Datagram Networks: Internet Protocol (IPv4)eem.eskisehir.edu.tr/isan/EEM 482/icerik/Network Layer...

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1 1 Datagram Networks: Internet Protocol (IPv4) The Internet Network layer: IP Internet Network Layer Components: IP protocol (addressing, datagram format and handling), Routing Protocols and ICMP protocol forwarding table 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 Network layer

Transcript of Datagram Networks: Internet Protocol (IPv4)eem.eskisehir.edu.tr/isan/EEM 482/icerik/Network Layer...

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Datagram Networks:Internet Protocol (IPv4)

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The Internet Network layer: IP• Internet Network Layer Components:

– IP protocol (addressing, datagram format and handling), Routing Protocols and ICMP protocol

forwardingtable

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

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IP Addressing: Introduction• IP address: 32-bit

identifier for host, router interface

• interface: connection between host/router and physical link– router’s typically have

multiple interfaces

– host may have multiple interfaces

– IP addresses associated with each interface

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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IP Addressing• IP address:

– network part (high order bits)

– host part (low order bits)

– NetworkID.HostID

• What’s an IP network ? – device interfaces with

same network part of IP address

– Hosts within the same IP network can reach each other without intervening router

network consisting of 3 IP networks:223.1.1.0, 223.1.2.0, 223.1.3.0

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

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IP Addressing

• IP networks are not restricted to Ethernet segments– Here we have 3 point-

to-point links and each have a different IP network defined over them.

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

Interconnected system consistingof 6 IP networks

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IP Addressing

• How to find IP networks?1. Detach each

interface from router, host

2. Create “islands of isolated IP networks

3. Each island defines an IP network

4. Internet consists of millions of such IP networks

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

Interconnected system consistingof 6 IP networks

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IP Addresses

0network host

10 network host

110 network host

1110 multicast address

A

B

C

D

class

1.0.0.0 to127.255.255.255

128.0.0.0 to191.255.255.255

192.0.0.0 to223.255.255.255

224.0.0.0 to239.255.255.255

32 bits

• given notion of “IP network”, let’s re-examine IP addresses: – We have “class-full” addressing

– Original Internet design

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IP addressing: CIDR• Classful addressing:

– inefficient use of address space, address space exhaustion• e.g., class B net allocated enough addresses for 65K hosts, even if

only 2K hosts in that network

– No longer used in the current Internet• Solution? Classless Inter Domain Routing (CIDR)

• CIDR: Classless InterDomain Routing– Standardized in 1993

– Network portion of address of arbitrary length

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

11001000 00010111 00010000 00000000

networkpart

hostpart

200.23.16.0/23

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Netmask• With CIDR a new way is needed to determine

the IP network given an IP address:– Solution: Define a netmask

– Given an IP address of the form networkID.hostID, the netmask of the IP address is obtained by putting all “1”s in the networkID portion and all “0”s in the hostID portion

11001000 00010111 00010000 00000000

networkpart

hostpart

200.23.16.0/23

11111111 11111111 11111110 00000000 = 255.255.254.0 Given an IPAddr and a netmask, we bit-wise AND IPAddr and

netmask to obtain the IP network. The rest is the hostID. NetworkID = IPAddr & Netmask

HostID = IPAddr & ~Netmask

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IP Layer Broadcast• Recall that a host can send a LL

broadcast message by putting FF-FF-FF-FF-FF-FF in destination MAC address

• How can a host send IP-layer broadcast packet? 2 ways:– Put 255.255.255.255 in destination IP

• Means that all IP hosts within the same LL broadcast domain will receive this IP datagram

– Make <hostID> all 1’s• Means that all hosts within the same IP subnet will receive

this datagram• Example: If IP subnet is 192.169.34.0, then a packet with a

destination IP: 192.169.34.255 will be received by all hosts whose IP subnet is 192.169.34.0

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IP addresses: how to get one?

Q: How does a host get an 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 shortly)

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Q: How does network get network 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

IP addresses: how to get one?

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Hierarchical addressing: route aggregation

“Send me anythingwith addresses beginning 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 7

Internet

Organization 1

ISPs-R-Us“Send me anythingwith addresses beginning 199.31.0.0/16”

200.23.20.0/23Organization 2

.

.

.

...

