1. Chapter 4 Network Layer A note on the use of these ppt slides:
We’re making these slides freely available to all (faculty, students, readers). They’re in PowerPoint form so you can add, modify, and delete slides (including this one) and slide content to suit your needs. They obviously represent a lot of work on our part. In return for use, we only ask the following:
If you use these slides (e.g., in a class) in substantially unaltered form, that you mention their source (after all, we’d like people to use our book!)
If you post any slides in substantially unaltered form on a www site, that you note that they are adapted from (or perhaps identical to) our slides, and note our copyright of this material.
Modified by Melissa, Brigitte, and Amy
Thanks and enjoy! JFK/KWR
All material copyright
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

2. 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

3. 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 service models
Routing algorithms commonly used to determine paths between source and destination
Use of hierarchy to address the problem of scale.
Explore the network layer of the Internet
Pieces of which are often collectively referred to as the IP layer named after the IP protocol which is actually only one piece of the network layer.
Network Layer
Network Layer
4-3 3

4. What is an IP Address?
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
Network Layer
Network Layer
4-4 5

5. 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
LAN
network consisting of 3 IP networks
(for IP addresses starting with 223,
first 24 bits are network address)
Network Layer
Network Layer
4-5 6

6. IP Addressing How to find the networks?
How to find the networks?
Detach each interface from router, host
create “islands of isolated networks
Interconnected
system consisting
of six networks
Network Layer
Network Layer
4-6 7

7. IP Addresses given notion of “network”, let’s re-examine IP addresses:
“class-full” addressing:
2^32 addresses = 4 billion addresses
32 bits
network
host A
class
126 (27-2)
16,777,214 to 10
network
host B
16,384 (214)
65,534 to
110
network
host C
2,097,151 (221-1) 254 to
1110
multicast address D to
Network Layer
Network Layer
4-7 8

8. 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
Network Layer
4-8 9

9. 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 /20
Organization /23
Organization /23
Organization /23
… … ….
Organization /23
Network Layer
Network Layer
4-9 10

10. 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
Network Layer
4-10 11

11. Masking the Network Portion
= Class B
Smith College =
Network Layer
Network Layer
4-11 12

12. Class B Sub-Netting: 2 bits
Original Mask New Mask
192
2 Bits Address Range Valid New Nets # Hosts per Net
Invalid None
Invalid None
TOTAL = 2 nets 124 hosts
Source: NetCom Systems <
Network Layer
Network Layer
4-12 13

13. Class B Sub-Netting: 3 bits
Original Mask New Mask
224
2 Bits Address Range Valid New Nets # Hosts per Net
000 0-Invalid None
Invalid None
TOTAL = 6 nets 180 hosts
Source: NetCom Systems <
Network Layer
Network Layer
4-13 14

14. Class “B” Sub-Netting A Network or Host Address
cannot contain all 0’s or all 1’s.
Source: NetCom Systems <
Network Layer
Network Layer
4-14 15

15. 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
/23
Network Layer
Network Layer
4-15 16

16. Hierarchical addressing: route aggregation
Hierarchical addressing allows efficient advertisement of routing
information:
Organization 0
/23
“Send me anything
with addresses
beginning
/20”
Organization 1
/23
Organization 2
/23 .
Fly-By-Night-ISP .
Internet
Organization 7
/23
“Send me anything
with addresses
beginning
/16”
ISPs-R-Us
Network Layer
Network Layer
4-16 17

17. Hierarchical addressing: more specific routes
ISPs-R-Us has a more specific route to Organization 1
Organization 0
/23
“Send me anything
with addresses
beginning
/20”
Organization 2
/23 .
Fly-By-Night-ISP .
Internet
Organization 7
/23
“Send me anything
with addresses
beginning /16
or /23”
ISPs-R-Us
Organization 1
/23
Network Layer
Network Layer
4-17 18

18. IP Address Quiz What IP address is contained in 11/8?
What IP address matches /32?
/ /32
/24
Network Layer
Network Layer
4-18 19

19. IP Address Quiz What IP address is contained in 86.32/12?
What IP address contains and ?
152.0/ /13
153.0/
Network Layer
Network Layer
4-19 20

20. Web Resources “whois” demo General info + Quizzes
General info + Quizzes
Network Layer
Network Layer
4-20 21

21. 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

22. Getting a datagram from source to dest.
forwarding table in A
Dest. Net. next router Nhops
IP datagram:
misc
fields
source
IP addr
dest
data A
* datagram remains unchanged E
Network Layer

