NAT and Firewall Traversal Recommendation

Q: What is NAT?

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.

Q: What is "Firewall and NAT traversal"?

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. Junction Network's OnSIP Hosted PBX service utilizes a remote "server side" solution to this technical issue.

Q: Should I set NAT traversal technologies such as STUN and ICE on my phones?

No.

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.

Q: Should I configure SIP or NAT traversal technologies on my firewall?

No.

Many of today's commercial routers implement SIP ALG (Application Level Gateway), coming with this feature enabled by default. While ALG could help in solving NAT related problems, the fact is that many routers' ALG implementations are wrong and break SIP.

An 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. 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.

We recommend that you disable all NAT traversal technologies including, but not limited to, SIP ALG (ALG), and SIP Stateful Packet Inspection (SPI), and SIP Transformations.

Q: Why do you recommend I turn these features off?

Junction Network's OnSIP Hosted PBX service utilizes a complete "server side" solution to NAT traversal. This solution operates under the assumption that the end user is not employing any "client side" NAT traversal technologies on their devices or firewalls. In some cases, our server side solution can be confused by changes made by client side technologies - the net effect being that NAT traversal fails.

Problems typically arise when client side NAT traversal technologies are either a) successful enough that they convince our server side solution that the end user device is not behind a NAT, but otherwise fail to work correctly or completely, or b) fail to work to the extent that our server side solution still recognizes that the end user device is behind a NAT but does not function correctly because the original SIP packet has been modified on the client side in some manner.

Furthermore, our server side solution optimizes call routing by making use of the IP packet header in conjunction with the internal IP address information inside the SIP packet body. For example, if we determine that both the caller and callee are behind the same NAT, the media will be routed directly between phones such that it never leaves the internal LAN (for example, extension to extension calls within the same office). Client side NAT traversal technologies which modify SIP packets can interfere with this process and in some cases can cause calls that could stay on the internal LAN to routed across the Internet.

That said, there are completely valid and workable circumstances where a network administrator may require local NAT traversal technologies to be deployed on their router/firewall. However, the only configuration that we currently support is our server side solution.