Networks - Chapter 4 - Network Layer 1spp

8/6/2019 Networks - Chapter 4 - Network Layer 1spp

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Network Layer 4-1

Chapter 4Network Layer

Computer Networking:A Top Down ApproachFeaturing the Internet,3rd edition.

Jim Kurose, Keith RossAddison-Wesley, July2004.

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 thefollowing:

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 1996-2004

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


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Network Layer 4-2

Chapter 4: Network Layer 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 4-3

Network layer delivers the segments from

the sending to the receivinghosts

on the sending side, itencapsulates the segmentsinto datagrams

on the receiving side, it

delivers the segments to thetransport layer

network layer protocols existin everyhost and router

the router examines theheader fields in all IPdatagrams passing through it

networkdata linkphysical


networkdata link

physical



network

data linkphysical


network

data linkphysical

applicationtransportnetworkdata linkphysical

applicationtransportnetwork

data linkphysical


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Network Layer 4-4

Key Network-Layer Functions forwarding:moving the

packets from the routerinput to appropriaterouter output locally

routing:determining theend-to-end route to betaken by the packetsfrom source to

destination using routingalgorithms& updatingthe forwarding tables

analogy:

routing: the process ofplanning a trip fromsource to destination

forwarding: the processof getting through asingle street interchange


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Network Layer 4-5

1

23

0111

value in arriving

packets header

routing algorithm

local forwarding tableheader value output link

0100

0101

0111

1001

3

2

2

1

Interplay between routing and forwarding


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Network Layer 4-6

Packet Switching DevicesA packet-switching device can be either:

A link-layer switch (or layer-2 switch): The forwarding decision is based on a value (usually the

physical address or MAC address) in the data-link layerheader (chap 5)

A network layer switch (or layer-3 switch or router): The forwarding decision is based on a value (usually thelogical address or IP address) in the network layer header

The routing algorithm, which can be centralized or

distributed, determines the entries of the routersforwarding table

The router receives routing protocol messages, which areused to configure the forwarding tables.


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Network Layer 4-7

Connection setup 3rd important function (after routing and

forwarding) in somenetwork architectures: ATM, frame relay, X.25 (virtual-circuit switching)

before the datagrams flow, the two hosts and

the intervening routers establish a virtualconnection i.e.; the routers get involved in the connection setup

network & transport layer connection service:Network layer: between two hosts

Transport layer: between two processes


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Network Layer 4-8

Network service modelQ: What is the service modelof the channeltransporting datagrams from a sender to a receiver?

Example services forindividual datagrams:

guaranteed delivery

guaranteed delay:e.g. delivery withless than 40 msec

Example services for aflow of datagrams:

in-order datagram

delivery guaranteed minimum

bandwidth to flow

guaranteed maximumjitter: restrictions onchanges in inter-packetspacing (maximum jitter)


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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

constant

rateguaranteed

rate

guaranteed

minimum

none

Loss

no

yes

yes

no

no

Order

no

yes

yes

yes

yes

Timing

no

yes

yes

no

no

Congestion

feedback

no

no

congestionno

congestion

yes

no

Guarantees ?

ATM: Asynchronous Transfer Mode

CBR: Constant Bit Rate, VBR: Variable Bit Rate

ABR: Available Bit Rate, UBR: Unspecified Bit Rate


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Network Layer 4-10






IPv6




4.6 Routing in the

Internet RIP

OSPF

BGP

4.7 Broadcast andmulticast routing


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Network Layer 4-11

Network-layer connection andconnection-less service

Datagram networks provide network-layer

connectionless service VC network provides network-layer connection

service

Analogous to the transport-layer services, but: Service: host-to-host (vs. process-to-process)

No choice: network provides one service or the other

but not both (vs. both are available at the same time) Implementation: in both the core and the end systems

(vs. in the end systems only)


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Network Layer 4-12

Virtual circuits

call setup for each call beforethe data can flow

call teardown for each call afterthe data transfer is complete

each packet carries a VC identifier (not a destination hostaddress)

everyrouter on the source-destination path maintains a statefor each passing connection

the link and router resources (e.g., bandwidth, buffers) may beallocatedto the VC

source-to-dest path behaves much like telephone circuit performance-wise

network actions along source-to-dest path


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Network Layer 4-13

VC implementationA VC consists of:

1. a path from the source to the destination2. VC numbers: one number for each link along path

3. entries in the forwarding tables of the routers

along the path packet belonging to VC carries a VC number.

the VC number must be changed on each link.

a new VC number comes from the forwarding table shorter VC field in the packet header

simpler VC setup: local independent decision


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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 2 171 97 3 87

Forwarding table inthe northwest router:

Routers maintain connection state information!


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Network Layer 4-15

Virtual circuits: signaling protocols end system and routers use signaling messages to

setup, maintain, and teardown VC

used in ATM, frame-relay, and X.25


data linkphysical


data linkphysical

1. Initiate call

2. incoming call

3. Accept call4. Call connected5. Data flow begins 6. Receive data


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Network Layer 4-16

Datagram networks

no call setup at the network layer

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

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

different paths depending on the network status and therouter decision making

applicationtransport


applicationtransportnetworkdata linkphysical

1. Send data 2. Receive data


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Network Layer 4-17

Forwarding tableDestination Address Range Link Interface

11001000 00010111 00010000 00000000 (200.23.16.0)

through 0

11001000 00010111 00010111 11111111 (200.23.23.255)

11001000 00010111 00011000 00000000 (200.23.24.0)through 1

11001000 00010111 00011000 11111111 (200.23.24.255)

11001000 00010111 00011001 00000000 (200.23.25.0)through 2

11001000 00010111 00011111 11111111 (200.23.31.255)

otherwise 3

32-bit IP Address 4

billion possible entries


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Network Layer 4-18

Longest prefix matchingPrefix Match Link Interface

11001000 00010111 00010 0

11001000 00010111 00011000 1

11001000 00010111 00011 2

otherwise 3

DA: 11001000 00010111 00011000 10101010

Examples

DA: 11001000 00010111 00010110 10100001 Which interface?

Which interface?

for the longest prefix matching to be effective, eachoutput link interface should be responsible forforwarding a large number of contiguous addresses this is the case with the Internet addresses as they are

assigned in a hierarchical fashion


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Network Layer 4-19

Datagram or VC networkInternet data exchange among computers

elastic services, no stricttiming requirements

smart end systems (e.g. PCs)

can adapt and perform

control and error recovery simple network core

complexity at the edge

quick and easy to add and

attach new services many link types

different characteristics

uniform service is difficult

ATM evolved from telephony

human conversation: strict timing and

reliability requirements

there is a need for

guaranteed service dumb end systems

telephones

complexity moved to

inside the network


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Network Layer 4-20






IPv6




4.6 Routing in the

Internet RIP

OSPF

BGP



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Network Layer 4-21

Router Architecture Overview

There are two key router functions: running routing algorithms/protocol (RIP, OSPF, BGP)

forwardingdatagrams from incoming to outgoing link


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Network Layer 4-22

Input Port Functions

Decentralized switching: given a packet dest., lookup the output port using

the forwarding table in input port memory an updated copy of the forwarding table is stored

in each input port avoiding a processingbottleneck at routing processor

goal: complete input port processing at line speed queuing: occurs if a new packet arrives faster

than the forwarding rate into switch fabric

Physical layer:bit-level reception

Data link layer:e.g., Ethernetsee chapter 5


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Network Layer 4-23

Speed of the input port processing example:

an OC-48 link runs at 2.5 Gbps. Therefore, with 256 bytepackets, the lookup speed should be about 1 million lookup/sec

lookup techniques to speed-up the datagram forwarding: binary tree: N steps to lookup N-bit addresses

even not fast enough for backbone routing requirements

Content Addressable Memory (CAM): constant time using cache memory other recent techniques: log(N) steps

queuing reasons at the input port: when a new packet is received and is ready to be forwarded

before the current packet is forwarded when the forwarding of the current packet is blocked by the

switching fabric (i.e.; fabric is busy forwarding another packetfrom another input port)


