MQTT V5.0通信协议详解

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"MQTT-V5.0通讯协议是OASIS标准的一项候选，详细阐述了MQTT通信协议的最新版本，适用于物联网(IoT)和其他低带宽、高延迟或不可靠网络环境的数据传输。该协议文档是英文内容，包含了多个版本的链接，包括最新版和先前版本的DOCX、HTML和PDF格式。由OASIS Message Queuing Telemetry Transport (MQTT) TC技术委员会负责，由Richard Coppen和Andrew Banks等人编辑。" MQTT（Message Queuing Telemetry Transport，消息队列遥测传输）是一种轻量级的发布/订阅模型的协议，最初设计用于远程位置的设备，尤其是那些连接受限或不可靠的环境。MQTT V5.0是其最新版本，相较于V3.1.1，它引入了许多增强功能和改进，以提高效率、安全性和灵活性。 1. **服务质量(QoS)**: MQTT协议提供三种服务质量级别：QoS 0（至多一次），QoS 1（至少一次）和QoS 2（恰好一次）。这些级别确保消息在不同网络条件下的可靠性。 2. **主题过滤器与主题名称**: 主题是MQTT的核心概念，用于区分消息。V5.0中，主题过滤器和主题名称更加灵活，支持更复杂的命名和匹配规则。 3. **身份验证与授权**: MQTT V5.0增加了多种身份验证方法，如TLS/SSL证书、OAuth 2.0令牌，以及自定义认证机制，提高了安全性。同时，协议支持在连接时设置权限，允许服务器控制客户端可以访问哪些主题。 4. **扩展性**: 新版本增加了更多可选的报头字段和属性，允许用户自定义更多的元数据，以适应各种应用场景。 5. **错误处理**: MQTT V5.0提供了更详细的错误码和原因短语，使得客户端和服务器之间的错误通信更加明确。 6. **保留消息**: 客户端可以请求服务器保留消息，以便新的订阅者能够接收到之前发布的消息。 7. **会话状态**: MQTT支持持久化会话，即使客户端断开连接后，仍能恢复之前的状态，确保消息不会丢失。 8. **用户属性(User Properties)**: V5.0引入了用户属性，这是附加的键值对，可以在发布和订阅操作中传递，增加数据交换的灵活性。 9. **内容编解码**: MQTT V5.0支持自定义内容编码，允许使用非UTF-8的编码方式，如二进制数据。 10. **主题别名**: 为了减少网络负载，协议支持使用主题别名，客户端和服务器之间可以协商一个简短的别名来代替长主题。 11. **订阅标识符**: 在多订阅者场景中，订阅标识符允许客户端追踪特定订阅的结果。 12. **发布确认(Publish Acknowledgements)**: 改进了QoS 1和QoS 2的确认机制，确保消息的正确传递。 MQTT V5.0不仅保留了V3.1.1的轻量级特性，还引入了丰富的增强功能，使其在物联网、工业自动化、移动应用等领域得到更广泛的应用。开发人员在使用MQTT V5.0时，可以根据实际需求选择合适的服务质量、认证方式和扩展特性，构建高效、安全的通信解决方案。

mqtt-v5.0-cos02 11 February 2019

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[USEUPRIVSH]

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U.S.-EU Privacy Shield Framework

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https://www.privacyshield.gov

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[RFC3986]

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Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform Resource Identifier (URI): Generic Syntax",

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STD 66, RFC 3986, DOI 10.17487/RFC3986, January 2005,

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http://www.rfc-editor.org/info/rfc3986

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[RFC1035]

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Mockapetris, P., "Domain names - implementation and specification", STD 13, RFC 1035,

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DOI 10.17487/RFC1035, November 1987,

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http://www.rfc-editor.org/info/rfc1035

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[RFC2782]

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Gulbrandsen, A., Vixie, P., and L. Esibov, "A DNS RR for specifying the location of services (DNS SRV)",

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RFC 2782, DOI 10.17487/RFC2782, February 2000,

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http://www.rfc-editor.org/info/rfc2782

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1.5 Data representation

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1.5.1 Bits

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Bits in a byte are labelled 7 to 0. Bit number 7 is the most significant bit, the least significant bit is

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assigned bit number 0.

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1.5.2 Two Byte Integer

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Two Byte Integer data values are 16-bit unsigned integers in big-endian order: the high order byte

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precedes the lower order byte. This means that a 16-bit word is presented on the network as Most

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Significant Byte (MSB), followed by Least Significant Byte (LSB).

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1.5.3 Four Byte Integer

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Four Byte Integer data values are 32-bit unsigned integers in big-endian order: the high order byte

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precedes the successively lower order bytes. This means that a 32-bit word is presented on the network

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as Most Significant Byte (MSB), followed by the next most Significant Byte (MSB), followed by the next

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most Significant Byte (MSB), followed by Least Significant Byte (LSB).

