Difference between ntp and ptp

• https://www.eecis.udel.edu/~mills/ptp.html
• http://www.fsmlabs.com/news/2012/06/28/choosing-between-ptp-and-ntp.html
• http://www.ntop.org/pf_ring/who-really-needs-sub-microsecond-packet-timestamps/
• http://vyodaiken.com/tag/timekeeper/
• http://www.thetradingmesh.com/pg/blog/mike/read/38896/beyond-latency-utilizing-ultra-accurate-network-timing-in-hft-systems