Download - Routing - Circuit Switching
#1EETS 8316/NTU CC725-N/TC/11-08-01
Routing - Circuit Switching
Telephone switching was hierarchical with only one route possible—Added redundant routes
—Evolved to full mesh
Subscribers
Central offices
Toll switches
#2EETS 8316/NTU CC725-N/TC/11-08-01
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
#3EETS 8316/NTU CC725-N/TC/11-08-01
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:
#4EETS 8316/NTU CC725-N/TC/11-08-01
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
Examplespanningtree
Examplenetwork
#5EETS 8316/NTU CC725-N/TC/11-08-01
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
#6EETS 8316/NTU CC725-N/TC/11-08-01
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
#7EETS 8316/NTU CC725-N/TC/11-08-01
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
#8EETS 8316/NTU CC725-N/TC/11-08-01
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
#9EETS 8316/NTU CC725-N/TC/11-08-01
Link-State Routing
Examples: OSPF (open shortest path first), IS-IS (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
#10EETS 8316/NTU CC725-N/TC/11-08-01
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)
A
5
23
1
2 3
1
1
5
2A
#11EETS 8316/NTU CC725-N/TC/11-08-01
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
#12EETS 8316/NTU CC725-N/TC/11-08-01
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
#13EETS 8316/NTU CC725-N/TC/11-08-01
Layered Routing (Cont)
Interior routers useintra-domain routing
Routing domain
Exterior routers use inter-domain routing
#14EETS 8316/NTU CC725-N/TC/11-08-01
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
#15EETS 8316/NTU CC725-N/TC/11-08-01
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
#16EETS 8316/NTU CC725-N/TC/11-08-01
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
#17EETS 8316/NTU CC725-N/TC/11-08-01
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.
#18EETS 8316/NTU CC725-N/TC/11-08-01
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
#19EETS 8316/NTU CC725-N/TC/11-08-01
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
Packet
Clusterhead
Clusterhead
Gateway Gateway
Clusterhead
#20EETS 8316/NTU CC725-N/TC/11-08-01
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
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Ad Hoc Networks - Table Driven Routing
Link stateupdates 1 sec
Link stateupdates 10 sec
Link stateupdates 50 sec
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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
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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
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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
#26EETS 8316/NTU CC725-N/TC/11-08-01
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