Also at: https://gitweb.torproject.org/user/mikeperry/torspec.git/blob/consensus-boot...

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Title: Faster Headless Consensus Bootstrapping Author: Mike Perry Created: 01-10-2012 Status: Open Target: 0.2.4.x+

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 100.

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 aggregate.

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 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 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.