ZyXEL Communications P-2000W V2 ユーザーズマニュアル

P2000W V2 Troubleshooting Guide

Fortunately, there are already ways to workaround the NAT for SIP. The most
commonly used way is listed below.

  SIP pass-through support on NAT router (require router vendor implement it).
  STUN.
  Outbound Proxy.

There are advantage and disadvantage of each technology.

For example SIP pass-through on NAT router this requires router vendor
implementation. From carrier point of view this does not seem feasible as not every
NAT router vendor implement this on their router and they do not have control of
which router their customer is using.

STUN is a simple traversal of user datagram protocol. It helps to traverse UDP
through NAT. STUN allows applications to discover the presence and type of NATs
between the host and public Internet.  It also provides the ability for application to
determine the public IP allocated to the host by NAT. STUN works with many
Existing NATs, and does not require special handling from NAT router.  As a result it
can make SIP pass-through NAT. Please refer to RFC 3489 for more details.

The outbound proxy is a normal SIP proxy. You configure your client, the phone or
software, to use the proxy for all SIP sessions, just like when you configure your Web
browser to use a Web proxy for all Web transactions. In some cases, the outbound
proxy is placed alongside the firewall and is the only way to let SIP traffic pass from
the internal network to the Internet. For more detail you can refer to RFC 3261 for
details.