path: root/proposals/259-guard-selection.txt

```
Filename: 259-guard-selection.txt
Title: New Guard Selection Behaviour
Author: Isis Lovecruft, George Kadianakis
Created: 2015-10-28
Status: Obsolete
Extends: 241-suspicious-guard-turnover.txt

This proposal was made obsolete by proposal #271.

§1. Overview

  In addition to the concerns regarding path bias attacks, namely that the
  space from which guards are selected by some specific client should not
  consist of the entirety of nodes with the Guard flag (cf. §1 of proposal
  #247), several additional concerns with respect to guard selection behaviour
  remain.  This proposal outlines a new entry guard selection algorithm, which
  additionally addresses the following concerns:

    - Heuristics and algorithms for determining how and which guard(s)
      is(/are) chosen should be kept as simple and easy to understand as
      possible.

    - Clients in censored regions or who are behind a fascist firewall who
      connect to the Tor network should not experience any significant
      disadvantage in terms of reachability or usability.

    - Tor should make a best attempt at discovering the most appropriate
      behaviour, with as little user input and configuration as possible.


§2. Design

  Alice, an OP attempting to connect to the Tor network, should undertake the
  following steps to determine information about the local network and to
  select (some) appropriate entry guards.  In the following scenario, it is
  assumed that Alice has already obtained a recent, valid, and verifiable
  consensus document.

  Before attempting the guard selection procedure, Alice initialises the guard
  data structures and prepopulates the guardlist structures, including the
  UTOPIC_GUARDLIST and DYSTOPIC_GUARDLIST (cf. §XXX).  Additionally, the
  structures have been designed to make updates efficient both in terms of
  memory and time, in order that these and other portions of the code which
  require an up-to-date guard structure are capable of obtaining such.

    0. Determine if the local network is potentially accessible.

       Alice should attempt to discover if the local network is up or down,
       based upon information such as the availability of network interfaces
       and configured routing tables.  See #16120. [0]

       [XXX: This section needs to be fleshed out more.  I'm ignoring it for
       now, but since others have expressed interest in doing this, I've added
       this preliminary step. —isis]

    1. Check that we have not already attempted to add too many guards
       (cf. proposal #241).

    2. Then, if the PRIMARY_GUARDS on our list are marked offline, the
       algorithm attempts to retry them, to ensure that they were not flagged
       offline erroneously when the network was down. This retry attempt
       happens only once every 20 mins to avoid infinite loops.

       [Should we do an exponential decay on the retry as s7r suggested? —isis]

    3. Take the list of all available and fitting entry guards and return the
       top one in the list.

    4. If there were no available entry guards, the algorithm adds a new entry
       guard and returns it.  [XXX detail what "adding" means]

    5. Go through the steps 1-4 above algorithm, using the UTOPIC_GUARDLIST.

       5.a. When the GUARDLIST_FAILOVER_THRESHOLD of the UTOPIC_GUARDLIST has
            been tried (without success), Alice should begin trying steps 1-4
            with entry guards from the DYSTOPIC_GUARDLIST as well.  Further,
            if no nodes from UTOPIC_GUARDLIST work, and it appears that the
            DYSTOPIC_GUARDLIST nodes are accessible, Alice should make a note
            to herself that she is possibly behind a fascist firewall.

       5.b. If no nodes from either the UTOPIC_GUARDLIST or the
            DYSTOPIC_GUARDLIST are working, Alice should make a note to
            herself that the network has potentially gone down.  Alice should
            then schedule, at exponentially decaying times, to rerun steps 0-5.
           
            [XXX Should we do step 0? Or just 1-4?  Should we retain any
            previous assumptions about FascistFirewall?  —isis]

    6. [XXX Insert potential other fallback mechanisms, e.g. switching to
       using bridges? —isis]


§3. New Data Structures, Consensus Parameters, & Configurable Variables

§3.1. Consensus Parameters & Configurable Variables

    Variables marked with an asterisk (*) SHOULD be consensus parameters.

    DYSTOPIC_GUARDS ¹ 
        All nodes listed in the most recent consensus which are marked with
        the Guard flag and which advertise their ORPort(s) on 80, 443, or any
        other addresses and/or ports controllable via the FirewallPorts and
        ReachableAddresses configuration options.

