Clock harmonization in deterministic networks

• US 10,348,481 B1
• Filed: 04/30/2018
• Issued: 07/09/2019
• Est. Priority Date: 04/30/2018
• Status: Active Grant

• Current Assignee: Cisco Technology, Inc. (Cisco Systems, Inc.)
• Original Assignee: Cisco Technology, Inc. (Cisco Systems, Inc.)
• Inventors: Wetterwald, Patrick, Thubert, Pascal, Levy-Abegnoli, Eric
• Primary Examiner(s): Neff, Michael R
• Application Number:
  US15/966,131
• Time in Patent Office: 435 Days
• Field of Search: 375359
• CPC Class Codes: H04J 3/0658 Clock or time synchronisation among packet nodes
• CPC Class Codes: H04J 3/0667 Bidirectional timestamps, e.g. NTP or PTP for compensation of clock drift and for compensation of propagation delays arrangements for monitoring round trip delays in packet switching networks H04L43/0864
• CPC Class Codes: H04L 7/0037 Delay of clock signal
• CPC Class Codes: H04L 7/0087 Preprocessing of received signal for synchronisation, e.g. by code conversion, pulse generation or edge detection

Abstract

In one embodiment, a device in a network receives a plurality of packets from one or more neighbors of the device. Each of the packets has a scheduled delivery time interval according to a deterministic communication schedule. The device determines an amount of clock drift for each of the one or more neighbors of the device by comparing arrival times of the received packets to their scheduled delivery time intervals according to the deterministic communication schedule. The device calculates a clock adjustment based on the amount of clock drift for each of the one or more neighbors. The device adjusts a clock of the device using the calculated clock adjustment.

Claims

1. A method comprising:

receiving, at a device in a network, a plurality of packets from one or more neighbors of the device, wherein each of the packets has a scheduled delivery time interval according to a deterministic communication schedule;

determining, by the device, an amount of clock drift for each of the one or more neighbors of the device by comparing arrival times of the received packets to their scheduled delivery time intervals according to the deterministic communication schedule;

determining, by the device, a distance from each of the one or more neighbors of the device to a clock reference in the network;

calculating, by the device, a clock adjustment based on the amount of clock drift for each of the one or more neighbors by applying, for each of the one or more neighbors, a weighting to the amount of clock drift for that neighbor, based on the determined distance from that neighbor to the clock reference in the network; and

adjusting, by the device, a clock of the device using the calculated clock adjustment.

2. The method as in claim 1, further comprising:

using, by the device, the adjusted clock to send a packet from the device to a neighbor of the device at a scheduled time according to the deterministic communication schedule.

3. The method as in claim 1, further comprising:

setting, by the device, the clock of the device based on a Network Time Protocol (NTP) or Precision Time Protocol (PTP) message received by the device, wherein the device adjusts the clock based on the average amount of clock drift for the one or more neighbors after setting the clock based on the NTP or PTP message.

4. The method as in claim 1, further comprising:

selecting, by the device, the one or more neighbors from among a set of neighbors of the device from which the device received packets, based on the selected one or more neighbors of the device being closer in the network to a clock reference than that of the device.

5. The method as in claim 1, wherein calculating the clock adjustment based on the amount of clock drift for each of the one or more neighbors comprises:

calculating the clock adjustment as an average clock drift for the one or more neighbors.

6. The method as in claim 1, wherein the device determines the distance from each of the one or more neighbors of the device to the clock reference based on routing protocol messaging in the network.

7. The method as in claim 1, further comprising:

filtering, by the device, the amount of clock drift of a particular neighbor of the node from being used to calculate the clock adjustment, when the amount of clock drift of the particular neighbor differs from that of other neighbors of the device by a threshold amount.

8. The method as in claim 1, wherein the device comprises an Ethernet switch.

9. An apparatus, comprising:

one or more network interfaces to communicate with a network;

a processor coupled to the network interfaces and adapted to execute one or more processes; and

a memory configured to store a process executable by the processor, the process when executed configured to;

receive a plurality of packets from one or more neighbors of the apparatus, wherein each of the packets has a scheduled delivery time interval according to a deterministic communication schedule;

determine an amount of clock drift for each of the one or more neighbors of the apparatus by comparing arrival times of the received packets to their scheduled delivery time intervals according to the deterministic communication schedule;

determine a distance from each of the one or more neighbors of the device to a clock reference in the network;

calculate a clock adjustment based on the amount of clock drift for each of the one or more neighbors by applying, for each of the one or more neighbors, a weighting to the amount of clock drift for that neighbor, based on the determined distance from that neighbor to the clock reference in the network; and

adjust a clock of the apparatus using the calculated clock adjustment.

10. The apparatus as in claim 9, wherein the process when executed is further configured to:

use the adjusted clock to send a packet from the apparatus to a neighbor of the apparatus at a scheduled time according to the deterministic communication schedule.

11. The apparatus as in claim 9, wherein the process when executed is further configured to:

set the clock of the apparatus based on a Network Time Protocol (NTP) or Precision Time Protocol (PTP) message received by the apparatus, wherein the apparatus adjusts the clock based on the average amount of clock drift for the one or more neighbors after setting the clock based on the NTP or PTP message.

12. The apparatus as in claim 9, wherein the process when executed is further configured to:

select the one or more neighbors from among a set of neighbors of the apparatus from which the apparatus received packets, based on the selected one or more neighbors of the apparatus being closer in the network to a clock reference than that of the apparatus.

13. The apparatus as in claim 9, wherein the apparatus calculates the clock adjustment based on the amount of clock drift for each of the one or more neighbors by:

calculating the clock adjustment as an average clock drift for the one or more neighbors.

14. The apparatus as in claim 9, wherein the apparatus determines the distance from each of the one or more neighbors of the apparatus to the clock reference based on routing protocol messaging in the network.

15. The apparatus as in claim 9, wherein the process when executed is further configured to:

filter the amount of clock drift of a particular neighbor of the node from being used to calculate the clock adjustment, when the amount of clock drift of the particular neighbor differs from that of other neighbors of the apparatus by a threshold amount.

16. The apparatus as in claim 9, wherein the apparatus comprises an Ethernet switch.

17. A tangible, non-transitory, computer-readable medium storing program instructions that cause a device in a network to execute a process comprising:

receiving, at the device, a plurality of packets from one or more neighbors of the device, wherein each of the packets has a scheduled delivery time interval according to a deterministic communication schedule;

determining, by the device, an amount of clock drift for each of the one or more neighbors of the device by comparing arrival times of the received packets to their scheduled delivery time intervals according to the deterministic communication schedule;

determine a distance from each of the one or more neighbors of the device to a clock reference in the network;

calculating, by the device, a clock adjustment based on the amount of clock drift for each of the one or more neighbors by applying, for each of the one or more neighbors, a weighting to the amount of clock drift for that neighbor, based on the determined distance from that neighbor to the clock reference in the network; and

adjusting, by the device, a clock of the device using the calculated clock adjustment.

18. The computer-readable medium as in claim 17, wherein the process further comprises:

using, by the device, the adjusted clock to send a packet from the device to a neighbor of the device at a scheduled time according to the deterministic communication schedule.

19. The computer-readable medium as in claim 17, wherein the process further determines the distance from each of the one or more neighbors of the apparatus to the clock reference based on routing protocol messaging in the network.

20. The computer-readable medium as in claim 17, wherein the process further comprises:

selecting, by the device, the one or more neighbors from among a set of neighbors of the apparatus from which the apparatus received packets, based on the selected one or more neighbors of the apparatus being closer in the network to a clock reference than that of the apparatus.