Comparison between DSR and AODV DSR Overview AODV Overview Similarity Difference Consequence DSR Overview Source routing routes are stored in a route cache data packets carry the source route in the packet header Route discovery Condition a node sends data to a destination for which it does not know the route Actions source floods the network with RREQ Each node receiving RREQ rebroadcasts it unless it is destination or it has the route to the destination in its cache Route reply A destination node or a node knowing the route to the destination in its cache replies with RREP RREQ and RREP are also source routed Route carried back by RREP is cached at the source DSR Overview II Error handling If any link on a source route is broken the source is notified by a RERR packet Source removes any route using this link from its cache A new Route Discovery process must be initiated by the source if the route is needed Optimizations Salvaging an intermediate node uses an alternative route from its cache when a data packet meets a failed link on its source route Gratuitous route repair A source node receiving RERR piggybacks the RERR in the following RREQ to clean the caches of other nodes that may use the failed link Promiscuous listening when a node overhears a packet not addressed to itself it checks whether the packet could be routed via itself to gain a shorter route If so sends a gratuitous RREP to the source with the new better route AODV Overview Discovers routes on an on demand basis via a similar route discovery process but uses a different mechanism to maintain routing info AODV uses routing table one entry per destination It relies on routing table entries to propagate a RREP back to the source and route data packets to the destination AODV uses sequence maintained at each destination to determine freshness of routing info And to prevent routing loops These sequence are carried by all routing packets AODV Overview II Maintain timer based states in each node A routing table entry is expired if not used recently A set of predecessor nodes is maintained for each routing table entry indicating neighbors that use the entry to route packets These nodes are notified with RERR when the next hop link breaks Each predecessor node forwards RERR to its predecessors erasing all routes using the broken link Optimization Expanding ring search control the RREQ flood in the route discovery process The search is controlled by the TTL field increasingly larger neighborhoods are searched to find the destination Common features of DSR and AODV Both discover routes only in the presence of data packets in the need for a route to the destination Route discovery is based on query and rely cycles and route information is stored in all intermediate nodes on the route Route table entires AODV route caches DSR High level difference DSR uses source routing but AODV uses a table driven routing framework and destination sequence to prevent loops and determine route freshness DSR does not rely on any timer based activities but AODV does DSR uses routing cache aggressively and maintains multiple routes per dest AODV uses one route per destination Differences between DSR and AODV I DSR has access to greater amount of routing information than AODV by the virtual of SR AODV can gather limited information DSR in a single query reply cycle source learns route to each intermediate node in the route in addition to the dest Each intermediate node also learns route to other nodes on the route Promiscuous listening also helps to learn the route to every node on the route AODV no source routing or promiscuous listening It causes AODV to rely on a route discovery flood more often generating more network overhead Difference II DSR uses route caching aggressively and replies to all requests reaching a destination from a single request cycle Source learns many alternative routes to the destination useful when the primary route fails This saves overhead due to discovery flood AODV maintains at most one entry per dest In the routing table The destination replies only once to the request arriving first and ignores the rest Difference III DSR does not have explicit mechanism to expire stale routes in the cache except that some are deleted by RERR or prefer fresher routes RERR backtracks the data packet that meets a failed link Nodes not on the upstream route of this data packet but using the failed link are not notified promptly AODV is more conservative the fresher route is always chosen The route deletion using RERR is also conservative Using the predecessor list RERR packets reach all nodes using a failed link on its route to any destination Consequence For application oriented metris delay and throughput DSR outperforms AODV in less stressful situations smaller of nodes and lower load and or mobility AODV outperforms DSR in more stress situations more load higher mobility DSR Aggressive use of caching lack of mechanism to expire stale cache DSR consistently generates less routing load than AODV Wireless TCP Why we study this work issues for TCP over wireless a sample of proposed solutions comparisons and comments Issues for Wireless TCP Different packet loss behavior violates the assumption of TCP that all packet losses are due to congestion control congestion induced loss new flow joins etc channel error induced loss bursty or random channel error handoff induced packet loss happens during handoff transition routing induced packet loss stale routing tables in a dynamic ad hoc network Uniform reaction to different losses in TCP in TCP reduce congestion window by half upon packet loss does one fit all work in the wireless scenario Solution Space Where you are allowed to modify add the design what is the information you can get Scenario 1 no change at any intermediate node e g base station change is only allowed at both the sender receiver sides however wireless link may provide information regarding whether it is wireless related loss or not Scenario 2 no change at any intermediate node no change at the receiver side only sender side is allowed to be modified no addition information feedback other than loss Solution Space contd Scenario 3 Intermediate node is allowed to change TCP sender receiver sides should be kept intact as much as possible Scenario 4 you can change anywhere TCP senders receivers as well as intermediate nodes Solutions End to end protocols Category 1 the network or receiver provides additional feedback