Network Layer

IP addressing, routers DHCP, NAT

So far,

• Networked application

• Need to know IP address of any hostname

• Use DNS,

• Given a port#, connect to a process running on a remote host

• Can use TCP or UDP transport protocol

Host needs a network Identifier

• To communicate over a link, hosts need a hardware identifier called the MAC address. Every Network Interface card has a hardware address (Bluetooth, wifi, ethernet etc).

• Hardware address is fixed, stored in hardware of the NIC

• To communicate over the Internet, host need a logical identifier called the IP address

• IP address can be static or dynamic, stored in the IP stack software

IPAddresses

IPV4

• 32 bits long

• Notation:

• Each byte is written in decimal in MSB order, separated by dots

• Example: 128.195.1.80

• Identifier for host, router interface

• Each interface needs to have an IP address

IPV6

• 128 bits long

• Notation:

• Divided into 16 bits segments, written in Hex digits separated by a colon

• 2001:0DB8:AC10:FE01:1C48:3A1C:A95A:B1C2

• FE80::1234:3456:5678:1A08

• Link local address: automatic way to configure IP address for local communication

4

5

Types of IPv4 Addresses

• Unicast Address

• Destination is a single host

• Multicast address

• Destination is a group of hosts

• Broadcast address

• 255.255.255.255

• Destination is all hosts

6

IPv4 Address Classes (old)

0 Net

32 bits

Type of Serv. Host

10 Net Host

110 Net Host

1110 Multicast address

11110 Reserved

A

B

C

D

E

Class

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IP Address Classes• Class A:

• For very large organizations

• 16 million hosts allowed

• Class B:

• For large organizations

• 65 thousand hosts allowed

• Class C

• For small organizations

• 255 hosts allowed

• Class D

• Multicast addresses

• No network/host hierarchy

8

IP Address Hierarchy

• Class A, B, C addresses support two levels of hierarchy

• However, the host portion can be further split into “subnets” by the address class owner

• more than 2 levels of hierarchy

9

Problems with Class-based Routing

• Too many small networks requiring multiple class C addresses

• Running out of class B addresses, not enough nets in class A

• Addressing strategy must allow for greater diversity of network sizes

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IP addressing: CIDRCIDR: 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

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CIDR

• An ISP can obtain a block of addresses and partition this further to its customers

• Say an ISP has 200.8.4.0/24 address (256 addresses). He has another customer who needs only 4 addresses from 200.8.4.24 then that block can be specified as 200.8.4.24/30

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Subnetting

Host idSubnet idNetwork id

Example: Class B address with 8-bit subnetting

165.230 .24 .8

16 bits 8 bits 8 bits

Example

Address:

13

Subnet Masks

Subnet masks allow hosts to determine if another

IP address is on the same subnet or the same

network

Host idSubnet idNetwork id

16 bits 8 bits 8 bits

1111111111111111 11111111 00000000Mask:

255.255 .255 .0

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Subnet Masks (cont’d)

Are IP addresses A and B on the same subnet?

1. Compute (A and M).

2. Compute (B and M).

3. If (A and M) = (B and M) then A and B are

on the same subnet.

Assume IP addresses A and B share subnet mask M.

Example: A and B are class B addresses

A = 165.230.82.52

B = 165.230.24.93

M = 255.255.255.0

Same network?

Same subnet?

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IP Addressing in network

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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The Internet Protocol (IP)• Provides delivery of packets from one host to any other host

in the Internet

• Internet packets are called “datagrams” and may be up to 64 kilobytes in length

• although they are typically much smaller

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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 ofservice

“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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IP Fragmentation & Reassembly

• network links have MTU (max.transfer size) - 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 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=370

fragflag=0

length=1040

One large datagram becomesseveral smaller datagrams

Example

❑ 4000 bytedatagram

❑ MTU = 1500 bytes

1480 bytes in data field

offset =1480/8

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IP Support Protocols

• ARP

• RARP

• ICMP

ARP

• A host can communicate with an other host connected on a link using

• MAC address

• IP address

• How does it know the IP address of its Peer?

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ARP

• Address Resolution Protocol

• Returns a MAC sublayer address or link layer address when given an Internet address

• After a packet reaches a router, the link layer header needs to be added to reflect the destination host on that link

• Need IP →MAC address translation

PreambleS D 0x0806 ARP PACKET

Type

ARP packet format

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Protocol Type : IPv4 0x0800, IPv6 0x86DD

Opcode ARP request:0

Opcode ARP reply:1

,

Source IP address

Source MAC address

Destination IP address

Destination MAC address

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ARP (cont’d)

Ethernet Address:

05:23:f4:3d:e1:04

IP Address:

128.195.1.20

Ethernet Address:

98:22:ee:f1:90:1a

IP Address:

128.195.1.38

Ethernet Address:

12:04:2c:6e:11:9c

IP Address:

128.195.1.122

Wants to transmit

to 128.195.1.38

ARP

ARP packet

containing “128.195.1.38?”

Ignored Answered

Proto=IPv4

0x0800

Sender H/W address

Sender IP address

Target H/W address

target IP address

Oper=1

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ICMP

• Protocol for error detection and reporting• tightly coupled with IP, unreliable

• ICMP messages delivered in IP packets

• ICMP functions:

• Announce network errors

• Announce network congestion

• Assist trouble shooting

• Announce timeouts

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ICMP MSG

IP header

Source, Destination Address, TTL, ...

