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.
USB
TCP or UDP
Host sends a command, which is an ascii string in a single packet no greater than 64 bytes.
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.
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.
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
Success. Stop.
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 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.
upload Read data from memory which was staged by the last
command, e.g. an oem command. The client will reply
with "DATA%08x" if it is ready to send %08x bytes of
data. If no data was staged in the last command,
the client must reply with "FAIL". After the client
successfully sends %08x bytes, the client shall send
a single packet starting with "OKAY". Clients
should not support "upload" unless it supports an
oem command that requires "upload" capabilities.
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.
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.
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.
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.
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.
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>
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:
+----------+----+-------+-------+--------------------+
| 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.
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.
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.
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.
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.
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
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]