[tor-bugs] #28942 [Circumvention/Snowflake]: Evaluate pion WebRTC

[Tor Bug Tracker & Wiki]

#28942: Evaluate pion WebRTC
--------------------------------------------+------------------------------
 Reporter:  backkem                         |          Owner:  cohosh
     Type:  enhancement                     |         Status:  accepted
 Priority:  Medium                          |      Milestone:
Component:  Circumvention/Snowflake         |        Version:
 Severity:  Normal                          |     Resolution:
 Keywords:  anti-censorship-roadmap-august  |  Actual Points:
Parent ID:                                  |         Points:  5
 Reviewer:                                  |        Sponsor:
                                            |  Sponsor28-must
--------------------------------------------+------------------------------

Comment (by cohosh):

 In addition to the issues above, which can be solved with the attached
 patch, the proxy is very slow to generate SDP answers causing the broker
 to timeout. Here are some logs:

 {{{
 2019/09/04 18:01:04 broker returns: 504
 2019/09/04 18:01:04 Received offer.
 2019/09/04 18:01:24 Setting remote description
 2019/09/04 18:01:24 sdp offer successfully received.
 2019/09/04 18:01:24 Generating answer...
 2019/09/04 18:01:24 error sending answer to client through broker: broker
 returned 410
 }}}

 I added the {Received offer} log message
 [https://github.com/cohosh/snowflake/blob/pion/proxy-go/snowflake.go#L349
 here] as a local change.

 You can see that it takes 20 seconds between receiving the offer and
 generating an answer and after further instrumenting, it appears to be
 caused by the `NewPeerConnection` function
 [https://github.com/cohosh/snowflake/blob/pion/proxy-go/snowflake.go#L280
 here].

 Note that the proxy-go instances don't use the trickle method and so ICE
 gathering needs to complete before this function returns. I'll try using
 the trickle method and setting an `OnICEGatheringStateChange` callback to
 set the answer when it completes.

 To be honest, starting ICE gathering in `NewPeerConnection` doesn't make
 sense to me from a design point of view. I would expect it to occur in
 `CreateAnswer` instead (with the trickle method it starts in
 `SetLocalDescription` which I also find unintuitive). The patch above also
 removes a call to `Gather` from `SetRemoteDescription` which also confused
 me.

--
Ticket URL: <https://trac.torproject.org/projects/tor/ticket/28942#comment:54>
Tor Bug Tracker & Wiki <https://trac.torproject.org/>
The Tor Project: anonymity online