xref: /xsrc/external/mit/libXdmcp/dist/doc/xdmcp.xml

<?xml version="1.0" encoding="UTF-8" ?>
<!DOCTYPE book PUBLIC "-//OASIS//DTD DocBook XML V4.3//EN"
                   "http://www.oasis-open.org/docbook/xml/4.3/docbookx.dtd">

<book id="xdmcp">

<bookinfo>
   <title>X Display Manager Control Protocol</title>
   <subtitle>X.Org Standard</subtitle>
   <releaseinfo>Version 1.1</releaseinfo>
   <authorgroup>
   <author>
      <firstname>Keith</firstname><surname>Packard</surname>
      <affiliation><orgname>
X Consortium,
Laboratory for Computer Science,
Massachusetts Institute of Technology
      </orgname></affiliation>
   </author>
   </authorgroup>

   <copyright><year>1989</year><holder>The Open Group</holder></copyright>
   <copyright><year>2004</year><holder>The Open Group</holder></copyright>
   <productnumber>X Version 11, Release 6.8</productnumber>

<legalnotice>



<para>
Permission is hereby granted, free of charge, to any person obtaining a copy
of this software and associated documentation files (the "Software"), to deal
in the Software without restriction, including without limitation the rights
to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
copies of the Software, and to permit persons to whom the Software is
furnished to do so, subject to the following conditions:
</para>
<para>
The above copyright notice and this permission notice shall be included in
all copies or substantial portions of the Software.
</para>
<para>
THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT.  IN NO EVENT SHALL THE
OPEN GROUP BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN
AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN
CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.
</para>
<para>
Except as contained in this notice, the name of The Open Group shall not be
used in advertising or otherwise to promote the sale, use or other dealings
in this Software without prior written authorization from The Open Group.
</para>

<para>
<emphasis remap='I'>X Window System</emphasis> is a trademark of The Open Group.
</para>
</legalnotice>
</bookinfo>

<chapter id="TITLE">
<title>TITLE</title>
<sect1 id="Purpose_and_Goals">
<title>Purpose and Goals</title>
<!-- .XS -->
<!-- (SN Purpose and Goals -->
<!-- .XE -->
<para>
<!-- .LP -->
The purpose of the X Display Manager Control Protocol (XDMCP)
is to provide a uniform mechanism for an autonomous
display to request login service from a remote host.
By autonomous, we mean
the display consists of hardware and processes that are independent of any
particular host where login service is desired.  (For example, the server
cannot simply be started by a
<function>fork/exec</function>
sequence on the host.)
An X terminal (screen, keyboard, mouse, processor, network interface)
is a prime example of an autonomous display.
</para>

<para>
From the point of view of the end user, it is very important to make
autonomous displays as easy to use as traditional hardwired character
terminals.  Specifically, you can typically just power on a hardwired
terminal and be greeted with a login prompt.  The same should be possible
with autonomous displays.  However, in a network environment with multiple
hosts, the end user may want to choose which host(s) to connect to.  In an
environment with many displays and many hosts, a site administrator may want
to associate particular collections of hosts with particular displays.  We
would like to support the following options:
</para>

<itemizedlist>
  <listitem>
    <para>
The display has a single, fixed host to which it should connect.  It should be
possible to power on the display and receive a login prompt, without user
intervention.
    </para>
  </listitem>
  <listitem>
    <para>
Any one of several hosts on a network or subnetwork may be acceptable
for accepting login from the display.
(For example, the user's file systems can be mounted onto
any such host, providing comparable environments.)  It should be possible
for the display to broadcast to find such hosts and to have the display
either automatically choose a host or present the possible hosts to the
user for selection.
    </para>
  </listitem>
  <listitem>
    <para>
The display has a fixed set of hosts that it can connect to.  It should be
possible for the display to have that set stored in RAM, but it should also be
possible for a site administrator to be able to maintain host sets for a
large number of displays using a centralized facility, without having to
interact (physically or electronically) with each individual display.
Particular hosts should be allowed to refuse login service, based on
whatever local criteria are desired.
    </para>
  </listitem>
</itemizedlist>

<para>
The control protocol should be designed in such a way that it can be used over
a reasonable variety of communication transport layers.  In fact, it is quite
desirable if every major network protocol family that supports the standard X
protocol is also capable of supporting XDMCP, because the end result of XDMCP
negotiation will be standard X protocol connections to the display.
However, because the number of displays per host may be large,
a connection-based protocol appears less desirable
than a connection-less protocol.  For this reason the protocol is designed
to use datagram services with the display responsible for sequencing and
retransmission.
</para>
<para>
<!-- .LP -->
To keep the burden on displays at a minimum (because display cost is not
a factor that can be ignored), it is desirable that displays not be required
to maintain permanent state (across power cycles) for the purposes
of the control protocol,
and it is desirable to keep required state at a minimum while the
display is powered on.
</para>
<para>
<!-- .LP -->
Security is an important consideration and must be an integral part of the
design.  The important security goals in the context of XDMCP are:
</para>
<itemizedlist>
  <listitem>
    <para>
It should be possible for the display to verify that it is communicating
with a legitimate host login service.  Because the user will present
credentials (for example, password) to this service,
it is important to avoid spoof attacks.
    </para>
  </listitem>
  <listitem>
    <para>
It should be possible for the display and the login service to negotiate the
authorization mechanism to be used for the standard X protocol.
    </para>
  </listitem>
  <listitem>
    <para>
It should be possible to provide the same level of security in verifying the
login service as is provided by the negotiated authorization mechanism.
    </para>
  </listitem>
  <listitem>
    <para>
Because there are no firm standards yet in the area of security,
XDMCP must be flexible enough to accomodate a variety of security mechanisms.
    </para>
  </listitem>
</itemizedlist>
</sect1>

<sect1 id="Overview_of_the_Protocol">
<title>Overview of the Protocol</title>
<!-- .XS -->
<!-- (SN Overview of the Protocol -->
<!-- .XE -->
<para>
<!-- .LP -->
XDMCP is designed to provide authenticated access to display management
services for remote displays.  A new network server, called a \fIDisplay
Manager\fP, will use XDMCP to communicate with displays to negotiate the
startup of X sessions.  The protocol allows the display to authenticate the
manager.  It also allows most of the configuration information to be
centralized with the manager and to ease the burden of system administration
in a large network of displays.
The essential goal is to provide plug-and-play
services similar to those provided in the familiar mainframe/terminal world.
</para>
<para>
<!-- .LP -->
Displays may be turned off by the user at any time.  Any existing session
running on a display that has been turned off must be identifiable.  This
is made possible by requiring a three-way handshake to start a session.  If
the handshake succeeds, any existing session is terminated immediately and a
new session started.  There is the problem (at least with TCP) that
connections may not be closed when the display is turned off.  In most
environments, the manager should reduce this problem by periodically XSync'ing
on its own connection, perhaps every five to ten minutes, and terminating the
session if its own connection ever closes.
</para>
<para>
<!-- .LP -->
Displays should not be required to retain permanent state for purposes of
the control protocol.  One solution to packets received out of sequence
would be to use monotonically increasing message identifiers in each message
to allow both sides to ignore messages that arrive out-of-sequence.  For
this to work, displays would at a minimum have to increment a stable crash
count each time they are powered on and use that number as part of a
larger sequence number.  But if displays cannot retain permanent state this
cannot work.  Instead, the manager assumes the responsibility for permanent
state by generating unique numbers that identify a particular session and
the protocol simply ignores packets that correspond to an invalid session.
</para>
<para>
<!-- .LP -->
The Manager must not be responsible for packet reception.  To prevent the
Manager from becoming stuck because of a hostile display, no portion of the
protocol requires the Manager to retransmit a packet.  Part of this means
that any valid packet that the Manager does receive must be
acknowledged in some way to prevent the display from continuously resending
packets.  The display can keep the protocol running as it will always know
when the Manager has received (at least one copy of) a packet.  On the
Manager side, this means that any packet may be received more than once (if
the response was lost) and duplicates must be ignored.
</para>
</sect1>

<sect1 id="Data_Types">
<title>Data Types</title>
<!-- .XS -->
<!-- (SN Data Types -->
<!-- .XE -->
<para>
<!-- .LP -->
XDMCP packets contain several types of data.  Integer values are always
stored most significant byte first in the packet ("Big Endian" order).
As XDMCP will not be used to transport large quantities of data, this
restriction will not substantially hamper the efficiency of any
implementation.  Also, no padding of any sort will occur within the packets.
</para>

<informaltable frame="none">
  <tgroup cols='3' align='left'>
  <colspec colname='c1' colsep="0"/>
  <colspec colname='c2' colsep="0"/>
  <colspec colname='c3' colsep="0"/>
  <thead>
    <row>
      <entry>Type Name</entry>
      <entry>Length (Bytes)</entry>
      <entry>Description</entry>
    </row>
  </thead>
  <tbody>
    <row rowsep="0">
      <entry>CARD8</entry>
      <entry>1</entry>
      <entry>A single byte unsigned integer</entry>
    </row>
    <row rowsep="0">
      <entry>CARD16</entry>
      <entry>2</entry>
      <entry>Two byte unsigned integer</entry>
    </row>
    <row rowsep="0">
      <entry>CARD32</entry>
      <entry>4</entry>
      <entry>Four byte unsigned integer</entry>
    </row>
    <row rowsep="0">
      <entry>ARRAY8</entry>
      <entry>n+2</entry>
      <entry>
This is actually a CARD16 followed by
a collection of CARD8.  The value of the CARD16
field (n) specifies the number of CARD8 values to follow
      </entry>
    </row>
    <row rowsep="0">
      <entry>ARRAY16</entry>
      <entry>2*m+1</entry>
      <entry>
This is a CARD8 (m) which specifies the
number of CARD16 values to follow
      </entry>
    </row>
    <row rowsep="0">
      <entry>ARRAY32</entry>
      <entry>4*l+1</entry>
      <entry>
This is a CARD8 (l) which specifies the
number of CARD32 values to follow
      </entry>
    </row>
    <row rowsep="0">
      <entry>ARRAYofARRAY8</entry>
      <entry>?</entry>
      <entry>
This is a CARD8 which specifies the
number of ARRAY8 values to follow.
      </entry>
    </row>
  </tbody>
  </tgroup>
</informaltable>
</sect1>

<sect1 id="Packet_Format">
<title>Packet Format</title>
<!-- .XS -->
<!-- (SN Packet Format -->
<!-- .XE -->
<para>
All XDMCP packets have the following information:
</para>

<informaltable frame="none">
  <tgroup cols='3' align='left'>
  <colspec colname='c1' colsep="0"/>
  <colspec colname='c2' colsep="0"/>
  <colspec colname='c3' colsep="0"/>
  <thead>
    <row>
      <entry>Length (Bytes)</entry>
      <entry>Field Type</entry>
      <entry>Description</entry>
    </row>
  </thead>
  <tbody>
    <row rowsep="0">
      <entry>2</entry>
      <entry>CARD16</entry>
      <entry>version number</entry>
    </row>
    <row rowsep="0">
      <entry>2</entry>
      <entry>CARD16</entry>
      <entry>opcode packet header</entry>
    </row>
    <row rowsep="0">
      <entry>2</entry>
      <entry>CARD16</entry>
      <entry>n = length of remaining data in bytes</entry>
    </row>
    <row rowsep="0">
      <entry>n</entry>
      <entry>???</entry>
      <entry>packet-specific data</entry>
    </row>
  </tbody>
  </tgroup>
</informaltable>

