From baa2d66fd60d0692a4d72398e35471c43bcd080e Mon Sep 17 00:00:00 2001 From: callebtc <93376500+callebtc@users.noreply.github.com> Date: Fri, 9 Jan 2026 20:13:25 +0700 Subject: [PATCH] update spec --- docs/SOURCE_ROUTING.md | 219 +++++++++++++++++++++-------------------- 1 file changed, 113 insertions(+), 106 deletions(-) diff --git a/docs/SOURCE_ROUTING.md b/docs/SOURCE_ROUTING.md index f1ee0853..bfb285ca 100644 --- a/docs/SOURCE_ROUTING.md +++ b/docs/SOURCE_ROUTING.md @@ -1,128 +1,135 @@ -# Source-Based Routing for BitChat Packets +# Source-Based Routing for BitChat Packets (v2) -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. +This document specifies the Source-Based Routing extension (v2) for the BitChat protocol. This upgrade enables efficient unicast routing across the mesh by allowing senders to specify an explicit path of intermediate relays. -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)`. This flag is **only valid for packet version >= 2**. -- Variable sections (when present, in order): - 1) `SenderID` (8 bytes) - 2) `RecipientID` (8 bytes) if `HAS_RECIPIENT` - 3) `Route` (if `HAS_ROUTE` AND version >= 2): `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. For v1 packets, the `HAS_ROUTE` flag MUST be ignored even if set, and the route field MUST NOT be present. This ensures strict backward compatibility. - -## Detailed Packet Structure (v1 vs v2) - -The Bitchat packet structure is designed to be compact and efficient for BLE transmission. - -### Fixed Header -The fixed header is present in all packets. Its size depends on the version. - -| Field | Size (v1) | Size (v2) | Description | -|---|---|---|---| -| Version | 1 byte | 1 byte | Protocol version (`0x01` or `0x02`). | -| Type | 1 byte | 1 byte | Message Type (e.g., `0x01` Announce, `0x02` Message). | -| TTL | 1 byte | 1 byte | Time-To-Live (hop limit). | -| Timestamp | 8 bytes | 8 bytes | `UInt64` (big-endian) creation time (ms since epoch). | -| Flags | 1 byte | 1 byte | Bitmask: `HAS_RECIPIENT(0x01)`, `HAS_SIGNATURE(0x02)`, `IS_COMPRESSED(0x04)`, `HAS_ROUTE(0x08)`. | -| Payload Length | **2 bytes** | **4 bytes** | `UInt16` (v1) or `UInt32` (v2) length of the *Payload* section only. **Does NOT include route or other headers.** | -| **Total Header** | **14 bytes** | **16 bytes** | | - -### Variable Sections (In Order) - -These fields follow the fixed header immediately. - -1. **Sender ID** (Fixed 8 bytes) - * Present in ALL packets. - * Derived from the sender's public key. - -2. **Recipient ID** (Optional, 8 bytes) - * Present only if `HAS_RECIPIENT` flag is set. - * Target peer ID for addressed messages. - -3. **Source Route** (Optional, Variable Length) - * **Condition:** Present **ONLY** if `HAS_ROUTE` flag is set **AND** `Version >= 2`. - * **Structure:** - * `Count` (1 byte): Number of hops (`N`). - * `Hops` (`N * 8` bytes): Sequence of 8-byte Peer IDs. - * **Note:** The size of this field (`1 + 8*N` bytes) is **NOT** included in the `Payload Length` field in the fixed header. It exists structurally between the Recipient ID and the Payload. - -4. **Payload** (Variable Length) - * Size is exactly the value specified in the `Payload Length` field of the fixed header. - * Contains the application data (e.g., encrypted message, announcement TLVs). - * If `IS_COMPRESSED` flag is set, the first 2 bytes are the original uncompressed size (UInt16), followed by the compressed bytes. - -5. **Signature** (Optional, 64 bytes) - * Present only if `HAS_SIGNATURE` flag is set. - * Ed25519 signature covering the entire packet (with TTL=0 and Signature excluded). +**Status:** Implemented in Android and iOS. Backward compatible (v1 clients ignore routing data). --- -## Route Field Encoding +## 1. Protocol Versioning & Layering -- Presence: Signaled by the `HAS_ROUTE (0x08)` bit in `flags` **AND** `version >= 2`. -- 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). +To support source routing and larger payloads, the packet format has been upgraded to **Version 2**. -## Sender Behavior +* **Version 1 (Legacy):** 2-byte payload length