Cisco Bug: CSCvi26101 - Large, Fragmented IP Packets Can Cause Converted MP4 and WAV Recording to Have Audio Out of Sync
Dec 12, 2018
- Cisco MediaSense
Known Affected Releases
Symptom: The RTP is written to the raw SMD file after the final IP packet from the fragment comes into MediaSense. If one of the RTP streams has any large, fragmented IP packets, that RTP stream will be delayed with respect to the other RTP streams. For example, if two RTP streams are being recorded, we expect each packet received to have the same amount of audio conversation since the same codec is used for all audio streams. If one stream has IP fragments due to the payload being very large, there will be more audio for that stream. For stream A using G711, there is 20ms of audio per packet. Say that 5 packets in a row are sent to MediaSense. For stream B also using G711, there is 20ms of audio per packet. Say that all 5 packets are sent to MediaSense but are part of the same IP fragment. MediaSense records the packet arrival on the SMD file for each stream in the following way: Stream A SMD file: 1. Packet 1 received at t=20ms 2. Packet 2 received at t=40ms 3. Packet 3 received at t=60ms 4. Packet 4 received at t=80ms 5. Packet 5 received at t=100ms Stream B: 1. Packets 1, 2, 3, 4, 5 received at t=100ms When converting the audio to MP4 or WAV format, the audio from each packet starts to play at the time that it was recorded as arriving on the SMD file. For stream B, all of the audio is played, but the audio from packet 1 is played starting at 100ms instead of t=20ms, which is the start time that we would expect it to be played if there was no IP packet fragmentation. When audio is played through a JNPL player, each IP fragment is played in real time arrival, so there is not delay for live streamed audio if there was IP fragmentation. Conditions: IP fragmentation occurring on one or more RTP streams but not all RTP streams that are part of the call. Delay is introduced in playing back audio in the streams that had the IP fragmentation. If the same/similar amount of RTP stream IP fragmentation is occurring on all streams, there is an equal/similar amount of delay recorded in the arrival times for the packets in the SMD file. No audio synchronization issues will be detected in this case.
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