Drawpile protocol
-----------------

Drawpile uses a message based protocol. Each message begins with a header
containing the length of the message, the type and the user ID.

Messages can be divided into different categories in three different ways:

 * Recordable/non-recordable messages
 * Transparent/opaque messages
 * Control/Meta/Command

Control messages are used for general client/server communication. They are not recordable.

Meta messages are used for things not directly related to drawing, such as user login
notifications and access control changes.

Command messages are used to perform drawing and other canvas manipulation.
The bulk of Drawpile traffic consists of command messages.

Transparent messages are those message types the server needs to understand. They include all
Control messages and some Meta messages. The server does not parse Opaque messages, but merely
passes them along to the clients. Most Meta and all Command messages are opaque.

Each message begins with a header indicating its length, type and user ID:

    ┌────────┬──────┬─────────────────────┐
    │ LENGTH │ TYPE │ USERID │ PAYLOAD... │
    └────────┴──────┴─────────────────────┘

The length field indicates the length of the payload data. The full length of
the message is 4 + payload length.

See `src/shared/net/message.h` for the full list of message types.

## Login process

The login handshake is a string based protocol that uses the LOGIN message type. The handshake begins when the user connects to the server port.

The server starts by sending a hello message, which includes the protocol version and the set of server feature flags. If the client does not support the protocol or does not understand the features, it should disconnect.

If the SECURE feature flag is set, the server will not let the login process continue until the client has upgraded to a secure connection.

Next, the client must authenticate. The client will first attempt a guest login, but if that is not possible, it will prompt the user for a password and retry. An authenticated user may be granted extra privileges by the server.

After authentication, the server continues will send a list of available sessions. The client can either join one of the sessions or host a new one. The server may send updated session information until the client has made a decision.

The built-in server only supports a single session per server instance. This is indicated by (the lack of) a feature flag. In this case, the client may join the only session automatically without prompting the user.

The server responds to the join/host command either by a success signal or an error code. If the command was accepted, the client leaves the login state and enters the session. In case of error, the server disconnects the client. Users joining a session will be assigned a user ID by the server. Hosting users can choose their own IDs.

When hosting a session, the hosting user must announce its full protocol version.
A joining client must not join a session with a mismatching minor version.
This way, a single server can support multiple client versions as long as the server part of the protocol versions match.

See `src/shared/server/loginhandler.h` for implementation details.

## Security

The server supports SSL/TLS encrypted connections. The client upgrades to a secure connection by sending the command "STARTTLS". The server will reply with a STARTTLS of its own and begins the SSL handshake.

Since most Drawpile users will likely run drawpile-srv on their home computers or small hosting services, Drawpile does not utilize PKI. The client accepts self-signed certificates and, when connecting to an IP address, certificates that do not match the hostname of the server. Instead, the client will remember the certificate associated with each hostname and warns if it changes.

## Session recording format

A session recording starts with a header that identifies the file type,
followed by messages in the same format as transmitted over the network.

There are two meta message types that are relevant only to recordings:

* Interval: this message is used to save timing information. Upon encountering an interval message, the client should pause for the given number of milliseconds.
* Marker: these are markers created by the user to annotate interesting points in the session. They can be used like bookmarks during playback.

A recording can be accompanied by an *index file*. The index file must have the same name as the recording file, except for the file extension, which must be "dpidx".

The format of the index file is not documented and it can change from version to version. However, the index can always be regenerated from the main recording. The index contains a map of message offsets and snapshots of drawingboard state to enable quick jumping in the recording.


## Protocol revision history

The protocol version number consists of four parts: namespace, server, major and minor version.

A server can support any client whose server number matches that of the server.
The namespace allows non-Drawpile clients to use the same server as well.

A change in the major number indicates changes to the message format (e.g. added or removed fields or message types.)
A change in the minor version indicates that the client interpretation of some commands has changed. (E.g. new
brush drawing algorithm that results in a different shape for the same settings.)

Clients can connect to any server sharing the same server protocol version number,
but all clients in the same session must share the exact version. Version numbers
are also used to determine whether a session recording is compatible with the user's client version.

