X Display Manager Control Protocol

X.Org Standard

Keith Packard

   Massachusetts Institute of Technology
   Laboratory for Computer Science
   X Consortium

   X Version 11

   Version 1.1

   Copyright © 1989, 2004 The Open Group

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   Table of Contents

   1. Purpose and Goals
   2. Overview of the Protocol
   3. Data Types
   4. Packet Format
   5. Protocol
   6. Session Termination
   7. State Diagrams
   8. Protocol Encoding
   9. Display Class Format
   10. Manufacturer Display ID Format
   11. Authentication

Chapter 1. Purpose and Goals

   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
   fork/exec sequence on the host.) An X terminal (screen,
   keyboard, mouse, processor, network interface) is a prime
   example of an autonomous display.

   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:
     * 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.
     * 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.
     * 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.

   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.

   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.

   Security is an important consideration and must be an integral
   part of the design. The important security goals in the context
   of XDMCP are:
     * 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.
     * It should be possible for the display and the login service
       to negotiate the authorization mechanism to be used for the
       standard X protocol.
     * It should be possible to provide the same level of security
       in verifying the login service as is provided by the
       negotiated authorization mechanism.
     * Because there are no firm standards yet in the area of
       security, XDMCP must be flexible enough to accomodate a
       variety of security mechanisms.

Chapter 2. Overview of the Protocol

   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.

   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.

   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.

   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.

Chapter 3. Data Types

   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.
   Type Name Length (Bytes) Description
   CARD8 1 A single byte unsigned integer
   CARD16 2 Two byte unsigned integer
   CARD32 4 Four byte unsigned integer
   ARRAY8 n+2 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
   ARRAY16 2*m+1 This is a CARD8 (m) which specifies the number of
   CARD16 values to follow
   ARRAY32 4*l+1 This is a CARD8 (l) which specifies the number of
   CARD32 values to follow
   ARRAYofARRAY8 ? This is a CARD8 which specifies the number of
   ARRAY8 values to follow.

Chapter 4. Packet Format

   All XDMCP packets have the following information:
   Length (Bytes) Field Type Description
   2              CARD16     version number
   2              CARD16     opcode packet header
   2              CARD16     n = length of remaining data in bytes
   n              ???        packet-specific data

   The fields are as follows:

   Version number

   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).

   Opcode

   This specifies what step of the protocol this packet represents
   and should contain one of the following values (encoding
   provided in section below): BroadcastQuery, Query,
   IndirectQuery, ForwardQuery, Willing, Unwilling, Request,
   Accept, Decline, Manage, Refuse, Failed, KeepAlive or Alive.

   Length of data in bytes

   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.

   Packets should be checked to make sure that they satisfy the
   following conditions:
    1. They must contain valid opcodes.
    2. The length of the remaining data should correspond to the
       sum of the lengths of the individual remaining data items.
    3. The opcode should be expected (a finite state diagram is
       given in a later section).
    4. If the packet is of type Manage or Refuse, the Session ID
       should match the value sent in the preceding Accept packet.

Chapter 5. Protocol

   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.
   Query
   BroadcastQuery
   IndirectQuery
     Display -> Manager
     Additional Fields:
       Authentication Names: ARRAYofARRAY8
         Specifies a list of authentication names that the display
   supports. The manager will choose one of these and return it in
   the Willing packet.
       Semantics


   A Query 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 Willing if it is
   willing to service the display or Unwilling if it is not.

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

   An IndirectQuery packet is sent to a well known manager that
   forwards the request to a larger collection of secondary
   managers using ForwardQuery 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 Willing packet in response to this packet.

   Each packet type has slightly different semantics:


   The Query packet is destined only for a single host. If the
   display is instructed to Query multiple managers, it will send
   multiple Query packets. The Query packet also demands a
   response from the manager, either Willing or Unwilling.

   The BroadcastQuery packet is sent to many hosts. Each manager
   that receives this packet will not respond with an Unwilling
   packet.

   The IndirectQuery packet is sent to only one manager with the
   request that the request be forwarded to a larger list of
   managers using ForwardQuery 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
   BroadcastQuery except that BroadcastQuery is not forwarded.
         Valid Responses:
       Willing, Unwilling
         Problems/Solutions:
       Problem:


   Not all managers receive the query packet.

