doc: document the Pseudo-encoding of LED state

VNC LED state Pseudo-encoding

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Introduction

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This document describes the Pseudo-encoding of LED state for RFB which

is the protocol used in VNC as reference link below:

http://tigervnc.svn.sourceforge.net/viewvc/tigervnc/rfbproto/rfbproto.rst?content-type=text/plain

When accessing a guest by console through VNC, there might be mismatch

between the lock keys notification LED on the computer running the VNC

client session and the current status of the lock keys on the guest

machine.

To solve this problem it attempts to add LED state Pseudo-encoding

extension to VNC protocol to deal with setting LED state.

Pseudo-encoding

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This Pseudo-encoding requested by client declares to server that it supports

LED state extensions to the protocol.

The Pseudo-encoding number for LED state defined as:

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Number  Name

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-261    'LED state Pseudo-encoding'

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LED state Pseudo-encoding

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The LED state Pseudo-encoding describes the encoding of LED state which

consists of 3 bits, from left to right each bit represents the Caps, Num,

and Scroll lock key respectively. '1' indicates that the LED should be

on and '0' should be off.

Some example encodings for it as following:

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Code    Description

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100     CapsLock is on, NumLock and ScrollLock are off

010     NumLock is on, CapsLock and ScrollLock are off

111     CapsLock, NumLock and ScrollLock are on

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