current state

notes/protocol.md

@@ -58,7 +58,7 @@ Currently carries the chunk manifest for the 3×3 neighbourhood around the playe
 
 The `entity_checksum` lets the client detect a lost `EntityPacket` without a dedicated
 ACK: if the checksum differs from the one computed over the last received entity update,
-the client knows to retransmit an `ActionPacket` (sequence preserved, no-op action) to
+the client knows to retransmit an `ActionPacket` (`target_tick = 0`, no-op action) to
 prompt the server to re-send the current entity state.
 
 ---
@@ -71,13 +71,27 @@ Sent by the client on player action or on a chunk cache miss.
 |--------|------|--------------|-----------------|--------------------------------|
 | 0      | 6    | Header       | `header`        | packet_type = 1                |
 | 6      | 8    | u64          | `auth_token`    | Token of the current session   |
-| 14     | 4    | u32          | `sequence`      | Monotonically increasing       |
+| 14     | 4    | u32          | `target_tick`   | Tick the action is scheduled for (see below); 0 = keep-alive/ack only |
 | 18     | 54   | ChunkEntry   | `cache[9]`      | Versions client currently holds |
 | 72     | 2    | PlayerAction | `player_action` | Derived from user input        |
 | 74     | ?    | ActionData   | `action_data`   | Dependent on PlayerAction      |
 
 **Minimum: 74 bytes** (no ActionData)
 
+**Tick-addressed scheduling.** Actions are scheduled onto the server's tick timeline
+instead of being consumed in arrival order. `target_tick` selects the movement window
+(`target_tick / TICKS_PER_MOVE`, rounded up) the action executes in:
+
+- A second action addressed to the same window **replaces** the first — this is how the
+  client retracts (NOOP) or changes a scheduled step until its window executes, and how
+  retransmits dedupe for free.
+- A **late** action (window already passed on arrival) moves to the next window, but
+  only if that slot is empty: late actions fill gaps, they never override newer intent.
+- Only the next `ACTION_WINDOW_HORIZON` windows are addressable; anything beyond is
+  dropped. Combined with one-action-per-window execution this bounds server memory and
+  movement speed regardless of client behavior.
+- `target_tick = 0` carries no scheduling intent (keep-alive / cache-ack packets).
+
 **ChunkEntry (6 bytes)**
 
 | Offset | Size | Type | Field      |
@@ -302,7 +316,7 @@ whatever is still missing. No deduplication logic is required.
 the client simply waits one tick (~100 ms).
 
 **Lost EntityPacket** — detected via the `entity_checksum` in the next `StatePacket`.
-The client retransmits a no-op `ActionPacket` (same sequence number, no-op action,
+The client retransmits a no-op `ActionPacket` (`target_tick = 0`, no-op action,
 current cache state); the server treats this as a normal diff request and re-sends the
 full entity update for the tick.