Advanced Computer Networking Guide

Advanced Computer Network
Subject Code-22520
Examination Scheme
Theory Practical
ESE PA Total ESE PA Total
Max Min Max Min Max Min Max Min Max Min Max Min
70 28 30* 00 100 40 25# 10 25 10 50 20

Syllabus
Unit
No.
Unit Title Total Marks
1 Network Layer and Protocols 08
2 Next Generation IP 10
3 Unicast and Multicast Routing Protocols 14
4 Transport Layer Protocols 18
5 Application Layer Protocols 20

Unit -1:
Network Layer and Protocols Total Marks-8
Topics and Sub-topics
1.1 IP Addressing : Address Space, Notations, Classfull addressing,
Classless addressing, Network Address Translation (NAT).
1.2 Internet Protocol (IP): Datagram format, Fragmentation, Options.
1.3 ICMPv4 : Messages, Debugging Tools, ICMP Checksum.
1.4 Mobile IP: Addressing, Agents, Three Phases, Inefficiency in
Mobile IP.
1.5 Virtual Private Network: VPN Technology.

. Revision
• Network- A network is a set of devices (often referred to as nodes) connected by communication
links.
• A node can be a computer, printer, or any other device capable of sending and/or receiving data
generated by other nodes on the network. A link can be a cable, air, optical fiber, or any medium which
can transport a signal carrying information.
• Network Criteria
1. Performance: Can be measured by transit time and response time.
• Transit time is the amount of time required for a message to travel from one device to
another.
• Response time is the elapsed time between an inquiry and a response.
• The performance of a network depends on the number of users, the type of transmission
medium, the capacities of the connected hardware and the efficiency of the software.
2. Reliability: Is measured by the frequency of failures, the time it takes a link to recover from
failure and the network robustness in a catastrophe.
3. Security: This refers to the ability to protect data from unauthorized access

. Protocols :
• A protocol is synonymous with rule. It consists of a set of rules that
govern data communications. It determines what is communicated,
how it is communicated and when it is communicated.
• Network Models:
1. OSI Reference Model
2. TCP/IP Model

. Network Layer
• Position of Network Layer

• Duties/Functions of Network Layer

• Duties/Functions of Network Layer Continue…
• Main responsibility of Network layer is to carry the data packets from the source
to the destination without changing or using it. If the packets are too large for
delivery, they are fragmented i.e., broken down into smaller packets.
1. Internetworking: It provides the logical connection between different types of
the network.
2. Addressing: Addressing is help us to identify each device present on the internet
uniquely. It is similar to a telephone system.
The address used in the network layer should be uniquely and universally
define the connection of a computer.

• Duties/Functions of Network Layer Continue….
3. Routing: In a network, there are multiple routes available from a source
to destination and one of them is to be chosen. The network layer
decides, Which route to be taken. This is called routing.
4. Packetizing: The network layer encapsulates the packets received from
upper-layer protocols and makes new packets. This is
called Packetizing. It is done by a network layer protocol
called IP (Internet Protocols).
5. Fragmenting: The sent datagram can travel through different networks.
Each router decapsulates the IP datagram from the received frame.
Then datagram is processed and encapsulated in another frame.

• Addressing Mechanism:
• It is the prime responsibility of the network layer to assign unique addresses
to different nodes in a network.
• Four levels of addresses are used in an Internet employing TCP/IP Protocol:
1. Physical Address or Link Address (MAC Address): It is the address of a node
defined by its LAN or WAN. A Media Access Control address is a hardware
identifier that uniquely identifies each device on a network. Often found on
a device’s NIC card. Physical address can be either unicast (one single
recipient) , multicast (a group of recipient), or broadcast ( to be received by
all systems in the network).
2. Logical Address : In the Internet a 32-bits or 128-bits address that can
uniquely defined a host connected to the internet. The logical addresses
can be either unicast, multicast or broadcast.

