Fastboot
--------

The fastboot protocol is a mechanism for communicating with bootloaders
over USB or ethernet.  It is designed to be very straightforward to implement,
to allow it to be used across a wide range of devices and from hosts running
Linux, macOS, or Windows.


## Basic Requirements

* USB
  * Two bulk endpoints (in, out) are required
  * Max packet size must be 64 bytes for full-speed, 512 bytes for
    high-speed and 1024 bytes for Super Speed USB.
  * The protocol is entirely host-driven and synchronous (unlike the
    multi-channel, bi-directional, asynchronous ADB protocol)

* TCP or UDP
  * Device must be reachable via IP.
  * Device will act as the server, fastboot will be the client.
  * Fastboot data is wrapped in a simple protocol; see below for details.


## Transport and Framing

1. Host sends a command, which is an ascii string in a single
   packet no greater than 64 bytes.

2. Client response with a single packet no greater than 64 bytes.
   The first four bytes of the response are "OKAY", "FAIL", "DATA",
   or "INFO".  Additional bytes may contain an (ascii) informative
   message.

   a. INFO -> the remaining 60 bytes are an informative message
      (providing progress or diagnostic messages).  They should
      be displayed and then step #2 repeats

   b. FAIL -> the requested command failed.  The remaining 60 bytes
      of the response (if present) provide a textual failure message
      to present to the user.  Stop.

   c. OKAY -> the requested command completed successfully.  Go to #5

   d. DATA -> the requested command is ready for the data phase.
      A DATA response packet will be 12 bytes long, in the form of
      DATA00000000 where the 8 digit hexadecimal number represents
      the total data size to transfer.

3. Data phase.  Depending on the command, the host or client will
   send the indicated amount of data.  Short packets are always
   acceptable and zero-length packets are ignored.  This phase continues
   until the client has sent or received the number of bytes indicated
   in the "DATA" response above.

4. Client responds with a single packet no greater than 64 bytes.
   The first four bytes of the response are "OKAY", "FAIL", or "INFO".
   Similar to #2:

   a. INFO -> display the remaining 60 bytes and return to #4

   b. FAIL -> display the remaining 60 bytes (if present) as a failure
      reason and consider the command failed.  Stop.

   c. OKAY -> success.  Go to #5

5. Success.  Stop.


## Example Session

    Host:    "getvar:version"        request version variable

    Client:  "OKAY0.4"               return version "0.4"

    Host:    "getvar:nonexistant"    request some undefined variable

    Client:  "FAILUnknown variable"  getvar failure; see getvar details below

    Host:    "download:00001234"     request to send 0x1234 bytes of data

    Client:  "DATA00001234"          ready to accept data

    Host:    < 0x1234 bytes >        send data

    Client:  "OKAY"                  success

    Host:    "flash:bootloader"      request to flash the data to the bootloader

    Client:  "INFOerasing flash"     indicate status / progress
             "INFOwriting flash"
             "OKAY"                  indicate success

    Host:    "powerdown"             send a command

    Client:  "FAILunknown command"   indicate failure


## Command Reference

* Command parameters are indicated by printf-style escape sequences.

* Commands are ascii strings and sent without the quotes (which are
  for illustration only here) and without a trailing 0 byte.

* Commands that begin with a lowercase letter are reserved for this
  specification.  OEM-specific commands should not begin with a
  lowercase letter, to prevent incompatibilities with future specs.

The various currently defined commands are:

    getvar:%s          Read a config/version variable from the bootloader.
                       The variable contents will be returned after the
                       OKAY response. If the variable is unknown, the bootloader
                       should return a FAIL response, optionally with an error
                       message.

