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4.2 Routing Algorithms

A routing algorithm decides which output link an incoming packet should be transmitted onEvery host is connected to a default routerIf the destination host is not directly connected, the packet is transferred to this default (first hop) router

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Simple routing

Destination directly connected?If not send packet to default router

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Shortest Path Routing

For a pair of communicating hosts, there is a shortest path between themShortness may be defined by:

Number of router/switch hopsGeographic distanceLink delayCost

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Routing Algorithms (cont’d)

Two types of routing algorithms:

Global Routing Algorithms– Complete state information is used in routing decisions

Decentalized Routing Algorithms– Local/neighborhood state

Hierarchical Routing is used to make these algorithms scale to large networks

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Routing Algorithms

Static routing algorithms do not base their routing decisions on the current state of the network

FloodingDynamic routing

Route path changes as link cost changes

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Shortest Path

A

B

C E

FD3

1

24

2

3

2

Edge Weight(Distance)

What is the shortest path between A and F?

1

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Computing the Shortest PathDijkstra’s Shortest Path Algorithm:

Step 1: Draw nodes as circles. Fill in a circle to mark it as a“permanent node.”Step 2: Set the current node equal to the source nodeStep 3: For the current node:

– Mark the cumulative distance from the current node to each non-permanent adjacent node. Also mark the name of the current node. Erase this marking if the adjacent node already has a shorter cumulative distance marked

– Mark the non-permanent node with the shortest listed cumulative distance as permanent and set the current node equal to it. Repeat step 3 until all nodes are marked permanent.

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Dijskstra’s Shortest Path AlgorithmExample

A

B

G

E F

C

H

D

7

4

2

2

6

1

2 3

2

3

2

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Graph model of a network

B

ED

C

FA

1

2

5

32 3

12

51

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Link State Routing

Each router measures the distance (in delay, hop count, etc.) between itself and its adjacent routers

The router builds a packet containing all these distances. The packet also contains a sequence number and an age field.

Each router distributes these packets using flooding

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Link State Routing (cont’d)

To control flooding, the sequence numbers are used by routers to discard flood packets they have already seen from a given routerThe age field in the packet is an expiration date. It specifies how long the information in the packet is good for.Once a router receives all the link state packets from the network, it can reconstruct the complete topology and compute a shortest path between itself and any other node using Dijsktra’s algorithm.

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Hierarchical Routing

All routing algorithms have difficulties as the network becomes largeFor large networks, the routing tables grow very quickly, and so does the number of flood packetsHow can this be reduced? Hierarchical routing

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Hierarchical Routing (cont’d)

Segment the network into regionsRouters in a single region know all the details about other routers in that region, but none of the details about routers in other regionsAnalogy: Telephone area codes

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Hierarchical Routing (cont’d)

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OSPF

Open Shortest Path FirstRouting algorithm now used in the InternetOSPF uses the Link State Routing algorithm with modifications to support:

Multiple distance metrics (geographical distance, delay, throughput)Support for real-time trafficHierarchical routingSecurity

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OSPF (cont’d)

OSPF divides the network into several hierarchies:

Autonomous Systems (AS’s)– groups of subnets

Areas– Groups of routers within an AS

Backbone Areas– Groups of routers that connect other areas together

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OSPF (cont’d)

AreaArea

BackboneArea

Autonomous System

Area

Area

BackboneArea

Autonomous System

Area

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OSPF (cont’d)

Routers are distinguished by the functions they perform

Internal routers– Only route packets within one area

Area border routers– Connect to areas together

Backbone routers– Reside only in the backbone area

AS boundary routers– Routers that connect to a router outside the AS

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OSPF: Modified Link State Routing

Recall:In link state routing, routers flood their routing information to all other routers in the network

In OSPF, routers only send their information to “adjacent routers”, not to all routers.Adjacent does NOT mean nearest-neighbor in OSPFOne router in each area is marked as the “designated router”Designated routers are considered adjacent to all other routers in the areaOSPF combines link state routing with centralized adaptive routing

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OSPF: Adjacency

A

B

C

D

E

DesignatedRouter

Area

F

To backbonearea

Example:C is “adjacent” to B but not to A or EB is “adjacent” to all routers in the area

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Distributed Routing Algorithms

Each router periodically exchanges routing information (e.g., estimated time delay, queue length, etc.) with its neighborsExamples:

Distance Vector RoutingRIP

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Distance Vectors

Each router maintains lists of best-known distances to all other known routers. These lists are called “vectors.”Each router is assumed to know the exact distance (in delay, hop count, etc.) to other routers directly connected to it.Periodically, vectors are exchanged between adjacent routers, and each router updates its vectors.

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Distance Vectors (cont’d)

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Problem: Count-to-Infinity

With distance vector routing, good news travels fast, but bad news travels slowlyWhen a router goes down, it takes can take a really long time before all the other routers become aware of it

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Count-to-InfinityA B C D E

1 2 3 4

3 2 3 4

3 4 3 4

5 4 5 4

5 6 5 6

7 6 7 6

Initially

After 1 exchange

After 2 exchanges

After 3 exchanges

After 4 exchanges

After 5 exchanges

etc… to infinity

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Improvements

Split HorizonDon’t tell neighbor about routes obtained from it

Or never advertise a route out of the interface you learnt it

Poison reverseAdvertise network unreachable back through the same interface you learnt about it

advertising all network IDs, but those network IDs learned in a given direction are advertised with a metric of 16, indicating that the network is unavailable.

Triggered updates as opposed to periodic updates

Path vectors, Store vectors or complete path as opposed to just next hop

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RIP

Route Information ProtocolOne of the routing algorithms used by the InternetBased on distance vector routingDid not scale well, and it suffered the count-to-infinity problemRIP is slowly being phased out

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Recent Developments: IPv6

IPv4 (the standard IP protocol) has limited address spaceMost importantly, IP is running out of addresses. 32 bits is not enough.Real-time traffic and mobile users are also becoming more common

IP version 6(Also called IPng, or IP next generation)

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IPv6: The Changes

Large address space:128-bit addresses (16 bytes)Allows up to 340,282,366,920,938,463,463,374,607,431,768,211,456unique addresses

Fixed length headers (40 bytes)Improves the speed of packet processing in routers

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IPv6 header

Flow label: can be used to label all packets with a certain property for special handlingDS/ECN for indicating type of service

Source Address

Version (4) DS/ECN (8) Flow Label (20)

Payload (16) Next header (8) Hop limit (8)

Destination Address

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IPv6: The Changes (cont’d)

Support for “flows”Flows help support real-time service in the InternetA “flow” is a number in the IPv6 header that can be used by routers to see which packets belong to the same streamGuarantees can then be assigned to certain flowsExample:

– Packets from flow 10 should receive rapid delivery– Packets from flow 12 should receive reliable delivery