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Chapter 7

X Display Manager Control Protocol

Version 1.0

X Consortium Standard

X Version 11, Release 6

Keith Packard

X Consortium

Laboratory for Computer Science

Massachusetts Institute of Technology

ABSTRACT

Since the X Display Manager (xdm) may be used to manage remote displays (such as X terminals), a protocol for requesting service over the network is needed. Since the display manager is a scarce resource, the X Display Manager Control Protocol (also called XDMCP) is designed to use unreliable datagrams and places the bulk of the burden for sequencing and retransmission on the display. A standard byte order and synchronous responses reduces the complexity of the protocol.

Copyright © 1989 X Consortium

The above copyright notice and this permission notice shall be included in all copies or substantial portions of the Software.

THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE X CONSORTIUM BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.

Except as contained in this notice, the name of the X Consortium shall not be used in advertising or otherwise to promote the sale, use or other dealings in this Software without prior written authorization from the X Consortium.

X Window System is a trademark of X Consortium, Inc.

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7.1.PURPOSE AND GOALS

The purpose of 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 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, since the end result of XDMCP negotiation will be standard X protocol connections to the display. However, since the number of displays per host may be large, a connection-based protocol appears less desirable than a connection-less protocol.

In order to keep the burden on displays at a minimum (since 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 of security goals in the context of XDMCP are:

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7.2 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 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, 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 which 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 which 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 which identify a particular session and the protocol simply ignores packets which 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 which 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.

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7.3 DATATYPES

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
(in bytes) 		Description
CARD8 1 A single byte unsigned integer
CARD16 2 Two byte unsigned integer
CARD32 4 Four byte unsigned integer
ARRAY8 n+2 This is actually a CARD16 followed by a collection of CARD8. The value of the CARD16 field (n) specifies the number of CARD8 values to follow
ARRAY16 2*m+1 This is a CARD8 (m) which specifies the number of CARD16 values to follow
ARRAY32 4*l+1 This is a CARD8 (l) which specifies the number of CARD32 values to follow
ARRAYofARRAY8 ? This is a CARD8 which specifies the number of ARRAY8 values to follow.
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7.4 PACKET FORMAT

All XDMCP packets have the following information:

Length in
Bytes		 Field
Type		 Description field  
2
2
2		 CARD16
CARD16
CARD16		 version number
opcode
n = length of remaining data in bytes 		packet header
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 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, 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. As every data item has either an explicit length, or an 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.
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7.5 PROTOCOL

Each of the opcodes is described below. Since 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
A list of authentication names which 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 sub-network). 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 which forwards the request to a larger collection of secondary managers using ForwardQuery packets. In this way, the collection of managers which 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 Querypacket also demands a response from the manager, either Willing or Unwilling.

The BroadcastQuery packet is sent to many hosts. Each manager which 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 BroadcastQueryis 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
The network address of the client display.
Client Port: ARRAY8
An identification of the client task on the client display.
Authentication Names: ARRAYofARRAY8
This is a duplicate of Authentication Names array which 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 which 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 which 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 which 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
This 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 Recquest packet. This choice should remain as constant as feasible so that displays which send multiple Query packets can use the Authentication Name from any Willing packet which arrives.

