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Chapter 4
Network Layer
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All material copyright 1996-2002
J.F Kurose and K.W. Ross, All Rights Reserved
Computer Networking:
A Top Down Approach
Featuring the Internet,
2nd edition.
Jim Kurose, Keith Ross
Addison-Wesley, July
2002.
Network Layer
4-1
Chapter 4 roadmap
4.1 Introduction and Network Service Models
4.2 Routing Principles
4.3 Hierarchical Routing
4.4 The Internet (IP) Protocol
 4.4.1 IPv4 addressing
 4.4.2 Moving a datagram from source to destination
 4.4.3 Datagram format
 4.4.4 IP fragmentation
 4.4.5 ICMP: Internet Control Message Protocol
 4.4.6 DHCP: Dynamic Host Configuration Protocol
 4.4.7 NAT: Network Address Translation
4.5 Routing in the Internet
4.6 What’s Inside a Router
4.7 IPv6
4.8 Multicast Routing
4.9 Mobility
Network Layer
4-2
4.4.1 IPv4 Addressing Roadmap
• Definition of an IP Address
• Definition of a Network from the IP perspective
• Address Assignment
ISPs
Networks
Hosts
• Classful Addressing
• Masking
• Subnetting
• Classful Addressing Alternative
• Hierarchial Addressing
• Quiz
Network Layer
4-3
What is an IP Address?
• IP address: 32-bit
identifier for host,
router interface
• interface: connection
between host/router
and physical link



223.1.1.1
223.1.2.1
223.1.1.2
223.1.1.4
223.1.1.3
223.1.2.9
223.1.3.27
223.1.2.2
router’s typically have
223.1.3.2
223.1.3.1
multiple interfaces
host may have multiple
interfaces
IP addresses associated
with each interface
223.1.1.1 = 11011111 00000001 00000001 00000001
223
1
1
Network Layer
1
4-4
IP Addressing
• IP address:
 network part (high order
bits)
 host part (low order bits)
• What’s a network ?
(from IP address
perspective)
 device interfaces with
same network part of IP
address
 can physically reach
each other without
intervening router
223.1.1.1
223.1.2.1
223.1.1.2
223.1.1.4
223.1.1.3
223.1.2.9
223.1.3.27
223.1.2.2
LAN
223.1.3.1
223.1.3.2
network consisting of 3 IP networks
(for IP addresses starting with 223,
first 24 bits are network address)
Network Layer
4-5
IP Addressing
223.1.1.2
How to find the networks?
• Detach each interface
from router, host
• create “islands of
isolated networks
223.1.1.1
223.1.1.4
223.1.1.3
223.1.9.2
223.1.7.0
223.1.9.1
223.1.7.1
223.1.8.1
223.1.8.0
223.1.2.6
Interconnected
system consisting
of six networks
223.1.2.1
223.1.3.27
223.1.2.2
223.1.3.1
223.1.3.2
Network Layer
4-6
IP Addresses
given notion of “network”, let’s re-examine IP addresses:
“class-full” addressing:
32 bits
class
A
0 network
10
16,777,214
network
16,384 (214)
C
110
1110
128.0.0.0 to
191.255.255.255
host
65,534
network
2,097,151 (221-1)
D
1.0.0.0 to
127.255.255.255
host
126 (27-2)
B
2^32 addresses = 4 billion addresses
multicast address
host
254
192.0.0.0 to
223.255.255.255
224.0.0.0 to
239.255.255.255
Network Layer
4-7
Address Assignment: To ISPs
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
4-8
Address Assignment: To Networks
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
Organization 1
Organization 2
...
11001000 00010111 00010000 00000000
11001000 00010111 00010010 00000000
11001000 00010111 00010100 00000000
…..
….
200.23.16.0/23
200.23.18.0/23
200.23.20.0/23
….
Organization 7
11001000 00010111 00011110 00000000
200.23.30.0/23
Network Layer
4-9
Address Assignment: To Hosts
Q: How does host get IP address?
