Transfer-Encoding

The Transfer-Encoding header specifies the form of encoding used to safely transfer the entity to the user.

Transfer-Encoding is a hop-by-hop header, that is applying to a message between two nodes, not to a resource itself. Each segment of a multi-node connection can use different Transfer-Encoding values. If you want to compress data over the whole connection, use the end-to-end header Content-Encoding header instead.

When present on a response to a HEAD request that has no body, it indicates the value that would have applied to the corresponding GET message.

Header type	Response header
Forbidden header name	yes

Syntax

Transfer-Encoding: chunked
Transfer-Encoding: compress
Transfer-Encoding: deflate
Transfer-Encoding: gzip
Transfer-Encoding: identity

// Several values can be listed, separated by a comma
Transfer-Encoding: gzip, chunked

Directives

chunked

Data is sent in a series of chunks. The Content-Length header is omitted in this case and at the beginning of each chunk you need to add the length of the current chunk in hexadecimal format, followed by '\r\n' and then the chunk itself, followed by another '\r\n'. The terminating chunk is a regular chunk, with the exception that its length is zero. It is followed by the trailer, which consists of a (possibly empty) sequence of entity header fields.

compress

A format using the Lempel-Ziv-Welch (LZW) algorithm. The value name was taken from the UNIX compress program, which implemented this algorithm.
Like the compress program, which has disappeared from most UNIX distributions, this content-encoding is used by almost no browsers today, partly because of a patent issue (which expired in 2003).

deflate

Using the zlib structure (defined in RFC 1950), with the deflate compression algorithm (defined in RFC 1951).

gzip

A format using the Lempel-Ziv coding (LZ77), with a 32-bit CRC. This is originally the format of the UNIX gzip program. The HTTP/1.1 standard also recommends that the servers supporting this content-encoding should recognize x-gzip as an alias, for compatibility purposes.

identity

Indicates the identity function (i.e. no compression, nor modification). This token, except if explicitly specified, is always deemed acceptable.

Examples

Chunked encoding

Chunked encoding is useful when larger amounts of data are sent to the client and the total size of the response may not be known until the request has been fully processed. For example, when generating a large HTML table resulting from a database query or when transmitting large images. A chunked response looks like this:

HTTP/1.1 200 OK 
Content-Type: text/plain 
Transfer-Encoding: chunked

7\r\n
Mozilla\r\n 
9\r\n
Developer\r\n
7\r\n
Network\r\n
0\r\n 
\r\n

Specifications

Specification	Title
RFC 7230, section 3.3.1: Transfer-Encoding	Hypertext Transfer Protocol (HTTP/1.1): Message Syntax and Routing

Browser compatibility

See also

• Accept-Encoding
• Content-Encoding
• Content-Length
• Header fields that regulate the use of trailers: TE (requests) and Trailer (responses).

• Chunked transfer encoding