Title: A Security Analysis of the NTP Protocol 
Authors: Matt Bishop   
File name: pubdocnetworkm.bishop.ntp.security.ps.Z 
Filesize: 47904 bytes 

Abstract: The Network Time Protocol is being
used throughout the Internet to provide an accurate time service.
This paper examines the security requirements of such a service,
analyzes the NTP protocol to determine how well it meets these
requirements, and suggests improvements where appropriate.