Transmit on time stamp is configured through the NTAPI configuration stream.
Configuration parameters
Transmit on time stamp is configured per port using the function NT_ConfigWrite() with a NtConfigTransmitOnTimestamp_s structure.
Field | Description |
---|---|
portNo |
Specifies the port for which to configure transmit on time stamp. |
data.timeDelta |
Specifies the delta in ns to add to time stamp before comparing to SmartNIC time. See Frame transmit condition for further explanation. |
data.enable |
Enables transmit on time stamp.
|
data.forceTxOnTs |
Forces transmit on time stamp, regardless of the txNow bit in the packet descriptor. To transmit PCAP frames on time stamp, set forceTxOnTs=true. |
data.timeDeltaAdjust |
Specifies the adjustment in ns to add to data.timeDelta whenever a txSetClock frame is encountered. The txSetClock bit is available in the Napatech standard packet descriptor; the PCAP descriptor does not have a txSetClock bit. Important: This behavior is not backwards compatible with 3GA. In 3GA,
a txSetClock frame actually sets the transmission clock to the
time stamp of the frame.
|
For the PCAP descriptor, which does not have a txNow bit, txNow is considered set.
Time stamp formats
The following time stamp formats are supported:
Descriptor | Time stamp format |
---|---|
Standard | Native Unix (10 ns) |
Native Unix ns (1 ns) | |
PCAP | PCAP-ns |
PCAP-μs |
Program flow for setting up transmit on time stamp
The general program flow for setting up transmit on time stamp will look like this:
- Read the time stamp from the first frame and convert the value to ns.
- Read the SmartNIC time and convert the value to ns.
- Calculate data.timeDelta: ∆ = SmartNIC time – first frame time stamp + additional offset.
- Configure transmit on time stamp per port:
- Use a config stream with NT_CONFIG_PARM_PORT_TRANSMIT_ON_TIMESTAMP parameters.
- Set calculated data.timeDelta.
- Set data.enable to enable the port for transmit on time stamp.
- Set data.forceTxOnTs to ignore the txNow bit in packet descriptor (which is assumed set for PCAP).
This program flow is illustrated in Example.
Program flow for replaying a capture file repeatedly
- Read the time stamp from the first and the last frame and convert the values to ns.
- Read the SmartNIC time and convert the value to ns.
- Calculate the estimated transmission time for last frame in ns.
- Calculate data.timeDeltaAdjust = last frame time stamp - first frame time stamp + estimated transmission time for last frame
- Calculate data.timeDelta: ∆ = SmartNIC time – first frame time + additional offset - data.timeDeltaAdjust.
- Configure transmit on time stamp per port:
- Use a config stream with NT_CONFIG_PARM_PORT_TRANSMIT_ON_TIMESTAMP parameters.
- Set calculated data.timeDelta.
- Set calculated data.timeDeltaAdjust.
- Set data.enable to enable the port for transmit on time stamp.
- At the start of each replay, set txSetClock for the first frame, using the macro NT_NET_SET_PKT_TXSETCLOCK().