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Johnathan Corgan 6a564e26ac Prepare the v0.5.0 release content

Everything the release needs except the version number, which stays at
0.5.0-dev until the tag.

The changelog entry covers only the work that is new on this line. The
point release's forty-six entries arrived under their own heading with the
forward merge and are left alone; the twenty that remained are regrouped by
topic and eight more added for changes no entry covered. Three of those
eight matter to someone upgrading. Five root modules and four re-exports
left the public library surface and Node::connections narrowed, none of it
recorded anywhere; the entry names what to use instead and distinguishes
the removed connection-phase enum from the Noise type of the same name,
which is a different type that still exists. Tracing targets moved, so an
existing RUST_LOG filter stops matching rather than erroring. And the
handshake resend interval key no longer governs the first resend, which is
now a constant, though it still governs later ones.

Seven more entries cover the work that landed after the first content pass
was written: the experimental native datagram API, the fipsctl probe
diagnostic, per-instance transport addressing, the app-owned UDP socket
seam, and the connect, disconnect and path-MTU fixes. The four bug fixes
among them all reach the deployed line, so the release notes no longer
claim this release carries exactly one fix for a shipped bug; it carries
four.

There is no security section, because after the split every security entry
belongs to the point release. The release notes say so plainly rather than
leaving a reader upgrading across both releases to conclude this one
carries no security work.

The notes are organized by audience, since the release spans OpenWrt
routers, embedders, FreeBSD, and the existing platforms, and a single list
serves none of them. The native datagram API is given a section of its own
rather than folded into the embedding seam: it is a client-facing API
rather than a way to host a node, and its one rule with no Berkeley-socket
counterpart, that the v1 wire carries no half-close, needs to be somewhere
a client author will read it. FreeBSD is advertised as supported on x86_64
only, stated wherever the platform appears. Android is advertised as an
embedding seam and not as a supported platform: a compile-gated library
surface with no artifact and no host application guide.

The configuration table rename is carried through every shipped file that
taught the old spelling: nine documentation files, the OpenWrt sample
config and a test generator, twenty-two sites in all. Guides written this
same cycle were among them, which is how the omission was found. The
documentation that arrived with the native API was checked for the same
omission and was already clean. The compatibility tests keep the old
spelling deliberately, since they exist to test the fold.

The changelog section is the fold of master's [Unreleased], not a snapshot
of it. An earlier version of this commit took a copy that then drifted, so
each section ended up holding a bullet the other did not and re-folding
them would have picked a winner silently. Both causes were fixed on master
instead — the NixOS module had never been recorded there, and the
pre-release batch of fixes was new — so [Unreleased] is a strict superset
and this is a copy rather than a merge. [0.5.0] carries all forty-six
bullets byte for byte, [Unreleased] is empty, and [0.4.2] is untouched,
checked by hashing it against master's copy.

The BLE work landed after the content pass and gets one summary entry in
the changelog and one section in the release notes rather than nine
bullets: the ble_available gate replacing target_os = "linux",
packet-boundary recovery for stream-oriented backends, peer recognition by
node identity instead of a rotating link address, the L2CAP PSM moving
into the backend seam and onto the advertisement, the embedder-supplied
Android radio, bounded probe retry, and inbound handshakes moved off the
accept loop.

The two release-notes copies no longer share their link paths. Relative
links resolve from one directory only, so the seven written for
docs/releases/ all 404ed from the root copy. The root copy now uses paths
from the repository root and the versioned copy keeps the ../ form; both
sets were resolved against the tree. The same two links are broken the
same way in the v0.4.0 through v0.4.2 notes, left as shipped history.

The contributor tallies are re-derived against maint..HEAD rather than
adjusted: twenty commits from outside the project and 171 from me, with
Arjen at fifteen and fr34aky at two. An earlier count of twelve and 138
was carried from a measurement taken three days before this content was
written, and the BLE branch widened the gap after it. Arjen's NixOS flake
module, the UDP sin6_scope_id fix and most of the BLE rework were
uncredited, as was fr34aky's L2CAP PSM seam. They want one last re-derive
at tag time if anything lands before the tag.

