D-Link DIR-850L User Manual page 92

Section 3 - Configuration
RTSP:
Allows application that uses Real Time Streaming Protocol to receive streaming media from the Internet. QuickTime and Real Player
are some of the common applications using this protocol.
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.
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