path: root/proposals/329-traffic-splitting.txt

Filename: 329-traffic-splitting.txt
Title: Overcoming Tor's Bottlenecks with Traffic Splitting
Author: David Goulet, Mike Perry
Created: 2020-11-25
Status: Draft

0. Status

  This proposal describes the Conflux [CONFLUX] system developed by
  Mashael AlSabah, Kevin Bauer, Tariq Elahi, and Ian Goldberg. It aims at
  improving Tor client network performance by dynamically splitting
  traffic between two circuits.


1. Overview

1.1. Multipath TCP Design Space

  In order to understand our improvements to Conflux, it is important to
  properly conceptualize what is involved in the design of multipath
  algorithms in general.
  
  The design space is broken into two orthogonal parts: congestion control
  algorithms that apply to each path, and traffic scheduling algorithms
  that decide which packets to send on each path.
  
  MPTCP specifies 'coupled' congestion control (see [COUPLED]). Coupled
  congestion control updates single-path congestion control algorithms to
  account for shared bottlenecks between the paths, so that the combined
  congestion control algorithms do not overwhelm any bottlenecks that
  happen to be shared between the multiple paths. Various ways of
  accomplishing this have been proposed and implemented in the Linux
  kernel.
  
  Because Tor's congestion control only concerns itself with bottlenecks in
  Tor relay queues, and not with any other bottlenecks (such as
  intermediate Internet routers), we can avoid this complexity merely by
  specifying that any paths that are constructed SHOULD NOT share any
  relays. In this way, we can proceed to use the exact same congestion
  control as specified in Proposal 324, for each path.
  
  For this reason, this proposal will focus on the traffic scheduling
  algorithms, rather than coupling. We propose three candidate algorithms
  that have been studied in the literature, and will compare their
  performance using simulation and consensus parameters.

1.2. Divergence from the initial Conflux design

  The initial [CONFLUX] paper doesn't provide any indications on how to
  handle the size of out-of-order cell queue, which we consider a
  potential dangerous memory DoS vector (see [MEMORY_DOS]). It also used
  RTT as the sole heuristic for selecting which circuit to send on, which
  may vary depending on the geographical locations of the participant
  relays, without considering their actual available circuit capacity
  (which will be available to us via Proposal 324). Additionally, since
  the publication of [CONFLUX], more modern packet scheduling algorithms
  have been developed, which aim to reduce out-of-order queue size.
  
  We propose mitigations for these issues using modern scheduling
  algorithms, as well as implementations options for avoiding the
  out-of-order queue at Exit relays. Additionally, we consider resumption,
  side channel, and traffic analysis risks and benefits in [RESUMPTION],
  [SIDE_CHANNELS] and [TRAFFIC_ANALYSIS].


2. Design

  The following section describes the Conflux design. Each sub-section is
  a building block to the multipath design that Conflux proposes.
  
  The circuit construction is as follow:
  
         Primary Circuit (lower RTT)
            +-------+      +--------+
            |Guard 1|----->|Middle 1|----------+
            +---^---+      +--------+          |
   +-----+      |                           +--v---+
   | OP  +------+                           | Exit |--> ...
   +-----+      |                           +--^---+
            +---v---+      +--------+          |
            |Guard 2|----->|Middle 2|----------+
            +-------+      +--------+
         Secondary Circuit (higher RTT)
  
  Both circuits are built using current Tor path selection, however they
  SHOULD NOT share the same Guard relay, or middle relay. By avoiding
  using the same relays in these positions in the path, we ensure
  additional path capacity, and eliminate the need to use more complicated
  'coupled' congestion control algorithms from the MPTCP
  literature[COUPLED].  This both simplifies design, and improves
  performance.
  
  Then, the OP needs to link the two circuits together, as described in
  [LINKING_CIRCUITS], [LINKING_EXIT], and [LINKING_SERVICE].
  
  For ease of explanation, the primary circuit is the circuit with lower
  RTT, and the secondary circuit is the circuit with higher RTT. Initial
  RTT is measured during circuit linking, as described in
  [LINKING_CIRCUITS].  RTT is continually measured using SENDME timing, as
  in Proposal 324.  This means that during use, the primary circuit and
  secondary circuit may switch roles, depending on unrelated network
  congestion caused by other Tor clients.
  
  We also support linking onion service circuits together. In this case,
  only two rendezvous circuits are linked. Each of these RP circuits will
  be constructed separately, and then linked. However, the same path
  constraints apply to each half of the circuits (no shared relays between
  the legs). If, by chance, the service and the client sides end up
  sharing some relays, this is not catastrophic. Multipath TCP researchers
  we have consulted (see [ACKNOWLEDGEMENTS]), believe Tor's congestion
  control from Proposal 324 to be sufficient in this rare case.
  
