X Version 11, Release 7.7
Version 1.1
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There are numerous possible protocols that can be used for communication among clients. They have many similarities and common needs, including authentication, version negotiation, data typing, and connection management. The Inter-Client Exchange (ICE) protocol is intended to provide a framework for building such protocols. Using ICE reduces the complexity of designing new protocols and allows the sharing of many aspects of the implementation.
Table of Contents
In discussing a variety of protocols -- existing, under development, and hypothetical -- it was noted that they have many elements in common. Most protocols need mechanisms for authentication, for version negotiation, and for setting up and taking down connections. There are also cases where the same two parties need to talk to each other using multiple protocols. For example, an embedding relationship between two parties is likely to require the simultaneous use of session management, data transfer, focus negotiation, and command notification protocols. While these are logically separate protocols, it is desirable for them to share as many pieces of implementation as possible.
The Inter-Client Exchange (ICE) protocol provides a generic framework for building protocols on top of reliable, byte-stream transport connections. It provides basic mechanisms for setting up and shutting down connections, for performing authentication, for negotiating versions, and for reporting errors. The protocols running within an ICE connection are referred to here as subprotocols. ICE provides facilities for each subprotocol to do its own version negotiation, authentication, and error reporting. In addition, if two parties are communicating using several different subprotocols, ICE will allow them to share the same transport layer connection.
Through some mechanism outside ICE, two parties make themselves known to each other and agree that they would like to communicate using an ICE subprotocol. ICE assumes that this negotation includes some notion by which the parties will decide which is the \*Qoriginating\*U party and which is the \*Qanswering\*U party. The negotiation will also need to provide the originating party with a name or address of the answering party. Examples of mechanisms by which parties can make themselves known to each other are the X selection mechanism, environment variables, and shared files.
The originating party first determines whether there is an existing ICE connection between the two parties. If there is, it can re-use the existing connection and move directly to the setup of the subprotocol. If no ICE connection exists, the originating party will open a transport connection to the answering party and will start ICE connection setup.
The ICE connection setup dialog consists of three major parts: byte order
exchange, authentication, and connection information exchange. The first
message in each direction is a
ByteOrder
message telling which byte order will be used by the sending party in
messages that it sends. After that, the originating party sends a
ConnectionSetup
message giving information about itself (vendor name and release number) and
giving a list of ICE version numbers it is capable of supporting and a list
of authentication schemes it is willing to accept. Authentication is
optional. If no authentication is required, the answering party responds
with a
ConnectionReply
message giving information about itself, and the connection setup is complete.
If the connection setup is to be authenticated, the answering party will
respond with an
AuthenticationRequired
message instead of a
ConnectionReply
message. The parties then exchange
AuthenticationReply
and
AuthenticationNextPhase
messages until authentication is complete, at which time the answering party
finally sends its
ConnectionReply
message.
Once an ICE connection is established (or an existing connection reused),
the originating party starts subprotocol negotiation by sending a
ProtocolSetup
message. This message gives the name of the subprotocol that the parties
have agreed to use, along with the ICE major opcode that the originating
party has assigned to that subprotocol. Authentication can also occur for
the subprotocol, independently of authentication for the connection.
Subprotocol authentication is optional. If there is no subprotocol
authentication, the answering party responds with a
ProtocolReply
message, giving the ICE major opcode that it has assigned
for the subprotocol.
Subprotocols are authenticated independently of each other, because they may
have differing security requirements. If there is authentication for this
particular subprotocol, it takes place before the answering party emits the
ProtocolReply
message, and it uses the
AuthenticationRequired
AuthenticationReply
and
AuthenticationNextPhase
messages, just as for the connection authentication. Only when subprotocol
authentication is complete does the answering party send its
ProtocolReply
message.
When a subprotocol has been set up and authenticated, the two parties can
communicate using messages defined by the subprotocol. Each message has two
opcodes: a major opcode and a minor opcode. Each party will send messages
using the major opcode it has assigned in its
ProtocolSetup
or
ProtocolReply
message. These opcodes will, in general, not be the same. For a particular
subprotocol, each party will need to keep track of two major opcodes: the
major opcode it uses when it sends messages, and the major opcode it expects
to see in messages it receives. The minor opcode values and semantics are
defined by each individual subprotocol.
