Disable dtls fragmentation for ClientHello messages
Set the handshake mtu to unlimited when encountering a ClienHello message and
reset it to its previous value after writing the record.
@@ -212,7 +212,10 @@ static int ssl_double_retransmit_timeout( mbedtls_ssl_context *ssl )
* delivered) of any compliant IPv4 (and IPv6) network, and should work
* on most non-IP stacks too. */
if( ssl->handshake->retransmit_timeout != ssl->conf->hs_timeout_min )
+ {
ssl->handshake->mtu = 508;
+ MBEDTLS_SSL_DEBUG_MSG( 2, ( "mtu autoreduction to %d bytes", ssl->handshake->mtu ) );
+ }
new_timeout = 2 * ssl->handshake->retransmit_timeout;
@@ -2945,6 +2948,7 @@ int mbedtls_ssl_resend( mbedtls_ssl_context *ssl )
int mbedtls_ssl_flight_transmit( mbedtls_ssl_context *ssl )
{
int ret;
+ uint16_t mtu_temp = 0;
MBEDTLS_SSL_DEBUG_MSG( 2, ( "=> mbedtls_ssl_flight_transmit" ) );
if( ssl->handshake->retransmit_state != MBEDTLS_SSL_RETRANS_SENDING )
@@ -2979,6 +2983,15 @@ int mbedtls_ssl_flight_transmit( mbedtls_ssl_context *ssl )
ssl_swap_epochs( ssl );
}
+ /* Disable handshake mtu for client hello message to avoid fragmentation.
+ * Setting it back after calling mbedtls_ssl_write_record */
+ if( ssl->out_msg[0] == MBEDTLS_SSL_HS_CLIENT_HELLO )
+ {
+ mtu_temp = ssl->handshake->mtu;
+ ssl->handshake->mtu = 0;
+ MBEDTLS_SSL_DEBUG_MSG( 2, ( "disabling fragmentation of ClientHello message" ) );
+ }
+
ret = ssl_get_remaining_payload_in_datagram( ssl );
if( ret < 0 )
return( ret );
@@ -3077,6 +3090,12 @@ int mbedtls_ssl_flight_transmit( mbedtls_ssl_context *ssl )
MBEDTLS_SSL_DEBUG_RET( 1, "mbedtls_ssl_write_record", ret );
return( ret );
}
+
+ if( mtu_temp != 0 )
+ {
+ ssl->handshake->mtu = mtu_temp;
+ mtu_temp = 0;
+ }
}
if( ( ret = mbedtls_ssl_flush_output( ssl ) ) != 0 )