  Only two circuits SHOULD be linked together. However, implementations
  SHOULD make it easy for researchers to *test* more than two paths, as
  this has been shown to assist in traffic analysis resistance[WTF_SPLIT].
  At minimum, this means not hardcoding only two circuits in the
  implementation.
  
  If the number of circuits exceeds the current number of guard relays,
  guard relays MAY be re-used, but implementations SHOULD use the same
  number of Guards as paths.
  
  Linked circuits MUST NOT be extended further once linked (ie:
  'cannibalization' is not supported).

2.1. Advertising support for conflux

  We propose a new protocol version in order to advertise support for
  circuit linking on the relay side:
  
     "Relay=4" -- Relay supports an 2 byte sequence number in a RELAY cell
                  header used for multipath circuit which are linked with the
                  new RELAY_CIRCUIT_LINK relay cell command.
  
  XXX: Advertise this in onion service descriptor.
  XXX: Onion service descriptor can advertise more than two circuits?

  The next section describes how the circuits are linked together.

2.2. Linking circuits [LINKING_CIRCUITS]

  To link circuits, we propose new relay commands that are sent on both
  circuits, as well as a response to confirm the join, and an ack of this
  response. These commands create a 3way handshake, which allows each
  endpoint to measure the initial RTT of each leg upon link, without
  needing to wait for any data.
  
  All three stages of this handshake are sent on *each* circuit leg to be
  linked.
  
  To save round trips, these cells SHOULD be combined with the initial
  RELAY_BEGIN cell on the faster circuit leg, using Proposal 325. See
  [LINKING_EXIT] and [LINKING_SERVICE] for more details on setup in each
  case.
  
  There are other ways to do this linking that we have considered, but
  they seem not to be significantly better than this method, especially
  since we can use Proposal 325 to eliminate the RTT cost of this setup
  before sending data. For those other ideas, see [ALTERNATIVE_LINKING]
  and [ALTERNATIVE_RTT], in the appendix.
  
  The first two parts of the handshake establish the link, and enable
  resumption:
  
     16 -- RELAY_CIRCUIT_LINK
  
           Sent from the OP to the exit/service in order to link
           circuits together at the end point.
  
     17 -- RELAY_CIRCUIT_LINKED
  
           Sent from the exit/service to the OP, to confirm the circuits
           were linked.
  
  These cells have the following contents:
  
    VERSION   [1 byte]
    PAYLOAD   [variable, up to end of relay payload]
  
  The VERSION tells us which circuit linking mechanism to use. At this
  point in time, only 0x01 is recognized and is the one described by the
  Conflux design.
  
  For version 0x01, the PAYLOAD contains:
  
     NONCE              [32 bytes]
     LAST_SEQNO_SENT    [8 bytes]
     LAST_SEQNO_RECV    [8 bytes]
  
  XXX: Should we let endpoints specify their preferred [SCHEDULING] alg
  here, to override consensus params? This has benefits: eg low-memory
  mobile clients can ask for an alg that is better for their reorder
  queues. But it also has complexity risk, if the other endpoint does not
  want to support it, because of its own memory issues.
  
  The NONCE contains a random 256-bit secret, used to associate the two
  circuits together. The nonce MUST NOT be shared outside of the circuit
  transmission, or data may be injected into TCP streams. This means it
  MUST NOT be logged to disk.
  
  The two sequence number fields are 0 upon initial link, but non-zero in
  the case of a resumption attempt (See [RESUMPTION]).
  
  If either circuit does not receive a RELAY_CIRCUIT_LINKED response, both
  circuits MUST be closed.
  
  The third stage of the handshake exists to help the exit/service measure
  initial RTT, for use in [SCHEDULING]:
  
     18 -- RELAY_CIRCUIT_LINKED_RTT_ACK
  
           Sent from the OP to the exit/service, to provide initial RTT
           measurement for the exit/service.
  
  For timeout of the handshake, clients SHOULD use the normal SOCKS/stream
  timeout already in use for RELAY_BEGIN.
  
  These three relay commands (RELAY_CIRCUIT_LINK, RELAY_CIRCUIT_LINKED,
  and RELAY_CIRCUIT_LINKED_RTT_ACK) are send on *each* leg, to allow each
  endpoint to measure the initial RTT of each leg.

2.2. Linking Circuits from OP to Exit [LINKING_EXIT]

  To link exit circuits, two circuits to the same exit are built. The
  client records the circuit build time of each.
  
