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Web & Networking › HTTP

Range Requests

Fetching part of a resource, for resumable downloads and video.

Backend Engineer · SeniorFrontend Engineer · SeniorData Engineer · Senior

Also known as: range requests, http range, partial content

Range requests fetch part of a resource: Range: bytes=1000-1999 returns that slice with 206 Partial Content and a Content-Range header. Servers advertising Accept-Ranges: bytes unlock resume-after-interruption, media seeking without full download, and parallel chunk fetching.

HEAD /big.zip → Accept-Ranges: bytes, 10 GB
GET /big.zip (Range: bytes=0-999999) → 206, first MB
…later… Range: bytes=1000000- → resume where it stopped

Ranges compose with validators: If-Range revalidates before serving the slice, so a changed file restarts instead of corrupting. CDNs and players depend on this machinery daily.

The classic mistakes:

  • No range support on large files. Forcing full downloads for a 2-hour video’s last 10 minutes (or a resumed 10GB transfer) wastes enormous bandwidth. Support ranges on anything big.
  • Ignoring If-Range. Serving a stale slice of a changed file corrupts the result silently. Validate before slicing.
  • Single-threaded giant downloads. One connection ramps slowly and stalls on loss; ranged parallel chunks saturate links (with server courtesy — bound concurrency).
  • Assuming all intermediaries pass ranges through. Some proxies collapse ranges or strip the headers; verify end to end, especially with signed URLs and auth.
  • Range math off-by-ones. Inclusive byte ranges (0-499 is 500 bytes) trip every implementer once. Test boundaries and suffix ranges (-500).
  • Multipart/byteranges complexity. Multi-range responses use multipart bodies few clients parse well. Single ranges cover nearly every real need.
  • Forgetting HEAD. Clients discover size and range support via HEAD; a HEAD that 404s or omits Accept-Ranges breaks resume before it starts.

How to serve them: advertise Accept-Ranges, honour Range with 206/Content-Range, validate with If-Range, and make HEAD accurate. It’s the protocol behind every seek bar and resumable download.