Tor design proposals

Filename: 281-bulk-md-download.txt
Title: Downloading microdescriptors in bulk
Author: Nick Mathewson
Created: 11-Aug-2017
Status: Reserve

1. Introduction

  This proposal describes a ways to download more microdescriptors
  at a time, using fewer bytes.

  Right now, to download N microdescriptors, the client must send
  about 44*N bytes in its HTTP request.  Because clients can request
  microdescriptors in any combination, the directory caches cannot
  pre-compress responses to these requests, and need to use less
  space-efficient on-the-fly compression algorithms.

  Under this proposal, clients simply say "Send me the
  microdescriptors I need", given what I know.

2. Combined microdescriptor downloads

2.1. By diff

  If a client has a consensus with base64 sha3-256 digest X, and it
  previously had a consensus with base64 sha3-256 digests Y then
  it may request all the microdescriptors listed in X but not Y,
  by asking for the resource:
      /tor/micro/diff/X/Y

  Clients SHOULD only ask for this resource compressed.

  Caches MUST NOT answer this request unless they recognize the
  consensus with digest X, and digest Y.

  If answering, caches MUST reply with all of the
  microdescriptors that the cache holds that were listed by
  consensus X, and MUST omit all the microdescriptors that were
  not listed in consensus Y.  (For the purposes of this proposal,
  microdescriptors are "the same" if they are textually identical
  and have the same digest.)

2.2. By consensus:

  If a client has fewer than NMNM% of the microdescriptors listed in a
  consensus X, it should fetch the resource
      /tor/micro/full/X

  Clients SHOULD only ask for this resource compressed.

  Caches MUST NOT answer this request unless they recognize the
  consensus with digest X. They should send all the microdescriptors
  they have that are listed in that consensus.

2.3. When to make these requests

  Clients should decide to use this format in preference to the old
  download-by-digest format if the consensus X lists their preferred
  directory cache as using a new DirCache subprotocol version. (See
  5 below.)

  When a client has some preferred directory caches that support
  this subprotocol and some that do not, it chooses one at random,
  and uses these requests if that one supports this subprotocol.

  (A client always has a consensus when it requests
  microdescriptors, so it will know whether any given cache supports
  these requests.)

3. Performance analysis

  This is a back-of-the-envelope analysis using a month's worth of
  consensus documents, and a randomly chosen sample of
  microdescriptors.


  On average, about 0.5% of the microdescriptors change between any
  two consensuses.  Call it 50.  That means 50*43 bytes == 2150
  bytes to request the microdescriptors.  It means ~24530 bytes of
  microdescriptors downloaded, compressed to ~13687 bytes by zstd.

  With this proposal, we're down to 86 bytes for the request, and we
  can precompute the compressed output, making it save to use lzma2,
  getting a compressed result more like 13362.

  It appears that this change would save about 15% for incremental
  microdescriptor downloads, most of that coming from the reduction
  in request size.

  For complete downloads, a complete set of microdescriptors is about
  7700 microdesciptors long.  That makes the total number of bytes
  for the requests 7700*43 == 331100 bytes.  The response, if
  compressed with lzma instead of zstd, would fall from 1659682 to
  1587804 bytes, for a total savings of 20%.


5. Compatibility

   Caches supporting this download protocol need to advertise
   support of a new DirCache subprotocol version.