  If the circuits are being built on-demand, for immediate use, the
  circuit with the lower build time SHOULD use Proposal 325 to append its
  first RELAY cell to the RELAY_COMMAND_LINK, on the circuit with the
  lower circuit build time. The exit MUST respond on this same leg.  After
  that, actual RTT measurements MUST be used to determine future
  transmissions, as specified in [SCHEDULING].
  
  The RTT times between RELAY_COMMAND_LINK and RELAY_COMMAND_LINKED are
  measured by the client, to determine each circuit RTT to determine
  primary vs secondary circuit use, and for packet scheduling.  Similarly,
  the exit measures the RTT times between RELAY_COMMAND_LINKED and
  RELAY_COMMAND_LINKED_RTT_ACK, for the same purpose.
  
2.3. Linking circuits to an onion service [LINKING_SERVICE]
  
  For onion services, we will only concern ourselves with linking
  rendezvous circuits.
  
  To join rendezvous circuits, clients make two introduce requests to a
  service's intropoint, causing it to create two rendezvous circuits, to
  meet the client at two separate rendezvous points. These introduce
  requests MUST be sent to the same intropoint (due to potential use of
  onionbalance), and SHOULD be sent back-to-back on the same intro
  circuit. They MAY be combined with Proposal 325.
  
  The first rendezvous circuit to get joined SHOULD use Proposal 325 to
  append the RELAY_BEGIN command, and the service MUST answer on this
  circuit, until RTT can be measured.
  
  Once both circuits are linked and RTT is measured, packet scheduling
  MUST be used, as per [SCHEDULING].
  
2.4. Congestion Control Application [CONGESTION_CONTROL]
  
  The SENDMEs for congestion control are performed per-leg. As data
  arrives, regardless of its ordering, it is counted towards SENDME
  delivery. In this way, 'cwnd - package_window' of each leg always
  reflects the available data to send on each leg. This is important for
  [SCHEDULING].
  
  The Congestion control Stream XON/XOFF can be sent on either leg, and
  applies to the stream's transmission on both legs.
  
2.5. Sequencing [SEQUENCING]
  
  With multiple paths for data, the problem of data re-ordering appears.
  In other words, cells can arrive out of order from the two circuits
  where cell N + 1 arrives before the cell N.
  
  Handling this reordering operates after congestion control for each
  circuit leg, but before relay cell command processing or stream data
  delivery.
  
  For the receiver to be able to reorder the receiving cells, a sequencing
  scheme needs to be implemented. However, because Tor does not drop or
  reorder packets inside of a circuit, this sequence number can be very
  small. It only has to signal that a cell comes after those arriving on
  another circuit.
  
  To achieve this, we add a small sequence number to the common relay
  header for all relay cells on linked circuits. This sequence number is
  meant to signal the number of cells sent on the *other* leg, so that
  each endpoint knows how many cells are still in-flight on another leg.
  It is different from the absolute sequence number used in
  [LINKING_CIRCUITS] and [RESUMPTION], but can be derived from that
  number, using relative arithmetic.
  
    Relay command   [1 byte]
    Recognized      [2 bytes]
    StreamID        [2 bytes]
    Digest          [4 bytes]
    Length          [2 bytes]
  > LongSeq         [1 bit]  # If this bit is set, use 31 bits for Seq
  > Sequencing      [7 or 31 bits]
    Data            [Remainder]
  
  The sequence number is only set for the first cell after the endpoint
  switches legs. In this case, LongSeq is set to 1, and the Sequencing
  field is 31 more bits. Otherwise it is a 1 byte 0 value.
  
  These fields MUST be present on ALL end-to-end relay cells on each leg
  that come from the endpoint, following a RELAY_CIRCUIT_LINK command.
  
  They are absent on 'leaky pipe' RELAY_COMMAND_DROP and
  RELAY_COMMAND_PADDING_NEGOTIATED cells that come from middle relays, as
  opposed to the endpoint, to support padding.
  
  When an endpoint switches legs, on the first cell in a new leg, LongSeq
  is set to 1, and the following 31 bits represent the *total* number of
  cells sent on the *other* leg, before the switch. The receiver MUST wait
  for that number of cells to arrive from the previous leg before
  delivering that cell.
  
  XXX: In the rare event that we send more than 2^31 cells (~1TB) on a
  single leg, do we force a switch of legs, or expand the field further?
  
  An alternative method of sequencing, that assumes that the endpoint
  knows when it is going to switch, the cell before it switches, is
  specified in [ALTERNATIVE_SEQUENCING]. Note that that method requires
  only 1 byte for sequence number and switch signaling, but requires that
  the sender know that it is planning to switch, the cell before it
  switches.  (This is possible with [BLEST_TOR], but [LOWRTT_TOR] can
  switch based on RTT change, so it may be one cell late in that case).

