commit d719ac402283b847b5f712a54ccdf118c7828fca Author: teor teor@torproject.org Date: Fri Jan 24 16:13:01 2020 +1000
Prop 311: IPv6 ORPort Reachability - Initial Draft
Related tickets: 24404 (proposal), 24403 (implementation). --- proposals/311-relay-ipv6-reachability.txt | 645 ++++++++++++++++++++++++++++++ 1 file changed, 645 insertions(+)
diff --git a/proposals/311-relay-ipv6-reachability.txt b/proposals/311-relay-ipv6-reachability.txt new file mode 100644 index 0000000..bb7062d --- /dev/null +++ b/proposals/311-relay-ipv6-reachability.txt @@ -0,0 +1,645 @@ +Filename: 311-relay-ipv6-reachability.txt +Title: Tor Relay IPv6 Reachability +Author: teor +Created: 22-January-2020 +Status: Draft +Ticket: #24404 + +0. Abstract + + We propose that Tor relays and bridges should check the reachability of + their IPv6 ORPort, before publishing their descriptor. To check IPv6 ORPort + reachability, relays and bridges need to be able to extend circuits via + other relays, and back to their own IPv6 ORPort. + +1. Introduction + + Tor relays and bridges currently check the reachability of their IPv4 + ORPort and DirPort before publishing them in their descriptor. But relays + and bridges do not test the reachability of their IPv6 ORPorts. + + However, Directory Authorities make direct connections to relay IPv4 and + IPv6 ORPorts, to test each relay's reachability. Once a relay has been + confirmed as reachable by a majority of authorities, it is included in the + consensus. (Currently, 6 out of 9 Directory Authorities perform IPv4 and + IPv6 reachability checks. The others just check IPv4.) + + The Bridge authority makes direct connections to bridge IPv4 ORPorts, to + test each bridge's reachability. Depending on its configuration, it may also + test IPv6 ORPorts. Once a bridge has been confirmed as reachable by the + bridge authority, it is included in the bridge networkstatus used by + BridgeDB. + + Many relay and bridge operators don't know when their relay's IPv6 ORPort is + unreachable. They may not find out until they check [Relay Search], or + their traffic may drop. For new operators, it might just look like Tor + simply isn't working, or it isn't using much traffic. IPv6 ORPort issues + are a significant source of relay and bridge operator support requests. + + Implementing IPv6 ORPort reachability checks will provide immediate, direct + feedback to operators in the relay or bridge's logs. It also enables future + work, such as automatically discovering relay and bridge addresses for IPv6 + ORPorts (see [Proposal 312]). + +2. Scope + + This proposal modifies Tor's behaviour as follows: + + Relays: + * circuit extension, + * OR connections for circuit extension, + * reachability testing. + + Bridges: + * reachability testing only. + + This proposal does not change client behaviour. + + When this proposal describes Tor's current behaviour, it covers all + supported Tor versions (0.3.5.7 to 0.4.2.5), as of January 2020. + +3. Allow Relay IPv6 Extends + + To check IPv6 ORPort reachability, relays and bridges need to be able to + extend circuits via other relays, and back to their own IPv6 ORPort. + + We propose that relays start to extend some circuits over IPv6 connections. + We do not propose any changes to bridge extend behaviour. + +3.1. Current IPv6 ORPort Implementation + + Currently, all relays and bridges must have an IPv4 ORPort. IPv6 ORPorts + are optional for relays and bridges. + + Tor supports making direct IPv6 OR connections: + * from directory authorities to relay ORPorts, + * from the bridge authority to bridge ORPorts, + * from clients to relay and bridge ORPorts. + + Tor relays and bridges accept IPv6 ORPort connections. But IPv6 ORPorts are + not currently included in extend requests to other relays. And even if an + extend cell contains an IPv6 ORPort, bridges and relays will not extend + via an IPv6 connection to another relay. + + Instead, relays will extend circuits: + * Using an existing authenticated connection to the requested relay + (which is typically over IPv4), or + * Over a new connection via the IPv4 ORPort in an extend cell. + + If a relay receives an extend cell that only contains an IPv6 ORPort, the + extend typically fails. + +3.2. Relays Extend to IPv6 ORPorts + + We propose that relays make some connections via the IPv6 ORPorts in + extend cells. + + Relays will extend circuits: + * using an existing authenticated connection to the requested relay + (which may be over IPv4 or IPv6), or + * over a new connection via the IPv4 or IPv6 ORPort in an extend cell. + + Since