master-degree-notes/Autonomous Networking/notes/4 WSN Routing.md

• routing technique is needed to establish multi-hop communication
• the routing strategy should ensure
  • mminimun energy consumption
  • maximization of the network lifetime

Ad Hoc Routing Protocols – Classification

• network topology
  • flat
  • hierarchical
• which data is used to identify nodes
  • arbitrary identifier
  • the position of a node
    • can be used to assist in geographical routing problems to decide next hop
    • scalable and suitable for sensor networks

Flat routing protocols

Three main categories

• Proactive protocols (table driven)
  • always tries to keep routing data up-to-date
  • active before tables are actually needed
    • routes are always already known
    • more bandwidth and energy usage
• Reactive protocols
  • route determined only when needed
  • operates on demand
    • when a route is needed, a kind of global search is started
      • causes delays if routes are not already cached
• Hybrid protocols
  • combination of these behaviors

Destination Sequence Distance Vector (DSDV)

• based on bellman-ford algorithm
• proactive protocol
• add aging information to avoid routing loops
• on topology change, send incremental route updates
• unstable route updates are delayed

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• to avoid loops, DSDV adds a sequence number to each routing table entry which is periodically updated. Routes with higher sequence number are preferred

Reactive protocols

Flooding

• copies of incoming packets are sent by every link except the one by which the packet is arrived
• generates a lot of superfluous traffic
• flooding is a reactive technique, and does not require costly topology maintenance and complex route discovery algorithms

Characteristics:

• derivery is guaranteed (e grazie al cazzo)
• one copy will arrive by the quickest possible route (wow)

Drawbacks:

• implosion: duplicated messages are broadcasted to the same node
• overlap: if two nodes share the same under observation region, both of them may sense the same stimuli at the same time. As a result, neighbor nodes receive duplicated messages
• resource blindness (no knowledge about the available resources)
• does not take into consideration all the available energy resources
• consumes a lot of energy

Gossiping

• nodes send the incoming packages to a randomly selected neighbor
• avoids implosion, but it takes long to propagate the message

Dynamic Source Routing (DSR)

• Source routing: Each data packet sent carries in its header the complete, ordered list of nodes through which the packet will pass
• The sender can select and control the routes used for its own packets and supports the use of multiple routes to any destination
• Including the route in the header of each packet, helps other nodes forwarding the packet to cache the routing information for future use

DSR is composed by two main mechanism:

• Route Discovery
  • mechanism by which a node S obtains the route to a destination node D
  • used only if S doesn't already know the route
  • every request contains an unique ID and a route record
    • S sends a broadcast Route Request packet
    • every node broadcasts the packet (with the same ID) appending their own address to the route record
    • when D receives the request, it sends a Route Reply back to the initiator S, with a copy of the route record
      • more than a route can be returned, making the protocol more resistent to changes in network topology
• Route Maintenance
  • mechanism by which a node S can detect (while sending a packet to D) if the network topology has changed and the route can't be used anymore

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Ad-hoc On Demand Distance Vector routing (AODV)

• a mix between DSR and DSDV
• nodes maintain routing tables
• sequence numbers added to handle stale caches (when routing info is too old)
• nodes remember from where a packet came and populate routing tables

We can see it as an improved DSDV, as it minimizes the number of required broadcast by creating routes on a demand basis.

• if the source node S does not have the route to D, S initiates a path discovery process to locate D
• the route request is broadcasted to the neighbors
• when a node receives a broadcast RREQ, it records in their table the address of the neighbor who sent the request
• when the destination or an intermediate node that can reach the destiation is reached, it replies with a route reply (RREP) packet back to the neighbor from which it first received the RREQ
• the RREQ is routed back along the reverse path
• nodes along the path updates their table

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Geographical routing

• routing tables contain information to which next hop a packet should be forwarded
• this can be explicitly constructed, or implicitly inferred from physical placement of nodes
  • by knowing the position of nodes, we can send the packet to a neighbor in the right direction
  • we can also do geocasting: sending to any node in a given area
  • to map a node ID to the node position we might need a location service

Strategies

• Most forward within range r
  • send to that neighbor that realizes the most forward progress towards destination, but staying in a range r (quello che stando nel range r si avvicina di più geograficamente parlando)
• Nearest node with (any) forward progress
  • the opposite as the previous strategy
  • minimizes transmission power
• Directional routing
  • choose next hop that is angularly closest to destination (closest to the connecting line to destination)
  • come se tracciassi una linea tra S e D e prendessi gli hop più vicini alla linea
  • problem: might result in loops!

Dead ends problem

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Conclusion

• there are many other protocols
• the best solution depends on network characteristics
  • mobility
  • node capabilities
• geographic approach allows to save more energy
• proactive approach is fast, but involves overhead
• reactive approach generate much less overhead, but it is slower