深入解析ITU-T X.224协议：连接模式传输服务

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"这篇文档详细解析了RDP（Remote Desktop Protocol）下的X224协议，该协议是连接模式协议规范的一部分，属于ITU-T（国际电信联盟电信标准部门）X系列推荐标准，用于开放系统互连中的连接模式传输服务。" 在远程桌面协议（RDP）中，X224协议扮演着关键角色。RDP是一种允许用户通过网络远程访问另一台计算机桌面环境的协议，广泛应用于Windows操作系统中。X224协议则负责在RDP会话中建立、管理和终止连接，提供可靠的数据传输服务。 X.224是ITU-T制定的一系列数据网络和开放系统通信标准之一。这个系列标准涵盖了从服务和设施、接口、传输、信号与切换，到网络方面、维护、行政安排，以及开放系统互连的各个方面。X系列的编号规则显示，X.200-X.299专注于模型和记法，X.210-X.219包含了连接模式协议的规范，而X.220-X.229则具体指代了X224所在的连接模式协议部分。 X224协议是OSI（开放系统互连）模型中传输层的一个子集，它提供了面向连接的服务，确保数据包按顺序到达且无丢失。在RDP中，X224协议负责建立控制通道，这一通道用来协商会话参数，如编码方式、安全性选项等，并管理数据传输的可靠性。此外，它还处理错误恢复、流量控制和拥塞控制，以确保高效且稳定的远程桌面连接。在实际应用中，X224协议通常与TPDU（Transport Protocol Data Unit）一起工作，TPDU是X224传输层的协议数据单元，它封装了RDP的上层数据，如会话初始化信息、屏幕更新指令、键盘和鼠标输入等。X224协议确保这些TPDU在不可靠的网络环境中也能被正确地传输和处理。在安全方面，虽然X224本身不包含直接的安全机制，但RDP可以与其他安全协议结合，如TLS（传输层安全）或NLA（网络级身份验证），来保护会话的隐私和完整性。这符合X系列标准中的X.260-X.269，这部分内容专门涉及安全协议。 X224协议是RDP中不可或缺的部分，它为远程桌面会话提供了可靠的连接基础，使得用户可以在远程位置像操作本地计算机一样操作远端设备。了解并掌握X224协议的工作原理，对于优化和调试RDP连接，以及增强远程访问的安全性具有重要意义。

ISO/IEC 8073 : 1997 (E)

8 ITU-T Rec. X.224 (1995 E)

5.3.1.2 Connection establishment

The purpose of connection establishment is to establish a transport connection between two TS-users. The following

functions of the Transport Layer during this phase match the TS-users’ requested quality of service with the services

offered by the Network Layer:

a) select the network service which best matches the requirement of the TS-user taking into account charges

for various services (see 6.5);

b) decide whether to multiplex multiple transport connections onto a single network connection only when

operating over CONS (see 6.5);

c) establish the optimum TPDU size (see 6.5);

d) select the functions that will be operational upon entering the data transfer phase (see 6.5);

e) map transport addresses onto network addresses;

f) provide a means to distinguish between two different transport connections (see 6.5);

g) transport of TS-user data (see 6.5);

h) exchange values of Inactivity timers (see 6.5).

5.3.1.3 Data transfer

The purpose of data transfer is to permit duplex transmission of TSDUs between the two TS-users connected by the

transport connection. This purpose is achieved by means of two-way simultaneous communication and by the following

functions, some of which are used or not used in accordance with the result of the selection performed in connection

establishment.

a) Concatenation and separation (see 6.4) – A function used to collect several TPDUs into a single NSDU

at the sending transport entity and to separate the TPDUs at the receiving transport entity.

b) Segmenting and reassembling (see 6.3) – A function used to segment a single data TSDU into

multiple TPDUs at the sending transport entity and to reassemble them into their original format at the

receiving transport entity.

