UNIT – V
MOBILE IP NETWORK LAYER
Ramesh Babu Palepu
Amrita Sai Institute of Science & Technology
We know that OSI is the basic model for
networking which has seven layers and
TCP/IP is the most prominent one today
with five layers. One of the layer in both the
models is the networks layer, which has
Internet Protocol (IP). This IP used in this
layer governs the way of networking in the
Internet and provides global internet
connectivity. In mobile communications the
mobile systems can also utilize Mobile
Internet Protocol for identification of nodes
as well as reliable packet transmission.
• First we consider the basic model of
networking OSI. Here the data transmitted
between the nodes is suitably encoded and
decoded at the both ends. The following are
OSI layers functions.
1.Physical Layer: It is used to sending and
receiving the data in the form of signals,
over the wire or fiber optics or wirelessly.
2.Data Link Layer: It is used to link source
and destination computers by using MAC
These two layers in OSI is usually associated
with physical communication media or radio
interface and switching infrastructure. 3. 3.
3. Network Layer: It facilitates the routing of
data through chain of routers. This layer
choosing a path among the large number of
paths available between source and
4. Transport Layer: It facilitates the
sequencing of data packets and enabling the
repeat transmission if required.
5. Session Layer: It is used to define the
transaction and session to establish end-toend connectivity.
6. Presentation Layer: It defines the data
7. Application Layer: It support various user
applications over the network, such as web
browsers, mail transfer, file transfer etc.
This layer may also defines the application
content and data of a port. Here each port
may support different applications.
• In the service point of view, we know that
to transmit the data we should establish the
• Sender side in OSI, the physical network
transmit the data from one application to
• At each layer the data is suitably encoded
by adding the headers and then transmitted
into the lower layers.
• At the receiver side, the encoded bits in the
header field are decoded at each layer before
the data passing to the next upper layer.
so at the both ends the corresponding layers
is take care of encoding, dividing, and
assembling of the data.
USER DATAGRAM PROTOCOL (UDP)
• The User Datagram Protocol (UDP) is a
protocol. It does not add any thing to the
services of IP except providing process-toprocess communication. It also poor in error
checking. It is very simple protocol with
interaction between the sender and receiver.
•UDP packets are called user datagrams,
which are having fixed size header of 8
bytes. The following diagram shows format
of user datagram.
•Source port Number: This port number
used by the process running on the sender
system. It is 16 bit long and ranges from 0 to
65,535. if the sender is a client then the port
number is a ephemeral port number.
• Suppose the sender is a server it port
number is well known port number.
• Destination Port Number: This port
number used by the process running on
the destination system. It also 16 bit. If the
destination system is a server, the port
number is well known port number,
otherwise it is a ephemeral port number.
• Length: It is also a 16 bit field that defines
total length of the packet including header
and data. The length of the packet from 0
to 65,535 bytes.
Actually the length of UDP packet is always
less than 65,535 bytes because UDP
datagram stored in IP datagram whose total
length also 65,535 bytes.
• Checksum: This field is used to detect
errors over the entire user datagram
including header and data.
UDP Operation: The User Datagram Protocol
supports the following services;
Connectionless services: We know that UDP
provide connectionless service, it means
each packet sent by UDP is an independent
one. There is no relationship between the
different user datagrams even if they are
coming from the same source process and
going to the same destination process.
• The user datagrams are not numbered and
there is no connection establishment and
connection termination mechanism. So the
datagrams between source and destination
can travel on different paths.
Flow and Error Control: The UDP is a very
simple and unreliable protocol. There is no
flow control and hence there is no window
mechanism. Sometimes the receiver may
overflow with incoming packets.
There is no error control mechanism in UDP
except checksum. So the sender does not
know whether the data is lost or duplicated.
• Suppose if the receiver detects the errors
through the checksum then simply that
packet is discarded.
Encapsulation and Decapsulation: To send
the data from one process to another
deccapsulates the packets in IP datagram.
Queuing: In UDP queues are associated with
ports. The following diagram shows queues
• At the client side when a transmission
process starts, it request a port number
from the operating system, at the time the
OS creates incoming and outgoing queues
and associate with that process.
Suppose if a process wants to
communicate with multiple processes, it
obtain only one port number and finally use
one outgoing and incoming queue.
• The queues are opened by the client are
identified by ephemeral port numbers. The
queues functions as long as process
running, when the process terminates, the
associate queues are destroyed.
