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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 Entities, Transfers, Coding Methods and Content Management
HTTP Data Length Issues, "Chunked" Transfers and Message Trailers
(Page 2 of 3)
Using "Chunked" Transfers
The problem of unknown message length
could be resolved by buffering the entire resource before transmission.
However, this would be wasteful of server memory and would delay the
transmission of the entity unnecessarilyno part could be sent
until the entire entity was ready. Instead, a special transfer encoding
method was developed to handle this particular problem of unsafe
transport: not knowing the length of a file. The method is called chunking.
When this technique is used, instead
of sending an entity as a raw sequence of bytes, it is broken into,
well, chunks. J
This allows HTTP to send a dynamically-generated resource, such as output
from a script, a piece at a time as the data becomes available from
the software processing it. To indicate that this method has been used,
the special header Transfer-Encoding: chunked is
placed in the message. A special format is also used for the body of
the HTTP message to delineate the chunks:
Basically, instead of putting the
whole entity in the body and indicating its length in a Content-Length
header, each chunk is placed in the body sequentially, each preceded
by the length of the chunk. The length is specified in hexadecimal,
and represented using ASCII characters. All chunk lengths and chunk
data are terminated with a CRLF sequence. The recipient
knows it has received the last chunk when it sees a chunk-length of
Key Concept: Since HTTP/1.1 uses persistent connections that allow multiple requests and responses to be sent over a TCP connection, clients and servers need some way to identify where one message ends and the next begins. The easier solution is to use the Content-Length header to indicate the size of a message, but this only works when the length of a message can be easily determined in advance. For dynamic content or other cases where message length cannot be easily computed before sending the data, the special chunked transfer encoding can be used, where the message body is sent as a sequence of chunks, each preceded by the length of the chunk.
When chunked transfer encoding is
used, the sender of the message may also choose to specify one or more
message trailers. These are the same as entity
headers, describing the contents of the
message body, but appear after the entity instead of before it. They
provide flexibility in the same way that chunking itself doesthey
allow a device to include an HTTP header that may contain information
that was not available when the HTTP message transmission began. A good
example would be an integrity check field calculated based on the byte
values of the entire entity.
Trailers are optional, and not always
be needed. When they are used, they are processed just like regular
entity headers. To give the recipient of a message a heads up
that trailers have been used, the special Trailer header should
be included at the start of the message, which lists the names of each
header that appears as a trailer.
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The TCP/IP Guide (http://www.TCPIPGuide.com)
Version 3.0 - Version Date: September 20, 2005
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