1 Spring Semester 2007, Dept. of Computer Science, Technion Internet Networking recitation #4 Mobile Ad-Hoc Networks AODV Routing.
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1 Spring Semester 2007, Dept. of Computer Science, Technion Internet Networking recitation #4 Mobile Ad-Hoc Networks AODV Routing
2 Internet Networking Mobile Ad-Hoc Networks (MANET) Wireless networks become an important part of the computers communication both globally and locally. 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. Ad-hoc networks consists of: Mobile hosts Non fixed communication infrastructure No base station or any central entity which can follow the locations of the nodes.
3 Internet Networking Mobile Ad-Hoc Networks (Cont.) Mobile Ad-Hoc Networks (Cont.) Mobile nodes are using multi-hop wireless links They 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. Such networks have been studied in the past in relation to defense research. Recently there has been a renewed interest in this field due to common availability of low-cost laptops and palmtops with radio interfaces. The challenge: development of dynamic routing protocols that can efficiently find routes between two communicating nodes the routing protocols must keep up with the high degree of node mobility that often changes the network topology drastically and unpredictably.
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 only 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 in order to maintain the most recent information between nodes. It is also used in order to prevent routing loops. All routing packets carry these sequence numbers. AODV maintain timer-based states in each node regarding utilization of individual routing table entries 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 “Route Request” (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 a destination sequence number higher than that enclosed in the request (and still “fresh enough” 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 and propagates along the reverse route in order to 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 # C’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.