spring 2000john kristoff1 network layer computer networks john ourada

Spring 2000 John Kristoff 1

Network Layer

Computer NetworksJohn Ourada


Where are we?


Will Layer 2 Networking Suffice?


Motivation

Connect various link technologies to form a larger internetwork

Universal addressing scheme required General purpose use Hides underlying technologies from end user Facilitate communicate between autonomous

domains Able to move packets between any host on the

internetwork


Connecting Heterogeneous Networks

Computer System used Special purpose Dedicated Works with LAN or WAN technologies Known as

routergateway


Illustration of a Router

Cloud denotes an arbitrary networkOne interface per network


Important Idea

A router can interconnect networks that use different technologies,

including different media and media access techniques, physical

addressing schemes or frame formats.


The Internet Concept


Key Functions of the Network Layer

Global AddressingFragmentationRouting

We’ll be primarily concerned with addressing and routing


Example Network Layer: Internet Protocol (IP)

Standardized by IETF as RFC 791Most popular Layer 3 protocolCore protocol used on the public InternetConnectionless protocol

datagrams contain identity of the destination each datagram sent/handled independently

Of utmost importance for this class!


IP Addressing

Provides an abstractionIndependent of hardware (MAC)

addressingUsed by

higher layer protocols applications


IP Address

Virtual only understood by software

Used for all communication across an internetwork

32-bit integerUnique value for each host/interface


IP Address Assignment

An IP address does not identify a specific computer. Instead, each IP

address identifies a connection between a computer and a network.

A computer with multiple network connections (e.g., a router) must be

assigned one IP address for each connection.


IP Address Details

Divided into two parts prefix identifies the network suffix identifies the host/interface

Global authority assigns unique prefix for the network

Local administrator assigns unique suffix for the host/interface


Class of IP Addresses (Historical)

Initial bits determined the class The class determines the boundary between prefix and suffix


Dotted Decimal Notation

Shorthand for IP addressesAllows humans to avoid binaryRepresents each octet in decimal

separated by dotsNOT the same as names like

www.depaul.edu


Examples of Dotted Decimal Notation

Four decimal values per 32-bit address Each decimal number

represents eight bits is between 0 and 255 inclusive


Classes and Network Size (Historical)

Maximum size determined by class of address Class A large Class B medium Class C small


Addressing Example


Illustration of Router Addresses

Address prefix identifies the networkNeed one address per router connection


Special Addresses

Network Address not used in packets Loopback addresses never leave the local computer


IP Addressing: Problems with Classes

Internet growthRouting table sizeExhaustion of addressesAdministration overheadMisappropriation of addresses


IP Addressing: Solutions

SubnettingVariable Length Subnet Mask (VLSM)SupernettingClassless InterDomain Routing

(CIDR)


Subnetting

Split the suffix into a local network portion and a smaller host id portion

Subnet mask becomes 255.255.255.0 for an 8-bit subnet mask


Subnetting

Subnet boundaries fall between any of the 32 bits in an IP address Can be complex and confusing, know binary if not not on 8-bit

