coap_net.c

/* coap_net.c -- CoAP context inteface
 *
 * Copyright (C) 2010--2024 Olaf Bergmann <bergmann@tzi.org> and others
 *
 * SPDX-License-Identifier: BSD-2-Clause
 *
 * This file is part of the CoAP library libcoap. Please see
 * README for terms of use.
 */

/**
 * @file coap_net.c
 * @brief CoAP context functions
 */

#include "coap3/coap_libcoap_build.h"
#include "coap3/coap_mutex_internal.h"

#include <ctype.h>
#include <stdio.h>
#ifdef HAVE_LIMITS_H
#include <limits.h>
#endif
#ifdef HAVE_UNISTD_H
#include <unistd.h>
#else
#ifdef HAVE_SYS_UNISTD_H
#include <sys/unistd.h>
#endif
#endif
#ifdef HAVE_SYS_TYPES_H
#include <sys/types.h>
#endif
#ifdef HAVE_SYS_SOCKET_H
#include <sys/socket.h>
#endif
#ifdef HAVE_SYS_IOCTL_H
#include <sys/ioctl.h>
#endif
#ifdef HAVE_NETINET_IN_H
#include <netinet/in.h>
#endif
#ifdef HAVE_ARPA_INET_H
#include <arpa/inet.h>
#endif
#ifdef HAVE_NET_IF_H
#include <net/if.h>
#endif
#ifdef COAP_EPOLL_SUPPORT
#include <sys/epoll.h>
#include <sys/timerfd.h>
#endif /* COAP_EPOLL_SUPPORT */
#ifdef HAVE_WS2TCPIP_H
#include <ws2tcpip.h>
#endif

#ifdef HAVE_NETDB_H
#include <netdb.h>
#endif

#ifdef WITH_LWIP
#include <lwip/pbuf.h>
#include <lwip/udp.h>
#include <lwip/timeouts.h>
#include <lwip/tcpip.h>
#endif

#ifndef INET6_ADDRSTRLEN
#define INET6_ADDRSTRLEN 40
#endif

#ifndef min
#define min(a,b) ((a) < (b) ? (a) : (b))
#endif

/**
 * The number of bits for the fractional part of ACK_TIMEOUT and
 * ACK_RANDOM_FACTOR. Must be less or equal 8.
 */
#define FRAC_BITS 6

/**
 * The maximum number of bits for fixed point integers that are used
 * for retransmission time calculation. Currently this must be @c 8.
 */
#define MAX_BITS 8

#if FRAC_BITS > 8
#error FRAC_BITS must be less or equal 8
#endif

/** creates a Qx.frac from fval in coap_fixed_point_t */
#define Q(frac,fval) ((uint16_t)(((1 << (frac)) * fval.integer_part) + \
                                 ((1 << (frac)) * fval.fractional_part + 500)/1000))

/** creates a Qx.FRAC_BITS from session's 'ack_random_factor' */
#define ACK_RANDOM_FACTOR                                        \
  Q(FRAC_BITS, session->ack_random_factor)

/** creates a Qx.FRAC_BITS from session's 'ack_timeout' */
#define ACK_TIMEOUT Q(FRAC_BITS, session->ack_timeout)

#ifndef WITH_LWIP

COAP_STATIC_INLINE coap_queue_t *
coap_malloc_node(void) {
  return (coap_queue_t *)coap_malloc_type(COAP_NODE, sizeof(coap_queue_t));
}

COAP_STATIC_INLINE void
coap_free_node(coap_queue_t *node) {
  coap_free_type(COAP_NODE, node);
}
#else /* !WITH_LWIP */

#include <lwip/memp.h>

COAP_STATIC_INLINE coap_queue_t *
coap_malloc_node() {
  return (coap_queue_t *)memp_malloc(MEMP_COAP_NODE);
}

COAP_STATIC_INLINE void
coap_free_node(coap_queue_t *node) {
  memp_free(MEMP_COAP_NODE, node);
}
#endif /* WITH_LWIP */

unsigned int
coap_adjust_basetime(coap_context_t *ctx, coap_tick_t now) {
  unsigned int result = 0;
  coap_tick_diff_t delta = now - ctx->sendqueue_basetime;

  if (ctx->sendqueue) {
    /* delta < 0 means that the new time stamp is before the old. */
    if (delta <= 0) {
      ctx->sendqueue->t -= delta;
    } else {
      /* This case is more complex: The time must be advanced forward,
       * thus possibly leading to timed out elements at the queue's
       * start. For every element that has timed out, its relative
       * time is set to zero and the result counter is increased. */

      coap_queue_t *q = ctx->sendqueue;
      coap_tick_t t = 0;
      while (q && (t + q->t < (coap_tick_t)delta)) {
        t += q->t;
        q->t = 0;
        result++;
        q = q->next;
      }

      /* finally adjust the first element that has not expired */
      if (q) {
        q->t = (coap_tick_t)delta - t;
      }
    }
  }

  /* adjust basetime */
  ctx->sendqueue_basetime += delta;

  return result;
}

int
coap_insert_node(coap_queue_t **queue, coap_queue_t *node) {
  coap_queue_t *p, *q;
  if (!queue || !node)
    return 0;

  /* set queue head if empty */
  if (!*queue) {
    *queue = node;
    return 1;
  }

  /* replace queue head if PDU's time is less than head's time */
  q = *queue;
  if (node->t < q->t) {
    node->next = q;
    *queue = node;
    q->t -= node->t;                /* make q->t relative to node->t */
    return 1;
  }

  /* search for right place to insert */
  do {
    node->t -= q->t;                /* make node-> relative to q->t */
    p = q;
    q = q->next;
  } while (q && q->t <= node->t);

  /* insert new item */
  if (q) {
    q->t -= node->t;                /* make q->t relative to node->t */
  }
  node->next = q;
  p->next = node;
  return 1;
}

COAP_API int
coap_delete_node(coap_queue_t *node) {
  int ret;
#if COAP_THREAD_SAFE
  coap_context_t *context;
#endif /* COAP_THREAD_SAFE */

  if (!node)
    return 0;
  if (!node->session)
    return coap_delete_node_lkd(node);

#if COAP_THREAD_SAFE
  /* Keep copy as node will be going away */
  context = node->session->context;
  (void)context;
#endif /* COAP_THREAD_SAFE */
  coap_lock_lock(context, return 0);
  ret = coap_delete_node_lkd(node);
  coap_lock_unlock(context);
  return ret;
}

int
coap_delete_node_lkd(coap_queue_t *node) {
  if (!node)
    return 0;

  coap_delete_pdu(node->pdu);
  if (node->session) {
    /*
     * Need to remove out of context->sendqueue as added in by coap_wait_ack()
     */
    if (node->session->context->sendqueue) {
      LL_DELETE(node->session->context->sendqueue, node);
    }
    coap_session_release_lkd(node->session);
  }
  coap_free_node(node);

  return 1;
}

void
coap_delete_all(coap_queue_t *queue) {
  if (!queue)
    return;

  coap_delete_all(queue->next);
  coap_delete_node_lkd(queue);
}

coap_queue_t *
coap_new_node(void) {
  coap_queue_t *node;
  node = coap_malloc_node();

  if (!node) {
    coap_log_warn("coap_new_node: malloc failed\n");
    return NULL;
  }

  memset(node, 0, sizeof(*node));
  return node;
}

coap_queue_t *
coap_peek_next(coap_context_t *context) {
  if (!context || !context->sendqueue)
    return NULL;

  return context->sendqueue;
}

coap_queue_t *
coap_pop_next(coap_context_t *context) {
  coap_queue_t *next;

  if (!context || !context->sendqueue)
    return NULL;

  next = context->sendqueue;
  context->sendqueue = context->sendqueue->next;
  if (context->sendqueue) {
    context->sendqueue->t += next->t;
  }
  next->next = NULL;
  return next;
}

#if COAP_CLIENT_SUPPORT
const coap_bin_const_t *
coap_get_session_client_psk_key(const coap_session_t *session) {

  if (session->psk_key) {
    return session->psk_key;
  }
  if (session->cpsk_setup_data.psk_info.key.length)
    return &session->cpsk_setup_data.psk_info.key;

  /* Not defined in coap_new_client_session_psk2() */
  return NULL;
}

const coap_bin_const_t *
coap_get_session_client_psk_identity(const coap_session_t *session) {

  if (session->psk_identity) {
    return session->psk_identity;
  }
  if (session->cpsk_setup_data.psk_info.identity.length)
    return &session->cpsk_setup_data.psk_info.identity;

  /* Not defined in coap_new_client_session_psk2() */
  return NULL;
}
#endif /* COAP_CLIENT_SUPPORT */

#if COAP_SERVER_SUPPORT
const coap_bin_const_t *
coap_get_session_server_psk_key(const coap_session_t *session) {

  if (session->psk_key)
    return session->psk_key;

  if (session->context->spsk_setup_data.psk_info.key.length)
    return &session->context->spsk_setup_data.psk_info.key;

  /* Not defined in coap_context_set_psk2() */
  return NULL;
}

const coap_bin_const_t *
coap_get_session_server_psk_hint(const coap_session_t *session) {

  if (session->psk_hint)
    return session->psk_hint;

  if (session->context->spsk_setup_data.psk_info.hint.length)
    return &session->context->spsk_setup_data.psk_info.hint;

  /* Not defined in coap_context_set_psk2() */
  return NULL;
}

COAP_API int
coap_context_set_psk(coap_context_t *ctx,
                     const char *hint,
                     const uint8_t *key,
                     size_t key_len) {
  int ret;

  coap_lock_lock(ctx, return 0);
  ret = coap_context_set_psk_lkd(ctx, hint, key, key_len);
  coap_lock_unlock(ctx);
  return ret;
}

int
coap_context_set_psk_lkd(coap_context_t *ctx,
                         const char *hint,
                         const uint8_t *key,
                         size_t key_len) {
  coap_dtls_spsk_t setup_data;

  coap_lock_check_locked(ctx);
  memset(&setup_data, 0, sizeof(setup_data));
  if (hint) {
    setup_data.psk_info.hint.s = (const uint8_t *)hint;
    setup_data.psk_info.hint.length = strlen(hint);
  }

  if (key && key_len > 0) {
    setup_data.psk_info.key.s = key;
    setup_data.psk_info.key.length = key_len;
  }

  return coap_context_set_psk2_lkd(ctx, &setup_data);
}

COAP_API int
coap_context_set_psk2(coap_context_t *ctx, coap_dtls_spsk_t *setup_data) {
  int ret;

  coap_lock_lock(ctx, return 0);
  ret = coap_context_set_psk2_lkd(ctx, setup_data);
  coap_lock_unlock(ctx);
  return ret;
}

int
coap_context_set_psk2_lkd(coap_context_t *ctx, coap_dtls_spsk_t *setup_data) {
  if (!setup_data)
    return 0;

  coap_lock_check_locked(ctx);
  ctx->spsk_setup_data = *setup_data;

  if (coap_dtls_is_supported() || coap_tls_is_supported()) {
    return coap_dtls_context_set_spsk(ctx, setup_data);
  }
  return 0;
}

COAP_API int
coap_context_set_pki(coap_context_t *ctx,
                     const coap_dtls_pki_t *setup_data) {
  int ret;

  coap_lock_lock(ctx, return 0);
  ret = coap_context_set_pki_lkd(ctx, setup_data);
  coap_lock_unlock(ctx);
  return ret;
}

int
coap_context_set_pki_lkd(coap_context_t *ctx,
                         const coap_dtls_pki_t *setup_data) {
  coap_lock_check_locked(ctx);
  if (!setup_data)
    return 0;
  if (setup_data->version != COAP_DTLS_PKI_SETUP_VERSION) {
    coap_log_err("coap_context_set_pki: Wrong version of setup_data\n");
    return 0;
  }
  if (coap_dtls_is_supported() || coap_tls_is_supported()) {
    return coap_dtls_context_set_pki(ctx, setup_data, COAP_DTLS_ROLE_SERVER);
  }
  return 0;
}
#endif /* ! COAP_SERVER_SUPPORT */

COAP_API int
coap_context_set_pki_root_cas(coap_context_t *ctx,
                              const char *ca_file,
                              const char *ca_dir) {
  int ret;

  coap_lock_lock(ctx, return 0);
  ret = coap_context_set_pki_root_cas_lkd(ctx, ca_file, ca_dir);
  coap_lock_unlock(ctx);
  return ret;
}

int
coap_context_set_pki_root_cas_lkd(coap_context_t *ctx,
                                  const char *ca_file,
                                  const char *ca_dir) {
  if (coap_dtls_is_supported() || coap_tls_is_supported()) {
    return coap_dtls_context_set_pki_root_cas(ctx, ca_file, ca_dir);
  }
  return 0;
}

void
coap_context_set_keepalive(coap_context_t *context, unsigned int seconds) {
  context->ping_timeout = seconds;
}

int
coap_context_set_cid_tuple_change(coap_context_t *context, uint8_t every) {
#if COAP_CLIENT_SUPPORT
  return coap_dtls_set_cid_tuple_change(context, every);
#else /* ! COAP_CLIENT_SUPPORT */
  (void)context;
  (void)every;
  return 0;
#endif /* ! COAP_CLIENT_SUPPORT */
}

void
coap_context_set_max_token_size(coap_context_t *context,
                                size_t max_token_size) {
  assert(max_token_size >= COAP_TOKEN_DEFAULT_MAX &&
         max_token_size <= COAP_TOKEN_EXT_MAX);
  context->max_token_size = (uint32_t)max_token_size;
}

void
coap_context_set_max_idle_sessions(coap_context_t *context,
                                   unsigned int max_idle_sessions) {
  context->max_idle_sessions = max_idle_sessions;
}

unsigned int
coap_context_get_max_idle_sessions(const coap_context_t *context) {
  return context->max_idle_sessions;
}

void
coap_context_set_max_handshake_sessions(coap_context_t *context,
                                        unsigned int max_handshake_sessions) {
  context->max_handshake_sessions = max_handshake_sessions;
}

unsigned int
coap_context_get_max_handshake_sessions(const coap_context_t *context) {
  return context->max_handshake_sessions;
}

static unsigned int s_csm_timeout = 30;

void
coap_context_set_csm_timeout(coap_context_t *context,
                             unsigned int csm_timeout) {
  s_csm_timeout = csm_timeout;
  coap_context_set_csm_timeout_ms(context, csm_timeout * 1000);
}

unsigned int
coap_context_get_csm_timeout(const coap_context_t *context) {
  (void)context;
  return s_csm_timeout;
}

void
coap_context_set_csm_timeout_ms(coap_context_t *context,
                                unsigned int csm_timeout_ms) {
  if (csm_timeout_ms < 10)
    csm_timeout_ms = 10;
  if (csm_timeout_ms > 10000)
    csm_timeout_ms = 10000;
  context->csm_timeout_ms = csm_timeout_ms;
}

unsigned int
coap_context_get_csm_timeout_ms(const coap_context_t *context) {
  return context->csm_timeout_ms;
}

void
coap_context_set_csm_max_message_size(coap_context_t *context,
                                      uint32_t csm_max_message_size) {
  assert(csm_max_message_size >= 64);
  context->csm_max_message_size = csm_max_message_size;
}

uint32_t
coap_context_get_csm_max_message_size(const coap_context_t *context) {
  return context->csm_max_message_size;
}

void
coap_context_set_session_timeout(coap_context_t *context,
                                 unsigned int session_timeout) {
  context->session_timeout = session_timeout;
}

unsigned int
coap_context_get_session_timeout(const coap_context_t *context) {
  return context->session_timeout;
}

int
coap_context_get_coap_fd(const coap_context_t *context) {
#ifdef COAP_EPOLL_SUPPORT
  return context->epfd;
#else /* ! COAP_EPOLL_SUPPORT */
  (void)context;
  return -1;
#endif /* ! COAP_EPOLL_SUPPORT */
}

int
coap_epoll_is_supported(void) {
#ifdef COAP_EPOLL_SUPPORT
  return 1;
#else /* ! COAP_EPOLL_SUPPORT */
  return 0;
#endif /* ! COAP_EPOLL_SUPPORT */
}

int
coap_threadsafe_is_supported(void) {
#ifdef COAP_THREAD_SAFE
  return 1;
#else /* ! COAP_THREAD_SAFE */
  return 0;
#endif /* ! COAP_THREAD_SAFE */
}

int
coap_ipv4_is_supported(void) {
#ifdef COAP_IPV4_SUPPORT
  return 1;
#else /* ! COAP_IPV4_SUPPORT */
  return 0;
#endif /* ! COAP_IPV4_SUPPORT */
}

int
coap_ipv6_is_supported(void) {
#ifdef COAP_IPV6_SUPPORT
  return 1;
#else /* ! COAP_IPV6_SUPPORT */
  return 0;
#endif /* ! COAP_IPV6_SUPPORT */
}

int
coap_client_is_supported(void) {
#ifdef COAP_CLIENT_SUPPORT
  return 1;
#else /* ! COAP_CLIENT_SUPPORT */
  return 0;
#endif /* ! COAP_CLIENT_SUPPORT */
}

int
coap_server_is_supported(void) {
#ifdef COAP_SERVER_SUPPORT
  return 1;
#else /* ! COAP_SERVER_SUPPORT */
  return 0;
#endif /* ! COAP_SERVER_SUPPORT */
}

int
coap_af_unix_is_supported(void) {
#ifdef COAP_AF_UNIX_SUPPORT
  return 1;
#else /* ! COAP_AF_UNIX_SUPPORT */
  return 0;
#endif /* ! COAP_AF_UNIX_SUPPORT */
}

void
coap_context_set_app_data(coap_context_t *context, void *app_data) {
  assert(context);
  context->app = app_data;
}

void *
coap_context_get_app_data(const coap_context_t *context) {
  assert(context);
  return context->app;
}

coap_context_t *
coap_new_context(const coap_address_t *listen_addr) {
  coap_context_t *c;

#if ! COAP_SERVER_SUPPORT
  (void)listen_addr;
#endif /* COAP_SERVER_SUPPORT */

  if (!coap_started) {
    coap_startup();
    coap_log_warn("coap_startup() should be called before any other "
                  "coap_*() functions are called\n");
  }

  c = coap_malloc_type(COAP_CONTEXT, sizeof(coap_context_t));
  if (!c) {
    coap_log_emerg("coap_init: malloc: failed\n");
    return NULL;
  }
  memset(c, 0, sizeof(coap_context_t));

  coap_lock_lock(c, coap_free_type(COAP_CONTEXT, c); return NULL);
#ifdef COAP_EPOLL_SUPPORT
  c->epfd = epoll_create1(0);
  if (c->epfd == -1) {
    coap_log_err("coap_new_context: Unable to epoll_create: %s (%d)\n",
                 coap_socket_strerror(),
                 errno);
    goto onerror;
  }
  if (c->epfd != -1) {
    c->eptimerfd = timerfd_create(CLOCK_REALTIME, TFD_NONBLOCK);
    if (c->eptimerfd == -1) {
      coap_log_err("coap_new_context: Unable to timerfd_create: %s (%d)\n",
                   coap_socket_strerror(),
                   errno);
      goto onerror;
    } else {
      int ret;
      struct epoll_event event;

      /* Needed if running 32bit as ptr is only 32bit */
      memset(&event, 0, sizeof(event));
      event.events = EPOLLIN;
      /* We special case this event by setting to NULL */
      event.data.ptr = NULL;

      ret = epoll_ctl(c->epfd, EPOLL_CTL_ADD, c->eptimerfd, &event);
      if (ret == -1) {
        coap_log_err("%s: epoll_ctl ADD failed: %s (%d)\n",
                     "coap_new_context",
                     coap_socket_strerror(), errno);
        goto onerror;
      }
    }
  }
#endif /* COAP_EPOLL_SUPPORT */

  if (coap_dtls_is_supported() || coap_tls_is_supported()) {
    c->dtls_context = coap_dtls_new_context(c);
    if (!c->dtls_context) {
      coap_log_emerg("coap_init: no DTLS context available\n");
      coap_free_context_lkd(c);
      return NULL;
    }
  }

  /* set default CSM values */
  c->csm_timeout_ms = 1000;
  c->csm_max_message_size = COAP_DEFAULT_MAX_PDU_RX_SIZE;

#if COAP_SERVER_SUPPORT
  if (listen_addr) {
    coap_endpoint_t *endpoint = coap_new_endpoint_lkd(c, listen_addr, COAP_PROTO_UDP);
    if (endpoint == NULL) {
      goto onerror;
    }
  }
#endif /* COAP_SERVER_SUPPORT */

  c->max_token_size = COAP_TOKEN_DEFAULT_MAX; /* RFC8974 */

  coap_lock_unlock(c);
  return c;

#if defined(COAP_EPOLL_SUPPORT) || COAP_SERVER_SUPPORT
onerror:
  coap_free_type(COAP_CONTEXT, c);
  return NULL;
#endif /* COAP_EPOLL_SUPPORT || COAP_SERVER_SUPPORT */
}

void
coap_set_app_data(coap_context_t *ctx, void *app_data) {
  assert(ctx);
  ctx->app = app_data;
}

void *
coap_get_app_data(const coap_context_t *ctx) {
  assert(ctx);
  return ctx->app;
}

COAP_API void
coap_free_context(coap_context_t *context) {
  if (!context)
    return;
  coap_lock_lock(context, return);
  coap_free_context_lkd(context);
  coap_lock_unlock(context);
}

void
coap_free_context_lkd(coap_context_t *context) {
  if (!context)
    return;

  coap_lock_check_locked(context);
#if COAP_SERVER_SUPPORT
  /* Removing a resource may cause a NON unsolicited observe to be sent */
  coap_delete_all_resources(context);
#endif /* COAP_SERVER_SUPPORT */

  coap_delete_all(context->sendqueue);

#ifdef WITH_LWIP
  context->sendqueue = NULL;
  if (context->timer_configured) {
    LOCK_TCPIP_CORE();
    sys_untimeout(coap_io_process_timeout, (void *)context);
    UNLOCK_TCPIP_CORE();
    context->timer_configured = 0;
  }
#endif /* WITH_LWIP */

#if COAP_ASYNC_SUPPORT
  coap_delete_all_async(context);
#endif /* COAP_ASYNC_SUPPORT */

#if COAP_OSCORE_SUPPORT
  coap_delete_all_oscore(context);
#endif /* COAP_OSCORE_SUPPORT */

#if COAP_SERVER_SUPPORT
  coap_cache_entry_t *cp, *ctmp;

  HASH_ITER(hh, context->cache, cp, ctmp) {
    coap_delete_cache_entry(context, cp);
  }
  if (context->cache_ignore_count) {
    coap_free_type(COAP_STRING, context->cache_ignore_options);
  }

  coap_endpoint_t *ep, *tmp;

  LL_FOREACH_SAFE(context->endpoint, ep, tmp) {
    coap_free_endpoint_lkd(ep);
  }
#endif /* COAP_SERVER_SUPPORT */

#if COAP_CLIENT_SUPPORT
  coap_session_t *sp, *rtmp;