limit. Ignores routing flags. +* **Version 2 (Current):** 4-byte payload length limit. Supports Source Routing. -- 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 the sender (your peerID) to the recipient (the destination peerID). The `BitchatPacket` already contains dedicated `senderID` and `recipientID` fields. The `Route` field's `hops` list **SHOULD** contain the sequence of intermediate peer IDs that the packet should traverse. It **SHOULD NOT** duplicate the `senderID` or `recipientID` if they are already present in the `BitchatPacket`'s dedicated fields. Instead, the `hops` list represents the explicit path *between* the sender and recipient, starting from the first relay and ending with the last relay before the recipient. -- Encoding: Ensure the packet `version` is set to 2 or higher. 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. +**Key Rule:** The `HAS_ROUTE (0x08)` flag is **only valid** if the packet `version >= 2`. Relays receiving a v1 packet must ignore this flag even if set. -## Relay Behavior +--- -When receiving a packet that is not addressed to you: +## 2. Packet Structure Comparison -1) If `HAS_ROUTE` is not set, or the route is empty, or the packet `version < 2`, relay using your normal broadcast logic (subject to TTL/probability policies). -2) If `HAS_ROUTE` is set AND `version >= 2`: - - **Route Sanity Check**: Before processing, the relay **MUST** validate the route. If the route contains duplicate hops (i.e., the same peer ID appears more than once), the packet **MUST** be dropped to prevent loops. - - If 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`), the packet has reached the end of its explicit route. The relay should then attempt to deliver it to the final `recipientID` if directly connected, but SHOULD NOT relay it further as a broadcast. +The following diagram illustrates the structural differences between a standard v1 packet and a source-routed v2 packet. -TTL handling remains unchanged: relays decrement TTL by 1 before forwarding (whether targeted or broadcast). If TTL reaches 0, do not relay. +### V1 Packet (Legacy) +```text ++-------------------+---------------------------------------------------------+ +| Fixed Header (14) | Variable Sections | ++-------------------+----------+-------------+------------------+-------------+ +| Ver: 1 (1B) | SenderID | RecipientID | Payload | Signature | +| Type, TTL, etc. | (8B) | (8B) | (Length in Head) | (64B) | +| Len: 2 Bytes | | (Optional) | | (Optional) | ++-------------------+----------+-------------+------------------+-------------+ +``` -## Receiver Behavior (Destination) +### V2 Packet (Source Routed) +```text ++-------------------+-----------------------------------------------------------------------------+ +| Fixed Header (16) | Variable Sections | ++-------------------+----------+-------------+-----------------------+------------------+-------------+ +| Ver: 2 (1B) | SenderID | RecipientID | SOURCE ROUTE | Payload | Signature | +| Type, TTL, etc. | (8B) | (8B) | (Variable) | (Length in Head) | (64B) | +| Len: 4 Bytes | | (Required*) | Only if HAS_ROUTE=1 | | (Optional) | ++-------------------+----------+-------------+-----------------------+------------------+-------------+ +``` -- 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. +**(*) Note:** A `Route` can be attached to **any** packet type that has a `RecipientID` (flag `HAS_RECIPIENT` set). -## Compatibility +### Fixed Header Differences -- Omission: If `HAS_ROUTE` is omitted, legacy behavior applies. Relays that don’t implement this feature will ignore the route entirely. -- Version Constraint: Implementations MUST NOT parse or act on the `Route` field in v1 packets, even if the `HAS_ROUTE` flag is set. This prevents potential parsing ambiguities with legacy clients. -- 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. +| Field | Size (v1) | Size (v2) | Description | +|---|---|---|---| +| **Version** | 1 byte | 1 byte | `0x01` vs `0x02` | +| **Payload Length** | **2 bytes** | **4 bytes** | `UInt32` in v2 to support large