bridges try to imitate client behaviour, they will not adopt this new + behaviour, until clients begin routinely connecting via IPv6. (See + [Proposal 306].) + +3.2.1. Making IPv6 ORPort Extend Connections + + Relays can make a new connection over IPv6 when: + * there is no existing authenticated connection to the requested relay, + and + * the extend cell contains an IPv6 ORPort. + + If these conditions are satisfied, and the extend cell also contains an + IPv4 ORPort, we propose that the relay choose between an IPv4 and an IPv6 + connection at random. + + If the extend cell does not contain an IPv4 ORPort, we propose that the + relay connects over IPv6. (Relays should support IPv6-only extend cells, + even though they are not used to test relay reachability in this proposal.) + + A successful IPv6 connection also requires that: + * the requested relay has an IPv6 ORPort. + But extending relays must not check the consensus for other relays' IPv6 + information. Consensuses may be out of date, particularly when relays are + doing reachability checks for new IPv6 ORPorts. + + See section 3.3.2 for other situations where IPv6 information may be + incorrect or unavailable. + +3.2.2. No Tor Client Changes + + Tor clients currently include IPv4 ORPorts in their extend cells, but they + do not include IPv6 ORPorts. + + We do not propose any client IPv6 extend cell changes at this time. + + The Tor network needs more IPv6 relays, before clients can safely use + IPv6 extends. (Relays do not require anonymity, so they can safely use + IPv6 extends to test their own reachability.) + + We also recommend prioritising client to relay IPv6 connections + (see [Proposal 306]) over relay to relay IPv6 connections. Because client + IPv6 connections have a direct impact on users. + +3.3. Alternative Extend Designs + + We briefly mention some potential extend designs, and the reasons that + they were not used in this proposal. + + (Some designs may be proposed for future Tor versions, but are not necessary + at this time.) + +3.3.1. Future Relay IPv6 Extend Behaviour + + Random selection of extend ORPorts is a simple design, which supports IPv6 + ORPort reachability checks. + + However, it is not the most efficient design when: + * both relays meet the requirements for IPv4 and IPv6 extends, + * a new connection is required, + * the relays have either IPv4 or IPv6 connectivity, but not both. + + In this very specific case, this proposal results in an average of 1 + circuit extend failure per new connection. (Because relays do not try to + connect to the other ORPort when the first one fails.) + + If relays try both the IPv4 and IPv6 ORPorts, then the circuit would + succeed. For example, relays could try the alternative port after a 250ms + delay, as in [Proposal 306]. This design results in an average circuit delay + of up to 125ms per new connection, rather than failure. + + However, partial relay connectivity should be uncommon. And relays keep + connections open long-term, so new relay connections are a small proportion + of extend requests. + + Therefore, we defer implementing any more complex designs. Since we propose + to use IPv6 extends to test relay reachability, occasional circuit extend + failures have a very minor impact. + +3.3.2. Future Bridge IPv6 Extend Behaviour + + When clients automatically connect to relay IPv4 and IPv6 ORPorts by + default, bridges should also adopt this behaviour. (For example, + see [Proposal 306].) + +3.3.3. Rejected Extend Designs + + Some designs may never be suitable for the Tor network. + + We rejected designs where relays check the consensus to see if other + relays support IPv6, because: + * relays may have different consensuses, + * the extend cell may have been created using a version of the + [Onion Service Protocol] which supports IPv6, or + * the extend cell may be from a relay that has just added IPv6, and is + testing the reachability of its own ORPort (see Section 4). + + We avoided designs where relays try to learn if other relays support IPv6, + because these designs: + * are more complex than random selection, + * potentially leak information between different client circuits, + * may enable denial of service attacks, where a flood of incorrect extend + cells causes a relay to believe that another relay is unreachable on an + ORPort that actually works, and + * require careful tuning to match the typical interval at which network + connectivity is actually changing. + +4. Check Relay and Bridge IPv6 ORPort Reachability + + We propose that relays and