network layer4-1 chapter 4 network layer these ppt slides are originally from the kurose and...

Network Layer 4-1

Chapter 4Network Layer

These ppt slides are originally from theKurose and Ross’s book. But some slides aredeleted and added for my own purpose, and some of them are modified.

Network Layer 4-2

Chapter 4: Network Layer

4. 1 Introduction 4.2 Virtual circuit

and datagram networks

4.3 What’s inside a router

4.4 IP: Internet Protocol Datagram format IPv4 addressing ICMP IPv6

4.5 Routing algorithms Link state Distance Vector Hierarchical routing

4.6 Routing in the Internet RIP OSPF BGP

4.7 Broadcast and multicast routing

Network Layer 4-3

Key Network-Layer Functions

routing: determine route taken by packets from source to dest.

Routing algorithms

Network Layer 4-4









Network Layer 4-5

Two packet switching technologies Virtual circuit

Path is determined before data is transferred.• Connection-oriented

All packets pass through the same path(circuit). Need signaling. X.25, Frame Relay, ATM Analogy: telephone network(circuit switching)

Datagram There is no determined path, but path is determined

per packet.• Connectionless

IP Analogy: telegram

Network Layer 4-6

Routing in IP datagram

How can routers determine the path of packets to destinations? Routers use the packet’s destination

address to forward the packet. Each router has a forwarding table that

maps destination addresses to link interfaces at which the packet will be delivered to the next-hop router on the path to the destination.

Network Layer 4-7

Routing Table Mask Destination address next-hop address flags

U(UP)- Router is operating now G(gateway)- destination is in other network H(host-specific) D(added by redirection) M(modified by redirection)

reference count: # of users to use this path use: # of packets to pass through this router interface

Network Layer 4-8

Router Operation

Receive an IP packet

1. For each entry in the routing tableApply the mask to packet destination addressIf (the result matches the value in the destination field)

If (the G flag is absent)Use packet destination address as next hop address

Send packet to fragmentation module with next hop addressReturn

2. If no match is found, send an ICMP error message

3. Return

Network Layer 4-9

193.14.5.160 193.14.5.192

R1

111.0.0.0R2

R3 R4

192.16.7.0 194.17.21.0R5

m1m3

m0

default router

193.14.5.165 193.14.5.197

111.25.19.20

111.15.17.32 111.20.18.14

182.16.7.5 194.17.21.14

192.16.7.52 194.17.21.68

111.30.31.18

194.17.21.16

Network Layer 4-10

Mask destination Next hop F ref. U I

255.0.0.0 111.0.0.0 - U 0 0 m0

255.255.255.224 193.14.5.160 - U 0 0 m2

255.255.255.224 193.14.5.192 - U 0 0 m1

……… ……….. ……… ….. … …. …

255.255.255.255 194.17.21.16 111.20.18.14 UGH 0 0 m0

255.255.255.0 192.16.7.0 111.15.17.12 UG 0 0 m0

255.255.255.0 194.17.21.0 111.20.18.14 UG 0 0 m0

0.0.0.0 0.0.0.0 111.30.31.18 UG 0 0 m0

Routing Table of R1

Network Layer 4-11









Network Layer 4-12

The Internet Network layer

forwardingtable

Host, router network layer functions:

Routing protocols•path selection•RIP, OSPF, BGP

IP protocol•addressing conventions•datagram format•packet handling conventions

ICMP protocol•error reporting•router “signaling”

Transport layer: TCP, UDP

Link layer

physical layer

Networklayer

Network Layer 4-13









Network Layer 4-14

IP datagram format

ver length

32 bits

data (variable length,typically a TCP

or UDP segment)

16-bit identifier

Internet checksum

time tolive

32 bit source IP address

IP protocol versionnumber

header length (bytes)

max numberremaining hops

(decremented at each router)

forfragmentation/reassembly

total datagramlength (bytes)

upper layer protocolto deliver payload to

head.len

type ofservice

“type” of data flgsfragment

offsetupper layer

32 bit destination IP address

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

how much overhead with TCP?

