如何定义一个HTTP响应呢？在RFC2616的第六章定义了HTTP Response的格式。请参考官网。
HTTP响应的定义格式：
Response = Status-Line
*(( general-header
| response-header
| entity-header ) CRLF)
CRLF
[ message-body ]
状态行的定义：
Status-Line = HTTP-Version SP Status-Code SP Reason-Phrase CRLF
状态代码和状态原因：
Status-Code =
"100" Continue
| "101" Switching Protocols
| "200" OK
| "201" Created
| "202" Accepted
| "203" Non-Authoritative Information
| "204" No Content
| "205" Reset Content
| "206" Partial Content
| "300" Multiple Choices
| "301" Moved Permanently
| "302" Found
| "303" See Other
| "304" Not Modified
| "305" Use Proxy
| "307" Temporary Redirect
| "400" Bad Request
| "401" Unauthorized
| "402" Payment Required
| "403" Forbidden
| "404" Not Found
| "405" Method Not Allowed
| "406" Not Acceptable
| "407" Proxy Authentication Required
| "408" Request Time-out
| "409" Conflict
| "410" Gone
| "411" Length Required
| "412" Precondition Failed
| "413" Request Entity Too Large
| "414" Request-URI Too Large
| "415" Unsupported Media Type
| "416" Requested range not satisfiable
| "417" Expectation Failed
| "500" Internal Server Error
| "501" Not Implemented
| "502" Bad Gateway
| "503" Service Unavailable
| "504" Gateway Time-out
| "505" HTTP Version not supported
| extension-code
extension-code = 3DIGIT
Reason-Phrase = *<TEXT, excluding CR, LF>
上面的状态码可以按第一个数字分为5大类别的状态：信息，成功，重定向，客户端错误，服务器端错误。如下：
- 1xx: Informational - Request received, continuing process
- 2xx: Success - The action was successfully received,
understood, and accepted
- 3xx: Redirection - Further action must be taken in order to
complete the request
- 4xx: Client Error - The request contains bad syntax or cannot
be fulfilled
- 5xx: Server Error - The server failed to fulfill an apparently
valid request
响应头字段的定义：
response-header = Accept-Ranges
| Age
| ETag
| Location
| Proxy-Authenticate
| Retry-After
| Server
| Vary
| WWW-Authenticate