTION_PROXY_URI
enumerator COAP_OPTION_PROXY_SCHEME
enumerator COAP_OPTION_SIZE1
enum coap_method

Available request methods.

To be used when creating a request or a response.

Values:

enumerator COAP_METHOD_GET
enumerator COAP_METHOD_POST
enumerator COAP_METHOD_PUT
enumerator COAP_METHOD_DELETE
enum coap_msgtype

CoAP packets may be of one of these types.

Values:

enumerator COAP_TYPE_CON

Confirmable message.

The packet is a request or response the destination end-point must acknowledge.

enumerator COAP_TYPE_NON_CON

Non-confirmable message.

The packet is a request or response that doesn’t require acknowledgements.

enumerator COAP_TYPE_ACK

Acknowledge.

Response to a confirmable message.

enumerator COAP_TYPE_RESET

Reset.

Rejecting a packet for any reason is done by sending a message of this type.

enum coap_response_code

Set of response codes available for a response packet.

To be used when creating a response.

Values:

enumerator COAP_RESPONSE_CODE_OK
enumerator COAP_RESPONSE_CODE_CREATED
enumerator COAP_RESPONSE_CODE_DELETED
enumerator COAP_RESPONSE_CODE_VALID
enumerator COAP_RESPONSE_CODE_CHANGED
enumerator COAP_RESPONSE_CODE_CONTENT
enumerator COAP_RESPONSE_CODE_CONTINUE
enumerator COAP_RESPONSE_CODE_BAD_REQUEST
enumerator COAP_RESPONSE_CODE_UNAUTHORIZED
enumerator COAP_RESPONSE_CODE_BAD_OPTION
enumerator COAP_RESPONSE_CODE_FORBIDDEN
enumerator COAP_RESPONSE_CODE_NOT_FOUND
enumerator COAP_RESPONSE_CODE_NOT_ALLOWED
enumerator COAP_RESPONSE_CODE_NOT_ACCEPTABLE
enumerator COAP_RESPONSE_CODE_INCOMPLETE
enumerator COAP_RESPONSE_CODE_PRECONDITION_FAILED
enumerator COAP_RESPONSE_CODE_REQUEST_TOO_LARGE
enumerator COAP_RESPONSE_CODE_UNSUPPORTED_CONTENT_FORMAT
enumerator COAP_RESPONSE_CODE_INTERNAL_ERROR
enumerator COAP_RESPONSE_CODE_NOT_IMPLEMENTED
enumerator COAP_RESPONSE_CODE_BAD_GATEWAY
enumerator COAP_RESPONSE_CODE_SERVICE_UNAVAILABLE
enumerator COAP_RESPONSE_CODE_GATEWAY_TIMEOUT
enumerator COAP_RESPONSE_CODE_PROXYING_NOT_SUPPORTED
enum coap_content_format

Set of Content-Format option values for CoAP.

To be used when encoding or decoding a Content-Format option.

Values:

enumerator COAP_CONTENT_FORMAT_TEXT_PLAIN
enumerator COAP_CONTENT_FORMAT_APP_LINK_FORMAT
enumerator COAP_CONTENT_FORMAT_APP_XML
enumerator COAP_CONTENT_FORMAT_APP_OCTET_STREAM
enumerator COAP_CONTENT_FORMAT_APP_EXI
enumerator COAP_CONTENT_FORMAT_APP_JSON
enumerator COAP_CONTENT_FORMAT_APP_CBOR
enum coap_block_size

Represents the size of each block that will be transferred using block-wise transfers [RFC7959]:

Each entry maps directly to the value that is used in the wire.

https://tools.ietf.org/html/rfc7959

Values:

enumerator COAP_BLOCK_16
enumerator COAP_BLOCK_32
enumerator COAP_BLOCK_64
enumerator COAP_BLOCK_128
enumerator COAP_BLOCK_256
enumerator COAP_BLOCK_512
enumerator COAP_BLOCK_1024

Functions

uint8_t coap_header_get_version(const struct coap_packet *cpkt)

Returns the version present in a CoAP packet.

Return

the CoAP version in packet

Parameters

  • cpkt: CoAP packet representation

uint8_t coap_header_get_type(const struct coap_packet *cpkt)

Returns the type of the CoAP packet.

Return

the type of the packet

Parameters

  • cpkt: CoAP packet representation

uint8_t coap_header_get_token(const struct coap_packet *cpkt, uint8_t *token)

Returns the token (if any) in the CoAP packet.

Return

Token length in the CoAP packet.