bridges check their own IPv6 ORPort reachability. + + To check IPv6 ORPort reachability, relays and bridges extend circuits via + other relays, and back to their own IPv6 ORPort. + + IPv6 reachability failures may result in a relay or bridge refusing to + publish its descriptor, if enough existing relays support IPv6 extends. + +4.1. Current Reachability Implementation + + Relays and bridges check the reachability of their IPv4 ORPorts and + DirPorts, and refuse to publish their descriptor if either reachability + check fails. + + IPv4 ORPort reachability checks succeed when any create cell is received on + any inbound OR connection. The check succeeds, even if the cell is from an + IPv6 ORPort, or a circuit built by a client. + + Directory Authorities make direct connections to relay IPv4 and IPv6 + ORPorts, to test each relay's reachability. Relays that fail either + reachability test, on enough directory authorities, are excluded from the + consensus. + + The Bridge authority makes direct connections to bridge IPv4 ORPorts, to + test each bridge's reachability. Depending on its configuration, it may also + test IPv6 ORPorts. Bridges that fail either reachability test are excluded + from BridgeDB. + +4.2. Checking IPv6 ORPort Reachability + + We propose that testing relays (and bridges) select some IPv6 extend-capable + relays for their reachability circuits, and include their own IPv4 and IPv6 + ORPorts in the final extend cells on those circuits. + + The final extending relay will extend to the testing relay: + * using an existing authenticated connection to the testing relay + (which may be over IPv4 or IPv6), or + * over a new connection via the IPv4 or IPv6 ORPort in the extend cell. + + The testing relay will confirm that test circuits can extend to both its + IPv4 and IPv6 ORPorts. + +4.2.1. Selecting the Final Extending Relay + + IPv6 ORPort reachability checks require an IPv6 extend-capable relay as + the second-last hop of reachability circuits. (The testing relay is the + last hop.) + + IPv6-extend capable relays must have: + * Relay subprotocol version 3 (or later), and + * an IPv6 ORPort. + (See section 5.1 for the definition of Relay subprotocol version 3.) + + The other relays in the path do not require any particular protocol + versions. + +4.2.2. Extending from the Second-Last Hop + + IPv6 ORPort reachability circuits should put the IPv4 and IPv6 ORPorts in + the extend cell for the final extend in reachability circuits. + + Supplying both ORPorts makes these extend cells indistinguishable from + future client extend cells. + + If reachability succeeds, the testing relay (or bridge) will accept the + final extend on one of its ORPorts, selected at random by the extending + relay (see section 3.2.1). + +4.2.3. Separate IPv4 and IPv6 Reachability Flags + + Testing relays (and bridges) will record reachability separately for IPv4 + and IPv6 ORPorts, based on the ORPort that the test circuit was received on. + +4.2.4. No Changes to DirPort Reachability + + We do not propose any changes to relay IPv4 DirPort reachability checks at + this time. + + The following configurations are currently not supported: + * bridge DirPorts, and + * relay IPv6 DirPorts. + Therefore, they are also out of scope for this proposal. + +4.3. Refuse to Publish Descriptor if IPv6 ORPort is Unreachable + + If an IPv6 ORPort reachability check fails, relays (and bridges) should log + a warning. + + In addition, if IPv6ORPort reachability checks are reliable, based on the + number of relays in the network that support IPv6 extends, relays should + refuse to publish their descriptor. + +4.3.1. Refusing to Publish the Descriptor + + We set a threshold of consensus relays for reliable IPv6 ORPort checks: + * at least 30 relays, and + * at least 1% of the total consensus weight, + must support IPv6 extends. + + We chose these parameters so that the number of relays is triple the + number of directory authorities, and the consensus weight is high enough + to support occasional reachability circuits. + + In small networks with: + * less than 2000 relays, or + * a total consensus weight of zero, + the threshold should be the minimum tor network size to test reachability: + * at least 2 relays, excluding this relay. + (Note: we may increase this threshold to 3 or 4 relays if we discover a + higher minimum during testing.) + + If the current consensus satisfies this threshold, testing relays (and + bridges) that fail IPv6 ORPort reachability checks should refuse to publish + their descriptors. + + To ensure an accurate