VERSION(5)VERSION(5)NAMEversion - negotiate protocol versionSYNOPSIS
size Tversion tag msize version[s]
size Rversion tag msize 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 ver‐
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 sin‐
gle 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 maxi‐
mum, 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
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 outstand‐
ing 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.