[SOLVED] algorithm Network Layer

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Network Layer

All material copyright 1996-2012
J.F Kurose and K.W. Ross, All Rights Reserved

George Parisis
School of Engineering and Informatics

University of Sussex

Network Layer 4-2

v introduction
v virtual circuit and datagram networks
v whats inside a router
v IP: Internet Protocol

datagram format
IPv4 addressing (NAT)
ICMP, IPv6

v routing algorithms
link state, distance vector
hierarchical routing

v routing in the Internet
RIP, OSPF
BGP

v broadcast routing

Outline

Network Layer 4-3

Hierarchical routing

scale: with 600 million
destinations:

v cant store all dests in
routing tables!

v routing table exchange
would swamp links!

administrative
autonomy

v internet = network of
networks

v each network admin may
want to control routing in
its own network

our routing study thus far idealization
v all routers identical
v network flat
not true in practice

Network Layer 4-4

v aggregate routers
into regions,
autonomous
systems (AS)

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

protocol
routers in different

AS can run different
intra-AS routing
protocol

gateway router:
v at edge of its own AS
v has link to router in

another AS

Hierarchical routing

Network Layer 4-5

3b

1d

3a
1c

2a AS3

AS1
AS2

1a

2c
2b

1b

Intra-AS
Routing
algorithm

Inter-AS
Routing
algorithm

Forwarding
table

3c

Interconnected ASes

v forwarding table
configured by both intra-
and inter-AS routing
algorithm
intra-AS sets entries

for internal dests
inter-AS & intra-AS

sets entries for
external dests

Network Layer 4-6

Inter-AS tasks
v suppose router in AS1

receives datagram
destined outside of
AS1:
router should forward

packet to gateway
router, but which
one?

AS1 must:
1. learn which dests are

reachable through
AS2, which through
AS3

2. propagate this
reachability info to all
routers in AS1

job of inter-AS routing!

AS3

AS2

3b

3c
3a

AS1

1c
1a

1d
1b

2a
2c

2b
other
networks

other
networks

Network Layer 4-7

Example: setting forwarding table in router
1d
v suppose AS1 learns (via inter-AS protocol) that subnet x

reachable via AS3 (gateway 1c), but not via AS2
inter-AS protocol propagates reachability info to all

internal routers
v router 1d determines from intra-AS routing info that its

interface Iis on the least cost path to 1c
installs forwarding table entry (x,I)

AS3

AS2

3b

3c
3a

AS1

1c
1a

1d
1b

2a
2c

2b
other
networks

other
networks

x

Network Layer 4-8

Example: choosing among multiple ASes
v now suppose AS1 learns from inter-AS protocol that

subnet x is reachable from AS3 and from AS2.
v to configure forwarding table, router 1d must determine

which gateway it should forward packets towards for
dest x
this is also job of inter-AS routing protocol!

AS3

AS2

3b

3c
3a

AS1

1c
1a

1d
1b

2a
2c

2b
other
networks

other
networks

x

?

Network Layer 4-9

learn from inter-AS
protocol that subnet
x is reachable via
multiple gateways

use routing info
from intra-AS

protocol to determine
costs of least-cost

paths to each
of the gateways

hot potato routing:
choose the gateway

that has the
smallest least cost

determine from
forwarding table the
interface I that leads

to least-cost gateway.
Enter (x,I) in

forwarding table

Example: choosing among multiple ASes

v now suppose AS1 learns from inter-AS protocol that
subnet x is reachable from AS3 and from AS2.

v to configure forwarding table, router 1d must determine
towards which gateway it should forward packets for
dest x
this is also job of inter-AS routing protocol!

v hot potato routing: send packet towards closest of two
routers.

Network Layer 4-10

v introduction
v virtual circuit and datagram networks
v whats inside a router
v IP: Internet Protocol

datagram format
IPv4 addressing (NAT)
ICMP, IPv6

v routing algorithms
link state, distance vector
hierarchical routing

v routing in the Internet
RIP, OSPF
BGP

v broadcast

Outline

Network Layer 4-11

Intra-AS Routing

v also known as interior gateway protocols
(IGP)

v most common intra-AS routing protocols:
RIP: Routing Information Protocol
OSPF: Open Shortest Path First
IGRP: Interior Gateway Routing Protocol

(Cisco proprietary)

Network Layer 4-12

RIP (Routing Information Protocol)
v included in BSD-UNIX distribution in 1982
v distance vector algorithm

distance metric: # hops (max = 15 hops), each link has cost 1
DVs exchanged with neighbors every 30 sec in response

message (aka advertisement)
each advertisement: list of up to 25 destination subnets (in IP

addressing sense)

D C

B A
u v

w

x

y
z

subnethops
u 1
v 2
w2
x 3
y 3
z 2

from router A to destination subnets:

Network Layer 4-13

RIP: example

destination subnetnextrouter# hops to dest
wA 2

yB 2
zB 7

x 1
.. .

routing table in router D

w x y
z

A

C

D B

Network Layer 4-14

w x y
z

A

C

D B

destination subnetnextrouter# hops to dest
wA 2

yB 2
zB 7

x 1
.. .

routing table in router D

A 5

dest nexthops
w 1
x 1
zC4
.

A-to-D advertisement
RIP: example

Network Layer 4-15

RIP: link failure, recovery
if no advertisement heard after 180 sec >

neighbor/link declared dead
routes via neighbor invalidated
new advertisements sent to neighbors
neighbors in turn send out new advertisements (if

tables changed)
poison reverse used to prevent ping-pong loops

(infinite distance = 16 hops)

Network Layer 4-16

RIP table processing

v RIP routing tables managed by application-
level process called route-d (daemon)

v advertisements sent in UDP packets,
periodically repeated

physical
link

network forwarding
(IP) table

transport
(UDP)

routed

physical
link

network
(IP)

transprt
(UDP)

routed

forwarding
table

Network Layer 4-17

OSPF (Open Shortest Path First)
v open: publicly available
v uses link state algorithm

LS packet dissemination
topology map at each node
route computation using Dijkstras algorithm

v OSPF advertisement carries one entry per
neighbour

v advertisements flooded to entire AS
carried in OSPF messages directly over IP (rather

than TCP or UDP

Network Layer 4-18

OSPF advanced features (not in
RIP)

v security: all OSPF messages authenticated (to
prevent malicious intrusion)

v multiple same-cost paths allowed (only one path
in RIP)

v for each link, multiple cost metrics for different
TOS (e.g., satellite link cost set low for best
effort ToS; high for real time ToS)

v hierarchical OSPF in large domains.

Network Layer 4-19

Hierarchical OSPF
boundary router

backbone router

area 1
area 2

area 3

backbone
area
border
routers

internal
routers

Network Layer 4-20

v two-level hierarchy: local area, backbone.
link-state advertisements only in area
each node has detailed area topology

v area border routers: summarize distancesto
nets in own area, advertise to other Area Border
routers.

v backbone routers: run OSPF routing limited to
backbone.

v boundary routers: connect to other ASs.

Hierarchical
OSPF

Network Layer 4-21

Summary

v Hierarchical routing
v Intra-AS routing in the Internet

RIP
OSPF

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[SOLVED] algorithm Network Layer
$25