The Alg Screen - ZyXEL Communications SBG5500 Series User Manual

8.6 The ALG Screen

Some NAT routers may include a SIP Application Layer Gateway (ALG). A SIP ALG allows SIP calls to pass
through NAT by examining and translating IP addresses embedded in the data stream. When the SBG
registers with the SIP register server, the SIP ALG translates the SBG's private IP address inside the SIP data
stream to a public IP address. You do not need to use STUN or an outbound proxy if your SBG is behind a
SIP ALG.
Use this screen to enable and disable the NAT, SIP (VoIP) and/or RTSP ALG in the SBG. To access this
screen, click Configuration > NAT > ALG.
Figure 105 Configuration > NAT > ALG
The following table describes the fields in this screen.
Table 52 Configuration > NAT > ALG
LABEL
NAT ALG
FTP ALG
TFTP ALG
RTSP ALG
SIP ALG
H.323 ALG
PPTP ALG
Apply
Reset
Chapter 8 Network Address Translation (NAT)
DESCRIPTION
Enable this to make sure applications such as FTP and file transfer in IM applications work
correctly with port-forwarding and address-mapping rules.
Turn on the FTP ALG to detect FTP (File Transfer Program) traffic and help build FTP sessions
through the SBG's NAT.
Turn on the TFTP ALG to detect TFTP (Trivial File Transfer Protocol) traffic and help build TFTP
sessions through the SBG's NAT.
Enable this to have the SBG detect RTSP traffic and help build RTSP sessions through its NAT. The
Real Time Streaming (media control) Protocol (RTSP) is a remote control for multimedia on the
Internet.
Enable this to make sure SIP (VoIP) works correctly with port-forwarding and address-mapping
rules.
Turn on the H.323 ALG to detect H.323 traffic (used for audio communications) and help build
H.323 sessions through the SBG's NAT.
Turn on the PPTP ALG to detect PPTP (Point-to-point Tunneling Protocol) traffic and help build
PPTP sessions through the SBG's NAT.
Click Apply to save your changes.
Click Reset to return the screen to its last-saved settings.
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