chapter4 5th april 2014 final2014

8/11/2019 Chapter4 5th April 2014 FINAL2014

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Network Layer 4-1

Chapter 4

Network Layer

A note on the use of these ppt slides:Were making these slides freely available to all (faculty, students, readers).

Theyre in PowerPoint form so you can add, modify, and delete slides

(including this one) and slide content to suit your needs. They obviously

represent a lotof work on our part. In return for use, we only ask the

following:

If you use these slides (e.g., in a class) in substantially unaltered form,that you mention their source (after all, wed like people to use our book!)

If you post any slides in substantially unaltered form on a www site, that

you note that they are adapted from (or perhaps identical to) our slides, and

note our copyright of this material.

Thanks and enjoy! JFK/KWR

All material copyright 1996-2009

J.F Kurose and K.W. Ross, All Rights Reserved

Computer Networking:A Top Down Approach5thedition.Jim Kurose, Keith RossAddison-Wesley, April2009.


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Network Layer 4-2

Chapter 4: Network Layer

Chapter goals: understand principles behind network layer

services:

network layer service models forwarding versus routing how a router works routing (path selection)

dealing with scale advanced topics: IPv6, mobility

instantiation, implementation in the Internet


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Network Layer 4-4

Network layer

transport segment from

sending to receiving host on sending side

encapsulates segmentsinto datagrams

on rcving side, deliversdatagrams to transportlayer

network layer protocols

in everyhost, router router examines header

fields in all IP datagramspassing through it

applicationtransport

networkdata linkphysical

applicationtransportnetwork

data linkphysical

networkdata linkphysical network

data linkphysical







network

data linkphysical

networkdata link

physicalnetworkdata linkphysical


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Network Layer 4-5

Two Key Network-Layer Functions

forwarding:movepackets from routersinput to appropriate

router output

routing:determineroute taken by

packets from sourceto dest.

routing algorithms

analogy:

routing:process of

planning trip fromsource to dest

forwarding:process

of getting throughsingle interchange


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Network Layer 4-6

1

23

0111

value in arriving

packets header

routing algorithm

local forwarding table

header value output link

0100

0101

01111001

3

2

21

Interplay between routing and forwarding


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Network Layer 4-7

Connection setup

3rdimportant function in somenetwork architectures:

ATM, frame relay, X.25

before datagrams flow, two end hosts andintervening

routers establish virtual connection routers get involved

network vs transport layer connection service:

network:between two hosts (may also involve

intervening routers in case of VCs) transport:between two processes


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Network Layer 4-8

Network service model

Q:What service modelfor channel transportingdatagrams from sender to receiver?

Example services for

individual datagrams: guaranteed delivery

guaranteed deliverywith less than 40 msec

delay

Example services for aflow of datagrams:

in-order datagramdelivery

guaranteed minimumbandwidth to flow

restrictions onchanges in inter-packet spacing


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Network Layer 4-9

Network layer service models:

Network

Architecture

Internet

ATM

ATM

ATM

ATM

Service

Model

best effort

CBR

VBR

ABR

UBR

Bandwidth

none

constantrate

guaranteed

rate

guaranteed

minimumnone

Loss

no

yes

yes

no

no

Order

no

yes

yes

yes

yes

Timing

no

yes

yes

no

no

Congestion

feedback

no (inferred

via loss)

nocongestion

no

congestion

yes

no

Guarantees ?


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Network Layer 4-10


4. 1 Introduction

4.2 Virtual circuit anddatagram networks

4.3 Whats inside arouter

4.4 IP: InternetProtocol Datagram format

IPv4 addressing

ICMP

IPv6

4.5 Routing algorithms Link state

Distance Vector

Hierarchical routing

4.6 Routing in theInternet RIP

OSPF

BGP 4.7 Broadcast and

multicast routing


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Network Layer 4-11

Network layer connection andconnection-less service

datagram network provides network-layerconnectionless service

VC network provides network-layer

connection service analogous to the transport-layer services,

but: service: host-to-host

no choice: network provides one or the other

implementation: in network core


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Network Layer 4-12

Virtual circuits

call setup, teardown for each call beforedata can flow

each packet carries VC identifier (not destination hostaddress)

everyrouter on source-dest path maintains state foreach passing connection

link, router resources (bandwidth, buffers) may beallocated to VC (dedicated resources = predictable service)

source-to-dest path behaves much like telephonecircuit performance-wise

network actions along source-to-dest path


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Network Layer 4-14

Forwarding table

12 22 32

12

3

VC number

interfacenumber

Incoming interface Incoming VC # Outgoing interface Outgoing VC #

1 12 3 222 63 1 183 7 2 17

1 97 3 87

Forwarding table innorthwest router:

Routers maintain connection state information!


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Network Layer 4-15

Virtual circuits: signaling protocols

used to setup, maintain teardown VC

used in ATM, frame-relay, X.25

not used in todays Internet

applicationtransportnetworkdata linkphysical

applicationtransport


1. Initiate call 2. incoming call3. Accept call4. Call connected

5. Data flow begins 6. Receive data


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Network Layer 4-16

Datagram networks

no call setup at network layer

routers: no state about end-to-end connections no network-level concept of connection

packets forwarded using destination host address packets between same source-dest pair may take

different paths

applicationtransportnetworkdata linkphysical

application

transportnetworkdata linkphysical

1. Send data 2. Receive data


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Network Layer 4-17

Forwarding table

Destination Address Range Link Interface

11001000 00010111 00010000 00000000through 0

11001000 00010111 00010111 11111111

11001000 00010111 00011000 00000000through 1

11001000 00010111 00011000 11111111

11001000 00010111 00011001 00000000through 2

11001000 00010111 00011111 11111111

otherwise 3

4 billionpossible entries


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Router?

Network Layer 4-18


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Network Layer 4-19

Longest prefix matching

Prefix Match Link Interface11001000 00010111 00010 0

11001000 00010111 00011000 111001000 00010111 00011 2

otherwise 3

DA: 11001000 00010111 00011000 10101010

Examples

DA: 11001000 00010111 00010110 10100001 Which interface?

Which interface?


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Network Layer 4-20

Datagram or VC network: why?