  SESSIONS_ITER_SAFE(context->sessions, sp, rtmp) {
    coap_session_release_lkd(sp);
  }
#endif /* COAP_CLIENT_SUPPORT */

  if (context->dtls_context)
    coap_dtls_free_context(context->dtls_context);
#ifdef COAP_EPOLL_SUPPORT
  if (context->eptimerfd != -1) {
    int ret;
    struct epoll_event event;

    /* Kernels prior to 2.6.9 expect non NULL event parameter */
    ret = epoll_ctl(context->epfd, EPOLL_CTL_DEL, context->eptimerfd, &event);
    if (ret == -1) {
      coap_log_err("%s: epoll_ctl DEL failed: %s (%d)\n",
                   "coap_free_context",
                   coap_socket_strerror(), errno);
    }
    close(context->eptimerfd);
    context->eptimerfd = -1;
  }
  if (context->epfd != -1) {
    close(context->epfd);
    context->epfd = -1;
  }
#endif /* COAP_EPOLL_SUPPORT */
#if COAP_SERVER_SUPPORT
#if COAP_WITH_OBSERVE_PERSIST
  coap_persist_cleanup(context);
#endif /* COAP_WITH_OBSERVE_PERSIST */
#endif /* COAP_SERVER_SUPPORT */
#if COAP_PROXY_SUPPORT
  coap_proxy_cleanup(context);
#endif /* COAP_PROXY_SUPPORT */

  coap_free_type(COAP_CONTEXT, context);
  coap_dump_memory_type_counts(COAP_LOG_DEBUG);
}

int
coap_option_check_critical(coap_session_t *session,
                           coap_pdu_t *pdu,
                           coap_opt_filter_t *unknown) {
  coap_context_t *ctx = session->context;
  coap_opt_iterator_t opt_iter;
  int ok = 1;
  coap_option_num_t last_number = -1;

  coap_option_iterator_init(pdu, &opt_iter, COAP_OPT_ALL);

  while (coap_option_next(&opt_iter)) {
    if (opt_iter.number & 0x01) {
      /* first check the known built-in critical options */
      switch (opt_iter.number) {
#if COAP_Q_BLOCK_SUPPORT
      case COAP_OPTION_Q_BLOCK1:
      case COAP_OPTION_Q_BLOCK2:
        if (!(ctx->block_mode & COAP_BLOCK_TRY_Q_BLOCK)) {
          coap_log_debug("disabled support for critical option %u\n",
                         opt_iter.number);
          ok = 0;
          coap_option_filter_set(unknown, opt_iter.number);
        }
        break;
#endif /* COAP_Q_BLOCK_SUPPORT */
      case COAP_OPTION_IF_MATCH:
      case COAP_OPTION_URI_HOST:
      case COAP_OPTION_IF_NONE_MATCH:
      case COAP_OPTION_URI_PORT:
      case COAP_OPTION_URI_PATH:
      case COAP_OPTION_URI_QUERY:
      case COAP_OPTION_ACCEPT:
      case COAP_OPTION_PROXY_URI:
      case COAP_OPTION_PROXY_SCHEME:
      case COAP_OPTION_BLOCK2:
      case COAP_OPTION_BLOCK1:
        break;
      case COAP_OPTION_OSCORE:
        /* Valid critical if doing OSCORE */
#if COAP_OSCORE_SUPPORT
        if (ctx->p_osc_ctx)
          break;
#endif /* COAP_OSCORE_SUPPORT */
      /* Fall Through */
      default:
        if (coap_option_filter_get(&ctx->known_options, opt_iter.number) <= 0) {
#if COAP_SERVER_SUPPORT
          if ((opt_iter.number & 0x02) == 0) {
            coap_opt_iterator_t t_iter;

            /* Safe to forward  - check if proxy pdu */
            if (session->proxy_session)
              break;
            if (COAP_PDU_IS_REQUEST(pdu) && ctx->proxy_uri_resource &&
                (coap_check_option(pdu, COAP_OPTION_PROXY_URI, &t_iter) ||
                 coap_check_option(pdu, COAP_OPTION_PROXY_SCHEME, &t_iter))) {
              pdu->crit_opt = 1;
              break;
            }
          }
#endif /* COAP_SERVER_SUPPORT */
          coap_log_debug("unknown critical option %d\n", opt_iter.number);
          ok = 0;

          /* When opt_iter.number cannot be set in unknown, all of the appropriate
           * slots have been used up and no more options can be tracked.
           * Safe to break out of this loop as ok is already set. */
          if (coap_option_filter_set(unknown, opt_iter.number) == 0) {
            break;
          }
        }
      }
    }
    if (last_number == opt_iter.number) {
      /* Check for duplicated option RFC 5272 5.4.5 */
      if (!coap_option_check_repeatable(opt_iter.number)) {
        ok = 0;
        if (coap_option_filter_set(unknown, opt_iter.number) == 0) {
          break;
        }
      }
    } else if (opt_iter.number == COAP_OPTION_BLOCK2 &&
               COAP_PDU_IS_REQUEST(pdu)) {
      /* Check the M Bit is not set on a GET request RFC 7959 2.2 */
      coap_block_b_t block;

      if (coap_get_block_b(session, pdu, opt_iter.number, &block)) {
        if (block.m) {
          size_t used_size = pdu->used_size;
          unsigned char buf[4];

          coap_log_debug("Option Block2 has invalid set M bit - cleared\n");
          block.m = 0;
          coap_update_option(pdu, opt_iter.number,
                             coap_encode_var_safe(buf, sizeof(buf),
                                                  ((block.num << 4) |
                                                   (block.m << 3) |
                                                   block.aszx)),
                             buf);
          if (used_size != pdu->used_size) {
            /* Unfortunately need to restart the scan */
            coap_option_iterator_init(pdu, &opt_iter, COAP_OPT_ALL);
            last_number = -1;
            continue;
          }
        }
      }
    }
    last_number = opt_iter.number;
  }

  return ok;
}

COAP_API coap_mid_t
coap_send_rst(coap_session_t *session, const coap_pdu_t *request) {
  coap_mid_t mid;

  coap_lock_lock(session->context, return COAP_INVALID_MID);
  mid = coap_send_rst_lkd(session, request);
  coap_lock_unlock(session->context);
  return mid;
}

coap_mid_t
coap_send_rst_lkd(coap_session_t *session, const coap_pdu_t *request) {
  return coap_send_message_type_lkd(session, request, COAP_MESSAGE_RST);
}

COAP_API coap_mid_t
coap_send_ack(coap_session_t *session, const coap_pdu_t *request) {
  coap_mid_t mid;

  coap_lock_lock(session->context, return COAP_INVALID_MID);
  mid = coap_send_ack_lkd(session, request);
  coap_lock_unlock(session->context);
  return mid;
}

coap_mid_t
coap_send_ack_lkd(coap_session_t *session, const coap_pdu_t *request) {
  coap_pdu_t *response;
  coap_mid_t result = COAP_INVALID_MID;

  coap_lock_check_locked(session->context);
  if (request && request->type == COAP_MESSAGE_CON &&
      COAP_PROTO_NOT_RELIABLE(session->proto)) {
    response = coap_pdu_init(COAP_MESSAGE_ACK, 0, request->mid, 0);
    if (response)
      result = coap_send_internal(session, response);
  }
  return result;
}

ssize_t
coap_session_send_pdu(coap_session_t *session, coap_pdu_t *pdu) {
  ssize_t bytes_written = -1;
  assert(pdu->hdr_size > 0);

  /* Caller handles partial writes */
  bytes_written = session->sock.lfunc[COAP_LAYER_SESSION].l_write(session,
                  pdu->token - pdu->hdr_size,
                  pdu->used_size + pdu->hdr_size);
  coap_show_pdu(COAP_LOG_DEBUG, pdu);
  return bytes_written;
}

static ssize_t
coap_send_pdu(coap_session_t *session, coap_pdu_t *pdu, coap_queue_t *node) {
  ssize_t bytes_written;

  if (session->state == COAP_SESSION_STATE_NONE) {
#if ! COAP_CLIENT_SUPPORT
    return -1;
#else /* COAP_CLIENT_SUPPORT */
    if (session->type != COAP_SESSION_TYPE_CLIENT)
      return -1;
#endif /* COAP_CLIENT_SUPPORT */
  }

  if (pdu->type == COAP_MESSAGE_CON &&
      (session->sock.flags & COAP_SOCKET_NOT_EMPTY) &&
      (session->sock.flags & COAP_SOCKET_MULTICAST)) {
    /* Violates RFC72522 8.1 */
    coap_log_err("Multicast requests cannot be Confirmable (RFC7252 8.1)\n");
    return -1;
  }

  if (session->state != COAP_SESSION_STATE_ESTABLISHED ||
      (pdu->type == COAP_MESSAGE_CON &&
       session->con_active >= COAP_NSTART(session))) {
    return coap_session_delay_pdu(session, pdu, node);
  }

  if ((session->sock.flags & COAP_SOCKET_NOT_EMPTY) &&
      (session->sock.flags & COAP_SOCKET_WANT_WRITE))
    return coap_session_delay_pdu(session, pdu, node);

  bytes_written = coap_session_send_pdu(session, pdu);
  if (bytes_written >= 0 && pdu->type == COAP_MESSAGE_CON &&
      COAP_PROTO_NOT_RELIABLE(session->proto))
    session->con_active++;

  return bytes_written;
}

COAP_API coap_mid_t
coap_send_error(coap_session_t *session,
                const coap_pdu_t *request,
                coap_pdu_code_t code,
                coap_opt_filter_t *opts) {
  coap_mid_t mid;

  coap_lock_lock(session->context, return COAP_INVALID_MID);
  mid = coap_send_error_lkd(session, request, code, opts);
  coap_lock_unlock(session->context);
  return mid;
}

coap_mid_t
coap_send_error_lkd(coap_session_t *session,
                    const coap_pdu_t *request,
                    coap_pdu_code_t code,
                    coap_opt_filter_t *opts) {
  coap_pdu_t *response;
  coap_mid_t result = COAP_INVALID_MID;

  assert(request);
  assert(session);

  response = coap_new_error_response(request, code, opts);
  if (response)
    result = coap_send_internal(session, response);

  return result;
}

COAP_API coap_mid_t
coap_send_message_type(coap_session_t *session, const coap_pdu_t *request,
                       coap_pdu_type_t type) {
  coap_mid_t mid;

  coap_lock_lock(session->context, return COAP_INVALID_MID);
  mid = coap_send_message_type_lkd(session, request, type);
  coap_lock_unlock(session->context);
  return mid;
}

coap_mid_t
coap_send_message_type_lkd(coap_session_t *session, const coap_pdu_t *request,
                           coap_pdu_type_t type) {
  coap_pdu_t *response;
  coap_mid_t result = COAP_INVALID_MID;

  coap_lock_check_locked(session->context);
  if (request && COAP_PROTO_NOT_RELIABLE(session->proto)) {
    response = coap_pdu_init(type, 0, request->mid, 0);
    if (response)
      result = coap_send_internal(session, response);
  }
  return result;
}

/**
 * Calculates the initial timeout based on the session CoAP transmission
 * parameters 'ack_timeout', 'ack_random_factor', and COAP_TICKS_PER_SECOND.
 * The calculation requires 'ack_timeout' and 'ack_random_factor' to be in
 * Qx.FRAC_BITS fixed point notation, whereas the passed parameter @p r
 * is interpreted as the fractional part of a Q0.MAX_BITS random value.
 *
 * @param session session timeout is associated with
 * @param r  random value as fractional part of a Q0.MAX_BITS fixed point
 *           value
 * @return   COAP_TICKS_PER_SECOND * 'ack_timeout' *
 *           (1 + ('ack_random_factor' - 1) * r)
 */
unsigned int
coap_calc_timeout(coap_session_t *session, unsigned char r) {
  unsigned int result;

  /* The integer 1.0 as a Qx.FRAC_BITS */
#define FP1 Q(FRAC_BITS, ((coap_fixed_point_t){1,0}))

  /* rounds val up and right shifts by frac positions */
#define SHR_FP(val,frac) (((val) + (1 << ((frac) - 1))) >> (frac))

  /* Inner term: multiply ACK_RANDOM_FACTOR by Q0.MAX_BITS[r] and
   * make the result a rounded Qx.FRAC_BITS */
  result = SHR_FP((ACK_RANDOM_FACTOR - FP1) * r, MAX_BITS);

  /* Add 1 to the inner term and multiply with ACK_TIMEOUT, then
   * make the result a rounded Qx.FRAC_BITS */
  result = SHR_FP(((result + FP1) * ACK_TIMEOUT), FRAC_BITS);

  /* Multiply with COAP_TICKS_PER_SECOND to yield system ticks
   * (yields a Qx.FRAC_BITS) and shift to get an integer */
  return SHR_FP((COAP_TICKS_PER_SECOND * result), FRAC_BITS);

#undef FP1
#undef SHR_FP
}

coap_mid_t
coap_wait_ack(coap_context_t *context, coap_session_t *session,
              coap_queue_t *node) {
  coap_tick_t now;

  node->session = coap_session_reference_lkd(session);

  /* Set timer for pdu retransmission. If this is the first element in
  * the retransmission queue, the base time is set to the current
  * time and the retransmission time is node->timeout. If there is
  * already an entry in the sendqueue, we must check if this node is
  * to be retransmitted earlier. Therefore, node->timeout is first
  * normalized to the base time and then inserted into the queue with
  * an adjusted relative time.
  */
  coap_ticks(&now);
  if (context->sendqueue == NULL) {
    node->t = node->timeout << node->retransmit_cnt;
    context->sendqueue_basetime = now;
  } else {
    /* make node->t relative to context->sendqueue_basetime */
    node->t = (now - context->sendqueue_basetime) +
              (node->timeout << node->retransmit_cnt);
  }

  coap_insert_node(&context->sendqueue, node);

  coap_log_debug("** %s: mid=0x%04x: added to retransmit queue (%ums)\n",
                 coap_session_str(node->session), node->id,
                 (unsigned)((node->timeout << node->retransmit_cnt) * 1000 /
                            COAP_TICKS_PER_SECOND));

  coap_update_io_timer(context, node->t);

  return node->id;
}

#if COAP_CLIENT_SUPPORT
/*
 * Sent out a test PDU for Extended Token
 */
static coap_mid_t
coap_send_test_extended_token(coap_session_t *session) {
  coap_pdu_t *pdu;
  coap_mid_t mid = COAP_INVALID_MID;
  size_t i;
  coap_binary_t *token;

  coap_log_debug("Testing for Extended Token support\n");
  /* https://rfc-editor.org/rfc/rfc8974#section-2.2.2 */
  pdu = coap_pdu_init(COAP_MESSAGE_CON, COAP_REQUEST_CODE_GET,
                      coap_new_message_id_lkd(session),
                      coap_session_max_pdu_size_lkd(session));
  if (!pdu)
    return COAP_INVALID_MID;

  token = coap_new_binary(session->max_token_size);
  if (token == NULL) {
    coap_delete_pdu(pdu);
    return COAP_INVALID_MID;
  }
  for (i = 0; i < session->max_token_size; i++) {
    token->s[i] = (uint8_t)(i + 1);
  }
  coap_add_token(pdu, session->max_token_size, token->s);
  coap_delete_binary(token);

  coap_insert_option(pdu, COAP_OPTION_IF_NONE_MATCH, 0, NULL);

  session->max_token_checked = COAP_EXT_T_CHECKING; /* Checking out this one */
  if ((mid = coap_send_internal(session, pdu)) == COAP_INVALID_MID)
    return COAP_INVALID_MID;
  session->remote_test_mid = mid;
  return mid;
}
#endif /* COAP_CLIENT_SUPPORT */

int
coap_client_delay_first(coap_session_t *session) {
#if COAP_CLIENT_SUPPORT
  if (session->type == COAP_SESSION_TYPE_CLIENT && session->doing_first) {
    int timeout_ms = 5000;
    coap_session_state_t current_state = session->state;

    if (session->delay_recursive) {
      assert(0);
      return 1;
    } else {
      session->delay_recursive = 1;
    }
    /*
     * Need to wait for first request to get out and response back before
     * continuing.. Response handler has to clear doing_first if not an error.
     */
    coap_session_reference_lkd(session);
    while (session->doing_first != 0) {
      int result = coap_io_process_lkd(session->context, 1000);

      if (result < 0) {
        session->doing_first = 0;
        session->delay_recursive = 0;
        coap_session_release_lkd(session);
        return 0;
      }

      /* coap_io_process_lkd() may have updated session state */
      if (session->state == COAP_SESSION_STATE_CSM &&
          current_state != COAP_SESSION_STATE_CSM) {
        /* Update timeout and restart the clock for CSM timeout */
        current_state = COAP_SESSION_STATE_CSM;
        timeout_ms = session->context->csm_timeout_ms;
        result = 0;
      }

      if (result < timeout_ms) {
        timeout_ms -= result;
      } else {
        if (session->doing_first == 1) {
          /* Timeout failure of some sort with first request */
          session->doing_first = 0;
          if (session->state == COAP_SESSION_STATE_CSM) {
            coap_log_debug("** %s: timeout waiting for CSM response\n",
                           coap_session_str(session));
            session->csm_not_seen = 1;
            coap_session_connected(session);
          } else {
            coap_log_debug("** %s: timeout waiting for first response\n",
                           coap_session_str(session));
          }
        }
      }
    }
    session->delay_recursive = 0;
    coap_session_release_lkd(session);
  }
#else /* ! COAP_CLIENT_SUPPORT */
  (void)session;
#endif /* ! COAP_CLIENT_SUPPORT */
  return 1;
}

/*
 * return  0 Invalid
 *         1 Valid
 */
int
coap_check_code_class(coap_session_t *session, coap_pdu_t *pdu) {

  /* Check validity of sending code */
  switch (COAP_RESPONSE_CLASS(pdu->code)) {
  case 0: /* Empty or request */
  case 2: /* Success */
  case 3: /* Reserved for future use */
  case 4: /* Client error */
  case 5: /* Server error */
    break;
  case 7: /* Reliable signalling */
    if (COAP_PROTO_RELIABLE(session->proto))
      break;
  /* Not valid if UDP */
  /* Fall through */
  case 1: /* Invalid */
  case 6: /* Invalid */
  default:
    return 0;
  }
  return 1;
}

#if COAP_CLIENT_SUPPORT
/*
 * If type is CON and protocol is not reliable, there is no need to set up
 * lg_crcv if it can be built up based on sent PDU if there is a
 * (Q-)Block2 in the response.  However, still need it for Observe, Oscore and
 * (Q-)Block1.
 */
static int
coap_check_send_need_lg_crcv(coap_session_t *session, coap_pdu_t *pdu) {
  coap_opt_iterator_t opt_iter;

  if (
#if COAP_OSCORE_SUPPORT
      session->oscore_encryption ||
#endif /* COAP_OSCORE_SUPPORT */
      ((pdu->type == COAP_MESSAGE_NON || COAP_PROTO_RELIABLE(session->proto)) &&
       COAP_PDU_IS_REQUEST(pdu) && pdu->code != COAP_REQUEST_CODE_DELETE) ||
      coap_check_option(pdu, COAP_OPTION_OBSERVE, &opt_iter) ||
#if COAP_Q_BLOCK_SUPPORT
      coap_check_option(pdu, COAP_OPTION_Q_BLOCK1, &opt_iter) ||
#endif /* COAP_Q_BLOCK_SUPPORT */
      coap_check_option(pdu, COAP_OPTION_BLOCK1, &opt_iter)) {
    return 1;
  }
  return 0;
}
#endif /* COAP_CLIENT_SUPPORT */

COAP_API coap_mid_t
coap_send(coap_session_t *session, coap_pdu_t *pdu) {
  coap_mid_t mid;

  coap_lock_lock(session->context, return COAP_INVALID_MID);
  mid = coap_send_lkd(session, pdu);
  coap_lock_unlock(session->context);
  return mid;
}

coap_mid_t
coap_send_lkd(coap_session_t *session, coap_pdu_t *pdu) {
  coap_mid_t mid = COAP_INVALID_MID;
#if COAP_CLIENT_SUPPORT
  coap_lg_crcv_t *lg_crcv = NULL;
  coap_opt_iterator_t opt_iter;
  coap_block_b_t block;
  int observe_action = -1;
  int have_block1 = 0;
  coap_opt_t *opt;
#endif /* COAP_CLIENT_SUPPORT */

  assert(pdu);

  coap_lock_check_locked(session->context);

  /* Check validity of sending code */
  if (!coap_check_code_class(session, pdu)) {
    coap_log_err("coap_send: Invalid PDU code (%d.%02d)\n",
                 COAP_RESPONSE_CLASS(pdu->code),
                 pdu->code & 0x1f);
    goto error;
  }
  pdu->session = session;
#if COAP_CLIENT_SUPPORT
  if (session->type == COAP_SESSION_TYPE_CLIENT &&
      !coap_netif_available(session)) {
    coap_log_debug("coap_send: Socket closed\n");
    goto error;
  }
  /*
   * If this is not the first client request and are waiting for a response
   * to the first client request, then drop sending out this next request
   * until all is properly established.
   */
  if (!coap_client_delay_first(session)) {
    goto error;
  }

  /* Indicate support for Extended Tokens if appropriate */
  if (session->max_token_checked == COAP_EXT_T_NOT_CHECKED &&
      session->max_token_size > COAP_TOKEN_DEFAULT_MAX &&
      session->type == COAP_SESSION_TYPE_CLIENT &&
      COAP_PDU_IS_REQUEST(pdu)) {
    if (COAP_PROTO_NOT_RELIABLE(session->proto)) {
      /*
       * When the pass / fail response for Extended Token is received, this PDU
       * will get transmitted.
       */
      if (coap_send_test_extended_token(session) == COAP_INVALID_MID) {
        goto error;
      }
    }
    /*
     * For reliable protocols, this will get cleared after CSM exchanged
     * in coap_session_connected()
     */
    session->doing_first = 1;
    if (!coap_client_delay_first(session)) {
      goto error;
    }
  }

  /*
   * Check validity of token length
   */
  if (COAP_PDU_IS_REQUEST(pdu) &&
      pdu->actual_token.length > session->max_token_size) {
    coap_log_warn("coap_send: PDU dropped as token too long (%zu > %" PRIu32 ")\n",
                  pdu->actual_token.length, session->max_token_size);
    goto error;
  }

  /* A lot of the reliable code assumes type is CON */
  if (COAP_PROTO_RELIABLE(session->proto) && pdu->type != COAP_MESSAGE_CON)
    pdu->type = COAP_MESSAGE_CON;

