[BUG] Backup groups: input rate estimation during the switch

[maxsharabayko]

Describe the bug

When SRT is configured with SRTO_MAXBW=0 and SRTO_INPUTBW=0, SRT socket estimates the input bitrate in runtime.
However, this estimation is done by a socket individually. When a backup link is activated, the group supplies the backup socket with buffered and not yet acknowledged packets, submitted all one by one without any pacing. This would result in an overestimated input BW, and lead to an undesirably high BW limit.

Expected behavior

(Maybe) Estimate input BW when a packet is submitted to a group, and use the value for member sockets.

Versions Affected

SRT Version: all versions since experimental bonding was introduced (v1.4.2 and later), main/backup mode.

Credits

@J-Rogmann for conducting BW-related switching experiments.

[J-Rogmann]

it looks like explicitly setting MAXBW doesn't help in case of switching from main to backup link. Please see following table:

I first ran into the issue when testing higher bandwidth streams with main/backup. Increasing rcvbuf helped. However, I also tried the same with a bandwidth limit of 10 Mbps on the 2 links and got pretty much the same results with MAXBWset or not set.

BW 1 (Mbps)	BW 2 (Mbps)	Streaming BW (Mbps)	works	packets dropped RCV
100	100	60 rcvbuf=37683200 sndbuf=37683200	yes	831
100	100	50 rcvbuf=37683200 sndbuf=37683200	yes	1154
100	100	50	no	connection died on both sockets
100	100	40 rcvbuf=37683200 sndbuf=37683200	yes	240
100	100	40	yes	446
		30 rcvbuf=37683200 sndbuf=37683200	yes	0
100	100	30	yes	2
100	100	20	yes	0
100	100	10	yes	0
10	10	9	yes	496
10	10	8	yes	555
10	10	7	yes	285
10	10	6	yes	349
10	10	5	yes	120
10	10	4	yes	0
10	10	3	yes	0
10	10	2	yes	0
10	10	1	yes	0
with MAXBW
10	10	9 maxbw=1250000	yes	490
10	10	8 maxbw=1250000	yes	389
10	10	7 maxbw=1250000	yes	482
10	10	6 maxbw=1250000	yes	332
10	10	5 maxbw=1250000	yes	119
10	10	4 maxbw=1250000	yes	1
10	10	3 maxbw=1250000	yes	0
10	10	2 maxbw=1250000	yes	0
10	10	1 maxbw=1250000	yes	0

[ethouris]

There could be done information interchange between the active link and the group, then forceful injecting this information into the newly activated socket so that it starts freshly with already prepared data over the other link.