SIP ALG - Why Does It Need to be Disabled? - Xinix

sip alg

Voice over Internet Protocol (VoIP) has transformed the whole communication system. When it is about VoIP, the significance of session initiation protocol (SIP) cannot be overstated. SIP Trunk is the protocol based on VoIP technology and is responsible for initiating, maintaining and terminating the communication session.

SIP ALG stands for Session Initiation Protocol Application Layer Gateway. This is one of the inbuilt features of network routers and operate as function of firewall. SIP ALG is intended to assist the traversal of sip traffic across the network address translation NAT devices. However, due to its intricate nature and complexity, SIP ALG does more harm than good. This guide will help you have a better understanding of application layer gateway sip and why you need to turn off SIP ALG?

What is SIP ALG?

SIP ALG is mostly an inbuilt function of most commercial networked routers. This feature serves as an independent firmware and function of the firewall. The goal is to inspect and modify the VoIP data packets in such a way to enhance quality and reliability of calls. SIP ALG works in client side local area network gateway, ensuring seamless connectivity and excellent call quality. Unfortunately, this well-intended feature mostly fails to serve the purpose. Instead of helping, if SIP ALG is enabled, it hinders call quality. Consequently, the user faces frequent issues including poor call quality, poor connectivity and so on.

Here is why it happens:

The underlying mechanism of a SIP call

VoIP calls based on SIP protocol, consists of a few steps:

  • Initiation – this is the invite step when a user tries to connect to another user
  • Response – following call invite, response is sent back to the user one who initiated the call
  • Answering –  the acknowledgment or acceptance of inbound call but not the actual call yet
  • Conformation – final acknowledgment or call receiving by the user who initiated the call
  • Call stage – it is when the connection is established and the users can communicate

Though it appears to be a lengthy process, in reality this is a matter of a few seconds. Simply put, the whole process is so delicate, intricate and yet super fast. Therefore, there is a huge probability that a part of the data packet is lost during ALG modification.

Understanding how SIP ALG works

It comprises of two distinct technologies:

  • SIP – Session Initiation Protocol, responsible for initiating, maintaining and terminating VoIP calls. It overlooks all significant aspects such as device registration, establishing connection and ensuring call quality.
  • ALG – routers segment the ISP and internal network via network address translation NAT. NAT is the process which enables a unified and unique IP address and also ensures optimal connectivity. For this purpose, it modifies local IP addresses. Here comes ALG which serves as a proxy and rewrites the destination IP address.

Every time the system acknowledges a connection attempt for sip call, the ALG modifies the IP address through a process called “scripting”. Here is what goes wrong. When scripting, most of the time a part of an IP address is often lost or gets changed. This results in poor call quality and poor performance.

In addition to SIP, all other signaling protocols such as RTP, RTSP and SDP, have the same issue because these are subgroups of SIP packets.

Now that you know the whole issue, here is why you need to disable SIP ALG:

Why do you need to turn off SIP ALG?

If you rely on cloud based telephony and you have not disabled this feature yet, here are some  major problems you might face:

  • Unable to register the call – in order to establish a stable connection, SIP requires multiple acknowledgments. If you do not turn off SIP ALG, it can result in failed registration and inability to make calls.
  • One-Way Audio – another issue resulting from SIP ALG working in the background. In this case, the data packets are lost and one of the users cannot hear the other.
  • Frequent call drops – due to packets being lost during transmission, users often face poor call quality. Call drops frequently, users may experience lapses in sound transmission, echo. Not only this, the user’s phone does not ring when called and sometimes calls go straight to voicemail even without any reason.
  • Unable to support advanced features –  when this feature is on, it disrupts connection as well as interfere with some of advanced sip features such as conference call, call transfer and so on.
  • Device incompatibility – sometimes the devices are designed to function without the aid of SIP ALG. In such a case, having it on will result in device compatibility issues.
  • Security threats – when data packets are being transferred and modified at such erratic pace, there might be security gaps. When data packets are modified and transferred, it might open additional ports which can be manipulated by hackers for unauthorized access and data breaches.
  • Lost connection – poor connectivity often end up in lost connection which is quite frustrating

In order to prevent and resolve these above-mentioned issues, service providers suggest to disable SIP ALG. This ensures optimal connectivity and call quality.

How to turn off SIP ALG?

You can always disable SIP ALG through the router’s software. For this purpose, you will need to log in into your router. In addition, the setting menu may vary depending on router manufacturer. Popular brands of routers include Cisco, TP-link, D-link, Asus, Linksys and Netgear.

Final thoughts

SIP ALG was originally designed to assist the communication. However, due to its intricacy and complexity, the well-intended feature does more harm than good. If not disabled, the user often faces multiple issues such as poor connectivity, frequent call dropping, failed registration, one-way audio and similar issues.

Simply put, VoIP telephony users need to turn it off in order to ensure optimal connectivity, seamless and uninterrupted communication and to achieve excellent call quality.

Read more: SIP Trunk Providers UK – Choose For Progress

7 Common VoIP Problems & Causes 


Should I enable or disable SIP ALG?

Ideally, SIP ALG should be turned off. This is because if enabled, users often face multiple issues with call quality.

What is the problem with SIP ALG?

Though the feature is intended to assist transmission of data packets across the NAT devices, it often does more harm than good. Users often face poor connectivity, frequent call dropping, one-way audio and similar issues that disrupt communication.  

Why is SIP Application Layer Gateway enabled?

The SIP application layer gateway serves as a function of the firewall and intended to inspect the data packets and modify them when required.

Is this sip application layer gateway feature a security risk?

When data packets are modified and transferred at a fast pace, it might open additional ports which can be manipulated by hackers for unauthorized access and data breaches. Therefore, sip application layer gateway can be a security risk.

Is it safe to turn off SIP?

The best way to optimize VoIP communication and to achieve excellent call quality with no interruption and distortion, it is recommended to turn off sip application layer gateway.

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