Call ID - unique identifier to group together messages. The To tag, From tag, and Call-ID identify a dialog. Call ID must be same for all request and responses within the dialog. Sip responses follow same path as corresponding requests in reverse order. Does this by using Via header. A response belongs to same transaction as request if: 1) Both request and response have the same value as the branch parameter in the top Via header 2) Both request and response have the same value as the method parameter in the Cseq header field