MQTT学习了解------阿冬专栏
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MQTT V3.1 Protocol Specification
- Authors:
- International Business Machines Corporation (IBM)
- Eurotech
Abstract
MQ Telemetry Transport (MQTT) is a lightweight broker-based publish/subscribe messaging protocol designed to be open, simple, lightweight and easy to implement. These characteristics make it ideal for use in constrained environments, for example, but not limited to:
- Where the network is expensive, has low bandwidth or is unreliable
- When run on an embedded device with limited processor or memory resources
Features of the protocol include:
- The publish/subscribe message pattern to provide one-to-many message distribution and decoupling of applications
- A messaging transport that is agnostic to the content of the payload
- The use of TCP/IP to provide basic network connectivity
- Three qualities of service for message delivery:
- "At most once", where messages are delivered according to the best efforts of the underlying TCP/IP network. Message loss or duplication can occur. This level could be used, for example, with ambient sensor data where it does not matter if an individual reading is lost as the next one will be published soon after.
- "At least once", where messages are assured to arrive but duplicates may occur.
- "Exactly once", where message are assured to arrive exactly once. This level could be used, for example, with billing systems where duplicate or lost messages could lead to incorrect charges being applied.
- A small transport overhead (the fixed-length header is just 2 bytes), and protocol exchanges minimised to reduce network traffic
- A mechanism to notify interested parties to an abnormal disconnection of a client using the Last Will and Testament feature
Copyright Notice
© 1999-2010 Eurotech, International Business Machines Corporation (IBM). All rights reserved.
Permission to copy and display the MQ Telemetry Transport specification (the "Specification"), in any medium without fee or royalty is hereby granted by Eurotech and International Business Machines Corporation (IBM) (collectively, the "Authors"), provided that you include the following on ALL copies of the Specification, or portions thereof, that you make:
- A link or URL to the Specification at one of the Authors' websites.
- The copyright notice as shown in the Specification.
The Authors each agree to grant you a royalty-free license, under reasonable, non-discriminatory terms and conditions to their respective patents that they deem necessary to implement the Specification. THE SPECIFICATION IS PROVIDED "AS IS," AND THE AUTHORS MAKE NO REPRESENTATIONS OR WARRANTIES, EXPRESS OR IMPLIED, INCLUDING, BUT NOT LIMITED TO, WARRANTIES OF MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE, NON-INFRINGEMENT, OR TITLE; THAT THE CONTENTS OF THE SPECIFICATION ARE SUITABLE FOR ANY PURPOSE; NOR THAT THE IMPLEMENTATION OF SUCH CONTENTS WILL NOT INFRINGE ANY THIRD PARTY PATENTS, COPYRIGHTS, TRADEMARKS OR OTHER RIGHTS. THE AUTHORS WILL NOT BE LIABLE FOR ANY DIRECT, INDIRECT, SPECIAL, INCIDENTAL OR CONSEQUENTIAL DAMAGES ARISING OUT OF OR RELATING TO ANY USE OR DISTRIBUTION OF THE SPECIFICATION.
The name and trademarks of the Authors may NOT be used in any manner, including advertising or publicity pertaining to the Specification or its contents without specific, written prior permission. Title to copyright in the Specification will at all times remain with the Authors.
No other rights are granted by implication, estoppel or otherwise.
Table of Contents
- 1. Introduction
- 1.1. Changes
- 2. Message format
- 2.1. Fixed header
- 2.2. Variable header
- 2.3. Payload
- 2.4. Message identifiers
- 2.5. MQTT and UTF-8
- 3. Command messages
- 3.1. CONNECT
- 3.2. CONNACK
- 3.3. PUBLISH
- 3.4. PUBACK
- 3.5. PUBREC
- 3.6. PUBREL
- 3.7. PUBCOMP
- 3.8. SUBSCRIBE
- 3.9. SUBACK
- 3.10. UNSUBSCRIBE
- 3.11. UNSUBACK
- 3.12. PINGREQ
- 3.13. PINGRESP
- 3.14. DISCONNECT
- 4. Flows
- 4.1. Quality of Service levels and flows
- 4.2. Message delivery retry
- 4.3. Message ordering
- Appendix A
1. Introduction
This specification is split into three main sections:
- the message format that is common to all packet types,
- the specific details of each packet type,
- how the packets flow between client and server.
Information on how topic wildcards are used is provided in the appendix.
1.1. Changes
The following are the changes between MQTT V3 and MQTT V3.1:
- User name and password can now be sent with a CONNECT packet
- New return codes on CONNACK packets, for security problems
- Clarification that clients are not informed of un-authorized PUBLISH or SUBSCRIBE commands, and that the normal MQTT flow should complete even though the command has not been performed.
- Strings in MQTT now support full UTF-8, instead of just the US-ASCII subset.
The protocol version number passed with CONNECT packets, is unchanged for this revision, and remains as the "3". Existing MQTT V3 server implementations should be able to accept connections from clients that support this revision, as long as they correctly respect the "Remaining Length" field, and therefore ignore the extra security information.
2. Message format
The message header for each MQTT command message contains a fixed header. Some messages also require a variable header and a payload. The format for each part of the message header is described in the following sections:
2.1. Fixed header
The message header for each MQTT command message contains a fixed header. The table below shows the fixed header format.
- Byte 1
Contains the Message Type and Flags (DUP, QoS level, and RETAIN) fields.
- Byte 2
(At least one byte) contains the Remaining Length field.
The fields are described in the following sections. All data values are in big-endian order: higher order bytes precede lower order bytes. A 16-bit word is presented on the wire as Most Significant Byte (MSB), followed by Least Significant Byte (LSB).
Message Type
Position: byte 1, bits 7-4.Represented as a 4-bit unsigned value. The enumerations for this version of the protocol are shown in the table below.
Flags
The remaining bits of byte 1 contain the fields DUP, QoS, and RETAIN. The bit positions are encoded to represent the flags as shown in the table below.
- DUP
Position: byte 1, bit 3.
This flag is set when the client or server attempts to re-deliver a PUBLISH, PUBREL, SUBSCRIBE or UNSUBSCRIBE message. This applies to messages where the value of QoS is greater than zero (0), and an acknowledgment is required. When the DUP bit is set, the variable header includes a Message ID.
The recipient should treat this flag as a hint as to whether the message may have been previously received. It should not be relied on to detect duplicates.
- QoS
Position: byte 1, bits 2-1.
This flag indicates the level of assurance for delivery of a PUBLISH message. The QoS levels are shown in the table below.
