Revert Backoff [v3]: Revert RTO on ICMP destination unreachable

Here, an ICMP host/network unreachable message, whose payload fits to
TCP's SND.UNA, is taken as an indication that the RTO retransmission has
not been lost due to congestion, but because of a route failure
somewhere along the path.
With true congestion, a router won't trigger such a message and the
patched TCP will operate as standard TCP.

This patch reverts one RTO backoff, if an ICMP host/network unreachable
message, whose payload fits to TCP's SND.UNA, arrives.
Based on the new RTO, the retransmission timer is reset to reflect the
remaining time, or - if the revert clocked out the timer - a retransmission
is sent out immediately.
Backoffs are only reverted, if TCP is in RTO loss recovery, i.e. if
there have been retransmissions and reversible backoffs, already.

Changes from v2:
1) Renaming of skb in tcp_v4_err() moved to another patch.
2) Reintroduced tcp_bound_rto() and __tcp_set_rto().
3) Fixed code comments.

Signed-off-by: Damian Lukowski <damian@tvk.rwth-aachen.de>
Acked-by: Ilpo Järvinen <ilpo.jarvinen@helsinki.fi>
Signed-off-by: David S. Miller <davem@davemloft.net>

3 files changed, 40 insertions, 4 deletions

diff --git a/net/ipv4/tcp_input.c b/net/ipv4/tcp_input.c
index 2bdb0da237e..af6d6fa00db 100644
--- a/net/ipv4/tcp_input.c
+++ b/net/ipv4/tcp_input.c
@@ -685,7 +685,7 @@ static inline void tcp_set_rto(struct sock *sk)
          *    is invisible. Actually, Linux-2.4 also generates erratic
          *    ACKs in some circumstances.
          */
-        inet_csk(sk)->icsk_rto = (tp->srtt >> 3) + tp->rttvar;
+        inet_csk(sk)->icsk_rto = __tcp_set_rto(tp);
 
         /* 2. Fixups made earlier cannot be right.
          *    If we do not estimate RTO correctly without them,
@@ -696,8 +696,7 @@ static inline void tcp_set_rto(struct sock *sk)
         /* NOTE: clamping at TCP_RTO_MIN is not required, current algo
          * guarantees that rto is higher.
          */
-        if (inet_csk(sk)->icsk_rto > TCP_RTO_MAX)
-                inet_csk(sk)->icsk_rto = TCP_RTO_MAX;
+        tcp_bound_rto(sk);
 }
 
 /* Save metrics learned by this TCP session.
diff --git a/net/ipv4/tcp_ipv4.c b/net/ipv4/tcp_ipv4.c
index 6ca1bc8c302..6755e29a6dd 100644
--- a/net/ipv4/tcp_ipv4.c
+++ b/net/ipv4/tcp_ipv4.c
@@ -332,12 +332,15 @@ void tcp_v4_err(struct sk_buff *icmp_skb, u32 info)
 {
         struct iphdr *iph = (struct iphdr *)icmp_skb->data;
         struct tcphdr *th = (struct tcphdr *)(icmp_skb->data + (iph->ihl << 2));
+        struct inet_connection_sock *icsk;
         struct tcp_sock *tp;
         struct inet_sock *inet;
         const int type = icmp_hdr(icmp_skb)->type;
         const int code = icmp_hdr(icmp_skb)->code;
         struct sock *sk;
+        struct sk_buff *skb;
         __u32 seq;
+        __u32 remaining;
         int err;
         struct net *net = dev_net(icmp_skb->dev);
 
@@ -367,6 +370,7 @@ void tcp_v4_err(struct sk_buff *icmp_skb, u32 info)
         if (sk->sk_state == TCP_CLOSE)
                 goto out;
 
+        icsk = inet_csk(sk);
         tp = tcp_sk(sk);
         seq = ntohl(th->seq);
         if (sk->sk_state != TCP_LISTEN &&
@@ -393,6 +397,39 @@ void tcp_v4_err(struct sk_buff *icmp_skb, u32 info)
                 }
 
                 err = icmp_err_convert[code].errno;
+                /* check if icmp_skb allows revert of backoff
+                 * (see draft-zimmermann-tcp-lcd) */
+                if (code != ICMP_NET_UNREACH && code != ICMP_HOST_UNREACH)
+                        break;
+                if (seq != tp->snd_una  || !icsk->icsk_retransmits ||
+                    !icsk->icsk_backoff)
+                        break;
+
+                icsk->icsk_backoff--;
+                inet_csk(sk)->icsk_rto = __tcp_set_rto(tp) <<
+                                         icsk->icsk_backoff;
+                tcp_bound_rto(sk);
+
+                skb = tcp_write_queue_head(sk);
+                BUG_ON(!skb);
+
+                remaining = icsk->icsk_rto - min(icsk->icsk_rto,
+                                tcp_time_stamp - TCP_SKB_CB(skb)->when);
+
+                if (remaining) {
+                        inet_csk_reset_xmit_timer(sk, ICSK_TIME_RETRANS,
+                                                  remaining, TCP_RTO_MAX);
+                } else if (sock_owned_by_user(sk)) {
+                        /* RTO revert clocked out retransmission,
+                         * but socket is locked. Will defer. */
+                        inet_csk_reset_xmit_timer(sk, ICSK_TIME_RETRANS,
+                                                  HZ/20, TCP_RTO_MAX);
+                } else {
+                        /* RTO revert clocked out retransmission.
+                         * Will retransmit now */
+                        tcp_retransmit_timer(sk);
+                }
+
                 break;
         case ICMP_TIME_EXCEEDED:
                 err = EHOSTUNREACH;
diff --git a/net/ipv4/tcp_timer.c b/net/ipv4/tcp_timer.c
index c520fb6e06d..408fa4b7b9b 100644
--- a/net/ipv4/tcp_timer.c
+++ b/net/ipv4/tcp_timer.c
@@ -279,7 +279,7 @@ static void tcp_probe_timer(struct sock *sk)
  *      The TCP retransmit timer.
  */
 
-static void tcp_retransmit_timer(struct sock *sk)
+void tcp_retransmit_timer(struct sock *sk)
 {
         struct tcp_sock *tp = tcp_sk(sk);
         struct inet_connection_sock *icsk = inet_csk(sk);