[BUG] Receiver buffer (msRcvBuf) remains at the same low level of 10ms

[mbakholdina]

Describe the bug

For test cases 1, 2, 4, 5 (see below), the level of receiver buffer (msRcvBuf) is not correct, converges to 10 ms and remains at this level for the whole experiment. For test cases 1, 4, we have latency configured incorrectly: for such a high RTT value it should be at least 3-4 times RTT. For test cases 2, 5, the latency is set to 4RTT, however, the receiver buffer rcvbuf is too small for such a sendrate and latency values. However, this isn't an expected behavior.

Also, for test cases 2, 5, the problem "No room to store incoming packet" can be easily catched. See issue #409, PR #1909.

The problem with receiver buffer level isn't a regression as we have the same test results for both v1.4.2 and the latest master (v1.4.3rc + some commits on top of).

For test cases 3, 6, when the latency and rcvbuf are configured correctly, there is no problems. However, for case 3, rcvbuf = 2812500 isn't enough. The msRcvBuf remains at the level of 2RTT instead of 1RTT. If we increase rcvbuf = 5625000 twice (case 6), the msRcvBuf remains at the level of 1RTT as expected. It means that the formula used for rcvbuf calculation (see below) should be made more precise.

Test Case	sendrate, Mbps	RTT, ms	latency, ms	rcvbuf, bytes	SRT version	Test Result
1	10	500	120 (default)	462500	v1.4.3rc	Rcv buffer remains at the same level of 10 ms
2	10	500	2000 (4RTT)	462500 (buffer size is too small for such latency and bitrate)	v1.4.3rc	No room to store incoming packet Rcv buffer remains at the same level of 10 ms
3	10	500	2000 (4RTT)	2812500	v1.4.3rc	No problems with buffer, msSndBuf is around 2RTT
4	10	500	120 (default)	462500	v1.4.2	Rcv buffer remains at the same level of 10 ms
5	10	500	2000 (4RTT)	462500 (buffer size is too small for such latency and bitrate)	v1.4.2	No room to store incoming packet Rcv buffer remains at the same level of 10 ms
6	10	500	2000 (4RTT)	5625000 (2 times higher than in 3)	v1.4.3rc	No problems with buffer, msSndBuf is around RTT

Experiment time = 120 s.

The value of rcvbuf is calculated using the following formula (see #703):
(latency_ms + rtt_ms / 2) * bps / 1000 / 8

Statistics of interest are byteAvailableRcvBuf, msRcvBuf.

Screenshot for test case 1 - At the beginning of the transmission.

Screenshot for test case 1 - During transmission.

To Reproduce

srt-xtransmit should be used for a reproduction:

# rcv
_build/bin/srt-xtransmit receive "srt://:4200?latency=2000&rcvbuf=2812500" --enable-metrics --metricsfile metrics-rcv.csv --metricsfreq 100ms --statsfile stats-rcv.csv --statsfreq 100ms -v

# snd
_build/bin/srt-xtransmit generate srt://192.168.2.2:4200?latency=2000 --sendrate 10Mbps --duration 120s --enable-metrics --statsfile stats-snd.csv --statsfreq 100ms -v

SRT version tested:

• v1.4.2
• v1.4.3rc + some commits on top of, the latest commit is
  commit c3864aa
  Author: Jose Santiago jsantiago@haivision.com
  Date: Mon Apr 12 03:49:26 2021 -0500

Experiments results are attached.
rcv-buffer-fullness.zip