                       Previous versions of this document indicated that getvar
                       should return an empty OKAY response for unknown
                       variables, so older devices might exhibit this behavior,
                       but new implementations should return FAIL instead.

    download:%08x      Write data to memory which will be later used
                       by "boot", "ramdisk", "flash", etc.  The client
                       will reply with "DATA%08x" if it has enough
                       space in RAM or "FAIL" if not.  The size of
                       the download is remembered.

    verify:%08x        Send a digital signature to verify the downloaded
                       data.  Required if the bootloader is "secure"
                       otherwise "flash" and "boot" will be ignored.

    flash:%s           Write the previously downloaded image to the
                       named partition (if possible).

    erase:%s           Erase the indicated partition (clear to 0xFFs)

    boot               The previously downloaded data is a boot.img
                       and should be booted according to the normal
                       procedure for a boot.img

    continue           Continue booting as normal (if possible)

    reboot             Reboot the device.

    reboot-bootloader
                       Reboot back into the bootloader.
                       Useful for upgrade processes that require upgrading
                       the bootloader and then upgrading other partitions
                       using the new bootloader.

    powerdown          Power off the device.


## Client Variables

The "getvar:%s" command is used to read client variables which
represent various information about the device and the software
on it.

The various currently defined names are:

    version             Version of FastBoot protocol supported.
                        It should be "0.4" for this document.

    version-bootloader  Version string for the Bootloader.

    version-baseband    Version string of the Baseband Software

    product             Name of the product

    serialno            Product serial number

    secure              If the value is "yes", this is a secure
                        bootloader requiring a signature before
                        it will install or boot images.

Names starting with a lowercase character are reserved by this
specification.  OEM-specific names should not start with lowercase
characters.


## TCP Protocol v1

The TCP protocol is designed to be a simple way to use the fastboot protocol
over ethernet if USB is not available.

The device will open a TCP server on port 5554 and wait for a fastboot client
to connect.

### Handshake
Upon connecting, both sides will send a 4-byte handshake message to ensure they
are speaking the same protocol. This consists of the ASCII characters "FB"
followed by a 2-digit base-10 ASCII version number. For example, the version 1
handshake message will be [FB01].

If either side detects a malformed handshake, it should disconnect.

The protocol version to use must be the minimum of the versions sent by each
side; if either side cannot speak this protocol version, it should disconnect.

### Fastboot Data
Once the handshake is complete, fastboot data will be sent as follows:

    [data_size][data]

Where data\_size is an unsigned 8-byte big-endian binary value, and data is the
fastboot packet. The 8-byte length is intended to provide future-proofing even
though currently fastboot packets have a 4-byte maximum length.

### Example
In this example the fastboot host queries the device for two variables,
"version" and "none".

    Host    <connect to the device on port 5555>
    Host    FB01
    Device  FB01
    Host    [0x00][0x00][0x00][0x00][0x00][0x00][0x00][0x0E]getvar:version
    Device  [0x00][0x00][0x00][0x00][0x00][0x00][0x00][0x07]OKAY0.4
    Host    [0x00][0x00][0x00][0x00][0x00][0x00][0x00][0x0B]getvar:none
    Device  [0x00][0x00][0x00][0x00][0x00][0x00][0x00][0x14]FAILUnknown variable
    Host    <disconnect>


## UDP Protocol v1

The UDP protocol is more complex than TCP since we must implement reliability
to ensure no packets are lost, but the general concept of wrapping the fastboot
protocol is the same.

Overview:
  1. As with TCP, the device will listen on UDP port 5554.
  2. Maximum UDP packet size is negotiated during initialization.
  3. The host drives all communication; the device may only send a packet as a
     response to a host packet.
  4. If the host does not receive a response in 500ms it will re-transmit.

### UDP Packet format

    +----------+----+-------+-------+--------------------+
    | Byte #   | 0  |   1   | 2 - 3 |  4+                |
    +----------+----+-------+-------+--------------------+
    | Contents | ID | Flags | Seq # | Data               |
    +----------+----+-------+-------+--------------------+

    ID      Packet ID:
              0x00: Error.
              0x01: Query.
              0x02: Initialization.
              0x03: Fastboot.

            Packet types are described in more detail below.

    Flags   Packet flags: 0 0 0 0 0 0 0 C
              C=1 indicates a continuation packet; the data is too large and will
                  continue in the next packet.

              Remaining bits are reserved for future use and must be set to 0.

    Seq #   2-byte packet sequence number (big-endian). The host will increment
            this by 1 with each new packet, and the device must provide the
            corresponding sequence number in the response packets.

    Data    Packet data, not present in all packets.

