tcp: ipv6: support RFC 6069 (TCP-LD)

Make tcp_ld_RTO_revert() helper available to IPv6, and
implement RFC 6069 :

Quoting this RFC :

3. Connectivity Disruption Indication

   For Internet Protocol version 6 (IPv6) [RFC2460], the counterpart of
   the ICMP destination unreachable message of code 0 (net unreachable)
   and of code 1 (host unreachable) is the ICMPv6 destination
   unreachable message of code 0 (no route to destination) [RFC4443].
   As with IPv4, a router should generate an ICMPv6 destination
   unreachable message of code 0 in response to a packet that cannot be
   delivered to its destination address because it lacks a matching
   entry in its routing table.
Signed-off-by: Eric Dumazet <edumazet@google.com>
Acked-by: Yuchung Cheng <ycheng@google.com>
Acked-by: Neal Cardwell <ncardwell@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

include/net/tcp.h

@@ -437,6 +437,7 @@ u16 tcp_get_syncookie_mss(struct request_sock_ops *rsk_ops,
 void tcp_v4_send_check(struct sock *sk, struct sk_buff *skb);
 void tcp_v4_mtu_reduced(struct sock *sk);
 void tcp_req_err(struct sock *sk, u32 seq, bool abort);
+void tcp_ld_RTO_revert(struct sock *sk, u32 seq);
 int tcp_v4_conn_request(struct sock *sk, struct sk_buff *skb);
 struct sock *tcp_create_openreq_child(const struct sock *sk,
 				      struct request_sock *req,

net/ipv4/tcp_ipv4.c

@@ -404,7 +404,7 @@ void tcp_req_err(struct sock *sk, u32 seq, bool abort)
 EXPORT_SYMBOL(tcp_req_err);
 
 /* TCP-LD (RFC 6069) logic */
-static void tcp_ld_RTO_revert(struct sock *sk, u32 seq)
+void tcp_ld_RTO_revert(struct sock *sk, u32 seq)
 {
 	struct inet_connection_sock *icsk = inet_csk(sk);
 	struct tcp_sock *tp = tcp_sk(sk);
@@ -441,6 +441,7 @@ static void tcp_ld_RTO_revert(struct sock *sk, u32 seq)
 		tcp_retransmit_timer(sk);
 	}
 }
+EXPORT_SYMBOL(tcp_ld_RTO_revert);
 
 /*
  * This routine is called by the ICMP module when it gets some

net/ipv6/tcp_ipv6.c

@@ -473,6 +473,15 @@ static int tcp_v6_err(struct sk_buff *skb, struct inet6_skb_parm *opt,
 		} else
 			sk->sk_err_soft = err;
 		goto out;
+	case TCP_LISTEN:
+		break;
+	default:
+		/* check if this ICMP message allows revert of backoff.
+		 * (see RFC 6069)
+		 */
+		if (!fastopen && type == ICMPV6_DEST_UNREACH &&
+		    code == ICMPV6_NOROUTE)
+			tcp_ld_RTO_revert(sk, seq);
 	}
 
 	if (!sock_owned_by_user(sk) && np->recverr) {