diff --git a/ChangeLog.d/fix_fragment_len_return.txt b/ChangeLog.d/fix_fragment_len_return.txt
new file mode 100644
index 000000000..bfbf93cd2
--- /dev/null
+++ b/ChangeLog.d/fix_fragment_len_return.txt
@@ -0,0 +1,5 @@
+Bugfix
+   * Fix the error returned when a client requests an invalid
+   * fragment length, as per RFC6066 section 4. We now return
+   * MBEDTLS_ERR_SSL_ILLEGAL_PARAMETER, and raise the corresponding
+   * alert.
diff --git a/ChangeLog.d/update_ssl_error_codes.txt b/ChangeLog.d/update_ssl_error_codes.txt
new file mode 100644
index 000000000..0630b5475
--- /dev/null
+++ b/ChangeLog.d/update_ssl_error_codes.txt
@@ -0,0 +1,3 @@
+Changes
+   * Various changes to which alert and/or error code may be returned
+   * during the TLS handshake.
diff --git a/library/ssl_cli.c b/library/ssl_cli.c
index 558755d4e..fc791d6bd 100644
--- a/library/ssl_cli.c
+++ b/library/ssl_cli.c
@@ -1389,8 +1389,8 @@ static int ssl_parse_renegotiation_info( mbedtls_ssl_context *ssl,
             mbedtls_ssl_send_alert_message(
                 ssl,
                 MBEDTLS_SSL_ALERT_LEVEL_FATAL,
-                MBEDTLS_SSL_ALERT_MSG_ILLEGAL_PARAMETER );
-            return( MBEDTLS_ERR_SSL_ILLEGAL_PARAMETER );
+                MBEDTLS_SSL_ALERT_MSG_HANDSHAKE_FAILURE );
+            return( MBEDTLS_ERR_SSL_HANDSHAKE_FAILURE );
         }
     }
     else
@@ -1403,8 +1403,8 @@ static int ssl_parse_renegotiation_info( mbedtls_ssl_context *ssl,
             mbedtls_ssl_send_alert_message(
                 ssl,
                 MBEDTLS_SSL_ALERT_LEVEL_FATAL,
-                MBEDTLS_SSL_ALERT_MSG_ILLEGAL_PARAMETER );
-            return( MBEDTLS_ERR_SSL_ILLEGAL_PARAMETER );
+                MBEDTLS_SSL_ALERT_MSG_HANDSHAKE_FAILURE );
+            return( MBEDTLS_ERR_SSL_HANDSHAKE_FAILURE );
         }
 
         ssl->secure_renegotiation = MBEDTLS_SSL_SECURE_RENEGOTIATION;
@@ -1453,7 +1453,7 @@ static int ssl_parse_cid_ext( mbedtls_ssl_context *ssl,
     {
         MBEDTLS_SSL_DEBUG_MSG( 1, ( "CID extension unexpected" ) );
         mbedtls_ssl_send_alert_message( ssl, MBEDTLS_SSL_ALERT_LEVEL_FATAL,
-                                        MBEDTLS_SSL_ALERT_MSG_ILLEGAL_PARAMETER );
+                                        MBEDTLS_SSL_ALERT_MSG_UNSUPPORTED_EXT );
         return( MBEDTLS_ERR_SSL_ILLEGAL_PARAMETER );
     }
 
@@ -1508,7 +1508,7 @@ static int ssl_parse_encrypt_then_mac_ext( mbedtls_ssl_context *ssl,
         mbedtls_ssl_send_alert_message(
             ssl,
             MBEDTLS_SSL_ALERT_LEVEL_FATAL,
-            MBEDTLS_SSL_ALERT_MSG_ILLEGAL_PARAMETER );
+            MBEDTLS_SSL_ALERT_MSG_UNSUPPORTED_EXT );
         return( MBEDTLS_ERR_SSL_ILLEGAL_PARAMETER );
     }
 