Each subprotocol will have one or more messages whose semantics are that the
subprotocol is to be shut down. Whether this is done unilaterally or is
performed through negotiation is defined by each subprotocol. Once a
subprotocol is shut down, its major opcodes are removed from
use; no further messages on this subprotocol should be sent until the
opcode is reestablished with
ProtocolSetup
ICE has a facility to negotiate the closing of the connection when there are
no longer any active subprotocols. When either party decides that no
subprotocols are active, it can send a
WantToClose
message. If the other party agrees to close the connection, it can simply
do so. If the other party wants to keep the connection open, it can
indicate its desire by replying with a
NoClose
message.
It should be noted that the party that initiates the connection isn't
necessarily the same as the one that initiates setting up a subprotocol.
For example, suppose party A connects to party B. Party A will issue the
ConnectionSetup
message and party B will respond with a
ConnectionReply
message. (The authentication steps are omitted here for brevity.)
Typically, party A will also issue the
ProtocolSetup
message and expect a
ProtocolReply
from party B. Once the connection is established, however, either party may
initiate the negotiation of a subprotocol. Continuing this example, party B
may decide that it needs to set up a subprotocol for communication with
party A. Party B would issue the
ProtocolSetup
message and expect a
ProtocolReply
from party A.
Table of Contents
ICE messages contain several types of data. Byte order is negotiated in the initial connection messages; in general data is sent in the sender's byte order and the receiver is required to swap it appropriately. In order to support 64-bit machines, ICE messages are padded to multiples of 8 bytes. All messages are designed so that fields are \*Qnaturally\*U aligned on 16-, 32-, and 64-bit boundaries. The following formula gives the number of bytes necessary to pad E bytes to the next multiple of b:
pad(E, b) = (b - (E mod b)) mod b
| Type Name | Description |
|---|---|
| CARD8 | 8-bit unsigned integer |
| CARD16 | 16-bit unsigned integer |
| CARD32 | 32-bit unsigned integer |
| BOOL |
|
| LPCE | A character from the X Portable Character Set in Latin Portable Character Encoding |
| Type Name | Type |
|---|---|
| VERSION | [Major, minor: CARD16] |
| STRING | LISTofLPCE |
LISTof<type> denotes a counted collection of <type>. The exact encoding varies depending on the context; see the encoding section.
All ICE messages include the following information:
| Field Type | Description |
|---|---|
| CARD8 | protocol major opcode |
| CARD8 | protocol minor opcode |
| CARD32 | length of remaining data in 8-byte units |
The fields are as follows:
Protocol major opcode | This specifies what subprotocol the message is intended for. Major opcode 0 is reserved for ICE control messages. The major opcodes of other subprotocols are dynamically assigned and exchanged at protocol negotiation time. |
Protocol minor opcode | This specifies what protocol-specific operation is to be performed. Minor opcode 0 is reserved for Errors; other values are protocol-specific. |
Length of data in 8-byte units |
This specifies the length of the information following the first 8 bytes.
Each message-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.
Messages that have too little or too much data indicate a serious
protocol failure, and should result in a |
Every message sent in a given direction has an implicit sequence number, starting with 1. Sequence numbers are global to the connection; independent sequence numbers are not maintained for each protocol.
Messages of a given major-opcode (i.e., of a given protocol) must be responded to (if a response is called for) in order by the receiving party. Messages from different protocols can be responded to in arbitrary order.
Minor opcode 0 in every protocol is for reporting errors. At most one error is generated per request. If more than one error condition is encountered in processing a request, the choice of which error is returned is implementation-dependent.
Error
offending-minor-opcode: | CARD8 |
severity: |
{CanContinue,
|
sequence-number: | CARD32 |
class: | CARD16 |
value(s): | <dependent on major/minor opcode and class> |
This message is sent to report an error in response to a message
from any protocol. The Error message
exists in all protocol major-opcode spaces; it
is minor-opcode zero in every protocol. The minor opcode of the
message that caused the error is reported, as well as the sequence
number of that message.
The severity indicates the sender's behavior following
the identification of the error. CanContinue
indicates the sender is willing to accept additional messages for this
protocol. FatalToProcotol
indicates the sender is unwilling to accept further messages for this
protocol but that messages for other protocols may be accepted.
FatalToConnection
indicates the sender is unwilling to accept any further
messages for any protocols on the connection. The sender
is required to conform to specified severity conditions
for generic and ICE (major opcode 0) errors; see
Generic Error Classes
ICE Error Classes
.
The class defines the generic class of
error. Classes are specified separately for each protocol (numeric
values can mean different things in different protocols). The error
values, if any, and their types vary with the specific error class
for the protocol.
