IETF RFC 3515 PDF

This document defines the REFER method. This Session Initiation Protocol (SIP) extension requests that the recipient REFER to a resource provided in the request. It provides a mechanism allowing the party sending the REFER to be notified of the outcome of the referenced request. This can be used to enable many applications, including call transfer.

In addition to the REFER method, this document defines the the refer event package and the Refer-To request header.