<para>
<!-- .LP -->
The fields are as follows:
</para>

<variablelist>
  <varlistentry>
    <term>Version number</term>
    <listitem>
      <para>
This specifies the version of XDMCP that generated this packet in
case changes in this protocol are required.  Displays and
managers may choose to support older versions for compatibility.
This field will initially be one (1).
      </para>
    </listitem>
  </varlistentry>
  <varlistentry>
    <term>Opcode</term>
    <listitem>
      <para>
This specifies what step of the protocol this packet represents and should
contain one of the following values (encoding provided in section below):
<emphasis role="bold">BroadcastQuery</emphasis>,
<emphasis role="bold">Query</emphasis>,
<emphasis role="bold">IndirectQuery</emphasis>,
<emphasis role="bold">ForwardQuery</emphasis>,
<emphasis role="bold">Willing</emphasis>,
<emphasis role="bold">Unwilling</emphasis>,
<emphasis role="bold">Request</emphasis>,
<emphasis role="bold">Accept</emphasis>,
<emphasis role="bold">Decline</emphasis>,
<emphasis role="bold">Manage</emphasis>,
<emphasis role="bold">Refuse</emphasis>,
<emphasis role="bold">Failed</emphasis>,
<emphasis role="bold">KeepAlive</emphasis>
or
<emphasis role="bold">Alive</emphasis>.
      </para>
    </listitem>
  </varlistentry>
  <varlistentry>
    <term>Length of data in bytes</term>
    <listitem>
      <para>
This specifies the length of the information following the first 6 bytes.
Each packet-type has a different format and will need to be separately
length-checked against this value.  Because every data item has either an
explicit or implicit length, this can be easily accomplished.
Packets that have too little or too much data should be ignored.
      </para>
    </listitem>
  </varlistentry>
</variablelist>
<para>
Packets should be checked to make sure that they satisfy the following
conditions:
</para>

<orderedlist>
  <listitem>
    <para>
They must contain valid opcodes.
    </para>
  </listitem>
  <listitem>
    <para>
The length of the remaining data should correspond to the sum of the
lengths of the individual remaining data items.
    </para>
  </listitem>
  <listitem>
    <para>
The opcode should be expected (a finite state diagram is given
in a later section).
    </para>
  </listitem>
  <listitem>
    <para>
If the packet is of type
<emphasis role="bold">Manage</emphasis> or
<emphasis role="bold">Refuse</emphasis>,
the Session ID should match the value sent in the preceding
<emphasis role="bold">Accept</emphasis> packet.
    </para>
  </listitem>
</orderedlist>
</sect1>

<sect1 id="Protocol">
<title>Protocol</title>
<!-- .XS -->
<!-- (SN Protocol -->
<!-- .XE -->
<para>
Each of the opcodes is described below.  Because a given packet type is only
ever sent one way, each packet description below indicates the direction.
Most of the packets have additional information included beyond the
description above.  The additional information is appended to the packet
header in the order described without padding, and the length field is
computed accordingly.
</para>

<informaltable frame="none">
  <tgroup cols='10' align='left'>
  <colspec colname='col1' colsep="0" colwidth="1*"/>
  <colspec colname='col2' colsep="0" colwidth="1*"/>
  <colspec colname='col3' colsep="0" colwidth="1*"/>
  <colspec colname='col4' colsep="0" colwidth="1*"/>
  <colspec colname='col5' colsep="0" colwidth="1*"/>
  <colspec colname='col6' colsep="0" colwidth="1*"/>
  <colspec colname='col7' colsep="0" colwidth="1*"/>
  <colspec colname='col8' colsep="0" colwidth="1*"/>
  <colspec colname='col9' colsep="0" colwidth="1*"/>
  <colspec colname='col10' colsep="0" colwidth="1*"/>
  <spanspec namest="col1" nameend="col10" spanname="col1_on" align="left"/>
  <spanspec namest="col2" nameend="col10" spanname="col2_on" align="left"/>
  <spanspec namest="col3" nameend="col10" spanname="col3_on" align="left"/>
  <spanspec namest="col4" nameend="col10" spanname="col4_on" align="left"/>
  <spanspec namest="col5" nameend="col10" spanname="col5_on" align="left"/>
  <tbody>
    <row rowsep="0">
      <entry spanname="col1_on"><emphasis role="bold">Query</emphasis></entry>
    </row>
    <row rowsep="0">
      <entry spanname="col1_on"><emphasis role="bold">BroadcastQuery</emphasis></entry>
    </row>
    <row rowsep="0">
      <entry spanname="col1_on"><emphasis role="bold">IndirectQuery</emphasis></entry>
    </row>
    <row rowsep="0">
      <entry></entry>
      <entry spanname="col2_on">Display -> Manager</entry>
    </row>
    <row rowsep="0">
      <entry></entry>
      <entry spanname="col2_on">Additional Fields:</entry>
    </row>
    <row rowsep="0">
      <entry></entry>
      <entry></entry>
      <entry spanname="col3_on">
<emphasis>Authentication Names</emphasis>: ARRAYofARRAY8
      </entry>
    </row>
    <row rowsep="0">
      <entry></entry>
      <entry></entry>
      <entry></entry>
      <entry spanname="col4_on">
Specifies a list of authentication names that the display supports.  The
manager will choose one of these and return it in the
<emphasis role="bold">Willing</emphasis> packet.
      </entry>
    </row>
<!-- AAAAAAAAAAAAA -->
    <row rowsep="0">
      <entry></entry>
      <entry></entry>
      <entry>Semantics</entry>
    </row>
    <row rowsep="0">
      <entry></entry>
      <entry></entry>
      <entry></entry>
      <entry spanname="col4_on">
<para>
A <function>Query</function>
packet is sent from the display to a specific host to ask if
that host is willing to provide management services to this display.  The
host should respond with
<function>Willing</function>
if it is willing to service the display or
<function>Unwilling</function>
if it is not.
</para>

<para>
A
<function>BroadcastQuery</function>
packet is similar to the
<function>Query</function>
packet except that it is intended to be received by all hosts on the network
(or subnetwork).  However, unlike
<function>Query</function>
requests, hosts that are not willing to service the display
should simply ignore
<function>BroadcastQuery</function>
requests.
</para>

<para>
An
<function>IndirectQuery</function>
packet is sent to a well known manager that forwards
the request to a larger collection of secondary managers using
<function>ForwardQuery</function>
packets.
In this way, the collection of managers that respond can be grouped
on other than network boundaries; the use of a central manager reduces
system administrative overhead.
The primary manager may also send a
<function>Willing</function>
packet in response to this packet.
</para>

<para>
Each packet type has slightly different semantics:
</para>
      </entry>
    </row>
    <row rowsep="0">
      <entry></entry>
      <entry></entry>
      <entry></entry>
      <entry></entry>
      <entry spanname="col5_on">
<para>
The
<function>Query</function>
packet is destined only for a single host.
If the display is instructed to
<function>Query</function>
multiple managers, it will send multiple
<function>Query</function>
packets.  The
<function>Query</function>
packet also demands a response from the manager, either
<function>Willing</function>
or
<function>Unwilling</function>.
    </para>
    <para>
The
<function>BroadcastQuery</function>
packet is sent to many hosts.
Each manager that receives this packet will not respond with an
<function>Unwilling</function>
packet.
    </para>
    <para>
The
<function>IndirectQuery</function>
packet is sent to only one manager with the request
that the request be forwarded to a larger list of managers using
<function>ForwardQuery</function>
packets.  This list is expected to be maintained at one
central site to reduce administrative overhead.
The function of this packet type is similar to
<function>BroadcastQuery except that</function>
<function>BroadcastQuery</function>
is not forwarded.
    </para>
      </entry>
    </row>
    <row rowsep="0">
      <entry></entry>
      <entry></entry>
      <entry></entry>
      <entry spanname="col4_on">Valid Responses:</entry>
    </row>
    <row rowsep="0">
      <entry></entry>
      <entry></entry>
      <entry spanname="col3_on">
<function>Willing</function>,
<function>Unwilling</function>
      </entry>
    </row>
    <row rowsep="0">
      <entry></entry>
      <entry></entry>
      <entry></entry>
      <entry spanname="col4_on">Problems/Solutions:</entry>
    </row>
    <row rowsep="0">
      <entry></entry>
      <entry></entry>
      <entry spanname="col3_on">Problem:</entry>
    </row>
    <row rowsep="0">
      <entry></entry>
      <entry></entry>
      <entry></entry>
      <entry spanname="col4_on">
<para>Not all managers receive the query packet.</para>
<para>Indication:</para>
      </entry>
    </row>
    <row rowsep="0">
      <entry></entry>
      <entry></entry>
      <entry></entry>
      <entry></entry>
      <entry spanname="col5_on">
None if
<function>BroadcastQuery</function>
or
<function>IndirectQuery</function>
was sent, else failure to receive
<function>Willing</function>.
      </entry>
    </row>
    <row rowsep="0">
      <entry></entry>
      <entry></entry>
      <entry></entry>
      <entry spanname="col4_on">Solution:</entry>
    </row>
    <row rowsep="0">
      <entry></entry>
      <entry></entry>
      <entry></entry>
      <entry></entry>
      <entry spanname="col5_on">
Repeatedly send the packet while waiting for user to choose a manager.
      </entry>
    </row>
    <row rowsep="0">
      <entry></entry>
      <entry></entry>
      <entry spanname="col3_on">
Timeout/Retransmission policy:
      </entry>
    </row>
    <row rowsep="0">
      <entry></entry>
      <entry></entry>
      <entry></entry>
      <entry spanname="col4_on">
An exponential backoff algorithm should be used here to reduce network load
for long-standing idle displays.  Start at 2 seconds, back off by factors of
2 to 32 seconds, and discontinue retransmit after 126 seconds.  The display
should reset the timeout when user-input is detected.  In this way, the
display will wakeup when touched by the user.
      </entry>
    </row>
    <row rowsep="0">
      <entry spanname="col1_on">
<function>ForwardQuery</function>
      </entry>
    </row>
    <row rowsep="0">
      <entry></entry>
      <entry spanname="col2_on">
<para>Primary Manager -&gt; Secondary Manager</para>
<para>Additional Fields:</para>
      </entry>
    </row>
    <row rowsep="0">
      <entry></entry>
      <entry></entry>
      <entry spanname="col3_on">
<emphasis remap='I'>Client Address</emphasis>: ARRAY8
      </entry>
    </row>
    <row rowsep="0">
      <entry></entry>
      <entry></entry>
      <entry></entry>
      <entry spanname="col4_on">
Specifies the network address of the client display.
      </entry>
    </row>
    <row rowsep="0">
      <entry></entry>
      <entry></entry>
      <entry spanname="col3_on">
<emphasis remap='I'>Client Port</emphasis>: ARRAY8
      </entry>
    </row>
    <row rowsep="0">
      <entry></entry>
      <entry></entry>
      <entry></entry>
      <entry spanname="col4_on">
Specifies an identification of the client task on the client display.
      </entry>
    </row>
    <row rowsep="0">
      <entry></entry>
      <entry></entry>
      <entry spanname="col3_on">
<emphasis remap='I'>Authentication Names</emphasis>: ARRAYofARRAY8
      </entry>
    </row>
    <row rowsep="0">
      <entry></entry>
      <entry></entry>
      <entry></entry>
      <entry spanname="col4_on">
Is a duplicate of Authentication Names array that was received
in the
<function>IndirectQuery</function>
packet.
      </entry>
    </row>
    <row rowsep="0">
      <entry></entry>
      <entry spanname="col2_on">
Semantics:
      </entry>
    </row>
    <row rowsep="0">
      <entry></entry>
      <entry></entry>
      <entry spanname="col3_on">
        <para>
When primary manager receives a
<function>IndirectQuery</function>
packet, it is responsible for sending
<function>ForwardQuery</function>
packets to an appropriate list of
managers that can provide service to the display using the same network
type as the one the original
<function>IndirectQuery</function>
packet was received from.
The Client Address and Client Port fields must contain an
address that the secondary manager can use to reach the display also using
this same network.  Each secondary manager sends a
<function>Willing</function>
packet to the display if it is willing to provide service.
        </para>

        <para>
<function>ForwardQuery</function>
packets are similar to
<function>BroadcastQuery</function>
packets in that managers that are not willing to service
particular displays should not send a
<function>Unwilling</function>
packet.
        </para>
      </entry>
    </row>
    <row rowsep="0">
      <entry></entry>
      <entry spanname="col2_on">
Valid Responses:
      </entry>
    </row>

    <row rowsep="0">
      <entry></entry>
      <entry></entry>
      <entry spanname="col3_on">
<function>Willing</function>
      </entry>
    </row>
    <row rowsep="0">
      <entry></entry>
      <entry spanname="col2_on">
Problems/Solutions:
      </entry>
    </row>
    <row rowsep="0">
      <entry></entry>
      <entry></entry>
      <entry spanname="col3_on">
Identical to
<function>BroadcastQuery</function>
      </entry>
    </row>
    <row rowsep="0">
      <entry></entry>
      <entry spanname="col2_on">
Timeout/Retransmission policy:
      </entry>
    </row>
    <row rowsep="0">
      <entry></entry>
      <entry></entry>
      <entry spanname="col3_on">
Like all packets sent from a manager, this packet should never be
retransmitted.
      </entry>
    </row>

