Ad Hoc Networks : Technologies and Protocols - University of ...

140 Transport Layer Protocols in Ad Hoc Networks
mechanisms. However, a drawback with TCP-ELFN is that the connections
could suffer from lower throughput in heavily loaded static scenarios [8]. This
is because the MAC layer is incapable of determining if a loss is due to route
failure, or simply due to high contention. Hence, in heavily loaded static scenarios,
ELFN messages would be generated even in the absence of route failures.
This would make the connection enter the “stand-by” mode and wait for a new
route computation to restore its state, thereby causing a degradation in throughput.
The choice of the probing interval is a critical parameter. While a large
value could increase the time for computation of a route, thereby reducing the
throughput, a small value could increase the contention in the network due to
the frequent probe packets that are generated. Furthermore, ELFN also tries
to mask out only the negative impacts arising from route failures caused due
to mobility by freezing TCP’s state during new route computations. However,
some of the basic characteristics of TCP that degrade its performance in ad-hoc
networks are still left un-addressed.
Related Work
There have been some recent works [8] [9] [10] [11] that discuss the effect of
mobility on TCP performance and suggest various transport layer mechanisms
to solve the problems caused due to mobility. [8] studies the performance of
ELFN on static and dynamic networks and corroborates the results obtained
in [2]. [9] discusses a mechanism called TCP-Feedback, which uses route failure
and re-establishment notifications to provide feedback to TCP, and thus
reduce the number of packet re-transmissions and TCP back- offs during route
calculation, to improve throughput. However, this mechanism is evaluated in
a simple one-hop wireless network. [10] studies the performance of TCP on
three different routing protocols and proposes a heuristic called fixed RTO,
which essentially freezes the TCP RTO value whenever there is a route loss.
It also evaluates the effectiveness of TCP’s selective and delayed acknowledgments
in improving the performance. [11] provides a transport layer solution to
improving TCP performance. It introduces a thin layer between the transport
and underlying routing layers, which puts TCP into persist mode whenever the
network gets disconnected or there are packet losses due to high bit error rate.
Thus, this thin layer acts as a shield to TCP, protecting it from the underlying
behavior of an ad-hoc network.
5.5 TCP-aware Cross-layered Solutions
An approach that falls under this category is the Atra framework proposed
by Anantharaman et. al. [12]. The framework comprises of mechanisms at
both the routing and medium access control layers and does not necessitate
any changes to TCP. The mechanisms in the Atra framework are based on the