QoS value bit 2 bit 1 Description 000At most onceFire and Forget<=1101At least onceAcknowledged delivery>=1210Exactly onceAssured delivery=1311Reserved- RETAIN
Position: byte 1, bit 0.
This flag is only used on PUBLISH messages. When a client sends a PUBLISH to a server, if the Retain flag is set (1), the server should hold on to the message after it has been delivered to the current subscribers.
When a new subscription is established on a topic, the last retained message on that topic should be sent to the subscriber with the Retain flag set. If there is no retained message, nothing is sent
This is useful where publishers send messages on a "report by exception" basis, where it might be some time between messages. This allows new subscribers to instantly receive data with the retained, or Last Known Good, value.
When a server sends a PUBLISH to a client as a result of a subscription that already existed when the original PUBLISH arrived, the Retain flag should not be set, regardless of the Retain flag of the original PUBLISH. This allows a client to distinguish messages that are being received because they were retained and those that are being received "live".
Retained messages should be kept over restarts of the server.
A server may delete a retained message if it receives a message with a zero-length payload and the Retain flag set on the same topic.
Remaining Length
Position: byte 2.
Represents the number of bytes remaining within the current message, including data in the variable header and the payload.
The variable length encoding scheme uses a single byte for messages up to 127 bytes long. Longer messages are handled as follows. Seven bits of each byte encode the Remaining Length data, and the eighth bit indicates any following bytes in the representation. Each byte encodes 128 values and a "continuation bit". For example, the number 64 decimal is encoded as a single byte, decimal value 64, hex 0x40. The number 321 decimal (= 65 + 2*128) is encoded as two bytes, least significant first. The first byte 65+128 = 193. Note that the top bit is set to indicate at least one following byte. The second byte is 2.
The protocol limits the number of bytes in the representation to a maximum of four. This allows applications to send messages of up to 268 435 455 (256 MB). The representation of this number on the wire is: 0xFF, 0xFF, 0xFF, 0x7F.
The table below shows the Remaining Length values represented by increasing numbers of bytes.
The algorithm for encoding a decimal number (X) into the variable length encoding scheme is as follows:
do digit = X MOD 128 X = X DIV 128 // if there are more digits to encode, set the top bit of this digit if ( X > 0 ) digit = digit OR 0x80 endif 'output' digitwhile ( X> 0 )where MOD is the modulo operator (% in C), DIV is integer division (/ in C), and OR is bit-wise or (| in C).
The algorithm for decoding the Remaining Length field is as follows:
multiplier = 1 value = 0 do digit = 'next digit from stream' value += (digit AND 127) * multiplier multiplier *= 128while ((digit AND 128) != 0)where AND is the bit-wise and operator (& in C).
When this algorithm terminates, value contains the Remaining Length in bytes.
Remaining Length encoding is not part of the variable header. The number of bytes used to encode the Remaining Length does not contribute to the value of the Remaining Length. The variable length "extension bytes" are part of the fixed header, not the variable header.
2.2. Variable header
Some types of MQTT command messages also contain a variable header component. It resides between the fixed header and the payload.
The variable length Remaining Length field is not part of the variable header. The bytes of the Remaining Length field do not contribute to the byte count of the Remaining Length value. This value only takes account of the variable header and the payload. See Fixed header for more information.
The format of the variable header fields are described in the following sections, in the order in which they must appear in the header:
Protocol name
The protocol name is present in the variable header of a MQTT CONNECT message. This field is a UTF-encoded string that represents the protocol name MQIsdp, capitalized as shown.
Protocol version
The protocol version is present in the variable header of a CONNECT message.
The field is an 8-bit unsigned value that represents the revision level of the protocol used by the client. The value of the Protocol version field for the current version of the protocol, 3 (0x03), is shown in the table below.
Connect flags
The Clean session, Will, Will QoS, and Retain flags are present in the variable header of a CONNECT message.
Clean session flag
Position: bit 1 of the Connect flags byte.
If not set (0), then the server must store the subscriptions of the client after it disconnects. This includes continuing to store QoS 1 and QoS 2 messages for the subscribed topics so that they can be delivered when the client reconnects. The server must also maintain the state of in-flight messages being delivered at the point the connection is lost. This information must be kept until the client reconnects.
If set (1), then the server must discard any previously maintained information about the client and treat the connection as "clean". The server must also discard any state when the client disconnects.
Typically, a client will operate in one mode or the other and not change. The choice will depend on the application. A clean session client will not receive stale information and it must resubscribe each time it connects. A non-clean session client will not miss any QoS 1 or QoS 2 messages that were published whilst it was disconnected. QoS 0 messages are never stored, since they are delivered on a best efforts basis.
This flag was formerly known as "Clean start". It has been renamed to clarify the fact it applies to the whole session and not just to the initial connect.
A server may provide an administrative mechanism for clearing stored information about a client which can be used when it is known that a client will never reconnect.
Bit 0 of this byte is not used in the current version of the protocol. It is reserved for future use.
Will flag
Position: bit 2 of the Connect flags byte.
The Will message defines that a message is published on behalf of the client by the server when either an I/O error is encountered by the server during communication with the client, or the client fails to communicate within the Keep Alive timer schedule. Sending a Will message is not triggered by the server receiving a DISCONNECT message from the client.
If the Will flag is set, the Will QoS and Will Retain fields must be present in the Connect flags byte, and the Will Topic and Will Message fields must be present in the payload.
The format of the Will flag is shown in the table below.
Bit 0 of this byte is not used in the current version of the protocol. It is reserved for future use.
Will QoS
Position: bits 4 and 3 of the Connect flags byte.
A connecting client specifies the QoS level in the Will QoS field for a Will message that is sent in the event that the client is disconnected involuntarily. The Will message is defined in the payload of a CONNECT message.
If the Will flag is set, the Will QoS field is mandatory, otherwise its value is disregarded.
The value of Will QoS is 0 (0x00), 1 (0x01), or 2 (0x02). The Will QoS flag is shown in the table below.
Bit 0 of this byte is not used in the current version of the protocol. It is reserved for future use.
Will Retain flag
Position: bit 5 of the Connect flags byte.
The Will Retain flag indicates whether the server should retain the Will message which is published by the server on behalf of the client in the event that the client is disconnected unexpectedly.
The Will Retain flag is mandatory if the Will flag is set, otherwise, it is disregarded. The format of the Will Retain flag is shown in the table below.
Bit 0 of this byte is not used in the current version of the protocol. It is reserved for future use.
User name and password flags
Position: bits 6 and 7 of the Connect flags byte.
A connecting client can specify a user name and a password, and setting the flag bits signifies that a User Name, and optionally a password, are included in the payload of aCONNECT message.
