Running a high-performance pluggable transports Tor bridge (FOCI 2023 short paper) - tor-relays

4 Sep 2023


      Linus Nordberg and I wrote a short paper that was presented at FOCI
2023. The topic is how to use all the available CPU capacity of a server
running a Tor relay.
This is how the Snowflake bridges are set up. It might also be useful
for anyone running a relay that is bottleneck on the CPU. If you have
ever run multiple relays on one IP address for better scaling (if you
are one of the relay operators affected by the recent
AuthDirMaxServersPerAddr change), you might want to experiment with this
setup. The difference is that all the instances of Tor have the same
relay fingerprint, so they operate like one big relay instead of many
small relays.
https://www.bamsoftware.com/papers/pt-bridge-hiperf/
...
The pluggable transports model in Tor separates the concerns of
anonymity and circumvention by running circumvention code in a
separate process, which exchanges information with the main Tor
process over local interprocess communication. This model leads to
problems with scaling, especially for transports, like meek and
Snowflake, whose blocking resistance does not rely on there being
numerous, independently administered bridges, but which rather forward
all traffic to one or a few centralized bridges. We identify what
bottlenecks arise as a bridge scales from 500 to 10,000 simultaneous
users, and then from 10,000 to 50,000, and show ways of overcoming
them, based on our experience running a Snowflake bridge. The key idea
is running multiple Tor processes in parallel on the bridge host, with
externally synchronized identity keys.