Lecture XVII: Distributed Systems Algorithms Inspired by Biology CMPT 431 Dr. Alexandra Fedorova

Problem Statement Load balancing in telecommunication networks Calls originate and end nodes and are destined to end nodes Calls are routed through intermediate switching stations or nodes Each node has a certain capacity – can support only a limited number of calls routed through it Many routes for each call Routing tables determine the route If the call is routed via a congested node, it must be dropped Goal: construct routing tables that minimize the number of dropped calls under changing load conditions

Potential Solutions Mobile agents Central controller: knows about the entire system, updates routing tables at nodes Nodes must communicate with the controller The controller is a single point of failure Use shortest-path routing Determine the shortest path from each source to each destination Construct routing tables to reflect shortest path routes (this can be done because network topology does not change) This will occupy the fewest nodes for each call, but will not necessarily result in routing along the least congested path Mobile agents Software agents (worms) move from node to node. Update routing tables based on their observations of the network

Structure of the Paper Schoonderwoerd et al. Ant-based load balancing in telecommunications networks Present a new solution – a new kind of distributed mobile agent Behaviour inspired by that observed in colonies of ants Evaluate A simulated network Measure the rate of dropped calls Compare with A different kind of mobile agent Static routing table

Inspired by Nature Ants are silly animals that accomplish sophisticated results as a team Regulating nests temperature within limits of 1˚C Forming bridges Raiding particular areas for food Building and protecting their nest Cooperating in carrying large items Finding the shortest routes from the nest to a food source Mobile agents: we want them to be silly (i.e., simple), but accomplish sophisticated things (load balancing in the communications network)

How Ants Cooperate Stigmetry – indirect communication through the environment Produce specific actions in response to local environmental stimuli These actions in turn affect the environment The modified environmental stimuli affect actions of the ants that come to that location Sematectonic stigmetry Produce the environmental change: i.e., deposit a ball of mud Causes other ants to repeat the action, i.e., deposit another ball of mud Sign-based stigmetry Deposit pheromones (smelly substances) that cause other ants to behave differently, responding to the presence of pheromones

Example: Laying a Trail (cont.) Ants lay pheromones as they travel along a trail A trail’s strength is determined by the amount of pheromones on the trail Amount of pheromones depends on: The rate at which pheromones are laid The amount of pheromones laid – how many ants laid them How much time has passed since the pheromones were last laid (pheromones evaporate over time) If many ants follow along the same trail the total amount of pheromones is high – the trail’s strength is high: Rate of deposit is high Pheromones laying is recent

Example: Laying a Trail (cont.) Ants started on the right Ants started on the left Shorter path has more pheromones

ABC: Ant-Based Control Routing tables are replaced with pheromone tables Each node in the network has a pheromone table for every other node Each table has an entry for each neighbour, indicating the probability of using that neighbour as the next hop Pheromone laying is updating probabilities

Updating Pheromone Tables At every time step ants can be launched from any node in the network The destination node is random Ants move from node to node, selecting the next node according to pheromone tables for their destination node At each node they update probabilities of the entry corresponding to their source node They increase the probability associated with the node where they came from

Updating Pheromone Tables (cont.) destination current location 2 source 1 3 4 Update routing table at node 1 for node 3 2 4 prob(2) = X prob(4) = Y increase by Δp the probability of taking 4 as next hop

Ageing and Delaying Ants Recall the system’s objectives: Find routes that are short; avoid routes that are congested This is accomplished by ageing and delaying ants Ageing ants: Age: the number of time steps the ant has travelled Δp (the amount by which you increase the probability) reduces progressively with the age of the ant This biases the system to “trust” ants who use shorter trails Delaying ants: Delay ants at nodes that are congested Degree of delay correlated with the degree of congestion This increases the age of ants travelling through congested nodes, so their pheromones have a smaller influence on pheromone tables Delays updates to pheromone tables leading to congested nodes

Routing Calls in ABC Network Route call to destination D At the current node, look up the pheromone table for node D Choose the neighbour corresponding to the highest probability in the table Use that node as the next hop The call is placed if the route is not congested, otherwise the call is dropped

Potential Problems Blocking problem Shortcut problem An available route is suddenly blocked It may take a while to find a new route Shortcut problem A better route becomes available It may take a while to adapt to the new route

Solving Blocking And Shortcut Problems Add a noise factor to ants movement protocol With probability f ant chooses a random path This ensures that Useless routes are used occasionally (so they can be rediscovered if they suddenly become good) Encourage more rapid discovery of a new route (if it becomes available)

ABC: Putting it All Together Ants are regularly launched with random destinations on every part of the system Ants walk according to probabilities in pheromone tables from their destination Ants update the probabilities in the pheromone table for their source location They increase the probability of selecting their previous node on the path as the next hop (to their source node) The increase in probability is a decreasing function of the ant’s age The ants are delayed on parts of the system that are congested

Other Mobile Agents Mobile software agent Travels from node to node Load management agent Parent agent Travels from node to node Updates routing table to find the least congested route Two variations: Largest minimum capacity (LMC) Minimum sum of squared utilizations (MSSU)

Network Simulation A software simulator Node representation: A node ID A capacity – number of simultaneous calls that the node can handle (40) Probability of being the end node (source or destination of a call) Spare capacity Routing table with n-1 entries, one for each node. A B D C Routing table at node C Destination Next hop A D B

Network Simulation (cont.) Calls are generated by a traffic generator Call parameters: source node, destination node, call duration (170 time steps average) Call is routed using routing tables, spare capacity of intermediate nodes is reduced If there is no spare capacity on the route, the call will fail

Experimental Setup Call probability set: a particular distribution of calls Adaptation period: run a load balancing mechanism Test period: measure network performance for the number of dropped calls

Results: Percentage of Dropped Calls What do these numbers indicate? Which load balancing method performed the best?

Results (cont.) Percentage of failed calls after stopping load balancing (call probabilities remain unchanged) What does this tell us about the system? It still makes sense to load balance, even if call probabilities don’t change, to adapt to dynamic changes in the network conditions

Results (cont.)

Results (cont.)

Summary In general ants performed better than other mobile agents ABC system stores information not only about good current routes, but about good recent alternative routes This allows it to adapt quickly to changes in network conditions Ants consume less network resources than mobile agents (ants don’t need to store info about all nodes visited) Ants can work concurrently without affecting each other; only one mobile agent can be active at once A failure of ant does not hurt the system – other ants will update pheromone tables: the failure of mobile agent affects launching of future agents, so the failure has to be detected