[PATCHv2] Revert TCP retransmission backoff on ICMP destination unreachable

[Damian Lukowski <damian@tvk.rwth-aachen.de>]

This patch implements the TCP improvement of the Internet Draft
"Make TCP more Robust to Long Connectivity Disruptions"
(http://tools.ietf.org/html/draft-zimmermann-tcp-lcd-01).

Exponential backoff is TCP's standard behaviour during long connectivity
disruptions, which is a countermeasure against network congestion.
If congestion can be excluded as the reason for RTO retransmission loss,
backoff is not desirable, as it yields longer TCP recovery times, when
the communication path is repaired shortly after an unsuccessful
retransmission probe.
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. On receipt of such a message, the timeout is
halved (in order to revert the doubling after the retransmission).
With true congestion, a router won't trigger such a message and the
patched TCP will operate as standard TCP.

Changes from v1:
1) Calling tcp_retransmit_timer() instead tcp_retransmit_skb().
   This fixes problems with SACK and a inconsistency with the draft.
2) If the socket is locked, retransmission is deferred by using the
   formula as in tcp_write_timer().
3) Removed the sysctl_tcp_retries modifications, which will be
   sent in a different patch request.
4) Recalculating the RTO from rtt, rttvar and icsk_backoff,
   not simply halving. This prevents, that an RTO of RTO_MAX-epsilon
   is "doubled" to RTO_MAX and then "reverted" to RTO_MAX/2 instead of
   RTO_MAX-epsilon. Currently, this is not what the draft specifies,
   but the draft will be changed accordingly.
5) Renamed skb to icmp_skb in tcp_v4_err in codebase code.
6) Different coding-style and indentation changes, as suggested
   by Ilpo Järvinen.
7) Using net-next-2.6 git-branch as codebase.

Thanks for the review, so far.

Signed-off-by: Damian Lukowski <damian@tvk.rwth-aachen.de>
---
 include/net/tcp.h    |    1 +
 net/ipv4/tcp_ipv4.c  |   54 ++++++++++++++++++++++++++++++++++++++++++-------
 net/ipv4/tcp_timer.c |    2 +-
 3 files changed, 48 insertions(+), 9 deletions(-)

diff --git a/include/net/tcp.h b/include/net/tcp.h
index 88af843..58c5b39 100644
--- a/include/net/tcp.h
+++ b/include/net/tcp.h
@@ -469,6 +469,7 @@ extern void __tcp_push_pending_frames(struct sock *sk, unsigned int cur_mss,
 				      int nonagle);
 extern int tcp_may_send_now(struct sock *sk);
 extern int tcp_retransmit_skb(struct sock *, struct sk_buff *);
+extern void tcp_retransmit_timer(struct sock *sk);
 extern void tcp_xmit_retransmit_queue(struct sock *);
 extern void tcp_simple_retransmit(struct sock *);
 extern int tcp_trim_head(struct sock *, struct sk_buff *, u32);
diff --git a/net/ipv4/tcp_ipv4.c b/net/ipv4/tcp_ipv4.c
index 6d88219..d1d73ac 100644
--- a/net/ipv4/tcp_ipv4.c
+++ b/net/ipv4/tcp_ipv4.c
@@ -328,26 +328,29 @@ static void do_pmtu_discovery(struct sock *sk, struct iphdr *iph, u32 mtu)
  *
  */
 
-void tcp_v4_err(struct sk_buff *skb, u32 info)
+void tcp_v4_err(struct sk_buff *icmp_skb, u32 info)
 {
-	struct iphdr *iph = (struct iphdr *)skb->data;
-	struct tcphdr *th = (struct tcphdr *)(skb->data + (iph->ihl << 2));
+	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(skb)->type;
-	const int code = icmp_hdr(skb)->code;
+	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(skb->dev);
+	struct net *net = dev_net(icmp_skb->dev);
 
-	if (skb->len < (iph->ihl << 2) + 8) {
+	if (icmp_skb->len < (iph->ihl << 2) + 8) {
 		ICMP_INC_STATS_BH(net, ICMP_MIB_INERRORS);
 		return;
 	}
 
 	sk = inet_lookup(net, &tcp_hashinfo, iph->daddr, th->dest,
-			iph->saddr, th->source, inet_iif(skb));
+			iph->saddr, th->source, inet_iif(icmp_skb));
 	if (!sk) {
 		ICMP_INC_STATS_BH(net, ICMP_MIB_INERRORS);
 		return;
@@ -367,6 +370,7 @@ void tcp_v4_err(struct sk_buff *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 +397,40 @@ void tcp_v4_err(struct sk_buff *skb, u32 info)
 		}
 
 		err = icmp_err_convert[code].errno;
+		/* check if ICMP unreach message allows revert of backoff */
+		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 = ((tp->srtt >> 3) + tp->rttvar) <<
+					 icsk->icsk_backoff;
+		if (inet_csk(sk)->icsk_rto > TCP_RTO_MAX)
+			inet_csk(sk)->icsk_rto = TCP_RTO_MAX;
+
+		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) {
+			/* RTO revert shortened timer. */
+			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 b144a26..a3ba494 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);
-- 
1.6.3.3