Parameters

  • cpkt: CoAP packet representation

  • token: Where to store the token

uint8_t coap_header_get_code(const struct coap_packet *cpkt)

Returns the code of the CoAP packet.

Return

the code present in the packet

Parameters

  • cpkt: CoAP packet representation

uint16_t coap_header_get_id(const struct coap_packet *cpkt)

Returns the message id associated with the CoAP packet.

Return

the message id present in the packet

Parameters

  • cpkt: CoAP packet representation

const uint8_t *coap_packet_get_payload(const struct coap_packet *cpkt, uint16_t *len)

Returns the data pointer and length of the CoAP packet.

Return

data pointer and length if payload exists NULL pointer and length set to 0 in case there is no payload

Parameters

  • cpkt: CoAP packet representation

  • len: Total length of CoAP payload

int coap_packet_parse(struct coap_packet *cpkt, uint8_t *data, uint16_t len, struct coap_option *options, uint8_t opt_num)

Parses the CoAP packet in data, validating it and initializing cpkt. data must remain valid while cpkt is used.

Return

0 in case of success or negative in case of error.

Parameters

  • cpkt: Packet to be initialized from received data.

  • data: Data containing a CoAP packet, its data pointer is positioned on the start of the CoAP packet.

  • len: Length of the data

  • options: Parse options and cache its details.

  • opt_num: Number of options

int coap_packet_init(struct coap_packet *cpkt, uint8_t *data, uint16_t max_len, uint8_t ver, uint8_t type, uint8_t token_len, uint8_t *token, uint8_t code, uint16_t id)

Creates a new CoAP Packet from input data.

Return

0 in case of success or negative in case of error.

Parameters

  • cpkt: New packet to be initialized using the storage from data.

  • data: Data that will contain a CoAP packet information

  • max_len: Maximum allowable length of data

  • ver: CoAP header version

  • type: CoAP header type

  • token_len: CoAP header token length

  • token: CoAP header token

  • code: CoAP header code

  • id: CoAP header message id

uint8_t *coap_next_token(void)

Returns a randomly generated array of 8 bytes, that can be used as a message’s token.

Return

a 8-byte pseudo-random token.

uint16_t coap_next_id(void)

Helper to generate message ids.

Return

a new message id

int coap_find_options(const struct coap_packet *cpkt, uint16_t code, struct coap_option *options, uint16_t veclen)

Return the values associated with the option of value code.

Return

The number of options found in packet matching code, negative on error.

Parameters

  • cpkt: CoAP packet representation

  • code: Option number to look for

  • options: Array of coap_option where to store the value of the options found

  • veclen: Number of elements in the options array

int coap_packet_append_option(struct coap_packet *cpkt, uint16_t code, const uint8_t *value, uint16_t len)

Appends an option to the packet.

Note: options must be added in numeric order of their codes. Otherwise error will be returned. TODO: Add support for placing options according to its delta value.

Return

0 in case of success or negative in case of error.

Parameters

  • cpkt: Packet to be updated

  • code: Option code to add to the packet, see coap_option_num

  • value: Pointer to the value of the option, will be copied to the packet

  • len: Size of the data to be added

unsigned int coap_option_value_to_int(const struct coap_option *option)

Converts an option to its integer representation.

Assumes that the number is encoded in the network byte order in the option.

Return

The integer representation of the option

Parameters

int coap_append_option_int(struct coap_packet *cpkt, uint16_t code, unsigned int val)

Appends an integer value option to the packet.

The option must be added in numeric order of their codes, and the least amount of bytes will be used to encode the value.

Return

0 in case of success or negative in case of error.

Parameters

  • cpkt: Packet to be updated

  • code: Option code to add to the packet, see coap_option_num

  • val: Integer value to be added

int coap_packet_append_payload_marker(struct coap_packet *cpkt)

Append payload marker to CoAP packet.

Return

0 in case of success or negative in case of error.

Parameters

  • cpkt: Packet to append the payload marker (0xFF)

int coap_packet_append_payload(struct coap_packet *cpkt, uint8_t *payload, uint16_t payload_len)

Append payload to CoAP packet.

Return

0 in case of success or negative in case of error.

Parameters

  • cpkt: Packet to append the payload

  • payload: CoAP packet payload

  • payload_len: CoAP packet payload len

int coap_handle_request(struct coap_packet *cpkt, struct coap_resource *resources, struct coap_option *options, uint8_t opt_num, struct sockaddr *addr, socklen_t addr_len)

When a request is received, call the appropriate methods of the matching resources.

Return

0 in case of success or negative in case of error.

