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Versions: 00 01 02 03 RFC 6143

Internet Engineering Task Force                            T. Richardson
Internet-Draft                                              RealVNC Ltd.
Intended status: Informational                                 J. Levine
Expires: May 22, 2009                               Taughannock Networks
                                                       November 18, 2008


                    The Remote Framebuffer Protocol
                          draft-levine-rfb-00

Status of this Memo

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   This Internet-Draft will expire on May 22, 2009.

Abstract

   RFB ("remote framebuffer") is a simple protocol for remote access to
   graphical user interfaces which allows a client to view and control a
   window system on another computer.  Because it works at the
   framebuffer level RFB is applicable to all windowing systems and
   applications.  This document describes the protocol used to
   communicate between an RFB client and RFB server.  RFB is the
   protocol used in VNC, Virtual Network Computing.





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Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  4
   2.  Initial Connection . . . . . . . . . . . . . . . . . . . . . .  4
   3.  Display Protocol . . . . . . . . . . . . . . . . . . . . . . .  5
   4.  Input Protocol . . . . . . . . . . . . . . . . . . . . . . . .  6
   5.  Representation of Pixel Data . . . . . . . . . . . . . . . . .  6
   6.  Protocol Versions and Extensions . . . . . . . . . . . . . . .  6
   7.  Protocol Messages  . . . . . . . . . . . . . . . . . . . . . .  7
     7.1.  Handshake Messages . . . . . . . . . . . . . . . . . . . .  8
       7.1.1.  ProtocolVersion Handshake  . . . . . . . . . . . . . .  8
       7.1.2.  Security Handshake . . . . . . . . . . . . . . . . . .  9
       7.1.3.  SecurityResult Handshake . . . . . . . . . . . . . . . 10
     7.2.  Security Types . . . . . . . . . . . . . . . . . . . . . . 11
       7.2.1.  None . . . . . . . . . . . . . . . . . . . . . . . . . 11
       7.2.2.  VNC Authentication . . . . . . . . . . . . . . . . . . 11
     7.3.  Initialization Messages  . . . . . . . . . . . . . . . . . 11
       7.3.1.  ClientInit . . . . . . . . . . . . . . . . . . . . . . 12
       7.3.2.  ServerInit . . . . . . . . . . . . . . . . . . . . . . 12
     7.4.  Pixel Format Data Structure  . . . . . . . . . . . . . . . 12
     7.5.  Client to Server Messages  . . . . . . . . . . . . . . . . 14
       7.5.1.  SetPixelFormat . . . . . . . . . . . . . . . . . . . . 14
       7.5.2.  SetEncodings . . . . . . . . . . . . . . . . . . . . . 15
       7.5.3.  FramebufferUpdateRequest . . . . . . . . . . . . . . . 15
       7.5.4.  KeyEvent . . . . . . . . . . . . . . . . . . . . . . . 16
       7.5.5.  PointerEvent . . . . . . . . . . . . . . . . . . . . . 18
       7.5.6.  ClientCutText  . . . . . . . . . . . . . . . . . . . . 19
     7.6.  Server to Client Messages  . . . . . . . . . . . . . . . . 19
       7.6.1.  FramebufferUpdate  . . . . . . . . . . . . . . . . . . 20
       7.6.2.  SetColorMapEntries . . . . . . . . . . . . . . . . . . 20
       7.6.3.  Bell . . . . . . . . . . . . . . . . . . . . . . . . . 21
       7.6.4.  ServerCutText  . . . . . . . . . . . . . . . . . . . . 21
     7.7.  Encodings  . . . . . . . . . . . . . . . . . . . . . . . . 22
       7.7.1.  Raw Encoding . . . . . . . . . . . . . . . . . . . . . 22
       7.7.2.  CopyRect Encoding  . . . . . . . . . . . . . . . . . . 22
       7.7.3.  RRE Encoding . . . . . . . . . . . . . . . . . . . . . 23
       7.7.4.  Hextile Encoding . . . . . . . . . . . . . . . . . . . 24
       7.7.5.  TRLE encoding  . . . . . . . . . . . . . . . . . . . . 26
       7.7.6.  ZRLE encoding  . . . . . . . . . . . . . . . . . . . . 29
     7.8.  Pseudo-Encodings . . . . . . . . . . . . . . . . . . . . . 30
       7.8.1.  Cursor pseudo-encoding . . . . . . . . . . . . . . . . 30
       7.8.2.  DesktopSize pseudo-encoding  . . . . . . . . . . . . . 30
   8.  Security . . . . . . . . . . . . . . . . . . . . . . . . . . . 31
   9.  Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 31
   10. References . . . . . . . . . . . . . . . . . . . . . . . . . . 31
   Appendix A.  Differences in earlier protocol versions  . . . . . . 31
     A.1.  Differences in the version 3.3 protocol  . . . . . . . . . 32
     A.2.  Differences in the version 3.7 protocol  . . . . . . . . . 32



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   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 32
   Intellectual Property and Copyright Statements . . . . . . . . . . 34

















































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1.  Introduction

   RFB ("remote framebuffer") is a simple protocol for remote access to
   graphical user interfaces.  Because it works at the framebuffer level
   it is applicable to all windowing systems and applications, including
   X11, Windows and Macintosh.  RFB is the protocol used in VNC (Virtual
   Network Computing).  The protocol is widely implemented and has had
   fairly good interoperability.

   The remote endpoint where the user sits (typically with a display,
   keyboard, and pointer) is called the RFB client or viewer.  The
   endpoint where changes to the framebuffer originate (i.e., the
   windowing system and applications) is known as the RFB server.

   RFB is a "thin client" protocol.  The emphasis in the design of the
   RFB protocol is to make very few requirements of the client.  In this
   way, clients can run on the widest range of hardware, and the task of
   implementing a client is made as simple as possible.

   The protocol also makes the client stateless.  If a client
   disconnects from a given server and subsequently reconnects to that
   same server, the state of the user interface is preserved.
   Furthermore, a different client endpoint can be used to connect to
   the same RFB server.  At the new endpoint, the user will see exactly
   the same graphical user interface as at the original endpoint.  In
   effect, the interface to the user's applications becomes completely
   mobile.  Wherever suitable network connectivity exists, the user can
   access their own personal applications, and the state of these
   applications is preserved between accesses from different locations.
   This provides the user with a familiar, uniform view of the computing
   infrastructure wherever they go.

   The RFB protocol has evolved over the past decade, and has been
   implemented several times, including at least one open source
   version.  This document describes the RFB protocol as actually
   implemented, so that future implementers can interoperate with
   existing clients and servers.


2.  Initial Connection

   An RFB server is typically a long-lived process that maintains the
   state of a framebuffer.  RFB clients typically connect, communicate
   with the server for a period of time to use and manipulate the
   framebuffer, then disconnect.  A subsequent RFB session will then
   pick up where a prior session left off, with the state of the
   framebuffer intact.




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   An RFB client contacts the server on TCP port 5900.  On systems with
   multiple RFB servers, server N typically listens on port 5900+N,
   analogous to the way that X Window servers listen on port 6000+N.

   Some browser-based clients use a Java application to run the RFB
   protocol.  RFB servers sometimes provide a simple HTTP server on port
   5800 that provides the requisite Java applet.