Internet (datagram) data exchange among

computers

elastic service, no stricttiming req.

smart end systems(computers)

can adapt, performcontrol, error recovery

simple inside network,complexity at edge

many link types

different characteristics

uniform service difficult

ATM (VC) evolved from telephony

human conversation:

strict timing, reliability

requirements need for guaranteed

service

dumb end systems

telephones

complexity insidenetwork


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Network Layer 4-21


4. 1 Introduction




IPv4 addressing

ICMP

IPv6


Distance Vector



OSPF


multicast routing


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Network Layer 4-22

Router Architecture Overview

Two key router functions: run routing algorithms/protocol (RIP, OSPF, BGP)

forwarding datagrams from incoming to outgoing link


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Network Layer 4-24

Three types of switching fabrics


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Network Layer 4-25

Switching Via Memory

First generation routers:

traditional computers with switching under directcontrol of CPU

packet copied to systems memory

speed limited by memory bandwidth (2 buscrossings per datagram)

Input

Port

Output

Port

Memory

System Bus


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27/155

Network Layer 4-27

Switching Via An InterconnectionNetwork

overcome bus bandwidth limitations

Banyan networks, other interconnection nets

initially developed to connect processors inmultiprocessor

advanced design: fragmenting datagram into fixedlength cells, switch cells through the fabric.

Cisco 12000: switches 60 Gbps through theinterconnection network


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Network Layer 4-28

Output Ports

Bufferingrequired when datagrams arrive from

fabric faster than the transmission rate Scheduling disciplinechooses among queued

datagrams for transmission


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Network Layer 4-29

Output port queueing

buffering when arrival rate via switch exceedsoutput line speed

queueing (delay) and loss due to output portbuffer overflow!


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Network Layer 4-30

How much buffering?

RFC 3439 rule of thumb: average bufferingequal to typical RTT (say 250 msec) timeslink capacity C

e.g., C = 10 Gps link: 2.5 Gbit buffer Recent recommendation: with Nflows,

buffering equal to RTT C.N


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Network Layer 4-31

Input Port Queuing

Fabric slower than input ports combined -> queueing

may occur at input queues Head-of-the-Line (HOL) blocking:queued datagram

at front of queue prevents others in queue frommoving forward

queueing delay and loss due to input buffer overflow!


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Network Layer 4-32


4. 1 Introduction




IPv4 addressing

ICMP

IPv6


Distance Vector



OSPF


multicast routing


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Network Layer 4-33

The Internet Network layer

forwardingtable

Host, router network layer functions:

Routing protocols

path selectionRIP, OSPF, BGP

IP protocoladdressing conventions

datagram formatpacket handling conventions

ICMP protocolerror reportingrouter signaling

Transport layer: TCP, UDP

Link layer

physical layer

Networklayer


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Network Layer 4-34


4. 1 Introduction




IPv4 addressing

ICMP

IPv6


Distance Vector



OSPF


multicast routing

IP d t f t


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Network Layer 4-35

IP datagram format

ver length

32 bits

data

(variable length,typically a TCP

or UDP segment)

16-bit identifier

headerchecksum

time tolive

32 bit source IP address

IP protocol versionnumber

header length(bytes)

max numberremaining hops

(decremented ateach router)

forfragmentation/reassembly

total datagramlength (bytes)

upper layer protocolto deliver payload to

head.len

type ofservice

type of dataflgs

fragmentoffset

upperlayer

32 bit destination IP address

Options (if any) E.g. timestamp,record routetaken, specifylist of routersto visit.

how much overheadwith TCP?

20 bytes of TCP

20 bytes of IP

= 40 bytes + app

layer overhead


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Network Layer 4-36

IP Fragmentation & Reassembly network links have MTU

(max.transfer size) - largestpossible link-level frame.

different link types,different MTUs

large IP datagram divided

(fragmented) within net one datagram becomes

several datagrams

reassembled only at finaldestination

IP header bits used toidentify, order relatedfragments

fragmentation:in:one large datagramout:3 smaller datagrams

reassembly


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Network Layer 4-37

IP Fragmentation and Reassembly

ID=x

offset=0

fragflag=0

length=4000

ID=x

offset=0

fragflag=1

length=1500

ID=x

offset=185

fragflag=1

length=1500

ID=x

offset=370

fragflag=0

length=1040

One large datagram becomesseveral smaller datagrams

Example

4000 bytedatagram

MTU = 1500 bytes

1480 bytes indata field

offset =1480/8


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Network Layer 4-38


4. 1 Introduction




IPv4 addressing

ICMP

IPv6


Distance Vector



OSPF


multicast routing


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Network Layer 4-39

IP Addressing: introduction

IP address:32-bitidentifier for host,router interface

interface:connection

between host/routerand physical link routers typically have

multiple interfaces

host typically has one

interface IP addresses

associated with eachinterface

223.1.1.1

223.1.1.2

223.1.1.3

223.1.1.4 223.1.2.9

223.1.2.2

223.1.2.1

223.1.3.2223.1.3.1

223.1.3.27

223.1.1.1 = 11011111 00000001 00000001 00000001

223 1 11


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Page 40

IP Address

Host addressNetwork addressIP address

IP address uniquely identifies a network device, it consistsof 32 binary digits.

IP address is often represented in a dotted decimalformat.

IP address is divided into two parts: Network address portion

Host address portion


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Page 41

Classification of IP Address

Class A

Class B

Class C

Class D

Class E

128.0.0.0~191.255.255.255

192.0.0.0~223.255.255.255

224.0.0.0~239.255.255.255

240.0.0.0~255.255.255.255

1 1 1 1 0 reserve

1.0.0.0~126.255.255.255

Host(24bit)0 Network(7bit)

Host(8bit)Network(21bit)1 1 0

1 1 1 0 Multicast address

Host(16bit)1 0 Network(14bit)


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Page 42

Special IP Address

Network part Host part Address type Usage

Any all 0Network

AddressRepresents a network Segment

Any all 1

Broadcast

Address

All nodes of a specifically

designated network segment

127 AnyLoopback

AddressLoop diagnostic functionality

all 0 All NetworksDesignates default routes

in Huawei Quidway routers

all 1Broadcast

Address

All nodes of a local network

segment


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Page 44

Subnet Mask Introduction

Subnet Masks manipulate the network and host address portion

used to determine the network portion of an IP address

The subnet mask format is same as the IP address format

Network and subnet of subnet mask identified in binary as aseries of 1 bits, the host bits are 0

For example Class Bs subnetmask is

255.255.0.0


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Network Layer 4-45

Subnets

IP address: subnet part (high

order bits)

host part (low orderbits)

