X Display Manager Control Protocol

X.Org Standard

Keith Packard

X Consortium, Laboratory for Computer Science, Massachusetts Institute of
Technology

Version 1.1

Copyright ? 1989 The Open Group

Copyright ? 2004 The Open Group

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Table of Contents

1. TITLE

    Purpose and Goals
    Overview of the Protocol
    Data Types
    Packet Format
    Protocol
    Session Termination
    State Diagrams
    Protocol Encoding
    Display Class Format
    Manufacturer Display ID Format
    Authentication

Chapter 1. TITLE

Table of Contents

Purpose and Goals
Overview of the Protocol
Data Types
Packet Format
Protocol
Session Termination
State Diagrams
Protocol Encoding
Display Class Format
Manufacturer Display ID Format
Authentication

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.

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.

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  Description
              (Bytes)
CARD8         1       A single byte unsigned integer
CARD16        2       Two byte unsigned integer
CARD32        4       Four byte unsigned integer
                      This is actually a CARD16 followed by a collection of
ARRAY8        n+2     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.

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.

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.

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.

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

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 IndirectQuery)
     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)

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.

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.

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}= encryption of plain text D by key ?

  * {?}*? = decryption of crypto text ? with key ?

  * ? = private key shared by display and manager

  * ? = 64 bit random number generated by display

  * ? = authentication data in XDMCP packets

  * ? = per-session private key, generated by manager

  * ? = 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}? = {D1 }? {D2 xor {D1 }? }?

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

?Request = {?}?

For the Accept packet, the manager decrypts the initial message and returns
?Accept:

? = {? Request } *?

? Accept = { ? + 1}?

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:

? Accept = {?}?

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

N mark = "X client identifier"

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

? = {?NT}?

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, ?, 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:

  * ? 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).