### Packet Types

    Query
          The host sends a query packet once on startup to sync with the device.
          The host will not know the current sequence number, so the device must
          respond to all query packets regardless of sequence number.

          The response data field should contain a 2-byte big-endian value
          giving the next expected sequence number.

    Init
          The host sends an init packet once the query response is returned. The
          device must abort any in-progress operation and prepare for a new
          fastboot session. This message is meant to allow recovery if a
          previous session failed, e.g. due to network error or user Ctrl+C.

          The data field contains two big-endian 2-byte values, a protocol
          version and the max UDP packet size (including the 4-byte header).
          Both the host and device will send these values, and in each case
          the minimum of the sent values must be used.

    Fastboot
          These packets wrap the fastboot protocol. To write, the host will
          send a packet with fastboot data, and the device will reply with an
          empty packet as an ACK. To read, the host will send an empty packet,
          and the device will reply with fastboot data. The device may not give
          any data in the ACK packet.

    Error
          The device may respond to any packet with an error packet to indicate
          a UDP protocol error. The data field should contain an ASCII string
          describing the error. This is the only case where a device is allowed
          to return a packet ID other than the one sent by the host.

### Packet Size
The maximum packet size is negotiated by the host and device in the Init packet.
Devices must support at least 512-byte packets, but packet size has a direct
correlation with download speed, so devices are strongly suggested to support at
least 1024-byte packets. On a local network with 0.5ms round-trip time this will
provide transfer rates of ~2MB/s. Over WiFi it will likely be significantly
less.

Query and Initialization packets, which are sent before size negotiation is
complete, must always be 512 bytes or less.

### Packet Re-Transmission
The host will re-transmit any packet that does not receive a response. The
requirement of exactly one device response packet per host packet is how we
achieve reliability and in-order delivery of packets.

For simplicity of implementation, there is no windowing of multiple
unacknowledged packets in this version of the protocol. The host will continue
to send the same packet until a response is received. Windowing functionality
may be implemented in future versions if necessary to increase performance.

The first Query packet will only be attempted a small number of times, but
subsequent packets will attempt to retransmit for at least 1 minute before
giving up. This means a device may safely ignore host UDP packets for up to 1
minute during long operations, e.g. writing to flash.

### Continuation Packets
Any packet may set the continuation flag to indicate that the data is
incomplete. Large data such as downloading an image may require many
continuation packets. The receiver should respond to a continuation packet with
an empty packet to acknowledge receipt. See examples below.

### Summary
The host starts with a Query packet, then an Initialization packet, after
which only Fastboot packets are sent. Fastboot packets may contain data from
the host for writes, or from the device for reads, but not both.

Given a next expected sequence number S and a received packet P, the device
behavior should be:

    if P is a Query packet:
      * respond with a Query packet with S in the data field
    else if P has sequence == S:
      * process P and take any required action
      * create a response packet R with the same ID and sequence as P, containing
        any response data required.
      * transmit R and save it in case of re-transmission
      * increment S
    else if P has sequence == S - 1:
      * re-transmit the saved response packet R from above
    else:
      * ignore the packet

### Examples

In the examples below, S indicates the starting client sequence number.

    Host                                    Client
    ======================================================================
    [Initialization, S = 0x55AA]
    [Host: version 1, 2048-byte packets. Client: version 2, 1024-byte packets.]
    [Resulting values to use: version = 1, max packet size = 1024]
    ID   Flag SeqH SeqL Data                ID   Flag SeqH SeqL Data
    ----------------------------------------------------------------------
    0x01 0x00 0x00 0x00
                                            0x01 0x00 0x00 0x00 0x55 0xAA
    0x02 0x00 0x55 0xAA 0x00 0x01 0x08 0x00
                                            0x02 0x00 0x55 0xAA 0x00 0x02 0x04 0x00