c) Splitting and recombining (see 6.23) – A function allowing, only when operating over CONS, the

simultaneous use of two or more network connections to support the same transport connection.

d) Flow control (see 6.16) – A function used to regulate the flow of TPDUs between two transport entities

on one transport connection.

e) Transport connection identification – A means to uniquely identify a transport connection between the

pair of transport entities supporting the connection during the lifetime of the transport connection.

f) Expedited data (see 6.11) – A function used to bypass the flow control of normal data TPDU. Expedited

data TPDU flow is controlled by separate flow control.

g) TSDU delimiting (see 6.3) – A function used to determine the beginning and ending of a TSDU.

5.3.1.4 Release

The purpose of release (see 6.7 and 6.8) is to provide disconnection of the transport connection, regardless of the current

activity.

5.4 Classes and options when operating over CONS

5.4.1 General

The functions of the Transport Layer have been organized into classes and options.

A class defines a set of functions. Options define those functions within a class which may or may not be used.

This Recommendation | International Standard defines five classes of protocol:

a) class 0: simple class;

b) class 1: basic error recovery class;

c) class 2: multiplexing class;

d) class 3: error recovery and multiplexing class;

e) class 4: error detection and recovery class.

ISO/IEC 8073 : 1997 (E)

ITU-T Rec. X.224 (1995 E) 9

NOTES

1 Transport connections of classes 2, 3 and 4 may be multiplexed together onto the same network connection.

2 Classes 0 to 3 do not specify mechanisms to detect unsignalled network transmission failures.

5.4.2 Negotiation

The use of classes and options is negotiated during connection establishment. The choice made by the transport entities

will depend upon:

a) the TS-users’ requirements expressed via T-CONNECT service primitives;

b) the quality of the available network services;

c) the user required service versus cost ratio acceptable to the TS-user.

5.4.3 Choice of network connection

The following list classifies network services in terms of quality with respect to error behaviour in relation to user

requirements; its main purpose is to provide a basis for the decision regarding which class of transport protocol should

be used in conjunction with given network connection:

a) Type A – Network connection with acceptable residual error rate (for example, not signalled by

disconnect or reset) and acceptable rate of signalled errors.

b) Type B – Network connections with acceptable residual error rate (for example, not signalled by

disconnect or reset) but unacceptable rate of signalled errors.

c) Type C – Network connections with unacceptable residual error rate.

It is assumed that each transport entity is aware of the quality of service provided by particular network connections.

5.4.4 Characteristics of class 0

Class 0 provides the simplest type of transport connection and is fully compatible with the Recommendation T.70 for

teletex terminals.

Class 0 has been designed to be used with type A network connections.

5.4.5 Characteristics of class 1

Class 1 provides a basic transport connection with minimal overheads.

The main purpose of the class is to recover from network disconnect or reset.

Selection of this class is usually based on reliability criteria. Class 1 has been designed to be used with type B network

connections.

5.4.6 Characteristics of class 2

5.4.6.1 General

Class 2 provides a way to multiplex several transport connections onto a single network connection. This class has been

designed to be used with type A network connections.

5.4.6.2 Use of explicit flow control

The objective is to provide flow control to help avoid congestion at transport-connection-end-points and on the network

connection. Typical use is when traffic is heavy and continuous, or when there is intensive multiplexing. Use of flow

control can optimize response times and resource utilization.

5.4.6.3 Non-use of explicit flow control

The objective is to provide a basic transport connection with minimal overheads suitable when explicit disconnection of

the transport connection is desirable. The option would typically be used for unsophisticated terminals, and when no

multiplexing onto network connections is required. Expedited data is never available.

5.4.7 Characteristics of class 3

Class 3 provides the characteristics of class 2 plus the ability to recover from network disconnect or reset. Selection of

this class is usually based upon reliability criteria. Class 3 has been designed to be used with type B network

connections.