    UTOPIC_GUARDS
        All nodes listed in the most recent consensus which are marked with
        the Guard flag and which do NOT advertise their ORPort(s) on 80, 443,
        or any other addresses and/or ports controllable via the FirewallPorts
        and ReachableAddresses configuration options.

    PRIMARY_GUARDS * 
       The number of first, active, PRIMARY_GUARDS on either the
       UTOPIC_GUARDLIST or DYSTOPIC_GUARDLIST as "primary". We will go to
       extra lengths to ensure that we connect to one of our primary guards,
       before we fall back to a lower priority guard. By "active" we mean that
       we only consider guards that are present in the latest consensus as
       primary.

    UTOPIC_GUARDS_ATTEMPTED_THRESHOLD *
    DYSTOPIC_GUARDS_ATTEMPTED_THRESHOLD *
       These thresholds limit the amount of guards from the UTOPIC_GUARDS and
       DYSTOPIC_GUARDS which should be partitioned into a single
       UTOPIC_GUARDLIST or DYSTOPIC_GUARDLIST respectively.  Thus, this
       represents the maximum percentage of each of UTOPIC_GUARDS and
       DYSTOPIC_GUARDS respectively which we will attempt to connect to.  If
       this threshold is hit we assume that we are offline, filtered, or under
       a path bias attack by a LAN adversary.

       There are currently 1600 guards in the network.  We allow the user to
       attempt 80 of them before failing (5% of the guards).  With regards to
       filternet reachability, there are 450 guards on ports 80 or 443, so the
       probability of picking such a guard here should be high.

       This logic is not based on bandwidth, but rather on the number of
       relays which possess the Guard flag.  This is for three reasons: First,
       because each possible *_GUARDLIST is roughly equivalent to others of
       the same category in terms of bandwidth, it should be unlikely [XXX How
       unlikely? —isis] for an OP to select a guardset which contains less
       nodes of high bandwidth (or vice versa).  Second, the path-bias attacks
       detailed in proposal #241 are best mitigated through limiting the
       number of possible entry guards which an OP might attempt to use, and
       varying the level of security an OP can expect based solely upon the
       fact that the OP picked a higher number of low-bandwidth entry guards
       rather than a lower number of high-bandwidth entry guards seems like a
       rather cruel and unusual punishment in addition to the misfortune of
       already having slower entry guards.  Third, we favour simplicity in the
       redesign of the guard selection algorithm, and introducing bandwidth
       weight fraction computations seems like an excellent way to
       overcomplicate the design and implementation.
       

§3.2. Data Structures

    UTOPIC_GUARDLIST
    DYSTOPIC_GUARDLIST
        These lists consist of a subset of UTOPIC_GUARDS and DYSTOPIC_GUARDS
        respectively.  The guards in these guardlists are the only guards to
        which we will attempt connecting.

        When an OP is attempting to connect to the network, she will construct
        hashring structure containing all potential guard nodes from both
        UTOPIC_GUARDS and DYSTOPIC_GUARDS.  The nodes SHOULD BE inserted into
        the structure some number of times proportional to their consensus
        bandwidth weight. From this, the client will hash some information
        about themselves [XXX what info should we use? —isis] and, from that,
        choose #P number of points on the ring, where #P is
        {UTOPIC,DYSTOPIC}_GUARDLIST_ATTEMPTED_THRESHOLD proportion of the
        total number of unique relays inserted (if a duplicate is selected, it
        is discarded).  These selected nodes comprise the
        {UTOPIC,DYSTOPIC}_GUARDLIST for (first) entry guards.  (We say "first"
        in order to distinguish between entry guards and the vanguards
        proposed for hidden services in proposal #247.)

        [Perhaps we want some better terminology for this.  Suggestions
        welcome. —isis]

        Each GUARDLIST SHOULD have the property that the total sum of
        bandwidth weights for the nodes contained within it is roughly equal
        to each other guardlist of the same type (i.e. one UTOPIC_GUARDLIST is
        roughly equivalent in terms of bandwidth to another UTOPIC_GUARDLIST,
        but necessarily equivalent to a DYSTOPIC_GUARDLIST).