• CIDRized addresses facilitate hierarchical routing• Fly-By-Night-ISP advertises that any IP datagram whose addresses

begin with 200.23.16.0/20 should be sent to it. The rest of the world need not that there are 8 other organizations each with its own IP network. This is called route aggregation

• Dividing an IP network into smaller IP networks as done in here is called subnetting. Each organization can further divide their IP address range into smaller IP subnets

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Hierarchical addressing: more specific routes

ISPs-R-Us has a more specific route to Organization 1. By the longest prefix matching rule, packets with destination addresses beginning with 200.23.18.0/23 are sent to ISPs-R-Us

“Send me anythingwith addresses beginning 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-Us“Send me anythingwith addresses beginning 199.31.0.0/16or 200.23.18.0/23”

200.23.20.0/23Organization 2

...

...

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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 root servers

– assigns domain names, resolves disputes

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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 at each router)

forfragmentation/reassembly

total datagramlength (bytes)

upper layer protocolto deliver payload to

head.

len

type of

service“type” of data

flgsfragment

offsetupperlayer

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

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Forwarding: Getting a datagram from source to dest.

IP datagram:

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

miscfields

sourceIP addr

destIP addr data

• datagram remains unchanged, as it travels from source to destination

Dest. Net. Next Hop Nhops

223.1.1 1223.1.2 223.1.1.4 2

223.1.3 223.1.1.4 2

forwarding table in A

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Starting at A, send IP datagram addressed to B:

• look up network address of B in forwarding table

• find B is on same net. as A

• B and A are directly connected– link layer will send datagram

directly to B inside link-layer frame. How?

Dest. Net. next router Nhops

223.1.1 1223.1.2 223.1.1.4 2

223.1.3 223.1.1.4 2

miscfields 223.1.1.1 223.1.1.2 data

forwarding table in A

Forwarding: Getting a datagram from source to dest.

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

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Delivering the packet from A to B

• Starting at A, given IP datagram addressed to B:– look up net. address of B, find

B on same net. as A

– link layer send datagram to B inside link-layer frame

– How does A know the MAC address of B? ARP protocol

B’s MACaddr

A’s MACaddr

A’s IPaddr

B’s IPaddr

IP payload

datagram

frame

frame source,dest address

datagram source,dest address

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

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ARP: Address Resolution Protocol

• Each IP node (Host, Router) on LAN has ARP module, table

• ARP Table: IP/MAC address mappings for some LAN nodes

< IP address; MAC address; TTL>

< ………………………….. >

– TTL (Time To Live): time after which address mapping will be forgotten (typically 20 min)

Question: how to determineMAC address of Bgiven B’s IP address?

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ARP protocol

• A knows B's IP address, wants to learn MAC address of B

• A broadcasts ARP query pkt, containing B's IP address – all machines on LAN receive ARP query

• B receives ARP packet, replies to A with its (B's) MAC address

• A caches (saves) IP-to-MAC address pairs until information becomes old (times out) – soft state: information that times out

(goes away) unless refreshed

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Dest. Net. next router Nhops

223.1.1 1223.1.2 223.1.1.4 2

223.1.3 223.1.1.4 2Starting at A, dest. E:• look up network address of E

in forwarding table

• E on different network

– A, E not directly attached

• routing table: next hop router to E is 223.1.1.4

• link layer sends datagram to router 223.1.1.4 inside link-layer frame

• datagram arrives at 223.1.1.4

• continued…..

miscfields 223.1.1.1 223.1.2.2 data

forwarding table in A

Forwarding: Getting a datagram from source to dest.

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

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Arriving at 223.1.4, destined for 223.1.2.2

• look up network address of E in router’s forwarding table

• E on same network as router’s

interface 223.1.2.9

– router, E directly attached

• link layer sends datagram to 223.1.2.2 inside link-layer

frame via interface 223.1.2.9

• datagram arrives at 223.1.2.2

miscfields 223.1.1.1 223.1.2.3 data Dest. Net router Nhops interface

223.1.1 - 1 223.1.1.4

223.1.2 - 1 223.1.2.9

223.1.3 - 1 223.1.3.27

forwarding table in router

Forwarding: Getting a datagram from source to dest.

