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Internet Engineering Task Force (IETF) T. Richardson
Request for Comments: 6143 J. Levine
Category: Informational RealVNC Ltd.
ISSN: 2070-1721 March 2011
The Remote Framebuffer Protocol
Abstract
RFB ("remote framebuffer") is a simple protocol for remote access to
graphical user interfaces that 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.
Status of This Memo
This document is not an Internet Standards Track specification; it is
published for informational purposes.
This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Not all documents
approved by the IESG are a candidate for any level of Internet
Standard; see Section 2 of RFC 5741.
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
http://www.rfc-editor.org/info/rfc6143.
Copyright Notice
Copyright (c) 2011 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(http://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Initial Connection . . . . . . . . . . . . . . . . . . . . . . 4
3. Display Protocol . . . . . . . . . . . . . . . . . . . . . . . 4
4. Input Protocol . . . . . . . . . . . . . . . . . . . . . . . . 5
5. Representation of Pixel Data . . . . . . . . . . . . . . . . . 5
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 . . . . . . . . . . . . . . . . . . 8
7.1.3. SecurityResult Handshake . . . . . . . . . . . . . . . 10
7.2. Security Types . . . . . . . . . . . . . . . . . . . . . . 10
7.2.1. None . . . . . . . . . . . . . . . . . . . . . . . . . 10
7.2.2. VNC Authentication . . . . . . . . . . . . . . . . . . 10
7.3. Initialization Messages . . . . . . . . . . . . . . . . . 11
7.3.1. ClientInit . . . . . . . . . . . . . . . . . . . . . . 11
7.3.2. ServerInit . . . . . . . . . . . . . . . . . . . . . . 11
7.4. Pixel Format Data Structure . . . . . . . . . . . . . . . 12
7.5. Client-to-Server Messages . . . . . . . . . . . . . . . . 13
7.5.1. SetPixelFormat . . . . . . . . . . . . . . . . . . . . 13
7.5.2. SetEncodings . . . . . . . . . . . . . . . . . . . . . 14
7.5.3. FramebufferUpdateRequest . . . . . . . . . . . . . . . 15
7.5.4. KeyEvent . . . . . . . . . . . . . . . . . . . . . . . 16
7.5.5. PointerEvent . . . . . . . . . . . . . . . . . . . . . 19
7.5.6. ClientCutText . . . . . . . . . . . . . . . . . . . . 19
7.6. Server-to-Client Messages . . . . . . . . . . . . . . . . 20
7.6.1. FramebufferUpdate . . . . . . . . . . . . . . . . . . 20
7.6.2. SetColorMapEntries . . . . . . . . . . . . . . . . . . 21
7.6.3. Bell . . . . . . . . . . . . . . . . . . . . . . . . . 22
7.6.4. ServerCutText . . . . . . . . . . . . . . . . . . . . 22
7.7. Encodings . . . . . . . . . . . . . . . . . . . . . . . . 22
7.7.1. Raw Encoding . . . . . . . . . . . . . . . . . . . . . 23
7.7.2. CopyRect Encoding . . . . . . . . . . . . . . . . . . 23
7.7.3. RRE Encoding . . . . . . . . . . . . . . . . . . . . . 23
7.7.4. Hextile Encoding . . . . . . . . . . . . . . . . . . . 24
7.7.5. TRLE . . . . . . . . . . . . . . . . . . . . . . . . . 27
7.7.6. ZRLE . . . . . . . . . . . . . . . . . . . . . . . . . 30
7.8. Pseudo-Encodings . . . . . . . . . . . . . . . . . . . . . 30
7.8.1. Cursor Pseudo-Encoding . . . . . . . . . . . . . . . . 30
7.8.2. DesktopSize Pseudo-Encoding . . . . . . . . . . . . . 31
8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 31
8.1. RFB Security Types . . . . . . . . . . . . . . . . . . . . 32
8.1.1. Registry Name . . . . . . . . . . . . . . . . . . . . 32
8.1.2. Registry Contents . . . . . . . . . . . . . . . . . . 32
8.2. Client-to-Server Message Types . . . . . . . . . . . . . . 32
8.2.1. Registry Name . . . . . . . . . . . . . . . . . . . . 32
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8.2.2. Registry Contents . . . . . . . . . . . . . . . . . . 32
8.3. Server-to-Client Message Types . . . . . . . . . . . . . . 33
8.3.1. Registry Name . . . . . . . . . . . . . . . . . . . . 33
8.3.2. Registry Contents . . . . . . . . . . . . . . . . . . 33
8.4. RFB Encoding Types . . . . . . . . . . . . . . . . . . . . 34
8.4.1. Registry Name . . . . . . . . . . . . . . . . . . . . 34
8.4.2. Registry Contents . . . . . . . . . . . . . . . . . . 34
9. Security . . . . . . . . . . . . . . . . . . . . . . . . . . . 36
10. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 36
11. References . . . . . . . . . . . . . . . . . . . . . . . . . . 36
11.1. Normative References . . . . . . . . . . . . . . . . . . . 36
11.2. Informative References . . . . . . . . . . . . . . . . . . 36
Appendix A. Differences in Earlier Protocol Versions . . . . . . 38
A.1. Differences in the Version 3.3 Protocol . . . . . . . . . 38
A.2. Differences in the Version 3.7 Protocol . . . . . . . . . 38
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. 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.
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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.
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.
Note that the only port number assigned by IANA for RFB is port 5900,
