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Review: “Measurement and Modeling of the Origins of Starvation of

Congestion-Controlled Flows in Wireless Mesh Networks”

Bhavesh Singh

2010CS50281

1. Summary

1.1 Motivation

The basic scenario of any CSMA-based Mesh Networks is sufficient to induce starvation.

Previous work showed that severe unfairness and even complete starvation can occur in

multi-hop wireless networks due to MAC behavior. TCP magnifies MAC unfair contention.

Though significant progress has been made in this area, no prior work identified severe

throughput imbalances in the basic scenario of mesh networks, in which a one-hop flow

contends with a two-hop flow for gateway access. The prior understanding of “why starvation

occurs” is incorrect and has yielded solutions that are not effective because it is believed that

TCP pacing/smart dropping with the optimal pacing rate solves this. It is also believed that

limiting or fixing TCP window to a small value is sufficient to induce fairness. These two beliefs

are not only the reason for starvation according to the authors.

1.2 Contribution

Their contributions are as follows -

Described the protocol origins of starvation as a compounding effect of three factors:

1. the MAC protocol induces bistability in which pairs of nodes alternate in capturing

system resources

2. despite the inherent symmetry of MAC bistability, the transport protocol induces

asymmetry in the time spent in each state and favors the one-hop flow

3. most critically, the multihop flow’s transmitter often incurs a high penalty in terms

of loss, delay, and consequently, throughput, in order to recapture system

resources

Demonstrated the existence of starvation under saturation conditions and show that only

a one-hop TCP flow in competition with a two-hop TCP flow is sufficient to induce

starvation.

Developed an analytical model both to study starvation and to devise a solution to

counter starvation.

Their counter-starvation policy completely solves the starvation problem.

Implement and empirically validate the solution on MirrorMesh, a network re-

deployment within the same urban environment.

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1.3 Methodology

Authors have presented an experimental demonstration of starvation in urban mesh

networks, an analysis of starvation’s cross layer protocol origins, an analytical model and a

counter-starvation policy, the experimental evaluation of such a policy. This methodology can

be briefly understood from the following points-

STARVATION IN URBAN MESH NETWORKS: First authors defined a basic topology of any

mesh network as shown in Fig. 1, in which two mesh nodes, A and B are located two and

one hops away from the gateway, GW, respectively. This topology is necessarily

embedded in any larger mesh network topology given that mesh networks are defined as

multihop wireless networks with gateways.

Then they experimentally demonstrate the potential for starvation in the TFA

network. TFA network is an operational mesh network that provides Internet access in a

densely populated urban neighborhood in Houston. They showed that the two-hop node

(A) “starves” when contending with the one-hop node (B).

STARVATION’S PROTOCOL ORIGINS: The collision avoidance mechanism in CSMA/CA

causes bistability, in which node pairs (A,B) and (B,GW) alternate in transmission of

multiple packet bursts. In order to understand the bistability, we first examine the

behavior of two flows in the scenario where the gateway node GW and two-hop node A

contend for transmitting TCP ACK and TCP DATA, respectively.

Due to such factors like bistability, asymmetry induced by sliding window and

some severe transition penalties, Node A faces severe starvation in the basic topology.

Then they also showed demonstration for the broader topology.

ANALYTICAL MODEL AND STARVATION SOLUTION: The analytical model was designed

with the following objectives-

o Isolate and capture the root cause of starvation

o Only model one aspect of congestion control

Sliding window

Technique used was Embedded Markov chain model.

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Counter-Starvation Policy: All nodes that are directly connected to a gateway, or

gateways in case of multiple gateways, should increase their minimum contention

window to a value greater than that of all other nodes.

o Simple to implement-no overhead or message exchange between nodes.

o Compliant with IEEE 802.11e EDCA.

Evaluation:

o Model

Static sliding window congestion control mechanism

o NS2

Fixed TCP congestion window (TCP mechanisms including timeouts and

cumulative ACKs)

o NS2

Legacy TCP New Reno (dynamic congestion window)

o TFA

Legacy TCP New Reno (dynamic congestion window + MAC and PHY

influences)

1.4 Conclusion

The interaction of one-hop TCP flows with two-hop TCP flows is sufficient to induce

starvation. They measured starvation in an operational multitier urban mesh network and

describe how the starvation’s originating factors stem from interaction between the

transport layer’s congestion control and the MAC layer’s collision avoidance. They analytically

model the system and utilize the model to devise a simple counter-starvation policy in which

nodes one hop away from the gateway increase their minimum contention window. They

finally implement and empirically validate the solution not only via simulation, but also on

MirrorMesh, a network redeployment within the same urban environment.

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2. Critique

2.1 Proposed model considers only one aspect of congestion control – ‘sliding window’

As described in paper, the DATA-ACK control loop in the transport layer is a key factor in

starvation. Consequently, they model only one aspect of congestion control, the sliding

window. Their counter starvation policy is also based on this single factor i.e. the size of the

contention window. They considered a fixed congestion control window and analytically

show that the combination of CSMA MAC and transport-layer sliding window congestion

control alone is sufficient to induce severe throughput imbalance. But apart from this, there

are other factors which are also responsible for starvation, for example asynchronization is

also one of the reason for starvation. The counter-starvation policy should have taken it into

account.

2.2 Limitation of counter-starvation policy is not mentioned

It is not clear from the paper that for which cases the counter-starvation policy will work.

Whether it is a generic solution for the starvation problem or it is limited to some specific

scenarios only. Apart from basic scenario as mentioned in paper, other scenarios are TCP

multi-stream, broader and denser multi-hop topologies etc. All the analysis which is done for

basic topology is not done for these cases in detail and the limitations of this policy is also not

mentioned.

3. Synthesis

The points which I mentioned in the second critique, that limitations of counter-starvation

policy is not mentioned, should be considered. The scenarios should be short-out where it

will not work and work-out some other solution for these scenarios. Similar analysis which

are done in paper should also be done for such cases.

The DATA-ACK control loop in the transport layer is a key factor in starvation.

Consequently, they model only one aspect of congestion control, the sliding window. Other

reasons for starvations should be considered to devise different solution. Some other

parameters are SIFS, DIFS etc., and asynchronization should be taken into account.