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fips/docs/design/fips-configuration.md
Johnathan Corgan 0a72317b59 Design documentation illustration pass and FLP→FMP rename 
Rename FIPS Link Protocol (FLP) to FIPS Mesh Protocol (FMP)

  The "Link Protocol" name understated the layer's scope — spanning tree
  construction, bloom filter routing, greedy forwarding, and mesh-wide
  coordination go well beyond link-level concerns. Rename fips-link-layer.md
  to fips-mesh-layer.md, update FLP→FMP throughout docs and source code
  (FLP_VERSION→FMP_VERSION, wire.rs, rx_loop.rs, spanning_tree.rs).

New SVG illustrations

  - Protocol stack: color-coded layer diagram replacing ASCII art
  - OSI mapping: side-by-side comparison with traditional networking layers
  - Bloom filter propagation: 6-node tree with sender-colored filter boxes
    showing split-horizon computation per link
  - Routing decision flowchart: 5-step priority chain with candidate ranking
    by tree distance and link performance
  - Coordinate discovery: sequence diagram showing LookupRequest propagation,
    response caching, and SessionSetup cache warming

Redesigned existing SVGs

  - Architecture overview: uniform node layout, U-shaped encrypted link
    connectors, separate end-to-end session line
  - Node architecture: split Router Core into FSP and FMP layers, reorganize
    transports into Overlay/Shared Medium/Point-to-Point categories
  - Identity derivation: wider boxes, visible encode arrow, dashed npub line

fips-intro.md revisions

  - Add inline references to prior work: Yggdrasil/Ironwood for coordinate
    routing, Noise Protocol Framework for IK handshakes, WireGuard for
    index-based session dispatch, Wikipedia for bloom filters, split-horizon,
    and greedy embedding
  - Add explanatory paragraphs after bloom filter diagram describing
    split-horizon filter computation and candidate selection behavior
  - Simplify transport abstraction language, remove I2P/LoRa references
  - Fix LookupRequest wording ("propagates" not "floods"), note intermediate
    node coordinate caching on lookup responses
  - Rewrite architecture overview prose to match redesigned diagrams
2026-02-21 22:05:44 +00:00