   In some cases, the initial roles of the client and server are
   reversed, with the RFB client listening on port 5500, and the RFB
   server contacting the RFB client.  Once the connection is
   established, the two sides take their normal roles, with the RFB
   server sending the first handshake message.

   IANA has allocated port 5900 to the RFB protocol; the other port
   numbers have been used informally and do not match IANA allocations.


3.  Display Protocol

   The display side of the protocol is based around a single graphics
   primitive: "put a rectangle of pixel data at a given x,y position".
   This might seem an inefficient way of drawing many user interface
   components.  However, allowing various different encodings for the
   pixel data gives us a large degree of flexibility in how to trade off
   various parameters such as network bandwidth, client drawing speed
   and server processing speed.

   A sequence of these rectangles makes a framebuffer update (or simply
   update).  An update represents a change from one valid framebuffer
   state to another, so in some ways is similar to a frame of video.
   The rectangles in an update are usually but not always disjoint.

   The update protocol is demand-driven by the client.  That is, an
   update is only sent from the server to the client in response to an
   explicit request from the client.  This gives the protocol an
   adaptive quality.  The slower the client and the network are, the
   lower the rate of updates.  With typical applications, changes to the
   same area of the framebuffer tend to happen soon after one another.
   With a slow client or network, transient states of the framebuffer
   can be ignored, resulting in less network traffic and less drawing
   for the client.

   After the initial handshake sequence, the protocol is asynchronous,
   with each side sending messages as needed.  The server must not send
   unsolicited updates.  An update must only be sent in response to a
   request from the client.  When several requests from the client are
   outstanding, a single update from the server may satisfy all of them.



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4.  Input Protocol

   The input side of the protocol is based on a standard workstation
   model of a keyboard and multi-button pointing device.  Input events
   are simply sent to the server by the client whenever the user presses
   a key or pointer button, or whenever the pointing device is moved.
   These input events can also be synthesised from other non-standard
   I/O devices.  For example, a pen-based handwriting recognition engine
   might generate keyboard events.


5.  Representation of Pixel Data

   Initial interaction between the RFB client and server involves a
   negotiation of the format and encoding of the pixel data that will be
   sent.  This negotiation has been designed to make the job of the
   client as easy as possible.  The server must always be able to supply
   pixel data in the form the client wants.  However if the client is
   able to cope equally with several different formats or encodings, it
   may choose one which is easier for the server to produce.

   Pixel format refers to the representation of individual colors by
   pixel values.  The most common pixel formats are 24-bit or 16-bit
   "true color", where bit-fields within the pixel value translate
   directly to red, green and blue intensities, and 8-bit "color map"
   (palette) where the pixel values are indices into a 256 entry table
   that contains the actual RGB intensities.

   Encoding refers to the way that a rectangle of pixel data will be
   sent to the client.  Every rectangle of pixel data is prefixed by a
   header giving the X,Y position of the rectangle on the screen, the
   width and height of the rectangle, and an encoding type which
   specifies the encoding of the pixel data.  The data itself then
   follows using the specified encoding.

   The encoding types defined at present are: Raw, CopyRect, RRE, TRLE,
   Hextile, and ZRLE.  In practice current servers use the ZRLE, TRLE,
   and CopyRect encodings since they provide the best compression for
   typical desktops.  Clients generally also support Hextile, which was
   often used by older RFB servers that didn't support TRLE.  See
   Section 7.7 for a description of each of the encodings.


6.  Protocol Versions and Extensions

   The RFB protocol has evolved through three published versions: 3.3,
   3.7, and 3.8.  This document primarily documents the final version
   3.8; differences from the earlier versions, which are minor, are



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   described in Appendix A.  Under no circumstances should an
   implementation use a protocol version number other than one defined
   in this document.  Over the years, different implementations of RFB
   have attempted to use different version numbers to add undocumented
   extensions, with the result being that to interoperate, any unknown
   3.x version must be treated as 3.3, so it is not possible to add a
   3.9 or higher version in a backward compatible fashion.  Future
   evolution of RFB will use 4.x version numbers.

   It is not necessary to change the protocol version number to extend
   the protocol.  The protocol can be extended within an existing
   version by:

   New encodings  A new encoding type can be added to the protocol
      relatively easily while maintaining compatibility with existing
      clients and servers.  Existing servers will simply ignore requests
      for a new encoding which they don't support.  Existing clients
      will never request the new encoding so will never see rectangles
      encoded that way.

   Pseudo encodings  In addition to genuine encodings, a client can
      request a "pseudo-encoding" to declare to the server that it
      supports a certain extension to the protocol.  A server which does
      not support the extension will simply ignore the pseudo-encoding.
      Note that this means the client must assume that the server does
      not support the extension until it gets some extension-specific
      confirmation from the server.  See Section 7.8 for a description
      of current pseudo-encodings.

   New security types  Adding a new security type gives full flexibility
      in modifying the behavior of the protocol without sacrificing
      compatibility with existing clients and servers.  A client and
      server which agree on a new security type can effectively talk
      whatever protocol they like after that - it doesn't necessarily
      have to be anything like the RFB protocol.

   RealVNC maintains a registry of encoding and security type IDs
   available at http://www.realvnc.com/rfbids.html.  Implementers may
   contact RealVNC to obtain an ID for a new type.


7.  Protocol Messages

   The RFB protocol can operate over any reliable transport, either
   byte-stream or message-based.  It usually operates over a TCP/IP
   connection.  There are three stages to the protocol.  First is the
   handshaking phase, the purpose of which is to agree upon the protocol
   version and the type of security to be used.  The second stage is an



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   initialisation phase where the client and server exchange ClientInit
   and ServerInit messages.  The final stage is the normal protocol
   interaction.  The client can send whichever messages it wants, and
   may receive messages from the server as a result.  All these messages
   begin with a message-type byte, followed by message-specific data.

   The following descriptions of protocol messages use the basic types
   U8, U16, U32, S8, S16, and S32.  These represent respectively 8, 16
   and 32-bit unsigned integers and 8, 16 and 32-bit signed integers.
   All multiple byte integers (other than pixel values themselves) are
   in big endian order (most significant byte first).  Some messages use
   arrays of the basic types, with the number of entries in the array
   determined from fields preceding the array.

   The type PIXEL means a pixel value of bytesPerPixel bytes, where
   bytesPerPixel is the number of bits-per-pixel divided by 8.  The
   bits-per-pixel is agreed by the client and server, either in the
   ServerInit message (Section 7.3.2) or a SetPixelFormat message
   (Section 7.5.1).  See Section 7.4 for the detailed description of the
   pixel format.

7.1.  Handshake Messages

   When an RFB client and server first connect, they exchange a sequence
   of handshake messages that determine the protocol version, what type
   of connection security if any to use, a password check if the
   security type requires it, and some initialization information.

7.1.1.  ProtocolVersion Handshake

   Handshaking begins by the server sending the client a ProtocolVersion
   message.  This lets the client know which is the highest RFB protocol
   version number supported by the server.  The client then replies with
   a similar message giving the version number of the protocol which
   should actually be used (which may be different to that quoted by the
   server).  A client should never request a protocol version higher
   than that offered by the server.  It is intended that both clients
   and servers may provide some level of backwards compatibility by this
   mechanism.

