fips/RELEASE-NOTES.md

FIPS v0.4.1

Released: 2026-07-19

v0.4.1 is a maintenance release on the v0.4.x line. It raises the default antipoison cap on inbound bloom filter announcements, removes a redundant spanning-tree metric counter, fixes two convergence and path-MTU bugs, and cuts per-packet CPU in the bloom and identity paths. There is no wire format change and no new feature surface.

v0.4.1 is wire-compatible with v0.4.0. Nodes can be upgraded one at a time with no coordinated restart, though one behavior change below is worth reading before you start a rolling upgrade.

At a glance

• node.bloom.max_inbound_fpr default moves from 0.10 to 0.20.
• The parent_switched metric counter is gone. Use parent_switches.
• Spanning tree no longer serves stale coordinates after a parent link is lost through peer removal.
• Discovery no longer loosens a path MTU clamp it had correctly tightened.
• Bloom probing and identity operations do measurably less work per call, with identical results.

Behavior changes worth flagging

The inbound filter FPR cap default doubles again

node.bloom.max_inbound_fpr goes from 0.10 to 0.20. The cap rejects inbound FilterAnnounce frames whose advertised false positive rate exceeds it. On the fixed 1 KB, k=5 filter, 0.10 corresponds to a fill of 0.631 and roughly 1,630 reachable entries, and the busiest nodes' aggregates had started reaching that ceiling as the mesh grew. 0.20 corresponds to a fill of 0.7248 and roughly 2,114 entries.

Be aware that this is the second time in two releases that this default has doubled, for the same reason both times. That is worth stating plainly rather than repeating the previous release's framing: raising the cap buys headroom, it does not fix anything. The real constraint is the fixed 1 KB filter size, which is a protocol constant. The structural remedy is the v2 filter work, where filter capacity scales with the mesh instead of being pinned. This release is an interim step to keep legitimate aggregates from being rejected until that lands. It is not the start of a pattern of raising the cap once per release, and if you are sizing capacity planning around this number, plan against the v2 work rather than against a third raise.

The antipoison property the cap exists for is preserved. A saturated or deliberately poisoned filter still presents an FPR near 100% and is still rejected.

This matters during a rolling upgrade. A v0.4.1 node accepts a FilterAnnounce with a derived FPR between 0.10 and 0.20; a v0.4.0 node drops the same frame, and the drop is silent on the wire with no NACK. The cap also gates the mesh size estimator, which declines to produce a value when any contributing filter is over the cap. So while a mesh is partly upgraded, upgraded and not-yet-upgraded nodes can legitimately report different mesh sizes, or one can report a size while the other reports unknown. This resolves once every node is on v0.4.1. If you want to avoid the window entirely, set node.bloom.max_inbound_fpr: 0.10 explicitly in your config before upgrading and remove it after the last node is done.

The parent_switched counter is removed

parent_switched was incremented on the line immediately before parent_switches at every site and never independently, so the two counters always held the same value. parent_switched is now gone from the tree metrics, the control socket snapshot, and the fipstop tree view. parent_switches remains and is unchanged.

If you scrape the control socket, or have dashboards or alerts referencing parent_switched, point them at parent_switches. Anything still asking for parent_switched will find nothing rather than a zero.

Notable bug fixes

Stale coordinates after losing a parent through peer removal

When a node's parent link dropped via peer removal, the node correctly reparented or self-rooted, but skipped the coordinate cache invalidation that every other position-change path performs. Cached entries for downstream destinations kept the node's old coordinate prefix. This did not self-correct the way a stale cache entry normally would: routing access refreshes an entry's TTL, so an entry that was actively being routed through never expired, and was only fixed by an unrelated fresh insert. Both invalidation classes now run on this path, matching the loop-detection branch.

Discovery could loosen a tightened path MTU clamp

An originator handling a LookupResponse overwrote its cached path MTU unconditionally. If a reactive MtuExceeded or PathMtuNotification had already taught it a tighter value, a later, looser discovery estimate would clobber that and re-loosen the clamp, risking a return to dropped oversized packets. The cached and received values are now compared and the tighter one is kept.

Upgrade notes

This is a drop-in upgrade from v0.4.0 with no wire format change, no config migration, and no coordinated restart. Upgrade nodes in whatever order you like.

Two things to do rather than assume:

1. If you monitor parent_switched, move to parent_switches before upgrading, or your dashboards will go blank rather than error.
2. During the rolling window, expect upgraded and not-yet-upgraded nodes to potentially disagree about mesh size, per the FPR cap section above. This is expected and self-resolves. Do not chase it as a bug unless it persists after every node reports 0.4.1.

If you have pinned node.bloom.max_inbound_fpr explicitly in your config, your setting is honored and nothing changes for you. The change only affects nodes taking the default.

Downgrading to v0.4.0 is supported and needs no special handling.

Getting v0.4.1

• Linux x86_64 / aarch64: .deb and tarball at the v0.4.1 release page.
• Arch Linux: fips from the AUR.
• macOS: .pkg at the v0.4.1 release page.
• Windows: ZIP at the v0.4.1 release page.
• OpenWrt: .ipk (OpenWrt 24.x and earlier) or .apk (OpenWrt 25+) at the v0.4.1 release page.
• From source: cargo build --release from a checkout of the v0.4.1 tag (Rust 1.94.1 per rust-toolchain.toml; libclang-dev is a required Linux build prerequisite).
• Nix / NixOS: nix build .#fips from a checkout of the v0.4.1 tag builds the binaries from source with the pinned toolchain and no manual prerequisites (see the Nix section of packaging/README.md).

The full per-commit changelog lives in CHANGELOG.md. Issues and discussion at github.com/jmcorgan/fips.

Contributors

Thanks to everyone who contributed code, packaging work, bug reports, or reviews to this release.

• @jcorgan: release shepherd, spanning-tree and discovery fixes, bloom and identity performance work, antipoison cap change, and testing.