Connected: An Internet Encyclopedia
3.6 Transfer Codings

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3.6 Transfer Codings

3.6 Transfer Codings

Transfer coding values are used to indicate an encoding transformation that has been, can be, or may need to be applied to an entity-body in order to ensure "safe transport" through the network. This differs from a content coding in that the transfer coding is a property of the message, not of the original entity.

          transfer-coding         = "chunked" | transfer-extension

          transfer-extension      = token

All transfer-coding values are case-insensitive. HTTP/1.1 uses transfer coding values in the Transfer-Encoding header field (section 14.40).

Transfer codings are analogous to the Content-Transfer-Encoding values of MIME , which were designed to enable safe transport of binary data over a 7-bit transport service. However, safe transport has a different focus for an 8bit-clean transfer protocol. In HTTP, the only unsafe characteristic of message-bodies is the difficulty in determining the exact body length (section 7.2.2), or the desire to encrypt data over a shared transport.

The chunked encoding modifies the body of a message in order to transfer it as a series of chunks, each with its own size indicator, followed by an optional footer containing entity-header fields. This allows dynamically-produced content to be transferred along with the information necessary for the recipient to verify that it has received the full message.

       Chunked-Body   = *chunk
                        "0" CRLF

       chunk          = chunk-size [ chunk-ext ] CRLF
                        chunk-data CRLF

       hex-no-zero    = <HEX excluding "0">

       chunk-size     = hex-no-zero *HEX
       chunk-ext      = *( ";" chunk-ext-name [ "=" chunk-ext-value ] )
       chunk-ext-name = token
       chunk-ext-val  = token | quoted-string
       chunk-data     = chunk-size(OCTET)

       footer         = *entity-header

The chunked encoding is ended by a zero-sized chunk followed by the footer, which is terminated by an empty line. The purpose of the footer is to provide an efficient way to supply information about an entity that is generated dynamically; applications MUST NOT send header fields in the footer which are not explicitly defined as being appropriate for the footer, such as Content-MD5 or future extensions to HTTP for digital signatures or other facilities.

An example process for decoding a Chunked-Body is presented in appendix 19.4.6.

All HTTP/1.1 applications MUST be able to receive and decode the "chunked" transfer coding, and MUST ignore transfer coding extensions they do not understand. A server which receives an entity-body with a transfer-coding it does not understand SHOULD return 501 (Unimplemented), and close the connection. A server MUST NOT send transfer-codings to an HTTP/1.0 client.

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Connected: An Internet Encyclopedia
3.6 Transfer Codings