[tor-dev] Proposal: Faster Headless Consensus Bootstrapping
mikeperry at torproject.org
Thu Oct 11 09:32:14 UTC 2012
Title: Faster Headless Consensus Bootstrapping
Author: Mike Perry
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 done in batches of three. Only one
connection will be performed to one of the canonical directory
authorities. Two connections will be performed to randomly chosen hard
coded directory mirrors.
If no connections complete within 5 seconds, another batch of three
connections will be launched. Otherwise, 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.
If at any time, the total outstanding bootstrap connection attempts
exceeds 15, no new connection attempts are to be launched until existing
connection attempts experience full timeout.
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.
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, rather than fixed at
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 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 initial TLS connections to the fallback dir mirrors
would be 2*C/100, or 10,000 for C=500,000 users. If no connections
complete before the five retries, this could reach as high as 50,000
connection attempts, but this is extremely unlikely to happen in full
However, in the no-consensus scenario today, the directory authorities
would already experience C/9 or 55,555 connection attempts. The
5-retry scheme increases their total maximum load to about 275,000
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
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 perform
multiple connections. Without refactoring, one approach would be
to make multiple calls to directory_initiate_command_routerstatus()
from directory_get_from_dirserver() if the purpose is
DIR_PURPOSE_FETCH_CONSENSUS and the only directory servers available
are the authorities and the fallback dir mirrors.
The code in directory_initiate_command_rend() would then need to be
altered to maintain a list of the dircons created for this purpose as
well as avoid immediately queuing the directory_send_command() request
for the DIR_PURPOSE_FETCH_CONSENSUS purpose. A flag would need to be set
on the dircon to be checked in connection_dir_finished_connecting().
The function connection_dir_finished_connecting() would need to be
altered to examine the list of pending dircons, determine if this one is
the first to complete, and if so, then call directory_send_command() to
download the consensus and close the other pending dircons.
An additional timer would need to be installed to re-call
update_consensus_networkstatus_downloads() or a related helper after 5
seconds. connection_dir_finished_connecting() would cancel this timer.
The helper would check the list of pending connections and ensure it
never exceeds 15.
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