    <row rowsep="0">
      <entry spanname="col1_on">
<function>Willing</function>
      </entry>
    </row>

    <row rowsep="0">
      <entry></entry>
      <entry spanname="col2_on">
        <para>
Manager -&gt; Display
        </para>
        <para>
Additional Fields:
        </para>
      </entry>
    </row>

    <row rowsep="0">
      <entry></entry>
      <entry></entry>
      <entry spanname="col3_on">
<emphasis remap='I'>Authentication Name</emphasis>: ARRAY8
      </entry>
    </row>

    <row rowsep="0">
      <entry></entry>
      <entry></entry>
      <entry></entry>
      <entry spanname="col4_on">
<para>
Specifies the authentication method, selected from the list offered in the
<function>Query ,</function>
<function>BroadcastQuery ,</function>
or
<function>IndirectQuery</function>
packet that the manger expects the display to use in the subsequent
<function>Request</function>
packet.
This choice should remain as constant as feasible so that displays that
send multiple
<function>Query</function>
packets can use the Authentication Name from any
<function>Willing</function>
packet that arrives.
</para>
<para>
The display is free to ignore managers that request an insufficient level
of authentication.
</para>
      </entry>
    </row>

    <row rowsep="0">
      <entry></entry>
      <entry></entry>
      <entry spanname="col3_on">
<emphasis remap='I'>Hostname</emphasis>: ARRAY8
      </entry>
    </row>

    <row rowsep="0">
      <entry></entry>
      <entry></entry>
      <entry></entry>
      <entry spanname="col4_on">
Is a human readable string describing the host from which the packet was sent.
The protocol specifies no interpretation of the data in this field.
      </entry>
    </row>

    <row rowsep="0">
      <entry></entry>
      <entry></entry>
      <entry spanname="col3_on">
<emphasis remap='I'>Status</emphasis>: ARRAY8
      </entry>
    </row>

    <row rowsep="0">
      <entry></entry>
      <entry></entry>
      <entry></entry>
      <entry spanname="col4_on">
Is a human readable string describing the status of the host.  This could
include load average/number of users connected or other information.  The
protocol specifies no interpretation of the data in this field.
      </entry>
    </row>


    <row rowsep="0">
      <entry></entry>
      <entry spanname="col2_on">
Semantics:
      </entry>
    </row>

    <row rowsep="0">
      <entry></entry>
      <entry></entry>
      <entry spanname="col3_on">
A
<function>Willing</function>
packet is sent by managers that may service connections from
this display.  It is sent in response to either a
<function>Query ,</function>
<function>BroadcastQuery ,</function>
or
<function>ForwardQuery</function>
but does not imply a commitment to provide service
(for example, it may later decide that it has accepted enough
connections already).
      </entry>
    </row>

    <row rowsep="0">
      <entry></entry>
      <entry spanname="col2_on">
Problems/Solutions:
      </entry>
    </row>

    <row rowsep="0">
      <entry></entry>
      <entry></entry>
      <entry spanname="col3_on">
Problem:
      </entry>
    </row>

    <row rowsep="0">
      <entry></entry>
      <entry></entry>
      <entry></entry>
      <entry spanname="col4_on">
<para>
<function>Willing</function>
not received by the display.
</para>
<para>
Indication:
</para>
      </entry>
    </row>

    <row rowsep="0">
      <entry></entry>
      <entry></entry>
      <entry></entry>
      <entry></entry>
      <entry spanname="col5_on">
None if
<function>BroadcastQuery</function>
or
<function>IndirectQuery</function>
was sent, else failure to receive
<function>Willing .</function>
      </entry>
    </row>

    <row rowsep="0">
      <entry></entry>
      <entry></entry>
      <entry></entry>
      <entry spanname="col4_on">
Solution:
      </entry>
    </row>

    <row rowsep="0">
      <entry></entry>
      <entry></entry>
      <entry></entry>
      <entry></entry>
      <entry spanname="col5_on">
The display should continue to send the query until a response is received.
      </entry>
    </row>



    <row rowsep="0">
      <entry></entry>
      <entry spanname="col2_on">
Timeout/Retransmission policy:
      </entry>
    </row>

    <row rowsep="0">
      <entry></entry>
      <entry></entry>
      <entry spanname="col3_on">
Like all packets sent from the manager to the display, this packet should
never be retransmitted.
      </entry>
    </row>

    <row rowsep="0">
      <entry spanname="col1_on">
<function>Unwilling</function>
      </entry>
    </row>

    <row rowsep="0">
      <entry></entry>
      <entry spanname="col2_on">
        <para>
Manager -&gt; Display
        </para>
        <para>
Additional Fields:
        </para>
      </entry>
    </row>

    <row rowsep="0">
      <entry></entry>
      <entry></entry>
      <entry spanname="col3_on">
The Hostname and Status fields as in the
<function>Willing</function>
packet.
The Status field should indicate to the user a reason
for the refusal of service.
      </entry>
    </row>

    <row rowsep="0">
      <entry></entry>
      <entry spanname="col2_on">
Semantics:
      </entry>
    </row>

    <row rowsep="0">
      <entry></entry>
      <entry></entry>
      <entry spanname="col3_on">
An
<function>Unwilling</function>
packet is sent by managers in response to direct
<function>Query</function>
requests (as opposed to
<function>BroadcastQuery</function>
or
<function>IndirectQuery</function>
requests) if the manager will not accept requests for management.
This is typically sent by managers that wish to only service
particular displays or that handle a limited number of displays at once.
      </entry>
    </row>

    <row rowsep="0">
      <entry></entry>
      <entry spanname="col2_on">
Problems/Solutions:
      </entry>
    </row>

    <row rowsep="0">
      <entry></entry>
      <entry></entry>
      <entry spanname="col3_on">
Problem:
      </entry>
    </row>

    <row rowsep="0">
      <entry></entry>
      <entry></entry>
      <entry></entry>
      <entry spanname="col4_on">
<para>
<function>Unwilling</function>
not received by the display.
</para>
<para>
Indication:
</para>
      </entry>
    </row>

    <row rowsep="0">
      <entry></entry>
      <entry></entry>
      <entry></entry>
      <entry></entry>
      <entry spanname="col5_on">
Display fails to receive
<function>Unwilling .</function>
      </entry>
    </row>

    <row rowsep="0">
      <entry></entry>
      <entry></entry>
      <entry></entry>
      <entry spanname="col4_on">
Solution:
      </entry>
    </row>

    <row rowsep="0">
      <entry></entry>
      <entry></entry>
      <entry></entry>
      <entry></entry>
      <entry spanname="col5_on">
The display should continue to send
<function>Query</function>
messages until a response is received.
      </entry>
    </row>

    <row rowsep="0">
      <entry></entry>
      <entry spanname="col2_on">
Timeout/Retransmission policy:
      </entry>
    </row>

    <row rowsep="0">
      <entry></entry>
      <entry></entry>
      <entry spanname="col3_on">
Like all packets sent from the manager to the display, this packet should
never be retransmitted.
      </entry>
    </row>
    <row rowsep="0">
      <entry spanname="col1_on">
<function>Request</function>
      </entry>
    </row>

    <row rowsep="0">
      <entry></entry>
      <entry spanname="col2_on">
        <para>
Display -&gt; Manager
        </para>
        <para>
Additional Fields:
        </para>
      </entry>
    </row>

    <row rowsep="0">
      <entry></entry>
      <entry></entry>
      <entry spanname="col3_on">
<emphasis remap='I'>Display Number</emphasis>: CARD16
      </entry>
    </row>

    <row rowsep="0">
      <entry></entry>
      <entry></entry>
      <entry></entry>
      <entry spanname="col4_on">
Specifies the index of this particular server for the host
on which the display is resident.
This value will be zero for most autonomous displays.
      </entry>
    </row>

    <row rowsep="0">
      <entry></entry>
      <entry></entry>
      <entry spanname="col3_on">
<emphasis remap='I'>Connection Types</emphasis>: ARRAY16
      </entry>
    </row>

    <row rowsep="0">
      <entry></entry>
      <entry></entry>
      <entry></entry>
      <entry spanname="col4_on">
Specifies an array indicating the stream services accepted by the display.
If the high-order byte in a particular entry is zero, the low-order byte
corresponds to an X-protocol host family type.
      </entry>
    </row>

    <row rowsep="0">
      <entry></entry>
      <entry></entry>
      <entry spanname="col3_on">
<emphasis remap='I'>Connection Addresses</emphasis>: ARRAYofARRAY8
      </entry>
    </row>

    <row rowsep="0">
      <entry></entry>
      <entry></entry>
      <entry></entry>
      <entry spanname="col4_on">
For each connection type in the previous array, the corresponding entry in
this array indicates the network address of the display device.
      </entry>
    </row>

    <row rowsep="0">
      <entry></entry>
      <entry></entry>
      <entry spanname="col3_on">
        <para>
<emphasis remap='I'>Authentication Name</emphasis>: ARRAY8
        </para>
        <para>
<emphasis remap='I'>Authentication Data</emphasis>: ARRAY8
        </para>
      </entry>
    </row>

    <row rowsep="0">
      <entry></entry>
      <entry></entry>
      <entry></entry>
      <entry spanname="col4_on">
Specifies the authentication protocol that the display expects
the manager to validate itself with.  The Authentication Data is
expected to contain data that the manager will interpret, modify
and use to authenticate itself.
      </entry>
    </row>

    <row rowsep="0">
      <entry></entry>
      <entry></entry>
      <entry spanname="col3_on">
<emphasis remap='I'>Authorization Names</emphasis>: ARRAYofARRAY8
      </entry>
    </row>

    <row rowsep="0">
      <entry></entry>
      <entry></entry>
      <entry></entry>
      <entry spanname="col4_on">
Specifies which types of authorization the display supports.  The
manager may decide to reject displays with which it cannot perform
authorization.
      </entry>
    </row>

    <row rowsep="0">
      <entry></entry>
      <entry></entry>
      <entry spanname="col3_on">
<emphasis remap='I'>Manufacturer Display ID</emphasis>: ARRAY8
      </entry>
    </row>

    <row rowsep="0">
      <entry></entry>
      <entry></entry>
      <entry></entry>
      <entry spanname="col4_on">
Can be used by the manager to determine how to decrypt the
Authentication Data field in this packet.  See the section below on
Manufacturer Display ID Format.
      </entry>
    </row>

    <row rowsep="0">
      <entry></entry>
      <entry spanname="col2_on">
Semantics:
      </entry>
    </row>

    <row rowsep="0">
      <entry></entry>
      <entry></entry>
      <entry spanname="col3_on">
A
<function>Request</function>
packet is sent by a display to a specific host to request a
session ID in preparation for a establishing a connection.  If the manager
is willing to service a connection to this display, it should return an
<function>Accept</function>
packet with a valid session ID and should be ready for a subsequent
<function>Manage</function>
request.  Otherwise, it should return a
<function>Decline</function>
packet.
      </entry>
    </row>

    <row rowsep="0">
      <entry></entry>
      <entry spanname="col2_on">
Valid Responses:
      </entry>
    </row>

    <row rowsep="0">
      <entry></entry>
      <entry></entry>
      <entry spanname="col3_on">
<function>Accept ,</function>
<function>Decline</function>
      </entry>
    </row>

    <row rowsep="0">
      <entry></entry>
      <entry spanname="col2_on">
Problems/Solutions:
      </entry>
    </row>

    <row rowsep="0">
      <entry></entry>
      <entry></entry>
      <entry spanname="col3_on">
Problem:
      </entry>
    </row>

    <row rowsep="0">
      <entry></entry>
      <entry></entry>
      <entry></entry>
      <entry spanname="col4_on">
Request not received by manager.
      </entry>
    </row>

    <row rowsep="0">
      <entry></entry>
      <entry></entry>
      <entry></entry>
      <entry spanname="col4_on">
Indication:
      </entry>
    </row>

