  Anonymity has become a significant issue in security field by recent advances
in information technology and internet. The main objective of anonymity is
hiding and concealing entities privacy inside a system. Many methods and
protocols have been proposed with different anonymity services to provide
anonymity requirements in various fields until now. Each anonymity method or
protocol is developed using particular approach. In this paper, first, accurate
and perfect definitions of privacy and anonymity are presented then most
important problems in anonymity field are investigated. Afterwards, the numbers
of main anonymity protocols are described with necessary details. Finally, all
findings are concluded and some more future perspectives are discussed.
