RFB protocol

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 Microsoft Windows, Mac OS X and the X Window System. RFB is the protocol used in Virtual Network Computing (VNC) and its derivatives.

Description

By default, a viewer/client uses TCP port 5900 to connect to a server (or 5800 for browser access), but can also be set to use any other port. Alternatively, a server can connect to a viewer in "listening mode" (by default on port 5500). One advantage of listening mode is that the server site does not have to configure its firewall/NAT to allow access on the specified ports; the burden is on the viewer, which is useful if the server site has no computer expertise, while the viewer user would be expected to be more knowledgeable.

Although RFB started as a relatively simple protocol, it has been enhanced with additional features (such as file transfers) and more sophisticated compression and security techniques as it has developed. To maintain seamless cross-compatibility between the many different VNC client and server implementations, the clients and servers negotiate a connection using the best RFB version, and the most appropriate compression and security options that they can both support.

History

RFB was originally developed at Olivetti Research Laboratory (ORL) as a remote display technology to be used by a simple thin client with ATM connectivity called a Videotile. In order to keep the device as simple as possible, RFB was developed and used in preference to any of the existing remote display technologies.

RFB found a second and more enduring use when VNC was developed. VNC was released as open source software and the RFB specification published on the web. Since then RFB has been a free protocol which anybody can use.

When ORL was closed in 2002 some of the key people behind VNC and RFB formed RealVNC, Ltd., in order to continue development of VNC and to maintain the RFB protocol. The current RFB protocol is published on the RealVNC website.

Protocol versions

Published versions of the RFB protocol are as follows:

Version Published Date Specification
RFB 3.3 ORL January 1998 The Remote Framebuffer Protocol 3.3
RFB 3.7 RealVNC Ltd August 2003 The Remote Framebuffer Protocol 3.7
RFB 3.8 (current) RealVNC Ltd June 2007 The Remote Framebuffer Protocol 3.8

Developers are free to add additional encoding and security types but they must book unique identification numbers for these with the maintainers of the protocol so that the numbers do not clash. Clashing type numbers would cause confusion when handshaking a connection and break cross-compatibility between implementations. The list of encoding and security types is maintained by RealVNC Ltd and is separate from the protocol specification so that new types can be added without requiring the specification to be reissued.

A community version of the RFB protocol specification which aims to document all existing extensions is hosted by the TigerVNC project.

Encoding types

Number Encoding
0x00000000 Raw
0x00000001 CopyRect
0x00000002 RRE (Rising Rectangle)
0x00000004 CoRRE (Compact Rising Rectangle)
0x00000005 Hextile
0x00000006 Zlib
0x00000007 Tight
0x00000008 ZlibHex
0x00000009 Ultra
0x00000010 ZRLE
0x00000011 ZYWRLE
0xFFFF0001 CacheEnable
0xFFFF0006 XOREnable
0xFFFF8000 ServerState (UltraVNC)
0xFFFF8001 EnableKeepAlive (UltraVNC)
0xFFFF8002 FTProtocolVersion (File Transfer Protocol Version - UltraVNC)
0xFFFFFF00–0xFFFFFF09 CompressLevel (Tight encoding)
0xFFFFFF10 XCursor
0xFFFFFF11 RichCursor
0xFFFFFF18 PointerPos
0xFFFFFF20 LastRect
0xFFFFFF21 NewFBSize
0xFFFFFFE0–0xFFFFFFE9 QualityLevel (Tight encoding)

Limitations

In terms of transferring clipboard data, "there is currently no way to transfer text outside the Latin-1 character set".[1]

The VNC protocol is pixel based. Although this leads to great flexibility (i.e. any type of desktop can be displayed), it is often less efficient than solutions that have a better understanding of the underlying graphic layout like X11 or desktop such as RDP. Those protocols send graphic primitives or high level commands in a simpler form (e.g. open window), whereas RFB just sends the raw pixel data, albeit compressed.

See also

References

  1. ↑ Richardson, Tristan (2010). "Sections 6.4.6, 6.5.4". The RFB Protocol - Version 3.8.

External links

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