netdev - [PATCH net-next v3] tcp: enable mid stream window clamp

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Message-ID: <20210825210117.1668371-1-ntspring@fb.com>
Date:   Wed, 25 Aug 2021 14:01:17 -0700
From:   Neil Spring <ntspring@...com>
To:     <davem@...emloft.net>, <edumazet@...gle.com>
CC:     <yoshfuji@...ux-ipv6.org>, <dsahern@...nel.org>, <kuba@...nel.org>,
        <netdev@...r.kernel.org>, <ncardwell@...gle.com>,
        <ycheng@...gle.com>, Neil Spring <ntspring@...com>
Subject: [PATCH net-next v3] tcp: enable mid stream window clamp

The TCP_WINDOW_CLAMP socket option is defined in tcp(7) to "Bound the size
of the advertised window to this value."  Window clamping is distributed
across two variables, window_clamp ("Maximal window to advertise" in
tcp.h) and rcv_ssthresh ("Current window clamp").

This patch updates the function where the window clamp is set to also
reduce the current window clamp, rcv_sshthresh, if needed.  With this,
setting the TCP_WINDOW_CLAMP option has the documented effect of limiting
the window.

Signed-off-by: Neil Spring <ntspring@...com>
---
v2: - fix email formatting

v3: - address comments by setting rcv_ssthresh based on prior window 

 net/ipv4/tcp.c | 1 +
 1 file changed, 1 insertion(+)

diff --git a/net/ipv4/tcp.c b/net/ipv4/tcp.c
index f931def6302e..e8b48df73c85 100644
--- a/net/ipv4/tcp.c
+++ b/net/ipv4/tcp.c
@@ -3338,6 +3338,7 @@ int tcp_set_window_clamp(struct sock *sk, int val)
 	} else {
 		tp->window_clamp = val < SOCK_MIN_RCVBUF / 2 ?
 			SOCK_MIN_RCVBUF / 2 : val;
+		tp->rcv_ssthresh = min(tp->rcv_wnd, tp->window_clamp);
 	}
 	return 0;
 }
-- 
2.30.2