auto import from //depot/cupcake/@135843
diff --git a/adb/OVERVIEW.TXT b/adb/OVERVIEW.TXT
new file mode 100644
index 0000000..6a5191a
--- /dev/null
+++ b/adb/OVERVIEW.TXT
@@ -0,0 +1,139 @@
+Implementation notes regarding ADB.
+
+I. General Overview:
+
+The Android Debug Bridge (ADB) is used to:
+
+- keep track of all Android devices and emulators instances
+ connected to or running on a given host developer machine
+
+- implement various control commands (e.g. "adb shell", "adb pull", etc..)
+ for the benefit of clients (command-line users, or helper programs like
+ DDMS). These commands are what is called a 'service' in ADB.
+
+As a whole, everything works through the following components:
+
+ 1. The ADB server
+
+ This is a background process that runs on the host machine. Its purpose
+ if to sense the USB ports to know when devices are attached/removed,
+ as well as when emulator instances start/stop.
+
+ It thus maintains a list of "connected devices" and assigns a 'state'
+ to each one of them: OFFLINE, BOOTLOADER, RECOVERY or ONLINE (more on
+ this below).
+
+ The ADB server is really one giant multiplexing loop whose purpose is
+ to orchestrate the exchange of data (packets, really) between clients,
+ services and devices.
+
+
+ 2. The ADB daemon (adbd)
+
+ The 'adbd' program runs as a background process within an Android device
+ or emulated system. Its purpose is to connect to the ADB server
+ (through USB for devices, through TCP for emulators) and provide a
+ few services for clients that run on the host.
+
+ The ADB server considers that a device is ONLINE when it has succesfully
+ connected to the adbd program within it. Otherwise, the device is OFFLINE,
+ meaning that the ADB server detected a new device/emulator, but could not
+ connect to the adbd daemon.
+
+ the BOOTLOADER and RECOVERY states correspond to alternate states of
+ devices when they are in the bootloader or recovery mode.
+
+ 3. The ADB command-line client
+
+ The 'adb' command-line program is used to run adb commands from a shell
+ or a script. It first tries to locate the ADB server on the host machine,
+ and will start one automatically if none is found.
+
+ then, the client sends its service requests to the ADB server. It doesn't
+ need to know.
+
+ Currently, a single 'adb' binary is used for both the server and client.
+ this makes distribution and starting the server easier.
+
+
+ 4. Services
+
+ There are essentially two kinds of services that a client can talk to.
+
+ Host Services:
+ these services run within the ADB Server and thus do not need to
+ communicate with a device at all. A typical example is "adb devices"
+ which is used to return the list of currently known devices and their
+ state. They are a few couple other services though.
+
+ Local Services:
+ these services either run within the adbd daemon, or are started by
+ it on the device. The ADB server is used to multiplex streams
+ between the client and the service running in adbd. In this case
+ its role is to initiate the connection, then of being a pass-through
+ for the data.
+
+
+II. Protocol details:
+
+ 1. Client <-> Server protocol:
+
+ This details the protocol used between ADB clients and the ADB
+ server itself. The ADB server listens on TCP:localhost:5037.
+
+ A client sends a request using the following format:
+
+ 1. A 4-byte hexadecimal string giving the length of the payload
+ 2. Followed by the payload itself.
+
+ For example, to query the ADB server for its internal version number,
+ the client will do the following:
+
+ 1. Connect to tcp:localhost:5037
+ 2. Send the string "000Chost:version" to the corresponding socket
+
+ The 'host:' prefix is used to indicate that the request is addressed
+ to the server itself (we will talk about other kinds of requests later).
+ The content length is encoded in ASCII for easier debugging.
+
+ The server should answer a request with one of the following:
+
+ 1. For success, the 4-byte "OKAY" string
+
+ 2. For failure, the 4-byte "FAIL" string, followed by a
+ 4-byte hex length, followed by a string giving the reason
+ for failure.
+
+ 3. As a special exception, for 'host:version', a 4-byte
+ hex string corresponding to the server's internal version number
+
+ Note that the connection is still alive after an OKAY, which allows the
+ client to make other requests. But in certain cases, an OKAY will even
+ change the state of the connection.
+
+ For example, the case of the 'host:transport:<serialnumber>' request,
+ where '<serialnumber>' is used to identify a given device/emulator; after
+ the "OKAY" answer, all further requests made by the client will go
+ directly to the corresponding adbd daemon.
+
+ The file SERVICES.TXT lists all services currently implemented by ADB.
+
+
+ 2. Transports:
+
+ An ADB transport models a connection between the ADB server and one device
+ or emulator. There are currently two kinds of transports:
+
+ - USB transports, for physical devices through USB
+
+ - Local transports, for emulators running on the host, connected to
+ the server through TCP
+
+ In theory, it should be possible to write a local transport that proxies
+ a connection between an ADB server and a device/emulator connected to/
+ running on another machine. This hasn't been done yet though.
+
+ Each transport can carry one or more multiplexed streams between clients
+ and the device/emulator they point to. The ADB server must handle
+ unexpected transport disconnections (e.g. when a device is physically
+ unplugged) properly.