AudioSocket

AudioSocket is a simple TCP-based protocol for sending and receiving realtime audio streams.

There exists a protocol definition (below), a Go library, and Asterisk application and channel interfaces.

NOTE: as of 2020-01-15, AudioSocket has been included in the upstream Asterisk system. While I am leaving the Asterisk patches here for use with previous versions, the Asterisk code in this repository should be considered obsolete. The Go code is up-to-date, maintained, and this is the primary source for it.

Protocol definition

The singular design goal of AudioSocket is to present the simplest possible audio streaming protocol, initially based on the constraints of Asterisk audio. Each packet contains a three-byte header and a variable payload. The header is composed of a one-byte type and a two-byte length indicator.

The minimum message length is three bytes: type and payload-length. Hangup indication, for instance, is 0x00 0x00 0x00.

Types

• 0x00 - Terminate the connection (socket closure is also sufficient)
• 0x01 - Payload will contain the UUID (16-byte binary representation) for the audio stream
• 0x10 - Payload is signed linear, 16-bit, 8kHz, mono PCM (little-endian)
• 0xff - An error has occurred; payload is the (optional) application-specific error code. Asterisk-generated error codes are listed below.

Payload length

The payload length is a 16-bit unsigned integer (big endian) indicating how many bytes are in the payload.

Payload

The content of the payload is defined by the header: type and length.

Asterisk error codes

Error codes are application-specific. The error codes for Asterisk are single-byte, bit-packed error codes:

• 0x01 - hangup of calling party
• 0x02 - frame forwarding error
• 0x04 - memory (allocation) error

Asterisk usage

There are two Asterisk implementations: a channel interface and a dialplan application interface. Each of these lends itself to simplify a different use-case, but they work in exactly the same way.

The following examples demonstrate an AudioSocket connection to a server at server.example.com running on TCP port 9092. The UUID (which is chosen arbitrarily) of the call is 40325ec2-5efd-4bd3-805f-53576e581d13.

Dialplan application:

exten = 100,1,Verbose("Call to AudioSocket via Dialplan Application")
 same = n,Answer()
 same = n,AudioSocket(40325ec2-5efd-4bd3-805f-53576e581d13,server.example.com:9092)
 same = n,Hangup()

Channel interface:

exten = 101,1,Verbose("Call to AudioSocket via Channel interface")
 same = n,Answer()
 same = n,Dial(AudioSocket/server.example.com:9092/40325ec2-5efd-4bd3-805f-53576e581d13)
 same = n,Hangup()