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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.
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