Precision Time ProtocolPrecision Time Protocol

IEEE1588v2

TICTOC BOF IETF Prague 2007

Ron Cohen Resolute Networks

ronc@resolutenetworks.com

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AgendaAgenda

• Status• PTPv1 overview• PTPv2 major additions• Transparent clocks• PTP and TICTOC

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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

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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

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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

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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

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Timing Protocol OperationTiming Protocol Operation

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Precision using HW time-stampingPrecision using HW time-stamping

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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

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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™)

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Sync and Delay-Req formatsSync and Delay-Req formats

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End-to-End Transparent ClocksEnd-to-End Transparent Clocks

E2E TCs cancel queuing and processing delays

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Peer-to-Peer Transparent ClocksPeer-to-Peer Transparent Clocks

P2P TCs cancel queuing, processing and propagation delays

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Peer Delay Measurement (Optional) Peer Delay Measurement (Optional)

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E2E TC Enabled PSNE2E TC Enabled PSN

Packet queuing and processing is removed

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P2P TC Enabled PSNP2P TC Enabled PSN

Topology change does not effect slave performance

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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

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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

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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

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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

Questions? Questions?

Ron CohenResolute Networks

ronc@resoluteNetworks.com

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NTP message formatNTP message format