    ----------------------------------------------------------------------
    [fastboot "getvar" commands, S = 0x0001]
    ID    Flags SeqH  SeqL  Data            ID    Flags SeqH  SeqL  Data
    ----------------------------------------------------------------------
    0x03  0x00  0x00  0x01  getvar:version
                                            0x03  0x00  0x00  0x01
    0x03  0x00  0x00  0x02
                                            0x03  0x00  0x00  0x02  OKAY0.4
    0x03  0x00  0x00  0x03  getvar:none
                                            0x03  0x00  0x00  0x03
    0x03  0x00  0x00  0x04
                                            0x03  0x00  0x00  0x04  FAILUnknown var

    ----------------------------------------------------------------------
    [fastboot "INFO" responses, S = 0x0000]
    ID    Flags SeqH  SeqL  Data            ID    Flags SeqH  SeqL  Data
    ----------------------------------------------------------------------
    0x03  0x00  0x00  0x00  <command>
                                            0x03  0x00  0x00  0x00
    0x03  0x00  0x00  0x01
                                            0x03  0x00  0x00  0x01  INFOWait1
    0x03  0x00  0x00  0x02
                                            0x03  0x00  0x00  0x02  INFOWait2
    0x03  0x00  0x00  0x03
                                            0x03  0x00  0x00  0x03  OKAY

    ----------------------------------------------------------------------
    [Chunking 2100 bytes of data, max packet size = 1024, S = 0xFFFF]
    ID   Flag SeqH SeqL Data                ID   Flag SeqH SeqL Data
    ----------------------------------------------------------------------
    0x03 0x00 0xFF 0xFF download:0000834
                                            0x03 0x00 0xFF 0xFF
    0x03 0x00 0x00 0x00
                                            0x03 0x00 0x00 0x00 DATA0000834
    0x03 0x01 0x00 0x01 <1020 bytes>
                                            0x03 0x00 0x00 0x01
    0x03 0x01 0x00 0x02 <1020 bytes>
                                            0x03 0x00 0x00 0x02
    0x03 0x00 0x00 0x03 <60 bytes>
                                            0x03 0x00 0x00 0x03
    0x03 0x00 0x00 0x04
                                            0x03 0x00 0x00 0x04 OKAY

    ----------------------------------------------------------------------
    [Unknown ID error, S = 0x0000]
    ID    Flags SeqH  SeqL  Data            ID    Flags SeqH  SeqL  Data
    ----------------------------------------------------------------------
    0x10  0x00  0x00  0x00
                                            0x00  0x00  0x00  0x00  <error message>

    ----------------------------------------------------------------------
    [Host packet loss and retransmission, S = 0x0000]
    ID    Flags SeqH  SeqL  Data            ID    Flags SeqH  SeqL  Data
    ----------------------------------------------------------------------
    0x03  0x00  0x00  0x00  getvar:version [lost]
    0x03  0x00  0x00  0x00  getvar:version [lost]
    0x03  0x00  0x00  0x00  getvar:version
                                            0x03  0x00  0x00  0x00
    0x03  0x00  0x00  0x01
                                            0x03  0x00  0x00  0x01  OKAY0.4

    ----------------------------------------------------------------------
    [Client packet loss and retransmission, S = 0x0000]
    ID    Flags SeqH  SeqL  Data            ID    Flags SeqH  SeqL  Data
    ----------------------------------------------------------------------
    0x03  0x00  0x00  0x00  getvar:version
                                            0x03  0x00  0x00  0x00 [lost]
    0x03  0x00  0x00  0x00  getvar:version
                                            0x03  0x00  0x00  0x00 [lost]
    0x03  0x00  0x00  0x00  getvar:version
                                            0x03  0x00  0x00  0x00
    0x03  0x00  0x00  0x01
                                            0x03  0x00  0x00  0x01  OKAY0.4

    ----------------------------------------------------------------------
    [Host packet delayed, S = 0x0000]
    ID    Flags SeqH  SeqL  Data            ID    Flags SeqH  SeqL  Data
    ----------------------------------------------------------------------
    0x03  0x00  0x00  0x00  getvar:version [delayed]
    0x03  0x00  0x00  0x00  getvar:version
                                            0x03  0x00  0x00  0x00
    0x03  0x00  0x00  0x01
                                            0x03  0x00  0x00  0x01  OKAY0.4
    0x03  0x00  0x00  0x00  getvar:version [arrives late with old seq#, is ignored]