RTP Timestamps and Sequence Numbers

D.

RTP (Real-time Transport Protocol) is a protocol used for transmitting audio and video over IP networks. RTP uses sequence numbers and timestamps to ensure that packets are delivered in the correct order and to detect any packet loss.

A. The sequence number is used to detect losses: The sequence number in RTP is a 16-bit field that is incremented by one for each RTP data packet transmitted in a session. The sequence number is used to detect losses in the data packets. When a receiver receives a data packet, it checks the sequence number to ensure that the packet has arrived in the correct order. If there is a gap in the sequence numbers, it indicates that one or more packets have been lost. The receiver can then request a retransmission of the missing packet(s).

B. Timestamps increase by the time "carrying" by a packet: The timestamp in RTP is a 32-bit field that is included in every RTP data packet. The timestamp represents the time at which the first sample in the packet was sampled. The timestamp increases by the time duration of the media carried in the packet. This allows the receiver to determine the playout time for the packet and to synchronize the media with other streams.

C. Sequence numbers increase by four for each RTP packet transmitted: This statement is incorrect. The sequence number in RTP is incremented by one for each RTP data packet transmitted, not by four.

D. The timestamp is used to place the incoming audio and video packets in the correct timing order (playout delay compensation): The timestamp in RTP is used to place incoming audio and video packets in the correct timing order, which is necessary for playout delay compensation. The receiver uses the timestamp to determine the arrival time of the packet and to calculate the amount of delay needed before the packet can be played out. This ensures that the media is played out in the correct order and at the correct time.

Therefore, option D is the correct answer to the given question.