About libdatachannel media-sender example

[JulisCheng2020]

Hi everyone,

I'm studying data channel. I get the lib of datachannel by vcpkg and build the media-sender example.
After ICE state changed to connected, I generate RTP stream by ffmpeg.
Then the example reports below errors when send data:

2024-02-29 14:08:29.002 DEBUG [41608] [rtc::impl::Init::doInit@113] Global initialization
2024-02-29 14:08:29.004 DEBUG [41608] [rtc::impl::Init::doInit@123] Spawning 12 threads
2024-02-29 14:08:29.034 DEBUG [47476] [rtc::impl::PollService::runLoop@164] Poll service started
2024-02-29 14:08:29.215 DEBUG [50708] [rtc::impl::Certificate::Generate@380] Generating certificate (OpenSSL)
2024-02-29 14:08:29.216 DEBUG [41608] [rtc::impl::IceTransport::IceTransport@58] Initializing ICE transport (libjuice)
2024-02-29 14:08:29.225 DEBUG [41608] [rtc::impl::PeerConnection::processLocalDescription@936] Adding media to local description, mid="video", removed=false
2024-02-29 14:08:29.239 INFO  [41608] [rtc::impl::PeerConnection::changeSignalingState@1228] Changed signaling state to have-local-offer
2024-02-29 14:08:29.240 INFO  [41608] [rtc::impl::PeerConnection::changeGatheringState@1215] Changed gathering state to in-progress
Gathering State: in-progress2024-02-29 14:08:29.240 INFO  [41608] [rtc::impl::IceTransport::LogCallback@363] juice: agent.c:1067: Changing state to gathering

