# US 9,438,364 B2 · Method for synchronizing distributed clocks by the precision time protocol, in a telecommunication network

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- Number: US 9,438,364 B2
- Document ID: US9438364B2
- Status: Granted
- Kind: B2
- Application number: 14/418,057
- Filed: 2013-06-27
- Published: 2015-06-18
- Granted: 2016-09-06
- Earliest claimed: 2012-08-22
- Assignees: Alcatel Lucent
- Applicants: Dinh Thai Bui; Michel Le Pallec
- Inventors: Dinh Thai Bui; Michel Le Pallec
- Web page: https://patentgrep.com/patent/US9438364B2

## CPC

- 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)}
- H04J 3/0658: {Clock or time synchronisation among packet nodes}
- H04J 3/0682: {by delay compensation, e.g. by compensation of propagation delay or variations thereof, by ranging}
- H04L 12/403: with centralised control, e.g. polling
- H04L 43/10: Active monitoring, e.g. heartbeat, ping or trace-route
- H04L 45/34: {Source routing}
- H04L 45/745: Address table lookup; Address filtering
- H04L 67/104: Peer-to-peer [P2P] networks
- H04J: MULTIPLEX COMMUNICATION (transmission in general H04B; peculiar to transmission of digital information H04L5/00; systems for the simultaneous or sequential transmission of more than one television signal H04N7/08; in exchanges H04Q11/00; stereophonic systems H04S)
- H04L: TRANSMISSION OF DIGITAL INFORMATION, e.g. TELEGRAPHIC COMMUNICATION (arrangements common to telegraphic and telephonic communication H04M)

## Abstract

The method includes sending a Sync message from a first peer-to-peer transparent clock to a second peer-to-peer transparent clock, estimating the path delay of the transmission path traveled by the synchronization message from the first to the second peer-to-peer transparent clocks, and taking this path delay into account for updating the time information carried by a synchronization message. The estimating includes creating a list of the network addresses of the network interfaces traversed by the synchronization message; ordering the first list into the order in which the network interfaces have been traversed by the Sync message; creating a second list by reversing the order of the first list; communicating the second list to the second peer-to-peer transparent clock; and using the mechanism available at the transport protocol level, to constrain the respective paths of Pdelay_Req and Pdelay_Resp messages so that their respective paths map to the second and first ordered lists of traversed interfaces.

## Claims

### Claim 1

A method for synchronizing distributed clocks by the Precision Time Protocol, in a telecommunication network, comprising the steps of:
sending a Sync message from one clock to another via a plurality of network nodes, a first of these nodes nodes comprising a first peer-to-peer transparent clock and a second of these nodes comprising a second peer-to peer transparent clock,
estimating the path delay of the transmission path travelled by the synchronization message from the first to the second peer-to-peer transparent clocks, by:
sending a Pdelay_Req message from the second peer-to-peer transparent clock to the first peer-to-peer transparent clock,
and sending a Pdelay_Resp message from the first peer-to-peer transparent clock to the second peer-to-peer transparent clock,
and then taking this path delay into account for updating the time information carried by a synchronization message, in the second peer-to-peer transparent clock;
where in, for estimating said path delay it comprises the steps of:
using a tool, available in the transport protocol, for creating in network node associated to the first peer-to-peer transparent clock, a list of the network addresses of the network interfaces transversed by the synchronization message along its transmission path between the first and the second peer-to-peer transparent clocks;
ordering the first list into the order in which the network interlaces have been traversed by the Sync message;
creating a second list by reversing the order of the first list;
communicating the second list to the second peer-to-peer transparent clock;
and using the mechanism available in the transport protocol, in network nodes associated respectively to the first and the second peer-to-peer transparent clocks to constrain the respective paths of Pdelay_Req and Pdelay_Resp messages so that their respective paths map to the second and first ordered lists of traversed interfaces.

### Claim 2 (depends on 1)

A method according to claim 1, wherein communicating the second list to the second peer-to-peer transparent clock comprises the step of putting the second list into a TLV field of a Pdelay_Resp message transmitted by the first peer-to-peer transparent clock to the second peer-to-peer transparent clock.

