README.md
1Fastboot
2--------
3
4The fastboot protocol is a mechanism for communicating with bootloaders
5over USB or ethernet. It is designed to be very straightforward to implement,
6to allow it to be used across a wide range of devices and from hosts running
7Linux, macOS, or Windows.
8
9
10## Basic Requirements
11
12* USB
13 * Two bulk endpoints (in, out) are required
14 * Max packet size must be 64 bytes for full-speed, 512 bytes for
15 high-speed and 1024 bytes for Super Speed USB.
16 * The protocol is entirely host-driven and synchronous (unlike the
17 multi-channel, bi-directional, asynchronous ADB protocol)
18
19* TCP or UDP
20 * Device must be reachable via IP.
21 * Device will act as the server, fastboot will be the client.
22 * Fastboot data is wrapped in a simple protocol; see below for details.
23
24
25## Transport and Framing
26
271. Host sends a command, which is an ascii string in a single
28 packet no greater than 4096 bytes.
29
302. Client response with a single packet no greater than 256 bytes.
31 The first four bytes of the response are "OKAY", "FAIL", "DATA",
32 "INFO" or "TEXT". Additional bytes may contain an (ascii) informative
33 message.
34
35 a. INFO -> the remaining 252 bytes are an informative message
36 (providing progress or diagnostic messages). They should
37 be displayed and then step #2 repeats. The print format is:
38 "(bootloader) " + InfoMessagePayload + '\n'
39
40 b. TEXT -> the remaining 252 bytes are arbitrary. They should
41 be displayed and then step #2 repeats.
42 It differs from info in that no formatting is applied.
43 The payload is printed as-is with no newline at the end.
44 Payload is expected to be NULL terminated.
45
46 c. FAIL -> the requested command failed. The remaining 252 bytes
47 of the response (if present) provide a textual failure message
48 to present to the user. Stop.
49
50 d. OKAY -> the requested command completed successfully. Go to #5
51
52 e. DATA -> the requested command is ready for the data phase.
53 A DATA response packet will be 12 bytes long, in the form of
54 DATA00000000 where the 8 digit hexadecimal number represents
55 the total data size to transfer.
56
573. Data phase. Depending on the command, the host or client will
58 send the indicated amount of data. Short packets are always
59 acceptable and zero-length packets are ignored. This phase continues
60 until the client has sent or received the number of bytes indicated
61 in the "DATA" response above.
62
634. Client responds with a single packet no greater than 256 bytes.
64 The first four bytes of the response are "OKAY", "FAIL",
65 "INFO" or "TEXT". Similar to #2:
66
67 a. INFO -> display the formatted remaining 252 bytes and return to #4
68
69 b. TEXT -> display the unformatted remaining 252 bytes and return to #4
70
71 c. FAIL -> display the remaining 252 bytes (if present) as a failure
72 reason and consider the command failed. Stop.
73
74 d. OKAY -> success. Go to #5
75
765. Success. Stop.
77
78
79## Example Session
80
81 Host: "getvar:version" request version variable
82
83 Client: "OKAY0.4" return version "0.4"
84
85 Host: "getvar:nonexistant" request some undefined variable
86
87 Client: "FAILUnknown variable" getvar failure; see getvar details below
88
89 Host: "download:00001234" request to send 0x1234 bytes of data
90
91 Client: "DATA00001234" ready to accept data
92
93 Host: < 0x1234 bytes > send data
94
95 Client: "OKAY" success
96
97 Host: "flash:bootloader" request to flash the data to the bootloader
98
99 Client: "INFOerasing flash" indicate status / progress
100 "INFOwriting flash"
101 "OKAY" indicate success
102
103 Host: "powerdown" send a command
104
105 Client: "FAILunknown command" indicate failure
106
107
108## Command Reference
109
110* Command parameters are indicated by printf-style escape sequences.
111
112* Commands are ascii strings and sent without the quotes (which are
113 for illustration only here) and without a trailing 0 byte.
114
115* Commands that begin with a lowercase letter are reserved for this
116 specification. OEM-specific commands should not begin with a
117 lowercase letter, to prevent incompatibilities with future specs.
