 HTTP message framing

HTTP message framing determines where an HTTP request or HTTP response ends and whether it is complete. The concrete rules depend on the protocol version. In HTTP/1.1, an empty line ends the header section, but the end of the message body requires the applicable length, transfer-coding or connection rules. A body can itself contain line breaks.^[ref]

Consider an ordinary successful response to GET whose text is café in UTF-8. Its four displayed letters occupy five octets, with hexadecimal values 63 61 66 c3 a9. With no transfer coding and no overriding bodyless-response rule, Content-Length: 5 therefore frames this body. Counting displayed characters would give the wrong boundary.^[ref]

An alternative is chunked transfer coding. Use Transfer-Encoding: chunked instead of the content-length field. The body can then be written in visible escape notation as 2\r\nca\r\n3\r\nfé\r\n0\r\n\r\n. Here \r\n denotes an actual carriage-return/line-feed pair, and the text is encoded in UTF-8. The first chunk supplies two octets; the second supplies three; the zero-sized chunk and empty trailer section finish the coding. This transfer-coded body occupies twenty octets and yields the same five content octets after decoding.^[ref]

Precedence matters. Responses to HEAD and responses with status 1xx, 204 or 304 have no message body even when fields appear to suggest a length. Some responses instead end with connection closure; requests are not close-delimited. Receiving fewer than the declared number of octets leaves a length-delimited message incomplete.^[ref]

Framing is separate from media type: a recipient first needs to delimit the transmitted data, then interpret its format. It also makes HTTP persistent connections possible, since a recipient can finish one exchange without treating connection closure as the only indication of completion.