From: "Alejandro Colomar (man-pages)" <alx.manpages@gmail.com>
To: Wei Wang <weiwan@google.com>
Cc: netdev@vger.kernel.org, Yuchung Cheng <ycheng@google.com>,
Michael Kerrisk <mtk.manpages@gmail.com>,
linux-man@vger.kernel.org
Subject: Re: [patch v2] tcp.7: Add description for TCP_FASTOPEN and TCP_FASTOPEN_CONNECT options
Date: Mon, 20 Sep 2021 21:50:32 +0200 [thread overview]
Message-ID: <c0617af0-fc29-5ac6-deb0-d0da3fcfc99d@gmail.com> (raw)
In-Reply-To: <20210917041702.167622-1-weiwan@google.com>
Hello Wei,
On 9/17/21 6:17 AM, Wei Wang wrote:
> TCP_FASTOPEN socket option was added by:
> commit 8336886f786fdacbc19b719c1f7ea91eb70706d4
> TCP_FASTOPEN_CONNECT socket option was added by the following patch
> series:
> commit 065263f40f0972d5f1cd294bb0242bd5aa5f06b2
> commit 25776aa943401662617437841b3d3ea4693ee98a
> commit 19f6d3f3c8422d65b5e3d2162e30ef07c6e21ea2
> commit 3979ad7e82dfe3fb94a51c3915e64ec64afa45c3
> Add detailed description for these 2 options.
> Also add descriptions for /proc entry tcp_fastopen and tcp_fastopen_key.
>
> Signed-off-by: Wei Wang <weiwan@google.com>
> Reviewed-by: Yuchung Cheng <ycheng@google.com>
Thanks for the patch (and the review, Yuchung)!
Please see some comments below.
Cheers,
Alex
> ---
> Change in v2: corrected some format issues
>
> man7/tcp.7 | 110 +++++++++++++++++++++++++++++++++++++++++++++++++++++
> 1 file changed, 110 insertions(+)
>
> diff --git a/man7/tcp.7 b/man7/tcp.7
> index 0a7c61a37..5a6fa7f50 100644
> --- a/man7/tcp.7
> +++ b/man7/tcp.7
> @@ -423,6 +423,28 @@ option.
> .\" Since 2.4.0-test7
> Enable RFC\ 2883 TCP Duplicate SACK support.
> .TP
> +.IR tcp_fastopen " (Bitmask; default: 0x1; since Linux 3.7)"
> +Enables RFC\ 7413 Fast Open support.
> +The flag is used as a bitmap with the following values:
> +.RS
> +.IP 0x1
> +Enables client side Fast Open support
> +.IP 0x2
> +Enables server side Fast Open support
> +.IP 0x4
> +Allows client side to transmit data in SYN without Fast Open option
> +.IP 0x200
> +Allows server side to accept SYN data without Fast Open option
> +.IP 0x400
> +Enables Fast Open on all listeners without
> +.B TCP_FASTOPEN
> +socket option
> +.RE
> +.TP
> +.IR tcp_fastopen_key " (since Linux 3.7)"
> +Set server side RFC\ 7413 Fast Open key to generate Fast Open cookie
> +when server side Fast Open support is enabled.
> +.TP
> .IR tcp_ecn " (Integer; default: see below; since Linux 2.4)"
> .\" Since 2.4.0-test7
> Enable RFC\ 3168 Explicit Congestion Notification.
> @@ -1202,6 +1224,94 @@ Bound the size of the advertised window to this value.
> The kernel imposes a minimum size of SOCK_MIN_RCVBUF/2.
> This option should not be used in code intended to be
> portable.
> +.TP
> +.BR TCP_FASTOPEN " (since Linux 3.6)"
> +This option enables Fast Open (RFC\ 7413) on the listener socket.
> +The value specifies the maximum length of pending SYNs
> +(similar to the backlog argument in
> +.BR listen (2)).
> +Once enabled,
> +the listener socket grants the TCP Fast Open cookie on incoming
> +SYN with TCP Fast Open option.
> +.IP
> +More importantly it accepts the data in SYN with a valid Fast Open cookie
> +and responds SYN-ACK acknowledging both the data and the SYN sequence.
