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 1/16 of burst time, so in this case 0,5s

Zero second - average-rate=(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 -> Actual-rate=4Mbps (2Mb per 0,5sek)

0,5 second - average-rate=(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 -> Actual-rate=4Mbps (2Mb per 0,5sek)

1,0 second - average-rate=(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 -> Actual-rate=4Mbps (2Mb per 0,5sek)

1,5 second - average-rate=(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 -> Actual-rate=4Mbps (2Mb per 0,5sek)

2,0 second - average-rate=(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 -> Actual-rate=4Mbps (2Mb per 0,5sek)

2,5 second - average-rate=(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 -> Actual-rate=4Mbps (2Mb per 0,5sek)

3,0 second - average-rate=(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 -> Actual-rate=2Mbps (1Mb per 0,5sek)

3,5 second - average-rate=(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 -> Actual-rate=2Mbps (1Mb per 0,5sek)

4,0 second - average-rate=(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 -> Actual-rate=2Mbps (1Mb per 0,5sek)

4,5 second - average-rate=(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 -> Actual-rate=2Mbps (1Mb per 0,5sek)

5,0 second - average-rate=(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 -> Actual-rate=2Mbps (1Mb per 0,5sek)

5,5 second - average-rate=(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 -> Actual-rate=2Mbps (1Mb per 0,5sek)

6,0 second - average-rate=(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 -> Actual-rate=2Mbps (1Mb per 0,5sek)

6,5 second - average-rate=(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 -> Actual-rate=2Mbps (1Mb per 0,5sek)

7,0 second - average-rate=(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 -> Actual-rate=2Mbps (1Mb per 0,5sek)

7,5 second - average-rate=(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 -> Actual-rate=2Mbps (1Mb per 0,5sek)

8,0 second - average-rate=(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 -> Actual-rate=2Mbps (1Mb per 0,5sek)

8,5 second - average-rate=(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 -> Actual-rate=2Mbps (1Mb per 0,5sek)

9,0 second - average-rate=(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 -> Actual-rate=2Mbps (1Mb per 0,5sek)

9,5 second - average-rate=(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 -> Actual-rate=2Mbps (1Mb per 0,5sek)

10,0 second - average-rate=(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 -> Actual-rate=2Mbps (1Mb per 0,5sek)

10,5 second - average-rate=(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 -> Actual-rate=2Mbps (1Mb per 0,5sek)

11,0 second - average-rate=(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 -> Actual-rate=2Mbps (1Mb per 0,5sek)

11,5 second - average-rate=(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 -> Actual-rate=2Mbps (1Mb per 0,5sek)

12,0 second - average-rate=(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 -> Actual-rate=2Mbps (1Mb per 0,5sek)

12,5 second - average-rate=(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 -> Actual-rate=2Mbps (1Mb per 0,5sek)

13,0 second - average-rate=(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 -> Actual-rate=0Mbps (0Mb per 0,5sek)

13,5 second - average-rate=(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 -> Actual-rate=0Mbps (0Mb per 0,5sek)

14,0 second - average-rate=(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 -> Actual-rate=0Mbps (0Mb per 0,5sek)

14,5 second - average-rate=(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 -> Actual-rate=0Mbps (0Mb per 0,5sek)

15,0 second - average-rate=(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 -> Actual-rate=0Mbps (0Mb per 0,5sek)

15,5 second - average-rate=(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 -> Actual-rate=0Mbps (0Mb per 0,5sek)

16,0 second - average-rate=(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 -> Actual-rate=0Mbps (0Mb per 0,5sek)

16,5 second - average-rate=(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 -> Actual-rate=0Mbps (0Mb per 0,5sek)

17,0 second - average-rate=(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 -> Actual-rate=2Mbps (1Mb per 0,5sek)

17,5 second - average-rate=(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 -> Actual-rate=4Mbps (2Mb per 0,5sek)

18,0 second - average-rate=(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 -> Actual-rate=4Mbps (2Mb per 0,5sek)

18,5 second - average-rate=(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 -> Actual-rate=4Mbps (2Mb per 0,5sek)

19,0 second - average-rate=(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 -> Actual-rate=4Mbps (2Mb per 0,5sek)

19,5 second - average-rate=(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 -> Actual-rate=2Mbps (1Mb per 0,5sek)

20,0 second - average-rate=(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 -> Actual-rate=2Mbps (1Mb per 0,5sek)

20,5 second - average-rate=(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 -> Actual-rate=2Mbps (1Mb per 0,5sek)

21,0 second - average-rate=(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 -> Actual-rate=2Mbps (1Mb per 0,5sek)

21,5 second - average-rate=(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 -> Actual-rate=2Mbps (1Mb per 0,5sek)

22,0 second - average-rate=(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 -> Actual-rate=2Mbps (1Mb per 0,5sek)

22,5 second - average-rate=(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 -> Actual-rate=2Mbps (1Mb per 0,5sek)

23,0 second - average-rate=(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 -> Actual-rate=2Mbps (1Mb per 0,5sek)

23,5 second - average-rate=(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 -> Actual-rate=2Mbps (1Mb per 0,5sek)

24,0 second - average-rate=(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 -> Actual-rate=2Mbps (1Mb per 0,5sek)

24,5 second - average-rate=(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 -> Actual-rate=2Mbps (1Mb per 0,5sek)

25,0 second - average-rate=(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 -> Actual-rate=2Mbps (1Mb per 0,5sek)

25,5 second - average-rate=(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 -> Actual-rate=2Mbps (1Mb per 0,5sek)

26,0 second - average-rate=(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 -> Actual-rate=2Mbps (1Mb per 0,5sek)

26,5 second - average-rate=(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 -> Actual-rate=2Mbps (1Mb per 0,5sek)

27,0 second - average-rate=(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 -> Actual-rate=2Mbps (1Mb per 0,5sek)

27,5 second - average-rate=(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 -> Actual-rate=2Mbps (1Mb per 0,5sek)

28,0 second - average-rate=(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 -> Actual-rate=2Mbps (1Mb per 0,5sek)

28,5 second - average-rate=(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 -> Actual-rate=2Mbps (1Mb per 0,5sek)

29,0 second - average-rate=(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 -> Actual-rate=2Mbps (1Mb per 0,5sek)

29,5 second - average-rate=(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 -> Actual-rate=2Mbps (1Mb per 0,5sek)

30,0 second - average-rate=(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 -> Actual-rate=2Mbps (1Mb per 0,5sek)

30,5 second - average-rate=(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 -> Actual-rate=0Mbps (0Mb per 0,5sek)

31,0 second - average-rate=(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 -> Actual-rate=0Mbps (0Mb per 0,5sek)