1. Precision Time Protocol
IEEE1588v2
TICTOC BOF IETF Prague 2007
Ron Cohen
Resolute Networks

2. Agenda Status PTPv1 overview PTPv2 major additions Transparent clocks
PTP and TICTOC

3. The Precision Time Protocol (PTP)
PTPv1 published in 2002
Industries involved:
v1: Industrial Automation, T&M, Military, Power Generation and Distribution
v2 : Audio-Visio Bridges (802.1AS), Telecom and Mobile
Symposia in 2003, 2004, 2005, and in Vienna
Products: Microprocessors, GPS Linked Clocks, Boundary Clocks, NIC Cards, Protocol Stacks, RF Instrumentation, Aircraft Flight Monitoring Instruments, etc.
Information:
Version 2 PAR approved March Technical work completed

4. PTPv1 objectives Sub-microsecond synchronization
Intended for relatively localized systems
Applicable to networks supporting multicast
Simple, administration free installation
Support heterogeneous systems of clocks with varying precision, resolution and stability
Minimal resource requirements on networks and host components

5. Protocol overview Timing Protocol
Align slaves to master time
Measure delay between master and slave
Measure per-link delay (v2)
Synchronization Hierarchy ‘routing’ Protocol
Automatic Best Master Clock Algorithm
Determines the master-slave synchronization clock tree hierarchy
Management Protocol
Configuration and performance monitoring

6. PTP Master-Slave Hierarchy
*Clock symbols taken from ITU-T SG15 ‘synchronization modeling components – time’ contribution #249 Geneva-2007 by Mike Gilson of BT

7. Timing Protocol Operation

8. Precision using HW time-stamping

9. 1-step and 2-step clocks
1-step clock updates accurate timestamp (t1) in Sync message
2-step clock sends accurate timestamp (t1) in a Follow_Up message
Simplify design while avoiding queuing noise
Ease integration of security extensions

10. PTPv2 major additions
Synchronization accuracies better than 1 nanosecond
Higher sampling/message rates
Unicast communication
Correction for asymmetry
Transparent clocks
Redundancy
Configurable synchronization hierarchy
Decouple sync messaging from hierarchy signaling
Formal mechanisms for message extensions
Mappings to UDP/IPv4&6, Ethernet w/o VLAN, (also DeviceNet™, PROFINET, ControlNet™)

11. Sync and Delay-Req formats

12. End-to-End Transparent Clocks
E2E TCs cancel queuing and processing delays

13. Peer-to-Peer Transparent Clocks
P2P TCs cancel queuing, processing and propagation delays

14. Peer Delay Measurement (Optional)

15. Packet queuing and processing is removed
E2E TC Enabled PSN
Packet queuing and processing is removed

16. Topology change does not effect slave performance
P2P TC Enabled PSN
Topology change does not effect slave performance

17. Comparison between switches
Boundary Clock
E2E TC
P2P TC
Clock
Synchronized
Syntonized
Topology limitations
√ None
Homogenous*
with 1:1 connection each link
State maintained
Per port state
Per unicast contract state
Temporal message state for 2-step TCs
Per link state
Slave scalability
√ Hierarchical
Master sees all slaves
√ Hierarchical (multicast Sync)
Linear scalability
Control wander accumulation √
Topology change
Measure new delay
√ Pre-compute link delays
*restriction can be relieved using P-delay over MPLS

18. PTP protocol extensions
Define a profile that selects optional feature set, default and range of values
Define TLV extension to messages
Define flag-fields to be carried in event messages
Define alternate management scheme (SNMP)
Define alternate synchronization hierarchy selection (routing) algorithm
Define additional transport mapping (PTP over MPLS)
Define alternate clock quality levels and attributes

19. PTP and TICTOC Requirements Description Services
Frequency, Phase, Time and meta-Time services
Precision
Nano-seconds accuracy demonstrated
Sub nano-seconds granularity
Incremental
Add Transparent clocks or Boundary clocks to improve performance if required in critical junctions
Performance
Telecom grade performance has been demonstrated over non-PTP-aware networks
Scalable
Use of boundary clocks and/or transparent clocks to scale and maintain performance limits
Robust to master failures
Smart slaves synchronizing to two domains or two masters in a single domain.
Alternate master takes over when master fails
Robust to topology changes
P2P mechanism allows to pre-compute link-delays to minimize effect of topology change

20. PTP and TICTOC Derived Requirements Description HW friendly
Correction field is scaled nano-seconds. All computations by transparent clocks are performed on correction field
Correction field is in a fixed position from beginning of PTP header
Single Timestamp in fixed position immediately after PTP header for master and slave HW setting
Heterogonous design options
2-step clocks simplify master designs
1-step transparent clocks do not need to maintain per master-slave message states
Slave friendly
Timescale (Timestamps) is continuous. Timestamps do not ‘jump’ or ‘miss’ one second when leap event occurs
Time-stamps do not roll over

21. Ron Cohen Resolute Networks ronc@resoluteNetworks.com
Questions?
Ron Cohen
Resolute Networks

22. NTP message format