Manual:Queues - Burst

Theory

Burst is a feature that allows to satisfy queue requirement for additional bandwidth even if required rate is bigger that MIR (max-limit) for a short period of time.

Burst can occur only if average-rate of the queue for the last burst-time seconds is smaller that burst-threshold. Burst must stop if average-rate of the queue for the last burst-time seconds is bigger or equal to burst-threshold

Burst mechanism is simple - if burst is allowed max-limit value is replaced by burst-limit value. When burst is disallowed max-limit value remains unchanged.

1. burst-limit (NUMBER) : maximal upload/download data rate which can be reached while the burst is allowed
2. burst-time (TIME) : period of time, in seconds, over which the average data rate is calculated. (This is NOT the time of actual burst)
3. burst-threshold (NUMBER) : basically this is value of burst on/off switch.
4. average-rate (read-only) : Every 1/16 part of the burst-time, the router calculates the average data rate of each class over the last burst-time seconds.
5. actual-rate (read-only) : actual traffic transfer rate of the queue

Example

Values: limit-at=1M , max-limit=2M , burst-threshold=1500k , burst-limit=4M

Client will try to download two 4MB (32Mb) files, first download will start at zero seconds, second download will start at 17th second. Traffic was unused for last minute.

Burst-time=16s

As we can see as soon as client requested bandwidth it was able to get 4Mpbs burst for 6 seconds. This is longest possible burst with given values (longest-burst-time = burst-threshold * burst-time / burst-limit). As soon as burst runs out rest of the file will be downloaded with 2Mbps. This way file was downloaded in 9 seconds without burst it would take 16 seconds. Burst have 7 seconds to recharge before next download will start.

Note that burst is still disallowed when download started and it kicks in only afterward - in the middle of download. So with this example we proved that burst may happen in the middle of download. Burst was ~4 seconds long and file was downloaded 4 seconds faster then without burst.