Uses of UDP: The following are some of the
uses of UDP protocol.
1.UDP is suitable for the processes that
request and response communication.
2.UDP is suitable for the processes with
internal flow and error control mechanisms.
3.UDP is a suitable transport protocol for
4.UDP is used for management processes
such as SNMP
5.UDP is used for some route updating
Transmission Control Protocol
• Transmission Control Protocol (TCP) is a
protocol. It creates virtual connection
between two TCPs which are in sender and
receiver. It also supports error control and
flow control mechanisms in transport level.
TCP Services: The following are the different
services offered by the TCP to its above layer
provides process-to-process communication
by using port numbers.
Stream Delivery Service: TCP is a stream
oriented protocol. It allows the sending
process to deliver the as a stream of bytes
and allows receiving process to obtain data
as a stream of bytes.
• TCP creates an environment in which the
two processes seem to be connected by an
imaginary tube that carries their data across
the internet. The following diagram show
Sending and Receiving Buffers: The sending
and receiving process may not happen at the
same speed in data transmission. So the
TCP need buffers for storage.
• There are two buffers, one is sending buffer
and another one is receiving buffer in each
• These buffers may also useful in error
control and congestion control.
• one of the method to implement the buffers
is circular array, each location capacity is
one byte. The following diagram shows
sending and receiving buffers.
• (Diagram 23.14)
• The above diagram shows movement of
data in one direction. At the sending side the
buffers has three kinds of chambers. Some
of the chambers are empty, which can be
filled by the sending process. Some of the
chambers have data that has been sent but
not yet acknowledged. TCP keep the data in
this chambers until it receives an
acknowledgement. Some of the chambers
have data that has to be sent by the TCP.
• Once the bytes are acknowledged, then the
chambers are recycled and available for use
of sending process.
• The buffers at the receiving side has two
parts, some of the chambers are empty to be
filled by data received from the network.
Other chambers have data that can be read
by the receiving process.
TCP Segment: TCP groups number of bytes
together into a packet called segment. TCP
adds header to each segment and send the
segment to IP layer for transmission. The
segments are encapsulated in IP datagram
and then transmitted.
• If the segments may be received out of
order, lost, corrupted and resent, all these
are handled by TCP.
• In TCP all the segments should not
necessarily same size.
Full duplex communication: TCP offers full
duplex service, in which data can flow in
both directions at the same time.
Connection oriented service: We know that
TCP is a connection oriented protocol. When
transmitting the data the following activities
1.The two TCPs establish a connection
2.Data are exchanged in both directions.
3.The connection is terminated.
Reliable Service: TCP is a reliable transport
mechanism to check safe arrival of data.
The TCP has several features, the following
are some of them;
Numbering system: The TCP keep track the
segments being transmitted or received, for
this purpose TCP has two fields, one is
numbers refer the byte numbers but not
• TCP numbered all the data bytes that are
transmitted. The numbering is independent
in each direction. The numbers does not
start from 0. instead TCP generates random
numbers between 0 to 232–1 for assign
number of first byte.
• After the bytes have been numbered, TCP
assigns a sequence number to each segment
that is being sent. The sequence number is
the number of the first byte carried in that
• If the segment carries use data and control
information it has sequence number. If the
segment does not carry user data, it does
not logically define sequence number.
• Some segments only carry the control
information need a sequence number to
allow an acknowledgement from the receiver.
acknowledgement number to conform the
acknowledgement number is the number of
next byte that a node expect to receive and
acknowledgement number is cumulative,
that is the receiver successfully received last
byte with some number then it adds 1 to it.
Flow control: TCP provides flow control. The
receiver controls, the volume of data to be
sent by the sender. This prevents the
receiver from the overflow. The TCP
numbering system may also used for byte
oriented flow control.
Error control: To provide reliable services,
TCP has error control mechanism. During
error detection total segment is considered
as a single data unit. Error control is byte
Congestion control: TCP considers the
congestion of network into account.
The amount of data sent by the sender not
only controlled by the receiver but also
determined by the level of congestion over
• A packet in TCP is called a segment. The
segment consists of 20 to 60 bytes header,
followed by data from application layer. If
the header is 20 bytes, there are no options,
suppose the header is 60 bytes it contains
options. The following diagram shows the
header format of TCP.
