Lucent This memo describes a private extension to SIP in the form of a P-Original-Dialog-ID header. The header is used to correlate two different dialogs across a B2BUA that is non-transparent to dialogs. The contents of the header identify the original dialog (Call-id, From tag, To tag) associated with the SIP entity that inserted the header into a request. When the same instance of the header is received at that SIP entity in a different request, then it is used to correlate the two different dialogs.