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# Connection

In native ADB architecture, once ADB **server** detects a new device, it initiates a connection by sending a `CNXN` [packet](./packet.md).

After optional [authentication](./authentication.md), 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](#protocol-version) | Highest supported version number |
| `arg1`    | See [Protocol Version](#protocol-version) | Maximum supported payload size   |
| `payload` | See [Device Banner](#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 | :x:            | :heavy_check_mark: |

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* **:**<br/>
  *DeviceIdentifier* **::** *ParameterList* **\0**

*DeviceIdentifier* **:**<br/>
  **host**<br/>
  **device**<br/>
  **bootloader**

*ParameterList* **:**<br/>
  *Parameter*<br/>
  *ParameterList* *Parameter*

*Parameter* **:**<br/>
  *ParameterName* **=** *ParameterValue* **;**

*ParameterName* **:**<br/>
  any character except **=** **,** **;**

*ParameterValue* **:**<br/>
  *ParameterStringValue*<br/>
  *ParameterListValue*

*ParameterStringValue* **:**<br/>
  any character except **=** **,** **;**

*ParameterListValue* **:**<br/>
  *ParameterStringValue*<br/>
  *ParameterListValue* **,** *ParameterStringValue*

A device banner sent by **server** may look like this:

```text
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
```

:::tip NULL Termination
The trailing `;` and `\0` are required in Android <9.
:::

A device banner sent by **daemon** may look like this:

```text
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

```uml
@startuml

participant Client
participant Daemon

Client -> Daemon: CNXN (host::)
...Optional Authentication...
Daemon -> Client: CNXN (device::)

@enduml
```