210-faster-headless-consensus-bootstrap.txt

Filename: 210-faster-headless-consensus-bootstrap.txt
Title: Faster Headless Consensus Bootstrapping
Author: Mike Perry, Tim Wilson-Brown, Peter Palfrader
Created: 01-10-2012
Last Modified: 02-10-2015
Status: Superseded
Target: 0.2.8.x+

Status-notes:

   * This has been partially superseded by the fallback directory code,
     and partially by the exponential-backoff code.

Overview and Motiviation

 This proposal describes a way for clients to fetch the initial
 consensus more quickly in situations where some or all of the directory
 authorities are unreachable. This proposal is meant to describe a
 solution for bug #4483.

Design: Bootstrap Process Changes

 The core idea is to attempt to establish bootstrap connections in
 parallel during the bootstrap process, and download the consensus from
 the first connection that completes.

 Connection attempts will be performed on an exponential backoff basis.
 Initially, connections will be performed to a randomly chosen hard
 coded directory mirror and a randomly chosen canonical directory
 authority. If neither of these connections complete, additional mirror
 and authority connections are tried. Mirror connections are tried at
 a faster rate than authority connections.

 Clients represent the majority of the load on the network. They can use
 directory mirrors to download their documents, as the mirrors download
 their documents from the authorities early in the consensus validity
 period.

 We specify that client mirror connections retry after one second, and
 then double the retry time with every connection attempt:
 0, 1, 2, 4, 8, 16, 32, ...
 (The timers currently implemented in Tor increment with every
 connection failure.)

 We specify that client directory authority connections retry after
 10 seconds, and then double the retry time with every connection:
 0, 10, 20, ...

 If a client has both an IPv4 and IPv6 address, it will try IPv4 and
 IPv6 mirrors and authorities on the following schedule:
 IPv4, IPv6, IPv4, IPv6, ...

 [ TODO: should we add random noise to these scheduled times? - teor
         Tor doesn’t add random noise to the current failure-based
         timers, but as failures are a network event, they are
         somewhat random/arbitrary already. These attempt-based timers
         will go off every few seconds, exactly erraon the second. ]

 (Relays can’t use directory mirrors to download their documents,
 as they *are* the directory mirrors.)

 The maximum retry time for all these timers is 3 days + 1 hour. This
 places a small load on the mirrors and authorities, while allowing a
 client that regains a network connection to eventually download a
 consensus.

 We try IPv4 first to avoid overloading IPv6-enabled authorities and
 mirrors. Each timing schedule uses a separate IPv4/IPv6 schedule.
 This ensures that clients try an IPv6 authority within the first
 10 seconds. This helps implement #8374 and related tickets.

 We don't want to keep on trying an IP version that always fails.
 Therefore, once sufficient IPv4 and IPv6 connections have been
 attempted, we select an IP version for new connections based on the ratio
 of their failure rates, up to a maximum of 1:5. This may not make a
 substantial difference to consensus downloads, as we only need one
 successful consensus download to bootstrap. However, it is important for
 future features like #17217, where clients try to automatically determine
 if they can use IPv4 or IPv6 to contact the Tor network.

 The retry timers and IP version schedules must reset on HUP and any
 network reachability events, so that clients that have unreliable networks
 can recover from network failures.
 [ TODO: Do we do this for any other timers?
         I think this needs another proposal, it’s out of scope here.
         - teor ]

 The first connection to complete will be used to download the consensus
 document and the others will be closed, after which bootstrapping will
 proceed as normal.

 We expect the vast majority of clients to succeed within 4 seconds,
 after making up to 4 connection attempts to mirrors and 1 connection
 attempt to an authority. Clients which can't connect in the first
 10 seconds, will try 1 more mirror, then try to contact another
 directory authority. We expect almost all clients to succeed within
 10 seconds. This is a much better success rate than the current Tor
 implementation, which fails k/n of clients if k of the n directory
 authorities are down. (Or, if the connection fails in certain ways,
 it will retry once, failing 1-(1-(k/n)^2).)

 If at any time, the total outstanding bootstrap connection attempts
 exceeds 10, no new connection attempts are to be launched until an
 existing connection attempt experiences full timeout. The retry time
 is not doubled when a connection is skipped.

 A benefit of connecting to directory authorities is that clients are
 warned if their clock is wrong. Starting the authority and fallback
 schedules at the same time should ensure that some clients check their
 clock with an authority at each bootstrap.

Design: Fallback Dir Mirror Selection

 The set of hard coded directory mirrors from #572 shall be chosen using
 the 100 Guard nodes with the longest uptime.

 The fallback weights will be set using each mirror's fraction of
 consensus bandwidth out of the total of all 100 mirrors, adjusted to
 ensure no fallback directory sees more than 10% of clients. We will
 also exclude fallback directories that are less than 1/1000 of the
 consensus weight, as they are not large enough to make it worthwhile
 including them.

 This list of fallback dir mirrors should be updated with every
 major Tor release. In future releases, the number of dir mirrors
 should be set at 20% of the current Guard nodes (approximately 200 as
 of October 2015), rather than fixed at 100.

