LECTURE 5

Internet Protocol

Chapter 18 Internetwork Protocols

(William Stallings, Data and Computer Communications, 7th
Edition)
Chapter 20 Network Layer: Internet Protocol
(Forouzan, Data Communications and Networking, 4th Edition)
1
Contents
• I. Internetworking
• II. IPv4
• III. IPv6
• IV. Transition form IPv4 to IPv6

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20-1 INTERNETWORKING

In this section, we discuss internetworking, connecting

networks together to make an internetwork or an
internet..
internet

Topics discussed in this section:

Need for Network Layer
Internet as a Datagram Network
Internet as a Connectionless Network

3
 Figure 20.1 Links between two hosts

•LAN/WAN only delivers frame within its own network

•Not capable of routing packet to another network on its own 4
Figure 20.2 Network layer in an internetwork

• Network layer provides a glue for sending packets from

one LAN/WAN to another LAN/WAN 5
Figure 20.3 Network layer at the source, router, and destination

• Create a packet with header • Verify destination address

(source and destination address) • Reassemble fragments
• Locate outgoing interface
6
Figure 20.3 Network layer at the source, router, and destination (2)

• Look up routing table and identify

outgoing interface
• Universal network layer address
is used to route packet from
source to destination
• Each packet is treated
independently and may be routed
differently
• Connectionless is much simpler in
a heterogeneous network

• Switching at the network layer in the Internet uses

the datagram approach to packet switching.
• Communication at the network layer in the Internet is
connectionless. 7
 20-2 IPv4

The Internet Protocol version 4 (IPv4

IPv4) is the delivery
mechanism used by the TCP/IP protocols.
protocols.
• Provide unreliable and connectionless datagram service
• Best effort delivery (like post office)
• Assume unreliable underlying network
• Additional service (TCP) can be used for reliable data transfer

Topics discussed in this section:

Datagram
Fragmentation
Checksum
Options
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Figure 20.4 Position of IPv4 in TCP/IP protocol suite

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Figure 20.5 IPv4 datagram format

• HLEN = Header length in 4-byte unit

• Total length includes both header and data in byte 10
Figure 20.6 Service type or differentiated services

• Legacy • Codepoint = 000xxx

• Precedence = datagram • Same interpretation
priority under congested as service type
situation • Used for service called
DiffServ to provide
quality of service

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 Table 20.1 Types of service

• The precedence subfield was part of version 4, but

never used
• Only 1 bit can have value of 1
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Table 20.2 Default types of service

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Table 20.3 Values for codepoints

Codepoint Assigning Authority

XXXXX0 Internet
XXXX11 Local
XXXX01 Temporary

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Figure 20.7 Encapsulation of a small datagram in an Ethernet frame

• The total length field defines the total length of

the datagram including the header
• Ethernet has a restriction on minimum frame
size (46 bytes)
• Padding is needed

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Figure 20.8 Protocol field and encapsulated data

• Identification, flags, and fragmentation offset are used

for fragmentation
• Time-to-live restricts maximum number of hops that a
packet can travel
• Protocol indicates which higher layer protocol (TCP, UDP,
ICMP, etc.) should receive this packet
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Table 20.4 Protocol values

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 Example 20.1

An IPv4 packet has arrived with the first 8 bits as shown:

01000010
The receiver discards the packet. Why?

Solution
There is an error in this packet. The 4 leftmost bits (0100)
show the version, which is correct. The next 4 bits (0010)
show an invalid header length (2 × 4 = 8). The minimum
number of bytes in the header must be 20. The packet has
been corrupted in transmission.
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Example 20.2

In an IPv4 packet, the value of HLEN is 1000 in binary.

How many bytes of options are being carried by this
packet?

Solution
The HLEN value is 8, which means the total number of
bytes in the header is 8 × 4, or 32 bytes. The first 20 bytes
are the base header, the next 12 bytes are the options.

