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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 Entities, Transfers, Coding Methods and Content Management
HTTP Data Transfer, Content Encodings and Transfer Encodings
(Page 2 of 3)
HTTP's Two-Level Encoding Scheme
This would seem to be an area where
HTTP was simpler than MIMEsince there is no need to encode the
entity, there is no need for the Content-Transfer-Encoding header,
and we have one less thing to worry about. Ha, nice try! J
It is true that HTTP could have simply been designed so
that all entities were just sent one byte at a time with no need to
specify encodings. But the developers of the protocol recognized that
this would have made the protocol inflexible. There are situations where
it might be useful to transform or encode an entity or message for transmission,
and then reverse the operation upon receipt.
This effort to make HTTP flexible
resulted in a system of representing encodings that is actually more
complicated than MIMEs. The key to understanding it is to recognize
that HTTP/1.1 actually splits MIMEs notion of a content
transfer encoding into two different encoding levels:
- Content Encoding: This is an encoding
that is applied specifically to the entity carried in an HTTP message,
to prepare or package it prior to transmission. Content encodings are
said to be end-to-end, because the encoding of the entity
is done once before it sent by the client or server, and only decoded
upon receipt by the ultimate recipient: server or client. When this
type of encoding is done, the method is identified in the special Content-Encoding
header. A client may also specify what
content encodings it can handle, using the Accept-Encoding header,
as we will see in the
topic on content negotiation.
- Transfer Encoding: This is an encoding
that is done specifically for the purpose of ensuring that data can
be safely transferred between devices. It is applied across an entire
HTTP message, and not specifically to the entity. This type of encoding
is hop-by-hop because a different transfer encoding may
be used for each hop of a message that is transmitted through many intermediaries
in the request/response
chain. The transfer encoding method, if
any, is indicated in the Transfer-Encoding general
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The TCP/IP Guide (http://www.TCPIPGuide.com)
Version 3.0 - Version Date: September 20, 2005
© Copyright 2001-2005 Charles M. Kozierok. All Rights Reserved.
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