Title: FlashFlow: A Secure Speed Test for Tor (Parent Proposal)
Author: Matthew Traudt, Aaron Johnson, Rob Jansen, Mike Perry
Created: 23 April 2020
Status: Draft

1. Introduction

FlashFlow is a new distributed bandwidth measurement system for Tor that consists of a single authority node ("coordinator") instructing one or more measurement nodes ("measurers") when and how to measure Tor relays. A measurement consists of the following steps:

  1. The measurement nodes demonstrate to the target relay permission to perform measurements.
  2. The measurement nodes open many TCP connections to the target relay and create a one-hop circuit to the target relay on each one.
  3. For 30 seconds the measurement nodes send measurement cells to the target relay and verify that the cells echoed back match the ones sent. During this time the relay caps the amount of background traffic it transfers. Background and measurement traffic are handled separately at the relay. Measurement traffic counts towards all the standard existing relay statistics.
  4. For every second during the measurement, the measurement nodes report to the authority node how much traffic was echoed back. The target relay also reports the amount of per-second background (non-measurement) traffic.
  5. The authority node sums the per-second reported throughputs into 30 sums (one for each second) and calculates the median. This is the estimated capacity of the relay.

FlashFlow performs a measurement of every relay according to a schedule described later in this document. Periodically it produces relay capacity estimates in the form of a v3bw file, which is suitable for direct consumption by a Tor directory authority. Alternatively an existing load balancing system such as Simple Bandwidth Scanner could be modified to use FlashFlow's v3bw file as input.

It is envisioned that each directory authority that wants to use FlashFlow will run their own FlashFlow deployment consisting of a coordinator that they run and one or more measurers that they trust (e.g. because they run them themselves), similar to how each runs their own Torflow/sbws. Section 5 of this proposal describes long term plans involving multiple FlashFlow deployments. FlashFlow coordinators do not need to communicate with each other.

FlashFlow is more performant than Torflow: FlashFlow takes 5 hours to measure the entire existing Tor network from scratch (with 3 Gbit/s measurer capacity) while Torflow takes 2 days; FlashFlow measures relays it hasn't seen recently as soon as it learns about them (i.e. every new consensus) while Torflow can take a day or more; and FlashFlow accurately measures new high-capacity relays the first time and every time while Torflow takes days/weeks to assign them their full fair share of bandwidth (especially for non-exits). FlashFlow is more secure than Torflow: FlashFlow allows a relay to inflate its measured capacity by up to 1.33x (configured by a parameter) while Torflow allows weight inflation by a factor of 89x [0] or even 177x [1].

After an overview in section 2 of the planned deployment stages, section 3, 4, and 5 discuss the short, medium, and long term deployment plans in more detail.

2. Deployment Stages

FlashFlow's deployment shall be broken up into three stages.

In the short term we will implement a working FlashFlow measurement system. This requires code changes in little-t tor and an external FlashFlow codebase. The majority of the implementation work will be done in the short term, and the product is a complete FlashFlow measurement system. Remaining pieces (e.g. better authentication) are added later for enhanced security and network performance.

In the medium term we will begin collecting data with a FlashFlow deployment. The intermediate results and v3bw files produced will be made available (semi?) publicly for study.

In the long term experiments will be performed to study ways of using FF v3bw files to improve load balancing. Two examples: (1) using FF v3bw files instead of sbws's (and eventually phasing out torflow/sbws), and (2) continuing to run sbws but use FF's results as a better estimate of relay capacity than observed bandwidth. Authentication and other FlashFlow features necessary to make it completely ready for full production deployment will be worked on during this long term phase.

3. FlashFlow measurement system: Short term

The core measurement mechanics will be implemented in little-t tor, but a separate codebase for the FlashFlow side of the measurement system will also be created. This section is divided into three parts: first a discussion of changes/additions that logically reside entirely within tor (essentially: relay-side modifications), second a discussion of the separate FlashFlow code that also requires some amount of tor changes (essentially: measurer-side and coordinator-side modifications), and third a security discussion.

3.1 Little-T Tor Components

The primary additions/changes that entirely reside within tor on the relay side:

  • New torrc options/consensus parameters.
  • New cell commands.
  • Pre-measurement handshaking (with a simplified authentication scheme).
  • Measurement mode, during which the relay will echo traffic with measurers, set a cap on the amount of background traffic it transfers, and report the amount of transferred background traffic.

