Network time protocol precision timestamping service
View Patent ↗Embodiments of the present invention set forth a method and system for reducing uncertainty in receive and transmit timestamps created by an NTP server. The uncertainty in the receive timestamps is removed by recording the time-of-arrival in the hardware clock of the NTP server before the incoming packets may be delayed by traversing the various layers of software in a timestamping system. The uncertainty in the transmit timestamps is removed by giving the outgoing packets a timestamp in the future using an estimate of the transmission latency calculated by the latency estimator filter. Subsequently, the actual time-of-departure is used to re-calculate and update the estimate of the transmission latency. In this fashion, superior control of the timestamping function may be implemented in existing NTP servers in a manner that retains interworking compatibility with the current NTP standards.
1. A method for establishing transmit timestamps to be inserted into outgoing packets, the method comprising:
creating a first outgoing packet;
requesting current time from a hardware clock;
inserting a sum of the current time and a correction term associated with an estimated latency into the first outgoing packet as the transmit timestamp;
placing the first outgoing packet with the transmit timestamp in an outbound queue;
monitoring network traffic flowing through a media independent interface (MII) channel to detect the first outgoing packet;
triggering the hardware clock to latch a time-of-departure of the detected first outgoing packet;
updating the correction term associated with an estimated latency based on the latched time-of-departure;
creating a second outgoing packet;
requesting current time from a hardware clock;
inserting a sum of the current time and the updated correction term into the second outgoing packet as the transmit timestamp; and
placing the second outgoing packet with the transmit timestamp in the outbound queue.
2. The method of claim 1 , wherein the outgoing packet is created as a reply to an incoming packet.
3. The method of claim 1 , wherein the correction term, k, is updated in the following manner:
Step 1: Move elements of an array of transmission latencies down by one, where μ i is a current transmission latency and μ i-1 is an immediately preceding transmission latency:
μ i-1 =μ i ;
Step 2: Update most recent transmission latency, where T tx is the transmit timestamp, and T 3 is the latched time-of-departure:
μ i =T tx −T 3 +k; and
Step 3: Calculate a new value for the correction term:
k =min(μ i-1 ,μ i ).
4. A precision timestamping service (PTS) system configured to insert a transmit timestamp into an outgoing packet, the system comprising:
a physical layer associated with an Ethernet PHY;
an Ethernet media access controller (MAC);
a media independent interface (MII) channel connecting the Ethernet PHY and the Ethernet MAC;
a hardware clock configured to provide current time and to latch a time-of-departure of the outgoing packet;
a network time protocol (NTP) application configured to create the outgoing packet, insert a sum of the current time and a correction term associated with an estimated latency into the outgoing packet as the transmit timestamp, and place the outgoing packet with the transmit timestamp in an outbound queue;
a passive tap unit configured to monitor network traffic flowing through the MII channel to detect the outgoing packet and trigger the hardware clock to latch the time-of-departure of the detected outgoing packet; and
a latency estimator filter configured to update the correction term associated with an estimated latency based on an offset between the transmit timestamp and the time-of-departure latched by the hardware clock.
5. The system of claim 4 , wherein the outgoing packet is created as a reply to an incoming packet from an NTP client.
6. The system of claim 4 , wherein the correction term, k, is updated in the following manner:
Step 1: Move elements of an array of transmission latencies down by one, where μ i is a current transmission latency and μ i-1 is an immediately preceding transmission latency:
μ i-1 =μ i ;
Step 2: Update most recent transmission latency, where T tx is the transmit timestamp, and T 3 is the latched time-of-departure:
μ i =T tx −T 3 +k; and
Step 3: Calculate a new value for the correction term:
k =min(μ i-1 ,μ i ).
7. A method for establishing timestamps to be inserted into packets, the method comprising:
creating an outgoing packet;
inserting a sum of a current time and a correction term associated with an estimated latency into the outgoing packet as a transmit timestamp;
monitoring network traffic flowing through a media independent interface (MII) channel to detect transmission of the outgoing packet through the MII;
triggering the hardware clock to latch a time-of-departure of the outgoing packet when the outgoing packet is transmitted through the MII; and
updating the correction term associated with an estimated latency based on the latched time-of-departure.
8. The method of claim 7 , further comprising:
monitoring network traffic flowing through the MII channel to detect an incoming packet;
triggering a hardware clock to latch a time-of-arrival of the detected incoming packet; and
inserting the latched time-of-arrival into the incoming packet as a receive timestamp.
9. The method of claim 7 , wherein the time indicated by the transmit timestamp is less than or greater than the latched time-of-departure.
10. The method of claim 7 , wherein the correction term, k, is updated in the following manner:
Step 1: Move elements of an array of transmission latencies down by one, where μ i is a current transmission latency and is an immediately preceding transmission latency:
μ i-1 =μ i ;
Step 2: Update most recent transmission latency, where T tx is the transmit timestamp, and T 3 is the latched time-of-departure:
μ i =T tx −T 3 +k; and
Step 3: Calculate a new value for the correction term:
k =min(μ i-1 ,μ i ).