On-demand routing protocols use route caches to make routing decisions. Due to mobility, cached routes easily become stale. To address the cache staleness issue, prior work in DSR used heuristics with ad hoc parameters to predict the lifetime of a link or a route. However, heuristics cannot accurately estimate timeouts because topology changes are unpredictable. In this paper, we propose proactively disseminating the broken link information to the nodes that have that link in their caches. We define a new cache structure called a cache table and present a distributed cache update algorithm. Each node maintains in its cache table the information necessary for cache updates. When a link failure is detected, the algorithm notifies all reachable nodes that have cached the link in a distributed manner. The algorithm does not use any ad hoc parameters, thus making route caches fully adaptive to topology changes. We show that the algorithm outperforms DSR with path caches and with Link-MaxLife, an adaptive timeout mechanism for link caches. We conclude that proactive cache updating is key to the adaptation of on-demand routing protocols to mobility.

On-demand routing protocols use route caches to make routing decisions. Due to mobility, cached routes easily become stale. To address the cache staleness issue, prior work in DSR used heuristics with ad hoc parameters to predict the lifetime of a link or a route. However, heuristics cannot accurately predict timeouts because topology changes are unpredictable. In this paper, we propose to proactively disseminate the broken link information to the nodes that have that link in their caches. We define a new cache structure called a cache table and present a distributed cache update algorithm. Each node maintains in its cache table the information necessary for cache updates. When a link failure is detected, the algorithm notifies all reachable nodes that have cached the link in a distributed manner. The algorithm does not use any ad hoc parameters, thus making route caches fully adaptive to topology changes. We show that the algorithm outperforms DSR with path caches and with Link-MaxLife, an adaptive timeout mechanism for link caches. We conclude that proactive cache updating is the key to the adaptation of on-demand routing protocols to mobility.

TCP performance is adversely affected by frequent route failures due to mobility in mobile ad hoc networks. Most of the recent attempts to improve TCP performance focused on transport layer mechanisms and several modifications to TCP were proposed to prevent it from invoking congestion control for packet losses caused by route failures. We present a new approach to improve TCP performance at the network layer: reducing route failures by making route caches in on-demand routing protocols adapt to topology changes quickly and efficiently. The route cache in on-demand routing protocols is used for routing decisions, however, due to frequent topology changes, cached routes easily become stale, seriously degrading TCP throughput. In our prior work, we proposed a distributed adaptive cache update algorithm to address the cache staleness issue in the dynamic source routing protocol (DSR), an important on-demand routing protocol. In this paper, we investigate the impact of this algorithm on TCP performance, without any modification to TCP. We show through detailed simulations that this algorithm significantly improves TCP throughput and reduces normalized routing overhead. We conclude that it is important to make route caches reflect topology changes quickly so that the effect of mobility on TCP is reduced.

TCP performance degrades significantly in mobile ad hoc networks because most of packet losses occur as a result of route failures. Prior work proposed to provide link failure feedback to TCP so that TCP can avoid responding to route failures as if congestion had occurred. However, after a link failure is detected, several packets will be dropped from the network interface queue; TCP will time out because of these losses. It will also time out for ACK losses caused by route failures. In this paper, we propose to make routing protocols aware of lost data packets and ACKs and help reduce TCP timeouts for mobility-induced losses. Toward this end, we present two mechanisms: early packet loss notification (EPLN) and best-effort ACK delivery (BEAD). EPLN seeks to notify TCP senders about lost data packets. For lost ACKs, BEAD attempts to retransmit ACKs at either intermediate nodes or TCP receivers. Both mechanisms extensively use cached routes, without initiating route discoveries at any intermediate node. We evaluate TCP-ELFN enhanced with the two mechanisms using two caching strategies for DSR, path caches and a distributed cache update algorithm proposed in our prior work. We show that TCP-ELFN with EPLN and BEAD significantly outperforms TCP-ELFN under both caching strategies. We conclude that cross-layer information awareness is key to making TCP efficient in the presence of mobility.