commit 7fa9903bd081a65ad1fe942fb86a3fbca329bbb1
author Andrzej Kurek <andrzej.kurek@arm.com> Wed Oct 03 09:16:44 2018 -0400
committer Andrzej Kurek <andrzej.kurek@arm.com> Wed Oct 03 09:16:44 2018 -0400
tree 2e8171626b92eee519fbd11d765c9c6502e8441a
parent db9ad837c99b74817eb3e22e68c9b8c9c2ec43f5

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.

library/ssl_tls.c

diff --git a/library/ssl_tls.c b/library/ssl_tls.c
index 8bd74db..38826f9 100644
--- a/library/ssl_tls.c
+++ b/library/ssl_tls.c
@@ -212,7 +212,10 @@
      * 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_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 @@
             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 @@
             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 )