3.1.1 Parameters

FlashFlow will require some consensus parameters/torrc options. Each has some default value if nothing is specified; the consensus parameter overrides this default value; the torrc option overrides both.

FFMeasurementsAllowed: A global toggle on whether or not to allow measurements. Even if all other settings would allow a measurement, if this is turned off, then no measurement is allowed. Possible values: 0,

  1. Default: 0 (disallowed).

FFAllowedCoordinators: The list of coordinator TLS certificate fingerprints that are allowed to start measurements. Relays check their torrc when they receive a connection from a FlashFlow coordinator to see if it's on the list. If they have no list, they check the consensus parameter. If nether exist, then no FlashFlow deployment is allowed to measure this relay. Default: empty list.

FFMeasurementPeriod: A relay should expect on average, to be measured by each FlashFlow deployment once each measurement period. A relay will not allow itself to be measured more than twice by a FlashFlow deployment in any time window of this length. Relays should not change this option unless they really know what they're doing. Changing it at the relay will not change how often FlashFlow will attempt to measure the relay. Possible values are in the range [1 hour, 1 month] inclusive. Default: 1 day.

FFBackgroundTrafficPercent: The maximum amount of regular non-measurement traffic a relay should handle while being measured, as a percent of total traffic (measurement + non-measurement). This parameter is a trade off between having to limit background traffic and limiting how much a relay can inflate its result by handling no background traffic but reporting that it has done so. Possible values are in the range [0, 99] inclusive. Default: 25 (a maximum inflation factor of 1.33).

FFMaxMeasurementDuration: The maximum amount of time, in seconds, that is allowed to pass from the moment the relay is notified that a measurement will begin soon and the end of the measurement. If this amount of time passes, the relay shall close all measurement connections and exit its measurement mode. Note this duration includes handshake time, thus it necessarily is larger than the expected actual measurement duration. Possible values are in the range [10, 120] inclusive. Default: 45.

3.1.2 New Cell Types

FlashFlow will introduce a new cell command MEASUREMENT.

The payload of each MEASUREMENT cell consists of:

Measure command [1 byte]
Data            [varied]

The measure commands are:

0 -- MEAS_PARAMS    [forward]
1 -- MEAS_PARAMS_OK [backward]
2 -- MEAS_BG        [backward]
3 -- MEAS_ERR       [forward and backward]

Forward cells are sent from the measurer/coordinator to the relay. Backward cells are sent from the relay to the measurer/coordinator.

MEAS_PARAMS and MEAS_PARAMS_OK are used during the pre-measurement stage to tell the target what to expect and for the relay to positively acknowledge the message. The target send a MEAS_BG cell once per second to report the amount of background traffic it is handling. MEAS_ERR cells are used to signal to the other party that there has been some sort of problem and that the measurement should be aborted. These measure commands are described in more detail in the next section.

FlashFlow also introduces a new relay command, MEAS_ECHO. Relay celsl with this relay command are the measurement traffic. The measurer generates and encrypts them, sends them to the target, the target decrypts them, then it sends them back. A variation where the measurer skips encryption of MEAS_ECHO cells in most cases is described in Appendix A, and was found to be necessary in paper prototypes to save CPU load at the measurer.

MEASUREMENT cells, on the other hand, are not encrypted (beyond the regular TLS on the connection).

3.1.3 Pre-Measurement Handshaking/Starting a Measurement

The coordinator establishes a one-hop circuit with the target relay and sends it a MEAS_PARAMS cell. If the target is unwilling to be measured at this time or if the coordinator didn't use a TLS certificate that the target trusts, it responds with an error cell and closes the connection. Otherwise it checks that the parameters of the measurement are acceptable (e.g. the version is acceptable, the duration isn't too long, etc.). If the target is happy, it sends a MEAS_PARAMS_OK, otherwise it sends a MEAS_ERR and closes the connection.

Upon learning the IP addresses of the measurers from the coordinator in the MEAS_PARAMS cell, the target whitelists their IPs in its DoS detection subsystem until the measurement ends (successfully or otherwise), at which point the whitelist is cleared.

