Synchronization of clocks between two communication terminals using TCP/IP

• US 9,203,934 B2
• Filed: 07/31/2012
• Issued: 12/01/2015
• Est. Priority Date: 08/01/2011
• Status: Active Grant

• Current Assignee: Salesforce.com, Inc.
• Original Assignee: Salesforce.com, Inc.
• Inventors: Spencer, Percy L.
• Primary Examiner(s): Baturay, Alicia
• Application Number:
  US13/563,450
• Publication Number:
  US 20130036239A1
• Time in Patent Office: 1,218 Days
• Field of Search: 709/228
• US Class Current: 1/1
• 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 69/16 Implementation or adaptation of Internet protocol [IP], of transmission control protocol [TCP] or of user datagram protocol [UDP]
• CPC Class Codes: H04L 69/28 Timers or timing mechanisms used in protocols

Abstract

Clocks of communication terminals can be synchronized using packets over TCP/IP. In one example, an Internet Protocol packet is received at the first terminal from the second terminal, the packet having a time stamp indicating the time at which it was sent from the second terminal. The time is determined at which the packet arrived at the first terminal and compared to the time stamp to determine a minimal feasible bias. The determined minimal feasible bias is compared to a stored bias and the stored bias is set to the determined minimal feasible bias if the stored bias is less than the minimal feasible bias. A clock time difference is then determined at the second terminal using the stored bias.

Claims

1. A method to determine a clock time difference between a first terminal and a second terminal, the method comprising:

sending a first Internet Protocol packet from the first terminal to the second terminal, the packet having a time stamp indicating the time at which it was sent from the first terminal;

receiving a second Internet Protocol packet at the first terminal from the second terminal, the packet having a time stamp indicating the time at which it was sent from the second terminal;

comparing the time stamp of the second packet and an arrival time at the first terminal to determine an inbound latency;

comparing the determined inbound latency to the stored bias;

setting the stored bias to the determined inbound latency if the stored bias is less than the determined inbound latency; and

determining a clock time difference at the second terminal using the stored bias.

2. The method of claim 1, wherein the packet from the first terminal is the same packet as the packet from the second terminal.

3. The method of claim 1, further comprising:

comparing the time stamp of the first packet and an arrival time at the second terminal to determine an outbound latency;

comparing the determined outbound latency to the stored bias; and

setting the stored bias to the determined outbound latency if the stored bias is greater than the determined outbound latency.

4. The method of claim 3, wherein the second packet received from the second terminal includes a time stamp indicating the time at which the first packet was received at the second terminal and wherein comparing the time stamp of the first packet and an arrival time at the second terminal comprises comparing the time stamp indicating the time at which the first packet was sent from the first terminal to the time stamp of the second packet indicating the time at which the first packet was received at the second terminal.

5. The method of claim 1, further comprising adjusting a clock of the first terminal using the stored bias.

6. The method of claim 1, further comprising determining a bandwidth of a connection between the first and the second terminal using the stored bias.

7. The method of claim 1, further comprising determining a maximum data rate through a connection between the first and second terminal using the stored bias.

8. The method of claim 1, wherein the first terminal is a video teleconferencing node and the second terminal is an upstream video teleconferencing node in a network of video teleconferencing nodes coupled through an Internet Protocol and wherein the stored bias is stored in a clock bias value register of the video teleconferencing node.

9. The method of claim 1, further comprising using the determined clock time difference to adjust an absolute time between the first terminal and the second terminal.

11. The method of claim 1, wherein the packet from the first terminal is the same packet as the packet from the second terminal.

10. A non-transitory machine-readable medium having instructions that when operated on by the machine cause the machine to perform operations comprising:

sending a first Internet Protocol packet from a first terminal to a second terminal, the packet having a time stamp indicating the time at which it was sent from the first terminal;

comparing the time stamp of the first packet and an arrival time at the second terminal to determine an outbound latency;

comparing the determined outbound latency to a stored bias; and

setting the stored bias to the determined outbound latency if the stored bias is greater than the determined outbound latency;

receiving a second Internet Protocol packet at the first terminal from the second terminal, the packet having a time stamp indicating the time at which it was sent from the second terminal;

comparing the time stamp of the second packet and an arrival time at the first terminal to determine an inbound latency;

comparing the determined inbound latency to the stored bias;

setting the stored bias to the determined inbound latency if the stored bias is less than the determined inbound latency; and

determining a clock time difference at the second terminal using the stored bias.

12. The medium of claim 10, the operations further comprising:

comparing the time stamp of the first packet and an arrival time at the second terminal to determine an outbound latency;

comparing the determined outbound latency to the stored bias; and

setting the stored bias to the determined outbound latency if the stored bias is greater than the determined outbound latency.

13. The medium of claim 12, wherein the second packet received from the second terminal includes a time stamp indicating the time at which the first packet was received at the second terminal and wherein comparing the time stamp of the first packet and an arrival time at the second terminal comprises comparing the time stamp indicating the time at which the first packet was sent from the first terminal to the time stamp of the second packet indicating the time at which the first packet was received at the second terminal.

14. The medium of claim 10, the operations further comprising adjusting a counter of the first terminal using the stored bias.

15. The medium of claim 10, the operations further comprising applying the bias to a second terminal reference maintained at the first terminal.

16. A conferencing node coupled to an upstream conferencing node, the conferencing nodes both having an internal clock, the conferencing node comprising:

a transmit port to send a first Internet Protocol packet from the conferencing node to the upstream conferencing node, the packet having a time stamp indicating the time at which it was sent from the conferencing node;

a receive port to receive a second Internet Protocol packet at the conferencing node from the upstream conferencing node, the packet having a time stamp indicating the time at which it was sent from the upstream conferencing node;

a register to store a clock bias value; and

a processor to compare the time stamp of the first packet and an arrival time at the upstream conferencing node to determine an outbound latency, to compare the determined outbound latency to the stored clock bias value, to set the stored bias to the determined outbound latency if the stored bias is greater than the determined outbound latency, to compare the time stamp of the second packet and an arrival time at the conferencing node to determine an inbound latency, to compare the determined inbound latency to the stored clock bias value, to set the stored clock bias value to the determined inbound latency if the stored clock bias value is less than the inbound latency, and to determine a clock time difference at the upstream conferencing node using the stored clock bias value bias.

17. The conferencing node of claim 16, the processor further:

comparing the time stamp of the first packet and an arrival time at the upstream conferencing node to determine an outbound latency;

comparing the determined outbound latency to the stored bias; and

setting the stored bias to the determined outbound latency if the stored bias is greater than the determined outbound latency.

18. The conferencing node of claim 16, wherein the second packet received from the upstream conferencing node includes a time stamp indicating the time at which the first packet was received at the upstream conferencing node and wherein comparing the time stamp of the first packet and an arrival time at the upstream conferencing node comprises comparing the time stamp indicating the time at which the first packet was sent from the conferencing node to the time stamp of the second packet indicating the time at which the first packet was received at the upstream conferencing node.

19. The conferencing node of claim 16, wherein the processor is further to use the clock time difference to adjust an absolute clock time between the conferencing node and the upstream conferencing node.