#if COAP_OSCORE_SUPPORT
  if (session->oscore_encryption) {
    if (session->recipient_ctx->initial_state == 1) {
      /*
       * Not sure if remote supports OSCORE, or is going to send us a
       * "4.01 + ECHO" etc. so need to hold off future coap_send()s until all
       * is OK. Continue sending current pdu to test things.
       */
      session->doing_first = 1;
    }
    /* Need to convert Proxy-Uri to Proxy-Scheme option if needed */
    if (COAP_PDU_IS_REQUEST(pdu) && !coap_rebuild_pdu_for_proxy(pdu)) {
      goto error;
    }
  }
#endif /* COAP_OSCORE_SUPPORT */

  if (!(session->block_mode & COAP_BLOCK_USE_LIBCOAP)) {
    return coap_send_internal(session, pdu);
  }

  if (COAP_PDU_IS_REQUEST(pdu)) {
    uint8_t buf[4];

    opt = coap_check_option(pdu, COAP_OPTION_OBSERVE, &opt_iter);

    if (opt) {
      observe_action = coap_decode_var_bytes(coap_opt_value(opt),
                                             coap_opt_length(opt));
    }

    if (coap_get_block_b(session, pdu, COAP_OPTION_BLOCK1, &block) &&
        (block.m == 1 || block.bert == 1)) {
      have_block1 = 1;
    }
#if COAP_Q_BLOCK_SUPPORT
    if (coap_get_block_b(session, pdu, COAP_OPTION_Q_BLOCK1, &block) &&
        (block.m == 1 || block.bert == 1)) {
      if (have_block1) {
        coap_log_warn("Block1 and Q-Block1 cannot be in the same request\n");
        coap_remove_option(pdu, COAP_OPTION_BLOCK1);
      }
      have_block1 = 1;
    }
#endif /* COAP_Q_BLOCK_SUPPORT */
    if (observe_action != COAP_OBSERVE_CANCEL) {
      /* Warn about re-use of tokens */
      if (session->last_token &&
          coap_binary_equal(&pdu->actual_token, session->last_token)) {
        coap_log_debug("Token reused - see https://rfc-editor.org/rfc/rfc9175.html#section-4.2\n");
      }
      coap_delete_bin_const(session->last_token);
      session->last_token = coap_new_bin_const(pdu->actual_token.s,
                                               pdu->actual_token.length);
    } else {
      /* observe_action == COAP_OBSERVE_CANCEL */
      coap_binary_t tmp;
      int ret;

      coap_log_debug("coap_send: Using coap_cancel_observe() to do OBSERVE cancellation\n");
      /* Unfortunately need to change the ptr type to be r/w */
      memcpy(&tmp.s, &pdu->actual_token.s, sizeof(tmp.s));
      tmp.length = pdu->actual_token.length;
      ret = coap_cancel_observe_lkd(session, &tmp, pdu->type);
      if (ret == 1) {
        /* Observe Cancel successfully sent */
        coap_delete_pdu(pdu);
        return ret;
      }
      /* Some mismatch somewhere - continue to send original packet */
    }
    if (!coap_check_option(pdu, COAP_OPTION_RTAG, &opt_iter) &&
        (session->block_mode & COAP_BLOCK_NO_PREEMPTIVE_RTAG) == 0 &&
        pdu->code != COAP_REQUEST_CODE_DELETE)
      coap_insert_option(pdu,
                         COAP_OPTION_RTAG,
                         coap_encode_var_safe(buf, sizeof(buf),
                                              ++session->tx_rtag),
                         buf);
  } else {
    memset(&block, 0, sizeof(block));
  }

#if COAP_Q_BLOCK_SUPPORT
  /* Indicate support for Q-Block if appropriate */
  if (session->block_mode & COAP_BLOCK_TRY_Q_BLOCK &&
      session->type == COAP_SESSION_TYPE_CLIENT &&
      COAP_PDU_IS_REQUEST(pdu)) {
    if (coap_block_test_q_block(session, pdu) == COAP_INVALID_MID) {
      goto error;
    }
    session->doing_first = 1;
    if (!coap_client_delay_first(session)) {
      /* Q-Block test Session has failed for some reason */
      set_block_mode_drop_q(session->block_mode);
      goto error;
    }
  }
#endif /* COAP_Q_BLOCK_SUPPORT */

#if COAP_Q_BLOCK_SUPPORT
  if (!(session->block_mode & COAP_BLOCK_HAS_Q_BLOCK))
#endif /* COAP_Q_BLOCK_SUPPORT */
  {
    /* Need to check if we need to reset Q-Block to Block */
    uint8_t buf[4];

    if (coap_get_block_b(session, pdu, COAP_OPTION_Q_BLOCK2, &block)) {
      coap_remove_option(pdu, COAP_OPTION_Q_BLOCK2);
      coap_insert_option(pdu, COAP_OPTION_BLOCK2,
                         coap_encode_var_safe(buf, sizeof(buf),
                                              (block.num << 4) | (0 << 3) | block.szx),
                         buf);
      coap_log_debug("Replaced option Q-Block2 with Block2\n");
      /* Need to update associated lg_xmit */
      coap_lg_xmit_t *lg_xmit;

      LL_FOREACH(session->lg_xmit, lg_xmit) {
        if (COAP_PDU_IS_REQUEST(&lg_xmit->pdu) &&
            lg_xmit->b.b1.app_token &&
            coap_binary_equal(&pdu->actual_token, lg_xmit->b.b1.app_token)) {
          /* Update the skeletal PDU with the block1 option */
          coap_remove_option(&lg_xmit->pdu, COAP_OPTION_Q_BLOCK2);
          coap_update_option(&lg_xmit->pdu, COAP_OPTION_BLOCK2,
                             coap_encode_var_safe(buf, sizeof(buf),
                                                  (block.num << 4) | (0 << 3) | block.szx),
                             buf);
          break;
        }
      }
    }
    if (coap_get_block_b(session, pdu, COAP_OPTION_Q_BLOCK1, &block)) {
      coap_remove_option(pdu, COAP_OPTION_Q_BLOCK1);
      coap_insert_option(pdu, COAP_OPTION_BLOCK1,
                         coap_encode_var_safe(buf, sizeof(buf),
                                              (block.num << 4) | (block.m << 3) | block.szx),
                         buf);
      coap_log_debug("Replaced option Q-Block1 with Block1\n");
      /* Need to update associated lg_xmit */
      coap_lg_xmit_t *lg_xmit;

      LL_FOREACH(session->lg_xmit, lg_xmit) {
        if (COAP_PDU_IS_REQUEST(&lg_xmit->pdu) &&
            lg_xmit->b.b1.app_token &&
            coap_binary_equal(&pdu->actual_token, lg_xmit->b.b1.app_token)) {
          /* Update the skeletal PDU with the block1 option */
          coap_remove_option(&lg_xmit->pdu, COAP_OPTION_Q_BLOCK1);
          coap_update_option(&lg_xmit->pdu, COAP_OPTION_BLOCK1,
                             coap_encode_var_safe(buf, sizeof(buf),
                                                  (block.num << 4) |
                                                  (block.m << 3) |
                                                  block.szx),
                             buf);
          /* Update as this is a Request */
          lg_xmit->option = COAP_OPTION_BLOCK1;
          break;
        }
      }
    }
  }

#if COAP_Q_BLOCK_SUPPORT
  if (COAP_PDU_IS_REQUEST(pdu) &&
      coap_get_block_b(session, pdu, COAP_OPTION_Q_BLOCK2, &block)) {
    if (block.num == 0 && block.m == 0) {
      uint8_t buf[4];

      /* M needs to be set as asking for all the blocks */
      coap_update_option(pdu, COAP_OPTION_Q_BLOCK2,
                         coap_encode_var_safe(buf, sizeof(buf),
                                              (0 << 4) | (1 << 3) | block.szx),
                         buf);
    }
  }
#endif /* COAP_Q_BLOCK_SUPPORT */

  /*
   * If type is CON and protocol is not reliable, there is no need to set up
   * lg_crcv here as it can be built up based on sent PDU if there is a
   * (Q-)Block2 in the response.  However, still need it for Observe, Oscore and
   * (Q-)Block1.
   */
  if (coap_check_send_need_lg_crcv(session, pdu)) {
    coap_lg_xmit_t *lg_xmit = NULL;

    if (!session->lg_xmit && have_block1) {
      coap_log_debug("PDU presented by app\n");
      coap_show_pdu(COAP_LOG_DEBUG, pdu);
    }
    /* See if this token is already in use for large body responses */
    LL_FOREACH(session->lg_crcv, lg_crcv) {
      if (coap_binary_equal(&pdu->actual_token, lg_crcv->app_token)) {
        /* Need to terminate and clean up previous response setup */
        LL_DELETE(session->lg_crcv, lg_crcv);
        coap_block_delete_lg_crcv(session, lg_crcv);
        break;
      }
    }

    if (have_block1 && session->lg_xmit) {
      LL_FOREACH(session->lg_xmit, lg_xmit) {
        if (COAP_PDU_IS_REQUEST(&lg_xmit->pdu) &&
            lg_xmit->b.b1.app_token &&
            coap_binary_equal(&pdu->actual_token, lg_xmit->b.b1.app_token)) {
          break;
        }
      }
    }
    lg_crcv = coap_block_new_lg_crcv(session, pdu, lg_xmit);
    if (lg_crcv == NULL) {
      goto error;
    }
    if (lg_xmit) {
      /* Need to update the token as set up in the session->lg_xmit */
      lg_xmit->b.b1.state_token = lg_crcv->state_token;
    }
  }
  if (session->sock.flags & COAP_SOCKET_MULTICAST)
    coap_address_copy(&session->addr_info.remote, &session->sock.mcast_addr);

#if COAP_Q_BLOCK_SUPPORT
  /* See if large xmit using Q-Block1 (but not testing Q-Block1) */
  if (coap_get_block_b(session, pdu, COAP_OPTION_Q_BLOCK1, &block)) {
    mid = coap_send_q_block1(session, block, pdu, COAP_SEND_INC_PDU);
  } else
#endif /* COAP_Q_BLOCK_SUPPORT */
    mid = coap_send_internal(session, pdu);
#else /* !COAP_CLIENT_SUPPORT */
  mid = coap_send_internal(session, pdu);
#endif /* !COAP_CLIENT_SUPPORT */
#if COAP_CLIENT_SUPPORT
  if (lg_crcv) {
    if (mid != COAP_INVALID_MID) {
      LL_PREPEND(session->lg_crcv, lg_crcv);
    } else {
      coap_block_delete_lg_crcv(session, lg_crcv);
    }
  }
#endif /* COAP_CLIENT_SUPPORT */
  return mid;

error:
  coap_delete_pdu(pdu);
  return COAP_INVALID_MID;
}

coap_mid_t
coap_send_internal(coap_session_t *session, coap_pdu_t *pdu) {
  uint8_t r;
  ssize_t bytes_written;
  coap_opt_iterator_t opt_iter;

  pdu->session = session;
  if (pdu->code == COAP_RESPONSE_CODE(508)) {
    /*
     * Need to prepend our IP identifier to the data as per
     * https://rfc-editor.org/rfc/rfc8768.html#section-4
     */
    char addr_str[INET6_ADDRSTRLEN + 8 + 1];
    coap_opt_t *opt;
    size_t hop_limit;

    addr_str[sizeof(addr_str)-1] = '\000';
    if (coap_print_addr(&session->addr_info.local, (uint8_t *)addr_str,
                        sizeof(addr_str) - 1)) {
      char *cp;
      size_t len;

      if (addr_str[0] == '[') {
        cp = strchr(addr_str, ']');
        if (cp)
          *cp = '\000';
        if (memcmp(&addr_str[1], "::ffff:", 7) == 0) {
          /* IPv4 embedded into IPv6 */
          cp = &addr_str[8];
        } else {
          cp = &addr_str[1];
        }
      } else {
        cp = strchr(addr_str, ':');
        if (cp)
          *cp = '\000';
        cp = addr_str;
      }
      len = strlen(cp);

      /* See if Hop Limit option is being used in return path */
      opt = coap_check_option(pdu, COAP_OPTION_HOP_LIMIT, &opt_iter);
      if (opt) {
        uint8_t buf[4];

        hop_limit =
            coap_decode_var_bytes(coap_opt_value(opt), coap_opt_length(opt));
        if (hop_limit == 1) {
          coap_log_warn("Proxy loop detected '%s'\n",
                        (char *)pdu->data);
          coap_delete_pdu(pdu);
          return (coap_mid_t)COAP_DROPPED_RESPONSE;
        } else if (hop_limit < 1 || hop_limit > 255) {
          /* Something is bad - need to drop this pdu (TODO or delete option) */
          coap_log_warn("Proxy return has bad hop limit count '%zu'\n",
                        hop_limit);
          coap_delete_pdu(pdu);
          return (coap_mid_t)COAP_DROPPED_RESPONSE;
        }
        hop_limit--;
        coap_update_option(pdu, COAP_OPTION_HOP_LIMIT,
                           coap_encode_var_safe8(buf, sizeof(buf), hop_limit),
                           buf);
      }

      /* Need to check that we are not seeing this proxy in the return loop */
      if (pdu->data && opt == NULL) {
        char *a_match;
        size_t data_len;

        if (pdu->used_size + 1 > pdu->max_size) {
          /* No space */
          return (coap_mid_t)COAP_DROPPED_RESPONSE;
        }
        if (!coap_pdu_resize(pdu, pdu->used_size + 1)) {
          /* Internal error */
          return (coap_mid_t)COAP_DROPPED_RESPONSE;
        }
        data_len = pdu->used_size - (pdu->data - pdu->token);
        pdu->data[data_len] = '\000';
        a_match = strstr((char *)pdu->data, cp);
        if (a_match && (a_match == (char *)pdu->data || a_match[-1] == ' ') &&
            ((size_t)(a_match - (char *)pdu->data + len) == data_len ||
             a_match[len] == ' ')) {
          coap_log_warn("Proxy loop detected '%s'\n",
                        (char *)pdu->data);
          coap_delete_pdu(pdu);
          return (coap_mid_t)COAP_DROPPED_RESPONSE;
        }
      }
      if (pdu->used_size + len + 1 <= pdu->max_size) {
        size_t old_size = pdu->used_size;
        if (coap_pdu_resize(pdu, pdu->used_size + len + 1)) {
          if (pdu->data == NULL) {
            /*
             * Set Hop Limit to max for return path.  If this libcoap is in
             * a proxy loop path, it will always decrement hop limit in code
             * above and hence timeout / drop the response as appropriate
             */
            hop_limit = 255;
            coap_insert_option(pdu, COAP_OPTION_HOP_LIMIT, 1,
                               (uint8_t *)&hop_limit);
            coap_add_data(pdu, len, (uint8_t *)cp);
          } else {
            /* prepend with space separator, leaving hop limit "as is" */
            memmove(pdu->data + len + 1, pdu->data,
                    old_size - (pdu->data - pdu->token));
            memcpy(pdu->data, cp, len);
            pdu->data[len] = ' ';
            pdu->used_size += len + 1;
          }
        }
      }
    }
  }

  if (session->echo) {
    if (!coap_insert_option(pdu, COAP_OPTION_ECHO, session->echo->length,
                            session->echo->s))
      goto error;
    coap_delete_bin_const(session->echo);
    session->echo = NULL;
  }
#if COAP_OSCORE_SUPPORT
  if (session->oscore_encryption) {
    /* Need to convert Proxy-Uri to Proxy-Scheme option if needed */
    if (COAP_PDU_IS_REQUEST(pdu) && !coap_rebuild_pdu_for_proxy(pdu))
      goto error;
  }
#endif /* COAP_OSCORE_SUPPORT */

  if (!coap_pdu_encode_header(pdu, session->proto)) {
    goto error;
  }

#if !COAP_DISABLE_TCP
  if (COAP_PROTO_RELIABLE(session->proto) &&
      session->state == COAP_SESSION_STATE_ESTABLISHED) {
    if (!session->csm_block_supported) {
      /*
       * Need to check that this instance is not sending any block options as
       * the remote end via CSM has not informed us that there is support
       * https://rfc-editor.org/rfc/rfc8323#section-5.3.2
       * This includes potential BERT blocks.
       */
      if (coap_check_option(pdu, COAP_OPTION_BLOCK1, &opt_iter) != NULL) {
        coap_log_debug("Remote end did not indicate CSM support for Block1 enabled\n");
      }
      if (coap_check_option(pdu, COAP_OPTION_BLOCK2, &opt_iter) != NULL) {
        coap_log_debug("Remote end did not indicate CSM support for Block2 enabled\n");
      }
    } else if (!session->csm_bert_rem_support) {
      coap_opt_t *opt;

      opt = coap_check_option(pdu, COAP_OPTION_BLOCK1, &opt_iter);
      if (opt && COAP_OPT_BLOCK_SZX(opt) == 7) {
        coap_log_debug("Remote end did not indicate CSM support for BERT Block1\n");
      }
      opt = coap_check_option(pdu, COAP_OPTION_BLOCK2, &opt_iter);
      if (opt && COAP_OPT_BLOCK_SZX(opt) == 7) {
        coap_log_debug("Remote end did not indicate CSM support for BERT Block2\n");
      }
    }
  }
#endif /* !COAP_DISABLE_TCP */

#if COAP_OSCORE_SUPPORT
  if (session->oscore_encryption &&
      pdu->type != COAP_MESSAGE_RST &&
      !(pdu->type == COAP_MESSAGE_ACK && pdu->code == COAP_EMPTY_CODE) &&
      !(COAP_PROTO_RELIABLE(session->proto) && pdu->code == COAP_SIGNALING_CODE_PONG)) {
    /* Refactor PDU as appropriate RFC8613 */
    coap_pdu_t *osc_pdu = coap_oscore_new_pdu_encrypted_lkd(session, pdu, NULL, 0);

    if (osc_pdu == NULL) {
      coap_log_warn("OSCORE: PDU could not be encrypted\n");
      if (coap_get_log_level() < COAP_LOG_DEBUG)
        coap_show_pdu(COAP_LOG_WARN, pdu);
      goto error;
    }
    bytes_written = coap_send_pdu(session, osc_pdu, NULL);
    coap_delete_pdu(pdu);
    pdu = osc_pdu;
  } else
#endif /* COAP_OSCORE_SUPPORT */
    bytes_written = coap_send_pdu(session, pdu, NULL);

  if (bytes_written == COAP_PDU_DELAYED) {
    /* do not free pdu as it is stored with session for later use */
    return pdu->mid;
  }
  if (bytes_written < 0) {
    goto error;
  }

#if !COAP_DISABLE_TCP
  if (COAP_PROTO_RELIABLE(session->proto) &&
      (size_t)bytes_written < pdu->used_size + pdu->hdr_size) {
    if (coap_session_delay_pdu(session, pdu, NULL) == COAP_PDU_DELAYED) {
      session->partial_write = (size_t)bytes_written;
      /* do not free pdu as it is stored with session for later use */
      return pdu->mid;
    } else {
      goto error;
    }
  }
#endif /* !COAP_DISABLE_TCP */

  if (pdu->type != COAP_MESSAGE_CON
      || COAP_PROTO_RELIABLE(session->proto)) {
    coap_mid_t id = pdu->mid;
    coap_delete_pdu(pdu);
    return id;
  }

  coap_queue_t *node = coap_new_node();
  if (!node) {
    coap_log_debug("coap_wait_ack: insufficient memory\n");
    goto error;
  }

  node->id = pdu->mid;
  node->pdu = pdu;
  coap_prng_lkd(&r, sizeof(r));
  /* add timeout in range [ACK_TIMEOUT...ACK_TIMEOUT * ACK_RANDOM_FACTOR] */
  node->timeout = coap_calc_timeout(session, r);
  return coap_wait_ack(session->context, session, node);
error:
  coap_delete_pdu(pdu);
  return COAP_INVALID_MID;
}

static int send_recv_terminate = 0;

void
coap_send_recv_terminate(void) {
  send_recv_terminate = 1;
}

COAP_API int
coap_send_recv(coap_session_t *session, coap_pdu_t *request_pdu,
               coap_pdu_t **response_pdu, uint32_t timeout_ms) {
  int ret;

  coap_lock_lock(session->context, return 0);
  ret = coap_send_recv_lkd(session, request_pdu, response_pdu, timeout_ms);
  coap_lock_unlock(session->context);
  return ret;
}

/*
 * Return 0 or +ve Time in function in ms after successful transfer
 *              -1 Invalid timeout parameter
 *              -2 Failed to transmit PDU
 *              -3 Nack or Event handler invoked, cancelling request
 *              -4 coap_io_process returned error (fail to re-lock or select())
 *              -5 Response not received in the given time
 *              -6 Terminated by user
 *              -7 Client mode code not enabled
 */
int
coap_send_recv_lkd(coap_session_t *session, coap_pdu_t *request_pdu,
                   coap_pdu_t **response_pdu, uint32_t timeout_ms) {
#if COAP_CLIENT_SUPPORT
  coap_mid_t mid = COAP_INVALID_MID;
  uint32_t rem_timeout = timeout_ms;
  uint32_t block_mode = session->block_mode;
  int ret = 0;
  coap_tick_t now;
  coap_tick_t start;
  coap_tick_t ticks_so_far;
  uint32_t time_so_far_ms;

  coap_ticks(&start);
  assert(request_pdu);

  coap_lock_check_locked(session->context);

  session->resp_pdu = NULL;
  session->req_token = coap_new_bin_const(request_pdu->actual_token.s,
                                          request_pdu->actual_token.length);

  if (timeout_ms == COAP_IO_NO_WAIT || timeout_ms == COAP_IO_WAIT) {
    ret = -1;
    goto fail;
  }
  if (session->state == COAP_SESSION_STATE_NONE) {
    ret = -3;
    goto fail;
  }

  session->block_mode |= COAP_BLOCK_SINGLE_BODY;
  session->doing_send_recv = 1;
  /* So the user needs to delete the PDU */
  request_pdu->ref++;
  mid = coap_send_lkd(session, request_pdu);
  if (mid == COAP_INVALID_MID) {
    if (!session->doing_send_recv)
      ret = -3;
    else
      ret = -2;
    goto fail;
  }

  /* Wait for the response to come in */
  while (rem_timeout > 0 && session->doing_send_recv && !session->resp_pdu) {
    if (send_recv_terminate) {
      ret = -6;
      goto fail;
    }
    ret = coap_io_process_lkd(session->context, rem_timeout);
    if (ret < 0) {
      ret = -4;
      goto fail;
    }
    /* timeout_ms is for timeout between specific request and response */
    coap_ticks(&now);
    ticks_so_far = now - session->last_rx_tx;
    time_so_far_ms = (uint32_t)((ticks_so_far * 1000) / COAP_TICKS_PER_SECOND);
    if (time_so_far_ms >= timeout_ms) {
      rem_timeout = 0;
    } else {
      rem_timeout = timeout_ms - time_so_far_ms;
    }
    if (session->state != COAP_SESSION_STATE_ESTABLISHED) {
      /* To pick up on (D)TLS setup issues */
      coap_ticks(&now);
      ticks_so_far = now - start;
      time_so_far_ms = (uint32_t)((ticks_so_far * 1000) / COAP_TICKS_PER_SECOND);
      if (time_so_far_ms >= timeout_ms) {
        rem_timeout = 0;
      } else {
        rem_timeout = timeout_ms - time_so_far_ms;
      }
    }
  }

  if (rem_timeout) {
    coap_ticks(&now);
    ticks_so_far = now - start;
    time_so_far_ms = (uint32_t)((ticks_so_far * 1000) / COAP_TICKS_PER_SECOND);
    ret = time_so_far_ms;
    /* Give PDU to user */
    *response_pdu = session->resp_pdu;
    session->resp_pdu = NULL;
    if (*response_pdu == NULL) {
      ret = -3;
    }
  } else {
    /* If there is a resp_pdu, it will get cleared below */
    ret = -5;
  }

fail:
  session->block_mode = block_mode;
  session->doing_send_recv = 0;
  if (session->resp_pdu && session->resp_pdu->ref)
    session->resp_pdu->ref--;
  coap_delete_pdu(session->resp_pdu);
  session->resp_pdu = NULL;
  coap_delete_bin_const(session->req_token);
  session->req_token = NULL;
  return ret;