If the User Name flag is set, the User Name field is mandatory, otherwise its value is disregarded. If the Password flag is set, the Password field is mandatory, otherwise its value is disregarded. It is not valid to supply a password without supplying a user name.
Bit 0 of this byte is not used in the current version of the protocol. It is reserved for future use.
Keep Alive timer
The Keep Alive timer is present in the variable header of a MQTT CONNECT message.
The Keep Alive timer, measured in seconds, defines the maximum time interval between messages received from a client. It enables the server to detect that the network connection to a client has dropped, without having to wait for the long TCP/IP timeout. The client has a responsibility to send a message within each Keep Alive time period. In the absence of a data-related message during the time period, the client sends a PINGREQ message, which the server acknowledges with a PINGRESP message.
If the server does not receive a message from the client within one and a half times the Keep Alive time period (the client is allowed "grace" of half a time period), it disconnects the client as if the client had sent a DISCONNECT message. This action does not impact any of the client's subscriptions. See DISCONNECT for more details.
If a client does not receive a PINGRESP message within a Keep Alive time period after sending a PINGREQ, it should close the TCP/IP socket connection.
The Keep Alive timer is a 16-bit value that represents the number of seconds for the time period. The actual value is application-specific, but a typical value is a few minutes. The maximum value is approximately 18 hours. A value of zero (0) means the client is not disconnected.
The format of the Keep Alive timer is shown in the table below. The ordering of the 2 bytes of the Keep Alive Timer is MSB, then LSB (big-endian).
Connect return code
The connect return code is sent in the variable header of a CONNACK message.
This field defines a one byte unsigned return code. The meanings of the values, shown in the tables below, are specific to the message type. A return code of zero (0) usually indicates success.
EnumerationHEXMeaning00x00Connection Accepted10x01Connection Refused: unacceptable protocol version20x02Connection Refused: identifier rejected30x03Connection Refused: server unavailable40x04Connection Refused: bad user name or password50x05Connection Refused: not authorized6-255 Reserved for future useTopic name
The topic name is present in the variable header of an MQTT PUBLISH message.
The topic name is the key that identifies the information channel to which payload data is published. Subscribers use the key to identify the information channels on which they want to receive published information.
The topic name is a UTF-encoded string. See the section on MQTT and UTF-8 for more information. Topic name has an upper length limit of 32,767 characters.
2.3. Payload
The following types of MQTT command message have a payload:
- CONNECT
- The payload contains one or more UTF-8 encoded strings. They specify a unqiue identifier for the client, a Will topic and message and the User Name and Password to use. All but the first are optional and their presence is determined based on flags in the variable header.
- SUBSCRIBE
- The payload contains a list of topic names to which the client can subscribe, and the QoS level. These strings are UTF-encoded.
- SUBACK
- The payload contains a list of granted QoS levels. These are the QoS levels at which the administrators for the server have permitted the client to subscribe to a particular Topic Name. Granted QoS levels are listed in the same order as the topic names in the corresponding SUBSCRIBE message.
The payload part of a PUBLISH message contains application-specific data only. No assumptions are made about the nature or content of the data, and this part of the message is treated as a BLOB.
If you want an application to apply compression to the payload data, you need to define in the application the appropriate payload flag fields to handle the compression details. You cannot define application-specific flags in the fixed or variable headers.
2.4. Message identifiers
The message identifier is present in the variable header of the following MQTT messages: PUBLISH, PUBACK, PUBREC, PUBREL, PUBCOMP, SUBSCRIBE, SUBACK, UNSUBSCRIBE, UNSUBACK.
The Message Identifier (Message ID) field is only present in messages where the QoS bits in the fixed header indicate QoS levels 1 or 2. See section on Quality of Service levels and flows for more information.
The Message ID is a 16-bit unsigned integer that must be unique amongst the set of "in flight" messages in a particular direction of communication. It typically increases by exactly one from one message to the next, but is not required to do so.
A client will maintain its own list of Message IDs separate to the Message IDs used by the server it is connected to. It is possible for a client to send a PUBLISH with Message ID 1 at the same time as receiving a PUBLISH with Message ID 1.
The ordering of the two bytes of the Message Identifier is MSB, then LSB (big-endian).
Do not use Message ID 0. It is reserved as an invalid Message ID.
2.5. MQTT and UTF-8
UTF-8 is an efficient encoding of Unicode character-strings that optimizes the encoding of ASCII characters in support of text-based communications.
In MQTT, strings are prefixed with two bytes to denote the length, as shown in the table below.
String Length is the number of bytes of encoded string characters, not the number of characters. For example, the string OTWP is encoded in UTF-8 as shown in the table below.
The Java writeUTF() and readUTF() data stream methods use this format.
2.6. Unused bits
Any bits marked as unused should be set to zero (0).
3. Command messages
- CONNECT
- CONNACK
- PUBLISH
- PUBACK
- PUBREC
- PUBREL
- PUBCOMP
- SUBSCRIBE
- SUBACK
- UNSUBSCRIBE
- UNSUBACK
- PINGREQ
- PINGRESP
- DISCONNECT
3.1. CONNECT - Client requests a connection to a server
When a TCP/IP socket connection is established from a client to a server, a protocol level session must be created using a CONNECT flow.
Fixed header
The fixed header format is shown in the table below.
The DUP, QoS, and RETAIN flags are not used in the CONNECT message.
Remaining Length is the length of the variable header (12 bytes) and the length of the Payload. This can be a multibyte field.
Variable header
An example of the format of the variable header is shown in the table below.
Password flag (1)
Will RETAIN (0)
Will QoS (01)
Will flag (1)
Clean Session (1)1100111xKeep Alive timerbyte 11Keep Alive MSB (0)00000000byte 12Keep Alive LSB (10)00001010
- User name flag
- Set (1).
- Password flag
- Set (1).
- Clean Session flag
- Set (1).
- Keep Alive timer
- Set to 10 seconds (0x000A).
- Will message
- Will flag is set (1)
- Will QoS field is 1
- Will RETAIN flag is clear (0)
Payload
The payload of the CONNECT message contains one or more UTF-8 encoded strings, based on the flags in the variable header. The strings, if present, must appear in the following order:- Client Identifier
The first UTF-encoded string. The Client Identifier (Client ID) is between 1 and 23 characters long, and uniquely identifies the client to the server. It must be unique across all clients connecting to a single server, and is the key in handling Message IDs messages with QoS levels 1 and 2. If the Client ID contains more than 23 characters, the server responds to the CONNECT message with a CONNACK return code 2: Identifier Rejected.
