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GATT(Generic Attribute Profile)，描述了一種使用ATT的服務(wù)框架
該框架定義了服務(wù)(Server)和服務(wù)屬性(characteristic)的過程(Procedure)及格式
Procedure定義了characteristic的發(fā)現(xiàn)、讀、寫、通知(Notifing)、指示(Indicating)
及配置characteristic的廣播 GATT可以被Application或其他Profile使用
其協(xié)議棧如下圖
--------------- ---------------
| Application | <----> | Application |
--------------- ---------------
| Attribute | | Attribute |
| Protocol | <----> | Protocol |
--------------- ---------------
| L2CAP | <----> | L2CAP |
--------------- ---------------
| Controller | <----> | Controller |
--------------- --------------- GATT可以配置為如下兩種角色(Role)
- Client : 命令、請求發(fā)起方
- Server : 命令、請求接收方 角色配置實(shí)例如下 _________ / ======= \ / __________\ | ___________ | Request /\ | |Computer | | ----------> / / | |(Client) | | <---------- / / Sensor(Server) | |_________| | Response / / \=____________/ \/ / ''''''''''' \ / ::::::::::::: \ (_________________) Computer是一個溫度服務(wù)客戶端, Sensor是溫度服務(wù)服務(wù)器
Computer向Sensor發(fā)起Procedure來讀Sensor的值 GATT對下層的需求如下
- Physical Link : 使用GAP Channel Establishment建立的ATT Bearer
- GATT Role : 不依賴于Coontroller角色(Master/Slave)
- Security : 對于LE,Security Features(Authorization、Authentication、Encryption)是可選的
對于BR/EDR, Encryption是強(qiáng)制的
- TX order : GATT中的多字節(jié)字段，采用Least Significant octet先發(fā)送(Little Endian) 2 內(nèi)容 2.1 Configured Broadcast對于LE物理鏈路，在Server廣播模式過程中
Client通過Configured Broadcast告知Server應(yīng)該在advertising data加入Characteristic Value 方法是Client設(shè)置指定bit位
廣播頻率則是Service、Characteristic行為定義的一部分 2.2 GATT Profile HierarchyGATT指定了數(shù)據(jù)交互的結(jié)構(gòu)(Structure)
這個結(jié)構(gòu)體定義了一些基本元素，如Service、Characteristic
這些元素存在于Attribute中 
GATT中最上層是Profile，Profile由一個或多個服務(wù)(Service)組成
服務(wù)是由Characteristics組成，或是其他服務(wù)的引用(Reference)
Characteristic包含一個值(Value)，可能包含該Value的相關(guān)信息 2.2.1 ServiceService是[數(shù)據(jù)]和與之關(guān)聯(lián)的[完成某個特定功能的行為]/[特性]的集合
在GATT中，一個服務(wù)由服務(wù)定義(Service Defintion)來實(shí)現(xiàn)
一個服務(wù)定義可能包含引用服務(wù)(Reference Service)、強(qiáng)制Characteristic和可選Characteristic 為了實(shí)現(xiàn)舊版本的兼容，新版本中服務(wù)定義只能增加新的引用服務(wù)或可選Characteristic
新版本中的服務(wù)定義禁止從舊的服務(wù)定義中改變行為 Server有兩類
- Primary Service : 擁有基本功能的服務(wù),可被其他服務(wù)包含,可以通過Primary Service Discovery過程來發(fā)現(xiàn)
- Secondary Service : 僅用來被Primary/Other Secondary Service、高層協(xié)議引用的服務(wù) 判讀一個服務(wù)是Primary or Secondary Service可通過高層協(xié)議強(qiáng)制規(guī)定 2.2.2 Included Service一個Included Service是一種引用已存在服務(wù)的方法，
具體辦法為在服務(wù)定義的開始加上Included Service的引用，
這樣整個Included Service定義成為新服務(wù)定義的一部分 2.2.3 CharacteristicCharacteristic由Characteristic Definition定義，
包含一個Characteristic聲明、Characteristic屬性、值、值的描述(Optional) 3 Service Interoperability Requirements 3.1 Service Definition服務(wù)定義(Service Definition)包含一個服務(wù)申明(Service Declaration)
可能包含Include Definitions和Characteristic Definitions
在下一個服務(wù)申明前或到達(dá)Maximum Attribute Handle時結(jié)束
服務(wù)定義在服務(wù)端上基于Attribute Handle順序呈現(xiàn) 服務(wù)定義中的Include Definitions和Characteristic Definitions被認(rèn)為是服務(wù)的一部分
服務(wù)定義中的順序?yàn)?