• (Diagram 23.16)
Source port address: This is a 16 bit field
that defines the port number of the
application program at the sender side.
Destination port address: This is a 16 bit
field that defines the port number of the
application program at the receiver side.
Sequence number: This is a 32 bit field that
represents number assigned to the first byte
of the segment.
• We know that TCP is a stream oriented
protocol, to ensure the connectivity each
transmitted byte has a number.
• During the connection establishment, each
party in use a random number generator to
create an initial sequence number (ISN),
which is different in each direction.
Acknowledgement number: This is a 32 bit
field represents the byte number that the
receiver is expecting to receive from the
• suppose the receiver successfully received
byte number x from the source, then it
sends x+1 as the acknowledgement number.
Header length: This is a 4 bit field indicates
the number of 4 byte words in TCP header.
We know that the length of TCP header is 20
to 60 bytes so the value of the field ranges
from 5 to 15.
Reserved: This is a 6 bit field reserved for
Control: This field contains six different
control bits or flags. One or more flags can
set at a time.
• These control bits are used for flow control,
connection abortion and mode of data
transfer in TCP.
• (Diagram 23.17)
Window size: This field defines the size of the
window, in bytes that the sender should
maintain. The length of this filed is 16 bits,
so the maximum size of the window is
65,535 bytes. This is the size of the receiving
window determined by the receiver.
Checksum: This 16 bit field contains
checksum to identify and correct the
Urgent pointer: This 16 bit field is valid
when urgent flag is set. This flag is used
when the segment contains urgent data.
Options: There can be up to 40 bytes of
optional information in TCP header.
• TCP is a connection oriented protocol. It
establishes a virtual path between the
source and destination. All the segments
belongs to a message are sent through this
virtual path. Using the single virtual path
facilitates acknowledgement process as well
as retransmission of damaged or lost
segments. If the segments arrives out of
order TCP holds it until the missing one
Connection establishment: TCP transmits
the data in full duplex mode. When two
TCPs in two machines are connected those
can transmit the data simultaneously. It
communication get approval from one
another before transmitting the data.
• Connection establishment in TCP is done
through three way handshaking. Let us
consider a client wants to communicate with
server by using TCP.
• Now the process starts with the server. The
server tells its TCP that is ready to accept
the connection, this is called passive open.
• Now the client program issues a request to
the server, is known as active open. It
represent, the client wish to connect to the
open server. Now the TCP starts the three
• To implement the three way handshaking
few fields of TCP segment are necessary,
acknowledgement number, control flags,
and window size.
• The following diagram shows the three way
1. The client sends the first segment, a SYN
segment, in which SYN flag is set. This
segment is for synchronization of sequence
numbers. It consumes one sequence
number. When data transmission starts the
sequence number is incremented by 1.
2. The server sends second segment
SYN+ACK by setting the two flag bits, this
segment has dual purpose, it servers as a
SYN for communication in other direction
and ACK for previous SYN segment. It
consumes one sequence number.
3. Now the client sends the third segment, it
acknowledges the receipt of second segment.
The ACK segment does not consume any
Simultaneous Open: It is a rare situation
occur when both sender and receiver issue
an active open. In this case both TCPs
transmit SYN+ACK segments to each other,
then a single connection established
Synchronization Flooding Attack: One more
establishment is SYN flood attack.
• This happen when an attacker sends a
large number of SYN segments to a server
and pretending them those are from
different client by using the fake IP address.
• Now the server allocate the necessary
resources and creating communication
tables for each fake request. This leads to
the server run out of the service. This attack
is known as the denial of service.
Data Transfer in TCP:
bidirectional data transfer can takes place.
The client and server send and receive data
• The following diagram shows data transfer
• Let we explain this concept with an
example, here the client sends the 2000
bytes of data in two segments, each one
have 1000 bytes and the server sends 2000
bytes of data in a single segment. Here we
should carefully observe the sequence and
acknowledgements are piggybacked.
• The data segments send by the client have
set the PUSH flag, so that the server knows
to deliver the data to the server process as
soon as they are received.
• There are many methods to terminate the
1.Three way handshaking.
2.Four way handshaking.
TCP Flow Control:
TCP uses sliding window protocol to handle
the flow control. We know that in sliding
• One is Go-Back-N and other is selective
• Some times TCP sliding window looks like
Go-Back-N because TCP does not have any
negative acknowledgements and some time
it follow selective repeat because the receiver
holds the out of order segments until
missing one arrives.
