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

Transport Layer for Ad-hoc Networks: Overview 135
link failure. This also contributes to the delay after which a source realizes that
a path is broken, consequently increasing the probability of timeouts. Figure
5.4(b) shows the latency involved in the MAC layer detecting a link failure. It
can be observed that for higher loads, the latency could be in the order of a few
seconds.
Both of the above factors directly contribute to more number of losses occuring
in the network, and thus impact the throughput performance of network
connections.
Impact of Routing Layer. The characteristics of the underlying routing
protocol have a significant impact on TCP’s performance. Some of the important
ones are outlined below.
In most of the reactive routing protocols (such as DSR), there is a provision
for the routing layer at the upstream node of a broken link to send back a path
failure message to the source. Once the source is informed of the path failure, it
initiates a new route computation. Any packet originating at the source during
this route-recomputation phase does not have a route. This directly increases
the fraction of time that packets in the routing layer spend without a route to the
destination during a connection’s lifetime. Further, the time taken to recompute
the new route also increases with increasing load. This can be observed in Figure
5.4(c) where the latency involved in route computation is presented.
In addition to the absolute impact of not having a route in the route computation
phase, TCP is also likely to experience timeouts during each route
computation time, especially in the heavy load scenario where route computation
time is around a couple of seconds. Furthermore, successive timeouts
and the resulting back-offs could potentially result in the stalling of the data
connection.
Finally, it is in the best interest of the connection to minimize the number of
route failures resulting from the routing protocol’s operation. This is because,
the number of route failures directly influence the above two factors. As the
number of route errors increases, the fraction of time a packet spends without a
route at the routing layer increases, consequently increasing the probability of
the expiry of TCP’s retransmission timer.
5.3 Transport Layer for Ad-hoc Networks: Overview
Existing approaches to improve transport layer performance over ad-hoc
networks fall under three broad categories: (i) Modifying TCP to handle the
characteristics of an ad-hoc network, (ii) Cross-layer TCP aware modifications
to the lower layers of the protocol stack to hide from TCP the vagaries of an
ad-hoc network, (iii) Built-from-scratch transport protocols that involve a fully
re-designed transport layer approach suited for ad-hoc networks. In the rest of