Editing HTTP (section)

=== Content retrieval optimizations ===
; HTTP/0.9
: A requested resource was always sent in its entirety.
; HTTP/1.0
: HTTP/1.0 added headers to manage resources cached by client in order to allow conditional GET requests; in practice a server has to return the entire content of the requested resource only if its last modified time is not known by client or if it changed since last full response to GET request. One of these headers, "Content-Encoding", was added to specify whether the returned content of a resource was or was not [[HTTP compression|compressed]].
: If the total length of the content of a resource was not known in advance (i.e. because it was dynamically generated, etc.) then the header <code>"Content-Length: number"</code> was not present in HTTP headers and the client assumed that when server closed the connection, the content had been sent in its entirety.  This mechanism could not distinguish between a resource transfer successfully completed and an interrupted one (because of a server / network error or something else).
; HTTP/1.1
: HTTP/1.1 introduced:
:* new headers to better manage the conditional retrieval of cached resources.
:* [[chunked transfer encoding]] to allow content to be streamed in chunks in order to reliably send it even when the server does not know its length in advance (i.e. because it is dynamically generated, etc.).
:* [[Byte serving|byte range serving]], where a client can request only one or more portions (ranges of bytes) of a resource (i.e. the first part, a part in the middle or in the end of the entire content, etc.) and the server usually sends only the requested part(s).  This is useful to resume an interrupted download (when a file is very large), when only a part of a content has to be shown or dynamically added to the already visible part by a browser (i.e. only the first or the following n comments of a web page) in order to spare time, bandwidth and system resources, etc.
; HTTP/2, HTTP/3
: Both HTTP/2 and HTTP/3 have kept the above mentioned features of HTTP/1.1.