2.6. Resumption [RESUMPTION]

  In the event that a circuit leg is destroyed, they MAY be resumed.
  
  Resumption is achieved by re-using the NONCE to the same endpoint
  (either [LINKING_EXIT] or [LINKING_SERVICE]). The resumed path need
  not use the same middle and guard relays as the destroyed leg(s), but
  SHOULD NOT share any relays with any existing legs(s).
  
  To provide resumption, endpoints store an absolute 64bit cell counter of
  the last cell they have sent on a conflux pair (their LAST_SEQNO_SENT),
  as well the last sequence number they have delivered in-order to edge
  connections corresponding to a conflux pair (their LAST_SEQNO_RECV).
  Additionally, endpoints MAY store the entire contents of unacked
  inflight cells (ie the 'package_window' from proposal 324), for each
  leg, along with information corresponding to those cells' absolute
  sequence numbers.
  
  These 64 bit absolute counters can wrap without issue, as congestion
  windows will never grow to 2^64 cells until well past the Singularity.
  However, it is possible that extremely long, bulk circuits could exceed
  2^64 total sent or received cells, so endpoints SHOULD handle wrapped
  sequence numbers for purposes of computing retransmit information. (But
  even this case is unlikely to happen within the next decade or so).
  
  Upon resumption, the LAST_SEQNO_SENT and LAST_SEQNO_RECV fields are used
  to convey the sequence numbers of the last cell the relay sent and
  received on that leg. The other endpoint can use these sequence numbers
  to determine if it received the in-flight data or not, or sent more data
  since that point, up to and including this absolute sequence number. If
  LAST_SEQNO_SENT has not been received, the endpoint MAY transmit the
  missing data, if it still has it buffered.
  
  Because both endpoints get information about the other side's absolute
  SENT sequence number, they will know exactly how many re-transmitted
  packets to expect, if the circuit is successfully resumed.

  Re-transmitters MUST NOT re-increment their absolute sent fields
  while re-transmitting.
  
  If it does not have this missing data due to memory pressure, that
  endpoint MUST destroy *both* legs, as this represents unrecoverable
  data loss.
  
  Otherwise, the new circuit can be re-joined, and its RTT can be compared
  to the remaining circuit to determine if the new leg is primary or
  secondary.
  
  It is even possible to resume conflux circuits where both legs have been
  collapsed using this scheme, if endpoints continue to buffer their
  unacked package_window data for some time after this close. However, see
  [TRAFFIC_ANALYSIS] for more details on the full scope of this issue.
  
  If endpoints are buffering package_window data, such data should be
  given priority to be freed in any oomkiller invocation. See [MEMORY_DOS]
  for more oomkiller information.


3. Traffic Scheduling [SCHEDULING]

  In order to load balance the traffic between the two circuits, the
  original conflux paper used only RTT. However, with Proposal 324, we
  will have accurate information on the instantaneous available bandwidth
  of each circuit leg, as 'cwnd - package_window' (see Section 3 of
  Proposal 324).
  
  Some additional RTT optimizations are also useful, to improve
  responsiveness and minimize out-of-order queue sizes.
  
  We specify two traffic schedulers from the multipath literature and
  adapt them to Tor: [LOWRTT_TOR] and [BLEST_TOR]. [LOWRTT_TOR] also has
  three variants, with different trade offs.
  
  However, see the [TRAFFIC_ANALYSIS] sections of this proposal for
  important details on how this selection can be changed, to reduce
  website traffic fingerprinting.

3.1. LowRTT Scheduling [LOWRTT_TOR]

  This scheduling algorithm is based on the original [CONFLUX] paper, with
  ideas from [MPTCP]'s minRTT/LowRTT scheduler.
  
  In this algorithm, endpoints send cells on the circuit with lower RTT
  (primary circuit). This continues while the congestion window on the
  circuit has available room: ie whenever cwnd - package_window > 0.
  
  Whenever the primary circuit's congestion window becomes full, the
  secondary circuit is used. We stop reading on the send window source
  (edge connection) when both congestion windows become full.
  
  In this way, unlike original conflux, we switch to the secondary circuit
  without causing congestion on the primary circuit. This improves both
  load times, and overall throughput.
  
  This behavior matches minRTT from [MPTCP], sometimes called LowRTT.
  
  It may be better to stop reading on the edge connection when the primary
  congestion window becomes full, rather than switch to the secondary
  circuit as soon as the primary congestion window becomes full. (Ie: only
  switch if the RTTs themselves change which circuit is primary). This is
  what was done in the original Conflux paper. This behavior effectively
  causes us to optimize for responsiveness and congestion avoidance,
  rather than throughput. For evaluation, we will control this switching
  behavior with a consensus parameter (see [CONSENSUS_PARAMETERS]).
  
