Hi,
My comments are inline.
Filename: 306-ipv6-happy-eyeballs.txt Title: A Tor Implementation of IPv6 Happy Eyeballs Author: Neel Chauhan Created: 25-Jun-2019 Supercedes: 299 Status: Open Ticket: https://trac.torproject.org/projects/tor/ticket/29801
- Introduction
As IPv4 address space becomes scarce, ISPs and organizations will
deploy
IPv6 in their networks. Right now, Tor clients connect to guards using IPv4 connectivity by default.
When networks first transition to IPv6, both IPv4 and IPv6 will be
enabled
on most networks in a so-called "dual-stack" configuration. This is
to not
break existing IPv4-only applications while enabling IPv6 connectivity. However, IPv6 connectivity may be unreliable and clients should be able to connect to the guard using the most reliable technology, whether
IPv4
or IPv6.
The big problem that happy eyeballs was meant to solve was that often you might have something announcing an IPv6 prefix but that routing was not properly configured, so while the operating system thought it had IPv6 Internet it was actually just broken. In some cases, the IPv6 Internet would be partitioned as there weren't enough backup routes to fail over to in times of outages. For most purposes, as I understand it, this means either IPv6 connectivity to a host is there or it's not. There's not really a middle ground where it sometimes works but is flaky (i.e. where you can maintain a connection but it has high packet loss).
In ticket #27490, we introduced the option ClientAutoIPv6ORPort which lets a client randomly choose between IPv4 or IPv6. However, this random decision does not take into account unreliable connectivity or falling back to the competing IP version should one be unreliable or unavailable.
One way to select between IPv4 and IPv6 on a dual-stack network is a so-called "Happy Eyeballs" algorithm as per RFC 8305. In one, a client attempts the preferred IP family, whether IPv4 or IPv6. Should it work, the client sticks with the preferred IP family. Otherwise, the client attempts the alternate version. This means if a dual-stack client has both IPv4 and IPv6, and IPv6 is unreliable, preferred or not, the client uses IPv4, and vice versa. However, if IPv4 and IPv6 are both equally reliable, and IPv6 is preferred, we use IPv6.
This sounds like a good candidate for a consensus parameter, such that we can switch the preference for all clients at once, not just the ones that have updated to the version we switch the preference in.
There may also be other ordering parameters for the address candidates. We might want to avoid using IPv6 addresses that are using 6to4 or Teredo as we *know* those are tunnels and thus have encapsulation overhead, higher latency, and funnel all the traffic through centralised (even if distributed) points in the network.
In Proposal 299, we have attempted a IP fallback mechanism using
failure
counters and preferring IPv4 and IPv6 based on the state of the
counters.
However, Prop299 was not standard Happy Eyeballs and an alternative, standards-compliant proposal was requested in [P299-TRAC] to avoid
issues
from complexity caused by randomness.
This proposal describes a Tor implementation of Happy Eyeballs and is intended as a successor to Proposal 299.
- Address Selection
To be able to handle Happy Eyeballs in Tor, we will need to modify the data structures used for connections to guards, namely the extend info structure.
The extend info structure should contain both an IPv4 and an IPv6
address.
This will allow us to try IPv4 and the IPv6 addresses should both be available on a relay and the client is dual-stack.
The Happy Eyeballs specification doesn't just talk about having one v4 and one v6 address. In some cases, relays may be multihomed and so may have multiple v4 or v6 addresses. We should be able to race all the candidates.
When parsing relay descriptors and filling in the extend info data structure, we need to fill in both the IPv4 and IPv6 address if
they both
are available. If only one family is available for a relay (IPv4 or
IPv6),
we should fill in the address for preferred family and leave the
alternate
family null.
To match the IETF protocol more closely, we should have a list of candidate addresses and order them according to our preferences.
- Connecting To A Relay
If there is an existing authenticated connection, we should use it similar to how we used it pre-Prop306.
If there is no existing authenticated connection for an extend info, we should attempt to connect using the first available, allowed, and preferred address.
We should also allow falling back to the alternate address. For this, three alternate designs will be given.
