> For the complete documentation index, see [llms.txt](https://http3-explained.haxx.se/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://http3-explained.haxx.se/en/why-quic/why-latency.md). # Reduced latency QUIC offers both 0-RTT and 1-RTT handshakes that reduce the time it takes to negotiate and setup a new connection. Compare with the 3-way handshake of TCP. In addition to that, QUIC offers "early data" support from the get-go which is done to allow more data and it is used more easily than TCP Fast Open. With the stream concept, another logical connection to the same host can be done at once without having to wait for the existing one to end first. ## TCP Fast Open is problematic TCP Fast Open was published as [RFC 7413](https://tools.ietf.org/html/rfc7413) in December 2014 and that specification describes how applications can pass data to the server to be delivered already in the first TCP SYN packet. Actual support for this feature in the wild has taken time and is riddled with problems even today in 2018. The TCP stack implementors have had issues and so have applications trying to take advantage of this feature - both in knowing in which OS version to try to activate it but also in figuring out how to gracefully back down and deal when problems arise. Several networks have been identified to interfere with TFO traffic and they have thus actively ruined such TCP handshakes. --- # Agent Instructions This documentation is published with GitBook. GitBook is the documentation platform designed so that both humans and AI agents can read, navigate, and reason over technical content effectively. Learn more at gitbook.com. ## Querying This Documentation If you need additional information that is not directly available in this page, you can query the documentation dynamically by asking a question. Perform an HTTP GET request on the current page URL with the `ask` query parameter, and the optional `goal` query parameter: ``` GET https://http3-explained.haxx.se/en/why-quic/why-latency.md?ask=&goal= ``` `ask` is the immediate question: it should be specific, self-contained, and written in natural language. `goal` is optional and describes the broader end goal you are ultimately trying to accomplish on behalf of the user. GitBook uses it to tailor the answer towards what is most useful for that goal. The response will contain a direct answer to the question and relevant excerpts and sources from the documentation. Use this mechanism when the answer is not explicitly present in the current page, you need clarification or additional context, or you want to retrieve related documentation sections.