This is described further in chapter 2. In this sense, the operating system is very like a collection of Objects accessed by standard Methods. The operating system monitor is a single monolithic monitor, which not only protects private data, but also privileged instructions. However, modern languages and compiler support mean that this is less necessary. These procedures in an operating system monitor are usually divided into those to do with file access, those to do with process manipulation, and those to do with InterProcess Communication.
Unreliable channels exhibit various different types of fault.
Messages may be lost, re-ordered, duplicated, changed to apparently correct but different messages and even created as if from nowhere by the channel. All of these problems may have to be overcome by the IPC mechanism. We can also classify IPC mechanisms by when the association between one process and another is made. This is orthogonal to the issue of reliability.