Standards compliance

All clients and servers MUST support HTTP 1.0. Clients and servers MAY support later versions of HTTP as well.

HTTP headers

Servers SHOULD set Content-Encoding to the algorithm used to compress the document(s) being served. Recognized algorithms are:

     - "identity"     -- RFC2616 section 3.5
     - "deflate"      -- RFC2616 section 3.5
     - "gzip"         -- RFC2616 section 3.5
     - "x-zstd"       -- The zstandard compression algorithm (
     - "x-tor-lzma"   -- The lzma compression algorithm, with a "preset"
                         value no higher than 6.

Clients SHOULD use Accept-Encoding on most directory requests to indicate which of the above compression algorithms they support. If they omit it (as Tor clients did before, then the server should serve only "deflate" or "identity" encoded documents, based on the presence or absence of the ".z" suffix on the requested URL.

Note that for anonymous directory requests (that is, requests made over multi-hop circuits, like those for onion service lookups) implementations SHOULD NOT advertise any Accept-Encoding values other than deflate. To do so would be to create a fingerprinting opportunity.

When receiving multiple documents, clients MUST accept compressed concatenated documents and concatenated compressed documents as equivalent.

Servers MAY set the Content-Length: header. When they do, it should match the number of compressed bytes that they are sending.

Servers MAY include an X-Your-Address-Is: header, whose value is the apparent IP address of the client connecting to them (as a dotted quad). For directory connections tunneled over a BEGIN_DIR stream, servers SHOULD report the IP from which the circuit carrying the BEGIN_DIR stream reached them.

Servers SHOULD disable caching of multiple network statuses or multiple server descriptors. Servers MAY enable caching of single descriptors, single network statuses, the list of all server descriptors, a v1 directory, or a v1 running routers document. XXX mention times.

HTTP status codes

Tor delivers the following status codes. Some were chosen without much thought; other code SHOULD NOT rely on specific status codes yet.

  200 -- the operation completed successfully
      -- the user requested statuses or serverdescs, and none of the ones we
         requested were found ( and earlier).

  304 -- the client specified an if-modified-since time, and none of the
         requested resources have changed since that time.

  400 -- the request is malformed, or
      -- the URL is for a malformed variation of one of the URLs we support,
      -- the client tried to post to a non-authority, or
      -- the authority rejected a malformed posted document, or

  404 -- the requested document was not found.
      -- the user requested statuses or serverdescs, and none of the ones
         requested were found ( and later).

  503 -- we are declining the request in order to save bandwidth
      -- user requested some items that we ordinarily generate or store,
         but we do not have any available.