Parameters

  • cpkt: Packet received

  • resources: Array of known resources

  • options: Parsed options from coap_packet_parse()

  • opt_num: Number of options

  • addr: Peer address

  • addr_len: Peer address length

static inline uint16_t coap_block_size_to_bytes(enum coap_block_size block_size)

Helper for converting the enumeration to the size expressed in bytes.

Return

The size in bytes that the block_size represents

Parameters

  • block_size: The block size to be converted

int coap_block_transfer_init(struct coap_block_context *ctx, enum coap_block_size block_size, size_t total_size)

Initializes the context of a block-wise transfer.

Return

0 in case of success or negative in case of error.

Parameters

  • ctx: The context to be initialized

  • block_size: The size of the block

  • total_size: The total size of the transfer, if known

int coap_append_block1_option(struct coap_packet *cpkt, struct coap_block_context *ctx)

Append BLOCK1 option to the packet.

Return

0 in case of success or negative in case of error.

Parameters

  • cpkt: Packet to be updated

  • ctx: Block context from which to retrieve the information for the Block1 option

int coap_append_block2_option(struct coap_packet *cpkt, struct coap_block_context *ctx)

Append BLOCK2 option to the packet.

Return

0 in case of success or negative in case of error.

Parameters

  • cpkt: Packet to be updated

  • ctx: Block context from which to retrieve the information for the Block2 option

int coap_append_size1_option(struct coap_packet *cpkt, struct coap_block_context *ctx)

Append SIZE1 option to the packet.

Return

0 in case of success or negative in case of error.

Parameters

  • cpkt: Packet to be updated

  • ctx: Block context from which to retrieve the information for the Size1 option

int coap_append_size2_option(struct coap_packet *cpkt, struct coap_block_context *ctx)

Append SIZE2 option to the packet.

Return

0 in case of success or negative in case of error.

Parameters

  • cpkt: Packet to be updated

  • ctx: Block context from which to retrieve the information for the Size2 option

int coap_get_option_int(const struct coap_packet *cpkt, uint16_t code)

Get the integer representation of a CoAP option.

Return

Integer value >= 0 in case of success or negative in case of error.

Parameters

  • cpkt: Packet to be inspected

  • code: CoAP option code

int coap_update_from_block(const struct coap_packet *cpkt, struct coap_block_context *ctx)

Retrieves BLOCK{1,2} and SIZE{1,2} from cpkt and updates ctx accordingly.

Return

0 in case of success or negative in case of error.

Parameters

  • cpkt: Packet in which to look for block-wise transfers options

  • ctx: Block context to be updated

size_t coap_next_block(const struct coap_packet *cpkt, struct coap_block_context *ctx)

Updates ctx so after this is called the current entry indicates the correct offset in the body of data being transferred.

Return

The offset in the block-wise transfer, 0 if the transfer has finished.

Parameters

  • cpkt: Packet in which to look for block-wise transfers options

  • ctx: Block context to be updated

void coap_observer_init(struct coap_observer *observer, const struct coap_packet *request, const struct sockaddr *addr)

Indicates that the remote device referenced by addr, with request, wants to observe a resource.

Parameters

  • observer: Observer to be initialized

  • request: Request on which the observer will be based

  • addr: Address of the remote device

bool coap_register_observer(struct coap_resource *resource, struct coap_observer *observer)

After the observer is initialized, associate the observer with an resource.

Return

true if this is the first observer added to this resource.

Parameters

  • resource: Resource to add an observer

  • observer: Observer to be added

void coap_remove_observer(struct coap_resource *resource, struct coap_observer *observer)

Remove this observer from the list of registered observers of that resource.

Parameters

  • resource: Resource in which to remove the observer

  • observer: Observer to be removed

struct coap_observer *coap_find_observer_by_addr(struct coap_observer *observers, size_t len, const struct sockaddr *addr)

Returns the observer that matches address addr.

Return

A pointer to a observer if a match is found, NULL otherwise.

Parameters

  • observers: Pointer to the array of observers

  • len: Size of the array of observers

  • addr: Address of the endpoint observing a resource

struct coap_observer *coap_observer_next_unused(struct coap_observer *observers, size_t len)

Returns the next available observer representation.

Return

A pointer to a observer if there’s an available observer, NULL otherwise.

Parameters

  • observers: Pointer to the array of observers

  • len: Size of the array of observers

void coap_reply_init(struct coap_reply *reply, const struct coap_packet *request)

Indicates that a reply is expected for request.

Parameters

  • reply: Reply structure to be initialized

  • request: Request from which reply will be based

int coap_pending_init(struct coap_pending *pending, const struct coap_packet *request, const struct sockaddr *addr, uint8_t retries)

Initialize a pending request with a request.