Table of Contents
Each of the ICE control opcodes is described below. Most of the messages have additional information included beyond the description above. The additional information is appended to the message header and the length field is computed accordingly.
In the following message descriptions, \*QExpected errors\*U indicates
errors that may occur in the normal course of events. Other errors
(in particular
BadMajor
BadMinor
BadState
BadLength
BadValue
ProtocolDuplicate and
MajorOpcodeDuplicate
might occur, but generally indicate a serious implementation failure on
the part of the errant peer.
ByteOrder
byte-order: |
{MSBfirst,
|
Both parties must send this message before sending any other, including errors. This message specifies the byte order that will be used on subsequent messages sent by this party.
Note: If the receiver detects an error in this message,
it must be sure to send its own
ByteOrder message before sending the
Error.
ConnectionSetup
versions: | LISTofVERSION |
must-authenticate: | BOOL |
authentication-protocol-names: | LISTofSTRING |
vendor: | STRING |
release: | STRING |
Responses: |
|
Expected errors: |
|
The party that initiates the connection (the one that does the
"connect()") must send this message as the second message (after
ByteOrder on startup.
Versions gives a list, in decreasing order of preference, of the protocol versions this party is capable of speaking. This document specifies major version 1, minor version 0.
If must-authenticate is True the initiating
party demands authentication; the accepting party
must pick an authentication scheme
and use it. In this case, the only valid response is
AuthenticationRequired
If must-authenticate is False the accepting
party may choose an authentication mechanism, use a host-address-based
authentication scheme, or skip authentication. When must-authenticate
is False ConnectionReply and
AuthenticationRequired are both valid responses.
If a host-address-based authentication scheme is used,
AuthenticationRejected and
AuthenticationFailed errors are possible.
Authentication-protocol-names specifies a (possibly null, if
must-authenticate is False
list of authentication protocols the party is willing to perform. If
must-authenticate is True
presumably the party will offer only authentication mechanisms
allowing mutual authentication.
Vendor gives the name of the vendor of this ICE implementation.
Release gives the release identifier of this ICE implementation.
AuthenticationRequired
authentication-protocol-index: | CARD8 |
data: | <specific to authentication protocol> |
Response: |
|
Expected errors: |
|
This message is sent in response to a ConnectionSetup
or ProtocolSetup
message to specify that authentication is to be done and what
authentication mechanism is to be used.
The authentication protocol is specified by a 0-based index into the list
of names given in the ConnectionSetup or
ProtocolSetup
Any protocol-specific data that might be required is also sent.
AuthenticationReply
data: | <specific to authentication protocol> |
Responses: |
|
Expected errors: |
|
This message is sent in response to an
AuthenticationRequired or
AuthenticationNextPhase message, to
supply authentication data as defined by the authentication protocol
being used.
Note that this message is sent by the party that initiated the current
negotiation -- the party that sent the
ConnectionSetup or
ProtocolSetup message.
AuthenticationNextPhase
indicates that more is to be done to complete the authentication.
If the authentication is complete,
ConnectionReply
is appropriate if the current authentication handshake is the result of a
ConnectionSetup and a
ProtocolReply
is appropriate if it is the result of a
ProtocolSetup.
AuthenticationNextPhase
data: | <specific to authentication protocol> |
Response: |
|
Expected errors: |
|
This message is sent in response to an
AuthenticationReply
message, to supply authentication data as defined by the authentication
protocol being used.
ConnectionReply
version-index: | CARD8 |
vendor: | STRING |
release: | STRING |
This message is sent in response to a
ConnectionSetup or
AuthenticationReply
message to indicate that the authentication handshake is complete.
Version-index gives a 0-based index into the list of versions offered in
the ConnectionSetup message; it specifies the
version of the ICE protocol that both parties
should speak for the duration of the connection.
Vendor gives the name of the vendor of this ICE implementation.
Release gives the release identifier of this ICE implementation.
ProtocolSetup
protocol-name: | STRING |
major-opcode: | CARD8 |
versions: | LISTofVERSION |
vendor: | STRING |
release: | STRING |
must-authenticate: | BOOL |
authentication-protocol-names: | LISTofSTRING |
Responses: |
|
Expected errors: |
|
This message is used to initiate negotiation of a protocol and establish any authentication specific to it.
Protocol-name gives the name of the protocol the party wishes to speak.
