Tradução em progresso.

Requisições de intervalo HTTP  permite enviar uma parte de uma mensagem HTTP do servidor para o cliente. Requisições de intervalo é útil  quando usadas em mídias grandes ou em download de arquivos com funções de pausa e retomada.

Checando se um servidor suporta resquisição por parte

Se o Accept-Ranges está na resposta HTTP(e o valor não é "none"), o servidor suporta requisições de intervalo. Você pode verificar isso fazendo uma emissão do HTTP HEAD com cURL, por exemplo.

curl -I http://i.imgur.com/z4d4kWk.jpg

HTTP/1.1 200 OK
...
Accept-Ranges: bytes
Content-Length: 146515

Na resposta, Accept-Ranges: bytes indica que bytes pode ser usado com unidade para definir um intervalo. Aqui, o cabeçalho Content-Length é também útil, pois diz qual é o tamanho total da imagem.

Se sites omitem o cabeçalho Accept-Ranges, é provavel que eles não suportem requisições parciais. Alguns sites também podem explicitamente enviar "none" como um valor, indicando que não suportam. Em alguns aplicativos, gerenciadores de download desabilitam seus botões de pausa nesse caso.

curl -I https://www.youtube.com/watch?v=EwTZ2xpQwpA

HTTP/1.1 200 OK
...
Accept-Ranges: none

Requisitando uma parte especifica de um servidor

Se o servidor suportar requisiçóes parciais, você pode fazer uma requisição usando o cabeçalho Range. Esse indica a(s) parte(s) de um documento que o servidor deve retornar.

Single part ranges

We can request a single range from a resource. Again, we can test a request by using cURL. The "-H" option will append a header line to the request, which in this case is the Range header requesting the first 1024 bytes.

curl http://i.imgur.com/z4d4kWk.jpg -i -H "Range: bytes=0-1023"

The issued request looks like this:

GET /z4d4kWk.jpg HTTP/1.1
Host: i.imgur.com
Range: bytes=0-1023

The server responses with the 206 Partial Content status:

HTTP/1.1 206 Partial Content
Content-Range: bytes 0-1023/146515
Content-Length: 1024
...
(binary content)

The Content-Length header now indicates the size of the requested range (and not the full size of the image). The Content-Range response header indicates where in the full resource this partial message belongs.

Multipart ranges

The Range header also allows you to get multiple ranges at once in a multipart document. The ranges are separated by a comma.

curl http://www.example.com -i -H "Range: bytes=0-50, 100-150"

The server responses with the 206 Partial Content status and a Content-Type: multipart/byteranges; boundary=3d6b6a416f9b5 header, indicating that a multipart byterange follows. Each part contains its own Content-Type and Content-Range fields and the required boundary parameter specifies the boundary string used to separate each body-part.

HTTP/1.1 206 Partial Content
Content-Type: multipart/byteranges; boundary=3d6b6a416f9b5
Content-Length: 282

--3d6b6a416f9b5
Content-Type: text/html
Content-Range: bytes 0-50/1270

<!doctype html>
<html>
<head>
    <title>Example Do
--3d6b6a416f9b5
Content-Type: text/html
Content-Range: bytes 100-150/1270

eta http-equiv="Content-type" content="text/html; c
--3d6b6a416f9b5--

Conditional range requests

When resuming to request more parts of a resource, you need to guarantee that the stored resource has not been modified since the last fragment has been received.

The If-Range HTTP request header makes a range request conditional: if the condition is fulfilled, the range request will be issued and the server sends back a 206 Partial Content answer with the appropriate body. If the condition is not fulfilled, the full resource is sent back, with a 200 OK status. This header can be used either with a Last-Modified validator, or with an ETag, but not with both.

If-Range: Wed, 21 Oct 2015 07:28:00 GMT 

Partial request responses

There are three relevant statuses, when working with range requests:

  • In case of a successful range request, the 206 Partial Content status is sent back from a server.
  • In case of a range request that is out of bounds (range values overlap the extent of the resource), the server responds with a 416 Requested Range Not Satisfiable status.
  • In case of no support of range requests, the 200 OK status is sent back from a server.

Comparison to chunked Transfer-Encoding

The Transfer-Encoding header allows chunked encoding, which is useful when larger amounts of data are sent to the client and the total size of the response is not known until the request has been fully processed. The server sends data to the client straight away without buffering the response or determining the exact length, which leads to improved latency. Range requests and chunking are compatible and can be used with or without each other.

See also

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