so RFB clients and servers should avoid using other port numbers
unless they are communicating with servers or clients known to use
the non-standard ports.
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.
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A sequence of these rectangles makes a framebuffer update (simply
referred to here as "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.
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 synthesized 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 that 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.
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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
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
that 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 that 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.
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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 that 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.
See Section 8 for information on obtaining an ID for a new encoding
or security 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
initialization 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.
Several message formats include padding bits or bytes. For maximum
compatibility, messages should be generated with padding set to zero,
but message recipients should not assume padding has any particular
value.
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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 that
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 since they do not implement the
different handshake in 3.7 or 3.8. 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
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 that it supports:
+--------------------------+-------------+--------------------------+
| No. of bytes | Type | Description |
| | [Value] | |
+--------------------------+-------------+--------------------------+
| 1 | U8 | number-of-security-types |
| number-of-security-types | U8 array | security-types |
+--------------------------+-------------+--------------------------+
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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.
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.
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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 |
+---------------+--------------+---------------+
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 |
+--------------+--------------+-------------+
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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 and
is not intended for use on untrusted networks. Many implementations
will want to use stronger security, such as running the session over
an encrypted channel provided by IPsec [RFC4301] or SSH [RFC4254].
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.
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, and 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:
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+--------------+--------------+------------------------------+
| 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 transmitted.
+--------------+--------------+-----------------+
| 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
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to get the red value in a pixel to the least significant bit. Green-
max, green-shift, blue-max, and 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
that are not directly composed from the red, green, and blue
intensities, but 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).
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
A SetPixelFormat message 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).
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RFC 6143 The Remote Framebuffer Protocol March 2011
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.
7.5.2. SetEncodings
A SetEncodings message 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 that 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 |
+--------------+--------------+---------------------+
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This is followed by number-of-encodings repetitions of the following:
+--------------+--------------+---------------+
| No. of bytes | Type [Value] | Description |
+--------------+--------------+---------------+
| 4 | S32 | encoding-type |
+--------------+--------------+---------------+
7.5.3. FramebufferUpdateRequest
A FramebufferUpdateRequest message 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.
If the client has lost the contents of a particular area that 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 |
+--------------+--------------+--------------+
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7.5.4. KeyEvent
A KeyEvent message indicates a key press or release. Down-flag is
non-zero (true) if the key is now pressed, and 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 the X Window System.
