DESCRIPTION

This man page focuses on the CoAP Session.

The CoAP stack’s global state is stored in a coap_context_t Context object. Resources, Endpoints and Sessions are associated with this context object. There can be more than one coap_context_t object per application, it is up to the application to manage each one accordingly.

A CoAP Session maintains the state of an ongoing connection between a Client and Server which is stored in a coap_session_t Session object.

The Session network traffic can be encrypted or un-encrypted if there is an underlying TLS library.

If TLS is going to be used for encrypting the network traffic, then the TLS information for Pre-Shared Keys (PSK) or Public Key Infrastructure (PKI) needs to be configured before any network traffic starts to flow. For Servers, this has to be done before the Endpoint is created, for Clients, this is done during the Client Session set up.

For Servers, all the encryption information is held internally by the TLS Context level and the CoAP Context level as the Server is listening for new incoming traffic based on the Endpoint definition. The TLS and CoAP session will not get built until the new traffic starts, which is done by the libcoap library, with the session having a reference count of 1.

For Clients, all the encryption information can be held at the TLS Context and CoAP Context levels, or at the TLS Session and CoAP Session levels. If defined at the Context level, then when Sessions are created, they will inherit the Context definitions, unless they have separately been defined for the Session level, in which case the Session version will get used. Typically the information will be configured at the Session level for Clients.

In principle the set-up sequence for CoAP Servers looks like (see coap_context(3) for further information on the functions)

coap_new_context()
coap_context_set_pki_root_cas() - if the root CAs need to be updated and PKI
coap_context_set_pki() and/or coap_context_set_psk() - if encryption is required
coap_new_endpoint()

Multiple endpoints can be set up per Context, each listening for a new traffic flow with different TCP/UDP protocols, TLS protocols, port numbers etc. When a new traffic flow is started, then the CoAP library will create and start a new server session.

In principle the set-up sequence for CoAP Clients looks like

coap_new_context()
coap_context_set_pki_root_cas() if the root CAs need to be updated and PKI
coap_new_client_session(), coap_new_client_session_pki() or coap_new_client_session_psk()

Multiple client sessions are supported per Context.

Different CoAP protocols can be defined for proto - the current supported list is:

COAP_PROTO_UDP
COAP_PROTO_DTLS
COAP_PROTO_TCP
COAP_PROTO_TLS

The coap_new_client_session() function initiates a new client session associated with context to the specified server using the CoAP protocol proto. If the port is not specified in server, then the default CoAP port is used. Normally local_if would be set to NULL, but by specifying local_if the source of the network session can be bound to a specific IP address or port. The session will initially have a reference count of 1.

The coap_new_client_session_pki() function, for a specific context, is used to configure the TLS context using the setup_data variables as defined in the coap_dtls_pki_t structure - see coap_encrytion(3). The session will initially have a reference count of 1.

The coap_new_client_session_psk() function, for a specific context, is used to configure the TLS context using the client identity, Pre-Shared Key key with length key_len. All 3 parameters must be defined, NULL is not valid. An empty string is not valid for identity. key_len must be greater than 0. This function also includes the server to connect to, optionally the local interface local_if to bind to and the CoAP protocol proto to use. The session will initially have a reference count of 1.

The coap_session_reference() is used to increment the reference count of the session. Incrementing the reference count by an application means that the library will not inadvertently remove the session when it has finished processing the session.

The coap_session_release() function must be used to decrement the session reference count, which when it gets to 0, will free off the session if this is a Client, which then clears all entries from the receive queue and send queue. If the reference count goes to 0 for a Server, then the session is added to a free pool ready for subsequent re-use. If the Server session is not used for 5 minutes, then it will get completely freed off.

The coap_sesson_set_default_mtu() function is used to set the MTU size (the maximum message size) of the data in a packet, excluding any IP or TCP/UDP overhead to mtu for the session.

The coap_session_max_pdu_size() funcition is used to get the maximum MTU size of the data for the session.

The coap_session_set_app_data() funstion is used to define a data pointer for the session which can then be retieved at a later date.

The coap_session_get_app_data() function is used to retrieve the data pointer previously defined by coap_session_set_app_data().

The coap_session_str() function is used to get a string containing the information about the session.

RETURN VALUES

coap_new_client_session(), coap_new_client_session_psk(), coap_new_client_session_pki() functions returns a newly created client session or NULL if there is a creation failure.

coap_session_reference() function returns a pointer to the session.

coap_session_get_app_data() function return a previously defined pointer.

coap_session_max_pdu_size() function returns the MTU size.

coap_session_str() function returns a description string of the session.

