# TCP Fast Open
#
# Linux:
# Both server and client-side TFO support must be enabled in the
# kernel, 'sudo sh -c "echo 3 > /proc/sys/net/ipv4/tcp_fastopen"'.
#
# A packet capture on the loopback interface will show the TFO
# option on the SYN, but the fast-output SMTP banner will not
# be seen unless you also deliberately emulate a long path:
# 'sudo tc qdisc add dev lo root netem delay 50ms'
# You'll need iproute-tc installed, for the tc command.
# You'll need kernel-modules-extra installed, or you get
# an unhelpful error from RTNETLINK.
# To tidy up:  'sudo tc qdisc delete dev lo root'
#
# MacOS:
# The kernel seems to have TFO enabled both ways as default.
# There is a net.inet.tcp.clear_tfocache parameter
## sysctl -w foo-val
#
# For network delays there is something called 'Network Link Conditioner'
# which might do the job.  But how to manipulate it?
#
#
# FreeBSD: it looks like you have to compile a custom kernel, with
# 'options TCP_RFC7413' in the config.  Also set
# 'net.inet.tcp.fastopen.server_enable=1' in /etc/sysctl.conf
# Seems to always claim TFO used by transport, if tried.
#
sudo perl
system ("tc qdisc add dev lo root netem delay 50ms");
****
#
#
# Disable the TFO blackhole detection, as we seem to be running foul of it.
# If bitten, we see the expected EINPROGRESS for sendto, yet no TFO cookie
# option on the SYN.
#
sudo perl
system ("[ -e /proc/sys/net/ipv4/tcp_fastopen_blackhole_timeout_sec ] && echo 0 > /proc/sys/net/ipv4/tcp_fastopen_blackhole_timeout_sec");
****
#
# First time runs will see a TFO request option only; subsequent
# ones should see the TFO cookie and fast-output SMTP banner
# (currently on a separate packet after the server SYN,ACK but before
# the client ACK).
#
# The client log => line  should have a "TFO" element.
# The server log <= line for a@test.ex  should not.
#
# First clear any previously-obtained cookie:
sudo perl
system ("ip tcp_metrics delete 127.0.0.1");
****
#
#
#
exim -DSERVER=server -bd -oX PORT_D
****
#
exim a@test.ex
Testing
****
sleep 3
#
# The server log <= line for b@test.ex  should have a "TFO" element, but
# this will only be obtained when the above delay is inserted into the
# loopback net path.
#
exim b@test.ex
Testing
****
sleep 3
#
#
sudo perl
system ("tc qdisc delete dev lo root");
system ("[ -e /proc/sys/net/ipv4/tcp_fastopen_blackhole_timeout_sec ] && echo 3600 > /proc/sys/net/ipv4/tcp_fastopen_blackhole_timeout_sec");
****
#
killdaemon
no_msglog_check