[anti-censorship-team] SCTP-Based Pluggable Transport Prototype

[David Fifield]

On Thu, Aug 12, 2021 at 12:07:05PM -0500, Alexander Mages wrote:
> The transport is functional but not yet tested with the Tor Browser. I
> prototyped this transport using Ncat for SCTP and Telnet for TCP. It is written
> in Python 3 and SCTP functionality was implemented using C/POSIX-like socket
> parameters, removing the necessity of a third-party library.
> 
> A write-up attached to this email denotes functionality, dependencies,
> limitations, and instructions for running the transport.

Thanks for making this contribution. I was able to get it running with a
few tweaks to the instructions (sent to you separately).

SCTP adds an interesting design dimension, because there's a separate
layer of streams within the overall TCP connection. As I understand it,
the sctPT client takes an incoming TCP connection and proxies it forward
as a single SCTP stream within a single SCTP connection. (This is how
ncat --sctp works.) An alternative design would be for the sctPT client
to establish a single connection to the sctPT server (it could do this
even before receiving any client TCP connections) and then forward each
incoming connection as a separate stream within that global connection.
See the distinction between the "one-to-one style" and "one-to-many
style" in https://linux.die.net/man/7/sctp.

As you discovered, pyptlib is unmaintained. It used to be that the
pluggable transports shipped with Tor Browser were written in Python,
but Python proved difficult to deploy and today it's more common to use
Go and goptlib. I am not sure whether it is possible to use SCTP sockets
from Go, but if it is, you can probably easily adapt the "dummy" example
transports from goptlib to use SCTP. (In the one-to-one style, at least;
one-to-many style would require additional refactoring.) You would have
to change the net.Dial("tcp") and net.ListenTCP("tcp") lines:
https://gitweb.torproject.org/pluggable-transports/goptlib.git/tree/examples/dummy-client/dummy-client.go?h=v1.1.0#n45
https://gitweb.torproject.org/pluggable-transports/goptlib.git/tree/examples/dummy-server/dummy-server.go?h=v1.1.0#n84

> The concept for this transport was derived from this wiki page:
> https://gitlab.torproject.org/legacy/trac/-/wikis/doc/PluggableTransports/ideas.

I would like to add a link to your work from this page, but
unfortunately since the change in wiki hosting from Trac to Gitlab, I am
not able to edit the page. Maybe we can make a new page under the
anti-censorship namespace.