Implement support for MTU setting

include/mbedtls/ssl.h

@@ -2430,6 +2430,7 @@ void mbedtls_ssl_conf_cert_req_ca_list( mbedtls_ssl_config *conf,
 #if defined(MBEDTLS_SSL_PROTO_DTLS)
 /**
  * \brief          Set the Maximum Tranport Unit (MTU).
+ *                 Special value: 0 means unset (no limit).
  *                 This represents the maximum size of a datagram payload
  *                 handled by the transport layer (usually UDP) as determined
  *                 by the network link and stack. In practice, this controls
@@ -2446,7 +2447,8 @@ void mbedtls_ssl_conf_cert_req_ca_list( mbedtls_ssl_config *conf,
  *
  * \note           Values larger than \c MBEDTLS_SSL_OUT_CONTENT_LEN have no
  *                 effect. This can only be used to decrease the maximum size
- *                 of detagrams sent.
+ *                 of datagrams sent. Values lower than record layer expansion
+ *                 are ignored.
  *
  * \param conf     SSL configuration
  * \param mtu      Value of the path MTU in bytes
@@ -2738,6 +2740,9 @@ const char *mbedtls_ssl_get_version( const mbedtls_ssl_context *ssl );
  * \brief          Return the (maximum) number of bytes added by the record
  *                 layer: header + encryption/MAC overhead (inc. padding)
  *
+ * \note           This function is not available (always returns an error)
+ *                 when record compression is enabled.
+ *
  * \param ssl      SSL context
  *
  * \return         Current maximum record expansion in bytes, or
@@ -2752,12 +2757,8 @@ int mbedtls_ssl_get_record_expansion( const mbedtls_ssl_context *ssl );
  *                 This is the value negotiated with peer if any,
  *                 or the locally configured value.
  *
- * \note           With DTLS, \c mbedtls_ssl_write() will return an error if
+ * \sa             mbedtls_ssl_conf_max_frag_len()
- *                 called with a larger length value.
+ * \sa             mbedtls_ssl_get_max_record_payload()
- *                 With TLS, \c mbedtls_ssl_write() will fragment the input if
- *                 necessary and return the number of bytes written; it is up
- *                 to the caller to call \c mbedtls_ssl_write() again in
- *                 order to send the remaining bytes if any.
  *
  * \param ssl      SSL context
  *
@@ -2766,6 +2767,34 @@ int mbedtls_ssl_get_record_expansion( const mbedtls_ssl_context *ssl );
 size_t mbedtls_ssl_get_max_frag_len( const mbedtls_ssl_context *ssl );
 #endif /* MBEDTLS_SSL_MAX_FRAGMENT_LENGTH */
 
+/**
+ * \brief          Return the current maximum outgoing record payload in bytes.
+ *                 This takes into account the config.h setting \c
+ *                 MBEDTLS_SSL_OUT_CONTENT_LEN, the configured and negotiated
+ *                 max fragment length extension if used, and for DTLS the
+ *                 path MTU as configured and current record expansion.
+ *
+ * \note           With DTLS, \c mbedtls_ssl_write() will return an error if
+ *                 called with a larger length value.
+ *                 With TLS, \c mbedtls_ssl_write() will fragment the input if
+ *                 necessary and return the number of bytes written; it is up
+ *                 to the caller to call \c mbedtls_ssl_write() again in
+ *                 order to send the remaining bytes if any.
+ *
+ * \note           This function is not available (always returns an error)
+ *                 when record compression is enabled.
+ *
+ * \sa             mbedtls_ssl_conf_mtu()
+ * \sa             mbedtls_ssl_get_max_frag_len()
+ * \sa             mbedtls_ssl_get_record_expansion()
+ *
+ * \param ssl      SSL context
+ *
+ * \return         Current maximum payload for an outgoing record,
+ *                 or a negative error code.
+ */
+int mbedtls_ssl_get_max_out_record_payload( const mbedtls_ssl_context *ssl );
+
 #if defined(MBEDTLS_X509_CRT_PARSE_C)
 /**
  * \brief          Return the peer certificate from the current connection

library/ssl_tls.c

@@ -2845,16 +2845,20 @@ int mbedtls_ssl_resend( mbedtls_ssl_context *ssl )
  */
 int mbedtls_ssl_flight_transmit( mbedtls_ssl_context *ssl )
 {
-#if defined(MBEDTLS_SSL_MAX_FRAGMENT_LENGTH)
+    const int ret_payload = mbedtls_ssl_get_max_out_record_payload( ssl );
-    const size_t max_record_content_len = mbedtls_ssl_get_max_frag_len( ssl );
+    const size_t max_record_payload = (size_t) ret_payload;
-#else
-    const size_t max_record_content_len = MBEDTLS_SSL_OUT_CONTENT_LEN;
-#endif
     /* DTLS handshake headers are 12 bytes */
-    const size_t max_hs_fragment_len = max_record_content_len - 12;
+    const size_t max_hs_fragment_len = max_record_payload - 12;
 