- Will Topic
If the Will Flag is set, this is the next UTF-8 encoded string. The Will Message is published to the Will Topic. The QoS level is defined by the Will QoS field, and the RETAIN status is defined by the Will RETAIN flag in the variable header.
- Will Message
If the Will Flag is set, this is the next UTF-8 encoded string. The Will Message defines the content of the message that is published to the Will Topic if the client is unexpectedly disconnected. This may be a zero-length message.
Although the Will Message is UTF-8 encoded in the CONNECT message, when it is published to the Will Topic only the bytes of the message are sent, not the first two length bytes. The message must therefore only consist of 7-bit ASCII characters.
- User Name
If the User Name flag is set, this is the next UTF-encoded string. The user name identifies the name of the user who is connecting, which can be used for authentication. It is recommended that user names are kept to 12 characters or fewer, but it is not required.
Note that, for compatibility with the original MQTT V3 specification, the Remaining Length field from the fixed header takes precedence over the User Name flag. Server implementations must allow for the possibility that the User Name flag is set, but the User Name string is missing. This is valid, and connections should be allowed to continue.
- Password
If the Password flag is set, this is the next UTF-encoded string. The password corresponding to the user who is connecting, which can be used for authentication. It is recommended that passwords are kept to 12 characters or fewer, but it is not required.
Note that, for compatibility with the original MQTT V3 specification, the Remaining Length field from the fixed header takes precedence over the Password flag. Server implementations must allow for the possibility that the Password flag is set, but the Password string is missing. This is valid, and connections should be allowed to continue.
Response
The server sends a CONNACK message in response to a CONNECT message from a client.
If the server does not receive a CONNECT message within a reasonable amount of time after the TCP/IP connection is established, the server should close the connection.
If the client does not receive a CONNACK message from the server within a reasonable amount of time, the client should close the TCP/IP socket connection, and restart the session by opening a new socket to the server and issuing a CONNECT message.
In both of these scenarios, a "reasonable" amount of time depends on the type of application and the communications infrastructure.
If a client with the same Client ID is already connected to the server, the "older" client must be disconnected by the server before completing the CONNECT flow of the new client.
If the client sends an invalid CONNECT message, the server should close the connection. This includes CONNECT messages that provide invalid Protocol Name or Protocol Version Numbers. If the server can parse enough of the CONNECT message to determine that an invalid protocol has been requested, it may try to send a CONNACK containing the "Connection Refused: unacceptable protocol version" code before dropping the connection.
3.2. CONNACK - Acknowledge connection request
The CONNACK message is the message sent by the server in response to a CONNECT request from a client.
Fixed header
The fixed header format is shown in the table below.
The DUP, QoS and RETAIN flags are not used in the CONNACK message.
Variable header
The variable header format is shown in the table below.
The values for the one byte unsigned Connect return code field are shown in the table below.
EnumerationHEXMeaning00x00Connection Accepted10x01Connection Refused: unacceptable protocol version20x02Connection Refused: identifier rejected30x03Connection Refused: server unavailable40x04Connection Refused: bad user name or password50x05Connection Refused: not authorized6-255 Reserved for future useReturn code 2 (identifier rejected) is sent if the unique client identifier is not between 1 and 23 characters in length.
Payload
There is no payload.
3.3. PUBLISH - Publish message
A PUBLISH message is sent by a client to a server for distribution to interested subscribers. Each PUBLISH message is associated with a topic name (also known as the Subject or Channel). This is a hierarchical name space that defines a taxonomy of information sources for which subscribers can register an interest. A message that is published to a specific topic name is delivered to connected subscribers for that topic.
If a client subscribes to one or more topics, any message published to those topics are sent by the server to the client as a PUBLISH message.
Fixed header
The table below shows the fixed header format.
- QoS level
- Set to 1. See QoS for more details.
- DUP flag
- Set to zero (0). This means that the message is being sent for the first time. See DUP for more details.
- RETAIN flag
Set to zero. This means do not retain. See Retain for more details.
- Remaining Length field
- The length of the variable header plus the length of the payload. It can be a multibyte field.
Variable header
The variable header contains the following fields:
- Topic name
- A UTF-encoded string.
This must not contain Topic wildcard characters.
When received by a client that subscribed using wildcard characters, this string will be the absolute topic specified by the originating publisher and not the subscription string used by the client.
- Message ID
- Present for messages with QoS level 1 and QoS level 2. See Message identifiers for more details.
The table below shows an example variable header for a PUBLISH message.
FieldValueTopic Name:"a/b"QoS level1Message ID:10The format of the variable header in this case is shown in the table below.
Payload
Contains the data for publishing. The content and format of the data is application specific. The Remaining Length field in the fixed header includes both the variable header length and the payload length. As such, it is valid for a PUBLISH to contain a 0-length payload.
Response
The response to a PUBLISH message depends on the QoS level. The table below shows the expected responses.
QoS LevelExpected responseQoS 0NoneQoS 1PUBACKQoS 2PUBRECActions
PUBLISH messages can be sent either from a publisher to the server, or from the server to a subscriber. The action of the recipient when it receives a message depends on the QoS level of the message:
- QoS 0
- Make the message available to any interested parties.
- QoS 1
- Log the message to persistent storage, make it available to any interested parties, and return a PUBACK message to the sender.
- QoS 2
- Log the message to persistent storage, do not make it available to interested parties yet, and return a PUBREC message to the sender.
If the server receives the message, interested parties means subscribers to the topic of the PUBLISH message. If a subscriber receives the message, interested parties means the application on the client which has subscribed to one or more topics, and is waiting for a message from the server.
See Quality of Service levels and flows for more details.
Note that if a server implementation does not authorize a PUBLISH to be made by a client, it has no way of informing that client. It must therefore make a positive acknowledgement, according to the normal QoS rules, and the client will not be informed that it was not authorized to publish the message.
3.4. PUBACK - Publish acknowledgment
A PUBACK message is the response to a PUBLISH message with QoS level 1. A PUBACK message is sent by a server in response to a PUBLISH message from a publishing client, and by a subscriber in response to a PUBLISH message from the server.
Fixed header
The table below shows the format of the fixed header.
- QoS level
- Not used.
- DUP flag
- Not used.
- RETAIN flag
- Not used.
- Remaining Length field
- This is the length of the variable header (2 bytes). It can be a multibyte field.
Variable header
Contains the Message Identifier (Message ID) for the PUBLISH message that is being acknowledged. The table below shows the format of the variable header.
Payload
There is no payload.
Actions
When the client receives the PUBACK message, it discards the original message, because it is also received (and logged) by the server.
