1 Spring Semester 2009, Dept. of Computer Science, Technion Internet Networking recitation #3 Mobile Ad-Hoc Networks AODV Routing

2 Internet Networking Mobile Ad-Hoc Networks (MANET)  Ad-hoc networks are useful for providing communication support where no fixed infrastructure exists or the deployment of a fixed infrastructure is not economically profitable.  Recently there has been a renewed interest in this field due to common availability of low-cost laptops and palmtops with radio interfaces.  Ad-hoc networks consists of:  Mobile hosts, each with a wireless communication device  Non fixed communication infrastructure  No base station or any central entity which can follow the locations of the nodes.  A transmission of a node s can be received by all nodes within its transmission range - r r d3 s d2 d1

3 Internet Networking Mobile Ad-Hoc Networks (Cont.) Mobile Ad-Hoc Networks (Cont.)  Mobile nodes form topology changing network without the aid of any central management.  All the nodes are free to move around  Each node is a router  The network is characterized by multi-hop connectivity.  A broadcast of routing messages may block other participants.  Failures are frequent  Due to node movement  Due to battery depletion  The challenge for routing protocols:  Finding efficient routes between two communicating nodes  Keeping up with the high degree of node mobility that often changes the network topology drastically and unpredictably. p q p q

4 Internet Networking Demand-driven Routing Protocols  Creation of routes only when desired by the source node  Done by Route Discovery Process  Route Discovery Process is completed when:  a route is found  all possible routes have been examined  Maintenance of a Route  Until the destination becomes inaccessible along every path from the source  Until the route is no longer desired

5 Internet Networking Ad hoc On Demand Vector Routing (AODV) RFC 3561  On demand – Only nodes on an active paths maintain routing information and exchange routing tables.  A node needs to discover and maintain a route to another node if :  the two nodes need to communicate.  it’s an intermediate forwarding station that maintains connectivity between two other nodes.  AODV makes use of sequence numbers created by the destination:  To maintain the most recent information between nodes.  To prevent routing loops.  All routing packets carry these sequence numbers.  AODV maintains timer-based states in each node  a routing table entry is “expired” if not used recently.

6 Internet Networking The protocol - “Route Request” (RREQ)  A source that needs a path to some destination broadcasts RREQ message enclosed by  a monotonically increasing “broadcast ID”,  a new “sequence number” of the sender and  a last known “sequence number” of the destination.  The RREQ is broadcast until it reaches a node that has a route to the destination with “fresh” information.  A RREQ propagating through the network establishes the next-hop information for the reverse route back to the source.

7 Internet Networking The protocol - “Route Response” (RREP)  A “Route Response” (RREP) is generated by the destination  propagates along the reverse route  establishes forward route information at the intermediate nodes.  Each node keeps the next hop for the destination.  Routing table information is restricted to the active nodes.  A neighbor is considered active if it originates or relays at least one packet for the destination within the most recent active timeout period.

8 Internet Networking Maintenance Maintenance  Failure of a link can be detected via hello messages or link layer detection.  When link goes down, the upstream nodes are notified of the failure  the destination is marked as unreachable in the routing tables of these nodes.  Timers:  Route request expiration timer  Route caching timeout  Active timeout period

9 Internet Networking Route Discovery Example S B DC A RREQ Node S needs a route in order to send data packet to D 1. It creates a Route Request (RREQ) along with:  D’s IP addr, last known D’s seq#.  S’s IP addr, new S’s seq#.  hopcount (=0). 2. It broadcasts RREQ to its neighbors 3. Node A receives RREQ  Makes reverse route entry for S dest = S, nexthop = S, hopcnt = 1  It has no route to D, so it continue to broadcast the RREQ

10 Internet Networking Route Discovery Example (Cont.)  The RREQ is broadcast to nodes B and C  Node C receives RREQ  Makes reverse route entry for S dest = S, nexthop = A, hopcnt = 2  It has a route to D whereas the seq# of this route is >= seq# in the RREQ, and it’s life-time counter > 0  Thus – it’s internal table holds a newer information S B DC A RREQ

11 Internet Networking Route Discovery Example (Cont.) n Node C creates a Route Reply (RREP) with:  D’s IP addr  D’s Sequence #  S’s Sequence #  S’s IP addr  hopcount to D (=1)  Lifetime  Send the RREP to A  Node A receives the RREP  Makes forward route entry to D dest = D, nexthop = C, hopcount = 2, Lifetime  Unicasts RREP to S  Node S receives RREP  Makes forward route entry to D dest = D, nexthop = A, hopcount = 3, Lifetime S B DC A RREP

12 Internet Networking Route Discovery with Multiple Routes Loop preventing  Node C receives replies from D and from B.  The reply from D was received before and was sent to A.  The reply from B will be ignored (it has the same sequence number). S B DC A RREP- Seq# = x E F G

13 Internet Networking Route Discovery Using Fresh Data  Sequence numbers are used as time stamps  Each message initiated by a node has its own Sequence number  The node that initiate the message increase the Sequence Number  A higher Sequence number means a newer route and allow nodes to compare how “fresh” is the information  RREQ with D-Seq#2 from S1 or S2 will cause route info to be sent till D. S1 B D A S2 RREQ D-Seq# =1 RREP D-Seq# =2 RREQ D-Seq# =1 RREQ D-Seq# =2

14 Internet Networking Maintenance  If a link breaks down the intermediate node tries to perform a local repair to the needed destinations.  If it fails, a Route Error message is sent to upstream neighbors.  It lists all the destinations which are now unreachable.  A “DestCount” field is used to indicates the number of unreachable destinations.  A Node that receives an RERR:  Checks whether the sender is its next hop towards the destination.  Deletes or invalidates the route towards the destination if needed.  Forwards the RERR upstream if needed  Rediscovers route if still needed

15 Internet Networking Advantages of AODV Protocol  Highly Scalable  Need for broadcast is minimized  Quick response to link breakage in active routes  Loop free routes  Prevents network flooding during discovery  Repairs breaks in active routes locally instead of notifying source.