• Addressing Mechanism: Continue…….
3. Port Address: In the TCP/IP architecture, the label assigned to a process is
called a port address. It is 16-bits in length.
4. Application Specific Addresses: they are designed for specific application
such as e-mail addresses and the Universal Resource Locator(URL) e.g.
www.msbte.com.

1.1 IP Addressing
• IP addressing is the method used to identify hosts and network devices.
• Usually, computers communicate through the Internet. The packet(data)
transmitted by the sender computer may pass through several LANs or
WANs before reaching the destination computer.
• For this level of communication, we need a global addressing scheme
what we call logical addressing.
• Two types of logical addresses are there:
1. IPv4 Addresses
2. IPv6 Addresses

1.1.1 IP Address
• An IP address is an address used to uniquely identify a device on a
network.
• An IP Address is 32-bit address consist of two parts:
1. Network ID- Identifies the network on which a host computer can be
found.
2. Host ID- Identifies a specific device on the network indicated by the
network ID.
e.g. 145.25.10.100

• IPv4 Addresses :
• An IPv4 address is a 32-bit address that uniquely and universally defines the
connection of a device (for example, a computer or a router) to the Internet.
• They are unique so that each address defines only one connection to the
Internet. Two devices on the Internet can never have the same IPV4
address at the same time.
• On the other hand, if a device operating at the network layer
has m connections to the Internet, it needs to have m addresses, for
example, a router.
• The IPv4 addresses are universal in the sense that the addressing system
must be accepted by any host that wants to be connected to the Internet.
That means global addressing.

1.1.2 Address Space
• IPv4 has a certain address space. An address space is the total number of
addresses used by the protocol. If a protocol uses N bits to define an address, the
address space is 2N
• IPv4 uses 32-bit address format, which means that the address space is 232 or
4,294,967,296 (more than 4 billion).
1.1.3.Notations
There are three notations to show an IPv4 address:
• Binary notation
• Dotted decimal notation
• Hexadecimal notation.

• IPv4 Addresses : (Continue…..)
1) Binary Notation(Base 2)
• In binary notation, the IPv4 address is displayed as 32 bits. Each octet is often
referred to as a byte. So it is common to hear an IPv4 address referred to a 4-
byte address. The following is an example of an IPv4 address in binary
notation: 01110111 10010101 00000001 00000011
2) Dotted-Decimal Notation(Base 256)
• IPV4 addresses are usually written in decimal form with a decimal point (dot)
separating the bytes since it’s more compatible. The following is an
example: 119.149.1.3 (above one and this one is same just different notation)
• Note: Each number in dotted-decimal notation is a value ranging from 0
to 255.

• IPv4 Addresses : (Continue…..)
3) Hexadecimal Notation(Base 16)
• Each hexadecimal digit is equivalent to four bits. i.e. a 32 bit address has
8 hexadecimal digits.
• This notation often used in network programming.
• E.g. 77.95.01.03 (above one and this one is same just different notation)

Dotted-decimal notation and binary notation for an IPv4 address

Change the following IPv4 addresses from binary notation to dotted-
decimal notation.
Example 1.1
Solution
We replace each group of 8 bits with its equivalent decimal number
and add dots for separation.

Change the following IPv4 addresses from dotted-decimal
notation to binary notation.
Example 1.2
Solution
We replace each decimal number with its binary
equivalent.

Find the error, if any, in the following IPv4 addresses.
Example 1.3
Solution
a. There must be no leading zero (045).
b. There can be no more than four numbers.
c. Each number needs to be less than or equal to 255.
d. A mixture of binary notation and dotted-decimal
notation is not allowed.

1.1.4 Classful Addressing:
The 32 bit IP address is divided into five sub-classes. These are:
• Class A
• Class B
• Class C
• Class D
• Class E
• Each of these classes has a valid range of IP addresses. Classes D and E are reserved
for multicast and experimental purposes respectively. The order of bits in the first
octet determine the classes of IP address.
• IPv4 address is divided into two parts:
• Network ID
• Host ID
• The class of IP address is used to determine the bits used for network ID and host ID
and the number of total networks and hosts possible in that particular class. Each ISP
or network administrator assigns IP address to each device that is connected to its
network.

