The Plan 9 remote file protocol.
Modern implementations include 9p2000.L, which is a Linux-specific extension on the base 9p2000 version of the protocol. Previous versions include 9p2000.u extension for UNIX systems, 9p2000 from Plan 9 4th Edition, and the original 9p protocol from earlier editions of Plan9.
On Inferno, it is called "Styx", but is not substantially different from 9p2000.
Implementations
The current 9p2000.L implementation is maintained here by LLNL. It is primarily written in C.
The protocol itself is describe here.
Messages are binary and represented in Little Endian order on the wire.
Performance Characteristics
9P is a pretty chatty protocol with a maximum of 4GB message sizes (in practices, 64kb is more common), no pipelining, and semantics that are unfavorable to performant systems. This makes 9P a poor choice for systems that need high throughput.
Some comments on 9p performance can be found here.