Major-opcode gives the opcode that the party will use in messages it sends.
Versions gives a list of version numbers, in decreasing order of preference, that the party is willing to speak.
Vendor and release are identification strings with semantics defined by the specific protocol being negotiated.
If must-authenticate is True,
the initiating party demands authentication; the accepting party
must pick an authentication scheme
and use it. In this case, the only valid response is
AuthenticationRequired
If must-authenticate is False,
the accepting party may choose an authentication mechanism, use a
host-address-based authentication scheme, or skip authentication.
When must-authenticate is False,
ProtocolReply and
AuthenticationRequired
are both valid responses. If a host-address-based authentication
scheme is used, AuthenticationRejected and
AuthenticationFailed errors are possible.
Authentication-protocol-names specifies a (possibly null, if
must-authenticate is False
list of authentication protocols the party is willing to perform. If
must-authenticate is True
presumably the party will offer only authentication mechanisms
allowing mutual authentication.
ProtocolReply
major-opcode: | CARD8 |
version-index: | CARD8 |
vendor: | STRING |
release: | STRING |
This message is sent in response to a ProtocolSetup
or AuthenticationReply
message to indicate that the authentication handshake is complete.
Major-opcode gives the opcode that this party will use in messages that it sends.
Version-index gives a 0-based index into the list of versions offered in the
ProtocolSetup message; it specifies the version
of the protocol that both parties should speak for the duration of
the connection.
Vendor and release are identification strings with semantics defined by the specific protocol being negotiated.
Ping
Response: |
|
This message is used to test if the connection is still functioning.
PingReply
This message is sent in response to a Ping
message, indicating that the connection is still functioning.
WantToClose
Responses: |
|
This message is used to initiate a possible close of the connection. The sending party has noticed that, as a result of mechanisms specific to each protocol, there are no active protocols left. There are four possible scenarios arising from this request:
The receiving side noticed too, and has already sent a
WantToClose On receiving a
WantToClose while already attempting to
shut down, each party should simply close the connection.
The receiving side hasn't noticed, but agrees. It closes the connection.
The receiving side has a ProtocolSetup
"in flight," in which case it is to ignore
WantToClose and the party sending
WantToClose is to abandon the shutdown attempt
when it receives the ProtocolSetup
The receiving side wants the connection kept open for some
reason not specified by the ICE protocol, in which case it
sends NoClose
See the state transition diagram for additional information.
NoClose
This message is sent in response to a WantToClose
message to indicate that the responding party does not want the
connection closed at this time. The receiving party should not close the
connection. Either party may again initiate
WantToClose at some future time.
These errors should be used by all protocols, as applicable.
For ICE (major opcode 0), FatalToProtocol
should be interpreted as FatalToConnection.
BadMinor
offending-minor-opcode: | <any> |
severity: |
|
values: | (none) |
Received a message with an unknown minor opcode.
BadState
offending-minor-opcode: | <any> |
severity: |
|
values: | (none) |
Received a message with a valid minor opcode which is not appropriate for the current state of the protocol.
BadLength
offending-minor-opcode: | <any> |
severity: |
|
values: | (none) |
Received a message with a bad length. The length of the message is longer or shorter than required to contain the data.
BadValue
offending-minor-opcode: | <any> |
severity: |
|
values: | CARD32 Byte offset to offending value in offending message. CARD32 Length of offending value. <varies> Offending value |
Received a message with a bad value specified.
These errors are all major opcode 0 errors.
BadMajor
offending-minor-opcode: | <any> |
severity: |
|
values: | CARD8 Opcode |
The opcode given is not one that has been registered.
NoAuthentication
offending-minor-opcode: |
|
severity: |
|
values: | (none) |
None of the authentication protocols offered are available.
NoVersion
offending-minor-opcode: |
|
severity: |
|
values: | (none) |
None of the protocol versions offered are available.
SetupFailed
offending-minor-opcode: |
|
severity: |
|
values: | STRING reason |
The sending side is unable to accept the new connection or new protocol for a reason other than authentication failure. Typically this error will be a result of inability to allocate additional resources on the sending side. The reason field will give a human-interpretable message providing further detail on the type of failure.
AuthenticationRejected
offending-minor-opcode: |
|
severity: |
|
values: | STRING reason |
Authentication rejected. The peer has failed to properly authenticate itself. The reason field will give a human-interpretable message providing further detail.