• Classful Addressing: (Continued…)
• Following figure shows the address space is divided into five classes.

Figure 3.2 Finding the classes in binary and dotted-decimal notation

• 1.2.1 IP Datagram format
• Packets in network (Internet) layer are called datagrams.
• A datagram is a variable length packet consisting of two parts namely,
header and data.
• The header is 20 to 60 bytes in length and contains information essential
to routing and delivery.

• 1.2.2 IP Header format Continue……
1. Version-
• Version is a 4 bit field that indicates the IP version used.
• The most popularly used IP versions are version-4 (IPv4) and version-6 (IPv6).
• Only IPv4 uses the above header.
• So, this field always contains the decimal value 4.
2. Header Length-
• Header length is a 4 bit field that contains the length of the IP header.
• It helps in knowing from where the actual data begins.
• Minimum And Maximum Header Length-
• The initial 5 rows of the IP header are always used.
• So, minimum length of IP header = 5 x 4 bytes = 20 bytes.
• The size of the 6th row representing the Options field vary.The size of Options field can go
up to 40 bytes. So, maximum length of IP header = 20 bytes + 40 bytes = 60 bytes.

3. Type Of Service-
• Type of service is a 8 bit field that is used for Quality of Service (QoS).
• 4 TOS bits and an unused bit that must be 0. The 4 TOS bits are:
1000 – Minimum delay
0100 – Maximum Throughput
0010 – Maximum Reliability
0001 – Minimize monetary cost
0000 – Normal service
4. Total Length-
• Total length is a 16 bit field that contains the total length of the datagram (in bytes).
• Total length = Header length + Payload length(Data Length)

5. Identification-
• Identification is a 16 bit field.
• It is used for the identification of the fragments of an original IP datagram.
• When an IP datagram is fragmented, Each fragmented datagram is assigned the same
identification number.
• This number is useful during the re assembly of fragmented datagrams.
• It helps to identify to which IP datagram, the fragmented datagram belongs to.
6. Flags :The router fragment activity is controlled by following 3 flags:
Sr. No. Flags Description
1 0 Reserved, must be zero
2 DF (Do not Fragment) 0 – Allow Fragmentation
1 – Do not Allow Fragmentation
3 MF (More Fragments) 0- this is the last fragment of datagram
1 – additional fragments will follow.

7. Fragment Offset-
• Fragment Offset is a 13 bit field.
• It indicates the position of a fragmented datagram in the original unfragmented
IP datagram.
• The first fragmented datagram has a fragment offset of zero.
• Fragment offset for a given fragmented datagram = Number of data bytes ahead
of it in the original unfragmented datagram
• Fragment offset field value = Fragment Offset / 8

8. Time To Live-
• Time to live (TTL) is a 8 bit field.
• It indicates the maximum number of hops a datagram can take to reach the destination.
• The main purpose of TTL is to prevent the IP datagrams from looping around forever in a routing loop.
• The value of TTL is decremented by 1 when-
 Datagram takes a hop to any intermediate device having network layer.
 Datagram takes a hop to the destination.
• If the value of TTL becomes zero before reaching the destination, then datagram is discarded.
9. Protocol-
• Protocol is a 8 bit field.
• It tells the network layer at the destination host to which protocol the IP datagram belongs to.
• In other words, it tells the next level protocol to the network layer at the destination side.
• Protocol number of ICMP is 1, IGMP is 2, TCP is 6 and UDP is 17.

