HTTP/3 represents a major evolution in web protocol design, building upon the foundations of HTTP/1.1 and HTTP/2 to address persistent bottlenecks and inefficiencies. To appreciate HTTP/3's improvements, consider a comparative analysis of three key protocol features: multiplexing, head-of-line blocking, and performance.

In HTTP/1.1, each request/response pair typically required a separate TCP connection, or relied on pipelining within a single connection. However, pipelining introduced its own problems, such as head-of-line blocking, where a delay in one response blocked all subsequent responses on that connection. HTTP/2 introduced multiplexing, allowing multiple streams within a single TCP connection, thus enabling concurrent requests and responses. Despite this, HTTP/2 was still vulnerable to TCP-level head-of-line blocking: if a single packet was lost, all streams sharing that connection had to wait for retransmission.

HTTP/3 fundamentally changes this landscape by operating over QUIC, a transport protocol built on UDP. QUIC supports true multiplexing by providing independent streams within a single connection, so packet loss on one stream does not block others. This design eliminates head-of-line blocking at the transport layer, significantly improving overall performance, especially in lossy network conditions. Additionally, HTTP/3 incorporates faster connection establishment, integrated security, and improved error recovery, offering a more responsive web experience.

To illustrate how HTTP/3 handles requests and responses using multiplexed streams, consider the following pseudo code that outlines the basic flow:

1. Client opens a QUIC connection to the server;
2. Client initiates multiple HTTP/3 streams for requests;
3. Each request is sent independently over its own stream;
4. Server processes requests concurrently and sends responses over the corresponding streams;
5. Streams can be closed independently without affecting others.

This pseudo code demonstrates the separation and concurrency that HTTP/3 enables:

connect_to_server_via_quic()
for each request in request_list:
    stream = open_new_stream()
    send_request(stream, request)
    response = receive_response(stream)
    close_stream(stream)
disconnect_from_server()

Delving deeper into HTTP/3's advanced features, you will find that header compression, stream prioritization, and error handling are integral to its efficiency and robustness.

HTTP/3 uses QPACK, a header compression mechanism designed specifically for multiplexed protocols. Unlike HTTP/2's HPACK, QPACK avoids head-of-line blocking by allowing header blocks to be decoded independently, even if some dynamic table updates are delayed. This reduces latency and ensures that streams are not held up waiting for header compression state to synchronize.

Stream prioritization in HTTP/3 enables clients to indicate the relative importance of different requests. This allows servers to allocate resources efficiently, ensuring that critical content loads first. Prioritization is communicated through explicit priority fields in the protocol, and can be dynamically adjusted as application needs change.

Error handling in HTTP/3 is designed for resilience. Each stream can signal errors independently using stream error codes, allowing other streams to continue unaffected. Connection-level errors can also be reported, prompting immediate termination of the entire connection if necessary. This granular approach to error management ensures that problems are isolated and do not cascade across unrelated streams.

To further clarify how HTTP/3 manages stream prioritization and error signaling, consider the following pseudo code:

1. Assign priority levels to outgoing streams;
2. When sending data, select streams based on priority;
3. If an error is detected on a stream, send an error code for that stream only;
4. If a critical error occurs, send a connection error code and terminate all streams.

Here is a simplified pseudo code representation:

for each stream in open_streams:
    set_stream_priority(stream, priority_level)
while connection_active:
    stream = select_highest_priority_stream()
    if error_on_stream(stream):
        send_stream_error(stream, error_code)
        close_stream(stream)
    else:
        send_data(stream)
if critical_connection_error():
    send_connection_error(connection_error_code)
    close_all_streams()