version – negotiate protocol version

size[4] Tversion tag[2] msize[4] version[s]
size[4] Rversion tag[2] msize[4] version[s]

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 9Pnnnn, 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.

The version message is generated by the fversion system call. It is also generated automatically, if required, by a mount or fauth system call on an uninitialized connection.