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 64 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. 166 167 168 169## Client Variables 170 171The "getvar:%s" command is used to read client variables which 172represent various information about the device and the software 173on it. 174 175The various currently defined names are: 176 177 version Version of FastBoot protocol supported. 178 It should be "0.4" for this document. 179 180 version-bootloader Version string for the Bootloader. 181 182 version-baseband Version string of the Baseband Software 183 184 product Name of the product 185 186 serialno Product serial number 187 188 secure If the value is "yes", this is a secure 189 bootloader requiring a signature before 190 it will install or boot images. 191 192 is-userspace If the value is "yes", the device is running 193 fastbootd. Otherwise, it is running fastboot 194 in the bootloader. 195 196Names starting with a lowercase character are reserved by this 197specification. OEM-specific names should not start with lowercase 198characters. 199 200## Logical Partitions 201 202There are a number of commands to interact with logical partitions: 203 204 update-super:%s:%s Write the previously downloaded image to a super 205 partition. Unlike the "flash" command, this has 206 special rules. The image must have been created by 207 the lpmake command, and must not be a sparse image. 208 If the last argument is "wipe", then all existing 209 logical partitions are deleted. If no final argument 210 is specified, the partition tables are merged. Any 211 partition in the new image that does not exist in the 212 old image is created with a zero size. 213 214 In all cases, this will cause the temporary "scratch" 215 partition to be deleted if it exists. 216 217 create-logical-partition:%s:%d 218 Create a logical partition with the given name and 219 size, in the super partition. 220 221 delete-logical-partition:%s 222 Delete a logical partition with the given name. 223 224 resize-logical-partition:%s:%d 225 Change the size of the named logical partition. 226 227In addition, there is a variable to test whether a partition is logical: 228 229 is-logical:%s If the value is "yes", the partition is logical. 230 Otherwise the partition is physical. 231 232## TCP Protocol v1 233 234The TCP protocol is designed to be a simple way to use the fastboot protocol 235over ethernet if USB is not available. 236 237The device will open a TCP server on port 5554 and wait for a fastboot client 238to connect. 239 240### Handshake 241Upon connecting, both sides will send a 4-byte handshake message to ensure they 242are speaking the same protocol. This consists of the ASCII characters "FB" 243followed by a 2-digit base-10 ASCII version number. For example, the version 1 244handshake message will be [FB01]. 245 246If either side detects a malformed handshake, it should disconnect. 247 248The protocol version to use must be the minimum of the versions sent by each 249side; if either side cannot speak this protocol version, it should disconnect. 250 251### Fastboot Data 252Once the handshake is complete, fastboot data will be sent as follows: 253 254 [data_size][data] 255 256Where data\_size is an unsigned 8-byte big-endian binary value, and data is the 257fastboot packet. The 8-byte length is intended to provide future-proofing even 258though currently fastboot packets have a 4-byte maximum length. 259 260### Example 261In this example the fastboot host queries the device for two variables, 262"version" and "none". 263 264 Host <connect to the device on port 5555> 265 Host FB01 266 Device FB01 267 Host [0x00][0x00][0x00][0x00][0x00][0x00][0x00][0x0E]getvar:version 268 Device [0x00][0x00][0x00][0x00][0x00][0x00][0x00][0x07]OKAY0.4 269 Host [0x00][0x00][0x00][0x00][0x00][0x00][0x00][0x0B]getvar:none 270 Device [0x00][0x00][0x00][0x00][0x00][0x00][0x00][0x14]FAILUnknown variable 271 Host <disconnect> 272 273 274## UDP Protocol v1 275 276The UDP protocol is more complex than TCP since we must implement reliability 277to ensure no packets are lost, but the general concept of wrapping the fastboot 278protocol is the same. 279 280Overview: 281 1. As with TCP, the device will listen on UDP port 5554. 282 2. Maximum UDP packet size is negotiated during initialization. 283 3. The host drives all communication; the device may only send a packet as a 284 response to a host packet. 285 4. If the host does not receive a response in 500ms it will re-transmit. 