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HTTP Data Length Issues, "Chunked" Transfers and Message Trailers
(Page 3 of 3)
Example Using the Content-Length Header and "Chunking"
Yes, I really did say that headers can actually be trailers, in which case a header called Trailer lists each header that is actually a trailer. Perhaps an example would help clarify matters somewhat? Suppose we have a server that contains a program that, when supplied with a file name, returns a simple HTML response that contains the size and last modification date of the file. This is obviously dynamic content, so the length of the response cannot be determined in advance.
If the server were to buffer the entire output of this program (since it is small) it could construct a conventional HTTP response using the Content-Length header, as shown in the sample output of Table 278. Instead, chunking allows the server to send out parts of the response as soon as they become available from the program. The equivalent output of that example using chunked transfers is shown in Table 279; notice that the Expires header is now a trailer, so it can be calculated based on the output of the program, and this is indicated by the “Trailer: Expires” header. Remember that the Content-Length header specifies the length as a decimal number while chunking specifies chunk lengths in hexadecimal; the chunks in this example are 41, 5, 35, 29 and 19 bytes, respectively.
HTTP/1.1 200
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HTTP/1.1 200
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Note: An HTTP/1.1 client can specify that it does not want to use persistent connections by including the “Connection: close” header in its request. In this case, the server does not have to use chunking in its response—since it will close the connection after the first response message, the client knows that everything it receives from the server is part of that response. However, some servers may use chunked transfers anyway, even in this situation.
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Key Concept: When chunked transfer encoding is used, the sender of the message may move certain headers from the start of the message to the end, where they are known as trailers. They are interpreted in the same way as normal headers by the recipient. The special Trailer header is used in such messages to tell the recipient to look for trailers after the body of the message.
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Version 3.0 - Version Date: September 20, 2005
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