#else /* !COAP_CLIENT_SUPPORT */

  (void)session;
  (void)timeout_ms;
  (void)request_pdu;
  coap_log_warn("coap_send_recv: Client mode not supported\n");
  *response_pdu =  NULL;
  return -7;

#endif /* ! COAP_CLIENT_SUPPORT */
}

coap_mid_t
coap_retransmit(coap_context_t *context, coap_queue_t *node) {
  if (!context || !node)
    return COAP_INVALID_MID;

  /* re-initialize timeout when maximum number of retransmissions are not reached yet */
  if (node->retransmit_cnt < node->session->max_retransmit) {
    ssize_t bytes_written;
    coap_tick_t now;
    coap_tick_t next_delay;

    node->retransmit_cnt++;
    coap_handle_event_lkd(context, COAP_EVENT_MSG_RETRANSMITTED, node->session);

    next_delay = (coap_tick_t)node->timeout << node->retransmit_cnt;
    if (context->ping_timeout &&
        context->ping_timeout * COAP_TICKS_PER_SECOND < next_delay) {
      uint8_t byte;

      coap_prng_lkd(&byte, sizeof(byte));
      /* Don't exceed the ping timeout value */
      next_delay = context->ping_timeout * COAP_TICKS_PER_SECOND - 255 + byte;
    }

    coap_ticks(&now);
    if (context->sendqueue == NULL) {
      node->t = next_delay;
      context->sendqueue_basetime = now;
    } else {
      /* make node->t relative to context->sendqueue_basetime */
      node->t = (now - context->sendqueue_basetime) + next_delay;
    }
    coap_insert_node(&context->sendqueue, node);

    if (node->is_mcast) {
      coap_log_debug("** %s: mid=0x%04x: mcast delayed transmission\n",
                     coap_session_str(node->session), node->id);
    } else {
      coap_log_debug("** %s: mid=0x%04x: retransmission #%d (next %ums)\n",
                     coap_session_str(node->session), node->id,
                     node->retransmit_cnt,
                     (unsigned)(next_delay * 1000 / COAP_TICKS_PER_SECOND));
    }

    if (node->session->con_active)
      node->session->con_active--;
    bytes_written = coap_send_pdu(node->session, node->pdu, node);

    if (node->is_mcast) {
      coap_session_connected(node->session);
      coap_delete_node_lkd(node);
      return COAP_INVALID_MID;
    }
    if (bytes_written == COAP_PDU_DELAYED) {
      /* PDU was not retransmitted immediately because a new handshake is
         in progress. node was moved to the send queue of the session. */
      return node->id;
    }

    if (bytes_written < 0)
      return (int)bytes_written;

    return node->id;
  }

  /* no more retransmissions, remove node from system */
  coap_log_warn("** %s: mid=0x%04x: give up after %d attempts\n",
                coap_session_str(node->session), node->id, node->retransmit_cnt);

#if COAP_SERVER_SUPPORT
  /* Check if subscriptions exist that should be canceled after
     COAP_OBS_MAX_FAIL */
  if (COAP_RESPONSE_CLASS(node->pdu->code) >= 2) {
    coap_handle_failed_notify(context, node->session, &node->pdu->actual_token);
  }
#endif /* COAP_SERVER_SUPPORT */
  if (node->session->con_active) {
    node->session->con_active--;
    if (node->session->state == COAP_SESSION_STATE_ESTABLISHED) {
      /*
       * As there may be another CON in a different queue entry on the same
       * session that needs to be immediately released,
       * coap_session_connected() is called.
       * However, there is the possibility coap_wait_ack() may be called for
       * this node (queue) and re-added to context->sendqueue.
       * coap_delete_node_lkd(node) called shortly will handle this and
       * remove it.
       */
      coap_session_connected(node->session);
    }
  }

  /* And finally delete the node */
  if (node->pdu->type == COAP_MESSAGE_CON) {
    coap_handle_nack(node->session, node->pdu, COAP_NACK_TOO_MANY_RETRIES, node->id);
  }
#if COAP_CLIENT_SUPPORT
  node->session->doing_send_recv = 0;
#endif /* COAP_CLIENT_SUPPORT */
  coap_delete_node_lkd(node);
  return COAP_INVALID_MID;
}

static int
coap_handle_dgram_for_proto(coap_context_t *ctx, coap_session_t *session, coap_packet_t *packet) {
  uint8_t *data;
  size_t data_len;
  int result = -1;

  coap_packet_get_memmapped(packet, &data, &data_len);
  if (session->proto == COAP_PROTO_DTLS) {
#if COAP_SERVER_SUPPORT
    if (session->type == COAP_SESSION_TYPE_HELLO)
      result = coap_dtls_hello(session, data, data_len);
    else
#endif /* COAP_SERVER_SUPPORT */
      if (session->tls)
        result = coap_dtls_receive(session, data, data_len);
  } else if (session->proto == COAP_PROTO_UDP) {
    result = coap_handle_dgram(ctx, session, data, data_len);
  }
  return result;
}

#if COAP_CLIENT_SUPPORT
void
coap_connect_session(coap_session_t *session, coap_tick_t now) {
#if COAP_DISABLE_TCP
  (void)now;

  session->sock.flags &= ~(COAP_SOCKET_WANT_CONNECT | COAP_SOCKET_CAN_CONNECT);
#else /* !COAP_DISABLE_TCP */
  if (coap_netif_strm_connect2(session)) {
    session->last_rx_tx = now;
    coap_handle_event_lkd(session->context, COAP_EVENT_TCP_CONNECTED, session);
    session->sock.lfunc[COAP_LAYER_SESSION].l_establish(session);
  } else {
    coap_handle_event_lkd(session->context, COAP_EVENT_TCP_FAILED, session);
    coap_session_disconnected_lkd(session, COAP_NACK_NOT_DELIVERABLE);
  }
#endif /* !COAP_DISABLE_TCP */
}
#endif /* COAP_CLIENT_SUPPORT */

static void
coap_write_session(coap_context_t *ctx, coap_session_t *session, coap_tick_t now) {
  (void)ctx;
  assert(session->sock.flags & COAP_SOCKET_CONNECTED);

  while (session->delayqueue) {
    ssize_t bytes_written;
    coap_queue_t *q = session->delayqueue;
    coap_log_debug("** %s: mid=0x%04x: transmitted after delay\n",
                   coap_session_str(session), (int)q->pdu->mid);
    assert(session->partial_write < q->pdu->used_size + q->pdu->hdr_size);
    bytes_written = session->sock.lfunc[COAP_LAYER_SESSION].l_write(session,
                    q->pdu->token - q->pdu->hdr_size + session->partial_write,
                    q->pdu->used_size + q->pdu->hdr_size - session->partial_write);
    if (bytes_written > 0)
      session->last_rx_tx = now;
    if (bytes_written <= 0 ||
        (size_t)bytes_written < q->pdu->used_size + q->pdu->hdr_size - session->partial_write) {
      if (bytes_written > 0)
        session->partial_write += (size_t)bytes_written;
      break;
    }
    session->delayqueue = q->next;
    session->partial_write = 0;
    coap_delete_node_lkd(q);
  }
}

void
coap_read_session(coap_context_t *ctx, coap_session_t *session, coap_tick_t now) {
#if COAP_CONSTRAINED_STACK
  /* payload and packet can be protected by global_lock if needed */
  static unsigned char payload[COAP_RXBUFFER_SIZE];
  static coap_packet_t s_packet;
#else /* ! COAP_CONSTRAINED_STACK */
  unsigned char payload[COAP_RXBUFFER_SIZE];
  coap_packet_t s_packet;
#endif /* ! COAP_CONSTRAINED_STACK */
  coap_packet_t *packet = &s_packet;

  assert(session->sock.flags & (COAP_SOCKET_CONNECTED | COAP_SOCKET_MULTICAST));

  packet->length = sizeof(payload);
  packet->payload = payload;

  if (COAP_PROTO_NOT_RELIABLE(session->proto)) {
    ssize_t bytes_read;
    memcpy(&packet->addr_info, &session->addr_info, sizeof(packet->addr_info));
    bytes_read = coap_netif_dgrm_read(session, packet);

    if (bytes_read < 0) {
      if (bytes_read == -2)
        /* Reset the session back to startup defaults */
        coap_session_disconnected_lkd(session, COAP_NACK_ICMP_ISSUE);
    } else if (bytes_read > 0) {
      session->last_rx_tx = now;
      /* coap_netif_dgrm_read() updates session->addr_info from packet->addr_info */
      coap_handle_dgram_for_proto(ctx, session, packet);
    }
#if !COAP_DISABLE_TCP
  } else if (session->proto == COAP_PROTO_WS ||
             session->proto == COAP_PROTO_WSS) {
    ssize_t bytes_read = 0;

    /* WebSocket layer passes us the whole packet */
    bytes_read = session->sock.lfunc[COAP_LAYER_SESSION].l_read(session,
                                                                packet->payload,
                                                                packet->length);
    if (bytes_read < 0) {
      coap_session_disconnected_lkd(session, COAP_NACK_NOT_DELIVERABLE);
    } else if (bytes_read > 2) {
      coap_pdu_t *pdu;

      session->last_rx_tx = now;
      /* Need max space incase PDU is updated with updated token etc. */
      pdu = coap_pdu_init(0, 0, 0, coap_session_max_pdu_rcv_size(session));
      if (!pdu) {
        return;
      }

      if (!coap_pdu_parse(session->proto, packet->payload, bytes_read, pdu)) {
        coap_handle_event_lkd(session->context, COAP_EVENT_BAD_PACKET, session);
        coap_log_warn("discard malformed PDU\n");
        coap_delete_pdu(pdu);
        return;
      }

      coap_dispatch(ctx, session, pdu);
      coap_delete_pdu(pdu);
      return;
    }
  } else {
    ssize_t bytes_read = 0;
    const uint8_t *p;
    int retry;

    do {
      bytes_read = session->sock.lfunc[COAP_LAYER_SESSION].l_read(session,
                                                                  packet->payload,
                                                                  packet->length);
      if (bytes_read > 0) {
        session->last_rx_tx = now;
      }
      p = packet->payload;
      retry = bytes_read == (ssize_t)packet->length;
      while (bytes_read > 0) {
        if (session->partial_pdu) {
          size_t len = session->partial_pdu->used_size
                       + session->partial_pdu->hdr_size
                       - session->partial_read;
          size_t n = min(len, (size_t)bytes_read);
          memcpy(session->partial_pdu->token - session->partial_pdu->hdr_size
                 + session->partial_read, p, n);
          p += n;
          bytes_read -= n;
          if (n == len) {
            if (coap_pdu_parse_header(session->partial_pdu, session->proto)
                && coap_pdu_parse_opt(session->partial_pdu)) {
              coap_dispatch(ctx, session, session->partial_pdu);
            }
            coap_delete_pdu(session->partial_pdu);
            session->partial_pdu = NULL;
            session->partial_read = 0;
          } else {
            session->partial_read += n;
          }
        } else if (session->partial_read > 0) {
          size_t hdr_size = coap_pdu_parse_header_size(session->proto,
                                                       session->read_header);
          size_t tkl = session->read_header[0] & 0x0f;
          size_t tok_ext_bytes = tkl == COAP_TOKEN_EXT_1B_TKL ? 1 :
                                 tkl == COAP_TOKEN_EXT_2B_TKL ? 2 : 0;
          size_t len = hdr_size + tok_ext_bytes - session->partial_read;
          size_t n = min(len, (size_t)bytes_read);
          memcpy(session->read_header + session->partial_read, p, n);
          p += n;
          bytes_read -= n;
          if (n == len) {
            /* Header now all in */
            size_t size = coap_pdu_parse_size(session->proto, session->read_header,
                                              hdr_size + tok_ext_bytes);
            if (size > COAP_DEFAULT_MAX_PDU_RX_SIZE) {
              coap_log_warn("** %s: incoming PDU length too large (%zu > %lu)\n",
                            coap_session_str(session),
                            size, COAP_DEFAULT_MAX_PDU_RX_SIZE);
              bytes_read = -1;
              break;
            }
            /* Need max space incase PDU is updated with updated token etc. */
            session->partial_pdu = coap_pdu_init(0, 0, 0,
                                                 coap_session_max_pdu_rcv_size(session));
            if (session->partial_pdu == NULL) {
              bytes_read = -1;
              break;
            }
            if (session->partial_pdu->alloc_size < size && !coap_pdu_resize(session->partial_pdu, size)) {
              bytes_read = -1;
              break;
            }
            session->partial_pdu->hdr_size = (uint8_t)hdr_size;
            session->partial_pdu->used_size = size;
            memcpy(session->partial_pdu->token - hdr_size, session->read_header, hdr_size + tok_ext_bytes);
            session->partial_read = hdr_size + tok_ext_bytes;
            if (size == 0) {
              if (coap_pdu_parse_header(session->partial_pdu, session->proto)) {
                coap_dispatch(ctx, session, session->partial_pdu);
              }
              coap_delete_pdu(session->partial_pdu);
              session->partial_pdu = NULL;
              session->partial_read = 0;
            }
          } else {
            /* More of the header to go */
            session->partial_read += n;
          }
        } else {
          /* Get in first byte of the header */
          session->read_header[0] = *p++;
          bytes_read -= 1;
          if (!coap_pdu_parse_header_size(session->proto,
                                          session->read_header)) {
            bytes_read = -1;
            break;
          }
          session->partial_read = 1;
        }
      }
    } while (bytes_read == 0 && retry);
    if (bytes_read < 0)
      coap_session_disconnected_lkd(session, COAP_NACK_NOT_DELIVERABLE);
#endif /* !COAP_DISABLE_TCP */
  }
}

#if COAP_SERVER_SUPPORT
static int
coap_read_endpoint(coap_context_t *ctx, coap_endpoint_t *endpoint, coap_tick_t now) {
  ssize_t bytes_read = -1;
  int result = -1;                /* the value to be returned */
#if COAP_CONSTRAINED_STACK
  /* payload and e_packet can be protected by global_lock if needed */
  static unsigned char payload[COAP_RXBUFFER_SIZE];
  static coap_packet_t e_packet;
#else /* ! COAP_CONSTRAINED_STACK */
  unsigned char payload[COAP_RXBUFFER_SIZE];
  coap_packet_t e_packet;
#endif /* ! COAP_CONSTRAINED_STACK */
  coap_packet_t *packet = &e_packet;

  assert(COAP_PROTO_NOT_RELIABLE(endpoint->proto));
  assert(endpoint->sock.flags & COAP_SOCKET_BOUND);

  /* Need to do this as there may be holes in addr_info */
  memset(&packet->addr_info, 0, sizeof(packet->addr_info));
  packet->length = sizeof(payload);
  packet->payload = payload;
  coap_address_init(&packet->addr_info.remote);
  coap_address_copy(&packet->addr_info.local, &endpoint->bind_addr);

  bytes_read = coap_netif_dgrm_read_ep(endpoint, packet);
  if (bytes_read < 0) {
    if (errno != EAGAIN) {
      coap_log_warn("*  %s: read failed\n", coap_endpoint_str(endpoint));
    }
  } else if (bytes_read > 0) {
    coap_session_t *session = coap_endpoint_get_session(endpoint, packet, now);
    if (session) {
      coap_log_debug("*  %s: netif: recv %4zd bytes\n",
                     coap_session_str(session), bytes_read);
      result = coap_handle_dgram_for_proto(ctx, session, packet);
      if (endpoint->proto == COAP_PROTO_DTLS && session->type == COAP_SESSION_TYPE_HELLO && result == 1)
        coap_session_new_dtls_session(session, now);
    }
  }
  return result;
}

static int
coap_write_endpoint(coap_context_t *ctx, coap_endpoint_t *endpoint, coap_tick_t now) {
  (void)ctx;
  (void)endpoint;
  (void)now;
  return 0;
}

#if !COAP_DISABLE_TCP
static int
coap_accept_endpoint(coap_context_t *ctx, coap_endpoint_t *endpoint,
                     coap_tick_t now, void *extra) {
  coap_session_t *session = coap_new_server_session(ctx, endpoint, extra);
  if (session)
    session->last_rx_tx = now;
  return session != NULL;
}
#endif /* !COAP_DISABLE_TCP */
#endif /* COAP_SERVER_SUPPORT */

COAP_API void
coap_io_do_io(coap_context_t *ctx, coap_tick_t now) {
  coap_lock_lock(ctx, return);
  coap_io_do_io_lkd(ctx, now);
  coap_lock_unlock(ctx);
}

void
coap_io_do_io_lkd(coap_context_t *ctx, coap_tick_t now) {
#ifdef COAP_EPOLL_SUPPORT
  (void)ctx;
  (void)now;
  coap_log_emerg("coap_io_do_io() requires libcoap not compiled for using epoll\n");
#else /* ! COAP_EPOLL_SUPPORT */
  coap_session_t *s, *rtmp;

  coap_lock_check_locked(ctx);
#if COAP_SERVER_SUPPORT
  coap_endpoint_t *ep, *tmp;
  LL_FOREACH_SAFE(ctx->endpoint, ep, tmp) {
    if ((ep->sock.flags & COAP_SOCKET_CAN_READ) != 0)
      coap_read_endpoint(ctx, ep, now);
    if ((ep->sock.flags & COAP_SOCKET_CAN_WRITE) != 0)
      coap_write_endpoint(ctx, ep, now);
#if !COAP_DISABLE_TCP
    if ((ep->sock.flags & COAP_SOCKET_CAN_ACCEPT) != 0)
      coap_accept_endpoint(ctx, ep, now, NULL);
#endif /* !COAP_DISABLE_TCP */
    SESSIONS_ITER_SAFE(ep->sessions, s, rtmp) {
      /* Make sure the session object is not deleted in one of the callbacks  */
      coap_session_reference_lkd(s);
      if ((s->sock.flags & COAP_SOCKET_CAN_READ) != 0) {
        coap_read_session(ctx, s, now);
      }
      if ((s->sock.flags & COAP_SOCKET_CAN_WRITE) != 0) {
        coap_write_session(ctx, s, now);
      }
      coap_session_release_lkd(s);
    }
  }
#endif /* COAP_SERVER_SUPPORT */

#if COAP_CLIENT_SUPPORT
  SESSIONS_ITER_SAFE(ctx->sessions, s, rtmp) {
    /* Make sure the session object is not deleted in one of the callbacks  */
    coap_session_reference_lkd(s);
    if ((s->sock.flags & COAP_SOCKET_CAN_CONNECT) != 0) {
      coap_connect_session(s, now);
    }
    if ((s->sock.flags & COAP_SOCKET_CAN_READ) != 0 && s->ref > 1) {
      coap_read_session(ctx, s, now);
    }
    if ((s->sock.flags & COAP_SOCKET_CAN_WRITE) != 0 && s->ref > 1) {
      coap_write_session(ctx, s, now);
    }
    coap_session_release_lkd(s);
  }
#endif /* COAP_CLIENT_SUPPORT */
#endif /* ! COAP_EPOLL_SUPPORT */
}

COAP_API void
coap_io_do_epoll(coap_context_t *ctx, struct epoll_event *events, size_t nevents) {
  coap_lock_lock(ctx, return);
  coap_io_do_epoll_lkd(ctx, events, nevents);
  coap_lock_unlock(ctx);
}

/*
 * While this code in part replicates coap_io_do_io_lkd(), doing the functions
 * directly saves having to iterate through the endpoints / sessions.
 */
void
coap_io_do_epoll_lkd(coap_context_t *ctx, struct epoll_event *events, size_t nevents) {
#ifndef COAP_EPOLL_SUPPORT
  (void)ctx;
  (void)events;
  (void)nevents;
  coap_log_emerg("coap_io_do_epoll() requires libcoap compiled for using epoll\n");
#else /* COAP_EPOLL_SUPPORT */
  coap_tick_t now;
  size_t j;

  coap_lock_check_locked(ctx);
  coap_ticks(&now);
  for (j = 0; j < nevents; j++) {
    coap_socket_t *sock = (coap_socket_t *)events[j].data.ptr;

    /* Ignore 'timer trigger' ptr  which is NULL */
    if (sock) {
#if COAP_SERVER_SUPPORT
      if (sock->endpoint) {
        coap_endpoint_t *endpoint = sock->endpoint;
        if ((sock->flags & COAP_SOCKET_WANT_READ) &&
            (events[j].events & EPOLLIN)) {
          sock->flags |= COAP_SOCKET_CAN_READ;
          coap_read_endpoint(endpoint->context, endpoint, now);
        }

        if ((sock->flags & COAP_SOCKET_WANT_WRITE) &&
            (events[j].events & EPOLLOUT)) {
          /*
           * Need to update this to EPOLLIN as EPOLLOUT will normally always
           * be true causing epoll_wait to return early
           */
          coap_epoll_ctl_mod(sock, EPOLLIN, __func__);
          sock->flags |= COAP_SOCKET_CAN_WRITE;
          coap_write_endpoint(endpoint->context, endpoint, now);
        }

#if !COAP_DISABLE_TCP
        if ((sock->flags & COAP_SOCKET_WANT_ACCEPT) &&
            (events[j].events & EPOLLIN)) {
          sock->flags |= COAP_SOCKET_CAN_ACCEPT;
          coap_accept_endpoint(endpoint->context, endpoint, now, NULL);
        }
#endif /* !COAP_DISABLE_TCP */

      } else
#endif /* COAP_SERVER_SUPPORT */
        if (sock->session) {
          coap_session_t *session = sock->session;