    <row rowsep="0">
      <entry></entry>
      <entry></entry>
      <entry></entry>
      <entry></entry>
      <entry spanname="col5_on">
Display timeout waiting for response.
      </entry>
    </row>

    <row rowsep="0">
      <entry></entry>
      <entry></entry>
      <entry></entry>
      <entry spanname="col4_on">
Solution:
      </entry>
    </row>

    <row rowsep="0">
      <entry></entry>
      <entry></entry>
      <entry></entry>
      <entry></entry>
      <entry spanname="col5_on">
Display resends
<function>Request</function>
message.
      </entry>
    </row>


    <row rowsep="0">
      <entry></entry>
      <entry></entry>
      <entry></entry>
      <entry spanname="col4_on">
Problem:
      </entry>
    </row>

    <row rowsep="0">
      <entry></entry>
      <entry></entry>
      <entry></entry>
      <entry></entry>
      <entry spanname="col5_on">
Message received out of order by manager.
      </entry>
    </row>

    <row rowsep="0">
      <entry></entry>
      <entry></entry>
      <entry></entry>
      <entry spanname="col4_on">
Indication:
      </entry>
    </row>

    <row rowsep="0">
      <entry></entry>
      <entry></entry>
      <entry></entry>
      <entry></entry>
      <entry spanname="col5_on">
None.
      </entry>
    </row>

    <row rowsep="0">
      <entry></entry>
      <entry></entry>
      <entry></entry>
      <entry spanname="col4_on">
Solution:
      </entry>
    </row>

    <row rowsep="0">
      <entry></entry>
      <entry></entry>
      <entry></entry>
      <entry></entry>
      <entry spanname="col5_on">
Each time a
<function>Request</function>
is sent, the manager sends the Session ID
associated with the next session in the
<function>Accept .</function>
If that next session is not yet started,
the manager will simply resend with the same Session ID.
If the session is in progress, the manager will reply
with a new Session ID; in which case, the
<function>Accept</function>
will be discarded by the display.
      </entry>
    </row>

    <row rowsep="0">
      <entry></entry>
      <entry spanname="col2_on">
Timeout/Retransmission policy:
      </entry>
    </row>

    <row rowsep="0">
      <entry></entry>
      <entry></entry>
      <entry spanname="col3_on">
Timeout after 2 seconds, exponential backoff to 32 seconds.
After no more than 126 seconds, give up and report an error to the user.
      </entry>
    </row>

    <row rowsep="0">
      <entry spanname="col1_on">
<function>Accept</function>
      </entry>
    </row>

    <row rowsep="0">
      <entry></entry>
      <entry spanname="col2_on">
        <para>
Manager -&gt; Display
        </para>
        <para>
Additional Fields:
        </para>
      </entry>
    </row>

    <row rowsep="0">
      <entry></entry>
      <entry></entry>
      <entry spanname="col3_on">
<emphasis remap='I'>Session ID</emphasis>: CARD32
      </entry>
    </row>

    <row rowsep="0">
      <entry></entry>
      <entry></entry>
      <entry></entry>
      <entry spanname="col4_on">
Identifies the session that can be started by the manager.
      </entry>
    </row>

    <row rowsep="0">
      <entry></entry>
      <entry></entry>
      <entry spanname="col3_on">
        <para>
<emphasis remap='I'>Authentication Name</emphasis>: ARRAY8
        </para>
        <para>
<emphasis remap='I'>Authentication Data</emphasis>: ARRAY8
        </para>
      </entry>
    </row>

    <row rowsep="0">
      <entry></entry>
      <entry></entry>
      <entry></entry>
      <entry spanname="col4_on">
Is the data sent back to the display to authenticate the manager.
If the Authentication Data is not the value expected by the display, it
should terminate the protocol at this point and display an error to the user.
      </entry>
    </row>

    <row rowsep="0">
      <entry></entry>
      <entry></entry>
      <entry spanname="col3_on">
        <para>
<emphasis remap='I'>Authorization Name</emphasis>: ARRAY8
        </para>
        <para>
<emphasis remap='I'>Authorization Data</emphasis>: ARRAY8
        </para>
      </entry>
    </row>

    <row rowsep="0">
      <entry></entry>
      <entry></entry>
      <entry></entry>
      <entry spanname="col4_on">
Is the data sent to the display to indicate the type of authorization the
manager will be using in the first call to
<function>XOpenDisplay</function>
after the
<function>Manage</function>
packet is received.
      </entry>
    </row>


    <row rowsep="0">
      <entry></entry>
      <entry spanname="col2_on">
Semantics:
      </entry>
    </row>

    <row rowsep="0">
      <entry></entry>
      <entry></entry>
      <entry spanname="col3_on">
<para>
An
<function>Accept</function>
packet is sent by a manager in response to a
<function>Request</function>
packet if the manager is willing to establish a connection for the display.
The Session ID is used to identify this connection from any preceding
ones and will be used by the display in its subsequent
<function>Manage</function>
packet.
The Session ID is a 32-bit number that is incremented each time an
<function>Accept</function>
packet is sent as it must be unique over a reasonably long period of time.
</para>
<para>
If the authentication information is invalid, a
<function>Decline</function>
packet will be returned with an appropriate
<function>Status</function>
message.
</para>
      </entry>
    </row>

    <row rowsep="0">
      <entry></entry>
      <entry spanname="col2_on">
Problems/Solutions:
      </entry>
    </row>

    <row rowsep="0">
      <entry></entry>
      <entry></entry>
      <entry spanname="col3_on">
Problem:
      </entry>
    </row>

    <row rowsep="0">
      <entry></entry>
      <entry></entry>
      <entry></entry>
      <entry spanname="col4_on">
<function>Accept</function>
or
<function>Decline</function>
not received by display.
      </entry>
    </row>

    <row rowsep="0">
      <entry></entry>
      <entry></entry>
      <entry></entry>
      <entry spanname="col4_on">
Indication:
      </entry>
    </row>

    <row rowsep="0">
      <entry></entry>
      <entry></entry>
      <entry></entry>
      <entry></entry>
      <entry spanname="col5_on">
Display timeout waiting for response to
<function>Request .</function>
      </entry>
    </row>

    <row rowsep="0">
      <entry></entry>
      <entry></entry>
      <entry></entry>
      <entry spanname="col4_on">
Solution:
      </entry>
    </row>

    <row rowsep="0">
      <entry></entry>
      <entry></entry>
      <entry></entry>
      <entry></entry>
      <entry spanname="col5_on">
Display resends
<function>Request</function>
message.
      </entry>
    </row>

    <row rowsep="0">
      <entry></entry>
      <entry></entry>
      <entry spanname="col3_on">
Problem:
      </entry>
    </row>

    <row rowsep="0">
      <entry></entry>
      <entry></entry>
      <entry></entry>
      <entry spanname="col4_on">
Message received out of order by display.
      </entry>
    </row>

    <row rowsep="0">
      <entry></entry>
      <entry></entry>
      <entry></entry>
      <entry spanname="col4_on">
Indication:
      </entry>
    </row>

    <row rowsep="0">
      <entry></entry>
      <entry></entry>
      <entry></entry>
      <entry></entry>
      <entry spanname="col5_on">
Display receives
<function>Accept</function>
after
<function>Manage</function>
has been sent.
      </entry>
    </row>

    <row rowsep="0">
      <entry></entry>
      <entry></entry>
      <entry></entry>
      <entry spanname="col4_on">
Solution:
      </entry>
    </row>

    <row rowsep="0">
      <entry></entry>
      <entry></entry>
      <entry></entry>
      <entry></entry>
      <entry spanname="col5_on">
Display discards
<function>Accept</function>
messages after it has sent a
<function>Manage</function>
message.
      </entry>
    </row>

    <row rowsep="0">
      <entry></entry>
      <entry spanname="col2_on">
Timeout/Retransmission policy:
      </entry>
    </row>

    <row rowsep="0">
      <entry></entry>
      <entry></entry>
      <entry spanname="col3_on">
Like all packets sent from the manager to the display, this packet should
never be retransmitted.
      </entry>
    </row>

    <row rowsep="0">
      <entry spanname="col1_on">
<function>Decline</function>
      </entry>
    </row>

    <row rowsep="0">
      <entry></entry>
      <entry spanname="col2_on">
        <para>
Manager -&gt; Display
        </para>
        <para>
Additional Fields:
        </para>
      </entry>
    </row>

    <row rowsep="0">
      <entry></entry>
      <entry></entry>
      <entry spanname="col3_on">
<emphasis remap='I'>Status</emphasis>: ARRAY8
      </entry>
    </row>

    <row rowsep="0">
      <entry></entry>
      <entry></entry>
      <entry></entry>
      <entry spanname="col4_on">
Is a human readable string indicating the reason for refusal of
service.
      </entry>
    </row>

    <row rowsep="0">
      <entry></entry>
      <entry></entry>
      <entry spanname="col3_on">
        <para>
<emphasis remap='I'>Authentication Name</emphasis>:
ARRAY8
        </para>
        <para>
<emphasis remap='I'>Authentication Data</emphasis>:
ARRAY8
        </para>
      </entry>
    </row>

    <row rowsep="0">
      <entry></entry>
      <entry></entry>
      <entry></entry>
      <entry spanname="col4_on">
Is the data sent back to the display to authenticate the manager.  If the
Authentication Data is not the value expected by the display, it
should terminate the protocol at this point and display an error to the user.
      </entry>
    </row>

    <row rowsep="0">
      <entry></entry>
      <entry spanname="col2_on">
Semantics:
      </entry>
    </row>

    <row rowsep="0">
      <entry></entry>
      <entry></entry>
      <entry spanname="col3_on">
A
<function>Decline</function>
packet is sent by a manager in response to a
<function>Request</function>
packet if the manager is unwilling to establish a connection for the
display.
This is allowed even if the manager had responded
<function>Willing</function>
to a previous query.
      </entry>
    </row>

    <row rowsep="0">
      <entry></entry>
      <entry spanname="col2_on">
Problems/Solutions:
      </entry>
    </row>

    <row rowsep="0">
      <entry></entry>
      <entry></entry>
      <entry spanname="col3_on">
Same as for
<function>Accept .</function>
      </entry>
    </row>

    <row rowsep="0">
      <entry></entry>
      <entry spanname="col2_on">
Timeout/Retransmission policy:
      </entry>
    </row>

    <row rowsep="0">
      <entry></entry>
      <entry></entry>
      <entry spanname="col3_on">
Like all packets sent from a manager to a display, this packet should never
be retransmitted.
      </entry>
    </row>

    <row rowsep="0">
      <entry spanname="col1_on">
<function>Manage</function>
      </entry>
    </row>

    <row rowsep="0">
      <entry></entry>
      <entry spanname="col2_on">
        <para>
Display -&gt; Manager
        </para>
        <para>
Additional Fields:
        </para>
      </entry>
    </row>

    <row rowsep="0">
      <entry></entry>
      <entry></entry>
      <entry spanname="col3_on">
<emphasis remap='I'>Session ID</emphasis>: CARD32
      </entry>
    </row>

    <row rowsep="0">
      <entry></entry>
      <entry></entry>
      <entry></entry>
      <entry spanname="col4_on">
Should contain the nonzero session ID returned in the
<function>Accept</function>
packet.
      </entry>
    </row>

    <row rowsep="0">
      <entry></entry>
      <entry></entry>
      <entry spanname="col3_on">
<emphasis remap='I'>Display Number</emphasis>: CARD16
      </entry>
    </row>

    <row rowsep="0">
      <entry></entry>
      <entry></entry>
      <entry></entry>
      <entry spanname="col4_on">
Must match the value sent in the previous
<function>Request</function>
packet.
      </entry>
    </row>

    <row rowsep="0">
      <entry></entry>
      <entry></entry>
      <entry spanname="col3_on">
<emphasis remap='I'>Display Class</emphasis>: ARRAY8
      </entry>
    </row>

    <row rowsep="0">
      <entry></entry>
      <entry></entry>
      <entry></entry>
      <entry spanname="col4_on">
Specifies the class of the display.
See the Display Class Format section,
which discusses the format of this field.
      </entry>
    </row>