ICMP MSG

Message type, Code, Checksum,

Data

IPV4 Header for ICMP

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1

ICMP header Protocol Field value=1

28

29

ICMP: Internet Control Message ProtocolType 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

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Specific uses of ICMP

• Echo request reply

• Can be used to check if a host is alive

• Destination unreachable

• Invalid address and/or port

• TTL expired

• Routing loops, or too far away

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Ping

• Uses ICMP echo request/reply

• Source sends ICMP echo request message to the destination address

• Destination replies with an ICMP echo reply message containing the data in the original echo request message

• Source can calculate round trip time (RTT) of packets

• If no echo reply comes back then the destination is unreachable

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Ping (cont’d)

R1 R2 R3A B

Tim

e

Echo request

Echo reply

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Traceroute

• Traceroute records the route that packets take

• A clever use of the TTL field

• When a router receives a packet, it decrements TTL

• If TTL=0, it sends an ICMP time exceeded message back to the sender

• To determine the route, progressively increase TTL

• Every time an ICMP time exceeded message is received, record the sender’s (router’s) address

• Repeat until the destination host is reached or an error message occurs

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Traceroute (cont’d)

R1 R2 R3A B

TTL=1, Dest = B,

port = invalid

TTL=2, Dest = B

TTL=3, Dest = B

TTL=4, Dest = B

Te (R1)

Te (R2)

Te (R3)

Pu (B)

Tim

e

Te = Time exceeded

Pu = Port unreachable

More Internet ProtocolsDHCP, NAT, IPv6

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DHCP (Ch 4.3)

• DHCP stands for dynamic host configuration protocol

• DHCP is client-server

• DHCP offers a number of more features

• Dynamic IP address allocation

• IP addresses can be leased for a certain time

• Useful where there are a limited number of IP addresses

• Useful for temporary connections (testing, laptops, mobile networks)

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DHCP (cont’d)

• DHCP has two components:

• A protocol for delivering bootstrap information from the server to the clients

• An algorithm for dynamically assigning addresses to clients

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Address Allocation Modes

• DHCP supports three modes of allocation

• Automatic allocation: Server assigns a permanent address to a host

• Dynamic allocation: Server assigns a host an IP address with a finite “lease”

• Manual allocation: Server assigns host an IP address chosen by the network administrator

IPV4 Header for DHCP

39

17

Source Port Destination port=67

DHCP PACKET

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DHCP Packets (cont’d)

Number of seconds Flags

Transaction ID

Request/Reply Hardware typeHardware address

length in bytesHop count

Client hardware address (16 bytes)

Your IP address

Server IP address

Gateway IP address

Server hostname (64 bytes)

Boot filename (128 bytes)

Options (312+ bytes)

Client IP address

0 7 8 15 16 23 24 31

Request=1

Reply=2

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Definitions of address fields

• ciaddr Client IP address; only filled in if client is in BOUND, RENEW or REBINDING state and can respond to ARP requests.

• yiaddr 'your' (client) IP address. The IP address, server is assigning to client

• siaddr IP address of server to use in the netx step of the bootstrap process; returned in DHCPOFFER, DHCPACK by server.

• giaddr Relay agent IP address, used in booting via a relay agent.

• chaddr Client hardware address used for identification.

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DHCP Packet Fields

• All fields are same as BOOTP except:

• Flags: One flag currently defined• Broadcast (bit 0): Clients can request that all DHCP server messages be broadcast to

it

• Options: • All DHCP packets must use the “DHCP message type” option, which defines the

“type” of DHCP message being sent:

• 1= DHCPDISCOVER

• 2= DHCPOFFER

• 3= DHCPREQUEST

• 4= DHCPDECLINE

• 5=DHCPACK

• 6=DHCPNACK

• 7=DHCP RELEASE

• 8=DHCP INFORM

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DHCP Message types

• DHCP message types

• DHCP Discover: Client broadcasts to locate a server

• DHCP Offer: Server responds with proposal of parameters

• DHCP Request: Client broadcasts its choice of server. All other servers are implicitly declined.

• DHCP ACK: Selected server responds to client with address

• DHCP NAK: Selected server rejects the client’s request

• DHCP Decline: Client declines server’s parameters

• DHCP Release: Client releases its assigned address

44

DHCP ProtocolServer 1 Server 2Client

Collects replies

Selects server 2

45

DHCP Protocol (cont’d)

• DHCP client broadcasts a DHCP Discover message

• Client may specify preference of a lease and/or IP address

• Many servers may respond with offers

• Client chooses one server from them

• Client broadcasts DHCP request with id of chosen server

• Selected server sends DHCP ACK or NAK

• Client begins using offered IP address once it receives ACK

• If the client finds a problem, it sends a DHCP Decline message to the server and starts over again

• Client may choose to release the address before lease expires by sending a DHCP Release message to the server

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DHCP Relay Agents

• Similar to BOOTP Relay Agents

• DHCP relay agents allow DHCP servers to handle requests from other subnets

DHCP

Relay

Agent

Client

IP

Gateway

Router

IP

Gateway

Router

DHCP

Server

47

Summary

• DHCP allow “ignorant” hosts to receive IP addresses (and more) at start-up time

• IP addresses don’t have to be manually configured into hosts

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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

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

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

• Features: local network uses just one IP address as far as outside world is concerned:

• range of addresses not needed from ISP: just one IP address 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

• 16-bit port-number field:

• 60,000 simultaneous connections with a single LAN-side address!

• NAT is controversial:

• routers should only process up to layer 3

• violates end-to-end argument

• NAT possibility must be taken into account by app designers, eg, P2P applications

• address shortage should instead be solved by IPv6