Tor relays and authorities can now serve clients an abbreviated version of the consensus document, containing only the changes since an older consensus document that the client holds. Clients now request these documents when available. When both client and server use this new protocol, they will use far less bandwidth (up to 94% less) to keep the client's consensus up-to-date. Implements proposal 140; closes ticket 13339. Based on work by Daniel Martí.
Tor can now compress directory traffic with lzma or with zstd compression algorithms, which can deliver better bandwidth performance. Because lzma is computationally expensive, it's only used for documents that can be compressed once and served many times. Support for these algorithms requires that tor is built with the libzstd and/or liblzma libraries available. Implements proposal 278; closes ticket 21662.
Relays now perform the more expensive compression operations, and consensus diff generation, in worker threads. This separation avoids delaying the main thread when a new consensus arrives.
Major features (experimental):
Tor can now build modules written in Rust. To turn this on, pass the "--enable-rust" flag to the configure script. It's not time to get excited yet: currently, there is no actual Rust functionality beyond some simple glue code, and a notice at startup to tell you that Rust is running. Still, we hope that programmers and packagers will try building Tor with Rust support, so that we can find issues and solve portability problems. Closes ticket 22106.
