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Lecture 13 (Network Layer) Outline

Internet Network Layer:

Internet Routing Protocols

IP Datagram Format IP Addressing

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13.1. Internet Network Layer

Host, router network layer functions:

routingtable

Routing protocolspath selectionRIP, OSPF, BGP

IP protocoldatagram formataddressing conventions

packet handling conventions

ICMP protocolerror reporting

router signaling

Transport layer: TCP, UDP

Link layer

physical layer

Networklayer

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13.2. Internet Routing Protocols

Our routing study thus far - idealization

all routers identical.

network flat.

not true in practice.

Two major issues: Scalability: with 50 million destinations:

cant store all destinations in routing tables!

routing table exchange would swamp links! Administrative autonomy:

each network admin may want to control routing in

its own network.

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13.2. Internet Routing Protocols

Hierarchical routings:

Aggregate routers into regions or autonomous

systems (AS).

a

b

b

a

aC

A

Bd

A.a

A.c

C.b

B.a

c

b

c

interior routers

border (gateway) routers

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13.2. Internet Routing Protocols

Hierarchical routings:

Interior routers:

routers in same AS.run same intra-AS routing protocol.

routers in different AS can run different intra-AS routing

protocol. Gateway (Border) routers:

special routers in AS

run intra-AS routing protocol with all other routers in AS.

also responsible for routing to destinations outside AS.

run inter-AS routing protocol with other gateway

routers.

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13.2. Internet Routing Protocols

Two-level routing:

Intra-domain or interior routing.

Routing withinASs. Inter-domain or exterior routing.

Routing betweenASs.

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13.2. Internet Routing Protocols

RIP: Routing Information Protocol

OSPF: Open Shortest Path First

BGP: Border Gateway Protocol

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13.2. Internet Routing Protocols

Routing Information Protocol (RIP):

Based on distance vector algorithm.

Link cost = 1 hop. Distance metric: # of hops (max = 15 hops)

Exchange distance vectors with its neighbors

every 30 seconds periodic exchange. This exchange is also called RIP advertisement.

Each advertisement can include route to up to 25

destination network.

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13.2. Internet Routing Protocols

Open Shortest Path First (OSPF):

Uses link state algorithm.

OSPF advertisement carries one cost entry perneighbor router.

If have n neighbors then have n entries.

Advertisements disseminated to entireAS (viaflooding).

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13.2. Internet Routing Protocols

Border Gateway Protocol (BGP):

The inter-domain protocol in use today.

Now in version 4, i.e. BGP-4 Supports arbitrary network topology

Based on Path Vector Routing.

similar to Distance Vector protocol.

each Border Gateway broadcast to neighbors

(peers) entire path (i.e. sequence of ASs) to

destination.

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13.2. Internet Routing Protocols

Path vector routing:

e.g. Router R1 sends a path vector messageadvertising the reach ability of N1.

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13.2. Internet Routing Protocols

Why different Intra- and Inter-AS routing ?

Policy:

Intra-AS: single admin, so no policy decisionsneeded.

Inter-AS: admin wants control over how its traffic

routed, who routes through its net. 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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13.2. Internet Routing Protocols

Policy example:

Suppose R2 sends its path to R3.

R3 may or may not accept path offered by R2policy (dont accept transit traffic).

R2 needs to find other route.

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13.2. Further Readings

Internet Network Layer:

Internet Routing Protocols : Section 21.7 21.8.

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Next Lecture

IP datagram

(Forouzan Section 24.2)