1Spring Semester 2009, Dept. of Computer Science, Technion

Internet NetworkingInternet Networkingrecitation #3recitation #3

Mobile Ad-Hoc NetworksMobile Ad-Hoc Networks

AODV RoutingAODV Routing

2

Internet Networking

Mobile Ad-Hoc Networks (MANET) 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 qp q

4

Internet Networking

Demand-driven Routing ProtocolsDemand-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) Ad hoc On Demand Vector Routing (AODV) RFC 3561RFC 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 - The protocol - “Route Request” (RREQ) “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 - The protocol - “Route Response” (RREP) “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

MaintenanceMaintenance

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 ExampleRoute 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.)Route Discovery Example (Cont.)

The RREQ is broadcast to nodes B and C

Node C receives RREQ

Makes reverse route entry for Sdest = 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.)Route Discovery Example (Cont.) 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 Ddest = D, nexthop = C, hopcount = 2, Lifetime

Unicasts RREP to S

Node S receives RREP

Makes forward route entry to Ddest = D, nexthop = A, hopcount = 3, Lifetime

S

B

DC

A RREP

RREP

12

Internet Networking

Route Discovery with Multiple Routes Loop preventingRoute 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

ARREP- Seq# = x

E F

GRREP- Seq# = x

RREP- Seq# = x

RREP- Seq# = x

RREP- Seq# = x

RREP- Seq# = x

RREP- Seq# = x

RREP- Seq# = x

13

Internet Networking

Route Discovery Using Fresh DataRoute 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

DA

S2

RREQD-Seq# =1

RREPD-Seq# =2

RREQD-Seq# =1

RREQ D-Seq# =2

14

Internet Networking

MaintenanceMaintenance

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 ProtocolAdvantages 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.