tcp: RFC7413 option support for Fast Open client

Fast Open has been using an experimental option with a magic number
(RFC6994). This patch makes the client by default use the RFC7413
option (34) to get and send Fast Open cookies.  This patch makes
the client solicit cookies from a given server first with the
RFC7413 option. If that fails to elicit a cookie, then it tries
the RFC6994 experimental option. If that also fails, it uses the
RFC7413 option on all subsequent connect attempts.  If the server
returns a Fast Open cookie then the client caches the form of the
option that successfully elicited a cookie, and uses that form on
later connects when it presents that cookie.

The idea is to gradually obsolete the use of experimental options as
the servers and clients upgrade, while keeping the interoperability
meanwhile.

Signed-off-by: Daniel Lee <Longinus00@gmail.com>
Signed-off-by: Yuchung Cheng <ycheng@google.com>
Signed-off-by: Neal Cardwell <ncardwell@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

1 files changed, 11 insertions, 2 deletions

diff --git a/net/ipv4/tcp_metrics.c b/net/ipv4/tcp_metrics.c
index 78ecc4a01712..a51d63a43e33 100644
--- a/net/ipv4/tcp_metrics.c
+++ b/net/ipv4/tcp_metrics.c
@@ -28,7 +28,8 @@ static struct tcp_metrics_block *__tcp_get_metrics(const struct inetpeer_addr *s
 
 struct tcp_fastopen_metrics {
         u16     mss;
-        u16     syn_loss:10;            /* Recurring Fast Open SYN losses */
+        u16     syn_loss:10,            /* Recurring Fast Open SYN losses */
+                try_exp:2;              /* Request w/ exp. option (once) */
         unsigned long   last_syn_loss;  /* Last Fast Open SYN loss */
         struct  tcp_fastopen_cookie     cookie;
 };
@@ -131,6 +132,8 @@ static void tcpm_suck_dst(struct tcp_metrics_block *tm,
         if (fastopen_clear) {
                 tm->tcpm_fastopen.mss = 0;
                 tm->tcpm_fastopen.syn_loss = 0;
+                tm->tcpm_fastopen.try_exp = 0;
+                tm->tcpm_fastopen.cookie.exp = false;
                 tm->tcpm_fastopen.cookie.len = 0;
         }
 }
@@ -713,6 +716,8 @@ void tcp_fastopen_cache_get(struct sock *sk, u16 *mss,
                         if (tfom->mss)
                                 *mss = tfom->mss;
                         *cookie = tfom->cookie;
+                        if (cookie->len <= 0 && tfom->try_exp == 1)
+                                cookie->exp = true;
                         *syn_loss = tfom->syn_loss;
                         *last_syn_loss = *syn_loss ? tfom->last_syn_loss : 0;
                 } while (read_seqretry(&fastopen_seqlock, seq));
@@ -721,7 +726,8 @@ void tcp_fastopen_cache_get(struct sock *sk, u16 *mss,
 }
 
 void tcp_fastopen_cache_set(struct sock *sk, u16 mss,
-                            struct tcp_fastopen_cookie *cookie, bool syn_lost)
+                            struct tcp_fastopen_cookie *cookie, bool syn_lost,
+                            u16 try_exp)
 {
         struct dst_entry *dst = __sk_dst_get(sk);
         struct tcp_metrics_block *tm;
@@ -738,6 +744,9 @@ void tcp_fastopen_cache_set(struct sock *sk, u16 mss,
                         tfom->mss = mss;
                 if (cookie && cookie->len > 0)
                         tfom->cookie = *cookie;
+                else if (try_exp > tfom->try_exp &&
+                         tfom->cookie.len <= 0 && !tfom->cookie.exp)
+                        tfom->try_exp = try_exp;
                 if (syn_lost) {
                         ++tfom->syn_loss;
                         tfom->last_syn_loss = jiffies;