diff --git a/examples/contiki/coap-observer.c b/examples/contiki/coap-observer.c
index b71cab4c..b399f84c 100644
--- a/examples/contiki/coap-observer.c
+++ b/examples/contiki/coap-observer.c
@@ -133,7 +133,7 @@ PROCESS_THREAD(coap_server_process, ev, data)
 
   request = coap_pdu_init(COAP_MESSAGE_CON, COAP_REQUEST_GET, 
 			  coap_new_message_id(coap_context), 
-			  COAP_PDU_SIZE_DYNAMIC);
+			  COAP_DEFAULT_MTU);
   
   coap_split_uri((unsigned char *)resource, strlen(resource), &uri);
 
diff --git a/examples/tiny.c b/examples/tiny.c
index 5a196a64..0f84b7a5 100644
--- a/examples/tiny.c
+++ b/examples/tiny.c
@@ -29,7 +29,7 @@ make_pdu( unsigned int value ) {
   static unsigned char buf[20];
   int len, ls;
 
-  if (!(pdu = coap_pdu_init(0, 0, 0, COAP_PDU_SIZE_DYNAMIC)))
+  if (!(pdu = coap_pdu_init(0, 0, 0, COAP_DEFAULT_MTU)))
     return NULL;
 
   pdu->type = COAP_MESSAGE_NON;
diff --git a/include/coap/pdu.h b/include/coap/pdu.h
index 06cb0b4f..30436ac6 100644
--- a/include/coap/pdu.h
+++ b/include/coap/pdu.h
@@ -32,8 +32,12 @@ struct coap_session_t;
 #define COAP_DEFAULT_MTU       1152
 #endif /* COAP_DEFAULT_MTU */
 #ifndef COAP_DEFAULT_MAX_PDU_RX_SIZE
+#if defined(WITH_CONTIKI) || defined(WITH_LWIP)
+#define COAP_DEFAULT_MAX_PDU_RX_SIZE 65804
+#else
 #define COAP_DEFAULT_MAX_PDU_RX_SIZE 8*1024*1024
-#endif /* COAP_DEFAULT_MTU */
+#endif
+#endif /* COAP_DEFAULT_MAX_PDU_RX_SIZE */
 
 #define COAP_DEFAULT_VERSION      1 /* version of CoAP supported */
 #define COAP_DEFAULT_SCHEME  "coap" /* the default scheme for CoAP URIs */
@@ -309,8 +313,6 @@ typedef uint8_t coap_proto_t;
 #define COAP_PROTO_TCP	  3
 #define COAP_PROTO_TLS	  4
 
-#define COAP_PDU_SIZE_DYNAMIC ((size_t)-1)
-
 /**
  * Creates a new CoAP PDU with at least enough storage space for the given
  * @p size maximum message size. The function returns a pointer to the
@@ -323,8 +325,6 @@ typedef uint8_t coap_proto_t;
  * @param code The message code.
  * @param tid  The transcation id to set or 0 if unknown / not applicable.
  * @param size The maximum allowed number of byte for the message.
- *             Use COAP_PDU_SIZE_DYNAMIC to get a variable size PDU that
- *             will grow automatically.
  * @return     A pointer to the new PDU object or @c NULL on error.
  */
 coap_pdu_t *
diff --git a/src/coap_session.c b/src/coap_session.c
index e29f5d15..42aa6a1d 100644
--- a/src/coap_session.c
+++ b/src/coap_session.c
@@ -133,13 +133,15 @@ void coap_session_free(coap_session_t *session) {
 }
 
 size_t coap_session_max_pdu_size(coap_session_t *session) {
-  if (session->mtu > 0)
-    return (size_t)(session->mtu - session->tls_overhead);
-  return COAP_PDU_SIZE_DYNAMIC;
+  return (size_t)(session->mtu - session->tls_overhead);
 }
 