boundaries


128 64 32 16 8 4 2 1

1

2021222324252627

0 0 0 1 1 0 0

140

1 1 0 0 0 0 0 0

192

0 0 1 1 1 0 0 0

56

0 0 1 0 1 1 0 1

45

1 1 1 1 1 1 1 1

255

1 1 1 1 1 1 1 1

255

1 1 1 1 1 1 1 1

255

0 0 0 0 0 0 0 0

0

IP Address

NetMask

1 0 0 0 1 1 0 0

140

1 1 0 0 0 0 0 0

192

0 0 1 1 1 0 0 0

56

0 0 0 0 0 0 0 0

0

Network Address

140.192.56.0/2424-bit mask8-bit subnet mask

140.192.56.45

1 0 0 0 1 1 0 0

140

1 1 0 0 0 0 0 0

192

0 0 1 1 1 0 0 0

56

0 0 1 0 1 1 0 1

45

1 1 1 1 1 1 1 1

255

1 1 1 1 1 1 1 1

255

1 1 1 1 0 0 0 0

240

0 0 0 0 0 0 0 0

0

1 0 0 0 1 1 0 0

140

1 1 0 0 0 0 0 0

192

0 0 1 1 0 0 0 0

48

0 0 0 0 0 0 0 0

0

140.192.48.0/2020-bit mask4-bit subnet mask

140.192.56.45

IP Address

NetMask

Network Address

Network Subnet Host

Network Subnet Host


128 64 32 16 8 4 2 1

1

2021222324252627

0 0 0 1 1 0 0

140

1 1 0 0 0 0 0 0

192 138 95

1 1 1 1 1 1 1 1

255

1 1 1 1 1 1 1 1

255

1 1 1 1 0 0 0 0

240

0 0 0 0 0 0 0 0

0

IP Address

NetMask

1 0 0 0 1 1 0 0

140

1 1 0 0 0 0 0 0

192Network Address

140.192.138.95

1 0 0 0 1 1 0 0

140

1 1 0 0 0 0 0 0

192

1 1 1 1 1 1 1 1

255

1 1 1 1 1 1 1 1

255 255 252

1 0 0 0 1 1 0 0

140

1 1 0 0 0 0 0 0

192

140.192.138.95

138 95


2 2

Subnet Mask Bits

2 3

2 4

2 5

2 6

2 7

2 8

2 9

2 10

2 11

2 12

2 13

2 14

4 -2 = 2

8 -2 = 6

16 -2 = 14

32 -2 = 30

64 -2 = 62

128 -2 = 126

256 -2 = 254

512 -2 = 510

1024 -2 = 1022

2048 -2 = 2046

4096 -2 = 4094

8192 -2 = 8190

16384 -2 = 16382

2

Bits Combo's N/A Net's

3

4

5

6

7

8

9

10

11

12

13

14

Bits Networks Hosts

4 14 Hosts409414

7 14 Hosts510126

12 14 Hosts624094

6 14 Hosts

10 14 Hosts

Class BSubnet Masks

Bits Networks Hosts

4 14 Hosts1414

2 14 Hosts622

3 14 Hosts

6 14 Hosts

Class CSubnet Masks

2

6

14

30

62

126

254

510

1022

2046

4094

8190

16382

Hosts

ClassB

Hosts

ClassC

2

6

14

30

62


VLSM

Variable Length Subnet Mask Can be complex and confusing, know binary! Use addresses more efficiently.


001 00000 = 32001 00001 = 33001 00010 = 34001 00011 = 35001 00100 = 36001 00101 = 37001 00110 = 38001 00111 = 39001 01000 = 40001 01001 = 41001 01010 = 42001 01011 = 43001 01100 = 44001 01101 = 45001 01110 = 46001 01111 = 47001 10000 = 48001 10001 = 49001 10010 = 50001 10011 = 51001 10100 = 52001 10101 = 53001 10110 = 54001 10111 = 55001 11000 = 56001 11001 = 57001 11010 = 58001 11011 = 59001 11100 = 60001 11101 = 61001 1111 0 = 62001 11111 = 63

10011 1001000010

00100

10001

01111

01110

01101

01100

01011 00101

00011

00001

0011001000

01001

01010

10000

00111

10011 1001000010

00100

10001

01111

01110

01101

01100

01011 00101

00011

00001

0011001000

01001

01010

10000

00111

10011 1001000010

00100

10001

01111

01110

01101

01100

01011 00101

00011

00001

0011001000

01001

01010

10000

00111

10011 1001000010

00100

10001

01111

01110

01101

01100

01011 00101

00011

00001

0011001000

01001

01010

10000

00111

001 - 00001 --- 001 - 11110Network 140.192.32.0/19Networks 140.192.33.0/24 --140.192.63.0/24