+--------------+--------------+--------------+
| 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'.
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o The difference between upper and lower case keysyms is
significant. This is unlike some of the keyboard processing in
the X Window System that treats them as the same. For example, a
server receiving an upper case 'A' keysym without any shift
presses should interpret it as an upper case '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 clients sending a
Control press followed by an 'a' press.
o On a client where modifiers like Control and Alt can also be used
to generate character-based keysyms, the client 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 client 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. Clients should
generate a shifted Tab rather than ISO_Left_Tab. However, to be
backwards-compatible with existing clients, servers should also
recognize 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.
o Some systems give a special interpretation to key combinations
such as Ctrl-Alt-Delete. RFB clients typically provide a menu or
toolbar function to send such key combinations. The RFB protocol
does not treat them specially; to send Ctrl-Alt-Delete, the client
sends the key presses for left or right Control, left or right
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Alt, and Delete, followed by the key releases. Many RFB servers
accept Shift-Ctrl-Alt-Delete as a synonym for Ctrl-Alt-Delete that
can be entered directly from the keyboard.
7.5.5. PointerEvent
A PointerEvent message 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 means up, 1 means down
(pressed).
On a conventional mouse, buttons 1, 2, and 3 correspond to the left,
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 |
+--------------+--------------+--------------+
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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
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 that 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 |
+--------------+--------------+---------------+
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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.
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 |
+--------------+--------------+-------------+
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7.6.3. Bell
A Bell message makes 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 |
+--------------+--------------+--------------+
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.
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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 that 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.
+--------------+--------------+----------------+
| 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
encodings 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 that can be
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RFC 6143 The Remote Framebuffer Protocol March 2011
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 |
+---------------+--------------+-------------------------+
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
encodings are more compact.
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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. 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.
However, the background pixel value may not be carried over if the
previous tile was raw. 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 foreground pixel value may not be
carried over if the previous tile was raw or had the SubrectsColored
bit set. It may, however, be carried over from a previous tile with
the AnySubrects bit clear, as long as that tile itself carried over a
valid foreground from its 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:
+--------------+--------------+---------------------+
| 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:
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BackgroundSpecified
If set, a pixel value of bytesPerPixel bytes follows and 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 and 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 and gives 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:
+--------------+--------------+------------------+
| 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 4 bits
of x-and-y-position specify the X position, the least significant
specify the Y position. The most significant 4 bits of width-and-
height specify the width minus 1, the least significant specify the
height minus 1.
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7.7.5. TRLE
TRLE stands for Tiled Run-Length Encoding, and combines tiling,
palettization, 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 as a special
case, it uses a more compact format if 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. If all of these
are the 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 7 bits indicate the size of the palette used: zero means
no palette, 1 means that the tile is of a single color, and 2 to 127
indicate a palette of that size. The special subencoding values 129
and 127 indicate that the palette is to be reused from the last tile
that had a palette, 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:
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 |
+-----------------------------+--------------+-------------+
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RFC 6143 The Remote Framebuffer Protocol March 2011
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 paletteSize 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 zero-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).
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 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
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RFC 6143 The Remote Framebuffer Protocol March 2011
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, 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 |
+--------------+--------------+--------------+
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 |
+-------------------------+--------------+-----------------------+
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7.7.6. ZRLE
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 begins with a 4-byte length
field, and is followed by that many bytes of zlib-compressed data. 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 |
+--------------+--------------+-------------+
| 4 | U32 | length |
| 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 rectangle. It need not flush the stream between
tiles within a rectangle. Since the zlibData for a single rectangle
can potentially be quite large, clients can incrementally decode and
interpret the zlibData but must not assume that encoded tile data is
byte aligned.
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 that 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
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RFC 6143 The Remote Framebuffer Protocol March 2011
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, the
number being div(width+7,8). Within each byte, the most significant
bit represents the leftmost pixel; a bit set to 1 means 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 that 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.