3.1. Proposed Designs
This subsection will have three proposed designs for connecting to
relays
via IPv4 and IPv6 in a Tor implementation of Happy Eyeballs. The
proposed
designs are as listed as follows:
Section 3.1.1: First Successful Authentication
Section 3.1.2: TCP Connection to Preferred Address On First
Authenticated
Connection
- Section 3.1.3: TCP Connection to Preferred Address On First TCP
Success
3.1.1. First Successful Authentication
In this design, Tor will first connect to the preferred address and attempt to authenticate. After a 1.5 second delay, Tor will connect to the alternate address and try to authenticate. On the first successful authenticated connection, we close the other connection.
This design places the least connection load on the network, but might add extra TLS load.
The delay seems arbitrary. OnionPerf collects data on latency in the Tor network, and could be used to inform better timing choices for the best end user performance (the happiest eyeballs).
If we choose to take this route, we should open new connections with a timeout of ~250ms, and only change the condition for deciding which is the connection we will use.
3.1.2. TCP Connection to Preferred Address On First Authenticated
Connection
This design attempts a TCP connection to a preferred address. On a failure or a 250 ms delay, we try the alternative address.
On the first successful TCP connection Tor attempts to authenticate immediately. On the authentication failure, or a 1.5 second delay, Tor closes the other connection.
This design is the most reliable for clients, but increases the connection load on dual-stack guards and authorities.
Creating TCP connections is not a huge issue, and we should be racing the connections with the ~250ms timeout anyway. All the designs will have this issue.
3.1.3. TCP Connection to Preferred Address On First TCP Success
In this design, we will connect via TCP to the first preferred address. On a failure or after a 250 ms delay, we attempt to connect via TCP to the alternate address. On a success, Tor attempts to authenticate and closes the other connection.
This design is the closest to RFC 8305 and is similar to how Happy Eyeballs is implemented in a web browser.
This is probably also the "simplest" to implement, as it means that the happy eyeballs algorithm is contained to the socket handling code.
I don't believe that requiring authentication to complete is going to do anything more than generate load on relays. Either the packet loss is high enough that the three way handshake fails, or there is low packet loss. I don't think the case where requiring an additional few packets make it through helps you choose a better connection is going to be that common.
Of course it is always possible to add a "PreferredAddressFamily" option to torrc for those that know they are on a bad IPv6 network.
3.2. Recommendations for Implementation of Section 3.1 Proposals
We should start with implementing and testing the implementation as described in Section 3.1.1 (First Successful Authentication), and then doing the same for the implementations described in 3.1.2 and 3.1.3 if desired or required.
I'd want to see some justification with some experimental (or even anecdotal) data as to why first successful authentication is the way to go. 3.1.3 is going to be the simpler option and, in my opinion, the best place to start.
3.1.3 can likely be implemented using exactly the algorithm in section 5 of RFC 8305, excluding portions relating to DNS because we already have all the candidates from the server descriptor.
- Handling Connection Successes And Failures
Should a connection to a guard succeed and is authenticated via TLS, we can then use the connection. In this case, we should cancel all other connection timers and in-progress connections. Cancelling the timers is so we don't attempt new unnecessary connections when our existing connection is successful, preventing denial-of-service risks.
However, if we fail all available and allowed connections, we
should tell
the rest of Tor that the connection has failed. This is so we can
attempt
another guard relay.
Some issues that come to mind:
- I wonder how many relay IPv6 addresses are actually using tunnels. At the levels of throughput they use, that overhead adds up. What is the additional bandwidth cost and what is the impact of reduced MSS? - What are the tunables? RFC8305 has some that would be applicable, and probably all of them could be consensus parameters if we wanted to tune them: * First Address Family Count * Connection Attempt Delay * Minimum Connection Attempt Delay * Maximum Connection Attempt Delay - How do we know what is going on? We do not collect metrics from clients about their usage, but we do collect metrics from relays. Are there any counters we should be adding to extra info descriptors to help us see whether or not this is working? Could clients help relays by reporting that a connection is being closed because they have another connection? (I don't know the answer, but RFC8305 does explicitly point out that it is a mitigation technique designed to hide problems from the user, which means that those problems might come back to haunt us later if we're not on top of them.)
Thanks, Iain.