














	     X Display Manager Control Protocol

			Version 1.1

		       X.Org Standard

		 X Version 11, Release 6.8




		       Keith Packard

			X Consortium
	      Laboratory for Computer Science
	   Massachusetts Institute of Technology

























































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XDMCP			 X Display Manager Control Protocol


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 typi-
cally 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 con-
nect 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:

o    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.

o    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 automat-
     ically choose a host or present the possible hosts to
     the user for selection.

o    The display has a fixed set of hosts that it can con-
     nect 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.  Particu-
     lar 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



			      1





X Display Manager Control Protocol		       XDMCP


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 ser-
vices 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 proto-
col, 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 inte-
gral part of the design.  The important security goals in
the context of XDMCP are:

o    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.

o    It should be possible for the display and the login
     service to negotiate the authorization mechanism to be
     used for the standard X protocol.

o    It should be possible to provide the same level of
     security in verifying the login service as is provided
     by the negotiated authorization mechanism.

o    Because there are no firm standards yet in the area of
     security, XDMCP must be flexible enough to accomodate a
     variety of security mechanisms.

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 Display Manager, will use XDMCP to communi-
cate 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 sys-
tem 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 requir-
ing a three-way handshake to start a session.  If the hand-
shake succeeds, any existing session is terminated immedi-
ately and a new session started.  There is the problem (at



			      2





XDMCP			 X Display Manager Control Protocol


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 ter-
minating 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 pack-
ets 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 perma-
nent state by generating unique numbers that identify a par-
ticular 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 hos-
tile display, no portion of the protocol requires the Man-
ager to retransmit a packet.  Part of this means that any
valid packet that the Manager does receive must be acknowl-
edged in some way to prevent the display from continuously
resending packets.  The display can keep the protocol run-
ning 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.

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 trans-
port 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




			      3





X Display Manager Control Protocol		       XDMCP


-----------------------------------------------------------------------------------
Type Name	Length (Bytes)	 Description
-----------------------------------------------------------------------------------
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.
-----------------------------------------------------------------------------------


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:

o    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 ver-
     sions for compatibility.  This field will initially be
     one (1).

o    Opcode

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

o    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.




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XDMCP			 X Display Manager Control Protocol


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.

5.  Protocol

Each of the opcodes is described below.  Because a given
packet type is only ever sent one way, each packet descrip-
tion 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 spe-
	  cific host to ask if that host is willing to pro-
	  vide 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).	How-
	  ever, 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 col-
	  lection of secondary managers using ForwardQuery
	  packets.  In this way, the collection of managers



			      5





X Display Manager Control Protocol		       XDMCP


	  that respond can be grouped on other than network
	  boundaries; the use of a central manager reduces
	  system administrative overhead.  The primary man-
	  ager may also send a Willing packet in response to
	  this packet.

	  Each packet type has slightly different semantics:

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

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

	  o    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 Broadcast-
	       Queryexcept 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 Will-
		    ing.
	       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



			      6





XDMCP			 X Display Manager Control Protocol


	       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 man-
	  ager 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, Broad-
	       castQuery, or IndirectQuery packet that the
	       manger expects the display to use in the sub-
	       sequent Request packet.	This choice should
	       remain as constant as feasible so that dis-
	       plays 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 authentica-
	       tion.
	  Hostname: ARRAY8
	       Is a human readable string describing the
	       host from which the packet was sent.  The
	       protocol specifies no interpretation of the



			      7





X Display Manager Control Protocol		       XDMCP


	       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 ser-
	  vice 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 Will-
		    ing.
	       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 dis-
	  play, 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 dis-
	  play, this packet should never be retransmitted.



			      8





XDMCP			 X Display Manager Control Protocol


Request
     Display -> Manager
     Additional Fields:
	  Display Number: CARD16
	       Specifies the index of this particular server
	       for the host on which the display is resi-
	       dent.  This value will be zero for most
	       autonomous displays.
	  Connection Types: ARRAY16
	       Specifies an array indicating the stream ser-
	       vices 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 authenti-
	       cate 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 Manu-
	       facturer Display ID Format.
     Semantics:
	  A Request packet is sent by a display to a spe-
	  cific host to request a session ID in preparation
	  for a establishing a connection.  If the manager
	  is willing to service a connection to this dis-
	  play, it should return an Accept packet with a
	  valid session ID and should be ready for a subse-
	  quent 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:



			      9





X Display Manager Control Protocol		       XDMCP


		    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 Authentica-
	       tion 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 Ses-
	  sion 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 appropri-
	  ate Status message.
     Problems/Solutions:



			     10





XDMCP			 X Display Manager Control Protocol


	  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 dis-
	  play, 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 Authentica-
	       tion 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



			     11





X Display Manager Control Protocol		       XDMCP


	       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 suc-
	  cess indication to both halves of the stream, and
	  positive failure indication to the connection ini-
	  tiator (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 match-
		    ing 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 time-
	       out occuring in the display.
	       Indication:
		    Manage packet received with Session ID
		    matching the session attempting to



			     12





XDMCP			 X Display Manager Control Protocol


		    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 neces-
		    sary.
     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 Ses-
	  sion 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 con-
		    nection, 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.




