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

134 Transport Layer Protocols in Ad Hoc Networks
will contend with the data stream on the forward path and reduce the rate enjoyed
by the data stream. (ii) If the forward and reverse paths are not the same,
the progress of the TCP connection will be dependent on both the forward path
and reverse path reliability. Thus, the chances of a connection stalling increase
when different paths are used. Note that even if the forward and reverse paths
are different, due to the shared channel in the vicinity of the sender and the
vicinity of the receiver, the data and ACK streams will still contend with each
other. Figure 5.4(a) shows the number of times the data stream and the ACK
stream experience independent path failures for the 1 flow scenario. It can
be observed that the forward and reverse paths experience the same order of
magnitude of failures.
TCP relies on retransmission timeouts as a backup loss detection mechanism.
As described in Section 5.2.2, the RTO value for a TCP connection can be
considerably inflated and vastly different from the optimal value. Figure 5.2(c)
presents the average of the maximum RTO values set by connections during
their lifetimes. It can be observed that for higher rates of mobility, the maximum
RTO values scale up to few tens of seconds. This is true even in the case of
heavy loads. This can result in significant time delays in loss recovery, and
hence result in gross under-utilization of the available bandwidth.
5.2.7 Absolute Impact of Losses
In the discussions thus far in the section, we have touched upon the negative
impact of mobility related losses on TCP’s performance. Losses, in addition
to being inaccurate indicators of congestion for TCP, also have an absolute
impact on the throughput performance of the TCP connection. In this section,
we profile some of the directly contributing causes for such losses.
Impact of MAC Layer. The MAC layer is responsible for detecting the
failure of a link due to congestion or mobility. Since the MAC layer (IEEE
802.11 DCF) has to go through the cycle of multiple retransmissions before
concluding link failure, there is a distinct component associated with the time
taken to actually detect link failure since the occurrence of the failure. Importantly,
the detection time increases with increasing load in the network. A
high MAC detection time will result in a higher likelihood of the TCP source
pumping in more packets (upto a window’s worth) into the broken path, with
all the packets being lost and the source eventually experiencing a timeout.
When a link failure is detected by the MAC layer, the link failure indication
(in DSR) is sent only to the source of the packet that triggered the detection.
If another source is using the same link in the path to its destination, the node
upstream of the link failure will wait till its MAC layer receives a packet from the
other source. Then the MAC layer will go through its cycle of retransmissions
to detect the link failure and only then would that source be informed of the