HTTP

Application layer protocol defines:

Types of messages exchanged.
Syntax of various message types(fields definition).
Semantics of fields.
Rules for when/how to send/respond to messages.

Protocol

Hypertext Transfer Protocol (RFC 2068):

HTTP -> Socket Interface -> TCP.
Stateless protocol.
HTTP/1.0 默认不开启长连接: 客户端与服务端必须同时发送 Connection: Keep-Alive.
HTTP/1.1 默认开启长连接:
- Keep-Alive: timeout=5, max=100: 表示 TCP 通道保持 5 秒, 最多接收 100 次请求.
- Keep-Alive 无法保证客户端和服务器之间的连接一定活跃.
HTTP 为无状态协议: 每个请求相互独立.

Connections

Non-persistent connections: 1 http request with 1 tcp connection.
Persistent connections: multiple http request with 1 tcp connection.

Message Format

HTTP request format:

request line -> (method field, object url field, protocol version)
header lines -> Host/Connections(close -> non-persistent connection)/User-agent/Accept-language
\r\n
entity body

HTTP response format:

status line -> (protocol version, status code, corresponding status message)
header lines -> Connections/Date/Server/Last-Modified/Content-Length(bytes)/Content-Type
\r\n
entity body

Process

Port to Transport Layer

Bandwidth-sensitive application: UDP.
Reliable-sensitive application: TCP.

Application	Application Layer	Transport Layer
Email	SMTP	TCP
Remote terminal access	Telnet	TCP
Web	HTTP/HTTPS	TCP
File transfer	FTP	TCP
Streaming multimedia	HTTP/HTTPS/RTP	TCP/UDP
Internet telephony	SIP/RTP	UDP

Address

IP (32 bits network layer): find specific host/end-system.
Port (16 bits transport layer): find specific process.

Status Codes

RFC 2616 defines the following status codes:

Informational responses: 100–199.
Successful responses: 200–299.
- 200 OK.
- 201 Created.
- 202 Accepted.
Redirects: 300–399.
- 301 Moved Permanently.
- 302 Found.
- 304 Not Modified.
- 307 Temporary Redirect.
- 308 Permanent Redirect.
Client errors: 400–499.
- 400 Bad Request.
- 401 Unauthorized.
- 403 Forbidden.
- 404 Not Found.
- 405 Method Not Allowed.
- 406 Not Acceptable.
Server errors: 500–599.
- 500 Internal Server Error.
- 501 Not Implemented.
- 502 Bad Gateway.
- 503 Service Unavailable.
- 504 Gateway Timeout.

HTTP Redirect

Use reasonable HTTP status codes:

200: general success.
201: successful creation.
301: moved permanently (SEO friendly).
302: moved temporarily.
304: not modified (HTTP cache).
400: bad requests from client.
401: unauthorized requests.
403: missing permissions.
404: missing resources.
429: too many requests.
5xx: internal errors (these should be avoided at all costs).

Performance

限制 Web 性能的主要因素是客户端与服务器之间的网络往返延迟 (RTT):

持久化连接以支持连接重用: N 次 HTTP 请求节省的总延迟时间为 (N-1) * RTT.
分块传输编码以支持流式响应.
请求管道以支持并行请求处理 (局限性较大):
- FIFO 管道, 队头请求会阻塞后续请求.
- 应用必须处理中断的连接并恢复.
- 应用必须处理中断请求的幂等问题.
- 应用必须保护自身不受出问题的代理的影响.
模拟多路复用: 并行使用多个 TCP 连接 (大多数现代浏览器支持每个主机打开 6 个连接).
利用多个 TCP 连接进行域名分区.
Resources bundling and inlining (但一定程度上放弃缓存粒度).
改进的更好的缓存机制.

Protocol​

Connections​

Message Format​

Process​

Port to Transport Layer​

Address​

Status Codes​

Performance​