          /* Make sure the session object is not deleted
             in one of the callbacks  */
          coap_session_reference_lkd(session);
#if COAP_CLIENT_SUPPORT
          if ((sock->flags & COAP_SOCKET_WANT_CONNECT) &&
              (events[j].events & (EPOLLOUT|EPOLLERR|EPOLLHUP|EPOLLRDHUP))) {
            sock->flags |= COAP_SOCKET_CAN_CONNECT;
            coap_connect_session(session, now);
            if (coap_netif_available(session) &&
                !(sock->flags & COAP_SOCKET_WANT_WRITE)) {
              coap_epoll_ctl_mod(sock, EPOLLIN, __func__);
            }
          }
#endif /* COAP_CLIENT_SUPPORT */

          if ((sock->flags & COAP_SOCKET_WANT_READ) &&
              (events[j].events & (EPOLLIN|EPOLLERR|EPOLLHUP|EPOLLRDHUP))) {
            sock->flags |= COAP_SOCKET_CAN_READ;
            coap_read_session(session->context, session, now);
          }

          if ((sock->flags & COAP_SOCKET_WANT_WRITE) &&
              (events[j].events & (EPOLLOUT|EPOLLERR|EPOLLHUP|EPOLLRDHUP))) {
            /*
             * Need to update this to EPOLLIN as EPOLLOUT will normally always
             * be true causing epoll_wait to return early
             */
            coap_epoll_ctl_mod(sock, EPOLLIN, __func__);
            sock->flags |= COAP_SOCKET_CAN_WRITE;
            coap_write_session(session->context, session, now);
          }
          /* Now dereference session so it can go away if needed */
          coap_session_release_lkd(session);
        }
    } else if (ctx->eptimerfd != -1) {
      /*
       * 'timer trigger' must have fired. eptimerfd needs to be read to clear
       *  it so that it does not set EPOLLIN in the next epoll_wait().
       */
      uint64_t count;

      /* Check the result from read() to suppress the warning on
       * systems that declare read() with warn_unused_result. */
      if (read(ctx->eptimerfd, &count, sizeof(count)) == -1) {
        /* do nothing */;
      }
    }
  }
  /* And update eptimerfd as to when to next trigger */
  coap_ticks(&now);
  coap_io_prepare_epoll_lkd(ctx, now);
#endif /* COAP_EPOLL_SUPPORT */
}

int
coap_handle_dgram(coap_context_t *ctx, coap_session_t *session,
                  uint8_t *msg, size_t msg_len) {

  coap_pdu_t *pdu = NULL;

  assert(COAP_PROTO_NOT_RELIABLE(session->proto));
  if (msg_len < 4) {
    /* Minimum size of CoAP header - ignore runt */
    return -1;
  }
  if ((msg[0] >> 6) != COAP_DEFAULT_VERSION) {
    /*
     * As per https://datatracker.ietf.org/doc/html/rfc7252#section-3,
     * this MUST be silently ignored.
     */
    coap_log_debug("coap_handle_dgram: UDP version not supported\n");
    return -1;
  }

  /* Need max space incase PDU is updated with updated token etc. */
  pdu = coap_pdu_init(0, 0, 0, coap_session_max_pdu_rcv_size(session));
  if (!pdu)
    goto error;

  if (!coap_pdu_parse(session->proto, msg, msg_len, pdu)) {
    coap_handle_event_lkd(session->context, COAP_EVENT_BAD_PACKET, session);
    coap_log_warn("discard malformed PDU\n");
    goto error;
  }

  coap_dispatch(ctx, session, pdu);
  coap_delete_pdu(pdu);
  return 0;

error:
  /*
   * https://rfc-editor.org/rfc/rfc7252#section-4.2 MUST send RST
   * https://rfc-editor.org/rfc/rfc7252#section-4.3 MAY send RST
   */
  coap_send_rst_lkd(session, pdu);
  coap_delete_pdu(pdu);
  return -1;
}

int
coap_remove_from_queue(coap_queue_t **queue, coap_session_t *session, coap_mid_t id,
                       coap_queue_t **node) {
  coap_queue_t *p, *q;

  if (!queue || !*queue)
    return 0;

  /* replace queue head if PDU's time is less than head's time */

  if (session == (*queue)->session && id == (*queue)->id) { /* found message id */
    *node = *queue;
    *queue = (*queue)->next;
    if (*queue) {          /* adjust relative time of new queue head */
      (*queue)->t += (*node)->t;
    }
    (*node)->next = NULL;
    coap_log_debug("** %s: mid=0x%04x: removed (1)\n",
                   coap_session_str(session), id);
    return 1;
  }

  /* search message id queue to remove (only first occurence will be removed) */
  q = *queue;
  do {
    p = q;
    q = q->next;
  } while (q && (session != q->session || id != q->id));

  if (q) {                        /* found message id */
    p->next = q->next;
    if (p->next) {                /* must update relative time of p->next */
      p->next->t += q->t;
    }
    q->next = NULL;
    *node = q;
    coap_log_debug("** %s: mid=0x%04x: removed (2)\n",
                   coap_session_str(session), id);
    return 1;
  }

  return 0;

}

void
coap_cancel_session_messages(coap_context_t *context, coap_session_t *session,
                             coap_nack_reason_t reason) {
  coap_queue_t *p, *q;

  while (context->sendqueue && context->sendqueue->session == session) {
    q = context->sendqueue;
    context->sendqueue = q->next;
    coap_log_debug("** %s: mid=0x%04x: removed (3)\n",
                   coap_session_str(session), q->id);
    if (q->pdu->type == COAP_MESSAGE_CON) {
      coap_handle_nack(session, q->pdu, reason, q->id);
    }
    coap_delete_node_lkd(q);
  }

  if (!context->sendqueue)
    return;

  p = context->sendqueue;
  q = p->next;

  while (q) {
    if (q->session == session) {
      p->next = q->next;
      coap_log_debug("** %s: mid=0x%04x: removed (4)\n",
                     coap_session_str(session), q->id);
      if (q->pdu->type == COAP_MESSAGE_CON) {
        coap_handle_nack(session, q->pdu, reason, q->id);
      }
      coap_delete_node_lkd(q);
      q = p->next;
    } else {
      p = q;
      q = q->next;
    }
  }
}

void
coap_cancel_all_messages(coap_context_t *context, coap_session_t *session,
                         coap_bin_const_t *token) {
  /* cancel all messages in sendqueue that belong to session
   * and use the specified token */
  coap_queue_t **p, *q;

  if (!context->sendqueue)
    return;

  p = &context->sendqueue;
  q = *p;

  while (q) {
    if (q->session == session &&
        (!token || coap_binary_equal(&q->pdu->actual_token, token))) {
      *p = q->next;
      coap_log_debug("** %s: mid=0x%04x: removed (6)\n",
                     coap_session_str(session), q->id);
      if (q->pdu->type == COAP_MESSAGE_CON && session->con_active) {
        session->con_active--;
        if (session->state == COAP_SESSION_STATE_ESTABLISHED)
          /* Flush out any entries on session->delayqueue */
          coap_session_connected(session);
      }
      coap_delete_node_lkd(q);
    } else {
      p = &(q->next);
    }
    q = *p;
  }
}

coap_pdu_t *
coap_new_error_response(const coap_pdu_t *request, coap_pdu_code_t code,
                        coap_opt_filter_t *opts) {
  coap_opt_iterator_t opt_iter;
  coap_pdu_t *response;
  size_t size = request->e_token_length;
  unsigned char type;
  coap_opt_t *option;
  coap_option_num_t opt_num = 0;        /* used for calculating delta-storage */

#if COAP_ERROR_PHRASE_LENGTH > 0
  const char *phrase;
  if (code != COAP_RESPONSE_CODE(508)) {
    phrase = coap_response_phrase(code);

    /* Need some more space for the error phrase and payload start marker */
    if (phrase)
      size += strlen(phrase) + 1;
  } else {
    /*
     * Need space for IP for 5.08 response which is filled in in
     * coap_send_internal()
     * https://rfc-editor.org/rfc/rfc8768.html#section-4
     */
    phrase = NULL;
    size += INET6_ADDRSTRLEN;
  }
#endif

  assert(request);

  /* cannot send ACK if original request was not confirmable */
  type = request->type == COAP_MESSAGE_CON ?
         COAP_MESSAGE_ACK : COAP_MESSAGE_NON;

  /* Estimate how much space we need for options to copy from
   * request. We always need the Token, for 4.02 the unknown critical
   * options must be included as well. */

  /* we do not want these */
  coap_option_filter_unset(opts, COAP_OPTION_CONTENT_FORMAT);
  coap_option_filter_unset(opts, COAP_OPTION_HOP_LIMIT);
  /* Unsafe to send this back */
  coap_option_filter_unset(opts, COAP_OPTION_OSCORE);

  coap_option_iterator_init(request, &opt_iter, opts);

  /* Add size of each unknown critical option. As known critical
     options as well as elective options are not copied, the delta
     value might grow.
   */
  while ((option = coap_option_next(&opt_iter))) {
    uint16_t delta = opt_iter.number - opt_num;
    /* calculate space required to encode (opt_iter.number - opt_num) */
    if (delta < 13) {
      size++;
    } else if (delta < 269) {
      size += 2;
    } else {
      size += 3;
    }

    /* add coap_opt_length(option) and the number of additional bytes
     * required to encode the option length */

    size += coap_opt_length(option);
    switch (*option & 0x0f) {
    case 0x0e:
      size++;
    /* fall through */
    case 0x0d:
      size++;
      break;
    default:
      ;
    }

    opt_num = opt_iter.number;
  }

  /* Now create the response and fill with options and payload data. */
  response = coap_pdu_init(type, code, request->mid, size);
  if (response) {
    /* copy token */
    if (!coap_add_token(response, request->actual_token.length,
                        request->actual_token.s)) {
      coap_log_debug("cannot add token to error response\n");
      coap_delete_pdu(response);
      return NULL;
    }

    /* copy all options */
    coap_option_iterator_init(request, &opt_iter, opts);
    while ((option = coap_option_next(&opt_iter))) {
      coap_add_option_internal(response, opt_iter.number,
                               coap_opt_length(option),
                               coap_opt_value(option));
    }

#if COAP_ERROR_PHRASE_LENGTH > 0
    /* note that diagnostic messages do not need a Content-Format option. */
    if (phrase)
      coap_add_data(response, (size_t)strlen(phrase), (const uint8_t *)phrase);
#endif
  }

  return response;
}

#if COAP_SERVER_SUPPORT
#define SZX_TO_BYTES(SZX) ((size_t)(1 << ((SZX) + 4)))

static void
free_wellknown_response(coap_session_t *session COAP_UNUSED, void *app_ptr) {
  coap_delete_string(app_ptr);
}

/*
 * Caution: As this handler is in libcoap space, it is called with
 * context locked.
 */
static void
hnd_get_wellknown_lkd(coap_resource_t *resource,
                      coap_session_t *session,
                      const coap_pdu_t *request,
                      const coap_string_t *query,
                      coap_pdu_t *response) {
  size_t len = 0;
  coap_string_t *data_string = NULL;
  coap_print_status_t result = 0;
  size_t wkc_len = 0;
  uint8_t buf[4];

  /*
   * Quick hack to determine the size of the resource descriptions for
   * .well-known/core.
   */
  result = coap_print_wellknown_lkd(session->context, buf, &wkc_len, UINT_MAX, query);
  if (result & COAP_PRINT_STATUS_ERROR) {
    coap_log_warn("cannot determine length of /.well-known/core\n");
    goto error;
  }

  if (wkc_len > 0) {
    data_string = coap_new_string(wkc_len);
    if (!data_string)
      goto error;

    len = wkc_len;
    result = coap_print_wellknown_lkd(session->context, data_string->s, &len, 0, query);
    if ((result & COAP_PRINT_STATUS_ERROR) != 0) {
      coap_log_debug("coap_print_wellknown failed\n");
      goto error;
    }
    assert(len <= (size_t)wkc_len);
    data_string->length = len;

    if (!(session->block_mode & COAP_BLOCK_USE_LIBCOAP)) {
      if (!coap_insert_option(response, COAP_OPTION_CONTENT_FORMAT,
                              coap_encode_var_safe(buf, sizeof(buf),
                                                   COAP_MEDIATYPE_APPLICATION_LINK_FORMAT), buf)) {
        goto error;
      }
      if (response->used_size + len + 1 > response->max_size) {
        /*
         * Data does not fit into a packet and no libcoap block support
         * +1 for end of options marker
         */
        coap_log_debug(".well-known/core: truncating data length to %zu from %zu\n",
                       len, response->max_size  - response->used_size - 1);
        len = response->max_size - response->used_size - 1;
      }
      if (!coap_add_data(response, len, data_string->s)) {
        goto error;
      }
      free_wellknown_response(session, data_string);
    } else if (!coap_add_data_large_response_lkd(resource, session, request,
                                                 response, query,
                                                 COAP_MEDIATYPE_APPLICATION_LINK_FORMAT,
                                                 -1, 0, data_string->length,
                                                 data_string->s,
                                                 free_wellknown_response,
                                                 data_string)) {
      goto error_released;
    }
  } else {
    if (!coap_insert_option(response, COAP_OPTION_CONTENT_FORMAT,
                            coap_encode_var_safe(buf, sizeof(buf),
                                                 COAP_MEDIATYPE_APPLICATION_LINK_FORMAT), buf)) {
      goto error;
    }
  }
  response->code = COAP_RESPONSE_CODE(205);
  return;

error:
  free_wellknown_response(session, data_string);
error_released:
  if (response->code == 0) {
    /* set error code 5.03 and remove all options and data from response */
    response->code = COAP_RESPONSE_CODE(503);
    response->used_size = response->e_token_length;
    response->data = NULL;
  }
}
#endif /* COAP_SERVER_SUPPORT */

/**
 * This function cancels outstanding messages for the session and
 * token specified in @p sent. Any observation relationship for
 * sent->session and the token are removed. Calling this function is
 * required when receiving an RST message (usually in response to a
 * notification) or a GET request with the Observe option set to 1.
 *
 * This function returns @c 0 when the token is unknown with this
 * peer, or a value greater than zero otherwise.
 */
static int
coap_cancel(coap_context_t *context, const coap_queue_t *sent) {
  int num_cancelled = 0;    /* the number of observers cancelled */

#ifndef COAP_SERVER_SUPPORT
  (void)sent;
#endif /* ! COAP_SERVER_SUPPORT */
  (void)context;

#if COAP_SERVER_SUPPORT
  /* remove observer for this resource, if any
   * Use token from sent and try to find a matching resource. Uh!
   */
  RESOURCES_ITER(context->resources, r) {
    coap_cancel_all_messages(context, sent->session, &sent->pdu->actual_token);
    num_cancelled += coap_delete_observer(r, sent->session, &sent->pdu->actual_token);
  }
#endif /* COAP_SERVER_SUPPORT */

  return num_cancelled;
}

#if COAP_SERVER_SUPPORT
/**
 * Internal flags to control the treatment of responses (specifically
 * in presence of the No-Response option).
 */
enum respond_t { RESPONSE_DEFAULT, RESPONSE_DROP, RESPONSE_SEND };

/*
 * Checks for No-Response option in given @p request and
 * returns @c RESPONSE_DROP if @p response should be suppressed
 * according to RFC 7967.
 *
 * If the response is a confirmable piggybacked response and RESPONSE_DROP,
 * change it to an empty ACK and @c RESPONSE_SEND so the client does not keep
 * on retrying.
 *
 * Checks if the response code is 0.00 and if either the session is reliable or
 * non-confirmable, @c RESPONSE_DROP is also returned.
 *
 * Multicast response checking is also carried out.
 *
 * NOTE: It is the responsibility of the application to determine whether
 * a delayed separate response should be sent as the original requesting packet
 * containing the No-Response option has long since gone.
 *
 * The value of the No-Response option is encoded as
 * follows:
 *
 * @verbatim
 *  +-------+-----------------------+-----------------------------------+
 *  | Value | Binary Representation |          Description              |
 *  +-------+-----------------------+-----------------------------------+
 *  |   0   |      <empty>          | Interested in all responses.      |
 *  +-------+-----------------------+-----------------------------------+
 *  |   2   |      00000010         | Not interested in 2.xx responses. |
 *  +-------+-----------------------+-----------------------------------+
 *  |   8   |      00001000         | Not interested in 4.xx responses. |
 *  +-------+-----------------------+-----------------------------------+
 *  |  16   |      00010000         | Not interested in 5.xx responses. |
 *  +-------+-----------------------+-----------------------------------+
 * @endverbatim
 *
 * @param request  The CoAP request to check for the No-Response option.
 *                 This parameter must not be NULL.
 * @param response The response that is potentially suppressed.
 *                 This parameter must not be NULL.
 * @param session  The session this request/response are associated with.
 *                 This parameter must not be NULL.
 * @return RESPONSE_DEFAULT when no special treatment is requested,
 *         RESPONSE_DROP    when the response must be discarded, or
 *         RESPONSE_SEND    when the response must be sent.
 */
static enum respond_t
no_response(coap_pdu_t *request, coap_pdu_t *response,
            coap_session_t *session, coap_resource_t *resource) {
  coap_opt_t *nores;
  coap_opt_iterator_t opt_iter;
  unsigned int val = 0;

  assert(request);
  assert(response);

  if (COAP_RESPONSE_CLASS(response->code) > 0) {
    nores = coap_check_option(request, COAP_OPTION_NORESPONSE, &opt_iter);

    if (nores) {
      val = coap_decode_var_bytes(coap_opt_value(nores), coap_opt_length(nores));

      /* The response should be dropped when the bit corresponding to
       * the response class is set (cf. table in function
       * documentation). When a No-Response option is present and the
       * bit is not set, the sender explicitly indicates interest in
       * this response. */
      if (((1 << (COAP_RESPONSE_CLASS(response->code) - 1)) & val) > 0) {
        /* Should be dropping the response */
        if (response->type == COAP_MESSAGE_ACK &&
            COAP_PROTO_NOT_RELIABLE(session->proto)) {
          /* Still need to ACK the request */
          response->code = 0;
          /* Remove token/data from piggybacked acknowledgment PDU */
          response->actual_token.length = 0;
          response->e_token_length = 0;
          response->used_size = 0;
          response->data = NULL;
          return RESPONSE_SEND;
        } else {
          return RESPONSE_DROP;
        }
      } else {
        /* True for mcast as well RFC7967 2.1 */
        return RESPONSE_SEND;
      }
    } else if (resource && session->context->mcast_per_resource &&
               coap_is_mcast(&session->addr_info.local)) {
      /* Handle any mcast suppression specifics if no NoResponse option */
      if ((resource->flags &
           COAP_RESOURCE_FLAGS_LIB_ENA_MCAST_SUPPRESS_2_XX) &&
          COAP_RESPONSE_CLASS(response->code) == 2) {
        return RESPONSE_DROP;
      } else if ((resource->flags &
                  COAP_RESOURCE_FLAGS_LIB_ENA_MCAST_SUPPRESS_2_05) &&
                 response->code == COAP_RESPONSE_CODE(205)) {
        if (response->data == NULL)
          return RESPONSE_DROP;
      } else if ((resource->flags &
                  COAP_RESOURCE_FLAGS_LIB_DIS_MCAST_SUPPRESS_4_XX) == 0 &&
                 COAP_RESPONSE_CLASS(response->code) == 4) {
        return RESPONSE_DROP;
      } else if ((resource->flags &
                  COAP_RESOURCE_FLAGS_LIB_DIS_MCAST_SUPPRESS_5_XX) == 0 &&
                 COAP_RESPONSE_CLASS(response->code) == 5) {
        return RESPONSE_DROP;
      }
    }
  } else if (COAP_PDU_IS_EMPTY(response) &&
             (response->type == COAP_MESSAGE_NON ||
              COAP_PROTO_RELIABLE(session->proto))) {
    /* response is 0.00, and this is reliable or non-confirmable */
    return RESPONSE_DROP;
  }

  /*
   * Do not send error responses for requests that were received via
   * IP multicast.  RFC7252 8.1
   */

  if (coap_is_mcast(&session->addr_info.local)) {
    if (request->type == COAP_MESSAGE_NON &&
        response->type == COAP_MESSAGE_RST)
      return RESPONSE_DROP;

    if ((!resource || session->context->mcast_per_resource == 0) &&
        COAP_RESPONSE_CLASS(response->code) > 2)
      return RESPONSE_DROP;
  }

  /* Default behavior applies when we are not dealing with a response
   * (class == 0) or the request did not contain a No-Response option.
   */
  return RESPONSE_DEFAULT;
}

static coap_str_const_t coap_default_uri_wellknown = {
  sizeof(COAP_DEFAULT_URI_WELLKNOWN)-1,
  (const uint8_t *)COAP_DEFAULT_URI_WELLKNOWN
};

/* Initialized in coap_startup() */
static coap_resource_t resource_uri_wellknown;

static void
handle_request(coap_context_t *context, coap_session_t *session, coap_pdu_t *pdu) {
  coap_method_handler_t h = NULL;
  coap_pdu_t *response = NULL;
  coap_opt_filter_t opt_filter;
  coap_resource_t *resource = NULL;
  /* The respond field indicates whether a response must be treated
   * specially due to a No-Response option that declares disinterest
   * or interest in a specific response class. DEFAULT indicates that
   * No-Response has not been specified. */
  enum respond_t respond = RESPONSE_DEFAULT;
  coap_opt_iterator_t opt_iter;
  coap_opt_t *opt;
  int is_proxy_uri = 0;
  int is_proxy_scheme = 0;
  int skip_hop_limit_check = 0;
  int resp = 0;
  int send_early_empty_ack = 0;
  coap_string_t *query = NULL;
  coap_opt_t *observe = NULL;
  coap_string_t *uri_path = NULL;
  int observe_action = COAP_OBSERVE_CANCEL;
  coap_block_b_t block;
  int added_block = 0;
  coap_lg_srcv_t *free_lg_srcv = NULL;
#if COAP_Q_BLOCK_SUPPORT
  int lg_xmit_ctrl = 0;
#endif /* COAP_Q_BLOCK_SUPPORT */
#if COAP_ASYNC_SUPPORT
  coap_async_t *async;
#endif /* COAP_ASYNC_SUPPORT */

  if (coap_is_mcast(&session->addr_info.local)) {
    if (COAP_PROTO_RELIABLE(session->proto) || pdu->type != COAP_MESSAGE_NON) {
      coap_log_info("Invalid multicast packet received RFC7252 8.1\n");
      return;
    }
  }
#if COAP_ASYNC_SUPPORT
  async = coap_find_async_lkd(session, pdu->actual_token);
  if (async) {
    coap_tick_t now;

    coap_ticks(&now);
    if (async->delay == 0 || async->delay > now) {
      /* re-transmit missing ACK (only if CON) */
      coap_log_info("Retransmit async response\n");
      coap_send_ack_lkd(session, pdu);
      /* and do not pass on to the upper layers */
      return;
    }
  }
#endif /* COAP_ASYNC_SUPPORT */

  coap_option_filter_clear(&opt_filter);
  opt = coap_check_option(pdu, COAP_OPTION_PROXY_SCHEME, &opt_iter);
  if (opt) {
    opt = coap_check_option(pdu, COAP_OPTION_URI_HOST, &opt_iter);
    if (!opt) {
      coap_log_debug("Proxy-Scheme requires Uri-Host\n");
      resp = 402;
      goto fail_response;
    }
    is_proxy_scheme = 1;
  }

  opt = coap_check_option(pdu, COAP_OPTION_PROXY_URI, &opt_iter);
  if (opt)
    is_proxy_uri = 1;

  if (is_proxy_scheme || is_proxy_uri) {
    coap_uri_t uri;

    if (!context->proxy_uri_resource) {
      /* Need to return a 5.05 RFC7252 Section 5.7.2 */
      coap_log_debug("Proxy-%s support not configured\n",
                     is_proxy_scheme ? "Scheme" : "Uri");
      resp = 505;
      goto fail_response;
    }
    if (((size_t)pdu->code - 1 <
         (sizeof(resource->handler) / sizeof(resource->handler[0]))) &&
        !(context->proxy_uri_resource->handler[pdu->code - 1])) {
      /* Need to return a 5.05 RFC7252 Section 5.7.2 */
      coap_log_debug("Proxy-%s code %d.%02d handler not supported\n",
                     is_proxy_scheme ? "Scheme" : "Uri",
                     pdu->code/100,  pdu->code%100);
      resp = 505;
      goto fail_response;
    }