    <row rowsep="0">
      <entry></entry>
      <entry spanname="col2_on">
Semantics:
      </entry>
    </row>

    <row rowsep="0">
      <entry></entry>
      <entry></entry>
      <entry spanname="col3_on">
A
<function>Manage</function>
packet is sent by a display to ask the manager to begin a
session on the display.  If the Session ID is correct the manager
should open a connection; otherwise, it should respond with a
<function>Refuse</function>
or
<function>Failed</function>
packet, unless the Session ID matches a currently
running session or a session that has not yet successfully opened the
display but has not given up the attempt.  In this latter case, the
<function>Manage</function>
packet should be ignored.
This will work as stream connections give positive success indication
to both halves of the stream, and positive failure indication
to the connection initiator (which will eventually generate a
<function>Failed</function>
packet).
      </entry>
    </row>

    <row rowsep="0">
      <entry></entry>
      <entry spanname="col2_on">
Valid Responses:
      </entry>
    </row>

    <row rowsep="0">
      <entry></entry>
      <entry></entry>
      <entry spanname="col3_on">
X connection with correct auth info,
<function>Refuse ,</function>
<function>Failed .</function>
      </entry>
    </row>

    <row rowsep="0">
      <entry></entry>
      <entry spanname="col2_on">
Problems/Solutions:
      </entry>
    </row>

    <row rowsep="0">
      <entry></entry>
      <entry></entry>
      <entry spanname="col3_on">
Problem:
      </entry>
    </row>

    <row rowsep="0">
      <entry></entry>
      <entry></entry>
      <entry></entry>
      <entry spanname="col4_on">
<function>Manage</function>
not received by manager.
      </entry>
    </row>

    <row rowsep="0">
      <entry></entry>
      <entry></entry>
      <entry></entry>
      <entry spanname="col4_on">
Indication:
      </entry>
    </row>

    <row rowsep="0">
      <entry></entry>
      <entry></entry>
      <entry></entry>
      <entry></entry>
      <entry spanname="col5_on">
Display timeout waiting for response.
      </entry>
    </row>

    <row rowsep="0">
      <entry></entry>
      <entry></entry>
      <entry></entry>
      <entry spanname="col4_on">
Solution:
      </entry>
    </row>

    <row rowsep="0">
      <entry></entry>
      <entry></entry>
      <entry></entry>
      <entry></entry>
      <entry spanname="col5_on">
Display resends
<function>Manage</function>
message.
      </entry>
    </row>


    <row rowsep="0">
      <entry></entry>
      <entry></entry>
      <entry spanname="col3_on">
Problem:
      </entry>
    </row>

    <row rowsep="0">
      <entry></entry>
      <entry></entry>
      <entry></entry>
      <entry spanname="col4_on">
<function>Manage</function>
received out of order by manager.
      </entry>
    </row>

    <row rowsep="0">
      <entry></entry>
      <entry></entry>
      <entry></entry>
      <entry spanname="col4_on">
Indication:
      </entry>
    </row>

    <row rowsep="0">
      <entry></entry>
      <entry></entry>
      <entry></entry>
      <entry></entry>
      <entry spanname="col5_on">
Session already in progress with matching Session ID.
      </entry>
    </row>

    <row rowsep="0">
      <entry></entry>
      <entry></entry>
      <entry></entry>
      <entry spanname="col4_on">
Solution:
      </entry>
    </row>

    <row rowsep="0">
      <entry></entry>
      <entry></entry>
      <entry></entry>
      <entry></entry>
      <entry spanname="col5_on">
<function>Manage</function>
packet ignored.
      </entry>
    </row>

    <row rowsep="0">
      <entry></entry>
      <entry></entry>
      <entry></entry>
      <entry spanname="col4_on">
Indication:
      </entry>
    </row>

    <row rowsep="0">
      <entry></entry>
      <entry></entry>
      <entry></entry>
      <entry></entry>
      <entry spanname="col5_on">
Session ID does not match next Session ID.
      </entry>
    </row>

    <row rowsep="0">
      <entry></entry>
      <entry></entry>
      <entry></entry>
      <entry spanname="col4_on">
Solution:
      </entry>
    </row>

    <row rowsep="0">
      <entry></entry>
      <entry></entry>
      <entry></entry>
      <entry></entry>
      <entry spanname="col5_on">
<function>Refuse</function>
message is sent.
      </entry>
    </row>


    <row rowsep="0">
      <entry></entry>
      <entry></entry>
      <entry spanname="col3_on">
Problem:
      </entry>
    </row>

    <row rowsep="0">
      <entry></entry>
      <entry></entry>
      <entry></entry>
      <entry spanname="col4_on">
Display cannot be opened on selected stream.
      </entry>
    </row>

    <row rowsep="0">
      <entry></entry>
      <entry></entry>
      <entry></entry>
      <entry spanname="col4_on">
Indication:
      </entry>
    </row>

    <row rowsep="0">
      <entry></entry>
      <entry></entry>
      <entry></entry>
      <entry></entry>
      <entry spanname="col5_on">
Display connection setup fails.
      </entry>
    </row>

    <row rowsep="0">
      <entry></entry>
      <entry></entry>
      <entry></entry>
      <entry spanname="col4_on">
Solution:
      </entry>
    </row>

    <row rowsep="0">
      <entry></entry>
      <entry></entry>
      <entry></entry>
      <entry></entry>
      <entry spanname="col5_on">
<function>Failed</function>
message is sent including a human readable reason.
      </entry>
    </row>

    <row rowsep="0">
      <entry></entry>
      <entry></entry>
      <entry spanname="col3_on">
Problem:
      </entry>
    </row>

    <row rowsep="0">
      <entry></entry>
      <entry></entry>
      <entry></entry>
      <entry spanname="col4_on">
Display open does not succeed before a second manage packet is received
because of a timeout occuring in the display.
      </entry>
    </row>

    <row rowsep="0">
      <entry></entry>
      <entry></entry>
      <entry></entry>
      <entry spanname="col4_on">
Indication:
      </entry>
    </row>

    <row rowsep="0">
      <entry></entry>
      <entry></entry>
      <entry></entry>
      <entry></entry>
      <entry spanname="col5_on">
<function>Manage</function>
packet received with Session ID matching the session
attempting to connect to the display.
      </entry>
    </row>

    <row rowsep="0">
      <entry></entry>
      <entry></entry>
      <entry></entry>
      <entry spanname="col4_on">
Solution:
      </entry>
    </row>

    <row rowsep="0">
      <entry></entry>
      <entry></entry>
      <entry></entry>
      <entry></entry>
      <entry spanname="col5_on">
<function>Manage</function>
packet is ignored.  As the stream connection will either
succeed, which will result in an active session, or the stream will
eventually give up hope of connecting and send a
<function>Failed</function>
packet; no response to this
<function>Manage</function>
packet is necessary.
      </entry>
    </row>

    <row rowsep="0">
      <entry></entry>
      <entry spanname="col2_on">
Timeout/Retransmission policy:
      </entry>
    </row>

    <row rowsep="0">
      <entry></entry>
      <entry></entry>
      <entry spanname="col3_on">
Timeout after 2 seconds, exponential backoff to 32 seconds.  After no more
than 126 seconds, give up and report an error to the user.
      </entry>
    </row>

    <row rowsep="0">
      <entry spanname="col1_on">
<function>Refuse</function>
      </entry>
    </row>

    <row rowsep="0">
      <entry></entry>
      <entry spanname="col2_on">
        <para>
Manager -&gt; Display
        </para>
        <para>
Additional Fields:
        </para>
      </entry>
    </row>

    <row rowsep="0">
      <entry></entry>
      <entry></entry>
      <entry spanname="col3_on">
<emphasis remap='I'>Session ID</emphasis>: CARD32
      </entry>
    </row>

    <row rowsep="0">
      <entry></entry>
      <entry></entry>
      <entry></entry>
      <entry spanname="col4_on">
Should be set to the Session ID received in the
<function>Manage</function>
packet.
      </entry>
    </row>


    <row rowsep="0">
      <entry></entry>
      <entry spanname="col2_on">
Semantics:
      </entry>
    </row>

    <row rowsep="0">
      <entry></entry>
      <entry></entry>
      <entry spanname="col3_on">
A
<function>Refuse</function>
packet is sent by a manager when the Session ID received in the
<function>Manage</function>
packet does not match the current Session ID.
The display should assume that it received an old
<function>Accept</function>
packet and should resend its
<function>Request</function>
packet.
      </entry>
    </row>

    <row rowsep="0">
      <entry></entry>
      <entry spanname="col2_on">
Problems/Solutions:
      </entry>
    </row>

    <row rowsep="0">
      <entry></entry>
      <entry></entry>
      <entry spanname="col3_on">
Problem:
      </entry>
    </row>

    <row rowsep="0">
      <entry></entry>
      <entry></entry>
      <entry></entry>
      <entry spanname="col4_on">
Error message is lost.
      </entry>
    </row>

    <row rowsep="0">
      <entry></entry>
      <entry></entry>
      <entry></entry>
      <entry spanname="col4_on">
Indication:
      </entry>
    </row>

    <row rowsep="0">
      <entry></entry>
      <entry></entry>
      <entry></entry>
      <entry></entry>
      <entry spanname="col5_on">
Display times out waiting for
new connection,
<function>Refuse</function>
or
<function>Failed .</function>
      </entry>
    </row>

    <row rowsep="0">
      <entry></entry>
      <entry></entry>
      <entry></entry>
      <entry spanname="col4_on">
Solution:
      </entry>
    </row>

    <row rowsep="0">
      <entry></entry>
      <entry></entry>
      <entry></entry>
      <entry></entry>
      <entry spanname="col5_on">
Display resends
<function>Manage</function>
message.
      </entry>
    </row>



    <row rowsep="0">
      <entry></entry>
      <entry spanname="col2_on">
Timeout/Retransmission policy:
      </entry>
    </row>

    <row rowsep="0">
      <entry></entry>
      <entry></entry>
      <entry spanname="col3_on">
Like all packets sent from a manager to a display, this packet should never be
retransmitted.
      </entry>
    </row>

    <row rowsep="0">
      <entry spanname="col1_on">
<function>Failed</function>
      </entry>
    </row>

    <row rowsep="0">
      <entry></entry>
      <entry spanname="col2_on">
        <para>
Manager -&gt; Display
        </para>
        <para>
Additional Fields:
        </para>
      </entry>
    </row>

    <row rowsep="0">
      <entry></entry>
      <entry></entry>
      <entry spanname="col3_on">
<emphasis remap='I'>Session ID</emphasis>: CARD32
      </entry>
    </row>

    <row rowsep="0">
      <entry></entry>
      <entry></entry>
      <entry></entry>
      <entry spanname="col4_on">
Should be set to the Session ID received in the
<function>Manage</function>
packet.
      </entry>
    </row>

    <row rowsep="0">
      <entry></entry>
      <entry></entry>
      <entry spanname="col3_on">
<emphasis remap='I'>Status</emphasis>: ARRAY8
      </entry>
    </row>

    <row rowsep="0">
      <entry></entry>
      <entry></entry>
      <entry></entry>
      <entry spanname="col4_on">
Is a human readable string indicating the reason for failure.
      </entry>
    </row>


    <row rowsep="0">
      <entry></entry>
      <entry spanname="col2_on">
Semantics:
      </entry>
    </row>

    <row rowsep="0">
      <entry></entry>
      <entry></entry>
      <entry spanname="col3_on">
A
<function>Failed</function>
packet is sent by a manager when it has problems establishing
the initial X connection in response to the
<function>Manage</function>
packet.
      </entry>
    </row>

    <row rowsep="0">
      <entry></entry>
      <entry spanname="col2_on">
Problems/Solutions
      </entry>
    </row>

    <row rowsep="0">
      <entry></entry>
      <entry></entry>
      <entry spanname="col3_on">
Same as for
<function>Refuse .</function>
      </entry>
    </row>

    <row rowsep="0">
      <entry spanname="col1_on">
<function>KeepAlive</function>
      </entry>
    </row>

    <row rowsep="0">
      <entry></entry>
      <entry spanname="col2_on">
        <para>
Display -&gt; Manager
        </para>
        <para>
Additional Fields:
        </para>
      </entry>
    </row>