• hard-coded by system admin in a file
Wintel: control-panel->network->configuration>tcp/ip->properties
• DHCP: Dynamic Host Configuration Protocol:
dynamically get address from as server
 “plug-and-play”

Network Layer 4-10
Masking the Network Portion
A
255.0.0.0
11111111 00000000 00000000 00000000
B
255.255.0.0
11111111 11111111 000 00000 00000000
C
255.255.255.0 11111111 11111111 11111111 00000000
131.229.157.11 = Class B
10000011 11100101 01100111 00001011
11111111 11111111 00000000 00000000
10000011 11100111 00000000 00000000
Smith College = 131.229.0.0
Network Layer
4-11
Class B Sub-Netting: 2 bits
Original Mask
255.255.
128 64 32 16 8 4 2 1
1
1 0 0 0000
New Mask
255.255.192
192
2 Bits
0 0
0 1
1 0
1 1
Address
0-Invalid
64
128
192-Invalid
Range
0-63
64-127
128-191
192-255
Valid
None
65-126
129-190
None
TOTAL =
New Nets
0
1
1
0
2 nets
# Hosts per Net
0
62
62
0
124 hosts
Source: NetCom Systems <www.netliban.com/download/tcpip%20address.doc>
Network Layer 4-12
Class B Sub-Netting: 3 bits
Original Mask
255.255.
128 64 32 16 8 4 2 1
1
1 1 0 0000
New Mask
255.255.224
224
2 Bits
000
001
010
111
100
101
110
111
Address
Range
0-Invalid
0-31
32
32-63
64
64-95
96
96-127
128
128-159
160
160-191
192
192-223
224-Invalid 224-255
Valid
None
33-62
65-94
97-126
129-158
161-190
193-222
None
TOTAL =
New Nets
0
1
1
1
1
1
1
0
6 nets
Source: NetCom Systems <www.netliban.com/download/tcpip%20address.doc>
# Hosts per Net
0
30
30
30
30
30
30
0
180 hosts
Network Layer 4-13
Class “B” Sub-Netting
Subnet
Bits
2
3
4
5
6
Subnets
2-2
2
6
14
30
62
Hosts
Per Net
62
30
14
6
2
Total
Hosts
Subnet Mask
124
180
196
180
124
255.255.192
255.255.224
255.255.240
255.255.248
255.255.252
A Network or Host Address
cannot contain all 0’s or all 1’s.
Source: NetCom Systems <www.netliban.com/download/tcpip%20address.doc>
Network Layer 4-14
The “Classful” Alternative: 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
• CIDR: Classless InterDomain Routing


network portion of address of arbitrary length
address format: a.b.c.d/x, where x is # bits in network portion of
address
network
part
host
part
11001000 00010111 00010000 00000000
200.23.16.0/23
Network Layer 4-15
Hierarchical addressing: route aggregation
Hierarchical addressing allows efficient advertisement of routing
information:
Organization 0
200.23.16.0/23
“Send me anything
with addresses
beginning
200.23.16.0/20”
Organization 1
200.23.18.0/23
Organization 2
200.23.20.0/23
Organization 7
.
.
.
.
.
.
Fly-By-Night-ISP
Internet
200.23.30.0/23
ISPs-R-Us
“Send me anything
with addresses
beginning
199.31.0.0/16”
Network Layer 4-16
Hierarchical addressing: more specific routes
ISPs-R-Us has a more specific route to Organization 1
Organization 0
200.23.16.0/23
Organization 2
200.23.20.0/23
Organization 7
.
.
.
“Send me anything
with addresses
beginning
200.23.16.0/20”
.
.
.
Fly-By-Night-ISP
Internet
200.23.30.0/23
ISPs-R-Us
Organization 1
200.23.18.0/23
“Send me anything
with addresses
beginning 199.31.0.0/16
or 200.23.18.0/23”
Network Layer 4-17
IP Address Quiz
What IP address is contained in 11/8?