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Network Layer 4-24

Three types of switching fabrics


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Network Layer 4-25

Switching Via MemoryFirst generation routers:

traditional computers with switching under direct control of the CPU

I/O ports functioned as traditional I/O devices input ports used interrupts to signal the packet arrival

packets are then copied to the system memory CPU reads the header and then forwards the packet to an output port

speed limited by memory bandwidth (2 bus crossings per datagram) therefore, the input-to-output forwarding speed is one-half of the memory

access speed

modern routers use this method as a shared-memory multiprocessors Examples: Cisco Catalyst 8500 Series & Bay Networks Accelar 1200 Series

Input

PortOutput

Port

Memory

System Bus


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Network Layer 4-26

Switching Via a Bus packet from input port memory to output port memory via ashared bus

one packet at a time can be transferred

bus contention: switching speed is limited by the busbandwidth (at least as fast as all input port together)

example: 1 Gbps bus, Cisco 1900: sufficient speed for accessand enterprise routers (not regional or backbone)


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Network Layer 4-27

Switching Via An Interconnection Network n x n crossbar switch with 2n busses

overcomes the bus bandwidth limitations (to a certain extent)

Advanced design: fragment the packets into fixed length cells,then switch the cells through the fabric to simplify andspeedup the switching

example: Cisco 12000: switches at up to 60 Gbps through theinterconnection network


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Network Layer 4-28

Output Ports

Buffering(queuing) required when packets arrive from fabricfaster than the transmission rate

Scheduling disciplinechooses among queued packets fortransmission


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Network Layer 4-29

Where does queuing and loss occur? the actual location of packet queuing and/or loss

depends on:

the traffic load (arrival rate, packet size, etc.) the relative speed of the switching fabric

the line speed

queuing reasons at the input port (review): when a new packet is received and is ready to be forwarded

before the current packet is forwarded

when the forwarding of the current packet is blocked bythe switching fabric

queuing reasons at the output port: when the arrival rate via the switching fabric exceeds the

output line speed


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Network Layer 4-30

Output port queueing Example:

3 input ports

3 output ports line speeds are identical = S

switch fabric speed = 3S

worst case: all packets at

input ports are destined tothe same output port

queuing delay and loss due tooutput port buffer overflow!


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Network Layer 4-31

Packet scheduling & queue management Which packet in the queue is selected next for transmission? Or

what is the scheduling scheme? Simply, First-Come-First-Served (FCFS) Weighted Fair Queuing (WFQ)

Fair share of outgoing link among end-to-end connections

What if the buffer is full? drop arriving packets (drop-tail policy) when full. But, can we do better?

Active Queue Management (AQM) algorithms:

Drop or mark the header of the arriving packet before the buffer isfilled. Why? A congestion indication to the sender Example: Random Early Detection (RED) algorithm:

Weighted average of the queue length is maintained If queue length is less than a min. threshold, accept packets

If queue length is greater than a max. threshold, mark or drop new packets If queue length is in between, drop or mark new packets based on a

probability that is a function of the queue length

Packet scheduling is very important for Quality-of-Service (QoS)guarantees


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Network Layer 4-32

Input Port Queuing

Fabric switch is slower than the input ports combined queuing may occur at the input queues

Head-of-the-Line (HOL) blocking: queued packet atthe front of the queue prevents others in the queuefrom moving forward


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Network Layer 4-33

Chapter 4: Network Layer 4.1 Introduction





IPv6




4.6 Routing in the

Internet RIP

OSPF

BGP



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Network Layer 4-34

The Internet Network layerHost, router network layer functions:


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Network Layer 4-35






IPv6




4.6 Routing in the

Internet RIP

OSPF

BGP


P d f


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Network Layer 4-36

IP datagram format

ver length

32 bits

data(variable length,typically a TCP

or UDP segment)

16-bit identifierInternetchecksum

time tolive

32 bit source IP address

IP protocol version number

header length (bytes)

max number remaininghops (decremented ateach router)

for

fragmentation/reassembly

total datagramlength (bytes)

upper layer protocol

to deliver payload to(glue between networkand transport layers)

headlen

type ofservice

type of dataflgs

fragment

offsetupperlayer

32 bit destination IP address

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

to visit, etc.

how much overheadwith TCP/IP typically?

