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<title>version(9P) - Plan 9 from User Space</title>
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<tr><td width=20><td><b>VERSION(9P)</b><td align=right><b>VERSION(9P)</b>
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<p><font size=+1><b>NAME </b></font><br>
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version – negotiate protocol version<br>
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<p><font size=+1><b>SYNOPSIS </b></font><br>
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<i>size</i>[4] <tt><font size=+1>Tversion</font></tt> <i>tag</i>[2] <i>msize</i>[4] <i>version</i>[<i>s</i>]<br>
<i>size</i>[4] <tt><font size=+1>Rversion</font></tt> <i>tag</i>[2] <i>msize</i>[4] <i>version</i>[<i>s</i>]<br>
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<p><font size=+1><b>DESCRIPTION </b></font><br>
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The <tt><font size=+1>version</font></tt> request negotiates the protocol version and message
size to be used on the connection and initializes the connection
for I/O. <tt><font size=+1>Tversion</font></tt> must be the first message sent on the 9P connection,
and the client cannot issue any further requests until it has
received the <tt><font size=+1>Rversion</font></tt> reply. The <i>tag</i> should be
<tt><font size=+1>NOTAG</font></tt> (value <tt><font size=+1>(ushort)~0</font></tt>) for a <tt><font size=+1>version</font></tt> message.
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The client suggests a maximum message size, <tt><font size=+1>msize</font></tt>, 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 <tt><font size=+1>size</font></tt> field and extending through the message,
but excludes enveloping transport protocols. The
server responds with its own maximum, <tt><font size=+1>msize</font></tt>, which must be less
than or equal to the client’s value. Thenceforth, both sides of
the connection must honor this limit.
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The <tt><font size=+1>version</font></tt> string identifies the level of the protocol. The string
must always begin with the two characters “<tt><font size=+1>9P</font></tt>”. If the server
does not understand the client’s version string, it should respond
with an <tt><font size=+1>Rversion</font></tt> message (not <tt><font size=+1>Rerror</font></tt>) with the <tt><font size=+1>version</font></tt> string
the 7 characters “<tt><font size=+1>unknown</font></tt>”.
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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 “<tt><font size=+1>9P2000</font></tt>”.
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 <tt><font size=+1>9P</font></tt><i>nnnn</i>, where <i>nnnn</i> is less than or equal to the digits sent
by the client.
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The client and server will use the protocol version defined by
the server’s response for all subsequent communication on the
connection.
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A successful <tt><font size=+1>version</font></tt> 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 <tt><font size=+1>version</font></tt> requests is
called a <i>session</i>.<br>
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<p><font size=+1><b>ENTRY POINTS </b></font><br>
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<i>Fsversion</i> (see <a href="../man3/9pclient.html"><i>9pclient</i>(3)</a>) generates <tt><font size=+1>version</font></tt> messages; it is
called automatically by <i>fsmount</i>.<br>
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