ISO/IEC 8073 : 1997 (E)

10 ITU-T Rec. X.224 (1995 E)

5.4.8 Characteristics of class 4

Class 4 provides the characteristics of class 3, plus the capability to detect and recover from errors which occur as a

result of the low grade of service available from the NS-provider. The kind of errors to be detected include:

– TPDU loss;

– TPDU delivery out of sequence;

– TPDU duplication; and

– TPDU corruption.

These errors may affect control TPDUs as well as data TPDUs.

This class also provides for increased throughput capability and additional resilience against network failure.

Class 4 has been designed to be used with type C network connections.

5.5 Characteristics of class 4 transport protocol when operating over CLNS

In operation over a connectionless-mode network service the class 4 transport procotol provides flow control between

communicating peer transport entities, the capability to detect and recover from errors which occur as a result of a low

grade of service available from the NS-provider, and resilience from failure of the peer entity. The kind of errors to be

detected include:

– TPDU loss;

– TPDU delivery out of sequence;

– TPDU duplication; and

– TPDU corruption.

These errors may affect control TPDUs as well as data TPDUs.

NOTE – The transport entity is incapable of distinguishing between failure of the network service and failure of the peer

entity, except optionally, by some local means, in the case of the failure of the local interface to the network service (e.g. in the failure

of the local transceiver on a local area network).

There is no indication given to the transport entity about the ability of the network entity to fulfill the service

requirements given in the N-UNIT-DATA primitive. However, it can be a local matter to make transport entities aware

of the availability and characteristics (QOS) of connectionless-mode network services, as the corresponding

NSAP associations, exist logically by the nature of the connectionless-mode network service and may be recognized by

network entities.

5.6 Model of the Transport Layer

A transport entity communicates with its TS-users through one or more TSAPs by means of the service primitives as

defined by the transport service definition (see ITU-T Rec. X.214 | ISO/IEC 8072). Service primitives will cause or be

the result of transport protocol data unit exchanges between the peer transport entities supporting a transport connection.

These protocol exchanges are effected using the services of the Network Layer as defined by the network service

definition (see CCITT Rec. X.213 | ISO/IEC 8348) through one or more NSAPs. (See Figure 2.)

Transport connection endpoints are identified in end systems by an internal, implementation dependent, mechanism so

that the TS-user and the transport entity can refer to each transport connection.

6 Elements of procedure

This clause contains elements of procedure which are used in the specification of protocol classes in clauses 7 to 12.

These elements are not meaningful on their own.

The procedures define the transfer of TPDUs whose structure and coding is specified in clause 13. Transport entities

shall accept and respond to any TPDU received in a valid NSDU and may issue TPDUs initiating specific elements of

procedure specified in this clause.

NOTE – Where network service primitives, TPDUs and parameters used are not significant for a particular element of

procedure, they have not been included in the specification.

ISO/IEC 8073 : 1997 (E)

ITU-T Rec. X.224 (1995 E) 11

T0719310-94/d02

TSAP TSAP

Transport entity Transport entity

NSAP NSAP

NOTE – For the purposes of illustration, this figure shows only one TSAP and one NSAP for each transport entity.

In certain instances, more than one TSAP and/or more than one NSAP may be associated with a particular transport

entity.

Figure 2 – Model of the Transport Layer

FIGURE 2...[D02] = 3 CM

6.1 Use of the network service

6.1.1 Assignment to network connection when operating over CONS

This procedure is used only when operating over the connection-mode network service.

6.1.1.1 Purpose

The procedure is used in all classes to assign transport connections to network connections.

6.1.1.2 Network service primitives

The procedure uses the following network service primitives:

a) N-CONNECT;

b) N-DISCONNECT.

6.1.1.3 Procedure

Each transport connection shall be assigned to a network connection. The initiator may assign the transport connection

to an existing network connection of which it is the owner or to a new network connection (see Note 1) which it creates

for this purpose.