Whats a subnet ? device interfaces with

same subnet part of IPaddress

can physically reacheach other withoutintervening router

223.1.1.1

223.1.1.2

223.1.1.3

223.1.1.4 223.1.2.9

223.1.2.2

223.1.2.1

223.1.3.2223.1.3.1

223.1.3.27

network consisting of 3 subnets

subnet


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Network Layer 4-46

Subnets 223.1.1.0/24 223.1.2.0/24

223.1.3.0/24

Recipe To determine the

subnets, detach eachinterface from its

host or router,creating islands ofisolated networks.Each isolated network

is called a subnet.Subnet mask: /24


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Network Layer 4-47

Subnets

How many?223.1.1.1

223.1.1.3

223.1.1.4

223.1.2.2223.1.2.1

223.1.2.6

223.1.3.2223.1.3.1

223.1.3.27

223.1.1.2

223.1.7.0

223.1.7.1223.1.8.0223.1.8.1

223.1.9.1

223.1.9.2

N t k Add ss d S b t


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Page 48

Network Address and SubnetMask

IP Address

Subnet Mask

Network Address:

192.168.1

255.255.255

192.168.1

.100

.0

.0


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Page 49

Binary to DecimalConversion

Decimal Summation is 255

11111111

1248163264128

8bit

Binary

20

21

22

24

25

26

27

23


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Page 50

Example of Conversion

10010111

1*10*20*41*80*161*321*641*128

233

+ + + + + + +64 32 0 0 10128 8

Example :

System Conversion of IP


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Page 51

System Conversion of IPAddress

IP address192.168.1.11(decimal)

byte8bits byte8bits byte8bits byte8bits

2726252423222120272625242322212027262524232221202726252423222120

192

168

1

11

1 1 0 0 0 0 0 0 1 0 1 0 1 0 0 0 0 0 0 0 0 0 0 1 0 0 0 0 1 0 1 1

binary digit

11000000.10101000.00000001.00001011


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Page 52

A. Classful Addressing

Classful Addressing uses default subnet masks, and thus nosubnet..

Classful Routing Updates

Subnet masks are not sent in routing updates

e.g. Class B segment 172.16.0.0 with mask 255.255.0.0

172.16.30.1/16 172.16.28.1/16

Ethernet

172.16.30.10/16


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Page 53

Calculation of Host Number

Host Number2n

Valid Host Number: 2n- 2

Subnet Mask

N bit

Network Portion Host Portion

11 1 111 1 111 1 111 000 0 000000 000 01 0

Example of Host Number


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Page 54

Example of Host NumberCalculation

IP Address192.168.1.100/28

/28=255.255.255.240

The binary representation of subnet mask:

28bits

networkportion

4bitsHost portion

The total number of host: 24

The valid number of host: 24-2

11111111111111111111111111110000


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B. Classless IP Addressing

Classless Routing Protocol Characteristics of classless routing

protocols: Routing updates include the subnet mask

Supports VLSM

Supports Route Summarization


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VLSMVLSMthe process

of sub-netting asubnetto fit yourneedsExample:

Subnet 10.1.0.0/16,8 more bits areborrowed again, tocreate 256 subnetswith a /24mask.

Mask allows for 254host addresses persubnet

Subnets range from:10.1.0.0 / 24 to10.1.255.0 / 24


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Page 57

Variable Length Subnet Mask(VLSM)

ISP announce192.168.1.0

192.168.1.32/27

192.168.1.64/27

192.168.1.96/27

192.168.1.128/27

192.168.1.160/30

192.168.1.164/30

192.168.1.168/30

192.168.1.172/30


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Page 58

Subnet Mask Representation

255 . 255 . 255 . 24011111111 11111111 11111111 11110000

192 . 168 . 1 . 7

11000000 10101000 00000001 00000111

8 + 8 + 8+ 4 = 28

192 . 168 . 1 . 7 / 28

IP Address

Subnet mask

Bits of subnet mask

Subnet mask representation


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Page 59

Calculation of Network address

192.168.1.0/28

11000000 10101000 00000001 00000000

IP Address is: 192.168.1.7/28

192 . 168 . 1 . 7

11000000 10101000 00000001 00000111

255 . 255 . 255 . 240

11111111 11111111 11111111 11110000

IP Address

Subnet Mask

Network Address

(Binary)

Network Address

Classless Inter Domain Routing


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Page 60

Internet

Classless Inter-Domain Routing(CIDR) CIDR reduces the scale of the routing table and enhances

network extensibility.

198.168.1.0

198.168.2.0

198.168.3.0

ISP

Announce route198.168.0.0/16

Classless Inter Domain Routing


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Classless Inter-Domain Routing(CIDR) CIDR

Uses IP addresses more efficiently through useof VLSM

VLSM is the process of subnetting a subnet

Allows for route summarization Route summarization is representing multiple

contiguous routes with a single route

Classless Routing Updates Subnet masks are included in updates


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Network Layer 4-62

IP addresses: how to get one?

Q:How does a hostget IP address?

hard-coded by system admin in a file

Windows: control-panel->network->configuration->tcp/ip->properties

UNIX: /etc/rc.config

DHCP:Dynamic Host Configuration Protocol:

dynamically get address from as server plug-and-play


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Network Layer 4-63

DHCP: Dynamic Host Configuration Protocol

Goal:allow host to dynamically obtain its IP address fromnetwork server when it joins networkCan renew its lease on address in use

Allows reuse of addresses (only hold address while connected an

on)Support for mobile users who want to join network (more shortly)

DHCP overview:

host broadcasts DHCP discover msg [optional]

DHCP server responds with DHCP offer msg[optional]

host requests IP address: DHCP request msg

DHCP server sends address: DHCP ack msg


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Network Layer 4-64

DHCP client-server scenario

223.1.1.1

223.1.1.2

223.1.1.3

223.1.1.4 223.1.2.9

223.1.2.2

223.1.2.1

223.1.3.2223.1.3.1

223.1.3.27

A

B

E

DHCPserver

arriving DHCP

clientneeds

address in this

network

DHCP client-server scenario


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Network Layer 4-65

DHCP client-server scenarioDHCP server: 223.1.2.5 arriving

client

time

DHCP discover

src : 0.0.0.0, 68

dest.: 255.255.255.255,67yiaddr: 0.0.0.0

transaction ID: 654

DHCP offer

src: 223.1.2.5, 67

dest: 255.255.255.255, 68

yiaddrr: 223.1.2.4

transaction ID: 654Lifetime: 3600 secs

DHCP request

src: 0.0.0.0, 68

dest:: 255.255.255.255, 67

yiaddrr: 223.1.2.4

transaction ID: 655Lifetime: 3600 secs

DHCP ACK

src: 223.1.2.5, 67

dest: 255.255.255.255, 68

yiaddrr: 223.1.2.4

transaction ID: 655

Lifetime: 3600 secs


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DHCP: example


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Network Layer 4-67

DHCP: example

connecting laptop needs itsIP address, addr of first-hop router, addr of DNSserver: use DHCP

router(runs DHCP)