3.5. PUBREC - Assured publish received (part 1)
A PUBREC message is the response to a PUBLISH message with QoS level 2. It is the second message of the QoS level 2 protocol flow. A PUBREC message is sent by the server in response to a PUBLISH message from a publishing client, or by a subscriber in response to a PUBLISH message from the server.
Fixed header
The table below shows the fixed header format.
- QoS level
- Not used.
- DUP flag
- Not used.
- RETAIN flag
- Not used.
- Remaining Length field
- The length of the variable header (2 bytes). It can be a multibyte field.
Variable header
The variable header contains the Message ID for the acknowledged PUBLISH. The table below shows the format of the variable header.
Payload
There is no payload.
Actions
When it receives a PUBREC message, the recipient sends a PUBREL message to the sender with the same Message ID as the PUBREC message.
3.6. PUBREL - Assured Publish Release (part 2)
A PUBREL message is the response either from a publisher to a PUBREC message from the server, or from the server to a PUBREC message from a subscriber. It is the third message in the QoS 2 protocol flow.
Fixed header
The table below shows the fixed header format.
- QoS level
PUBREL messages use QoS level 1 as an acknowledgement is expected in the form of a PUBCOMP. Retries are handled in the same way as PUBLISH messages.
- DUP flag
Set to zero (0). This means that the message is being sent for the first time. See DUP for more details.
- RETAIN flag
- Not used.
- Remaining Length field
- The length of the variable header (2 bytes). It can be a multibyte field.
Variable header
The variable header contains the same Message ID as the PUBREC message that is being acknowledged. The table below shows the format of the variable header.
Payload
There is no payload.
Actions
When the server receives a PUBREL message from a publisher, the server makes the original message available to interested subscribers, and sends a PUBCOMP message with the same Message ID to the publisher. When a subscriber receives a PUBREL message from the server, the subscriber makes the message available to the subscribing application and sends a PUBCOMP message to the server.
3.7. PUBCOMP - Assured publish complete (part 3)
This message is either the response from the server to a PUBREL message from a publisher, or the response from a subscriber to a PUBREL message from the server. It is the fourth and last message in the QoS 2 protocol flow.
Fixed header
The table below shows the fixed header format.
- QoS level
- Not used.
- DUP flag
- Not used.
- RETAIN flag
- Not used.
- Remaining Length field
- The length of the variable header (2 bytes). It can be a multibyte field.
Variable header
The variable header contains the same Message ID as the acknowledged PUBREL message.
Payload
There is no payload.
Actions
When the client receives a PUBCOMP message, it discards the original message because it has been delivered, exactly once, to the server.
3.8. SUBSCRIBE - Subscribe to named topics
The SUBSCRIBE message allows a client to register an interest in one or more topic names with the server. Messages published to these topics are delivered from the server to the client as PUBLISH messages. The SUBSCRIBE message also specifies the QoS level at which the subscriber wants to receive published messages.
Fixed header
The table below shows the fixed header format.
- QoS level
- SUBSCRIBE messages use QoS level 1 to acknowledge multiple subscription requests. The corresponding SUBACK message is identified by matching the Message ID. Retries are handled in the same way as PUBLISH messages.
- DUP flag
Set to zero (0). This means that the message is being sent for the first time. See DUP for more details.
- RETAIN flag
- Not used.
- Remaining Length field
- The length of the payload. It can be a multibyte field.
Variable header
The variable header contains a Message ID because a SUBSCRIBE message has a QoS level of 1. See Message identifiers for more details.
The table below shows an example format for the variable header with a Message ID of 10.
Payload
The payload of a SUBSCRIBE message contains a list of topic names to which the client wants to subscribe, and the QoS level at which the client wants to receive the messages. The strings are UTF-encoded, and the QoS level occupies 2 bits of a single byte. The topic strings may contain special Topic wildcard characters to represent a set of topics. These topic/QoS pairs are packed contiguously as shown in the example payload in the table below.
Topic name"a/b"Requested QoS1Topic name"c/d"Requested QoS2Topic names in a SUBSCRIBE message are not compressed.
The format of the example payload is shown in the table below.
Assuming that the requested QoS level is granted, the client receives PUBLISH messages at less than or equal to this level, depending on the QoS level of the original message from the publisher. For example, if a client has a QoS level 1 subscription to a particular topic, then a QoS level 0 PUBLISH message to that topic is delivered to the client at QoS level 0. A QoS level 2 PUBLISH message to the same topic is downgraded to QoS level 1 for delivery to the client.
A corollary to this is that subscribing to a topic at QoS level 2 is equivalent to saying "I would like to receive messages on this topic at the QoS at which they are published".
This means a publisher is responsible for determining the maximum QoS a message can be delivered at, but a subscriber is able to downgrade the QoS to one more suitable for its usage. The QoS of a message is never upgraded.
The Requested QoS field is encoded in the byte following each UTF-encoded topic name as shown in the table below.
The upper 6 bits of this byte are not used in the current version of the protocol. They are reserved for future use.
A request with both QoS level bits set should be considered an invalid packet and the connection closed.
Response
When it receives a SUBSCRIBE message from a client, the server responds with a SUBACK message.
A server may start sending PUBLISH messages due to the subscription before the client receives the SUBACK message.
Note that if a server implementation does not authorize a SUBSCRIBE request to be made by a client, it has no way of informing that client. It must therefore make a positive acknowledgement with a SUBACK, and the client will not be informed that it was not authorized to subscribe.
A server may chose to grant a lower level of QoS than the client requested. This could happen if the server is not able to provide the higher levels of QoS. For example, if the server does not provider a reliable persistence mechanism it may chose to only grant subscriptions at QoS 0.
3.9. SUBACK - Subscription acknowledgement
A SUBACK message is sent by the server to the client to confirm receipt of a SUBSCRIBE message.
A SUBACK message contains a list of granted QoS levels. The order of granted QoS levels in the SUBACK message matches the order of the topic names in the correspondingSUBSCRIBE message.
Fixed header
The table below shows the fixed header format.
- QoS level
- Not used.
- DUP flag
- Not used.
- RETAIN flag
- Not used.
- Remaining Length field
- The length of the payload. It can be a multibyte field.
Variable header
The variable header contains the Message ID for the SUBSCRIBE message that is being acknowledged. The table below shows the format of the variable header.Payload
The payload contains a vector of granted QoS levels. Each level corresponds to a topic name in the corresponding SUBSCRIBE message. The order of QoS levels in the SUBACK message matches the order of topic name and Requested QoS pairs in the SUBSCRIBE message. The Message ID in the variable header enables you to match SUBACK messages with the corresponding SUBSCRIBE messages.
