About SIP HNT
SIP HNT is a technique the Oracle® Enterprise Session Border Controller uses to provide persistent reachability for SIP UAs located in private Local Area Networks (LANs) behind Nat/firewall devices. It relies on frequent, persistent messaging to ensure that the binding on the intermediary NAT device is not torn down because of inactivity. HNT does not require support for the NAT in the SIP endpoint.
The following diagram illustrates SIP HNT traversal.z
![](img/GUID-E9B61952-5A62-45EC-89DA-2EE53A86C7B1-default.jpg)
The Oracle® Enterprise Session Border Controller’s HNT function allows endpoints located behind NATs to communicate; providing means to traverse NATs. The Oracle® Enterprise Session Border Controller interacts with endpoints (using SIP) to allow persistent inbound and outbound signaling and media communications through these NATs.
The Oracle® Enterprise Session Border Controller automatically detects when an intermediate NAT exists between the UA and the Oracle® Enterprise Session Border Controller by comparing the Layer 3 IP address of a REGISTER message with the IP address indicated within the UA. The Oracle® Enterprise Session Border Controller sends signaling responses to the address and port that the request came from, rather than the address and port indicated in the request. The Via header in the request message indicates where the response should be sent.