[patch v3] tcp.7: Add description for TCP_FASTOPEN and TCP_FASTOPEN_CONNECT options

 man7/tcp.7 | 125 +++++++++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 125 insertions(+)

diff --git a/man7/tcp.7 b/man7/tcp.7
index 0a7c61a37..bdd4a33ca 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 +1224,109 @@ 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 on 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 certain system calls might set
+different
+.I errno
+values.
+With cookie present,
+.BR write (2)
+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.
+.IP
+The application should  either set
+.B TCP_FASTOPEN_CONNECT
+socket option before
+.BR write (2)
+or
+.BR sendmsg (2)
+,
+or call
+.BR write (2)
+or
+.BR sendmsg (2)
+with
+.B MSG_FASTOPEN
+flag directly,
+instead of both on the same connection.
+.IP
+Here is the typical call flow with this new option:
+.IP
+.in +4n
+.EX
+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);
+.EE
+.in
+.IP
 .SS Sockets API
 TCP provides limited support for out-of-band data,
 in the form of (a single byte of) urgent data.
--

2.33.0.685.g46640cef36-goog

Thanks!
Alex


---

     tcp.7: Minor tweaks to Wei's patch

     - wsfix
     - Boldface literals (see groff_man(7)).
     - Replace '\ ' by \~, per Branden's advise.
     - Use phrasal semantic newlines.
     - Put '...' in a C comment, to avoid interfering with groff.
       It has the side effect that the code example is pure C now.
     - Remove incorrect trailing '.IP'.

     Cc: Wei Wang [off-list ref]
     Signed-off-by: Alejandro Colomar [off-list ref]

diff --git a/man7/tcp.7 b/man7/tcp.7
index 264e3ccc4..69d85c05a 100644
--- a/man7/tcp.7
+++ b/man7/tcp.7

@@ -423,26 +423,31 @@ 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.
+.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
+.TP
+.B 0x1
  Enables client side Fast Open support
-.IP 0x2
+.TP
+.B 0x2
  Enables server side Fast Open support
-.IP 0x4
+.TP
+.B 0x4
  Allows client side to transmit data in SYN without Fast Open option
-.IP 0x200
+.TP
+.B 0x200
  Allows server side to accept SYN data without Fast Open option
-.IP 0x400
+.TP
+.B 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
+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)"

@@ -1226,19 +1231,19 @@ 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.
+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.
+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.
+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

@@ -1252,18 +1257,18 @@ or
  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).
+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 on if a Fast Open cookie is available
-for the destination.
+would behave differently depending on
+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.
+except the SYN would include an empty Fast Open cookie option
+to solicit a cookie.
  .IP
  If a cookie is available,
  .BR connect (2)

@@ -1297,13 +1302,12 @@ without
  .BR write (2)
  will cause the blocking socket to be blocked forever.
  .IP
-The application should  either set
+The application should either set
  .B TCP_FASTOPEN_CONNECT
  socket option before
  .BR write (2)
  or
-.BR sendmsg (2)
-,
+.BR sendmsg (2),
  or call
  .BR write (2)
  or

@@ -1322,11 +1326,10 @@ 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);
  .EE
  .in
-.IP
  .SS Sockets API
  TCP provides limited support for out-of-band data,
  in the form of (a single byte of) urgent data.