1 files changed, 17 insertions, 6 deletions

diff --git a/net/ipv4/tcp_output.c b/net/ipv4/tcp_output.c
index de54f02f10a9..e4c5ac9fe89b 100644
--- a/net/ipv4/tcp_output.c
+++ b/net/ipv4/tcp_output.c
@@ -362,6 +362,17 @@ struct tcp_out_options {
         __u32 tsval, tsecr;     /* need to include OPTION_TS */
 };
 
+/* Beware: Something in the Internet is very sensitive to the ordering of
+ * TCP options, we learned this through the hard way, so be careful here.
+ * Luckily we can at least blame others for their non-compliance but from
+ * inter-operatibility perspective it seems that we're somewhat stuck with
+ * the ordering which we have been using if we want to keep working with
+ * those broken things (not that it currently hurts anybody as there isn't
+ * particular reason why the ordering would need to be changed).
+ *
+ * At least SACK_PERM as the first option is known to lead to a disaster
+ * (but it may well be that other scenarios fail similarly).
+ */
 static void tcp_options_write(__be32 *ptr, struct tcp_sock *tp,
                               const struct tcp_out_options *opts,
                               __u8 **md5_hash) {
@@ -376,6 +387,12 @@ static void tcp_options_write(__be32 *ptr, struct tcp_sock *tp,
                 *md5_hash = NULL;
         }
 
+        if (unlikely(opts->mss)) {
+                *ptr++ = htonl((TCPOPT_MSS << 24) |
+                               (TCPOLEN_MSS << 16) |
+                               opts->mss);
+        }
+
         if (likely(OPTION_TS & opts->options)) {
                 if (unlikely(OPTION_SACK_ADVERTISE & opts->options)) {
                         *ptr++ = htonl((TCPOPT_SACK_PERM << 24) |
@@ -392,12 +409,6 @@ static void tcp_options_write(__be32 *ptr, struct tcp_sock *tp,
                 *ptr++ = htonl(opts->tsecr);
         }
 
-        if (unlikely(opts->mss)) {
-                *ptr++ = htonl((TCPOPT_MSS << 24) |
-                               (TCPOLEN_MSS << 16) |
-                               opts->mss);
-        }
-
         if (unlikely(OPTION_SACK_ADVERTISE & opts->options &&
                      !(OPTION_TS & opts->options))) {
                 *ptr++ = htonl((TCPOPT_NOP << 24) |