    <row rowsep="0">
      <entry></entry>
      <entry></entry>
      <entry spanname="col3_on">
<emphasis remap='I'>Display Number</emphasis>: CARD16
      </entry>
    </row>

    <row rowsep="0">
      <entry></entry>
      <entry></entry>
      <entry></entry>
      <entry spanname="col4_on">
Set to the display index for the display host.
      </entry>
    </row>

    <row rowsep="0">
      <entry></entry>
      <entry></entry>
      <entry spanname="col3_on">
<emphasis remap='I'>Session ID</emphasis>: CARD32
      </entry>
    </row>

    <row rowsep="0">
      <entry></entry>
      <entry></entry>
      <entry></entry>
      <entry spanname="col4_on">
Should be set to the Session ID received in the
<function>Manage</function>
packet during the negotiation for the current session.
      </entry>
    </row>

    <row rowsep="0">
      <entry></entry>
      <entry spanname="col2_on">
Sematics:
      </entry>
    </row>

    <row rowsep="0">
      <entry></entry>
      <entry></entry>
      <entry spanname="col3_on">
        <para>
A
<function>KeepAlive</function>
packet can be sent at any time during the session by a
display to discover if the manager is running.
The manager should respond with
<function>Alive</function>
whenever it receives this type of packet.
        </para>
        <para>
This allows the display to discover when the manager host
is no longer running.
A display is not required to send
<function>KeepAlive</function>
packets and, upon lack of receipt of
<function>Alive</function>
packets, is not required to perform any specific action.
        </para>
        <para>
The expected use of this packet is to terminate an active session when the
manager host or network link fails.  The display should keep track of the
time since any packet has been received from the manager host and use
<function>KeepAlive</function>
packets when a substantial time has elapsed since the
most recent packet.
        </para>
      </entry>
    </row>

    <row rowsep="0">
      <entry></entry>
      <entry spanname="col2_on">
Valid Responses:
      </entry>
    </row>

    <row rowsep="0">
      <entry></entry>
      <entry></entry>
      <entry spanname="col3_on">
<function>Alive</function>
      </entry>
    </row>

    <row rowsep="0">
      <entry></entry>
      <entry spanname="col2_on">
Problems/Solutions:
      </entry>
    </row>

    <row rowsep="0">
      <entry></entry>
      <entry></entry>
      <entry spanname="col3_on">
Problem:
      </entry>
    </row>

    <row rowsep="0">
      <entry></entry>
      <entry></entry>
      <entry></entry>
      <entry spanname="col4_on">
Manager does not receive the packet or display does not receive the response.
      </entry>
    </row>


    <row rowsep="0">
      <entry></entry>
      <entry></entry>
      <entry></entry>
      <entry spanname="col4_on">
Indication:
      </entry>
    </row>

    <row rowsep="0">
      <entry></entry>
      <entry></entry>
      <entry></entry>
      <entry></entry>
      <entry spanname="col5_on">
No
<function>Alive</function>
packet is returned.
      </entry>
    </row>

    <row rowsep="0">
      <entry></entry>
      <entry></entry>
      <entry></entry>
      <entry spanname="col4_on">
Solution:
      </entry>
    </row>

    <row rowsep="0">
      <entry></entry>
      <entry></entry>
      <entry></entry>
      <entry></entry>
      <entry spanname="col5_on">
Retransmit the packet with an exponential backoff; start at 2 seconds and
assume the host is not up after no less than 30 seconds.
      </entry>
    </row>

    <row rowsep="0">
      <entry spanname="col1_on">
<function>Alive</function>
      </entry>
    </row>

    <row rowsep="0">
      <entry></entry>
      <entry spanname="col2_on">
        <para>
Manager -&gt; Display
        </para>
        <para>
Additional Fields:
        </para>
      </entry>
    </row>

    <row rowsep="0">
      <entry></entry>
      <entry></entry>
      <entry spanname="col3_on">
<emphasis remap='I'>Session Running</emphasis>: CARD8
      </entry>
    </row>

    <row rowsep="0">
      <entry></entry>
      <entry></entry>
      <entry></entry>
      <entry spanname="col4_on">
Indicates that the session identified by Session ID is
currently active.  The value is zero if no session is active
or one if a session
is active.
      </entry>
    </row>

    <row rowsep="0">
      <entry></entry>
      <entry></entry>
      <entry spanname="col3_on">
<emphasis remap='I'>Session ID</emphasis>: CARD32
      </entry>
    </row>

    <row rowsep="0">
      <entry></entry>
      <entry></entry>
      <entry></entry>
      <entry spanname="col4_on">
Specifies the ID of the currently running session; if any.
When no session is active this field should be zero.
      </entry>
    </row>


    <row rowsep="0">
      <entry></entry>
      <entry spanname="col2_on">
Semantics:
      </entry>
    </row>

    <row rowsep="0">
      <entry></entry>
      <entry></entry>
      <entry spanname="col3_on">
An
<function>Alive</function>
packet is sent in response to a
<function>KeepAlive</function>
request.
If a session is currently active on the display, the manager includes the
Session ID in the packet.  The display can use this information to
determine the status of the manager.
      </entry>
    </row>
  </tbody>
  </tgroup>
</informaltable>

</sect1>

<sect1 id="Session_Termination">
<title>Session Termination</title>
<para>
When the session is over, the initial connection with the display (the one
that acknowledges the
<function>Manage</function>
packet) will be closed by the manager.
If only a single session was active on the display,
all other connections should be closed by the display
and the display should be reset.  If multiple sessions
are active simultaneously and the display can identify which connections
belong to the terminated sesssion, those connections should be closed.
Otherwise, all connections should be closed and the display reset only when
all sessions have been terminated (that is, all initial connections closed).
</para>

<para>
The session may also be terminated at any time by the display if the
managing host no longer responds to
<function>KeepAlive</function>
packets.
The exact time-outs for sending
<function>KeepAlive</function>
packets is not specified in this protocol as the trade off
should not be fixed between loading an otherwise idle system with spurious
<function>KeepAlive</function>
packets and not noticing that the manager host is down for a long time.
</para>
</sect1>

<sect1 id="State_Diagrams">
<title>State Diagrams</title>
<!-- .XS -->
<!-- (SN State Diagrams -->
<!-- .XE -->
<para>
<!-- .LP -->
The following state diagrams are designed to cover all actions of both
the display and the manager.  Any packet that is received out-of-sequence
will be ignored.
</para>
<para>
<!-- .LP -->
Display:
</para>

<variablelist>
  <varlistentry>
    <term><emphasis remap='I'>start</emphasis>:</term>
    <listitem>
      <para>
User-requested connect to one host -&gt; <emphasis remap='I'>query</emphasis>
      </para>
      <para>
User-requested connect to some host -&gt; <emphasis remap='I'>broadcast</emphasis>
      </para>
      <para>
User-requested connect to site host-list -&gt; <emphasis remap='I'>indirect</emphasis>
      </para>
    </listitem>
  </varlistentry>
  <varlistentry>
    <term><emphasis remap='I'>query</emphasis>:</term>
    <listitem>
      <para>
Send <function>Query</function> packet
-&gt; <emphasis remap='I'>collect-query</emphasis>
      </para>
    </listitem>
  </varlistentry>
  <varlistentry>
    <term><emphasis remap='I'>collect-query</emphasis>:</term>
    <listitem>
      <para>
Receive <function>Willing</function> -&gt;
<emphasis remap='I'>start-connection</emphasis>
      </para>
      <para>
Receive <function>Unwilling</function> -&gt;
<emphasis remap='I'>stop-connection</emphasis>
      </para>
      <para>
Timeout -&gt; <emphasis remap='I'>query</emphasis>
      </para>
    </listitem>
  </varlistentry>

  <varlistentry>
    <term><emphasis remap='I'>broadcast</emphasis>:</term>
    <listitem>
      <para>
Send <function>BroadcastQuery</function> packet
      </para>
      <para>
-&gt; <emphasis remap='I'>collect-broadcast-query</emphasis>
      </para>
    </listitem>
  </varlistentry>
  <varlistentry>
    <term><emphasis remap='I'>collect-broadcast-query</emphasis>:</term>
    <listitem>
      <para>
Receive <function>Willing</function> -&gt;
<emphasis remap='I'>update-broadcast-willing</emphasis>
      </para>
      <para>
User-requested connect to one host -&gt;
<emphasis remap='I'>start-connection</emphasis>
      </para>
      <para>
Timeout -&gt; <emphasis remap='I'>broadcast</emphasis>
      </para>
    </listitem>
  </varlistentry>
  <varlistentry>
    <term><emphasis remap='I'>update-broadcast-willing</emphasis>:</term>
    <listitem>
      <para>
Add new host to the host list presented to the user
      </para>
      <para>
-&gt; <emphasis remap='I'>collect-broadcast-query</emphasis>
      </para>
    </listitem>
  </varlistentry>
  <varlistentry>
    <term><emphasis remap='I'>indirect</emphasis>:</term>
    <listitem>
      <para>
Send <function>IndirectQuery</function> packet
      </para>
      <para>
-&gt; <emphasis remap='I'>collect-indirect-query</emphasis>
      </para>
    </listitem>
  </varlistentry>
  <varlistentry>
    <term><emphasis remap='I'>collect-indirect-query</emphasis>:</term>
    <listitem>
      <para>
Receive <function>Willing</function> -&gt;
<emphasis remap='I'>update-indirect-willing</emphasis>
      </para>
      <para>
User-requested connect to one host -&gt;
<emphasis remap='I'>start-connection</emphasis>
      </para>
      <para>
Timeout -&gt; <emphasis remap='I'>indirect</emphasis>
      </para>
    </listitem>
  </varlistentry>
  <varlistentry>
    <term><emphasis remap='I'>update-indirect-willing</emphasis>:</term>
    <listitem>
      <para>
Add new host to the host list presented to the user
      </para>
      <para>
-&gt; <emphasis remap='I'>collect-indirect-query</emphasis>
      </para>
    </listitem>
  </varlistentry>

  <varlistentry>
    <term><emphasis remap='I'>start-connection</emphasis>:</term>
    <listitem>
      <para>
Send <function>Request</function> packet
      </para>
      <para>
-&gt; <emphasis remap='I'>await-request-response</emphasis>
      </para>
    </listitem>
  </varlistentry>
  <varlistentry>
    <term><emphasis remap='I'>await-request-response</emphasis>:</term>
    <listitem>
      <para>
Receive <function>Accept</function> -&gt;
<emphasis remap='I'>manage</emphasis>
      </para>
      <para>
Receive <function>Decline</function> -&gt;
<emphasis remap='I'>stop-connection</emphasis>
      </para>
      <para>
Timeout -&gt; <emphasis remap='I'>start-connection</emphasis>
      </para>
    </listitem>
  </varlistentry>
  <varlistentry>
    <term><emphasis remap='I'>manage</emphasis>:</term>
    <listitem>
      <para>
Save Session ID
      </para>
      <para>
Send <function>Manage</function> packet with Session ID
      </para>
      <para>
-&gt; <emphasis remap='I'>await-manage-response</emphasis>
      </para>
    </listitem>
  </varlistentry>
  <varlistentry>
    <term><emphasis remap='I'>await-manage-response</emphasis>:</term>
    <listitem>
      <para>
Receive <function>XOpenDisplay :</function> -&gt;
<emphasis remap='I'>run-session</emphasis>
      </para>
      <para>
Receive <function>Refuse</function> with matching Session ID
-&gt; <emphasis remap='I'>start-connection</emphasis>
      </para>
      <para>
Receive <function>Failed</function> with matching Session ID
-&gt; <emphasis remap='I'>stop-connection</emphasis>
      </para>
      <para>
Timeout -&gt; <emphasis remap='I'>manage</emphasis>
      </para>
    </listitem>
  </varlistentry>
  <varlistentry>
    <term><emphasis remap='I'>stop-connection</emphasis>:</term>
    <listitem>
      <para>
Display cause of termination to user
      </para>
      <para>
-&gt; <emphasis remap='I'>start</emphasis>
      </para>
    </listitem>
  </varlistentry>
  <varlistentry>
    <term><emphasis remap='I'>run-session</emphasis>:</term>
    <listitem>
      <para>
Decide to send <function>KeepAlive</function> packet -&gt;
<emphasis remap='I'>keep-alive</emphasis>
      </para>
      <para>
wait close of first display connection
      </para>
      <para>
-&gt; <emphasis remap='I'>reset-display</emphasis>
      </para>
    </listitem>
  </varlistentry>
  <varlistentry>
    <term><emphasis remap='I'>keep-alive</emphasis>:</term>
    <listitem>
      <para>
Send <function>KeepAlive</function> packet with current Session ID
      </para>
      <para>
-&gt; <emphasis remap='I'>await-alive</emphasis>
      </para>
    </listitem>
  </varlistentry>
  <varlistentry>
    <term><emphasis remap='I'>await-alive</emphasis>:</term>
    <listitem>
      <para>
Receive <function>Alive</function> with matching Session ID -&gt;
<emphasis remap='I'>run-session</emphasis>
      </para>
      <para>
Receive <function>Alive</function> with nonmatching Session ID
or FALSE Session Running -&gt; <emphasis remap='I'>reset-display</emphasis>
      </para>
      <para>
Final timeout without receiving <function>Alive</function>
packet -&gt; <emphasis remap='I'>reset-display</emphasis>
      </para>
      <para>
Timeout -&gt; <emphasis remap='I'>keep-alive</emphasis>
      </para>
    </listitem>
  </varlistentry>
  <varlistentry>
    <term><emphasis remap='I'>reset-display</emphasis>:</term>
    <listitem>
      <para>
(if possible) -&gt; close all display connections associated with this session
      </para>
      <para>
Last session -&gt; close all display connections
      </para>
      <para>
-&gt; <emphasis remap='I'>start</emphasis>
      </para>
    </listitem>
  </varlistentry>
</variablelist>


