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SIP Parameters : P-Preferred-Identity
< Based on 3GPP 24.229 5.1.2A.1.1 General >
the UE may insert a P-Preferred-Identity header field in any initial request for a dialog or request for a standalone transaction as a hint for creation of an asserted identity (contained in the P-Asserted-Identity header field) within the IM CN subsystem.
The UE shall determine the public user identity to be used for this request as follows:
1) if a P-Preferred-Identity was included, then use that as the public user identity for this request; or
2) if no P-Preferred-Identity was included, then use the default public user identity for the security association or TLS session and the associated contact address as the public user identity for this request;
< Based on RFC 3325 >
The P-Preferred-Identity header field is used from a user agent to a trusted proxy to carry the identity. The user sending the SIP message wishes to be used for the P-Asserted-Header field value that the trusted element will insert.
A P-Preferred-Identity header field value MUST consist of exactly one name-addr or addr-spec. There may be one or two P-Preferred-Identity values. If there is one value, it MUST be a sip, sips, or tel URI.
If there are two values, one value MUST be a sip or sips URI and the other MUST be a tel URI. It is worth noting that proxies can (and will) remove this header field.
If a P-Preferred-Identity header field is present in the message that a proxy receives from an entity that it does not trust, the proxy MAY use this information as a hint suggesting which of multiple valid identities for the authenticated user should be asserted.
A user agent only sends a P-Preferred-Identity header field to proxy servers in a Trust Domain; user agents MUST NOT populate the P-Preferred-Identity header field in a message that is not sent directly to a proxy that is trusted by the user agent.