• The following diagram shows working of
sliding window in TCP.
• The window has three activities, open,
closed and shrunk, these activities are in
the control of the receiver, the sender should
obey the commands of the receiver in this
• Open window means moving the window’s
right wall to the right. This operation allows
more data bytes are eligible for sending.
• Closing the window means moving the left
wall of the window to the right. This means,
the sender already acknowledge some bytes
of data, so those are removed from the
• Shrunk means moving the window’s right
wall to the left. This action is generally not
allowed because it reject the eligibility of
byte for sending and left wall cannot moves
left because already send bytes again get the
eligibility for sending. (Data duplication may
• The size of the sliding window is
determined by the two parameters receiver
window size and congestion window size.
Here receiver window size is determined by
the receiver and congestion window size is
determined by the network.
• First we discuss general congestion control
mechanisms in the Network. The congestion
is a situation in which the network is unable
to deliver the data to the destination.
• Congestion control refers techniques and
mechanisms that can either prevent the
congestion before it happens or remove the
congestion after it has happened.
• In broad sense we can divide the
congestion control in two categories;
1.Open loop congestion control.
2.Closed loop congestion control.
Open loop congestion control: The open loop
congestion control policies are applied to
prevent the congestion before it happens. In
this situation congestion control is handled by
either sender or receiver.
retransmission is an unavoidable situation in
the networks. The sender retransmits the data
when a packet is lost or corrupted. The
congestion over the network. So we choose good
retransmission policy. In the retransmission
policy, retransmission timers are adjusted to
optimize the efficiency and prevent the
Window policy: The type of window used at
the sender side is also effect the congestion.
To prevent the congestion selective repeat
the window is best one.
acknowledgement policy imposed by the
receiver may also effect the congestion.
• There are several mechanisms in
1. The receiver not immediately acknowledge
each and every packet send by the sender, it
may slow down the sender and prevent the
2. The receiver send an acknowledgement
only if it has a packet to be sent or special
3. The receiver may acknowledge only N
packets at a time.
Discarding policy: The good discarding
policy may prevent the congestion as well as
keep the integrity of the data. For example,
discarding less sensitive packets in audio
transmission when congestion is likely to
happen, is not harmful to audio quality and
the congestion is alleviated.
Admission policy: In this policy switches in
the network, first check the resource
requirement and current flow of the network
and then admit the future transmissions.
The admission policy give much importance
to the quality of service.
Closed loop congestion control: Closed loop
congestion control mechanism try to
alleviate congestion after it happens.
congestion node stops receiving data from
the immediate upstream node, it may cause
the upstream nodes become congested.
• so the upstream nodes reject the data from
backpressure is propagates in opposite
direction of data flow finally reaches the
source. Now the source node identifies the
Choke Packet: The drawback in the
regarding the congestion.
• In Choke packet method a packet is sent
by the congested node to the source directly.
Implicit signaling: In this method, no
communication between the congested node
and the source. Here the source guesses
that there is a congestion in the network
from other symptoms.
For example, suppose the source sends the
packets and there is no acknowledgement
for a while indicates the congestion over the
Explicit signaling: This method is different
from choke packet but explicitly send a
congestion signal to the source or
• In explicit signaling, the signal may also
carry the data. The signaling is two types;
1.Backward signaling: A signal bit sent in
the direction opposite to the data flow. The
bit can warn the sender and slow down the
2.Forward signaling: A signal bit sent in the
direction of data packet. This bit inform
about the congestion to the destination. Now
acknowledgements or follow some other
policies to mitigate the congestion.
TCP Congestion Control:
Congestion Window: We know that the size
of the sender window is determined by the
receiver and we also consider the current
transmission state of the network. Now the
sender has two pieces of information, that is
congestion window size. So the actual size of
the sender window is the minimum of these
Actual window size = Minimum(rwnd, cwnd)
Congestion policy: TCPs congestion handling
policy is implemented in three phases; Slow
start, Congestion detection, and Congestion
• At first the sender starts with very slow
rate of transmission, but increases the rate
rapidly to reach threshold. When threshold
is reached, the data rate is reduced to avoid
congestion. Finally if congestion is detected,
the sender goes back to slow-start or
congestion avoidance phase based on how
congestion is detected.
