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A registration is a binding between a SIP URI, or Address Of Record, and one or more contact URIs, which are additional resources to contact to reach the user identified by the Address Of Record.

The SBC supports

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 supports UE subscribing to registration event package as defined in RFC 3680. This event defines a message-body in NOTIFY which carries UE contact details. SBC relays The

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 relays the message body appropriately. The SBC also

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 also performs V4-V6 interworking (if necessary) for SUBSCRIBE session setup.

If Contact header is transparently passed, there is no special handling needed at P-CSCF. Else, to support this package SBC intercepts

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 intercepts and interprets NOTIFY’s message-body and changes the Contact header. This feature assumes that Contact header transparency is enabled along with it.

The SBC can

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 can receive SUBSCRIBE message (for registration event package) from a registered UE and relay the message based on stored Service-Route header and perform V6-V4 interworking if access side supports V6 and IMS CN supports V4 (i.e. stored Service-Route header points to V4). For this request, the SBC can

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 can:

• Act as a B2B UA and change the Contact header to point to itself towards the egress call-leg
• Optionally insert Record-Route header in the egress SIP request
• Relay 200 OK or any other final response towards UE, after interworking 200 OK’s Contact header appropriately

Similarly, SBC can the

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 can receive NOTIFY (for registration event package) towards a registered UE and relay the message towards UE if the contact header transparency is enabled. Note that in the NOTIFY request, the UE IP address is received in Request-URI and the top-most Route header points to SBCthe

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.

When REGISTER is interworked from V6 to V4 or vice versa, or if the contact header transparency is disabled, the SBC changes

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 changes the Contact header as it egresses the REGISTER request. The Contact address is changed to point to itself. Hence a NOTIFY is received for registration event package in such a scenario, SBC modifies The

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 modifies the Contact header in NOTIFY message-body and send it towards UE.

• NOTIFY message body sent by S-CSCF contains all the contact headers registered for the public identity. The contact headers may also include the contacts registered by UE devices through different P-CSCF. The SBC modifies

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   modifies only the contact headers pointing to itself to UE IP address and leaves other contact headers unchanged.
• The

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   modifies SBC modifies the contact address to all the contact address registered by UE if the multiple contact header registration (by single REGISTER) is supported by the SBC

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  .

When the SBC acting

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 acting as P-CSCF performs IP interworking, it receives NOTIFY (for registration event package) towards a registered UE and relay the same towards UE.

• In this case, the SBC interworks

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   interworks the IP address in NOTIFY’s message-body. The IP address in the message-body points to SBCthe

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  . This changes to UE’s IP address before relaying NOTIFY towards the UE.
• In this case, if UE has registered multiple contact addresses then SBC interworks the

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   interworks the IP address pointing to it to all the contact headers registered by UE.

The SBC also

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 also supports multiple contact address registrations by UE.

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Info
For configuration details, seerefer to: CLI: Common IP Attributes - SIP - CLI EMA: Common Ip Attributes - Relay Flags To learn more about registrations, see SIP Registration.