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

144 Transport Layer Protocols in Ad Hoc Networks
The proactive route error mechanism can prove to be disadvantageous
when a link failure has occurred due to congestion. Consider an example
where a link between nodes A and B is traversed by 2 TCP connections
and In the default set-up, when a packet belonging to experiences
congestion related link failure, only would be informed of the
link failure prompting to choose a different route and thus relieving
congestion along the original path for However, when the proactive
route error mechanism is used, both and will be informed of the
route failure making both of them to recompute their routes (although the
same path might be chosen all over again). However, the characteristic
of the default set-up to let route requests through in preference to data
packets results in routes being chosen irrespective of the congestion along
the path. Hence, in the example considered there is nothing to prevent
flow from choosing the same path again even under the default set-up.
Performance
While the symmetric route pinning mechanism reduces the probability of
route failures, the route failure prediction mechanism reduces the occurrence
of route failures by predicting them proactively. These two mechanisms directly
reduce the number of retranmission timeouts in TCP. However, when a route
failure does occur, the proactive route error mechanism reduces the delay in
informing the sources of the route failure, thereby reducing the probability of a
retransmission timeout. Hence, these three mechanisms in concert reduce the
number of retransmission timeouts experienced by the connection as observed
in Figure 5.7(a) when compared to the default TCP case in Figure 5.2(c) for
the 1 connection scenario. A direct benefit of the reduction in the number
of timeouts, is the resulting reduction in the loss percentage of packets and
the consequent increase in throughput as observed in Figures 5.7 (b) and (c)
respectively.
Trade-offs
The lower layer mechanisms in the ATRA framework help improve TCP’s
performance without requiring any changes to the transport layer, by appropriately
taking actions to mask out the negative impacts of route failures caused due
to mobility. However, this is achieved at the cost of lower layers being required
to be TCP-aware in their operations. Also, the route prediction mechanism
relies on the signal strength of the received packets, it is challenge to make
such predictions accurate under conditions of signal fading due to obstacles,
multipath, etc. Finally, several characteristics of TCP that are by themselves
inappropriate for operation over ad-hoc networks, are not addressed by this
framework.