SIP ALG: Everything You Need to Know.

What is SIP ALG?

SIP (Session Initiation Protocol) ALG (Application Layer Gateway) is a type of feature found in many routers and firewalls that is designed to help with the configuration and management of SIP-based VoIP (Voice over Internet Protocol) communication.

Real-time sessions involving video, phone, message, and other communications applications can be started, maintained, and ended using the SIP protocol. SIP ALG is a software component that operates in a network device, generally a router or firewall, that permits the change of network address translation (NAT) protocols as well as SIP packet headers.

The main function of SIP ALG is to monitor SIP packets and modify them as necessary to allow communication between two SIP devices that are behind a NAT. NAT is a technology that allows multiple devices on a local network to share a single IP address when communicating with devices on the internet. However, it can cause problems for SIP communication, as it can modify SIP packets in such a way that they become unusable.

SIP ALG’s primary job is to keep track of SIP packets and alter them as necessary to enable communication between two SIP devices that are situated behind NATs. NAT is a mechanism that enables numerous local network devices to share a single IP address while interacting with internet-connected devices. Yet, because it has the ability to alter SIP packets in ways that render them useless, it may provide issues for SIP communication. But SIP ALG can also cause issues if not properly configured. It can interfere with SIP traffic, leading to call drops, one-way audio, or failed call setups. It can also conflict with other security features in a network, such as firewalls or intrusion prevention systems.

To avoid these problems, some experts recommend disabling SIP ALG altogether or configuring it properly. If you decide to enable SIP ALG, it is important to ensure that it is configured correctly and that it is not interfering with other network traffic or security features. SIP ALG is an important feature that helps to manage SIP-based VoIP communication in networks using NAT. However, it can also cause problems if not properly configured, and as such, it is important to understand how it works and how to properly configure it.

What are the functions of SIP ALG?

SIP ALG (Application Layer Gateway) is a feature found in some routers that is designed to help manage SIP (Session Initiation Protocol) traffic. SIP is a protocol used for initiating and terminating VoIP (Voice over Internet Protocol) calls.

The functions of SIP ALG include-

  1. NAT traversal: SIP ALG helps to overcome issues caused by Network Address Translation (NAT), which is used to allow multiple devices on a local network to share a single public IP address. SIP ALG can modify SIP packets so that they contain the correct IP addresses and ports, enabling SIP traffic to pass through the NAT device.
  2. Call establishment: SIP ALG helps to establish and manage VoIP calls by modifying SIP packets as they pass through the router. This includes translating IP addresses, setting up port forwarding rules, and opening and closing ports as needed.
  3. SIP header manipulation: SIP ALG can modify certain fields in the SIP headers, such as the “contact” field, to ensure that VoIP calls are properly routed and established.
  4. Call termination: SIP ALG can also help terminate VoIP calls by closing ports and removing port forwarding rules once the call is complete.
  5. Security: SIP ALG can also provide some security features, such as protecting against denial-of-service (DoS) attacks and preventing unauthorized access to the network

It’s worth noting that SIP ALG can sometimes cause problems with VoIP calls, such as call drops, poor call quality, and one-way audio. In some cases, it may be necessary to disable SIP ALG in order to ensure reliable VoIP communications.

SIP ALG Signs Impact VoIP Calls 

Session Initiation Protocol (SIP) is a protocol used to establish, modify, and terminate VoIP (Voice over Internet Protocol) calls. SIP Application Layer Gateway (ALG) is a feature that is commonly enabled on routers and firewalls to help with NAT (Network Address Translation) traversal. It works by modifying the SIP packets that are being transmitted between two devices on the network, in order to allow the VoIP traffic to pass through the NAT without interruption.

