ISO/IEC 20922:2016 情報技術—メッセージキューテレメトリトランスポート（MQTT）v3.1.1 | ページ 3

1.5.1 ビット

バイト内のビットには 7 ～ 0 のラベルが付けられます。ビット番号 7 は最上位ビットで、最下位ビットにはビット番号 0 が割り当てられます。

1 Introduction

1.1 Organization of MQTT

This specification is split into seven chapters:

• Chapter 1 - Introduction
• Chapter 2 - MQTT Control Packet format
• Chapter 3 - MQTT Control Packets
• Chapter 4 - Operational behavior
• Chapter 5 - Security
• Chapter 6 - Using WebSocket as a network transport
• Chapter 7 - Conformance Targets

1.2 Terminology

The key words"MUST","MUST NOT","REQUIRED","SHALL","SHALL NOT","SHOULD","SHOULD NOT","RECOMMENDED","MAY", and"OPTIONAL" in this specification are to be interpreted as described in IETF RFC 2119 [RFC2119].

Network Connection:

A construct provided by the underlying transport protocol that is being used by MQTT.

• It connects the Client to the Server.
• It provides the means to send an ordered, lossless, stream of bytes in both directions.

For examples see Section 4.2.

Application Message:

The data carried by the MQTT protocol across the network for the application. When Application

Messages are transported by MQTT they have an associated Quality of Service and a Topic Name.

Client:

A program or device that uses MQTT. A Client always establishes the Network Connection to the It can

• Publish Application Messages that other Clients might be interested in.
• Subscribe to request Application Messages that it is interested in receiving.
• Unsubscribe to remove a request for Application Messages.
• Disconnect from the Server.

Server:

A program or device that acts as an intermediary between Clients which publish Application Messages and Clients which have made Subscriptions. A Server

• Accepts Network Connections from Clients.
• Accepts Application Messages published by Clients.
• Processes Subscribe and Unsubscribe requests from Clients.
• Forwards Application Messages that match Client Subscriptions.

Subscription:

A Subscription comprises a Topic Filter and a maximum QoS. A Subscription is associated with a single Session. A Session can contain more than one Subscription. Each Subscription within a session has a different Topic Filter.

Topic Name:

The label attached to an Application Message which is matched against the Subscriptions known to the Server. The Server sends a copy of the Application Message to each Client that has a matching Subscription.

Topic Filter:

An expression contained in a Subscription, to indicate an interest in one or more topics. A Topic Filter can include wildcard characters.

Session:

A stateful interaction between a Client and a Server . Some Sessions last only as long as the Network Connection, others can span multiple consecutive Network Connections between a Client and a Server.

MQTT Control Packet:

A packet of information that is sent across the Network Connection. The MQTT specification defines fourteen different types of Control Packet, one of which (the PUBLISH packet) is used to convey Application Messages.

1.3 Normative references

1.4 Non normative references

1.5 Data representations

1.5.2 Integer data values

Integer data values are 16 bits in big-endian order: the high order byte precedes the lower order byte. This means that a 16-bit word is presented on the network as Most Significant Byte (MSB), followed by Least Significant Byte (LSB).

1.5.3 UTF-8 encoded strings

Text fields in the Control Packets described later are encoded as UTF-8 strings. UTF-8 [RFC3629] is an efficient encoding of Unicode [Unicode] characters that optimizes the encoding of ASCII characters in support of text-based communications.

Each of these strings is prefixed with a two byte length field that gives the number of bytes in a UTF-8 encoded string itself, as illustrated in Figure 1.1 Structure of UTF-8 encoded strings below. Consequently there is a limit on the size of a string that can be passed in one of these UTF-8 encoded string components; you cannot use a string that would encode to more than 65535 bytes.

Unless stated otherwise all UTF-8 encoded strings can have any length in the range 0 to 65535 bytes.

The character data in a UTF-8 encoded string MUST be well-formed UTF-8 as defined by the Unicode specification [Unicode] and restated in RFC 3629 [RFC3629]. In particular this data MUST NOT include encodings of code points between U+D800 and U+DFFF. If a Server or Client receives a Control Packet containing ill-formed UTF-8 it MUST close the Network Connection [MQTT-1.5.3-1].

A UTF-8 encoded string MUST NOT include an encoding of the null character U+0000. If a receiver (Server or Client) receives a Control Packet containing U+0000 it MUST close the Network Connection [MQTT-1.5.3-2].

The data SHOULD NOT include encodings of the Unicode [Unicode] code points listed below. If a receiver (Server or Client) receives a Control Packet containing any of them it MAY close the Network Connection:

U+0001..U+001F control characters

U+007F..U+009F control characters

Code points defined in the Unicode specification [Unicode] to be non-characters (for example U+0FFFF)

A UTF-8 encoded sequence 0xEF 0xBB 0xBF is always to be interpreted to mean U+FEFF ("ZERO WIDTH NO-BREAK SPACE") wherever it appears in a string and MUST NOT be skipped over or stripped off by a packet receiver [MQTT-1.5.3-3].

1.5.3.1 Non normative example

For example, the string A which is LATIN CAPITAL Letter A followed by the code point U+2A6D4 (which represents a CJK IDEOGRAPH EXTENSION B character) is encoded as follows:

1.6 Editing conventions

Text highlighted in Yellow within this specification identifies conformance statements. Each conformance statement has been assigned a reference in the format [MQTT-x.x.x-y].