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Computer Communication & Networks

Lecture 22

Network Layer: Delivery, Forwarding, Routing (contd.)

http://web.uettaxila.edu.pk/CMS/coeCCNbsSp09/index.asp

Waleed Ejazwaleed.ejaz@uettaxila.edu.pk

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Network Layer

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Network Layer Topics to CoverLogical Addressing

Internet Protocol

Address Mapping

Delivery, Forwarding, Routing

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Two-node Instability: Counting to Infinity Problem

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Split Horizon and Split Horizon with Poisoned Reverse

• Split Horizon– min cost to a given destination is not sent to a neighbor if the neighbor is the next node along the shortest path– a route is not broadcast on the interface through which the node has learnt it

• Split Horizon with Poisoned Reverse– send infinity

1 2 3 41 1 1

N1 N2 N3 N4

Initial (2,3) (3,2) (4,1) (4,0)

1 (2,3) (3,2) (-1,) (4,0)

2 (2,3) (-1,) (-1,) (4,0)

3 (-1,) (-1,) (-1,) (4,0)

• Reverse Route– a route pointing to the node where it has arrived– it creates potential cycle

R1R2

Subnet N

Reverse Route

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RIP ( Routing Information Protocol) Uses the distance-vector algorithm

DC

BA

u v

w

x

yz

destination hops u 1 v 2 w 2 x 3 y 3 z 2

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Runs on top of UDP, port number 520 Metric: number of hops Max limited to 15

suitable for small networks (local area environments)

value of 16 is reserved to represent infinity small number limits the count-to-infinity problem

Routing Information Protocol (RIP)

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RIP Operation

Router sends update message to neighbors every 30 sec A router expects to receive an update message from each

of its neighbors within 180 seconds in the worst case If router does not receive update message from neighbor

X within this limit, it assumes the link to X has failed and sets the corresponding minimum cost to 16 (infinity)

Uses split horizon with poisoned reverse Convergence speeded up by triggered updates

neighbors notified immediately of changes in distance vector table

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Example of a Domain using RIP

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Fixes some of the deficiencies in RIP Enables each router to learn complete network topology Each router monitors the link state to each neighbor and

floods the link-state information to other routers Each router builds an identical link-state database Allows router to build shortest path tree with router as root OSPF typically converges faster than RIP when there is a

failure in the network

Open Shortest Path First

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Path Vector Routing

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EGP: Exterior Gateway Protocol designed for tree-structured Internet

concerned with reachability, not optimal

routes

BGP – Border Gateway Protocol

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Exterior Gateway Protocols Within each AS, there is a consistent set of routes

connecting the constituent networks EGP enables two AS’s to exchange routing

information about: The networks that are contained within each AS The AS’s that can be reached through each AS

EGP path selection guided by policy rather than path optimality Trust, peering arrangements, etc

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EGP Example

AS1

AS2

AS3

R1

R2 R3

R4N1

N1 reachable through AS3

• R4 advertises that network N1 can be reached through AS3

• R3 examines announcement & applies policy to decide whether it will forward packets to N1 through R4

• If yes, routing table updated in R3 to indicate R4 as next hop to N1

• IGP propagates N1 reachability information through AS2

Only EGP routers are

shown

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EGP Example

AS1

AS2

AS3

R1

R2 R3

R4N1

N1 reachable through AS2

• EGP routers within an AS, e.g. R3 and R2, are kept consistent

• Suppose AS2 willing to handle transit packets from AS1 to N1

• R2 advertises to AS1 the reachability of N1 through AS2

• R1 applies its policy to decide whether to send to N1 via AS2

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EGP Requirements

Scalability to global Internet Provide connectivity at global scale

Link-state does not scale

Fully distributed

EGP path selection guided by policy rather

than path optimality Trust, peering arrangements, etc

EGP should allow flexibility in choice of paths

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Internet inter-AS routing: BGP

BGP provides each AS a means to:

1. Obtain subnet reachability information from neighboring ASs.

2. Propagate the reachability information to all routers internal to the AS.

3. Determine “good” routes to subnets based on reachability information and policy.

Allows a subnet to advertise its existence to rest of the Internet: “I am here”

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Initial routing tables in path vector routing

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Stabilized tables for three autonomous systems

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BGP Policy Examples of policy:

Never use AS X

Never use AS X to get to a destination in AS Y

Never use AS X and AS Y in the same path

Import policies to accept, deny, or set preferences on route

advertisements from neighbors

Export policies to determine which routes should be

advertised to which neighbors

A route is advertised only if AS is willing to carry traffic on

that route

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Why different Intra- and Inter-AS routing ?

Policy: Inter-AS: admin wants control over how its traffic routed, who

routes through its net. Intra-AS: single admin, so no policy decisions needed

Scale: hierarchical routing saves table size, reduced update traffic

Performance: Intra-AS: can focus on performance Inter-AS: policy may dominate over performance

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