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TCP itself is a strictly sequenced protocol. This works OK in theory but the design of HTTP/2 failed to consider the possibility of Head of Line Blocking at the TCP layer. A single TCP connection can handle a theoretically unlimited number of concurrent request and response flows. Rather than transmitting requests and responses as contiguous streams, HTTP/2 divides those into discrete chunks called frames that can be interlaced with other frames. HTTP/2 attempted resolving this issue by introducing multiplexing. This is known as Head of Line Blocking and the source of significant latency and poor use of network bandwidth. That means if you want to send one 10 megabyte file and then one 2 megabyte file, the former would have to be fully transmitted before the latter could even be started.
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While it is possible to send multiple requests or responses over a single TCP connection, one complete message must be fully transmitted before beginning to send the next complete message. In HTTP/1, all messages exchanged between the client and server are in the form of contiguous, uninterrupted blocks of data. Understanding why this is important is critical to understanding why QUIC is superior to TCP. In effect, it reimplements the majority of TCP on top of UDP with one key difference: unlike TCP, it is still possible to transmit packets out of sequence. The QUIC protocol defines a layer on top of UDP that introduces error handling, reliability, flow control, and built-in security (via TLS 1.3) to UDP. UDP does not include any of the reliability and sequencing guarantees of TCP that are strictly required for a higher-level protocol such as HTTP.
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UDP is notoriously unreliable, with packets frequently being lost, reordered, duplicated, and so forth. This new UDP-based transport protocol is intended to ultimately replace the use of TCP for all HTTP traffic.Īnyone familiar with UDP should, at this point, rightfully question why it is being used here. In March of last year, thanks to support from NearForm and Protocol Labs, I started the process of implementing Node.js support for the new QUIC protocol.