IPv4 uses a datagram format with a header and data. The header contains information for routing and delivery and is 20-60 bytes. It includes fields for the version, length, identification, fragmentation, protocol, and source/destination addresses. Datagrams can be fragmented into smaller pieces if their size exceeds the MTU of a network. Fragments are reassembled at the destination using the identification field. The time to live field limits the number of hops a packet can make to prevent endless routing.

1. Internet Protocol Version 4 (IPv4)

2. Learning Outcome
• At the end of this lecture, students will be
able to,
– Explain the format of an IPv4 datagram.
– Describe Fragmentation and reassembly of
datagrams.
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3. • Where IP is positioned in the TCP/IP
protocol suite?
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Recall
Fig. 1. Network Layer
IGMP
IP
ARP
ICMP

4. Datagrams
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• Datagram – packets in Internet i.e. Network Layer.
• Variable-length packet.
• Includes two parts-
– Header
– Data
• Header is 20 to 60 bytes in length which contains
information which is essential for routing and
delivery.

5. Internet Protocol(IP)
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• Major protocol in TCP/IP Protocol suite.
• Works at Network Layer of OSI model and at Internet
Layer of TCP/IP model.
• Responsibility
– Identification of hosts based on their logical addresses
– Route data among them over the network
• Provides mechanism for identification of hosts by using an
IP addressing scheme.
Fig. 2. IP Datagram
DataHeader
20-60 bytes
20 - 65535 bytes

6. IP Header Format
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Fig. 3. IP Header Format
VER
4 bits
HLEN
4 bits
Service type
8 bits
Total length
16 bits
Identification
16 bits
Flags
3 bits
Fragmentation offset
13 bits
Time to live
8 bits
Protocol
8 bits
Header checksum
16 bits
Source IP address
Destination IP address
Options + padding
(0 to 40 bytes)
0 3 4 7 8 15 16 31

7. 7
IP Header Format
• VER(4 bits)
– Version number of internet protocol used.
• HLEN(4 bits)
– Length of the entire IP header.
• Service type(8 bits)
– Services code points which indicates type of service.
• Total length(16 bits)
– Length of entire IP packet including IP header and IP payload.
• Identification(16 bits)
– Used to identify the all the fragments of original IP packet.
– IP packet is fragmented during the transmission.

8. 8
IP Header Format
• Flags(3 bits)
– If IP packet is too large then these flags tells if they can be
fragmented or not.
• Fragmentation offset(13 bits)
– Used to indicate the exact position of the fragment in the original
IP packet.
• Time to live(8 bits)
– Tells network that how many hops (routers) that packet can cross.
• Protocol(8 bits)
– Tells network layer at destination host that to which protocol this
packet belongs to i.e. next level protocol.(1-ICMP,6-TCP,17-UDP)

9. 9
IP Header Format
• Header checksum(16 bits)
– Keeps the checksum value for entire header.
– Used to check if the received packet is error free or not.
• Source IP address(32 bits)
– Address of the sender of the packet.
• Destination IP address(32 bits)
– Address of the receiver of the packet.
• Options
– It may contains values for options such as Security, Record route,
Time stamp, etc.

10. 10
• Datagram travel through different networks.
• Each network has its own MTU(Maximum Transfer Unit).
• If MTU is smaller than the received IP Packet size then
fragmentation is done by the router.
• Every router does the decapsulation of the IP datagram
from the frame it receives, process it, and again
encapsulate it into the another frame.
• The format and size of the received and sent frame
depends on the protocol used by the physical network
through which frame has just traveled or the frame is going
to travel respectively.
Fragmentation

11. 11
4020
14,567 1 000
Bytes 0000-3999
Fragmentation Example
1420
14,567 1 000
Bytes 0000-1399
1420
14,567 1 175
Bytes 1400-2799
1220
14,567 0 350
Bytes 2800-3999
820
14,567 1 175
Bytes 1400-2199
620
14,567 1 275
Bytes 2200-2799
Original Datagram
Fragment 2.2
Fragment 2.1
Fragment 3
Fragment 2
Fragment 1

12. 12
• Reconstruction of original datagram from multiple
fragments.
• Is done at the end nodes or destination only.
• Which reduces unnecessary work where large packets are
fragmented multiple times if reassembly is done at every
node.
• Need buffer for reassembly at receiver side.
• Fragments may arrive in any order. Some fragments may
never arrive then all fragments received with same
identification number are discarded.
Reassembly

13. References
• “Computer Networks” by Andrew S.
Tanenbaum 4th edition by Pearson.
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14. Thank You
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