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Lecture 1
Recap: “Operational” view of Internet Internet: “network of
networks” Requires sending,
receiving of messages
protocols control sending, receiving of messages e.g., TCP, IP, HTTP, Skype,
Ethernet etc.
Design of protocols is the key for Internet
Home network
Institutional network
Mobile network
Global ISP
Regional ISP
1-1
application support host/network applications Email, FTP, HTTP (HTML)
transport process-process data transfer TCP, UDP
network routing of datagrams from src. to destn. IP address, routing protocols
link data transfer between neighboring network
elements Ethernet, PPP
physical bits “on the wire”
1-2
Internet protocol stack
application
transport
network
link
physical
Lecture 3
Network Layer 4-3
Network layer
Network layer protocols in every host, router
Router examines header fields in all IP datagrams passing through it
application
transportnetworkdata linkphysical
application
transportnetworkdata linkphysical
networkdata linkphysical network
data linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysicalnetwork
data linkphysical
Network Layer 4-4
Key Network-Layer Functions
Forwarding: move packets from router’s input to appropriate router output
Routing: determine route taken by packets from source to dest.
analogy:
forwarding: process of getting through single interchange
routing: process of planning trip from source to dest
Network Layer 4-5
Key Network-Layer Function
requirement:
Need to know the addresses Zip codes ~ e.g., 10019
What is the address for computers / routers?
IP Address
Network Layer 4-6
7
IP Address
An IP address is a 32-bit sequence of 1s and 0s. To make the IP address easier to use, the address is
usually written as four decimal numbers separated by periods.
This way of writing the address is called the dotted decimal format.
11011111 00000001 00000001 00000001
223 1 11
Network Layer 4-8
IP Addressing example network IP address: 32-bit
identifier for host, router interface
interface: connection between host/router and physical link router’s typically have
multiple interfaces host typically has one
interface 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
9
A quick look at Binary and Decimal Number format
Decimal (base 10)
Uses positional representation Each digit corresponds to a power of 10
based on its position in the number The powers of 10 increment from 0, 1,
2, etc. as you move right to left
1,234 = 1 * 103 + 2 * 102 + 3 * 101 + 4 * 100
Binary (base 2) Two digits: 0, 1 To make the binary numbers more
readable, the digits are often put in groups of 4 or 8
1010 = 1 * 23 + 0 * 22 + 1 * 21 + 0 * 20
= 8 + 2 = 10
1100 1001 = 1 * 27 + 1 * 26 + 1 * 23 + 1 * 20
= 128 + 64 + 8 + 1 = 201
Conversion From binary to decimal
Use positional representation as shown in last slide
From decimal to binary (tricky!) Keep dividing by 2 Remainders give the digits, starting from
lowest power
Let’s look at some examples… Now we are ready for IP addressing
IP address Class Handouts…
(provided in class)
Network Layer 4-13
Every IP address has two parts: 1. Network part2. Host part
IP addresses are divided into classes A,B and C to define -- large, -- medium, and -- small networks.
The Class D address class was created to enable multicasting.
Class E addresses reserved for future and research.
IP Address
IP Address classes
Some special IP addresses
Network Layer 4-16
Network Layer 4-17
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 useAllows reuse 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 [optional] DHCP server responds with “DHCP offer” msg [optional] host requests IP address: “DHCP request” msg DHCP server sends address: “DHCP ack” msg
Network Layer 4-18
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
BE
DHCP server
arriving DHCP client needsaddress in thisnetwork
Network Layer 4-19
DHCP client-server scenarioDHCP server: 223.1.2.5 arriving
client
time
DHCP discover
src : 0.0.0.0, 68 dest.: 255.255.255.255,67yiaddr: 0.0.0.0transaction ID: 654
DHCP offer
src: 223.1.2.5, 67 dest: 255.255.255.255, 68yiaddrr: 223.1.2.4transaction ID: 654Lifetime: 3600 secs
DHCP request
src: 0.0.0.0, 68 dest:: 255.255.255.255, 67yiaddrr: 223.1.2.4transaction ID: 655Lifetime: 3600 secs
DHCP ACK
src: 223.1.2.5, 67 dest: 255.255.255.255, 68yiaddrr: 223.1.2.4transaction ID: 655Lifetime: 3600 secs IP: 223.1.2.4
Numerical example
A software company has 100 employees. What would be the ideal class from which the company would
choose its network IP to prevent wastage of IP addresses? How many bits would be assigned for network part and how many bits would be assigned for host part?
The company suddenly goes through increase in number of employees from 100 to 2040.
What would be the ideal class from which the company would choose its network IP to prevent wastage of IP addresses?
How many bits would be assigned for network part and how many bits would be assigned for host part?
Solve!
Network Layer 4-20
Network Layer 4-21
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
Back to the previous numerical example? How many address wastage?
11001000 00010111 00010000 00000000
subnetpart
hostpart
200.23.16.0/21
Network Address Translation (NAT)
Network Layer 4-22
Home network
local network(e.g., home network)
rest ofInternet
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
NAT: Network Address Translation
Advantages:
local network uses just one IP address as far as outside world is concerned: min. IP address wastage
can change addresses of devices in local network without notifying outside world: flexibility
devices inside local net not explicitly addressable, visible by outside world (a security plus).
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, 80
2
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 arrives dest. address: 138.76.29.7, 5001
4: NAT routerchanges datagramdest addr from138.76.29.7, 5001 to 10.0.0.1, 3345
NAT: Network Address Translation
16-bit port-number field: 60,000 simultaneous connections with a
single LAN-side address!
NAT traversal problem client wants to connect to
server with address 10.0.0.1 server address 10.0.0.1
local to LAN (client can’t use it as destination addr)
only one externally visible NATted address: 138.76.29.7
solution 1: statically configure NAT to forward incoming connection requests at given port to server
10.0.0.1
10.0.0.4
NAT router
138.76.29.7
Client?
NAT traversal problem solution 2: relaying (used in Skype)
NATed client establishes connection to relay
External client connects to relay relay bridges packets between connections
138.76.29.7
Client
10.0.0.1
NAT router
1. connection torelay initiatedby NATted host
2. connection torelay initiatedby client
3. relaying established
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