2024-02-29 14:08:29.242 INFO  [41608] [rtc::impl::IceTransport::LogCallback@363] juice: agent.c:1067: Changing state to connecting
2024-02-29 14:08:29.243 INFO  [41608] [rtc::impl::PeerConnection::changeIceState@1197] Changed ICE state to checking
2024-02-29 14:08:29.243 INFO  [41608] [rtc::impl::PeerConnection::changeState@1179] Changed state to connecting
State: 2024-02-29 14:08:29.243 INFO  [41608] [rtc::impl::IceTransport::LogCallback@363] juice: agent.c:2395: Candidate gathering done
connecting
2024-02-29 14:08:29.243 INFO  [41608] [rtc::impl::PeerConnection::changeGatheringState@1215] Changed gathering state to complete
Gathering State: complete
RTP video stream expected on localhost:6000
Please copy/paste the answer provided by the browser:
{"sdp":"v=0\r\no=rtc 1943048762 0 IN IP4 127.0.0.1\r\ns=-\r\nt=0 0\r\na=group:BUNDLE video\r\na=group:LS video\r\na=msid-semantic:WMS *\r\na=setup:actpass\r\na=ice-ufrag:cSJL\r\na=ice-pwd:vN7No3JtW3ivXzfUwTiaro\r\na=ice-options:ice2,trickle\r\na=fingerprint:sha-256 51:67:DC:95:86:9D:18:51:E2:30:E6:A8:F4:57:1E:AD:EF:96:8B:E1:0D:E1:C0:F0:02:CA:E0:66:BE:B0:1A:0D\r\nm=video 51191 UDP/TLS/RTP/SAVPF 96\r\nc=IN IP4 172.27.48.1\r\na=mid:video\r\na=sendonly\r\na=ssrc:42 cname:video-send\r\na=rtcp-mux\r\na=rtpmap:96 H264/90000\r\na=rtcp-fb:96 nack\r\na=rtcp-fb:96 nack pli\r\na=rtcp-fb:96 goog-remb\r\na=fmtp:96 profile-level-id=42e01f;packetization-mode=1;level-asymmetry-allowed=1\r\na=candidate:1 1 UDP 2122317823 172.27.48.1 51191 typ host\r\na=candidate:2 1 UDP 2122317567 192.168.12.237 51191 typ host\r\na=end-of-candidates\r\n","type":"offer"}
{"type":"answer","sdp":"v=0\r\no=- 3803703392084638642 2 IN IP4 127.0.0.1\r\ns=-\r\nt=0 0\r\na=group:BUNDLE video\r\na=msid-semantic: WMS\r\nm=video 9 UDP/TLS/RTP/SAVPF 96\r\nc=IN IP4 0.0.0.0\r\na=rtcp:9 IN IP4 0.0.0.0\r\na=candidate:2692239017 1 udp 2113937151 f3441e2b-82f0-4edd-b232-01f180112c96.local 59952 typ host generation 0 network-cost 999\r\na=ice-ufrag:zM1X\r\na=ice-pwd:aVohsLr4TwRF8BLrUsPu5BQa\r\na=ice-options:trickle\r\na=fingerprint:sha-256 E2:54:59:3E:1D:C5:A0:A6:9B:FB:A4:CB:D7:EA:E6:DB:F8:DD:11:BD:05:E8:4C:A7:41:68:B9:1D:F7:DE:AB:77\r\na=setup:active\r\na=mid:video\r\na=recvonly\r\na=rtcp-mux\r\na=rtpmap:96 H264/90000\r\na=rtcp-fb:96 goog-remb\r\na=rtcp-fb:96 nack\r\na=rtcp-fb:96 nack pli\r\na=fmtp:96 level-asymmetry-allowed=1;packetization-mode=1;profile-level-id=42e01f\r\n"}
2024-02-29 14:40:48.301 INFO  [41608] [rtc::impl::IceTransport::LogCallback@363] juice: agent.c:2379: Connectivity timer started
2024-02-29 14:40:48.302 INFO  [41608] [rtc::impl::PeerConnection::changeSignalingState@1228] Changed signaling state to stable
2024-02-29 14:40:49.352 INFO  [30820] [rtc::impl::IceTransport::LogCallback@363] juice: agent.c:1067: Changing state to connected
2024-02-29 14:40:49.352 INFO  [30820] [rtc::impl::PeerConnection::changeIceState@1197] Changed ICE state to connected
2024-02-29 14:40:49.352 INFO  [30820] [rtc::impl::PeerConnection::initDtlsTransport@253] This connection requires media support
2024-02-29 14:40:49.353 DEBUG [30820] [rtc::impl::DtlsTransport::DtlsTransport@733] Initializing DTLS transport (OpenSSL)
2024-02-29 14:40:49.377 WARN  [30820] [rtc::impl::DtlsTransport::DtlsTransport@805] AES-GCM for SRTP is not supported, falling back to default profile
2024-02-29 14:40:49.378 DEBUG [30820] [rtc::impl::DtlsSrtpTransport::DtlsSrtpTransport@68] Initializing DTLS-SRTP transport
2024-02-29 14:40:49.379 DEBUG [30820] [rtc::impl::DtlsTransport::start@837] Starting DTLS transport
2024-02-29 14:40:49.385 INFO  [30820] [rtc::impl::IceTransport::LogCallback@363] juice: agent.c:1067: Changing state to completed
2024-02-29 14:40:49.385 INFO  [30820] [rtc::impl::PeerConnection::changeIceState@1197] Changed ICE state to completed
2024-02-29 14:40:49.424 INFO  [8004] [rtc::impl::IceTransport::LogCallback@363] juice: udp.c:256: IP Differentiated Services are not supported on Windows
2024-02-29 14:40:49.442 INFO  [44940] [rtc::impl::DtlsTransport::doRecv@960] DTLS handshake finished
2024-02-29 14:40:49.442 INFO  [44940] [rtc::impl::DtlsSrtpTransport::postHandshake@301] Deriving SRTP keying material (OpenSSL)
2024-02-29 14:40:49.442 DEBUG [44940] [rtc::impl::DtlsSrtpTransport::postHandshake@307] SRTP profile is: SRTP_AES128_CM_SHA1_80
2024-02-29 14:40:49.444 INFO  [44940] [rtc::impl::PeerConnection::changeState@1179] Changed state to connected
State: connected
2024-02-29 14:41:01.974 WARN  [41608] [rtc::impl::IceTransport::LogCallback@363] juice: conn_poll.c:418: Send failed, datagram is too large
2024-02-29 14:41:01.975 WARN  [41608] [rtc::impl::IceTransport::LogCallback@363] juice: conn_poll.c:418: Send failed, datagram is too large
2024-02-29 14:41:01.975 WARN  [41608] [rtc::impl::IceTransport::LogCallback@363] juice: conn_poll.c:418: Send failed, datagram is too large
2024-02-29 14:41:01.976 WARN  [41608] [rtc::impl::IceTransport::LogCallback@363] juice: conn_poll.c:418: Send failed, datagram is too large
2024-02-29 14:41:01.976 WARN  [41608] [rtc::impl::IceTransport::LogCallback@363] juice: conn_poll.c:418: Send failed, datagram is too large

Thanks

[paullouisageneau]

As the error indicates, datagrams are too large. This is because encryption adds a few bytes so you need to allow for a sufficient margin.

You should reduce the size of RTP packets produced by ffmpeg by appending for instance ?pkt_size=1200 to the output URL.