Download Link-State Routing

Routing - Circuit Switching
 Telephone
switching was hierarchical with only
one route possible
— Added redundant routes
— Evolved to full mesh
Toll switches
Central offices
Subscribers
EETS 8316/NTU CC725-N/TC/11-08-01
#1
Routing - Packet Switching
 Data
networks have used various routing
protocols
— Flooding
— Source routing
— Distributed shortest-path routing
• Distance-vector routing
• Link-state routing
 Layered
routing is used in Internet for
scalability
EETS 8316/NTU CC725-N/TC/11-08-01
#2
Flooding
 Packet
is copied at every router and broadcast
to neighbors
— Useful when info must be shared at all routers, eg,
routing updates
 Advantage:
no knowledge of network topology
is necessary
— Well suited to highly dynamic networks, eg,
battlefield
 Disadvantage:
Number of message copies
may be huge without additional mechanisms,
eg:
EETS 8316/NTU CC725-N/TC/11-08-01
#3
Flooding (Cont)
— Messages have hop limit to restrict number of
copies
— Messages are uniquely identified so routers can
stop unnecessary copies
— Routers can establish minimum spanning tree that
message copies will follow
• Overlay tree that covers every router exactly
once
Example
network
EETS 8316/NTU CC725-N/TC/11-08-01
Example
spanning
tree
#4
Source Routing
 Sender
determines route and specifies it in
packet header
— Example: IP source routing option
— Usually centralized route server maintains network
topology info and collect updates from all routers
 Advantage:
sender can select optimal route or
test certain routes
 Disadvantages:
— Centralized routing is not scalable to large networks
— Difficult to carry long routes in packet header
EETS 8316/NTU CC725-N/TC/11-08-01
#5
Distributed Shortest-Path Routing
 Distributed
shortest-path routing: each router
builds up routing table independently and
shares routing updates with other routers
— Routing table entries specify how to forward
packets to destination addresses/networks
 Two
main types of routing protocols known
from Internet experience:
— Distance-vector or Bellman-Ford
— Link-state
EETS 8316/NTU CC725-N/TC/11-08-01
#6
Distance-Vector Routing (Bellman-Ford)
 Example:
RIP (routing information protocol)
 Each
router maintains a table (vector) of
reachable destinations and corresponding
shortest distances to each destination
— Periodically shares routing table with its neighbors
 Each
router modifies its routing table with the
received updates
— Shortest distance to every destination is chosen
— New reachable destinations are added,
unreachable destinations are deleted
EETS 8316/NTU CC725-N/TC/11-08-01
#7
Distance-Vector Routing (Cont)
 RIP
was first routing protocol used in Internet
but difficulties were found
— Slow to propagate routing updates through large
network
— Routers can have inconsistent or even incorrect
information
— Scalability problem: vector of destinations becomes
longer as network is larger  routing updates are
larger
EETS 8316/NTU CC725-N/TC/11-08-01
#8
Link-State Routing
 Examples:
OSPF (open shortest path first), ISIS (intermediate system - intermediate system)
 Each
router maintains a graph representing
complete network topology
— Monitors state of its links with neighboring routers
— Floods link-state updates to all routers periodically
 Each
router receives link-state updates from
other routers and revises its graph
— Shortest routes can be computed systematically by
Dijkstra’s algorithm
EETS 8316/NTU CC725-N/TC/11-08-01
#9
Link-State Routing (Cont)
 Example:
topology graph is given, minimum
spanning tree generated by Dijkstra’s algorithm
is all shortest routes from source node A (link
labels are distances)
5
3
2
2
A
1
3
5
A
1
2
1
EETS 8316/NTU CC725-N/TC/11-08-01
#10
Link-State Routing (Cont)
 Advantages:
— Every router should have consistent, complete
topology info  consistent, optimal route selection
— Routing updates do not grow with size of network
(link-state updates depend on number of links per
router)
 Disadvantage:
flooding link-state updates can
result in large traffic volume and traffic
oscillations
— Limit updates to only significant changes
— Flooding is limited by layered routing
EETS 8316/NTU CC725-N/TC/11-08-01
#11
Layered Routing
 Internet
is too large for routing as “flat network”
where every router is aware of every other
router
 Layered
routing: routers are grouped together
into “routing domains” or “areas”
— At lowest layer, interior routers within each routing
domain use an intra-domain routing protocol with
each other
— Exterior routers represent each routing domain in
higher layer inter-domain routing protocol
— Interior routers are aware only of routing domain
EETS 8316/NTU CC725-N/TC/11-08-01
#12
Layered Routing (Cont)
Routing domain
Interior routers use
intra-domain routing
Exterior routers use
inter-domain routing
EETS 8316/NTU CC725-N/TC/11-08-01
#13
Ad Hoc Networks
 Wireless
networks may be classified as
— Infrastructured: cellular systems consist of fixed
(wireline) network with mobile users attached to
network edge through fixed base stations
• Handoff is necessary and critical
— Ad hoc: no fixed infrastructure
• All nodes can be mobile and autonomous
• Connections are dynamic and arbitrary
• Examples: battlefield,, emergency rescue
operations, WLAN, PAN
EETS 8316/NTU CC725-N/TC/11-08-01
#14
Ad Hoc Networks (Cont)
 IETF
(Internet Engineering Task Force) Mobile
Ad Hoc Networks (MANET) working group is
studying proposals
— MANETs consist of mobile routers and mobile hosts
(can be physically same)
— Mobile hosts are temporarily or permanently
attached to a mobile router
— Inter-router connectivity can change frequently
• Routing protocol must adapt to continually
changing topology
• Addresses cannot be based on location
EETS 8316/NTU CC725-N/TC/11-08-01
#15
Ad Hoc Networks - Routing
 Routing
protocols
— Table driven: nodes maintain and update routing
tables
• Cost in overhead traffic for distributing routing
updates
• Routes are always ready when needed for data
— On-demand (source initiated): routing tables are not
maintained cont., routes are found only when
needed for data transmission
• Cost in overhead traffic only when data is ready
for transmission
• Routing tables are smaller
EETS 8316/NTU CC725-N/TC/11-08-01
#16
Ad Hoc Networks - Table Driven Routing
 Destination
sequenced distance-vector (DSDV)
routing
— As in Bellman-Ford, each node maintains routing
table of reachable destinations and their distances
(number of hops)
— Routing table info. is periodically broadcast for
consistency, using two packet types:
• Full dump packets contain complete routing info.