AuthenticationFailed
offending-minor-opcode: |
|
severity: |
|
values: | STRING reason |
Authentication failed. AuthenticationFailed
does not imply that the authentication was rejected, as
AuthenticationRejected
does. Instead it means that the sender was unable to complete
the authentication for some other reason. (For instance, it
may have been unable to contact an authentication server.)
The reason field will give a human-interpretable message
providing further detail.
ProtocolDuplicate
offending-minor-opcode: |
|
severity: |
|
values: | STRING protocol name |
The protocol name was already registered. This is fatal to
the "new" protocol being set up by ProtocolSetup
but it does not affect the existing registration.
MajorOpcodeDuplicate
offending-minor-opcode: |
|
severity: |
|
values: | CARD8 opcode |
The major opcode specified was already registered. This is
fatal to the \*Qnew\*U protocol being set up by
ProtocolSetup but it does not affect the
existing registration.
UnknownProtocol
offending-minor-opcode: |
|
severity: |
|
values: | STRING protocol name |
The protocol specified is not supported.
Here are the state diagrams for the party that initiates the connection:
start:
connect to other end, send ByteOrder ConnectionSetup -> conn_wait
conn_wait:
receive ConnectionReply -> stasis
receive AuthenticationRequired -> conn_auth1
receive Error -> quit
receive <other>, send Error -> quit
conn_auth1:
if good auth data, send AuthenticationReply -> conn_auth2
if bad auth data, send Error -> quit
conn_auth2:
receive ConnectionReply -> stasis
receive AuthenticationNextPhase -> conn_auth1
receive Error -> quit
receive <other>, send Error -> quit
Here are top-level state transitions for the party that accepts connections.
listener:
accept connection -> init_wait
init_wait:
receive ByteOrder ConnectionSetup -> auth_ask
receive <other>, send Error -> quit
auth_ask:
send ByteOrder ConnectionReply
-> stasis
send AuthenticationRequired -> auth_wait
send Error -> quit
auth_wait:
receive AuthenticationReply -> auth_check
receive <other>, send Error -> quit
auth_check:
if no more auth needed, send ConnectionReply -> stasis
if good auth data, send AuthenticationNextPhase -> auth_wait
if bad auth data, send Error -> quit
Here are the top-level state transitions for all parties after the initial connection establishment subprotocol.
Note: this is not quite the truth for branches out from stasis, in that multiple conversations can be interleaved on the connection.
stasis:
send ProtocolSetup -> proto_wait
receive ProtocolSetup -> proto_reply
send Ping -> ping_wait
receive Ping send PingReply -> stasis
receive WantToClose -> shutdown_attempt
receive <other>, send Error -> stasis
all protocols shut down, send WantToClose -> close_wait
proto_wait:
receive ProtocolReply -> stasis
receive AuthenticationRequired -> give_auth1
receive Error give up on this protocol -> stasis
receive WantToClose -> proto_wait
give_auth1:
if good auth data, send AuthenticationReply -> give_auth2
if bad auth data, send Error give up on this protocol -> stasis
receive WantToClose -> give_auth1
give_auth2:
receive ProtocolReply -> stasis
receive AuthenticationNextPhase -> give_auth1
receive Error give up on this protocol -> stasis
receive WantToClose -> give_auth2
proto_reply:
send ProtocolReply -> stasis
send AuthenticationRequired -> take_auth1
send Error give up on this protocol -> stasis
take_auth1:
receive AuthenticationReply -> take_auth2
receive Error give up on this protocol -> stasis
take_auth2:
if good auth data \(-> take_auth3
if bad auth data, send Error give up on this protocol -> stasis
take_auth3:
if no more auth needed, send ProtocolReply -> stasis
if good auth data, send AuthenticationNextPhase -> take_auth1
if bad auth data, send Error give up on this protocol -> stasis
ping_wait:
receive PingReply -> stasis
quit:
-> close connection
Here are the state transitions for shutting down the connection:
shutdown_attempt:
if want to stay alive anyway, send NoClose -> stasis
else -> quit
close_wait:
receive ProtocolSetup -> proto_reply
receive NoClose -> stasis
receive WantToClose -> quit
connection close -> quit
Table of Contents
In the encodings below, the first column is the number of bytes occupied. The second column is either the type (if the value is variable) or the actual value. The third column is the description of the value (e.g., the parameter name). Receivers must ignore bytes that are designated as unused or pad bytes.
This document describes major version 1, minor version 0 of the ICE protocol.