10. Header Checksum-
• Header checksum is a 16 bit field.
• It contains the checksum value of the entire header.
• The checksum value is used for error checking of the header.
• At each hop,
 The header checksum is compared with the value contained in this field.
 If header checksum is found to be mismatched, then the datagram is discarded.
 Router updates the checksum field whenever it modifies the datagram header.
• The fields that may be modified are-
 TTL
 Options
 Datagram Length
 Header Length
 Fragment Offset

11. Source IP Address-
• Source IP Address is a 32 bit field.
• It contains the logical address of the sender of the datagram.
12. Destination IP Address-
• Destination IP Address is a 32 bit field.
• It contains the logical address of the receiver of the datagram.
13. Options-
• Options is a field whose size vary from 0 bytes to 40 bytes.
• This field is used for several purposes such as-
1. Record route
2. Source routing
3. Padding

1. Record Route-
• A record route option is used to record the IP Address of the routers through
which the datagram passes on its way.
• When record route option is set in the options field, IP Address of the router gets
recorded in the Options field.
2. Source Routing-
• A source routing option is used to specify the route that the datagram must take
to reach the destination.
• This option is generally used to check whether a certain path is working fine or
not.
• Source routing may be loose or strict.

3. Padding-
• Addition of dummy data to fill up unused space in the transmission unit and make it
conform to the standard size is called as padding.
• Options field is used for padding.
Example-
• When header length is not a multiple of 4, extra zeroes are padded in the Options
field.
• By doing so, header length becomes a multiple of 4.
• If header length = 30 bytes, 2 bytes of dummy data is added to the header.
• This makes header length = 32 bytes.
• Then, the value 32 / 4 = 8 is put in the header length field.
• In worst case, 3 bytes of dummy data might have to be padded to make the header
length a multiple of 4.

• 1.2.2 Fragmentation
• IP Fragmentation is a process of dividing the datagram into fragments
during its transmission.
• It is done by intermediary devices such as routers at the destination host
at network layer.

• 1.2.2 Fragmentation Continue…………
• Need-
• Each network has its maximum transmission unit (MTU).
• It dictates the maximum size of the packet that can be transmitted through it.
• Data packets of size greater than MTU can not be transmitted through the network.
• So, datagrams are divided into fragments of size less than or equal to MTU.
• Datagram Fragmentation-
• When router receives a datagram to transmit further, it examines the following-
1. Size of the datagram
2. MTU of the destination network
3. DF bit value in the IP header

Then, following cases are possible-
Case-01:
• Size of the datagram is found to be smaller than or equal to MTU.
• In this case, router transmits the datagram without any fragmentation.
Case-02:
• Size of the datagram is found to be greater than MTU and DF bit set to 1.
• In this case, router discards the datagram.
Case-03:
• Size of the datagram is found to be greater than MTU and DF bit set to 0.
• In this case, router divides the datagram into fragments of size less than or equal to MTU.
• Router attaches an IP header with each fragment making the following changes in it.
• Then, router transmits all the fragments of the datagram.

• Changes Made By Router-
Router makes the following changes in IP header of each fragment-
1. It changes the value of total length field to the size of fragment.
2. It sets the MF bit to 1 for all the fragments except the last one.
3. For the last fragment, it sets the MF bit to 0.
4. It sets the fragment offset field value.
5. It recalculates the header checksum.
• Reassembly of Fragments:
• It takes place only at destination and not at routers since packets take
independent path, so all may not meet at a router and hence need of
fragmentation may arise again.

• Reassembly Algorithm-
Receiver applies the following steps for reassembly of all the fragments-
1. It identifies whether datagram is fragmented or not using MF bit and Fragment
offset field.
2. It identifies all the fragments belonging to the same datagram using identification
field.
3. It identifies the first fragment. Fragment with offset field value = 0 is the first
fragment.
4. It identifies the subsequent fragments using total length, header length and
fragment offset.
5. It repeats step-04 until MF bit = 0.