20 bytes of TCP 20 bytes of IP = 40 bytes +

app layer overhead

Network Layer 4-15

Header Fields(1)

version(4 bits) The current IP is version 4.

head length (4 bits) fixed part(20 bytes) + option (in 32-bit words) Ex, fixed part = 5 (160 bits)

Type of services(TOS) or Differentiated services(DS) Defines the class of datagram for quality of service.

total length (8 bits) Header + data (unit: bytes) Data 의 Size = TOTAL LENGTH - HLEN

• Maximum size of IP Datagram is 65,535 bytes long.

Network Layer 4-16

Header Fields(2)

Identification (16 bits) It identifies the datagram from a source. Normally it is incremented by 1 every

datagram. Flags (3 bits)

Two flag bits are defined:• “more fragmentation” 비트• “don’t fragment” 비트

Fragmentation offset (13 bits)

Network Layer 4-17

MTU and Fragmentation

Maximum Transmission Unit(MTU) Each physical network has its own maximum length of data

unit(PDU) it can process.• Ex, Ethernet: 1518 bytes, ATM: 53 bytes

Thus the size of the IP datagram should not be larger than MTU of the physical network it is passing through.

Fragmentation If the datagram size is larger than MTU of the physical network, the

datagram must be fragmented.

Net 1(MTU=1000)

Net 2(MTU=400)

1000 bytes 1000 bytes

Network Layer 4-18

DATA 1 DATA 2 DATA 3

400 Octet400 Octet 400 Octet400 Octet 200 Octet200 Octet

DATA 1

DATA 2

DATA 3

Original datagram (1000 octets)

Fragment offset = 0, More Fragment Bit = 1

Fragment offset = 400/8=50, More Fragment Bit = 1

Fragment offset = 800/8=100, More Fragment Bit = 0

Fragmentation

0 399

400 799

800 999

Network Layer 4-19

Header Fields(3)

Time-to-live (8 bits) It is used to control the max. # of hops(routers)

visited by the datagram. Normally it starts with 32 or 64. Each router that

processes the datagram decrements this number by 1.

If the value reaches 0, the router discards this datagram. And the router sends the ICMP message to the source host.

Protocol (8 bits) It defines the higher layer protocol that uses the

services of the IP layer.

Network Layer 4-20

Protocol Values

1 2

6 17

89

Network Layer 4-21

Checksum It is used for the error detection for the IP header.

So the checksum covers only the header. If the checksum is incorrect, the receiver discards

this datagram. Sender

packet is divided into k sections, each of n bits(16 bits) all sections are added together using 1’s complement the final result is complemented to make the checksum

Receiver packet is divided into k sections, each of n bits all sections are added together using 1’s complement the result is complemented If the final result is 0, then no error

Network Layer 4-22

n bits

n bits

all 0s

n bits

n bits

n bits

Section 1

Section 2

checksum

Section n

Sum

checksum

….….

complement

sender

n bits

n bits

n bits

n bits

n bits

All 0s?

Section 1

Section 2

checksum

Section n

Sum

checksum

….….

complement

receiver

n bits

checksum

packet

Network Layer 4-23

Example

Network Layer 4-24

Option

Options(variable) It is used for testing and debugging. The following fields are defined.

• Record route• loose source routing• strict source routing• timestamp

Network Layer 4-25









Network Layer 4-26

Addressing

Internet architecture Classful addressing Subnetting Private addresses and NAT Classless addressing

Network Layer 4-27

Internet Architecture

The internet consists of many heterogeneous networks. A network of networks

The Internet is a special internet which connects all networks based on the TCP/IP protocol.

In Internet, internetworking devices called routers(or gateways) provide interconnections among all networks.

Network Layer 4-28

Net 1 Net 2 Net 3R1 R2

Internet

real structure

router physicalnet

host

Internet as a universal network

host

user’s view

Virtual, uniform one network

Network Layer 4-29

IP address

IP addresses denote the connection(interface) to the Internet. cf> telephone address, ethernet address

IP addresses are unique. Each address defines only on connection to

the Internet. IP addresses are global(universal).

Any host connected to the Internet must use the same IP addressing scheme.

Network Layer 4-30

IP address

An IP address is a 32 bits long. It consists of two parts: net-id and host-

id.

