SIP HNT Forced Unregistration
If you use HNT and experience the issue explained in this section, consider using the Oracle® Enterprise Session Border Controller (E-SBC) forced unregistration feature. When this feature is enabled and a registration entry for an endpoint expires, the E-SBC notifies the soft switch to remove this binding using REGISTER message. In that REGISTER message, the expires header will be set to 0 and the expires parameter in the Contact header will also be set to 0.
The benefits of using forced unregistration include:
- Leveraging existing HNT configuration to provide near real-time information about the UA’s status to the registrar/softswitch
- Preserving resource utilization for the E-SBC and the softswitch by deleting a contact binding that is no longer valid or needed
- Preventing extra bindings from being generated at the softswitch (e.g., in instances when the UA or NAT restart)
This feature applies to:
- HNT endpoints with registration caching enabled by default, and when the nat-traversal parameter in the SIP interface configuration is set to always
- non-HNT endpoints with registration caching enabled, when the registration-interval parameter in the SIP interface configuration is used in the expires header sent to the UA in the 200 OK
When to Use Forced Unregistration
For typical HNT use, it is common that the registration interval between the client UA and the Oracle® Enterprise Session Border Controller (E-SBC) is between 60 and 120 seconds. This differs significantly from the re-registration interval between the E-SBC and the and the registrar, which varies from approximately 30 to 60 minutes.
If the UA fails to refresh its registration, the contact binding at the E-SBC is deleted after the registration expires. This expiration is determined by the expires= header in the 200 OK. The binding at the real registrar will remain intact. This creates a discrepancy between the real state of the UA and state of the softswitch. In the best case scenario, the contact binding expires at the softswitch after a few minutes.
For network management, this discrepancy can be problematic because the service provider would be unaware of the UA’s status until the binding expires at the softswitch. This can take a considerable amount of time to happen.
In addition, the E-SBC encodes a cookie in the userinfo of the Contact header in the REGISTER message. This is a function of the source IPv4 address and port from which the request came, i.e., the ephemeral port in the NAT for DSL scenarios. Therefore, additional bindings that remain for long periods of time are created at the registrar if, for example, the:
- UA reboots
- Ethernet link between the UA and the DSL router is lost for over two minutes
- DSL crashes
- DSL/ATM layer between the DSL router
Caution for Using Forced Unregistration
You should use caution when applying SIP HNT forced unregistration for the following reasons:
- It can have an impact on the performance of your Oracle® Enterprise Session Border Controller and the registrar, especially when you have a large number of HNT endpoints in your configuration that become unavailable simultaneously.
- It is possible that the registrar might become vulnerable to overload in the case where the registrar must authenticate a large number of register messages generated when HNT endpoints are de-registered. It is possible that the cached registration credentials might become “stale” over time (e.g., the nonce value usually has a limited lifetime). Without proper credentials, the registrar will reject the de-registrations.
Given these concerns, we recommend that you consult with your Oracle systems engineer before adopting the use of forced unregistration.