  Because of potential side channel risk (see [SIDE_CHANNELS]), a third
  variant of this algorithm, where the primary circuit is chosen during
  the [LINKING_CIRCUITS] handshake and never changed, is also possible
  to control via consensus parameter.

3.2. BLEST Scheduling [BLEST_TOR]

  [BLEST] attempts to predict the availability of the primary circuit, and
  use this information to reorder transmitted data, to minimize
  head-of-line blocking in the recipient (and thus minimize out-of-order
  queues there).
  
  BLEST_TOR uses the primary circuit until the congestion window is full.
  Then, it uses the relative RTT times of the two circuits to calculate
  how much data can be sent on the secondary circuit faster than if we
  just waited for the primary circuit to become available. 
  
  This is achieved by computing two variables at the sender:   
  
    rtts = secondary.currRTT / primary.currRTT
    primary_limit = primary.cwnd + (rtts-1)/2)*rtts
  
  Note: This (rtts-1)/2 factor represents anticipated congestion window
  growth over this period.. it may be different for Tor, depending on CC
  alg.
  
  If primary_limit < secondary.cwnd - (secondary.package_window + 1), then
  there is enough space on the secondary circuit to send data faster than
  we could than waiting for the primary circuit.
  
  XXX: Note that BLEST uses total_send_window where we use secondary.cwnd
  in this check. total_send_window is min(recv_win, CWND). But since Tor
  does not use receive windows and instead uses stream XON/XOFF, we only
  use CWND. There is some concern this may alter BLEST's buffer
  minimization properties, but since receive window only matter if
  the application is slower than Tor, and XON/XOFF will cover that case,
  hopefully this is fine. If we need to, we could turn [REORDER_SIGNALING]
  into a receive window indication of some kind, to indicate remaining
  buffer size.
  
  Otherwise, if the primary_limit condition is not hit, cease reading on
  source edge connections until SENDME acks come back.
  
  Here is the pseudocode for this:
  
    while source.has_data_to_send():
      if primary.cwnd > primary.package_window:
        primary.send(source.get_packet())
        continue
    
      rtts = secondary.currRTT / primary.currRTT
      primary_limit = (primary.cwnd + (rtts-1)/2)*rtts
    
      if primary_limit < secondary.cwnd - (secondary.package_window+1):
        secondary.send(source.get_packet())
      else:
        break # done for now, wait for SENDME to free up CWND and restart
  
  Note that BLEST also has a parameter lambda that is updated whenever HoL
  blocking occurs. Because it is expensive and takes significant time to
  signal this over Tor, we omit this.
  
  XXX: See [REORDER_SIGNALING] section if we want this lambda feedback.

3.3. Reorder queue signaling [REORDER_SIGNALING]

  Reordering is fairly simple task. By following using the sequence
  number field in [SEQUENCING], endpoints can know how many cells are
  still in flight on the other leg.
  
  To reorder them properly, a buffer of out of order cells needs to be
  kept. On the Exit side, this can quickly become overwhelming
  considering ten of thousands of possible circuits can be held open
  leading to gigabytes of memory being used. There is a clear potential
  memory DoS vector in this case, covered in more detail in
  [MEMORY_DOS].

  Luckily, [BLEST_TOR] and the form of [LOWRTT_TOR] that only uses the
  primary circuit will minimize or eliminate this out-of-order buffer.
  
  XXX: The remainder of this section may be over-complicating things... We
  only need these concepts if we want to use BLEST's lambda feedback. Though
  turning this into some kind of receive window that indicates remaining
  reorder buffer size may also help with the total_send_window also noted
  in BLEST_TOR.
  
  The default for this queue size is governed by the 'cflx_reorder_client'
  and 'cflx_reorder_srv' consensus parameters (see [CONSENSUS_PARAMS]).
  'cflx_reorder_srv' applies to Exits and onion services. Both parameters
  can be overridden by Torrc, to larger or smaller than the consensus
  parameter. (Low memory clients may want to lower it; SecureDrop onion
  services or other high-upload services may want to raise it).
  
  When the reorder queue hits this size, a RELAY_CONFLUX_XOFF is sent down
  the circuit leg that has data waiting in the queue and use of that leg
  SHOULD cease, until it drains to half of this value, at which point an
  RELAY_CONFLUX_XON is sent. Note that this is different than the stream
  XON/XOFF from Proposal 324.
  
  XXX: [BLEST] actually does not cease use of a path in this case, but
  instead uses this signal to adjust the lambda parameter, which biases
  traffic away from that leg.