   The only published protocol versions at this time are 3.3, 3.7, and
   3.8.  Other version numbers are reported by some servers and clients,
   but should be interpreted as 3.3.  Addition of a new encoding or
   pseudo-encoding type does not require a change in protocol version,
   since a server can simply ignore encodings it does not understand.

   The ProtocolVersion message consists of 12 bytes interpreted as a
   string of ASCII characters in the format "RFB xxx.yyy\n" where xxx



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   and yyy are the major and minor version numbers, left padded with
   zeros:

       RFB 003.008\n (hex 52 46 42 20 30 30 33 2e 30 30 38 0a)

7.1.2.  Security Handshake

   Once the protocol version has been decided, the server and client
   must agree on the type of security to be used on the connection.  The
   server lists the security types which it supports:

   +--------------------------+-------------+--------------------------+
   | No. of bytes             | Type        | Description              |
   |                          | [Value]     |                          |
   +--------------------------+-------------+--------------------------+
   | 1                        | U8          | number-of-security-types |
   | number-of-security-types | U8 array    | security-types           |
   +--------------------------+-------------+--------------------------+

   If the server listed at least one valid security type supported by
   the client, the client sends back a single byte indicating which
   security type is to be used on the connection:

              +--------------+--------------+---------------+
              | No. of bytes | Type [Value] | Description   |
              +--------------+--------------+---------------+
              | 1            | U8           | security-type |
              +--------------+--------------+---------------+

   If number-of-security-types is zero, then for some reason the
   connection failed (e.g. the server cannot support the desired
   protocol version).  This is followed by a string describing the
   reason (where a string is specified as a length followed by that many
   ASCII characters):

             +---------------+--------------+---------------+
             | No. of bytes  | Type [Value] | Description   |
             +---------------+--------------+---------------+
             | 4             | U32          | reason-length |
             | reason-length | U8 array     | reason-string |
             +---------------+--------------+---------------+

   The server closes the connection after sending the reason-string.








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             The security types defined in this document are:

                      +--------+--------------------+
                      | Number | Name               |
                      +--------+--------------------+
                      | 0      | Invalid            |
                      | 1      | None               |
                      | 2      | VNC Authentication |
                      +--------+--------------------+

   Other security types exist but are not publicly documented.

   Once the security-type has been decided, data specific to that
   security-type follows (see Section 7.2 for details).  At the end of
   the security handshaking phase, the protocol normally continues with
   the SecurityResult message.

   Note that after the security handshaking phase, it is possible that
   further communication is over an encrypted or otherwise altered
   channel if the two ends agree on an extended security type beyond the
   ones described here.

7.1.3.  SecurityResult Handshake

   The server sends a word to inform the client whether the security
   handshaking was successful.

               +--------------+--------------+-------------+
               | No. of bytes | Type [Value] | Description |
               +--------------+--------------+-------------+
               | 4            | U32          | status:     |
               |              | 0            | OK          |
               |              | 1            | failed      |
               +--------------+--------------+-------------+

   If successful, the protocol passes to the initialization phase
   (Section 7.3).

   If unsuccessful, the server sends a string describing the reason for
   the failure, and then closes the connection:

             +---------------+--------------+---------------+
             | No. of bytes  | Type [Value] | Description   |
             +---------------+--------------+---------------+
             | 4             | U32          | reason-length |
             | reason-length | U8 array     | reason-string |
             +---------------+--------------+---------------+




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7.2.  Security Types

   Two security types are defined here.

7.2.1.  None

   No authentication is needed.  The protocol continues with the
   SecurityResult message.

7.2.2.  VNC Authentication

   VNC authentication is to be used.  The server sends a random 16-byte
   challenge:

               +--------------+--------------+-------------+
               | No. of bytes | Type [Value] | Description |
               +--------------+--------------+-------------+
               | 16           | U8           | challenge   |
               +--------------+--------------+-------------+

   The client encrypts the challenge with DES, using a password supplied
   by the user as the key.  To form the key, the password is truncated
   to eight characters, or padded with null bytes on the right.  The
   client then sends the resulting 16-byte response:

               +--------------+--------------+-------------+
               | No. of bytes | Type [Value] | Description |
               +--------------+--------------+-------------+
               | 16           | U8           | response    |
               +--------------+--------------+-------------+

   The protocol continues with the SecurityResult message.

   This type of authentication is known to be cryptographically weak for
   the modern Internet; many implementations will want to use a stronger
   security.

7.3.  Initialization Messages

   Once the client and server agree on and perhaps validate a security
   type, the protocol passes to the initialization stage.  The client
   sends a ClientInit message.  Then the server sends a ServerInit
   message.








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7.3.1.  ClientInit

               +--------------+--------------+-------------+
               | No. of bytes | Type [Value] | Description |
               +--------------+--------------+-------------+
               | 1            | U8           | shared-flag |
               +--------------+--------------+-------------+

   Shared-flag is non-zero (true) if the server should try to share the
   desktop by leaving other clients connected, zero (false) if it should
   give exclusive access to this client by disconnecting all other
   clients.

7.3.2.  ServerInit

   After receiving the ClientInit message, the server sends a ServerInit
   message.  This tells the client the width and height of the server's
   framebuffer, its pixel format and the name associated with the
   desktop:

      +--------------+--------------+------------------------------+
      | No. of bytes | Type [Value] | Description                  |
      +--------------+--------------+------------------------------+
      | 2            | U16          | framebuffer-width in pixels  |
      | 2            | U16          | framebuffer-height in pixels |
      | 16           | PIXEL_FORMAT | server-pixel-format          |
      | 4            | U32          | name-length                  |
      | name-length  | U8 array     | name-string                  |
      +--------------+--------------+------------------------------+

   Server-pixel-format specifies the server's natural pixel format.
   This pixel format will be used unless the client requests a different
   format using the SetPixelFormat message (Section 7.5.1).

7.4.  Pixel Format Data Structure

   Several server to client messages include a PIXEL_FORMAT, a 16 byte
   structure that describes the way a pixel is transmited.













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             +--------------+--------------+-----------------+
             | No. of bytes | Type [Value] | Description     |
             +--------------+--------------+-----------------+
             | 1            | U8           | bits-per-pixel  |
             | 1            | U8           | depth           |
             | 1            | U8           | big-endian-flag |
             | 1            | U8           | true-color-flag |
             | 2            | U16          | red-max         |
             | 2            | U16          | green-max       |
             | 2            | U16          | blue-max        |
             | 1            | U8           | red-shift       |
             | 1            | U8           | green-shift     |
             | 1            | U8           | blue-shift      |
             | 3            |              | padding         |
             +--------------+--------------+-----------------+

   Bits-per-pixel is the number of bits used for each pixel value on the
   wire.  This must be greater than or equal to the depth which is the
   number of useful bits in the pixel value.  Currently bits-per-pixel
   must be 8, 16 or 32.  Big-endian-flag is non-zero (true) if multi-
   byte pixels are interpreted as big endian.  Although the depth should
   be consistent with the bits-per-pixel and the various -max values,
   clients do not use it when interpreting pixel data.

