蓝牙---BLE GATT介绍

GATT(Generic Attribute Profile)，描述了一种使用ATT的服务框架该框架定义了服务(Server)和服务属性(characteristic)的过程(Procedure)及格式 Procedure定义了characteristic的发现、读、写、通知(Notifing)、指示(Indicating) 及配置characteristic的广播

GATT可以被Application或其他Profile使用其协议栈如下图 --------------- --------------- | Application | <----> | Application | --------------- --------------- | Attribute | | Attribute | | Protocol | <----> | Protocol | --------------- --------------- | L2CAP | <----> | L2CAP | --------------- --------------- | Controller | <----> | Controller | --------------- ---------------

GATT可以配置为如下两种角色(Role) - Client : 命令、请求发起方 - Server : 命令、请求接收方

角色配置实例如下

     _________ 
    / =======  
   / __________ 
  | ___________ |     Request      / 
  | |Computer | |   ---------->   / / 
  | |(Client) | |   <----------  / / Sensor(Server) 
  | |_________| |     Response  / / 
  =____________/               / 
  / """""""""""   
 / :::::::::::::    
(_________________)

Computer是一个温度服务客户端, Sensor是温度服务服务器 Computer向Sensor发起Procedure来读Sensor的值

GATT对下层的需求如下 - Physical Link : 使用GAP Channel Establishment建立的ATT Bearer - GATT Role : 不依赖于Coontroller角色(Master/Slave) - Security : 对于LE,Security Features(Authorization、Authentication、Encryption)是可选的对于BR/EDR, Encryption是强制的 - TX order : GATT中的多字节字段，采用Least Significant octet先发送(Little Endian)

2.1 Configured Broadcast

对于LE物理链路，在Server广播模式过程中 Client通过Configured Broadcast告知Server应该在advertising data加入Characteristic Value

方法是Client设置指定bit位广播频率则是Service、Characteristic行为定义的一部分

2.2 GATT Profile Hierarchy

GATT指定了数据交互的结构(Structure) 这个结构体定义了一些基本元素，如Service、Characteristic 这些元素存在于Attribute中

GATT中最上层是Profile，Profile由一个或多个服务(Service)组成服务是由Characteristics组成，或是其他服务的引用(Reference) Characteristic包含一个值(Value)，可能包含该Value的相关信息

2.2.1 Service

Service是[数据]和与之关联的[完成某个特定功能的行为]/[特性]的集合在GATT中，一个服务由服务定义(Service Defintion)来实现一个服务定义可能包含引用服务(Reference Service)、强制Characteristic和可选Characteristic

为了实现旧版本的兼容，新版本中服务定义只能增加新的引用服务或可选Characteristic 新版本中的服务定义禁止从旧的服务定义中改变行为

Server有两类 - Primary Service : 拥有基本功能的服务,可被其他服务包含,可以通过Primary Service Discovery过程来发现 - Secondary Service : 仅用来被Primary/Other Secondary Service、高层协议引用的服务

判读一个服务是Primary or Secondary Service可通过高层协议强制规定

2.2.2 Included Service

一个Included Service是一种引用已存在服务的方法，具体办法为在服务定义的开始加上Included Service的引用，这样整个Included Service定义成为新服务定义的一部分

2.2.3 Characteristic

Characteristic由Characteristic Definition定义，包含一个Characteristic声明、Characteristic属性、值、值的描述(Optional)

3 Service Interoperability Requirements

3.1 Service Definition

服务定义(Service Definition)包含一个服务申明(Service Declaration) 可能包含Include Definitions和Characteristic Definitions 在下一个服务申明前或到达Maximum Attribute Handle时结束服务定义在服务端上基于Attribute Handle顺序呈现

服务定义中的Include Definitions和Characteristic Definitions被认为是服务的一部分服务定义中的顺序为 Service Declaration ~ Include Definitions(>=0) ~ Characteristic Definitions(>=0)

Service Declaration如下

Attribute Handle	Attribute Type	Attribute Value	Attribute Permission
0xNNNN	0x2800 – UUID for <Primary Service> 0x2801 – UUID for <Secondary Service>	16-bit Bluetooth UUID 128-bit UUID for Service	Read Only, No Authentication, No Authorization

规则如下: - 当多个服务存在时使用16-bit Bluetooth UUID服务定义的服务应该分组(如按顺序排列) 同理，使用128-bit UUID的服务定义的服务也分组 - 一个设备或高层协议可能有多个服务定义，同时多个服务定义含有相同的Service UUID - 服务端的所有Attributes应该包含一个服务声明或存在一个服务定义 - 服务端的服务定义可能无序;Client不应该认为服务端的服务定义一定是有序的

