commit b8eec192f6c0150186c07ae1fbc2ea103cd38be1
author Manuel Pégourié-Gonnard <manuel.pegourie-gonnard@arm.com> Mon Aug 20 09:34:02 2018 +0200
committer Manuel Pégourié-Gonnard <manuel.pegourie-gonnard@arm.com> Wed Aug 22 10:50:30 2018 +0200
tree 48acefd79f8fea297e62efd72f736e2efefb4086
parent 68ae351dbec53e8e6b5eae3ff1392952055f1a2b

Implement PMTU auto-reduction in handshake

library/ssl_tls.c

diff --git a/library/ssl_tls.c b/library/ssl_tls.c
index faa9467..30c1a78 100644
--- a/library/ssl_tls.c
+++ b/library/ssl_tls.c
@@ -108,6 +108,15 @@
     if( ssl->handshake->retransmit_timeout >= ssl->conf->hs_timeout_max )
         return( -1 );
 
+    /* Implement the final paragraph of RFC 6347 section 4.1.1.1
+     * in the following way: after the initial transmission and a first
+     * retransmission, back off to a temporary estimated MTU of 508 bytes.
+     * This value is guaranteed to be deliverable (if not guaranteed to be
+     * 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;
+
     new_timeout = 2 * ssl->handshake->retransmit_timeout;
 
     /* Avoid arithmetic overflow and range overflow */
@@ -7088,6 +7097,20 @@
 }
 #endif /* MBEDTLS_SSL_MAX_FRAGMENT_LENGTH */
 
+#if defined(MBEDTLS_SSL_PROTO_DTLS)
+static size_t ssl_get_current_mtu( const mbedtls_ssl_context *ssl )
+{
+    if( ssl->handshake == NULL || ssl->handshake->mtu == 0 )
+        return( ssl->mtu );
+
+    if( ssl->mtu == 0 )
+        return( ssl->handshake->mtu );
+
+    return( ssl->mtu < ssl->handshake->mtu ?
+            ssl->mtu : ssl->handshake->mtu );
+}
+#endif /* MBEDTLS_SSL_PROTO_DTLS */
+
 int mbedtls_ssl_get_max_out_record_payload( const mbedtls_ssl_context *ssl )
 {
     size_t max_len = MBEDTLS_SSL_OUT_CONTENT_LEN;
@@ -7105,9 +7128,9 @@
 #endif
 
 #if defined(MBEDTLS_SSL_PROTO_DTLS)
-    if( ssl->mtu != 0 )
+    if( ssl_get_current_mtu( ssl ) != 0 )
     {
-        const size_t mtu = ssl->mtu;
+        const size_t mtu = ssl_get_current_mtu( ssl );
         const int ret = mbedtls_ssl_get_record_expansion( ssl );
         const size_t overhead = (size_t) ret;
 
@@ -7123,7 +7146,7 @@
         if( max_len > mtu - overhead )
             max_len = mtu - overhead;
     }
-#endif
+#endif /* MBEDTLS_SSL_PROTO_DTLS */
 
     return( (int) max_len );
 }