The initiator shall not assign or reassign the transport connection to an existing network connection if the protocol

class(es) proposed for the class in use for the transport connection, (is) (are) incompatible with the current usage of the

network connection with respect to multiplexing (see Note 2).

During the resynchronization (see 6.14) and reassignment after failure (see 6.12) procedures, the transport entity may

reassign a transport connection to another network connection joining the same NSAPs, provided that it is the owner of

the network connection and that the transport connection is assigned to only one network connection at any given time.

During the splitting procedure (see 6.23), a transport entity may assign a transport connection to any additional network

connection joining the same NSAPs, provided that it is the owner of the network connection and that either the network

connection does not have another transport connection assigned to it or multiplexing is possible on the network

connection.

The responder becomes aware of the assignment when it receives:

a) a CR-TPDU during the connection establishment procedure (see 6.5); or

b) an RJ-TPDU or a retransmitted CR or DR TPDU during the resynchronization (see 6.14) and

reassignment after failure (see 6.12) procedures; or

c) any TPDU when splitting (see 6.23) is used.

ISO/IEC 8073 : 1997 (E)

12 ITU-T Rec. X.224 (1995 E)

NOTES

1 When a new network connection is created, the quality of service requested is a local matter, although it will

normally be related to the requirements of transport connection(s) expected to be assigned to it.

2 An existing network connection may also not be suitable if, for example, the quality of service requested for the

transport connection cannot be attained by using or enhancing the network connection.

3 A network connection with no transport connection(s) assigned to it, may be available after initial establishment, or

because all of the transport connections previously assigned to it have been released. It is recommended that only the owner of such a

network connection should release it. Furthermore, it is recommended that it not be released immediately after the transmission of the

final TPDU of a transport connection, either a DR-TPDU in response to CR-TPDU or a DC-TPDU in response to DR-TPDU. An

appropriate delay will allow the TPDU concerned to reach the other transport entity allowing the freeing of any resources associated

with the transport connection concerned.

4 After the failure of a network connection, transport connections which were previously multiplexed together may be

assigned to different network connections, and vice versa.

5 The transport protocol identification procedures specified in Recommendation X.264 may need to be considered in

conjunction with this procedure.

6.1.2 Transmission over CLNS

This procedure is used only when operating over the connectionless-mode network service.

6.1.2.1 Purpose

The procedure is used to transmit TPDUs over the connectionless-mode network service.

6.1.2.2 Network service primitives

The procedure makes use of the following network service primitive:

– N-UNIT-DATA.

6.1.2.3 Procedure

Each TPDU shall be transmitted in a single invocation of the connectionless-mode network service, over a pre-existing

association between a pair of NSAPs. The association is considered by transport entities as permanently established and

available.

6.2 Transport Protocol Data Unit (TPDU) transfer

6.2.1 Purpose

The TPDU transfer procedure is used in all classes to convey transport protocol data units in user data fields of network

service primitives.

6.2.2 Network service primitives

The procedure uses the following network service primitives when operating over CONS:

a) N-DATA;

b) N-EXPEDITED DATA.

The procedure uses the following network service primitive when operating over CLNS:

– N-UNIT-DATA.

6.2.3 Procedure

The Transport Protocol Data Units (TPDUs) defined for the protocol, are listed in 4.2.

When operating over CLNS, the transport entities shall transmit and receive all TPDUs as NS-user data parameters of

N-UNIT-DATA primitives.

When operating over CONS and when the network expedited variant has been selected for class 1, the transport entities

shall transmit and receive ED and EA-TPDUs as NS-user data parameters of N-EXPEDITED DATA primitives.

In all other cases, transport entities shall transmit and receive TPDUs as NS-user data parameters of N-DATA

primitives.

When a TPDU is put into an NS-user data parameter, the significance of the bits within an octet and the order of octets

within a TPDU shall be defined in 13.2.

NOTE – TPDUs may be concatenated (see 6.4).

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