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# FIPS Configuration
FIPS uses YAML-based configuration with a cascading multi-file priority system.
All parameters have sensible defaults; a node can run with no configuration file
at all (it will generate an ephemeral identity and listen on default addresses).
## Configuration Loading
### Search Paths
When started without the `-c` flag, FIPS searches for `fips.yaml` in these
locations, lowest to highest priority:
| Priority | Path | Purpose |
|----------|------|---------|
| 1 (lowest) | `/etc/fips/fips.yaml` | System-wide defaults |
| 2 | `~/.config/fips/fips.yaml` | User preferences |
| 3 | `~/.fips.yaml` | Legacy user config |
| 4 (highest) | `./fips.yaml` | Deployment-specific overrides |
All found files are loaded and merged in priority order. Values from higher
priority files override those from lower priority files. This allows a system
administrator to set site-wide defaults in `/etc/fips/fips.yaml` while
individual deployments override specific values in `./fips.yaml`.
### CLI Option
```text
fips -c /path/to/config.yaml
```
When `-c` is specified, only that file is loaded (search paths are skipped).
### Partial Configuration
Every field has a built-in default. A configuration file only needs to specify
values that differ from defaults. For example, a minimal config might contain
only the identity and peer list, inheriting all other defaults.
## YAML Structure
The configuration is organized into five top-level sections:
```yaml
node: # Node behavior, protocol parameters, and tuning
tun: # TUN virtual interface
dns: # DNS responder for .fips domain
transports: # Network transports (UDP, future: TCP, Tor)
peers: # Static peer list
```
All tunable protocol parameters live under `node.*`, organized as sysctl-style
dotted paths. The top-level sections (`tun`, `dns`, `transports`, `peers`)
handle infrastructure concerns only.
## Node Parameters (`node.*`)
### Identity (`node.identity.*`)
| Parameter | Type | Default | Description |
|-----------|------|---------|-------------|
| `node.identity.nsec` | string | *(generate random)* | Hex-encoded secret key. If omitted, an ephemeral identity is generated on each start. |
### General
| Parameter | Type | Default | Description |
|-----------|------|---------|-------------|
| `node.leaf_only` | bool | `false` | Leaf-only mode: node does not forward traffic or participate in routing |
| `node.tick_interval_secs` | u64 | `1` | Periodic maintenance tick interval (retry checks, timeout cleanup, tree refresh) |
| `node.base_rtt_ms` | u64 | `100` | Initial RTT estimate for new links before measurements converge |
| `node.heartbeat_interval_secs` | u64 | `10` | Heartbeat send interval per peer for liveness detection |
| `node.link_dead_timeout_secs` | u64 | `30` | No-traffic timeout before a peer is declared dead and removed |
### Resource Limits (`node.limits.*`)
Controls capacity for connections, peers, and links.
| Parameter | Type | Default | Description |
|-----------|------|---------|-------------|
| `node.limits.max_connections` | usize | `256` | Max handshake-phase connections |
| `node.limits.max_peers` | usize | `128` | Max authenticated peers |
| `node.limits.max_links` | usize | `256` | Max active links |
| `node.limits.max_pending_inbound` | usize | `1000` | Max pending inbound handshakes |
### Rate Limiting (`node.rate_limit.*`)
Handshake rate limiting protects against DoS on the Noise IK handshake path.
| Parameter | Type | Default | Description |
|-----------|------|---------|-------------|
| `node.rate_limit.handshake_burst` | u32 | `100` | Token bucket burst capacity |
| `node.rate_limit.handshake_rate` | f64 | `10.0` | Tokens per second refill rate |
| `node.rate_limit.handshake_timeout_secs` | u64 | `30` | Stale handshake cleanup timeout |
| `node.rate_limit.handshake_resend_interval_ms` | u64 | `1000` | Initial handshake message resend interval |
| `node.rate_limit.handshake_resend_backoff` | f64 | `2.0` | Resend backoff multiplier (1s, 2s, 4s, 8s, 16s with defaults) |
| `node.rate_limit.handshake_max_resends` | u32 | `5` | Max resends per handshake attempt |
### Retry / Backoff (`node.retry.*`)
Connection retry with exponential backoff.
| Parameter | Type | Default | Description |
|-----------|------|---------|-------------|
| `node.retry.max_retries` | u32 | `5` | Max connection retry attempts |
| `node.retry.base_interval_secs` | u64 | `5` | Base backoff interval |
| `node.retry.max_backoff_secs` | u64 | `300` | Cap on exponential backoff (5 minutes) |
Auto-reconnect (triggered by MMP link-dead removal) uses the same backoff
parameters but bypasses `max_retries`, retrying indefinitely. See
`peers[].auto_reconnect` below.
