ODMRP-ASYM(On Demand Multicast Routing Protocol) For Linux Implementation CS218, Fall 2003, Professor Gerla Eric Bostrom, Jason Lin Tutor: Joon-Sang Park University of California, Los Angeles

Asymmetric Link Problem in On- demand routing Asymmetric link  link with different characteristics in the two directions  link that connects A to B, but not B to A Sources of asymmetric Links  External interference  Directional antennas  Power control Asymmetric link problem in on-demand routing  Reverse Path technique does not work! Two ways to solve the problem  Eliminate or unitize

Asymmetric Links: To eliminate or utilize? Problem with eliminating asymmetric links:  Network partition! Asymmetric links’ impact on network topology is not negligible Our challenge: Exploit asymmetric links!

ODMRP: Forwarding Group Forwarding Group: All the nodes inside the “bubble” forward the M- cast packets via “restricted” flooding Multicast Tree replaced by Multicast “Mesh” Topology Flooding redundancy helps overcome displacements and fading FG nodes selected by tracing shortest paths between M-cast members FG Forwarding Group

ODMRP: Route construction Similar to other on-demand routing protocols Consists of a query and a reply phase A source periodically transmits Join Query packets when it has data to send Join Query packets can carry data payload to eliminate route acquisition latency Intermediate nodes forward the packet and set up path back to the source (backward learning) The destination sends a Join Reply in response to a Join Query

ODMRP-ASYM: Overview ODMRP-ASYM: ODMRP with ASYMmetric link support Observation: A loop exists when there are two non- identical asymmetric paths between a pair of nodes. Our strategy: Detect the loop and mark it! S D AB C E Asymmetric links Loop! Path from S to D Path from D to S

ODMRP with asymmetric links Source S floods Join Query toward destination D Reserve path for relaying Join Reply from D to S is blocked at B due to link asymmetry S D AB C E Join Query Join Reply

ODMRP-ASYM: Loop Detection S D AB C E On detecting asymmetric link, node B initiates Loop Detect Packet (LDP) flooding A LDP goes around a loop while collecting node IDs in the loop and returns to B LDP CEA

ODMRP-ASYM: Loop Marking S D AB C E Once found a loop, B circulates Loop Mark Packet (LMP) to set forwarding group flag on (on node A) The node closest to the source in the loop (node A) resumes relaying Join Reply to the source Route Constructed! LMP CEA Join Reply

ODMRP-ASYM: Detecting asymmetric links In ODMRP, every Join Reply is to be ACKed as the reliable delivery of Join Query is critical for establishing forwarding path. If Join Reply is not ACKed, it is retransmitted at most two times. A link is considered to be asymmetric and initiate the loop detection if a Join Reply is not ACKed even for the second retransmission.

ODMRP-ASYM: Linux Implementation Existing Code for ODMRP in Linux  Written by many different people  Not the entire protocol was implemented to specification Defined our packet types and added handler code Hardest part: set up and configure the testbed to run our code

Existing Code Written by a number of different people over a larger period of time. Poor commenting Not fully compliant to specification  Acks and timeouts / some hacks Large project with many files

Loop Discovery Packet Definition

Our Implementation Acks and Timeouts still yet to be done  Should have been done in ODMRP  We used a single ack timeout with passive ack Loops limited to internal nodes  ODMRP actually 2 programs, sender receiver and routing nodes. We only modified the routing nodes protocol  Easily extendible to the end nodes Implementation supports only 1 loop currently Loop Marking Packet in progress

Testbed Consists of 5 dell Lattitude laptops with b Lucent wireless pcmcia cards installed. Asymmetric Links are made using iptables packet filtering tool S D AB C Asymmetric links Loop! Path from S to D Path from D toS

Conclusion ODMRP-ASYM: extending ODMRP to asymmetric links Implementation on Linux is near completion We are having some problems with the testbed configurations, acking and timeouts are not fully compliant to specification

Papers Lee, S.-J., Su, W., Gerla, M.: On- Demand Multicast Routing Protocol in Multihop Wireless Mobile Networks. ACM/Kluwer Mobile Networks and Applications, Implementation of On-Demand Multicast Routing Protocol (ODMRP) in Linux. Dept. of Computer Science, UCLA M. Gerla, Y. Lee, J. Park, and Y. Yi. ODMRP (On Demand Multicast Routing Protocol) on a Topology with Asymmetric Links. Computer Science Dept. UCLA

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