sent infrequently
• Incremental packets contain only new routing
info.
EETS 8316/NTU CC725-N/TC/11-08-01
#17
Ad Hoc Networks - Table Driven Routing
 Clusterhead
gateway switch routing
— Nodes are grouped into clusters and elect a cluster
head by a cluster head selection algorithm
— DSDV routing is used within clusters
— Between clusters, packets are routed to cluster
head  gateway  cluster head
• Gateways are nodes within communications
range of 2 or more cluster heads
— Packets may have to hop through multiple cluster
heads and gateways to final destination
EETS 8316/NTU CC725-N/TC/11-08-01
#18
Ad Hoc Networks - Table Driven Routing
— Each node maintains “cluster member table” of
cluster heads for every destination node
• Nodes broadcast their cluster member table and
update their own tables
Gateway
Packet
Cluster
head
EETS 8316/NTU CC725-N/TC/11-08-01
Gateway
Cluster
head
Cluster
head
#19
Ad Hoc Networks - Table Driven Routing
 Fisheye
link-state routing
— Similar to link-state routing but link-state updates
are not broadcast to every node
• Broadcast link-state updates to nodes one hop
away every 1 sec, nodes 2 hops away every 10
sec, nodes 3 hops away every 50 sec,...
— Routes are more accurate to nearby nodes, but
approximate to distant nodes
• Packets will converge to more accurate routes
to destination as get closer to destination
EETS 8316/NTU CC725-N/TC/11-08-01
#20
Ad Hoc Networks - Table Driven Routing
Link state
updates 10 sec
Link state
updates 1 sec
Link state
updates 50 sec
EETS 8316/NTU CC725-N/TC/11-08-01
#21
Ad Hoc Networks - Source Initiated Routing
 Ad
hoc on-demand distance-vector routing
— Similar to DSDV routing
— Source node with data ready to send initiates a path
discovery process
• Broadcasts route request (RREQ0 packet to its
neighbors. they broadcast to their neighbors,
and so on
• Eventually, destination node responds with route
reply (RREP) packet in reverse direction
• Nodes build up routing tables
EETS 8316/NTU CC725-N/TC/11-08-01
#22
Ad Hoc Networks - Source Initiated Routing
D
S
RREQ
RREP
D
S
EETS 8316/NTU CC725-N/TC/11-08-01
#23
Ad Hoc Networks - Source Initiated Routing
 Dynamic
source routing
— Nodes maintain route caches
— If route to destination is unknown in its route cache,
node will initiate route discovery
• Broadcasts route request packet with
destination address and source address
• Other nodes will add their address into packet
and continue broadcast to other nodes
• Route reply is returned by destination or a node
that knows a route to destination
EETS 8316/NTU CC725-N/TC/11-08-01
#24
Ad Hoc Networks - Source Initiated Routing
 Associativity-based
routing
— Routes are selected base on new metric: degree of
association stability
• Each node periodically generates a beacon to
advertise presence
• Neighboring nodes will increment associativity
tick of this node in their associativity tables for
each beacon received
• Association stability is defined by connection
stability between two nodes over space and time
– High association stability may mean low node mobility
EETS 8316/NTU CC725-N/TC/11-08-01
#25
Ad Hoc Networks - Source Initiated Routing
— Route discovery:
• Node will broadcast query (BQ message) to
destination
• Intermediate nodes will add their address and
associativity ticks into message
• Destination node receives packets with
associativity ticks of nodes along routes taken,
and chooses route with best degree of
association stability
• Destination node returns BQ-REPLY back to
source along chosen route
• Intermediate nodes will record this chosen route
EETS 8316/NTU CC725-N/TC/11-08-01
#26