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HNT - nat traversal. SBC compare L3 address to the address in the sip request. If the Via and Contact header address match, but are different from L3 address, then this indicates nat'ed endpoint.
SBC will reply back to L3 address.

How to keep the nat binding (in nat device) open? - by returning short expiration time in register response - it will force endoint to send frequent REGISTER request.
SBC keeps registration state. - How to define the real register? Configure the register host in sip-config, setting the ip and port there. (or with sip nat - see guide).

When a successful response it received from registrar, SBC caches registration. The cached reg lives for length of time indicated by expiration period defined in the REGISTER response from registrar.
The response sent back to endpoint has a shorter expiration time - defined by sip config nat interval (in sip-interface object), and which causes endpoint to send another REG msg within that interval.
IF the endpoint send another REGISTER msg before the cached registration expires, SBC directly responds to endpoint. (it not forward to 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 Communications 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.

Once the SBC caches the registration locally:
The subsequent REGISTER requests from UA wil be procesed on the SBC, not on registrar, until the endpoint needs to re-register with the registrar, or fails to re-register, within the expiration time.

For each subsequent REGISTER from UA:
- if it contain the same SIP URI (address and port) in Via header as the one in the Via header in original REGISTER, the SBC refreshes the entry for the UA in registration cache, instead of creating a new entry.
- if it contain the same SIP URI (address and port) in Via header as the one in the Via header in original REGISTER, but with a differnet Call-ID from the one in original REGISTER, SBC still refreshes the entry for UA in reg chche, instead of creating a new one.

If SBC receive a REGISTER at or less than half of Expires time specified by registrar, such as 1800 seconds, SBC re-originnates the REGISTER and sends it to the registrar to keep the UA's registration in the Registrar.