Final answer:
The 20-bit field used to mark individual traffic flows with unique values is the Flow Label field in the IPv6 protocol.
Step-by-step explanation:
The new field with a length of 20 bits that is used to mark individual traffic flows with unique values is the Flow Label field in the IPv6 protocol. This field is designed to classify packets of a communication session for quality-of-service management by networking devices. It ensures that packets of the same flow are kept together when traveling through the Internet, enabling more efficient handling of data transfers, reducing latency, and ensuring a smoother communication experience for end-users.