PTP support on SONiC (#232)

doc/ptp/ptp.md

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+PTP support on SONiC
+
+# Introduction
+
+## Precision Time Protocol (PTP)
+
+- Standardized protocol, IEEE1588
+- Synchronize the clocks in local computing systems and devices
+- Microsecond to sub-microsecond accuracy and precision
+- Administration free 
+	- Capability to autonomously decide time server(master)
+	- called Best Master Clock Algorithm (BMCA)
+
+## How PTP works
+
+Time offset between master and slave clocks is calculated based on timestamps at packet sending and receiving.
+
+path delay = ((t4-t1) - (t3-t2))/2
+
+offset from master clock = (t2 - t1) - path delay
+
+![PTP Messages for boundry clock](ptp-messages.jpg)
+
+Achieving high precision requires hardware timestamping
+
+- Hardware assisted timestamp at PHY or MAC layer
+- Get time from PTP Hardware Clock (PHC) on NIC
+
+
+## PTP on Linux
+
+- PTP protocol is implemented on user-land
+- Kernel feature for PTP
+	- Socket option SO_TIMESTAMPING for packet timestamping
+		- Hardware timestamping
+	- PHC subsystem
+		- Allow to access PHC via clock_gettime/settime/adjtime system calls
+
+## LinuxPTP
+
+- ptp4l
+	- Implementation of PTP (Ordinary Clock, Boundary Clock)
+- phc2sys
+	- Synchronize two clocks (typically PHC and system clock)
+
+
+ptp4l adjust PHC, phc2sys adjusts system clock.
+
+### ptp4l servo
+
+ptp4l has PI (proportional-integral) controller servo
+- A kind of feedback loop
+- Determine frequency set to PHC
+
+![ptp servo](ptp-servo.PNG)
+
+
+# LinuxPTP on SONiC
+
+Requirements:
+- Trap PTP packet/events
+	- https://github.com/opencomputeproject/SAI/pull/841
+- NETDEV device support SO_TIMESTAMP option
+	- https://www.kernel.org/doc/Documentation/networking/timestamping.txt
+	- Section 3.1
+- Develop PHC driver
+	- https://www.kernel.org/doc/Documentation/ptp/ptp.txt
+
+
+# Reference
+
+- [Precision Time Protocol on Linux ~ Introduction to linuxptp](https://events.static.linuxfound.org/sites/events/files/slides/lcjp14_ichikawa_0.pdf) 
+- [IEEE 1588 PTP on Cisco Nexus 3100 Platform and 9000 Series Switches](https://www.cisco.com/c/en/us/products/collateral/switches/nexus-9000-series-switches/white-paper-c11-733921.html)