This solution operates under the assumption that the end user is not employing any "client side" NAT traversal technologies on their devices or firewalls. OnSIP utilizes a complete "server-side" solution to NAT traversal. Why do you recommend I turn these features off? So unless you know the SIP ALG on your router/firewall works (the SIP ALG on a Cisco router for example), we recommend that you disable it and all NAT traversal technologies including, but not limited to, SIP ALG (ALG), and SIP Stateful Packet Inspection (SPI), and SIP Transformations. A NAT router with a built-in SIP ALG can re-write information within the SIP messages (SIP headers and SDP body) making signaling and audio traffic between the client behind NAT and the SIP endpoint possible.
While ALG could help in solving NAT related problems, the fact is that many routers' ALG implementations are wrong and break SIP.Īn ALG understands the protocol used by the specific applications that it supports (in this case SIP) and does a protocol stateful packet inspection (SPI) of traffic through it. Unfortunately, many of today's lower end commercial routers implement SIP ALG (Application Layer Gateway)/SIP Helper, coming with this feature enabled by default. If you are using a Cisco ASA Router which is known to have a quality SIP ALG (sometimes referred to as SIP Helper) implementation that works well generally then enabling the SIP ALG/SIP Helper will generally work and not cause any issues. Should I configure SIP or NAT traversal technologies on my firewall? While there are some perfectly valid circumstances where configuring NAT traversal technologies on your local device is desired, unless you have a concrete reason to do so and clearly understand what you are doing, we strongly recommend that you disable all NAT traversal technologies including, but not limited to, STUN, ICE, and hard coding external addresses. Should I set NAT traversal technologies such as STUN and ICE on my phones? OnSIP Hosted PBX service utilizes a remote "server side" solution to this technical issue.
One of the technical challenges to implementing a SIP based VoIP solution is making everything work when a firewall and/or NAT is deployed between devices exchanging data. NAT (Network Address Translation) is a technology most commonly used by firewalls and routers to allow multiple devices on a LAN with 'private' IP addresses to share a single public IP address.
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