Hello Isis, thanks for the proposal. Looks good! I think adding the logic for greater reachability of people behind FascistFirewalls makes sense and will be very helpful. Some comments on the proposal inlined below: ----

Filename: xxx-guard-selection.txt Title: New Guard Selection Behaviour Author: Isis Lovecruft, George Kadianakis

Thanks for adding me as a co-author! Would also be great if you could link to my original post for reference: https://lists.torproject.org/pipermail/tor-dev/2015-August/009328.html

Created: 2015-10-28 Status: Draft Extends: 241-suspicious-guard-turnover.txt

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§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).

I'd suggest you extract all the functionality and variables you need from prop#241 and incorporate them to your proposal. Otherwise it's not immediately obvious how the two proposals interact, and which parts of #prop241 are still active. Personally, I think I only used CANDIDATE_THRESHOLD from #prop241 which I renamed to GUARDS_ATTEMPTED_THRESHOLD.

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.

It's interesting that these two sets DYSTOPIC_GUARDS and UTOPIC_GUARDS are disjoint. This means that there will be no 80/443 relays on the UTOPIC guardlist. This means that 80/443 guards will only be used by people under FascistFirewall; which makes them a bit like bridges in a way, and also has significant effects on our load balancing. Are we sure we want these two sets to be disjoint? I could imagine an alternative design where we still have the two guard lists, but we also allow 80/443 relays to be in UTOPIC_GUARDS. If you were lucky and you had 80/443 guards in your UTOPIC guard list you don't need to go down to the DYSTOPIC guard list. Or something like this.

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 guard here should be high.

oops "guard guard"

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.)

I don't entirely understand why we prefer a hash ring over a simple list here for sampling guards. I was imagining that when we need a new guard, we would just put all guards in a list, and sample a random guard weighted by bandwidth. I think this is what the current code is doing in smartlist_choose_node_by_bandwidth_weights() and it seems easy!

[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).

Why is it important for guard lists to have about the same total bandwidth? Also, will this be enforced somehow, or is it a result of how the hash ring is designed?

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

These operations sound good!

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.

Why is shifting-resistance important here? I'm not sure what you mean "reduce the resources required for hashring update". This is something that happens about once every hour right?

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 convert 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 B disappears:

Hm, maybe you mean "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....