   If true-color-flag is non-zero (true) then the last six items specify
   how to extract the red, green and blue intensities from the pixel
   value.  Red-max is the maximum red value and must be 2^N - 1 where N
   is the number of bits used for red.  Note the -max values are always
   in big endian order.  Red-shift is the number of shifts needed to get
   the red value in a pixel to the least significant bit.  Green-max,
   green-shift and blue-max, blue-shift are similar for green and blue.
   For example, to find the red value (between 0 and red-max) from a
   given pixel, do the following:

   o  Swap the pixel value according to big-endian-flag, e.g., if big-
      endian-flag is zero (false) and host byte order is big endian,
      then swap.

   o  Shift right by red-shift.

   o  AND with red-max (in host byte order).

   If true-color-flag is zero (false) then the server uses pixel values
   which are not directly composed from the red, green and blue
   intensities, but which serve as indices into a color map.  Entries in
   the color map are set by the server using the SetColorMapEntries
   message (See Section 7.6.2).




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7.5.  Client to Server Messages

   The client to server message types defined in this document are:

                   +--------+--------------------------+
                   | Number | Name                     |
                   +--------+--------------------------+
                   | 0      | SetPixelFormat           |
                   | 2      | SetEncodings             |
                   | 3      | FramebufferUpdateRequest |
                   | 4      | KeyEvent                 |
                   | 5      | PointerEvent             |
                   | 6      | ClientCutText            |
                   +--------+--------------------------+

   Other message types exist but are not publicly documented.  Before
   sending a message other than those described in this document a
   client must have determined that the server supports the relevant
   extension by receiving an appropriate extension-specific confirmation
   from the server.

7.5.1.  SetPixelFormat

   Sets the format in which pixel values should be sent in
   FramebufferUpdate messages.  If the client does not send a
   SetPixelFormat message then the server sends pixel values in its
   natural format as specified in the ServerInit message
   (Section 7.3.2).

   If true-color-flag is zero (false) then this indicates that a "color
   map" is to be used.  The server can set any of the entries in the
   color map using the SetColorMapEntries message (Section 7.6.2).
   Immediately after the client has sent this message the contents of
   the color map are undefined, even if entries had previously been set
   by the server.

              +--------------+--------------+--------------+
              | No. of bytes | Type [Value] | Description  |
              +--------------+--------------+--------------+
              | 1            | U8 [0]       | message-type |
              | 3            |              | padding      |
              | 16           | PIXEL_FORMAT | pixel-format |
              +--------------+--------------+--------------+

               PIXEL_FORMAT is as described in Section 7.4.






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7.5.2.  SetEncodings

   Sets the encoding types in which pixel data can be sent by the
   server.  The order of the encoding types given in this message is a
   hint by the client as to its preference (the first encoding specified
   being most preferred).  The server may or may not choose to make use
   of this hint.  Pixel data may always be sent in raw encoding even if
   not specified explicitly here.

   In addition to genuine encodings, a client can request "pseudo-
   encodings" to declare to the server that it supports certain
   extensions to the protocol.  A server which does not support the
   extension will simply ignore the pseudo-encoding.  Note that this
   means the client must assume that the server does not support the
   extension until it gets some extension-specific confirmation from the
   server.

   See Section 7.7 for a description of each encoding and Section 7.8
   for the meaning of pseudo-encodings.

           +--------------+--------------+---------------------+
           | No. of bytes | Type [Value] | Description         |
           +--------------+--------------+---------------------+
           | 1            | U8 [2]       | message-type        |
           | 1            |              | padding             |
           | 2            | U16          | number-of-encodings |
           +--------------+--------------+---------------------+

   followed by number-of-encodings repetitions of the following:

              +--------------+--------------+---------------+
              | No. of bytes | Type [Value] | Description   |
              +--------------+--------------+---------------+
              | 4            | S32          | encoding-type |
              +--------------+--------------+---------------+

7.5.3.  FramebufferUpdateRequest

   Notifies the server that the client is interested in the area of the
   framebuffer specified by x-position, y-position, width and height.
   The server usually responds to a FramebufferUpdateRequest by sending
   a FramebufferUpdate.  A single FramebufferUpdate may be sent in reply
   to several FramebufferUpdateRequests.

   The server assumes that the client keeps a copy of all parts of the
   framebuffer in which it is interested.  This means that normally the
   server only needs to send incremental updates to the client.




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   If the client has lost the contents of a particular area which it
   needs, then the client sends a FramebufferUpdateRequest with
   incremental set to zero (false).  This requests that the server send
   the entire contents of the specified area as soon as possible.  The
   area will not be updated using the CopyRect encoding.

   If the client has not lost any contents of the area in which it is
   interested, then it sends a FramebufferUpdateRequest with incremental
   set to non-zero (true).  If and when there are changes to the
   specified area of the framebuffer, the server will send a
   FramebufferUpdate.  Note that there may be an indefinite period
   between the FramebufferUpdateRequest and the FramebufferUpdate.

   In the case of a fast client, the client may want to regulate the
   rate at which it sends incremental FramebufferUpdateRequests to avoid
   excessive network traffic.

              +--------------+--------------+--------------+
              | No. of bytes | Type [Value] | Description  |
              +--------------+--------------+--------------+
              | 1            | U8 [3]       | message-type |
              | 1            | U8           | incremental  |
              | 2            | U16          | x-position   |
              | 2            | U16          | y-position   |
              | 2            | U16          | width        |
              | 2            | U16          | height       |
              +--------------+--------------+--------------+

7.5.4.  KeyEvent

   A key press or release.  Down-flag is non-zero (true) if the key is
   now pressed, zero (false) if it is now released.  The key itself is
   specified using the "keysym" values defined by the X Window System,
   even if the client or server is not running X.

              +--------------+--------------+--------------+
              | No. of bytes | Type [Value] | Description  |
              +--------------+--------------+--------------+
              | 1            | U8 [4]       | message-type |
              | 1            | U8           | down-flag    |
              | 2            |              | padding      |
              | 4            | U32          | key          |
              +--------------+--------------+--------------+

   For most ordinary keys, the keysym is the same as the corresponding
   ASCII value.  For full details, see [XLIBREF] or see the header file
   <X11/keysymdef.h> in the X Window System distribution.  Some other
   common keys are:



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                 +-----------------+--------------------+
                 | Key name        | Keysym value (hex) |
                 +-----------------+--------------------+
                 | BackSpace       | 0xff08             |
                 | Tab             | 0xff09             |
                 | Return or Enter | 0xff0d             |
                 | Escape          | 0xff1b             |
                 | Insert          | 0xff63             |
                 | Delete          | 0xffff             |
                 | Home            | 0xff50             |
                 | End             | 0xff57             |
                 | Page Up         | 0xff55             |
                 | Page Down       | 0xff56             |
                 | Left            | 0xff51             |
                 | Up              | 0xff52             |
                 | Right           | 0xff53             |
                 | Down            | 0xff54             |
                 | F1              | 0xffbe             |
                 | F2              | 0xffbf             |
                 | F3              | 0xffc0             |
                 | F4              | 0xffc1             |
                 | ...             | ...                |
                 | F12             | 0xffc9             |
                 | Shift (left)    | 0xffe1             |
                 | Shift (right)   | 0xffe2             |
                 | Control (left)  | 0xffe3             |
                 | Control (right) | 0xffe4             |
                 | Meta (left)     | 0xffe7             |
                 | Meta (right)    | 0xffe8             |
                 | Alt (left)      | 0xffe9             |
                 | Alt (right)     | 0xffea             |
                 +-----------------+--------------------+