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1.5.4 UTF-8 Encoded String

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Text fields within the MQTT Control Packets described later are encoded as UTF-8 strings. UTF-8

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[RFC3629] is an efficient encoding of Unicode [Unicode] characters that optimizes the encoding of ASCII

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characters in support of text-based communications.

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mqtt-v5.0-cos02 11 February 2019

Each of these strings is prefixed with a Two Byte Integer length field that gives the number of bytes in a

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UTF-8 encoded string itself, as illustrated in Figure 1.1 Structure of UTF-8 Encoded Strings below.

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Consequently, the maximum size of a UTF-8 Encoded String is 65,535 bytes.

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Unless stated otherwise all UTF-8 encoded strings can have any length in the range 0 to 65,535 bytes.

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Figure 1-1 Structure of UTF-8 Encoded Strings

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Bit

byte 1

String length MSB

byte 2

String length LSB

byte 3 ….

UTF-8 encoded character data, if length > 0.

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The character data in a UTF-8 Encoded String MUST be well-formed UTF-8 as defined by the Unicode

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specification [Unicode] and restated in RFC 3629 [RFC3629]. In particular, the character data MUST NOT

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include encodings of code points between U+D800 and U+DFFF [MQTT-1.5.4-1]. If the Client or Server

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receives an MQTT Control Packet containing ill-formed UTF-8 it is a Malformed Packet. Refer to section

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4.13 for information about handling errors.

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A UTF-8 Encoded String MUST NOT include an encoding of the null character U+0000. [MQTT-1.5.4-2].

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If a receiver (Server or Client) receives an MQTT Control Packet containing U+0000 it is a Malformed

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Packet. Refer to section 4.13 for information about handling errors.

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The data SHOULD NOT include encodings of the Unicode [Unicode] code points listed below. If a

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receiver (Server or Client) receives an MQTT Control Packet containing any of them it MAY treat it as a

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Malformed Packet. These are the Disallowed Unicode code points. Refer to section 5.4.9 for more

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information about handling Disallowed Unicode code points.

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• U+0001..U+001F control characters

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• U+007F..U+009F control characters

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• Code points defined in the Unicode specification [Unicode] to be non-characters (for example

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U+0FFFF)

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A UTF-8 encoded sequence 0xEF 0xBB 0xBF is always interpreted as U+FEFF ("ZERO WIDTH NO-

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BREAK SPACE") wherever it appears in a string and MUST NOT be skipped over or stripped off by a

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packet receiver [MQTT-1.5.4-3].

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Non-normative example

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For example, the string A𪛔 which is LATIN CAPITAL Letter A followed by the code point U+2A6D4

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(which represents a CJK IDEOGRAPH EXTENSION B character) is encoded as follows:

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Figure 1-2 UTF-8 Encoded String non-normative example

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Bit

byte 1

String Length MSB (0x00)

mqtt-v5.0-cos02 11 February 2019

encodedByte = encodedByte OR 128

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endif

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'output' encodedByte

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while (X > 0)

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Where MOD is the modulo operator (% in C), DIV is integer division (/ in C), and OR is bit-wise or

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(| in C).

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Non-normative comment

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The algorithm for decoding a Variable Byte Integer type is as follows:

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multiplier = 1

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value = 0

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encodedByte = 'next byte from stream'

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value += (encodedByte AND 127) * multiplier

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if (multiplier > 128*128*128)

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throw Error(Malformed Variable Byte Integer)

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multiplier *= 128

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while ((encodedByte AND 128) != 0)

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where AND is the bit-wise and operator (& in C).

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When this algorithm terminates, value contains the Variable Byte Integer value.

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1.5.6 Binary Data

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Binary Data is represented by a Two Byte Integer length which indicates the number of data bytes,

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followed by that number of bytes. Thus, the length of Binary Data is limited to the range of 0 to 65,535

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Bytes.

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1.5.7 UTF-8 String Pair

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A UTF-8 String Pair consists of two UTF-8 Encoded Strings. This data type is used to hold name-value

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pairs. The first string serves as the name, and the second string contains the value.

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Both strings MUST comply with the requirements for UTF-8 Encoded Strings [MQTT-1.5.7-1]. If a receiver

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(Client or Server) receives a string pair which does not meet these requirements it is a Malformed Packet.

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Refer to section 4.13 for information about handling errors.

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1.6 Security

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MQTT Client and Server implementations SHOULD offer Authentication, Authorization and secure

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communication options, such as those discussed in Chapter 5. Applications concerned with critical

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infrastructure, personally identifiable information, or other personal or sensitive information are strongly

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advised to use these security capabilities.

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