Issue of RtcpNackResponder incoming()

[zhan6841]

Issue of RtcpNackResponder incoming()

[paullouisageneau]

It will trigger an SRTP error status srtp_err_status_replay_fail on the receiver side (which is also implemented w/ libdatachannel), indicating the sequence number of an SSRC does not strictly increase, if I understand it correctly. This happens in DtlsSrtpTransport::recvMedia() and a related discussion is here #866

libsrtp does not rely on strictly increasing sequence number, as the network might reorder packets. Instead, it maintains a replay window to mark which packets were received. The window size is set to 1024 in libdatachannel:

libdatachannel/src/impl/dtlssrtptransport.cpp

Line 347 in b5518ff

inbound.window_size = 1024;

If a retransmitted packet triggers srtp_err_status_replay_fail on receiver side, it means the packet was actually received before, so the retransmission was useless and the retransmitted packet can be dropped. This is harmless and can happen under normal operation.

Issue #866 is a different thing, it is about sending (not receiving) with the same SSRC and sequence number going in the past because of a inconsistent RTP flow on user side. It'll typically cause libsrtp to fail with err_status_replay_old as the sequence is out of the replay window.

[zhan6841]

Thanks for the explanation. I think I figured the real issue out after reading it! It's not due to libdatachannel but my own implementation.