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HTTP Status Code: 100 - CONTINUE

100 - CONTINUE is an Informational Code for HTTP. This article contains all information and use case about 100 - CONTINUE status code.

The client SHOULD continue with its request. This interim response is used to inform the client that the initial part of the request has been received and has not yet been rejected by the server. The client SHOULD continue by sending the remainder of the request or, if the request has already been completed, ignore this response. The server MUST send a final response after the request has been completed.

The initial part of a request has been received and has not yet been rejected by the server. The server intends to send a final response after the request has been fully received and acted upon.

When the request contains an Expect header field that includes a 100-continue expectation, the 100 response indicates that the server wishes to receive the request payload body1. The client ought to continue sending the request and discard the 100 response.

If the request did not contain an Expect header field containing the 100-continue expectation, the client can simply discard this interim response.


Wikipedia

This means that the server has received the request headers, and that the client should proceed to send the request body (in the case of a request for which a body needs to be sent; for example, a POST request). If the request body is large, sending it to a server when a request has already been rejected based upon inappropriate headers is inefficient. To have a server check if the request could be accepted based on the request's headers alone, a client must send Expect: 100-continue as a header in its initial request and check if a 100 Continue status code is received in response before continuing (or receive 417 Expectation Failed and not continue).


Use of the 100 (Continue) Status

The purpose of the 100 (Continue) status 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 with the "100-continue" expectation. 
- A client MUST NOT send an Expect request-header field 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.


Client Behavior if Server Prematurely Closes Connection

If an HTTP/1.1 client sends a request which includes a request body, but which does not include an Expect request-header field with the "100-continue" expectation, and if the client is not directly connected to an HTTP/1.1 origin server, and if the client sees the connection close before receiving any status from the server, the client SHOULD retry the request. If the client does retry this request, it MAY use the following "binary exponential backoff" algorithm to be assured of obtaining a reliable response:
1. Initiate a new connection to the server 
2. Transmit the request-headers 
3. Initialize a variable R to the estimated round-trip time to the server (e.g., based on the time it took to establish the connection), or to a constant value of 5 seconds if the round- trip time is not available. 
4. Compute T = R * (2**N), where N is the number of previous retries of this request. 
5. Wait either for an error response from the server, or for T seconds (whichever comes first) 
6. If no error response is received, after T seconds transmit the body of the request. 
7. If client sees that the connection is closed prematurely, repeat from step 1 until the request is accepted, an error response is received, or the user becomes impatient and terminates the retry process. 

If at any point an error status is received, the client - SHOULD NOT continue and - SHOULD close the connection if it has not completed sending the request message.
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