    /* Need to check if authority is the proxy endpoint RFC7252 Section 5.7.2 */
    if (is_proxy_uri) {
      if (coap_split_proxy_uri(coap_opt_value(opt),
                               coap_opt_length(opt), &uri) < 0) {
        /* Need to return a 5.05 RFC7252 Section 5.7.2 */
        coap_log_debug("Proxy-URI not decodable\n");
        resp = 505;
        goto fail_response;
      }
    } else {
      memset(&uri, 0, sizeof(uri));
      opt = coap_check_option(pdu, COAP_OPTION_URI_HOST, &opt_iter);
      if (opt) {
        uri.host.length = coap_opt_length(opt);
        uri.host.s = coap_opt_value(opt);
      } else
        uri.host.length = 0;
    }

    resource = context->proxy_uri_resource;
    if (uri.host.length && resource->proxy_name_count &&
        resource->proxy_name_list) {
      size_t i;

      if (resource->proxy_name_count == 1 &&
          resource->proxy_name_list[0]->length == 0) {
        /* If proxy_name_list[0] is zero length, then this is the endpoint */
        i = 0;
      } else {
        for (i = 0; i < resource->proxy_name_count; i++) {
          if (coap_string_equal(&uri.host, resource->proxy_name_list[i])) {
            break;
          }
        }
      }
      if (i != resource->proxy_name_count) {
        /* This server is hosting the proxy connection endpoint */
        if (pdu->crit_opt) {
          /* Cannot handle critical option */
          pdu->crit_opt = 0;
          resp = 402;
          goto fail_response;
        }
        is_proxy_uri = 0;
        is_proxy_scheme = 0;
        skip_hop_limit_check = 1;
      }
    }
    resource = NULL;
  }

  if (!skip_hop_limit_check) {
    opt = coap_check_option(pdu, COAP_OPTION_HOP_LIMIT, &opt_iter);
    if (opt) {
      size_t hop_limit;
      uint8_t buf[4];

      hop_limit =
          coap_decode_var_bytes(coap_opt_value(opt), coap_opt_length(opt));
      if (hop_limit == 1) {
        /* coap_send_internal() will fill in the IP address for us */
        resp = 508;
        goto fail_response;
      } else if (hop_limit < 1 || hop_limit > 255) {
        /* Need to return a 4.00 RFC8768 Section 3 */
        coap_log_info("Invalid Hop Limit\n");
        resp = 400;
        goto fail_response;
      }
      hop_limit--;
      coap_update_option(pdu, COAP_OPTION_HOP_LIMIT,
                         coap_encode_var_safe8(buf, sizeof(buf), hop_limit),
                         buf);
    }
  }

  uri_path = coap_get_uri_path(pdu);
  if (!uri_path)
    return;

  if (!is_proxy_uri && !is_proxy_scheme) {
    /* try to find the resource from the request URI */
    coap_str_const_t uri_path_c = { uri_path->length, uri_path->s };
    resource = coap_get_resource_from_uri_path_lkd(context, &uri_path_c);
  }

  if ((resource == NULL) || (resource->is_unknown == 1) ||
      (resource->is_proxy_uri == 1)) {
    /* The resource was not found or there is an unexpected match against the
     * resource defined for handling unknown or proxy URIs.
     */
    if (resource != NULL)
      /* Close down unexpected match */
      resource = NULL;
    /*
     * Check if the request URI happens to be the well-known URI, or if the
     * unknown resource handler is defined, a PUT or optionally other methods,
     * if configured, for the unknown handler.
     *
     * if a PROXY URI/Scheme request and proxy URI handler defined, call the
     *  proxy URI handler.
     *
     * else if unknown URI handler defined and COAP_RESOURCE_HANDLE_WELLKNOWN_CORE
     *  set, call the unknown URI handler with any unknown URI (including
     *  .well-known/core) if the appropriate method is defined.
     *
     * else if well-known URI generate a default response.
     *
     * else if unknown URI handler defined, call the unknown
     *  URI handler (to allow for potential generation of resource
     *  [RFC7272 5.8.3]) if the appropriate method is defined.
     *
     * else if DELETE return 2.02 (RFC7252: 5.8.4.  DELETE).
     *
     * else return 4.04.
     */

    if (is_proxy_uri || is_proxy_scheme) {
      resource = context->proxy_uri_resource;
    } else if (context->unknown_resource != NULL &&
               context->unknown_resource->flags & COAP_RESOURCE_HANDLE_WELLKNOWN_CORE &&
               ((size_t)pdu->code - 1 <
                (sizeof(resource->handler) / sizeof(coap_method_handler_t))) &&
               (context->unknown_resource->handler[pdu->code - 1])) {
      resource = context->unknown_resource;
    } else if (coap_string_equal(uri_path, &coap_default_uri_wellknown)) {
      /* request for .well-known/core */
      resource = &resource_uri_wellknown;
    } else if ((context->unknown_resource != NULL) &&
               ((size_t)pdu->code - 1 <
                (sizeof(resource->handler) / sizeof(coap_method_handler_t))) &&
               (context->unknown_resource->handler[pdu->code - 1])) {
      /*
       * The unknown_resource can be used to handle undefined resources
       * for a PUT request and can support any other registered handler
       * defined for it
       * Example set up code:-
       *   r = coap_resource_unknown_init(hnd_put_unknown);
       *   coap_register_request_handler(r, COAP_REQUEST_POST,
       *                                 hnd_post_unknown);
       *   coap_register_request_handler(r, COAP_REQUEST_GET,
       *                                 hnd_get_unknown);
       *   coap_register_request_handler(r, COAP_REQUEST_DELETE,
       *                                 hnd_delete_unknown);
       *   coap_add_resource(ctx, r);
       *
       * Note: It is not possible to observe the unknown_resource, a separate
       *       resource must be created (by PUT or POST) which has a GET
       *       handler to be observed
       */
      resource = context->unknown_resource;
    } else if (pdu->code == COAP_REQUEST_CODE_DELETE) {
      /*
       * Request for DELETE on non-existant resource (RFC7252: 5.8.4.  DELETE)
       */
      coap_log_debug("request for unknown resource '%*.*s',"
                     " return 2.02\n",
                     (int)uri_path->length,
                     (int)uri_path->length,
                     uri_path->s);
      resp = 202;
      goto fail_response;
    } else { /* request for any another resource, return 4.04 */

      coap_log_debug("request for unknown resource '%*.*s', return 4.04\n",
                     (int)uri_path->length, (int)uri_path->length, uri_path->s);
      resp = 404;
      goto fail_response;
    }

  }

#if COAP_OSCORE_SUPPORT
  if ((resource->flags & COAP_RESOURCE_FLAGS_OSCORE_ONLY) && !session->oscore_encryption) {
    coap_log_debug("request for OSCORE only resource '%*.*s', return 4.04\n",
                   (int)uri_path->length, (int)uri_path->length, uri_path->s);
    resp = 401;
    goto fail_response;
  }
#endif /* COAP_OSCORE_SUPPORT */
  if (resource->is_unknown == 0 && resource->is_proxy_uri == 0) {
    /* Check for existing resource and If-Non-Match */
    opt = coap_check_option(pdu, COAP_OPTION_IF_NONE_MATCH, &opt_iter);
    if (opt) {
      resp = 412;
      goto fail_response;
    }
  }

  /* the resource was found, check if there is a registered handler */
  if ((size_t)pdu->code - 1 <
      sizeof(resource->handler) / sizeof(coap_method_handler_t))
    h = resource->handler[pdu->code - 1];

  if (h == NULL) {
    resp = 405;
    goto fail_response;
  }
  if (pdu->code == COAP_REQUEST_CODE_FETCH) {
    opt = coap_check_option(pdu, COAP_OPTION_CONTENT_FORMAT, &opt_iter);
    if (opt == NULL) {
      /* RFC 8132 2.3.1 */
      resp = 415;
      goto fail_response;
    }
  }
  if (context->mcast_per_resource &&
      (resource->flags & COAP_RESOURCE_FLAGS_HAS_MCAST_SUPPORT) == 0 &&
      coap_is_mcast(&session->addr_info.local)) {
    resp = 405;
    goto fail_response;
  }

  response = coap_pdu_init(pdu->type == COAP_MESSAGE_CON ?
                           COAP_MESSAGE_ACK : COAP_MESSAGE_NON,
                           0, pdu->mid, coap_session_max_pdu_size_lkd(session));
  if (!response) {
    coap_log_err("could not create response PDU\n");
    resp = 500;
    goto fail_response;
  }
  response->session = session;
#if COAP_ASYNC_SUPPORT
  /* If handling a separate response, need CON, not ACK response */
  if (async && pdu->type == COAP_MESSAGE_CON)
    response->type = COAP_MESSAGE_CON;
#endif /* COAP_ASYNC_SUPPORT */
  /* A lot of the reliable code assumes type is CON */
  if (COAP_PROTO_RELIABLE(session->proto) && response->type != COAP_MESSAGE_CON)
    response->type = COAP_MESSAGE_CON;

  if (!coap_add_token(response, pdu->actual_token.length,
                      pdu->actual_token.s)) {
    resp = 500;
    goto fail_response;
  }

  query = coap_get_query(pdu);

  /* check for Observe option RFC7641 and RFC8132 */
  if (resource->observable &&
      (pdu->code == COAP_REQUEST_CODE_GET ||
       pdu->code == COAP_REQUEST_CODE_FETCH)) {
    observe = coap_check_option(pdu, COAP_OPTION_OBSERVE, &opt_iter);
  }

  /*
   * See if blocks need to be aggregated or next requests sent off
   * before invoking application request handler
   */
  if (session->block_mode & COAP_BLOCK_USE_LIBCOAP) {
    uint32_t block_mode = session->block_mode;

    if (pdu->code == COAP_REQUEST_CODE_FETCH ||
        resource->flags & COAP_RESOURCE_FLAGS_FORCE_SINGLE_BODY)
      session->block_mode |= COAP_BLOCK_SINGLE_BODY;
    if (coap_handle_request_put_block(context, session, pdu, response,
                                      resource, uri_path, observe,
                                      &added_block, &free_lg_srcv)) {
      session->block_mode = block_mode;
      goto skip_handler;
    }
    session->block_mode = block_mode;

    if (coap_handle_request_send_block(session, pdu, response, resource,
                                       query)) {
#if COAP_Q_BLOCK_SUPPORT
      lg_xmit_ctrl = 1;
#endif /* COAP_Q_BLOCK_SUPPORT */
      goto skip_handler;
    }
  }

  if (observe) {
    observe_action =
        coap_decode_var_bytes(coap_opt_value(observe),
                              coap_opt_length(observe));

    if (observe_action == COAP_OBSERVE_ESTABLISH) {
      coap_subscription_t *subscription;

      if (coap_get_block_b(session, pdu, COAP_OPTION_BLOCK2, &block)) {
        if (block.num != 0) {
          response->code = COAP_RESPONSE_CODE(400);
          goto skip_handler;
        }
#if COAP_Q_BLOCK_SUPPORT
      } else if (coap_get_block_b(session, pdu, COAP_OPTION_Q_BLOCK2,
                                  &block)) {
        if (block.num != 0) {
          response->code = COAP_RESPONSE_CODE(400);
          goto skip_handler;
        }
#endif /* COAP_Q_BLOCK_SUPPORT */
      }
      subscription = coap_add_observer(resource, session, &pdu->actual_token,
                                       pdu);
      if (subscription) {
        uint8_t buf[4];

        coap_touch_observer(context, session, &pdu->actual_token);
        coap_add_option_internal(response, COAP_OPTION_OBSERVE,
                                 coap_encode_var_safe(buf, sizeof(buf),
                                                      resource->observe),
                                 buf);
      }
    } else if (observe_action == COAP_OBSERVE_CANCEL) {
      coap_delete_observer_request(resource, session, &pdu->actual_token, pdu);
    } else {
      coap_log_info("observe: unexpected action %d\n", observe_action);
    }
  }

  /* TODO for non-proxy requests */
  if (resource == context->proxy_uri_resource &&
      COAP_PROTO_NOT_RELIABLE(session->proto) &&
      pdu->type == COAP_MESSAGE_CON) {
    /* Make the proxy response separate and fix response later */
    send_early_empty_ack = 1;
  }
  if (send_early_empty_ack) {
    coap_send_ack_lkd(session, pdu);
    if (pdu->mid == session->last_con_mid) {
      /* request has already been processed - do not process it again */
      coap_log_debug("Duplicate request with mid=0x%04x - not processed\n",
                     pdu->mid);
      goto drop_it_no_debug;
    }
    session->last_con_mid = pdu->mid;
  }
#if COAP_WITH_OBSERVE_PERSIST
  /* If we are maintaining Observe persist */
  if (resource == context->unknown_resource) {
    context->unknown_pdu = pdu;
    context->unknown_session = session;
  } else
    context->unknown_pdu = NULL;
#endif /* COAP_WITH_OBSERVE_PERSIST */

  /*
   * Call the request handler with everything set up
   */
  if (resource == &resource_uri_wellknown) {
    /* Leave context locked */
    coap_log_debug("call handler for pseudo resource '%*.*s' (3)\n",
                   (int)resource->uri_path->length, (int)resource->uri_path->length,
                   resource->uri_path->s);
    h(resource, session, pdu, query, response);
  } else {
    coap_log_debug("call custom handler for resource '%*.*s' (3)\n",
                   (int)resource->uri_path->length, (int)resource->uri_path->length,
                   resource->uri_path->s);
    coap_lock_callback_release(context,
                               h(resource, session, pdu, query, response),
                               /* context is being freed off */
                               goto finish);
  }

  /* Check validity of response code */
  if (!coap_check_code_class(session, response)) {
    coap_log_warn("handle_request: Invalid PDU response code (%d.%02d)\n",
                  COAP_RESPONSE_CLASS(response->code),
                  response->code & 0x1f);
    goto drop_it_no_debug;
  }

  /* Check if lg_xmit generated and update PDU code if so */
  coap_check_code_lg_xmit(session, pdu, response, resource, query);

  if (free_lg_srcv) {
    /* Check to see if the server is doing a 4.01 + Echo response */
    if (response->code ==  COAP_RESPONSE_CODE(401) &&
        coap_check_option(response, COAP_OPTION_ECHO, &opt_iter)) {
      /* Need to keep lg_srcv around for client's response */
    } else {
      LL_DELETE(session->lg_srcv, free_lg_srcv);
      coap_block_delete_lg_srcv(session, free_lg_srcv);
    }
  }
  if (added_block && COAP_RESPONSE_CLASS(response->code) == 2) {
    /* Just in case, as there are more to go */
    response->code = COAP_RESPONSE_CODE(231);
  }

skip_handler:
  if (send_early_empty_ack &&
      response->type == COAP_MESSAGE_ACK) {
    /* Response is now separate - convert to CON as needed */
    response->type = COAP_MESSAGE_CON;
    /* Check for empty ACK - need to drop as already sent */
    if (response->code == 0) {
      goto drop_it_no_debug;
    }
  }
  respond = no_response(pdu, response, session, resource);
  if (respond != RESPONSE_DROP) {
#if (COAP_MAX_LOGGING_LEVEL >= _COAP_LOG_DEBUG)
    coap_mid_t mid = pdu->mid;
#endif
    if (COAP_RESPONSE_CLASS(response->code) != 2) {
      if (observe) {
        coap_remove_option(response, COAP_OPTION_OBSERVE);
      }
    }
    if (COAP_RESPONSE_CLASS(response->code) > 2) {
      if (observe)
        coap_delete_observer(resource, session, &pdu->actual_token);
      if (response->code != COAP_RESPONSE_CODE(413))
        coap_remove_option(response, COAP_OPTION_BLOCK1);
    }

    /* If original request contained a token, and the registered
     * application handler made no changes to the response, then
     * this is an empty ACK with a token, which is a malformed
     * PDU */
    if ((response->type == COAP_MESSAGE_ACK)
        && (response->code == 0)) {
      /* Remove token from otherwise-empty acknowledgment PDU */
      response->actual_token.length = 0;
      response->e_token_length = 0;
      response->used_size = 0;
      response->data = NULL;
    }

    if (!coap_is_mcast(&session->addr_info.local) ||
        (context->mcast_per_resource &&
         resource &&
         (resource->flags & COAP_RESOURCE_FLAGS_LIB_DIS_MCAST_DELAYS))) {
      /* No delays to response */
#if COAP_Q_BLOCK_SUPPORT
      if (session->block_mode & COAP_BLOCK_USE_LIBCOAP &&
          !lg_xmit_ctrl && response->code == COAP_RESPONSE_CODE(205) &&
          coap_get_block_b(session, response, COAP_OPTION_Q_BLOCK2, &block) &&
          block.m) {
        if (coap_send_q_block2(session, resource, query, pdu->code, block,
                               response,
                               COAP_SEND_INC_PDU) == COAP_INVALID_MID)
          coap_log_debug("cannot send response for mid=0x%x\n", mid);
        response = NULL;
        if (query)
          coap_delete_string(query);
        goto finish;
      }
#endif /* COAP_Q_BLOCK_SUPPORT */
      if (coap_send_internal(session, response) == COAP_INVALID_MID) {
        coap_log_debug("cannot send response for mid=0x%04x\n", mid);
      }
    } else {
      /* Need to delay mcast response */
      coap_queue_t *node = coap_new_node();
      uint8_t r;
      coap_tick_t delay;

      if (!node) {
        coap_log_debug("mcast delay: insufficient memory\n");
        goto drop_it_no_debug;
      }
      if (!coap_pdu_encode_header(response, session->proto)) {
        coap_delete_node_lkd(node);
        goto drop_it_no_debug;
      }

      node->id = response->mid;
      node->pdu = response;
      node->is_mcast = 1;
      coap_prng_lkd(&r, sizeof(r));
      delay = (COAP_DEFAULT_LEISURE_TICKS(session) * r) / 256;
      coap_log_debug("   %s: mid=0x%04x: mcast response delayed for %u.%03u secs\n",
                     coap_session_str(session),
                     response->mid,
                     (unsigned int)(delay / COAP_TICKS_PER_SECOND),
                     (unsigned int)((delay % COAP_TICKS_PER_SECOND) *
                                    1000 / COAP_TICKS_PER_SECOND));
      node->timeout = (unsigned int)delay;
      /* Use this to delay transmission */
      coap_wait_ack(session->context, session, node);
    }
  } else {
    coap_log_debug("   %s: mid=0x%04x: response dropped\n",
                   coap_session_str(session),
                   response->mid);
    coap_show_pdu(COAP_LOG_DEBUG, response);
drop_it_no_debug:
    coap_delete_pdu(response);
  }
  if (query)
    coap_delete_string(query);
#if COAP_Q_BLOCK_SUPPORT
  if (coap_get_block_b(session, pdu, COAP_OPTION_Q_BLOCK1, &block)) {
    if (COAP_PROTO_RELIABLE(session->proto)) {
      if (block.m) {
        /* All of the sequence not in yet */
        goto finish;
      }
    } else if (pdu->type == COAP_MESSAGE_NON) {
      /* More to go and not at a payload break */
      if (block.m && ((block.num + 1) % COAP_MAX_PAYLOADS(session))) {
        goto finish;
      }
    }
  }
#endif /* COAP_Q_BLOCK_SUPPORT */

#if COAP_Q_BLOCK_SUPPORT || COAP_THREAD_SAFE
finish:
#endif /* COAP_Q_BLOCK_SUPPORT || COAP_THREAD_SAFE */
  coap_delete_string(uri_path);
  return;

fail_response:
  coap_delete_pdu(response);
  response =
      coap_new_error_response(pdu, COAP_RESPONSE_CODE(resp),
                              &opt_filter);
  if (response)
    goto skip_handler;
  coap_delete_string(uri_path);
}
#endif /* COAP_SERVER_SUPPORT */

#if COAP_CLIENT_SUPPORT
static void
handle_response(coap_context_t *context, coap_session_t *session,
                coap_pdu_t *sent, coap_pdu_t *rcvd) {

  /* Set in case there is a later call to coap_update_token() */
  rcvd->session = session;

  /* In a lossy context, the ACK of a separate response may have
   * been lost, so we need to stop retransmitting requests with the
   * same token. Matching on token potentially containing ext length bytes.
   */
  if (rcvd->type != COAP_MESSAGE_ACK)
    coap_cancel_all_messages(context, session, &rcvd->actual_token);

  /* Check for message duplication */
  if (COAP_PROTO_NOT_RELIABLE(session->proto)) {
    if (rcvd->type == COAP_MESSAGE_CON) {
      if (rcvd->mid == session->last_con_mid) {
        /* Duplicate response: send ACK/RST, but don't process */
        if (session->last_con_handler_res == COAP_RESPONSE_OK)
          coap_send_ack_lkd(session, rcvd);
        else
          coap_send_rst_lkd(session, rcvd);
        return;
      }
      session->last_con_mid = rcvd->mid;
    } else if (rcvd->type == COAP_MESSAGE_ACK) {
      if (rcvd->mid == session->last_ack_mid) {
        /* Duplicate response */
        return;
      }
      session->last_ack_mid = rcvd->mid;
    }
  }
  /* Check to see if checking out extended token support */
  if (session->max_token_checked == COAP_EXT_T_CHECKING &&
      session->remote_test_mid == rcvd->mid) {

    if (rcvd->actual_token.length != session->max_token_size ||
        rcvd->code == COAP_RESPONSE_CODE(400) ||
        rcvd->code == COAP_RESPONSE_CODE(503)) {
      coap_log_debug("Extended Token requested size support not available\n");
      session->max_token_size = COAP_TOKEN_DEFAULT_MAX;
    } else {
      coap_log_debug("Extended Token support available\n");
    }
    session->max_token_checked = COAP_EXT_T_CHECKED;
    session->doing_first = 0;
    return;
  }
#if COAP_Q_BLOCK_SUPPORT
  /* Check to see if checking out Q-Block support */
  if (session->block_mode & COAP_BLOCK_PROBE_Q_BLOCK &&
      session->remote_test_mid == rcvd->mid) {
    if (rcvd->code == COAP_RESPONSE_CODE(402)) {
      coap_log_debug("Q-Block support not available\n");
      set_block_mode_drop_q(session->block_mode);
    } else {
      coap_block_b_t qblock;

      if (coap_get_block_b(session, rcvd, COAP_OPTION_Q_BLOCK2, &qblock)) {
        coap_log_debug("Q-Block support available\n");
        set_block_mode_has_q(session->block_mode);
      } else {
        coap_log_debug("Q-Block support not available\n");
        set_block_mode_drop_q(session->block_mode);
      }
    }
    session->doing_first = 0;
    return;
  }
#endif /* COAP_Q_BLOCK_SUPPORT */

  if (session->block_mode & COAP_BLOCK_USE_LIBCOAP) {
    /* See if need to send next block to server */
    if (coap_handle_response_send_block(session, sent, rcvd)) {
      /* Next block transmitted, no need to inform app */
      coap_send_ack_lkd(session, rcvd);
      return;
    }

