This study aims to examine the use cases and potential requirements that allow different Public User Identities of the form sip:user@domain belonging to the same domain to be:

• assigned by multiple network operators, where an individual Public User Identity is served by only a single network operator for all IMS services;
• provisioned by network operators in the case where other URI schemes have already been, or are going to be, provisioned by different service providers in the internet.

The Public User Identities could be SIP URIs associated with or derived from user identities of services from the Internet domain. This study also aims to clarify the relationships (e.g., business, interworking) between the Domain Name Owner, the assignee of sip:user@domain where it is not the same as the domain owner, and operators sharing the domain name. Any potential regulatory aspects will also be considered.

The following documents contain provisions which, through reference in this text, constitute provisions of the present document.

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IMS network independent PUIs based on Internet domain names are being explored to satisfy customer requirements for PUIs based on their domains rather than PUIs based on E.164 telephone numbers or operator owned domains. Currently, alphanumeric SIP URIs from the same domain, e.g. user.name@operator.com, can only be provided by a single operator. If the operator that provides URIs for domain @operator.com has subsidiaries in different countries, those subsidiaries cannot provide URIs for the same domain @operator.com. Permitting an operator's national/regional subsidiaries in different countries/regions to provide URIs for the same domain will allow operators to keep a single domain name for their international subscriber base. Although domains typically refer to an operator, e.g. @operator.com, users may wish to use URIs based on their own domains, rather than SIP URIs based on E.164 numbering or operator-owned domains. This is especially true in the case of large corporations as they would prefer to use their own domain name, e.g. @company.com. For the enterprise case, in general there are two scenarios that need to be considered. In the first scenario, a corporation has IMS-based services provided by different operators within one country. In the second scenario, a multinational corporation has IMS-based services provided on an operator-per-country basis. It follows that allowing different operators within one country and also from different countries to provide URIs for the same domain will provide increased flexibility. Implementing such PUIs in a secure fashion will present some novel challenges. Current and proposed implementations in [2] and [3] rely on a private DNS infrastructure to resolve PUIs to the serving operator which depends on information about E.164 and E.212 numbering resource assignment to operators and operator ownership of other domains.