Inter-Client Exchange (ICE) Protocol

X Consortium Standard

Robert Scheifler

X Consortium

Jordan Brown

Quarterdeck Office Systems

X Version 11, Release 7.7

Version 1.1

Copyright © 1993, 1994 X Consortium

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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.

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Table of Contents

1. Purpose and Goals
2. Overview of the Protocol
3. Data Types

    Primitive Types
    Complex Types
    Message Format

4. Overall Protocol Description
5. ICE Control Subprotocol -- Major Opcode 0

    Generic Error Classes
    ICE Error Classes

6. State Diagrams
7. Protocol Encoding

    Primitives
    Enumerations
    Compound Types
    ICE Minor opcodes
    Message Encoding
    Error Class Encoding

        Generic Error Class Encoding
        ICE-specific Error Class Encoding

A. Modification History

    Release 6 to Release 6.1
    Release 6.1 to Release 6.3

B. ICE X Rendezvous Protocol

    Introduction
    Overview of ICE X Rendezvous
    Registering Known Protocols
    Initiating the Rendezvous
    ICE Subprotocol Versioning

Chapter 1. Purpose and Goals

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.

Chapter 2. Overview of the Protocol

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 “originating” party and which is the “answering” 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.

Chapter 3. Data Types

Table of Contents

Primitive Types
Complex Types
Message Format

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 “naturally” 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

Primitive Types

┌────────┬────────────────────────────────────────────────────────────────────┐
│Type    │Description                                                         │
│Name    │                                                                    │
├────────┼────────────────────────────────────────────────────────────────────┤
│CARD8   │8-bit unsigned integer                                              │
├────────┼────────────────────────────────────────────────────────────────────┤
│CARD16  │16-bit unsigned integer                                             │
├────────┼────────────────────────────────────────────────────────────────────┤
│CARD32  │32-bit unsigned integer                                             │
├────────┼────────────────────────────────────────────────────────────────────┤
│BOOL    │False or True                                                       │
├────────┼────────────────────────────────────────────────────────────────────┤
│LPCE    │A character from the X Portable Character Set in Latin Portable     │
│        │Character Encoding                                                  │
└────────┴────────────────────────────────────────────────────────────────────┘

Complex Types

┌─────────┬──────────────────────┐
│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.

Message Format

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   This specifies what subprotocol the message is intended for. Major
major      opcode 0 is reserved for ICE control messages. The major opcodes of
opcode     other subprotocols are dynamically assigned and exchanged at
           protocol negotiation time.

Protocol   This specifies what protocol-specific operation is to be performed.
minor      Minor opcode 0 is reserved for Errors; other values are
opcode     protocol-specific.

           This specifies the length of the information following the first 8
Length of  bytes. Each message-type has a different format, and will need to be
data in    separately length-checked against this value. As every data item has
8-byte     either an explicit length, or an implicit length, this can be easily
units      accomplished. Messages that have too little or too much data
           indicate a serious protocol failure, and should result in a
           BadLength error.

Chapter 4. Overall Protocol Description

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, FatalToProtocol FatalToConnection

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.

Chapter 5. ICE Control Subprotocol -- Major Opcode 0

Table of Contents

Generic Error Classes
ICE Error Classes

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, “Expected errors” 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, LSBfirst

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

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:                     ConnectionReply, AuthenticationRequired (See
                               note)

Expected errors:               NoVersion, SetupFailed, NoAuthentication,
                               AuthenticationRejected, AuthenticationFailed

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:                      AuthenticationReply

Expected errors:               AuthenticationRejected, AuthenticationFailed

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:       AuthenticationNextPhase, ConnectionReply, ProtocolReply

Expected errors: AuthenticationRejected, AuthenticationFailed, SetupFailed

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:        AuthenticationReply

Expected errors: AuthenticationRejected, AuthenticationFailed

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:                     AuthenticationRequired, ProtocolReply

                               UnknownProtocol, NoVersion, SetupFailed,
Expected errors:               NoAuthentication, AuthenticationRejected,
                               AuthenticationFailed

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: PingReply

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: WantToClose, NoClose, ProtocolSetup

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:

 1. 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.

 2. The receiving side hasn't noticed, but agrees. It closes the connection.

 3. 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

 4. 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.

Generic Error Classes

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:               FatalToProtocol or CanContinue (protocol's discretion)

values:                 (none)

Received a message with an unknown minor opcode.

BadState

offending-minor-opcode: <any>

severity:               FatalToProtocol or CanContinue (protocol's discretion)

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:               FatalToProtocol or CanContinue (protocol's discretion)

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:               CanContinue

                        CARD32 Byte offset to offending value in offending
values:                 message. CARD32 Length of offending value. <varies>
                        Offending value

Received a message with a bad value specified.

ICE Error Classes

These errors are all major opcode 0 errors.

BadMajor

offending-minor-opcode: <any>

severity:               CanContinue

values:                 CARD8 Opcode

The opcode given is not one that has been registered.

NoAuthentication

offending-minor-opcode: ConnectionSetup, ProtocolSetup

severity:               ConnectionSetup \(-> FatalToConnection ProtocolSetup \
                        (-> FatalToProtocol

values:                 (none)

None of the authentication protocols offered are available.

NoVersion

offending-minor-opcode: ConnectionSetup, ProtocolSetup

severity:               ConnectionSetup \(-> FatalToConnection ProtocolSetup \
                        (-> FatalToProtocol

values:                 (none)

None of the protocol versions offered are available.