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 Example 20.3

In an IPv4 packet, the value of HLEN is 5, and the value of

the total length field is 0x0028. How many bytes of data
are being carried by this packet?

Solution
The HLEN value is 5, which means the total number of
bytes in the header is 5 × 4, or 20 bytes (no options). The
total length is 40 bytes, which means the packet is carrying
20 bytes of data (40 − 20).

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 Example 20.4

An IPv4 packet has arrived with the first few hexadecimal

digits as shown.
0x45000028000100000102 . . .
How many hops can this packet travel before being
dropped? The data belong to what upper-layer protocol?

Solution
To find the time-to-live field, we skip 8 bytes. The time-to-
live field is the ninth byte, which is 01. This means the
packet can travel only one hop. The protocol field is the
next byte (02), which means that the upper-layer protocol is
IGMP. 21
Figure 20.9 Maximum transfer unit (MTU)

• Maximum datagram size is restricted by

hardware and software used in the network

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Table 20.5 MTUs for some networks

• Transmission is more efficient with large packet

• But if necessary, datagram can be fragmented
several times before reaching the destination 23
Figure 20.10 Flags used in fragmentation

• Fragment is necessary because format and size

of frame depend on physical layer protocol
• M = 1 means more fragment
• Fragmentation offset indicates offset of the data
in original datagram measured in units of 8-byte

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Figure 20.11 Fragmentation example

• Required part of header must be copied to all fragments

• Identification/Source address must be unique
— All fragments share the same identification number
• Flag, fragmentation offset, and total length are changed
— And checksum recalculated
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Figure 20.12 Detailed fragmentation example

Fragment can also

be fragmented
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 Example 20.5

A packet has arrived with an M bit value of 0. Is this the

first fragment, the last fragment, or a middle fragment? Do
we know if the packet was fragmented?

Solution
If the M bit is 0, it means that there are no more fragments;
the fragment is the last one. However, we cannot say if the
original packet was fragmented or not. A non-fragmented
packet is considered the last fragment.

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 Example 20.6

A packet has arrived with an M bit value of 1. Is this the

first fragment, the last fragment, or a middle fragment? Do
we know if the packet was fragmented?

Solution
If the M bit is 1, it means that there is at least one more
fragment. This fragment can be the first one or a middle
one, but not the last one. We don’t know if it is the first one
or a middle one; we need more information (the value of
the fragmentation offset).
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 Example 20.7

A packet has arrived with an M bit value of 1 and a

fragmentation offset value of 0. Is this the first fragment,
the last fragment, or a middle fragment?

Solution
Because the M bit is 1, it is either the first fragment or a
middle one. Because the offset value is 0, it is the first
fragment.

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Example 20.8

A packet has arrived in which the offset value is 100. What

is the number of the first byte? Do we know the number of
the last byte?

Solution
To find the number of the first byte, we multiply the offset
value by 8. This means that the first byte number is 800. We
cannot determine the number of the last byte unless we
know the length.

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 Example 20.9

A packet has arrived in which the offset value is 100, the

value of HLEN is 5, and the value of the total length field
is 100. What are the numbers of the first byte and the last
byte?
Solution
The first byte number is 100 × 8 = 800. The total length is
100 bytes, and the header length is 20 bytes (5 × 4), which
means that there are 80 bytes in this datagram. If the first
byte number is 800, the last byte number must be 879.

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 Example 20.10

Figure 20.13 shows an example of a checksum calculation

for an IPv4 header without options. The header is divided
into 16-bit sections. All the sections are added and the sum
is complemented. The result is inserted in the checksum
field.

