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+# Lockable HTTP Tarball Protocol
+
+Tarball flakes can be served as regular tarballs via HTTP or the file
+system (for `file://` URLs). Unless the server implements the Lockable
+HTTP Tarball protocol, it is the responsibility of the user to make sure that
+the URL always produces the same tarball contents.
+
+An HTTP server can return an "immutable" HTTP URL appropriate for lock
+files. This allows users to specify a tarball flake input in
+`flake.nix` that requests the latest version of a flake
+(e.g. `https://example.org/hello/latest.tar.gz`), while `flake.lock`
+will record a URL whose contents will not change
+(e.g. `https://example.org/hello/<revision>.tar.gz`). To do so, the
+server must return an [HTTP `Link` header](https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/Link) with the `rel` attribute set to
+`immutable`, as follows:
+
+```
+Link: <flakeref>; rel="immutable"
+```
+
+(Note the required `<` and `>` characters around *flakeref*.)
+
+*flakeref* must be a tarball flakeref. It can contain flake attributes
+such as `narHash`, `rev` and `revCount`. If `narHash` is included, its
+value must be the NAR hash of the unpacked tarball (as computed via
+`nix hash path`). Nix checks the contents of the returned tarball
+against the `narHash` attribute. The `rev` and `revCount` attributes
+are useful when the tarball flake is a mirror of a fetcher type that
+has those attributes, such as Git or GitHub. They are not checked by
+Nix.
+
+```
+Link: <https://example.org/hello/442793d9ec0584f6a6e82fa253850c8085bb150a.tar.gz
+  ?rev=442793d9ec0584f6a6e82fa253850c8085bb150a
+  &revCount=835
+  &narHash=sha256-GUm8Uh/U74zFCwkvt9Mri4DSM%2BmHj3tYhXUkYpiv31M%3D>; rel="immutable"
+```
+
+(The linebreaks in this example are for clarity and must not be included in the actual response.)
+
+For tarball flakes, the value of the `lastModified` flake attribute is
+defined as the timestamp of the newest file inside the tarball.