Protocol stacks
A protocol stack translates one session interface to another (or the same) session interface. For example, a terminal component may provide a command-line application with a service for obtaining textual user input and for printing text. To implement this service, the terminal uses a GUI session. Figure 1 depicts the relationship between the GUI server, the terminal, and its client application. For realizing the output of a stream of characters on screen, it implements a parser for escape sequences, maintains a state machine for the virtual terminal, and renders the pixel representation of characters onto the virtual framebuffer of the GUI session. For the textual user input, it responds to key presses reported by the GUI session's input stream by converting input events into a character stream as understood by terminal applications. When viewed from the outside of the component, the terminal translates a terminal session to a GUI session.
Similar to a device driver, a protocol stack typically serves a single client. In contrast to device drivers, however, protocol stacks are not bound to physical devices. Therefore, a protocol stack can be instantiated any number of times. For example, if multiple terminals are needed, one terminal component could be instantiated per terminal. Because each terminal uses an independent instance of the protocol stack, a bug in the protocol stack of one terminal does not affect any other terminal. However complex the implementation of the protocol stack may be, it is not prone to leaking information to another terminal because it is connected to a single client only. The leakage of information is constrained to interfaces used by the individual instance. Hence, in cases like this, the protocol-stack component is suitable for hosting highly complex untrusted code if such code cannot be avoided.
Note that the example above cannot be generalized for all protocol stacks. There are protocol stacks that are critical for the confidentiality of information. For example, an in-band encryption component may translate plain-text network traffic to encrypted network traffic designated to be transported over a public network. Even though the component is a protocol stack, it may still be prone to leaking unencrypted information to the public network.
Whereas protocol stacks are not necessarily critical for integrity and confidentiality, they are almost universally critical for availability.