Connection

In native ADB architecture, once ADB server detects a new device, it initiates a connection by sending a CNXN packet.

After optional authentication, the daemon responses with a CNXN packet to indicate success.

CNXN Packet

Field	Value	Description
command	0x4e584e43	CNXN in UTF-8 encoding
arg0	See Protocol Version	Highest supported version number
arg1	See Protocol Version	Maximum supported payload size
payload	See Device Banner	Device banner

Protocol Version

ADB now has two versions, here are the differences:

Version number	0x01000000	0x01000001
Android version	<9	>=9
Max payload size in each packet	4 * 1024 bytes	1024 * 1024 bytes
Does not validate the checksum field	❌	✔️

In the first upstream (server to daemon) CNXN packet:

• arg0 field SHOULD contains this server's version (it can be lower, just will be slower).
• arg1 MUST NOT exceed the max payload size for that version in above table.
• checksum MUST NOT be omitted. The version of daemon is not yet to know. When connecting to an older version, it may still validate the checksum can reject the connection.

In the response (daemon to server) CNXN packet:

• arg0 field SHOULD contains this daemon's version.
• arg1 MUST NOT exceed the max payload size for that version in above table.
• checksum can be omitted if the server's version is at least 0x01000001. "Omit" means filling with anything, not removing the field entirely.

After exchanging CNXN packets, both sides pick the lowest version (whether can omit checksum) and lowest max payload size to use in further packets.

Device Banner

Device banners describe device information and capabilities. It has the following format:

DeviceBanner :
  DeviceIdentifier :: ParameterList \0

DeviceIdentifier :
  host
  device
  bootloader

ParameterList :
  Parameter
  ParameterList Parameter

Parameter :
  ParameterName = ParameterValue ;

ParameterName :
  any character except = , ;

ParameterValue :
  ParameterStringValue
  ParameterListValue

ParameterStringValue :
  any character except = , ;

ParameterListValue :
  ParameterStringValue
  ParameterListValue , ParameterStringValue

A device banner sent by server may look like this:

host::features=shell_v2,cmd,stat_v2,ls_v2,fixed_push_mkdir,apex,abb,fixed_push_symlink_timestamp,abb_exec,remount_shell,track_app,sendrecv_v2,sendrecv_v2_brotli,sendrecv_v2_lz4,sendrecv_v2_zstd,sendrecv_v2_dry_run_send;\0

NULL Termination

The trailing ; and \0 are required in Android <9.

A device banner sent by daemon may look like this:

device::ro.product.name=venus;ro.product.model=M2011K2C;ro.product.device=venus;features=sendrecv_v2_brotli,remount_shell,sendrecv_v2,abb_exec,fixed_push_mkdir,fixed_push_symlink_timestamp,abb,shell_v2,cmd,ls_v2,apex,stat_v2

Sequence Diagram