At a high level SIP ALG (Application Level Gateway) is a feature in which the network device (router, access point, or any Laye2 or Layer 3 device) manipulates the payload section of a SIP Packet to change the Private address to be Public address. As the phone (or softphone) is not aware of the public address, all payload information would reference the device's Private Address. Network devices with ALG Enabled attempt to "correct" this by opening all SIP packets and manipulating the payload (body) of the packets by replacing private addresses with the public/Natted IP of the edge device and the Natted port. Unfortunately, some devices do not properly manipulate these packets causing them to be invalid or contain incorrect information.
When SIP ALG re-writes SIP packet headings and payloads, the process can disrupt the delivery process. This can make the device believe that it is not behind a NAT, when in fact it is. If ALG disrupts a call, it can lead to incoming call failure, and phones that unregister themselves, one-way audio, hold issues, and more. For this reason, 8x8 recommends that this function be disabled.
SIP ALG is not fatal in and of itself. There are times when SIP ALGs do not cause problems and only affects 8x8 service when SRTP is DISABLED or during the device activation process.
For more information on 8x8's recommendations see X Series Technical Requirements
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Paul Chase
8x8 Network Engineer (Pre-Sales and Deployment)
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