20 bytes of TCP

20 bytes of IP

= 40 bytes + applayer overhead

Headerchecksum


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Network Layer 4-37

IP Fragmentation & Reassembly network links have MTU (Max.Transmission Unit), which is thelargest possible link-level frame

different link types and hencedifferent MTUs along the route

Ethernet: 1500 bytes

Some WANs: 576 bytes

large IP datagram is divided(fragmented) within the network

one datagram becomes severaldatagrams

reassembled only at the finaldestination (keep core simple)

IP header bits are used toidentify and order the relatedfragments

fragmentation:in: one large datagramout: 3 smaller datagrams

reassembly


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Network Layer 4-38

IP Fragmentation and ReassemblyID=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=370fragflag

=0length=1040

One large datagram becomesseveral smaller datagrams

Example

4000 byte datagram

3980 byte payload 20 bytes IP header

MTU = 1500 bytes

1480 bytes in data field(must be multiple of 8 bytesexcept for last fragment)

offset = 1480/8

offset = (1480 + 1480)/8

3980 - 1480 - 1480 = 1020bytes in the data field


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Network Layer 4-39






IPv6

4.5 Routing algorithms Link state

Distance Vector


4.6 Routing in the

Internet RIP

OSPF

BGP



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Network Layer 4-40

IP Addressing: introduction IP address: 32-bit ID for the

interfaceof the host or router interface:is the connection

between the host or routerand the physical link routers typically have multiple

interfaces a host typically has one

interface one IP addresses is associated

with each interface a portion of the interfaces IP

address is determined by thesubnet it is connected to

IP addresses in the globalInternet should be unique(except for interfaces behindthe NATs)

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

2231 11

Dotted-Decimal Notation


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Network Layer 4-41

Subnets

the IP address consists of: subnet part: the x most

significant bits

host part: 32-x leastsignificant bits

whats a subnet ? a network of devices with

their interfaces having thesame subnet part of IPaddress

the devices on the samesubnet can physically reach

each other without anintervening router

Connected by a data-linklayer hub or switch

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

subnet subnet


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Network Layer 4-42

Subnets

223.1.1.0/24223.1.2.0/24

223.1.3.0/24

Recipe To determine the subnets,

detach each interfacefrom its host or router,creating islands of isolatednetworks. Each isolatednetwork is called a subnet.

The subnet mask indicatesthe number of most

significant bits used toidentify the subnet part ofthe IP address

Subnet mask: /24 256 addresses


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Network Layer 4-43

SubnetsHow 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

223.1.7.1

223.1.8.0223.1.8.1

223.1.9.1

223.1.9.2


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Network Layer 4-44

IP addressing: Classful Addressing the old method of IP addressing

subnet addresses must be 1, 2, or 3 bytes

subnet address classes: Class A: a.b.c.d/8 (over 16 million addresses)

Class B: a.b.c.d/16 (over 65 thousand addresses)

Class C: a.b.c.d/24 (only 256 addresses) what if an organization needs only 500 addresses?

Problems with classful addressing: fast depletion of class B address space

poor utilization of the assigned number space


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Network Layer 4-45

IP addressing: CIDR strategyCIDR: Classless InterDomain Routing

subnet portion of address can be of an arbitrary length

address format: a.b.c.d/x, where x is # bits in subnet portion of

address (also called the prefix or the network prefix) the prefix represents the network portion of the IP address

IP addresses are usually assigned to organizations in blocks ofcontiguous addresses that share a common prefix

only the x bits are considered by routers outside the organizationsnetwork

the remaining 32-x bits are used to identify device interfaces withinthe organizations network (may have additional subnetting structure)

11001000 00010111 00010000 00000000

subnetpart

hostpart

200.23.16.0/23


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Network Layer 4-46

IP addresses: how to get one?Q: How does a hostget an IP address?

manually configured by system admin in a system file

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

UNIX: /etc/rc.config

DHCP: Dynamic Host Configuration Protocol: dynamically get anIP address from a server

plug-and-play

efficient IP address utilization

efficient for mobile hosts such as laptops

(more in next chapter)


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Network Layer 4-47

IP addresses: how to get one?Q: How does a networkget the subnet part of IP

address?

