1 Implementation notes regarding ADB. 2 3 I. General Overview: 4 5 The Android Debug Bridge (ADB) is used to: 6 7 - keep track of all Android devices and emulators instances 8 connected to or running on a given host developer machine 9 10 - implement various control commands (e.g. "adb shell", "adb pull", etc.) 11 for the benefit of clients (command-line users, or helper programs like 12 DDMS). These commands are called 'services' in ADB. 13 14 As a whole, everything works through the following components: 15 16 1. The ADB server 17 18 This is a background process that runs on the host machine. Its purpose 19 is to sense the USB ports to know when devices are attached/removed, 20 as well as when emulator instances start/stop. 21 22 It thus maintains a list of "connected devices" and assigns a 'state' 23 to each one of them: OFFLINE, BOOTLOADER, RECOVERY or ONLINE (more on 24 this below). 25 26 The ADB server is really one giant multiplexing loop whose purpose is 27 to orchestrate the exchange of data (packets, really) between clients, 28 services and devices. 29 30 31 2. The ADB daemon (adbd) 32 33 The 'adbd' program runs as a background process within an Android device 34 or emulated system. Its purpose is to connect to the ADB server 35 (through USB for devices, through TCP for emulators) and provide a 36 few services for clients that run on the host. 37 38 The ADB server considers that a device is ONLINE when it has successfully 39 connected to the adbd program within it. Otherwise, the device is OFFLINE, 40 meaning that the ADB server detected a new device/emulator, but could not 41 connect to the adbd daemon. 42 43 The BOOTLOADER and RECOVERY states correspond to alternate states of 44 devices when they are in the bootloader or recovery mode. 45 46 3. The ADB command-line client 47 48 The 'adb' command-line program is used to run adb commands from a shell 49 or a script. It first tries to locate the ADB server on the host machine, 50 and will start one automatically if none is found. 51 52 Then, the client sends its service requests to the ADB server. 53 54 Currently, a single 'adb' binary is used for both the server and client. 55 this makes distribution and starting the server easier. 56 57 58 4. Services 59 60 There are essentially two kinds of services that a client can talk to. 61 62 Host Services: 63 These services run within the ADB Server and thus do not need to 64 communicate with a device at all. A typical example is "adb devices" 65 which is used to return the list of currently known devices and their 66 states. They are a few other services though. 67 68 Local Services: 69 These services either run within the adbd daemon, or are started by 70 it on the device. The ADB server is used to multiplex streams 71 between the client and the service running in adbd. In this case 72 its role is to initiate the connection, then of being a pass-through 73 for the data. 74 75 76 II. Protocol details: 77 78 1. Client <-> Server protocol: 79 80 This details the protocol used between ADB clients and the ADB 81 server itself. The ADB server listens on TCP:localhost:5037. 82 83 A client sends a request using the following format: 84 85 1. A 4-byte hexadecimal string giving the length of the payload 86 2. Followed by the payload itself. 87 88 For example, to query the ADB server for its internal version number, 89 the client will do the following: 90 91 1. Connect to tcp:localhost:5037 92 2. Send the string "000Chost:version" to the corresponding socket 93 94 The 'host:' prefix is used to indicate that the request is addressed 95 to the server itself (we will talk about other kinds of requests later). 96 The content length is encoded in ASCII for easier debugging. 97 98 The server should answer a request with one of the following: 99 100 1. For success, the 4-byte "OKAY" string 101 102 2. For failure, the 4-byte "FAIL" string, followed by a 103 4-byte hex length, followed by a string giving the reason 104 for failure. 105 106 3. As a special exception, for 'host:version', a 4-byte 107 hex string corresponding to the server's internal version number 108 109 Note that the connection is still alive after an OKAY, which allows the 110 client to make other requests. But in certain cases, an OKAY will even 111 change the state of the connection. 112 113 For example, the case of the 'host:transport:<serialnumber>' request, 114 where '<serialnumber>' is used to identify a given device/emulator; after 115 the "OKAY" answer, all further requests made by the client will go 116 directly to the corresponding adbd daemon. 117 118 The file SERVICES.TXT lists all services currently implemented by ADB. 119 120 121 2. Transports: 122 123 An ADB transport models a connection between the ADB server and one device 124 or emulator. There are currently two kinds of transports: 125 126 - USB transports, for physical devices through USB 127 128 - Local transports, for emulators running on the host, connected to 129 the server through TCP 130 131 In theory, it should be possible to write a local transport that proxies 132 a connection between an ADB server and a device/emulator connected to/ 133 running on another machine. This hasn't been done yet though. 134 135 Each transport can carry one or more multiplexed streams between clients 136 and the device/emulator they point to. The ADB server must handle 137 unexpected transport disconnections (e.g. when a device is physically 138 unplugged) properly. 139
