Introduction
SIP ALG stands for Session Initiation Protocol Application Layer Gateway. It was originally created to help VOIP (Voice Over Internet Protocol) phones function properly behind a NAT (Network Address Translation) device such as a broadband router or firewall. While the intent was to facilitate seamless communication, SIP ALG often causes more problems than it solves in today‘s network environments.
VOIP phones connect to a SIP server and send a REGISTER request, which includes the phone‘s IP address so the SIP server knows where to direct incoming calls. Problems arise when the phone is on a private network but the SIP server is cloud-based. In such cases, the SIP server receives the phone‘s private IP address (e.g., 192.168.1.100) and attempts to route the calls within its own private network instead of the public IP address of the phone‘s network.
SIP ALG attempts to resolve this issue by modifying the SIP REGISTER packet, replacing the private IP address of the phone with the public IP address of the router. The expectation is that the SIP server will then send incoming calls to the public IP address of the router, which will forward the traffic to the phone‘s private IP address.
Problems Caused By SIP ALG
However, replacing the private IP in the SIP packet with the router‘s public IP makes it difficult to distinguish between multiple phones on the same network, as they share the same public IP. Consequently, customers often report that only one phone receives calls while the others do not. Additionally, SIP ALG implementations are frequently flawed, causing SIP packet headers to be improperly formatted. Modern cloud-based VOIP systems do not rely on SIP ALG, as they employ their own methods for NAT traversal, which can fail when SIP ALG is enabled.
Common symptoms of SIP ALG issues include:
- Incoming calls only ring on one phone, or ring the wrong phone.
- One-way audio, where one party can hear the other, but not vice versa.
- Calls dropping when attempting to transfer them to another phone.
Recommendations
To ensure proper functionality, most cloud phone systems (including ours) require SIP ALG to be disabled. The NICC Standards forum also recommends disabling SIP ALG in their guidance published here.
How To Turn Off SIP ALG
SIP ALG can be disabled through your broadband router‘s settings. Look for options labeled "SIP" or "Gateway" settings. Consult the documentation for your router‘s make and model if you have trouble finding the settings. Unfortunately, the SIP ALG setting is not always user-configurable, in which case you will need to contact your internet service provider to have them disable it.
If It Causes Problems, Why Does My Router Have SIP ALG Turned On?
This is a good question. The straightforward answer is that router manufacturers may not be aware of the problems SIP ALG can cause. A more cynical perspective is that internet providers might enable SIP ALG deliberately to disrupt VOIP phone services other than their own, which would be an anti-competitive practice and a violation of net neutrality principles. However, it is challenging to prove such intent, and Hanlon‘s razor might apply: "Never attribute to malice that which is adequately explained by stupidity."
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Updated on 18/06/2024 by David Wood
As senior cloud solutions architect at TelephoneSystems.cloud, with a degree in computing, I bring over 29 years of extensive experience in cloud technology.