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Active Queue Management

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Active Queue Management

- Queues
- Queues are to absorb bursts of packets.
- They are required for statistical multiplexing.
- Queuing theory shows how far higher throughput is possible if queues are included.

M/M/1/K queue =>

If packets arrive at rate , the rate of lost packets is P(full queue).

Rate that packets are transmitted is (1-P(full queue))

Fractional reduction of packets transmitted is (1-P(full queue))

Conclusion: With longer queues we get higher good-put.

- TCP
- TCP fills the queue to determine the bandwidth.
- The transmission time of a short file is dependent on the RTT, which depends on the queue occupancy.
- Faster networks are achieved by smaller queues. (?)

TCP fills queues.

Long queues cause delay.

Small queue capacity decreases good-put.

TCP uses the queues in a funny way.

Maybe the router can detect that TCP has reached the link bit-rate and then drop packets to inform TCP. This could give small queue occupancy but still have big queues to absorb bursts.

- Maximize throughput
- If the queue is empty, then the link is idle -> a reduction in throughput.
- Due to random variations in packet arrivals, the queue occupancy will vary.
- If the mean queue occupancy is large, then it is less likely that the queue will ever wander to empty.
- In the single bottleneck case, if the queue capacity is the same as the bottleneck delay product, then the queue will never empty.

- Minimize queue size
- Voice-over-IP requires a delay of no more than 250ms. A 10Mbps link with 1500B packets. A queue of about 200 packets will cause 250ms queuing delay.
- Of course a few late packets are permissible, but a long string of late packets would degrade the quality.
- Average Google web page is 40KB = 26 packets = 6 RTT (including SYN, but not including DNS).
- 25ms RTT => 150 ms transfer time ~ 0
- 250 ms RTT => 1.5 s transfer time not zero.

- Objective: full throughput but with small queuing delay. Maximize the time in which the queue is not empty with a bound on the queue occupancy, or on the probability of the queue exceeding some occupancy.

- No bias against bursty traffic.
- TCP sends packets in bursts.
- The burst size is approximately the size of the cwnd.
- Flows with large RTT will send a large number of packets in a burst.
- Drop tail queues will tend to drop packets that arrive in bursts. Hence, these flows are treated unfairly.
- AQM should attempt to solve this problem.

- If several flows share the same bottleneck and have the same RTT, they will synchronize. This synchronization will give the optimal throughput.
- If the RTTs are different, then weird synchronizations can happen where some flows get a huge amount of the bandwidth.
- In simulation, such synchronization is very difficult to avoid.
- If packets are dropped randomly, as most AQM schemes do, then there is not synchronization.
- However, if there are short flows, then synchronization does not occur.
- End-host random delay also can get rid of synchronization.
- It is not known if synchronization occurs in real networks.
- It appears to be more likely in high-speed networks.

- If a flow sends packets very fast, the queue will fill and all flows sharing the bottleneck will receive drops. TCP flows will decrease their bit-rate.
- If a non-TCP flow sends too fast, the TCP flows will starve.
- The router should detect such situations and drop the non-TCP flow’s packets.

- AQM/TCP is a closed-loop feedback system, hence stability is a concern.
- If the system is unstable, then the queue occupancy and flow bit-rates might wildly oscillate.
- On the other hand, as long as the queue never empties and never fills too far, then stability doesn’t matter.
- A huge amount of research has focused on the stability issue. But only on the infinitely long flow case.

- Network operators should not need a Ph.D. to setup the AQM (routing is hard enough).
- Complicated systems are difficult to understand and might bring unforeseen problems/weaknesses. The Internet is necessary for economic vitality.
- Complicated protocols are likely to be implemented incorrectly. This might cause other unforeseen problems.
- Complicated schemes are viewed as being non-robust and hence are not trusted by network operators.
- Reliability takes precedence over performance.

Smoothed/filtered queue occupancy

- Drop tail - the first and the most widely used.
- RED (random early discard/detection) Sally Floyd and Van Jacobson, 1993

Kth packet arrives at k

Gentle RED

RED

1

1

0.8

0.8

0.6

f(q) - marking probability

0.6

f(q) - marking probability

0.4

0.4

maxp

maxp

0.2

0.2

0

0

0

10

20

30

40

50

60

0

10

20

30

40

50

60

q

minth

maxth

qmax

q

minth

maxth

qmax

- RED’s parameters are difficult to adjust. They seem to depend on the number of flows and the RTT of the flows.
- ARED dynamically adjusts maxp to account for random variations in the traffic.

m denotes the sample, T denotes the sample period, usually 0.5 seconds

The ARED paper says that maxp is AIMD, but this is only true of maxp<4%!

TCP model:

This is a nonlinear ODE, near an operating point, a linear model can be found.

A proportional integral controller is used, i.e. the transfer function from queue occupancy to dropping probability is

Queue dynamics

A variable r is maintained

q* is the desired queue occpancy (0?)

(k) is the arrival rate during the kth sample

C is link capacity

T is the sample period

Loss probability during the kth sample interval is

Two queues: a virtual one and a real one

When a packet arrives, it is put in the real queue and a token is put in the virtual queue. Packets in the real queue are served at the rate of the real link.

Tokens in the virtual queue are served at the virtual bit-rate (given below).

If the virtual queue fills, then the real packet is dropped.

- Objective is to keep queue empty.
- Most AQM methods could be extended to the virtual queue case.

- - user parameter
- - desired utilization
(t) - arrival rate at time t

- Monitored the UNC network to determine realistic network traffic.
- Applied this type of traffic to an experimental network (not simulations, well not really).
- Conclusions
- At 80% utilization, all AQM schemes performed the same.
- At 90% utilization, if ECN was not used, then all AQM schemes yields the same performance.
- At 90% utilization, if ECN is used, then PI and REM work the best
- ARED works the worst.

- The experiment is a bit funny because all AQM methods tested were designed for long-lived flows and yet the test was for short-lived flows.

- Experimental set-up

- Traffic – From a large data collection project, the file size and “think” times for HTTP traffic was determined.
- Calibration – With the bottleneck set at 1Gbps, the number of “users” was found that produce an average demand of 80Mbps, 90Mbps, 98Mbps and 105Mbps.
- Experiment – With a 1Mbps bottleneck, the number of users was set according to the demands found during calibration. PI, REM, and ARED were tested at each demand.

- Without ECN droptail, PI, and REM all perform the same. ARED is a bit worst. PI is slightly better.
- The highest utilization and smallest loss probability is with drop tail and a large queue.

At 105% demand, the difference between drop-tail and PI/REM and larger

- ARED is an ad hoc scheme
- REM and PI are better thoughtout and seem to work better.
- But, without ECN these schemes don’t bring any benefit. One possible reason is that for large drop probability, TCP times out and no scheme considers the effect of timeout.
- Another problem with PI and REM is they are developed for long-lived flows and the experiment as well as the Internet is has a significant amount of traffic from short-flows.