Keeping the NAT Binding Open

Additional measures are also required to keep the NAT binding open because most NAT bindings are discarded after approximately a minute of inactivity. The Oracle® Enterprise Session Border Controller keeps the SIP NAT binding open by returning a short expiration time in REGISTER responses that forces the endpoint to send frequent REGISTER requests.

In order to keep the NAT binding open for SIP, the Oracle® Enterprise Session Border Controller maintains the registration state. When an endpoint first registers, the Oracle® Enterprise Session Border Controller forwards that REGISTER message on to the real registrar. You can define the real registrar using either of the following methods:

• Configure the SIP config registrar host and registrar port to indicate the real registrar.
• Map the SIP config registrar host and registrar port values to the SIP NAT home proxy address and home proxy port values. Then configure the SIP NAT’s external proxy address and external proxy port values to correspond to the real registrar.

  Note:

  A registrar can be located in a SIP NAT realm.

When a successful response is received, the Oracle® Enterprise Session Border Controller caches the registration to memory. This cached registration lives for the length of time indicated by the expiration period defined in the REGISTER response message from the registrar. The response sent back to the endpoint has a shorter expiration time (defined by the SIP config’s NAT interval) that causes the endpoint to send another REGISTER message within that interval. If the endpoint sends another REGISTER message before the cached registration expires, the Oracle® Enterprise Session Border Controller responds directly to the endpoint. It does not forward the message to the real registrar.

If the cached registration expires within the length of time indicated by the NAT interval, the REGISTER message is forwarded to the real registrar. If the Oracle® Enterprise Session Border Controller does not receive another REGISTER message from the endpoint within the length of time indicated by the NAT interval, it discards the cached registration.

The Contact Uniform Resource Identifier (URI) in the REGISTER message sent to the registrar by the Oracle® Enterprise Session Border Controller points at the Oracle® Enterprise Session Border Controller so that the proxy associated with the real registrar sends inbound requests to the Oracle® Enterprise Session Border Controller. This way, the inbound requests can be forwarded to the endpoint through the NAT binding.

The following example illustrates the SIP HNT registration call flow for the SIP HNT feature.

The following example illustrates the SIP HNT invitation call flow for the SIP HNT feature.