• 1.2.3 Options
• The IP header has 2 parts namely , a fixed part and a variable part.
• Fixed part is 20 byte long, and variable part is comprises the options, which can
be maximum of 40 bytes.
• It is not required for a datagram. They can be used for network testing and
debugging.
• Figure a shows format of option.
8 Bits 8 Bits Variable Length
Type Length Value
1
(Copy)
2
(Class)
5
(Number)

• 1.2.3 Options Continue….
• The various fields in option format are as follows:
1. Type :
• It is 8 bit long and contains three subfields namely copy, class and number.
a) Copy (1 bit)
b) Class (2 bits)
c) Number (5 bits)
a) Copy :
Copy Meaning
0 Copy Option field only in first fragment.
1 Copy Option field in all fragments.

b) Class : It is 2-bit subfield used to define purpose of option.
Class Meaning
00 Datagram Control
01 Not defined or reserved
10 Debugging and management
11 Not defined or reserved

c) Number : This is 5- bit subfield used for defining the type of option.
• This subfield has 32 possible values but currently only 6 types are defined.
Number Meaning
00000 End of option.
00001 No option.
00011 Loose source route.
00100 Timestamp.
00111 Record root.
01001 Strict source route.

2. Length:
• This 8-bit field is used for defining the total length of option with the type field
and length field included.
• The length field will not be present in all the option types.
3. Value:
• This is variable length field which contains the specific data which is required by
that option.
• The value field will not be present in all the option types.

1.3 ICMPv4
• Internet Control Message Protocol (ICMP) works in the network layer of the OSI
model and the internet layer of the TCP/IP model.
• It is used to send control messages to network devices and hosts.
• Routers and other network devices monitor the operation of the network. When
an error occurs, these devices send a message using ICMP.
• Messages that can be sent include "destination unreachable", "time exceeded",
and "echo requests".
• ICMP is a network layer protocol.
• ICMP messages are not passed directly to the data link layer. The message is first
encapsulated inside the IP datagram before going to the lower layer.

1.3 ICMPv4 Continue….
1.3.1 Functions of ICMP:
1. Announce Network Errors : Such as host or entire portion of the network being
unreachable, due to some type of failure.
2. Announce Network Congestion : When router begins buffering too many
packets, due to inability to transmit them as fast as they are being received, it
will generate ICMP source quench messages. Directed at the sender, these
messages should cause the rate of packet transmission to be slowed.
3. Announce Troubleshooting: ICMP supports Echo function, which just sends a
packet on a roundtrip between two hosts. Ping tool will transmit a series of
packets, measuring average round-trip times and computing loss percentages.
4. Announce Timeouts: If an IP packet’s TTL field drops to zero, the router
discarding the packet will often generate an ICMP packet announcing this fact.

1.3.2 ICMP Message Encapsulation:
• ICMP is a network layer protocol.
• ICMP messages are not passed directly to the data link layer. The message is first
encapsulated inside the IP datagram before going to the lower layer.
• ICMP Message Format: Following figure shows the general ICMP message
format.

• Fields in message format of ICMPv4 are explained below:
1. Type − The type field identifies the type of the message.
2. Code − The code field in ICMP describes the purpose of the message.
3. Checksum − The checksum field is used to validate ICMP messages.
4. The Rest of header is specific for each message type.
5. Data in error messages carries information for finding original packet that had error. Data
in query messages carries extra information based on type of query.
1.3.2.1 Types of ICMP Messages:
• Error-reporting message − This message report problems that a router or a host
(destination) may encounter when it processes an IP packet.
• Query Message − The query messages, which occur in pairs, helps a host or a network
manager to get specific information from a router or another host.
• Following table lists the ICMP messages in each category.

Sr.
No.
Message Type Description
Error Reporting Messages:
1. Destination unreachable 3 Packet could not be delivered.
2. Source quench 4 Choke Packet
3. Redirect 5 Teach a router about
geography.
4. Time exceeded 11 Time to live field hit 0.
5. Parameter Problem 12 Invalid header field.
Query Messages:
1. Echo Request 8 or 0 Ask a machine if it is alive
2. Echo Reply 8 or 0 Yes, I am alive
3. Timestamp Request 13 or 14 Same as Echo request, but with
timestamp.
4. Timestamp Reply 13 or 14 Same as Echo reply, but with
timestamp.