DHCP

UDP

IPEth

Phy

DHCP

DHCP

DHCP

DHCP

DHCP

DHCPUDP

IP

Eth

Phy

DHCP

DHCP

DHCP

DHCPDHCP

DHCP request encapsulatedin UDP, encapsulated in IP,encapsulated in 802.1Ethernet

Ethernet frame broadcast(dest: FFFFFFFFFFFF) on LAN,received at router runningDHCP server

Ethernet demuxed to IPdemuxed, UDP demuxed toDHCP

168.1.1.1

DHCP: example


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Network Layer 4-68

DCP server formulatesDHCP ACK containing

clients IP address, IPaddress of first-hoprouter for client, name &IP address of DNS server

router(runs DHCP)

DHCP

UDP

IPEth

Phy

DHCP

DHCP

DHCP

DHCP

DHCPUDP

IP

Eth

Phy

DHCP

DHCP

DHCP

DHCP

DHCP

encapsulation of DHCPserver, frame forwardedto client, demuxing up toDHCP at client

client now knows its IPaddress, name and IPaddress of DSN server, IPaddress of its first-hop

router

DHCP: example

DHCP i h k M t B t R l (2)


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Network Layer 4-69

DHCP: wiresharkoutput (home LAN)

Message type: Boot Reply (2)Hardware type: EthernetHardware address length: 6Hops: 0Transaction ID: 0x6b3a11b7Seconds elapsed: 0Bootp flags: 0x0000 (Unicast)

Client IP address: 192.168.1.101 (192.168.1.101)Your (client) IP address: 0.0.0.0 (0.0.0.0)Next server IP address: 192.168.1.1 (192.168.1.1)Relay agent IP address: 0.0.0.0 (0.0.0.0)Client MAC address: Wistron_23:68:8a (00:16:d3:23:68:8a)Server host name not givenBoot file name not givenMagic cookie: (OK)Option: (t=53,l=1) DHCP Message Type = DHCP ACKOption: (t=54,l=4) Server Identifier = 192.168.1.1Option: (t=1,l=4) Subnet Mask = 255.255.255.0Option: (t=3,l=4) Router = 192.168.1.1Option: (6) Domain Name Server

Length: 12; Value: 445747E2445749F244574092;IP Address: 68.87.71.226;IP Address: 68.87.73.242;IP Address: 68.87.64.146

Option: (t=15,l=20) Domain Name = "hsd1.ma.comcast.net."

reply

Message type: Boot Request (1)Hardware type: EthernetHardware address length: 6Hops: 0Transaction ID: 0x6b3a11b7Seconds elapsed: 0Bootp flags: 0x0000 (Unicast)Client IP address: 0.0.0.0 (0.0.0.0)Your (client) IP address: 0.0.0.0 (0.0.0.0)Next server IP address: 0.0.0.0 (0.0.0.0)

Relay agent IP address: 0.0.0.0 (0.0.0.0)Client MAC address: Wistron_23:68:8a (00:16:d3:23:68:8a)Server host name not givenBoot file name not givenMagic cookie: (OK)Option: (t=53,l=1) DHCP Message Type = DHCP RequestOption: (61) Client identifier

Length: 7; Value: 010016D323688A;Hardware type: EthernetClient MAC address: Wistron_23:68:8a (00:16:d3:23:68:8a)

Option: (t=50,l=4) Requested IP Address = 192.168.1.101Option: (t=12,l=5) Host Name = "nomad"Option: (55) Parameter Request List

Length: 11; Value: 010F03062C2E2F1F21F92B1 = Subnet Mask; 15 = Domain Name3 = Router; 6 = Domain Name Server44 = NetBIOS over TCP/IP Name Server

request


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Network Layer 4-70

IP addresses: how to get one?

Q:How does networkget subnet part of IPaddr?

A:gets allocated portion of its provider ISPsaddress space

ISP's block 11001000 00010111 00010000 00000000 200.23.16.0/20

Organization 0 11001000 00010111 00010000 00000000 200.23.16.0/23

Organization 1 11001000 00010111 00010010 00000000 200.23.18.0/23

Organization 2 11001000 00010111 00010100 00000000 200.23.20.0/23

... .. . .

Organization 7 11001000 00010111 00011110 00000000 200.23.30.0/23


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Network Layer 4-71

Hierarchical addressing: route aggregation

Send me anythingwith addressesbeginning200.23.16.0/20

200.23.16.0/23

200.23.18.0/23

200.23.30.0/23

Fly-By-Night-ISP

Organization 0

Organization 7Internet

Organization 1

ISPs-R-UsSend me anythingwith addressesbeginning199.31.0.0/16

200.23.20.0/23Organization 2

...

...

Hierarchical addressing allows efficient advertisement of routinginformation:

Hi hi l dd i ifi


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Network Layer 4-72

Hierarchical addressing: more specificroutes

ISPs-R-Us has a more specific route to Organization 1

Send me anything

with addressesbeginning200.23.16.0/20

200.23.16.0/23

200.23.18.0/23

200.23.30.0/23

Fly-By-Night-ISP

Organization 0

Organization 7Internet

Organization 1

ISPs-R-Us Send me anythingwith addressesbeginning 199.31.0.0/16or 200.23.18.0/23

200.23.20.0/23Organization 2

...

...


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Network Layer 4-73

IP addressing: the last word...

Q:How does an ISP get block of addresses?A:ICANN: Internet Corporation for Assigned

Names and Numbers

allocates addressesmanages DNS

assigns domain names, resolves disputes

N N k dd l


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Network Layer 4-74

NAT: Network Address Translation

10.0.0.1

10.0.0.2

10.0.0.3

10.0.0.4

138.76.29.7

local network(e.g., home network)

10.0.0/24

rest ofInternet

Datagrams with source or

destination in this networkhave 10.0.0/24 address forsource, destination (as usual)

Alldatagrams leavinglocal

network have samesingle sourceNAT IP address: 138.76.29.7,different source port numbers

N T N k dd T l i


75/155

Network Layer 4-75


Motivation:local network uses just one IP address asfar as outside world is concerned:

range of addresses not needed from ISP: just one IPaddress for all devices

can change addresses of devices in local networkwithout notifying outside world

can change ISP without changing addresses ofdevices in local network

devices inside local net not explicitly addressable,visible by outside world (a security plus).