Upon receiving a MEAS_PARAMS_OK from the target, the coordinator will instruct the measurers to open their circuits (one circuit per connection) with the target. If the coordinator or any measurer receives a MEAS_ERR, it reports the error to the coordinator and considers the measurement a failure. It is also a failure if any measurer is unable to open at least half of its circuits with the target.

The payload of MEAS_PARAMS cells [XXX more may need to be added]:

- meas_duration [2 bytes] [1, 600]
- num_measurers [1 byte] [1, 10]
- measurer_info [num_measurers times]

meas_duration is the duration, in seconds, that the actual measurement will last. num_measurers is how many link_specifier structs follow containing information on the measurers that the relay should expect. Future versions of FlashFlow and MEAS_PARAMS will use TLS certificates instead of IP addresses. [XXX probably need diff layout to allow upgrade to TLS certs instead of link_specifier structs. probably using ext-type-length-value like teor suggests] [XXX want to specify number of conns to expect from each measurer here?]

MEAS_PARAMS_OK has no payload: it's just padding bytes to make the cell PAYLOAD_LEN (509) bytes long.

The payload of MEAS_ECHO cells:

- arbitrary bytes [PAYLOAD_LEN bytes]

The payload of MEAS_BG cells [XXX more for extra info? like CPU usage]:

- second        [2 byte] [1, 600]
- sent_bg_bytes [4 bytes] [0, 2^32-1]
- recv_bg_bytes [4 bytes] [0, 2^32-1]

second is the number of seconds since the measurement began. MEAS_BG cells are sent once per second from the relay to the FlashFlow coordinator. The first cell will have this set to 1, and each subsequent cell will increment it by one. sent_bg_bytes is the number of background traffic bytes sent in the last second (since the last MEAS_BG cell). recv_bg_bytes is the same but for received bytes.

The payload of MEAS_ERR cells [XXX need field for more info]:

- err_code [1 byte] [0, 255]

The error code is one of:

[... XXX TODO ...]
255 -- OTHER

3.1.4 Measurement Mode

The relay considers the measurement to have started the moment it receives the first MEAS_ECHO cell from any measurer. At this point, the relay

  • Starts a repeating 1s timer on which it will report the amount of background traffic to the coordinator over the coordinator's connection.
  • Enters "measurement mode" and limits the amount of background traffic it handles according to the torrc option/consensus parameter.

The relay decrypts and echos back all MEAS_ECHO cells it receives on measurement connections until it has reported its amount of background traffic the same number of times as there are seconds in the measurement (e.g. 30 per-second reports for a 30 second measurement). After sending the last MEAS_BG cell, the relay drops all buffered MEAS_ECHO cells, closes all measurement connections, and exits measurement mode.

During the measurement the relay targets a ratio of background traffic to measurement traffic as specified by a consensus parameter/torrc option. For a given ratio r, if the relay has handled x cells of measurement traffic recently, Tor then limits itself to y = xr/(1-r) cells of non-measurement traffic this scheduling round. If x is very small, the relay will perform the calculation s.t. x is the number of cells required to produce 10 Mbit/s of measurement traffic, thus ensuring some minimum amount of background traffic is allowed.

[XXX teor suggests in [4] that the number 10 Mbit/s could be derived more intelligently. E.g. based on AuthDirFastGuarantee or AuthDirGuardBWGuarantee]

3.2 FlashFlow Components

The FF coordinator and measurer code will reside in a FlashFlow repository separate from little-t tor.

There are three notable parameters for which a FF deployment must choose values. They are:

  • The number of sockets, s, the measurers should open, in aggregate, with the target relay. We suggest s=160 based on the FF paper.
  • The bandwidth multiplier, m. Given an existing capacity estimate for a relay, z, the coordinator will instruct the measurers to, in aggregate, send m*z Mbit/s to the target relay. We recommend m=2.25.
  • The measurement duration, d. Based on the FF paper, we recommend d=30 seconds.

The rest of this section first discusses notable functions of the FlashFlow coordinator, then goes on to discuss FF measurer code that will require supporting tor code.

3.2.1 FlashFlow Coordinator

The coordinator is responsible for scheduling measurements, aggregating results, and producing v3bw files. It needs continuous access to new consensus files, which it can obtain by running an accompanying Tor process in client mode.