   Indication:
           None if BroadcastQuery or IndirectQuery was sent, else
   failure to receive Willing.
         Solution:
           Repeatedly send the packet while waiting for user to
   choose a manager.
       Timeout/Retransmission policy:
         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.
   ForwardQuery


   Primary Manager -> Secondary Manager

   Additional Fields:
       Client Address: ARRAY8
         Specifies the network address of the client display.
       Client Port: ARRAY8
         Specifies an identification of the client task on the
   client display.
       Authentication Names: ARRAYofARRAY8
         Is a duplicate of Authentication Names array that was
   received in the IndirectQuery packet.
     Semantics:


   When primary manager receives a IndirectQuery packet, it is
   responsible for sending ForwardQuery packets to an appropriate
   list of managers that can provide service to the display using
   the same network type as the one the original IndirectQuery
   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 Willing packet to the display if it
   is willing to provide service.

   ForwardQuery packets are similar to BroadcastQuery packets in
   that managers that are not willing to service particular
   displays should not send a Unwilling packet.
     Valid Responses:
       Willing
     Problems/Solutions:
       Identical to BroadcastQuery
     Timeout/Retransmission policy:
       Like all packets sent from a manager, this packet should
   never be retransmitted.
   Willing


   Manager -> Display

   Additional Fields:
       Authentication Name: ARRAY8


   Specifies the authentication method, selected from the list
   offered in the Query , BroadcastQuery , or IndirectQuery packet
   that the manger expects the display to use in the subsequent
   Request packet. This choice should remain as constant as
   feasible so that displays that send multiple Query packets can
   use the Authentication Name from any Willing packet that
   arrives.

   The display is free to ignore managers that request an
   insufficient level of authentication.
       Hostname: ARRAY8
         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.
       Status: ARRAY8
         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.
     Semantics:
       A Willing packet is sent by managers that may service
   connections from this display. It is sent in response to either
   a Query , BroadcastQuery , or ForwardQuery but does not imply a
   commitment to provide service (for example, it may later decide
   that it has accepted enough connections already).
     Problems/Solutions:
       Problem:


   Willing not received by the display.

   Indication:
           None if BroadcastQuery or IndirectQuery was sent, else
   failure to receive Willing .
         Solution:
           The display should continue to send the query until a
   response is received.
     Timeout/Retransmission policy:
       Like all packets sent from the manager to the display, this
   packet should never be retransmitted.
   Unwilling


   Manager -> Display

   Additional Fields:
       The Hostname and Status fields as in the Willing packet.
   The Status field should indicate to the user a reason for the
   refusal of service.
     Semantics:
       An Unwilling packet is sent by managers in response to
   direct Query requests (as opposed to BroadcastQuery or
   IndirectQuery 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.
     Problems/Solutions:
       Problem:


   Unwilling not received by the display.

   Indication:
           Display fails to receive Unwilling .
         Solution:
           The display should continue to send Query messages
   until a response is received.
     Timeout/Retransmission policy:
       Like all packets sent from the manager to the display, this
   packet should never be retransmitted.
   Request


   Display -> Manager

   Additional Fields:
       Display Number: CARD16
         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.
       Connection Types: ARRAY16
         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.
       Connection Addresses: ARRAYofARRAY8
         For each connection type in the previous array, the
   corresponding entry in this array indicates the network address
   of the display device.


   Authentication Name: ARRAY8

   Authentication Data: ARRAY8
         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.
       Authorization Names: ARRAYofARRAY8
         Specifies which types of authorization the display
   supports. The manager may decide to reject displays with which
   it cannot perform authorization.
       Manufacturer Display ID: ARRAY8
         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.
     Semantics:
       A Request 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 Accept packet with a valid
   session ID and should be ready for a subsequent Manage request.
   Otherwise, it should return a Decline packet.
     Valid Responses:
       Accept , Decline
     Problems/Solutions:
       Problem:
         Request not received by manager.
         Indication:
           Display timeout waiting for response.
         Solution:
           Display resends Request message.
         Problem:
           Message received out of order by manager.
         Indication:
           None.
         Solution:
           Each time a Request is sent, the manager sends the
   Session ID associated with the next session in the Accept . 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 Accept will be discarded by the display.
     Timeout/Retransmission policy:
       Timeout after 2 seconds, exponential backoff to 32 seconds.
   After no more than 126 seconds, give up and report an error to
   the user.
   Accept


   Manager -> Display

   Additional Fields:
       Session ID: CARD32
         Identifies the session that can be started by the
   manager.


   Authentication Name: ARRAY8

   Authentication Data: ARRAY8
         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.


