Describe packets

API.md

@@ -1,8 +1,8 @@
 # The Forst API
 
 A client communicates with a Forst server using JSON packets over one or more
-WebSocket connections. A normal client does not need more than one connection,
-but can still use more if it is more convenient (e.g. multiple browser tabs).
+WebSocket connections. While a single connection should usually suffice, some
+clients may prefer multiple connections (e.g. multiple browser tabs).
 
 <!-- Implementing a client should be easy, especially implementing a bot. -->
 <!-- The API must be usable in the context of a web browser. -->
@@ -22,6 +22,37 @@ phase, the client can enter and exit rooms, as well as interact with other
 clients in entered rooms. The server will also send status updates (events) for
 entered rooms.
 
+## Packets
+
+The server and client communicate using packets. A packet is a textual WebSocket
+message containing a JSON object.
+
+There are three types of packets: **Events**, **commands**, and **replies**.
+
+- An **event** is sent from the server to the client whenever something happens
+  that the client should know about.
+- A **command** is sent from the client to the server. It instructs the server
+  to perform an action on behalf of the client.
+- The server must respond to a command with a **reply**. It describes the result
+  of executing the command.
+
+Every packet has a `type` field that describes the type of the packet. It must
+have one of the following values:
+
+- `"event"` if the packet is an **event**
+- `"command"` if the packet is a **command**
+- `"reply"` if the packet is a **reply**
+
+Every packet has a `name` field and a `data` field. The `name` field contains
+the name of the event or command. The `data` field contains a json object that
+is the payload of the event, command, or reply.
+
+**Command** and **reply** packets have an optional field `id` of type string
+that can be used by the client to associate replies with commands. When the
+client sends a command, it may include an arbitrary id. In its reply to the
+command, the server must include the exact same id. If the client omitted the
+id, the server must omit it as well.
+
 ## Primitive types
 
 ### EventId
@@ -49,8 +80,6 @@ A user id is a string matching `[a-z0-9-]+`. It is usually four characters long
 for accounts and eight characters plus a dash for other sessions, though this is
 not required. A user id uniquely identifies a user.
 
-<!-- Dashes are optional and carry no meaning. They can be added and removed at will. -->
-
 ## Auth phase commands
 
 ### `c-auth-cookie`