timeout_at is pre-send anchored (F-04), so it can precede the same
probe's post-send sent_at when probe_timeout is small relative to the
send-call duration. Rolling-window eviction assumes non-decreasing
event timestamps, so clamp the internal StatsEvent::Loss windowing
value without changing the public EchoLoss.timeout_at field. Also
correct an AGENTS.md claim that sent_at feeds the downstream one-way
delay, which it does not.
The post-send sent_at change does shift the upstream IPDV component's
endpoint, since ReplySample copies sent_at into client_send_mono/wall
for send_ipdv_ns. Document the shift instead of claiming IPDV is
unaffected.
Moves the client userspace probe sent_at measurement to immediately
after a successful UDP send, improving raw RTT and the userspace
upstream-OWD fallback by excluding the socket send-call interval and
the small amount of local work preceding it.
All fallible probe-commit preflight (timeout deadline checked_add,
capacity/collision checks) still runs before the send, from a private
pre-send anchor (SessionMachine::finalize_probe_commit). The public
sent_at is now supplied afterward to an infallible
SessionMachine::commit_probe_sent, so DurationOverflow still guarantees
nothing was transmitted and a WouldBlock try_send still commits
nothing.
The operational probe timeout deadline stays anchored to the pre-send
timestamp on purpose, not to the new post-send sent_at, preserving
existing timeout/loss timing. Linux kernel TX_SOFTWARE plausibility
keeps its own pre-send wall-clock lower bound
(PendingProbe::tx_not_before_wall) instead of the post-send sent_at,
since a legitimate TX_SOFTWARE timestamp can be generated while the
send syscall is still in flight and would otherwise be wrongly
rejected.
Also flags (without touching production code) that irtt-stats'
ReplySample::from_reply_parts feeds the public sent_at directly into
IpdvSample.client_send_mono/client_send_wall_ns, so the upstream/send
IPDV component naturally shifts along with the sent_at semantics
change; per scope this is reported rather than compensated for.
Nils Andreas Svee