### Cache Parameters (`node.cache.*`)
Controls caching of tree coordinates and identity mappings.
| Parameter | Type | Default | Description |
|-----------|------|---------|-------------|
| `node.cache.coord_size` | usize | `50000` | Max entries in coordinate cache |
| `node.cache.coord_ttl_secs` | u64 | `300` | Coordinate cache entry TTL (5 minutes) |
| `node.cache.identity_size` | usize | `10000` | Max entries in identity cache (LRU, no TTL) |
### Discovery Protocol (`node.discovery.*`)
Controls flood-based node discovery (LookupRequest/LookupResponse).
| Parameter | Type | Default | Description |
|-----------|------|---------|-------------|
| `node.discovery.ttl` | u8 | `64` | Hop limit for LookupRequest flood |
| `node.discovery.timeout_secs` | u64 | `10` | Lookup completion timeout |
| `node.discovery.recent_expiry_secs` | u64 | `10` | Dedup cache expiry for recent request IDs |
### Spanning Tree (`node.tree.*`)
Controls tree construction and parent selection.
| Parameter | Type | Default | Description |
|----------------------------------------|-------|---------|--------------------------------------------------|
| `node.tree.announce_min_interval_ms` | u64 | `500` | Per-peer TreeAnnounce rate limit |
| `node.tree.parent_switch_threshold` | usize | `1` | Min depth improvement required to switch parents |
### Bloom Filter (`node.bloom.*`)
| Parameter | Type | Default | Description |
|-----------|------|---------|-------------|
| `node.bloom.update_debounce_ms` | u64 | `500` | Debounce interval for filter update propagation |
Bloom filter size (1 KB), hash count (5), and size classes are protocol
constants and not configurable.
### Session / Data Plane (`node.session.*`)
Controls end-to-end session behavior and packet queuing.
| Parameter | Type | Default | Description |
|-----------|------|---------|-------------|
| `node.session.default_ttl` | u8 | `64` | Default SessionDatagram TTL |
| `node.session.pending_packets_per_dest` | usize | `16` | Queue depth per destination during session establishment |
| `node.session.pending_max_destinations` | usize | `256` | Max destinations with pending packets |
| `node.session.idle_timeout_secs` | u64 | `90` | Idle session timeout; established sessions with no application data for this duration are removed. MMP reports (SenderReport, ReceiverReport, PathMtuNotification) do not count as activity |
| `node.session.coords_warmup_packets` | u8 | `5` | Number of initial data packets per session that include the CP flag for transit cache warmup; also the reset count on CoordsRequired/PathBroken receipt |
| `node.session.coords_response_interval_ms` | u64 | `2000` | Minimum interval (ms) between standalone CoordsWarmup responses to CoordsRequired/PathBroken signals per destination |
The anti-replay window size (2048 packets) is a compile-time constant and not
configurable.
### Link-Layer MMP (`node.mmp.*`)
Metrics Measurement Protocol for per-peer link measurement. See
[fips-mesh-layer.md](fips-mesh-layer.md) for behavioral details.
| Parameter | Type | Default | Description |
|-----------|------|---------|-------------|
| `node.mmp.mode` | string | `"full"` | Operating mode: `full` (sender + receiver reports), `lightweight` (receiver reports only), or `minimal` (spin bit + CE echo only, no reports) |
| `node.mmp.log_interval_secs` | u64 | `30` | Periodic operator log interval for link metrics |
| `node.mmp.owd_window_size` | usize | `32` | One-way delay trend ring buffer size |
### Session-Layer MMP (`node.session_mmp.*`)
Metrics Measurement Protocol for end-to-end session measurement. Configured
independently from link-layer MMP because session reports are routed through
every transit link, consuming bandwidth proportional to path length.
| Parameter | Type | Default | Description |
|-----------|------|---------|-------------|
| `node.session_mmp.mode` | string | `"full"` | Operating mode: `full`, `lightweight`, or `minimal` |
| `node.session_mmp.log_interval_secs` | u64 | `30` | Periodic operator log interval for session metrics |
| `node.session_mmp.owd_window_size` | usize | `32` | One-way delay trend ring buffer size |
### Internal Buffers (`node.buffers.*`)
Channel sizes affecting throughput and memory. Primarily useful for performance
tuning under high load or on memory-constrained devices.
| Parameter | Type | Default | Description |
|-----------|------|---------|-------------|
| `node.buffers.packet_channel` | usize | `1024` | Transport to Node packet channel capacity |
| `node.buffers.tun_channel` | usize | `1024` | TUN to Node outbound channel capacity |
| `node.buffers.dns_channel` | usize | `64` | DNS to Node identity channel capacity |
## TUN Interface (`tun.*`)
| Parameter | Type | Default | Description |
|-----------|------|---------|-------------|