• 15.31.184.254
14.50.97.103
• 8.85.29.189
11.150.208.105
What IP address matches 7.74.34.159/32?
• 7.74.39/24
7.74.34.52/32
• 7.74.34/24
Network Layer 4-18
IP Address Quiz
What IP address is contained in 86.32/12?
• 86.33.224.123
86.79.65.216
86.58.119.74
86.68.206.154
What IP address contains 152.7.77.159 and
152.31.47.252?
• 152.0/11
152.40/13
• 153.0/9
152.64.12
Network Layer 4-19
Web Resources
• “whois” demo
• www.iana.org/ipaddress/ip-addresses.htm
• General info + Quizzes
• www.freesoft.org/CIE/Course/Subnet/index.htm
Network Layer 4-20
Chapter 4 roadmap
4.1 Introduction and Network Service Models
4.2 Routing Principles
4.3 Hierarchical Routing
4.4 The Internet (IP) Protocol
 4.4.1 IPv4 addressing
 4.4.2 Moving a datagram from source to destination
 4.4.3 Datagram format
 4.4.4 IP fragmentation
 4.4.5 ICMP: Internet Control Message Protocol
 4.4.6 DHCP: Dynamic Host Configuration Protocol
 4.4.7 NAT: Network Address Translation
4.5 Routing in the Internet
4.6 What’s Inside a Router
4.7 IPv6
4.8 Multicast Routing
4.9 Mobility
Network Layer 4-21
Getting a datagram from source to dest.
forwarding table in A
Dest. Net. next router Nhops
223.1.1
223.1.2
223.1.3
IP datagram:
misc source dest
fields IP addr IP addr
data
* datagram remains
unchanged
A
223.1.1.4
223.1.1.4
1
2
2
223.1.1.1
223.1.2.1
223.1.1.2
223.1.1.4
223.1.2.9
223.1.2.2
223.1.1.3
223.1.3.1
223.1.3.27
E
223.1.3.2
Network Layer 4-22
Getting a datagram from source to dest.
misc
data
fields 223.1.1.1 223.1.1.3
Starting at Amelia, send IP
datagram addressed to
Marga:
Amelia’s Forwarding Table
Dest. Net. next router Nhops
223.1.1
223.1.2
223.1.3
223.1.1.4
223.1.1.4
Amelia
223.1.1.1
look up net. address of Marga in
forwarding table
Marga is on same net. as Amelia
 link layer will send datagram
directly to Marga inside link-layer
frame
1
2
2
223.1.2.1
223.1.1.2
223.1.1.4
223.1.2.9
Marga
223.1.1.3
223.1.3.1
223.1.2.2
223.1.3.27
223.1.3.2
Network Layer 4-23
Getting a datagram from source to dest.
Amelia’s Forwarding Table
misc
data
fields 223.1.1.1 223.1.2.2
Starting at Amelia, dest. to
Sonya :
look up network address of
Sonya in forwarding table
Sonya on different network
i.e. not directly attached
routing table: next hop router
to Sonya is 223.1.1.4
link layer sends datagram to router
223.1.1.4 inside link-layer frame
Dest. Net. next router Nhops
223.1.1
223.1.2
223.1.3
223.1.1.4
223.1.1.4
1
2
2
Amelia
223.1.1.1
223.1.2.1
223.1.1.2
223.1.1.4
223.1.2.9
Sonya
223.1.2.2
223.1.1.3
223.1.3.1
223.1.3.27
223.1.3.2
 datagram arrives at 223.1.1.4
 continued…..