   The interpretation of keysyms is a complex area.  In order to be as
   widely interoperable as possible the following guidelines should be
   followed:

   o  The "shift state" (i.e. whether either of the Shift keysyms is
      down) should only be used as a hint when interpreting a keysym.
      For example, on a US keyboard the '#' character is shifted, but on
      a UK keyboard it is not.  A server with a US keyboard receiving a
      '#' character from a client with a UK keyboard will not have been
      sent any shift presses.  In this case, it is likely that the
      server will internally need to simulate a shift press on its local
      system in order to get a '#' character and not a '3'.

   o  The difference between upper and lower case keysyms is
      significant.  This is unlike some of the keyboard processing in



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      the X Window System which treats them as the same.  For example, a
      server receiving an uppercase 'A' keysym without any shift presses
      should interpret it as an uppercase 'A'.  Again this may involve
      an internal simulated shift press.

   o  Servers should ignore "lock" keysyms such as CapsLock and NumLock
      where possible.  Instead they should interpret each character-
      based keysym according to its case.

   o  Unlike Shift, the state of modifier keys such as Control and Alt
      should be taken as modifying the interpretation of other keysyms.
      Note that there are no keysyms for ASCII control characters such
      as Ctrl-A - these should be generated by viewers sending a Control
      press followed by an 'a' press.

   o  On a viewer where modifiers like Control and Alt can also be used
      to generate character-based keysyms, the viewer may need to send
      extra "release" events in order that the keysym is interpreted
      correctly.  For example, on a German PC keyboard, Ctrl-Alt-Q
      generates the '@' character.  In this case, the viewer needs to
      send simulated release events for Control and Alt in order that
      the '@' character is interpreted correctly, since Ctrl-Alt-@ may
      mean something completely different to the server.

   o  There is no universal standard for "backward tab" in the X Window
      System.  On some systems shift+tab gives the keysym
      "ISO_Left_Tab", on others it gives a private "BackTab" keysym and
      on others it gives "Tab" and applications tell from the shift
      state that it means backward-tab rather than forward-tab.  In the
      RFB protocol the latter approach is preferred.  Viewers should
      generate a shifted Tab rather than ISO_Left_Tab. However, to be
      backwards-compatible with existing viewers, servers should also
      recognise ISO_Left_Tab as meaning a shifted Tab.

   o  Modern versions of the X Window system handle keysyms for Unicode
      characters, consisting of the Unicode character with the hex
      1000000 bit set.  For maximum compatibility, if a key has both a
      Unicode and a legacy encoding, clients should send the legacy
      encoding.

7.5.5.  PointerEvent

   Indicates either pointer movement or a pointer button press or
   release.  The pointer is now at ( x-position, y-position), and the
   current state of buttons 1 to 8 are represented by bits 0 to 7 of
   button-mask respectively, 0 meaning up, 1 meaning down (pressed).

   On a conventional mouse, buttons 1, 2 and 3 correspond to the left,



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   middle and right buttons on the mouse.  On a wheel mouse, each step
   of the wheel upwards is represented by a press and release of button
   4, and each step downwards is represented by a press and release of
   button 5.

              +--------------+--------------+--------------+
              | No. of bytes | Type [Value] | Description  |
              +--------------+--------------+--------------+
              | 1            | U8 [5]       | message-type |
              | 1            | U8           | button-mask  |
              | 2            | U16          | x-position   |
              | 2            | U16          | y-position   |
              +--------------+--------------+--------------+

7.5.6.  ClientCutText

   RFB provides limited support for synchronizing the "cut buffer" of
   selected text between client and server.  This message tells the
   server that the client has new ISO 8859-1 (Latin-1) text in its cut
   buffer.  Ends of lines are represented by the newline character (hex
   0a) alone.  No carriage-return (hex 0d) is used.  There is no way to
   transfer text outside the Latin-1 character set.

              +--------------+--------------+--------------+
              | No. of bytes | Type [Value] | Description  |
              +--------------+--------------+--------------+
              | 1            | U8 [6]       | message-type |
              | 3            |              | padding      |
              | 4            | U32          | length       |
              | length       | U8 array     | text         |
              +--------------+--------------+--------------+

7.6.  Server to Client Messages

   The server to client message types defined in this document are:

                      +--------+--------------------+
                      | Number | Name               |
                      +--------+--------------------+
                      | 0      | FramebufferUpdate  |
                      | 1      | SetColorMapEntries |
                      | 2      | Bell               |
                      | 3      | ServerCutText      |
                      +--------+--------------------+

   Other private message types exist but are not publicly documented.
   Before sending a message other than those described in this document
   a server must have determined that the client supports the relevant



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   extension by receiving some extension-specific confirmation from the
   client - usually a request for a given pseudo-encoding.

7.6.1.  FramebufferUpdate

   A framebuffer update consists of a sequence of rectangles of pixel
   data which the client should put into its framebuffer.  It is sent in
   response to a FramebufferUpdateRequest from the client.  Note that
   there may be an indefinite period between the
   FramebufferUpdateRequest and the FramebufferUpdate.

          +--------------+--------------+----------------------+
          | No. of bytes | Type [Value] | Description          |
          +--------------+--------------+----------------------+
          | 1            | U8 [0]       | message-type         |
          | 1            |              | padding              |
          | 2            | U16          | number-of-rectangles |
          +--------------+--------------+----------------------+

   This header is followed by number-of-rectangles rectangles of pixel
   data.  Each rectangle starts with a rectangle header:

              +--------------+--------------+---------------+
              | No. of bytes | Type [Value] | Description   |
              +--------------+--------------+---------------+
              | 2            | U16          | x-position    |
              | 2            | U16          | y-position    |
              | 2            | U16          | width         |
              | 2            | U16          | height        |
              | 4            | S32          | encoding-type |
              +--------------+--------------+---------------+

   The rectangle header is followed by the pixel data in the specified
   encoding.  See Section 7.7 for the format of the data for each
   encoding and Section 7.8 for the meaning of pseudo-encodings.

7.6.2.  SetColorMapEntries

   When the pixel format uses a "color map", this message tells the
   client that the specified pixel values should be mapped to the given
   RGB values.  Note that this message may only update part of the color
   map.  This message should not be sent by the server until after the
   client has sent at least one FramebufferUpdateRequest, and only when
   the agreed pixel format uses a color map.

   Color map values are always 16 bits, with the range of values running
   from 0 to 65535, regardless of the display hardware in use.  The
   color map value for white, for example, is 65535,65535,65535.



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   The message starts with a header describing the range of colormap
   entries to be updated.

            +--------------+--------------+------------------+
            | No. of bytes | Type [Value] | Description      |
            +--------------+--------------+------------------+
            | 1            | U8 [1]       | message-type     |
            | 1            |              | padding          |
            | 2            | U16          | first-color      |
            | 2            | U16          | number-of-colors |
            +--------------+--------------+------------------+

   This header is followed by number-of-colors RGB values, each of which
   is in this format:

               +--------------+--------------+-------------+
               | No. of bytes | Type [Value] | Description |
               +--------------+--------------+-------------+
               | 2            | U16          | red         |
               | 2            | U16          | green       |
               | 2            | U16          | blue        |
               +--------------+--------------+-------------+

7.6.3.  Bell

   Make an audible signal on the client if it provides one.

