MQTT 5.0官方草案：物联网通信最新标准

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MQTT (Message Queuing Telemetry Transport) 是一种轻量级的、基于发布/订阅模式的网络通信协议，主要用于设备之间的低带宽、高效率的通信，特别适合于物联网(IoT)环境。此文档是 MQTT 版本 5.0 的 Committee Specification Draft 02 (CSRD02)，发布日期为 2017 年 10 月 26 日。这是当前最新版的 MQTT 规范，旨在提供更高效、安全和可扩展的连接，相比之前的版本进行了许多改进。在 MQTT 5.0 中，有几个关键特点值得关注： 1. **协议结构优化**：5.0 版本可能对消息格式和控制报文的定义进行了简化，提高了性能和解析效率。这包括可能对连接管理、认证、质量保证(QoS)等核心功能进行了重新设计，以适应现代 IoT 设备和应用场景的需求。 2. **安全性增强**：5.0 版本可能会引入更强的身份验证机制和加密手段，以保护数据传输的安全性，例如支持TLS/SSL作为安全通道，以及可能增加了对证书和加密算法的支持。 3. **QoS 改进**：服务质量(QoS)等级在早期版本中是 MQTT 的一个重要特性，5.0 可能对 QoS 模型做了优化，允许更高的可靠性和数据完整性保障，同时保持低延迟。 4. **客户端和服务器交互**：新版本可能会提供更多的灵活性和定制选项，以便于开发者构建具有不同需求的应用。这可能包括扩展连接选项、更好的错误处理机制和更丰富的控制信号。 5. **可扩展性和互操作性**：5.0 针对大规模部署和云原生应用进行了优化，增强了与其他标准的兼容性，如CoAP（Constrained Application Protocol）等，便于与不同平台和服务集成。 6. **标准化过程**：文档提供了多个官方版本链接，表明 OASIS MQTT TC 对规范的公开讨论和审查过程持续进行，反映了不断演进的技术共识和最佳实践。 MQTT 5.0 的发布标志着这一协议正在朝着更加现代化、安全和可扩展的方向发展，对于物联网开发者来说，理解和掌握这一新版本将有助于他们在实际项目中更好地利用 MQTT 进行设备间的数据传输和通信。

mqtt-v5.0-csprd02 26 October 2017

[RFC3986]

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

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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, DOI

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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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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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mqtt-v5.0-csprd02 26 October 2017

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.

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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)

byte 2

String Length LSB (0x05)

byte 3

‘A’ (0x41)

byte 4

(0xF0)

mqtt-v5.0-csprd02 26 October 2017

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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1.7 Editing convention

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Text highlighted in Yellow within this specification identifies conformance statements. Each conformance

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statement has been assigned a reference in the format [MQTT-x.x.x-y] where x.x.x is the section number

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and y is a statement counter within the section.

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