Implement DTLS restart

[Antonito]

pion/srtp#148 is needed before being able to merge

The unit test needs pion/transport#141

Fixes #1636

I'm not sure about if t.state != DTLSTransportStateNew && t.state != DTLSTransportStateClosed, one other option would be to set the state to New before restarting, but I don't want to trigger a StateChanged event – which could be wrong.
Turns out that's what libwebrtc does

Also the condition on which DTLS is restarted might be too permissive – but it doesn't seem to break any test so far.

[codecov]

Codecov Report

Merging #1846 (ce2fd9b) into master (1765e9b) will decrease coverage by 6.91%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #1846      +/-   ##
==========================================
- Coverage   76.89%   69.98%   -6.92%     
==========================================
  Files          87       64      -23     
  Lines        8937     3258    -5679     
==========================================
- Hits         6872     2280    -4592     
+ Misses       1647      860     -787     
+ Partials      418      118     -300     

Flag	Coverage Δ
go	?
wasm	69.98% <ø> (ø)

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
constants.go	0.00% <0.00%> (-100.00%)	⬇️
icecandidatepair.go	0.00% <0.00%> (-81.82%)	⬇️
rtpcodec.go	13.33% <0.00%> (-80.00%)	⬇️
stats.go	0.00% <0.00%> (-75.00%)	⬇️
track_local.go	0.00% <0.00%> (-66.67%)	⬇️
internal/mux/muxfunc.go	9.09% <0.00%> (-63.64%)	⬇️
icemux.go	0.00% <0.00%> (-54.55%)	⬇️
atomicbool.go	57.14% <0.00%> (-42.86%)	⬇️
sdpsemantics.go	0.00% <0.00%> (-34.49%)	⬇️
internal/mux/endpoint.go	24.13% <0.00%> (-34.49%)	⬇️
... and 36 more

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[Sean-Der]

@Antonito This is absolutely amazing work, your PRs are the best I have seen in months. Gives me the energy to get back into it :)

I think we could make the test a lot simpler.

• Create two PeerConnections and connect them.
• Send one RTP packet
• Create a new PeerConnection and generate an offer, and send it to one of the PeerConnections in the first flow.
• Send one RTP packet

I think that gets us the coverage we need and keeps the LoC low still? Push back if I am wrong!

[Antonito]

@Antonito This is absolutely amazing work, your PRs are the best I have seen in months. Gives me the energy to get back into it :)

I think we could make the test a lot simpler.

• Create two PeerConnections and connect them.
• Send one RTP packet
• Create a new PeerConnection and generate an offer, and send it to one of the PeerConnections in the first flow.
• Send one RTP packet

I think that gets us the coverage we need and keeps the LoC low still? Push back if I am wrong!

So, I tried to rewrite the unit test in a simpler way, and it uncovered several problems. I'm not sure if there are caused by the DTLS restart or were already there (I'm thinking the later, for most of them).

• Sending a single RTP packet doesn't trigger an OnTrack event on the other peer. Sending two in a row doesn't either, but sending 1 / waiting a few ms / sendign another one works.
• When ran multiple times in a row, the test sometimes logs pc ERROR: 2021/06/28 13:30:33 Incoming unhandled RTP ssrc(1595441956), OnTrack will not be fired. single media section has an explicit SSRC. I initially thought this was a difference in behavior with the test I already pushed (since with your method, the first peer connection doesn't go to disconnected or failed before the other one connects), but turns out it's also happening with this one – I just had lucky runs. It doesn't seem to be a race condition, tsan didn't detect anything.
• I found out weird behaviors with DataChannels
  • I added a call to CreateDataChannel, behavior unchanged (as expected)
  • I then set an empty OnDataChannel callback -> this causes the test to sometimes hang (again I'm not doing anything more with DataChannels here)
  • I then tried to send data from ClientB and read with ClientA1 and ClientA2, sometime it hangs, but when it doesn't:
    • As expected, ClientA1 can read
    • After ClientA2 connects, ClientB tries to send data – which causes a io: read/write on closed pipe error log
    • ClientA2 can't read, since ClientB can't send anything (this is most likely an issue with DTLS restart)

[Antonito]

I rewrote the unit test without pion/transport, and split it in two very similar tests, for debugging purpose:

• TestPeerConnection_DTLS_Restart_MediaOnly
• TestPeerConnection_DTLS_Restart_MediaAndDataChannel

Before merging, we should only keep TestPeerConnection_DTLS_Restart_MediaAndDataChannel.

SCTP restart is now implemented and DataChannels are re-opened, if needed.

There seems to be some kind of race condition, but I really don't think it's related to the stuff this PR modifies – I think the unit tests just show an already existing problem.
It's what I was describing above, and now have a bit more validation of what's really happening.

The TestPeerConnection_DTLS_Restart_MediaOnly behaves correctly, in go test. To make it work with tsan, I had to add an extra:

webrtc/peerconnection_media_test.go

Line 1329 in 0d1e50c

triggerMedia()

I think this is a consequence of the first point I was developing in my previous message (about RTP packets).

TestPeerConnection_DTLS_Restart_MediaAndDataChannel behaves correctly with go test, but hangs with tsan. I also tried to add a few extra RTP packets, but it doesn't change any thing. Somehow, the hang seems to be caused by the simple fact of calling CreateDataChannel and OnDataChannel.

[Antonito]

Following up discussion with @davidzhao, I updated both this PR and pion/srtp#148.

In order to get some progress on this, I think we should first focus on merging pion/srtp#148 – we need to figure out @Sean-Der 's concerns about rolloverCounter and lastSequenceNumber. I was careful and really don't think this one introduces any security issue, but wouldn't mind a second opinion :)

As for this PR, it requires a bit more work, there are still issues with TestPeerConnection_DTLS_Restart_MediaAndDataChannel, and I'm far less confident about security issues here. Unfortunately I don't have much time to dedicate to this for now, I'd gladly welcome any help 😁

[davidzhao]

Thanks for picking this back up @Antonito! We'd love to collaborate on this. Will have a look in a few weeks.