              +--------------+--------------+--------------+
              | No. of bytes | Type [Value] | Description  |
              +--------------+--------------+--------------+
              | 1            | U8 [2]       | message-type |
              +--------------+--------------+--------------+

7.6.4.  ServerCutText

   The server has new ISO 8859-1 (Latin-1) text in its cut buffer.  Ends
   of lines are represented by the newline character (hex 0a) alone.  No
   carriage-return (hex 0d) is used.  There is no way to transfer text
   outside the Latin-1 character set.

              +--------------+--------------+--------------+
              | No. of bytes | Type [Value] | Description  |
              +--------------+--------------+--------------+
              | 1            | U8 [3]       | message-type |
              | 3            |              | padding      |
              | 4            | U32          | length       |
              | length       | U8 array     | text         |
              +--------------+--------------+--------------+



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7.7.  Encodings

   The encodings defined in this document are:

                 +--------+-----------------------------+
                 | Number | Name                        |
                 +--------+-----------------------------+
                 | 0      | Raw                         |
                 | 1      | CopyRect                    |
                 | 2      | RRE                         |
                 | 5      | Hextile                     |
                 | 15     | TRLE                        |
                 | 16     | ZRLE                        |
                 | -239   | Cursor pseudo-encoding      |
                 | -223   | DesktopSize pseudo-encoding |
                 +--------+-----------------------------+

   Other encoding types exist but are not publicly documented.

7.7.1.  Raw Encoding

   The simplest encoding type is raw pixel data.  In this case the data
   consists of width*height pixel values (where width and height are the
   width and height of the rectangle).  The values simply represent each
   pixel in left-to-right scan line order.  All RFB clients must be able
   to handle pixel data in this raw encoding, and RFB servers should
   only produce raw encoding unless the client specifically asks for
   some other encoding type.

        +----------------------------+--------------+-------------+
        | No. of bytes               | Type [Value] | Description |
        +----------------------------+--------------+-------------+
        | width*height*bytesPerPixel | PIXEL array  | pixels      |
        +----------------------------+--------------+-------------+

7.7.2.  CopyRect Encoding

   The CopyRect (copy rectangle) encoding is a very simple and efficient
   encoding which can be used when the client already has the same pixel
   data elsewhere in its framebuffer.  The encoding on the wire simply
   consists of an X,Y coordinate.  This gives a position in the
   framebuffer from which the client can copy the rectangle of pixel
   data.  This can be used in a variety of situations, the most common
   of which are when the user moves a window across the screen, and when
   the contents of a window are scrolled.






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             +--------------+--------------+----------------+
             | No. of bytes | Type [Value] | Description    |
             +--------------+--------------+----------------+
             | 2            | U16          | src-x-position |
             | 2            | U16          | src-y-position |
             +--------------+--------------+----------------+

   For maximum compatibility the source rectangle of a CopyRect should
   not include pixels updated by previous entries in the same
   FramebufferUpdate message.

7.7.3.  RRE Encoding

   Note: RRE encoding is obsolescent.  In general, ZRLE and TRLE
   encoding are more compact.

   RRE stands for rise-and-run-length encoding.  As its name implies, it
   is essentially a two-dimensional analogue of run-length encoding.
   RRE-encoded rectangles arrive at the client in a form which can be
   rendered immediately by the simplest of graphics engines.  RRE is not
   appropriate for complex desktops, but can be useful in some
   situations.

   The basic idea behind RRE is the partitioning of a rectangle of pixel
   data into rectangular subregions (subrectangles) each of which
   consists of pixels of a single value and the union of which comprises
   the original rectangular region.  The near-optimal partition of a
   given rectangle into such subrectangles is relatively easy to
   compute.

   The encoding consists of a background pixel value, Vb (typically the
   most prevalent pixel value in the rectangle) and a count N, followed
   by a list of N subrectangles, each of which consists of a tuple
   <v,x,y,w,h> where v (which should be different from Vb) is the pixel
   value, (x,y) are the coordinates of the subrectangle relative to the
   top-left corner of the rectangle, and (w,h) are the width and height
   of the subrectangle.  The client can render the original rectangle by
   drawing a filled rectangle of the background pixel value and then
   drawing a filled rectangle corresponding to each subrectangle.

   On the wire, the data begins with the header:

        +---------------+--------------+-------------------------+
        | No. of bytes  | Type [Value] | Description             |
        +---------------+--------------+-------------------------+
        | 4             | U32          | number-of-subrectangles |
        | bytesPerPixel | PIXEL        | background-pixel-value  |
        +---------------+--------------+-------------------------+



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   This is followed by number-of-subrectangles instances of the
   following structure:

          +---------------+--------------+---------------------+
          | No. of bytes  | Type [Value] | Description         |
          +---------------+--------------+---------------------+
          | bytesPerPixel | PIXEL        | subrect-pixel-value |
          | 2             | U16          | x-position          |
          | 2             | U16          | y-position          |
          | 2             | U16          | width               |
          | 2             | U16          | height              |
          +---------------+--------------+---------------------+

7.7.4.  Hextile Encoding

   Note: Hextile encoding is obsolescent.  In general, ZRLE and TRLE
   encoding are more compact.

   Hextile is a variation on RRE.  Rectangles are split up into 16x16
   tiles, allowing the dimensions of the subrectangles to be specified
   in 4 bits each, 16 bits in total.  The rectangle is split into tiles
   starting at the top left going in left-to-right, top-to-bottom order.
   The encoded contents of the tiles simply follow one another in the
   predetermined order.  If the width of the whole rectangle is not an
   exact multiple of 16 then the width of the last tile in each row will
   be correspondingly smaller.  Similarly if the height of the whole
   rectangle is not an exact multiple of 16 then the height of each tile
   in the final row will also be smaller.

   Each tile is either encoded as raw pixel data, or as a variation on
   RRE.  Each tile has a background pixel value, as before.  However,
   the background pixel value does not need to be explicitly specified
   for a given tile if it is the same as the background of the previous
   tile.  If all of the subrectangles of a tile have the same pixel
   value, this can be specified once as a foreground pixel value for the
   whole tile.  As with the background, the foreground pixel value can
   be left unspecified, meaning it is carried over from the previous
   tile.

   The data consists of each tile encoded in order.  Each tile begins
   with a subencoding type byte, which is a mask made up of a number of
   bits:









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           +--------------+--------------+---------------------+
           | No. of bytes | Type [Value] | Description         |
           +--------------+--------------+---------------------+
           | 1            | U8           | subencoding-mask:   |
           |              | [1]          | Raw                 |
           |              | [2]          | BackgroundSpecified |
           |              | [4]          | ForegroundSpecified |
           |              | [8]          | AnySubrects         |
           |              | [16]         | SubrectsColored     |
           +--------------+--------------+---------------------+

   If the Raw bit is set then the other bits are irrelevant;
   width*height pixel values follow (where width and height are the
   width and height of the tile).  Otherwise the other bits in the mask
   are as follows:

   BackgroundSpecified  If set, a pixel value of bytesPerPixel bytes
      follows which specifies the background color for this tile.  The
      first non-raw tile in a rectangle must have this bit set.  If this
      bit isn't set then the background is the same as the last tile.

