netdev - [PATCH net] tcp: note that tcp_rmem[1] has a limited range

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Date:   Mon,  3 Jan 2022 16:37:22 -0800
From:   Ivan Babrou <ivan@...udflare.com>
To:     netdev@...r.kernel.org
Cc:     linux-kernel@...r.kernel.org, kernel-team@...udflare.com,
        "David S . Miller" <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>,
        Jonathan Corbet <corbet@....net>,
        Eric Dumazet <edumazet@...gle.com>,
        Ivan Babrou <ivan@...udflare.com>
Subject: [PATCH net] tcp: note that tcp_rmem[1] has a limited range

The value of rcv_ssthresh is limited to tcp_rwin, which is limited
to 64k at the handshake time, since window scaling is not allowed there.

Let's add a note to the docs that increasing tcp_rmem[1] does not have
any effect on raising the initial value of rcv_ssthresh past 64k.

Link: https://lkml.org/lkml/2021/12/22/652
Signed-off-by: Ivan Babrou <ivan@...udflare.com>
---
 Documentation/networking/ip-sysctl.rst | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/Documentation/networking/ip-sysctl.rst b/Documentation/networking/ip-sysctl.rst
index 2572eecc3e86..16528bc92e65 100644
--- a/Documentation/networking/ip-sysctl.rst
+++ b/Documentation/networking/ip-sysctl.rst
@@ -683,7 +683,8 @@ tcp_rmem - vector of 3 INTEGERs: min, default, max
 	default: initial size of receive buffer used by TCP sockets.
 	This value overrides net.core.rmem_default used by other protocols.
 	Default: 131072 bytes.
-	This value results in initial window of 65535.
+	This value results in initial window of 65535. Increasing this value
+	won't raise the initial advertised window above 65535.
 
 	max: maximal size of receive buffer allowed for automatically
 	selected receiver buffers for TCP socket. This value does not override
-- 
2.34.1