1 0 0 0 1 1 0 0

140

1 1 0 0 0 0 0 0

192 Host

Big Circles Little Circles


E 1/1

E 1/1

E 1/0S 2/0

E 1/1

E 1/1

E 1/0

S 2/0 E 1/1

E 1/1

E 1/0

S 2/0

E 1/1

E 1/1

E 1/0S 2/0

S 1/0

S 1/1 S 1/3

S 1/2

R2 R4

R5R3

R1

140.192.33.1

140.192.34.1

140.192.35.1

140.192.49.1

140.192.50.1

140.192.51.1

140.192.17.10

140.192.17.6

140.192.17.5

140.192.17.9

140.192.17.13

140.192.17.17

140.192.17.14

140.192.17.18

140.192.65.1

140.192.66.1

140.192.67.1

140.192.81.1

140.192.82.1

140.192.83.1

140.192.32.0/20

140.192.16.0/20

140.192.64.0/20

140.192.80.0/20140.192.48.0/20

Option 2Variable length mask using 20-bits, 24-bits, and 30-bits


Supernetting

Combine multiple smaller address classes into a larger block

1 1 0 1 0 0 0 0

208

1 1 0 0 1 1 1 1

207

0 0 1 1 0 1 0 0

52

0 0 0 0 0 0 0 0

0

1 1 0 1 0 0 0 0

208

1 1 0 0 1 1 1 1

207

0 0 1 1 0 1 0 1

53

0 0 0 0 0 0 0 0

0

1 1 0 1 0 0 0 0

208

1 1 0 0 1 1 1 1

207

0 0 1 1 0 1 1 0

54

0 0 0 0 0 0 0 0

0

208.207.52.0/24

208.207.53.0/24

208.207.54.0/24

1 1 0 1 0 0 0 0

208

1 1 0 0 1 1 1 1

207

0 0 1 1 0 1 1 1

55

0 0 0 0 0 0 0 0

0 208.207.55.0/24

1 1 0 1 0 0 0 0

208

1 1 0 0 1 1 1 1

207

0 0 1 1 0 1 0 0

52

0 0 0 0 0 0 0 0

0 208.207.52.0/22


CIDR

Classless Inter-domain RoutingEmploy supernetting information in

IP routersAdvertise smaller CIDR blocksDecreases the routing table size


IP Packet (datagram) Format


IP Datagrams

Can be delayedDuplicatedDelivered out of orderLostCan change routes from packet to

packetAre connectionless


IP Routing

Performed by routersTable-drivenForwarding on a hop-by-hop basisDestination address used for route

determination


Routing/Forwarding Overview

Strip off layer 2 headers/trailersExtract destination address field, DLook up D in the routing tableFind next hop address, NSend datagram to NAdd on layer 2 headers/trailers


Routing Basic Operation

A.344321

A.243483

B.294923

B.564002

D.33984

D.901834

C.458732

C.886202

RouterD.1

A.1 B.1

C.1

Basic Routing


Routing Basic Operation

DA ProtocolP. DA

Netw orkSA

P. DAHost

P. SANetw ork

P. SAHost

Data FCS

P. DANetw ork

P. DAHost

P. SANetw ork

P. SAHost

Data

Layer 2

Layer 3

1234 JIP A3256 34 C 45 Data FCS

A 34 C 45 Data

Layer 2

Layer 3


Basic Routing Operations

Netw ork

A

B

Interface

0

1

C 2

D 3

Routing Table

Netw ork.Host

A.34

A.24

Layer 2

4321

3483

B.29 4923

B.56 4002

Layer 2 <--> Layer 3 Table

C.45 8732

C.88 6202

D.3 3948

D.90 1834


Basic Routing Operations

A.344321

A.243483

B.294923

B.564002

D.33984

D.901834

C.458732

C.886202

RouterD.15890

A.12398

B.18034

C.13012

Basic Routing


3012 JIP A8732 34 C 45 Data FCS

A 34 C 45 Data

4321 JIP A2398 34 C 45 Data FCS

From C.45 to A.34C.45 know s that A.34 isn't on the same net and sends it to router at C.1Note DA for layer 2

Inside the router the Layer 2 headers and trailers are removed leaving only thelayer 3 packet.The router looks up the packet's DA in the routing table and forw ards to theappropriate interface.