> +.BR accept (2)
> +returns a socket that is available for read and write when the handshake
> +has not completed yet.
> +Thus the data exchange can commence before the handshake completes.
> +This option requires enabling the server-side support on sysctl
> +.IR net.ipv4.tcp_fastopen
> +(see above).
> +For TCP Fast Open client-side support,
> +see
> +.BR send (2)
> +.B MSG_FASTOPEN
> +or
> +.B TCP_FASTOPEN_CONNECT
> +below.
> +.TP
> +.BR TCP_FASTOPEN_CONNECT " (since Linux 4.11)"
> +This option enables an alternative way to perform Fast Open on the active
> +side (client).
> +When this option is enabled,
> +.BR connect (2)
> +would behave differently depending if a Fast Open cookie is available for
> +the destination.
> +.IP
> +If a cookie is not available (i.e. first contact to the destination),
> +.BR connect (2)
> +behaves as usual by sending a SYN immediately,
> +except the SYN would include an empty Fast Open cookie option to solicit a
> +cookie.
> +.IP
> +If a cookie is available,
> +.BR connect (2)
> +would return 0 immediately but the SYN transmission is defered.
> +A subsequent
> +.BR write (2)
> +or
> +.BR sendmsg (2)
> +would trigger a SYN with data plus cookie in the Fast Open option.
> +In other words,
> +the actual connect operation is deferred until data is supplied.
> +.IP
> +.B Note:
> +While this option is designed for convenience,
> +enabling it does change the behaviors and might set new
> +.I errnos
typo?
errno values?
> +of socket calls.
The above is not very clear to me.
> +With cookie present,
> +.BR write (2)
> +/
Does this mean an "or"? If so, prefer the "or".
> +.BR sendmsg (2)
> +must be called right after
> +.BR connect (2)
> +in order to send out SYN+data to complete 3WHS and establish connection.
> +Calling
> +.BR read (2)
> +right after
> +.BR connect (2)
> +without
> +.BR write (2)
> +will cause the blocking socket to be blocked forever.
> +The application should use either
> +.B TCP_FASTOPEN_CONNECT
> +or
> +.BR send (2)
This is not clear to me. So TCP_FASTOPEN_CONNECT can use write(2) and
sendmsg(2) (mentioned above), and TCP_FASTOPEN can only use send(2)? Or
what did you mean?
> +with
> +.B MSG_FASTOPEN ,
> +instead of both on the same connection.
From "The application ...":
Does this have relation with the text just above it? It appears to me
to be a more generic statement that both options shouldn't be mixed, so
maybe a new paragraph is more appropriate.
> +.IP
> +Here is the typical call flow with this new option:
> + s = socket();
> + setsockopt(s, IPPROTO_TCP, TCP_FASTOPEN_CONNECT, 1, ...);
> + connect(s);
> + write(s); // write() should always follow connect() in order to
> + // trigger SYN to go out
> + read(s)/write(s);
> + ... > + close(s);
See man-pages(7):
Indentation of structure definitions, shell session logs, and
so on
When structure definitions, shell session logs, and so on
are included in running text, indent them by 4 spaces
(i.e., a block enclosed by .in +4n and .in), format them
using the .EX and EE macros, and surround them with suit‐
able paragraph markers (either .PP or .IP). For example:
.PP
.in +4n
.EX
int
main(int argc, char *argv[])
{
return 0;
}
.EE
.in
.PP
> .SS Sockets API
> TCP provides limited support for out-of-band data,
> in the form of (a single byte of) urgent data.
>
--
Alejandro Colomar
Linux man-pages comaintainer; https://www.kernel.org/doc/man-pages/
http://www.alejandro-colomar.es/
next prev parent reply other threads:[~2021-09-21 1:38 UTC|newest]
Thread overview: 3+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-09-17 4:17 [patch v2] tcp.7: Add description for TCP_FASTOPEN and TCP_FASTOPEN_CONNECT options Wei Wang
2021-09-20 19:50 ` Alejandro Colomar (man-pages) [this message]
2021-09-24 22:01 ` Wei Wang
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