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@xxxxxxxxxx> --- 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
