What Is Sip ALG (Application Layer Gateway) Voip firewall

SIP ALG (Application Layer Gateway) is a security component, commonly found in a router or firewall device .
An ALG is created in the same way as a proxy policy and offers similar configuration options, SIP Application Layer Gateway (ALG) provides functionality to allow VoIP traffic to pass both from the private to public and public to private side of the firewall when using Network Address and Port Translation (NAPT), SIP ALG inspects and modifies SIP traffic to allow SIP traffic to pass through the firewall.
Many of today's commercial routers implement SIP ALG, coming with this feature enabled by default. 

Basics SIP ALG operations
1-Control SIP call activity,The call duration and inactivity media timeout features help you to conserve network resources and maximize throughput.
2-Protect the SIP proxy server from denial-of-service (DoS) flood attacks
3-Enable unknown messages to pass when the session is in Network Address Translation (NAT) mode and route mode.

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Anonymous said...

I've had 2 routers with SIP ALG in them and had to disable this feature on both routers. What it does for me is disable inbound VoIP traffic. The caller can hear me, but I cannot hear them. It's too bad it causes these issues because the internet is riddled with posts where people are having this very problem.

Anonymous said...

Anyone else having the same problem. This is called one way voice. One way to fix this is forward the RTP ports to the static ip address assigned to your voip adapter.

Anonymous said...

I work for a SIP trunking provider and we advise our customers to disable SIP ALG. The intention is good but most SIP ALGs are flawed and cause problems such as you have mentioned.

Good reference:http://www.voip-info.org/wiki/view/Routers+SIP+ALG

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