diff --git a/library/ssl_srv.c b/library/ssl_srv.c
index 2c801ef76..75beb6e93 100644
--- a/library/ssl_srv.c
+++ b/library/ssl_srv.c
@@ -201,8 +201,8 @@ static int ssl_parse_renegotiation_info( mbedtls_ssl_context *ssl,
         {
             MBEDTLS_SSL_DEBUG_MSG( 1, ( "non-matching renegotiation info" ) );
             mbedtls_ssl_send_alert_message( ssl, MBEDTLS_SSL_ALERT_LEVEL_FATAL,
-                                            MBEDTLS_SSL_ALERT_MSG_ILLEGAL_PARAMETER );
-            return( MBEDTLS_ERR_SSL_ILLEGAL_PARAMETER );
+                                            MBEDTLS_SSL_ALERT_MSG_HANDSHAKE_FAILURE );
+            return( MBEDTLS_ERR_SSL_HANDSHAKE_FAILURE );
         }
     }
     else
@@ -212,8 +212,8 @@ static int ssl_parse_renegotiation_info( mbedtls_ssl_context *ssl,
         {
             MBEDTLS_SSL_DEBUG_MSG( 1, ( "non-zero length renegotiation info" ) );
             mbedtls_ssl_send_alert_message( ssl, MBEDTLS_SSL_ALERT_LEVEL_FATAL,
-                                            MBEDTLS_SSL_ALERT_MSG_DECODE_ERROR );
-            return( MBEDTLS_ERR_SSL_DECODE_ERROR );
+                                            MBEDTLS_SSL_ALERT_MSG_HANDSHAKE_FAILURE );
+            return( MBEDTLS_ERR_SSL_HANDSHAKE_FAILURE );
         }
 
         ssl->secure_renegotiation = MBEDTLS_SSL_SECURE_RENEGOTIATION;
diff --git a/library/ssl_tls.c b/library/ssl_tls.c
index 97f9c1b84..1cfda4a08 100644
--- a/library/ssl_tls.c
+++ b/library/ssl_tls.c
@@ -1898,7 +1898,7 @@ static int ssl_parse_certificate_chain( mbedtls_ssl_context *ssl,
             mbedtls_ssl_send_alert_message( ssl,
                               MBEDTLS_SSL_ALERT_LEVEL_FATAL,
                               MBEDTLS_SSL_ALERT_MSG_DECODE_ERROR );
-            return( MBEDTLS_SSL_ALERT_MSG_DECODE_ERROR );
+            return( MBEDTLS_ERR_SSL_DECODE_ERROR );
         }
         /* In theory, the CRT can be up to 2**24 Bytes, but we don't support
          * anything beyond 2**16 ~ 64K. */
@@ -1907,8 +1907,8 @@ static int ssl_parse_certificate_chain( mbedtls_ssl_context *ssl,
             MBEDTLS_SSL_DEBUG_MSG( 1, ( "bad certificate message" ) );
             mbedtls_ssl_send_alert_message( ssl,
                             MBEDTLS_SSL_ALERT_LEVEL_FATAL,
-                            MBEDTLS_SSL_ALERT_MSG_HANDSHAKE_FAILURE );
-            return( MBEDTLS_ERR_SSL_HANDSHAKE_FAILURE );
+                            MBEDTLS_SSL_ALERT_MSG_UNSUPPORTED_CERT );
+            return( MBEDTLS_ERR_SSL_BAD_CERTIFICATE );
         }
 
         /* Read length of the next CRT in the chain. */
@@ -1943,8 +1943,8 @@ static int ssl_parse_certificate_chain( mbedtls_ssl_context *ssl,
                 MBEDTLS_SSL_DEBUG_MSG( 1, ( "new server cert during renegotiation" ) );
                 mbedtls_ssl_send_alert_message( ssl,
                                                 MBEDTLS_SSL_ALERT_LEVEL_FATAL,
-                                                MBEDTLS_SSL_ALERT_MSG_ILLEGAL_PARAMETER );
-                return( MBEDTLS_ERR_SSL_ILLEGAL_PARAMETER );
+                                                MBEDTLS_SSL_ALERT_MSG_ACCESS_DENIED );
+                return( MBEDTLS_ERR_SSL_BAD_CERTIFICATE );
             }
 
             /* Now we can safely free the original chain. */
@@ -2929,8 +2929,8 @@ int mbedtls_ssl_parse_finished( mbedtls_ssl_context *ssl )
     {
         MBEDTLS_SSL_DEBUG_MSG( 1, ( "bad finished message" ) );
         mbedtls_ssl_send_alert_message( ssl, MBEDTLS_SSL_ALERT_LEVEL_FATAL,
-                                        MBEDTLS_SSL_ALERT_MSG_ILLEGAL_PARAMETER );
-        return( MBEDTLS_ERR_SSL_ILLEGAL_PARAMETER );
+                                        MBEDTLS_SSL_ALERT_MSG_DECRYPT_ERROR );
+        return( MBEDTLS_ERR_SSL_HANDSHAKE_FAILURE );
     }
 
 #if defined(MBEDTLS_SSL_RENEGOTIATION)