 SIP ALG can sometimes have a negative impact on VoIP calls. Here are some signs that SIP ALG might be causing issues with your VoIP calls:

  1. One-way audio: This is a common issue when SIP ALG is enabled. It can cause the caller or receiver to only hear the other person, but the other person cannot hear them. This is usually caused by the NAT not correctly handling the RTP (Real-time Transport Protocol) packets, which are responsible for transmitting the actual voice data.
  2. Dropped calls: Another sign that SIP ALG might be causing issues is dropped calls. If a call is dropped after a few seconds, it could be because SIP ALG is not correctly handling the session establishment process. This could be due to a misconfiguration of the SIP ALG or a compatibility issue with the VoIP service provider.
  3. Call quality issues: SIP ALG can also cause call quality issues, such as poor audio quality or delay in voice transmission. This can be due to the SIP ALG modifying the SIP packets in a way that is not compatible with the VoIP service provider, or due to the SIP ALG not correctly handling the NAT traversal.
  4. Registration issues: SIP ALG can also cause issues with the registration of the VoIP device with the service provider. This can cause the device to not be able to make or receive calls. This could be due to the SIP ALG modifying the registration packets in a way that is not compatible with the service provider.

If SIP ALG is improperly configured or incompatible with the VoIP service provider, it may negatively affect VoIP calls. If you notice any of the aforementioned symptoms, you should consider turning off the SIP ALG on your firewall or router to check if the quality of your VoIP conversations is affected.

How to turn off your SIP ALG?

SIP ALG (Session Initiation Protocol Application Layer Gateway) is a feature found in many routers that can cause problems with VoIP (Voice over IP) services. Turning off SIP ALG may be necessary to resolve issues with voice quality, call drops, or other VoIP-related problems.

Here are the steps to turn off SIP ALG:

Step 1- Access your router’s web-based configuration page. You can do this by opening a web browser and typing your router’s IP address in the address bar. You can find your router’s IP address by looking in the documentation that came with your router or by doing a web search for your router model.

Step 2- Log in to your router’s configuration page. You will need to enter the username and password for your router. If you haven’t changed the default login credentials, you can find them in the documentation or by doing a web search for your router model.

Step 3- Find the SIP ALG setting. The location of this setting may vary depending on your router model and firmware version, but it is usually found in the advanced settings section of the router configuration page.

Step 4- Disable the SIP ALG setting. This may be labeled as “SIP ALG”, “SIP Helper”, or “SIP Transformations”. Select the option to disable this setting and save the changes.

Step 5- Restart your router. This will apply the changes you made to the SIP ALG setting.

Note: Keep in mind that the location and labeling of the SIP ALG setting may vary depending on your router model and firmware version. If you are unsure about how to turn off SIP ALG on your router, consult the documentation that came with your router or contact the manufacturer’s customer support for assistance.


To conclude I would like to state some Fun and interesting facts on  SIP ALG:

Session Initiation Protocol Application Layer Gateway is a feature that’s commonly found on many routers and firewalls. Here are some fun and interesting facts about SIP ALG:

  • SIP ALG is designed to help with the implementation of VoIP (Voice over Internet Protocol) technology. It does this by allowing SIP packets to pass through network address translation (NAT) and firewall devices.
  • SIP ALG can be both helpful and problematic. On one hand, it can improve the quality of VoIP calls by providing better network support. On the other hand, it can also cause issues like dropped calls, one-way audio, and poor call quality if not configured properly.
  • Many VoIP providers recommend disabling SIP ALG because of the potential problems it can cause. If you’re experiencing issues with your VoIP calls, disabling SIP ALG is often one of the first steps recommended by support technicians.
  • SIP ALG is not a standard, and implementations can vary widely between devices and vendors. This can make troubleshooting and configuration difficult.
  • SIP ALG is not just used for VoIP traffic. It can also be used for other real-time communication protocols, such as video conferencing and instant messaging.
  • SIP ALG is sometimes referred to as H.323 ALG, because H.323 is another real-time communication protocol that it can support.
  • SIP ALG is not the only solution for supporting VoIP traffic through firewalls and NAT devices. Other solutions include STUN (Simple Traversal of UDP through NATs) and TURN (Traversal Using Relays around NAT).
  • SIP ALG is not just found on routers and firewalls. It can also be found on some endpoint devices, such as IP phones and softphones.
  • Disabling SIP ALG is not always a straightforward process. It can sometimes require logging into the router or firewall’s configuration interface and manually turning it off.
  • In some cases, disabling SIP ALG can actually cause more problems than it solves. This is why it’s important to consult with a VoIP expert before making any changes to your network configuration.

I hope you enjoyed reading the article and I was able to cover all the necessary information.

Frequently Asked Questions:

Loader image

Add a Comment

Your email address will not be published. Required fields are marked *