118
119The various currently defined commands are:
120
121 getvar:%s Read a config/version variable from the bootloader.
122 The variable contents will be returned after the
123 OKAY response. If the variable is unknown, the bootloader
124 should return a FAIL response, optionally with an error
125 message.
126
127 Previous versions of this document indicated that getvar
128 should return an empty OKAY response for unknown
129 variables, so older devices might exhibit this behavior,
130 but new implementations should return FAIL instead.
131
132 download:%08x Write data to memory which will be later used
133 by "boot", "ramdisk", "flash", etc. The client
134 will reply with "DATA%08x" if it has enough
135 space in RAM or "FAIL" if not. The size of
136 the download is remembered.
137
138 upload Read data from memory which was staged by the last
139 command, e.g. an oem command. The client will reply
140 with "DATA%08x" if it is ready to send %08x bytes of
141 data. If no data was staged in the last command,
142 the client must reply with "FAIL". After the client
143 successfully sends %08x bytes, the client shall send
144 a single packet starting with "OKAY". Clients
145 should not support "upload" unless it supports an
146 oem command that requires "upload" capabilities.
147
148 flash:%s Write the previously downloaded image to the
149 named partition (if possible).
150
151 erase:%s Erase the indicated partition (clear to 0xFFs)
152
153 boot The previously downloaded data is a boot.img
154 and should be booted according to the normal
155 procedure for a boot.img
156
157 continue Continue booting as normal (if possible)
158
159 reboot Reboot the device.
160
161 reboot-bootloader
162 Reboot back into the bootloader.
163 Useful for upgrade processes that require upgrading
164 the bootloader and then upgrading other partitions
165 using the new bootloader.
166
167
168## Flashing Logic
169
170Fastboot binary will follow directions listed out fastboot-info.txt
171build artifact for fastboot flashall && fastboot update comamnds.
172This build artifact will live inside of ANDROID_PRODUCT_OUT &&
173target_files_package && updatepackage.
174
175
176The currently defined commands are:
177
178 flash %s Flash a given partition. Optional arguments include
179 --slot-other, {filename_path}, --apply-vbmeta
180
181 reboot %s Reboot to either bootloader or fastbootd
182
183 update-super Updates the super partition
184
185 if-wipe Conditionally run some other functionality if
186 wipe is specified
187
188 erase %s Erase a given partition (can only be used in conjunction)
189 with if-wipe -> eg. if-wipe erase cache
190
191Flashing Optimization:
192
193 After generating the list of tasks to execute, Fastboot will try and
194 optimize the flashing of the dynamic partitions by constructing an
195 optimized flash super task. Fastboot will explicitly pattern match the
196 following commands and try and concatenate it into this task. (doing so
197 will allow us to avoid the reboot into userspace fastbootd which takes
198 significant time)
199
200 //Optimizable Block
201 reboot fastboot
202 update-super ---> generate optimized flash super task
203 $FOR EACH {dynamic partition}
204 flash {dynamic partition}
205
206## Client Variables
207
208The "getvar:%s" command is used to read client variables which
209represent various information about the device and the software
210on it.
211
212The various currently defined names are:
213
214 version Version of FastBoot protocol supported.
215 It should be "0.4" for this document.
216
217 version-bootloader Version string for the Bootloader.
218
219 version-baseband Version string of the Baseband Software
220
221 product Name of the product
222
223 serialno Product serial number
224
225 secure If the value is "yes", this is a secure
226 bootloader requiring a signature before
227 it will install or boot images.
228
229 is-userspace If the value is "yes", the device is running
230 fastbootd. Otherwise, it is running fastboot
231 in the bootloader.
232
233Names starting with a lowercase character are reserved by this
234specification. OEM-specific names should not start with lowercase
235characters.
236
237## Logical Partitions
238
239There are a number of commands to interact with logical partitions:
240
241 update-super:%s:%s Write the previously downloaded image to a super
242 partition. Unlike the "flash" command, this has
243 special rules. The image must have been created by
244 the lpmake command, and must not be a sparse image.
