Aiortc-datachannel-only

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Data channels
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* Do not raise congestion window when it is not fully utilized.
* Fix Highest TSN Newly Acknowledged logic for striking lost chunks.
* Do not limit congestion window to 120kB, limit burst size instead.

Media
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* Skip RTX packets with an empty payload.

Examples
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* `apprtc` : make the initiator send messages using an HTTP POST instead of WebSocket.
* `janus` : new example to connect to the Janus WebRTC server.
* `server` : add cartoon effect to video transforms.

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DTLS
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* Do not use DTLSv1_get_timeout after DTLS handshake completes.

Data channels
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* Add setter for :attr:`RTCDataChannel.bufferedAmountLowThreshold`.
* Use `crc32c` package instead of `crcmod`, it provides better performance.
* Improve parsing and serialization code performance.
* Disable logging code if it is not used to improve performance.

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DTLS
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* Do not bomb if SRTP is received before DTLS handshake completes.

Data channels
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* Implement unordered delivery, so that the `ordered` option is honoured.
* Implement partial reliability, so that the `maxRetransmits` and `maxPacketLifeTime` options are honoured.

Media
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* Put all tracks in the same stream for now, fixes breakage introduced in 0.9.14.
* Use case-insensitive comparison for codec names.
* Use a=msid attribute in SDP instead of SSRC-level attributes.

Examples
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* `server` : make it possible to select unreliable mode for data channels.
* `server` : print the round-trip time for data channel messages.

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DTLS
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* Log OpenSSL errors if the DTLS handshake fails.
* Fix DTLS handshake in server mode with OpenSSL < 1.1.0.

Media
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* Add :meth:`RTCRtpReceiver.getCapabilities` and :meth:`RTCRtpSender.getCapabilities`.
* Add :meth:`RTCRtpReceiver.getSynchronizationSources`.
* Add :meth:`RTCRtpTransceiver.setCodecPreferences`.

Examples
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* `server` : make it possible to force audio codec.
* `server` : shutdown cleanly on Chrome which lacks :meth:`RTCRtpTransceiver.stop`.

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Data channels
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* Emit a warning if the crcmod C extension is not present.

Media
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* Support subsequent offer / answer exchanges.
* Route RTCP parameters to RTP receiver and sender independently.
* Fix a regression when the remote SSRC are not known.
* Fix VP8 descriptor parsing errors detected by fuzzing.
* Fix H264 descriptor parsing errors detected by fuzzing.

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Media
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* Add support for RTX retransmission packets.
* Fix RTP and RTCP parsing errors detected by fuzzing.
* Use case-insensitive comparison for hash algorithm in SDP, fixes interoperability with Asterisk.
* Offer NACK PLI and REMB feedback mechanisms for H.264.