NAT N k Add T l i


76/155

Network Layer 4-76

NAT: Network Address TranslationImplementation:NAT router must:

outgoing datagrams:replace(source IP address, port#) of every outgoing datagram to (NAT IP address,new port #). . . remote clients/servers will respond using (NAT

IP address, new port #) as destination addr.

remember (in NAT translation table) every (sourceIP address, port #) to (NAT IP address, new port #)translation pair

incoming datagrams:replace(NAT IP address, newport #) in dest fields of every incoming datagramwith corresponding (source IP address, port #)stored in NAT table

N T N k dd T l i


77/155

Network Layer 4-77


10.0.0.1

10.0.0.2

10.0.0.3

S: 10.0.0.1, 3345D: 128.119.40.186, 80

1

10.0.0.4

138.76.29.7

1: host 10.0.0.1sends datagram to128.119.40.186, 80

NAT translation tableWAN side addr LAN side addr

138.76.29.7, 5001 10.0.0.1, 3345

S: 128.119.40.186, 80D: 10.0.0.1, 3345 4

S: 138.76.29.7, 5001D: 128.119.40.186, 802

2: NAT routerchanges datagramsource addr from10.0.0.1, 3345 to138.76.29.7, 5001,updates table

S: 128.119.40.186, 80D: 138.76.29.7, 5001 33: Reply arrivesdest. address:138.76.29.7, 5001

4: NAT routerchanges datagramdest addr from138.76.29.7, 5001 to 10.0.0.1, 3345

NAT N k Add T l i


78/155

Network Layer 4-78


16-bit port-number field: 60,000 simultaneous connections with a single

LAN-side address!

NAT is controversial: routers should only process up to layer 3

violates end-to-end argument NAT possibility must be taken into account by app

designers, eg, P2P applications

address shortage should instead be solved byIPv6


79/155

NAT t l bl


80/155

Network Layer 4-80

NAT traversal problem

solution 2: Universal Plug andPlay (UPnP) Internet GatewayDevice (IGD) Protocol. AllowsNATted host to: learn public IP address

(138.76.29.7) add/remove port mappings

(with lease times)

i.e., automate static NAT portmap configuration

10.0.0.1

10.0.0.4

NATrouter

138.76.29.7

IGD

NAT t l bl


81/155

Network Layer 4-81

NAT traversal problem

solution 3: relaying (used in Skype)NATed client establishes connection to relay

External client connects to relay

relay bridges packets between to connections

138.76.29.7Client

10.0.0.1

NATrouter

1.connection torelay initiatedby NATted host

2.connection torelay initiatedby client

3.relayingestablished

Ch t 4 N t k L


82/155

Network Layer 4-82


4. 1 Introduction 4.2 Virtual circuit and

datagram networks

4.3 Whats inside a

router 4.4 IP: Internet

Protocol Datagram format

IPv4 addressing ICMP

IPv6


Distance Vector



OSPF


multicast routing

ICMP I t t C t l M P t l


83/155

Network Layer 4-83

ICMP: Internet Control Message Protocol

used by hosts & routers tocommunicate network-levelinformation

error reporting:unreachable host, network,port, protocol

echo request/reply (usedby ping)

network-layer above IP:

ICMP msgs carried in IPdatagrams

ICMP message:type, code plusfirst 8 bytes of IP datagramcausing error

Type Code description

0 0 echo reply (ping)

3 0 dest. network unreachable

3 1 dest host unreachable

3 2 dest protocol unreachable

3 3 dest port unreachable3 6 dest network unknown

3 7 dest host unknown

4 0 source quench (congestion

control - not used)

8 0 echo request (ping)

9 0 route advertisement10 0 router discovery

11 0 TTL expired

12 0 bad IP header

T t d ICMP


84/155

Network Layer 4-84

Traceroute and ICMP

Source sends series ofUDP segments to dest First has TTL =1

Second has TTL=2, etc.

Unlikely port number

When nth datagram arrivesto nth router: Router discards datagram

And sends to source anICMP message (type 11,

code 0) Message includes name of

router& IP address

When ICMP messagearrives, source calculatesRTT

Traceroute does this 3times

Stopping criterion UDP segment eventually

arrives at destination host

Destination returns ICMPhost unreachable packet(type 3, code 3)

When source gets thisICMP, stops.

Ch pt 4: N t k L


85/155

Network Layer 4-85



datagram networks

4.3 Whats inside a




IPv6


Distance Vector



OSPF


multicast routing


86/155

IPv6 Header (Cont)


87/155

Network Layer 4-87

IPv6 Header (Cont)

Priority: identify priority among datagrams in flowFlow Label:identify datagrams in same flow.(concept offlow not well defined).

Next header:identify upper layer protocol for data


88/155

Transition From IPv4 To IPv6


89/155

Network Layer 4-89

Transition From IPv4 To IPv6

Not all routers can be upgraded simultaneous no flag days

How will the network operate with mixed IPv4 andIPv6 routers?

Tunneling:IPv6 carried as payload in IPv4datagram among IPv4 routers

Tunneling


90/155

Network Layer 4-90

TunnelingA B E F

IPv6 IPv6 IPv6 IPv6

tunnelLogical view:

Physical view:A B E F

IPv6 IPv6 IPv6 IPv6IPv4 IPv4

Tunneling


91/155

Network Layer 4-91

TunnelingA B E F

IPv6 IPv6 IPv6 IPv6

tunnelLogical view:

Physical view:A B E F

IPv6 IPv6 IPv6 IPv6

C D

IPv4 IPv4

Flow: XSrc: ADest: F

data


data


data

Src:BDest: E


data

Src:BDest: E

A-to-B:IPv6

E-to-F:IPv6

B-to-C:IPv6 inside

IPv4

B-to-C:IPv6 inside

IPv4



92/155

Network Layer 4-92



datagram networks

4.3 Whats inside a




IPv6


Distance Vector



OSPF


multicast routing

Interplay between routing, forwarding


93/155

Network Layer 4-93

1

23

0111

value in arriving

packets header

routing algorithm

local forwarding table

header value output link

0100

0101

01111001

3

2

21

p y g, g

I li


94/155

Page 94

In reality

[RTB]display ip routing-table

Routing Tables: Public

Destinations : 10 Routes : 10

Destination/Mask Proto Pre Cost NextHop Interface

10.1.1.0/30 Direct 0 0 10.1.1.2 Serial0/0/0

10.1.1.1/32 Direct 0 0 10.1.1.1 Serial0/0/0

127.0.0.1/32 Direct 0 0 127.0.0.1 InLoopBack0

172.16.1.1/32 OSPF 10 1562 10.1.2.2 Serial0/0/1

192.168.2.0/24 RIP 100 1 10.1.1.1 Serial0/0/0

10.1.1.0/8

RTB

10.1.2.0/24

.1

.1

OSPF

RIP

RTA

RTC

172.16.1.1/32

192.168.2.1/24

.2

.2

Graph abstraction


95/155

Network Layer 4-95

u

yx

wv

z2

21

3

1

1

2

53

5

Graph: G = (N,E)

