- tributed via re-invitation. Similarly, modified media types and distribution parameters like the definition of new mixers or network and multicast specific parameters are negotiated along the lines. Any failure of a re-INVITE will lead to session continuation with the previously established parameter set. Exist- ing conferences therefore cannot be disturbed or destroyed by inappropriate re-INVITE requests. 6.2.2.3 REFER Method The SIP REFER method [24] allows a user agent to request another user agent to access a resource it refers to. The URI of the referred resource is given within the refer-to argument: REFER sips: lucy@psychic.org SIP/2.0 Via: SIP/2.0/TLS alpha.brown.com: 5061; branch=z9hG4bknashds Max-Forwards: 70 From: Charlie <sips: charlie@brown.com>; tag=23431 To: Lucy <sips :lucy@psychic.org>;tag=1234567 Call-ID: 787878@alpha.brown.co CSeq: 9380 REFER Refer-To: <sips:hypnotic-talks@circles.com> Content-Length: 0 A REFER request can be sent either inside or outside an existing dialog and also provides mechanisms to notify the originator of the outcome of his referenced request. Initially created for call transfer, it is used in conferencing to add third parties. Any client may send a REFER request to a partner, asking him to send an INVITE to an established conference URI. Equally, the initiator may send a REFER to the conference focus asking it to invite the partner. The latter way provides the benefit of allowing a client to add a conference-unaware user agent that does not support the REFER method. Analogously, a client may ask the conference focus via REFER to terminate conference membership for a third party. © 2009 by Taylor & Francis Group, LLC Group Conference Management with SIP REFER requests may be cascaded in the following way. A participant, who wishes that a focus or another participant refers a third party into the con- ference by sending a REFER method, may express this by adding an escaped refer-to header field within its refer-to argument: REFER sips : hypnotic-talks@circles.co SIP/2.