LISTof<type> indicates some number of repetitions of <type>, with no additional padding. The number of repetitions must be specified elsewhere in the message.
| Type Name | Length (bytes) | Description |
|---|---|---|
| CARD8 | 1 | 8-bit unsigned integer |
| CARD16 | 2 | 16-bit unsigned integer |
| CARD32 | 4 | 32-bit unsigned integer |
| LPCE | 1 | A character from the X Portable Character Set in Latin Portable Character Encoding |
| Type Name | Length (bytes) | Type | Description |
|---|---|---|---|
| VERSION | |||
| 2 | CARD16 | Major version number | |
| 2 | CARD16 | Minor version number | |
| STRING | |||
| 2 | CARD16 | length of string in bytes | |
| n | LISTofLPCE | string | |
| p | unused, p = pad(n+2, 4) |
| Message Name | Encoding |
|---|---|
| Error | 0 |
| ByteOrder | 1 |
| ConnectionSetup | 2 |
| AuthenticationRequired | 3 |
| AuthenticationReply | 4 |
| AuthenticationNextPhase | 5 |
| ConnectionReply | 6 |
| ProtocolSetup | 7 |
| ProtocolReply | 8 |
| Ping | 9 |
| PingReply | 10 |
| WantToClose | 11 |
| NoClose | 12 |
Error
1 CARD8 major-opcode
1 0 Error
2 CARD16 class
4 (n+p)/8+1 length
1 CARD8 offending-minor-opcode
1 severity:
0 CanContinue
1 FatalToProtocol
2 FatalToConnection
2 unused
4 CARD32 sequence number of erroneous message
n <varies> value(s)
p pad, p = pad(n,8)
ByteOrder
1 0 ICE
1 1 ByteOrder
1 byte-order:
0 LSBfirst
1 MSBfirst
1 unused
4 0 length
ConnectionSetup
1 0 ICE
1 2 ConnectionSetup
1 CARD8 Number of versions offered
1 CARD8 Number of authentication protocol names offered
4 (i+j+k+m+p)/8+1 length
1 BOOL must-authenticate
7 unused
i STRING vendor
j STRING release
k LISTofSTRING authentication-protocol-names
m LISTofVERSION version-list
p unused, p = pad(i+j+k+m,8)
AuthenticationRequired
1 0 ICE
1 3 AuthenticationRequired
1 CARD8 authentication-protocol-index
1 unused
4 (n+p)/8+1 length
2 n length of authentication data
6 unused
n <varies> data
p unused, p = pad(n,8)
AuthenticationReply
1 0 ICE
1 4 AuthenticationReply
2 unused
4 (n+p)/8+1 length
2 n length of authentication data
6 unused
n <varies> data
p unused, p = pad(n,8)
AuthenticationNextPhase
1 0 ICE
1 5 AuthenticationNextPhase
2 unused
4 (n+p)/8+1 length
2 n length of authentication data
6 unused
n <varies> data
p unused, p = pad(n,8)
ConnectionReply
1 0 ICE
1 6 ConnectionReply
1 CARD8 version-index
1 unused
4 (i+j+p)/8 length
i STRING vendor
j STRING release
p unused, p = pad(i+j,8)
ProtocolSetup
1 0 ICE
1 7 ProtocolSetup
1 CARD8 major-opcode
1 BOOL must-authenticate
4 (i+j+k+m+n+p)/8+1 length
1 CARD8 Number of versions offered
1 CARD8 Number of authentication protocol names offered
6 unused
i STRING protocol-name
j STRING vendor
k STRING release
m LISTofSTRING authentication-protocol-names
n LISTofVERSION version-list
p unused, p = pad(i+j+k+m+n,8)
ProtocolReply
1 0 ICE
1 8 ProtocolReply
1 CARD8 version-index
1 CARD8 major-opcode
4 (i+j+p)/8 length
i STRING vendor
j STRING release
p unused, p = pad(i+j, 8)
Ping
1 0 ICE
1 9 Ping
2 0 unused
4 0 length
PingReply
1 0 ICE
1 10 PingReply
2 0 unused
4 0 length
WantToClose
1 0 ICE
1 11 WantToClose
2 0 unused
4 0 length
NoClose
1 0 ICE
1 12 NoClose
2 0 unused
4 0 length
Generic errors have classes in the range 0x8000-0xFFFF, and subprotocol-specific errors are in the range 0x0000-0x7FFF.