| `tun.enabled` | bool | `false` | Enable TUN virtual interface |
| `tun.name` | string | `"fips0"` | Interface name |
| `tun.mtu` | u16 | `1280` | Interface MTU (IPv6 minimum) |
## DNS Responder (`dns.*`)
Resolves `<npub>.fips` queries to FIPS IPv6 addresses. Resolution is pure
computation (npub to public key to address); resolved identities are registered
with the node for routing.
| Parameter | Type | Default | Description |
|-----------|------|---------|-------------|
| `dns.enabled` | bool | `false` | Enable DNS responder |
| `dns.bind_addr` | string | `"127.0.0.1"` | Bind address |
| `dns.port` | u16 | `5354` | Listen port |
| `dns.ttl` | u32 | `300` | AAAA record TTL in seconds |
The `dns.ttl` value should not exceed `node.cache.coord_ttl_secs` to avoid
stale address mappings.
## Transports (`transports.*`)
### UDP (`transports.udp.*`)
| Parameter | Type | Default | Description |
|-----------|------|---------|-------------|
| `transports.udp.bind_addr` | string | `"0.0.0.0:4000"` | UDP bind address and port |
| `transports.udp.mtu` | u16 | `1280` | Transport MTU |
| `transports.udp.recv_buf_size` | usize | `2097152` | UDP socket receive buffer size in bytes (2 MB). Linux kernel doubles the requested value internally. Host `net.core.rmem_max` must be >= this value. |
| `transports.udp.send_buf_size` | usize | `2097152` | UDP socket send buffer size in bytes (2 MB). Host `net.core.wmem_max` must be >= this value. |
## Peers (`peers[]`)
Static peer list. Each entry defines a peer to connect to.
| Parameter | Type | Default | Description |
|-----------|------|---------|-------------|
| `peers[].npub` | string | *(required)* | Peer's Nostr public key (npub-encoded) |
| `peers[].alias` | string | *(none)* | Human-readable name for logging |
| `peers[].addresses[].transport` | string | *(required)* | Transport type (`udp`) |
| `peers[].addresses[].addr` | string | *(required)* | Transport address (e.g., `"10.0.0.2:4000"`) |
| `peers[].addresses[].priority` | u8 | `100` | Address priority (lower = preferred) |
| `peers[].connect_policy` | string | `"auto_connect"` | Connection policy: `auto_connect`, `on_demand`, or `manual` |
| `peers[].auto_reconnect` | bool | `true` | Automatically reconnect after MMP link-dead removal (exponential backoff, unlimited retries) |
## Minimal Example
A typical node configuration enabling TUN, DNS, and a single peer:
```yaml
node:
identity:
nsec: "0102030405060708090a0b0c0d0e0f101112131415161718191a1b1c1d1e1f20"
tun:
enabled: true
name: fips0
mtu: 1280
dns:
enabled: true
bind_addr: "127.0.0.1"
port: 53
transports:
udp:
bind_addr: "0.0.0.0:4000"
mtu: 1197
peers:
- npub: "npub1tdwa4vjrjl33pcjdpf2t4p027nl86xrx24g4d3avg4vwvayr3g8qhd84le"
alias: "node-b"
addresses:
- transport: udp
addr: "172.20.0.11:4000"
connect_policy: auto_connect
```
All `node.*` parameters use their defaults. To override specific values, add
only the relevant sections:
```yaml
node:
identity:
nsec: "..."
limits:
max_peers: 64
retry:
max_retries: 10
max_backoff_secs: 600
cache:
coord_size: 100000
```
## Complete Reference
The full YAML structure with all defaults:
```yaml
node:
identity:
nsec: null # hex secret key (null = generate ephemeral)
leaf_only: false
tick_interval_secs: 1
base_rtt_ms: 100
heartbeat_interval_secs: 10
link_dead_timeout_secs: 30
limits:
max_connections: 256
max_peers: 128
max_links: 256
max_pending_inbound: 1000
rate_limit:
handshake_burst: 100
handshake_rate: 10.0
handshake_timeout_secs: 30
handshake_resend_interval_ms: 1000
handshake_resend_backoff: 2.0
handshake_max_resends: 5
retry:
max_retries: 5
base_interval_secs: 5
max_backoff_secs: 300
cache:
coord_size: 50000
coord_ttl_secs: 300
identity_size: 10000
discovery:
ttl: 64
timeout_secs: 10
recent_expiry_secs: 10
tree:
announce_min_interval_ms: 500
parent_switch_threshold: 1
bloom:
update_debounce_ms: 500
session:
default_ttl: 64
pending_packets_per_dest: 16
pending_max_destinations: 256
idle_timeout_secs: 90
coords_warmup_packets: 5
coords_response_interval_ms: 2000
mmp:
mode: full # full | lightweight | minimal
log_interval_secs: 30
owd_window_size: 32
session_mmp:
mode: full # full | lightweight | minimal
log_interval_secs: 30
owd_window_size: 32
buffers:
packet_channel: 1024
tun_channel: 1024
dns_channel: 64
tun:
enabled: false
name: "fips0"
mtu: 1280
dns:
enabled: false
bind_addr: "127.0.0.1"
port: 5354
ttl: 300
transports:
udp:
bind_addr: "0.0.0.0:4000"
mtu: 1280
recv_buf_size: 2097152 # 2 MB (kernel doubles to 4 MB actual)
send_buf_size: 2097152 # 2 MB
peers: # static peer list
# - npub: "npub1..."
# alias: "node-b"
# addresses:
# - transport: udp
# addr: "10.0.0.2:4000"
# priority: 100
# connect_policy: auto_connect
# auto_reconnect: true # reconnect after link-dead removal
```
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