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Open Time Server

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Spec Revision 1.0

The Open Time Server (OTS) is an Open, Scalable and Validated reference architecture that can be deployed in Data Centers or in an edge environments.
This spec can be accessed using


ACAtomic Clock
COTSCommodity off-the-shelf
GNSSGlobal Navigation Satellite System
NICNetwork Interface Card
NTPNetwork Time Protocol
OCPOpen Compute Project
OCXOOven-Controlled Oscillator
OTSOpen Time Server (Previously known as an Open Grandmaster or a Leader)
PHCPTP Hardware Clock
PTMPrecision Time Measurements
PTPPrecision Time Protocol
TAPTime Appliance Project
TCXOTemperature-compensated Oscillator
ToDTime of Day

Table 1. Abbreviations


OCP TAP is targeting to ease the addition of Time-sync as a service to the datacenter. The Project targets are to define the service requirements, deployment, and design of an open reference design.

The time-sync service is relying on a synchronization technology, for now, we are adopting PTP (IEEE 1588) with some addition to that and NTP.

PTP architecture is scalable and defines the time source from an entity called the Time Server clock (or stratum 1 in NTP terms). The Open Time Server is distributing time to the entire network and usually gets its timing from an external source, (GNSS signal).

The current state-of-the-art Open Time Server implementations suffer from a few drawbacks that we wish to accommodate:

  • They are HW appliances that usually target different GTM than a DC
  • They expose none standard and inconsistent Interfaces and SW feature-sets
  • Development cycles and the effort needed to add new features are long and expensive
  • It doesn’t rely on open-source software
  • The accuracy/stability grades aren’t in line with DC requirements

High-Level Architecture

In general, the OTS is divided into 3 HW components:

  1. COTS server
  2. Commodity NIC
  3. Time Card

Open Time Server System Diagram

Figure 1. Open Time Server System Diagram

The philosophy behind this fragmentation is very clear, and each decision, modification that will be made, must look-out to this philosophy:
  • COTS servers keep their “value for money” due to huge market requirements. They are usually updated with the latest OS version, security patches, and newer technology, faster than HW appliances.
  • Modern Commodity NICs already support HW timestamp, lead the market with Ethernet and PCIe latest Speeds and Feeds. Modern NIC also supports a wide range of OS versions and comes with a great software ecosystem. NIC + COTS server will allow the OTS to run a full software (and even open source one) PTP and NTP stack.
  • Timecard will be the smallest (conceptually) possible HW board, which will provide the GNSS signal input and stable frequency input. Isolating these functions in a timecard will allow OTS to choose the proper timecard for their needs (accuracy, stability, cost, etc) and remain with the same SW, interface, and architecture.

Open Time Server Concept

Figure 2. Open Time Server Concept

General Idea is the Time Card is connected via PCIe to the server and provides Time Of Day (TOD) via /dev/ptpX interface.
Using this interface phc2sys continuously synchronizes PHC on the network card from the atomic clock on the Time Card. This provides precision < 1us.
For the extremely high precision 1PPS output of the Time Card will be connected to the 1PPS input of the NIC, providing <100ns precision.

Responsibilities and Requirements

COTS Server

  • Run commodity OS
  • PCIe as an interconnect
  • [optional] PTM Support

Network Interface Card

  • Hardware timestamps
  • [optional] PPS in/out
  • [optional] PTM Support
  • [optional] Time of day tunnel from timecard to SW

Time Card

  • Holdover
  • GNSS in
  • PPS in/out
  • Leap second awareness
  • Time of day
  • [optional] PTM Support

Detailed Architecture

Real life assembly components can be,

Open Time Server Assembly

Figure 3. Open Time Server Assembly

COTS Server


Most of the general purpose hardware can be used.
vt-d CPU support is required.
If multi-CPU system is chosen - we recommend connecting the Time Card and the Network Card to the same PCIe lane.
Synchronization between multiple CPUs will add an extra offset.

We've tested and proved setup is working with the following platform spec:

  • HPE ProLiant DL380 Gen10
  • OCP Tioga Pass
  • SuperMicro 6019U-TRT - single or dual CPU
  • SuperMicro 5019GP-TT - single socket with maximum throughput
  • Intel NUC 9 Pro


Please detailed software description document

  • Linux operating system with the ocp_ptp driver (included in Linux kernel 5.12 and newer). Driver may require vt-d CPU flag enabled in BIOS
  • NTP server - Chrony/NTPd reading /dev/ptpX of the Time Card
  • PTP server - ptp4u or ptp4l reading /dev/ptpX of the NIC
    • phc2sys/ts2phc to copy clock values from the Time Card to the NIC


Most of the general purpose hardware can be used.
For the improved precision of NTP or PTP there can be extra requirements below.


  • Standard PCIe Stand-up HHHL Half-Height, Half-Length -or- OCP NIC 3.0
  • Single Slot - Passive Cooling Solution
  • Support for Standard PCIe Tall and Short brackets -or- OCP NIC 3.0 ejector types

PCIe Interface

  • PCIe Gen3.0/Gen4.0 X n lanes on Gold-fingers, where n = at least 8

Network Ports

  • Single or Dual-port Ethernet

Hardware timestamps

NIC should timestamp all ingress packets.
Non PTP packets can be batch and have a common TS in the SW descriptor, as long as they are not distant more than TBD nanosecond.
NIC should timestamp all PPP egress packets.

  • PHC
  • PTM
  • 1PPS input
  • [optional] 10MHz input which can be used as frequency input to the TSU unit
  • [optional] Multi-host support

PPS out

  • PPS Out Rise/Fall Time < 5 nano Sec
  • PPS Out Delay < 400 pico Sec
  • PPS Out Jitter < 250 femto Sec
  • PPS Out Impedance = 50 Ohm
  • PPS Out frequency 1Hz - 10MHz


  • PPS In Delay < 400 pico Sec
  • PPS In Jitter < 250 femto Sec
  • PPS In Impedance = 50 Ohm
  • PPS In frequency 1Hz - 10MHz


Time Card

Please see Time Card details architecture document or simply visit

General Idea is this card will be connected via PCIe to the server and provide Time Of Day (TOD) via /dev/ptpX interface. Using this interface phc2sys will continuously synchronize PHC on the network card from the atomic clock on the Time Card. This provides precision < 1us.

For the extremely high precision 1PPS output of the Time Card should be connected to the 1PPS input of the NIC. In this setup ts2phc can provide < 100ns precision.


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