8. IANA Considerations
IANA has allocated port 5900 to the RFB protocol. The other port
numbers mentioned in Section 2 are called out for historical context
and do not match IANA allocations.
Future assignments to the IANA registries created by this
specification are to be made through either "Expert Review" or "IESG
Approval" (if there is no currently appointed expert) as defined in
[RFC5226].
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RFC 6143 The Remote Framebuffer Protocol March 2011
8.1. RFB Security Types
8.1.1. Registry Name
The name of this registry is "Remote Framebuffer Security Types".
8.1.2. Registry Contents
IANA established a registry for security types that are used with the
RFB protocol.
The initial entries in the registry are:
+------------+-------------------------+-----------------------+
| Number | Name | References |
+------------+-------------------------+-----------------------+
| 0 | Invalid | (this document) |
| 1 | None | (this document) |
| 2 | VNC Authentication | (this document) |
| 3 to 15 | RealVNC | (historic assignment) |
| 16 | Tight | (historic assignment) |
| 17 | Ultra | (historic assignment) |
| 18 | TLS | (historic assignment) |
| 19 | VeNCrypt | (historic assignment) |
| 20 | GTK-VNC SASL | (historic assignment) |
| 21 | MD5 hash authentication | (historic assignment) |
| 22 | Colin Dean xvp | (historic assignment) |
| 128 to 255 | RealVNC | (historic assignment) |
+------------+-------------------------+-----------------------+
8.2. Client-to-Server Message Types
8.2.1. Registry Name
The name of this registry is "Remote Framebuffer Client-to-Server
Message Types".
8.2.2. Registry Contents
IANA established a registry for client-to-server message types that
are used with the RFB protocol.
The initial entries in the registry are:
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RFC 6143 The Remote Framebuffer Protocol March 2011
+--------+------------------------------+-----------------------+
| Number | Name | References |
+--------+------------------------------+-----------------------+
| 0 | SetPixelFormat | (this document) |
| 2 | SetEncodings | (this document) |
| 3 | FramebufferUpdateRequest | (this document) |
| 4 | KeyEvent | (this document) |
| 5 | PointerEvent | (this document) |
| 6 | ClientCutText | (this document) |
| 127 | VMWare | (historic assignment) |
| 128 | Nokia Terminal Mode Spec | (historic assignment) |
| 249 | OLIVE Call Control | (historic assignment) |
| 250 | Colin Dean xvp | (historic assignment) |
| 251 | Pierre Ossman SetDesktopSize | (historic assignment) |
| 252 | tight | (historic assignment) |
| 253 | gii | (historic assignment) |
| 254 | VMWare | (historic assignment) |
| 255 | Anthony Liguori | (historic assignment) |
+--------+------------------------------+-----------------------+
8.3. Server-to-Client Message Types
8.3.1. Registry Name
The name of this registry is "Remote Framebuffer Server-to-Client
Message Types".
8.3.2. Registry Contents
IANA established a registry for server-to-client message types that
are used with the RFB protocol.
The initial entries in the registry are:
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RFC 6143 The Remote Framebuffer Protocol March 2011
+--------+--------------------------+-----------------------+
| Number | Name | References |
+--------+--------------------------+-----------------------+
| 0 | FramebufferUpdate | (this document) |
| 1 | SetColourMapEntries | (this document) |
| 2 | Bell | (this document) |
| 3 | ServerCutText | (this document) |
| 127 | VMWare | (historic assignment) |
| 128 | Nokia Terminal Mode Spec | (historic assignment) |
| 249 | OLIVE Call Control | (historic assignment) |
| 250 | Colin Dean xvp | (historic assignment) |
| 252 | tight | (historic assignment) |
| 253 | gii | (historic assignment) |
| 254 | VMWare | (historic assignment) |
| 255 | Anthony Liguori | (historic assignment) |
+--------+--------------------------+-----------------------+
8.4. RFB Encoding Types
8.4.1. Registry Name
The name of this registry is "Remote Framebuffer Encoding Types".
