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.
Migrate the Linux client's receive kernel timing from SO_TIMESTAMPNS to
SO_TIMESTAMPING with RX_SOFTWARE + SOFTWARE, reported via
SCM_TIMESTAMPING instead of SCM_TIMESTAMPNS. Linux documents fallback
quirks when SO_TIMESTAMP[NS] and SO_TIMESTAMPING are both enabled on a
socket, so this establishes one unambiguous software-timestamp facility
before client TX timestamp work builds on the same API.
ReceiveMeta.kernel_rx_timestamp semantics, PacketMeta propagation, and
all PR #100 downstream OWD/RTT/IPDV behavior are unchanged. The receive
control buffer is now sized deliberately from cmsg_space::<T>() instead
of an unexplained fixed constant.
Also add safe, test-only Linux primitives that a follow-up change needs
for client TX timestamping: SO_TIMESTAMPING TX flag construction,
nonblocking MSG_ERRQUEUE parsing, and extended-error classification
that distinguishes a genuine send-timestamp completion from a malformed
timestamp record and from a real socket/network error. Production
sockets do not enable SOF_TIMESTAMPING_TX_SOFTWARE and nothing drains
the error queue in production; these primitives only run under
#[cfg(test)].
Eliminate unsafe code blocks and manual pointer arithmetic in Linux-specific receive paths by utilizing nix for socket options and control message parsing. This improves safety and maintainability while preserving the existing kernel RX timestamp and traffic class functionality.
Introduce a managed group runner to support concurrent probing of multiple targets over a single UDP socket with coordinated send pacing. Refactor the existing client logic into a reusable SessionRuntime to support both individual and grouped session management.
Nils Andreas Svee