			     13





X Display Manager Control Protocol		       XDMCP


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 man-
	  ager is running.  The manager should respond with
	  Alive whenever it receives this type of packet.

	  This allows the display to discover when the man-
	  ager 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 sub-
	  stantial time has elapsed since the most recent
	  packet.
     Valid Responses:
	  Alive
     Problems/Solutions:
	  Problem:
	       Manager does not receive the packet or dis-
	       play does not receive the response.
	       Indication:
		    No Alive packet is returned.
	       Solution:
		    Retransmit the packet with an exponen-
		    tial 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 Ses-
	       sion ID is currently active.  The value is
	       zero if no session is active or one if a ses-
	       sion is active.
	  Session ID: CARD32
	       Specifies the ID of the currently running
	       session; if any.  When no session is active



			     14





XDMCP			 X Display Manager Control Protocol


	       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.

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 dis-
play 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.

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 -> indi-
	  rect


     query:
	  Send Query packet
	  -> collect-query


     collect-query:
	  Receive Willing -> start-connection
	  Receive Unwilling -> stop-connection



			     15





X Display Manager Control Protocol		       XDMCP


	  Timeout -> query


     broadcast:
	  Send BroadcastQuery packet
	  -> collect-broadcast-query


     collect-broadcast-query:
	  Receive Willing -> update-broadcast-willing
	  User-requested connect to one host -> start-con-
	  nection
	  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-con-
	  nection
	  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



			     16





XDMCP			 X Display Manager Control Protocol


     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
	  Await 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-ses-
	  sion
	  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



			     17





X Display Manager Control Protocol		       XDMCP


     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 redi-
	  rect 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 dis-
	  play -> 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-ses-
	  sion



			     18





XDMCP			 X Display Manager Control Protocol


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

8.  Protocol Encoding

When XDMCP is implemented on top of the Internet User Data-
gram 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





			     19





X Display Manager Control Protocol		       XDMCP


     --------------------------
     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
     Alive	      14
     --------------------------


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



-----------
   A previous version of this document incorrectly
reversed the opcodes of Alive and KeepAlive.



			     20





XDMCP			 X Display Manager Control Protocol


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)
  2 x m 	 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)



			     21





X Display Manager Control Protocol		       XDMCP


  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)



			     22





XDMCP			 X Display Manager Control Protocol


  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)


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 dis-
play user to avoid unexpected configuration errors.

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 Man-
ufacturer 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



			     23





X Display Manager Control Protocol		       XDMCP


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 modifi-
able by the display user.

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 ver-
ify that the source of the various packets is a trusted man-
ager.  These packets will contain authentication informa-
tion.  As an example of such a system, the following discus-
sion 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.

o    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




			     24





XDMCP			 X Display Manager Control Protocol


		    =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 mes-
sage 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 authoriza-
tion protocol, the client computes the following:

		    N=X client identifier




			     25





X Display Manager Control Protocol		       XDMCP


		    T=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 connec-
tion setup to the server.  The server receives the packet,
decrypts the contents.	To accept the connection, the fol-
lowing must hold:

o     must match the value generated for the most recent
     XDMCP negotiation.

o    T must be within 1200 seconds of the internally stored
     time.  If no time been received before, the current
     time is set to T.

o    No packet containing the same pair (N, T) can have been
     received in the last 1200 seconds (20 minutes).

































			     26





XDMCP			 X Display Manager Control Protocol


			Table of Contents


     1. Purpose and Goals  . . . . . . . . . . . . . . .   1
     2. Overview of the Protocol . . . . . . . . . . . .   2
     3. Data Types . . . . . . . . . . . . . . . . . . .   3
     4. Packet Format  . . . . . . . . . . . . . . . . .   4
     5. Protocol . . . . . . . . . . . . . . . . . . . .   5
     6. Session Termination  . . . . . . . . . . . . . .  15
     7. State Diagrams . . . . . . . . . . . . . . . . .  15
     8. Protocol Encoding  . . . . . . . . . . . . . . .  19
     9. Display Class Format . . . . . . . . . . . . . .  23
     10. Manufacturer Display ID Format  . . . . . . . .  23
     11. Authentication  . . . . . . . . . . . . . . . .  24











































			     27


