Relying on Safe Distance to Achieve Strong Partitionable Group Membership in Ad Hoc Networks Authors: Q. Huang, C. Julien, G. Roman Presented By: Jeff Joyner

Overview Group Membership Explanation Problems in Group Membership Solutions to Group Membership Issues Safe Distance Explanation System Implementation Conclusions

Strong Group Membership “Service that establishes and maintains some kind of agreement over time among participating components about who is currently in the group, despite the presence of failures in the corresponding distributed system”

Need For Consistent Group Membership Views Any situation that requires the presence of two entities to implement a task requires a consistent membership view –Electronic witness –Intelligent transportation systems –Military battle group systems

Problems with Consistent Group Membership Views Ad Hoc Networks have high rates of disconnections among group members To maintain a consistent group membership view, there is a need to detect a future disconnection before it occurs Determining a future disconnection enables a group to merge or split in advance to maintain a consistent membership view –Safe Distance algorithm accomplishes this goal

Safe Distance Goal Provide the ability to maintain a consistent global data structure in a mobile environment where hosts join and leave the network randomly and conduct reliable transactions with other hosts in the group membership view

Safe Distance Importance All members of a group must have a common group membership view Disconnections are common in ad hoc networks Fixed networks eventually recover from a disconnection Ad hoc networks usually do not recover from a disconnection Safe Distance is necessary to detect the disconnection before it happens and maintain a consistent group membership view

Safe Distance Maximum distance between hosts that a communication task is guaranteed to complete before disconnection Two hosts agree on membership in the same group when they are at distance: r <= R – 2v * (t + t’)

Safe Distance Responsibilities Determines when two groups can be merged Determines when a group must be split to maintain the requirements for a consistent group membership view

Safe Group Definition Two members of a group are connected on a path in which all consecutive hosts are at a safe distance

Safe Distance Example

Safe Distance Equation r <= R – 2v * (t + t’) –R = Transmitter communication range –2v = Worst-case movement where two hosts are moving in opposite directions at max speed –t = Upper bound time for network latency –t’ = Time for group-level operation to complete

System Model Assumptions No host crash failures and no failures based on network congestion Underlying communication system is reliable and timely All hosts have the same communication radius All hosts know their physical location at all times No knowledge of mobility patterns Speed is bound by Vmax

Group Leader Responsibilities Maintain data map containing the current location of all hosts in the group Process to see if current group members are still within safe distance of each other Check to see if new hosts are in the region and can be safely added to the group Maintain a list of groups that are close enough to be considered for merging

Group Discovery Protocol Periodic hello messages are sent out by hosts that contain location information and a group identifier Hosts in each group use safe distance to determine possible merge candidates Discoveries are reported to the group leader Duplicate discoveries reported to the group leader are discarded

Reconfiguration Protocol Agreement is reached for which hosts and groups are participating Formal notification is sent to involved hosts Barrier synchronization performed –Flush messages in transit –Timeout delay Messages from “future” configuration must be delayed until reconfiguration is complete

Merging Process Group leader discovers another group in its vicinity through group discovery protocol Group leader initiates the merger by sending a merge-request message to the group leader of the 2 nd group 2 nd Group Leader –Accepts request => sends back ACK with configuration sequence number –Rejects request => sends back NACK, aborts merge

Merging Process(2) If ACK received, new configuration number is generated by adding 1 to the larger of the two group member’s configuration numbers Group leader sends a merge-commit to 2 nd group leader and a merge-order to its members which contains –New group membership list –New configuration number –New leader identity

Merging Process(3) After hosts receive merge-order, a flush-message is sent to all original group members to perform barrier synchronization Group members stop sending in the old configuration until all flush-messages have been received If new configuration messages are received before the host has entered the new configuration, the messages must be postponed as future messages If two groups initiate merging simultaneously, the highest group leader id becomes the initiator

Merging Example

Merging Example (2)

Partitioning Group leader determines unsafe situation exists Group leader issues split-order message to all group members which contains –New leader id –New membership list –New configuration number Host with the lowest id in each subgroup becomes the group leader for the subgroup After receiving split-order message, the host enters message flushing phase

Partitioning(2) Group members wait for all messages sent in the previous configuration before moving into the new configuration Group leader can initiate a split that will partition the group into two or more groups in order to maintain safe distance property

Partitioning Example

Implementation Java-based Integrated with LIME middleware Event model is used as a basis for listeners that are monitoring Group Member updates Beacon threads are used to send periodic “hello” messages Poller threads are used to listen for nearby “hello” messages

Implementation(2) Local state actions in system implementation –NeighborGreetings –LocationUpdate –Merge –Partition

Implementation(3) Arrival message actions in system implementation –NeighborHello –InformLeader –Merge-Request –Merge-Commit –Merge-Order –Split-Order

Safe Distance Choices Too conservative of an equation could cause the groups to be too small or useful for some applications An equation that is not conservative enough could pose danger to the correctness of the group A proper balance must be achieved to make the safe distance algorithm safe and useful

Safe Distance Improvements Include additional sensor information from each group member in the updates to the group leader Base the safe distance equation on more information such as transmission power and speed rather than just physical location Use parameter values that are more accurate rather than overestimates for the safe distance equation

Conclusions Safe Distance accomplishes the goal of detecting a future disconnection in advance Safe Distance maintains a consistent group membership view as required by mobile applications Safe Distance as proposed in this paper leaves room for future improvements to the algorithm

References Q. Huang, C. Julien, and G. Roman, “Relying on Safe Distance to Achieve Strong Partitionable Group Membership in Ad Hoc Networks”, IEEE Transactions On Mobile Computing, Vol. 3, No. 2, pp