Development: DesktopSize

When we tried to document a proper behaviour for the DesktopSize extension, there were already multiple implementations in the wild and we knew they weren't behaving the same way. So a survey was done of the major players to get some idea of what the safest behaviour would be.

The things we checked were:

• Is the framebuffer retained?
• How is the following screen update handled (incremental or not)?
• Where is the DesktopSize rectangle positioned in the data stream.

Clients

Name	Framebuffer	Incremental
RealVNC 4	Retains	Incremental
gtk-vnc	Drops	Incremental
UltraVNC	Drops	Non-incremental
TightVNC	Drops	Incremental

The dangerous combination here is dropping the framebuffer and then asking for a non-incremental update as that assumes that the server will treat the entire screen as modified (which might not be true).

Servers

Name	Incremental	Position
RealVNC 4	Non-incremental	Last
Vino	Non-incremental	Separate
UltraVNC	Non-incremental	Separate
TightVNC	Non-incremental	Separate

The unsafe behaviours here are to send incremental updates (as some clients assume this never happens), or to send the DesktopSize rectangle as anything other than the last rectangle in an update (some older clients assumes this behaviour).

Result

The resulting documentation ended up like this:

DesktopSize Pseudo-encoding

A client which requests the DesktopSize pseudo-encoding is declaring that it is capable of coping with a change in the framebuffer width and/or height.

The server changes the desktop size by sending a pseudo-rectangle with the DesktopSize pseudo-encoding. The pseudo-rectangle's x-position and y-position are ignored, and width and height indicate the new width and height of the framebuffer. There is no further data associated with the pseudo-rectangle.

The semantics of the DesktopSize pseudo-encoding were originally not clearly defined and as a results there are multiple differing implementations in the wild. Both the client and server need to take special steps to ensure maximum compatibility.

In the initial implementation the DesktopSize pseudo-rectangle was sent in its own update without any modifications to the framebuffer data. The client would discard the framebuffer contents upon receiving this pseudo-rectangle and the server would consider the entire framebuffer to be modified.

A later implementation sent the DesktopSize pseudo-rectangle together with modifications to the framebuffer data. It also expected the client to retain the framebuffer contents as those modifications could be from after the framebuffer resize had occurred on the server.

The semantics defined here retain compatibility with both of two older implementations.

Server Semantics

The update containing the pseudo-rectangle should not contain any rectangles that change the framebuffer data as that will most likely be discarded by the client and will have to be resent later.

The server should assume that the client discards the framebuffer data when receiving a DesktopSize pseudo-rectangle. It should therefore not use any encoding that relies on the previous contents of the framebuffer. The server should also consider the entire framebuffer to be modified.

Some early client implementations require the DesktopSize pseudo-rectangle to be the very last rectangle in an update. Servers should make every effort to support these.

The server should only send a DesktopSize pseudo-rectangle when an actual change of the framebuffer dimensions has occurred. Some clients respond to a DesktopSize pseudo-rectangle in a way that could send the system into an infinite loop if the server sent out the pseudo-rectangle for anything other than an actual change.

Client Semantics

The client should assume that the server expects the framebuffer data to be retained when the framebuffer dimensions change. This requirement can be satisfied either by actually retaining the framebuffer data, or by making sure that incremental is set to non-zero in the next FramebufferUpdateRequest.

The principle of one framebuffer update being a transition from one valid state to another does not hold for updates with the DesktopSize pseudo-rectangle as the framebuffer contents can temporarily be partially or completely undefined. Clients should try to handle this gracefully, e.g. by showing a black framebuffer or delay the screen update until a proper update of the framebuffer contents has been received.