TCP_DEFER_ACCEPT documentation should be more precise about rounding to retransmits

TCP_DEFER_ACCEPT documentation should be more precise about rounding to retransmits

[Josh Triplett]

`tcp(7)` says:
> Takes an integer value (seconds), this can bound the maximum number of attempts TCP will make to complete the connection.

However, it doesn't say *how* it bounds the attempts. From the kernel
code, it appears to round up to the first retransmit time that's larger
than the specified bound. Could the manpage please document this?

(Discovered when adding Rust bindings to this, and trying to figure out
and document the rounding behavior.)

- Josh Triplett

Re: TCP_DEFER_ACCEPT documentation should be more precise about rounding to retransmits

[Alejandro Colomar]

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Hi Josh,

On 2026-04-14T10:06:17-0700, Josh Triplett wrote:
> `tcp(7)` says:
> > Takes an integer value (seconds), this can bound the maximum number of attempts TCP will make to complete the connection.
> 
> However, it doesn't say *how* it bounds the attempts. From the kernel
> code, it appears to round up to the first retransmit time that's larger
> than the specified bound. Could the manpage please document this?

Would you mind sending a patch?  (And if appropriate, CC someone.)


Have a lovely night!
Alex

> 
> (Discovered when adding Rust bindings to this, and trying to figure out
> and document the rounding behavior.)
> 
> - Josh Triplett
> 

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

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[PATCH] Document TCP_DEFER_ACCEPT rounding

[Josh Triplett]

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Patch attached.

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From ad2b798376326d40944f6998270a755f61c57815 Mon Sep 17 00:00:00 2001
Message-ID: <ad2b798376326d40944f6998270a755f61c57815.1776374539.git.josh@joshtriplett.org>
From: Josh Triplett <josh@joshtriplett.org>
Date: Thu, 16 Apr 2026 14:21:34 -0700
Subject: [PATCH] Document `TCP_DEFER_ACCEPT` rounding

Signed-off-by: Josh Triplett <josh@joshtriplett.org>
---
 man/man7/tcp.7 | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/man/man7/tcp.7 b/man/man7/tcp.7
index ff46ea7bc..26a99b879 100644
--- a/man/man7/tcp.7
+++ b/man/man7/tcp.7
@@ -1091,6 +1091,8 @@ 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.
+The kernel will round this value up to the next larger TCP retransmit
+duration.
 .IP
 On a client socket,
 if nonzero,
-- 
2.53.0