Anonymous Gossip: Improving Multicast Reliability in Mobile Ad-Hoc Networks Ranveer Chandra (joint work with Venugopalan Ramasubramanian and Ken Birman)

What is an Ad-Hoc Network? Wireless medium Mobile nodes No fixed infrastructure for communication Applications: relief operations, warfront!!

Warfront BAS

The Model A graph with ‘n’ nodes A node can move in any direction with any speed Connectivity is based on transmission power and land features Frequently changing connectivity and neighborhood of the nodes.

The Model A B C

Salient Features Dynamic Topologies Bandwidth Constrained Links Energy Constrained Operation Limited Security

A Future Network Ad Hoc Network Fixed Network Base Station Mobile Host Wired Link Wireless Link Ad Hoc Network Switch

Issues in Multicast Routing Minimize state stored in each node Reduce the number of messages exchanged Active adaptability of nodes to mobility, power Local effect of link breakages

Multicast Routing Protocols Tree based: MAODV, AMRIS Multicast performed over an underlying tree structure. Mesh based: ODMRP, MCEDAR Multicast over a mesh, or presence of alternate paths None of them try to recover packets lost during reconfiguration

MAODV: Route Discovery Source Node sends RREQ Sets ‘J’ Flag if joining Retry for some time if unsuccessful Become group leader if still unsuccessful Other nodes set up reverse route entries Multicast Tree Group Member Tree Member

MAODV: Route Reply Only tree members can respond to a join request. RREP is generated and unicast to the source RREP has address of group member and distance from closest tree member Intermediate nodes also update their tables on receiving an RREP. Multicast Tree Group Member Tree Member

MAODV: Route Activation Selects route based on seq # and hopcount Unicasts a MACT to the selected next hop On receiving MACT, the node updates entry in multicast route table Unicasts its own MACT if it is not a tree member. Multicast Tree Group Member Tree Member

MAODV: Other Considerations Link breakages Partitioned Trees Leaving a group Details in draft-ietf-manet-aodv-05.txt Charles Perkins and Elizabeth Royer, March 2000

Desired Properties Improved delivery rate Reduced variation in the number of packets received by the group members.

A Modified Protocol Borrows ideas from Bimodal Multicast Proceed as a combination of 2 sub- protocols i. Any existing protocol is used to multicast messages (MAODV in our case) ii. The Gossip Protocol recovers lost messages.

The Gossip Protocol  A node randomly selects a group member.  Sends a message history  The receiver checks to see if it has any extra messages  The nodes then exchange messages and recover the ones that are lost.

Gossip Protocol: Issues  How does a node know the group membership?  With whom does it gossip?  What is the direction of information exchange?  How is message history maintained?

Group Membership Maintaining group membership:  Wired Networks: Easy because of domain sub-domain hierarchy  Ad-Hoc Networks: Very expensive to maintain information about all the group members.  Is it necessary to know the other group members?

Anonymous Gossip Randomly select one of the neighbors in the multicast tree. Construct a gossip message and send it along the selected node. On receiving a gossip message either forward it along the outgoing links or accept it with some probability if it is a group member.

Anonymous Gossip A B C D E

Ensuring Locality of Gossip Gossiping with a near member Ensures reduced traffic Gossiping with a distant node Able to recover messages that were lost in an entire locality. Gossip locally with a very high probability and occasionally with distant nodes

Ensuring Locality of Gossip Each node maintains a field called ‘nearest_member’ Has the information of the nearest member by taking the link along the next hop node. The probability of taking the next hop is inversely proportional to the ‘nearest_member’ value.

Example A B C D E F G H

Probability Function k0 k1 k2 kN Probability that nbr(i) is chosen as the next hop for gossip: 1 – ki/(k1+k2+....kN)

Tree Overloading All gossip messages are sent along the multicast tree: Extra traffic on these links makes the tree congested Shorter routes may exist During tree repair no gossip messages can be sent Cached Gossip with some probability!!!

Cached Gossip Maintain a member cache: Add a member on receiving a reply of an anonymous gossip request. Delete a member if no route to it is known or it does not reply to a certain number of gossip requests. Each entry is a three tuple of the address, distance and ‘last_gossip_time’ to the node.

Sending Gossip Requests In each gossip round:  Use anonymous gossip with some probability.  If cached gossip is chosen:  Select near members with a very high probability.  Among them select the one with the least ‘last_gossip_time’.

Cached Gossip A B C D E (E, 2) (C, 2)

Other Data Structures Data Structures at each group member:  History Table: A bounded FIFO buffer of received messages.  Lost Table: Fixed size buffer to store sequence numbers of lost messages.  Lost Buffer: The most recent entries of the Lost Table.

Data Structures History Table: Msg1, Msg5, Msg7, … Msgn Lost Table: ……. Lost Buffer: 2 3 Expected Sequence Number: n+1 Example:

Gossip Request Message Group Address Source Address Lost Buffer Size of Lost Buffer Expected Sequence Number

The Protocol Each group member periodically sends a gossip request message On receiving a gossip request the receiver checks to see if it has a copy of the requested messages. It then unicasts any messages found back to the requester. * Push would be expensive

The Protocol History table: msg1, msg4, msg5 Lost Table: 2, 3 Lost Buffer: 3 Expected Sequence Number: 6 History table: msg1… msg6 Lost Table: Lost Buffer: Expected Sequence Number: 7 Gossip Request A B Gossip Reply Msgs 6, 3

Simulation Results Used GloMoSim AG is implemented over MAODV and improvement is measured. 200m x 200m area 40 nodes and 13 group members Random waypoint mobility model One node sent 2201 packets to the multicast group over 440 seconds.

Packet Delivery vs Transmission Range Low transmission power => less connectivity and hence reduced performance.

Packet Delivery vs Maximum Speed Increased Speed => frequent link breakages and hence reduced performance

Packet Delivery vs Number of Nodes

Results Gossip significantly improves the number of packets delivered. The variation in the number of packets received by the different group members is reduced Resulting protocol is more scalable.

Conclusions and Future Work A more reliable underlying multicast protocol would yield much better results. Anonymous Gossip can be implemented over any multicast protocol without much overhead. Is AG well suited for the internet?