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How does a remote display protocol actually work?

IT should know how remote display protocols, which transmit the desktop image from the data center to the end user, work because of the critical role they play in VDI.

A remote display protocol provides a structure for transmitting a remote desktop to a client device in a location separate from the host running the desktop.

The protocol is also responsible for transferring user input such as keyboard entries and touchscreen actions back to the remote desktop. In this way, the user can interact with the desktop and its applications as though he were working with them directly.

IT usually installs dedicated software on the client device to interface with the protocol and render the desktop on the device's monitor. In some cases, IT can use an HTML5 browser instead of the dedicated client software, depending on the protocol.

The remote display protocol process

Typically, remote display protocols do much more than simply transfer data between the host and client. Most protocols use technologies such as encoding, compression and deduplication to minimize the amount of data they have to transmit and maximize performance. Each protocol follows its own rules for delivering these services.

As part of this process, the protocols commonly use codecs -- computer programs that encode and compress data streams for transmission -- to transmit and render the desktop images. A codec can also decode and decompress data streams for viewing and editing.

One codec that many remote display protocols such as VMware's Blast Extreme use is H.264 -- also referred to as AVC/H.264 or Advanced Video Coding, MPEG-4 Part 10. The H.264 codec makes it possible to deliver high-quality video data across a wide range of networks and systems, even at relatively low bit rates.

An RDP quiz for enterprise VDI shops

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Remote display protocols are not limited to the H.264 codec or to only one codec. Some protocols use multiple codecs, implementing them according to their specific circumstances.

How does a remote display protocol transmit data?

When transmitting data across networks, most remote display protocols use the Transmission Control Protocol (TCP), the User Datagram Protocol (UDP) or a combination of both. TCP includes a control mechanism to confirm that recipients properly receive packets. UDP does not. Because of this, UDP incurs lower overhead than TCP, making it faster and more lightweight, but UDP can also cause data loss. In some cases, such as with streaming video, this is often not an issue.

Some remote display protocols, including Citrix HDX and Microsoft RemoteFX, let administrators choose between TCP and UDP, often defaulting to one or the other. When the option to choose is available, administrators should account for network restrictions, supported workloads and whether data loss is acceptable.

In some cases, the remote display protocol can redirect a portion of the desktop processing to the client device to improve performance, which is especially useful for multimedia. This type of redirection is viable only on client devices that have the necessary processing capabilities, however.

Most remote display protocols also support USB redirection, which allows users to plug peripheral devices into their systems and then access those devices through their remote desktops.

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