        For space and time efficiency reasons, implementations of the
        GUARDLISTs SHOULD support prepopulation(), update(), insert(), and
        remove() functions.  A second data structure design consideration is
        that the amount of "shifting" — that is, the differential between
        constructed hashrings as nodes are inserted or removed (read: ORs
        falling in and out of the network consensus) — SHOULD be minimised in
        order to reduce the resources required for hashring update upon
        receiving a newer consensus.

        The implementation we propose is to use a Consistent Hashring,
        modified to dynamically allocate replications in proportion to
        fraction of total bandwidth weight.  As with a normal Consistent
        Hashring, replications determine the number times the relay is
        inserted into the hashring.  The algorithm goes like this:

          router          ← ⊥
          key             ← 0
          replications    ← 0
          bw_weight_total ← 0
          while router ∈ GUARDLIST:
           | bw_weight_total ← bw_weight_total + BW(router)
          while router ∈ GUARDLIST:
           | replications ← FLOOR(CONSENSUS_WEIGHT_FRACTION(BW(router), bw_total) * T)
           | factor ← (S / replications)
           | while replications != 0:
           |  | key ← (TOINT(HMAC(ID)[:X] * replications * factor) mod S
           |  | INSERT(key, router)
           |  | replications <- replications - 1

        where:
 
          - BW is a function for extracting the value of an OR's `w bandwith=`
            weight line from the consensus,
          - GUARDLIST is either UTOPIC_GUARDLIST or DYSTOPIC_GUARDLIST,
          - CONSENSUS_WEIGHT_FRACTION is a function for computing a router's
            consensus weight in relation to the summation of consensus weights
            (bw_total),
          - T is some arbitrary number for translating a router's consensus
            weight fraction into the number of replications,
          - H is some collision-resistant hash digest,
          - S is the total possible hash space of H (e.g. for SHA-1, with
            digest sizes of 160 bits, this would be 2^160),
          - HMAC is a keyed message authentication code which utilises H,
          - ID is an hexadecimal string containing the hash of the router's
            public identity key,
          - X is some (arbitrary) number of bytes to (optionally) truncate the
            output of the HMAC to,
          - S[:X] signifies truncation of S, some array of bytes, to a
            sub-array containing X bytes, starting from the first byte and
            continuing up to and including the Xth byte, such that the
            returned sub-array is X bytes in length.
          - INSERT is an algorithm for inserting items into the hashring,
          - TOINT converts hexadecimal to decimal integers,
 
        For routers A and B, where B has a little bit more bandwidth than A,
        this gets you a hashring which looks like this:

                           B-´¯¯`-BA
                        A,`        `.
                        /            \
                       B|            |B
                        \            /
                         `.        ,´A
                          AB--__--´B
 
        When B disappears, A remains in the same positions:

                           _-´¯¯`-_A
                        A,`        `.
                        /            \
                        |            |
                        \            /
                         `.        ,´A
                          A`--__--´
                                
        And similarly if A disappears:

                           B-´¯¯`-B
                         ,`        `.
                        /            \
                       B|            |B
                        \            /
                         `.        ,´
                           B--__--´B
 
        Thus, no "shifting" problems, and recalculation of the hashring when a
        new consensus arrives via the update() function is much more time
        efficient.

        Alternatively, for a faster and simpler algorithm, but non-uniform
        distribution of the keys, one could remove the "factor" and replace
        the derivation of "key" in the algorithm above with:

                key ← HMAC(ID || replications)[:X]

        A reference implementation in Python is available². [1]


§4. Footnotes

¹ "Dystopic" was chosen because those are the guards you should choose from if
  you're behind a FascistFirewall.

² One tiny caveat being that the ConsistentHashring class doesn't dynamically
  assign replication count by bandwidth weight; it gets initialised with the
  number of replications.  However, nothing in the current implementation
  prevents you from doing:
      >>> h = ConsistentHashring('SuperSecureKey', replications=6)
      >>> h.insert(A)
      >>> h.replications = 23
      >>> h.insert(B)
      >>> h.replications = 42
      >>> h.insert(C)


§5. References

  [0]: https://trac.torproject.org/projects/tor/ticket/16120
  [1]: https://gitweb.torproject.org/user/isis/bridgedb.git/tree/bridgedb/hashring.py?id=949d33e8#n481


-*- coding: utf-8 -*-
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