The Session Initiation Protocol (SIP) has a mechanism for conveying the identity of the originator of a request by means of the P-Asserted-Identity and P-Preferred-Identity header fields. These header fields are specified for use in requests using a number of SIP methods, in particular the INVITE method. However, RFC 3325 does not specify the insertion of the P-Asserted-Identity header field by a trusted User Agent Client (UAC), does not specify the use of P-Asserted-Identity and P-Preferred-Identity header fields with certain SIP methods such as UPDATE, REGISTER, MESSAGE, and PUBLISH, and does not specify how to handle an unexpected number of URIs or unexpected URI schemes in these header fields. This document extends RFC 3325 to cover these situations. This document is not an Internet Standards Track specification; it is published for informational purposes.