user IDs. As such, sessions destined to the user can be placed with any of the registered public user IDs. The serving proxy and application server(s) in the termination network may apply certain filtering rules or services based on the public user ID contained in the Request-URI. The UA may also apply certain filtering rules or services based on the called public user ID.
Therefore, it must be possible for all sessions to deliver the dialed public user ID to the terminating entities, such as the serving proxy, application servers, and terminating UA.
4.11. Identifiers Used for Routing
Routing of SIP signaling within IMS must use SIP URLs as defined in SIP [2]. E.164 [34] format public user identities must not be used for routing within IMS, and session requests based on E.164 format public user identities will require conversion into SIP URI format for internal IMS usage.
We believe that this requirement is achieved by translating E.164 numbers into SIP URIs. A database, such as ENUM [9], might do the job.
4.12. Hiding Requirements
Although the requirements included in this section are not optional, the hiding feature is optional to use through configuration. This means that a network operator can, at his desire, switch the hiding functionality on or off.
4.12.1. Hiding of the Network Structure
A network operator need not be required to reveal the internal network structure to another network (in Via, Route, or other headers) that may contain indication of the number of SIP proxies, domain name of the SIP proxies, capabilities of the SIP proxies, or capacity of the network.
4.12.2. Hiding of IP Addresses
A network need not be required to expose the explicit IP addresses of the nodes within the network (excluding firewalls and border gateways).
4.12.3. SIP Hiding Proxy
In order to support the hiding requirements, a SIP hiding proxy may be included in the SIP signaling path. This additional proxy may be
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