4. Security Considerations

4.1. Memory Denial of Service [MEMORY_DOS]

  Both reorder queues and retransmit buffers inherently represent a memory
  denial of service condition.
  
  For [RESUMPTION] retransmit buffers, endpoints that support this feature
  SHOULD free retransmit information as soon as they get close to memory
  pressure. This prevents resumption while data is in flight, but will not
  otherwise harm operation.
  
  For reorder buffers, adversaries can potentially impact this at any
  point, but most obviously and most severely from the client position.
  
  In particular, clients can lie about sequence numbers, sending cells
  with sequence numbers such that the next expected sequence number is
  never sent. They can do this repeatedly on many circuits, to exhaust
  memory at exits.
  
  One option is to only allow actual traffic splitting in the downstream
  direction, towards clients, and always use the primary circuit for
  everything in the upstream direction. However, the ability to support
  conflux from the client to the exit shows promise against traffic
  analysis (see [WTF_SPLIT]).
  
  The other option is to use [BLEST_TOR] from clients to exits, as it has
  predictable interleaved cell scheduling, and minimizes reorder queues at
  exits. If the ratios prescribed by that algorithm are not followed
  within some bounds, the other endpoint can close both circuits, and free
  the queue memory.
  
  This still leaves the possibility that intermediate relays may block a
  leg, allowing cells to traverse only one leg, thus still accumulating at
  the reorder queue. Clients can also spoof sequence numbers similarly, to
  make it appear that they are following [BLEST_TOR], without actually
  sending any data on one of the legs.
  
  To handle either of these cases, when a relay is under memory pressure,
  the circuit OOM killer SHOULD free and close circuits with the oldest
  reorder queue data, first. This heuristic was shown to be best during
  the [SNIPER] attack OOM killer iteration cycle.

4.2. Side Channels [SIDE_CHANNELS]

  Two potential side channels may be introduced by the use of Conflux:
     1. RTT leg-use bias by altering SENDME latency
     2. Location info leaks through the use of both leg's latencies
  
  For RTT and leg-use bias, Guard relays could delay legs to introduce a
  pattern into the delivery of cells at the exit relay, by varying the
  latency of SENDME cells (every 100th cell) to change the distribution of
  traffic to send information. This attack could be performed in either
  direction of traffic, to bias traffic load off of a particular Guard.
  If an adversary controls both Guards, it could in theory send a binary
  signal more easily, by alternating delays on each.
  
  However, this risk weighs against the potential benefits against traffic
  fingerprinting, as per [WTF_SPLIT]. Additionally, even ignoring
  cryptographic tagging attacks, this side channel provides significantly
  lower information over time than inter-packet-delay based side channels
  that are already available to Guards and routers along the path to the
  Guard.
  
  Tor currently provides no defenses against already existing
  single-circuit delay-based side channels, though both circuit padding
  and [BACKLIT] are potential options it could conceivably deploy. The
  [BACKLIT] paper also has an excellent review of the various methods that
  have been studied for such single circuit side channels, and the
  [BACKLIT] style RTT monitoring could be used to protect against these
  conflux side channels as well. Circuit padding can also help to obscure
  which cells are SENDMEs, since circuit padding is not counted towards
  SENDME totals.
  
  The second class of side channel is where the Exit relay may be able to
  use the two legs to further infer more information about client
  location. See [LATENCY_LEAK] for more details. It is unclear at this
  time how much more severe this is for two paths than just one.
  
  We preserve the ability to disable conflux to and from Exit relays
  using consensus parameters, if these side channels prove more severe,
  or if it proves possible possible to mitigate single-circuit side
  channels, but not conflux side channels.
  
  In all cases, all of these side channels appear less severe for onion
  service traffic, due to the higher path variability due to relay
  selection, as well as the end-to-end nature of conflux in that case.
  Thus, we separate our ability to enable/disable conflux for onion
  services from Exits.

4.3. Traffic analysis [TRAFFIC_ANALYSIS]

  Even though conflux shows benefits against traffic analysis in
  [WTF_SPLIT], these gains may be moot if the adversary is able to perform
  packet counting and timing analysis at guards to guess which specific
  circuits are linked.  In particular, the 3 way handshake in
  [LINKING_CIRCUITS] may be quite noticeable.
  
  As one countermeasure, it may be possible to eliminate the third leg
  (RELAY_CIRCUIT_LINKED_RTT_ACK) by computing the exit/service RTT via
  measuring the time between CREATED/REND_JOINED and RELAY_CIRCUIT_LINK,
  but this will introduce cross-component complexity into Tor's protocol
  that could quickly become unwieldy and fragile.
  
