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SessionDatagram redesign: add src_addr, reclassify error signals

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Add src_addr to SessionDatagram envelope (34-byte header: msg_type +
src_addr + dest_addr + hop_limit) so transit routers can route error
signals back to the packet's originator.

Reclassify CoordsRequired/PathBroken as link-layer error signals
(plaintext inside SessionDatagram) rather than e2e encrypted session
messages. Transit routers generate these when forwarding fails and
route them to src_addr; if source is also unreachable, drop silently.

Remove redundant src_addr/dest_addr/hop_limit from SessionSetup,
SessionAck, and DataPacket (now in envelope). DataPacket header
reduced from 36 to 4 bytes. Remove PathBroken.original_src.

Fix routing loop vulnerability: gate bloom filter path on having
cached dest_coords to prevent blind forwarding between peers.
Simplify select_best_candidate() to require coordinates.

Fix gossip protocol type codes (0x11->0x20, 0x12->0x30, 0x13->0x31)
for consistency across all design docs.

All 5 design docs updated and cross-checked for consistency.
335 tests pass, zero warnings.
This commit is contained in:
Johnathan Corgan
2026-02-12 11:32:45 +00:00
parent 2f8e97c0ab
commit d41009b778
9 changed files with 535 additions and 349 deletions
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34
docs/design/fips-intro.md
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@@ -462,28 +462,40 @@ FIPS uses a discriminator-based wire format for efficient message dispatch.
Exchanged between directly connected peers, encrypted with link session keys:
| Type | Name | Purpose |
|------|----------------|--------------------------------------------|
| 0x10 | TreeAnnounce | Spanning tree state (parent, ancestry) |
| 0x20 | FilterAnnounce | Bloom filter reachability update |
| 0x30 | LookupRequest | Query for node's tree coordinates |
| 0x31 | LookupResponse | Response with coordinates and proof |
| 0x40 | SessionDatagram| Carries end-to-end encrypted payloads |
| 0x50 | Disconnect | Orderly disconnect notification |
| Type | Name | Purpose |
|------|----------------|------------------------------------------------------|
| 0x10 | TreeAnnounce | Spanning tree state (parent, ancestry) |
| 0x20 | FilterAnnounce | Bloom filter reachability update |
| 0x30 | LookupRequest | Query for node's tree coordinates |
| 0x31 | LookupResponse | Response with coordinates and proof |
| 0x40 | SessionDatagram| Carries session-layer payloads through the mesh |
| 0x50 | Disconnect | Orderly disconnect notification |
### Session Layer Messages
### SessionDatagram Payload Messages
Carried inside SessionDatagram, encrypted end-to-end between source and
destination:
Carried inside SessionDatagram (0x40), which provides src_addr, dest_addr, and
hop_limit for multi-hop forwarding.
**Session-layer messages** (end-to-end encrypted between source and destination):
| Type | Name | Purpose |
|------|----------------|--------------------------------------------|
| 0x00 | SessionSetup | Establish routing session with coordinates |
| 0x01 | SessionAck | Acknowledge session establishment |
| 0x10 | DataPacket | Encrypted application data (IPv6 payload) |
**Link-layer error signals** (plaintext, generated by transit routers):
| Type | Name | Purpose |
|------|----------------|--------------------------------------------|
| 0x20 | CoordsRequired | Router cache miss—need fresh coordinates |
| 0x21 | PathBroken | Greedy routing failed—need re-lookup |
Error signals are sent back to the original source using the `src_addr` from
the SessionDatagram that triggered the error. They are plaintext because the
transit router has no end-to-end session with the source; link-layer encryption
protects them hop-by-hop.
See [fips-wire-protocol.md](fips-wire-protocol.md) for wire format details and
[fips-gossip-protocol.md](fips-gossip-protocol.md) for gossip message formats.