The display is free to ignore managers which request an insufficient level of authentication.
Hostname: ARRAY8
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
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 (e.g. 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.
TimeoutlRetransmission policy:
Like all packets sent from the manager to the display, this packet should never be retransmitted.
Unwilling
Manager Display
Additional Fields:
The same 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 which 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
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
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: ARRAY of ARRAY8
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
This specifies the authentication protocol that the display expects the manager to validate itself with. The Authentication Data is expected to contain data which the manager will interpret, modify and use to authenticate itself.
Authorization Names: ARRAY of ARRAY8
This array 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
This field can be used by the manager to determine how to decrypt the Authentication Data field in this packet. See Section 7.10, 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 Acknowledge. 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 Acknowledge 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
This identifies the session which can be started by the manager.
Authentication Name: ARRAY8
Authentication Data: ARRAY8
This data is 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
This data is sent to the display to indicate the type of authorization the manager will be using in the first XOpenDisplay request 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 which is incremented each time an Accept packet is sent as it must be reasonably unique over a 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
This is a human readable string indicating the reason for refusal of service.
Authentication Name: ARRAY8
Authentication Data: ARRAY8
This data is 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
This field should contain the non-zero session id returned in the Accept packet.
Display Number: CARD16
This field must match the value sent in the previous Request packet.
Display Class: ARRAY8
This array specifies the class of the display. Please refer to Section 7.9, Display Class Format 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 which 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 doesn't match next Session ID
Solution:
Refuse message is sent.
Problem:
Display cannot be opened on selected stream.
Indication:
open display 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 occurring 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 t is 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:
This field 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 OpenDisplay, 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
This field should be set to the Session ID received in the Manage packet.
Status: ARRAY8
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.
Sess ion ID: CARD32
This field 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
ProblemSolutions:
Problem:
Manager doesn't receive the packet or display doesn't receive the response.
Indication:
No Alive packet 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
This field indicates that the session indicated by Session ID is currently active. The value is 0 if no session is active, 1 if a session is active.
Session ID: CARD32
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 KeepAliverequest. 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.
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7.6 SESSION TERMINATION

When the session is over, the initial connection with the display (the one which ack's 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 session, those connections should be closed. Otherwise all connections should be closed and the display reset only when all sessions have been terminated (i.e. 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.

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7.7 STATE DIAGRAMS

These state diagrams are designed to cover all actions of both the display and the manager. Any packet which 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 Unwilling 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

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-session

Receive Alive with non-matching 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. 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

fifinish-session:
XCloseDisplay

idle

send-alive:
Send Alive packet containing current status.

idle
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7.8 PROTOCOL ENCODING

When XDMCP is implemented on top of UDP (the Internet User Datagram Protocol), port number 177 is to be used.
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
Alive 13
KeepAlive 14

Per packet information follows:

Query
BroadcastQuery
IndirectQuery
These packets are identical except for the opcode field.
Length Type Description
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

ForwardQuery

Length Type Description
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

Length Type Description
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

Length Type Description
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

Length Type Description
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)
2m 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

Length Type Description
2 CARD16 version number (always 1)
2 CARD16 opcode (always Accept)
2 CARD16 length(l2 + 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

Length Type Description
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

Length Type Description
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

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

Failed

Length Type Description
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

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

Alive

Length Type Description
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)

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7.9 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 Manufacturer ID is a string which should be registered with the X Consortium. The Model Number 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. This string should probably not be specifiable by the display user to avoid unexpected configuration errors.

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7.10 MANUFACTURER DISPLAY ID FORMAT

To authenticate the manager, the display and manager will share a private key. The manager, then, must be able to discover which key to use for a particular device. The Manufacturer Display ID field of the Request packet is intended for this purpose. Typically, the manager host will contain a map between this number and the key. This field is intended to be unique per display, possibly the ethernet address of the

display in the form:
-Ethernet-8:0:2b:a:f:d2
or string of the form:
ManufacturerlD-ModelNumber-SerialNumber

where 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.

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7.11 AUTHENTICATION

In an environment where authentication is not needed, XDMCP can disable authentication by having the display send empty Authentication Name and Authentication Data fields in the Request packet. In this case, the manager will not attempt to authenticate itself. Other authentication protocols may be developed, depending on local needs.

In an unsecure environment, the display must be able to verify that the source of the various packets is a trusted manager. These packets will contain authentication information. As an example of such a system, the following discussion describes the "XDM-AUTHENTICATION-I" 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). I.e. the last octet will be incremented, subsequent carries propagate towards the first octet.

Assumptions:
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.

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 DES; 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:

Requcst={}

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

Accepl = {}

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

N=X client identifier

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

={NT}

For TCP connections N is 48 bits long and contains the 32 bit IP 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 which 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:

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

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