 sent instead of an INVITE, which is the default method if none is present. An example of the Refer-To header field in compact form with an HTTP URL is: r: http://www.artech-house.com> 6.2.21 Referred-By The Referred-By header field [20] is an optional header field in a REFER request and a request triggered by a REFER. It provides the recipient of a trig- gered request information that the request was generated as a result of a REFER and the originator of the REFER. This information can be presented to the user or have policy applied in deciding how the UA should handle the request. An unsigned Referred-By has the form: Referred-By: <sip:user@host.com> However, as this header field could be modified or fabricated, a more secure usage involves the addition of a Referred-By security token. The token is carried as a message body whose content id (cid) is indicated in the Referred- - By header field. The token is an S/MIME signature over a mes- sage/sipfrag, which contains, at a minimum, the From, Date, SIP Header Fields Call-ID, - Refer-To, and Referred-By header fields from the REFER request. An example part of a REFER request is shown here: Referred-By: sip:referror@referror.example cid=%3C39438823.FWQF33093@referror.example%3 Content-Type: multipart/mixed; boundary=unique-boundary-1 Content-Length: (appropriate value) -unique-boundary-1 Content-Type: multipart/signed; protocol="application/pkcs7-signature"; micalg=sha1; boundary=divider42 Content-ID: <39438823.FWQF33093@referror.example Content-Length: (appropriate value) -divider42 Content-Type: message/sipfrag Content-Disposition: auth-id; handling=optional From: sip:referror@referror.example Date: Thu, 14 Feb 2003 16:23:03 GMT Call-ID: 2203900ef0299349d9209f023a Refer-To: sip:refertarget@target.examp Referred-By: sip:referror@referror.example ;cid=3C39438823.FWQF33093@referror.example3E -divider42 Content-Type: application/pkcs7-signature; name=smime.p7s Content-Transfer-Encoding base64 Content-Disposition: attachment; filename=smime.p7s; handling=required (ap