   ForegroundSpecified  If set, a pixel value of bytesPerPixel bytes
      follows which specifies the foreground color to be used for all
      subrectangles in this tile.

      If this bit is set then the SubrectsColored bit must be zero.

   AnySubrects  If set, a single byte follows giving the number of
      subrectangles following.  If not set, there are no subrectangles
      (i.e. the whole tile is just solid background color).

   SubrectsColored  If set then each subrectangle is preceded by a pixel
      value giving the color of that subrectangle, so a subrectangle is:

          +---------------+--------------+---------------------+
          | No. of bytes  | Type [Value] | Description         |
          +---------------+--------------+---------------------+
          | bytesPerPixel | PIXEL        | subrect-pixel-value |
          | 1             | U8           | x-and-y-position    |
          | 1             | U8           | width-and-height    |
          +---------------+--------------+---------------------+

      If not set, all subrectangles are the same color, the foreground
      color; if the ForegroundSpecified bit wasn't set then the
      foreground is the same as the last tile.  A subrectangle is:






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            +--------------+--------------+------------------+
            | No. of bytes | Type [Value] | Description      |
            +--------------+--------------+------------------+
            | 1            | U8           | x-and-y-position |
            | 1            | U8           | width-and-height |
            +--------------+--------------+------------------+

   The position and size of each subrectangle is specified in two bytes,
   x-and-y-position and width-and-height.  The most-significant four
   bits of x-and-y-position specify the X position, the least-
   significant specify the Y position.  The most-significant four bits
   of width-and-height specify the width minus one, the least-
   significant specify the height minus one.

7.7.5.  TRLE encoding

   TRLE stands for Tiled Run-Length Encoding, and combines tiling,
   palettisation and run-length encoding.  The rectangle is divided into
   tiles of 16x16 pixels in left-to-right, top-to-bottom order, similar
   to hextile.  If the width of the rectangle is not an exact multiple
   of 16 then the width of the last tile in each row is smaller, and if
   the height of the rectangle is not an exact multiple of 16 then the
   height of each tile in the final row is smaller.

   TRLE makes use of a new type CPIXEL (compressed pixel).  This is the
   same as a PIXEL for the agreed pixel format, except where true-color-
   flag is non-zero, bits-per-pixel is 32, depth is 24 or less and all
   of the bits making up the red, green and blue intensities fit in
   either the least significant 3 bytes or the most significant 3 bytes.
   In this case a CPIXEL is only 3 bytes long, and contains the least
   significant or the most significant 3 bytes as appropriate.
   bytesPerCPixel is the number of bytes in a CPIXEL.

   Each tile begins with a subencoding type byte.  The top bit of this
   byte is set if the tile has been run-length encoded, clear otherwise.
   The bottom seven bits indicate the size of the palette used: zero
   means no palette, one means that the tile is of a single color, and 2
   to 127 indicate a palette of that size.  The special values 129 and
   127 indicate that the palette is to be reused from the previous tile,
   with and without RLE respectively.

   Note: in this discussion, the div(a,b) function means the result of
   dividing a/b truncated to an integer.

   The possible values of subencoding are:






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   0  Raw pixel data. width*height pixel values follow (where width and
      height are the width and height of the tile):

       +-----------------------------+--------------+-------------+
       | No. of bytes                | Type [Value] | Description |
       +-----------------------------+--------------+-------------+
       | width*height*BytesPerCPixel | CPIXEL array | pixels      |
       +-----------------------------+--------------+-------------+

   1  A solid tile consisting of a single color.  The pixel value
      follows:

              +----------------+--------------+-------------+
              | No. of bytes   | Type [Value] | Description |
              +----------------+--------------+-------------+
              | bytesPerCPixel | CPIXEL       | pixelValue  |
              +----------------+--------------+-------------+

   2 to 16  Packed palette types.  The palleteSize is the value of the
      subencoding, which is followed by the palette, consisting of
      paletteSize pixel values.  The packed pixels follow, with each
      pixel represented as a bit field yielding a 0-based index into the
      palette.  For paletteSize 2, a 1-bit field is used, for
      paletteSize 3 or 4 a 2-bit field is used, and for paletteSize from
      5 to 16 a 4-bit field is used.  The bit fields are packed into
      bytes, with the most significant bits representing the leftmost
      pixel (i.e. big endian).  For tiles not a multiple of 8, 4 or 2
      pixels wide (as appropriate), padding bits are used to align each
      row to an exact number of bytes.

       +----------------------------+--------------+--------------+
       | No. of bytes               | Type [Value] | Description  |
       +----------------------------+--------------+--------------+
       | paletteSize*bytesPerCPixel | CPIXEL array | palette      |
       | m                          | U8 array     | packedPixels |
       +----------------------------+--------------+--------------+

      where m is the number of bytes representing the packed pixels.
      For paletteSize of 2 this is div(width+7,8)*height, for
      paletteSize of 3 or 4 this is div(width+3,4)*height, or for
      paletteSize of 5 to 16 this is div(width+1,2)*height.

   17 to 126  Unused.  (Packed palettes of these sizes would offer no
      advantage over palette RLE).







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   127  Packed palette with the palette reused from the previous tile.
      The subencoding byte is followed by the packed pixels as described
      above for packed palette types.

   128  Plain RLE.  The data consists of a number of runs, repeated
      until the tile is done.  Runs may continue from the end of one row
      to the beginning of the next.  Each run is a represented by a
      single pixel value followed by the length of the run.  The length
      is represented as one or more bytes.  The length is calculated as
      one more than the sum of all the bytes representing the length.
      Any byte value other than 255 indicates the final byte.  So for
      example length 1 is represented as [0], 255 as [254], 256 as
      [255,0], 257 as [255,1], 510 as [255,254], 511 as [255,255,0] and
      so on.

    +-------------------------+--------------+-----------------------+
    | No. of bytes            | Type [Value] | Description           |
    +-------------------------+--------------+-----------------------+
    | bytesPerCPixel          | CPIXEL       | pixelValue            |
    | div(runLength - 1, 255) | U8 array     | 255                   |
    | 1                       | U8           | (runLength-1) mod 255 |
    +-------------------------+--------------+-----------------------+

   129  Palette RLE with the palette reused from the previous tile.
      Followed by a number of runs, repeated until the tile is done, as
      described below for 130 to 255.

   130 to 255  Palette RLE.  Followed by the palette, consisting of
      paletteSize = (subencoding - 128) pixel values:

        +----------------------------+--------------+-------------+
        | No. of bytes               | Type [Value] | Description |
        +----------------------------+--------------+-------------+
        | paletteSize*bytesPerCPixel | CPIXEL array | palette     |
        +----------------------------+--------------+-------------+

      Following the palette is, as with plain RLE, of a number of runs,
      repeated until the tile is done.  A run of length one is
      represented simply by a palette index:

              +--------------+--------------+--------------+
              | No. of bytes | Type [Value] | Description  |
              +--------------+--------------+--------------+
              | 1            | U8           | paletteIndex |
              +--------------+--------------+--------------+






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      A run of length more than one is represented by a palette index
      with the top bit set, followed by the length of the run as for
      plain RLE.