     MBEDTLS_SSL_DEBUG_MSG( 2, ( "=> mbedtls_ssl_flight_transmit" ) );
 
+    if( ret_payload < 0 )
+    {
+        MBEDTLS_SSL_DEBUG_RET( 1, "mbedtls_ssl_get_max_out_record_payload",
+                                  ret_payload );
+        return( ret_payload );
+    }
+
     if( ssl->handshake->retransmit_state != MBEDTLS_SSL_RETRANS_SENDING )
     {
         MBEDTLS_SSL_DEBUG_MSG( 2, ( "initialise fligh transmission" ) );
@@ -7008,6 +7012,7 @@ int mbedtls_ssl_get_record_expansion( const mbedtls_ssl_context *ssl )
 #if defined(MBEDTLS_ZLIB_SUPPORT)
     if( ssl->session_out->compression != MBEDTLS_SSL_COMPRESS_NULL )
         return( MBEDTLS_ERR_SSL_FEATURE_UNAVAILABLE );
+    }
 #endif
 
     switch( mbedtls_cipher_get_cipher_mode( &transform->cipher_ctx_enc ) )
@@ -7055,10 +7060,45 @@ size_t mbedtls_ssl_get_max_frag_len( const mbedtls_ssl_context *ssl )
         max_len = ssl_mfl_code_to_length( ssl->session_negotiate->mfl_code );
     }
 
-    return max_len;
+    return( max_len );
 }
 #endif /* MBEDTLS_SSL_MAX_FRAGMENT_LENGTH */
 
+int mbedtls_ssl_get_max_out_record_payload( const mbedtls_ssl_context *ssl )
+{
+    size_t max_len = MBEDTLS_SSL_OUT_CONTENT_LEN;
+
+#if defined(MBEDTLS_SSL_MAX_FRAGMENT_LENGTH)
+    const size_t mfl = mbedtls_ssl_get_max_frag_len( ssl );
+
+    if( max_len > mfl )
+        max_len = mfl;
+#endif
+
+#if defined(MBEDTLS_SSL_PROTO_DTLS)
+    if( ssl->conf->mtu != 0 )
+    {
+        const size_t mtu = ssl->conf->mtu;
+        const int ret = mbedtls_ssl_get_record_expansion( ssl );
+        const size_t overhead = (size_t) ret;
+
+        if( ret < 0 )
+            return( ret );
+
+        if( mtu <= overhead )
+        {
+            MBEDTLS_SSL_DEBUG_MSG( 1, ( "MTU too low for record expansion" ) );
+            return( MBEDTLS_ERR_SSL_FEATURE_UNAVAILABLE );
+        }
+
+        if( max_len > mtu - overhead )
+            max_len = mtu - overhead;
+    }
+#endif
+
+    return( (int) max_len );
+}
+
 #if defined(MBEDTLS_X509_CRT_PARSE_C)
 const mbedtls_x509_crt *mbedtls_ssl_get_peer_cert( const mbedtls_ssl_context *ssl )
 {
@@ -7610,12 +7650,15 @@ int mbedtls_ssl_read( mbedtls_ssl_context *ssl, unsigned char *buf, size_t len )
 static int ssl_write_real( mbedtls_ssl_context *ssl,
                            const unsigned char *buf, size_t len )
 {
-    int ret;
+    int ret = mbedtls_ssl_get_max_out_record_payload( ssl );
-#if defined(MBEDTLS_SSL_MAX_FRAGMENT_LENGTH)
+    const size_t max_len = (size_t) ret;
-    size_t max_len = mbedtls_ssl_get_max_frag_len( ssl );
+
-#else
+    if( ret < 0 )
-    size_t max_len = MBEDTLS_SSL_OUT_CONTENT_LEN;
+    {
-#endif /* MBEDTLS_SSL_MAX_FRAGMENT_LENGTH */
+        MBEDTLS_SSL_DEBUG_RET( 1, "mbedtls_ssl_get_max_out_record_payload", ret );
+        return( ret );
+    }
+
     if( len > max_len )
     {
 #if defined(MBEDTLS_SSL_PROTO_DTLS)