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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 Status Code Format, Status Codes and Reason Phrases
(Page 4 of 4)
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.
Note: In some cases, servers send these preliminary replies even when they are not supposed to, so clients must be prepared to deal with them (they are simply discarded, since they contain no information).
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The TCP/IP Guide (http://www.TCPIPGuide.com)
Version 3.0 - Version Date: September 20, 2005
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