  Additionally, the conflux handshake may make onion services stand out
  more, regardless of the number of stages in the handshake. For this
  reason, it may be more wise to simply address these issues with circuit
  padding machines during circuit setup (see padding-spec.txt).
  
  Additional traffic analysis considerations arise when combining conflux
  with padding, for purposes of mitigating traffic fingerprinting. For
  this, it seems wise to treat the packet schedulers as another piece of a
  combined optimization problem in tandem with optimizing padding
  machines, perhaps introducing randomness or fudge factors their
  scheduling, as a parameterized distribution. For details, see
  https://github.com/torproject/tor/blob/master/doc/HACKING/CircuitPaddingDevelopment.md
  
  Finally, conflux may exacerbate forms of confirmation-based traffic
  analysis that close circuits to determine concretely if they were in
  use, since closing either leg might cause resumption to fail. TCP RST
  injection can perform this attack on the side, without surveillance
  capability. [RESUMPTION] with buffering of the inflight unacked
  package_window data, for retransmit, is a partial mitigation, if
  endpoints buffer this data for retransmission for a brief time even if
  both legs close. This seems more feasible for onion services, which are
  more vulnerable to this attack. However, if the adversary controls the
  client, they will notice the resumption re-link, and still obtain
  confirmation that way.
  
  It seems the only way to fully mitigate these kinds of attacks is with
  the Snowflake pluggable transport, which provides its own resumption and
  retransmit behavior. Additionally, Snowflake's use of UDP DTLS also
  protects against TCP RST injection, which we suspect to be the main
  vector for such attacks.
  
  In the future, a DTLS or QUIC transport for Tor such as masque could
  provide similar RST injection resistance, and resumption at Guard/Bridge
  nodes, as well.


5. System Interactions

  - congestion control
  - EWMA and KIST
  - CBT and number of guards
  - Onion service circ obfuscation
  - Future UDP (may increase need for UDP to buffer before dropping)
  - Padding (no sequence numbers on padding cells, as per [SEQUENCING])
    - Also, any padding machines may need re-tuning
  - No 'cannibalization' of linked circuits


6. Consensus and Torrc Parameters [CONSENSUS]

  - conflux_circs
    - Number of conflux circuits

  - conflux_sched_exits, conflux_sched_clients, conflux_sched_service
    - Three forms of LOWRTT_TOR, and BLEST_TOR

  - ConfluxOnionService
  - ConfluxOnionCircs


7. Tuning Experiments [EXPERIMENTS]

  - conflux_sched & conflux_exits
    - Exit reorder queue size
    - Responsiveness vs throughput tradeoff?
  - Congestion control
  - EWMA and KIST
  - num guards & conflux_circs


Appended A [ALTERNATIVES]

A.1 BEGIN/END sequencing [ALTERNATIVE_SEQUENCING]

  In this method of signaling, we increment the sequence number by 1 only
  when we switch legs, and use BEGIN/END "bookends" to know that all data
  on a leg has been received.
  
  To achieve this, we add a small sequence number to the common relay
  header for all relay cells on linked circuits, as well as a field to
  signal the beginning of a sequence, intermediate data, and the end of a
  sequence.
  
    Relay command   [1 byte]
    Recognized      [2 bytes]
    StreamID        [2 bytes]
    Digest          [4 bytes]
    Length          [2 bytes]
  > Switching       [2 bits]    # 01 = BEGIN, 00 = CONTINUE, 10 = END
  > Sequencing      [6 bits]
    Data            [PAYLOAD_LEN - 12 - Length bytes]
  
  These fields MUST be present on ALL end-to-end relay cells on each leg
  that come from the endpoint, following a RELAY_CIRCUIT_LINK command.
  
  They are absent on 'leaky pipe' RELAY_COMMAND_DROP and
  RELAY_COMMAND_PADDING_NEGOTIATED cells that come from middle relays, as
  opposed to the endpoint, to support padding.
  
  Sequence numbers are incremented by one when an endpoint switches legs
  to transmit a cell. This number will wrap; implementations MUST treat
  0 as the next sequence after 2^6-1. Because we do not expect to support
  significantly more than 2 legs, and much fewer than 63, this is not an
  issue.
  
  The first cell on a new circuit MUST use the BEGIN code for switching.
  Cells are delivered from that circuit until an END switching signal is
  received, even if cells arrive first on another circuit with the next
  sequence number before and END switching field. Recipients MUST only
  deliver cells with a BEGIN, if their Sequencing number is one more than
  the last END.

A.2 Alternative Link Handshake [ALTERNATIVE_LINKING]

  The circuit linking in [LINKING_CIRCUITS] could be done as encrypted
  ntor onionskin extension fields, similar to those used by v3 onions.
  