A: gets allocated a 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

Hi hi l dd ssi : t ti


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Network Layer 4-48

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 7

Internet

Organization 1

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

200.23.20.0/23Organization 2

.

.

.

...

hierarchical addressing: addresses are assigned in contiguous blocks to ISPsand then from ISPs to client organizations: allows for efficient advertisement of routing info

route (or address) aggregation is the ability to use a single prefix toadvertise multiple networks: works very well with hierarchical addressing

Hi hi l dd i ifi


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Network Layer 4-49

Hierarchical addressing: more specific routes

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

Send me anythingwith addressesbeginning

200.23.16.0/20

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 addressesbeginning 199.31.0.0/16or 200.23.18.0/23

200.23.20.0/23

Organization 2

.

..

...

Fly-By-Night acquires ISPs-R-Us and connect Organization 1 through it: Organization 1 renumbers all its routers and hosts (very costly solution)

Organization 1 keeps the same numbers, which are specifically advertised byISPs-R-Us benefiting from the longest-prefix-match routing feature


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Network Layer 4-50

IP addressing: the last word...

Q: How does an ISP get block of addresses?

A: ICANN: Internet Corporation for AssignedNames and Numbers

allocates addresses

manages DNS assigns domain names, resolves disputes


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Network Layer 4-51

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)

10.0.0.0/24

rest ofInternet

Datagrams with source ordestination in this networkhave 10.0.0.0/24 address forsource, destination (as usual)

Alldatagrams leavinglocalnetwork have same single sourceNAT IP address: 138.76.29.7 butwith different source port numbers

Note: 10.0.0.0/8 range is reserved for private networks


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Network Layer 4-52

NAT: Network Address Translation Motivation: the local network uses just one IP address

as far as outside world is concerned: no need to be allocated a range of addresses from the ISP: -

just one IP address is used for all devices

can change addresses of the devices in the local networkwithout notifying the outside world

can change the ISP without changing the addresses of thedevices in the local network

devices inside the local network are not explicitly addressableor visible by the outside world (a security plus).


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Network Layer 4-53

NAT: Network Address TranslationImplementation: NAT router must: change the header of the outgoing datagrams: replace(source

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

new port #) as destination address

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

change the header of the incoming datagrams: replace(NAT IPaddress, new port #) in dest fields of every incoming datagramwith corresponding (source IP address, port #) stored in NATtable


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Network Layer 4-54


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 datagram

source addr from10.0.0.1, 3345 to138.76.29.7, 5001,updates table

S: 128.119.40.186, 80D: 138.76.29.7, 5001 3

3: Reply arrivesdest. address:138.76.29.7, 5001

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


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Network Layer 4-55


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

side address!

NAT is controversial: port addresses are meant to be used for addressing

processes not hosts causes trouble to servers running within the local network

routers should only process up to layer 3 violates end-to-end argument

NAT possibility must be taken into account by networkapplication designers, e.g., P2P applications

address shortage should instead be solved by IPv6


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Network Layer 4-56





IPv4 addressing

ICMP

IPv6


Distance Vector


4.6 Routing in the

Internet RIP

OSPF

BGP


l l


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Network Layer 4-57

ICMP: Internet Control Message Protocol

used by hosts & routers tocommunicate network-levelinformation

error reporting: unreachablehost, network, port, protocol

echo request/reply (used byping)

network-layer above IP:

ICMP msgs carried in IPdatagrams

ICMP datagrams aredecapsulated and demuxed

to the ICMP ICMP message: type & code plus

first 8 bytes of IP datagramcausing error

Type Code description

0 0 echo reply (ping)

3 0 dest. network unreachable

3 1 dest host unreachable3 2 dest protocol unreachable

3 3 dest port unreachable

3 6 dest network unknown

3 7 dest host unknown

4 0 source quench (congestioncontrol - not used)

8 0 echo request (ping)

9 0 route advertisement

10 0 router discovery

11 0 TTL expired12 0 bad IP header


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Network Layer 4-58

Traceroute and ICMP Source sends series of

UDP segments to dest

First has TTL =1 Second has TTL=2, etc.