The coordinator has the following functions, which will be described in this section:

  • result aggregation.
  • schedule measurements.
  • v3bw file generation. Aggregating Results

Every second during a measurement, the measurers send the amount of verified measurement traffic they have received back from the relay. Additionally, the relay sends a MEAS_BG cell each second to the coordinator with amount of non-measurement background traffic it is sending and receiving.

For each second's reports, the coordinator sums the measurer's reports. The coordinator takes the minimum of the relay's reported sent and received background traffic. If, when compared to the measurer's reports for this second, the relay's claimed background traffic is more than what's allowed by the background/measurement traffic ratio, then the coordinator further clamps the relay's report down. The coordinator adds this final adjusted amount of background traffic to the sum of the measurer's reports.

Once the coordinator has done the above for each second in the measurement (e.g. 30 times for a 30 second measurement), the coordinator takes the median of the 30 per-second throughputs and records it as the estimated capacity of the target relay. Measurement Schedule

The short term implementation of measurement scheduling will be simpler than the long term one due to (1) there only being one FlashFlow deployment, and (2) there being very few relays that support being measured by FlashFlow. In fact the FF coordinator will maintain a list of the relays that have updated to support being measured and have opted in to being measured, and it will only measure them.

The coordinator divides time into a series of 24 hour periods, commonly referred to as days. Each period has measurement slots that are longer than a measurement lasts (30s), say 60s, to account for pre- and post-measurement work. Thus with 60s slots there's 1,440 slots in a day.

At the start of each day the coordinator considers the list of relays that have opted in to being measured. From this list of relays, it repeatedly takes the relay with the largest existing capacity estimate. It selects a random slot. If the slot has existing relays assigned to it, the coordinator makes sure there is enough additional measurer capacity to handle this relay. If so, it assigns this relay to this slot. If not, it keeps picking new random slots until one has sufficient additional measurer capacity.

Relays without existing capacity estimates are assumed to have the 75th percentile capacity of the current network.

If a relay is not online when it's scheduled to be measured, it doesn't get measured that day. Example

Assume the FF deployment has 1 Gbit/s of measurer capacity. Assume the chosen multiplier m=2. Assume there are only 5 slots in a measurement period.

Consider a set of relays with the following existing capacity estimates and that have opted in to being measured by FlashFlow.

  • 500 Mbit/s
  • 300 Mbit/s
  • 250 Mbit/s
  • 200 Mbit/s
  • 100 Mbit/s
  • 50 Mbit/s

The coordinator takes the largest relay, 500 Mbit/s, and picks a random slot for it. It picks slot 3. The coordinator takes the next largest, 300, and randomly picks slot 2. The slots are now:

   0   |   1   |   2   |   3   |   4
       |       |  300  |  500  |
       |       |       |       |

The coordinator takes the next largest, 250, and randomly picks slot 2. Slot 2 already has 600 Mbit/s of measurer capacity reserved (300*m); given just 1000 Mbit/s of total measurer capacity, there is just 400 Mbit/s of spare capacity while this relay requires 500 Mbit/s. There is not enough room in slot 2 for this relay. The coordinator picks a new random slot, 0.

   0   |   1   |   2   |   3   |   4
  250  |       |  300  |  500  |
       |       |       |       |

The next largest is 200 and the coordinator randomly picks slot 2 again (wow!). As there is just enough spare capacity, the coordinator assigns this relay to slot 2.

   0   |   1   |   2   |   3   |   4
  250  |       |  300  |  500  |
       |       |  200  |       |

The coordinator randomly picks slot 4 for the last remaining relays, in that order.

   0   |   1   |   2   |   3   |   4
  250  |       |  300  |  500  |  100
       |       |  200  |       |   50 Generating V3BW files

Every hour the FF coordinator produces a v3bw file in which it stores the latest capacity estimate for every relay it has measured in the last week. The coordinator will create this file on the host's local file system. Previously-generated v3bw files will not be deleted by the coordinator. A symbolic link at a static path will always point to the latest v3bw file.

$ ls -l
v3bw -> v3bw.2020-03-01-05-00-00

[XXX Either FF should auto-delete old ones, logrotate config should be provided, a script provided, or something to help bwauths not accidentally fill up their disk]

[XXX What's the approxmiate disk usage for, say, a few years of these?]