EXAMPLES

CoAP Client Non-Encrypted Setup

#include <coap2/coap.h>

#include <netinet/in.h>

static coap_session_t *
setup_client_session (struct in_addr ip_address) {
  coap_session_t *session;
  coap_address_t server;
  /* See coap_context(3) */
  coap_context_t *context = coap_new_context(NULL);

  if (!context)
    return NULL;

  coap_address_init(&server);
  server.addr.sa.sa_family = AF_INET;
  server.addr.sin.sin_addr = ip_address;
  server.addr.sin.sin_port = htons (5683);

  session = coap_new_client_session(context, NULL, &server, COAP_PROTO_UDP);
  if (!session) {
    coap_free_context(context);
    return NULL;
  }
  /* The context is in session->context */
  return session;
}

CoAP Client PKI Setup

#include <coap2/coap.h>

#include <netinet/in.h>

static int
verify_cn_callback(const char *cn,
                   const uint8_t *asn1_public_cert,
                   size_t asn1_length,
                   coap_session_t *session,
                   unsigned depth,
                   int validated,
                   void *arg
) {
  /* Check that the CN is valid */

  /* ... */

  return 1;
}

static coap_session_t *
setup_client_session_pki (struct in_addr ip_address,
                          const char *public_cert_file,
                          const char *private_key_file,
                          const char *ca_file
) {
  coap_session_t *session;
  coap_address_t server;
  coap_dtls_pki_t dtls_pki;
  /* See coap_context(3) */
  coap_context_t *context = coap_new_context(NULL);

  if (!context)
    return NULL;

  coap_address_init(&server);
  server.addr.sa.sa_family = AF_INET;
  server.addr.sin.sin_addr = ip_address;
  server.addr.sin.sin_port = htons (5684);

  memset (&dtls_pki, 0, sizeof (dtls_pki));

  /* See coap_encryption(3) */
  dtls_pki.version                 = COAP_DTLS_PKI_SETUP_VERSION;
  dtls_pki.verify_peer_cert        = 1;
  dtls_pki.require_peer_cert       = 1;
  dtls_pki.allow_self_signed       = 1;
  dtls_pki.allow_expired_certs     = 1;
  dtls_pki.cert_chain_validation   = 1;
  dtls_pki.cert_chain_verify_depth = 1;
  dtls_pki.check_cert_revocation   = 1;
  dtls_pki.allow_no_crl            = 1;
  dtls_pki.allow_expired_crl       = 1;
  dtls_pki.validate_cn_call_back   = verify_cn_callback;
  dtls_pki.cn_call_back_arg        = NULL;
  dtls_pki.validate_sni_call_back  = NULL;
  dtls_pki.sni_call_back_arg       = NULL;
  dtls_pki.additional_tls_setup_call_back = NULL;
  dtls_pki.sni                     = NULL;
  dtls_pki.pki_key.key_type        = COAP_PKI_KEY_PEM;
  dtls_pki.pki_key.key.pem.ca_file = ca_file;
  dtls_pki.pki_key.key.pem.public_cert = public_cert_file;
  dtls_pki.pki_key.key.pem.private_key = private_key_file;

  session = coap_new_client_session_pki(context, NULL, &server,
                                        COAP_PROTO_DTLS, &dtls_pki);
  if (!session) {
    coap_free_context(context);
    return NULL;
  }
  /* The context is in session->context */
  return session;
}

CoAP Client PSK Setup

#include <coap2/coap.h>

#include <netinet/in.h>

static coap_session_t *
setup_client_session_psk (struct in_addr ip_address,
                          const char *identity,
                          const uint8_t *key,
                          unsigned key_len
) {
  coap_session_t *session;
  coap_address_t server;
  /* See coap_context(3) */
  coap_context_t *context = coap_new_context(NULL);

  if (!context)
    return NULL;

  coap_address_init(&server);
  server.addr.sa.sa_family = AF_INET;
  server.addr.sin.sin_addr = ip_address;
  server.addr.sin.sin_port = htons (5684);

  session = coap_new_client_session_psk(context, NULL, &server,
                                        COAP_PROTO_DTLS, identity, key, key_len);
  if (!session) {
    coap_free_context(context);
    return NULL;
  }
  /* The context is in session->context */
  return session;
}

CoAP Client Setup

#include <coap2/coap.h>

#include <netinet/in.h>

static coap_session_t *
setup_client_session (struct in_addr ip_address) {
  coap_session_t *session;
  coap_address_t server;
  /* See coap_context(3) */
  coap_context_t *context = coap_new_context(NULL);

  if (!context)
    return NULL;

  coap_address_init(&server);
  server.addr.sa.sa_family = AF_INET;
  server.addr.sin.sin_addr = ip_address;
  server.addr.sin.sin_port = htons (5683);

  session = coap_new_client_session(context, NULL, &server,
                                        COAP_PROTO_DTLS);
  if (!session) {
    coap_free_context(context);
    return NULL;
  }
  /* The context is in session->context */
  return session;
}

SEE ALSO

coap_context(3), coap_resource(3), coap_encryption(3) and coap_tls_library(3)

FURTHER INFORMATION

See "RFC7252: The Constrained Application Protocol (CoAP)" for further information.

BUGS

Please report bugs on the mailing list for libcoap: libcoap-developers@lists.sourceforge.net

AUTHORS

The libcoap project <libcoap-developers@lists.sourceforge.net>