286 287### UDP Packet format 288 289 +----------+----+-------+-------+--------------------+ 290 | Byte # | 0 | 1 | 2 - 3 | 4+ | 291 +----------+----+-------+-------+--------------------+ 292 | Contents | ID | Flags | Seq # | Data | 293 +----------+----+-------+-------+--------------------+ 294 295 ID Packet ID: 296 0x00: Error. 297 0x01: Query. 298 0x02: Initialization. 299 0x03: Fastboot. 300 301 Packet types are described in more detail below. 302 303 Flags Packet flags: 0 0 0 0 0 0 0 C 304 C=1 indicates a continuation packet; the data is too large and will 305 continue in the next packet. 306 307 Remaining bits are reserved for future use and must be set to 0. 308 309 Seq # 2-byte packet sequence number (big-endian). The host will increment 310 this by 1 with each new packet, and the device must provide the 311 corresponding sequence number in the response packets. 312 313 Data Packet data, not present in all packets. 314 315### Packet Types 316 317 Query 318 The host sends a query packet once on startup to sync with the device. 319 The host will not know the current sequence number, so the device must 320 respond to all query packets regardless of sequence number. 321 322 The response data field should contain a 2-byte big-endian value 323 giving the next expected sequence number. 324 325 Init 326 The host sends an init packet once the query response is returned. The 327 device must abort any in-progress operation and prepare for a new 328 fastboot session. This message is meant to allow recovery if a 329 previous session failed, e.g. due to network error or user Ctrl+C. 330 331 The data field contains two big-endian 2-byte values, a protocol 332 version and the max UDP packet size (including the 4-byte header). 333 Both the host and device will send these values, and in each case 334 the minimum of the sent values must be used. 335 336 Fastboot 337 These packets wrap the fastboot protocol. To write, the host will 338 send a packet with fastboot data, and the device will reply with an 339 empty packet as an ACK. To read, the host will send an empty packet, 340 and the device will reply with fastboot data. The device may not give 341 any data in the ACK packet. 342 343 Error 344 The device may respond to any packet with an error packet to indicate 345 a UDP protocol error. The data field should contain an ASCII string 346 describing the error. This is the only case where a device is allowed 347 to return a packet ID other than the one sent by the host. 348 349### Packet Size 350The maximum packet size is negotiated by the host and device in the Init packet. 351Devices must support at least 512-byte packets, but packet size has a direct 352correlation with download speed, so devices are strongly suggested to support at 353least 1024-byte packets. On a local network with 0.5ms round-trip time this will 354provide transfer rates of ~2MB/s. Over WiFi it will likely be significantly 355less. 356 357Query and Initialization packets, which are sent before size negotiation is 358complete, must always be 512 bytes or less. 359 360### Packet Re-Transmission 361The host will re-transmit any packet that does not receive a response. The 362requirement of exactly one device response packet per host packet is how we 363achieve reliability and in-order delivery of packets. 364 365For simplicity of implementation, there is no windowing of multiple 366unacknowledged packets in this version of the protocol. The host will continue 367to send the same packet until a response is received. Windowing functionality 368may be implemented in future versions if necessary to increase performance. 369 370The first Query packet will only be attempted a small number of times, but 371subsequent packets will attempt to retransmit for at least 1 minute before 372giving up. This means a device may safely ignore host UDP packets for up to 1 373minute during long operations, e.g. writing to flash. 374 375### Continuation Packets 376Any packet may set the continuation flag to indicate that the data is 377incomplete. Large data such as downloading an image may require many 378continuation packets. The receiver should respond to a continuation packet with 379an empty packet to acknowledge receipt. See examples below. 380 381### Summary 382The host starts with a Query packet, then an Initialization packet, after 383which only Fastboot packets are sent. Fastboot packets may contain data from 384the host for writes, or from the device for reads, but not both. 385 386Given a next expected sequence number S and a received packet P, the device 387behavior should be: 388 389 if P is a Query packet: 390 * respond with a Query packet with S in the data field 391 else if P has sequence == S: 392 * process P and take any required action 393 * create a response packet R with the same ID and sequence as P, containing 394 any response data required. 