[gareth-morgan-nv]

Summary

Lets the sender answer a four-timestamp clock exchange over UDP on its CLOCK_MONOTONIC, so a receiving host can measure the offset between its clock and the sender's. It is independent of the camera backend and of the AVS1 timing record, and off by default.

What changed

• ClockExchange: the 13-byte request (Q, u32 seq, i64 t1) and 29-byte reply (R, seq, t1 echoed, t2, t3), little-endian; t2 and t3 are the sender's CLOCK_MONOTONIC in nanoseconds.
• ClockResponder: a stateless UDP responder started with the sender and kept for the life of the process, whether or not a receiver is connected. t2 is read as soon as the receive returns and t3 just before the reply is sent. Anything that is not exactly one request is ignored.
• network.clock_port (0 = off, the default) sets the port. The README documents the exchange and how a requester computes round-trip delay and offset from its own t1 and t4.

Validation

• clang-format-14 and pre-commit clean on the changed files.
• aarch64 cross-compile: syntax check and full link.
• New unit tests clock_exchange and clock_responder, plus the config key, built and run by hand on a PC.
• Native build and unit tests on a Jetson Orin (R39.2.1), including both new tests (17 of 17 camera-free tests on a build that also contained the sender-timing PR).
• Probed from a PC over WiFi against a running sender: 299 of 299 requests answered, round-trip delay floor about 2 ms, and malformed datagrams got no reply. A later run with a real receiver sent 161 requests and its best delay was 2.27 ms.

Known limitations / follow-ups

• The responder has no authentication and exposes only the sender's own clock readings, like the AVS1 stream beside it; keep the port on a trusted camera network.
• The offset estimate is wrong by up to half of any difference between the two network directions; this change serves the clocks and makes no claim about accuracy beyond that. A requester should accept only replies matching a request it still has outstanding and reset its estimate when the AVS1 session_id changes or the connection is re-established; the responder keeps no session state.

Test plan

• Query the responder from a second host and confirm the offset estimate is stable across a sender restart.
• CI.

🤖 Generated with Claude Code

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