Unlikely port number

When nth datagram arrives

to nth router: Router discards datagram

And sends to source aTTL Expired ICMPmessage (type 11, code 0)

Message includes name &IP address of the router

When ICMP messagearrives, source calculates

RTT Traceroute does this 3

times

Stopping criterion

UDP segment eventuallyarrives at destination host

Destination returns ICMPport unreachable packet

(type 3, code 3) When source gets this

ICMP, it stops.


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Network Layer 4-59





IPv4 addressing

ICMP

IPv6


Distance Vector


4.6 Routing in the

Internet RIP

OSPF

BGP



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Network Layer 4-60

IPv6 Initial motivation: 32-bit address space is

soon to be completely allocated.

Additional motivation: header format helps speed processing/forwarding

header changes are to facilitate QoS

IPv6 datagram format:

fixed-length 40 byte header

no fragmentation allowed at intermediate routers


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Network Layer 4-62

Other Changes from IPv4 Fragmentation/reassembly:not allowed any more

Too large packets are dropped by the router

A Packet Too Big ICMP error message is sent to source

Checksum: removed entirely to reduce processingtime at each hop

Options:allowed, but outside of header, indicatedby Next Header field

ICMPv6:new version of ICMP additional message types, e.g. Packet Too Big

multicast group management functions


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Network Layer 4-63

Transition From IPv4 To IPv6 Not all routers can be upgraded simultaneously

no flag day (all machines are turned off & upgraded together)

How will the network operate with mixed IPv4 and IPv6 routers?

Dual-stack (IPv6/IPv4) nodes: should be able to know if other nodes are dual-stack: DNS

two IPv6-capable nodes may end-up communicating using IPv4 ifan intermediate node in between is not

Tunneling:IPv6 carried as payload in IPv4 datagramamong IPv4 routers

Tunneling


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Network Layer 4-64

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: A

Dest: F

data

Flow: XSrc: A

Dest: F

data

Flow: XSrc: ADest: F

data

Src:B

Dest: E Flow: XSrc: ADest: F

data

Src:B

Dest: E

A-to-B:IPv6

E-to-F:IPv6

B-to-C:IPv6 inside

IPv4

B-to-C:IPv6 inside

IPv4


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Interplay between routing and forwarding


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Network Layer 4-66

1

23

0111

value in arriving

packets header

routing algorithm

local forwarding tableheader value output link

0100

0101

0111

1001

3

2

2

1

Graph abstraction


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Network Layer 4-67

u

yx

wv

z2

2

13

1

1

2

53

5

Graph: G = (N,E)

N = set of nodes (or routers ) = { u, v, w, x, y, z }

E = set of edges (or 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


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Network Layer 4-68

Graph abstract on costs

u

yx

wv

z

2

21

3

1

12

53

5 c(a,b) = cost of link (a,b)

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

cost could always be 1, orinversely related tobandwidth, or inverselyrelated to congestion

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

Default, first-hop,

or source router

Destination router

Routing Algorithm classification


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Network Layer 4-69

Routing Algorithm classification

Global or decentralized?Global:

all routers have complete

topology & link cost info link state or LS algorithms

Decentralized:

router only knows thephysically-connected neighbors& the link costs to them

iterative process ofcomputation, exchange of info

with neighbors distance vector or DS

algorithms

Static or dynamic?Static:

routes change very slowly

over time e.g. manually updated tables

Dynamic:

routes change more quickly

periodic update

in response to topologyor link cost changes

adapts to network status

susceptible to routing loopsand oscillation in routes

Chapter 4: Network Layer


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Network Layer 4-70


4. 1 Introduction




IPv4 addressing

ICMP

IPv6


Distance Vector Hierarchical routing

4.6 Routing in the

Internet RIP

OSPF

BGP


A Link-State (LS) Routing Algorithm


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Network Layer 4-71

A Link State (LS) Routing Algorithm

Dijkstras algorithm network topology and link

costs are known to all nodes

accomplished via linkstate broadcast

all nodes have the sameinformation

computes least cost pathsfrom one node (source) toall other nodes gives the forwarding table

