> 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/zh/h3/h3-h2.md).

# 与HTTP/2的比较

HTTP/3面向QUIC设计，QUIC是一个自己处理数据流的传输层协议。

HTTP/2面向TCP设计，因此数据流在HTTP层处理。

## 相似之处

这两个协议为客户端提供了几乎相同的功能集。

* 两者都提供数据流
* 两者都提供服务器推送
* 两者都有头部压缩，QPACK与HPACK的设计非常类似
* 两者都通过单一连接上的数据流提供复用
* 两者都提供数据流的优先度设置

## 不同之处

两个协议的主要不同点在于细节，不同之处主要由HTTP/3使用的QUIC带来。

* 得益于QUIC的0-RTT握手，HTTP/3可以提供更好的早期数据支持，而TCP快速打开和TLS通常只能传输更少的数据，且经常存在问题。
* 得益于QUIC，HTTP/3的握手速度比TCP+TLS快得多。
* HTTP/3不存在明文的不安全版本。尽管在互联网上很少见，HTTP/2还是可以不配合HTTPS来实现和使用。
* 通过ALPN拓展，HTTP/2可以直接在TLS握手时进行协商。HTTP/3基于QUIC，所以需要凭借响应中的 `Alt-Svc:` 头部来向客户端宣告。


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