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

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– A creates IP packet with source A, destination E – A uses ARP to get R’s MAC address for 111.111.111.110

– A creates Ethernet frame with R's MAC as dest, Ethernet frame contains A-to-E IP datagram

– A’s data link layer sends Ethernet frame to R – R’s data link layer receives Ethernet frame – R removes IP datagram from Ethernet frame, sees its destined

to E– R uses ARP to get E’s MAC address – R creates frame containing A-to-E IP datagram sends to E

A

RE

Another IP Packet Forwarding Example

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IP packet forwarding algorithmD = destination IP addressBool found = false;For each forwarding table entry (SubnetNumber, SubnetMask, NextHop) do {

D1 = SubnetMask & D;if (D1 == SubnetNumber) {

if (NextHop is an Interface) {Deliver the datagram directly to the destination within a LL frame

} else {Deliver the datagram to NextHop (a router)

} //end-elsefound = true;break;

} //end-if} //end-for

If (!found) {if (there is a <default> router) Deliver the datagram to the <default> routerelse “Report: Destination unreachable”

} //end-if

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IP packet forwarding: ExampleSubnet Number SubnetMask NextHop

128.96.34.0 255.255.255.128 Interface0

128.96.34.128 255.255.255.128 Interface1

128.96.33.0 255.255.255.0 R2

<default> R3

• Assume Destination IP = 128.96.34.68– D1 = 128.96.34.68 & 255.255.255.128 = 128.96.34.0, which is equal to

128.99.34.0 Deliver the datagram to the destination over Interface0

• Assume Destination IP = 128.96.34.150– D1 = 128.96.34.150 & 255.255.255.128 = 128.96.34.128.

– D1 = 128.96.34.150 & 255.255.255.128 = 128.96.34.128, which is equal to 128.96.34.128 Deliver datagram to the destination over Interface1

• Assume Destination IP = 128.96.35.44– IP subnet will not match any of the known IP subnetsDeliver the

packet to the <default> router, R3

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IP Fragmentation & Reassembly• network links have MTU

(max.transfer unit) - largest possible link-level frame.

– different link types, different MTUs

• large IP datagram divided (“fragmented”) within net

– one datagram becomes several datagrams

– “reassembled” only at final destination

– IP header bits used to identify, order related fragments

fragmentation: in: one large datagramout: 3 smaller datagrams

reassembly

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IP Fragmentation & Reassembly

ID=x

offset=0

fragflag=0

length=4000

ID=x

offset=0

fragflag=1

length=1500

ID=x

offset=1480

fragflag=1

length=1500

ID=x

offset=2960

fragflag=0

length=1040

One large datagram becomesseveral smaller datagrams

Example

• 4000 byte datagram– 3980 byte payload

• MTU = 1500 bytes– 1st packet payload:

1480 bytes

– 2nd packet payload: 1480 bytes

– 3rd packet payload: 1020 bytes

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Host IP Address Configuration

Q: How does a host get an 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 shortly)

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DHCP: Dynamic Host Configuration Protocol

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

Allows re use 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

– DHCP server responds with “DHCP offer” msg

– host requests IP address: “DHCP request” msg

– DHCP server sends address: “DHCP ack” msg

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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

DHCP server

arriving DHCP

client needs

address in this

network

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DHCP client-server scenarioDHCP server: 223.1.2.5 arriving

client

time

DHCP discover

src : 0.0.0.0, 68

dest.: 255.255.255.255,67

yiaddr: 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: 654

Lifetime: 3600 secs

DHCP request

src: 0.0.0.0, 68

dest:: 255.255.255.255, 67

yiaddrr: 223.1.2.4

transaction ID: 655

Lifetime: 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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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 for source, destination (as usual)

All datagrams leaving localnetwork have same single source

NAT IP address: 138.76.29.7,different source port numbers

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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).

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NAT: Network Address Translation

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 10.0.0.1 sends datagram to 128.119.40.186, 80

NAT translation tableWAN side addr LAN side addr

138.76.29.7, 5001 10.0.0.1, 3345…… ……

S: 128.119.40.186, 80 D: 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, 80 D: 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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NAT: Network Address Translation

Implementation: 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 (source IP address, port #) to (NAT IP address, new port #) translation pair

– incoming datagrams: replace (NAT IP address, new port #) in dest fields of every incoming datagram with corresponding (source IP address, port #) stored in NAT table

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NAT: Network Address Translation• 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

– address shortage should instead be solved by IPv6

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ICMP: Internet Control Message Protocol

• Used by hosts, routers, gateways to communication network-level information

– error reporting: unreachable host, network, port, protocol

– echo request/reply (used by ping)

• ICMP runs over IP:

– ICMP msgs carried in IP datagrams

• ICMP message: type, code plus first 8 bytes of IP datagram causing 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 unreachable

3 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 advertisement

10 0 router discovery

11 0 TTL expired

12 0 bad IP header