9P

9P, also known as the 9P protocol or Plan 9 File Protocol, is a network protocol developed for the Plan 9 from Bell Labs operating system family. It provides a uniform interface for accessing resources—files, devices, and services—across a network as if they were part of a single hierarchical file system. This abstraction enables remote resources to be mounted and accessed with simple read/write semantics.

Interaction is message-based. A client issues a sequence of requests carrying a file identifier (fid) that references an open object, and the server replies with the data and status. Core operations include attach to connect a client to a namespace, walk to traverse path components, open and create to obtain an opened file, read and write, stat to get metadata, clunk to release a fid, and walk results. FIDs and QIDs (unique identifiers for objects) are used to maintain context across messages. 9P supports mounting a server's namespace at a point in a client’s namespace, enabling flexible resource sharing.

Originally defined as 9P2000 in Plan 9, the protocol has been implemented in several variants and extensions to support different environments and security requirements. The transport typically uses TCP but can run over other channels. Implementations exist on Plan 9, Inferno, and ported to various Unix-like systems and embedded environments.

9P is notable for its simplicity and compact wire format, which lower the barrier to implementing servers in diverse environments and for enabling transparent networking of resources. It has influenced other distributed file system designs and remains a reference in discussions of network file systems.