version(9P) - Plan 9 from User Space

| VERSION(9P) | VERSION(9P)
| NAME
| version – negotiate protocol version
SYNOPSIS
| size[4] `Tversion` tag[2] msize[4] version[s] size[4] `Rversion` tag[2] msize[4] version[s]
DESCRIPTION
| The `version` request negotiates the protocol version and message size to be used on the connection and initializes the connection for I/O. `Tversion` must be the first message sent on the 9P connection, and the client cannot issue any further requests until it has received the `Rversion` reply. The tag should be `NOTAG` (value `(ushort)~0`) for a `version` message.
The client suggests a maximum message size, `msize`, that is the maximum length, in bytes, it will ever generate or expect to receive in a single 9P message. This count includes all 9P protocol data, starting from the `size` field and extending through the message, but excludes enveloping transport protocols. The server responds with its own maximum, `msize`, which must be less than or equal to the client’s value. Thenceforth, both sides of the connection must honor this limit.
The `version` string identifies the level of the protocol. The string must always begin with the two characters “`9P`”. If the server does not understand the client’s version string, it should respond with an `Rversion` message (not `Rerror`) with the `version` string the 7 characters “`unknown`”.
The server may respond with the client’s version string, or a version string identifying an earlier defined protocol version. Currently, the only defined version is the 6 characters “`9P2000`”. Version strings are defined such that, if the client string contains one or more period characters, the initial substring up to but not including any single period in the version string defines a version of the protocol. After stripping any such period-separated suffix, the server is allowed to respond with a string of the form `9P`nnnn, where nnnn is less than or equal to the digits sent by the client.
The client and server will use the protocol version defined by the server’s response for all subsequent communication on the connection.
A successful `version` request initializes the connection. All outstanding I/O on the connection is aborted; all active fids are freed (‘clunked’) automatically. The set of messages between `version` requests is called a session.
ENTRY POINTS
| Fsversion (see 9pclient(3)) generates `version` messages; it is called automatically by fsmount.