man/man7/tcp.7: Document effect of TCP_DEFER_ACCEPT on clientsocket

man/man7/tcp.7: Document effect of TCP_DEFER_ACCEPT on clientsocket

[Ran Benita]

TCP_DEFER_ACCEPT has an interesting little-known effect on client
sockets. I don't think it's very useful but should still be documented.

See here for reference:
<https://elixir.bootlin.com/linux/v6.19.6/source/net/ipv4/tcp_input.c#L6781-L6796>

Signed-off-by: Ran Benita <ran@unusedvar.com>
---
man/man7/tcp.7 | 5 +++++
1 file changed, 5 insertions(+)

diff --git a/man/man7/tcp.7 b/man/man7/tcp.7
index adfd6623e..85f6c631c 100644
--- a/man/man7/tcp.7
+++ b/man/man7/tcp.7
@@ -1091,6 +1091,11 @@ Allow a listener to be awakened only when data arrives on the socket.
Takes an integer value (seconds), this can
bound the maximum number of attempts TCP will make to
complete the connection.
+.IP
+On a client socket, if non-zero, delays sending the final ACK of the three-way
+handshake until the socket has data to send, in which case the ACK is
+piggybacked on the first data segment, or a delayed-ACK timeout is reached.
+.IP
This option should not be used in code intended to be portable.
.TP
.BR TCP_INFO " (since Linux 2.4)"
-- 
2.53.0

Re: man/man7/tcp.7: Document effect of TCP_DEFER_ACCEPT on clientsocket

[Alejandro Colomar]

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Hi Ran,

On 2026-03-13T00:25:53+0200, Ran Benita wrote:
> TCP_DEFER_ACCEPT has an interesting little-known effect on client
> sockets. I don't think it's very useful but should still be documented.
> 
> See here for reference:
> <https://elixir.bootlin.com/linux/v6.19.6/source/net/ipv4/tcp_input.c#L6781-L6796>
> 
> Signed-off-by: Ran Benita <ran@unusedvar.com>

Thanks!  Please see some comments below.


Have a lovely night!
Alex

> ---
> man/man7/tcp.7 | 5 +++++
> 1 file changed, 5 insertions(+)
> 
> diff --git a/man/man7/tcp.7 b/man/man7/tcp.7
> index adfd6623e..85f6c631c 100644
> --- a/man/man7/tcp.7
> +++ b/man/man7/tcp.7
> @@ -1091,6 +1091,11 @@ Allow a listener to be awakened only when data arrives on the socket.
> Takes an integer value (seconds), this can
> bound the maximum number of attempts TCP will make to
> complete the connection.
> +.IP
> +On a client socket, if non-zero, delays sending the final ACK of the three-way

s/non-zero/nonzero/

$ grep -rho 'non[^ ]*zero' | sort | uniq -c
     72 non-zero
      4 non\-zero
   2119 nonzero
      3 nonzero	non-zero
      2 nonzero/non-zero
      2 nonzero\h'|\n[3cl1]u'non-zero


> +handshake until the socket has data to send, in which case the ACK is
> +piggybacked on the first data segment, or a delayed-ACK timeout is reached.

Please use semantic newlines.  See man-pages(7):

$ MANWIDTH=72 man man-pages | awk '/Use semantic newlines/,/^$/'
   Use semantic newlines
     In the source of a manual page, new sentences should be started on
     new lines, long sentences should be split  into  lines  at  clause
     breaks  (commas,  semicolons, colons, and so on), and long clauses
     should be split at phrase boundaries.  This convention,  sometimes
     known as "semantic newlines", makes it easier to see the effect of
     patches, which often operate at the level of individual sentences,
     clauses, or phrases.

> +.IP
> This option should not be used in code intended to be portable.
> .TP
> .BR TCP_INFO " (since Linux 2.4)"
> -- 
> 2.53.0
> 

-- 
<https://www.alejandro-colomar.es>

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[PATCH v2] man/man7/tcp.7: Document effect of TCP_DEFER_ACCEPT on client socket

[Ran Benita]

TCP_DEFER_ACCEPT has an interesting little-known effect on client
sockets. I don't think it's very useful but should still be documented.

See here for reference:
<https://elixir.bootlin.com/linux/v6.19.6/source/net/ipv4/tcp_input.c#L6781-L6796>

Signed-off-by: Ran Benita <ran@unusedvar.com>
---
Thanks for taking a look. I fixed the following:
- Change non-zero -> nonzero
- Use semantic newlines

 man/man7/tcp.7 | 6 ++++++
 1 file changed, 6 insertions(+)

diff --git a/man/man7/tcp.7 b/man/man7/tcp.7
index adfd6623e..80b91ffda 100644
--- a/man/man7/tcp.7
+++ b/man/man7/tcp.7
@@ -1091,6 +1091,12 @@ Allow a listener to be awakened only when data arrives on the socket.
 Takes an integer value (seconds), this can
 bound the maximum number of attempts TCP will make to
 complete the connection.
+.IP
+On a client socket, if nonzero,
+delays sending the final ACK of the three-way handshake until the socket has data to send,
+in which case the ACK is piggybacked on the first data segment,
+or a delayed-ACK timeout is reached.
+.IP
 This option should not be used in code intended to be portable.
 .TP
 .BR TCP_INFO " (since Linux 2.4)"
-- 
2.53.0

Re: [PATCH v2] man/man7/tcp.7: Document effect of TCP_DEFER_ACCEPT on client socket

[Alejandro Colomar]

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Hi Ran,

On 2026-03-13T11:26:12+0200, Ran Benita wrote:
> TCP_DEFER_ACCEPT has an interesting little-known effect on client
> sockets. I don't think it's very useful but should still be documented.
> 
> See here for reference:
> <https://elixir.bootlin.com/linux/v6.19.6/source/net/ipv4/tcp_input.c#L6781-L6796>
> 
> Signed-off-by: Ran Benita <ran@unusedvar.com>

Thanks!  I've applied the patch.


Have a lovely night!
Alex

> ---
> Thanks for taking a look. I fixed the following:
> - Change non-zero -> nonzero
> - Use semantic newlines
> 
>  man/man7/tcp.7 | 6 ++++++
>  1 file changed, 6 insertions(+)
> 
> diff --git a/man/man7/tcp.7 b/man/man7/tcp.7
> index adfd6623e..80b91ffda 100644
> --- a/man/man7/tcp.7
> +++ b/man/man7/tcp.7
> @@ -1091,6 +1091,12 @@ Allow a listener to be awakened only when data arrives on the socket.
>  Takes an integer value (seconds), this can
>  bound the maximum number of attempts TCP will make to
>  complete the connection.
> +.IP
> +On a client socket, if nonzero,
> +delays sending the final ACK of the three-way handshake until the socket has data to send,
> +in which case the ACK is piggybacked on the first data segment,
> +or a delayed-ACK timeout is reached.
> +.IP
>  This option should not be used in code intended to be portable.
>  .TP
>  .BR TCP_INFO " (since Linux 2.4)"
> -- 
> 2.53.0
> 

-- 
<https://www.alejandro-colomar.es>

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