Increase control channel packet size for faster handshakes

Instead of limiting the control channel TCP/UDP packet payload size at
'100 bytes + real control channel overhead' (~140 bytes ethernet payload),
increase the max TCP/UDP payload size to '1250 bytes - calculated overhead'
(~1210 bytes ethernet payload).

Note that this patch does *not* yield an optimal solution, but it is a
simple and rather safe change that will improve connection setup times
significantly.

v2: use the mininum value of --link-mtu and 1250 to give the user a way to
    reduce control packet size if really needed.

trac #545

Signed-off-by: Steffan Karger <steffan@karger.me>
Acked-by: Gert Doering <gert@greenie.muc.de>
Message-Id: <1435693496-10931-1-git-send-email-steffan@karger.me>
URL: http://article.gmane.org/gmane.network.openvpn.devel/9841
Signed-off-by: Gert Doering <gert@greenie.muc.de>

diff --git a/src/openvpn/ssl.c b/src/openvpn/ssl.c
index ebb2f0db93ea1646efb3d3afb07cb167e6f3409d..4e44410ef6c7ffcfd5eb1f9ce1feca21a973f623 100644 (file)
--- a/src/openvpn/ssl.c
+++ b/src/openvpn/ssl.c
@@ -298,8 +298,9 @@ tls_init_control_channel_frame_parameters(const struct frame *data_channel_frame
   reliable_ack_adjust_frame_parameters (frame, CONTROL_SEND_ACK_MAX);
   frame_add_to_extra_frame (frame, SID_SIZE + sizeof (packet_id_type));
 
-  /* set dynamic link MTU to minimum value */
-  frame_set_mtu_dynamic (frame, 0, SET_MTU_TUN);
+  /* set dynamic link MTU to cap control channel packets at 1250 bytes */
+  ASSERT (TUN_LINK_DELTA (frame) < min_int (frame->link_mtu, 1250));
+  frame->link_mtu_dynamic = min_int (frame->link_mtu, 1250) - TUN_LINK_DELTA (frame);
 }
 
 void