> 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/quic/quic-connections.md). # Connections A QUIC connection is a single conversation between two QUIC endpoints. QUIC's connection establishment combines version negotiation with the cryptographic and transport handshakes to reduce connection establishment latency. To actually send data over such a connection, one or more streams have to be created and used. ## Connection ID Each connection possesses a set of connection identifiers, or connection IDs, each of which can be used to identify the connection. Connection IDs are independently selected by endpoints; each endpoint selects the connection IDs that its peer uses. The primary function of these connection IDs is to ensure that changes in addressing at lower protocol layers (UDP, IP, and below) do not cause packets for a QUIC connection to be delivered to the wrong endpoint. By taking advantage of the connection ID, connections can thus migrate between IP addresses and network interfaces in ways TCP never could. For instance, migration allows an in-progress download to move from a cellular network connection to a faster wifi connection when the user moves their device into a location offering wifi. Similarly, the download can proceed over the cellular connection if wifi becomes unavailable. ## Port numbers QUIC is built atop UDP, so a 16 bit port number field is used to differentiate incoming connections. ## Version negotiation A QUIC connection request originating from a client will tell the server which QUIC protocol version it wants to speak, and the server will respond with a list of supported versions for the client to select from. --- # 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/quic/quic-connections.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.