Average rate is calculated every 1/16 of burst time, so in this case 1s

Time	average-rate	burst	actual-rate
0	(0+0+0+0+0+0+0+0+0+0+0+0+0+0+0+0)/16=0Kbps	average-rate < burst-threshold → Burst is allowed	4Mbps
1	(0+0+0+0+0+0+0+0+0+0+0+0+0+0+0+4)/16=250Kbps	average-rate < burst-threshold → Burst is allowed	4Mbps
2	(0+0+0+0+0+0+0+0+0+0+0+0+0+0+4+4)/16=500Kbps	average-rate < burst-threshold → Burst is allowed	4Mbps
3	(0+0+0+0+0+0+0+0+0+0+0+0+0+4+4+4)/16=750Kbps	average-rate < burst-threshold → Burst is allowed	4Mbps
4	(0+0+0+0+0+0+0+0+0+0+0+0+4+4+4+4)/16=1000Kbps	average-rate < burst-threshold → Burst is allowed	4Mbps
5	(0+0+0+0+0+0+0+0+0+0+0+4+4+4+4+4)/16=1250Kbps	average-rate < burst-threshold → Burst is allowed	4Mbps
6	(0+0+0+0+0+0+0+0+0+0+4+4+4+4+4+4)/16=1500Kbps	average-rate = burst-threshold → Burst not allowed	2Mbps
7	(0+0+0+0+0+0+0+0+0+4+4+4+4+4+4+2)/16=1625Kbps	average-rate > burst-threshold → Burst not allowed	2Mbps
8	(0+0+0+0+0+0+0+0+4+4+4+4+4+4+2+2)/16=1750Kbps	average-rate > burst-threshold → Burst not allowed	2Mbps
9	(0+0+0+0+0+0+0+4+4+4+4+4+4+2+2+2)/16=1750Kbps	average-rate > burst-threshold → Burst not allowed	2Mbps
10	(0+0+0+0+0+0+4+4+4+4+4+4+2+2+2+2)/16=1875Kbps	average-rate > burst-threshold → Burst not allowed	0Mbps
11	(0+0+0+0+0+4+4+4+4+4+4+2+2+2+2+0)/16=1875Kbps	average-rate > burst-threshold → Burst not allowed	0Mbps
12	(0+0+0+0+4+4+4+4+4+4+2+2+2+2+0+0)/16=1875Kbps	average-rate > burst-threshold → Burst not allowed	0Mbps
13	(0+0+0+4+4+4+4+4+4+2+2+2+2+0+0+0)/16=1875Kbps	average-rate > burst-threshold → Burst not allowed	0Mbps
14	(0+0+4+4+4+4+4+4+2+2+2+2+0+0+0+0)/16=1875Kbps	average-rate > burst-threshold → Burst not allowed	0Mbps
15	(0+4+4+4+4+4+4+2+2+2+2+0+0+0+0+0)/16=1875Kbps	average-rate > burst-threshold → Burst not allowed	0Mbps
16	(4+4+4+4+4+4+2+2+2+2+0+0+0+0+0+0)/16=1875Kbps	average-rate > burst-threshold → Burst not allowed	0Mbps
17	(4+4+4+4+4+2+2+2+2+0+0+0+0+0+0+0)/16=1625Kbps	average-rate > burst-threshold → Burst not allowed	2Mbps
18	(4+4+4+4+2+2+2+2+0+0+0+0+0+0+0+2)/16=1500Kbps	average-rate = burst-threshold → Burst not allowed	2Mbps
19	(4+4+4+2+2+2+2+0+0+0+0+0+0+0+2+2)/16=1375Kbps	average-rate < burst-threshold → Burst is allowed	4Mbps
20	(4+4+2+2+2+2+0+0+0+0+0+0+0+2+2+4)/16=1375Kbps	average-rate < burst-threshold → Burst is allowed	4Mbps
21	(4+2+2+2+2+0+0+0+0+0+0+0+2+2+4+4)/16=1375Kbps	average-rate < burst-threshold → Burst is allowed	4Mbps
22	(2+2+2+2+0+0+0+0+0+0+0+2+2+4+4+4)/16=1375Kbps	average-rate < burst-threshold → Burst is allowed	4Mbps
23	(2+2+2+0+0+0+0+0+0+0+2+2+4+4+4+4)/16=1500Kbps	average-rate = burst-threshold → Burst not allowed	2Mbps
24	(2+2+0+0+0+0+0+0+0+2+2+4+4+4+4+2)/16=1500Kbps	average-rate = burst-threshold → Burst not allowed	2Mbps
25	(2+0+0+0+0+0+0+0+2+2+4+4+4+4+2+2)/16=1500Kbps	average-rate = burst-threshold → Burst not allowed	2Mbps
26	(0+0+0+0+0+0+0+2+2+4+4+4+4+2+2+2)/16=1500Kbps	average-rate = burst-threshold → Burst not allowed	2Mbps
27	(0+0+0+0+0+0+2+2+4+4+4+4+2+2+2+2)/16=1625Kbps	average-rate > burst-threshold → Burst not allowed	2Mbps
28	(0+0+0+0+0+2+2+4+4+4+4+2+2+2+2+2)/16=1750Kbps	average-rate > burst-threshold → Burst not allowed	2Mbps
29	(0+0+0+0+2+2+4+4+4+4+2+2+2+2+2+2)/16=1875Kbps	average-rate > burst-threshold → Burst not allowed	0Mbps
30	(0+0+0+2+2+4+4+4+4+2+2+2+2+2+2+0)/16=1875Kbps	average-rate > burst-threshold → Burst not allowed	0Mbps
31	(0+0+2+2+4+4+4+4+2+2+2+2+2+2+0+0)/16=1875Kbps	average-rate > burst-threshold → Burst not allowed	0Mbps

Burst-time=8s

If we decrease burst-time to 8 seconds - we are able to see that in this case bursts are only at the beginning of downloads

Average rate is calculated every 1/16th of burst time, so in this case every 0.5 seconds.

