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The TCP/IP Guide  9  TCP/IP Application Layer Protocols, Services and Applications (OSI Layers 5, 6 and 7)       9  TCP/IP Key Applications and Application Protocols            9  TCP/IP File and Message Transfer Applications and Protocols (FTP, TFTP, Electronic Mail, USENET, HTTP/WWW, Gopher)                 9  TCP/IP World Wide Web (WWW, "The Web") and the Hypertext Transfer Protocol (HTTP)                      9  TCP/IP Hypertext Transfer Protocol (HTTP)                           9  HTTP Messages, Message Formats, Methods and Status Codes

HTTP Methods 1 2 3 4 HTTP Message Headers

The 100 (Continue) Preliminary Reply

Phew. Now, let's go back to the top, status code 100. Normally, a client sends a complete request to the server, and waits for a response to it (while optionally pipelining additional requests). In certain circumstances, however, the client might wish to check in advance if the server is willing to accept the request before it bothers sending the whole message. This is not a common occurrence, because most requests are quite small, which makes it not worth the bother. However, in cases where a user wants to submit a very large amount of data to an online program, or use PUT to store a large file, for example, checking with the server first can be a useful optimization.

In this situation, the client sends a request containing the special header “Expect: 100-continue”. Assuming that the server supports the feature, it will process the request's headers and immediately send back the “100 Continue” preliminary reply. This tells the client to continue sending the rest of the request. The server then processes it and responds normally. If the server doesn't send the 100 response after a certain amount of time, the client will typically just send the rest of the request anyway.

HTTP Methods 1 2 3 4 HTTP Message Headers