3.2.2 FlashFlow Measurer

The measurers take commands from the coordinator, connect to target relays with many sockets, send them traffic, and verify the received traffic is the same as what was sent.

Notable new things that internal tor code will need to do on the measurer (client) side:

  1. Open many TLS+TCP connections to the same relay on purpose. Open many connections

FlashFlow prototypes needed to "hack in" a flag in the open-a-connection-with-this-relay function call chain that indicated whether or not we wanted to force a new connection to be created. Most of Tor doesn't care if it reuses an existing connection, but FF does want to create many different connections. The cleanest way to accomplish this will be investigated.

On the relay side, these measurer connections do not count towards DoS detection algorithms.

3.3 Security

In this section we discuss the security of various aspects of FlashFlow and the tor changes it requires.

3.3.1 Weight Inflation

Target relays are an active part of the measurement process; they know they are getting measured. While a relay cannot fake the measurement traffic, it can trivially stop transferring client background traffic for the duration of the measurement yet claim it carried some. More generally, there is no verification of the claimed amount of background traffic during the measurement. The relay can claim whatever it wants, but it will not be trusted above the ratio the FlashFlow deployment is configured to know. This places an easy to understand, firm, and (if set as we suggest) low cap on how much a relay can inflate its measured capacity.

Consider a background/measurement ratio of 1/4, or 25%. Assume the relay in question has a hard limit on capacity (e.g. from its NIC) of 100 Mbit/s. The relay is supposed to use up to 25% of its capacity for background traffic and the remaining 75%+ capacity for measurement traffic. Instead the relay ceases carrying background traffic, uses all 100 Mbit/s of capacity to handle measurement traffic, and reports ~33 Mbit/s of background traffic (33/133 = ~25%). FlashFlow would trust this and consider the relay capable of 133 Mbit/s. (If the relay were to report more than ~33 Mbit/s, FlashFlow limits it to just ~33 Mbit/s.) With r=25%, FlashFlow only allows 1.33x weight inflation.

Prior work shows that Torflow allows weight inflation by a factor of 89x [0] or even 177x [1].

The ratio chosen is a trade-off between impact on background traffic and security: r=50% allows a relay to double its weight but won't impact client traffic for relays with steady state throughput below 50%, while r=10% allows a very low inflation factor but will cause throttling of client traffic at far more relays. We suggest r=25% (and thus 1/(1-0.25)=1.33x inflation) for a reasonable trade-off between performance and security.

It may be possible to catch relays performing this attack, especially if they literally drop all background traffic during the measurement: have the measurer (or some party on its behalf) create a regular stream through the relay and measure the throughput on the stream before/during/after the measurement. This can be explored longer term.

3.3.2 Incomplete Authentication

The short term FlashFlow implementation has the relay set two torrc options if they would like to allow themselves to be measured: a flag allowing measurement, and the list of coordinator TLS certificate that are allowed to start a measurement.

The relay drops MEAS_PARAMS cells from coordinators it does not trust, and immediately closes the connection after that. A FF coordinator cannot convince a relay to enter measurement mode unless the relay trusts its TLS certificate.

A trusted coordinator specifies in the MEAS_PARAMS cell the IP addresses of the measurers the relay shall expect to connect to it shortly. The target adds the measurer IP addresses to a whitelist in the DoS connection limit system, exempting them from any configured connection limit. If a measurer is behind a NAT, an adversary behind the same NAT can DoS the relay's available sockets until the end of the measurement. The adversary could also pretend to be the measurer. Such an adversary could induce measurement failures and inaccuracies. (Note: the whitelist is cleared after the measurement is over.)

4. FlashFlow measurement system: Medium term

The medium term deployment stage begins after FlashFlow has been implemented and relays are starting to update to a version of Tor that supports it.

New link- and relay-subprotocol versions will be used by the relay to indicate FF support. E.g. at the time of writing, the next relay subprotocol version is 4 [3].

We plan to host a FlashFlow deployment consisting of a FF coordinator and a single FF measurer on a single 1 Gbit/s machine. Data produced by this deployment will be made available (semi?) publicly, including both v3bw files and intermediate results.

Any development changes needed during this time would go through separate proposals.

