socket: Add support for TCP defer accept

TCP_DEFER_ACCEPT Allow a listener to be awakened only when data
arrives on the socket. If TCP_DEFER_ACCEPT set on a server-side
listening socket, the TCP/IP stack will not to wait for the final
ACK packet and not to initiate the process until the first packet
of real data has arrived. After sending the SYN/ACK, the server will
then wait for a data packet from a client. Now, only three packets
will be sent over the network, and the connection establishment delay
will be significantly reduced.

man/systemd.socket.xml

@@ -545,6 +545,47 @@
                                 for details.).</para></listitem>
                         </varlistentry>
 
+                        <varlistentry>
+                                <term><varname>DeferAccept=</varname></term>
+
+                                <listitem><para>Takes time (in
+                                seconds) as argument. If set, the
+                                listening process will be awakened
+                                only when data arrives on the socket,
+                                and not immediately when connection is
+                                established. When this option is set,
+                                the
+                                <constant>TCP_DEFER_ACCEPT</constant>
+                                socket option will be used (see
+                                <citerefentry><refentrytitle>tcp</refentrytitle><manvolnum>7</manvolnum></citerefentry>),
+                                and the kernel will ignore initial ACK
+                                packets without any data. The argument
+                                specifies the approximate amount of
+                                time the kernel should wait for
+                                incoming data before falling back to
+                                the normal behaviour of honouring
+                                empty ACK packets. This option is
+                                beneficial for protocols where the
+                                client sends the data first (e.g.
+                                HTTP, in contrast to SMTP), because
+                                the server process will not be woken
+                                up unnecessarily before it can take
+                                any action.
+                                </para>
+
+                                <para>If the client also uses the
+                                <constant>TCP_DEFER_ACCEPT</constant>
+                                option, the latency of the initial
+                                connection may be reduced, because the
+                                kernel will send data in the final
+                                packet establishing the connection
+                                (the third packet in the "three-way
+                                handshake").</para>
+
+                                <para>Disabled by default.</para>
+                                </listitem>
+                        </varlistentry>
+
                         <varlistentry>
                                 <term><varname>ReceiveBuffer=</varname></term>
                                 <term><varname>SendBuffer=</varname></term>