A. Error Reporting Messages:
1. Source Quench − It requests to decrease the traffic rate of message sending from source to
destination.
2. Time Exceeded − When fragments are lost in a network the fragments hold by the router will be
dropped and then ICMP will take the source IP from the discarded packet and inform the source, that
datagram is discarded due to the time to live field reaches zero, by sending time exceeded message.
3. Destination Unreachable − This error message indicates that the destination host, network, or port
number that is specified in the IP packet is unreachable. This may happen due to the destination host
device is down, an intermediate router is unable to find a path to forward the packet, and a firewall is
configured to block connections from the source of the packet.
4. Parameter Problem – indicates that an illegal values has been detected in the header field indicates a
bug in the sending host’s IP software.
5. Redirect Message − A redirect error message is used when a router needs to tell a sender that it
should use a different path for a specific destination. It occurs when the router knows a shorter path to
the destination.

B. Query Messages:
• The query messages occur in pairs and provide specific information from the intermediate
devices to the source device.
1. Echo-Request and Echo-Reply Message: These messages help resolve network diagnostic
issues.
2. Timestamp Request and Reply: These time stamp requests and reply messages help
determine the round trip time between the devices.

1.3.3 Debugging Tools:
• 2 Tools are used.
1. Ping-
• It sends out pings, also referred to as echo request messages -- and then measures the
amount of time it takes the message to reach its destination and return to the source.
• These replies are called echo reply messages. Pings are useful for gathering latency
information about a specific device. Unlike traceroute, though, ping does not provide
picture maps of the routing layout.
• The ping can calculate the Round-trip time.
Round-trip time(RTT) = Departure time of packet – Arrival time of packet
e.g. ping google.com
• It tells the number of packets received, the total time, and the RTT minimum, maximum
and average.

2. Traceroute or Tracert :
• From source to the destination to trace the path of packet in Unix, the
traceroute command and in windows tracert command can be used.
• The traceroute utility is used to display the physical routing path between two
internet devices communicating with each other.
• It maps out the journey from one router to another -- sometimes called a hop.
• Using traceroute to diagnose network problems can help administrators locate
the source of a network delay.
• Traceroute uses two error messages namely, time exceeded and destination
unreachable.
• E.g. tracert google.com

1.4 Mobile IP
• Mobile IP is a communication protocol that enables a host to move from one network to
another while still being connected to its home network.
• Mobile IP allows a mobile host to communicate with the remote host being in a foreign
network.
• It is the extension of Internetworking Protocol (IP) that boosts mobile communication.
• Why do we need Mobile IP?
• Mobile IP is an advanced version of IP.
• IP addresses were designed for the stationary host that always remains attached to one
specific network.
• The prefix of the IP address identifies the network to which the host belongs. The suffix of
the IP address identifies the particular host in the corresponding network.

1.4 Mobile IP Continue…..
• What if the host moved to another network?
Its IP address would no longer be valid and the host would not be able to
communicate with any remote host.
• Thus, we need to modify the structure of the IP address. The two proposed
solutions to modify IP address are:
1. Changing the IP Address:
• Changing the IP address each time the host enters a new network.
• This attempt was not as successful as it has several drawbacks.
• Every time the computer enters a new network it has to reboot. We have to keep
revising the DNS table to let every host on the internet have knowledge about
changes. Each time we have to change the configuration files. If the host moves
to another network in-between the transmission of data, the data exchange
would be interrupted.

2. Two Addresses :
• This solution is more feasible here, a mobile host will have two addresses:
a) Home Address and b) Care of Address/Temporary address.
• The home address is an original IP address of the mobile host, and the temporary
address is called as the care of address.
• A temporary address (care-of address) that would identify the host in the foreign
network. The care-of address will keep on changing whenever the host will move
to a new network.
• The second solution contributes to the design of Mobile IP. It allows a host to
move to another network by maintaining its original IP address and still is able to
communicate with the remote hosts over the internet.

