How is a network traffic flow uniquely identified?

What features/statistics of network traffic uniquely identifies a specific flow?

My initial thoughts were unique identifiers such as all packets going from eth_src to eth_dst and in and out of the same ports.

I have been using the Ryu Traffic Monitor to try and get my head around flows, it seems to use the in_port and eth_dst:

    for stat in sorted([flow for flow in body if flow.priority == 1],
                       key=lambda flow: (flow.match['in_port'],
                                         flow.match['eth_dst'])):
        self.logger.info('%016x %8x %17s %8x %8d %8d',
                         ev.msg.datapath.id,
                         stat.match['in_port'], stat.match['eth_dst'],
                         stat.instructions[0].actions[0].port,
                         stat.packet_count, stat.byte_count)

Solution

Further investigation has helped me understand this more:

A network traffic flow is a flow of traffic from a source to a destination.

The answer to my specific question (which is in relation to transport protocols) is a network flow is defined as a 5-tuple consisting of a source IP address/port number, destination IP address/port number and the protocol in use. Ethernet addresses may also be added in here.

https://www.techopedia.com/definition/28190/5-tuple

edit: Thanks to Ron for mentioning that this is only valid for transport protocols that use ports