3.2 Include Definition

一个Include Definition只包含一个Include Declaration

Include Declaration如下

Attribute Handle	Attribute Type	Attribute Value			Attribute Permission
0xNNNN	0x2802 – UUID for<Include>	Included Service Attribute Handle	End Group Handle	Service UUID	Read Only, No Authentication, No Authorization

其中，仅当UUID是16-bit Bluetooth UUID时才存在

如果一个Service的Include Definition(A)是引用其他Server的Include Definition(B) 那么Include Definition(B)不应该引用Include Definition(A)，否则就是循环引用(Circular Reference)

当一个Client检测到循环引用或detects nested include declarations to a greater level than it expects Client应当终止本次通信(ATT Bearer)

3.3 Characteristic Definition

Characteristic Definition包含如下内容 - Characteristic Declaration : First - Characteristic Value declaration : Second - Characteristic Descriptor Declarations(Optional) : Last(含多个时顺序不关紧要)

Characteristic Definitions在服务端以Attribute Handle排序

以上每个Declaration包含在一个单独的Attribute中

3.3.1 Characteristic Declaration

Characteristic Declaration如下

Attribute Handle	Attribute Type	Attribute Value			Attribute Permission
0xNNNN	0x2803 – UUID for Characteristic	Characteristic Properties	Characteristic Value Attribute Handle	Characteristic UUID	Read Only, No Authentication, No Authorization

中，Attribute Value只读 Attribute Value字段如下

Attribute Value	Size	Description
Characteristic Properties	1 octets	Bit field of characteristic properties
Characteristic Value Handle	2 octets	Handle of the Attribute containing the value of this characteristic
Characteristic UUID	2/16 octets	16-bit Bluetooth UUID or 128-bit UUID for Characteristic Value

一个Service可能含有相同Characteristic UUID的Characteristic Definitions

在一个服务定义中，一些强制的Characteristics应该位于Include Declarations之后、其他可选characteristics之前而Client不应该认为Characteristic是有序的同时，16-bit Bluetooth UUIDs和128-bit Bluetooth UUIDs应该分别分组

Characteristic Properties以bit位的方式决定如下内容 - Characteristic Value的使用方式 - Characteristic Descriptors的访问方式

Characteristic Properties bit field的详细内容如下 Notice: 多个比特位可以同时设置

Properties	Value	Description
Broadcast	0x01	If set, permits broadcasts of the Characteristic Value using Characteristic Configuration Descriptor
Read	0x02	If set, permits reads of the Characteristic Value using procedures defined in Section 4.8
Write Without Response	0x04	If set, permit writes of the Characteristic Value without response using procedures defined in Section 4.9.1
Write	0x08	If set, permits writes of the Characteristic Value with response using procedures defined in Section 4.9.3 or Section 4.9.4
Notify	0x10	If set, permits notifications of a Characteristic Value without acknowledgement using the procedure defined in Section 4.10
Indicate	0x20	If set, permits indications of a Characteristic Value with acknowledgement using the procedure defined in Section 4.11
Authenticated Signed Writes	0x40	If set, permits signed writes to the Characteristic Value using the procedure defined in Section 4.9.2
Extended Properties	0x80	If set, additional characteristic properties are defined in the Characteristic Extended Properties Descriptor defined in Section 3.3.3.1

3.3.2 Characteristic Value Declaration

Characteristic Value Declaration如下

Attribute Handle	Attribute Type	Attribute Value	Attribute Permissions
0xNNNN	0xuuuu – 16-bit Bluetooth UUID or 128-bit UUID for Characteristic UUID	Characteristic Value	Higher layer profile or implementation specific

3.3.3 Characteristic Descriptor Declarations

Characteristic Descriptor Declarations包含了Characteristic Value相关信息

GATT定义了一系列的标准Characteristic Descriptors供高层协议使用高层协议也可以定义协议相关的Characteristic Descriptors

Characteristic Descriptors在服务端上是无序的，Client不应该理所当然 Characteristic Descriptors Declarations Permissions由高层协议定义或协议相关的 Client不应该理所当然地认为是可读的

Characteristic Descriptor Declarations包括如下内容(详细见规范)

- Characteristic Extended Properties - Characteristic User Description - Client Characteristic Configuration - Server Characteristic Configuration - Characteristic Presentation Format - Characteristic Aggregate Format