Network Layer 4-24
Getting a datagram from source to dest.
forwarding table in router
misc
data
fields 223.1.1.1 223.1.2.2
Arriving at 223.1.4,
destined for 223.1.2.2
 look up network address of
Dest. Net router Nhops interface
223.1.1
223.1.2
223.1.3
-
1
1
1
router’s interface 223.1.2.9
router, Sonya directly attached
 link layer sends datagram to
223.1.2.2 inside link-layer frame via
interface 223.1.2.9
 Sonya receives datagram
223.1.3.27
Amelia
223.1.1.1
Sonya in forwarding table
 Sonya on same network as
223.1.1.4
223.1.2.9
223.1.2.1
223.1.1.2
223.1.1.4
223.1.2.9
Sonya
223.1.2.2
223.1.1.3
223.1.3.1
223.1.3.27
223.1.3.2
Network Layer 4-25
Smith Routers
Quad
Science
Center
Neilson
Cutter
Stoddard
Seelye
UMass
MHC
Network Layer 4-26
IP datagram format
IP protocol version
number
header length
“type” of data
max number
remaining hops
(decremented at
each router)
upper layer protocol
32 bits
head. type of
length
ver
len service
fragment
flgs
16-bit identifier
offset
upper
time to
Internet
layer
live
checksum
total datagram
length
fragmentation
32 bit source IP address
32 bit destination IP address
Options (if any)
data
(variable length,
typically a TCP
or UDP segment)
E.g. record route
taken, specify
list of routers
to visit.
Network Layer 4-27
Recall IP datagram format
head. type of
length
ver
len service
fragment
flgs
16-bit identifier
offset
upper
time to
Internet
layer
live
checksum
misc source
dest
fields IP addr IP addr
source IP address
data
destination IP address
Options (if any)
data
(variable length,
typically a TCP
or UDP segment)
Network Layer 4-28
How Much Overhead with TCP?
Consider All overhead up till now:
• Application Layer
application
transport
• Transport Layer
• Network Layer
network
link
*Consider application layer at
end, start with transport layer
physical
Network Layer 4-29
Transport Layer
32 bits
20 bytes of TCP, no
options included
source port #
dest port #
sequence number
acknowledgement number
head not
UAP R S F
len used
checksum
Receive window
Urg data pnter
Options (none)
data
(variable length)
Network Layer 4-30
Network Layer
 20 bytes minimum
for IP Datagram
 Also, no Options
 Note: our TCP
segment header
information follows our
IP datagram header
information
ver head. type of
len service
16-bit identifier
upper
time to
layer
live
length
flgs
fragment
offset
Internet
checksum
source IP address
destination IP address
Options (none)
data
(our TCP segment)
Network Layer 4-31
Total TCP Overhead
20 bytes of TCP header
application
20 bytes IP Datagram header
transport
= 40 bytes + application layer
network
overhead
link
physical
Network Layer 4-32
IP Fragmentation & Reassembly
network links have MTU
(max.transfer size)
different link types,
different MTUs
fragmentation:
in: one large datagram
out: 3 smaller datagrams
large IP datagram
data
header
divided (“fragmented”)



one datagram becomes
several datagrams
“reassembled” only at final
destination
IP header bits used to
identify, the order of
fragments
reassembly
Network Layer 4-33
IP Fragmentation and Reassembly
Recall the IP Datagram
included: ID, Flag, and
offset information for
fragmentation.
Example
 4000 byte datagram
 MTU = 1500 bytes
length ID fragflag
=4000 =7
=0
offset
=0
One large datagram becomes
several smaller datagrams
length ID fragflag
=1500 =7
=1
offset
=0
length ID fragflag offset
=1500 =7
=1
=1480
length ID fragflag offset
=1040 =7
=0
=2960
Applet:http://wps.aw.com/wps/media/objects/221/227091/applets/ip/ipfra
gmentation.html
Network Layer 4-34
Quiz: Four Questions
Q: Suppose an application generates chunks of 60 bytes
of data every second, and each chunk gets encapsulated in
a TCP segment and then an IP datagram. What percentage
of each datagram will contain application data?
A. 40%
B. 20%
C. 80%
D. 60%
Ans: D or 60%, this because we will gain 40bytes more from TCP
and IP datagram segments, thus our whole chunk of information
is 100bytes, therefore the application we started with or 60bytes
is 60/100 or 60%.
