Avoid fragmentation when using UDP

We'll have to revive UDP because we experienced congestion with TCP.
This should make UDP efficient in good environment.
MTU discovery is required however to enable UDP by default.

re6st/plib.py

@@ -23,11 +23,19 @@ def openvpn(iface, encrypt, *args, **kw):
     logging.debug('%r', args)
     return utils.Popen(args, **kw)
 
+# tested in a LAN with a switch and wired/wireless interfaces (mtu 1500)
+ovpn_link_mtu_dict = {'udp': 1490, 'udp6': 1450}
 
 def server(iface, max_clients, dh_path, pipe_fd, port, proto, encrypt, *args, **kw):
     client_script = '%s %s' % (ovpn_server, pipe_fd)
     if pipe_fd is not None:
         args = ('--client-disconnect', client_script) + args
+    try:
+        args = ('--link-mtu', str(ovpn_link_mtu_dict[proto]),
+                # mtu-disc ignored for udp6 due to a bug in OpenVPN
+                '--mtu-disc', 'yes') + args
+    except KeyError:
+        proto += '-server'
     return openvpn(iface, encrypt,
         '--tls-server',
         '--mode', 'server',
@@ -35,14 +43,20 @@ def server(iface, max_clients, dh_path, pipe_fd, port, proto, encrypt, *args, **
         '--dh', dh_path,
         '--max-clients', str(max_clients),
         '--port', str(port),
-        '--proto', proto + '-server' if proto in ('tcp', 'tcp6') else proto,
+        '--proto', proto,
         *args, **kw)
 
 
 def client(iface, address_list, encrypt, *args, **kw):
     remote = ['--nobind', '--client']
+    # XXX: We'd like to pass <connection> sections at command-line.
+    link_mtu = set()
     for ip, port, proto in address_list:
         remote += '--remote', ip, port, proto
+        link_mtu.add(ovpn_link_mtu_dict.get(proto))
+    link_mtu, = link_mtu
+    if link_mtu:
+        remote += '--link-mtu', str(link_mtu), '--mtu-disc', 'yes'
     remote += args
     return openvpn(iface, encrypt, *remote, **kw)