 void coap_session_set_mtu(coap_session_t *session, unsigned mtu) {
-  session->mtu = (uint16_t)mtu;
+#if defined(WITH_CONTIKI) || defined(WITH_LWIP)
+  if (mtu >= 65805)
+    mtu = 65804;
+#endif
+  session->mtu = mtu;
   if (session->tls_overhead >= session->mtu) {
     session->tls_overhead = session->mtu;
     coap_log(LOG_ERR, "DTLS overhead exceeds MTU\n");
diff --git a/src/net.c b/src/net.c
index 05b32642..3cd9ed53 100644
--- a/src/net.c
+++ b/src/net.c
@@ -1942,8 +1942,8 @@ handle_signaling(coap_context_t *context, coap_session_t *session,
   if (pdu->code == COAP_SIGNALING_CSM) {
     while ((option = coap_option_next(&opt_iter))) {
       if (opt_iter.type == COAP_SIGNALING_OPTION_MAX_MESSAGE_SIZE) {
-	session->mtu = coap_decode_var_bytes(coap_opt_value(option),
-	  coap_opt_length(option));
+	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) {
         /* ... */
       }
diff --git a/src/pdu.c b/src/pdu.c
index a37c0592..67c5aefa 100644
--- a/src/pdu.c
+++ b/src/pdu.c
@@ -84,18 +84,13 @@ coap_pdu_init(uint8_t type, uint8_t code, uint16_t tid, size_t size) {
   coap_pdu_t *pdu;
   uint8_t *buf;
 
-#ifdef WITH_CONTIKI
-  if (size == COAP_PDU_SIZE_DYNAMIC)
-    size = COAP_RXBUFFER_SIZE - COAP_PDU_MAX_UDP_HEADER_SIZE;
-  assert(size <= COAP_RXBUFFER_SIZE - COAP_PDU_MAX_UDP_HEADER_SIZE);
-  if (size > COAP_RXBUFFER_SIZE - COAP_PDU_MAX_UDP_HEADER_SIZE)
-    return NULL;
-#endif
-
   pdu = coap_malloc_type(COAP_PDU, sizeof(coap_pdu_t));
   if (!pdu) return NULL;
 
 #if defined(WITH_CONTIKI) || defined(WITH_LWIP)
+  assert(size < 65805);
+  if (size >= 65805)
+    return NULL;
   pdu->max_hdr_size = COAP_PDU_MAX_UDP_HEADER_SIZE;
 #else
   pdu->max_hdr_size = COAP_PDU_MAX_TCP_HEADER_SIZE;
@@ -108,16 +103,8 @@ coap_pdu_init(uint8_t type, uint8_t code, uint16_t tid, size_t size) {
     pdu = NULL;
   }
   pdu->token = pdu->pbuf->payload + pdu->max_hdr_size;
-  if (size == COAP_PDU_SIZE_DYNAMIC)
-    size = pbuf->tot_len - pdu->max_hdr_size;
-  assert(size <= pbuf->tot_len - pdu->max_hdr_size);
-  if (size > pbuf->tot_len - pdu->max_hdr_size)
-    return NULL;
 #else /* WITH_LWIP */
-  if (size != COAP_PDU_SIZE_DYNAMIC)
-    pdu->alloc_size = size;
-  else
-    pdu->alloc_size = 256;
+  pdu->alloc_size = min(size, 256);
   buf = coap_malloc_type(COAP_PDU_BUF, pdu->alloc_size + pdu->max_hdr_size);
   if (buf == NULL) {
     coap_free_type(COAP_PDU, pdu);
@@ -192,6 +179,11 @@ coap_pdu_check_resize(coap_pdu_t *pdu, size_t size) {
     size_t new_size = max(256, pdu->alloc_size * 2);
     while (size > new_size)
       new_size *= 2;
+    if (pdu->max_size && new_size > pdu->max_size) {
+      new_size = pdu->max_size;
+      if (new_size < size)
+	return 0;
+    }
     if (!coap_pdu_resize(pdu, new_size))
       return 0;
   }
diff --git a/tests/test_error_response.c b/tests/test_error_response.c
index b21f0be3..dde9ffb2 100644
--- a/tests/test_error_response.c
+++ b/tests/test_error_response.c
@@ -321,7 +321,7 @@ t_error_response8(void) {
 
 static int
 t_error_response_tests_create(void) {
-  pdu = coap_pdu_init(0, 0, 0, COAP_PDU_SIZE_DYNAMIC);
+  pdu = coap_pdu_init(0, 0, 0, COAP_DEFAULT_MTU);
 
   return pdu == NULL;
 }
diff --git a/tests/test_pdu.c b/tests/test_pdu.c
index b887e6e4..a83851d2 100644
--- a/tests/test_pdu.c
+++ b/tests/test_pdu.c
@@ -618,7 +618,7 @@ t_encode_pdu11(void) {
 
 static int
 t_pdu_tests_create(void) {
-  pdu = coap_pdu_init(0, 0, 0, COAP_PDU_SIZE_DYNAMIC);
+  pdu = coap_pdu_init(0, 0, 0, COAP_DEFAULT_MTU);
 
   return pdu == NULL;
 }