• Entities of Mobile IP - The functional entities in the Mobile IP protocol are:
1. Home Network is a network to which the mobile host actually belongs. It is a
permanent network of the mobile host.
2. Foreign Network is a new network into which the mobile host has moved.
3. Remote Network is a network which is neither the home network nor the foreign
network.
4. Mobile Host is a host of the home network which has moved to the foreign network.
5. Remote Host is a host in a remote network.
6. Home Agent is a router attached to the home network that allows the mobile host to
send and receive data from the remote host over the internet.
7. Foreign Agent is a router attached to the foreign network that allows a mobile host to
send and receive data from a remote host over the internet.
8. Care-of address is a temporary IP address provided by the foreign agent to the mobile
host till it is in the foreign network.
9. Home Address is the address of the mobile host in its home network.

• Working
• Mobile IP communication protocol allows the mobile host to communicate with
the remote host even if it is in a network other than its home network.
If a mobile host wants to communicate with the remote host being itself in the
foreign network, then it has to go through three phases:
1. Phase I : Agent Discovery (Step 1 to 4)
• Agent discovery is the first phase and requires the involvement of a mobile host,
a home agent and a foreign agent. This phase also has two sub-phases as
described below:
• The mobile host has to discover the home agent’s address before it moves away
from the home network (step 1 and 2)
• As the mobile host moves to a new network (foreign network), it has to discover
the foreign agent’s address and also the care-of address (step 3 and 4).

• The agent discovery phase includes two types of messages: agent advertisement
and agent solicitation.
a) Agent Advertisement
• A simple router advertises its existence on the network with the ‘ICMP router
advertisement’ packet. If the router is playing the role of an agent, it ‘appends’
the agent advertisement message to the ICMP advertisement packet.
• If the advertisement is done by a foreign agent, then it sends a ‘list of addresses’
available as care-of addresses. From this list of addresses, the mobile host has to
choose one. The announcement of the care-of address chosen by the mobile
host is done in the registration request.

b) Agent Solicitation
• In case, if a host in the network doesn’t receive the ‘router ICMP advertisement’
packet. The host can initiate itself by sending the ‘router ICMP solicitation
packet’. If a mobile host has not received the ‘agent advertisement’, it can use
the ‘router ICMP solicitation packet’ to send the ‘agent solicitation’ message.
2. Phase-II : Registration (steps 5 to 8):
• This is the second phase of mobile communication. The mobile host first moves
to the foreign network and discovers the foreign agent ( Phase-I).
• After this is must go to registration phase, which corresponds to steps 5 to 8.
• The four aspects of registration are as follows:
i. Registration of mobile host with foreign agent (step 5).
ii. Registration of mobile host with its home agent. This is normally done by the
foreign agent on behalf of mobile host (step 6).

iii. The reply packet is sent from the ‘home agent’ to the ‘mobile host’ via a ‘foreign
agent’. This registration reply has the confirmation of whether the request is accepted
or denied. (step 7 and 8).
• The mobile host has to renew its registration if it has expired. While returning back to
the home network the mobile host has to cancel or deregister its registration.
3. Phase-III : Data Transfer:
• Now finally, after the agent discovery and registration process, the mobile host is in
the foreign network and can communicate with the remote host.
i. From Remote Host to Home Agent :
• Consider that the ‘remote host’ wants to send a data packet to the ‘mobile host’
unaware that the mobile host is not in its home network.
• The remote host would definitely send the packet with its own address in the source
address field and the mobile host’s home address in the destination address field of
the packet.

