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

148 Transport Layer Protocols in Ad Hoc Networks
of loss. The is due to the inability of the MAC layer to distinguish the cause
of the loss (congestion, random wireless errors or mobility-induced), which
makes loss an inappropriate indicator of congestion. ATP uses a three phase
rate adaptation mechanism instead of the LIMD mechanism of TCP. This is because
connections in ad-hoc networks are vulnerable to route failures. Hence
TCP’s multiplicative decrease is unwarranted during route failures where most
of the time a new route is chosen. Furthermore, the linear increase causes slow
convergence to the optimal operating bandwidth. Finally, the coarse grained receiver
feedback in ATP eliminates the data connection’s dependence on ACKs
and thereby the strong coupling between the forward and reverse paths.
Performance
The results presented in Figure 5.8 are for a single connection scenario. ATP’s
rate based transmissions eliminate the negative impacts resulting from burstiness
of packet transmissions. This can be observed from the sequence number
progression for the ATP flow in Figure 5.8(a) where the packet transmissions are
more uniformly spaced out when compared to the bursty transmissions in the
case of default TCP in Figure 5.2(a) for the same scenario. ATP overcomes the
under-utilization of network resources, resulting from the use of slow- starts
in ad-hoc networks, by employing the quick start mechanism. Though, the
quick start mechanism probes for the available network bandwidth along the
path within a single rtt, it is still a bandwidth estimation phase and hence we
are interested in the under-utilization of network resources resulting from the
use of quick start. The result in Figure 5.8(b) indicates that the total amount
of time spent by the ATP connection in the quick start phase is an order of
magnitude less than that spent by a default TCP flow in slow start for the same
scenario. Finally, the various design elements of ATP help it obtain a significant
throughput improvement over the default TCP case, as shown in Figure 5.8(c).
Trade-offs
ATP, being a protocol tailored to the characteristics of ad-hoc networks,
attains significant performance improvement. However, its inter-operability
with TCP is of prime concern if a mobile host using ATP also wants to be a part
of the static Internet. The inter-operability of ATP with TCP is not evident, and
is currently being investigated [13]. Further, while the strong coupling between
the data and ACK paths in TCP is alleviated by the use of coarse-grained receiver
feedback, ATP’s rate adaptation mechanism still relies on receiver feedback, and
the tuning of ATP’s receiver feedback rate is still not fully addressed [13].
Related Work
An example of another protocol whose design is tailored to a specific target
environment is the satellite transport protocol (STP) [14]. It has been designed