TCP Fast Open

3 files changed, 30 insertions, 1 deletions

diff --git a/doc/doc-docbook/spec.xfpt b/doc/doc-docbook/spec.xfpt
index 45d845718..478b5e1a4 100644
--- a/doc/doc-docbook/spec.xfpt
+++ b/doc/doc-docbook/spec.xfpt
@@ -23986,11 +23986,30 @@ unauthenticated. See also &%hosts_require_auth%&, and chapter
 .cindex CHUNKING "enabling, in client"
 .cindex BDAT "SMTP command"
 .cindex "RFC 3030" "CHUNKING"
-This option provides a list of server to which, provided they announce
+This option provides a list of servers to which, provided they announce
 CHUNKING support, Exim will attempt to use BDAT commands rather than DATA.
 BDAT will not be used in conjuction with a transport filter.
 .wen
 
+.new
+.option hosts_try_fastopen smtp "host list!!" unset
+.option "fast open, TCP" "enabling, in client"
+.option "TCP Fast Open" "enabling, in client"
+.option "RFC 7413" "TCP Fast Open"
+This option provides a list of servers to which, provided
+the facility is supported by this system, Exim will attempt to
+perform a TCP Fast Open.
+No data is sent on the SYN segment but, if the remote server also
+supports the facility, it can send its SMTP banner immediately after
+the SYN,ACK segment.  This can save up to one round-trip time.
+
+The facility is only active for previously-contacted servers,
+as the initiator must present a cookie in the SYN segment.
+
+On (at least some) current Linux distributions the facility must be enabled
+in the kernel by the sysadmin before the support is usable.
+.wen
+
 .option hosts_try_prdr smtp "host list&!!" *
 .cindex "PRDR" "enabling, optional in client"
 This option provides a list of servers to which, provided they announce
diff --git a/doc/doc-txt/NewStuff b/doc/doc-txt/NewStuff
index cca958f79..83ee4c3af 100644
--- a/doc/doc-txt/NewStuff
+++ b/doc/doc-txt/NewStuff
@@ -54,6 +54,15 @@ Version 4.88
 14. If built with EXPERIMENTAL_QUEUEFILE, a queuefile transport, for writing
     out copies of the message spool files for use by 3rd-party scanners.
 
+15. A new option on the smtp transport, hosts_try_fastopen.  If the system
+    supports it (on Linux it must be enabled in the kernel by the sysadmin)
+    try to use RFC 7413 "TCP Fast Open".  No data is sent on the SYN segment
+    but it permits a peer that also supports the facility to send its SMTP
+    banner immediately after the SYN,ACK segment rather then waiting for
+    another ACK - so saving up to one roundtrip time.  Because it requires
+    previous communication with the peer (we save a cookie from it) this
+    will only become active on frequently-contected destinations.
+
 
 Version 4.87
 ------------
diff --git a/doc/doc-txt/OptionLists.txt b/doc/doc-txt/OptionLists.txt
index 53c07436a..dc16b6dee 100644
--- a/doc/doc-txt/OptionLists.txt
+++ b/doc/doc-txt/OptionLists.txt
@@ -300,6 +300,7 @@ hosts_require_ocsp                   host list       unset         smtp
 hosts_require_tls                    host list       unset         smtp              3.20
 hosts_treat_as_local                 domain list     unset         main              1.95
 hosts_try_auth                       host list       unset         smtp              4.00
+hosts_try_fastopen                   host list       unset         smtp              4.88
 hosts_try_prdr                       host list       unset         smtp              4.82 if experimental_prdr
 ibase_servers                        string          unset         main              4.23
 ignore_bounce_errors_after           time            0s            main              4.00