Precision Time ProtocolPrecision Time Protocol
IEEE1588v2
TICTOC BOF IETF Prague 2007
Ron Cohen Resolute Networks
2
AgendaAgenda
• Status• PTPv1 overview• PTPv2 major additions• Transparent clocks• PTP and TICTOC
3
The Precision Time Protocol (PTP)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 2006. 2007 in Vienna• Products: Microprocessors, GPS Linked Clocks, Boundary Clocks,
NIC Cards, Protocol Stacks, RF Instrumentation, Aircraft Flight Monitoring Instruments, etc.
• Information: http://ieee1588.nist.gov• Version 2 PAR approved March 2005. Technical work
completed
4
PTPv1 objectivesPTPv1 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 overviewProtocol 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 HierarchyPTP 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 OperationTiming Protocol Operation
8
Precision using HW time-stampingPrecision using HW time-stamping
9
1-step and 2-step clocks1-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 additionsPTPv2 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 formatsSync and Delay-Req formats
12
End-to-End Transparent ClocksEnd-to-End Transparent Clocks
E2E TCs cancel queuing and processing delays
13
Peer-to-Peer Transparent ClocksPeer-to-Peer Transparent Clocks
P2P TCs cancel queuing, processing and propagation delays
14
Peer Delay Measurement (Optional) Peer Delay Measurement (Optional)
15
E2E TC Enabled PSNE2E TC Enabled PSN
Packet queuing and processing is removed
16
P2P TC Enabled PSNP2P TC Enabled PSN
Topology change does not effect slave performance
17
Comparison between switchesComparison between switchesBoundary Clock E2E TC P2P TC
Clock Synchronized Syntonized Syntonized
Topology limitations
√ None √ 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• Temporal
message state for 2-step TCs
Slave scalability
√ Hierarchical Master sees all slaves
√ Hierarchical (multicast Sync)
Linear scalability
Control wander accumulation
√ √
Topology change
Measure new delay Measure new delay √ Pre-compute link delays
*restriction can be relieved using P-delay over MPLS
18
PTP protocol extensions 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 TICTOCPTP and TICTOCRequirements Description
Services Frequency, Phase, Time and meta-Time services
Precision Nano-seconds accuracy demonstratedSub 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 TICTOCPTP and TICTOCDerivedRequirements
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 designs1-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
22
NTP message formatNTP message format