N = set of routers = { u, v, w, x, y, z }

E = set of links ={ (u,v), (u,x), (v,x), (v,w), (x,w), (x,y), (w,y), (w,z), (y,z) }

Graph abstraction

Remark: Graph abstraction is useful in other network contexts

Example: P2P, where N is set of peers and E is set of TCP connections

Graph abstraction: costs


96/155

Network Layer 4-96

Graph abstraction: costs

u

yx

wv

z2

21

3

1

1

2

53

5 c(x,x) = cost of link (x,x)

- e.g., c(w,z) = 5

cost could always be 1, orinversely related to bandwidth,or inversely related tocongestion

Cost of path (x1, x2, x3,, xp) = c(x1,x2) + c(x2,x3) + + c(xp-1,xp)

Question: Whats the least-cost path between u and z ?

Routing algorithm: algorithm that finds least-cost path

Routing Algorithm classification


97/155

Network Layer 4-97

Routing Algorithm classification

Global or decentralizedinformation?

Global:

all routers have completetopology, link cost info

link state algorithmsDecentralized:

router knows physically-connected neighbors, linkcosts to neighbors

iterative process ofcomputation, exchange ofinfo with neighbors

distance vector algorithms

Static or dynamic?Static: routes change slowly

over time

Dynamic: routes change more

quickly

periodic update

in response to linkcost changes



98/155

Network Layer 4-98



datagram networks

4.3 Whats inside a




IPv6


Distance Vector



OSPF


multicast routing

A Link-State Routing Algorithm


99/155

Network Layer 4-99

A Link-State Routing Algorithm

Dijkstras algorithm net topology, link costs

known to all nodes

accomplished via linkstate broadcast

all nodes have same info

computes least cost pathsfrom one node (source) toall other nodes

gives forwarding tablefor that node

iterative: after kiterations, know least costpath to k dest.s

Notation: c(x,y):link cost from node

x to y; = if not directneighbors

D(v):current value of costof path from source todest. v

p(v):predecessor nodealong path from source to v

N':set of nodes whoseleast cost path definitivelyknown

Dijsktras Algorithm


100/155

Network Layer4-100

Dijsktra s Algorithm

1Init ial ization:

2 N' = {u}

3 for all nodes v

4 if v adjacent to u

5 then D(v) = c(u,v)

6 else D(v) =

78 Loop

9 find w not in N' such that D(w) is a minimum

10 add w to N'

11 update D(v) for all v adjacent to w and not in N' :

12 D(v) = min( D(v), D(w) + c(w,v) )13 /* new cost to v is either old cost to v or known

14 shortest path cost to w plus cost from w to v */

15 un t i l al l nodes in N'

Dijkstras algorithm: example


101/155

Network Layer 4-101

Dijkstra s algorithm: example

Step0

1

2

3

4

5

N'u

ux

uxy

uxyv

uxyvw

uxyvwz

D(v),p(v)2,u

2,u

2,u

D(w),p(w)5,u

4,x

3,y

3,y

D(x),p(x)1,u D(y),p(y)2,x

D(z),p(z)

4,y

4,y

4,y

u

yx

wv

z2

21

3

1

1

2

53

5


102/155


103/155



104/155

Network Layer4-104



datagram networks

4.3 Whats inside a




IPv6


Distance Vector



OSPF

BGP

4.7 Broadcast andmulticast routing

Distance Vector Algorithm


105/155

Network Layer4-105


Bellman-Ford Equation (dynamic programming)Define

dx(y) := cost of least-cost path from x to y

Then

dx(y) = min {c(x,v) + dv(y) }

where min is taken over all neighbors v of x

v

Bellman-Ford example


106/155

Network Layer4-106

Bellman Ford example

u

yx

wv

z2

21

3

1

1

2

53

5 Clearly, dv(z) = 5, dx(z) = 3, dw(z) = 3

du(z) = min { c(u,v) + dv(z),c(u,x) + dx(z),c(u,w) + dw(z) }

= min {2 + 5,1 + 3,

5 + 3} = 4Node that achieves minimum is nexthop in shortest path forwarding table

B-F equation says:



107/155

Network Layer4-107


Dx(y)= estimate of least cost from x to yNode x knows cost to each neighbor v:

c(x,v)

Node x maintains distance vector Dx=[Dx(y): y N ]

Node x also maintains its neighborsdistance vectors For each neighbor v, x maintains

Dv= [Dv(y): y N ]

Distance vector algorithm (4)


108/155

Network Layer4-108

Distance vector algorithm (4)

Basic idea: From time-to-time, each node sends its own

distance vector estimate to neighbors Asynchronous

When a node x receives new DV estimate fromneighbor, it updates its own DV using B-F equation:

Dx(y)minv{c(x,v) + Dv(y)} for each node y N

Under minor, natural conditions, the estimateDx(y) converge to the actual least costdx(y)


109/155

node x table

Dx(y) = min{c(x,y) + Dy(y), c(x,z) + Dz(y)}= min{2+0 , 7+1} = 2

Dx(z) = min{c(x,y) +Dy(z), c(x,z) + Dz(z)}

= min{2+1 , 7+0} = 3


110/155

Network Layer 4-110

x y z

xyz

0 2 7

from

cost to

from

from

x y z

xyz

0

from

cost to

x y z

xyz

cost to

x y zx

yz

7 1 0

cost to

2 0 1

2 0 17 1 0

time

x z12

7

y

node x table

node y table

node z table

m n{ , 7 }

32

node x table

Dx(y) = min{c(x,y) + Dy(y), c(x,z) + Dz(y)}= min{2+0 , 7+1} = 2

Dx(z) = min{c(x,y) +Dy(z), c(x,z) + Dz(z)}

= min{2+1 , 7+0} = 3


111/155

Network Layer 4-111

x y z

xyz

0 2 7

from

cost to

from

from

x y z

xyz

0 2 3

from

cost tox y z

xyz

0 2 3

from

cost to

x y z

xyz

cost tox y z

xyz

0 2 7

from

cost to

x y zx

yz

0 2 3

from

cost to

x y zx

yz

0 2 3

from

cost to

x y zx

yz

0 2 7

from

cost to

x y zx

yz

7 1 0

cost to

2 0 1

2 0 17 1 0

2 0 17 1 0

2 0 13 1 0

2 0 13 1 0

2 0 1

3 1 0

2 0 1

3 1 0

time

x z12

7

y

node x table

node y table

node z table

{ , }

Distance Vector: link cost changes


112/155

Network Layer 4-112

D stance Vector l nk cost changes

Link cost changes: node detects local link cost change

updates routing info, recalculatesdistance vector

if DV changes, notify neighbors

goodnewstravelsfast

x z14

50

y1

At time t0, ydetects the link-cost change, updates its DV,and informs its neighbors.

