HTTP: The Definitive Guide

Range Requests

We now understand how a client can ask a server to send it a resource only if the client’s copy of the resource is no longer valid. HTTP goes further: it allows clients to actually request just part or a range of a document.

Imagine if you were three-fourths of the way through downloading the latest hot software across a slow modem link, and a network glitch interrupted your connection. You would have been waiting for a while for the download to complete, and now you would have to start all over again, hoping the same thing does not happen again.

With range requests, an HTTP client can resume downloading an entity by asking for the range or part of the entity it failed to get (provided that the object did not change at the origin server between the time the client first requested it and its subsequent range request). For example:

GET /bigfile.html HTTP/1.1
Host: www.joes-hardware.com
Range: bytes=4000-
User-Agent: Mozilla/4.61 [en] (WinNT; I)
...

In this example, the client is requesting the remainder of the document after the first 4,000 bytes (the end bytes do not have to be specified, because the size of the document may not be known to the requestor). Range requests of this form can be used for a failed request where the client received the first 4,000 bytes before the failure. The Range header also can be used to request multiple ranges (the ranges can be specified in any order and may overlap)—for example, imagine a client connecting to multiple servers simultaneously, ...