<para>
Manager:
</para>

<variablelist>
  <varlistentry>
    <term><emphasis remap='I'>idle</emphasis>:</term>
    <listitem>
      <para>
Receive <function>Query</function> -&gt;
<emphasis remap='I'>query-respond</emphasis>
      </para>
      <para>
Receive
<function>BroadcastQuery</function>
-&gt; <emphasis remap='I'>broadcast-respond</emphasis>
      </para>
      <para>
Receive
<function>IndirectQuery</function>
-&gt; <emphasis remap='I'>indirect-respond</emphasis>
      </para>
      <para>
Receive
<function>ForwardQuery</function>
-&gt; <emphasis remap='I'>forward-respond</emphasis>
Receive
      </para>
      <para>
<function>Request</function>
-&gt; <emphasis remap='I'>request-respond</emphasis>
      </para>
      <para>
Receive
<function>Manage</function>
-&gt; <emphasis remap='I'>manage</emphasis>
      </para>
      <para>
An active session terminates
-&gt; <emphasis remap='I'>finish-session</emphasis>
      </para>
      <para>
Receive <function>KeepAlive</function>
-&gt; <emphasis remap='I'>send-alive</emphasis>
      </para>
      <para>
-&gt; <emphasis remap='I'>idle</emphasis>
      </para>
    </listitem>
  </varlistentry>
  <varlistentry>
    <term><emphasis remap='I'>query-respond</emphasis>:</term>
    <listitem>
      <para>
If willing to manage -&gt; <emphasis remap='I'>send-willing</emphasis>
      </para>
      <para>
-&gt; <emphasis remap='I'>send-unwilling</emphasis>
      </para>
    </listitem>
  </varlistentry>
  <varlistentry>
    <term><emphasis remap='I'>broadcast-respond</emphasis>:</term>
    <listitem>
      <para>
If willing to manage -&gt; <emphasis remap='I'>send-willing</emphasis>
      </para>
      <para>
-&gt; <emphasis remap='I'>idle</emphasis>
      </para>
    </listitem>
  </varlistentry>
  <varlistentry>
    <term><emphasis remap='I'>indirect-respond</emphasis>:</term>
    <listitem>
      <para>
Send <function>ForwardQuery</function>
packets to all managers on redirect list
      </para>
      <para>
If willing to manage -&gt; <emphasis remap='I'>send-willing</emphasis>
      </para>
      <para>
-&gt; <emphasis remap='I'>idle</emphasis>
      </para>
    </listitem>
  </varlistentry>
  <varlistentry>
    <term><emphasis remap='I'>forward-respond</emphasis>:</term>
    <listitem>
      <para>
Decode destination address, if willing to manage -&gt;
<emphasis remap='I'>send-willing</emphasis>
      </para>
      <para>
-&gt; <emphasis remap='I'>idle</emphasis>
      </para>
    </listitem>
  </varlistentry>
  <varlistentry>
    <term><emphasis remap='I'>send-willing</emphasis>:</term>
    <listitem>
      <para>
Send <function>Willing</function> packet
      </para>
      <para>
-&gt; <emphasis remap='I'>idle</emphasis>
      </para>
    </listitem>
  </varlistentry>
  <varlistentry>
    <term><emphasis remap='I'>send-unwilling</emphasis>:</term>
    <listitem>
      <para>
Send <function>Unwilling</function> packet
-&gt; <emphasis remap='I'>idle</emphasis>
      </para>
    </listitem>
  </varlistentry>
  <varlistentry>
    <term><emphasis remap='I'>request-respond</emphasis>:</term>
    <listitem>
      <para>
If manager is willing to allow a session on display
-&gt; <emphasis remap='I'>accept-session</emphasis>
      </para>
      <para>
-&gt; <emphasis remap='I'>decline-session</emphasis>
      </para>
    </listitem>
  </varlistentry>
  <varlistentry>
    <term><emphasis remap='I'>accept-session</emphasis>:</term>
    <listitem>
      <para>
Generate Session ID and save Session ID, display address, and
display number somewhere
      </para>
      <para>
Send <function>Accept</function> packet
      </para>
      <para>
-&gt; <emphasis remap='I'>idle</emphasis>
      </para>
    </listitem>
  </varlistentry>
  <varlistentry>
    <term><emphasis remap='I'>decline-session</emphasis>:</term>
    <listitem>
      <para>
Send <function>Decline</function> packet
      </para>
      <para>
-&gt; <emphasis remap='I'>idle</emphasis>
      </para>
    </listitem>
  </varlistentry>
  <varlistentry>
    <term><emphasis remap='I'>manage</emphasis>:</term>
    <listitem>
      <para>
If Session ID matches saved Session ID -&gt;
<emphasis remap='I'>run-session</emphasis>
      </para>
      <para>
If Session ID matches Session ID of session in process of
starting up, or currently active session -&gt;
<emphasis remap='I'>idle</emphasis>
      </para>
      <para>
-&gt; <emphasis remap='I'>refuse</emphasis>
      </para>
    </listitem>
  </varlistentry>
  <varlistentry>
    <term><emphasis remap='I'>refuse</emphasis>:</term>
    <listitem>
      <para>
Send <function>Refuse</function> packet
      </para>
      <para>
-&gt; <emphasis remap='I'>idle</emphasis>
      </para>
    </listitem>
  </varlistentry>
  <varlistentry>
    <term><emphasis remap='I'>run-session</emphasis>:</term>
    <listitem>
      <para>
Terminate any session in progress
      </para>
      <para>
<function>XOpenDisplay</function>
      </para>
      <para>
Open display succeeds -&gt;
<emphasis remap='I'>start-session</emphasis>
      </para>
      <para>
-&gt; <emphasis remap='I'>failed</emphasis>
      </para>
    </listitem>
  </varlistentry>
  <varlistentry>
    <term><emphasis remap='I'>failed</emphasis>:</term>
    <listitem>
      <para>
Send <function>Failed</function> packet
      </para>
      <para>
-&gt; <emphasis remap='I'>idle</emphasis>
      </para>
    </listitem>
  </varlistentry>
  <varlistentry>
    <term><emphasis remap='I'>start-session</emphasis>:</term>
    <listitem>
      <para>
Start a new session
      </para>
      <para>
-&gt; <emphasis remap='I'>idle</emphasis>
      </para>
    </listitem>
  </varlistentry>
  <varlistentry>
    <term><emphasis remap='I'>finish-session</emphasis>:</term>
    <listitem>
      <para>
<function>XCloseDisplay</function>
      </para>
      <para>
-&gt; <emphasis remap='I'>idle</emphasis>
      </para>
    </listitem>
  </varlistentry>
  <varlistentry>
    <term><emphasis remap='I'>send-alive</emphasis>:</term>
    <listitem>
      <para>
Send <function>Alive</function> packet containing current status
      </para>
      <para>
-&gt; <emphasis remap='I'>idle</emphasis>
      </para>
    </listitem>
  </varlistentry>
</variablelist>

</sect1>

<sect1 id="Protocol_Encoding">
<title>Protocol Encoding</title>
<para>
When XDMCP is implemented on top of the Internet User Datagram Protocol (UDP),
port number 177 is to be used. When using UDP over IPv4, Broadcast Query
packets are sent via UDP broadcast.  When using UDP over IPv6, Broadcast Query
packets are sent via multicast, either to an address in the IANA registered
XDMCP multicast address range of
FF0<emphasis remap='I'>X</emphasis>:0:0:0:0:0:0:12B
(where the <emphasis remap='I'>X</emphasis> is replaced by a valid scope id)
or to a locally assigned
multicast address. The version number in all packets will be 1.
Packet opcodes are 16-bit integers.
</para>

<informaltable frame="none">
  <tgroup cols='2' align='left'>
  <colspec colname='c1' colsep="0"/>
  <colspec colname='c2' colsep="0"/>
  <thead>
    <row>
      <entry>Packet Name</entry>
      <entry>Encoding</entry>
    </row>
  </thead>
  <tbody>
    <row rowsep="0">
      <entry>BroadcastQuery</entry>
      <entry>1</entry>
    </row>
    <row rowsep="0">
      <entry>Query</entry>
      <entry>2</entry>
    </row>
    <row rowsep="0">
      <entry>IndirectQuery</entry>
      <entry>3</entry>
    </row>
    <row rowsep="0">
      <entry>ForwardQuery</entry>
      <entry>4</entry>
    </row>
    <row rowsep="0">
      <entry>Willing</entry>
      <entry>5</entry>
    </row>
    <row rowsep="0">
      <entry>Unwilling</entry>
      <entry>6</entry>
    </row>
    <row rowsep="0">
      <entry>Request</entry>
      <entry>7</entry>
    </row>
    <row rowsep="0">
      <entry>Accept</entry>
      <entry>8</entry>
    </row>
    <row rowsep="0">
      <entry>Decline</entry>
      <entry>9</entry>
    </row>
    <row rowsep="0">
      <entry>Manage</entry>
      <entry>10</entry>
    </row>
    <row rowsep="0">
      <entry>Refuse</entry>
      <entry>11</entry>
    </row>
    <row rowsep="0">
      <entry>Failed</entry>
      <entry>12</entry>
    </row>
    <row rowsep="0">
      <entry>KeepAlive</entry>
      <entry>13
<footnote><para>
A previous version of this document incorrectly reversed the opcodes of
<emphasis role="bold">Alive</emphasis> and
<emphasis role="bold">KeepAlive</emphasis>.
</para></footnote>
      </entry>
    </row>
    <row rowsep="0">
      <entry>Alive</entry>
      <entry>14
<footnote><para>
A previous version of this document incorrectly reversed the opcodes of
<emphasis role="bold">Alive</emphasis> and
<emphasis role="bold">KeepAlive</emphasis>.
</para></footnote>
      </entry>
    </row>
  </tbody>
  </tgroup>
</informaltable>

<para>
Per packet information follows:
</para>

<para>
<function>Query</function>,
<function>BroadcastQuery</function>,
<function>IndirectQuery</function>
</para>

<literallayout class="monospaced">
     2     CARD16     version number (always 1)
     2     CARD16     opcode (always Query, BroadcastQuery or IndirectQuery)
     2     CARD16     length
     1     CARD8      number of Authentication Names sent (m)
     2     CARD16     length of first Authentication Name (m<emphasis role="subscript">1</emphasis>)
     m<emphasis role="subscript">1</emphasis>   CARD8     first Authentication Name
     ...          Other Authentication Names
</literallayout>

<para>
Note that these three packets are identical except for the opcode field.
</para>

