draft proposal: download server descriptors on demand

[Peter Palfrader]

Hi,

this is my working draft for the just in time download of SDs proposal.
It's still rough, it has a few easier things not speced out, like how we
want to convey load balancing weights in the consensus, and doesn't yet
contain much useful about how we will do exit selection.

Nick, if I could get a proposal number for this I could edit it in svn.

-- 
weasel
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Filename: xxx-jit-sd-downloads
Title: Download server descriptors on demand
Version: $Revision$
Last-Modified: $Date$
Author: Peter Palfrader
Created: 15-Jun-2008
Status: Draft

1. Overview

  Downloading all server descriptors is the most expensive part
  of bootstrapping a Tor client.  These server descriptors currently
  amount to about 1.5 Megabytes of data, and this size will grow
  linearly with network size.

  Fetching all these server descriptors takes a long while for people
  behind slow network connections.  It is also a considerable load on
  our network of directory mirrors.

  This document describes proposed changes to the Tor network and
  directory protocol so that clients will no longer need to download
  all server descriptors.

  These changes consist of moving load balancing information into
  network status documents, implementing a means to download server
  descriptors on demand in an anonymity-preserving way, and dealing
  with exit node selection.

2. What is in a server descriptor

  When a Tor client starts the first thing it will try to get is a
  current network status document, a consensus signed by a majority
  of directory authorities.  This document is currently about 100
  Kilobytes in size, tho it will grow linearly with network size.
  This document lists all servers currently running on the network.
  The Tor client will then try to get a server descriptor for each
  of the running servers.  All server descriptors currently amount
  to about 1.5 Metabytes of downloads.

  A Tor client learns several things about a server from its descriptor.
  Some of these it already learned from the network status document
  published by the authorities, but the server descriptor contains it
  again in a single statement signed by the server itself, not just by
  the directory authorities.

  Tor clients use the information from server descriptors for
  different purposes, which are considered in the following sections.

  #three ways:  One, to determine if a server will be able to handle
  #this client's request; two, to actually communicate or use the server;
  #three, for load balancing decisions.
  #
  #These three points are considered in the following subsections.

2.1 Load balancing

  The Tor load balancing mechanism is quite complex in its details, but
  it has a simple goal: The more traffic a server can handle the more
  traffic it should get.  That means the more traffic a server can
  handle the more likely a client will use it.

  For this purpose each server descriptor has bandwidth information
  which tries to convey a server's capacity to clients.

  Currently we weigh servers differently for different purposes.  There
  is a weigh for when we use a server as a guard node (our entry to the
  Tor network), there is one weigh we assign servers for exit duties,
  and a third for when we need intermediate (middle) nodes.

2.2 Exit information

  When a Tor wants to exit to some resource on the internet it will
  build a circuit to an exit node that allows access to that resource's
  IP address and TCP Port.

  When building that circuit the client can make sure that the circuit
  ends at a server that will be able to fulfill the request because the
  client already learned of all the servers' exit policies from their
  descriptors.

2.3 Capability information

  Server descriptors contain information about the specific version or
  the Tor protocol they understand [proposal 105].

  Furthermore the server descriptor also contains the exact version of
  the Tor software that the server is running and some decisions are
  made based on the server version number (for instance a Tor client
  will only make conditional consensus requests [proposal from 13 Apr
  2008 that never got a number] when talking to Tor servers version
  0.2.1.1-alpha or later).

2.4 Contact/key information

  A server descriptor lists a server's IP address and TCP ports on which
  it accepts onion and directory connections.  Furthermore it contains
  the onion key, a short lived RSA key to which clients encrypt CREATE
  cells.

2.5 Identity information

  A Tor client learns the digest of a server's key from the network
  status document.  Once it has a server descriptor this descriptor
  contains the full RSA identity key of the server.  Clients verify
  that 1) the digest of the identity key matches the expected digest
  it got from the consensus, and 2) that the signature on the descriptor
  from that key is valid.