threshold, testing relays should exclude: + * the testing relay itself, and + * relays that they will not use in testing circuits, + from the: + * relay count, and + * the numerator of the threshold percentage. + + Typically, relays will be excluded if they are in the testing relay's: + * family, + * IPv4 address /16 network, + * IPv6 address /32 network (a requirement as of Tor 0.4.0.1-alpha), + unless EnforceDistinctSubnets is 0. + + As a useful side-effect, these different thresholds for each relay family + will reduce the likelihood of the network flapping around the threshold. + + If flapping has an impact on the network health, directory authorities + should set the AssumeIPv6Reachable consensus parameter. (See the next + section.) + +4.3.2. Add AssumeIPv6Reachable Option + + We add an AssumeIPv6Reachable torrc option and consensus parameter. + + If IPv6 ORPort checks have bugs that impact the health of the network, + they can be disabled by setting AssumeIPv6Reachable=1 in the consensus + parameters. + + If IPv6 ORPort checks have bugs that impact a particular relay (or bridge), + they can be disabled by setting "AssumeIPv6Reachable 1" in the relay's + torrc. + + This option disables IPv6 ORPort reachability checks, so relays publish + their descriptors if their IPv4 ORPort reachability checks succeed. + + The default for the torrc option is "auto", which checks the consensus + parameter. If the consensus parameter is not set, the default is "0". + + "AssumeReachable 1" overrides all values of "AssumeIPv6Reachable", + disabling both IPv4 and IPv6 ORPort reachability checks. + +4.4. Optional Efficiency and Reliability Changes + + We propose some optional changes for efficiency and reliability, and + describe their impact. + + Some of these changes may be more appropriate in future releases, or + along with other proposed features. + +4.4.1. Extend IPv6 From All Supported Second-Last Hops + + The testing relay (or bridge) puts both IPv4 and IPv6 ORPorts in its final + extend cell, and the receiving ORPort is selected at random by the + extending relay (see sections 3.2.1 and 4.2). Therefore, approximately half + of IPv6 ORPort reachability circuits will actually end up confirming IPv4 + ORPort reachability. + + We propose this optional change, to improve the rate of IPv6 ORPort + reachability checks: + + If the second-last hop of an IPv4 ORPort reachability circuit supports IPv6 + extends, testing relays may put the IPv4 and IPv6 ORPorts in the extend + cell for the final extend. + + As the number of relays that support IPv6 extends increases, this change + will increase the number of IPv6 reachability confirmations. In the ideal + case, where the entire network supports IPv4 and IPv6 extends, IPv4 and IPv6 + ORPort reachability checks would require a similar number of circuits. + +4.4.2. Close Existing Connections Before Testing Reachability + + When a busy relay is performing reachability checks, it may already have + established inbound or outbound connections to the second-last hop in its + reachability test circuits. The extending relay may use these connections + for the extend, rather than opening a connection to the target ORPort + (see sections 3.2 and 4.2.2). + + Bridges only establish outbound connections to other relays, and only over + IPv4 (except for reachability test circuits). So they are still potentially + affected by this issue. + + We propose these optional changes, to improve the efficiency of IPv4 and + IPv6 ORPort reachability checks: + + Testing relays (and bridges): + * close any outbound connections to the second-last hop of reachability + circuits, and + * close inbound connections to the second-last hop of reachability + circuits, if those connections are not using the target ORPort. + + Even though it is unlikely that bridges will have inbound connections to + a non-target ORPort, bridges should still do inbound connection checks, for + consistency. + + These changes are particularly important if a relay is connected to all + other relays in the network, but only over IPv4. (Or in the future, only + over IPv6.) + + We expect that these changes will slightly increase the number of relay + re-connections, but reduce the number of reachability test circuits + required to confirm reachability. + +4.4.3. Accurately Identifying Test Circuits + + The testing relay (or bridge) may confirm that the create cells it is + receiving are from its own test circuits, and that test circuits are + capable of returning create cells to the origin. + + Currently, relays confirm