Time	average-rate	burst	actual-rate
0.0	(0+0+0+0+0+0+0+0+0+0+0+0+0+0+0+0)/8=0Kbps	average-rate < burst-threshold → Burst is allowed	4Mbps (2Mb per 0,5sek)
0.5	(0+0+0+0+0+0+0+0+0+0+0+0+0+0+0+2)/8=250Kbps	average-rate < burst-threshold → Burst is allowed	4Mbps (2Mb per 0,5sek)
1.0	(0+0+0+0+0+0+0+0+0+0+0+0+0+0+2+2)/8=500Kbps	average-rate < burst-threshold → Burst is allowed	4Mbps (2Mb per 0,5sek)
1.5	(0+0+0+0+0+0+0+0+0+0+0+0+0+2+2+2)/8=750Kbps	average-rate < burst-threshold → Burst is allowed	4Mbps (2Mb per 0,5sek)
2.0	(0+0+0+0+0+0+0+0+0+0+0+0+2+2+2+2)/8=1000Kbps	average-rate < burst-threshold → Burst is allowed	4Mbps (2Mb per 0,5sek)
2.5	(0+0+0+0+0+0+0+0+0+0+0+2+2+2+2+2)/8=1250Kbps	average-rate < burst-threshold → Burst is allowed	4Mbps (2Mb per 0,5sek)
3.0	(0+0+0+0+0+0+0+0+0+0+2+2+2+2+2+2)/8=1500Kbps	average-rate = burst-threshold → Burst not allowed	2Mbps (1Mb per 0,5sek)
3.5	(0+0+0+0+0+0+0+0+0+2+2+2+2+2+2+1)/8=1625Kbps	average-rate > burst-threshold → Burst not allowed	2Mbps (1Mb per 0,5sek)
4.0	(0+0+0+0+0+0+0+0+2+2+2+2+2+2+1+1)/8=1750Kbps	average-rate > burst-threshold → Burst not allowed	2Mbps (1Mb per 0,5sek)
4.5	(0+0+0+0+0+0+0+2+2+2+2+2+2+1+1+1)/8=1875Kbps	average-rate > burst-threshold → Burst not allowed	2Mbps (1Mb per 0,5sek)
5.0	(0+0+0+0+0+0+2+2+2+2+2+2+1+1+1+1)/8=2000Kbps	average-rate > burst-threshold → Burst not allowed	2Mbps (1Mb per 0,5sek)
5.5	(0+0+0+0+0+2+2+2+2+2+2+1+1+1+1+1)/8=2125Kbps	average-rate > burst-threshold → Burst not allowed	2Mbps (1Mb per 0,5sek)
6.0	(0+0+0+0+2+2+2+2+2+2+1+1+1+1+1+1)/8=2250Kbps	average-rate > burst-threshold → Burst not allowed	2Mbps (1Mb per 0,5sek)
6.5	(0+0+0+2+2+2+2+2+2+1+1+1+1+1+1+1)/8=2375Kbps	average-rate > burst-threshold → Burst not allowed	2Mbps (1Mb per 0,5sek)
7.0	(0+0+2+2+2+2+2+2+1+1+1+1+1+1+1+1)/8=2500Kbps	average-rate > burst-threshold → Burst not allowed	2Mbps (1Mb per 0,5sek)
7.5	(0+2+2+2+2+2+2+1+1+1+1+1+1+1+1+1)/8=2625Kbps	average-rate > burst-threshold → Burst not allowed	2Mbps (1Mb per 0,5sek)
8.0	(2+2+2+2+2+2+1+1+1+1+1+1+1+1+1+1)/8=2750Kbps	average-rate > burst-threshold → Burst not allowed	2Mbps (1Mb per 0,5sek)
8.5	(2+2+2+2+2+1+1+1+1+1+1+1+1+1+1+1)/8=2625Kbps	average-rate > burst-threshold → Burst not allowed	2Mbps (1Mb per 0,5sek)
9.0	(2+2+2+2+1+1+1+1+1+1+1+1+1+1+1+1)/8=2500Kbps	average-rate > burst-threshold → Burst not allowed	2Mbps (1Mb per 0,5sek)
9.5	(2+2+2+1+1+1+1+1+1+1+1+1+1+1+1+1)/8=2375Kbps	average-rate > burst-threshold → Burst not allowed	2Mbps (1Mb per 0,5sek)
10.0	(2+2+1+1+1+1+1+1+1+1+1+1+1+1+1+1)/8=2250Kbps	average-rate > burst-threshold → Burst not allowed	2Mbps (1Mb per 0,5sek)