A sweep of all 99 tracked markdown files against the tree corrected
fifty-three of them. Four told the reader to run a build.sh that does not
exist; the only harness builder is testing/scripts/build.sh. The BLE build
prerequisites were described as optional on the strength of a probe that
build.rs does not perform, and bluez was named a build prerequisite when
libdbus-sys asks only for libdbus-1-dev and pkg-config and bluez is the
runtime daemon. Link cost is the primary sort key in next-hop ranking, not
reserved for future use; Ethernet runs on macOS as well as Linux; the BLE
MTU is the L2CAP CoC MTU rather than a negotiated ATT_MTU; effective
Ethernet MTU is 1497; the LAN discovery subsystem is src/mdns and eight
citations still named a src/discovery that never existed here. The
connectivity states in three tutorials were invented, and their jq filters
matched nothing including healthy peers. One command filtered on a literal
fd97: address prefix, which only the first byte of fixes, so it returned
empty for all but one reader in 256 and every later step using the
variable failed silently. transports.tor.advertise_on_nostr was
undocumented despite being validated against node.rendezvous.nostr.enabled.

The transport design document gains the BLE section it never had, written
from the source: the backend cascade and its compile_error tripwire, the
platform gate, the PSM advertisement wire layout and the byte budget that
forces a 16-bit service-data key, and the probe and admission bounds.

Three source files carried the same class of staleness and are corrected
with the documentation: the OpenWrt ipk usage line and Makefile error text
both named a packaging/openwrt that does not exist, and chaos.sh parsed
--subnet without listing it.

Folded in with the content commit, having been prepared alongside it:

The three GitHub Action pins that had gone stale. Every third-party
action is pinned to a commit SHA, nothing reports that a pin has aged,
and re-resolving all ten against their tags found dorny/test-reporter@v2,
taiki-e/install-action@v2 and vmactions/freebsd-vm@v1 had moved. The
three install-action@nextest references stay unpinned, since that action
reads the tool to install from the ref name. check-action-pins.sh passes
at 75 references and all nine workflow files parse.

The lockfile refresh, which is the mutating half of the dependency sweep.
Thirty-six packages move to their latest semver-compatible versions and
every one is transitive; nothing declared in Cargo.toml changes version.
No advisory forces any of them. It was taken before the validation
battery, because a gate run against a lockfile that later moves proves
nothing about what ships.

The sha2 0.10 to 0.11, hkdf 0.12 to 0.13 and bech32 0.11 to 0.12 majors,
three of the four deferred at v0.4.0 for change surface rather than
security. All three land with no source change. sha2 and hkdf must move
together, since both depend on digest 0.11, and neither changes an
algorithm. That matters because the chaining-key KDF in the Noise
handshake is built on Hkdf::<Sha256>, where an output change would be a
wire break rather than a compile error; no known-answer vectors exist for
that path, so the wire-compatibility gate is what covers it. secp256k1
0.31 is deliberately absent, since nostr's own requirement would leave
two copies of the ECC library in the tree.

The README support matrix, rebuilt as one feature table broken out by
Linux variety. A single Linux column hid that Debian, Ubuntu, Arch and
NixOS are one glibc build differing in packaging, that OpenWrt is musl
and drops BLE, and that Android is not a daemon platform. Transport rows
sort by how many platforms carry them. A Native API row reads its
platform set from the cfg gates. The installer row becomes a package
format row naming the artifact, and only the .deb is exercised per
release.

Four changelog and release-note gaps the BLE re-walk found: a Bluetooth
LE bullet stranded inside the released 0.4.2 section, a missing Fixed
entry for the scan and probe loop counting a pool-refused connection as
an established link, the unnamed embedder call that installs an
application-owned radio, and the fact that stopping the transport now
stops scanning as well as advertising.

Three release-document gaps found walking the unsurveyed commits: the UDP
reuse-flag fix stated in the direction opposite to the one it was made,
with the silent second-daemon bind it prevents left unsaid; the corrected
native-API socket paragraph carried into both release-note copies, which
still named SOCK_SEQPACKET on FreeBSD and two kernels where three are
handled; and the coordinate-cache hardening, which shipped with no text
anywhere despite adding four operator-visible status fields. That last
entry states plainly that the checks are mitigations and not a closure,
since the coordinate is still not authenticated.

Also folded in, the documentation pass that followed the content commit:

A stage-pipeline diagram for the probe, embedded in the fipsctl
reference under the five-stage list. It draws the five stages left to
right with each stage's failure reasons below it, and the bypass that
skips both lookup stages when the coordinates are cached or the target
is a direct peer. Its branches come from the probe state machine rather
than from the report, so the path stage is drawn as the one failure that
does not stop the probe.

A rewrite of the README's "What FIPS does" section. It now opens with
what a machine running FIPS gets, rather than with the two deployment
modes, and gives the self-organizing and permissionless property its own
paragraph since it holds for both modes.

A regrouping of the README's feature list into the mesh, getting traffic
onto it, and running a node, with a bullet added for the native datagram
API, which had none despite sitting in the support matrix. The Quick
start now leads with the released packages rather than a source build.
It also fixes a real defect: the package enables fips.service and
fips-dns.service and starts neither on a fresh install, so .fips name
resolution was silently dead until the next reboot and neither page said
to start the service.

