When to Use a SIP ALG Detector Test and What To Do Next

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So you’ve just installed your VoIP phone system, but now you’re facing issues. Maybe your calls are dropping or the audio is acting up.

If that’s the case, there’s a good chance the SIP ALG in your network devices is messing with your VoIP calls.

A Session Initiation Protocol Application Layer Gateway (SIP ALG) is a feature usually found in routers and firewalls. It’s designed to inspect and modify VoIP traffic to assist in the traversal of a Network Address Translation (NAT) device.

It does this by translating the private Internal Protocol (IP) addresses and ports of devices inside your network to the public IP address and corresponding port, making sure calls can travel properly.

That said, while SIP ALG is intended to help with VoIP calls, it can sometimes cause issues. That’s because it can alter the IP addresses and ports in the packet headers. As a result, you may notice a disruption in your connection or call quality.

In that case, you may want to use a SIP ALG Detector test, which is a program that lets you discover the presence of SIP ALG on your network. That way, you can disable it from your VoIP systems so you can avoid call disruptions.

Differences between SIP and VoIP graphic from Nextiva.

Why Run A SIP ALG Detector?

If this all sounds confusing, don’t worry. Once you get past the technical jargon, the idea behind running a SIP ALG detector is simple.

First, let’s back up for a second and talk about Session Initiation Protocol (SIP).

SIP is a signaling protocol that makes online communication methods, like VoIP calls and video conferencing, possible. Basically, it defines how data is transmitted and received over a network so devices can communicate properly and calls run smoothly.

Many networks use NAT to conserve public IP addresses. SIP messages, which contain IP addresses, may need translation as they traverse from private networks to the public internet. SIP ALG helps in correctly translating these addresses, allowing for successful communication.

Besides translation, the SIP ALG can dynamically allocate ports for multiple streams of communication, such as voice and video. When ports are open, they’re available for communication.

Unfortunately, SIP ALG can cause issues with VoIP calls, specifically affecting the quality and reliability. Common problems often found from this VoIP security feature include:

  • Call Dropping: SIP ALG may improperly modify SIP packets, leading to call drops or terminations. Oftentimes, SIP ALG interferes with the natural flow of communication between VoIP devices.
  • One-Way Audio: Incorrect handling of SIP packets by SIP ALG can result in one-way audio issues. That means that one party can hear the other, but the other side can’t hear anything.
  • Audio Quality Problems: SIP ALG interference may lead to degraded audio quality during VoIP calls. This can manifest as choppy or distorted sound, making it difficult for users to understand each other.
  • Registration Failures: VoIP devices often need to register with servers to establish their presence on the network. SIP ALG issues can interfere with the registration process, leading to failures and an inability to make or receive calls.
  • Failed Connections: SIP ALG may disrupt the dynamic port allocation process, resulting in failed connections between VoIP devices.

But how do you know the SIP ALG is the culprit for your call issues? Generally, they’re easy to spot. Sudden call disconnections, audio imbalances, and registration failures are all telltale signs of a SIP ALG issue.

The good news is that all you have to do is disable the SIP ALG in your routers.

Generally, disabling the SIP ALG is fairly straightforward when you’re on a home network. In many cases, it’s just disabling the SIP ALG in a single router.

But it gets harder when dealing with a larger network. SIP ALG can sneak into various devices like routers, firewalls, and even some modems, making it a bit trickier to locate and disable.

Routers are the usual suspects, but firewalls and certain modems can also harbor this feature.

To troubleshoot the issue, you first need to know whether SIP ALG is in your router. To do this, you’ll need to use a SIP ALG detector, which is a tool to help users determine whether SIP ALG is enabled on their network devices.

The SIP ALG detector works by sending test SIP messages through the network to observe how they’re processed by the network devices. Then, it analyzes the response. It detects the presence of SIP ALG if it makes modifications—like changes to IP addresses or ports—to the data packets as they pass through.

Where To Get SIP ALG Detector

Getting a SIP ALG detector can potentially save your VoIP calls and remove the headaches of dropped or failed calls. If you’re looking for a detector, there are several options to choose from:

  • Online Tools: Some websites offer SIP ALG detection services. Simply input your network details, and the tool scans for the presence of SIP ALG. It’s quick, easy, and doesn’t require any downloads.
  • Downloadable Apps: Another route is downloading dedicated SIP ALG detector apps. These applications are designed to run on your device and perform a thorough check of your network. If you have a specific network setup that is housed locally, downloadable apps work great since they’re installed directly on your device.
  • Built-in Features in Network Devices: Some routers and network devices come equipped with built-in SIP ALG detection features. Check your device’s settings or documentation to see if this option is available. It might save you the trouble of using external tools.

We recommend Wireshark since they offer some of the best SIP testing resources. Wireshark is an open-source and free network protocol analyzer that offers SIP testing, so you can identify whether SIP ALG is enabled.

Another option is Network Miner. They’re incredibly beginner-friendly and easy to use. On top of that, they take a deep dive into the data you’ve captured. That way, you can see how the SIP ALG is modifying the data packets and disrupting your calls.

Ideally, look for options from well-known network security companies, router manufacturers, or VoIP service providers. Keep an eye out for user reviews to gauge the reliability of a specific tool.

A Detector Won’t Disable SIP ALG

While a SIP ALG detector alerts you to the presence of SIP ALG on the network, you’ll need to manually disable the feature by following the device manufacturer’s instructions.

If you want to disable SIP ALG, the first thing you need to do is to log into your router or firewall. The specific instructions are dependent on the device you’re using.

In many cases, you’ll have to look under your security settings and uncheck the SIP ALG option. From there, simply reboot the device. That being said, some advanced corporate firewalls may require further action, such as port forwarding.

In a nutshell, the SIP ALG detector helps to find the presence of SIP ALG on a network. The responsibility then falls on you to manually implement the necessary steps, as outlined by the device manufacturers, to ensure smoother VoIP calls on your network.

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