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Figure 20.5 IPv4 datagram format

• HLEN = Header length in 4-byte unit

• Total length includes both header and data in byte 33
Figure 20.13 Example of checksum calculation in IPv4

• Checksum is
used to detect
error in header
• Save on
processing
time

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Figure 20.14 Taxonomy of options in IPv4

Filler

Padding

• Strict source route: All routers must be visited

• Loose source route: Can also visit other routers
• Timestamp records time of processing by router 35
 20-3 IPv6

The network layer protocol in the TCP/IP protocol

suite is currently IPv
IPv4
4. Although IPv
IPv44 is well designed,
data communication has evolved since the inception of
IPv44 in the 1970s
IPv 1970s. IPv4
IPv4 has some deficiencies that
make it unsuitable for the fast
fast--growing Internet
Internet..

Topics discussed in this section:

Advantages
Packet Format
Extension Headers

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IPv6 Advantages
• Larger address space
• Better header format allows routers to ignore options
• New options
• Allowance for protocol extensions
• Support for resource allocation and reservation
— Enabling source to request special handling
— Good for real-time audio and video
• Support for more security
— Encryption and Authentication options
• Integrate other protocols (ARP, RARP, IGMP) into
ICMPv6

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Figure 20.15 IPv6 datagram header and payload

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Figure 20.16 Format of an IPv6 datagram

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IPv6 Flow Label
• Flow label provides special handling for a
particular data flow
• Flow shares the same path, uses the same
resource, and has the same kind of security
—Separate protocol is needed to setup how router treat
each flow, such as Real-Time Protocol (RTP) or
Resource Reservation Protocol (RSVP)
—Special treatment for each flow such as high
bandwidth, large buffer, long processing time, and
resource reservation
• Flow can also be used to speed up processing
• Use a random 24-bit number or zero if not
supported
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IPv6 Fields
• Payload length excludes base header
• Next header indicates optional extension
headers of header of encapsulated packet (UDP)
—Also exist in each extension header
• Hop limit is the same as Time-to-live field
• Source and destination addresses are 128 bits
—Destination address = next router for source routing

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Table 20.6 Next header codes for IPv6

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IPv6 Priority
• Indicate packet priority with respect to other
packets from the same source
• Two sets of values
—Congestion-controlled traffic if source can adapt itself
to traffic slowdown when there is congestion
—Noncongestion-controlled traffic if retransmission is
mostly impossible

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Table 20.7 Priorities for congestion-controlled traffic

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Table 20.8 Priorities for noncongestion-controlled traffic

• Data with least redundancy indicates that

discarding a packet can lead to large drop in
quality
• Used for low-fidelity audio or low bit-rate video

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Table 20.9 Comparison between IPv4 and IPv6 packet headers

lower

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Figure 20.17 Extension header types

Need to pass information

to all routers visited
> 65535 Bytes
Strict & loose source routes

Validate sender &

Ensure data integrity

• Only original source can fragment packet

• Prevented by using path MTU discovery technique
• Intermediate router is not permitted to look at
Destination option 47
Table 20.10 Comparison between IPv4 options and IPv6 extension headers

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20-4 Transition from IPv4 to IPv6

Because of the huge number of systems on the

Internet, the transition from IPv4 IPv4 to IPv
IPv66 cannot
happen suddenly.
suddenly. It takes a considerable amount of
time before every system in the Internet can move from
IPv44 to IPv
IPv IPv66. The transition must be smooth to prevent
any problems between IPvIPv4 4 and IPv6
IPv6 systems.
systems.

Topics discussed in this section:

Dual Stack
Tunneling
Header Translation
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Figure 20.18 Three transition strategies

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Figure 20.19 Dual stack

• Use IPv4 stack if DNS return IPv4 address

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Figure 20.20 Tunneling strategy

• Protocol value of IPv4 header is set to 41 (IPv6)

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Figure 20.21 Header translation strategy

• Majority of users has moved to IPv6

• Use mapped IPv6 address

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Figure 19.18 Reserved addresses in IPv6

• Compatible is used when two IPv6 nodes communicate via

IPv4 network
• Mapped is used when IPv6 node communicates with IPv4
node 54
Table 20.11 Header translation

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