At time t1, zreceives the update from yand updates its table.It computes a new least cost to x and sends its neighbors its DVAt time t2, yreceives zs update and updates its distance table.ys least costs do not change and hence y does notsend anymessage to z.

Distance Vector: link cost changes


113/155

Network Layer 4-113

D V gLink cost changes:

good news travels fast bad news travels slow -

count to infinityproblem! **

44 iterations before

algorithm stabilizes: seetext

Poisoned reverse: If Z routes through Y to get

to X :

Z tells Y its (Zs) distanceto X is infinite (so Y wontroute to X via Z)

will this completely solvecount to infinity problem?

Read ebook page 387 and 388 tounderstand this

x z14

50

y60


114/155



115/155

Network Layer 4-115

Chapter 4 Network Layer


datagram networks

4.3 Whats inside a




IPv6


Distance Vector



OSPF

BGP


Hierarchical Routing


116/155

Network Layer 4-116

g

scale:with 200 milliondestinations:

cant store all dests inrouting tables!

routing table exchangewould swamp links!

administrative autonomy internet = network of

networks

each network admin may

want to control routing in itsown network

Our routing study thus far - idealization all routers identical

network flat

nottrue in practice

Hierarchical Routing


117/155

Network Layer 4-117

g

aggregate routers intoregions,autonomoussystems (AS)

routers in same AS run

same routing protocol intra-AS routingprotocol

routers in different AScan run different intra-

AS routing protocol

Gateway router Direct link to router in

another AS

Interconnected ASes


118/155

Network Layer4-118

3b

1d

3a

1c2aAS3

AS1

AS21a

2c2b

1b

Intra-AS

Routing

algorithm

Inter-AS

Routing

algorithm

Forwarding

table

3c

forwarding tableconfigured by bothintra- and inter-ASrouting algorithm intra-AS sets entries

for internal dests

inter-AS & intra-Assets entries forexternal dests

Interconnected ASes


119/155

Page 119

Interconnected ASes

IGPsRIP OSPF ISIS

EGPsBGP

AS100 AS200

Inter-AS (Exterior Gateway Protocol) tasksAS1 must:


120/155

Network Layer4-120

3b

1d

3a

1c2aAS3

AS1

AS21a

2c2b

1b

3c

suppose router in AS1receives datagram

destined outside ofAS1: router should

forward packet togateway router, butwhich one?

1. learn which dests arereachable through

AS2, which throughAS3

2. propagate thisreachability info to allrouters in AS1

Job of inter-AS routing!

Example: Setting forwarding table in router 1d


121/155

Network Layer4-121

suppose AS1 learns (via inter-AS protocol) that subnetxreachable via AS3 (gateway 1c) but not via AS2.

inter-AS protocol propagates reachability info to allinternal routers.

router 1d determines from intra-AS routing info thatits interface I is on the least cost path to 1c.

installs forwarding table entry (x,I)

3b

1d

3a

1c2aAS3

AS1

AS21a

2c2b

1b

3c

x

Example: Choosing among multiple ASes


122/155

Network Layer4-122

now suppose AS1 learns from inter-AS protocol that

subnet xis reachable from AS3 andfrom AS2. to configure forwarding table, router 1d mustdetermine towards which gateway it should forwardpackets for dest x. this is also job of inter-AS routing protocol!

3b

1d

3a

1c 2aAS3

AS1

AS21a

2c

2b

1b

3cx

Example: Choosing among multiple ASes


123/155

Network Layer4-123

Learn from inter-AS

protocol that subnetx 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 gatewaythat has the

smallest least cost

Determine from

forwarding table the

interface I that leadsto least-cost gateway.

Enter (x,I) in

forwarding table

now suppose AS1 learns from inter-AS protocol that

subnet xis reachable from AS3 andfrom AS2. to configure forwarding table, router 1d mustdetermine towards which gateway it should forwardpackets for dest x. this is also job of inter-AS routing protocol!

hot potato routing:send packet towards closest oftwo routers.



124/155

Network Layer4-124

p y


datagram networks

4.3 Whats inside a




IPv6


Distance Vector



OSPF

BGP


Intra-AS Routing


125/155

Network Layer4-125

g

also known as Interior Gateway Protocols (IGP) most common Intra-AS routing protocols:

RIP: Routing Information Protocol

OSPF: Open Shortest Path First

IGRP: Interior Gateway Routing Protocol (Ciscoproprietary)



126/155

Network Layer4-126

p y


datagram networks

4.3 Whats inside a




IPv6


Distance Vector



OSPF

BGP


RIP ( Routing Information Protocol)


127/155

Network Layer4-127

g

RIPv1 , RIPv2 distance vector algorithm

included in BSD-UNIX Distribution in 1982

distance metric: # of hops (max = 15 hops)

DC

BA

u v

w

x

yz

destination hopsu 1v 2w 2

x 3y 3z 2

From router A to subnets:

RIPv1 vs. RIPv2


128/155

Page 128

RIPv1 is a classful routing protocol, does notsupport VLSM and CIDR.

Sends messages via broadcast.

Doesnt support authentication.

RIPv2 is a classless routing protocol, supportsVLSM, route aggregation and CIDR.

Supports messages sent via broadcast ormulticast address (240.0.0.9).

Supports plain text and MD5 authentication.

RIP advertisements


129/155

Network Layer4-129

distance vectors:exchanged amongneighbors every 30 sec via ResponseMessage (also called advertisement)

each advertisement: list of up to 25destination subnets within AS

RIP: Example


130/155

Network Layer4-130

p

Destination Network Next Router Num. of hops to dest.

w A 2y B 2z B 7x -- 1. . ....

w x yz

A

C

D B

Routing/Forwarding table in D

RIP: Example

D t N t h


131/155

Network Layer 4-131

Destination Network Next Router Num. of hops to dest.

w A 2

y B 2z B A 7 5x -- 1. . ....