### Claim 3 (depends on 1)

A method according to claim 1, wherein communicating the second list to the second peer-to-peer transparent clock comprises the step of putting the second list into a Precision Time Protocol management message transmitted by the first peer-to-peer transparent clock to the second peer-to-peer transparent clock.

### Claim 4 (depends on 1)

A method according to claim 1, for a homogeneous environment where UDP/IP encapsulation of PTPv2 messages is supported by all network nodes, wherein,
for estimating said path delay it comprises the step of using the IP TRACEROUTE command with the IP destination address set to the address of a slave clock,
for communicating the second ordered list of IP interface addresses to the second peer-to-peer transparent clock it comprises the step of inserting it into a PTP message encapsulated over an IP data packet,
and for constraining Pdelay_Req and Pdelay_Resp respective paths, it comprises the step of using RSVP-TE signaling with EXPLICIT_ROUTE OBJECT.

### Claim 5 (depends on 1)

A method according to claim 1, for a homogeneous environment where Ethernet encapsulation of PTPv2 messages is supported by all network nodes, wherein,
for estimating said path delay it comprises using the Ethernet OAM TRACEROUTE command, and using a medium access control destination address set to the one of a slave clock,
for communicating the second ordered list of IP interface addresses to the second peer-to-peer transparent clock it comprises the step of inserting it into a PTP message encapsulated over an Ethernet data packet,
and for constraining Pdelay_Req and Pdelay_Resp respective paths, it comprises the step of using Virtual LAN or RSVP-TE with EXPLICIT_ROUTE OBJECT.

### Claim 6 (depends on 1)

A method according to claim 1, for a heterogeneous environment composed of three successive technology domains D1, D2, D3 with D1 and D3 separated by D2, and with D1 and D3 implementing the first encapsulation technology and D2 implementing the second encapsulation technology,
and where D1 and D2 are separated by the first node comprising a first peer-to-peer transparent clock and D2 and D3 are separated by the second node comprising a second peer-to-peer transparent clock,
wherein,
for monitoring the Sync message first technology destination address, the first node IPRB′ uses tools related to the first technology protocols,
for discovering the Sync message first technology next hop address, the first node IPRB′ performs a lookup with the Sync message first technology destination address into its routing or switching tables related to the first technology,
for obtaining the second technology address associated to the Sync message first technology next hop address, the first node uses address resolution tools amongst tools allowing for the interworking before the two technologies,
for estimating said path delay across D2 it comprises the step of using the second technology OAM TRACEROUTE command, with the destination address set to the second technology address obtained in the previous step,
for communicating the second ordered list of second technology interface addresses to the second peer-to-peer transparent clock it comprises the step of inserting it into a PIP message encapsulated accordingly to the second encapsulation technology,
and for constraining Pdelay_Req and Pdelay_Resp respective paths, it comprises the step of using path constraining tools implemented within the second technology.

### Claim 7 (depends on 6)

A method according to claim 6 where the first encapsulation technology is IP-based implementing the IEEE standard 1588_2008 annex D and the second technology is Ethernet-based implementing the IEEE standard 1588_2008 annex F jointly with the IEEE 802.1q.

### Claim 8 (depends on 6)

A method according to claim 6 where the first encapsulation technology is IP-based implementing the IEEE standard 1588_2008 annex E and the second technology is Ethernet-based implementing IEEE standard 1588_2008 annex F jointly with IEEE 802.1q.

### Claim 9 (depends on 1)

An interworking agent comprising a set of machine executable program instructions, which, when executed on a processor, cause the processor to perform all the method steps of the method according to claim 1.

### Claim 10 (depends on 1)

A digital data storage medium storing a set of machine executable program instructions, which, when executed on a processor, cause the processor to perform all the method steps of the method according to claim 1.

### Claim 11 (depends on 1)

A computer program product comprising processor-executable instructions for performing a method when the program is run on a processor, the method comprising the steps of claim 1.