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 |
規(guī)則如下:
- 當(dāng)多個服務(wù)存在時
使用16-bit Bluetooth UUID服務(wù)定義的服務(wù)應(yīng)該分組(如按順序排列)
同理，使用128-bit UUID的服務(wù)定義的服務(wù)也分組
- 一個設(shè)備或高層協(xié)議可能有多個服務(wù)定義，同時多個服務(wù)定義含有相同的Service UUID
- 服務(wù)端的所有Attributes應(yīng)該包含一個服務(wù)聲明或存在一個服務(wù)定義
- 服務(wù)端的服務(wù)定義可能無序;Client不應(yīng)該認(rèn)為服務(wù)端的服務(wù)定義一定是有序的 3.2 Include Definition一個Include Definition只包含一個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，僅當(dāng)UUID是16-bit Bluetooth UUID時才存在 如果一個Service的Include Definition(A)是引用其他Server的Include Definition(B)
那么Include Definition(B)不應(yīng)該引用Include Definition(A)，否則就是循環(huán)引用(Circular Reference) 當(dāng)一個Client檢測到循環(huán)引用或detects nested include declarations to a greater level than it expects
Client應(yīng)當(dāng)終止本次通信(ATT Bearer) 3.3 Characteristic DefinitionCharacteristic Definition包含如下內(nèi)容
- Characteristic Declaration : First
- Characteristic Value declaration : Second
- Characteristic Descriptor Declarations(Optional) : Last(含多個時順序不關(guān)緊要) Characteristic Definitions在服務(wù)端以Attribute Handle排序 以上每個Declaration包含在一個單獨(dú)的Attribute中 3.3.1 Characteristic DeclarationCharacteristic 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 |
其中，Attribute Value只讀
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 |
一個Service可能含有相同Characteristic UUID的Characteristic Definitions 在一個服務(wù)定義中，一些強(qiáng)制的Characteristics應(yīng)該位于Include Declarations之后、其他可選characteristics之前
而Client不應(yīng)該認(rèn)為Characteristic是有序的
同時，16-bit Bluetooth UUIDs和128-bit Bluetooth UUIDs應(yīng)該分別分組 Characteristic Properties以bit位的方式?jīng)Q定如下內(nèi)容
- Characteristic Value的使用方式
- Characteristic Descriptors的訪問方式 Characteristic Properties bit field的詳細(xì)內(nèi)容如下
Notice: 多個比特位可以同時設(shè)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 DeclarationsCharacteristic Descriptor Declarations包含了Characteristic Value相關(guān)信息 GATT定義了一系列的標(biāo)準(zhǔn)Characteristic Descriptors供高層協(xié)議使用
高層協(xié)議也可以定義協(xié)議相關(guān)的Characteristic Descriptors Characteristic Descriptors在服務(wù)端上是無序的，Client不應(yīng)該理所當(dāng)然
Characteristic Descriptors Declarations Permissions由高層協(xié)議定義或協(xié)議相關(guān)的
Client不應(yīng)該理所當(dāng)然地認(rèn)為是可讀的 Characteristic Descriptor Declarations包括如下內(nèi)容(詳細(xì)見規(guī)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 OverviewGATT中定義了11項(xiàng)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 每個Feature都有對應(yīng)的過程和子過程，這些過程描述了如何使用ATT來實(shí)現(xiàn)各自的功能 4.2 Feature Support and Procedure Mapping詳細(xì)的對應(yīng)列表如下表 No | Feature | Sub-Procedure | Ref | Client
Support | Server
Support | 1 | Server Configuration | Exchange MTU | 4.3.1 | O | O | 2 | Primary Service Discovery | Discover All Primary Services | 4.4.1 | O | M | Discover Primary Services By
Service UUID | 4.4.2 | O | M | 3 | Relationship Discovery | Find Included Services | 4.5.1 | O | M | 4 | Characteristic
Discovery | Discover All Characteristic of a Service | 4.6.1 | O | M | Discover Characteristic by UUID | 4.6.2 | O | M | 5 | Characteristic
Descriptor Discovery | Discover All Characteristic Descriptors | 4.7.1 | O | M | 6 | Characteristic Value Read | Read Characteristic Value | 4.8.1 | O | M | Read Using Characteristic UUID | 4.8.1 | O | M | Read Long Characteristic Values | 4.8.2 | O | O | Read Multiple Characteristic Values | 4.8.3 | O | O | 7 | Characteristic Value Write | Write Without Response | 4.9.1 | O | C.1 | Signed Write Without Response | 4.9.2 | O | O | Write Characteristic Value | 4.9.3 | O | C.2 | Write Long Characteristic Values | 4.9.4 | O | O | Characteristic Value Reliable Writes | 4.9.5 | O | O | 8 | Characteristic Value Notification | Notifications | 4.10.1 | O | O | 9 | Characteristic Value Indication | Indications | 4.11.1 | M | C.3 | 10 | Characteristic
