bitchat-android/docs/SOURCE_ROUTING.md

Source-Based Routing for BitChat Packets

This document specifies an optional source-based routing extension to the BitChat packet format. A sender may attach a hop-by-hop route (list of peer IDs) to instruct relays on the intended path. Relays that support this feature will try to forward to the next hop directly; otherwise, they fall back to regular broadcast relaying.

Status: optional and backward-compatible.

Layering Overview

• Outer packet: BitChat binary packet with unchanged fixed header (version/type/ttl/timestamp/flags/payloadLength).
• Flags: adds a new bit HAS_ROUTE (0x08).
• Variable sections (when present, in order):
  1. SenderID (8 bytes)
  2. RecipientID (8 bytes) if HAS_RECIPIENT
  3. Route (if HAS_ROUTE): count (1 byte) + count * 8 bytes hop IDs
  4. Payload (with optional compression preamble)
  5. Signature (64 bytes) if HAS_SIGNATURE

Unknown flags are ignored by older implementations (they will simply not see a route and continue broadcasting as before).

Route Field Encoding

• Presence: Signaled by the HAS_ROUTE (0x08) bit in flags.
• Layout (immediately after optional RecipientID):
  • count: 1 byte (0..255)
  • hops: concatenation of count peer IDs, each encoded as exactly 8 bytes
• Peer ID encoding (8 bytes): same as used elsewhere in BitChat (16 hex chars → 8 bytes; left-to-right conversion; pad with 0x00 if shorter). This matches the on‑wire senderID/recipientID encoding.
• Size impact: 1 + 8*N bytes, where N = count.
• Empty route: HAS_ROUTE with count = 0 is treated as no route (relays ignore it).

Sender Behavior

• Applicability: Intended for addressed packets (i.e., where recipientID is set and is not the broadcast ID). For broadcast packets, omit the route.
• Path computation: Use Dijkstra’s shortest path (unit weights) on your internal mesh topology to find a route from src (your peerID) to dst (recipient peerID). The hop list SHOULD include the full path [src, ..., dst].
• Encoding: Set HAS_ROUTE, write count = path.length, then the 8‑byte hop IDs in order. Keep count <= 255.
• Signing: The route is covered by the Ed25519 signature (recommended):
  • Signature input is the canonical encoding with signature omitted and ttl = 0 (TTL excluded to allow relay decrement) — same rule as base protocol.

Relay Behavior

When receiving a packet that is not addressed to you:

1. If HAS_ROUTE is not set, or the route is empty, relay using your normal broadcast logic (subject to TTL/probability policies).
2. If HAS_ROUTE is set and your peer ID appears at index i in the hop list:
   • If there is a next hop at i+1, attempt a targeted unicast to that next hop if you have a direct connection to it.
     • If successful, do NOT broadcast this packet further.
     • If not directly connected (or the send fails), fall back to broadcast relaying.
   • If you are the last hop (no i+1), proceed with standard handling (e.g., if not addressed to you, do not relay further).

TTL handling remains unchanged: relays decrement TTL by 1 before forwarding (whether targeted or broadcast). If TTL reaches 0, do not relay.

Receiver Behavior (Destination)

• This extension does not change how addressed packets are handled by the final recipient. If the packet is addressed to you (recipientID == myPeerID), process it normally (e.g., decrypt Noise payload, verify signatures, etc.).
• Signature verification MUST include the route field when present; route tampering will invalidate the signature.

Compatibility

• Omission: If HAS_ROUTE is omitted, legacy behavior applies. Relays that don’t implement this feature will ignore the route entirely, because they won’t set or check HAS_ROUTE.
• Partial support: If any relay on the path cannot directly reach the next hop, it will fall back to broadcast relaying; delivery is still probabilistic like the base protocol.

Minimal Example (conceptual)

• Header (fixed 13 bytes): unchanged.
• Variable sections (ordered):
  • SenderID(8)
  • RecipientID(8) (if present)
  • HAS_ROUTE set → count=3, hops = [H0 H1 H2] where each Hk is 8 bytes
  • Payload (optionally compressed)
  • Signature (64)

Where H0 is the sender’s peer ID, H2 is the recipient’s peer ID, and H1 is an intermediate relay. The receiver verifies the signature over the packet encoding (with ttl = 0 and signature omitted), which includes the hops when HAS_ROUTE is set.

Operational Notes

• Routing optimality depends on the freshness and completeness of the topology your implementation has learned (e.g., via gossip of direct neighbors). Recompute routes as needed.
• Route length should be kept small to reduce overhead and the probability of missing a direct link at some hop.
• Implementations may introduce policy controls (e.g., disable source routing, cap max route length).