Hi,
Here is an initial draft of Proposal 311: Relay IPv6 Reachability.
This proposal includes: * relay IPv6 ORPort extends, and * relay IPv6 ORPort reachability checks.
This is the first of 3 proposals: * Proposal 311: Relay IPv6 Reachability * Proposal 312: Automatic Relay IPv6 Addresses * Proposal 313: Relay IPv6 Statistics (I haven't written the others yet!)
I also want to make some minor changes to Proposal 306, so that bridge IPv6 behaviour stays in sync with client IPv6 behaviour.
There are still a few TODO items in the proposal, mostly about Tor's current behaviour. If you know the answers, please let me know.
The full text is included below, and it is also available as a GitHub pull request: https://github.com/torproject/torspec/pull/103
The related tickets are #24404 (proposal) and #24403 (implementation): https://trac.torproject.org/projects/tor/ticket/24404 https://trac.torproject.org/projects/tor/ticket/24403
Please feel free to reply on this list, or via GitHub pull request comments.
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
--
(End)
On Fri, Jan 24, 2020 at 1:31 AM teor teor@riseup.net wrote:
Hello, Teor! I hope this reply finds you well.
This proposal seems like a good start: It's solid, readable, and I think it would work. There are few places where I have questions or suggested clarifications, and a few where I think we should consider other designs as well.
I'm going to only quote the parts I'm commenting on.
Also we should probably have the condition, "The relay itself supports IPv6, or thinks that it does"?
[...]
Let me sketch another alternative design here:
Suppose that a relay's extend cell contains the IPv4 address and the IPv6 address in their _preferred order_. So if the party generating the extend cell would prefer an IPv4 connection, it puts the IPv4 addess first; if it would prefer an IPv6 connection, it puts the IPv6 address first.
The relay that receives the extend cell could respond in several ways: * One possibility (similar to your 3.2.1) is to choose at random, with a higher probability given to the first option. * One possibility (similar to your 3.3.1) is to try the first, and then try the second if the first one fails.
I think this scheme has some advantage, in that it lets the self-testing relay say "please try IPv6 if you can" or "please try IPv4 if you can" in a reliable way, and lets us migrate from the current behavior to the 3.3.1 behavior down the road.
[...]
Is this the behavior we want? Another behavior would be to omit our IPv6 orport from our descriptor if it is not reachable. That way, relay operators could configure their relays to just listen on IPv6 unconditionally, and let the relay figure out whether it should advertise IPv6 or not. This behavior seems preferable to me, since it has less chance of breaking a relay that would otherwise "work".
As a third possibility, we could have an option to say whether we should publish a descriptor if some of our addresses are not reachable.
[...]
Should we have some way to AssumeReachable our IPv4 address only, and not our IPv6 address? Or is that too much?
[...]
If we make these changes, I think we should track both whether we are "maybe reachable" (under the current definition of 'reachable') and "definitely reachable" (based on the new definition). We should log different messages depending on whether we are "maybe reachable" but these new tests fail, or whether we are completely unreachable.
One possibility: we could store a set of our test circuits' extend cells g^X values, and then check incoming cells create cells against that set.
There are probably fancier designs based on EC trickery or tweaks to the ntor protocol, but I think we'd best avoid them.
I have another concern here: if this option is truly relay-only, it should probably have a name that indicates the fact.
As an aside: I think in the long run, the future is for clients to treat relays' link specifiers as completely opaque when they are making extend cells.
That is, if a client is extending to some relay X, it should know "here is a blob that you use as X's link specifiers" -- it can be the authorities' job to decide which IP addresses and which identity keys such a blob needs to contain.
(Of course, a client still needs to know a relay's address when making a connection itself, but that's more in prop306 territory.)
I don't fully understand; this reads as ambiguous to me. Does "Relay=3" mean "IF I have an IPv6 address, I support IPv6 extends and checks"? Or does it mean "I have an IPv6 address, and I support IPv6 extends and checks"?
I would argue for the former interpretation, since no other subprotocol version is specified to depend on the relay's network configuration.
I think that this is saying too much; we should leave it out. Clients on the current Tor network MAY include IPv6 ORPorts, after all: we don't want to retroactively specify that any implementations that _do_ include them are noncompliant with subprotocol v2.
Is this always true? Would "may not" or "do not always" be more correct?
Should this be "self-checks" instead of "checks" for clarity?
Also, I wonder whether having self-checking be part of the relay protocol is correct.
This "all" should probably be removed or clarified. How about "all relays and bridges running software that supports it, whether or not they have IPv6 ORPorts configured."
This last seems like it's saying that bridges SHOULD or MUST refuse to extend over IPv6; it might be better to say that they MAY, so that if client behavior changes, bridges can change too.
I think that the "pick a random ORPort" behavior should be specified here too, as part of relay subprotocol v3, if we keep it.
yrs,
Hi Nick,
Thanks for your proposal review.
I've made most of the changes you've suggested, you can see the latest version of the proposal here: https://github.com/torproject/torspec/pull/103/files
I'll send another full text to the list when all the reviews are done.