Descriptor Value Read | Read Characteristic Descriptors | 4.12.1 | O | O | Read Long Characteristic Descriptors | 4.12.2 | O | O | 11 | Characteristic
Descriptor Value Write | Write Characteristic Descriptors | 4.12.3 | O | O | Write Long Characteristic Descriptors | 4.12.4 | O | O | C1: Write Without Response is mandatory if Signed Write Without Response is supported otherwise optional C2: Write Characteristic Value is mandatory if Write Long Characteristic Values is supported otherwise optional C3: If Service Change Characteristic is present, this feature is mandatory, otherwise optional. |
4.3 Server Configuration該過程可被Client用來配置Attribute Protocol的MTU大小 4.3.1 Exchange MTUClient使用該子過程來設(shè)置適配雙方均支持的最大ATT_MTU 在BR/EDR物理鏈路中不應(yīng)該使用該過程，而應(yīng)該使用L2CAP Channel Configuration Procedures 該過程對應(yīng)于ATT的MTU Exchange Request/Response
見<Bluetooth ATT介紹 - 4.2 MTU Exchange> 4.4 Primary Service DiscoveryClient使用該過程來發(fā)現(xiàn)服務(wù)端的Primary Services
一旦發(fā)現(xiàn)服務(wù)存在，可通過其他過程來訪問Primary Services的附加信息(關(guān)聯(lián)主服務(wù)和次服務(wù))
可使用的其他過程包括Characteristic Discovery和Relationship Discovery 該過程包括兩個子過程:
- Discover All Primary Services
- Discover Primary Services by Service UUID 在BR/EDR物理鏈路上則使用SDP service discovery來發(fā)現(xiàn)服務(wù) 4.4.1 Discover All Primary ServicesClient使用該子過程來發(fā)現(xiàn)服務(wù)端的所有Primary Services 該子過程使用ATT的Read By Group Type Request,同時設(shè)置如下參數(shù)
- Starting Handle : 0x0001
- Ending Handle : 0xFFFF
- Attribute Type : UUID for <Primary Service> 可能的回應(yīng)有
- Read By Group Type Response
- Error Response Read By Group Type Response返回三元組列表
三元組包括
- Attribute Handle : 服務(wù)聲明的Handle
- End Group Handle : 服務(wù)定義中最后一個Attribute的Handle
- Attribute Value : 服務(wù)端支持的服務(wù)的Service UUID 當(dāng)收到Error Response<Error Code: Attribute Not Found>時
則表明該過程已經(jīng)完成 當(dāng)Client找到自己所需要的服務(wù)時,可以終止該過程 Note: 3.1中已指出Service Declaration是可讀,并且不需要認(rèn)證或授權(quán)
因此權(quán)限相關(guān)的錯誤不會發(fā)生
下圖是一個實(shí)例圖 
4.4.2 Discover Primary Service by Service UUID當(dāng)Client只知道Service UUID時,可以使用該子過程來發(fā)現(xiàn)對應(yīng)的主服務(wù) 該子過程使用ATT的Find By Type Value Request,同時設(shè)置參數(shù)如下
- Starting Handle : 0x0001
- Ending Handle : 0xFFFF
- Attribute Value : 16-bit Bluetooth UUID or 128-bit UUID
- Attribute Type : UUID for <Primary Service> 可能的回應(yīng)有
- Find By Type Value Response
- Error Response Find By Type Value Response返回Attribute Handle ranges列表
Attribute Handle range即服務(wù)定義的Starting Handle和Ending Handle
如果Attribute Handle range中的End Found Handle不是0xFFFF
那么Client將會再請求一次Req
同時將Starting Handle設(shè)置為收到的最后一個Attribute Handle+1 終止規(guī)則和權(quán)限問題同Discover All Primary Services 下圖是一個實(shí)例圖 
4.5 Relationship DiscoveryClient使用該過程來發(fā)現(xiàn)和其他服務(wù)的服務(wù)關(guān)系 4.5.1 Find Include ServicesClient使用該子過程來發(fā)現(xiàn)一個服務(wù)定義包含的服務(wù)申明 該子過程使用ATT的Read By Type Request,同時設(shè)置參數(shù)如下
- Starting Handle : 所要查找服務(wù)的Starting Handle
- Ending Handle : 所要查找服務(wù)的Ending Handle
- Attribute Type : UUID for <Include> 可能的回應(yīng)有
- Find By Type Response
- Error Response Find By Type Response返回[Attribute Handle, Attribute Value]集合對
Attribute Value由所包含服務(wù)申明的Attribute Handle和End Group Handle組成
當(dāng)UUID為16-bit Bluetooth UUID時,那么它也將包含在Rsp中 該Req應(yīng)該被再次請求，同時設(shè)置Starting Handle為為收到的最后一個Attribute Handle+1 當(dāng)Rsp中包含的服務(wù)申明中Attribute Handle等于Req的Ending Handle時,
該子過程被認(rèn)為完成(當(dāng)然Attribute Not Found-Error Rsp也是) 當(dāng)Include Service使用128-bit UUID時
使用Read Request來獲取Include Service UUID
其中Attribute Handle參數(shù)設(shè)置為Include Service的Attribute Handle 權(quán)限規(guī)則同上面 下圖是一個實(shí)例圖 
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