245 If the last argument is "wipe", then all existing
246 logical partitions are deleted. If no final argument
247 is specified, the partition tables are merged. Any
248 partition in the new image that does not exist in the
249 old image is created with a zero size.
250
251 In all cases, this will cause the temporary "scratch"
252 partition to be deleted if it exists.
253
254 create-logical-partition:%s:%d
255 Create a logical partition with the given name and
256 size, in the super partition.
257
258 delete-logical-partition:%s
259 Delete a logical partition with the given name.
260
261 resize-logical-partition:%s:%d
262 Change the size of the named logical partition.
263
264In addition, there is a variable to test whether a partition is logical:
265
266 is-logical:%s If the value is "yes", the partition is logical.
267 Otherwise the partition is physical.
268
269## TCP Protocol v1
270
271The TCP protocol is designed to be a simple way to use the fastboot protocol
272over ethernet if USB is not available.
273
274The device will open a TCP server on port 5554 and wait for a fastboot client
275to connect.
276
277### Handshake
278Upon connecting, both sides will send a 4-byte handshake message to ensure they
279are speaking the same protocol. This consists of the ASCII characters "FB"
280followed by a 2-digit base-10 ASCII version number. For example, the version 1
281handshake message will be [FB01].
282
283If either side detects a malformed handshake, it should disconnect.
284
285The protocol version to use must be the minimum of the versions sent by each
286side; if either side cannot speak this protocol version, it should disconnect.
287
288### Fastboot Data
289Once the handshake is complete, fastboot data will be sent as follows:
290
291 [data_size][data]
292
293Where data\_size is an unsigned 8-byte big-endian binary value, and data is the
294fastboot packet. The 8-byte length is intended to provide future-proofing even
295though currently fastboot packets have a 4-byte maximum length.
296
297### Example
298In this example the fastboot host queries the device for two variables,
299"version" and "none".
300
301 Host <connect to the device on port 5555>
302 Host FB01
303 Device FB01
304 Host [0x00][0x00][0x00][0x00][0x00][0x00][0x00][0x0E]getvar:version
305 Device [0x00][0x00][0x00][0x00][0x00][0x00][0x00][0x07]OKAY0.4
306 Host [0x00][0x00][0x00][0x00][0x00][0x00][0x00][0x0B]getvar:none
307 Device [0x00][0x00][0x00][0x00][0x00][0x00][0x00][0x14]FAILUnknown variable
308 Host <disconnect>
309
310
311## UDP Protocol v1
312
313The UDP protocol is more complex than TCP since we must implement reliability
314to ensure no packets are lost, but the general concept of wrapping the fastboot
315protocol is the same.
316
317Overview:
318 1. As with TCP, the device will listen on UDP port 5554.
319 2. Maximum UDP packet size is negotiated during initialization.
320 3. The host drives all communication; the device may only send a packet as a
321 response to a host packet.
322 4. If the host does not receive a response in 500ms it will re-transmit.
323
324### UDP Packet format
325
326 +----------+----+-------+-------+--------------------+
327 | Byte # | 0 | 1 | 2 - 3 | 4+ |
328 +----------+----+-------+-------+--------------------+
329 | Contents | ID | Flags | Seq # | Data |
330 +----------+----+-------+-------+--------------------+
331
332 ID Packet ID:
333 0x00: Error.
334 0x01: Query.
335 0x02: Initialization.
336 0x03: Fastboot.
337
338 Packet types are described in more detail below.
339
340 Flags Packet flags: 0 0 0 0 0 0 0 C
341 C=1 indicates a continuation packet; the data is too large and will
342 continue in the next packet.
343
344 Remaining bits are reserved for future use and must be set to 0.
345
346 Seq # 2-byte packet sequence number (big-endian). The host will increment
347 this by 1 with each new packet, and the device must provide the
348 corresponding sequence number in the response packets.
349
350 Data Packet data, not present in all packets.
351
352### Packet Types
353
354 Query
355 The host sends a query packet once on startup to sync with the device.
356 The host will not know the current sequence number, so the device must
357 respond to all query packets regardless of sequence number.
358
359 The response data field should contain a 2-byte big-endian value
360 giving the next expected sequence number.
