Exam 300-815 topic 1 question 12 discussion

Actual exam question from Cisco's 300-815

Question #: 12
Topic #: 1

[All 300-815 Questions]

Which description of RTP timestamps or sequence numbers is true?

A. The sequence number is used to detect losses.
B. Timestamps increase by the time "carrying" by a packet.
C. Sequence numbers increase by four for each RTP packet transmitted.
D. The timestamp is used to place the incoming audio and video packets in the correct timing order (playout delay compensation).

Show Suggested Answer

Suggested Answer: D 🗳️
Reference:
https://www.cs.columbia.edu/~hgs/rtp/faq.html

by AgshinA at March 9, 2021, 10:23 a.m.

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AgshinA

Highly Voted 4 years, 3 months ago

The timestamp is used to place the incoming audio and video packets in the correct timing order (playout delay compensation). The sequence number is mainly used to detect losses. Sequence numbers increase by one for each RTP packet transmitted, timestamps increase by the time "covered" by a packet. For video formats where a video frame is split across several RTP packets, several packets may have the same timestamp. In some cases such as carrying DTMF (touch tone) data (RFC 2833), RTP timestamps may not be monotonic. D is corect

upvoted 7 times

stocaxx

3 years, 4 months ago

so also A is correct. The sequence number is mainly used to detect losses

upvoted 1 times

Mert_kerna

3 years, 1 month ago

Right, but this is a Cisco exam. If it's not an absolute answer to the exact question, and there's another answer that is more correct, it's wrong.

upvoted 2 times

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Piji

Most Recent 4 months, 2 weeks ago

Selected Answer: D

Why D is correct: The RTP timestamp is primarily used for playout delay compensation, which means it ensures that audio and video packets are played back at the correct time. It helps synchronize media streams and smooth out network jitter (variations in packet arrival time). Without timestamps, audio and video playback would be choppy or out of sync. Why A is not the best choice: The RTP sequence number does help detect packet loss, but its primary function is not related to timing or playback order. The sequence number only detects missing packets (e.g., gaps in numbering), but it does not help with timing adjustments or delay compensation. If packets arrive out of order, the sequence number alone cannot fix playback timing—timestamps are required for that.

upvoted 1 times

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