 [TODO: change the script to dynamically calculate an upper limit.]

Performance: Additional Load with Current Parameter Choices

 This design and the connection count parameters were chosen such that
 no additional bandwidth load would be placed on the directory
 authorities. In fact, the directory authorities should experience less
 load, because they will not need to serve the entire consensus document
 for a connection in the event that one of the directory mirrors complete
 their connection before the directory authority does.

 However, the scheme does place additional TLS connection load on the
 fallback dir mirrors. Because bootstrapping is rare, and all but one of
 the TLS connections will be very short-lived and unused, this should not
 be a substantial issue.

 The dangerous case is in the event of a prolonged consensus failure
 that induces all clients to enter into the bootstrap process. In this
 case, the number of TLS connections to the fallback dir mirrors within
 the first second would be 2*C/100, or 40,000 for C=2,000,000 users. If
 no connections complete before the 10 retries, 7 of which go to
 mirrors, this could reach as high as 140,000 connection attempts, but
 this is extremely unlikely to happen in full aggregate.

 However, in the no-consensus scenario today, the directory authorities
 would already experience 2*C/9 or 444,444 connection attempts. (Tor
 currently tries 2 authorities, before delaying the next attempt.) The
 10-retry scheme, 3 of which go to authorities, increases their total
 maximum load to about  666,666 connection attempts, but again this is
 unlikely to be reached in aggregate. Additionally, with this scheme,
 even if the dirauths are taken down by this load, the dir mirrors
 should be able to survive it.

Implementation Notes: Code Modifications

 The implementation of the bootstrap process is unfortunately mixed 
 in with many types of directory activity.

 The process starts in update_consensus_networkstatus_downloads(),
 which initiates a single directory connection through
 directory_get_from_dirserver(). Depending on bootstrap state,
 a single directory server is selected and a connection is
 eventually made through directory_initiate_command_rend().

 There appear to be a few options for altering this code to retry multiple
 simultaneous connections. It looks like we can modify
 update_consensus_networkstatus_downloads() to make connections more often
 if the purpose is DIR_PURPOSE_FETCH_CONSENSUS and there is no valid
 (reasonably live) consensus. We can make multiple connections from
 update_consensus_networkstatus_downloads(), as the sockets are non-blocking.
 (This socket appears to be non-blocking on Unixes (SOCK_NONBLOCK & O_NONBLOCK)
 and Windows (FIONBIO).) As long as we can tolerate a timer resolution of
 ~1 second (due to the use of second_elapsed_callback and time_t), this
 requires no additional timers or callbacks. We can make 1 connection for each
 schedule per second, for a maximum of 2 per second.

 The schedules can be specified in:
 TestingClientBootstrapConsensusAuthorityDownloadSchedule
 TestingClientBootstrapConsensusFallbackDownloadSchedule
 (Similar to the existing TestingClientConsensusDownloadSchedule.)

 TestingServerIPVersionPreferenceSchedule
 (Consisting of a CSV like “4,6,4,6”, or perhaps “0,1,0,1”.)

 update_consensus_networkstatus_downloads() checks the list of pending
 connections and, if it is 10 or greater, skip the connection attempt,
 and leave the retry time constant.

 The code in directory_send_command() and connection_finished_connecting()
 would need to be altered to check that we are not already downloading the
 consensus. If we’re not, then download the consensus on this connection, and
 close any other pending consensus dircons.

 We might also need to make similar changes in authority_certs_fetch_missing(),
 as we can’t use a consensus until we have enough authority certificates.
 However, Tor already makes multiple requests (one per certificate), and only
 needs a majority of certificates to validate a consensus. Therefore, we will
 only need to modify authority_certs_fetch_missing() if clients download a
 consensus, then end up getting stuck downloading certificates. (Current tests
 show bootstrapping working well without any changes to authority certificate
 fetches.)

Reliability Analysis

 We make the pessimistic assumptions that 50% of connections to directory
 mirrors fail, and that 20% of connections to authorities fail. (Actual
 figures depend on relay churn, age of the fallback list, and authority
 uptime.)

 We expect the first 10 connection retry times to be:
 (Research shows users tend to lose interest after 40 seconds.)
 Mirror:   0s  1s  2s    4s    8s           16s             32s
 Auth:     0s                        10s            20s
 Success: 90% 95% 97% 98.7% 99.4% 99.89% 99.94% 99.988% 99.994%

 97%    of clients succeed in the first 2 seconds.
 99.4%  of clients succeed without trying a second authority.
 99.89% of clients succeed in the first 10 seconds.
  0.11% of clients remain, but in this scenario, 2 authorities are
        unreachable, so the client is most likely blocked from the Tor
        network. Alternately, they will likely succeed on relaunch.

 The current implementation makes 1 or 2 authority connections within the
 first second, depending on exactly how the first connection fails. Under
 the 20% authority failure assumption, these clients would have a success
 rate of either 80% or 96% within a few seconds. The scheme above has a
 greater success rate in the first few seconds, while spreading the load
 among a larger number of directory mirrors. In addition, if all the
 authorities are blocked, current clients will inevitably fail, as they
 do not have a list of directory mirrors.