HTTP/1.0 was finalized in 1996. Every request to the same server requires a separate TCP connection which is expensive to establish.

HTTP/1.1 (1997) introduced 𝗽𝗲𝗿𝘀𝗶𝘀𝘁𝗲𝗻𝘁 𝗰𝗼𝗻𝗻𝗲𝗰𝘁𝗶𝗼𝗻𝘀, which allow a TCP connection to be reused for multiple requests and responses. This reduces the latency in setting up new connections for each request. But HTTP/1.1 doesn’t solve the 𝗵𝗲𝗮𝗱-𝗼𝗳-𝗹𝗶𝗻𝗲 (𝗛𝗢𝗟) 𝗯𝗹𝗼𝗰𝗸𝗶𝗻𝗴 problem.

Although HTTP/1.1 enabled pipelining, where multiple requests could be sent out without waiting for each response, the responses still had to be processed and sent back in the order the requests were received. This ordering requirement caused HOL blocking - if the first request took a long time, all later requests had to wait.

HTTP/2 (2015) introduced 𝗛𝗧𝗧𝗣 𝘀𝘁𝗿𝗲𝗮𝗺𝘀 - an abstraction that allows 𝗺𝘂𝗹𝘁𝗶𝗽𝗹𝗲𝘅𝗶𝗻𝗴 different HTTP exchanges onto the same TCP connection. Streams don’t need to be sent in order. This eliminates HOL blocking at the application layer. But HOL still exists at the TCP transport layer.

HTTP/3 draft was published in 2020. It uses 𝗤𝗨𝗜𝗖 instead of TCP as the 𝘁𝗿𝗮𝗻𝘀𝗽𝗼𝗿𝘁 𝗽𝗿𝗼𝘁𝗼𝗰𝗼𝗹, removing HOL blocking in the transport layer.

QUIC uses UDP. It introduces 𝘀𝘁𝗿𝗲𝗮𝗺𝘀 at the transport layer. QUIC streams share one connection, so no new handshakes or slow starts are necessary to create new streams. QUIC streams are delivered independently so packet loss usually doesn’t affect other streams.