361
362 Init
363 The host sends an init packet once the query response is returned. The
364 device must abort any in-progress operation and prepare for a new
365 fastboot session. This message is meant to allow recovery if a
366 previous session failed, e.g. due to network error or user Ctrl+C.
367
368 The data field contains two big-endian 2-byte values, a protocol
369 version and the max UDP packet size (including the 4-byte header).
370 Both the host and device will send these values, and in each case
371 the minimum of the sent values must be used.
372
373 Fastboot
374 These packets wrap the fastboot protocol. To write, the host will
375 send a packet with fastboot data, and the device will reply with an
376 empty packet as an ACK. To read, the host will send an empty packet,
377 and the device will reply with fastboot data. The device may not give
378 any data in the ACK packet.
379
380 Error
381 The device may respond to any packet with an error packet to indicate
382 a UDP protocol error. The data field should contain an ASCII string
383 describing the error. This is the only case where a device is allowed
384 to return a packet ID other than the one sent by the host.
385
386### Packet Size
387The maximum packet size is negotiated by the host and device in the Init packet.
388Devices must support at least 512-byte packets, but packet size has a direct
389correlation with download speed, so devices are strongly suggested to support at
390least 1024-byte packets. On a local network with 0.5ms round-trip time this will
391provide transfer rates of ~2MB/s. Over WiFi it will likely be significantly
392less.
393
394Query and Initialization packets, which are sent before size negotiation is
395complete, must always be 512 bytes or less.
396
397### Packet Re-Transmission
398The host will re-transmit any packet that does not receive a response. The
399requirement of exactly one device response packet per host packet is how we
400achieve reliability and in-order delivery of packets.
401
402For simplicity of implementation, there is no windowing of multiple
403unacknowledged packets in this version of the protocol. The host will continue
404to send the same packet until a response is received. Windowing functionality
405may be implemented in future versions if necessary to increase performance.
406
407The first Query packet will only be attempted a small number of times, but
408subsequent packets will attempt to retransmit for at least 1 minute before
409giving up. This means a device may safely ignore host UDP packets for up to 1
410minute during long operations, e.g. writing to flash.
411
412### Continuation Packets
413Any packet may set the continuation flag to indicate that the data is
414incomplete. Large data such as downloading an image may require many
415continuation packets. The receiver should respond to a continuation packet with
416an empty packet to acknowledge receipt. See examples below.
417
418### Summary
419The host starts with a Query packet, then an Initialization packet, after
420which only Fastboot packets are sent. Fastboot packets may contain data from
421the host for writes, or from the device for reads, but not both.
422
423Given a next expected sequence number S and a received packet P, the device
424behavior should be:
425
426 if P is a Query packet:
427 * respond with a Query packet with S in the data field
428 else if P has sequence == S:
429 * process P and take any required action
430 * create a response packet R with the same ID and sequence as P, containing
431 any response data required.
432 * transmit R and save it in case of re-transmission
433 * increment S
434 else if P has sequence == S - 1:
435 * re-transmit the saved response packet R from above
436 else:
437 * ignore the packet
438
439### Examples
440
441In the examples below, S indicates the starting client sequence number.
442
443 Host Client
444 ======================================================================
445 [Initialization, S = 0x55AA]
446 [Host: version 1, 2048-byte packets. Client: version 2, 1024-byte packets.]