3.4 Summary of GATT Profile Attribute Types

Attribute Type	UUID	Description
<Primary Service>	0x2800	Primary Service Declaration
<Secondary Service>	0x2801	Secondary Service Declaration
<Include>	0x2802	Include Declaration
<Characteristic>	0x2803	Characteristic Declaration
<Characteristic Extended Properties>	0x2900	Characteristic Extended Properties
<Characteristic User Description>	0x2901	Characteristic User Description Descriptor
<Client Characteristic Configuration>	0x2902	Client Characteristic Configuration Descriptor
<Server Characteristic Configuration>	0x2903	Server Characteristic Configuration Descriptor
<Characteristic Format>	0x2904	Characteristic Format Descriptor
<Characteristic Aggregate Format>	0x2905	Characteristic Aggregate Format Descriptor

4. GATT Feature Requirements

4.1 Overview

GATT中定义了11项Feature 1. Server Configuration 2. Primary Service Discovery 3. Relationship Discovery 4. Characteristic Discovery 5. Characteristic Descriptor Discovery 6. Reading a Characteristic Value 7. Writing a Characteristic Value 8. Notification of a Characteristic Value 9. Indication of a Characteristic Value 10. Reading a Characteristic Descriptor 11. Writing a Characteristic Descriptor

每个Feature都有对应的过程和子过程，这些过程描述了如何使用ATT来实现各自的功能

4.3 Server Configuration

该过程可被Client用来配置Attribute Protocol的MTU大小

4.3.1 Exchange MTU

Client使用该子过程来设置适配双方均支持的最大ATT_MTU

在BR/EDR物理链路中不应该使用该过程，而应该使用L2CAP Channel Configuration Procedures

该过程对应于ATT的MTU Exchange Request/Response 见<Bluetooth ATT介绍 - 4.2 MTU Exchange>

4.4 Primary Service Discovery

Client使用该过程来发现服务端的Primary Services 一旦发现服务存在，可通过其他过程来访问Primary Services的附加信息(关联主服务和次服务) 可使用的其他过程包括Characteristic Discovery和Relationship Discovery

该过程包括两个子过程: - Discover All Primary Services - Discover Primary Services by Service UUID

在BR/EDR物理链路上则使用SDP service discovery来发现服务

4.4.1 Discover All Primary Services

Client使用该子过程来发现服务端的所有Primary Services

该子过程使用ATT的Read By Group Type Request,同时设置如下参数 - Starting Handle : 0x0001 - Ending Handle : 0xFFFF - Attribute Type : UUID for <Primary Service>

可能的回应有 - Read By Group Type Response - Error Response

Read By Group Type Response返回三元组列表三元组包括 - Attribute Handle : 服务声明的Handle - End Group Handle : 服务定义中最后一个Attribute的Handle - Attribute Value : 服务端支持的服务的Service UUID

当收到Error Response<Error Code: Attribute Not Found>时则表明该过程已经完成

当Client找到自己所需要的服务时,可以终止该过程

Note: 3.1中已指出Service Declaration是可读,并且不需要认证或授权因此权限相关的错误不会发生

下图是一个实例图

4.4.2 Discover Primary Service by Service UUID

当Client只知道Service UUID时,可以使用该子过程来发现对应的主服务

该子过程使用ATT的Find By Type Value Request,同时设置参数如下 - Starting Handle : 0x0001 - Ending Handle : 0xFFFF - Attribute Value : 16-bit Bluetooth UUID or 128-bit UUID - Attribute Type : UUID for <Primary Service>

可能的回应有 - Find By Type Value Response - Error Response

Find By Type Value Response返回Attribute Handle ranges列表 Attribute Handle range即服务定义的Starting Handle和Ending Handle 如果Attribute Handle range中的End Found Handle不是0xFFFF 那么Client将会再请求一次Req 同时将Starting Handle设置为收到的最后一个Attribute Handle+1

终止规则和权限问题同Discover All Primary Services

下图是一个实例图

4.5 Relationship Discovery

Client使用该过程来发现和其他服务的服务关系

4.5.1 Find Include Services

Client使用该子过程来发现一个服务定义包含的服务申明

该子过程使用ATT的Read By Type Request,同时设置参数如下 - Starting Handle : 所要查找服务的Starting Handle - Ending Handle : 所要查找服务的Ending Handle - Attribute Type : UUID for <Include>

可能的回应有 - Find By Type Response - Error Response

Find By Type Response返回[Attribute Handle, Attribute Value]集合对 Attribute Value由所包含服务申明的Attribute Handle和End Group Handle组成当UUID为16-bit Bluetooth UUID时,那么它也将包含在Rsp中

该Req应该被再次请求，同时设置Starting Handle为为收到的最后一个Attribute Handle+1

当Rsp中包含的服务申明中Attribute Handle等于Req的Ending Handle时, 该子过程被认为完成(当然Attribute Not Found-Error Rsp也是)

当Include Service使用128-bit UUID时使用Read Request来获取Include Service UUID 其中Attribute Handle参数设置为Include Service的Attribute Handle

权限规则同上面

下图是一个实例图