A rewrite of the release notes. They opened with seven subsections of
upgrade caveats and reached the first feature two hundred lines in; they
now open with a summary of the release and elaborate below it in the
same order. Android is stated as supported through an embedded crate
rather than as a standalone daemon, consistently across all three
documents. The OpenWrt pair is corrected: it is 802.11s between routers
with FIPS supplying encryption, authentication and routing, plus a
convention of an open !FIPS SSID a client joins over WiFi, not meshing
over a router's own radios. The probe's path output is described as the
least-common-ancestor walk, which is the worst-case fallback route
rather than the route a packet takes. Detail that did not change what a
reader does was cut from the notes and kept in the changelog.

2026-08-30 10:42:59 +00:00

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Metrics Measurement Protocol (MMP)

The Metrics Measurement Protocol provides per-link and per-session quality metrics — SRTT, loss, jitter, goodput, ETX, and one-way delay trend — using only counter and timestamp fields already present in the FMP and FSP wire formats. No additional probing traffic is required. The same algorithms and report message format are used at both layers; only the routing scope and configuration namespace differ.

This document is the canonical home for the MMP design. For the link-layer instance's role inside FMP, see fips-mesh-layer.md. For the session-layer instance's role inside FSP, see fips-session-layer.md. For the byte-level SenderReport and ReceiverReport layouts, see ../reference/wire-formats.md.

Two Layers, One Protocol

MMP runs at two layers:

Link-layer MMP: One instance per active FMP peer link. Reports are exchanged peer-to-peer between direct neighbors and measure the quality of that single hop.
Session-layer MMP: One instance per established FSP session. Reports are encrypted end-to-end and forwarded through every transit link, measuring end-to-end quality independent of hop count.

The algorithms (SRTT estimation, jitter computation, loss inference, ETX) are identical at both layers. The differences are configuration namespace, report intervals, and routing scope. See Layer Differences below.

Metrics Tracked

MMP computes the following metrics from the per-frame counter and timestamp fields:

SRTT — Smoothed round-trip time (Jacobson/RFC 6298, α=1/8). Derived from timestamp-echo in ReceiverReports with dwell-time compensation.
Loss rate — Bidirectional loss inferred from counter gaps. Tracked as both instantaneous (per-interval) and long-term EWMA.
Jitter — Interarrival jitter (RFC 3550 algorithm) in microseconds.
Goodput — Bytes per second of payload data (excludes MMP reports).
OWD trend — One-way delay trend (µs/s, signed). Indicates congestion buildup before loss occurs.
ETX — Expected Transmission Count, computed from bidirectional delivery ratios. Used in cost-based parent selection via link_cost = etx * (1.0 + srtt_ms / 100.0), and in bloom-filter candidate ranking inside find_next_hop() (the same link_cost is the primary key when choosing among bloom-filter peers, with tree distance as the tie-breaker).
Dual EWMA trends — Short-term (α=1/4) and long-term (α=1/32) trend indicators for both RTT and loss, enabling change detection.

Session-layer MMP additionally tracks the observed forward-path MTU; see fips-mtu.md for the end-to-end path-MTU mechanism.

Operating Modes

MMP supports three modes:

Mode	Reports Exchanged	Metrics Available
Full (default)	SenderReport + ReceiverReport	All metrics including RTT, loss, jitter, goodput, OWD trend
Lightweight	ReceiverReport only	Loss (from counter gaps), jitter, OWD trend. No RTT.
Minimal	None	Spin bit and CE echo flags only. No computed metrics.

The mode is configured per layer (node.mmp.mode and node.session_mmp.mode).

Report Scheduling

Reports are sent at RTT-adaptive intervals computed as clamp(2 × SRTT, low, high). A cold-start interval is used until SRTT has converged.

Layer	Adaptive bounds	Cold-start
Link	`[1s, 5s]`	200 ms (first 5 samples)
Session	`[500ms, 10s]`	1 s

The session-layer bounds are higher because session reports are encrypted and forwarded through every transit link, so bandwidth cost is proportional to path length.

Spin Bit and RTT

The SP (spin bit) flag in the FMP inner header follows the QUIC spin bit pattern: reflected on receive, toggled on send when the reflected value matches the last sent value. The spin bit state machine runs for TX reflection, but RTT samples from the spin bit are discarded. In a mesh protocol where frames are sent irregularly (tree announces, bloom filters, MMP reports on different timers), inter-frame processing delays inflate spin bit RTT measurements unpredictably. Timestamp-echo from ReceiverReports (with dwell-time compensation) is the sole SRTT source.

