1 files changed, 6 insertions, 1 deletions

diff --git a/net/ipv4/tcp_input.c b/net/ipv4/tcp_input.c
index 5dfbc333e79a..6660ce2a7333 100644
--- a/net/ipv4/tcp_input.c
+++ b/net/ipv4/tcp_input.c
@@ -6263,6 +6263,11 @@ static inline void pr_drop_req(struct request_sock *req, __u16 port, int family)
  * congestion control: Linux DCTCP asserts ECT on all packets,
  * including SYN, which is most optimal solution; however,
  * others, such as FreeBSD do not.
+ *
+ * Exception: At least one of the reserved bits of the TCP header (th->res1) is
+ * set, indicating the use of a future TCP extension (such as AccECN). See
+ * RFC8311 §4.3 which updates RFC3168 to allow the development of such
+ * extensions.
  */
 static void tcp_ecn_create_request(struct request_sock *req,
                                    const struct sk_buff *skb,
@@ -6282,7 +6287,7 @@ static void tcp_ecn_create_request(struct request_sock *req,
         ecn_ok_dst = dst_feature(dst, DST_FEATURE_ECN_MASK);
         ecn_ok = net->ipv4.sysctl_tcp_ecn || ecn_ok_dst;
 
-        if ((!ect && ecn_ok) || tcp_ca_needs_ecn(listen_sk) ||
+        if (((!ect || th->res1) && ecn_ok) || tcp_ca_needs_ecn(listen_sk) ||
             (ecn_ok_dst & DST_FEATURE_ECN_CA) ||
             tcp_bpf_ca_needs_ecn((struct sock *)req))
                 inet_rsk(req)->ecn_ok = 1;