23. Getting a datagram from source to dest.
Amelia’s Forwarding Table
misc
fields
Dest. Net. next router Nhops
data
Starting at Amelia, send IP datagram addressed to Marga:
Amelia
look up net. address of Marga in
forwarding table
Marga is on same net. as Amelia
Marga
link layer will send datagram directly to Marga inside link-layer frame
Network Layer

24. Getting a datagram from source to dest.
Amelia’s Forwarding Table
misc
fields
Dest. Net. next router Nhops
data
Starting at Amelia, dest. to Sonya :
look up network address of
Sonya in forwarding table
Amelia
Sonya on different network
i.e. not directly attached
routing table: next hop router
to Sonya is
Sonya
link layer sends datagram to router
inside link-layer frame
datagram arrives at
continued…..
Network Layer

25. Getting a datagram from source to dest.
forwarding table in router
Dest. Net router Nhops interface
misc
fields
data
Arriving at ,
destined for
Amelia
look up network address of
Sonya in forwarding table
Sonya on same network as
router’s interface
router, Sonya directly attached
Sonya
link layer sends datagram to inside link-layer frame via interface
Sonya receives datagram
Network Layer

26. Smith Routers Quad Cutter Science Center Neilson Stoddard Seelye UMass
MHC
Network Layer

27. 32 bit destination IP address
IP datagram format
IP protocol version
number
32 bits
total datagram
length
header length
head.
len
type of
service
ver
length
fragmentation
“type” of data
fragment
offset
16-bit identifier
flgs
max number
remaining hops
(decremented at
each router)
time to
live
upper
layer
Internet
checksum
32 bit source IP address
32 bit destination IP address
upper layer protocol
Options (if any)
E.g. record route
taken, specify
list of routers
to visit.
data
(variable length,
typically a TCP
or UDP segment)
Network Layer

28. Recall IP datagram format
head.
len
type of
service
ver
length
fragment
offset
16-bit identifier
flgs
time to
live
upper
layer
Internet
checksum
misc
fields
source
IP addr
dest
data
source IP address
destination IP address
Options (if any)
data
(variable length,
typically a TCP
or UDP segment)
Network Layer

29. How Much Overhead with TCP?
Consider All overhead up till now:
application
transport
network
link
physical
• Application Layer
• Transport Layer
• Network Layer
*Consider application layer at
end, start with transport layer
Network Layer

30. acknowledgement number
Transport Layer
source port #
dest port #
32 bits
data
(variable length)
sequence number
acknowledgement number
Receive window
Urg data pnter
checksum F S R P A U
head
len
not
used
Options (none)
20 bytes of TCP, no
options included
Network Layer

31. destination IP address
Network Layer
head.
len
type of
service
ver
20 bytes minimum
for IP Datagram
Also, no Options
Note: our TCP segment header information follows our IP datagram header information
length
fragment
offset
16-bit identifier
flgs
time to
live
upper
layer
Internet
checksum
source IP address
destination IP address
Options (none)
data
(our TCP segment)
Network Layer

32. Total TCP Overhead application 20 bytes of TCP header transport
network
link
physical
20 bytes of TCP header
20 bytes IP Datagram header
= 40 bytes + application layer
overhead
Network Layer

33. 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
divided (“fragmented”)
data
header
one datagram becomes several datagrams
“reassembled” only at final destination
IP header bits used to identify, the order of fragments
reassembly
Network Layer

34. IP Fragmentation and Reassembly
Recall the IP Datagram included: ID, Flag, and offset information for fragmentation. ID =7
offset =0
fragflag
length
=4000 =1
=1500
=1480
=2960
=1040
One large datagram becomes
several smaller datagrams
Example
4000 byte datagram
MTU = 1500 bytes
Applet:
Network Layer

35. 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

36. Are you Challenged Yet?
Q: Consider sending a 1300 byte datagram into a link that has an MTU of 500 bytes. 
Three fragments are created with offsets 0,
Three fragments are created, with offsets 0, 460 and 920.
Three fragments are created with offsets 0, 480 and 960.
  None of the above
ANS: C
Network Layer

37. You have a 50% Chance! Q: With a datagram network layer, each packet  
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

