Network
An X client cannot generally be detached from one server and reattached to another unless its code specifically provides for it (emacs is one of the few common programs with this ability). By extension, moving an entire session from one X server to another is generally not possible. However, approaches like Virtual Network Computing (VNC), NX and Xpra allow a virtual session to be reached from different X servers (in a manner similar to GNU Screen in relation to terminals), and other applications and toolkits provide related facilities. Workarounds like x11vnc (VNC :0 viewers) and NX (nxagent's shadow mode) also exist to make the current X-server screen available via VNC and NX respectively. This ability allows the user interface (mouse, keyboard, monitor) of a running application to be switched from one location to another without stopping and restarting the application. This can be important in some applications, such as process monitoring and control.
Network traffic between an X server and remote X clients is not encrypted by default. An attacker with a packet sniffer can intercept it, making it possible to view anything displayed to or sent from the user's screen. The most common way to encrypt X traffic is to establish a Secure Shell (SSH) tunnel for communication.
Like all thin clients, when using X across a network, bandwidth limitations can impede the use of bitmap-intensive applications that require rapidly updating large portions of the screen with low latency, such as 3D animation or photo editing. Even a relatively small uncompressed 640x480x24bit 30fps video stream can easily outstrip the bandwidth of a 100Mbit network for a single client. Early versions of X would suffer even when not using the local network, such as when using a linux workstation with the X server and client on the same system, due to the network-centric protocol which would still utilize an internal network model within the computer, via localhost or unix sockets. In contrast, modern versions of X generally have extensions such as MESA allowing local display of a local program's graphics to be optimized to bypass the network model and directly control the video card, for use of full-screen video, rendered 3D applications, and other such applications.
Read more about this topic: X Servers, Limitations and Criticism
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