    /* Need to see if needing to request next block */
    if (coap_handle_response_get_block(context, session, sent, rcvd,
                                       COAP_RECURSE_OK)) {
      /* Next block transmitted, ack sent no need to inform app */
      return;
    }
  }
  if (session->doing_first)
    session->doing_first = 0;

  /* Call application-specific response handler when available. */
  if (session->doing_send_recv && session->req_token &&
      coap_binary_equal(session->req_token, &rcvd->actual_token)) {
    /* processing coap_send_recv() call */
    session->resp_pdu = rcvd;
    rcvd->ref++;
    coap_send_ack_lkd(session, rcvd);
    session->last_con_handler_res = COAP_RESPONSE_OK;
  } else if (context->response_handler) {
    coap_response_t ret;

    coap_lock_callback_ret_release(ret, context,
                                   context->response_handler(session, sent, rcvd,
                                                             rcvd->mid),
                                   /* context is being freed off */
                                   return);
    if (ret == COAP_RESPONSE_FAIL && rcvd->type != COAP_MESSAGE_ACK) {
      coap_send_rst_lkd(session, rcvd);
      session->last_con_handler_res = COAP_RESPONSE_FAIL;
    } else {
      coap_send_ack_lkd(session, rcvd);
      session->last_con_handler_res = COAP_RESPONSE_OK;
    }
  } else {
    coap_send_ack_lkd(session, rcvd);
    session->last_con_handler_res = COAP_RESPONSE_OK;
  }
}
#endif /* COAP_CLIENT_SUPPORT */

#if !COAP_DISABLE_TCP
static void
handle_signaling(coap_context_t *context, coap_session_t *session,
                 coap_pdu_t *pdu) {
  coap_opt_iterator_t opt_iter;
  coap_opt_t *option;
  int set_mtu = 0;

  coap_option_iterator_init(pdu, &opt_iter, COAP_OPT_ALL);

  if (pdu->code == COAP_SIGNALING_CODE_CSM) {
    if (session->csm_not_seen) {
      coap_tick_t now;

      coap_ticks(&now);
      /* CSM timeout before CSM seen */
      coap_log_warn("***%s: CSM received after CSM timeout\n",
                    coap_session_str(session));
      coap_log_warn("***%s: Increase timeout in coap_context_set_csm_timeout_ms() to > %d\n",
                    coap_session_str(session),
                    (int)(((now - session->csm_tx) * 1000) / COAP_TICKS_PER_SECOND));
    }
    if (session->max_token_checked == COAP_EXT_T_NOT_CHECKED) {
      session->max_token_size = COAP_TOKEN_DEFAULT_MAX;
    }
    while ((option = coap_option_next(&opt_iter))) {
      if (opt_iter.number == COAP_SIGNALING_OPTION_MAX_MESSAGE_SIZE) {
        coap_session_set_mtu(session, coap_decode_var_bytes(coap_opt_value(option),
                                                            coap_opt_length(option)));
        set_mtu = 1;
      } else if (opt_iter.number == COAP_SIGNALING_OPTION_BLOCK_WISE_TRANSFER) {
        session->csm_block_supported = 1;
      } else if (opt_iter.number == COAP_SIGNALING_OPTION_EXTENDED_TOKEN_LENGTH) {
        session->max_token_size =
            coap_decode_var_bytes(coap_opt_value(option),
                                  coap_opt_length(option));
        if (session->max_token_size < COAP_TOKEN_DEFAULT_MAX)
          session->max_token_size = COAP_TOKEN_DEFAULT_MAX;
        else if (session->max_token_size > COAP_TOKEN_EXT_MAX)
          session->max_token_size = COAP_TOKEN_EXT_MAX;
        session->max_token_checked = COAP_EXT_T_CHECKED;
      }
    }
    if (set_mtu) {
      if (session->mtu > COAP_BERT_BASE && session->csm_block_supported)
        session->csm_bert_rem_support = 1;
      else
        session->csm_bert_rem_support = 0;
    }
    if (session->state == COAP_SESSION_STATE_CSM)
      coap_session_connected(session);
  } else if (pdu->code == COAP_SIGNALING_CODE_PING) {
    coap_pdu_t *pong = coap_pdu_init(COAP_MESSAGE_CON, COAP_SIGNALING_CODE_PONG, 0, 1);
    if (context->ping_handler) {
      coap_lock_callback(context,
                         context->ping_handler(session, pdu, pdu->mid));
    }
    if (pong) {
      coap_add_option_internal(pong, COAP_SIGNALING_OPTION_CUSTODY, 0, NULL);
      coap_send_internal(session, pong);
    }
  } else if (pdu->code == COAP_SIGNALING_CODE_PONG) {
    session->last_pong = session->last_rx_tx;
    if (context->pong_handler) {
      coap_lock_callback(context,
                         context->pong_handler(session, pdu, pdu->mid));
    }
  } else if (pdu->code == COAP_SIGNALING_CODE_RELEASE
             || pdu->code == COAP_SIGNALING_CODE_ABORT) {
    coap_session_disconnected_lkd(session, COAP_NACK_RST);
  }
}
#endif /* !COAP_DISABLE_TCP */

static int
check_token_size(coap_session_t *session, const coap_pdu_t *pdu) {
  if (COAP_PDU_IS_REQUEST(pdu) &&
      pdu->actual_token.length >
      (session->type == COAP_SESSION_TYPE_CLIENT ?
       session->max_token_size : session->context->max_token_size)) {
    /* https://rfc-editor.org/rfc/rfc8974#section-2.2.2 */
    if (session->max_token_size > COAP_TOKEN_DEFAULT_MAX) {
      coap_opt_filter_t opt_filter;
      coap_pdu_t *response;

      memset(&opt_filter, 0, sizeof(coap_opt_filter_t));
      response = coap_new_error_response(pdu, COAP_RESPONSE_CODE(400),
                                         &opt_filter);
      if (!response) {
        coap_log_warn("coap_dispatch: cannot create error response\n");
      } else {
        /*
         * Note - have to leave in oversize token as per
         * https://rfc-editor.org/rfc/rfc7252#section-5.3.1
         */
        if (coap_send_internal(session, response) == COAP_INVALID_MID)
          coap_log_warn("coap_dispatch: error sending response\n");
      }
    } else {
      /* Indicate no extended token support */
      coap_send_rst_lkd(session, pdu);
    }
    return 0;
  }
  return 1;
}

void
coap_dispatch(coap_context_t *context, coap_session_t *session,
              coap_pdu_t *pdu) {
  coap_queue_t *sent = NULL;
  coap_pdu_t *response;
  coap_opt_filter_t opt_filter;
  int is_ping_rst;
  int packet_is_bad = 0;
#if COAP_OSCORE_SUPPORT
  coap_opt_iterator_t opt_iter;
  coap_pdu_t *dec_pdu = NULL;
#endif /* COAP_OSCORE_SUPPORT */
  int is_ext_token_rst;

  pdu->session = session;
  coap_show_pdu(COAP_LOG_DEBUG, pdu);

  /* Check validity of received code */
  if (!coap_check_code_class(session, pdu)) {
    coap_log_info("coap_dispatch: Received invalid PDU code (%d.%02d)\n",
                  COAP_RESPONSE_CLASS(pdu->code),
                  pdu->code & 0x1f);
    packet_is_bad = 1;
    if (pdu->type == COAP_MESSAGE_CON) {
      coap_send_message_type_lkd(session, pdu, COAP_MESSAGE_RST);
    }
    /* find message id in sendqueue to stop retransmission */
    coap_remove_from_queue(&context->sendqueue, session, pdu->mid, &sent);
    goto cleanup;
  }

  coap_option_filter_clear(&opt_filter);

#if COAP_OSCORE_SUPPORT
  if (!COAP_PDU_IS_SIGNALING(pdu) &&
      coap_option_check_critical(session, pdu, &opt_filter) == 0) {
    if (pdu->type == COAP_MESSAGE_NON) {
      coap_send_rst_lkd(session, pdu);
      goto cleanup;
    } else if (pdu->type == COAP_MESSAGE_CON) {
      if (COAP_PDU_IS_REQUEST(pdu)) {
        response =
            coap_new_error_response(pdu, COAP_RESPONSE_CODE(402), &opt_filter);

        if (!response) {
          coap_log_warn("coap_dispatch: cannot create error response\n");
        } else {
          if (coap_send_internal(session, response) == COAP_INVALID_MID)
            coap_log_warn("coap_dispatch: error sending response\n");
        }
      } else {
        coap_send_rst_lkd(session, pdu);
      }
    }
    goto cleanup;
  }

  if (coap_check_option(pdu, COAP_OPTION_OSCORE, &opt_iter) != NULL) {
    int decrypt = 1;
#if COAP_SERVER_SUPPORT
    coap_opt_t *opt;
    coap_resource_t *resource;
    coap_uri_t uri;
#endif /* COAP_SERVER_SUPPORT */

    if (COAP_PDU_IS_RESPONSE(pdu) && !session->oscore_encryption)
      decrypt = 0;

#if COAP_SERVER_SUPPORT
    if (decrypt && COAP_PDU_IS_REQUEST(pdu) &&
        coap_check_option(pdu, COAP_OPTION_PROXY_SCHEME, &opt_iter) != NULL &&
        (opt = coap_check_option(pdu, COAP_OPTION_URI_HOST, &opt_iter))
        != NULL) {
      /* Need to check whether this is a direct or proxy session */
      memset(&uri, 0, sizeof(uri));
      uri.host.length = coap_opt_length(opt);
      uri.host.s = coap_opt_value(opt);
      resource = context->proxy_uri_resource;
      if (uri.host.length && resource && resource->proxy_name_count &&
          resource->proxy_name_list) {
        size_t i;
        for (i = 0; i < resource->proxy_name_count; i++) {
          if (coap_string_equal(&uri.host, resource->proxy_name_list[i])) {
            break;
          }
        }
        if (i == resource->proxy_name_count) {
          /* This server is not hosting the proxy connection endpoint */
          decrypt = 0;
        }
      }
    }
#endif /* COAP_SERVER_SUPPORT */
    if (decrypt) {
      /* find message id in sendqueue to stop retransmission and get sent */
      coap_remove_from_queue(&context->sendqueue, session, pdu->mid, &sent);
      if ((dec_pdu = coap_oscore_decrypt_pdu(session, pdu)) == NULL) {
        if (session->recipient_ctx == NULL ||
            session->recipient_ctx->initial_state == 0) {
          coap_log_warn("OSCORE: PDU could not be decrypted\n");
        }
        coap_delete_node_lkd(sent);
        return;
      } else {
        session->oscore_encryption = 1;
        pdu = dec_pdu;
      }
      coap_log_debug("Decrypted PDU\n");
      coap_show_pdu(COAP_LOG_DEBUG, pdu);
    }
  }
#endif /* COAP_OSCORE_SUPPORT */

  switch (pdu->type) {
  case COAP_MESSAGE_ACK:
    /* find message id in sendqueue to stop retransmission */
    coap_remove_from_queue(&context->sendqueue, session, pdu->mid, &sent);

    if (sent && session->con_active) {
      session->con_active--;
      if (session->state == COAP_SESSION_STATE_ESTABLISHED)
        /* Flush out any entries on session->delayqueue */
        coap_session_connected(session);
    }
    if (coap_option_check_critical(session, pdu, &opt_filter) == 0) {
      packet_is_bad = 1;
      goto cleanup;
    }

#if COAP_SERVER_SUPPORT
    /* if sent code was >= 64 the message might have been a
     * notification. Then, we must flag the observer to be alive
     * by setting obs->fail_cnt = 0. */
    if (sent && COAP_RESPONSE_CLASS(sent->pdu->code) == 2) {
      coap_touch_observer(context, sent->session, &sent->pdu->actual_token);
    }
#endif /* COAP_SERVER_SUPPORT */

    if (pdu->code == 0) {
#if COAP_Q_BLOCK_SUPPORT
      if (sent) {
        coap_block_b_t block;

        if (sent->pdu->type == COAP_MESSAGE_CON &&
            COAP_PROTO_NOT_RELIABLE(session->proto) &&
            coap_get_block_b(session, sent->pdu,
                             COAP_PDU_IS_REQUEST(sent->pdu) ?
                             COAP_OPTION_Q_BLOCK1 : COAP_OPTION_Q_BLOCK2,
                             &block)) {
          if (block.m) {
#if COAP_CLIENT_SUPPORT
            if (COAP_PDU_IS_REQUEST(sent->pdu))
              coap_send_q_block1(session, block, sent->pdu,
                                 COAP_SEND_SKIP_PDU);
#endif /* COAP_CLIENT_SUPPORT */
            if (COAP_PDU_IS_RESPONSE(sent->pdu))
              coap_send_q_blocks(session, sent->pdu->lg_xmit, block,
                                 sent->pdu, COAP_SEND_SKIP_PDU);
          }
        }
      }
#endif /* COAP_Q_BLOCK_SUPPORT */
#if COAP_CLIENT_SUPPORT
      /*
       * In coap_send(), lg_crcv was not set up if type is CON and protocol is not
       * reliable to save overhead as this can be set up on detection of a (Q)-Block2
       * response if the response was piggy-backed. Here, a separate response
       * detected and so the lg_crcv needs to be set up before the sent PDU
       * information is lost.
       *
       * lg_crcv was not set up if not a CoAP request or if DELETE.
       *
       * lg_crcv was always set up in coap_send() if Observe, Oscore and (Q)-Block1
       * options.
       */
      if (sent &&
          !coap_check_send_need_lg_crcv(session, pdu) &&
          COAP_PDU_IS_REQUEST(sent->pdu)) {
        /*
         * lg_crcv was not set up in coap_send(). It could have been set up
         * the first separate response.
         * See if there already is a lg_crcv set up.
         */
        coap_lg_crcv_t *lg_crcv;
        uint64_t token_match =
            STATE_TOKEN_BASE(coap_decode_var_bytes8(sent->pdu->actual_token.s,
                                                    sent->pdu->actual_token.length));

        LL_FOREACH(session->lg_crcv, lg_crcv) {
          if (token_match == STATE_TOKEN_BASE(lg_crcv->state_token) ||
              coap_binary_equal(&sent->pdu->actual_token, lg_crcv->app_token)) {
            break;
          }
        }
        if (!lg_crcv) {
          /*
           * Need to set up a lg_crcv as it was not set up in coap_send()
           * to save time, but server has not sent back a piggy-back response.
           */
          lg_crcv = coap_block_new_lg_crcv(session, sent->pdu, NULL);
          if (lg_crcv) {
            LL_PREPEND(session->lg_crcv, lg_crcv);
          }
        }
      }
#endif /* COAP_CLIENT_SUPPORT */
      /* an empty ACK needs no further handling */
      goto cleanup;
    } else if (COAP_PDU_IS_REQUEST(pdu)) {
      /* This is not legitimate - Request using ACK - ignore */
      coap_log_debug("dropped ACK with request code (%d.%02d)\n",
                     COAP_RESPONSE_CLASS(pdu->code),
                     pdu->code & 0x1f);
      packet_is_bad = 1;
      goto cleanup;
    }

    break;

  case COAP_MESSAGE_RST:
    /* We have sent something the receiver disliked, so we remove
     * not only the message id but also the subscriptions we might
     * have. */
    is_ping_rst = 0;
    if (pdu->mid == session->last_ping_mid &&
        context->ping_timeout && session->last_ping > 0)
      is_ping_rst = 1;

#if COAP_Q_BLOCK_SUPPORT
    /* Check to see if checking out Q-Block support */
    if (session->block_mode & COAP_BLOCK_PROBE_Q_BLOCK &&
        session->remote_test_mid == pdu->mid) {
      coap_log_debug("Q-Block support not available\n");
      set_block_mode_drop_q(session->block_mode);
    }
#endif /* COAP_Q_BLOCK_SUPPORT */

    /* Check to see if checking out extended token support */
    is_ext_token_rst = 0;
    if (session->max_token_checked == COAP_EXT_T_CHECKING &&
        session->remote_test_mid == pdu->mid) {
      coap_log_debug("Extended Token support not available\n");
      session->max_token_size = COAP_TOKEN_DEFAULT_MAX;
      session->max_token_checked = COAP_EXT_T_CHECKED;
      session->doing_first = 0;
      is_ext_token_rst = 1;
    }

    if (!is_ping_rst && !is_ext_token_rst)
      coap_log_alert("got RST for mid=0x%04x\n", pdu->mid);

    if (session->con_active) {
      session->con_active--;
      if (session->state == COAP_SESSION_STATE_ESTABLISHED)
        /* Flush out any entries on session->delayqueue */
        coap_session_connected(session);
    }

    /* find message id in sendqueue to stop retransmission */
    coap_remove_from_queue(&context->sendqueue, session, pdu->mid, &sent);

    if (sent) {
      if (!is_ping_rst)
        coap_cancel(context, sent);

      if (!is_ping_rst && !is_ext_token_rst) {
        if (sent->pdu->type==COAP_MESSAGE_CON) {
          coap_handle_nack(sent->session, sent->pdu, COAP_NACK_RST, sent->id);
        }
      } else if (is_ping_rst) {
        if (context->pong_handler) {
          coap_lock_callback(context,
                             context->pong_handler(session, pdu, pdu->mid));
        }
        session->last_pong = session->last_rx_tx;
        session->last_ping_mid = COAP_INVALID_MID;
      }
    } else {
#if COAP_SERVER_SUPPORT
      /* Need to check is there is a subscription active and delete it */
      RESOURCES_ITER(context->resources, r) {
        coap_subscription_t *obs, *tmp;
        LL_FOREACH_SAFE(r->subscribers, obs, tmp) {
          if (obs->pdu->mid == pdu->mid && obs->session == session) {
            /* Need to do this now as session may get de-referenced */
            coap_session_reference_lkd(session);
            coap_delete_observer(r, session, &obs->pdu->actual_token);
            coap_handle_nack(session, NULL, COAP_NACK_RST, pdu->mid);
            coap_session_release_lkd(session);
            goto cleanup;
          }
        }
      }
#endif /* COAP_SERVER_SUPPORT */
      coap_handle_nack(session, NULL, COAP_NACK_RST, pdu->mid);
    }
    goto cleanup;

  case COAP_MESSAGE_NON:
    /* check for unknown critical options */
    if (coap_option_check_critical(session, pdu, &opt_filter) == 0) {
      packet_is_bad = 1;
      coap_send_rst_lkd(session, pdu);
      goto cleanup;
    }
    if (!check_token_size(session, pdu)) {
      goto cleanup;
    }
    break;

  case COAP_MESSAGE_CON:        /* check for unknown critical options */
    if (!COAP_PDU_IS_SIGNALING(pdu) &&
        coap_option_check_critical(session, pdu, &opt_filter) == 0) {
      packet_is_bad = 1;
      if (COAP_PDU_IS_REQUEST(pdu)) {
        response =
            coap_new_error_response(pdu, COAP_RESPONSE_CODE(402), &opt_filter);

        if (!response) {
          coap_log_warn("coap_dispatch: cannot create error response\n");
        } else {
          if (coap_send_internal(session, response) == COAP_INVALID_MID)
            coap_log_warn("coap_dispatch: error sending response\n");
        }
      } else {
        coap_send_rst_lkd(session, pdu);
      }
      goto cleanup;
    }
    if (!check_token_size(session, pdu)) {
      goto cleanup;
    }
    break;
  default:
    break;
  }

  /* Pass message to upper layer if a specific handler was
    * registered for a request that should be handled locally. */
#if !COAP_DISABLE_TCP
  if (COAP_PDU_IS_SIGNALING(pdu))
    handle_signaling(context, session, pdu);
  else
#endif /* !COAP_DISABLE_TCP */
#if COAP_SERVER_SUPPORT
    if (COAP_PDU_IS_REQUEST(pdu))
      handle_request(context, session, pdu);
    else
#endif /* COAP_SERVER_SUPPORT */
#if COAP_CLIENT_SUPPORT
      if (COAP_PDU_IS_RESPONSE(pdu))
        handle_response(context, session, sent ? sent->pdu : NULL, pdu);
      else
#endif /* COAP_CLIENT_SUPPORT */
      {
        if (COAP_PDU_IS_EMPTY(pdu)) {
          if (context->ping_handler) {
            coap_lock_callback(context,
                               context->ping_handler(session, pdu, pdu->mid));
          }
        } else {
          packet_is_bad = 1;
        }
        coap_log_debug("dropped message with invalid code (%d.%02d)\n",
                       COAP_RESPONSE_CLASS(pdu->code),
                       pdu->code & 0x1f);

        if (!coap_is_mcast(&session->addr_info.local)) {
          if (COAP_PDU_IS_EMPTY(pdu)) {
            if (COAP_PROTO_NOT_RELIABLE(session->proto)) {
              coap_tick_t now;
              coap_ticks(&now);
              if (session->last_tx_rst + COAP_TICKS_PER_SECOND/4 < now) {
                coap_send_message_type_lkd(session, pdu, COAP_MESSAGE_RST);
                session->last_tx_rst = now;
              }
            }
          } else {
            if (pdu->type == COAP_MESSAGE_CON)
              coap_send_message_type_lkd(session, pdu, COAP_MESSAGE_RST);
          }
        }
      }

cleanup:
  if (packet_is_bad) {
    if (sent) {
      coap_handle_nack(session, sent->pdu, COAP_NACK_BAD_RESPONSE, sent->id);
    } else {
      coap_handle_event_lkd(context, COAP_EVENT_BAD_PACKET, session);
    }
  }
  coap_delete_node_lkd(sent);
#if COAP_OSCORE_SUPPORT
  coap_delete_pdu(dec_pdu);
#endif /* COAP_OSCORE_SUPPORT */
}