for that node

iterative: after kiterations,it knows the least cost pathto kdestinations

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

x to y; c(X,Y) = if X and Y

are not direct neighbors D(v): the current value of the

cost of the path from source todestination v

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

N': the set of nodes whoseleast cost path are definitively

known

Dijsktras Algorithm


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Network Layer 4-72

Dijsktra s Algorithm

1 Initialization:

2 N' = {u}

3 for all nodes v

4 if v adjacent to u5 then D(v) = c(u,v)

6 else D(v) =

7

8 Loop9 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 known14 least-cost path cost to w plus cost from w to v */

15 until all nodes in N'

Dijkstras algorithm: example


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Network Layer 4-73

Dijkstra s algorithm example

Step

0

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,y4,y

4,y

u

yx

wv

z2

2

13

1

1

2

53

5

Dijkstras algorithm: example (2)


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Network Layer 4-74

Dijkstra s algorithm example (2)

u

yx

wv

z

Resulting shortest-path tree from u:

v

xy

w

z

(u,v)

(u,x)(u,x)

(u,x)

(u,x)

destination link

Resulting forwarding table in u:


75/92



76/92

Network Layer 4-76

p L y

4. 1 Introduction




IPv4 addressing

ICMP

IPv6



4.6 Routing in the

Internet RIP

OSPF

BGP


Distance Vector (DV) Algorithm


77/92

Network Layer 4-77

( ) g

Based on Bellman-Ford Equation (dynamic programming)

Definedx(y) : is the total cost of least-cost path from x to y

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

where min is taken over all neighbors v of x

v

Distributed: each node receives info from direct neighbors, runs thealgorithm, and redistribute the results back to direct neighbors

Iterative: same process continues until no more changes are possible Self-terminating: the algorithm finally converges Asynchronous: does not require all nodes to operate simultaneously

Bellman-Ford example


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Network Layer 4-78

u

yx

wv

z

2

21

3

1

12

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}

= 4

The node that achieves the minimum cost is the nexthop in the path in forwarding table

Using B-F equation:

Distance Vector Algorithm


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Network Layer 4-79

g

Dx(y) = estimate of least cost from x to y

Distance vector: Dx = [Dx(y): y

N ] Least cost estimate to every node in the network

Node x:

knows cost to each neighbor v: c(x,v)maintains Dx = [Dx(y): y N ]

maintains its neighbors distance vectors For each neighbor v, x maintains Dv = [Dv(y): y N ]

Distance vector algorithm (4)


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Network Layer 4-80

g ( )

Basic idea:

Each node periodically sends its own distancevector estimate to neighbors

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 normal conditions, the estimateDx(y)

converge to the actual least costdx(y)

Distance Vector Algorithm (5)


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Network Layer 4-81

Iterative, asynchronous:each local iteration caused by:

local link cost change

DV update message from aneighbor

Distributed:

each node notifies neighborsonlywhen its DV changes neighbors then notify their

neighbors if necessary

waitfor (change in local linkcost or msg from neighbor)

recomputeestimates

if DV to any dest has

changed, notifyneighbors

Each node:

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


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Network Layer 4-82

x y z

xy

z

0 2 7

from

cost to

from

from

x y z

xy

z

0 2 3

from

cost tox y z

xy

z

0 2 3

from

cost to

x y z

x

yz

cost tox y z

x

yz

0 2 7

fro

m

cost to

x y z

x

yz

0 2 3

from

cost to

x y z

xyz

0 2 3

from

cost tox y z

xyz

0 2 7

from

cost to

x y z

xyz

7 1 0

cost to

2 0 1

2 0 1

7 1 0

2 0 17 1 0

2 0 13 1 0

2 0 1

3 1 0

2 0 1

3 1 0

2 0 1

3 1 0

time

xz

12

7

y

node y table

node z table

Distance Vector: link cost changes


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Network Layer 4-83

Link cost changes: node detects local link cost change

updates routing info, recalculatesdistance vector

if DV changes, notify neighbors

goodnewstravels

fast

x z

14

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 DV.

