User's Manual

AirStream 4000/4001 WiMAX CPE User Manual

9.8 STUN Configuration

STUN (Simple Traversal of User Datagram Protocol (UDP) Through Network Address Translators (NATs)) is a

network protocol allowing clients behind NAT (or multiple NATs) to find out its public address, the type of NAT

it is behind and the internet side port associated by the NAT with a particular local port. This information is used

to set up UDP communication between two hosts that are both behind NAT routers.

Protocols like SIP use UDP packets for the transfer of sound and/or video and/or real-time text signaling traffic

over the Internet. Unfortunately as both endpoints are often behind NAT, a connection cannot be set up in the

traditional way. This is where STUN is useful.

STUN is a client-server protocol. A VoIP phone or software package may include a STUN client, which will

send a request to a STUN server. The server then reports back to the STUN client what the public IP address of

the NAT router is, and what port was opened by the NAT to allow incoming traffic back in to the network.

By default, a STUN server in the public domain larry.gloo.net has been configured as the secondary server. The

Service Provider may specify address of its own STUN server, if implemented, as a primary server.

The response also allows the STUN client to determine what type of NAT is in use, as different types of NATs

handle incoming UDP packets differently. There are four main types of NAT:

 full cone NAT,

 restricted cone NAT, and

 port restricted cone NAT.

 symmetric NAT (also known as bi-directional NAT)

The CPE has built in intelligence to work with three of four main types: full cone NAT, restricted cone NAT, and

port restricted cone NAT, by pre-adjust the outgoing SIP messages and voice using public IP address and source

port discovered.

The CPE will not work with symmetric NAT (also known as bi-directional NAT).