tcp: remove thin_dupack feature

Thin stream DUPACK is to start fast recovery on only one DUPACK
provided the connection is a thin stream (i.e., low inflight).  But
this older feature is now subsumed with RACK. If a connection
receives only a single DUPACK, RACK would arm a reordering timer
and soon starts fast recovery instead of timeout if no further
ACKs are received.

The socket option (THIN_DUPACK) is kept as a nop for compatibility.
Note that this patch does not change another thin-stream feature
which enables linear RTO. Although it might be good to generalize
that in the future (i.e., linear RTO for the first say 3 retries).

Signed-off-by: Yuchung Cheng <ycheng@google.com>
Signed-off-by: Neal Cardwell <ncardwell@google.com>
Acked-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

Documentation/networking/ip-sysctl.txt

@@ -703,18 +703,6 @@ tcp_thin_linear_timeouts - BOOLEAN
 	Documentation/networking/tcp-thin.txt
 	Default: 0
 
-tcp_thin_dupack - BOOLEAN
-	Enable dynamic triggering of retransmissions after one dupACK
-	for thin streams. If set, a check is performed upon reception
-	of a dupACK to determine if the stream is thin (less than 4
-	packets in flight). As long as the stream is found to be thin,
-	data is retransmitted on the first received dupACK. This
-	improves retransmission latency for non-aggressive thin
-	streams, often found to be time-dependent.
-	For more information on thin streams, see
-	Documentation/networking/tcp-thin.txt
-	Default: 0
-
 tcp_limit_output_bytes - INTEGER
 	Controls TCP Small Queue limit per tcp socket.
 	TCP bulk sender tends to increase packets in flight until it

include/linux/tcp.h

@@ -220,7 +220,7 @@ struct tcp_sock {
 		unused:5;
 	u8	nonagle     : 4,/* Disable Nagle algorithm?             */
 		thin_lto    : 1,/* Use linear timeouts for thin streams */
-		thin_dupack : 1,/* Fast retransmit on first dupack      */
+		unused1	    : 1,
 		repair      : 1,
 		frto        : 1;/* F-RTO (RFC5682) activated in CA_Loss */
 	u8	repair_queue;

net/ipv4/sysctl_net_ipv4.c

@@ -536,13 +536,6 @@ static struct ctl_table ipv4_table[] = {
 		.mode           = 0644,
 		.proc_handler   = proc_dointvec
 	},
-	{
-		.procname       = "tcp_thin_dupack",
-		.data           = &sysctl_tcp_thin_dupack,
-		.maxlen         = sizeof(int),
-		.mode           = 0644,
-		.proc_handler   = proc_dointvec
-	},
 	{
 		.procname	= "tcp_early_retrans",
 		.data		= &sysctl_tcp_early_retrans,

net/ipv4/tcp.c

@@ -2474,9 +2474,6 @@ static int do_tcp_setsockopt(struct sock *sk, int level,
 	case TCP_THIN_DUPACK:
 		if (val < 0 || val > 1)
 			err = -EINVAL;
-		else {
-			tp->thin_dupack = val;
-		}
 		break;
 
 	case TCP_REPAIR:
@@ -2966,8 +2963,9 @@ static int do_tcp_getsockopt(struct sock *sk, int level,
 	case TCP_THIN_LINEAR_TIMEOUTS:
 		val = tp->thin_lto;
 		break;
+
 	case TCP_THIN_DUPACK:
-		val = tp->thin_dupack;
+		val = 0;
 		break;
 
 	case TCP_REPAIR:

net/ipv4/tcp_input.c

@@ -95,9 +95,6 @@ int sysctl_tcp_rfc1337 __read_mostly;
 int sysctl_tcp_max_orphans __read_mostly = NR_FILE;
 int sysctl_tcp_frto __read_mostly = 2;
 int sysctl_tcp_min_rtt_wlen __read_mostly = 300;
-
-int sysctl_tcp_thin_dupack __read_mostly;
-
 int sysctl_tcp_moderate_rcvbuf __read_mostly = 1;
 int sysctl_tcp_early_retrans __read_mostly = 3;
 int sysctl_tcp_invalid_ratelimit __read_mostly = HZ/2;
@@ -2170,16 +2167,6 @@ static bool tcp_time_to_recover(struct sock *sk, int flag)
 	if (tcp_dupack_heuristics(tp) > tp->reordering)
 		return true;
 
-	/* If a thin stream is detected, retransmit after first
-	 * received dupack. Employ only if SACK is supported in order
-	 * to avoid possible corner-case series of spurious retransmissions
-	 * Use only if there are no unsent data.
-	 */
-	if ((tp->thin_dupack || sysctl_tcp_thin_dupack) &&
-	    tcp_stream_is_thin(tp) && tcp_dupack_heuristics(tp) > 1 &&
-	    tcp_is_sack(tp) && !tcp_send_head(sk))
-		return true;
-
 	return false;
 }