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.
Upstream one-way delay now prefers the correlated Linux TX_SOFTWARE wall
timestamp retained by PendingProbe when it is locally plausible (sent_at
<= kernel_tx <= received_at, all on the client's own wall clock), falling
back to the userspace sent_at.wall sample otherwise. RTT, downstream OWD,
IPDV, and timeout/scheduling semantics are unchanged.
On Linux, best-effort upgrade the socket's SO_TIMESTAMPING flags to
TX_SOFTWARE | OPT_ID | OPT_TSONLY after a successful Open and before the
first Echo probe. Promote the #103 error-queue foundation to production:
MSG_ERRQUEUE is drained opportunistically with a fixed per-call budget at
several natural choke points (after a probe send, before processing a
reply, before evaluating timeouts), and each automatic OPT_ID timestamp is
correlated directly with wire_seq — safe because Linux rolls back its
speculative sk_tskey increment on every send failure, wire_seq and the
kernel counter start together at every Open, and a socket can never be
reopened for a second session. The observed kernel TX wall timestamp is
retained on PendingProbe (surviving pending -> timed-out retention) as
dormant metadata only; RTT, OWD, and IPDV are unchanged and continue to use
the userspace send timestamp exclusively. Capability failure is fully
non-fatal and RX timestamping is unaffected either way. Non-Linux and
non-ancillary builds are unchanged.
Nils Andreas Svee