Opcode表示该PDU的类型，包括command, request, response, indication,notification or confirmation这六种。
Logical Attribute Representation：
The Attribute Handle is an index corresponding to a specific Attribute. The Attribute Type is a UUID that describes the Attribute Value. The Attribute Value is the data described by the Attribute Type and indexed by the Attribute Handle. The Attributes are ordered by increasing Attribute Handle values.Attribute Permissions is part of the Attribute that cannot be read from or written to using the Attribute Protocol.
多个service组成完整的profile，一个service可能包含几个referenced services, 一些mandatory characteristics 和optional characteristics.有两种类型的service：primary service and secondary service。A primary service can be included by another service，A secondary service is a service that is only intended to be referenced from a primary service or another secondary service or other higher layer specification.
A service definition shall contain a service declaration and may contain include definitions and characteristic definitions.
组织顺序：service declaration-->include definitions-->characteristic definitions
A characteristic definition shall contain a characteristic declaration, a Characteristic Value declaration and may contain characteristic descriptor declarations.
The Characteristic Value declaration shall exist immediately following the characteristic declaration
(3.1) Characteristic Declaration
(3.2)Characteristic Value Declaration
(3.3)Characteristic Descriptor Declarations
Used to contain related information about the Characteristic Value.Only one Characteristic Extended Properties.
declaration shall exist in a characteristic definition.
(3.3.1)Characteristic Extended Properties
(3.3.2）Characteristic User Description
(3.3.3)Client Characteristic Configuration
（3.3.4）Server Characteristic Configuration
(3.3.5)Characteristic Presentation Format
(3.3.6)Characteristic Aggregate Format
Only one Characteristic Aggregate Format declaration shall exist in a characteristic definition.The Characteristic Aggregate Format value is composed of a list of Attribute Handles of Characteristic Presentation Format declarations, where each Attribute. Handle points to a Characteristic Presentation Format declaration.
可以理解为是多个Characteristic Presentation Format的集合，而且可以使用其他characteristic definition的characteristic presentation。
3.4 GATT PROFILE ATTRIBUTE TYPES总结
主要有以上几种GATT Attribute Type。
5.1 SERVER CONFIGURATION
This sub-procedure is used by the client to set the ATT_MTU to the maximum possible value that can be supported by both devices.
5.2 PRIMARY SERVICE DISCOVERY
两个sub-procedures:Discover All Primary Services and Discover Primary Services by Service UUID
This procedure is used by a client to discover primary services on a server.
5.3 RELATIONSHIP DISCOVERY
一个sub-procedure：Find Included Services.
There is one sub-procedure that can be used for relationship discovery: Find Included Services.
5.4 CHARACTERISTIC DISCOVERY
两个sub-procedures:Discover All Characteristics of a Service and Discover Characteristics by UUID.
This procedure is used by a client to discover service characteristics on a server.
5.5 CHARACTERISTIC DESCRIPTOR DISCOVERY
一个sub-procedure：Discover All Characteristic Descriptors.
This procedure is used by a client to discover characteristic descriptors of a characteristic.
5.6 CHARACTERISTIC VALUE READ
四个sub-procedures:Read Characteristic Value, Read Using Characteristic UUID, Read Long CharacteristicValues, and Read Multiple Characteristic Values.
This procedure is used to read a Characteristic Value from a server.
5.7 CHARACTERISTIC VALUE WRITE
5个sub-procedures: Write Without Response, Signed Write Without Response, Write Characteristic
Value, Write Long Characteristic Values and Reliable Writes.
This procedure is used to write a Characteristic Value to a server.
5.8 CHARACTERISTIC VALUE NOTIFICATION
This procedure is used to notify a client of the value of a Characteristic Value from a server.
5.9 CHARACTERISTIC VALUE INDICATIONS
This procedure is used to indicate the Characteristic Value from a server to a client.
5.10 CHARACTERISTIC DESCRIPTORS
两个sub-procedures: Read Characteristic Descriptors and Write Characteristic Descriptors.
This procedure is used to read and write characteristic descriptors on a server.
SERVICE CHANGED：一个重要的characteristic，be used to indicate to connected devices that services have changed。
There is only one instance of the Service Changed characteristic within the GATT service definition.
GATT over BR/EDR：