I've also responded to your comments below individually:
Thanks, I included that condition later in the proposal, but missed it here: https://github.com/torproject/torspec/pull/103/commits/9204293f9b1e106ca73c4...
I've added this alternate design as section 3.3.3, and renumbered the previous section 3.3.3 to 3.4: https://github.com/torproject/torspec/pull/103/commits/654bcb16815c0c57d5c55...
Apart from relay reachability self-tests, I'm not sure if there are any other use cases where preferring IPv4 or IPv6 extends is necessary. Most clients shouldn't care, they just want to get to an exit safely. (And most clients can't care, because existing connections may be used, and they may be over IPv4 or IPv6.)
But I wanted to include this design in the proposal, in case we find that it is necessary during testing.
In general, when I've asked similar questions before, relay operators and directory authority operators have wanted relays to fail fast, rather than continuing to work with a partly-broken config.
The last time I asked, it was about directory authority IPv6 ORPort reachability checks to relays. The directory authority operators told me that relays which fail IPv6 reachability checks should not be in the consensus, so relay operators notice, and fix their relays.
But in this case, I think we should make an exception for relays that automatically guess their IPv6 address (rather than having it configured explicitly by the operator). Since IPv6 address guesses will be a new feature, and they happen automatically when operators upgrade tor, we should just warn if a relay appears to have an IPv6 address, but it is not actually reachable.
Relays that explicitly configure an IPv6 address should continue to fail fast and fail loudly (in their logs). They were always going to fail anyway, once they published their IPv6 address, and the directory authorities found them unreachable.
With the new design above, I don't think that's necessary: relay operators can simply remove their configured IPv6 ORPort address, and tor will start publishing its descriptor.
See the changes here: https://github.com/torproject/torspec/pull/103/commits/2e82cbd45679e9e17390e...
There's one configuration where it might be needed: * a reachable IPv4 address, * a reachable IPv6 address, * but the IPv6 address is unsuitable for an ORPort.
In that case, tor would automatically discover both addresses, publish them both, and the relay operator could not turn off IPv6.
So I think a better way to fix this issue is to turn off guessing the IPv6 address. I've added a new AddressDisableIPv6 as an required part of proposal 312: https://github.com/torproject/torspec/pull/105/files#diff-28c992d72bedaa9378...
Done in: https://github.com/torproject/torspec/pull/103/commits/6dceff0658e122ac00713...
Also done in: https://github.com/torproject/torspec/pull/103/commits/6dceff0658e122ac00713...
There are probably fancier designs based on EC trickery or tweaks to the ntor protocol, but I think we'd best avoid them.
I think you're right.
Changed to RelaySendIPv6Extends in: https://github.com/torproject/torspec/pull/103/commits/f874424ebc9dafb7c721e...
Seems fine to me, but I don't think we're looking that far into the future in this proposal.
It was always meant to be configuration-independent. It was documented as configuration-independent in section 5.1, I've now made those changes in the introduction to section 5 as well: https://github.com/torproject/torspec/pull/103/commits/2662c688170696fbed2ba...
I made these useful features "may not" in: https://github.com/torproject/torspec/pull/103/commits/2662c688170696fbed2ba...
It's always true of the tor implementation that we maintain. I made these useful features "may not" in: https://github.com/torproject/torspec/pull/103/commits/2662c688170696fbed2ba...
I don't think the rest of the network cares about self-checks, so I removed them from this section: https://github.com/torproject/torspec/pull/103/commits/2662c688170696fbed2ba...
I also reviewed the rest of the proposal, it's obvious when we're talking about self-checks, and when we're talking about authority checks.
I clarified that the protocol applies to some tor versions, regardless of their current configuration: https://github.com/torproject/torspec/pull/103/commits/2662c688170696fbed2ba...
I clarified that bridges may not extend: https://github.com/torproject/torspec/pull/103/commits/2662c688170696fbed2ba...
Although, we'll need bridge clients to start sending IPv6 ORPorts in EXTEND2 cells, before bridges can extend over IPv6. But these changes may not need a new protocol version. (We could use a consensus parameter instead.)
I added the random selection, and added a TODO to check the final implementation after we merge: https://github.com/torproject/torspec/pull/103/commits/2662c688170696fbed2ba...
T
On Wed, Jan 29, 2020 at 8:56 AM teor teor@riseup.net wrote:
Thanks for the followup, Teor! I have only one comment on the changes:
I made these useful features "may not" in: https://github.com/torproject/torspec/pull/103/commits/2662c688170696fbed2ba...
We should avoid "may not" in standards documents because it can be ambiguous. "Nick may not eat breakfast" can mean either that I'm not allowed to eat breakfast, or that it's possible I won't eat breakfast.
I think this is the reason why RFC 2119 does not include "MAY NOT" as an option. Let's use "should not" or "or "may refrain from" or "might not" as appropriate?
best wishes,
-
Nick Mathewson -
Nick Mathewson -
teor