reachability if any create cell is received on + any inbound connection (see section 4.1). Relays do not check that the + circuit is a reachability test circuit, and they do not wait to receive the + return created cell. This behaviour has resulted in difficult to diagnose + bugs on some rare relay configurations. + + We propose these optional changes, to improve the efficiency of IPv4 and + IPv6 ORPort reachability checks: + + Testing relays may: + * check that the create cell is received from a test circuit + (by comparing the received cell to the cells sent by test circuits), + * check that the create cell is received on an inbound connection + (this is existing behaviour), + * if the create cell from a test circuit is received on an outbound + connection, destroy the circuit (rather than returning a created cell), + and + * check that the created cell is returned to the relay on a test circuit + (by comparing the remote address of the final hop on the circuit, to + the local IPv4 and IPv6 ORPort addresses). + + TODO: work out how to efficiently match inbound create cells to test + circuits. + +4.4.4. Allowing More Relay IPv6 Extends + + Currently, clients, relays, and bridges do not include IPv6 ORPorts in their + extend cells. + + In this proposal, we only make relays (and bridges) extend over IPv6 on + the final hop of test circuits. This limited use of IPv6 extends means that + IPv6 connections will still be uncommon. + + We propose these optional changes, to increase the number of IPv6 + connections between relays: + + To increase the number of IPv6 connections, relays that support IPv6 + extends may want to use them for all hops of their own circuits. Relays + make their own circuits for reachability tests, bandwidth tests, and + ongoing preemptive circuits. (Bridges can not change their behaviour, + because they try to imitate clients.) + + We propose a torrc option and consensus parameter SendIPv6CircuitExtends, + which is only supported on relays (and not bridges or clients). This option + makes relays send IPv4 and IPv6 ORPorts in all their extend cells, when + supported by the extending and receiving relay. (See section 3.2.1.) + + TODO: Is there a shorter name, that isn't easily confused with enabling + support for other nodes to perform IPv6 extends via this relay? + + The default value for this option is "auto", which checks the consensus + parameter. If the consensus parameter is not set, it defaults to "0" in + the initial release. + + Once IPv6 extends have had enough testing, we may enable + SendIPv6CircuitExtends on the network. The consensus parameter will be set + to 1. The default will be changed to "1" (if the consensus parameter is not + set). + + We defer any client (and bridge) changes to a separate proposal, to be + implemented when there are more IPv6 relays in the network. But we note + that relay IPv6 extends will provide some cover traffic when clients + eventually use IPv6 extends in their circuits. + + As a useful side effect, increasing the number of IPv6 connections in the + network makes it more likely that an existing connection can be used for + the final hop of a relay IPv6 ORPort reachability check. + +4.5. Alternate Reachability Designs + + We briefly mention some potential reachability designs, and the reasons that + they were not used in this proposal. + +4.5.1. Removing IPv4 ORPorts from Extend Cells + + We avoid designs that only include IPv6 ORPorts in extend cells, and remove + IPv4 ORPorts. + + Only including the IPv6 ORPort would provide slightly more specific + reachability check circuits. However, we don't need IPv6-only designs, + because relays continue trying different reachability circuits until they + confirm reachability. + + IPv6-only designs also make it easy to distinguish relay reachability extend + cells from other extend cells. This distinguisher will become more of an + issue as IPv6 extends become more common in the network (see sections 4.2.2 + and 4.4.4). + + Removing the IPv4 ORPort also provides no fallback, if the IPv6 ORPort is + actually unreachable. IPv6-only failures do not affect reachability checks, + but they will become important in the future, as other circuit types start + using IPv6 extends. + + IPv6-only reachability designs also increase the number of special cases in + the implementation. (And the likelihood of subtle bugs.) + + These designs may be appropriate in future, when there are IPv6-only bridges + or relays. + +5. New Relay Subprotocol Version + + We reserve Tor subprotocol "Relay=3" for: + * relays that