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.
References are either specific (identified by date of publication, edition number, version number, etc.) or non specific.
For a specific reference, subsequent revisions do not apply.
For a non-specific reference, the latest version applies. In the case of a reference to a 3GPP document (including a GSM document), a non-specific reference implicitly refers to the latest version of that document in the same Release as the present document.
For the purposes of the present document, the terms and definitions given in TR 21.905 and the following apply. A term defined in the present document takes precedence over the definition of the same term, if any, in TR 21.905.
The entity that hosts and manages the Domain Name on behalf of the Domain Name Owner on the Internet.
As defined in  and .
Domain Name Owner:
The entity that is noted in the Internet (i.e. ICANN or one of its subsidiaries) as owning the Domain Name.
IMS Intermediate Network:
An network that provides an interconnect between two operator's networks, an operator's network and another Intermediate Network, or between two other Intermediate Networks.
IMS Network Independent Public User Identity (INIPUI):
A Public User Identity in the form of a SIP URI where the Domain Name part is a Shared Domain Name.
IMS Network Independent Public User Identity (INIPUI) User:
An entity that is identified by an IMS Network-Independent Public User Identity.
IMS Network-Independent Public User Identity (INIPUI) Host:
The entity that hosts and manages the INIPUIs.
IMS Network-Network Interface (NNI):
The connection between two operators, that can consist of zero or more Intermediate Networks, and which is totally transparent to the End User. An example of a common NNI used by mobile operators is the IP Packet eXchange (IPX).
IMS Network-Independent Public User Identity (INIPUI) Registry:
An entity that provides mapping of IMS Network-Independent Public User Identities and IMS NNI-Routable Identifiers.
IMS NNI-Routable Identifier:
An identifier that uniquely identifies the terminating network (and together with the username, identifies a unique subscriber belonging to that network).
An IMS operator who provides IMS-based Services for a Shared Domain Name.
IP Packet eXchange (IPX):
The IPX is an inter-operator IP backbone network that is transparent to subscribers. The commercial details of it are defined in  and the technical details are defined in .
Shared Domain Name:
The Domain Name in the IMS Network-Independent Public User Identity, and which is served by multiple INIPUI Operators.
For the purposes of the present document, the abbreviations given in TR 21.905 and the following apply. An abbreviation defined in the present document takes precedence over the definition of the same abbreviation, if any, in TR 21.905.
Internet Corporation for Assigned Names and Numbers
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. firstname.lastname@example.org, 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  and  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.