Table of Contents
Release 6.1 added the ICE X rendezvous protocol (Appendix B) and updated the document version to 1.1.
Table of Contents
The ICE X rendezvous protocol is designed to answer the need posed in Section 2 for one mechanism by which two clients interested in communicating via ICE are able to exchange the necessary information. This protocol is appropriate for any two ICE clients who also have X connections to the same X server.
The ICE X Rendezvous Mechanism requires clients willing to act as ICE
originating parties to pre-register the ICE subprotocols they support in an
ICE_PROTOCOLS property on their top-level window. Clients willing to
act as ICE answering parties then send an ICE_PROTOCOLS X
ClientMessage
event to the ICE originating parties. This
ClientMessage
event identifies
the ICE network IDs of the ICE answering party as well as the ICE
subprotocol it wishes to speak. Upon receipt of this message the ICE
originating party uses the information to establish an ICE connection
with the ICE answering party.
Clients willing to act as ICE originating parties preregister
the ICE subprotocols they support in a list of atoms held by an
ICE_PROTOCOLS property on their top-level window. The name of each
atom listed in ICE_PROTOCOLS must be of the form
ICE_INITIATE_pname where
pname is the name of the ICE
subprotocol the ICE originating party is willing to speak, as would be
specified in an ICE
ProtocolSetup
message.
Clients with an ICE_INITIATE_pname atom
in the ICE_PROTOCOLS property on their top-level windows must respond to
ClientMessage events of
type ICE_PROTOCOLS specifying ICE_INITIATE_
pname. If a client does not
want to respond to these client message events, it should
remove the ICE_INITIATE_pname
atom from its ICE_PROTOCOLS property
or remove the ICE_PROTOCOLS property entirely.
To initiate the rendezvous a client acting as an ICE answering
party sends an X
ClientMessage
event of type ICE_PROTOCOLS to an ICE
originating party. This ICE_PROTOCOLS client message contains the
information the ICE originating party needs to identify the ICE
subprotocol the two parties will use as well as the ICE network
identification string of the ICE answering party.
Before the ICE answering party sends the client message event it must define a text property on one of its windows. This text property contains the ICE answering party's ICE network identification string and will be used by ICE originating parties to determine the ICE answering party's list of ICE network IDs.
The property name will normally be ICE_NETWORK_IDS, but may be any name of the ICE answering party's choosing. The format for this text property is as follows:
| Field | Value |
|---|---|
| type | XA_STRING |
| format | 8 |
| value | comma-separated list of ICE network IDs |
Once the ICE answering party has established this text property on one
of its windows, it initiates the rendezvous by sending an
ICE_PROTOCOLS
ClientMessage
event to an ICE originating party's
top-level window. This event has the following format
and must only be sent to windows that have pre-registered the ICE
subprotocol in an ICE_PROTOCOLS property on their top-level window.
| Field | Value |
|---|---|
| message_type | Atom = "ICE_PROTOCOLS" |
| format | 32 |
| data.l[0] | Atom identifying the ICE subprotocol to speak |
| data.l[1] | Timestamp |
| data.l[2] | ICE answering party's window ID with ICE network IDs text property |
| data.l[3] | Atom naming text property containing the ICE answering party's ICE network IDs |
| data.l[4] | Reserved. Must be 0. |
The name of the atom in data.l[0] must be of the form ICE_INITIATE_pname, where pname is the name of the ICE subprotocol the ICE answering party wishes to speak.
When an ICE originating party receives a
ClientMessage
event of type
ICE_PROTOCOLS specifying ICE_INITIATE_pname
it can initiate an ICE connection with the ICE answering party.
To open this connection the client retrieves the ICE answering
party's ICE network IDs from the window specified in data.l[2] using
the text property specified in data.l[3].
If the connection attempt fails for any reason, the client must
respond to the client message event by sending a return
ClientMessage
event to the window specified in data.l[2]. This return
event has the following format:
| Field | Value |
|---|---|
| message_type | Atom = "ICE_INITIATE_FAILED" |
| format | 32 |
| data.l[0] | Atom identifying the ICE subprotocol requested |
| data.l[1] | Timestamp |
| data.l[2] | Initiating party's window ID (holding ICE_PROTOCOLS) |
| data.l[3] | int: reason for failure |
| data.l[4] | Reserved, must be 0 |
The values of data.l[0] and data.l[1] are copied directly from the client message event the client received.