Network Layer 4-35
Are you Challenged Yet?
Q: Consider sending a 1300 byte datagram
into a link that has an MTU of 500 bytes.
A. Three fragments are created with offsets
0, 500 1000
B. Three fragments are created, with offsets
0, 460 and 920.
C. Three fragments are created with offsets
0, 480 and 960.
D.
None of the above
ANS: C
Network Layer 4-36
You have a 50% Chance!
Q: With a datagram network layer, each packet
carries the address of the destination host. (Note: If
your answer is incorrect we will have to redo the entire lecture)
True
False
Ans: True
Network Layer 4-37
Don’t Forget Brigitte’s Lecture
Q:The network portion of an IP address is the
same for all the hosts on the same IP network.
True
False
Ans: True
Network Layer 4-38
Chapter 4 roadmap
4.1 Introduction and Network Service Models
4.2 Routing Principles
4.3 Hierarchical Routing
4.4 The Internet (IP) Protocol
 4.4.1 IPv4 addressing
 4.4.2 Moving a datagram from source to destination
 4.4.3 Datagram format
 4.4.4 IP fragmentation
 4.4.5 ICMP: Internet Control Message Protocol
 4.4.6 DHCP: Dynamic Host Configuration Protocol
 4.4.7 NAT: Network Address Translation
4.5 Routing in the Internet
4.6 What’s Inside a Router
4.7 IPv6
4.8 Multicast Routing
4.9 Mobility
Network Layer 4-39
The Internet Network layer (recap)
Host, router network layer functions:
Transport layer: TCP, UDP
Network
layer
IP protocol
•addressing conventions
•datagram format
•packet handling conventions
Routing protocols
•path selection
•RIP, OSPF, BGP
forwarding
table
ICMP protocol
•error reporting
•router “signaling”
Link layer
physical layer
4-30
Network Layer 4-40
ICMP: Internet Control Message Protocol
 ICMP is used by the following
for communication in the
network-layer:
o Hosts
o Routers
o Gateways
 RFC 792
 Typically used for error reporting
o Unreachable host, network,
port protocol
o Echo request/reply (in ping)
telnet server
IP router
TCP
software
telnet client
Network Layer 4-41
IP datagram format
32 bits
head. type of
length
ver
len service
fragment
flgs
16-bit identifier
offset
upper
time to
Internet
layer
live
checksum
32 bit source IP address
32 bit destination IP address
Options (if any)
how much overhead
with TCP?
 20 bytes of TCP
 20 bytes of IP
 = 40 bytes + app
layer overhead
data
(variable length,
typically a TCP
or UDP segment)
Network Layer 4-42
ICMP: Internet Control Message Protocol
 ICMP messages are carried in
IP datagrams, so they are
architecturally below IP.
 ICMP message contain:
o Type and code field
o First eight bytes of IP
datagram that caused the
ICMP message generation
 Not only for error messaging
o ping
o traceroute
Type
0
3
3
3
3
3
3
4
Code
0
0
1
2
3
6
7
0
8
9
10
11
12
0
0
0
0
0
description
echo reply (ping)
dest. network unreachable
dest host unreachable
dest protocol unreachable
dest port unreachable
dest network unknown
dest host unknown
source quench (congestion
control - not used)
echo request (ping)
route advertisement
router discovery
TTL expired
bad IP header
Network Layer 4-43
DHCP: Dynamic Host Configuration Protocol

DHCP assigns IP addresses to
hosts dynamically.
 client-server protocol
o client = newly arriving host
wanting to obtain network
config info (including IP
address)
o If DHCP server exists, all is
good
o If DHCP server does not
exist, DHCP relay agent
(typically router) is needed.