   Authorization Name: ARRAY8

   Authorization Data: ARRAY8
         Is the data sent to the display to indicate the type of
   authorization the manager will be using in the first call to
   XOpenDisplay after the Manage packet is received.
     Semantics:


   An Accept packet is sent by a manager in response to a Request
   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 Manage packet. The Session ID is a 32-bit number
   that is incremented each time an Accept packet is sent as it
   must be unique over a reasonably long period of time.

   If the authentication information is invalid, a Decline packet
   will be returned with an appropriate Status message.
     Problems/Solutions:
       Problem:
         Accept or Decline not received by display.
         Indication:
           Display timeout waiting for response to Request .
         Solution:
           Display resends Request message.
       Problem:
         Message received out of order by display.
         Indication:
           Display receives Accept after Manage has been sent.
         Solution:
           Display discards Accept messages after it has sent a
   Manage message.
     Timeout/Retransmission policy:
       Like all packets sent from the manager to the display, this
   packet should never be retransmitted.
   Decline


   Manager -> Display

   Additional Fields:
       Status: ARRAY8
         Is a human readable string indicating the reason for
   refusal of service.


   Authentication Name: ARRAY8

   Authentication Data: ARRAY8
         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.
     Semantics:
       A Decline packet is sent by a manager in response to a
   Request packet if the manager is unwilling to establish a
   connection for the display. This is allowed even if the manager
   had responded Willing to a previous query.
     Problems/Solutions:
       Same as for Accept .
     Timeout/Retransmission policy:
       Like all packets sent from a manager to a display, this
   packet should never be retransmitted.
   Manage


   Display -> Manager

   Additional Fields:
       Session ID: CARD32
         Should contain the nonzero session ID returned in the
   Accept packet.
       Display Number: CARD16
         Must match the value sent in the previous Request packet.
       Display Class: ARRAY8
         Specifies the class of the display. See the Display Class
   Format section, which discusses the format of this field.
     Semantics:
       A Manage 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 Refuse or Failed 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 Manage 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 Failed packet).
     Valid Responses:
       X connection with correct auth info, Refuse , Failed .
     Problems/Solutions:
       Problem:
         Manage not received by manager.
         Indication:
           Display timeout waiting for response.
         Solution:
           Display resends Manage message.
       Problem:
         Manage received out of order by manager.
         Indication:
           Session already in progress with matching Session ID.
         Solution:
           Manage packet ignored.
         Indication:
           Session ID does not match next Session ID.
         Solution:
           Refuse message is sent.
       Problem:
         Display cannot be opened on selected stream.
         Indication:
           Display connection setup fails.
         Solution:
           Failed message is sent including a human readable
   reason.
       Problem:
         Display open does not succeed before a second manage
   packet is received because of a timeout occuring in the
   display.
         Indication:
           Manage packet received with Session ID matching the
   session attempting to connect to the display.
         Solution:
           Manage 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
   Failed packet; no response to this Manage packet is necessary.
     Timeout/Retransmission policy:
       Timeout after 2 seconds, exponential backoff to 32 seconds.
   After no more than 126 seconds, give up and report an error to
   the user.
   Refuse


   Manager -> Display

   Additional Fields:
       Session ID: CARD32
         Should be set to the Session ID received in the Manage
   packet.
     Semantics:
       A Refuse packet is sent by a manager when the Session ID
   received in the Manage packet does not match the current
   Session ID. The display should assume that it received an old
   Accept packet and should resend its Request packet.
     Problems/Solutions:
       Problem:
         Error message is lost.
         Indication:
           Display times out waiting for new connection, Refuse or
   Failed .
         Solution:
           Display resends Manage message.
     Timeout/Retransmission policy:
       Like all packets sent from a manager to a display, this
   packet should never be retransmitted.
   Failed


   Manager -> Display

   Additional Fields:
       Session ID: CARD32
         Should be set to the Session ID received in the Manage
   packet.
       Status: ARRAY8
         Is a human readable string indicating the reason for
   failure.
     Semantics:
       A Failed packet is sent by a manager when it has problems
   establishing the initial X connection in response to the Manage
   packet.
     Problems/Solutions
       Same as for Refuse .
   KeepAlive


   Display -> Manager

   Additional Fields:
       Display Number: CARD16
         Set to the display index for the display host.
       Session ID: CARD32
         Should be set to the Session ID received in the Manage
   packet during the negotiation for the current session.
     Sematics:


   A KeepAlive 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 Alive whenever it receives this type of
   packet.