support for IPv6 extends, and + * relays and bridges that support IPv6 ORPort reachability checks, + as described in this proposal. + +5.1. Tor Specification Changes + + We propose the following changes to the [Tor Specification], once this + proposal is implemented. + + Adding a new Relay subprotocol version lets testing relays identify other + relays that support IPv6 extends. It also allows us to eventually recommend + or require support for IPv6 extends on all relays (and bridges). + + Append to the Relay version 2 subprotocol specification: + + Relay=2 has limited IPv6 support: + * Clients do not include IPv6 ORPorts in EXTEND2 cells. + * Relays do not not initiate IPv6 connections in response to + EXTEND2 cells containing IPv6 ORPorts. + However, relays accept inbound connections to their IPv6 ORPorts, + and will extend circuits via those connections. + + "3" -- relays support extending over IPv6 connections in response to an + EXTEND2 cell containing an IPv6 ORPort. Relays and bridges perform + IPv6 ORPort reachability checks. Client behaviour does not change. + + This subprotocol is advertised by all relays and bridges, regardless + of their configured ORPorts. But relays without a configured IPv6 + ORPort may refuse to extend over IPv6. And bridges always refuse to + extend over IPv6, because they try to imitate client behaviour. + + A successful IPv6 extend requires: + * Relay subprotocol version 3 (or later) and an IPv6 ORPort on the + extending relay, + * an IPv6 ORPort in the EXTEND2 cell, and + * an IPv6 ORPort on the accepting relay. + (Because different tor instances can have different views of the + network, these checks should be done when the path is selected. + Extending relays should only check local IPv6 information, before + attempting the extend.) + + This subprotocol version is described in proposal 311, and + implemented in Tor 0.4.4.1-alpha. (TODO: check version after code is + merged). + +6. Test Plan + + We provide a quick summary of our testing plans. + +6.1. Test IPv6 ORPort Reachability and Extends + + We propose to test these changes using chutney networks with AssumeReachable + disabled. (Chutney currently enables AssumeReachable by default.) + + We also propose to test these changes on the public network with a small + number of relays and bridges. + + Once these changes are merged, volunteer relay and bridge operators will be + able to test them by: + * compiling from source, + * running nightly builds, or + * running alpha releases. + +6.2. Test Existing Features + + We will modify and test these existing features: + * IPv4 ORPort reachability checks + + We do not plan on modifying these existing features: + * Relay reachability retries + TODO: Do relays re-check their own reachability? How often? + * Relay canonical connections + * "Too many connections" warning logs + But we will test that they continue to function correctly, and fix any bugs + triggered by the modifications in this proposal. + +6.3. Test Legacy Relay Compatibility + + We will also test IPv6 extends from newer relays (which implement this + proposal) to older relays (which do not). Although this proposal does not + create these kinds of circuits, we need to check for bugs and excessive + logs in older tor versions. + +8. Ongoing Monitoring + + To monitor the impact of these changes, relays should collect basic IPv4 + and IPv6 connection and bandwidth statistics (see [Proposal 313]). + + We may also collect separate statistics on connections from: + * clients (and bridges, because they act like clients), and + * other relays (and authorities, because they act like relays). + + We do not propose to collect additional statistics on: + * bridges, + * circuit counts, or + * failure rates. + Collecting statistics like these could impact user privacy. + +References: + +[Onion Service Protocol]: In particular, Version 3 of the Onion Service Protocol supports IPv6: https://gitweb.torproject.org/torspec.git/tree/rend-spec-v3.txt + +[Proposal 306]: One possible design for automatic client IPv4 and IPv6 connections is at https://gitweb.torproject.org/torspec.git/tree/proposals/306-ipv6-happy-eyeb... (TODO: modify to include bridge changes with client changes) + +[Proposal 312]: https://gitweb.torproject.org/torspec.git/tree/proposals/312-auto-relay-ipv6... (TODO) + +[Proposal 313]: https://gitweb.torproject.org/torspec.git/tree/proposals/313-relay-ipv6-stat... (TODO) + +[Relay Search] https://metrics.torproject.org/rs.html + +[Tor Specification]: https://gitweb.torproject.org/torspec.git/tree/tor-spec.txt