#if COAP_MAX_LOGGING_LEVEL >= _COAP_LOG_DEBUG
static const char *
coap_event_name(coap_event_t event) {
  switch (event) {
  case COAP_EVENT_DTLS_CLOSED:
    return "COAP_EVENT_DTLS_CLOSED";
  case COAP_EVENT_DTLS_CONNECTED:
    return "COAP_EVENT_DTLS_CONNECTED";
  case COAP_EVENT_DTLS_RENEGOTIATE:
    return "COAP_EVENT_DTLS_RENEGOTIATE";
  case COAP_EVENT_DTLS_ERROR:
    return "COAP_EVENT_DTLS_ERROR";
  case COAP_EVENT_TCP_CONNECTED:
    return "COAP_EVENT_TCP_CONNECTED";
  case COAP_EVENT_TCP_CLOSED:
    return "COAP_EVENT_TCP_CLOSED";
  case COAP_EVENT_TCP_FAILED:
    return "COAP_EVENT_TCP_FAILED";
  case COAP_EVENT_SESSION_CONNECTED:
    return "COAP_EVENT_SESSION_CONNECTED";
  case COAP_EVENT_SESSION_CLOSED:
    return "COAP_EVENT_SESSION_CLOSED";
  case COAP_EVENT_SESSION_FAILED:
    return "COAP_EVENT_SESSION_FAILED";
  case COAP_EVENT_PARTIAL_BLOCK:
    return "COAP_EVENT_PARTIAL_BLOCK";
  case COAP_EVENT_XMIT_BLOCK_FAIL:
    return "COAP_EVENT_XMIT_BLOCK_FAIL";
  case COAP_EVENT_SERVER_SESSION_NEW:
    return "COAP_EVENT_SERVER_SESSION_NEW";
  case COAP_EVENT_SERVER_SESSION_DEL:
    return "COAP_EVENT_SERVER_SESSION_DEL";
  case COAP_EVENT_BAD_PACKET:
    return "COAP_EVENT_BAD_PACKET";
  case COAP_EVENT_MSG_RETRANSMITTED:
    return "COAP_EVENT_MSG_RETRANSMITTED";
  case COAP_EVENT_OSCORE_DECRYPTION_FAILURE:
    return "COAP_EVENT_OSCORE_DECRYPTION_FAILURE";
  case COAP_EVENT_OSCORE_NOT_ENABLED:
    return "COAP_EVENT_OSCORE_NOT_ENABLED";
  case COAP_EVENT_OSCORE_NO_PROTECTED_PAYLOAD:
    return "COAP_EVENT_OSCORE_NO_PROTECTED_PAYLOAD";
  case COAP_EVENT_OSCORE_NO_SECURITY:
    return "COAP_EVENT_OSCORE_NO_SECURITY";
  case COAP_EVENT_OSCORE_INTERNAL_ERROR:
    return "COAP_EVENT_OSCORE_INTERNAL_ERROR";
  case COAP_EVENT_OSCORE_DECODE_ERROR:
    return "COAP_EVENT_OSCORE_DECODE_ERROR";
  case COAP_EVENT_WS_PACKET_SIZE:
    return "COAP_EVENT_WS_PACKET_SIZE";
  case COAP_EVENT_WS_CONNECTED:
    return "COAP_EVENT_WS_CONNECTED";
  case COAP_EVENT_WS_CLOSED:
    return "COAP_EVENT_WS_CLOSED";
  case COAP_EVENT_KEEPALIVE_FAILURE:
    return "COAP_EVENT_KEEPALIVE_FAILURE";
  default:
    return "???";
  }
}
#endif /* COAP_MAX_LOGGING_LEVEL >= _COAP_LOG_DEBUG */

COAP_API int
coap_handle_event(coap_context_t *context, coap_event_t event,
                  coap_session_t *session) {
  int ret;

  coap_lock_lock(context, return 0);
  ret = coap_handle_event_lkd(context, event, session);
  coap_lock_unlock(context);
  return ret;
}

int
coap_handle_event_lkd(coap_context_t *context, coap_event_t event,
                      coap_session_t *session) {
  int ret = 0;

  coap_log_debug("***EVENT: %s\n", coap_event_name(event));

  if (context->handle_event) {
    coap_lock_callback_ret(ret, context, context->handle_event(session, event));
#if COAP_PROXY_SUPPORT
    if (event == COAP_EVENT_SERVER_SESSION_DEL)
      coap_proxy_remove_association(session, 0);
#endif /* COAP_PROXY_SUPPORT */
#if COAP_CLIENT_SUPPORT
    switch (event) {
    case COAP_EVENT_DTLS_CLOSED:
    case COAP_EVENT_TCP_CLOSED:
    case COAP_EVENT_SESSION_CLOSED:
    case COAP_EVENT_OSCORE_DECRYPTION_FAILURE:
    case COAP_EVENT_OSCORE_NOT_ENABLED:
    case COAP_EVENT_OSCORE_NO_PROTECTED_PAYLOAD:
    case COAP_EVENT_OSCORE_NO_SECURITY:
    case COAP_EVENT_OSCORE_INTERNAL_ERROR:
    case COAP_EVENT_OSCORE_DECODE_ERROR:
    case COAP_EVENT_WS_PACKET_SIZE:
    case COAP_EVENT_WS_CLOSED:
    case COAP_EVENT_BAD_PACKET:
      /* Those that are deemed fatal to end sending a request */
      session->doing_send_recv = 0;
      break;
    case COAP_EVENT_DTLS_CONNECTED:
    case COAP_EVENT_DTLS_RENEGOTIATE:
    case COAP_EVENT_DTLS_ERROR:
    case COAP_EVENT_TCP_CONNECTED:
    case COAP_EVENT_TCP_FAILED:
    case COAP_EVENT_SESSION_CONNECTED:
    case COAP_EVENT_SESSION_FAILED:
    case COAP_EVENT_PARTIAL_BLOCK:
    case COAP_EVENT_XMIT_BLOCK_FAIL:
    case COAP_EVENT_SERVER_SESSION_NEW:
    case COAP_EVENT_SERVER_SESSION_DEL:
    case COAP_EVENT_MSG_RETRANSMITTED:
    case COAP_EVENT_WS_CONNECTED:
    case COAP_EVENT_KEEPALIVE_FAILURE:
    default:
      break;
    }
#endif /* COAP_CLIENT_SUPPORT */
  }
  return ret;
}

COAP_API int
coap_can_exit(coap_context_t *context) {
  int ret;

  coap_lock_lock(context, return 0);
  ret = coap_can_exit_lkd(context);
  coap_lock_unlock(context);
  return ret;
}

int
coap_can_exit_lkd(coap_context_t *context) {
  coap_session_t *s, *rtmp;
  if (!context)
    return 1;
  coap_lock_check_locked(context);
  if (context->sendqueue)
    return 0;
#if COAP_SERVER_SUPPORT
  coap_endpoint_t *ep;

  LL_FOREACH(context->endpoint, ep) {
    SESSIONS_ITER(ep->sessions, s, rtmp) {
      if (s->delayqueue)
        return 0;
      if (s->lg_xmit)
        return 0;
    }
  }
#endif /* COAP_SERVER_SUPPORT */
#if COAP_CLIENT_SUPPORT
  SESSIONS_ITER(context->sessions, s, rtmp) {
    if (s->delayqueue)
      return 0;
    if (s->lg_xmit)
      return 0;
  }
#endif /* COAP_CLIENT_SUPPORT */
  return 1;
}
#if COAP_SERVER_SUPPORT
#if COAP_ASYNC_SUPPORT
coap_tick_t
coap_check_async(coap_context_t *context, coap_tick_t now) {
  coap_tick_t next_due = 0;
  coap_async_t *async, *tmp;

  LL_FOREACH_SAFE(context->async_state, async, tmp) {
    if (async->delay != 0 && async->delay <= now) {
      /* Send off the request to the application */
      handle_request(context, async->session, async->pdu);

      /* Remove this async entry as it has now fired */
      coap_free_async_lkd(async->session, async);
    } else {
      if (next_due == 0 || next_due > async->delay - now)
        next_due = async->delay - now;
    }
  }
  return next_due;
}
#endif /* COAP_ASYNC_SUPPORT */
#endif /* COAP_SERVER_SUPPORT */

int coap_started = 0;

#if COAP_THREAD_SAFE
/*
 * Global lock for multi-thread support
 */
coap_lock_t global_lock;
#endif /* COAP_THREAD_SAFE */

void
coap_startup(void) {
  coap_tick_t now;
#ifndef WITH_CONTIKI
  uint64_t us;
#endif /* !WITH_CONTIKI */

  if (coap_started)
    return;
  coap_started = 1;

#if COAP_THREAD_SAFE
  coap_lock_init();
#endif /* COAP_THREAD_SAFE */

#if defined(HAVE_WINSOCK2_H)
  WORD wVersionRequested = MAKEWORD(2, 2);
  WSADATA wsaData;
  WSAStartup(wVersionRequested, &wsaData);
#endif
  coap_clock_init();
  coap_ticks(&now);
#ifndef WITH_CONTIKI
  us = coap_ticks_to_rt_us(now);
  /* Be accurate to the nearest (approx) us */
  coap_prng_init_lkd((unsigned int)us);
#else /* WITH_CONTIKI */
  coap_start_io_process();
#endif /* WITH_CONTIKI */
  coap_memory_init();
  coap_dtls_startup();
#ifdef WITH_LWIP
  coap_io_lwip_init();
#endif /* WITH_LWIP */
#if COAP_SERVER_SUPPORT
  static coap_str_const_t well_known = { sizeof(".well-known/core")-1,
                                         (const uint8_t *)".well-known/core"
                                       };
  memset(&resource_uri_wellknown, 0, sizeof(resource_uri_wellknown));
  resource_uri_wellknown.handler[COAP_REQUEST_GET-1] = hnd_get_wellknown_lkd;
  resource_uri_wellknown.flags = COAP_RESOURCE_FLAGS_HAS_MCAST_SUPPORT;
  resource_uri_wellknown.uri_path = &well_known;
#endif /* COAP_SERVER_SUPPORT */
  send_recv_terminate = 0;
}

void
coap_cleanup(void) {
  if (!coap_started)
    return;
  coap_started = 0;
#if defined(HAVE_WINSOCK2_H)
  WSACleanup();
#elif defined(WITH_CONTIKI)
  coap_stop_io_process();
#endif
#ifdef WITH_LWIP
  coap_io_lwip_cleanup();
#endif /* WITH_LWIP */
  coap_dtls_shutdown();

  coap_debug_reset();
}

void
coap_register_response_handler(coap_context_t *context,
                               coap_response_handler_t handler) {
#if COAP_CLIENT_SUPPORT
  context->response_handler = handler;
#else /* ! COAP_CLIENT_SUPPORT */
  (void)context;
  (void)handler;
#endif /* COAP_CLIENT_SUPPORT */
}

void
coap_register_nack_handler(coap_context_t *context,
                           coap_nack_handler_t handler) {
  context->nack_handler = handler;
}

void
coap_register_ping_handler(coap_context_t *context,
                           coap_ping_handler_t handler) {
  context->ping_handler = handler;
}

void
coap_register_pong_handler(coap_context_t *context,
                           coap_pong_handler_t handler) {
  context->pong_handler = handler;
}

COAP_API void
coap_register_option(coap_context_t *ctx, uint16_t type) {
  coap_lock_lock(ctx, return);
  coap_register_option_lkd(ctx, type);
  coap_lock_unlock(ctx);
}

void
coap_register_option_lkd(coap_context_t *ctx, uint16_t type) {
  coap_option_filter_set(&ctx->known_options, type);
}

#if ! defined WITH_CONTIKI && ! defined WITH_LWIP && ! defined RIOT_VERSION
#if COAP_SERVER_SUPPORT
COAP_API int
coap_join_mcast_group_intf(coap_context_t *ctx, const char *group_name,
                           const char *ifname) {
  int ret;

  coap_lock_lock(ctx, return -1);
  ret = coap_join_mcast_group_intf_lkd(ctx, group_name, ifname);
  coap_lock_unlock(ctx);
  return ret;
}

int
coap_join_mcast_group_intf_lkd(coap_context_t *ctx, const char *group_name,
                               const char *ifname) {
#if COAP_IPV4_SUPPORT
  struct ip_mreq mreq4;
#endif /* COAP_IPV4_SUPPORT */
#if COAP_IPV6_SUPPORT
  struct ipv6_mreq mreq6;
#endif /* COAP_IPV6_SUPPORT */
  struct addrinfo *resmulti = NULL, hints, *ainfo;
  int result = -1;
  coap_endpoint_t *endpoint;
  int mgroup_setup = 0;

  /* Need to have at least one endpoint! */
  assert(ctx->endpoint);
  if (!ctx->endpoint)
    return -1;

  /* Default is let the kernel choose */
#if COAP_IPV6_SUPPORT
  mreq6.ipv6mr_interface = 0;
#endif /* COAP_IPV6_SUPPORT */
#if COAP_IPV4_SUPPORT
  mreq4.imr_interface.s_addr = INADDR_ANY;
#endif /* COAP_IPV4_SUPPORT */

  memset(&hints, 0, sizeof(hints));
  hints.ai_socktype = SOCK_DGRAM;

  /* resolve the multicast group address */
  result = getaddrinfo(group_name, NULL, &hints, &resmulti);

  if (result != 0) {
    coap_log_err("coap_join_mcast_group_intf: %s: "
                 "Cannot resolve multicast address: %s\n",
                 group_name, gai_strerror(result));
    goto finish;
  }

  /* Need to do a windows equivalent at some point */
#ifndef _WIN32
  if (ifname) {
    /* interface specified - check if we have correct IPv4/IPv6 information */
    int done_ip4 = 0;
    int done_ip6 = 0;
#if defined(ESPIDF_VERSION)
    struct netif *netif;
#else /* !ESPIDF_VERSION */
#if COAP_IPV4_SUPPORT
    int ip4fd;
#endif /* COAP_IPV4_SUPPORT */
    struct ifreq ifr;
#endif /* !ESPIDF_VERSION */

    /* See which mcast address family types are being asked for */
    for (ainfo = resmulti; ainfo != NULL && !(done_ip4 == 1 && done_ip6 == 1);
         ainfo = ainfo->ai_next) {
      switch (ainfo->ai_family) {
#if COAP_IPV6_SUPPORT
      case AF_INET6:
        if (done_ip6)
          break;
        done_ip6 = 1;
#if defined(ESPIDF_VERSION)
        netif = netif_find(ifname);
        if (netif)
          mreq6.ipv6mr_interface = netif_get_index(netif);
        else
          coap_log_err("coap_join_mcast_group_intf: %s: "
                       "Cannot get IPv4 address: %s\n",
                       ifname, coap_socket_strerror());
#else /* !ESPIDF_VERSION */
        memset(&ifr, 0, sizeof(ifr));
        strncpy(ifr.ifr_name, ifname, IFNAMSIZ - 1);
        ifr.ifr_name[IFNAMSIZ - 1] = '\000';

#ifdef HAVE_IF_NAMETOINDEX
        mreq6.ipv6mr_interface = if_nametoindex(ifr.ifr_name);
        if (mreq6.ipv6mr_interface == 0) {
          coap_log_warn("coap_join_mcast_group_intf: "
                        "cannot get interface index for '%s'\n",
                        ifname);
        }
#elif defined(__QNXNTO__)
#else /* !HAVE_IF_NAMETOINDEX */
        result = ioctl(ctx->endpoint->sock.fd, SIOCGIFINDEX, &ifr);
        if (result != 0) {
          coap_log_warn("coap_join_mcast_group_intf: "
                        "cannot get interface index for '%s': %s\n",
                        ifname, coap_socket_strerror());
        } else {
          /* Capture the IPv6 if_index for later */
          mreq6.ipv6mr_interface = ifr.ifr_ifindex;
        }
#endif /* !HAVE_IF_NAMETOINDEX */
#endif /* !ESPIDF_VERSION */
#endif /* COAP_IPV6_SUPPORT */
        break;
#if COAP_IPV4_SUPPORT
      case AF_INET:
        if (done_ip4)
          break;
        done_ip4 = 1;
#if defined(ESPIDF_VERSION)
        netif = netif_find(ifname);
        if (netif)
          mreq4.imr_interface.s_addr = netif_ip4_addr(netif)->addr;
        else
          coap_log_err("coap_join_mcast_group_intf: %s: "
                       "Cannot get IPv4 address: %s\n",
                       ifname, coap_socket_strerror());
#else /* !ESPIDF_VERSION */
        /*
         * Need an AF_INET socket to do this unfortunately to stop
         * "Invalid argument" error if AF_INET6 socket is used for SIOCGIFADDR
         */
        ip4fd = socket(AF_INET, SOCK_DGRAM, 0);
        if (ip4fd == -1) {
          coap_log_err("coap_join_mcast_group_intf: %s: socket: %s\n",
                       ifname, coap_socket_strerror());
          continue;
        }
        memset(&ifr, 0, sizeof(ifr));
        strncpy(ifr.ifr_name, ifname, IFNAMSIZ - 1);
        ifr.ifr_name[IFNAMSIZ - 1] = '\000';
        result = ioctl(ip4fd, SIOCGIFADDR, &ifr);
        if (result != 0) {
          coap_log_err("coap_join_mcast_group_intf: %s: "
                       "Cannot get IPv4 address: %s\n",
                       ifname, coap_socket_strerror());
        } else {
          /* Capture the IPv4 address for later */
          mreq4.imr_interface = ((struct sockaddr_in *)&ifr.ifr_addr)->sin_addr;
        }
        close(ip4fd);
#endif /* !ESPIDF_VERSION */
        break;
#endif /* COAP_IPV4_SUPPORT */
      default:
        break;
      }
    }
  }
#else /* _WIN32 */
  /*
   * On Windows this function ignores the ifname variable so we unset this
   * variable on this platform in any case in order to enable the interface
   * selection from the bind address below.
   */
  ifname = 0;
#endif /* _WIN32 */

  /* Add in mcast address(es) to appropriate interface */
  for (ainfo = resmulti; ainfo != NULL; ainfo = ainfo->ai_next) {
    LL_FOREACH(ctx->endpoint, endpoint) {
      /* Only UDP currently supported */
      if (endpoint->proto == COAP_PROTO_UDP) {
        coap_address_t gaddr;

        coap_address_init(&gaddr);
#if COAP_IPV6_SUPPORT
        if (ainfo->ai_family == AF_INET6) {
          if (!ifname) {
            if (endpoint->bind_addr.addr.sa.sa_family == AF_INET6) {
              /*
               * Do it on the ifindex that the server is listening on
               * (sin6_scope_id could still be 0)
               */
              mreq6.ipv6mr_interface =
                  endpoint->bind_addr.addr.sin6.sin6_scope_id;
            } else {
              mreq6.ipv6mr_interface = 0;
            }
          }
          gaddr.addr.sin6.sin6_family = AF_INET6;
          gaddr.addr.sin6.sin6_port = endpoint->bind_addr.addr.sin6.sin6_port;
          gaddr.addr.sin6.sin6_addr = mreq6.ipv6mr_multiaddr =
                                          ((struct sockaddr_in6 *)ainfo->ai_addr)->sin6_addr;
          result = setsockopt(endpoint->sock.fd, IPPROTO_IPV6, IPV6_JOIN_GROUP,
                              (char *)&mreq6, sizeof(mreq6));
        }
#endif /* COAP_IPV6_SUPPORT */
#if COAP_IPV4_SUPPORT && COAP_IPV6_SUPPORT
        else
#endif /* COAP_IPV4_SUPPORT && COAP_IPV6_SUPPORT */
#if COAP_IPV4_SUPPORT
          if (ainfo->ai_family == AF_INET) {
            if (!ifname) {
              if (endpoint->bind_addr.addr.sa.sa_family == AF_INET) {
                /*
                 * Do it on the interface that the server is listening on
                 * (sin_addr could still be INADDR_ANY)
                 */
                mreq4.imr_interface = endpoint->bind_addr.addr.sin.sin_addr;
              } else {
                mreq4.imr_interface.s_addr = INADDR_ANY;
              }
            }
            gaddr.addr.sin.sin_family = AF_INET;
            gaddr.addr.sin.sin_port = endpoint->bind_addr.addr.sin.sin_port;
            gaddr.addr.sin.sin_addr.s_addr = mreq4.imr_multiaddr.s_addr =
                                                 ((struct sockaddr_in *)ainfo->ai_addr)->sin_addr.s_addr;
            result = setsockopt(endpoint->sock.fd, IPPROTO_IP, IP_ADD_MEMBERSHIP,
                                (char *)&mreq4, sizeof(mreq4));
          }
#endif /* COAP_IPV4_SUPPORT */
          else {
            continue;
          }

        if (result == COAP_SOCKET_ERROR) {
          coap_log_err("coap_join_mcast_group_intf: %s: setsockopt: %s\n",
                       group_name, coap_socket_strerror());
        } else {
          char addr_str[INET6_ADDRSTRLEN + 8 + 1];

          addr_str[sizeof(addr_str)-1] = '\000';
          if (coap_print_addr(&gaddr, (uint8_t *)addr_str,
                              sizeof(addr_str) - 1)) {
            if (ifname)
              coap_log_debug("added mcast group %s i/f %s\n", addr_str,
                             ifname);
            else
              coap_log_debug("added mcast group %s\n", addr_str);
          }
          mgroup_setup = 1;
        }
      }
    }
  }
  if (!mgroup_setup) {
    result = -1;
  }

finish:
  freeaddrinfo(resmulti);

  return result;
}

void
coap_mcast_per_resource(coap_context_t *context) {
  context->mcast_per_resource = 1;
}

#endif /* ! COAP_SERVER_SUPPORT */

#if COAP_CLIENT_SUPPORT
int
coap_mcast_set_hops(coap_session_t *session, size_t hops) {
  if (session && coap_is_mcast(&session->addr_info.remote)) {
    switch (session->addr_info.remote.addr.sa.sa_family) {
#if COAP_IPV4_SUPPORT
    case AF_INET:
      if (setsockopt(session->sock.fd, IPPROTO_IP, IP_MULTICAST_TTL,
                     (const char *)&hops, sizeof(hops)) < 0) {
        coap_log_info("coap_mcast_set_hops: %zu: setsockopt: %s\n",
                      hops, coap_socket_strerror());
        return 0;
      }
      return 1;
#endif /* COAP_IPV4_SUPPORT */
#if COAP_IPV6_SUPPORT
    case AF_INET6:
      if (setsockopt(session->sock.fd, IPPROTO_IPV6, IPV6_MULTICAST_HOPS,
                     (const char *)&hops, sizeof(hops)) < 0) {
        coap_log_info("coap_mcast_set_hops: %zu: setsockopt: %s\n",
                      hops, coap_socket_strerror());
        return 0;
      }
      return 1;
#endif /* COAP_IPV6_SUPPORT */
    default:
      break;
    }
  }
  return 0;
}
#endif /* COAP_CLIENT_SUPPORT */

#else /* defined WITH_CONTIKI || defined WITH_LWIP */
COAP_API int
coap_join_mcast_group_intf(coap_context_t *ctx COAP_UNUSED,
                           const char *group_name COAP_UNUSED,
                           const char *ifname COAP_UNUSED) {
  return -1;
}

int
coap_mcast_set_hops(coap_session_t *session COAP_UNUSED,
                    size_t hops COAP_UNUSED) {
  return 0;
}

void
coap_mcast_per_resource(coap_context_t *context COAP_UNUSED) {
}
#endif /* defined WITH_CONTIKI || defined WITH_LWIP */