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hi, i have a pcap containing RTP packets. in each packets of this pcap, marker bit is set and sequence number increments for each packet. is it rigth that marker bit is set in each packet?

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asked 13 Jul, 06:34

ghader's gravatar image

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edited 13 Jul, 06:36

The marker bit is subject to much discussion. The relevance of the marker bit is defined by the so-called RTP profile used. That profile is the set of rules which apply to that RTP stream. It defines for instance the codec used to encode the audio and/or video. It defines how events in the media stream are signalled in the packets, etc.

A common use of the marker bit is for synchronisation. eg. when the profile allows for silence suppression (that is: no packets are transmitted when there's silence) it is helpful for the decoder to know when speech resumes. This is flagged by the marker bit. You might ask 'but when there's silence no packets are transmitted, so why do you need the marker bit?' That is because intermediate RTP packets are used to update the background noise level.

Another example of the use of the marker bit is to signal the start of a new video frame in a sequence of RTP packets carrying video data. This is then according to a video RTP profile.

So depending on the profile used, this may or may not be as specified. On the other hand, many applications don't depend heavily on the marker bit. It varies.

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answered 13 Jul, 07:15

Jaap's gravatar image

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question asked: 13 Jul, 06:34

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last updated: 13 Jul, 07:15

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