    +-------------------------+--------------+-----------------------+
    | No. of bytes            | Type [Value] | Description           |
    +-------------------------+--------------+-----------------------+
    | 1                       | U8           | paletteIndex + 128    |
    | div(runLength - 1, 255) | U8 array     | 255                   |
    | 1                       | U8           | (runLength-1) mod 255 |
    +-------------------------+--------------+-----------------------+

7.7.6.  ZRLE encoding

   ZRLE stands for Zlib (see [RFC1950] and [RFC1951]) Run-Length
   Encoding, and combines an encoding similar to TRLE with zlib
   compression.  On the wire, the rectangle consists of zlib-compressed
   data which continues until the end of the message.  A single zlib
   "stream" object is used for a given RFB protocol connection, so that
   ZRLE rectangles must be encoded and decoded strictly in order.

               +--------------+--------------+-------------+
               | No. of bytes | Type [Value] | Description |
               +--------------+--------------+-------------+
               | length       | U8 array     | zlibData    |
               +--------------+--------------+-------------+

   The zlibData when uncompressed represents tiles in left-to-right,
   top-to-bottom order, similar to TRLE, but with a tile size of 64x64
   pixels.  If the width of the rectangle is not an exact multiple of 64
   then the width of the last tile in each row is smaller, and if the
   height of the rectangle is not an exact multiple of 64 then the
   height of each tile in the final row is smaller.

   The tiles are encoded in exactly the same way as TRLE, except that
   subencoding may not take the values 127 or 129, i.e. palettes cannot
   be reused between tiles.

   The server flushes the zlib stream to a byte boundary at the end of
   each ZRLE encoded message.  It need not flush the stream between
   tiles within a message.  Since the zlibData for a single message can
   potentially be quite large, clients can incrementally decode and
   interpret the zlibData but must not assume that encoded tile data is
   byte aligned.







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7.8.  Pseudo-Encodings

   An update rectangle with a "pseudo-encoding" does not directly
   represent pixel data but instead allows the server to send arbitrary
   data to the client.  How this data is interpreted depends on the
   pseudo-encoding.

7.8.1.  Cursor pseudo-encoding

   A client which requests the Cursor pseudo-encoding is declaring that
   it is capable of drawing a pointer cursor locally.  This can
   significantly improve perceived performance over slow links.  The
   server sets the cursor shape by sending a rectangle with the Cursor
   pseudo-encoding as part of an update.  The rectangle's x-position and
   y-position indicate the hotspot of the cursor, and width and height
   indicate the width and height of the cursor in pixels.  The data
   consists of width*height raw pixel values followed by a shape
   bitmask, with one bit corresponding to each pixel in the cursor
   rectangle.  The bitmask consists of left-to-right, top-to-bottom scan
   lines, where each scan line is padded to a whole number of bytes
   div(width+7,8).  Within each byte the most significant bit represents
   the leftmost pixel, with a 1-bit meaning the corresponding pixel in
   the cursor is valid.

       +----------------------------+--------------+---------------+
       | No. of bytes               | Type [Value] | Description   |
       +----------------------------+--------------+---------------+
       | width*height*bytesPerPixel | PIXEL array  | cursor-pixels |
       | div(width+7,8)*height      | U8 array     | bitmask       |
       +----------------------------+--------------+---------------+

7.8.2.  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 height.  The server changes the desktop size by sending a
   rectangle with the DesktopSize pseudo-encoding as the last rectangle
   in an update.  The 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 rectangle.  After
   changing the desktop size, the server must assume that the client no
   longer has the previous framebuffer contents.  This will usually
   result in a complete update of the framebuffer at the next update.
   However for maximum interoperability with existing servers the client
   should preserve the top-left portion of the framebuffer between the
   old and new sizes.



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8.  Security

   The RFB protocol as defined here provides no security beyond the
   optional and cryptographically weak password check described in
   Section 7.2.2.  In particular, it provides no protection against
   observation of or tampering with the data stream.

   Security methods beyond those described here may be used to protect
   the integrity of the data.  The client and server might agree to use
   an extended security type to encrypt the session, or the session
   might be transmitted over a secure channel such as IPSEC [RFC4301] or
   SSH [RFC4254].


9.  Acknowledgements

   James Weatherall, Andy Harter and Ken Wood also contributed to the
   design of the RFB protocol.


10.  References

   [RFC1950]  Deutsch, L. and J-L. Gailly, "ZLIB Compressed Data Format
              Specification version 3.3", RFC 1950, May 1996.

   [RFC1951]  Deutsch, P., "DEFLATE Compressed Data Format Specification
              version 1.3", RFC 1951, May 1996.

   [RFC4254]  Ylonen, T. and C. Lonvick, "The Secure Shell (SSH)
              Connection Protocol", RFC 4254, January 2006.

   [RFC4301]  Kent, S. and K. Seo, "Security Architecture for the
              Internet Protocol", RFC 4301, December 2005.

   [XLIBREF]  Nye, A., "XLIB Reference Manual R5", June 1994.


Appendix A.  Differences in earlier protocol versions

   For maximum interoperability, clients and servers should be prepared
   to fall back to the earlier 3.3 and 3.7 versions of the RFB protocol.
   Any version reported other than 3.7 or 3.8 should be treated as 3.3.

   All of the differences occur in the initial handshake phase.  Once
   the session reaches the ClientInit and ServerInit messages, all three
   protocol versions are identical.  Even within a protocol version,
   clients and servers may support different subsets of the encoding and
   pseudo-encoding types.



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A.1.  Differences in the version 3.3 protocol

   The ProtocolVersion message is:

       RFB 003.003\n (hex 52 46 42 20 30 30 33 2e 30 30 33 0a)

   In the security handshake (Section 7.1.2), rather than a two-way
   negotiation the server decides the security type and sends a single
   word:

              +--------------+--------------+---------------+
              | No. of bytes | Type [Value] | Description   |
              +--------------+--------------+---------------+
              | 4            | U32          | security-type |
              +--------------+--------------+---------------+

   The security-type may only take the value 0, 1 or 2.  A value of 0
   means that the connection has failed and is followed by a string
   giving the reason, as described in Section 7.1.2.

   If the security-type is 1, for no authentication, the server does not
   send the SecurityResult message but proceeds directly to the
   initialization messages. (Section 7.3)

   In VNC Authentication (Section 7.2.2), if the authentication fails,
   the server sends the SecurityResult message, but does not send an
   error message before closing the connection.

A.2.  Differences in the version 3.7 protocol

   The ProtocolVersion message is:

       RFB 003.007\n (hex 52 46 42 20 30 30 33 2e 30 30 37 0a)

   After the security handshake, if the security-type is 1, for no
   authentication, the server does not send the SecurityResult message
   but proceeds directly to the initialization messages. (Section 7.3)

   In VNC Authentication (Section 7.2.2), if the authentication fails,
   the server sends the SecurityResult message, but does not send an
   error message before closing the connection.










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Authors' Addresses

   Tristan Richardson
   RealVNC Ltd.
   Betjeman House, 104 Hills Road
   Cambridge  CB2 1LQ
   UK

   Phone: +44 1223 310410
   Email: standards@realvnc.com
   URI:   http://www.realvnc.com


   John Levine
   Taughannock Networks
   PO Box 727
   Trumansburg, NY  14886

   Phone: +1 607 330 5711
   Email: standards@taugh.com
   URI:   http://www.taugh.com






























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Full Copyright Statement

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