diff --git a/include/coap2/coap_session.h b/include/coap2/coap_session.h
index 039965e3..afb37bef 100644
--- a/include/coap2/coap_session.h
+++ b/include/coap2/coap_session.h
@@ -63,7 +63,7 @@ typedef struct coap_session_t {
   coap_session_state_t state;       /**< current state of relationaship with peer */
   unsigned ref;                     /**< reference count from queues */
   unsigned tls_overhead;            /**< overhead of TLS layer */
-  unsigned mtu;                     /**< path or CSM mtu */
+  uint64_t mtu;                     /**< path or CSM mtu */
   coap_address_t local_if;          /**< optional local interface address */
   UT_hash_handle hh;
   coap_addr_tuple_t addr_info;      /**< key: remote/local address info */
@@ -74,6 +74,7 @@ typedef struct coap_session_t {
   void *tls;                        /**< security parameters */
   uint16_t tx_mid;                  /**< the last message id that was used in this session */
   uint8_t con_active;               /**< Active CON request sent */
+  uint8_t csm_block_supported;      /**< CSM TCP blocks supported */
   coap_tid_t last_ping_mid;         /**< the last keepalive message id that was used in this session */
   struct coap_queue_t *delayqueue;  /**< list of delayed messages waiting to be sent */
   size_t partial_write;             /**< if > 0 indicates number of bytes already written from the pdu at the head of sendqueue */
diff --git a/include/coap2/pdu.h b/include/coap2/pdu.h
index 6b4d9aef..2487bc31 100644
--- a/include/coap2/pdu.h
+++ b/include/coap2/pdu.h
@@ -45,10 +45,10 @@ struct coap_session_t;
 
 #ifndef COAP_DEFAULT_MAX_PDU_RX_SIZE
 #if defined(WITH_CONTIKI) || defined(WITH_LWIP)
-#define COAP_DEFAULT_MAX_PDU_RX_SIZE (COAP_MAX_MESSAGE_SIZE_TCP16+4)
+#define COAP_DEFAULT_MAX_PDU_RX_SIZE (COAP_MAX_MESSAGE_SIZE_TCP16+4UL)
 #else
 /* 8 MiB max-message-size plus some space for options */
-#define COAP_DEFAULT_MAX_PDU_RX_SIZE (8*1024*1024+256)
+#define COAP_DEFAULT_MAX_PDU_RX_SIZE (8UL*1024*1024+256)
 #endif
 #endif /* COAP_DEFAULT_MAX_PDU_RX_SIZE */
 
diff --git a/src/coap_session.c b/src/coap_session.c
index 61047485..a5bfcf7d 100644
--- a/src/coap_session.c
+++ b/src/coap_session.c
@@ -314,11 +314,14 @@ void coap_session_send_csm(coap_session_t *session) {
   session->partial_write = 0;
   if (session->mtu == 0)
     session->mtu = COAP_DEFAULT_MTU;  /* base value */
-  pdu = coap_pdu_init(COAP_MESSAGE_CON, COAP_SIGNALING_CSM, 0, 16);
+  pdu = coap_pdu_init(COAP_MESSAGE_CON, COAP_SIGNALING_CSM, 0, 20);
   if ( pdu == NULL
     || coap_add_option(pdu, COAP_SIGNALING_OPTION_MAX_MESSAGE_SIZE,
          coap_encode_var_safe(buf, sizeof(buf),
                                 COAP_DEFAULT_MAX_PDU_RX_SIZE), buf) == 0
+    || coap_add_option(pdu, COAP_SIGNALING_OPTION_BLOCK_WISE_TRANSFER,
+         coap_encode_var_safe(buf, sizeof(buf),
+                                0), buf) == 0
     || coap_pdu_encode_header(pdu, session->proto) == 0
   ) {
     coap_session_disconnected(session, COAP_NACK_NOT_DELIVERABLE);
diff --git a/src/net.c b/src/net.c
index 458e4895..18c60a4a 100644
--- a/src/net.c
+++ b/src/net.c
@@ -1003,6 +1003,25 @@ coap_send(coap_session_t *session, coap_pdu_t *pdu) {
     goto error;
   }
 
+#if !COAP_DISABLE_TCP
+  if (COAP_PROTO_RELIABLE(session->proto) && !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://tools.ietf.org/html/rfc8323#section-5.3.2
+     * Note that this also includes BERT which is application specific.
+     */
+    coap_opt_iterator_t opt_iter;
+
+    if (coap_check_option(pdu, COAP_OPTION_BLOCK1, &opt_iter) != NULL) {
+      goto error;
+    }
+    if (coap_check_option(pdu, COAP_OPTION_BLOCK2, &opt_iter) != NULL) {
+      goto error;
+    }
+  }
+#endif /* !COAP_DISABLE_TCP */
+
   bytes_written = coap_send_pdu( session, pdu, NULL );
 
   if (bytes_written == COAP_PDU_DELAYED) {
@@ -1337,6 +1356,14 @@ coap_read_session(coap_context_t *ctx, coap_session_t *session, coap_tick_t now)
           if (n == len) {
             size_t size = coap_pdu_parse_size(session->proto, session->read_header,
               hdr_size);
+            if (size > COAP_DEFAULT_MAX_PDU_RX_SIZE) {
+              coap_log(LOG_WARNING,
+                       "** %s: incoming PDU length too large (%zu > %lu)\n",
+                       coap_session_str(session),
+                       size, COAP_DEFAULT_MAX_PDU_RX_SIZE);
+              bytes_read = -1;
+              break;
+            }
             session->partial_pdu = coap_pdu_init(0, 0, 0, size);
             if (session->partial_pdu == NULL) {
               bytes_read = -1;
@@ -2395,7 +2422,7 @@ handle_signaling(coap_context_t *context, coap_session_t *session,
         coap_session_set_mtu(session, coap_decode_var_bytes(coap_opt_value(option),
           coap_opt_length(option)));
       } else if (opt_iter.type == COAP_SIGNALING_OPTION_BLOCK_WISE_TRANSFER) {
-        /* ... */
+        session->csm_block_supported = 1;
       }
     }
     if (session->state == COAP_SESSION_STATE_CSM)