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The following commit(s) were added to refs/heads/main by this push: new 1db24dd prop329: Remove a whole lot of white spaces 1db24dd is described below

commit 1db24dddbf2203f2a465747302f133ef3ed6eed4 Author: David Goulet dgoulet@torproject.org AuthorDate: Mon Nov 28 09:19:46 2022 -0500

prop329: Remove a whole lot of white spaces

Signed-off-by: David Goulet dgoulet@torproject.org --- proposals/329-traffic-splitting.txt | 268 ++++++++++++++++++------------------ 1 file changed, 134 insertions(+), 134 deletions(-)

diff --git a/proposals/329-traffic-splitting.txt b/proposals/329-traffic-splitting.txt index 85d704b..9bb72be 100644 --- a/proposals/329-traffic-splitting.txt +++ b/proposals/329-traffic-splitting.txt @@ -19,11 +19,11 @@ Status: Draft 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 @@ -31,14 +31,14 @@ Status: Draft 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 @@ -55,7 +55,7 @@ Status: Draft (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, @@ -67,9 +67,9 @@ Status: Draft

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|----------+ @@ -81,7 +81,7 @@ Status: Draft |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 @@ -89,10 +89,10 @@ Status: Draft '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 @@ -100,7 +100,7 @@ Status: Draft 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 @@ -109,17 +109,17 @@ Status: Draft 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).

@@ -127,11 +127,11 @@ Status: Draft

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?

@@ -144,77 +144,77 @@ Status: Draft 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. @@ -223,66 +223,66 @@ Status: Draft

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 @@ -290,7 +290,7 @@ Status: Draft 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] @@ -299,27 +299,27 @@ Status: Draft

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 @@ -331,12 +331,12 @@ Status: Draft 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 @@ -345,14 +345,14 @@ Status: Draft 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 @@ -360,27 +360,27 @@ Status: Draft 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. @@ -393,14 +393,14 @@ Status: Draft 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. @@ -409,21 +409,21 @@ Status: Draft

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 @@ -432,7 +432,7 @@ Status: Draft 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 @@ -444,25 +444,25 @@ Status: Draft 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: - + 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 @@ -472,29 +472,29 @@ Status: Draft 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] @@ -502,7 +502,7 @@ Status: Draft 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 @@ -512,26 +512,26 @@ Status: Draft

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. @@ -543,38 +543,38 @@ Status: Draft

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 @@ -585,7 +585,7 @@ Status: Draft 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 @@ -593,14 +593,14 @@ Status: Draft 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 @@ -610,17 +610,17 @@ Status: Draft 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. @@ -634,18 +634,18 @@ Status: Draft 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 @@ -653,7 +653,7 @@ Status: Draft 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/CircuitPaddingDeve... - + 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 @@ -665,13 +665,13 @@ Status: Draft 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. @@ -718,12 +718,12 @@ 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] @@ -732,20 +732,20 @@ A.1 BEGIN/END sequencing [ALTERNATIVE_SEQUENCING]

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 @@ -757,7 +757,7 @@ 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 @@ -771,7 +771,7 @@ A.2 Alternative Link Handshake [ALTERNATIVE_LINKING] 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 @@ -782,31 +782,31 @@ 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.

@@ -814,15 +814,15 @@ 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!