17.4. Network Address Translation

17.4. Network Address Translation
	Chapter 17. VoIP

Network Addres Translation (NAT) is common on many Internet connections in homes and offices. It means that the office uses private IP addresses (e.g. 192.168.1.1) and these are mapped (translated) to one external address.

NAT is known to cause a lot of problems with a wide variety of applications and protocols. One of those that suffers a lot from the problems of NAT is VoIP. There are, sometimes, ways of making this work, but it usually a compromise of some sort and prone to problems.

The FB2900 provides some key ways to tackle the issues of NAT.

An FB2500/FB2700 can be used as a gateway device in a home or office - using PPPoE to connect to the Internet. This means the FireBrick has a real external IP address without NAT. The FireBrick can then connect to SIP handsets on the LAN using private IP addresses. The FireBrick provides a gateway for VoIP with no NAT implications.
Note
Some ISPs may use Carrier Grade NAT (CGNAT), and so a real IP address may involve additional cost.
The FireBrick can make use of the current Internet Protcol (IPv6). At present there are few carriers and handsets that work with IPv6, but this is improving all of the time. IPv6 avoids the need for NAT. The FireBrick acts as a media gateway which makes firewalling rules simple even when using IPv6, and allows IPv4 and IPv6 devices to interwork with no problems.
The FireBrick, when acting as a call server outside of any NAT connected telephones, can handle cases where devices are behind a simple port mapping NAT which has a timeout of at least 60 seconds. It recognises REGISTER and INVITE requests that appear to be behind NAT and will treat the requester IP/port as the contact rather than the stated contact in the message. At RTP level it will send all audio to the same IP and port from which it is received rather than the endpoint defined in the SDP. Registrations that seem to be from NAT connections will receive a null UDP packet at approximately 60 second intervals to keep the control session open on the NAT router. This is not foolproof but works in most cases with simple NAT gateways (including where a FireBrick is doing the NAT). It obviously also works where a NAT device is doing full ALG and changing the control messages and RTP accordingly.


17.3. Home/office phone system		17.5. Number plan

17.4. Network Address Translation

Note