   This allows the display to discover when the manager host is no
   longer running. A display is not required to send KeepAlive
   packets and, upon lack of receipt of Alive packets, is not
   required to perform any specific action.

   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 KeepAlive packets when a
   substantial time has elapsed since the most recent packet.
     Valid Responses:
       Alive
     Problems/Solutions:
       Problem:
         Manager does not receive the packet or display does not
   receive the response.
         Indication:
           No Alive packet is returned.
         Solution:
           Retransmit the packet with an exponential backoff;
   start at 2 seconds and assume the host is not up after no less
   than 30 seconds.
   Alive


   Manager -> Display

   Additional Fields:
       Session Running: CARD8
         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.
       Session ID: CARD32
         Specifies the ID of the currently running session; if
   any. When no session is active this field should be zero.
     Semantics:
       An Alive packet is sent in response to a KeepAlive 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.

Chapter 6. Session Termination

   When the session is over, the initial connection with the
   display (the one that acknowledges the Manage 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).

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

Chapter 7. State Diagrams

   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.

   Display:

   start:

   User-requested connect to one host -> query

   User-requested connect to some host -> broadcast

   User-requested connect to site host-list -> indirect

   query:

   Send Query packet -> collect-query

   collect-query:

   Receive Willing -> start-connection

   Receive Unwilling -> stop-connection

   Timeout -> query

   broadcast:

   Send BroadcastQuery packet

   -> collect-broadcast-query

   collect-broadcast-query:

   Receive Willing -> update-broadcast-willing

   User-requested connect to one host -> start-connection

   Timeout -> broadcast

   update-broadcast-willing:

   Add new host to the host list presented to the user

   -> collect-broadcast-query

   indirect:

   Send IndirectQuery packet

   -> collect-indirect-query

   collect-indirect-query:

   Receive Willing -> update-indirect-willing

   User-requested connect to one host -> start-connection

   Timeout -> indirect

   update-indirect-willing:

   Add new host to the host list presented to the user

   -> collect-indirect-query

   start-connection:

   Send Request packet

   -> await-request-response

   await-request-response:

   Receive Accept -> manage

   Receive Decline -> stop-connection

   Timeout -> start-connection

   manage:

   Save Session ID

   Send Manage packet with Session ID

   -> await-manage-response

   await-manage-response:

   Receive XOpenDisplay : -> run-session

   Receive Refuse with matching Session ID -> start-connection

   Receive Failed with matching Session ID -> stop-connection

   Timeout -> manage

   stop-connection:

   Display cause of termination to user

   -> start

   run-session:

   Decide to send KeepAlive packet -> keep-alive

   wait close of first display connection

   -> reset-display

   keep-alive:

   Send KeepAlive packet with current Session ID

   -> await-alive

   await-alive:

   Receive Alive with matching Session ID -> run-session

   Receive Alive with nonmatching Session ID or FALSE Session
   Running -> reset-display

   Final timeout without receiving Alive packet -> reset-display

   Timeout -> keep-alive

   reset-display:

   (if possible) -> close all display connections associated with
   this session

   Last session -> close all display connections

   -> start

   Manager:

   idle:

   Receive Query -> query-respond

   Receive BroadcastQuery -> broadcast-respond

   Receive IndirectQuery -> indirect-respond

   Receive ForwardQuery -> forward-respond Receive

   Request -> request-respond

   Receive Manage -> manage

   An active session terminates -> finish-session

   Receive KeepAlive -> send-alive

   -> idle

   query-respond:

   If willing to manage -> send-willing

   -> send-unwilling

   broadcast-respond:

   If willing to manage -> send-willing

   -> idle

   indirect-respond:

   Send ForwardQuery packets to all managers on redirect list

   If willing to manage -> send-willing

   -> idle

   forward-respond:

   Decode destination address, if willing to manage ->
   send-willing

   -> idle

   send-willing:

   Send Willing packet

   -> idle

   send-unwilling:

   Send Unwilling packet -> idle

   request-respond:

   If manager is willing to allow a session on display ->
   accept-session

   -> decline-session

   accept-session:

   Generate Session ID and save Session ID, display address, and
   display number somewhere

   Send Accept packet

   -> idle

   decline-session:

   Send Decline packet

   -> idle

   manage:

   If Session ID matches saved Session ID -> run-session

   If Session ID matches Session ID of session in process of
   starting up, or currently active session -> idle

   -> refuse

   refuse:

   Send Refuse packet

   -> idle

   run-session:

