6 files changed, 42 insertions, 1 deletions

diff --git a/Documentation/networking/ip-sysctl.txt b/Documentation/networking/ip-sysctl.txt
index f147310d9af4..2571a62d923e 100644
--- a/Documentation/networking/ip-sysctl.txt
+++ b/Documentation/networking/ip-sysctl.txt
@@ -499,6 +499,18 @@ 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
+
 UDP variables:
 
 udp_mem - vector of 3 INTEGERs: min, pressure, max
diff --git a/include/linux/tcp.h b/include/linux/tcp.h
index 3ba8b074612f..a778ee024590 100644
--- a/include/linux/tcp.h
+++ b/include/linux/tcp.h
@@ -104,6 +104,7 @@ enum {
 #define TCP_MD5SIG              14      /* TCP MD5 Signature (RFC2385) */
 #define TCP_COOKIE_TRANSACTIONS 15      /* TCP Cookie Transactions */
 #define TCP_THIN_LINEAR_TIMEOUTS 16      /* Use linear timeouts for thin streams*/
+#define TCP_THIN_DUPACK         17      /* Fast retrans. after 1 dupack */
 
 /* for TCP_INFO socket option */
 #define TCPI_OPT_TIMESTAMPS     1
@@ -343,7 +344,8 @@ struct tcp_sock {
         u8      frto_counter;   /* Number of new acks after RTO */
         u8      nonagle     : 4,/* Disable Nagle algorithm?             */
                 thin_lto    : 1,/* Use linear timeouts for thin streams */
-                unused      : 3;
+                thin_dupack : 1,/* Fast retransmit on first dupack      */
+                unused      : 2;
 
 /* RTT measurement */
         u32     srtt;           /* smoothed round trip time << 3        */
diff --git a/include/net/tcp.h b/include/net/tcp.h
index 6278fc734abd..56f0aec40ed6 100644
--- a/include/net/tcp.h
+++ b/include/net/tcp.h
@@ -245,6 +245,7 @@ extern int sysctl_tcp_slow_start_after_idle;
 extern int sysctl_tcp_max_ssthresh;
 extern int sysctl_tcp_cookie_size;
 extern int sysctl_tcp_thin_linear_timeouts;
+extern int sysctl_tcp_thin_dupack;
 
 extern atomic_t tcp_memory_allocated;
 extern struct percpu_counter tcp_sockets_allocated;
diff --git a/net/ipv4/sysctl_net_ipv4.c b/net/ipv4/sysctl_net_ipv4.c
index e6a2460587d4..c1bc074f61b7 100644
--- a/net/ipv4/sysctl_net_ipv4.c
+++ b/net/ipv4/sysctl_net_ipv4.c
@@ -582,6 +582,13 @@ 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       = "udp_mem",
                 .data           = &sysctl_udp_mem,
diff --git a/net/ipv4/tcp.c b/net/ipv4/tcp.c
index 21bae9afefea..5901010fad55 100644
--- a/net/ipv4/tcp.c
+++ b/net/ipv4/tcp.c
@@ -2236,6 +2236,13 @@ static int do_tcp_setsockopt(struct sock *sk, int level,
                         tp->thin_lto = val;
                 break;
 
+        case TCP_THIN_DUPACK:
+                if (val < 0 || val > 1)
+                        err = -EINVAL;
+                else
+                        tp->thin_dupack = val;
+                break;
+
         case TCP_CORK:
                 /* When set indicates to always queue non-full frames.
                  * Later the user clears this option and we transmit
diff --git a/net/ipv4/tcp_input.c b/net/ipv4/tcp_input.c
index 3fddc69ccccc..788851ca8c5d 100644
--- a/net/ipv4/tcp_input.c
+++ b/net/ipv4/tcp_input.c
@@ -89,6 +89,8 @@ int sysctl_tcp_frto __read_mostly = 2;
 int sysctl_tcp_frto_response __read_mostly;
 int sysctl_tcp_nometrics_save __read_mostly;
 
+int sysctl_tcp_thin_dupack __read_mostly;
+
 int sysctl_tcp_moderate_rcvbuf __read_mostly = 1;
 int sysctl_tcp_abc __read_mostly;
 
@@ -2447,6 +2449,16 @@ static int tcp_time_to_recover(struct sock *sk)
                 return 1;
         }
 
+        /* 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 1;
+
         return 0;
 }