10.5	(2+1+1+1+1+1+1+1+1+1+1+1+1+1+1+1)/8=2125Kbps	average-rate > burst-threshold → Burst not allowed	2Mbps (1Mb per 0,5sek)
11.0	(1+1+1+1+1+1+1+1+1+1+1+1+1+1+1+1)/8=2000Kbps	average-rate > burst-threshold → Burst not allowed	2Mbps (1Mb per 0,5sek)
11.5	(1+1+1+1+1+1+1+1+1+1+1+1+1+1+1+1)/8=2000Kbps	average-rate > burst-threshold → Burst not allowed	2Mbps (1Mb per 0,5sek)
12.0	(1+1+1+1+1+1+1+1+1+1+1+1+1+1+1+1)/8=2000Kbps	average-rate > burst-threshold → Burst not allowed	2Mbps (1Mb per 0,5sek)
12.5	(1+1+1+1+1+1+1+1+1+1+1+1+1+1+1+1)/8=2000Kbps	average-rate > burst-threshold → Burst not allowed	2Mbps (1Mb per 0,5sek)
13.0	(1+1+1+1+1+1+1+1+1+1+1+1+1+1+1+1)/8=2000Kbps	average-rate > burst-threshold → Burst not allowed	0Mbps (0Mb per 0,5sek)
13.5	(1+1+1+1+1+1+1+1+1+1+1+1+1+1+1+0)/8=1875Kbps	average-rate > burst-threshold → Burst not allowed	0Mbps (0Mb per 0,5sek)
14.0	(1+1+1+1+1+1+1+1+1+1+1+1+1+1+0+0)/8=1750Kbps	average-rate > burst-threshold → Burst not allowed	0Mbps (0Mb per 0,5sek)
14.5	(1+1+1+1+1+1+1+1+1+1+1+1+1+0+0+0)/8=1625Kbps	average-rate > burst-threshold → Burst not allowed	0Mbps (0Mb per 0,5sek)
15.0	(1+1+1+1+1+1+1+1+1+1+1+1+0+0+0+0)/8=1500Kbps	average-rate > burst-threshold → Burst not allowed	0Mbps (0Mb per 0,5sek)
15.5	(1+1+1+1+1+1+1+1+1+1+1+0+0+0+0+0)/8=1375Kbps	average-rate < burst-threshold → Burst is allowed	0Mbps (0Mb per 0,5sek)
16.0	(1+1+1+1+1+1+1+1+1+1+0+0+0+0+0+0)/8=1250Kbps	average-rate < burst-threshold → Burst is allowed	0Mbps (0Mb per 0,5sek)
16.5	(1+1+1+1+1+1+1+1+1+0+0+0+0+0+0+0)/8=1125Kbps	average-rate < burst-threshold → Burst is allowed	0Mbps (0Mb per 0,5sek)
17.0	(1+1+1+1+1+1+1+1+0+0+0+0+0+0+0+0)/8=1000Kbps	average-rate < burst-threshold → Burst is allowed	2Mbps (1Mb per 0,5sek)
17.5	(1+1+1+1+1+1+1+0+0+0+0+0+0+0+0+1)/8=1000Kbps	average-rate < burst-threshold → Burst is allowed	4Mbps (2Mb per 0,5sek)
18.0	(1+1+1+1+1+1+0+0+0+0+0+0+0+0+1+2)/8=1125Kbps	average-rate < burst-threshold → Burst is allowed	4Mbps (2Mb per 0,5sek)
18.5	(1+1+1+1+1+0+0+0+0+0+0+0+0+1+2+2)/8=1250Kbps	average-rate < burst-threshold → Burst is allowed	4Mbps (2Mb per 0,5sek)
19.0	(1+1+1+1+0+0+0+0+0+0+0+0+1+2+2+2)/8=1375Kbps	average-rate < burst-threshold → Burst is allowed	4Mbps (2Mb per 0,5sek)
19.5	(1+1+1+0+0+0+0+0+0+0+0+1+2+2+2+2)/8=1500Kbps	average-rate = burst-threshold → Burst not allowed	2Mbps (1Mb per 0,5sek)
20.0	(1+1+0+0+0+0+0+0+0+0+1+2+2+2+2+1)/8=1500Kbps	average-rate = burst-threshold → Burst not allowed	2Mbps (1Mb per 0,5sek)
20.5	(1+0+0+0+0+0+0+0+0+1+2+2+2+2+1+1)/8=1500Kbps	average-rate = burst-threshold → Burst not allowed	2Mbps (1Mb per 0,5sek)
