1 files changed, 19 insertions, 10 deletions

diff --git a/net/ipv4/tcp_input.c b/net/ipv4/tcp_input.c
index 24f1630b2afb..031cf72cd05c 100644
--- a/net/ipv4/tcp_input.c
+++ b/net/ipv4/tcp_input.c
@@ -5378,8 +5378,8 @@ static bool tcp_rcv_fastopen_synack(struct sock *sk, struct sk_buff *synack,
 {
         struct tcp_sock *tp = tcp_sk(sk);
         struct sk_buff *data = tp->syn_data ? tcp_write_queue_head(sk) : NULL;
-        u16 mss = tp->rx_opt.mss_clamp;
-        bool syn_drop;
+        u16 mss = tp->rx_opt.mss_clamp, try_exp = 0;
+        bool syn_drop = false;
 
         if (mss == tp->rx_opt.user_mss) {
                 struct tcp_options_received opt;
@@ -5391,16 +5391,25 @@ static bool tcp_rcv_fastopen_synack(struct sock *sk, struct sk_buff *synack,
                 mss = opt.mss_clamp;
         }
 
-        if (!tp->syn_fastopen)  /* Ignore an unsolicited cookie */
+        if (!tp->syn_fastopen) {
+                /* Ignore an unsolicited cookie */
                 cookie->len = -1;
+        } else if (tp->total_retrans) {
+                /* SYN timed out and the SYN-ACK neither has a cookie nor
+                 * acknowledges data. Presumably the remote received only
+                 * the retransmitted (regular) SYNs: either the original
+                 * SYN-data or the corresponding SYN-ACK was dropped.
+                 */
+                syn_drop = (cookie->len < 0 && data);
+        } else if (cookie->len < 0 && !tp->syn_data) {
+                /* We requested a cookie but didn't get it. If we did not use
+                 * the (old) exp opt format then try so next time (try_exp=1).
+                 * Otherwise we go back to use the RFC7413 opt (try_exp=2).
+                 */
+                try_exp = tp->syn_fastopen_exp ? 2 : 1;
+        }
 
-        /* The SYN-ACK neither has cookie nor acknowledges the data. Presumably
-         * the remote receives only the retransmitted (regular) SYNs: either
-         * the original SYN-data or the corresponding SYN-ACK is lost.
-         */
-        syn_drop = (cookie->len <= 0 && data && tp->total_retrans);
-
-        tcp_fastopen_cache_set(sk, mss, cookie, syn_drop);
+        tcp_fastopen_cache_set(sk, mss, cookie, syn_drop, try_exp);
 
         if (data) { /* Retransmit unacked data in SYN */
                 tcp_for_write_queue_from(data, sk) {