Knowledge Base

Backwards timestamps in TCP packets   ________________________________________________________________________ Applicable Environment ________________________________________________________________________ Topic: Backwards time in TCP packet frame arrival timestamps SDP: 8.0.0, 7.1.0 Target: Any supported target ________________________________________________________________________ Issue ________________________________________________________________________ TCP packet frame arrival timestamps seem to go backwards in time when recorded with tcpdump. ________________________________________________________________________ Solution ________________________________________________________________________ To resolve this issue, start io-pkt-* with the strict_ts option. The strict_ts option guarantees monotonic timestamps at the cost of sacrificing microsecond timestamp precision for millisecond precision. As per the documentation: https://www.qnx.com/developers/docs/7.1/#com.qnx.doc.neutrino.utilities/topic/i/io-pkt.html "strict_ts: (QNX Neutrino 7.0.1 or later) Use the io-pkt timer for timestamping BPF. This results in a best-case precision of 1 millisecond, but the timestamps are guaranteed to be monotonic. By default ClockCycles() is used to give timestamps that are precise down to 1 microsecond, at the risk of occasional nonmonotonic timestamps as the ClockCycles() clock is recalibrated against the io-pkt timer." ________________________________________________________________________ NOTE: This entry has been validated against the SDP version listed above. Use caution when considering this advice for any other SDP version. For supported releases, please reach out to QNX Technical Support if you have any questions/concerns. ________________________________________________________________________ Related Attachments  None Found

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