The table below shows the Granted QoS field encoded in a byte.
The upper 6 bits of this byte are not used in the current version of the protocol. They are reserved for future use.
The table below shows an example payload.
Granted QoS0Granted QoS2The table below shows the format of this payload.
3.10. UNSUBSCRIBE - Unsubscribe from named topics
An UNSUBSCRIBE message is sent by the client to the server to unsubscribe from named topics.
Fixed header
The table below shows an example fixed header format.
- QoS level
- UNSUBSCRIBE messages use QoS level 1 to acknowledge multiple unsubscribe requests. The corresponding UNSUBACK message is identified by the Message ID. Retries are handled in the same way as PUBLISH messages.
- DUP flag
Set to zero (0). This means that the message is being sent for the first time. See DUP for more details.
- RETAIN flag
- Not used.
- Remaining Length
- This is the length of the Payload. It can be a multibyte field.
Variable header
The variable header contains a Message ID because an UNSUBSCRIBE message has a QoS level of 1. See Message identifiers for more details.
The table below shows an example format for the variable header with a Message ID of 10.
Payload
The client unsubscribes from the list of topics named in the payload. The strings are UTF-encoded and are packed contiguously. Topic names in a UNSUBSCRIBE message are not compressed. The table below shows an example payload.
Topic Name"a/b"Topic Name"c/d"The table below shows the format of this payload.
Response
The server sends an UNSUBACK to a client in response to an UNSUBSCRIBE message.
3.11. UNSUBACK - Unsubscribe acknowledgment
The UNSUBACK message is sent by the server to the client to confirm receipt of an UNSUBSCRIBE message.
Fixed header
The table below shows the fixed header format.
- QoS level
- Not used.
- DUP flag
- Not used.
- RETAIN flag
- Not used.
- Remaining Length
- The length of the Variable Header (2 bytes).
Variable header
The variable header contains the Message ID for the UNSUBSCRIBE message that is being acknowledged. The table below shows the format of the variable header.
Payload
There is no payload.
3.12. PINGREQ - PING request
The PINGREQ message is an "are you alive?" message that is sent from a connected client to the server.
See Keep Alive timer for more details.
Fixed header
The table below shows the fixed header format.
The DUP, QoS, and RETAIN flags are not used.
Variable header
There is no variable header.
Payload
There is no payload.
Response
The response to a PINGREQ message is a PINGRESP message.
3.13. PINGRESP - PING response
A PINGRESP message is the response sent by a server to a PINGREQ message and means "yes I am alive".
See Keep Alive timer for more details.
Fixed header
The table below shows the fixed header format:
The DUP, QoS, and RETAIN flags are not used.
Payload
There is no payload.
Variable header
There is no variable header.
3.14. DISCONNECT - Disconnect notification
The DISCONNECT message is sent from the client to the server to indicate that it is about to close its TCP/IP connection. This allows for a clean disconnection, rather than just dropping the line.
If the client had connected with the clean session flag set, then all previously maintained information about the client will be discarded.
A server should not rely on the client to close the TCP/IP connection after receiving a DISCONNECT.
Fixed header
The fixed header format is shown in the table below.
The DUP, QoS, and RETAIN flags are not used in the DISCONNECT message.
Payload
There is no payload.
Variable header
There is no variable header.
4. Flows
4.1. Quality of Service levels and flows
MQTT delivers messages according to the levels defined in a Quality of Service (QoS). The levels are described below:
- QoS level 0: At most once delivery
- The message is delivered according to the best efforts of the underlying TCP/IP network. A response is not expected and no retry semantics are defined in the protocol. The message arrives at the server either once or not at all.
The table below shows the QoS level 0 protocol flow.
Client Message and direction Server QoS = 0PUBLISH---------->Action: Publish message to subscribers - QoS level 1: At least once delivery
- The receipt of a message by the server is acknowledged by a PUBACK message. If there is an identified failure of either the communications link or the sending device, or the acknowledgement message is not received after a specified period of time, the sender resends the message with the DUP bit set in the message header. The message arrives at the server at least once. Both SUBSCRIBE and UNSUBSCRIBE messages use QoS level 1.
A message with QoS level 1 has a Message ID in the message header.
The table below shows the QoS level 1 protocol flow.
Client Message and direction Server QoS = 1
DUP = 0
Message ID = xAction: Store message
PUBLISH---------->Actions:Store message
- Publish message to subscribers
Delete message
<----------If the client does not receive a PUBACK message (either within a time period defined in the application, or if a failure is detected and the communications session is restarted), the client may resend the PUBLISH message with the DUP flag set.
When it receives a duplicate message from the client, the server republishes the message to the subscribers, and sends another PUBACK message.
- QoS level 2: Exactly once delivery
- Additional protocol flows above QoS level 1 ensure that duplicate messages are not delivered to the receiving application. This is the highest level of delivery, for use when duplicate messages are not acceptable. There is an increase in network traffic, but it is usually acceptable because of the importance of the message content.
A message with QoS level 2 has a Message ID in the message header.
The table below shows the QoS level 2 protocol flow. There are two semantics available for how a PUBLISH flow should be handled by the recipient. They affect the point within the flow that the message is made available to the subscribers. The choice of semantic is implementation specific and does not affect the guarantees of a QoS level 2 flow.
Client Message and direction Server QoS = 2
DUP = 0
Message ID = xAction: Store message
PUBLISH---------->Action: Store messageor
Actions:- Store message ID
- Publish message to subscribers
<----------Message ID = xMessage ID = xPUBREL---------->Actions:- Publish message to subscribers
- Delete message
or
Action: Delete message IDAction: Discard messagePUBCOMP<----------Message ID = xIf a failure is detected, or after a defined time period, the protocol flow is retried from the last unacknowledged protocol message; either the PUBLISH or PUBREL. SeeMessage delivery retry for more details. The additional protocol flows ensure that the message is delivered to subscribers once only.
Assumptions for QoS levels 1 and 2
In any network, it is possible for devices or communication links to fail. If this happens, one end of the link might not know what is happening at the other end; these are known as in doubt windows. In these scenarios assumptions have to be made about the reliability of the devices and networks involved in message delivery.
MQTT assumes that the client and server are generally reliable, and that the communications channel is more likely to be unreliable. If the client device fails, it is typically a catastrophic failure, rather than a transient one. The possibility of recovering data from the device is low. Some devices have non-volatile storage, for example flash ROM. The provision of more persistent storage on the client device protects the most critical data from some modes of failure.
