Advanced Computer Networks: Lecture 19 - Dr. Amir Qayyum

CS716
Advanced Computer Networks
By Dr. Amir Qayyum
1


Lecture No. 19


Fragmentation and Reassembly 
Example
H1

R1

R2

R3

H8
Start of header
Ident= x

1

Offset= 0

Rest of header

ETH IP (1400)

FDDI IP (1400)

PPP IP (512)

ETH IP (512)

PPP IP (512)

ETH IP (512)

PPP IP (376)

ETH IP (376)

512 data bytes
Start of header
Ident= x

1 Offset= 64
Rest of header
512 data bytes

Start of header
Ident= x

0

Offset= 0

Rest of header

1400 data bytes

Start of header
Ident= x

0 Offset= 128
Rest of header
376 data bytes

3


Datagram Forwarding
Network Number

Next Hop

1

R3

2

R1

3

Interface 1

4

Interface 0

4


Datagram Forwarding
Network #

Netmask

18.0.0.0
128.32.0.0
0.0.0.0

255.0.0.0
255.255.0.0
0.0.0.0

dest: 18.26.10.0

mask with 255.0.0.0

          Next Hop/Port
1
2
3
matched! send to port 1

dest: 128.16.14.0 mask with 255.0.0.0 not matched

mask with 255.255.0.0 not matched
matched! send to port 3
mask with 0.0.0.0
5


ARP Packet Format
0

8

16

Hardware type = 1
HLen = 48

31
Protocol

PLen = 32

Type = 0x0800

Operation

SourceHardwareAddr (bytes 0

– 3)

SourceHardwareAddr (bytes 4

– 5) SourceProtocolAddr (bytes 0

– 1)

SourceProtocolAddr (bytes 2

– 3) TargetHardwareAddr (bytes 0

– 1)

TargetHardwareAddr (bytes 2
TargetProtocolAddr (bytes 0

– 5)
– 3)

6


ARP in ATM
10.0.0.2
H1

R
12.0.0.3

10.0.0.1

12.0.0.5

H2

LIS 10
LIS 12

ATM network

7


Internet Control Message Protocol 
(ICMP)
• IP companion protocol (not necessary)
• Handles error and control messages
FTP

HTTP

NV

TFTP

UDP

TCP

ICMP

IP

FDDI

Ethernet

…

ATM
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ICMP Message

• Sent to the source when a node is unable to 
process IP datagram successfully
• Error messages
–
–
–
–
–

Destination unreachable (protocol, port, or host)
Reassembly failed
IP Checksum failed; or invalid header
TTL exceeded (so datagrams don’t cycle forever)
Cannot fragment

• Control messages

– Echo (ping) request and reply

– Redirect (from router to source host, to change 
route)

9


Dynamic Host Configuration 
Protocol­ DHCP
• DHCP server is required to provide 
configuration information to each host
– Each host retrieve this information on bootup

• DHCP server can be configured manually, or 
it may allocate addresses on­demand
– Addresses are “leased” for some period of time

• Each host is not configured for DHCP server, 
it performs a DHCP server discovery
– A broadcast discovery message is sent by the host 
10
and a unicast reply is sent by the server


DHCP Server
Unicast to server

Broadcast

DHCP
relay

Other networks

DHCP
server

Host

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