<literallayout class="monospaced">
<function>ForwardQuery</function>
     2     CARD16     version number (always 1)
     2     CARD16     opcode (always ForwardQuery)
     2     CARD16     length
     2     CARD16     length of Client Address (m)
     m     CARD8     Client Address
     2     CARD16     length of Client Port (n)
     n     CARD8     Client Port
     1     CARD8     number of Authentication Names sent (o)
     2     CARD16     length of first Authentication Name (o<emphasis role="subscript">1</emphasis>)
     o<emphasis role="subscript">1</emphasis>     CARD8     first Authentication Name
     ...          Other Authentication Names
</literallayout>

<literallayout class="monospaced">
<function>Willing</function>
     2     CARD16     version number (always 1)
     2     CARD16     opcode (always Willing)
     2     CARD16     length (6 + m + n + o)
     2     CARD16     Length of Authentication Name (m)
     m     CARD8      Authentication Name
     2     CARD16     Hostname length (n)
     n     CARD8      Hostname
     2     CARD16     Status length (o)
     o     CARD8      Status
</literallayout>

<literallayout class="monospaced">
<function>Unwilling</function>
     2     CARD16     version number (always 1)
     2     CARD16     opcode (always Unwilling)
     2     CARD16     length (4 + m + n)
     2     CARD16     Hostname length (m)
     m     CARD8      Hostname
     2     CARD16     Status length (n)
     n     CARD8      Status
</literallayout>

<literallayout class="monospaced">
<function>Request</function>
     2     CARD16     version number (always 1)
     2     CARD16     opcode (always Request)
     2     CARD16     length
     2     CARD16     Display Number
     1     CARD8      Count of Connection Types (m)
     2xm   CARD16     Connection Types
     1     CARD8      Count of Connection Addresses (n)
     2     CARD16     Length of first Connection Address (n<emphasis role="subscript">1</emphasis>)
     n<emphasis role="subscript">1</emphasis>     CARD8      First Connection Address
     ...              Other connection addresses
     2     CARD16     Length of Authentication Name (o)
     o     CARD8      Authentication Name
     2     CARD16     Length of Authentication Data (p)
     p     CARD8      Authentication Data
     1     CARD8      Count of Authorization Names (q)
     2     CARD16     Length of first Authorization Name (q<emphasis role="subscript">1</emphasis>)
     q<emphasis role="subscript">1</emphasis>     CARD8      First Authorization Name
     ...              Other authorization names
     2     CARD16     Length of Manufacturer Display ID (r)
     r     CARD8      Manufacturer Display ID
</literallayout>

<literallayout class="monospaced">
<function>Accept</function>
     2     CARD16     version number (always 1)
     2     CARD16     opcode (always Accept)
     2     CARD16     length (12 + n + m + o + p)
     4     CARD32     Session ID
     2     CARD16     Length of Authentication Name (n)
     n     CARD8      Authentication Name
     2     CARD16     Length of Authentication Data (m)
     m     CARD8      Authentication Data
     2     CARD16     Length of Authorization Name (o)
     o     CARD8      Authorization Name
     2     CARD16     Length of Authorization Data (p)
     p     CARD8      Authorization Data
</literallayout>

<literallayout class="monospaced">
<function>Decline</function>
     2     CARD16     version number (always 1)
     2     CARD16     opcode (always Decline)
     2     CARD16     length (6 + m + n + o)
     2     CARD16     Length of Status (m)
     m     CARD8      Status
     2     CARD16     Length of Authentication Name (n)
     n     CARD8      Authentication Name
     2     CARD16     Length of Authentication Data (o)
     o     CARD8      Authentication Data
</literallayout>

<literallayout class="monospaced">
<function>Manage</function>
     2     CARD16     version number (always 1)
     2     CARD16     opcode (always Manage)
     2     CARD16     length (8 + m)
     4     CARD32     Session ID
     2     CARD16     Display Number
     2     CARD16     Length of Display Class (m)
     m     CARD8      Display Class
</literallayout>

<literallayout class="monospaced">
<function>Refuse</function>
     2     CARD16     version number (always 1)
     2     CARD16     opcode (always Refuse)
     2     CARD16     length (4)
     4     CARD32     Session ID
</literallayout>

<literallayout class="monospaced">
<function>Failed</function>
     2     CARD16     version number (always 1)
     2     CARD16     opcode (always Failed)
     2     CARD16     length (6 + m)
     4     CARD32     Session ID
     2     CARD16     Length of Status (m)
     m     CARD8      Status
</literallayout>

<literallayout class="monospaced">
<function>KeepAlive</function>
     2     CARD16     version number (always 1)
     2     CARD16     opcode (always KeepAlive)
     2     CARD16     length (6)
     2     CARD16     Display Number
     4     CARD32     Session ID
</literallayout>

<literallayout class="monospaced">
<function>Alive</function>
     2     CARD16     version number (always 1)
     2     CARD16     opcode (always Alive)
     2     CARD16     length (5)
     1     CARD8      Session Running (0: not running 1: running)
     4     CARD32     Session ID (0: not running)
</literallayout>
</sect1>

<sect1 id="Display_Class_Format">
<title>Display Class Format</title>
<!-- .XS -->
<!-- (SN Display Class Format -->
<!-- .XE -->
<para>
The Display Class field of the
<function>Manage</function>
packet is used by the display manager to collect common sorts of
displays into manageable groups.  This field is a string encoded of
ISO-LATIN-1 characters in the following format:
</para>

<literallayout class="monospaced">
<emphasis remap='I'>ManufacturerID</emphasis>-<emphasis remap='I'>ModelNumber</emphasis>
</literallayout>

<para>
Both elements of this string must exclude characters of the set
{ <function>-</function>,
<function>.</function>,
<function>:</function>,
<function>*</function>,
<function>?</function>,
<emphasis remap='I'>&lt;space&gt;</emphasis> }.
The ManufacturerID is a string that should be registered
with the X Consortium.
The ModelNumber is designed to identify characteristics of the display
within the manufacturer's product line.
This string should be documented in the users manual for the
particular device and  should probably not be specifiable by the
display user to avoid unexpected configuration errors.
</para>
</sect1>

<sect1 id="Manufacturer_Display_ID_Format">
<title>Manufacturer Display ID Format</title>
<!-- .XS -->
<!-- (SN Manufacturer Display ID Format -->
<!-- .XE -->
<para>
To authenticate the manager, the display and manager will share a private
key.
The manager, then, must be able to discover which key to use for a
particular device.
The Manufacturer Display ID field of the
<function>Request</function>
packet is intended for this purpose.  Typically, the manager host will
contain a map between this number and the key.  This field is intended to be
unique per display, possibly the ethernet address of the display in the form:
</para>

<literallayout class="monospaced">
-Ethernet-8:0:2b:a:f:d2
</literallayout>

<para>
It can also be a string of the form:
</para>

<literallayout class="monospaced">
<emphasis remap='I'>ManufacturerID</emphasis>-<emphasis remap='I'>ModelNumber</emphasis>-<emphasis remap='I'>SerialNumber</emphasis>
</literallayout>

<para>
The ManufacturerID, ModelNumber and SerialNumber are encoded using
ISO-LATIN-1 characters, excluding {
<function>-</function>,
<function>.</function>,
<function>*</function>,
<function>?</function>,
<emphasis remap='I'>&lt;space&gt;</emphasis> }
</para>

<para>
When the display is shipped to a customer, it should include both the
Manufacturer Display ID and the private key in the documentation set.
This information should not be modifiable by the display user.
</para>

</sect1>

<sect1 id="Authentication">
<title>Authentication</title>
<!-- .XS -->
<!-- (SN Authentication -->
<!-- .XE -->
<para>
In an environment where authentication is not needed, XDMCP can disable
authentication by having the display send empty Authentication Name
and Authentication Data fields in the
<function>Request</function>
packet.
In this case, the manager will not attempt to authenticate itself.
Other authentication protocols may be developed, depending on local needs.
</para>

<para>
In an unsecure environment, the display must be able to verify that the
source of the various packets is a trusted manager.  These packets will
contain authentication information.  As an example of such a system, the
following discussion describes the "XDM-AUTHENTICATION-1" authentication
system.  This system uses a 56-bit shared private key, and 64 bits of
authentication data.  An associated example X authorization protocol
"XDM-AUTHORIZATION-1" will also be discussed.  The 56-bit key is represented
as a 64-bit number in network order (big endian).  This means that the first
octet in the representation will be zero.  When incrementing a 64-bit value,
the 8 octets of data will be interpreted in network order (big endian).
That is, the last octet will be incremented, subsequent carries propogate
towards the first octet.
</para>

<para>
Assumptions:
</para>

<orderedlist>
  <listitem>
    <para>
The display and manager share a private key.  This key could be programmed
into the display by the manufacturer and shipped with the unit.  It must not
be available from the display itself, but should allow the value to be
modified in some way.  The system administrator would be responsible for
managing a database of terminal keys.
    </para>
  </listitem>
  <listitem>
    <para>
The display can generate random authentication numbers.
    </para>
  </listitem>
</orderedlist>

<para>
Some definitions first:
</para>

<!-- FIXME: -->
<!-- double chk math symbols, use mathml to display them? -->
<itemizedlist>
  <listitem>
    <para>
{D}= encryption of plain text D by key κ
    </para>
  </listitem>
  <listitem>
    <para>
{&#x394;}*&#x3BA; = decryption of crypto text &#x394; with key &#x3BA;
    </para>
  </listitem>
  <listitem>
    <para>
&tau; = private key shared by display and manager
    </para>
  </listitem>
  <listitem>
    <para>
&rho; = 64 bit random number generated by display
    </para>
  </listitem>
  <listitem>
    <para>
&alpha; = authentication data in XDMCP packets
    </para>
  </listitem>
  <listitem>
    <para>
&sigma; = per-session private key, generated by manager
    </para>
  </listitem>
  <listitem>
    <para>
&beta; = authorization data
    </para>
  </listitem>
</itemizedlist>

<para>
Encryption will use the Data Encryption Standard (DES, FIPS 46-3); blocks
shorter than 64 bits will be zero-filled
on the right to 64 bits.  Blocks longer than 64 bits will use block chaining:
</para>
<para>
{D}&#x3BA; = {D1 }&#x3BA; {D2 xor {D1 }&#x3BA; }&#x3BA;
</para>

<para>
The display generates the first authentication data in the
<function>Request</function>
packet:
</para>

<para>
&#945;Request = {&#961;}τ

</para>

<para>
For the <function>Accept</function>
packet, the manager decrypts the initial message and returns
αAccept:

</para>

<para>
&rho; = {&alpha; Request } *&tau;
</para>

<para>
&alpha; Accept = { &rho; + 1}&tau;
</para>

<para>
The <function>Accept</function>
packet also contains the authorization intended for use by
the X server.  A description of authorization type "XDM-AUTHORIZATION-1"
follows.
</para>

<para>
The <function>Accept</function>
packet contains the authorization name
"XDM-AUTHORIZATION-1".  The authorization data is the string:
</para>
<para>
β Accept = {σ}τ
</para>

<para>
To create authorization information for connection setup with the X server
using the XDM-AUTHORIZATION-1 authorization protocol, the client computes the
following:
</para>
<para>
N mark = "X client identifier"
</para>
<para>
T lineup = "Current time in seconds on client host (32 bits)"
</para>
<para>
β = {ρNT}σ
</para>

<para>
For TCP connections @N@ is 48 bits long and contains the 32-bit IPv4 address of
the client host followed by the 16-bit port number of the client socket.
Formats for other connections must be registered.
The resulting value, β, is 192 bits of authorization data that is sent
in the connection setup to the server.  The server receives the packet,
decrypts the contents.  To accept the connection, the following must hold:
</para>

<itemizedlist>
  <listitem>
    <para>
ρ must match the value generated for the most recent XDMCP negotiation.
    </para>
  </listitem>
  <listitem>
    <para>
T must be within 1200 seconds of the internally stored time.  If no time
been received before, the current time is set to @T@.
    </para>
  </listitem>
  <listitem>
    <para>
No packet containing the same pair (N, T) can have been received
in the last 1200 seconds (20 minutes).
    </para>
  </listitem>
</itemizedlist>
</sect1>
</chapter>
</book>