SIP ALG and how it effects VoIP services.
Session Initiation Protocol Application Layer Gateway (SIP ALG) is a common feature in routers and firewalls that modifies SIP packets to allow VoIP traffic to pass through network address translation (NAT) enabled devices. While it can be helpful, SIP ALG can also cause issues that can affect VoIP service quality. In this article, we will discuss SIP ALG and how to detect it.
What is SIP ALG?
SIP ALG is a software component in a router or firewall that modifies SIP packets. When a VoIP device initiates a session, it sends a SIP message to the VoIP service provider. If the VoIP device is behind a NAT-enabled router or firewall, the SIP message will contain the private IP address of the device instead of the public IP address, which can cause issues. SIP ALG modifies the SIP message to replace the private IP address with the public IP address of the router or firewall, enabling smooth communication between the VoIP device and service provider.
How can SIP ALG affect VoIP services?
SIP ALG can cause several issues with VoIP services. One issue is that it can modify SIP messages incorrectly, leading to poor call quality or dropped calls. It can also introduce latency into the VoIP traffic by adding extra processing time for each SIP message modification. Additionally, SIP ALG may interfere with the ability of VoIP devices from different vendors to communicate with each other properly.
How to detect SIP ALG?
There are several ways to detect SIP ALG. The first is to check the router or firewall settings. SIP ALG is usually enabled by default, so it should be listed in the settings. However, some routers or firewalls may not have an option to disable SIP ALG, so checking the settings may not always be effective. We recommend to detect SIP ALG is to run a packet capture using a tool like this. This tool captures and displays all network traffic, including SIP messages, allowing you to see if SIP ALG is modifying the messages. To do this, connect your computer to the same network as the VoIP device and run the tool while making a VoIP call. If SIP ALG is enabled, you will see modifications to the SIP messages in the packet capture.
When you download and install the SIP ALG Detector provided in this article you can simply launch the detector and if it says positive you will need to contact your ISP and have it turned off. If you see the test results report False then your network is clear.
Example if No SIP ALG is detected on the network:
Conclusion
SIP ALG is a feature in routers and firewalls that modifies SIP packets to allow VoIP traffic to pass through NAT-enabled devices. While it can be helpful, SIP ALG can cause issues with VoIP services. Detecting SIP ALG can be done by checking router or firewall settings, running a packet capture, or checking with the VoIP service provider. If SIP ALG is causing issues with your VoIP service, disabling it may improve call quality and reliability.
Download the SIP ALG detector here! Or contact us at helpdesk@netcomplete.net and we can schedule a session to remote into your PC and run the tool.
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