Duplicate or regressed ReceiverReports are ignored before any RTT, loss, goodput, or ETX update. If receiver-side dwell time exceeds the wire field, the report keeps its counters but sends a zero timestamp echo so the sender cannot form an invalid RTT sample.

The spin bit lives in the link-layer FMP inner header, so this mechanism applies to link-layer MMP only. Session-layer MMP carries its spin bit in the FSP encrypted inner header but uses it the same way: reflected for diagnostic visibility, not used for SRTT.

ECN Congestion Signaling

The CE (Congestion Experienced) flag (bit 1 in the FMP flags byte) provides hop-by-hop congestion signaling through the mesh. Transit nodes detect congestion on outgoing links and set CE on forwarded packets; once set, the flag stays set for all subsequent hops to the destination.

Congestion detection triggers on any of:

Outgoing link MMP loss rate ≥ node.ecn.loss_threshold (default 5%)
Outgoing link MMP ETX ≥ node.ecn.etx_threshold (default 3.0)
Kernel receive buffer drops detected on any local transport (via SO_RXQ_OVFL on UDP)

CE relay: The forwarding path computes outgoing_ce = incoming_ce || local_congestion. Once CE is set on a packet, it remains set for the rest of the forward path.

IPv6 ECN-CE marking: When a CE-flagged DataPacket arrives at its final destination, the IPv6 Traffic Class ECN bits are marked CE (0b11) before TUN delivery — but only for ECN-capable packets (ECT(0) or ECT(1)). Not-ECT packets are never marked per RFC 3168. The host TCP stack then echoes ECE in ACKs, triggering sender cwnd reduction through standard congestion control.

Session-layer tracking: The ecn_ce_count field in MMP ReceiverReports tracks CE-flagged packets received per link, providing end-to-end visibility into congestion propagation.

ECN signaling is a link-layer mechanism. Session-layer MMP only observes the CE counter as part of the report stream; CE marking is not generated end-to-end. Tuning parameters live under node.ecn.* in ../reference/configuration.md.

Send Failure Backoff (Session Layer Only)

When a session MMP report cannot be delivered (destination unreachable, no route), the sender applies exponential backoff to the probe interval — a standard distributed-systems pattern for transient failure handling:

Each consecutive failure doubles the interval: 2x, 4x, 8x, 16x, 32x
Backoff caps at 32x the base interval (5 consecutive failures)
A successful send resets to the normal SRTT-based interval
Debug logging is suppressed after 3 consecutive failures; a summary is logged when the destination becomes reachable again

This prevents wasted CPU and log noise when a session's remote endpoint has departed the network but the local session has not yet timed out. Link-layer MMP has no equivalent — link-layer reports are peer-to-peer over an authenticated link, so delivery failure is indistinguishable from link death and the link-liveness mechanism takes over.

Layer Differences

Aspect	Link layer	Session layer
Routing scope	Peer-to-peer (one hop)	End-to-end (forwarded through every hop)
Configuration namespace	`node.mmp.*`	`node.session_mmp.*`
Report bounds	`[1s, 5s]`	`[500ms, 10s]`
Cold-start interval	200 ms (first 5 samples)	1 s
Bandwidth cost	One link	Proportional to path length
Send-failure backoff	Not applicable	Yes
Path-MTU echo	Not applicable	PathMtuNotification (see fips-mtu.md)
Idle-timeout interaction	None	Reports do not reset session idle timer

Idle Timeout Interaction (Session Layer Only)

MMP reports (SenderReport, ReceiverReport) and PathMtuNotification do not reset the session idle timer. Only application data (DataPacket, type 0x10) resets last_activity. This ensures sessions with no application traffic tear down after node.session.idle_timeout_secs (default 90s), while MMP continues providing measurement data up to the teardown moment.

Operator Logging

Both layers emit periodic metrics at debug level, so node.log_level (or RUST_LOG) must be set to debug to see them. The interval is node.mmp.log_interval_secs for link-layer (default 30s) and node.session_mmp.log_interval_secs for session-layer (default 30s).

Link-layer:

MMP link metrics peer=node-b rtt=2.3ms loss=0.2% jitter=0.1ms goodput=76.0MB/s tx_pkts=1234 rx_pkts=5678

Session-layer:

MMP session metrics session=npub1tdwa...84le rtt=4.3ms loss=0.6% jitter=0.2ms goodput=71.3MB/s mtu=1472 tx_pkts=1234 rx_pkts=5678

Teardown logs include final SRTT, loss rate, jitter, ETX, goodput, and cumulative tx/rx packet and byte counts.

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