The request’s fields are copied into the pending struct, so request doesn’t have to live for as long as the pending struct lives, but “data” that needs to live for at least that long.

Return

0 in case of success or negative in case of error.

Parameters

  • pending: Structure representing the waiting for a confirmation message, initialized with data from request

  • request: Message waiting for confirmation

  • addr: Address to send the retransmission

  • retries: Maximum number of retransmissions of the message.

struct coap_pending *coap_pending_next_unused(struct coap_pending *pendings, size_t len)

Returns the next available pending struct, that can be used to track the retransmission status of a request.

Return

pointer to a free coap_pending structure, NULL in case none could be found.

Parameters

struct coap_reply *coap_reply_next_unused(struct coap_reply *replies, size_t len)

Returns the next available reply struct, so it can be used to track replies and notifications received.

Return

pointer to a free coap_reply structure, NULL in case none could be found.

Parameters

  • replies: Pointer to the array of coap_reply structures

  • len: Size of the array of coap_reply structures

struct coap_pending *coap_pending_received(const struct coap_packet *response, struct coap_pending *pendings, size_t len)

After a response is received, returns if there is any matching pending request exits. User has to clear all pending retransmissions related to that response by calling coap_pending_clear().

Return

pointer to the associated coap_pending structure, NULL in case none could be found.

Parameters

  • response: The received response

  • pendings: Pointer to the array of coap_reply structures

  • len: Size of the array of coap_reply structures

struct coap_reply *coap_response_received(const struct coap_packet *response, const struct sockaddr *from, struct coap_reply *replies, size_t len)

After a response is received, call coap_reply_t handler registered in coap_reply structure.

Return

Pointer to the reply matching the packet received, NULL if none could be found.

Parameters

  • response: A response received

  • from: Address from which the response was received

  • replies: Pointer to the array of coap_reply structures

  • len: Size of the array of coap_reply structures

struct coap_pending *coap_pending_next_to_expire(struct coap_pending *pendings, size_t len)

Returns the next pending about to expire, pending->timeout informs how many ms to next expiration.

Return

The next coap_pending to expire, NULL if none is about to expire.

Parameters

bool coap_pending_cycle(struct coap_pending *pending)

After a request is sent, user may want to cycle the pending retransmission so the timeout is updated.

Return

false if this is the last retransmission.

Parameters

  • pending: Pending representation to have its timeout updated

void coap_pending_clear(struct coap_pending *pending)

Cancels the pending retransmission, so it again becomes available.

Parameters

  • pending: Pending representation to be canceled

void coap_pendings_clear(struct coap_pending *pendings, size_t len)

Cancels all pending retransmissions, so they become available again.

Parameters

void coap_reply_clear(struct coap_reply *reply)

Cancels awaiting for this reply, so it becomes available again. User responsibility to free the memory associated with data.

Parameters

  • reply: The reply to be canceled

void coap_replies_clear(struct coap_reply *replies, size_t len)

Cancels all replies, so they become available again.

Parameters

  • replies: Pointer to the array of coap_reply structures

  • len: Size of the array of coap_reply structures

int coap_resource_notify(struct coap_resource *resource)

Indicates that this resource was updated and that the notify callback should be called for every registered observer.

Return

0 in case of success or negative in case of error.

Parameters

  • resource: Resource that was updated

bool coap_request_is_observe(const struct coap_packet *request)

Returns if this request is enabling observing a resource.

Return

True if the request is enabling observing a resource, False otherwise

Parameters

  • request: Request to be checked

int coap_well_known_core_get(struct coap_resource *resource, struct coap_packet *request, struct coap_packet *response, uint8_t *data, uint16_t len)
struct coap_resource
#include <coap.h>

Description of CoAP resource.

CoAP servers often want to register resources, so that clients can act on them, by fetching their state or requesting updates to them.

struct coap_observer
#include <coap.h>

Represents a remote device that is observing a local resource.

struct coap_packet
#include <coap.h>

Representation of a CoAP Packet.

struct coap_option
#include <coap.h>
struct coap_pending
#include <coap.h>

Represents a request awaiting for an acknowledgment (ACK).

struct coap_reply
#include <coap.h>

Represents the handler for the reply of a request, it is also used when observing resources.

struct coap_block_context
#include <coap.h>

Represents the current state of a block-wise transaction.

struct coap_core_metadata
#include <coap_link_format.h>

In case you want to add attributes to the resources included in the ‘well-known/core’ “virtual” resource, the ‘user_data’ field should point to a valid coap_core_metadata structure.