5. FlashFlow measurement system: Long term

In the long term, finishing-touch development work will be done, including adding better authentication and measurement scheduling, and experiments will be run to determine the best way to integrate FlashFlow into the Tor ecosystem.

Any development changes needed during this time would go through separate proposals.

5.1 Authentication to Target Relay

Short term deployment already had FlashFlow coordinators using TLS certificates when connecting to relays, but in the long term, directory authorities will vote on the consensus parameter for which coordinators should be allowed to perform measurements. The voting is done in the same way they currently vote on recommended tor versions.

FlashFlow measurers will be updated to use TLS certificates when connecting to relays too. FlashFlow coordinators will update the contents of MEAS_PARAMS cells to contain measurer TLS certificates instead of IP addresses, and relays will update to expect this change.

5.2 Measurement Scheduling

Short term deployment only has one FF deployment running. Long term this may no longer be the case because, for example, more than one directory authority decides to adopt it and they each want to run their own deployment. FF deployments will need to coordinate between themselves to not measure the same relay at the same time, and to handle new relays as they join during the middle of a measurement period (during the day).

The measurement scheduling process shall be non-interactive. All the inputs (e.g. the shared random value, the identities of the coords, the relays currently in the network) are publicly known to (at least) the bwauths, thus each individual bwauth can calculate same multi-coord measurement schedule.

The following is quoted from Section 4.3 of the FlashFlow paper.

To measure all relays in the network, the BWAuths periodically
determine the measurement schedule. The schedule determines when and
by whom a relay should be measured. We assume that the BWAuths have
sufficiently synchronized clocks to facilitate coordinating their
schedules. A measurement schedule is created for each measurement
period, the length p of which determines how often a relay is
measured. We use a measurement period of p = 24 hours.

To help avoid active denial-of-service attacks on targeted relays,
the measurement schedule is randomized and known only to the
BWAuths. Before the next measurement period starts, the BWAuths
collectively generate a random seed (e.g. using Tor’s
secure-randomness protocol). Each BWAuth can then locally determine
the shared schedule using pseudorandom bits extracted from that
seed. The algorithm to create the schedule considers each
measurement period to be divided into a sequence of t-second
measurement slots. For each old relay, slots for each BWAuth to
measure it are selected uniformly at random without replacement
from all slots in the period that have sufficient unallocated
measurement capacity to accommodate the measurement. When a new
relay appears, it is measured separately by each BWAuth in the first
slots with sufficient unallocated capacity. Note that this design
ensures that old relays will continue to be measured, with new
relays given secondary priority in the order they arrive.

[XXX Teor leaves good ideas in his tor-dev@ post [5], including a good plain language description of what the FF paper quotes says, and a recommendation on which consensus to use when making a new schedule]

A problem arises when two relays are hosted on the same machine but measured at different times: they both will be measured to have the full capacity of their host. At the very least, the scheduling algo should schedule relays with the same IP to be measured at the same time. Perhaps better is measuring all relays in the same MyFamily, same ipv4/24, and/or same ipv6/48 at the same time. What specifically to do here is left for medium/long term work.

5.3 Experiments

[XXX todo]

5.4 Other Changes/Investigations/Ideas

  • How can FlashFlow data be used in a way that doesn't lead to poor load balancing given the following items that lead to non-uniform client behavior:
    • Guards that high-traffic HSs choose (for 3 months at a time)
    • Guard vs middle flag allocation issues
    • New Guard nodes (Guardfraction)
    • Exit policies other than default/all
    • Directory activity
    • Total onion service activity
    • Super long-lived circuits
  • Add a cell that the target relay sends to the coordinator indicating its CPU and memory usage, whether it has a shortage of sockets, how much bandwidth load it has been experiencing lately, etc. Use this information to lower a relays weight, never increase.
  • If FlashFlow and sbws work together (as opposed to FlashFlow replacing sbws), consider logic for how much sbws can increase/decrease FF results
  • Coordination of multiple FlashFlow deployments: scheduling of measurements, seeding schedule with shared random value.
  • Other background/measurement traffic ratios. Dynamic? (known slow relay => more allowed bg traffic?)
  • Catching relays inflating their measured capacity by dropping background traffic.
  • What to do about co-located relays. Can they be detected reliably? Should we just add a torrc option a la MyFamily for co-located relays?
  • What is the explanation for dennis.jackson's scary graphs in this [2] ticket? Was it because of the speed test? Why? Will FlashFlow produce the same behavior?

