ࡱ> ,.+bjbjUU >??   KMMMMMMBJMMbKKA7x0... ,MM. :Resumo Os protocolos de roteamento para Redes Tolerantes a Atrasos e Desconexes (Delay and Disruption Tolerant Networks - DTNs) so suscetveis a comportamentos maliciosos dos ns. Particularmente, no ataque de falsificao de reconhecimentos positivos, ns maliciosos forjam reconhecimentos positivos, ou ACKs, com o objetivo de causar impacto negativo no desempenho da rede removendo mensagens que ainda no chegaram ao destino dos buffers dos ns. Este trabalho prope novas contramedidas contra o ataque de falsificao de reconhecimentos positivos em DTNs. As contramedidas so denominadas DRAC (Drop Acknowledged Messages First) e DRAC-SF (Drop Acknowledged Messages First and Stop Forwarding) e funcionam como se segue. Quando ACKs so recebidos, as propostas no descartam imediatamente as mensagens para as quais estes reconhecimentos foram gerados. Ao invs disso, elas alteram a prioridade destas mensagens na fila de descarte de modo que estas mensagens tero prioridade para serem descartadas em caso de estouro do buffer. Adicionalmente, a contramedida DRAC-SF para de encaminhar e replicar mensagens para as quais ACKs foram recebidos. Ressalta-se que DRAC e DRAC-SF no se baseiam em nenhum mtodo de autenticao, visto que no tentam identificar quais so os ns maliciosos. A anlise considera quatro cenrios reais de mobilidade, sete protocolos de roteamento e dois modelos de ataque distintos. Os resultados mostram que as propostas reduzem a eficincia do ataque de falsificao de reconhecimentos positivos. Adicionalmente, cabe destacar o desempenho da proposta DRAC-SF, que supera o desempenho da principal contramedida existente na literatura em 88% dos cenrios avaliados, alcanando taxas de entrega superiores em at 135%. Palavras-chave: Redes tolerantes a atrasos e desconexes; reconhecimentos positivos; segurana. Abstract Routing protocols for Delay and Disruption Tolerant Networks (DTNs) are prone to suffer with malicious behavior of nodes. Particularly, in the acknowledgment counterfeiting attack, malicious nodes forge positive acknowledgments, also known as ACKs, in order to negatively impact network performance by removing from nodes' buffers messages which were not yet delivered to their destinations. This thesis proposes new countermeasures against the acknowledgement counterfeiting attack in DTNs. The proposed countermeasures are named DRAC (DRop ACknowledged messages first) and DRAC-SF (DRop ACknowledged messages first and Stop Forwarding) and work as follows. When an ACK is received, DRAC and DRAC-SF do not immediately drop the message for which this ACK was generated. Instead, in case of buffer overflow drop priority is given to this message. Additionally, DRAC-SF also stops forwarding and replicating messages for which ACKs were received. We emphasize that DRAC and DRAC-SF do not rely in any authentication method because both countermeasures do not try to identify malicious nodes. The analysis considers traces of four real networks, seven routing protocols and two distinct attacker models. Results show our proposals decrease the efficiency of the acknowledgement counterfeiting attack. In addition, we highlight that DRAC-SF outperforms the main countermeasure against this kind of attack found in literature in 88% of the evaluated scenarios, providing higher delivery rates up to 135%. Keywords: Delay and Disruption Tolerant Networks; positive acknowledgements; security. Sy 4 [ {  ><DLMN-6߾ߠߐߌ{g\hEHh mH sH &hEHh 5OJQJ\^JmH sH  h 5CJ OJQJ\^JaJ h hEHh 5OJQJ\^Jh OJQJ^J&hEHh 6OJQJ]^JmH sH  hEHh OJQJ^JmH sH hEHh 6OJQJ]^JhEHh OJQJ^J&hEHh 5CJ OJQJ\^JaJ <=>?@ABCDMN,-gdEH $dha$gdEH $dha$gdEH<P1h:p . 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