Protocol stability promises:

 * No breaking changes between patch releases (e.g. 2.0.0 and 2.0.1)
 * Client compatability may only be broken between minor number bumps (e.g. 2.0.0 and 2.1.0)
 * Server compatibility may only be broken between major number bumps (e.g. 2.0.0 and 3.0.0)
 * New server features may be added at any time, but they should not break older clients,
   nor should a missing feature break newer clients.

### Protocol dp:4.21.2 (2.1.9)
 * User 0 (server) is now always treated as Operator tier. (Change for experimental smart server)

### Protocol dp:4.21.2 (2.1.2)
 * Added FIXED flag to LayerAttributes. (Minor protocol stability promise break, but 2.1.1 is still a beta version)

### Protocol dp:4.21.2 (2.1.0)

 * Changed PutImage pixel format to ARGB32_Premultiplied
 * Added PutTile command
 * Removed ToolChange and PenMove commands
 * Added DrawDabsClassic and DrawDabsPixel commands
 * Added CanvasBackground command
 * Added TrustedUsers command
 * General session lock is now applied with the LayerACL command by using layer ID 0
 * Removed "lock new users by default" feature
 * Replaced SessionACL message with FeatureAccessLevels message
 * Added sublayer field to LayerAttributes command
 * Added flags field and "censored" flag to LayerAttributes command
 * Added PrivateChat transparent meta message

### Protocol dp:4.20.1 (2.0.9)

 * Added `Filtered` message type. Fully backward compatible.

### Protocol dp:4.20.1 (2.0.0)

 * Protocol major revision 4. Lots of changes!
 * Message header now includes the user ID

### Protocol 15.6 (1.0.0)

 * Added "Behind" blending mode
 * Added "Color erase" blending mode
 * PutImage now uses regular brush/layer blending modes

### Protocol 15.5 (0.9.10)

 * Merged LayerCopy with LayerCreate

### Protocol 14.5 (0.9.9)

 * Implemented LayerCopy
 * Fully backward compatible

### Protocol 13.5 (0.9.8)

 * Added ERASE and UNDER composition modes to PutImage
 * Fully backward compatible

### Protocol 13.4 (0.9.7)

 * Layer IDs and annotation IDs are now 16 bits long and assigned by the client.

### Protocol 12.4 (0.9.6)

 * Added Ping message type
 * Added smudge fields to ToolChange message
 * Brush size now means brush diameter instead of radius
 * Login protocol change: session ID prefixed with '!' means ID was user specified

### Protocol 11.3 (0.9.5)

 * Added preserve alpha blending mode
 * Fully backwards compatible

### Protocol 11.2 (0.9.3)

 * Added ACTION flag to Chat message.
 * Fully backwards compatible: No server changes needed and older clients will simply ignore the flag.

### Protocol 11.2 (0.9.2)

 * Various changes to the login process to fix bugs and to support authenticated logins
 * Added maxUsers field and PERSISTENT flag to SessionConf message
 * Increased SessionConf::attrs length to 16 bits to make room for more future attributes
 * Session ID format change: replaced numeric IDs with strings of format [a-zA-Z0-9:-]{1,64}
 * Added "flags" field to Chat message
 * Changed some operator commands
 * Increased UserAttr::attrs length to 16 bits to make room for more future attributes
 * Added MOD and AUTH UserAttr attributes
 * Use int32 coordinates in PutImage, CanvasResize and FillRect
 * Breaks compatiblity with previous version
 * First version to support session hibernation

### Protocol 10.2 (0.9.0)

 * New login process to support multiple sessions on a single server
 * Added Disconnect message
 * No change in recording format

### Protocol 9.2: (0.8.6)

 * MovePointer command coordinates now use 1/4 pixel resolution (same as normal brushes)
 * MovePointers were not recorded in earlier versions, so backwards compatibility is maintained.
 * New meta command: Marker
 * Client side change: minor changes to stroke rendering
 * Recordings are backward compatible with 7.1 and 8.1, but strokes may be drawn slightly differently

### Protocol 8.1:

 * New command: FillRect
 * New meta command: MovePointer
 * Fully backward compatible with 7.1

### Protocol 7.1:

 * First protocol revision that can be recorded.