  This approach has at least four problems:
    i). For onion services, since onionskins traverse the intro circuit
        and return on the rend circuit, this handshake cannot measure
        RTT there.
   ii). Since these onionskins are larger, and have no PFS, an adversary
        at the middle relay knows that the onionskin is for linking, and
        can potentially try to obtain the onionskin key for attacks on
        the link.
  iii). It makes linking circuits more fragile, since they could timeout
        due to CBT, or other issues during construction.
   iv). The overhead in processing this onionskin in onionskin queues
        adds additional time for linking, even in the Exit case, making
        that RTT potentially noisy.
  
  Additionally, it is not clear that this approach actually saves us
  anything in terms of setup time, because we can optimize away the
  linking phase using Proposal 325, to combine initial RELAY_BEGIN cells
  with RELAY_CIRCUIT_LINK.

A.3. Alternative RTT measurement [ALTERNATIVE_RTT]

  Instead of measuring RTTs during [LINKING_CIRCUITS], we could create
  PING/PONG cells, whose sole purpose is to allow endpoints to measure
  RTT.
  
  This was rejected for several reasons. First, during circuit use, we
  already have SENDMEs to measure RTT. Every 100 cells (or
  'circwindow_inc' from Proposal 324), we are able to re-measure RTT based
  on the time between that Nth cell and the SENDME ack. So we only need
  PING/PONG to measure initial circuit RTT.
  
  If we were able to use onionskins, as per [ALTERNATIVE_LINKING] above,
  we might be able to specify a PING/PONG/PING handshake solely for
  measuring initial RTT, especially for onion service circuits.
  
  The reason for not making a dedicated PING/PONG for this purpose is that
  it is context-free. Even if we were able to use onionskins for linking
  and resumption, to avoid additional data in handshake that just measures
  RTT, we would have to enforce that this PING/PONG/PING only follows the
  exact form needed by this proposal, at the expected time, and at no
  other points.
  
  If we do not enforce this specific use of PING/PONG/PING, it becomes
  another potential side channel, for use in attacks such as [DROPMARK].
  
  In general, Tor is planning to remove current forms of context-free and
  semantic-free cells from its protocol:
  https://gitlab.torproject.org/tpo/core/torspec/-/issues/39
  
  We should not add more.


Appendix B: Acknowledgments [ACKNOWLEDGEMENTS]

  Thanks to Per Hurtig for helping us with the framing of the MPTCP
  problem space.
  
  Thanks to Simone Ferlin for clarifications on the [BLEST] paper, and for
  pointing us at the Linux kernel implementation.
  
  Extreme thanks goes again to Toke Høiland-Jørgensen, who helped
  immensely towards our understanding of how the BLEST condition relates
  to edge connection pushback, and for clearing up many other
  misconceptions we had.
  
  Finally, thanks to Mashael AlSabah, Kevin Bauer, Tariq Elahi, and Ian
  Goldberg, for the original [CONFLUX] paper!


References:

[CONFLUX]
   https://freehaven.net/anonbib/papers/pets2013/paper_65.pdf

[BLEST]
  https://olivier.mehani.name/publications/2016ferlin_blest_blocking_estimation_mptcp_scheduler.pdf
  https://opus.lib.uts.edu.au/bitstream/10453/140571/2/08636963.pdf
  https://github.com/multipath-tcp/mptcp/blob/mptcp_v0.95/net/mptcp/mptcp_blest.c

[WTF_SPLIT]
   https://www.comsys.rwth-aachen.de/fileadmin/papers/2020/2020-delacadena-trafficsliver.pdf

[COUPLED]
   https://datatracker.ietf.org/doc/html/rfc6356
   https://www.researchgate.net/profile/Xiaoming_Fu2/publication/230888515_Delay-based_Congestion_Control_for_Multipath_TCP/links/54abb13f0cf2ce2df668ee4e.pdf?disableCoverPage=true
   http://staff.ustc.edu.cn/~kpxue/paper/ToN-wwj-2020.04.pdf
   https://www.thinkmind.org/articles/icn_2019_2_10_30024.pdf
   https://arxiv.org/pdf/1308.3119.pdf

[BACKLIT]
   https://www.freehaven.net/anonbib/cache/acsac11-backlit.pdf

[LATENCY_LEAK]
   https://www.freehaven.net/anonbib/cache/ccs07-latency-leak.pdf
   https://www.robgjansen.com/publications/howlow-pets2013.pdf

[SNIPER]
   https://www.freehaven.net/anonbib/cache/sniper14.pdf

[DROPMARK]
   https://www.petsymposium.org/2018/files/papers/issue2/popets-2018-0011.pdf