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传统应用
互联网最古老的应用至今仍最具启发性：电子邮件拆分为报文格式——首部与正文采用 ASCII，并经 MIME 扩展以承载编码后的二进制内容；传输协议 SMTP 在服务器之间推送邮件；检索协议 IMAP 让读者在服务器上管理邮箱。而万维网则把一种文档语言与请求-响应协议相配：URL 为资源命名，使文档可以相互链接；HTTP 在 TCP 之上承载面向文本的请求和响应，按设计保持无状态，方法、状态码、持久连接和缓存代理共同使整个系统得以扩展。这两种设计表明，应用层协议如何把传输层的字节流和数据报变成人们使用的东西。
1. Mail in Three Pieces
Mail in Three Pieces
2. Naming Things So They Can Link
Naming Things So They Can Link
3. Stateless Requests, Scalable Web
Stateless Requests, Scalable Web
4. From Documents to Services
From Documents to Services
5. Three agreements
What are the three separable pieces of Internet email architecture?
6. The ASCII costume
How does MIME carry a photograph through a system designed for ASCII text?
7. Why retrieval is separate
Why does IMAP keep the mailbox on the server rather than downloading everything to one machine?
8. The name before the protocol
What makes URLs the web's foundational invention?
9. The stateless bet
Why is HTTP designed stateless, and what follows?
10. The caching multiplier
How do caching proxies multiply the web's capacity?
11. The format's name
Which standard defines email messages as an ASCII header of structured lines plus a body?
12. The push protocol
Which protocol transfers mail from the sender's server toward the receiver's in a store-and-forward conversation?
13. The three-digit verdict
In an HTTP response, what does the three-digit status code convey?
14. The standing channel
What HTTP feature amortizes TCP connection setup across the many objects of one page?