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

Modified TCP 137
MAC layers without changing TCP, some of the inherent problems resulting
from TCP’s key design elements cannot be addressed.
Finally, the third approach is to consider a transport layer design that is
drawn from scratch, and tailored specifically for the characteristics of
an ad-hoc network. We refer to this approach as the Ad-hoc Transport
Protocol approach. An obvious drawback of such an approach is that
hosts in the ad-hoc network will now possess a transport layer protocol
that is different from TCP. While this is not a problem in stand-alone
dedicated ad-hoc networks such as those in military applications, it is an
issue when ad-hoc networks are seen to “hang-off” from the Internet. As
we show later in the chapter, such an approach can provide connections
with the best possible performance. Such an approach can also be used
as a bench-mark for the earlier two approaches.
Figure 5.5 illustrates the layers that exhibit changes in the protocol stack
for the three classes of approaches. In the following sections, we present one
instance of each of the above classes of approaches in detail. We also discuss
other related work under the three approach categories.
5.4 Modified TCP
The first approach involves effecting changes to the TCP protocol with support
from the underlying layers in order to mask out the problems arising from
mobility. A protocol that falls in this class is the ELFN (Explicit Link Failure
Notification) protocol proposed by Holland et al. [2]. ELFN employs simple
support from the network and lower layers to achieve the purpose. The bulk
of the mechanisms in ELFN reside in the transport layer, and can be viewed as
being additional to those already present in TCP. The objective of ELFN is to
provide the TCP sender with information about link and route failures so that it
can avoid responding to the failures as if congestion occurred, and consequently
reduce any unnecessary degradation in performance. In the rest of the paper
we refer to this approach as simply TCP-ELFN.
Mechanisms
The salient features of TCP-ELFN are:
When a link failure occurs, the node upstream of the failure link sends
back an ELFN message to the source of every TCP connection using that
link. A link failure is said to occur when the MAC layer is unable to
successfully deliver a packet across the link after trying for a threshold
number of times. The notification message is sent by modifying DSR’s
route failure message to carry a payload similar to the “host unreachable”
ICMP message.