Http Message Length

The transfer-length of a message is the length of the message-body as it appears in the message; that is, after any transfer-codings have been applied. When a message-body is included with a message, the transfer-length of that body is determined by one of the following (in order of precedence):

1.Any response message which "MUST NOT" include a message-body (such as the 1xx, 204, and 304 responses and any response to a HEAD request) is always terminated by the first empty line after the header fields, regardless of the entity-header fields present in the message.

2.If a Transfer-Encoding header field (section 14.41) is present and has any value other than "identity", then the transfer-length is defined by use of the "chunked" transfer-coding (section 3.6), unless the message is terminated by closing the connection.

3.If a Content-Length header field (section 14.13) is present, its decimal value in OCTETs represents both the entity-length and the transfer-length. The Content-Length header field MUST NOT be sent if these two lengths are different (i.e., if a Transfer-Encoding

     header field is present). If a message is received with both a      Transfer-Encoding header field and a Content-Length header field,      the latter MUST be ignored. 

4.If the message uses the media type "multipart/byteranges", and the transfer-length is not otherwise specified, then this self- delimiting media type defines the transfer-length. This media type MUST NOT be used unless the sender knows that the recipient can parse it; the presence in a request of a Range header with multiple byte- range specifiers from a 1.1 client implies that the client can parse multipart/byteranges responses.

       A range header might be forwarded by a 1.0 proxy that does not        understand multipart/byteranges; in this case the server MUST        delimit the message using methods defined in items 1,3 or 5 of        this section. 

5.By the server closing the connection. (Closing the connection cannot be used to indicate the end of a request body, since that would leave no possibility for the server to send back a response.)

For compatibility with HTTP/1.0 applications, HTTP/1.1 requests containing a message-body MUST include a valid Content-Length header field unless the server is known to be HTTP/1.1 compliant. If a request contains a message-body and a Content-Length is not given, the server SHOULD respond with 400 (bad request) if it cannot determine the length of the message, or with 411 (length required) if it wishes to insist on receiving a valid Content-Length.

All HTTP/1.1 applications that receive entities MUST accept the "chunked" transfer-coding (section 3.6), thus allowing this mechanism to be used for messages when the message length cannot be determined in advance.

Messages MUST NOT include both a Content-Length header field and a non-identity transfer-coding. If the message does include a non- identity transfer-coding, the Content-Length MUST be ignored.

When a Content-Length is given in a message where a message-body is allowed, its field value MUST exactly match the number of OCTETs in the message-body. HTTP/1.1 user agents MUST notify the user when an invalid length is received and detected.

Chunked transfer encoding

From Wikipedia, the free encyclopedia

Chunked transfer encoding is a data transfer mechanism in the Hypertext Transfer Protocol (HTTP) that allows HTTP data to be reliably delivered between a web server and a clientapplication, usually a web browser, without knowing in advance of transmission the size of the entire message body. This is achieved by splitting the data payload of the message in small parts (chunks) and transmitting with each chunk its size. The data transfer is terminated by a final chunk of length zero. This makes it possible to transmit dynamically generated content in web pages. The chunked method is only available in version 1.1 of the HTTP protocol (HTTP/1.1).

Without chunked transfer encoding, the size of data delivered in HTTP responses must be indicated by the Content-Length header field to allow clients to determine the end of transmission.

Format

If a Transfer-Encoding field with a value of chunked is specified in an HTTP message (either a request sent by a client or the response from the server), the body of the message consists of an unspecified number of chunks, a terminating last-chunk, an optional trailer of entity-header fields, and a final CRLF sequence.

Each chunk starts with the number of octets of the data it embeds expressed in hexadecimal followed by optional parameters (chunk extension) and a terminating CRLF (carriage returnand line feed) sequence, followed by the chunk data. The chunk is terminated by CRLF. If chunk extensions are provided, the chunk size is terminated by a semicolon followed with the extension name and an optional equal sign and value.

The last chunk is a zero-length chunk, with the chunk size coded as 0, but without any chunk data section.

The final chunk may be followed by an optional trailer of additional entity-header fields that are normally delivered in the HTTP header to allow the delivery of data that can only be computed after all chunk data has been generated. The sender may indicate in a Trailer header field which additional fields it will send in the trailer after the chunks.

[edit]Example

[edit]Encoded response

HTTP/1.1 200 OK Content-Type: text/plain Transfer-Encoding: chunked  25 This is the data in the first chunk  1C and this is the second one  3 con 8 sequence 0  

[edit]Anatomy of encoded response

The first two chunks contain explicit \r\n characters in the chunk data.

"This is the data in the first chunk\r\n"      (37 chars => hex: 0x25) "and this is the second one\r\n"               (28 chars => hex: 0x1C) "con"                                          (3  chars => hex: 0x03) "sequence"                                     (8  chars => hex: 0x08) 

The response ends with a zero-length last chunk: "0\r\n" and the final "\r\n".

[edit]Decoded data

This is the data in the first chunk and this is the second one consequence