files. **Excludes** route/IDs/sig. | +| **Total Size** | **14 bytes** | **16 bytes** | V2 header is 2 bytes larger. | -## Minimal Example (conceptual) +--- -- Header (fixed 13 bytes): unchanged. -- Variable sections (ordered): - - `SenderID(8)` - - `RecipientID(8)` (if present) - - `HAS_ROUTE` set → `count=1`, `hops = [H1]` where `H1` is 8 bytes - - Payload (optionally compressed) - - Signature (64) +## 3. Source Route Specification -In this example, `SENDER_ID` is the sender, `RECIPIENT_ID` is the final recipient, and `H1` is the single intermediate relay. The `hops` list explicitly defines the path *between* the sender and recipient. The receiver verifies the signature over the packet encoding (with `ttl = 0` and `signature` omitted), which includes the `hops` when `HAS_ROUTE` is set. +The `Source Route` field is a variable-length list of **intermediate hops** that the packet must traverse. -## Operational Notes +* **Location:** Immediately follows `RecipientID`. +* **Structure:** + * `Count` (1 byte): Number of intermediate hops (`N`). + * `Hops` (`N * 8` bytes): Sequence of Peer IDs. -- 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). +### Intermediate Hops Only +The route list MUST contain **only** the intermediate relays between the sender and the recipient. +* **DO NOT** include the `SenderID` (it is already in the packet). +* **DO NOT** include the `RecipientID` (it is already in the packet). +**Example:** +Topology: `Alice (Sender) -> Bob -> Charlie -> Dave (Recipient)` +* Packet `SenderID`: Alice +* Packet `RecipientID`: Dave +* Packet `Route`: `[Bob, Charlie]` (Count = 2) + +--- + +## 4. Topology Discovery (Gossip) + +To calculate routes, nodes need a view of the network topology. This is achieved via a **Neighbor List** extension to the `IdentityAnnouncement` packet. + +* **Mechanism:** `IdentityAnnouncement` packets contain a TLV (Type-Length-Value) payload. +* **New TLV Type:** `0x04` (Direct Neighbors). +* **Content:** A list of Peer IDs that the announcing node is directly connected to. + +**TLV Structure (Type 0x04):** +```text +[Type: 0x04] [Length: 1B] [Count: 1B] [NeighborID1 (8B)] [NeighborID2 (8B)] ... +``` +Nodes receiving this TLV update their local mesh graph, linking the sender to the listed neighbors. + +--- + +## 5. Fragmentation & Source Routing + +When a large source-routed packet (e.g., File Transfer) exceeds the MTU and requires fragmentation: + +1. **Version Inheritance:** All fragments MUST be marked as **Version 2**. +2. **Route Inheritance:** All fragments MUST contain the **exact same Route field** as the parent packet. + +**Why?** If fragments were sent as v1 packets or without routes, they would fall back to flooding, negating the bandwidth benefits of source routing for large data transfers. + +--- + +## 6. Security & Signing + +Source routing is fully secured by the existing Ed25519 signature scheme. + +* **Scope:** The signature covers the **entire packet structure** (Header + Sender + Recipient + Route + Payload). +* **Verification:** The receiver verifies the signature against the `SenderID`'s public key. +* **Integrity:** Any tampering with the route list by malicious relays will invalidate the signature, causing the packet to be dropped by the destination. + +**Signature Input Construction:** +Serialize the packet exactly as transmitted, but temporarily set `TTL = 0` and remove the `Signature` bytes. + +--- + +## 7. Relay Logic + +When a node receives a packet **not** addressed to itself: + +1. **Check Route:** + * Is `Version >= 2`? + * Is `HAS_ROUTE` flag set? + * Is the route list non-empty? +2. **If YES (Source Routed):** + * Find local Peer ID in the route list at index `i`. + * **Next Hop:** The peer at `i + 1`. + * **Last Hop:** If `i` is the last index, the Next Hop is the `RecipientID`. + * **Action:** Attempt to unicast (`sendToPeer`) to the Next Hop. + * **Fallback:** If the Next Hop is unreachable, **fall back to broadcast/flood** to ensure delivery. +3. **If NO (Standard):** + * Flood the packet to all connected neighbors (subject to TTL and probability rules).