3. Doing away with the need for all SDs

3.1 Load balancing info in consensus documents

  One of the reasons why clients download all server descriptors is for
  doing load proper load balancing as described in 2.1.  In order for
  clients to not require all server descriptors this information will
  have to move into the network status document.

  [XXX Two open questions here:
   a) how do we arrive at a consensus weight?
   b) how to represent weights in the consensus?
      Maybe "s Guard=0.13 Exit=0.02 Middle=0.00 Stable.."
  ]

3.2 Fetching descriptors on demand

  As described in 2.4 a descriptor lists IP address, OR- and Dir-Port,
  and the onion key for a server.

  A client already knows the IP address and the ports from the consensus
  documents, but without the onion key it will not be able to send
  CREATE/EXTEND cells for that server.  Since the client needs the onion
  key it needs the descriptor.

  If a client only downloaded a few descriptors in an observable manner
  then that would leak which nodes it was going to use.

  This proposal suggests the following:

  1) when connecting to a guard node for which the client does not
     yet have a cached descriptor it requests the descriptor it
     expects by hash.  (The consensus document that the client holds
     has a hash for the descriptor of this server.  We want exactly
     that descriptor, not a different one.)

     [XXX: How?  We could either come up with a new cell type,
      RELAY_REQUEST_SD that takes only a hash (of the SD), or use
      RELAY_BEGIN_DIR.  The former is probably smarter since we will
      want to use it later on as well, and there we will require
      padding.]

     A client MAY cache the descriptor of the guard node so that it does
     not need to request it every single time it contacts the guard.

  2) when a client wants to extend a circuit that currently ends in
     server B to a new next server C, the client will send a
     RELAY_REQUEST_SD cell to server B.  This cell contains in its
     payload the hash of a server descriptor the client would like
     to obtain (C's server descriptor).  The server sends back the
     descriptor and the client can now form a valid EXTEND/CREATE cell
     encrypted to C's onion key.

     Clients MUST NOT cache such descriptors.  If they did they might
     leak that they already extended to that server at least once
     before.

  Replies to RELAY_REQUEST_SD requests need to be padded to some
  constant upper limit in order to conceal a client's destination
  from anybody who might be counting cells/bytes.

  [XXX: detailed spec of RELAY_REQUEST_SD cell and its reply]
  [XXX: figure out a decent padding size]

3.3 Protocol versions

  [XXX: find out where we need "opt protocols Link 1 2 Circuit 1"
  information described in 2.3 above.  If we need it, it might have
  to go into the consensus document.]

  [XXX: Similarly find out where we need the version number of a
  remote tor server.  This information is in the consensus, but
  maybe we use it in some place where having it signed by the
  server in question is really important?]

3.4 Exit selection

  Currently finding an appropriate exit node for a user's request is
  easy for a client because it has complete knowledge of all the exit
  policies of all servers on the network.

  [XXX: I have no finished ideas here yet.
    - if clients only rely on the current exit flag they will
      a) never use servers for exit purposes that don't have it,
      b) will have a hard time finding a suitable exit node for
         their weird port that only a few servers allow.
    - the authorities could create a new summary document that
      lists all the exit policies and their nodes (by fingerprint).
      I need to find out how large that document would be.
    - can we make the "Exit" flag more useful?  can we come
      up with some "standard policies" and have operators pick
      one of the standards?
  ]

4. Future possibilities

  This proposal still requires that all servers have the descriptors of
  every other node in the network in order to answer RELAY_REQUEST_SD
  cells.  These cells are sent when a circuit is extended from ending at
  node B to a new node C.  In that case B would have to answer a
  RELAY_REQUEST_SD cell that asks for C's server descriptor (by SD digest).

  In order to answer that request B obviously needs a copy of C's server
  descriptor.  In the future we might amend RELAY_REQUEST_SD cells to
  contain also the expected IP address and OR-port of the server C (the
  client learns them from the network status document), so that B no
  longer needs to know all the descriptors of the entire network but
  instead can simply go and ask C for its descriptor before passing it
  back to the client.