[C++] How to set up an SCTP Datachannel ?

[Nicolas]

Hi,

I am trying to modify the peerconnection client example to create an SCTP datachannel. I want it to work between two native clients.

I create the peerconnection object with the constraint kEnableDtlsSrtp, and then I call its CreateDataChannel method. Then sdp are exchanged and when the answer is received, I get an error in SetRemoteDescription: "Failed to set data receive codecs". Immediately after SetRemoteDescription I get a segmentation fault on session_description->type().

If I set "local_port_" manually to the SCTP port number in SctpDataMediaChannel::SetRecvCodecs, I can establish a connection and exchange data using the datachannel, but I get an error (in srtpfilter.cc) "Failed to unprotect SRTP packet, err=2", and (in channel.cc) "Failed to unprotect audio RTP packet:" and "Failed to unprotect video RTP packet:".

Why do I get an error if I do not set "local_port_" like that ? Why do I still get errors if I do ?

Thanks !


Nicolas

[Pau Ceano]

I am in a very similar situation, any advances?

[Jiayang Liu]

Could you attach the offer and answer SDP?

[Pau Ceano]

Sure,
I start communication from my app (a modification of peer_connection_client) and use WebRTCPeerconnection JS example.
Using the same testbench RTP Data channel works well.

Offer:
v=0
o=- 4589503729374787877 2 IN IP4 127.0.0.1
s=-
t=0 0
a=msid-semantic: WMS
m=application 1 DTLS/SCTP 5000
c=IN IP4 0.0.0.0
a=ice-ufrag:txUovafjLjoESJL9
a=ice-pwd:aTRxo6zFYlclnbs2wfQrV/PX
a=ice-options:google-ice
a=fingerprint:sha-1 C4:95:AA:1A:34:C7:D4:0D:DA:71:C0:C5:6E:8A:F6:75:C6:5B:00:88
a=setup:actpass
a=mid:data
a=sctpmap:5000 webrtc-datachannel 1024

Answer (I use ):
v=0
o=- 372770647812769445 2 IN IP4 127.0.0.1
s=-
t=0 0
a=msid-semantic: WMS
m=application 1 DTLS/SCTP 5000
c=IN IP4 0.0.0.0
a=ice-ufrag:jaF+SpqRCiDsZR82
a=ice-pwd:CUOXeU8BpV9vmV/wRNuELnXE
a=fingerprint:sha-256 78:2B:42:F5:0C:2E:15:F6:D2:60:D4:7B:30:12:58:00:70:D4:98:E5:7E:0A:82:08:55:AA:26:DC:A1:0B:96:15
a=setup:active
a=mid:data
a=sctpmap:5000 webrtc-datachannel 1024

And I get same errors explained by Nicolas. Resumed:
Setting local data description
Warning(channel.cc:2265): Failed to set local data description
Error(channel.cc:659): Failure in SetLocalContent with action 0
Error(webrtcsession.cc:253): Failed to set local offer sdp: Session error code: ERROR_CONTENT. Session error description: Failed to set data receive codecs..

Thanks for your help,

Pau

[Philipp Hancke]

I have seen that error recently when one side was using rtp data channels (i.e. was passing RtpDataChannels:true to the peerconnection constructor).

Arguably, the error message might be improved then, but RTP data channels are not worth the effort ;-)

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[Pau Ceano]

Thks, I double-checked that and from the oferer I am only defining EnableDtlsSrtp flag  in the constraints. The receiver is WebRTCPeerconnection and has the RTP check box unchecked.

[Jiayang Liu]

I think this is similar to what Lally saw when he developed the end-to-end tests: he solved it by serializing the SDP to a string and parsing it to get the codec (i.e. SCTP port) populated.

You can try that to unblock yourself. Meanwhile please file a bug at https://code.google.com/p/webrtc/issues/list. 

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[Pau Ceano]

Thanks for the answer but I do not totally understand.
You talk about the offer SDP?
Where I have to define the codec?
is Lally workaround documented?

thks.

[Jiayang Liu]

No documentation as his work is in progress, I think.

You need to modify the code in https://code.google.com/p/chromium/codesearch#chromium/src/third_party/libjingle/source/talk/examples/peerconnection/client/conductor.cc&l=476:

do not do: peer_connection_->SetLocalDescription(
      DummySetSessionDescriptionObserver::Create(), desc);
but do: 
peer_connection_->SetLocalDescription(
      DummySetSessionDescriptionObserver::Create(), CreateSessionDescription("offer", sdp));

[Pau Ceano]

thks. Now it is working.

[Jiayang Liu]

Could you file a bug?

[Pau Ceano]

sure, doing it.