447 [Resulting values to use: version = 1, max packet size = 1024]
448 ID Flag SeqH SeqL Data ID Flag SeqH SeqL Data
449 ----------------------------------------------------------------------
450 0x01 0x00 0x00 0x00
451 0x01 0x00 0x00 0x00 0x55 0xAA
452 0x02 0x00 0x55 0xAA 0x00 0x01 0x08 0x00
453 0x02 0x00 0x55 0xAA 0x00 0x02 0x04 0x00
454
455 ----------------------------------------------------------------------
456 [fastboot "getvar" commands, S = 0x0001]
457 ID Flags SeqH SeqL Data ID Flags SeqH SeqL Data
458 ----------------------------------------------------------------------
459 0x03 0x00 0x00 0x01 getvar:version
460 0x03 0x00 0x00 0x01
461 0x03 0x00 0x00 0x02
462 0x03 0x00 0x00 0x02 OKAY0.4
463 0x03 0x00 0x00 0x03 getvar:none
464 0x03 0x00 0x00 0x03
465 0x03 0x00 0x00 0x04
466 0x03 0x00 0x00 0x04 FAILUnknown var
467
468 ----------------------------------------------------------------------
469 [fastboot "INFO" responses, S = 0x0000]
470 ID Flags SeqH SeqL Data ID Flags SeqH SeqL Data
471 ----------------------------------------------------------------------
472 0x03 0x00 0x00 0x00 <command>
473 0x03 0x00 0x00 0x00
474 0x03 0x00 0x00 0x01
475 0x03 0x00 0x00 0x01 INFOWait1
476 0x03 0x00 0x00 0x02
477 0x03 0x00 0x00 0x02 INFOWait2
478 0x03 0x00 0x00 0x03
479 0x03 0x00 0x00 0x03 OKAY
480
481 ----------------------------------------------------------------------
482 [Chunking 2100 bytes of data, max packet size = 1024, S = 0xFFFF]
483 ID Flag SeqH SeqL Data ID Flag SeqH SeqL Data
484 ----------------------------------------------------------------------
485 0x03 0x00 0xFF 0xFF download:0000834
486 0x03 0x00 0xFF 0xFF
487 0x03 0x00 0x00 0x00
488 0x03 0x00 0x00 0x00 DATA0000834
489 0x03 0x01 0x00 0x01 <1020 bytes>
490 0x03 0x00 0x00 0x01
491 0x03 0x01 0x00 0x02 <1020 bytes>
492 0x03 0x00 0x00 0x02
493 0x03 0x00 0x00 0x03 <60 bytes>
494 0x03 0x00 0x00 0x03
495 0x03 0x00 0x00 0x04
496 0x03 0x00 0x00 0x04 OKAY
497
498 ----------------------------------------------------------------------
499 [Unknown ID error, S = 0x0000]
500 ID Flags SeqH SeqL Data ID Flags SeqH SeqL Data
501 ----------------------------------------------------------------------
502 0x10 0x00 0x00 0x00
503 0x00 0x00 0x00 0x00 <error message>
504
505 ----------------------------------------------------------------------
506 [Host packet loss and retransmission, S = 0x0000]
507 ID Flags SeqH SeqL Data ID Flags SeqH SeqL Data
508 ----------------------------------------------------------------------
509 0x03 0x00 0x00 0x00 getvar:version [lost]
510 0x03 0x00 0x00 0x00 getvar:version [lost]
511 0x03 0x00 0x00 0x00 getvar:version
512 0x03 0x00 0x00 0x00
513 0x03 0x00 0x00 0x01
514 0x03 0x00 0x00 0x01 OKAY0.4
515
516 ----------------------------------------------------------------------
517 [Client packet loss and retransmission, S = 0x0000]
518 ID Flags SeqH SeqL Data ID Flags SeqH SeqL Data
519 ----------------------------------------------------------------------
520 0x03 0x00 0x00 0x00 getvar:version
521 0x03 0x00 0x00 0x00 [lost]
522 0x03 0x00 0x00 0x00 getvar:version
523 0x03 0x00 0x00 0x00 [lost]
524 0x03 0x00 0x00 0x00 getvar:version
525 0x03 0x00 0x00 0x00
526 0x03 0x00 0x00 0x01
527 0x03 0x00 0x00 0x01 OKAY0.4
528
529 ----------------------------------------------------------------------
530 [Host packet delayed, S = 0x0000]
531 ID Flags SeqH SeqL Data ID Flags SeqH SeqL Data
532 ----------------------------------------------------------------------
533 0x03 0x00 0x00 0x00 getvar:version [delayed]
534 0x03 0x00 0x00 0x00 getvar:version
535 0x03 0x00 0x00 0x00
536 0x03 0x00 0x00 0x01
537 0x03 0x00 0x00 0x01 OKAY0.4
538 0x03 0x00 0x00 0x00 getvar:version [arrives late with old seq#, is ignored]
539