Ericsson Ericsson Nokia SIP has a mechanism for conveying the identity of the referrer of a request by means of the Referred-By header field. This header field may be used when exploding a SIP MESSAGE request to a pre-defined group URI and when exploding a SIP INVITE request to an ad-hoc group or to a pre-defined group URI. The Referred-By header is only included if the P-Asserted-Identity header field or From header field in the exploded SIP requests needs to carry another value, e.g. the URI of a pre-defined group, or a conference focus URI. In those cases, the Referred-By header field in the resulting exploded requests is set to the P-Asserted-Identity header field or to the From header field of the original SIP request received before exploding to convey to the receiver the identity of the original inviting sender. RFC 3892 restricts the value of the header to only one SIP URI. However the P-Asserted-Identity header field currently allows two URI values and may be expanded in the future to carry more than two values as described in draft-ietf-sipping-update-pai-09. This document extends the Referred-By definition to support more than one value as well.