NAT (Network Address Translation) is a firewall or router function that operates by rewriting the IP addresses in the IP headers as packets pass from one interface to the other. When a packet, for example, is sent from the inside, the source IP address and port are rewritten from the private IP address space into the address space on the outside (Internet).
NAT rewrites the addresses but leaves the packets themselves untouched. This kind of translation works fine for many protocols, but causes a lot of trouble for SIP packets that contain address information in their content (for example an INVITE request from one IP address to another).
NAT traversal solves this problem, providing a “view from the outside” that makes it possible to replace the IP address in the SIP requests with the address shown on the other side of the firewall.
Note that in some cases NAT traversal is not necessary. Some public service providers of IP telephony keep track of the actual IP address used to register a phone, and the one used in the SIP requests from the same phone, and then replaces the addresses in the SIP messages.
|STUN||STUN (Simple Traversal of UDP through NATs) is a protocol that assists devices behind a NAT firewall or router with their packet routing. STUN is commonly used in real-time voice, video, messaging, and other interactive IP communication applications.
The protocol allows applications operating through a NAT to discover the presence and specific type of NAT and obtain the mapped (public) IP address (NAT address) and port number that the NAT has allocated for the application’s UDP (User Datagram Protocol) connections to remote hosts. The protocol requires assistance from a 3rd-party network server (STUN server).
STUN should be activated if an external SIP server cannot connect to the Avaya B179 behind a firewall NAT function and the SIP server supports STUN. A suitable STUN server is usually provided by the VoIP service provider.
Note: STUN might also be referred to as Session Traversal Utilities for NAT.
|STUN host||The IP address or public name of the STUN server.|
|Offer ICE||ICE (Interactive Connectivity Establishment), is a STUN addition that provides various techniques to allow SIP-based VoIP devices to successfully traverse the variety of firewalls that may exist between the devices. The protocol provides a mechanism for both endpoints to identify the most optimal path for the media traffic to follow.|
|TURN||TURN (Traversal Using Relay NAT) TURN is an extension of the STUN protocol that enables NAT traversal when both endpoints are behind symmetric NAT. With TURN, media traffic for the session will have to go to a relay server. Since relaying is expensive, in terms of bandwidth that must be provided by the provider, and additional delay for the media traffic, TURN is normally used as a last resort when endpoints cannot communicate directly.|
|TURN User||User authentication name on the TURN server.|
|TURN host||The IP address or public name of the TURN server.|
|Password||User authentication password on the TURN server.|