Fundamentals of Computer NetworksECE 478/578
Lecture #17: Routing 2Instructor: Loukas Lazos
Dept of Electrical and Computer EngineeringUniversity of Arizona
Broadcast RoutingBroadcast Communication: All nodes of the network must receive the same
messageTV, radio are by default broadcast operations
Unicasting to each nodeSource needs transmit message N timesAddresses of all recipients must be knownA total of N messages must traverse the network
FloodingInefficient use of bandwidth – Nodes receive the same message multiple timesDoes not scale with network size N
Spanning Tree RoutingBuild a (minimum) spanning tree from source to all nodesSource need send the message only onceMessage is relayed a total of (N - 1) times
2
Building a Spanning TreeKruskal’s Algorithm: Greedy approach
Add the edge with the smallest weight that adds connects one or two new nodes togetherRepeat until all nodes are added
BroadcastingAll edges have the same weightAny spanning tree is a minimum spanning treeSpanning tree is the same regardless of the source of the message
3
Reverse Path ForwardingIf packet arrives from a “preferred router” forward, otherwise discard
Preferred router: router on the reverse shortest path to the source
4
Source
Broadcasting in Wireless Multi-hop NetsMust take into account energyThe Broadcast advantage
SBn > SAn + ABn, broadcast to A and A will forward to BElse use one transmission to B
BIP: Broadcast Incremental Power
5
S
B
AS
A
B
C
DE
• T={S}• Add node j, which minimally increases the
power required to reach new node• Repeat until all nodes are added to T• Greedy algorithm, that leads to a non-
optimal solution
Routing for Mobile HostsHost moves from one network to another
If IP is changed, certain services may not work
6
Optimizing Routing for Mobile Hosts
7
Routing in Ad Hoc NetworksAd Hoc Distance Vector Routing (AODV)
Source node floods a RREQ message Every receiving hosts keeps a reverse path to where the packet came fromOnce the message reaches the destination, it replies with a RREPRREP is forwarded on a unicast path back to the source
8
Message FormatRoute Request Message
Destination Sequence Number: Freshness of route to dest.Source Sequence Number: Freshness of RREQ
Route Reply Message
9
Route MaintenanceActive neighbors: Neighbors that maintain active paths
10
Dynamic Source Routing (DSR)The source determines the sequence of nodes that the packet must follow based
onCashed InformationA route discovery phase
Advantages of DSRNo periodic route updates as in DV, saves energy resourcesNo need for bidirectional links as in AODVCan quickly adapt to changes
Disadvantages of DSRRoute is specified on the packet header that can grow fairly longUses flooding for routing discovery as well
11
Route Discovery in DSRSender broadcasts a route request (RREQ) packet
Similar format to AODV packet
Each Intermediate node<Source address, request id>
If same, discard
The address of the intermediate node is already in the route recordThis is a loop - discard
This node is the targetSend a route reply (RREP)
Else Append node’s address to the route record, and re-broadcast
12
Example: Routing with DSR
13
A
BC
E
D
F
H
G
I
A
A
A, B
A, BA, D
A, D, F
A, D
A, D, F, H
A, D, GA, B, D, G
A, B, C
A, B, C, E
Geographical RoutingRoute based on geographical location
Send packet to the neighbor closest to the destination, based on geographic coordinates
No need for route discovery, or routing tables
14
A
BC
E
D
F
H
G
I
Geographical Routing May Get StuckA node is closest to the destination, yet there is no transition to the
destination
15
Avoid local MinimaCreate a planar graph of the network
Route in the perimeter until you hit the straight line
16
Top Related