Beyond the basic failure of the communications link, the failure mode matrix becomes complex, resulting in more scenarios than the specification for MQTT can handle.
4.2. Message delivery retry
Although TCP normally guarantees delivery of packets, there are certain scenarios where an MQTT message may not be received. In the case of MQTT messages that expect a response (QoS >0 PUBLISH, PUBREL, SUBSCRIBE, UNSUBSCRIBE), if the response is not received within a certain time period, the sender may retry delivery. The sender should set the DUP flag on the message.
The retry timeout should be a configurable option. However care must be taken to ensure message delivery does not timeout while it is still being sent. For example, sending a large message over a slow network will naturally take longer than a small message over a fast network. Repeatedly retrying a timed-out message could often make matters worse so a strategy of increasing the timeout value across multiple retries should be used.
When a client reconnects, if it is not marked clean session, both the client and server should redeliver any previous in-flight messages.
Other than this "on reconnect" retry behaviour, clients are not required to retry message delivery. Brokers, however, should retry any unacknowledged message.
4.3. Message ordering
Message ordering can be affected by a number of factors, including how many in-flight PUBLISH flows a client allows and whether the client is single- or multi-threaded. For purposes of discussion, clients are assumed to be single-threaded at the point packets are written to and read from the network.
For an implementation to provide any guarantees regarding the ordering of messages it must ensure each stage of the message delivery flows are completed in the order they were started. For example, in a series of QoS level 2 flows, the PUBREL flows must be sent in the same order as the original PUBLISH flows:
---------->PUBLISH 2
---------->PUBLISH 3
----------> PUBREC 1<----------PUBREC 2
<---------- PUBREL 1----------> PUBREC 3<---------- PUBREL 2----------> PUBCOMP 1<---------- PUBREL 3----------> PUBCOMP 2<----------PUBCOMP 3
<---------- The number of in-flight messages permitted also has an effect on the type of guarantees that can be made:
With an in-flight window of 1, each delivery flow is completed before the next one starts. This guarantees messages are delivered in the order they were submitted.
With an in-flight window greater than 1, message ordering can only be guaranteed within the QoS level.
Appendix A - Topic wildcards
A subscription may contain special characters, which allow you to subscribe to multiple topics at once.
The topic level separator is used to introduce structure into the topic, and can therefore be specified within the topic for that purpose. The multi-level wildcard and single-level wildcard can be used for subscriptions, but they cannot be used within a topic by the publisher of a message.
- Topic level separator
- The forward slash (/) is used to separate each level within a topic tree and provide a hierarchical structure to the topic space. The use of the topic level separator is significant when the two wildcard characters are encountered in topics specified by subscribers.
- Multi-level wildcard
The number sign (#) is a wildcard character that matches any number of levels within a topic. For example, if you subscribe to finance/stock/ibm/#, you receive messages on these topics:
finance/stock/ibm finance/stock/ibm/closingprice finance/stock/ibm/currentprice
The multi-level wildcard can represent zero or more levels. Therefore, finance/# can also match the singular finance, where # represents zero levels. The topic level separator is meaningless in this context, because there are no levels to separate.
The multi-level wildcard can be specified only on its own or next to the topic level separator character. Therefore, # and finance/# are both valid, but finance# is not valid. The multi-level wildcard must be the last character used within the topic tree. For example, finance/# is valid but finance/#/closingprice is not valid.
- Single-level wildcard
The plus sign (+) is a wildcard character that matches only one topic level. For example, finance/stock/+ matches finance/stock/ibm and finance/stock/xyz, but notfinance/stock/ibm/closingprice. Also, because the single-level wildcard matches only a single level, finance/+ does not match finance.
The single-level wildcard can be used at any level in the topic tree, and in conjunction with the multilevel wildcard. It must be used next to the topic level separator, except when it is specified on its own. Therefore, + and finance/+ are both valid, but finance+ is not valid. The single-level wildcard can be used at the end of the topic tree or within the topic tree. For example, finance/+ and finance/+/ibm are both valid.
Topic semantics and usage
When you build an application, the design of the topic tree should take into account the following principles of topic name syntax and semantics:
- A topic must be at least one character long.
- Topic names are case sensitive. For example, ACCOUNTS and Accounts are two different topics.
- Topic names can include the space character. For example, Accounts payable is a valid topic.
- A leading "/" creates a distinct topic. For example, /finance is different from finance. /finance matches "+/+" and "/+", but not "+".
- Do not include the null character (Unicode
\x0000) in any topic.
The following principles apply to the construction and content of a topic tree:
- The length is limited to 64k but within that there are no limits to the number of levels in a topic tree.
- There can be any number of root nodes; that is, there can be any number of topic trees.
MQTT是轻量级基于代理的发布/订阅的消息传输协议，它可以通过很少的代码和带宽和远程设备连接。例如通过卫星和代理连接，通过拨号和医疗保健提供者连接，以及在一些自动化或小型设备上，而且由于小巧，省电，协议开销小和能高效的向一和多个接收者传递信息，故同样适用于称动应用设备上。
MQTT(Message Queuing Telemetry Transport, 消息队列遥测传输)是一种轻量级基于代理的发布/订阅的消息传输协议, 其设计思想是开放、简单、轻量、
易于实现, 适合在带宽、计算和处理能力受限的环境下工作[1]例如, 网络代价昂贵, 带宽低、不可靠的环境或者处理器和内存资源有限的环境. 这使得MQTT
成为M2M、物联网以及移动应用的理想选择. MQTT技术于1999年, 由IBM的Andy Stanford-Clark博士以及Arcom公司的ArlenNipper博士发明, 其支持所有平
台, 几乎可以把所有联网设备与外部连接起来, 被用作传感器和驱动器的通信协议. 也被称为SCADA协议(Supervisory Control And Data Acquisition, 数据采集① 收稿时间:2013-08-24;收到修改稿时间:2013-09-25 与监视控制), 成为物联网协议的重要部分. 目前, 美国结构化信息标准发展委员会(OASIS)成立专门的MQTT技术委员会, 旨在负责制定基于MQTT技术的物联网标准协议. OASIS 专门的MQTT技术委员会将以MQTT协议为基础, 进一步研究与规范更为开放、简单、轻量的, 适合在有限网络和多平台环境中使用的MQTT物联网协议.