   Terminate any session in progress

   XOpenDisplay

   Open display succeeds -> start-session

   -> failed

   failed:

   Send Failed packet

   -> idle

   start-session:

   Start a new session

   -> idle

   finish-session:

   XCloseDisplay

   -> idle

   send-alive:

   Send Alive packet containing current status

   -> idle

Chapter 8. Protocol Encoding

   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 FF0X:0:0:0:0:0:0:12B (where the X 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.
   Packet Name    Encoding
   BroadcastQuery 1
   Query          2
   IndirectQuery  3
   ForwardQuery   4
   Willing        5
   Unwilling      6
   Request        7
   Accept         8
   Decline        9
   Manage         10
   Refuse         11
   Failed         12
   KeepAlive      13 ^[a]
   Alive          14 ^[b]

   ^[a] A previous version of this document incorrectly reversed
   the opcodes of Alive and KeepAlive.

   ^[b] A previous version of this document incorrectly reversed
   the opcodes of Alive and KeepAlive.

   Per packet information follows:

   Query, BroadcastQuery, IndirectQuery
     2     CARD16     version number (always 1)
     2     CARD16     opcode (always Query, BroadcastQuery or IndirectQu
ery)
     2     CARD16     length
     1     CARD8      number of Authentication Names sent (m)
     2     CARD16     length of first Authentication Name (m1)
     m1   CARD8     first Authentication Name
     ...          Other Authentication Names

   Note that these three packets are identical except for the
   opcode field.
ForwardQuery
     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 (o1)
     o1     CARD8     first Authentication Name
     ...          Other Authentication Names

Willing
     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

Unwilling
     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

Request
     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 (n1)
     n1     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 (q1)
     q1     CARD8      First Authorization Name
     ...              Other authorization names
     2     CARD16     Length of Manufacturer Display ID (r)
     r     CARD8      Manufacturer Display ID

Accept
     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

Decline
     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

Manage
     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

Refuse
     2     CARD16     version number (always 1)
     2     CARD16     opcode (always Refuse)
     2     CARD16     length (4)
     4     CARD32     Session ID

Failed
     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

KeepAlive
     2     CARD16     version number (always 1)
     2     CARD16     opcode (always KeepAlive)
     2     CARD16     length (6)
     2     CARD16     Display Number
     4     CARD32     Session ID

Alive
     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)

Chapter 9. Display Class Format

   The Display Class field of the Manage 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:
ManufacturerID-ModelNumber

   Both elements of this string must exclude characters of the set
   { -, ., :, *, ?, <space> }. 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.

Chapter 10. Manufacturer Display ID Format

   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 Request 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:
-Ethernet-8:0:2b:a:f:d2

   It can also be a string of the form:
ManufacturerID-ModelNumber-SerialNumber

   The ManufacturerID, ModelNumber and SerialNumber are encoded
   using ISO-LATIN-1 characters, excluding { -, ., *, ?, <space> }

   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.

Chapter 11. Authentication

   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
   Request packet. In this case, the manager will not attempt to
   authenticate itself. Other authentication protocols may be
   developed, depending on local needs.

   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.

   Assumptions:
    1. 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.
    2. The display can generate random authentication numbers.

   Some definitions first:
     * {D}^k = encryption of plain text D by key k
     * {D}*^k = decryption of crypto text D with key k
     * t = private key shared by display and manager
     * r = 64 bit random number generated by display
     * a = authentication data in XDMCP packets
     * s = per-session private key, generated by manager
     * b = authorization data

   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:

   {D}^k = {D[1]}^k {D[2] xor {D[1]}^k}^k

   The display generates the first authentication data in the
   Request packet:

   a[Request] = {r}^t

   For the Accept packet, the manager decrypts the initial message
   and returns a[Accept]:

   r = {a[Request]}*^t

   a[Accept] = { r + 1}^t

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

   The Accept packet contains the authorization name
   "XDM-AUTHORIZATION-1". The authorization data is the string:

   b[Accept] = {s}^t

   To create authorization information for connection setup with
   the X server using the XDM-AUTHORIZATION-1 authorization
   protocol, the client computes the following:

   N = X client identifier

   T = Current time in seconds on client host (32 bits)

   b = {rNT}^s

   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, b, 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:
     * r must match the value generated for the most recent XDMCP
       negotiation.
     * T must be within 1200 seconds of the internally stored
       time. If no time been received before, the current time is
       set to T.
     * No packet containing the same pair (N, T) can have been
       received in the last 1200 seconds (20 minutes).
