HTTP/3 represents a major evolution in web protocol architecture, moving away from the traditional use of TCP as the underlying transport layer. Instead, HTTP/3 is built on top of QUIC, a protocol designed to address many of the limitations of TCP, especially in the context of modern web applications. The shift from TCP to QUIC is significant because QUIC operates over UDP, allowing for faster connection establishment, improved stream multiplexing, and enhanced reliability in the face of network changes. This architecture enables HTTP/3 to provide a more responsive and robust experience for users, particularly on mobile and high-latency networks.

To illustrate how HTTP/3 leverages QUIC, consider the following pseudo code for establishing a QUIC connection and managing multiple streams. This captures the basic flow of operations without getting into protocol-specific details:

function establish_quic_connection(server_address):
    send_initial_packet(server_address)
    receive_server_response()
    perform_tls_handshake()
    if handshake_successful:
        return quic_connection_object

function open_stream(quic_connection_object):
    stream_id = allocate_new_stream_id()
    create_stream_object(stream_id)
    return stream_object

function send_data(stream_object, data):
    packet = build_quic_packet(stream_object.stream_id, data)
    send_packet_over_udp(packet)

In this model, multiple streams can be created and managed independently over a single QUIC connection, allowing for efficient multiplexing of HTTP requests and responses.

QUIC introduces several advanced features that set it apart from traditional TCP-based transport. One key capability is connection migration, which allows an ongoing QUIC connection to survive changes in network interfaces, such as when a device switches from Wi-Fi to cellular data. Another important feature is 0-RTT (zero round-trip time) connection establishment, which enables clients to send data immediately upon connecting, reducing latency for repeat connections. Perhaps most crucially, QUIC has built-in encryption by default, integrating TLS 1.3 directly into the protocol and ensuring that all data transmitted is protected without any additional setup. These features collectively make QUIC—and by extension, HTTP/3—more resilient, secure, and efficient for modern web communication.

Handling packets and managing streams in QUIC involve specific logic to support its advanced features. The following pseudo code outlines the core steps in processing incoming packets and managing active streams:

function receive_quic_packet(packet):
    if packet_is_encrypted(packet):
        decrypted_packet = decrypt_packet(packet)
        stream_id, data = parse_stream_frame(decrypted_packet)
        deliver_data_to_application(stream_id, data)
    else:
        handle_handshake_or_retry(packet)

function manage_streams(quic_connection_object):
    for stream in quic_connection_object.active_streams:
        if stream.has_data_to_send():
            packet = build_quic_packet(stream.stream_id, stream.get_next_data_chunk())
            send_packet_over_udp(packet)
        if stream.is_closed():
            release_stream_resources(stream)

This approach ensures that data for each stream is handled independently, and encryption is always enforced throughout the connection lifecycle.