o DHCP relay agent knows
address of a DHCP server for
that network

Client-server protocol (cont)
o Renew its lease time
o Allows reuse of addresses
o Support for mobile users who
want to join networks
(continued later)
Network Layer 4-44
DHCP client-server scenario
A
B
223.1.2.1
DHCP
server
223.1.1.1
223.1.1.2
223.1.1.4
223.1.2.9
223.1.2.2
223.1.1.3
223.1.3.1
223.1.3.27
223.1.3.2
E
arriving DHCP
client needs
address in this
network
Network Layer 4-45
DHCP: Dynamic Host Configuration Protocol
Overview of DHCP protocol steps
 DHCP server discovery
o First task = find DHCP
server
o Send DHCP discover
message
 DHCP server offer(s)
o DHCP offer message sent
back to client
o address lease time – valid
time of IP address (may
be renewed)
Overview of DHCP protocol steps
 DHCP request
o New client will choose
among offers and respond
with a request message
o New client echoes config
parameters to server it
chooses
 DHCP ACK
o Server responds with an
acknowledgement,
confirming configuration
parameters
Network Layer 4-46
DHCP: Dynamic Host Configuration Protocol
Network Layer 4-47
DHCP client-server scenario
DHCP server: 131.229.64.27
DHCP discover
arriving
client
src : 0.0.0.0, 68
dest.: 255.255.255.255,67
yiaddr: 0.0.0.0
transaction ID: 654
DHCP offer
src: 131.229.64.27, 67
dest: 255.255.255.255, 68
yiaddrr: 131.229.183.50
transaction ID: 654
Lifetime: 3600 secs
DHCP request
time
src: 0.0.0.0, 68
dest:: 255.255.255.255, 67
yiaddrr: 131.229.183.50
transaction ID: 655
Lifetime: 3600 secs
DHCP ACK
src: 131.229.64.27, 67
dest: 255.255.255.255, 68
yiaddrr: 131.229.183.50
transaction ID: 655
Lifetime: 3600 secs
Network Layer 4-48
DHCP: Dynamic Host Configuration Protocol
Network Layer 4-49
NAT: Network Address Translation
rest of
Internet
local network
(e.g., home network)
192.168.0/24
192.168.0.1
192.168.0.108
192.168.0.109
12.222.208.63
192.168.0.110
All datagrams leaving local
network have same single source
NAT IP address: 12.222.208.63,
different source port numbers
Datagrams with source or
destination in this network
have 192.168.0/24 address for
source, destination (as usual)
Network Layer 4-50
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 4-51
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
Network Layer 4-52
NAT: Network Address Translation
NAT translation table
WAN side addr
LAN side addr
2: NAT router
changes datagram
12.222.208.63, 5001
source addr from
……
192.168.0.108, 3345 to
12.222.208.63, 5001,
updates table
2
1: host 192.168.0.108
sends datagram to
www.yahoo.com, 80
192.168.0.108, 3345
……
S: 192.168.0.108, 3345
D: www.yahoo.com, 80
192.168.0.108
1
S: 12.222.208.63, 5001
D: www.yahoo.com, 80
138.76.29.7
S: www.yahoo.com, 80
D: 12.222.208.63, 5001
3
3: Reply arrives
dest. address:
12.222.208.63, 5001
192.168.0.109
10.0.0.4
S: www.yahoo.com, 80
D: 192.168.0.108, 3345
4
4: NAT router
192.168.0.110
changes datagram
dest addr from
12.222.208.63, 5001 to 192.168.0.108, 3345
Network Layer 4-53
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
Network Layer 4-54
NAT Manipulation in the IP datagram
NAT
translation
original
head. type of
length
ver
len service
fragment
flgs
16-bit identifier
offset
upper
time to
Internet
layer
live
checksum
head. type of
length
len service
fragment
16-bit identifier flgs
offset
upper
time to
Internet
layer
live
checksum
ver
192.168.0.108:3345
12.222.208.63:5001
www.yahoo.com:80
www.yahoo.com:80
Options (if any)
Options (if any)
data
(variable length,
typically a TCP
or UDP segment)
data
(variable length,
typically a TCP
or UDP segment)
Network Layer 4-55