At the interface, layer 2 headers and trailers are added back.DA is the address of the destination host.SA is the address of the router.FCS is recalculated.


TCP/IP Routing

140.192.10.50060CA23BE45

140.192.10.250060CA34CD29

140.192.100.340060CA4AD2EE

140.192.100.80060CAAABBCC

140.192.201.220060CA3499CC

140.192.201.1260060CA3499DE

140.192.34.340060CA114499

140.192.34.350060CA7819AA

Router140.192.201.1

00C0C1AA3410

140.192.10.100C0C1AA3411

140.192.100.100C0C1AA3412

140.192.34.100C0C1AA3413

IP Routing


TCP/IP Routing

DA Protocol P. DASA P. SA Data FCS

Data

Layer 2

Layer 3

00C0C1AA3413 IP 140.192.10.50060CA114499 Data FCS

Layer 2

Layer 3

140.192.34.34

140.192.10.5 Data140.192.34.34

P. DA P. SA


TCP/IP RoutingFrom 140.192.34.34 to 140.192.10.5140.192.34.34 know s that 140.192.10.5 isn't on the same net and sends it to router at 140.192.34.1Note DA for layer 2

Inside the router the Layer 2 headers and trailers are removed leaving only thelayer 3 packet.The router looks up the packet's DA in the routing table and forw ards to theappropriate interface.

At the interface, layer 2 headers and trailers are added back.DA is the address of the destination host.SA is the address of the router.FCS is recalculated.

00C0C1AA3413 IP 140.192.10.50060CA114499 Data FCS140.192.34.34

140.192.10.5 Data140.192.34.34

0060CA23BE45 IP 140.192.10.500C0C1AA3411 Data FCS140.192.34.34


TCP/IP Routing

Netw ork

140.192.10.0

140.192.100.0

Interface

0

1

140.192.201.0 2

140.192.34.0 3

Routing Table

Netw ork.Host

140.192.10.5

140.192.10.25

Layer 2

0060CA23BE45

0060CA34CD29

140.192.100.34 0060CA4AD2EE

140.192.100.8 0060CAAABBCC

Layer 2 <--> Layer 3 TableARP Table

140.192.201.22 0060CA3499CC

140.192.201.126 0060CA3499DE

140.192.34.34 0060CA114499

140.192.34.35 0060CA7819AA


ARP Protocol

ARP: Address Resolution Protocol Resolves IP address to MAC address Node sends broadcast looking for another

node140.192.23.1 broadcasts looking for 140.192.23.23

Node replies with MAC address140.192.23.23 replies with 00600A34AA3C

ARP Table: contains records of learned relationships.


Example IP Routing Table

Table (b) is for center router in (a)


Routing Table Size

Since each destination in a routing table corresponds to a network, the number of entries in a routing table

is proportional to the number of networks in the internetwork.


Key Concept

The destination address in a datagram header always refers to the ultimate destination. When a router forwards the datagram to another router, the

address of the next hop does not appear in the datagram header.


Routing Protocol Requirements

Efficient routing table sizeEfficient routing control messagesRobustness and reliability

prevent loops avoid black holes reconvergence time is short


Source of Route Table Information

Manual Table created by hand Useful in small networks Useful if routes never change

Automatic software creates/updates tables Needed in large networks Changes routes when failures occur


Compute Shortest/Best Path

Possible metric geographic distance economic cost capacity


Algorithms for Computing Shortest Path

Distance Vector Exchange routing tables with

neighboring routers e.g., RIP, RIPv2

Link State Routers exchange link status

information e.g., OSPF


Distance Vector

Routers periodically advertise and learn about IP networks

Cost of the route is based on hops to the network (number of routers to pass)

Recalculation occurs when links fail


Count to Infinity Problem

What happens when link 1<->5 goes down? Does 5 think it can get to 1 through 2?


