69D-Link COVR-1900 User Manual
Section 3 - Configuration
Advanced Settings... - Application Level Gateway (ALG) Configuration
PPTP Allows  multiple machines on  the LAN  to connect to their 
corporate network using the PPTP protocol.
IPSec (VPN) Allows multiple VPN  clients to connect  to their corp
orate 
network using IPSec. Some VPN clients support traversal of 
IPSec  through NAT. This Application Level Gateway (ALG) 
may interfere with the operation of such VPN clients. If you 
are having trouble connecting with your corporate network, 
try  turning  this  ALG off.  Please  check  with  the  system 
administrator of your corporate network whether your VPN 
client supports NAT traversal.
RTSP Allows applications that uses Real Time Streaming Protocol 
(R
TSP) to receive streaming media from the Internet.
SIP Allows devices and applications  using VoIP (Voice over IP) 
to communicate across NAT.  Some VoIP applications  and 
devices have the ability to discover NAT devices and work 
around them. This ALG may interfere with the operation of 
such devices. If you are having trouble making VoIP calls, try 
turning this ALG off.