# Ethernet protocol (GV)

## Introduction

This part contains a description of the protocol implemented by Aravis for the control of the gigabit ethenet cameras. This protocol is a standard used by many video cameras manufacturers for their products. It is a closed standard which should not be confused by what is used on 'IP cameras'. In the case of 'IP cameras' the video stream is a compressed stream (jpeg, h264...) send over HTTP or RTSP, whereas in Aravis, the video stream is a raw stream, mainly targeted to industrial applications (though the protocol supports non raw payload, it is not yet implemented in Aravis).

The protocol implemented by Aravis library is based on UDP packets transmited over an ethernet link. Two different types of packets are used: GVCP packets and GVSP packets. GVCP packets are used for the control of the devices, and GVSP for the transmission of the data stream.

This documentation is the result of a reverse engineering, which means it is far from complete. A more complete description of the ethernet protocol can be found in the GVSP and GVCP dissectors found in Wireshark sources.

## GVCP packets

Devices are waiting for GVCP packets listening to the GVCP port (3956). When the client sends a command GVCP packet to the device, the device sends back an acknowledge packets in return. Each command/acknowledge pair is identified by a 16 bit value, which allows to check if an acknowledge packet corresponds to the previsoulsy sent command. 0 is an error value for this identifier. If an acknowledge is not received after a given timeout period, another command packet is sent, until a maximum number of retries is reached.

The content of a GVCP packet is composed by a 64 bit headers, followed or not by a data byte array, depending on the GVCP packet type. Multibyte values are in big endian encoding.

### Discovery

A device discovery mechanism is implemented using a discovery GVCP packet, broadcasted by the client to the GVCP port. Each available device responds to this discovery packet with an acknowledge packet, containing a description of the device (device name, model name, manufacturer name, MAC Address...).

### Read and write register

These packets are used for the read and write of 32 bit registers, accessed using 32 bit adresses. For the read command, a list of addresses is sent ot the client, which returns a list of 32 bit values. For the write command, a list of address/value pairs is sent to the client, which returns a list of 32 bit values.

Address and data are encoded in the packet as big endian values.

### Read and write memory

Write memory packet data area consists in a 32 bit address, followed by the data to write. Client returns an acknowledge packet with the target address.

For read memory command, an address/size pair is sent to the device, which returns the content of the given memory area.

## GVSP packets

GVSP packets are sent by the device for the video stream transmission. Depending on the device, it is possible to have more than one stream simultaneously, each directed to a different client port. Stream is splitted in frames. For each frame, identified by a frame id, three different packets are used: a leader packet, one or more data packets, and a trailer packet. Each packet in a frame is identified by a packet id, starting from 0.

In the video stream reception code, client must take care to check if all frame packets are present, and ask for packet resend for missing ones, using a GVCP packet resend command.