At 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


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Network Layer 4-84

Link cost changes: good news travels fast

bad news travels slow - count toinfinity problem!

44 iterations before algorithmstabilizes

Poisoned-reverse solution: If Z routes through Y to get to X :

Z tells Y its (Zs) distance to X is

infinity (so Y wont route to X via Z) will this completely solve count to

infinity problem? No.

x z

14

50

y60

before the change y x = 4 z y x = 5

after the change, y doesntknow that z routes to x via y Y z x = 1 + 5 = 6 A routing loop is created Y informs z of the change

z changes to z y x = 6 + 1 = 7 z informs y of the change

and so on ..

Comparison of LS and DV algorithms


85/92

Network Layer 4-85

Message complexity LS: with n nodes, E links,

O(nE) msgs sent

DV: exchange betweenneighbors only

convergence time varies

Speed of Convergence

LS: O(n2

) algorithm requiresO(nE) msgs

may have oscillations

DV: convergence time varies

may be routing loops

count-to-infinity problem

Robustness: what happens ifrouter malfunctions?

LS (more robust):

node can advertiseincorrect linkcost

each node computes onlyits owntable

DV (less robust): DV node can advertise

incorrectpathcost

each nodes table used byothers

error propagate thrunetwork



86/92

Network Layer 4-86

4. 1 Introduction

4.2 Virtual circuit and

datagram networks 4.3 Whats inside a

router


IPv4 addressing

ICMP IPv6



4.6 Routing in theInternet RIP

OSPF

BGP



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Hierarchical RoutingS l i


88/92

Network Layer 4-88

Solution: organize routers into autonomous systems (AS)

each AS is a group of routers that are under the sameadmin control

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

routers in different AS can run different intra-AS routingprotocol

Gateway router direct link to a routers in another AS

Interconnected ASes


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Network Layer 4-89

3b

1d

3a

1c2aAS3

AS1

AS21a

2c2b

1b

Intra-ASRouting

algorithm

Inter-ASRouting

algorithm

Forwarding

table

3c

Forwarding table isconfigured by both intra-and inter-AS routing

algorithm Intra-AS sets entries for

internal dests

Inter-AS & Intra-AS setsentries for external dests

same inter-AS routingprotocol in all ASs

Inter-AS tasks S s t i AS1 AS1 ds t :


90/92

Network Layer 4-90

3b

1d

3a

1c2aAS3

AS1

AS21a

2c2b

1b

3c

Suppose a router in AS1receives a datagram ofwhich the dest is outsideof AS1

Router should forwardpacket towards one of thegateway routers, butwhich one?

AS1 needs to:1. learn which dests are

reachable through AS2and which through AS3

2. propagate thisreachability info to allrouters in AS1

A job of the inter-AS routing!

Example: Setting forwarding table in router 1d


91/92

Network Layer 4-91

Suppose AS1 learns from the inter-AS protocolthat subnet xis reachable from AS3 (gateway 1c)but not from AS2.

Intra-AS protocol propagates reachability info toall internal routers.

Router 1d determines from intra-AS routing info

that its interface I is on the least cost path to 1c. Puts in forwarding table entry (x,I).

Example: Choosing among multiple ASes


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Network Layer 4-92

Learn from inter-AS

protocol that subnet

x is reachable viamultiple gateways

Use routing info

from intra-AS

protocol to determine

costs of least-costpaths to each

of the gateways

Hot potato routing:

Choose the gateway

that has thesmallest least cost

Determine from

forwarding table the

interface I that leads

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

forwarding table

Now suppose AS1 learns from the inter-AS protocolthat subnet xis reachable from AS3 andfrom AS2.

To configure forwarding table, router 1d mustdetermine towards which gateway it should forwardpackets for dest x.

Hot potato routing: send packet towards closest

(the one with least-cost) of two routers.

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