Solving the Count to Infinity Problem

Hold down Wait for a period of time before switching paths.

Advertise route cost as infinity. Based on timers.

Report the entire path Guarantees no loops, but expensive.

Split horizon Do not advertise routes to neighbors if the route was

received from that neighbor. Not foolproof.


Other Distance Vector Improvements

Triggered updates Advertise changes as soon as you learn of them. May

help convergence time. May create routing instability for flapping routes.

Poison reverse Used with split horizon. Report infinity rather than

nothing at all.

Diffusing Update ALgorithm (DUAL) Somewhat like hold down, but routers are alerted of

broken paths. Complex. Not popular.


Link State

Routers distribute link cost and topology information to all other routers in its area.

All routers have complete information about the network.

Each router computes its own optimal path to destinations.

Ensures loop free environments.


Network Layer: Final Notes

ICMPARPFragmentationBOOTP/DHCP


BOOTP

BOOTP: boot protocol (RFC 951)BOOTP is based on UDP so it uses IP

for transport and is routeable.


BOOTP: the way it works

Workstation broadcasts BOOTP request containing its MAC address on power-up

BOOTP Server responds with:Host IP address

File server address, Boot file name

DNS servers, subnet mask, router address

Routers may forward BOOTP requests, depending on configuration.

Interface command: ip helper address 140.192.1.50


BOOTP: configuration

Network manager sets up a static table mapping MAC addresses to IP addresses in each BOOTP server.loop.dummy:\ :sm=255.255.255.128:\ :bf=null:\ :ds=140.192.1.50,140.192.8.250:

# subnet 140.192.10.0 -- acs in ac350.ac350.dummy:\ :tc=.loop.dummy:gw=140.192.10.120:

#:140.192.10.11--140.192.10.14 for Netware server in AC subnetdept13.acs.depaul.edu:tc=.ac350.dummy:ht=ethernet:ha=00A024E281E0:ip=140.192.10.13:dept16.acs.depaul.edu:tc=.ac350.dummy:ht=ethernet:ha=00608CEB7F0E:ip=140.192.10.16:


DHCP

DHCP: Dynamic Host Configuration Protocol (RFC 1531)

Superset of BOOTP, provides the same service with more options.

New servers are able to work with DNS also.


DHCP: the way it works

IP Addresses bound to workstations dynamically. Workstation broadcasts DHCPDISCOVER

message on power-up.

Several DHCP Servers may respond with DHCPOFFER messages containing:

IP address, subnet mask

Router address

Renewal Time


DHCP

Workstation responds to one offer with DHCPREQUEST.Request may include items like: DNS

servers, time servers, boot files,

DHCP Server now binds IP address and replies with DHCPACK message with requested options.


DHCP

Manager assigns multiple ranges of IP addresses to each DHCP server and server manages distribution to clients.

Client must renew IP address at regular intervals indicated by Renewal Time.


DHCP: configuration

server-identifier 140.192.1.52;

# option definitions common to all supported networks...option domain-name "depaul.edu";option domain-name-servers 140.192.1.50,140.192.8.250;option subnet-mask 255.255.255.128;default-lease-time 43200;max-lease-time 86400;

shared-network RESNET {

# option definitions common to this shared network. option subnet-mask 255.255.255.128; default-lease-time 6000; max-lease-time 72000;


DHCP: configuration

# primary ip address for the interface subnet 140.192.216.0 netmask 255.255.255.128 { option broadcast-address 140.192.216.127; option routers 140.192.216.1; }

# The other subnet that shares this physical network subnet 140.192.211.0 netmask 255.255.255.128 { range 140.192.211.11 140.192.211.126; option broadcast-address 140.192.211.127; option routers 140.192.211.1; }

# The other subnet that shares this physical network subnet 140.192.211.128 netmask 255.255.255.128 { range 140.192.211.130 140.192.211.254; option broadcast-address 140.192.211.255; option routers 140.192.211.129; }}

spring 2000john kristoff1 network layer computer networks john ourada

Documents

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