[TCP]: DSACK signals data receival, be conservative

In case a DSACK is received, it's better to lower cwnd as it's
a sign of data receival.

Signed-off-by: Ilpo Järvinen <ilpo.jarvinen@helsinki.fi>
Signed-off-by: David S. Miller <davem@davemloft.net>

1 files changed, 4 insertions, 1 deletions

diff --git a/net/ipv4/tcp_input.c b/net/ipv4/tcp_input.c
index c3124e6de1d3..f030435e0eb4 100644
--- a/net/ipv4/tcp_input.c
+++ b/net/ipv4/tcp_input.c
@@ -103,6 +103,7 @@ int sysctl_tcp_abc __read_mostly;
 #define FLAG_SLOWPATH           0x100 /* Do not skip RFC checks for window update.*/
 #define FLAG_ONLY_ORIG_SACKED   0x200 /* SACKs only non-rexmit sent before RTO */
 #define FLAG_SND_UNA_ADVANCED   0x400 /* Snd_una was changed (!= FLAG_DATA_ACKED) */
+#define FLAG_DSACKING_ACK       0x800 /* SACK blocks contained DSACK info */
 
 #define FLAG_ACKED              (FLAG_DATA_ACKED|FLAG_SYN_ACKED)
 #define FLAG_NOT_DUP            (FLAG_DATA|FLAG_WIN_UPDATE|FLAG_ACKED)
@@ -966,12 +967,14 @@ tcp_sacktag_write_queue(struct sock *sk, struct sk_buff *ack_skb, u32 prior_snd_
 
         /* Check for D-SACK. */
         if (before(ntohl(sp[0].start_seq), TCP_SKB_CB(ack_skb)->ack_seq)) {
+                flag |= FLAG_DSACKING_ACK;
                 found_dup_sack = 1;
                 tp->rx_opt.sack_ok |= 4;
                 NET_INC_STATS_BH(LINUX_MIB_TCPDSACKRECV);
         } else if (num_sacks > 1 &&
                         !after(ntohl(sp[0].end_seq), ntohl(sp[1].end_seq)) &&
                         !before(ntohl(sp[0].start_seq), ntohl(sp[1].start_seq))) {
+                flag |= FLAG_DSACKING_ACK;
                 found_dup_sack = 1;
                 tp->rx_opt.sack_ok |= 4;
                 NET_INC_STATS_BH(LINUX_MIB_TCPDSACKOFORECV);
@@ -1858,7 +1861,7 @@ static void tcp_cwnd_down(struct sock *sk, int flag)
         struct tcp_sock *tp = tcp_sk(sk);
         int decr = tp->snd_cwnd_cnt + 1;
 
-        if ((flag&FLAG_ANY_PROGRESS) ||
+        if ((flag&(FLAG_ANY_PROGRESS|FLAG_DSACKING_ACK)) ||
             (IsReno(tp) && !(flag&FLAG_NOT_DUP))) {
                 tp->snd_cwnd_cnt = decr&1;
                 decr >>= 1;