HTTP状态码100的用法

The purpose of the 100 (Continue) status (see section 10.1.1) is to allow a client that is sending a request message with a request body to determine if the origin server is willing to accept the request (based on the request headers) before the client sends the request body. In some cases, it might either be inappropriate or highly inefficient for the client to send the body if the server will reject the message without looking at the body.

Requirements for HTTP/1.1 clients:

      - If a client will wait for a 100 (Continue) response before
        sending the request body, it MUST send an Expect request-header
        field (section 14.20) with the "100-continue" expectation.

      - A client MUST NOT send an Expect request-header field (section
        14.20) with the "100-continue" expectation if it does not intend
        to send a request body.

Because of the presence of older implementations, the protocol allows ambiguous situations in which a client may send "Expect: 100- continue" without receiving either a 417 (Expectation Failed) status or a 100 (Continue) status. Therefore, when a client sends this header field to an origin server (possibly via a proxy) from which it has never seen a 100 (Continue) status, the client SHOULD NOT wait for an indefinite period before sending the request body.

Requirements for HTTP/1.1 origin servers:

      - Upon receiving a request which includes an Expect request-header
        field with the "100-continue" expectation, an origin server MUST
        either respond with 100 (Continue) status and continue to read
        from the input stream, or respond with a final status code. The
        origin server MUST NOT wait for the request body before sending
        the 100 (Continue) response. If it responds with a final status
        code, it MAY close the transport connection or it MAY continue
        to read and discard the rest of the request.  It MUST NOT
        perform the requested method if it returns a final status code.

      - An origin server SHOULD NOT send a 100 (Continue) response if
        the request message does not include an Expect request-header
        field with the "100-continue" expectation, and MUST NOT send a
        100 (Continue) response if such a request comes from an HTTP/1.0
        (or earlier) client. There is an exception to this rule: for
        compatibility with RFC 2068, a server MAY send a 100 (Continue)
        status in response to an HTTP/1.1 PUT or POST request that does
        not include an Expect request-header field with the "100-
        continue" expectation. This exception, the purpose of which is
        to minimize any client processing delays associated with an
        undeclared wait for 100 (Continue) status, applies only to
        HTTP/1.1 requests, and not to requests with any other HTTP-
        version value.

      - An origin server MAY omit a 100 (Continue) response if it has
        already received some or all of the request body for the
        corresponding request.

      - An origin server that sends a 100 (Continue) response MUST
        ultimately send a final status code, once the request body is
        received and processed, unless it terminates the transport
        connection prematurely.

      - If an origin server receives a request that does not include an
        Expect request-header field with the "100-continue" expectation,
        the request includes a request body, and the server responds
        with a final status code before reading the entire request body
        from the transport connection, then the server SHOULD NOT close
        the transport connection until it has read the entire request,
        or until the client closes the connection. Otherwise, the client
        might not reliably receive the response message. However, this
        requirement is not be construed as preventing a server from
        defending itself against denial-of-service attacks, or from
        badly broken client implementations.

Requirements for HTTP/1.1 proxies:

      - If a proxy receives a request that includes an Expect request-
        header field with the "100-continue" expectation, and the proxy
        either knows that the next-hop server complies with HTTP/1.1 or
        higher, or does not know the HTTP version of the next-hop
        server, it MUST forward the request, including the Expect header
        field.

      - If the proxy knows that the version of the next-hop server is
        HTTP/1.0 or lower, it MUST NOT forward the request, and it MUST
        respond with a 417 (Expectation Failed) status.

      - Proxies SHOULD maintain a cache recording the HTTP version
        numbers received from recently-referenced next-hop servers.

      - A proxy MUST NOT forward a 100 (Continue) response if the
        request message was received from an HTTP/1.0 (or earlier)
        client and did not include an Expect request-header field with
        the "100-continue" expectation. This requirement overrides the
        general rule for forwarding of 1xx responses (see section 10.1).

转自：http://www.w3.org/Protocols/rfc2616/rfc2616-sec8.html#sec8.2.3

不甚理解。