SetupFailed

offending-minor-opcode: ConnectionSetup, ProtocolSetup, AuthenticationReply

                        ConnectionSetup \(-> FatalToConnection ProtocolSetup \
severity:               (-> FatalToProtocol AuthenticationReply \(->
                        FatalToConnection if authenticating a connection,
                        otherwise FatalToProtocol

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: AuthenticationReply, AuthenticationRequired,
                        AuthenticationNextPhase

severity:               FatalToProtocol

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: AuthenticationReply, AuthenticationRequired,
                        AuthenticationNextPhase

severity:               FatalToProtocol

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: ProtocolSetup

severity:               FatalToProtocol (but see note)

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: ProtocolSetup

severity:               FatalToProtocol (but see note)

values:                 CARD8 opcode

The major opcode specified was already registered. This is fatal to the “new”
protocol being set up by ProtocolSetup but it does not affect the existing
registration.

UnknownProtocol

offending-minor-opcode: ProtocolSetup

severity:               FatalToProtocol

values:                 STRING protocol name

The protocol specified is not supported.

Chapter 6. State Diagrams

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

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

Chapter 7. Protocol Encoding

Table of Contents

Primitives
Enumerations
Compound Types
ICE Minor opcodes
Message Encoding
Error Class Encoding

    Generic Error Class Encoding
    ICE-specific Error Class Encoding

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.

Primitives

┌──────┬──────────┬───────────────────────────────────────────────────────────┐
│Type  │Length    │Description                                                │
│Name  │(bytes)   │                                                           │
├──────┼──────────┼───────────────────────────────────────────────────────────┤
│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                                │
└──────┴──────────┴───────────────────────────────────────────────────────────┘

Enumerations

┌─────────┬─────┬────────────┐
│Type Name│Value│Description │
├─────────┼─────┼────────────┤
│BOOL     │0    │False       │
├─────────┼─────┼────────────┤
│         │1    │True        │
└─────────┴─────┴────────────┘

Compound Types

┌─────────┬──────────────┬──────────┬─────────────────────────┐
│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)  │
└─────────┴──────────────┴──────────┴─────────────────────────┘

ICE Minor opcodes

┌───────────────────────┬────────┐
│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      │
└───────────────────────┴────────┘

Message Encoding

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

Error Class Encoding

Generic errors have classes in the range 0x8000-0xFFFF, and
subprotocol-specific errors are in the range 0x0000-0x7FFF.

Generic Error Class Encoding

┌──────────┬────────┐
│Class     │Encoding│
├──────────┼────────┤
│BadMinor  │0x8000  │
├──────────┼────────┤
│BadState  │0x8001  │
├──────────┼────────┤
│BadLength │0x8002  │
├──────────┼────────┤
│BadValue  │0x8003  │
└──────────┴────────┘

ICE-specific Error Class Encoding

┌──────────────────────┬────────┐
│Class                 │Encoding│
├──────────────────────┼────────┤
│BadMajor              │0       │
├──────────────────────┼────────┤
│NoAuthentication      │1       │
├──────────────────────┼────────┤
│NoVersion             │2       │
├──────────────────────┼────────┤
│SetupFailed           │3       │
├──────────────────────┼────────┤
│AuthenticationRejected│4       │
├──────────────────────┼────────┤
│AuthenticationFailed  │5       │
├──────────────────────┼────────┤
│ProtocolDuplicate     │6       │
├──────────────────────┼────────┤
│MajorOpcodeDuplicate  │7       │
├──────────────────────┼────────┤
│UnknownProtocol       │8       │
└──────────────────────┴────────┘

Appendix A. Modification History

Table of Contents

Release 6 to Release 6.1
Release 6.1 to Release 6.3

Release 6 to Release 6.1

Release 6.1 added the ICE X rendezvous protocol (Appendix B) and updated the
document version to 1.1.

Release 6.1 to Release 6.3

Release 6.3 added the listen on well known ports feature.

Appendix B. ICE X Rendezvous Protocol

Table of Contents

Introduction
Overview of ICE X Rendezvous
Registering Known Protocols
Initiating the Rendezvous
ICE Subprotocol Versioning

Introduction

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.

Overview of ICE X Rendezvous

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.

Registering Known Protocols

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.

Initiating the Rendezvous

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                                    │
├────────────────────┼────────┼───────────────────────────────────────────────┤
│                    │        │The client was unable to open the connection   │
│                    │        │(e.g. a call to IceOpenConnection() failed). If│
│OpenFailed          │1       │the client is able to distinguish              │
│                    │        │authentication or authorization errors from    │
│                    │        │general errors, then the preferred reply is    │
│                    │        │AuthenticationFailed for authorization errors. │
├────────────────────┼────────┼───────────────────────────────────────────────┤
│                    │        │Authentication or authorization of the         │
│                    │        │connection or protocol setup was refused. This │
│AuthenticationFailed│2       │reply will be given only if the client is able │
│                    │        │to distinguish it from OpenFailed otherwise    │
│                    │        │OpenFailed will be returned.                   │
├────────────────────┼────────┼───────────────────────────────────────────────┤
│                    │        │The client was unable to initiate the specified│
│SetupFailed         │3       │protocol on the connection (e.g. a call to     │
│                    │        │IceProtocolSetup() failed).                    │
├────────────────────┼────────┼───────────────────────────────────────────────┤
│                    │        │The client does not recognize the requested    │
│UnknownProtocol     │4       │protocol. (This represents a semantic error on │
│                    │        │the part of the answering party.)              │
├────────────────────┼────────┼───────────────────────────────────────────────┤
│                    │        │The client was in the process of removing      │
│                    │        │ICE_INITIATE_pname from its ICE_PROTOCOLS list │
│Refused             │5       │when the client message was sent; the client no│
│                    │        │longer is willing to establish the specified   │
│                    │        │ICE communication.                             │
└────────────────────┴────────┴───────────────────────────────────────────────┘

Note

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.

ICE Subprotocol Versioning

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.

Note

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.

