Today, we’re going to go over SIP ALG. This will be a slightly more technical blog than usual. We reccommend you show this to your IT professional at your company because the information can be very helpful in fixing certain VoIP issues.
SIP ALG (Application Layer Gateway) is a mechanism found in most routers that rewrites packets transmitted across the device. Certain protocols are processed by the application layer gateway (ALG) and rewritten to make for improved flow through a firewall or when NAT (Network Address Translation) is employed. The SIP protocol is one of various protocols controlled by this system.
One of the frequently brought up issues with VoIP relates to audio transmission and presence of a firewall and/or NAT traversal being configured. A lot of the time, a properly configured system may still have audio issues when making or receiving calls where only one party is heard during a call. Implementing the necessary changes to disable SIP ALG can usually resolve these issues.
What is SIP ALG and Why is it Bad?
The tricky thing with SIP ALG is the fact that many times, packet rewriting causes undesirable operation. The intent of the technology was to assist the packet flow of SIP and other packets and help solve NAT related problems. In this case, the ALG’s function is to perform a packet level inspection (SPI) of traffic coming through it. SIP messages would then be re-written by SIP ALG to allow the correct communication of signaling and voice traffic between endpoints and effective NAT traversal. However, the frequent result in lower end routers is a hindrance for data transmission due to poor implementations of ALG that break SIP. A lot of the time, the issues many experience relate to one-way or no audio, depending on who initiates the call.
In most cases, it is recommended that SIP ALG, SPI and SIP transformations are disabled.
Stay tuned for our guide on working with SIP ALG for specific routers
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