path: root/net/ipv4
diff options
authorIlpo Järvinen <ilpo.jarvinen@helsinki.fi>2008-10-23 14:06:35 -0700
committerDavid S. Miller <davem@davemloft.net>2008-10-23 14:06:35 -0700
commitfd6149d332973bafa50f03ddb0ea9513e67f4517 (patch)
tree6f15537b56e2d565bbff797d5c96a98e5cda87ec /net/ipv4
parentb63365a2d60268a3988285d6c3c6003d7066f93a (diff)
tcp: Restore ordering of TCP options for the sake of inter-operability
This is not our bug! Sadly some devices cannot cope with the change of TCP option ordering which was a result of the recent rewrite of the option code (not that there was some particular reason steming from the rewrite for the reordering) though any ordering of TCP options is perfectly legal. Thus we restore the original ordering to allow interoperability with/through such broken devices and add some warning about this trap. Since the reordering just happened without any particular reason, this change shouldn't cost us anything. There are already couple of known failure reports (within close proximity of the last release), so the problem might be more wide-spread than a single device. And other reports which may be due to the same problem though the symptoms were less obvious. Analysis of one of the case revealed (with very high probability) that sack capability cannot be negotiated as the first option (SYN never got a response). Signed-off-by: Ilpo Järvinen <ilpo.jarvinen@helsinki.fi> Reported-by: Aldo Maggi <sentiniate@tiscali.it> Tested-by: Aldo Maggi <sentiniate@tiscali.it> Signed-off-by: David S. Miller <davem@davemloft.net>
Diffstat (limited to 'net/ipv4')
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) |

Privacy Policy