38. 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

39. 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

40. The Internet Network layer (recap)
Host, router network layer functions:
Transport layer: TCP, UDP
IP protocol
addressing conventions
datagram format
packet handling conventions
Routing protocols
path selection
RIP, OSPF, BGP
Network
layer
forwarding
table
ICMP protocol
error reporting
router “signaling”
Network layer protocol, commonly known as IP.
Network-layer addressing
Fields in the datagram
Actions taken by routers and end systems on a datagram based on values in these fields.
There are 2 versions of Internet Protocol which we will be studying in this class: v4 (today) v6 (chapter 4.7)
Path determination component that determines the route a datagram follows from source to destination.
In sec 4.2 we saw that routing protocols compute the forwarding tables that are used to route packets through the network.
Path determination component in 4.5
Facility to report errors in datagrams
Respond to requests for certain network-layer information.
Internet Control Message Protocol (ICMP) at the end of this section
Link layer
physical layer
Network Layer
Network Layer
4-30 4

41. ICMP: Internet Control Message Protocol
ICMP is used by the following for communication in the network-layer:
Hosts
Routers
Gateways
RFC 792
Typically used for error reporting
Unreachable host, network, port protocol
Echo request/reply (in ping)
telnet server
IP router
telnet client
telnet client
TCP
software
Network Layer

42. 32 bit destination IP address
IP datagram format
32 bits
head.
len
type of
service
ver
length
fragment
offset
16-bit identifier
flgs
time to
live
upper
layer
Internet
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

43. ICMP: Internet Control Message Protocol
ICMP messages are carried in IP datagrams, so they are architecturally below IP.
ICMP message contain:
Type and code field
First eight bytes of IP datagram that caused the ICMP message generation
Not only for error messaging
ping
traceroute
Type Code 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

44. DHCP: Dynamic Host Configuration Protocol
DHCP assigns IP addresses to hosts dynamically.
client-server protocol
client = newly arriving host wanting to obtain network config info (including IP address)
If DHCP server exists, all is good
If DHCP server does not exist, DHCP relay agent (typically router) is needed.
DHCP relay agent knows address of a DHCP server for that network
Client-server protocol (cont)
Renew its lease time
Allows reuse of addresses
Support for mobile users who want to join networks (continued later)
Network Layer

45. DHCP client-server scenario
server B
arriving DHCP
client needs
address in this
network E
Network Layer

46. DHCP: Dynamic Host Configuration Protocol
Overview of DHCP protocol steps
DHCP server discovery
First task = find DHCP server
Send DHCP discover message
DHCP server offer(s)
DHCP offer message sent back to client
address lease time – valid time of IP address (may be renewed)
Overview of DHCP protocol steps
DHCP request
New client will choose among offers and respond with a request message
New client echoes config parameters to server it chooses
DHCP ACK
Server responds with an acknowledgement, confirming configuration parameters
Network Layer

47. DHCP: Dynamic Host Configuration Protocol
Network Layer

48. DHCP client-server scenario
DHCP server:
arriving
client
DHCP discover
src : , 68
dest.: ,67
yiaddr:
transaction ID: 654
DHCP offer
src: , 67
dest: , 68
yiaddrr:
transaction ID: 654
Lifetime: 3600 secs
DHCP request
src: , 68
dest:: , 67
yiaddrr:
transaction ID: 655
Lifetime: 3600 secs
time
DHCP ACK
src: , 67
dest: , 68
yiaddrr:
transaction ID: 655
Lifetime: 3600 secs
Network Layer

49. DHCP: Dynamic Host Configuration Protocol
Network Layer

50. NAT: Network Address Translation
rest of
Internet
local network
(e.g., home network)
/24
All datagrams leaving local
network have same single source NAT IP address: ,
different source port numbers
Datagrams with source or
destination in this network
have /24 address for
source, destination (as usual)
Network Layer

51. 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

52. 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

53. NAT: Network Address Translation
NAT translation table
WAN side addr LAN side addr
1: host
sends datagram to 80
2: NAT router
changes datagram
source addr from
, 3345 to
, 5001,
updates table
, , 3345
…… ……
S: , 3345
D: 80 1
S: 80
D: , 3345 4
S: , 5001
D: 80 2
S: 80
D: , 5001 3
4: NAT router
changes datagram
dest addr from
, 5001 to , 3345
3: Reply arrives
dest. address:
, 5001
Network Layer

54. 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

55. NAT Manipulation in the IP datagram
translation
original
head.
len
type of
service
head.
len
type of
service
ver
ver
length
length
fragment
offset
fragment
offset
16-bit identifier
flgs
16-bit identifier
flgs
time to
live
upper
layer
Internet
checksum
time to
live
upper
layer
Internet
checksum
:3345
:5001
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