8.4.2. Registry Contents
IANA established a registry for encoding types that are used with the
RFB protocol.
The initial entries in the registry are:
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RFC 6143 The Remote Framebuffer Protocol March 2011
+-------------------+----------------------------+------------------+
| Number | Name | References |
+-------------------+----------------------------+------------------+
| 0 | Raw | (this document) |
| 1 | CopyRect | (this document) |
| 2 | RRE | (this document) |
| 5 | Hextile | (this document) |
| 16 | ZRLE | (this document) |
| -239 | Cursor pseudo-encoding | (this document) |
| -223 | DesktopSize | (this document) |
| | pseudo-encoding | |
| 4 | CoRRE | (historic |
| | | assignment) |
| 6 | zlib | (historic |
| | | assignment) |
| 7 | tight | (historic |
| | | assignment) |
| 8 | zlibhex | (historic |
| | | assignment) |
| 15 | TRLE | (this document) |
| 17 | Hitachi ZYWRLE | (historic |
| | | assignment) |
| 1024 to 1099 | RealVNC | (historic |
| | | assignment) |
| -1 to -222 | tight options | (historic |
| | | assignment) |
| -224 to -238 | tight options | (historic |
| | | assignment) |
| -240 to -256 | tight options | (historic |
| | | assignment) |
| -257 to -272 | Anthony Liguori | (historic |
| | | assignment) |
| -273 to -304 | VMWare | (historic |
| | | assignment) |
| -305 | gii | (historic |
| | | assignment) |
| -306 | popa | (historic |
| | | assignment) |
| -307 | Peter Astrand DesktopName | (historic |
| | | assignment) |
| -308 | Pierre Ossman | (historic |
| | ExtendedDesktopSize | assignment) |
| -309 | Colin Dean xvp | (historic |
| | | assignment) |
| -310 | OLIVE Call Control | (historic |
| | | assignment) |
| -412 to -512 | TurboVNC fine-grained | (historic |
| | quality level | assignment) |
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RFC 6143 The Remote Framebuffer Protocol March 2011
| -523 to -524 | Nokia Terminal Mode Spec | (historic |
| | | assignment) |
| -763 to -768 | TurboVNC subsampling level | (historic |
| | | assignment) |
| 0x574d5600 to | VMWare | (historic |
| 0x574d56ff | | assignment) |
+-------------------+----------------------------+------------------+
9. 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. It has typically
been used on secure physical or virtual networks.
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].
10. Acknowledgements
James Weatherall, Andy Harter, and Ken Wood also contributed to the
design of the RFB protocol.
RFB and VNC are registered trademarks of RealVNC Ltd. in the U.S. and
in other countries.
11. References
11.1. Normative 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.
[XLIBREF] Nye, A., "XLIB Reference Manual R5", June 1994.
11.2. Informative References
[RFC4254] Ylonen, T. and C. Lonvick, "The Secure Shell (SSH)
Connection Protocol", RFC 4254, January 2006.
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[RFC4301] Kent, S. and K. Seo, "Security Architecture for the
Internet Protocol", RFC 4301, December 2005.
[RFC5226] Narten, T. and H. Alvestrand, "Guidelines for Writing an
IANA Considerations Section in RFCs", BCP 26, RFC 5226,
May 2008.
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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.
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)
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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.
Authors' Addresses
Tristan Richardson
RealVNC Ltd.
Betjeman House, 104 Hills Road
Cambridge CB2 1LQ
UK
Phone: +44 1223 310400
EMail: standards@realvnc.com
URI: http://www.realvnc.com
John Levine
RealVNC Ltd.
Phone: +44 1223 790005
EMail: standards@taugh.com
URI: http://jl.ly
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