21.0	(0+0+0+0+0+0+0+0+1+2+2+2+2+1+1+1)/8=1500Kbps	average-rate = burst-threshold → Burst not allowed	2Mbps (1Mb per 0,5sek)
21.5	(0+0+0+0+0+0+0+1+2+2+2+2+1+1+1+1)/8=1625Kbps	average-rate > burst-threshold → Burst not allowed	2Mbps (1Mb per 0,5sek)
22.0	(0+0+0+0+0+0+1+2+2+2+2+1+1+1+1+1)/8=1750Kbps	average-rate > burst-threshold → Burst not allowed	2Mbps (1Mb per 0,5sek)
22.5	(0+0+0+0+0+1+2+2+2+2+1+1+1+1+1+1)/8=1875Kbps	average-rate > burst-threshold → Burst not allowed	2Mbps (1Mb per 0,5sek)
23.0	(0+0+0+0+1+2+2+2+2+1+1+1+1+1+1+1)/8=2000Kbps	average-rate > burst-threshold → Burst not allowed	2Mbps (1Mb per 0,5sek)
23.5	(0+0+0+1+2+2+2+2+1+1+1+1+1+1+1+1)/8=2125Kbps	average-rate > burst-threshold → Burst not allowed	2Mbps (1Mb per 0,5sek)
24.0	(0+0+1+2+2+2+2+1+1+1+1+1+1+1+1+1)/8=2250Kbps	average-rate > burst-threshold → Burst not allowed	2Mbps (1Mb per 0,5sek)
24.5	(0+1+2+2+2+2+1+1+1+1+1+1+1+1+1+1)/8=2375Kbps	average-rate > burst-threshold → Burst not allowed	2Mbps (1Mb per 0,5sek)
25.0	(1+2+2+2+2+1+1+1+1+1+1+1+1+1+1+1)/8=2500Kbps	average-rate > burst-threshold → Burst not allowed	2Mbps (1Mb per 0,5sek)
25.5	(2+2+2+2+1+1+1+1+1+1+1+1+1+1+1+1)/8=2500Kbps	average-rate > burst-threshold → Burst not allowed	2Mbps (1Mb per 0,5sek)
26.0	(2+2+2+1+1+1+1+1+1+1+1+1+1+1+1+1)/8=2375Kbps	average-rate > burst-threshold → Burst not allowed	2Mbps (1Mb per 0,5sek)
26.5	(2+2+1+1+1+1+1+1+1+1+1+1+1+1+1+1)/8=2250Kbps	average-rate > burst-threshold → Burst not allowed	2Mbps (1Mb per 0,5sek)
27.0	(2+1+1+1+1+1+1+1+1+1+1+1+1+1+1+1)/8=2125Kbps	average-rate > burst-threshold → Burst not allowed	2Mbps (1Mb per 0,5sek)
27.5	(1+1+1+1+1+1+1+1+1+1+1+1+1+1+1+1)/8=2000Kbps	average-rate > burst-threshold → Burst not allowed	2Mbps (1Mb per 0,5sek)
28.0	(1+1+1+1+1+1+1+1+1+1+1+1+1+1+1+1)/8=2000Kbps	average-rate > burst-threshold → Burst not allowed	2Mbps (1Mb per 0,5sek)
28.5	(1+1+1+1+1+1+1+1+1+1+1+1+1+1+1+1)/8=2000Kbps	average-rate > burst-threshold → Burst not allowed	2Mbps (1Mb per 0,5sek)
29.0	(1+1+1+1+1+1+1+1+1+1+1+1+1+1+1+1)/8=2000Kbps	average-rate > burst-threshold → Burst not allowed	2Mbps (1Mb per 0,5sek)
29.5	(1+1+1+1+1+1+1+1+1+1+1+1+1+1+1+1)/8=2000Kbps	average-rate > burst-threshold → Burst not allowed	2Mbps (1Mb per 0,5sek)
30.0	(1+1+1+1+1+1+1+1+1+1+1+1+1+1+1+1)/8=2000Kbps	average-rate > burst-threshold → Burst not allowed	2Mbps (1Mb per 0,5sek)
30.5	(1+1+1+1+1+1+1+1+1+1+1+1+1+1+1+1)/8=2000Kbps	average-rate > burst-threshold → Burst not allowed	0Mbps (0Mb per 0,5sek)
31.0	(1+1+1+1+1+1+1+1+1+1+1+1+1+1+1+0)/8=1875Kbps	average-rate > burst-threshold → Burst not allowed	0Mbps (0Mb per 0,5sek)