相信在想深入学习这协议必是奔着解决某个问题而来的，上面给出了适用的场景，我之所以想深入的学习和了解这个协议，理由如下：
1、可以实现手机消息推送（PUSH）
2、协议简单，最小的头部只需2个字节，特别适合于嵌入式中。
3、这是个了解什么是协议绝好的例子。相比于其它复杂的协议例如tcp，http协议，至少说明文档看的下去。
在这里，我以推送为例子说明，虽然现在现成的推送解决方案已经比较成熟，但是这个Repeat ReInvent the Whell还是要做一下，什么都是拿来主义，和搬运工有什么区别。
一、需要的环境：
1、PHP+Apache或Nginx
2、安装开源代理程序Mosquitto，这里用其做为代理服务器，负责连接和分发。
安装方法很简单，http://mosquitto.org/files/ binary是编译好的，source是源码安装需要的（make & make install 就行）
唯 一要配置的就是在解压后的config.mk,安装完后设置文件是mosquitto.conf
当然主要是设置是否支持ssl，还有就是config.mk最下面的安装位置的设定。这里一切默认。
默认启动是绑定的IP是本地IP，端口是1883可以在mosquitto.conf里设置（要去掉前面的#字注释），linux 中 -c 可以指定设置文件并运行
比 如： mosquitto -c /etc/mosquitto.conf
二、协议初解
先说一下整个协议的构造，整体上协议可拆分为：
固定头部+可变头部+消息体
协议说白了就是对于双方通信的一个约定，比如传过来一段字符流，第1个字节表示什么，第2个字节表示什么。。。。一个约定。
所以在固定头部的构造如下：
1、MessageType(0和15保留，共占4个字节)
public $operations=array( "MQTT_CONNECT"=>1,//请求连接 "MQTT_CONNACK"=>2,//请求应答 "MQTT_PUBLISH"=>3,//发布消息 "MQTT_PUBACK"=>4,//发布应答 "MQTT_PUBREC"=>5,//发布已接收，保证传递1 "MQTT_PUBREL"=>6,//发布释放，保证传递2 "MQTT_PUBCOMP"=>7,//发布完成，保证传递3 "MQTT_SUBSCRIBE"=>8,//订阅请求 "MQTT_SUBACK"=>9,//订阅应答 "MQTT_UNSUBSCRIBE"=>10,//取消订阅 "MQTT_UNSUBACK"=>11,//取消订阅应答 "MQTT_PINGREQ"=>12,//ping请求 "MQTT_PINGRESP"=>13,//ping响应 "MQTT_DISCONNECT"=>14//断开连接 );
2、DUP flag
其是用来在保证消息传输可靠的，如果设置为1，则在下面的变长头部里多加MessageId,并需要回复确认，保证消息传输完成，但不能用于检测消息重复发送。
3、Qos
主要用于PUBLISH（发布态）消息的，保证消息传递的次数。
00表示最多一次 即<=1
01表示至少一次 即>=1
10表示一次，即==1
11保留后用
4、Retain
主要用于PUBLISH(发布态)的消息，表示服务器要保留这次推送的信息，如果有新的订阅者出现，就把这消息推送给它。如果不设那么推送至当前订阅的就释放了。
5、固定头部的byte 2
是用来保存接下去的变长头部+消息体的总大小的。
但是不是并不是直接保存的，同样也是可以扩展的，其机制是，前7位用于保存长度，后一部用做标识。
我举个例了，即如果计算出后面的大小为0<length<=127的，正常保存
如果是127<length<16383的，则需要二个字节保存了，将第一个字节的最大的一位置1,表示未完。然后第二个字节继续存。
拿130来说，第一个字节存10000011,第二个字节存000000001，也就是0x83,0x01,把两个字节连起来看，第二个字节权重从2的8次开始。
同起可以加第3个字节，最多可以加至第4个字节。故MQTT协议最多可以实现268 435 455 (0xFF, 0xFF, 0xFF, 0x7F)将近256M的数据。可谓能伸能缩。
可变头部
这个是可变头部的全貌。
1、首先最上面的8个字节是Protocol Name(编码名)，UTF编码的字符“MQIsdp”，头两个是编码名提长为6。
这里多说一些，接下去的协议多采用这种方式组合，即头两个字节表示下一部分的长，然后后面跟上内容。这里头两个字节长为6，下面跟6个字符“MQIsdp”。
2、Protocol Version，协议版本号，v3 也是固定的。
3、Connect Flag，连接标识，有点像固定头部的。8位分别代表不同的标志。第1个字节保留。
Clean Session,Will flag，Will Qos, Will Retain都是相对于CONNECT消息来说的。
Clean Session:0表示如果订阅的客户机断线了，那么要保存其要推送的消息，如果其重新连接时，则将这些消息推送。
1表示消除，表示客户机是第一次连接，消息所以以前的连接信息。
Will Flag，表示如果客户机在不是在发送DISCONNECT消息中断，比如IO错误等，将些置为1,要求重传。并且下且的WillQos和WillRetain也要设置，消息体中的Topic和MessageID也要设置，就是表示发生了错误，要重传。
Will Qos，在CONNECT非正常情况下设置，一般如果标识了WillFlag，那么这个位置也要标识。
Will RETAIN：同样在CONNECT中，如果标识了WillFlag,那么些位也一定要标识
usename flag和passwordflag，用来标识是否在消息体中传递用户和密码，只有标识了，消息体中的用户名和密码才用效，只标记密码而不标记用户名是不合法的。
4、Keep Alive，表示响应时间，如果这个时间内，连接或发送操作未完成，则断开tcp连接，表示离线。
5、Connect Return Code即通常于CONNACK消息中，表示返回的连接情况，我可以通过此检验连接情况。
6、Topic Name，订阅消息标识，MQTT是基于订阅/发布的消息，那么这个就是消息订阅的标识，像新闻客户端里的订阅不同的栏目一样。用于区别消息的推送类别。
主要用于PUBLISH和SUBSCRIBE中。最大可支持32767个字符，即4个字节。
消息体（PayLoad）
只有3种消息有消息体CONNECT，SUBSCRIBE，SUBACK
CONNECT主要是客户机的ClientID，订阅的Topic和Message以及用户名和密码，其于变长头部中的will是对应的。
SUBSCRIBE是包含了一系列的要订阅的主题以及QOS。
SUBACK是用服务器对于SUBSCRIBE所申请的主题及QOS进行确认和回复。
而PUBLISH是消息体中则保存推送的消息，以二进制形式，当然这里的编辑可自定义。
7、Message Identifier
包含于PUBLISH, PUBACK, PUBREC, PUBREL, PUBCOMP, SUBSCRIBE, SUBACK, UNSUBSCRIBE, UNSUBACK.
其为16位字符表示，用于在Qos为1或2时标识Message的，保证Message传输的可靠性。
至于具体的消息例子，我们在后面的代码中慢慢体现。
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