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. 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 
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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 redeployment within the same urban environment.
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2. 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 objectiveso 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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3. 
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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4. 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.
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