[systemd-devel] [PATCH 3/3] socket: Add support for TCP defer accept

[Susant Sahani]

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 | 16 ++++++++++++++++
 src/core/dbus-socket.c |  1 +
 src/core/socket.c      | 11 +++++++++++
 src/core/socket.h      |  1 +
 4 files changed, 29 insertions(+)

diff --git a/man/systemd.socket.xml b/man/systemd.socket.xml
index e6bbb2e..9ce94aa 100644
--- a/man/systemd.socket.xml
+++ b/man/systemd.socket.xml
@@ -539,6 +539,22 @@
                         </varlistentry>
 
                         <varlistentry>
+                                <term><varname>DeferAccept=</varname></term>
+                                <listitem><para>Takes time (in seconds) as argument
+                                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.
+                                This controls the TCP_DEFER_ACCEPT socket option (see
+                                <citerefentry><refentrytitle>socket</refentrytitle><manvolnum>7</manvolnum></citerefentry>
+                                Defaults to
+                                <option>disabled</option>.</para></listitem>
+                        </varlistentry>
+
+                        <varlistentry>
                                 <term><varname>Priority=</varname></term>
                                 <listitem><para>Takes an integer
                                 argument controlling the priority for
diff --git a/src/core/dbus-socket.c b/src/core/dbus-socket.c
index f9ef7ef..1142ca5 100644
--- a/src/core/dbus-socket.c
+++ b/src/core/dbus-socket.c
@@ -101,6 +101,7 @@ const sd_bus_vtable bus_socket_vtable[] = {
         SD_BUS_PROPERTY("KeepAliveInterval", "t", bus_property_get_usec, offsetof(Socket, keep_alive_interval), SD_BUS_VTABLE_PROPERTY_CONST),
         SD_BUS_PROPERTY("KeepAliveProbes", "i", bus_property_get_int, offsetof(Socket, keep_alive_cnt), SD_BUS_VTABLE_PROPERTY_CONST),
         SD_BUS_PROPERTY("FastOpen" , "b", bus_property_get_bool, offsetof(Socket, fast_open), SD_BUS_VTABLE_PROPERTY_CONST),
+        SD_BUS_PROPERTY("DeferAccept" , "t", bus_property_get_usec, offsetof(Socket, defer_accept), SD_BUS_VTABLE_PROPERTY_CONST),
         SD_BUS_PROPERTY("Priority", "i", bus_property_get_int, offsetof(Socket, priority), SD_BUS_VTABLE_PROPERTY_CONST),
         SD_BUS_PROPERTY("ReceiveBuffer", "t", bus_property_get_size, offsetof(Socket, receive_buffer), SD_BUS_VTABLE_PROPERTY_CONST),
         SD_BUS_PROPERTY("SendBuffer", "t", bus_property_get_size, offsetof(Socket, send_buffer), SD_BUS_VTABLE_PROPERTY_CONST),
diff --git a/src/core/socket.c b/src/core/socket.c
index b798d4e..32cadf9 100644
--- a/src/core/socket.c
+++ b/src/core/socket.c
@@ -610,6 +610,11 @@ static void socket_dump(Unit *u, FILE *f, const char *prefix) {
                         "%sKeepAliveProbes: %u\n",
                         prefix, s->keep_alive_cnt);
 
+        if(s->defer_accept)
+                fprintf(f,
+                        "%sDeferAccept: %lo\n",
+                        prefix, s->defer_accept / USEC_PER_SEC);
+
         LIST_FOREACH(port, p, s->ports) {
 
                 if (p->type == SOCKET_SOCKET) {
@@ -831,6 +836,12 @@ static void socket_apply_socket_options(Socket *s, int fd) {
                         log_warning_unit(UNIT(s)->id, "TCP_FASTOPEN failed: %m");
         }
 
+        if (s->defer_accept) {
+                int value = s->defer_accept / USEC_PER_SEC;
+                if (setsockopt(fd, SOL_TCP, TCP_DEFER_ACCEPT, &value, sizeof(value)) < 0)
+                        log_warning_unit(UNIT(s)->id, "TCP_DEFER_ACCEPT failed: %m");
+        }
+
         if (s->broadcast) {
                 int one = 1;
                 if (setsockopt(fd, SOL_SOCKET, SO_BROADCAST, &one, sizeof(one)) < 0)
diff --git a/src/core/socket.h b/src/core/socket.h
index 9cb82fa..7452d27 100644
--- a/src/core/socket.h
+++ b/src/core/socket.h
@@ -104,6 +104,7 @@ struct Socket {
         usec_t timeout_usec;
         usec_t keep_alive_time;
         usec_t keep_alive_interval;
+        usec_t defer_accept;
 
         ExecCommand* exec_command[_SOCKET_EXEC_COMMAND_MAX];
         ExecContext exec_context;
-- 
1.9.3