[tor-relays] libzstd and/or liblzma

[Nick Mathewson]

On Tue, Jun 6, 2017 at 8:08 PM, Felix <zwiebel at quantentunnel.de> wrote:
> Hi everybody
>
> Can somebody please help me understand the change log for 3.1.x: "Support
> for these algorithms requires that tor is built with the libzstd and/or
> liblzma libraries available." Is it AND or OR or something whatever
> different ?


Hi!  Let me try to clear it up:

If Tor is built with liblzma available, it will use liblzma when
appropriate.  The lzma format is expensive to calculate, but it
provides very good compression, so we only use it for cases when we
can compress something once and server it many many times -- like
consensus documents, or consensus diffs.

If Tor is built with libzstd available, it will use libzstd when
appropriate. The zstd format is cheap to calculate, but appears to
provide better compression than zlib on our data.

If Tor is built with both libraries available, it will use either one
when appropriate.

Of course, we can only use these compression formats when both sides
support them.

best,
--
Nick