NetXtreme Time Sync Assist
BCM5718 Programmer’s Guide
Broadcom
®
January 29, 2016 • 5718-PG108-R
Page 153
PTP Link Delay Measurement
At the completion of the Delay Request/Response exchange, the delay requester uses four timestamps (t1, t2,
t3, t4) to compute the link delay. The link delay is computed as the average of the two one-way delays using the
following formula:
T delay = [(t2 – t1) + (t4 – t3)] / 4
Assume that both nodes contain a NetXtreme Time-Synch Capable NIC.
describes the roles the PTP
software component and the Time-Synch capable NIC hardware play during the above exchange.
PTP Time Synchronization Messaging
The slave device then uses the link delay (Tdelay) and the Sync Message timestamps (t1, t2) to calculate the
time offset it needs to compensate its local clock by using the following equation:
Tslave-offset = t2 – t1 – Tdelay
Assume that both nodes contain a NetXtreme Time-Synch Capable NIC.
describes the
roles the PTP software component and Time-Synch capable NIC hardware play during above exchange.
Table 43: PTP Link Delay Measure Roles
Delay Requester
Delay Responder
Host Software
NIC Hardware
NIC Hardware
Host Software
1
TX PTP Delay Request
packet- mark capture
→
Capture TX stamp (t1)
→
–
–
2
–
–
→
Receive and identify
PTP delay request
packet, then capture RX
Time Stamp (t2)
→
Receive packet, Read
RX Time Stamp Reg.
3a
–
–
Capture TX Time Stamp
(t3)
←
TX PTP Delay Response
packet with embedded
(t2) value — mark
capture
←
4
←
Receive Packet, Read
RX Time Stamp Reg
Receive and identify PTP
Delay Response packet,
then capture RX Time
Stamp (t4)
←
–
–
3b
Receive packet
–
←
(pass through)
TX PTP Delay Response
Follow-up packet with
embedded (t3) value
5
Collect t1, t2, t3, and t4 to
compute T delay.
–
–
–