Routing/Forwarding table in D

w x y

z

A

C

D B

Dest Next hopsw - 1x - 1

z C 4. ...

Advertisementfrom A to D

RIP: Link Failure and Recovery


132/155

Network Layer4-132

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 (iftables changed)

link failure info quickly (?) propagates to entire net

poison reverseused to prevent ping-pong loops

(infinite distance = 16 hops)

RIP link failure and


133/155

Page 133

Recovery

E0

S0 S0

S1

S0

E0

11.2.0.

0

11.3.0.

0

A

B

C

11.1.0.0

11.4.0.

0If the hop count is16, itmeans this route is

unreachable

16S011.4.0.0

1S011.3.0.0

0S011.2.0.0

0E011.1.0.0

countinterfac

e

destination

Routing Table

1S011.1.0.0

16S111.4.0.

0

0S111.3.0.0

0S011.2.0.0

countinterface

interface

Routing Table

2S011.1.0.0

1S011.2.0.0

16E011.4.0.0

0S011.3.0.0

countinterfac

e

destinatio

n

Routing Table

RIP Table processing


134/155

Network Layer4-134

RIP routing tables managed by application-levelprocess called route-d (daemon)

advertisements sent in UDP packets, periodicallyrepeated

physical

link

network forwarding(IP) table

Transprt(UDP)

routed

physical

link

network(IP)

Transprt(UDP)

routed

forwardingtable



135/155

Network Layer4-135


datagram networks

4.3 Whats inside a




IPv6


Distance Vector


4.6 Routing in theInternet RIPv1 & RIPv2

OSPF

BGP



136/155

OSPF advanced features (not inRIPv1)


137/155

Network Layer4-137

RIPv1)

security:all OSPF messages authenticated (toprevent malicious intrusion)

multiple same-cost paths allowed (only one path inRIP)

For each link, multiple cost metrics for differentTOS (e.g., satellite link cost set low for best effort;high for real time)

integrated uni- and multicastsupport:

Multicast OSPF (MOSPF) uses same topology database as OSPF

hierarchicalOSPF in large domains.

Hierarchical OSPF


138/155

Network Layer4-138

Hierarchical OSPF


139/155

Network Layer4-139

two-level hierarchy:local area, backbone. Link-state advertisements only in area

each nodes has detailed area topology; only knowdirection (shortest path) to nets in other areas.

area border routers:summarize distances to netsin own area, advertise to other Area Border routers.

backbone routers:run OSPF routing limited tobackbone.

boundary routers:connect to other ASs.



140/155

Network Layer4-140


datagram networks

4.3 Whats inside a




IPv6


Distance Vector



OSPF

BGP


Internet inter-AS (EGP) routing:BGP


141/155

Network Layer 4-141

BGP

BGP (Border Gateway Protocol):thedefacto standard BGP provides each AS a means to:

1. Obtain subnet reachability information from

neighboring ASs.2. Propagate reachability information to all AS-

internal routers.3. Determine good routes to subnets based on

reachability information and policy.

allows subnet to advertise its existence torest of Internet: I am here


142/155

Distributing reachability infoi BGP i b t 3 d 1 AS3 d


143/155

Network Layer4-143

using eBGP session between 3a and 1c, AS3 sendsprefix reachability info to AS1. 1c can then use iBGP do distribute new prefix

info to all routers in AS1 1b can then re-advertise new reachability info

to AS2 over 1b-to-2a eBGP session

when router learns of new prefix, it creates entryfor prefix in its forwarding table.

3b

1d

3a

1c2aAS3

AS1

AS21a

2c

2b

1b

3ceBGP session

iBGP session

Path attributes & BGP routes


144/155

Network Layer4-144

advertised prefix includes BGP attributes. prefix + attributes = route

two important attributes: AS-PATH:contains ASs through which prefix

advertisement has passed: e.g, AS 67, AS 17NEXT-HOP:indicates specific internal-AS router

to next-hop AS. (may be multiple links fromcurrent AS to next-hop-AS)

when gateway router receives routeadvertisement, uses import policytoaccept/decline.

BGP route selection


145/155

Network Layer4-145

router may learn about more than 1 routeto some prefix. Router must select route.

elimination rules:1. local preference value attribute: policy

decision

2. shortest AS-PATH

3. closest NEXT-HOP router: hot potato routing

4. additional criteria

BGP messages


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Network Layer4-146

BGP messages exchanged using TCP. BGP messages:

OPEN:opens TCP connection to peer andauthenticates sender

UPDATE:advertises new path (or withdraws old) KEEPALIVEkeeps connection alive in absence of

UPDATES; also ACKs OPEN request

NOTIFICATION:reports errors in previous msg;

also used to close connection

BGP routing policy


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Network Layer4-147

A,B,C are provider networks

X,W,Y are customer (of provider networks)

X is dual-homed:attached to two networks

X does not want to route from B via X to C

.. so X will not advertise to B a route to C

A

B

C

WX

Y

legend:

customernetwork:

provider

network

BGP routing policy (2)


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Network Layer4-148

A advertises path AW to B

B advertises path BAW to X

Should B advertise path BAW to C?

No way! B gets no revenue for routing CBAWsince neither W nor C are Bs customers

B wants to force C to route to w via A

B wants to route only to/from its customers!

A

B

C

WX

Y

legend:

customernetwork:

provider

network

Why different Intra- and Inter-AS routing ?


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Network Layer4-149

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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Network Layer4-150


datagram networks

4.3 Whats inside a




IPv6


Distance Vector



OSPF

BGP


Broadcast Routing

deliver packets from source to all other nodes


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Network Layer 4-151

R1

R2

R3 R4

source

duplication

R1

R2

R3 R4

in-network

duplication

duplicate

creation/transmissionduplicate

duplicate

deliver packets from source to all other nodes

source duplication is inefficient:

source duplication: how does sourcedetermine recipient addresses?

In-network duplication


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Network Layer4-152

flooding: when node receives brdcst pckt,sends copy to all neighbors Problems: cycles & broadcast storm

controlled flooding: node only brdcsts pkt

if it hasnt brdcst same packet beforeNode keeps track of pckt ids already brdcstedOr reverse path forwarding (RPF): only forward

pckt if it arrived on shortest path between

node and source spanning tree

No redundant packets received by any node


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Chapter 4: summary


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datagram networks

4.3 Whats inside a





Distance Vector



OSPF

BGP

4.7 Broadcast andl i i

chapter4 5th april 2014 final2014

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