References

http://www.nsa.gov/snac/

http://www.ntp.org/ntpfaq/NTP-s-def.htm

http://www.cisco.com/en/US/tech/tk648/tk361/technologies_tech_note09186a0080120f48.shtml

http://www.cisco.com/en/US/products/ps6350/products_configuration_guide_chapter09186a008044036d.html

http://www.cisco.com/en/US/products/ps6350/products_configuration_guide_chapter09186a008030c799.html

http://www.cisco.com/en/US/tech/tk869/tk769/technologies_white_paper09186a0080117070.shtml

http://www.cisco.com/en/US/products/sw/iosswrel/ps5207/products_feature_guide09186a008022a7ce.html