The value in data.l[2] is the id of the window to which the ICE_PROTOCOLS.ICE_INITIATE_pname client message event was sent.
Data.l[3] has one of the following values:
| Value | Encoding | Description |
|---|---|---|
| OpenFailed | 1 |
The client was unable to open the connection
(e.g. a call to IceOpenConnection() failed). If the
client is able to distinguish authentication or
authorization errors from general errors, then
the preferred reply is AuthenticationFailed
for authorization errors.
|
| AuthenticationFailed | 2 | Authentication or authorization of the
connection or protocol setup was refused.
This reply will be given only if the client is
able to distinguish it from
OpenFailed
otherwise
OpenFailed
will be returned. |
| SetupFailed | 3 | The client was unable to initiate the specified protocol on the connection (e.g. a call to IceProtocolSetup() failed). |
| UnknownProtocol | 4 | The client does not recognize the requested protocol. (This represents a semantic error on the part of the answering party.) |
| Refused | 5 | The client was in the process of removing ICE_INITIATE_pname from its ICE_PROTOCOLS list when the client message was sent; the client no longer is willing to establish the specified ICE communication. |
Clients willing to act as ICE originating parties must update the ICE_PROTOCOLS property on their top-level windows to include the ICE_INITIATE_pname atom(s) identifying the ICE subprotocols they speak. The method a client uses to update the ICE_PROTOCOLS property to include ICE_INITIATE_pname atoms is implementation dependent, but the client must ensure the integrity of the list to prevent the accidental omission of any atoms previously in the list.
When setting up the ICE network IDs text property on one of its windows, the ICE answering party can determine its comma-separated list of ICE network IDs by calling IceComposeNetworkIdList() after making a call to IceListenForConnections(). The method an ICE answering party uses to find the top-level windows of clients willing to act as ICE originating parties is dependent upon the nature of the answering party. Some may wish to use the approach of requiring the user to click on a client's window. Others wishing to find existing clients without requiring user interaction might use something similar to the XQueryTree() method used by several freely-available applications. In order for the ICE answering party to become automatically aware of new clients willing to originate ICE connections, the ICE answering party might register for SubstructureNotify events on the root window of the display. When it receives a SubstructureNotify event, the ICE answering party can check to see if it was the result of the creation of a new client top-level window with an ICE_PROTOCOLS property.
In any case, before attempting to use this ICE X Rendezvous Mechanism
ICE answering parties wishing to speak ICE subprotocol
pname should
check for the ICE_INITIATE_pname atom
in the ICE_PROTOCOLS property on
a client's top-level window. A client that does not include an
ICE_INITIATE_pname atom in a
ICE_PROTOCOLS property on some top-level window should be assumed to ignore
ClientMessage
events of type
ICE_PROTOCOLS specifying ICE_INITIATE_pname
for ICE subprotocol pname.
Although the version of the ICE subprotocol could be passed in the
client message event, ICE provides more a flexible version negotiation
mechanism than will fit within a single
ClientMessage
event. Because
of this, ICE subprotocol versioning is handled within the ICE protocol
setup phase.
Clients wish to communicate with each other via an ICE subprotocol known as "RAP V1.0". In RAP terminology one party, the "agent", communicates with other RAP-enabled applications on demand. The user may direct the agent to establish communication with a specific application by clicking on the application's window, or the agent may watch for new application windows to be created and automatically establish communication.
During startup the ICE answering party (the agent) first calls IceRegisterForProtocolReply() with a list of the versions (i.e., 1.0) of RAP the agent can speak. The answering party then calls IceListenForConnections() followed by IceComposeNetworkIdList() and stores the resulting ICE network IDs string in a text property on one of its windows.
When the answering party (agent) finds a client with which it wishes to speak, it checks to see if the ICE_INITIATE_RAP atom is in the ICE_PROTOCOLS property on the client's top-level window. If it is present the agent sends the client's top-level window an ICE_PROTOCOLS client message event as described above. When the client receives the client message event and is willing to originate an ICE connection using RAP, it performs an IceRegisterForProtocolSetup() with a list of the versions of RAP the client can speak. The client then retrieves the agent's ICE network ID from the property and window specified by the agent in the client message event and calls IceOpenConnection(). After this call succeeds the client calls IceProtocolSetup() specifying the RAP protocol. During this process, ICE calls the RAP protocol routines that handle the version negotiation.
Note that it is not necessary for purposes of this rendezvous that the client application call any ICElib functions prior to receipt of the client message event.