Appendix A: Save CPU at measurer by not encrypting all MEAS_ECHO cells

Verify echo cells

A parameter will exist to tell the measurers with what frequency they shall verify that cells echoed back to them match what was sent. This parameter does not need to exist outside of the FF deployment (e.g. it doesn't need to be a consensus parameter).

The parameter instructs the measurers to check 1 out of every N cells.

The measurer keeps a count of how many measurement cells it has sent. It also logically splits its output stream of cells into buckets of size N. At the start of each bucket (when num_sent % N == 0), the measurer chooses a random index in the bucket. Upon sending the cell at that index (num_sent % N == chosen_index), the measurer records the cell.

The measurer also counts cells that it receives. When it receives a cell at an index that was recorded, it verifies that the received cell matches the recorded sent cell. If they match, no special action is taken. If they don't match, the measurer indicates failure to the coordinator and target relay and closes all connections, ending the measurement.


Consider bucket_size is 1000. For the moment ignore cell encryption.

We start at idx=0 and pick an idx in [0, 1000) to record, say 640. At idx=640 we record the cell. At idx=1000 we choose a new idx in [1000, 2000) to record, say 1236. At idx=1236 we record the cell. At idx=2000 we choose a new idx in [2000, 3000). Etc.

There's 2000+ cells in flight and the measurer has recorded two items:

- (640, contents_of_cellA)
- (1236, contents_of_cellB)

Consider the receive side now. It counts the cells it receives. At receive idx=640, it checks the received cell matches the saved cell from before. At receive idx=1236, it again checks the received cell matches. Etc.


A malicious relay may want to skip decryption of measurement cells to save CPU cycles and obtain a higher capacity estimate. More generally, it could generate fake measurement cells locally, ignore the measurement traffic it is receiving, and flood the measurer with more traffic that it (the measurer) is even sending.

The security of echo cell verification is discussed in section 3.3.1.


A smaller bucket size means more cells are checked and FF is more likely to detect a malicious target. It also means more bookkeeping overhead (CPU/RAM).

An adversary that knows bucket_size and cheats on one item out of every bucket_size items will have a 1/bucket_size chance of getting caught in the first bucket. This is the worst case adversary. While cheating on just a single item per bucket yields very little advantage, cheating on more items per bucket increases the likelihood the adversary gets caught. Thus only the worst case is considered here.

In general, the odds the adversary can successfully cheat in a single bucket are


Thus the odds the adversary can cheat in X consecutive buckets are


In our case, X will be highly varied: Slow relays won't see very many buckets, but fast relays will. The damage to the network a very slow relay can do by faking being only slightly faster is limited. Nonetheless, for now we motivate the selection of bucket_size with a slow relay:

  • Assume a very slow relay of 1 Mbit/s capacity that will cheat 1 cell in each bucket. Assume a 30 second measurement.
  • The relay will handle 1*30 = 30 Mbit of traffic during the measurement, or 3.75 MB, or 3.75 million bytes.
  • Cells are 514 bytes. Approximately (e.g. ignoring TLS) 7300 cells will be sent/recv over the course of the measurement.
  • A bucket_size of 50 results in about 146 buckets over the course of the 30s measurement.
  • Therefore, the odds of the adversary cheating successfully as (49/50)^(146), or about 5.2%.

This sounds high, but a relay capable of double the bandwidth (2 Mbit/s) will have (49/50)^(2*146) or 0.2% odds of success, which is quite low.

Wanting a <1% chance that a 10 Mbit/s relay can successfully cheat results in a bucket size of approximately 125:

  • 10*30 = 300 Mbit of traffic during 30s measurement. 37.5 million bytes.
  • 37,500,000 bytes / 514 bytes/cell = ~73,000 cells
  • bucket_size of 125 cells means 73,000 / 125 = 584 buckets
  • (124/125)^(584) = 0.918% chance of successfully cheating

Slower relays can cheat more easily but the amount of extra weight they can obtain is insignificant in absolute terms. Faster relays are essentially unable to cheat.