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Subject[PATCH] tcp: use RTAX_CWND for outgoing connections properly
For outgoing connections, the initial value of cwnd is always set to 2 (in 
tcp_v4_init_sock()) regardless of setting of RTAX_CWND. For incoming
connections, this is handled properly in tcp_init_metrics().

As a result of this, Linux TCP stack always uses cwnd == 2 at the beginning of
outgoing TCP session (i.e. waits for ACK after 2 packets once the connection
has been established) and grows it in accordance with slow-start algorithm
only after it receives ACK for first two packets.

When slow-start triggers later during the connection (e.g. after idle),
cwnd is properly re-initialized to RTAX_CWND value (if specified) through
tcp_cwnd_restart() -> tcp_init_cwnd().

Initialize tp->snd_cwnd properly so that RTAX_CWND value is being used
also in the slow-start phase for the first packets in the connection.

Signed-off-by: Jiri Kosina <>
net/ipv4/tcp_output.c | 2 ++
1 files changed, 2 insertions(+), 0 deletions(-)

diff --git a/net/ipv4/tcp_output.c b/net/ipv4/tcp_output.c
index 61c2463..6dbc55b 100644
--- a/net/ipv4/tcp_output.c
+++ b/net/ipv4/tcp_output.c
@@ -2572,6 +2572,8 @@ static void tcp_connect_init(struct sock *sk)
dst_metric(dst, RTAX_INITRWND));

+ tp->snd_cwnd = tcp_init_cwnd(tp, dst);
tp->rx_opt.rcv_wscale = rcv_wscale;
tp->rcv_ssthresh = tp->rcv_wnd;


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