Research On Improving TCP Performance Using Cross-layer Mechanism In Wireless Ad Hoc Networks

Wireless Ad Hoc network provides the free interconnection of mobile nodes in environments without infrastructure. Because of the wide use both in military field and civilian life, it attracts wide attention of academia. Its unique characteristics bring challenges for many researches, in which TCP is an important area. In ad hoc network, most packet losses are caused by route failure. TCP still interprets such losses as congestion and invokes congestion control yielding poor performance. Existing literatures try to improve TCP performance through cross-layer method, and has achieved some results. A typical cross-layer proposal EPLN uses feedback from the network layer to differentiate the causes of packet loss and adjusts the congestion control mechanism accordingly. However, this proposal is limited to interaction between TCP and network layer, without using information from MAC layer which has a serious impact on the accuracy of packet loss reason estimation. EPLN freezes TCP on receiving packet loss feedback and restores transmission with the state before freezing when network condition gets better. But it is still possible that TCP send excessive packet into network causing packet losses and performance degradation.This thesis presents a proposal called ENEPLN based on EPLN. The new proposal tries to share information among MAC layer, routing layer and transport layer to improve TCP performance. When packet lost, specific algorithm is used to estimate whether it is caused by node mobility based on recorded history signals at MAC layer, then the result is passed to network layer. Routing protocol DSR sends Route Error as a notification to the source of TCP with the packet loss reason piggybacked in it. The source decides whether to remove the failed route according to the reason in the notification, and notify TCP about packet loss through ICMP packets. At transport layer, when TCP recovers from freeze state, it resumes data transmission with a proper rate calculated based on round-trip hops count of current source route.We implemented ENEPLN in NS2 and ran a lot of simulations to validate our proposal. Simulation results show that ENEPLN reduces average route discovery times, reduces average TCP slow-start times and improves TCP throughput by 10% to 20% in static scenes as well as small improvement in dynamic scenes.