• As the mobile host is not it’s the home network, the packet sent by the remote
host is received by the home agent on behalf of the mobile host.
• the mobile communication between the remote host and home agent has been
marked by a thick path as “1” in fig.
• Figure :Data transfer from remote host to home agent

ii. From Home to Foreign Agent :
• The ‘home agent’ encapsulates the received IP datagram into another IP
datagram (with the home agent’s address in the source address field and the
foreign agent address in the destination address field) and relays it to the foreign
agent as shown by the thick path marked by “2” in above fig.
iii. From Foreign Agent to Mobile Host :
• The foreign agent receives the packet, removes the encapsulation and sees the
home address of the mobile host in the destination address field of the original
packet sent by the remote host.
• The foreign agent reviews its registry table, observes which care-of address has
been registered to the corresponding home address, and then forwards the
corresponding mobile host packet as shown by the thick path marked by “3” in
above fig.

iv. From Mobile Host to Remote Host :
• if a mobile host being in the foreign network wants to reply or communicate with
the remote host, it simply prepares a packet and puts its home address in the
source address field of the packet and the remote host’s address in the
destination address field of the packet. The mobile host then directly sends the
packet from the foreign network to the remote host as shown by thick path “4”
in fig.
• Transparency :
• The movement of a mobile host from one network to another is transparent to
the entire internet as the internet is totally unaware of the movements of the
host.

• Advantages and Disadvantages of Mobile IP
• Advantages
1. A user with its network devices can move to any other network without losing its
connection with its home address.
2. Mobile IP provides transparency while the data transfer process. It hides the fact that
the mobile host is not in its home network and is communicating from a foreign
network.
• Disadvantages
1. When the ‘remote host’ and ‘mobile host’ both are in a foreign network and still the
data transfer is occurring through the ‘home agent’ then the data packet has to travel
more distance though both the host are in the same network.
2. As we have seen above, if the mobile host in the foreign network wants to send the
data packet to the remote host it sends it directly from the foreign network with its
home address as the source and the remote host address in the destination.
But, if a remote host wants to send a packet to a mobile host in a foreign network, the
data packet has to travel to the mobile host via its home agent. So, here it has to
travel the extra distance.

• Inefficiency in Mobile IP:
• The communication done with the help of mobile IP can be moderately and
severely inefficient.
• The case of moderate inefficiency is called as the Triangle Routing, or Dog Leg
Routing whereas the case of severe inefficiency called as Double Crossing or 2X.
1. Double Crossing or 2X :
• Consider a situation in which a remote host wants to communicates with a
mobile host that has moved to the same network as the remote host as shown in
fig.
• A mobile host can send a packet directly to the remote host. Therefore there is
no efficiency; the communication is local.
• But if the remote host sends a packet to the mobile host then it cannot do so
directly (via the dotted direct path in fig. 1.4.3)

• Instead the remote host has to send the packet first home agent (path 1) and
home agent will route the packet to the mobile host(path 2).
• In this case the packet crosses the Internet twice. Thus resources are used twice
unnecessarily in this communication which reduces the efficiency severely.
• Hence the double cross case is called as the case of severe efficiency.
Figure 1.4.3 Double Crossing or 2X.

2. Triangle Routing or Dog Leg Routing:
• It is a case of moderate inefficiency.
• Occurs when the remote host wants to send a packet to the mobile host that is
not attached to the same network as the mobile host as shown in fig.
• When the mobile host sends a packet to the remote host it can do so directly.
There is no efficiency .
• But when the remote host sends a packet to the mobile host the packet goes
from the remote host to the home agent and then to the mobile host as shown
in fig. 1.4.4.
• Thus the packet has to travel along the two sides of a triangle instead of only
one which is the direct path shown by dotted line in fig.

Figure 1.4.4 Triangle Routing
• Solution :
• The remote host must know the mobile host’ s care-of address.
• Send packet using the mobile host’ s care-of address.
• The home agent can tell the remote host about this information by the update
binding packet.
• However, when the mobile host moves, its care-of address may be changed n
The home agent needs to send a warning packet to the remote host to inform it.

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