Net id Host id

32 bits

Denote a specificphysical networkIn the Internet

Denote a connection in a specificphysical network

Network Layer 4-31

Dotted decimal notation

Network Layer 4-32

Types of IP addresses

Network Layer 4-33

Range of Each Class

Network Layer 4-34

Net-id and host-id of each class

Network Layer 4-35

Network Address

Network Layer 4-36

Sample Network

Network Layer 4-37

Special IP addresses

0’s mean “this”, 1’s mean “all”

Limited broadcast( local net)2

all 0s

all 0s

all 1s

all 1snet

host

127 anything (often 1)

This host1

Host on this net1

Directed broadcast for net2

Loopback3

Notes:1 Allowed only at system startup(bootstrap) and is never a valid destination address.2 Never a valid source address.3 Should never appear on a network. Used in testing.

Network Layer 4-38

Subnetting

IP address is designed with two levels of hierarchy: netid and hostid.

Sometimes the block of a network address assigned to an organization need to be divided for efficient routing, so that an organization has several subnetworks(subnets).

Subnetting provides a mean to have three levels of hierarchy.

Network Layer 4-39

H1

128.10.1.1

H2

128.10.1.2

Network 128.10.1.0

H3

128.10.2.1

H4

128.10.2.2

Network 128.10.2.0

REST OF THEINTERNET

all traffic to128.10.X.X

R

All packets with 128.10.x.x arecoming to this organization from outside.

Network Layer 4-40

Address with subnet-id

Network Layer 4-41

Subnet Mask

The subnet mask specifies how many bits in the host-id are assigned for subnet-id.

Network Layer 4-42

Subnet Example

Network ID Subnet ID Host ID

16 8 8

11111111 11111111 11111111 00000000

Network ID Subnet ID Host ID

16 10 6

11111111 11111111 11111111 11 000000

0xffffff00 = 255.255.255.0

0xffffffc0 = 255.255.255.192

Network Layer 4-43

Private Address

The private addresses are not recognized in the global Internet.

IP private addresses can only be used in local networks.

The blocks of IP private addresses 10.0.0.0 ~ 10.255.255.255 172.16.0.0 ~ 172.31.255.255 192.168.0.0 ~ 192.168.255.255

The local sites with private addresses can be connected to the global Internet through NAT.

Network Layer 4-44

NAT: Network Address Translation

Motivation: local network uses just one IP address as far as outside word is concerned: no need to be allocated range of addresses from

ISP: - just one IP address is used for all devices can change addresses of devices in local network

without notifying outside world can change ISP without changing addresses of

devices in local network devices inside local net not explicitly

addressable, visible by outside world (a security plus).

Network Layer 4-45

Network Address Translation(NAT)

Network Layer 4-46

Network Layer 4-47

Translation Table

How does a NAT router know the destination address for a packet coming from the outside? For many-to-many relation between the private address

network and global Internet servers, we need the following translation table.

Private Address

Private Port

ExternalAddress

External Port

TransportProtocol

172.18.3.1 1400 25.8.3.2 80 TCP

172.18.3.2 1401 25.8.3.2 80 TCP

... ... ... ... ...

Network Layer 4-48

Classless Interdomain Routing(CIDR)

Classful address IP addresses have three classes depending on the

leftmost bit(s). So the length of the net-id has fixed depending on

the classes. Classless address

IP addresses has no classes. The length of net-id is variable.

The length of net-id is specified for each IP address. CIDR uses classless address.

Network Layer 4-49

IP addressing: CIDR

CIDR: Classless InterDomain Routing subnet portion of address of arbitrary length address format: a.b.c.d/x, where x is # bits in

subnet portion of address

11001000 00010111 00010000 00000000

subnetpart

hostpart

200.23.16.0/23

Network Layer 4-50

CIDR notation

In CIDR, an IP address is expressed as: Network address/length

Ex) 234.170.168.0/21–(234.170.168.0, 234.170.175.255) block Dotted decimal 32-bit binary equivalentlowest 234.170.168.0 11101010 10101010 10101000 00000000highest 234.170.175.255 11101010 10101010 10101111 11111111CIDR mask 11111111 11111111 11111000 00000000

Network Layer 4-51

Ex) 198.32.0.0/16 (198.32.0.0, 198.32.255.255) block

lowest 198.32.0.0 11000110 00100000 00000000 00000000Highest 198.32.255.255 11000110 00100000 11111111 11111111

Network Layer 4-52

An ISP has the address block of 206.0.64.0/18 (64/24s).If a customer requests 800 IP addresses, then the ISP can assign the address block of 206.0.68.0/22 (4 contiguous/24s) to the customer.

