http状态码

状态代码有三位数字组成，第一个数字定义了响应的类别，共分五种类别:
1xx：指示信息--表示请求已接收，继续处理
2xx：成功--表示请求已被成功接收、理解、接受
3xx：重定向--要完成请求必须进行更进一步的操作
4xx：客户端错误--请求有语法错误或请求无法实现
5xx：服务器端错误--服务器未能实现合法的请求

常见状态码：

200 OK                        //客户端请求成功
400 Bad Request               //客户端请求有语法错误，不能被服务器所理解
401 Unauthorized              //请求未经授权，这个状态代码必须和WWW-Authenticate报头域一起使用 
403 Forbidden                 //服务器收到请求，但是拒绝提供服务
404 Not Found                 //请求资源不存在，eg：输入了错误的URL
500 Internal Server Error     //服务器发生不可预期的错误
503 Server Unavailable        //服务器当前不能处理客户端的请求，一段时间后可能恢复正常

8.2.3 for detailed discussion of the use and handling of this status code.

14.19.

14.27) to make the request conditional.

The response MUST include the following header fields:

      - Either a Content-Range header field (section 14.16) indicating
        the range included with this response, or a multipart/byteranges
        Content-Type including Content-Range fields for each part. If a
        Content-Length header field is present in the response, its
        value MUST match the actual number of OCTETs transmitted in the
        message-body.

      - Date

      - ETag and/or Content-Location, if the header would have been sent
        in a 200 response to the same request

      - Expires, Cache-Control, and/or Vary, if the field-value might
        differ from that sent in any previous response for the same
        variant

If the 206 response is the result of an If-Range request that used a strong cache validator (see section 13.3.3), the response SHOULD NOT include other entity-headers. If the response is the result of an If-Range request that used a weak validator, the response MUST NOT include other entity-headers; this prevents inconsistencies between cached entity-bodies and updated headers. Otherwise, the response MUST include all of the entity-headers that would have been returned with a 200 (OK) response to the same request.

A cache MUST NOT combine a 206 response with other previously cached content if the ETag or Last-Modified headers do not match exactly, see 13.5.4.

A cache that does not support the Range and Content-Range headers MUST NOT cache 206 (Partial) responses.

14.19), caches will operate correctly.

      - ETag and/or Content-Location, if the header would have been sent
        in a 200 response to the same request

      - Expires, Cache-Control, and/or Vary, if the field-value might
        differ from that sent in any previous response for the same
        variant

If the conditional GET used a strong cache validator (see section 13.3.3), the response SHOULD NOT include other entity-headers. Otherwise (i.e., the conditional GET used a weak validator), the response MUST NOT include other entity-headers; this prevents inconsistencies between cached entity-bodies and updated headers.

If a 304 response indicates an entity not currently cached, then the cache MUST disregard the response and repeat the request without the conditional.

If a cache uses a received 304 response to update a cache entry, the cache MUST update the entry to reflect any new field values given in the response.

14.8). If the request already included Authorization credentials, then the 401 response indicates that authorization has been refused for those credentials. If the 401 response contains the same challenge as the prior response, and the user agent has already attempted authentication at least once, then the user SHOULD be presented the entity that was given in the response, since that entity might include relevant diagnostic information. HTTP access authentication is explained in "HTTP Authentication: Basic and Digest Access Authentication" [43].

14.33) containing a challenge applicable to the proxy for the requested resource. The client MAY repeat the request with a suitable Proxy-Authorization header field (section 14.34). HTTP access authentication is explained in "HTTP Authentication: Basic and Digest Access Authentication" [43].

14.16). This response MUST NOT use the multipart/byteranges content- type.

3.1, other than with this error message. The response SHOULD contain an entity describing why that version is not supported and what other protocols are supported by that server.