395 * transmit R and save it in case of re-transmission 396 * increment S 397 else if P has sequence == S - 1: 398 * re-transmit the saved response packet R from above 399 else: 400 * ignore the packet 401 402### Examples 403 404In the examples below, S indicates the starting client sequence number. 405 406 Host Client 407 ====================================================================== 408 [Initialization, S = 0x55AA] 409 [Host: version 1, 2048-byte packets. Client: version 2, 1024-byte packets.] 410 [Resulting values to use: version = 1, max packet size = 1024] 411 ID Flag SeqH SeqL Data ID Flag SeqH SeqL Data 412 ---------------------------------------------------------------------- 413 0x01 0x00 0x00 0x00 414 0x01 0x00 0x00 0x00 0x55 0xAA 415 0x02 0x00 0x55 0xAA 0x00 0x01 0x08 0x00 416 0x02 0x00 0x55 0xAA 0x00 0x02 0x04 0x00 417 418 ---------------------------------------------------------------------- 419 [fastboot "getvar" commands, S = 0x0001] 420 ID Flags SeqH SeqL Data ID Flags SeqH SeqL Data 421 ---------------------------------------------------------------------- 422 0x03 0x00 0x00 0x01 getvar:version 423 0x03 0x00 0x00 0x01 424 0x03 0x00 0x00 0x02 425 0x03 0x00 0x00 0x02 OKAY0.4 426 0x03 0x00 0x00 0x03 getvar:none 427 0x03 0x00 0x00 0x03 428 0x03 0x00 0x00 0x04 429 0x03 0x00 0x00 0x04 FAILUnknown var 430 431 ---------------------------------------------------------------------- 432 [fastboot "INFO" responses, S = 0x0000] 433 ID Flags SeqH SeqL Data ID Flags SeqH SeqL Data 434 ---------------------------------------------------------------------- 435 0x03 0x00 0x00 0x00 <command> 436 0x03 0x00 0x00 0x00 437 0x03 0x00 0x00 0x01 438 0x03 0x00 0x00 0x01 INFOWait1 439 0x03 0x00 0x00 0x02 440 0x03 0x00 0x00 0x02 INFOWait2 441 0x03 0x00 0x00 0x03 442 0x03 0x00 0x00 0x03 OKAY 443 444 ---------------------------------------------------------------------- 445 [Chunking 2100 bytes of data, max packet size = 1024, S = 0xFFFF] 446 ID Flag SeqH SeqL Data ID Flag SeqH SeqL Data 447 ---------------------------------------------------------------------- 448 0x03 0x00 0xFF 0xFF download:0000834 449 0x03 0x00 0xFF 0xFF 450 0x03 0x00 0x00 0x00 451 0x03 0x00 0x00 0x00 DATA0000834 452 0x03 0x01 0x00 0x01 <1020 bytes> 453 0x03 0x00 0x00 0x01 454 0x03 0x01 0x00 0x02 <1020 bytes> 455 0x03 0x00 0x00 0x02 456 0x03 0x00 0x00 0x03 <60 bytes> 457 0x03 0x00 0x00 0x03 458 0x03 0x00 0x00 0x04 459 0x03 0x00 0x00 0x04 OKAY 460 461 ---------------------------------------------------------------------- 462 [Unknown ID error, S = 0x0000] 463 ID Flags SeqH SeqL Data ID Flags SeqH SeqL Data 464 ---------------------------------------------------------------------- 465 0x10 0x00 0x00 0x00 466 0x00 0x00 0x00 0x00 <error message> 467 468 ---------------------------------------------------------------------- 469 [Host packet loss and retransmission, S = 0x0000] 470 ID Flags SeqH SeqL Data ID Flags SeqH SeqL Data 471 ---------------------------------------------------------------------- 472 0x03 0x00 0x00 0x00 getvar:version [lost] 473 0x03 0x00 0x00 0x00 getvar:version [lost] 474 0x03 0x00 0x00 0x00 getvar:version 475 0x03 0x00 0x00 0x00 476 0x03 0x00 0x00 0x01 477 0x03 0x00 0x00 0x01 OKAY0.4 478 479 ---------------------------------------------------------------------- 480 [Client packet loss and retransmission, S = 0x0000] 481 ID Flags SeqH SeqL Data ID Flags SeqH SeqL Data 482 ---------------------------------------------------------------------- 483 0x03 0x00 0x00 0x00 getvar:version 484 0x03 0x00 0x00 0x00 [lost] 485 0x03 0x00 0x00 0x00 getvar:version 486 0x03 0x00 0x00 0x00 [lost] 487 0x03 0x00 0x00 0x00 getvar:version 488 0x03 0x00 0x00 0x00 489 0x03 0x00 0x00 0x01 490 0x03 0x00 0x00 0x01 OKAY0.4 491 492 ---------------------------------------------------------------------- 493 [Host packet delayed, S = 0x0000] 494 ID Flags SeqH SeqL Data ID Flags SeqH SeqL Data 495 ---------------------------------------------------------------------- 496 0x03 0x00 0x00 0x00 getvar:version [delayed] 497 0x03 0x00 0x00 0x00 getvar:version 498 0x03 0x00 0x00 0x00 499 0x03 0x00 0x00 0x01 500 0x03 0x00 0x00 0x01 OKAY0.4 501 0x03 0x00 0x00 0x00 getvar:version [arrives late with old seq#, is ignored] 502