What is the advantage of using CIDR comparing with the case of assigningone class B address or 4 class C addresses?

ISP’s block 11001110.00000000.01000000.00000000 206.0.64.0/18

client’s block 11001110.00000000.01000100.00000000 206.0.68.0/22

Class C(0) 11001110.00000000.01000100.00000000 206.0.68.0/24Class C(1) 11001110.00000000.01000101.00000000 206.0.69.0/24Class C(2) 11001110.00000000.01000110.00000000 206.0.70.0/24Class C(3) 11001110.00000000.01000111.00000000 206.0.71.0/24

Example Example

Network Layer 4-53

IP addresses: how to get one?

Q: How does host get IP address?

hard-coded by system admin in a file Wintel: 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”

(more in next chapter)

Network Layer 4-54

IP addresses: how to get one?

Q: How does network get subnet part of IP addr?

A: gets allocated portion of its provider ISP’s address 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

Network Layer 4-55

IP addressing: the last word...

Q: How does an ISP get block of addresses?

A: ICANN: Internet Corporation for Assigned

Names and Numbers allocates addresses manages DNS assigns domain names, resolves disputes

Network Layer 4-56









Network Layer 4-57

ICMP: Internet Control Message Protocol

used by hosts & routers to communicate network-level information error reporting:

unreachable host, network, port, protocol

echo request/reply (used by ping)

network-layer “above” IP: ICMP msgs carried in IP

datagrams ICMP message: type, code

plus first 8 bytes of IP datagram causing error

Type Code description0 0 echo reply (ping)3 0 dest. network unreachable3 1 dest host unreachable3 2 dest protocol unreachable3 3 dest port unreachable3 6 dest network unknown3 7 dest host unknown4 0 source quench (congestion control - not used)8 0 echo request (ping)9 0 route advertisement10 0 router discovery11 0 TTL expired12 0 bad IP header

Network Layer 4-58

Error Reporting messages(1)

Network Layer 4-59


Destination Unreachable When a router or host can’t route a

datagram, it discards the datagram and sends a destination unreachable message to the source.

Source quench When a router or host discards datagrams

due to congestion, It sends a source quench message to the source.

Network Layer 4-60


Time exceeded When TTL value is 0, a router discards the datagram

and send a Time exceeded message the original source host.

When all fragmented datagrams of one datagram do not arrive at the destination within a certain time limit, this message is generated.

Parameter problem When any ambiguous or missing values in the

header of the datagram are detected,

Network Layer 4-61


Redirection The host may send a datagram which is

destined for a wrong router because of its old routing table. In this case the router will forward this datagram to the correct router and at the same time it sends a redirection message back to the host.

Network Layer 4-62

Query Messages(1)

Network Layer 4-63

Query Messages(2)

Error Request and Reply Used for diagnostic purpose

Time stamp Request and Reply When routers or hosts determine the round-

trip time needed for a datagram to travel between them

Address Mask Request and Reply When a host want to know the subnet mask

of the network it belongs to

Network Layer 4-64

Query Messages(3)

Router Solicitation and Advertisement When a host want to know a default router,

it broadcasts a router solicitation message. A router can send a router advertisement

message periodically even if no host has solicited.

Network Layer 4-65

ICMP message encapsulation

Network Layer 4-66

Example of ICMP Use

Ping Traceroute MTU discovery

Network Layer 4-67

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 an

ICMP message (type 11, code 0)

Message includes name of router& IP address

When ICMP message arrives, source calculates RTT

Traceroute does this 3 times

Stopping criterion UDP segment eventually

arrives at destination host

Destination returns ICMP “host unreachable” packet (type 3, code 3)

When source gets this ICMP, stops.

network layer4-1 chapter 4 network layer these ppt slides are originally from the kurose and...

Documents

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