Slow Start : Exponential Increase: One of the
TCP congestion control algorithm is slow
start. In this method the size of the
maximum segment size (MSS). The size of
MSS determined during the connection
The following diagram shows slow start,
• Here the size of window increases one MSS
each time, an acknowledgement is received.
That is the window starts slowly but grows
• we will explain with an example, initially
the sender starts with one segment, after
getting the acknowledgement of segment-1,
the congestion window size is incremented
by 1. which mean now the congestion
window size is 2 now these two segments
can be sent. When acknowledgement is
received the size of window increased by 4
segments and soon.
• The slow start not continue indefinitely, a
threshold is stop this phase. The sender
maintain ‘ssthresh’ variable, when window
size reached threshold, slow start stops and
next phase starts.
Congestion avoidance: To avoid congestion
before it happen, we should slow down the
exponential growth, this is done by
congestion avoidance algorithm.
• The congestion avoidance algorithm makes
additive increase instead of exponential
increase. After the slow start phase the
additive increase phase starts.
Decrease: If congestion occurs, the
congestion window size must decreased.
Now the sender should retransmit the
retransmission occurs in two cases;
1.When timer times out.
2.When three ACKs are received.
In both cases the size of threshold is
dropped to half.
• We know that the host or node within the
network should not get any packet if it
moves out of the network.
• In mobile environment the nodes are
always in moving between several areas,
and our goal is, where ever the node is,
should get the services of the network. To
make it possible the node should have a
topologically correct address (IP).
• There are several ways to assign a
topologically correct address (IP) to a
• One quick solution to assign topologically
correct IP to the mobile node use of DHCP.
So, the mobile node visits the new location
it gets the new IP. Here the problem is, no
body knows the new IP, it is impossible to
find a node on network, which just
changed its address.
• The next solution is use of Dynamic DNS,
which maps the logical name with IP
address. But as per the considerations of
the DDNS, the IP has no special interest
for services offering.
• The DNS should take certain time to map
the logical name to an IP address. This
approach does not work, because, the
mobile node moves quit often and DNS not
designed for frequent updates of routing
• There is one more sever problem, that is
changing of the IP address causes
breaking of TCP connection. We know that
a TCP connection is identified by a socket
• Another solution for assigning topologically
correct address is, creation of specific
routers for mobile nodes.
• But it is possible, theoretically onlychanging routing tables all over the world
to create specific routes to a mobile node.
Here we does not scale how many routers
to be fit, because large volumes of nodes
day by day connected to the network.
• One more basic problem with this
approach is, the routers are basically
designed for fast forwarding the packets,
but not for updating of routing tables.
• So the above trails are not working to
assign a correct IP address to a mobile
node, now we can design a new standard
with following requirements.
Specifications (Requirements) of Mobile IP:
1. Compatibility: We know that today all the
computers that connects to the internet
are working on the TCP/IP. The new
standard cannot change the applications
or network protocols are already in use
and it should work with current operating
system. It should not introduce new
software for the routers, but it may
capabilities. It should not require special
media or MAC/LLC protocols, it should
compatible with non mobile IP addresses.
2. Transparency: We know that the mobility
is invisible for the many higher layer
protocols, so higher layers should continue
its work even if the mobile node change its
point of attachment to the network.
Suppose the interruption of service
(disconnection) is too long, TCP is unable
to procure the connectivity. The new
technology should aware the higher layers
about the changing the point of
attachment of a mobile node as per the
3. Scalability and efficiency: The new
technology should not vulnerable the
efficiency of the network. It may not generate
too many messages flooding the network and
special care should taken on lower
bandwidths. The mobile IP should scalable for
large number of participants in the whole
4. Security: We know that mobility has many
security problems, here the minimum
requirement is all the information regarding
mobile IP management should authenticate.
The IP layer sure that if it forward a packet to
a mobile host this host should receive the
• The IP layer should guarantee that the IP
address of a receiver is correct. It should
preventing the fake IP addresses or
Entities and Terminology:
Mobile Node (MN): A Mobile node is an end
system or a router that can change its
point of attachment with the network from
time to time. The MN keeps its IP address
and can continuously communicate with
other systems within the network.
correspondent node. It is fixed or mobile.
Home Network: The home network is subnet
the MN belongs to with respect of its IP
address. No mobile IP support is needed in
Foreign Network: The foreign network is the
current subnet the MN visits and which is
not the home network.
Foreign Agent (FA): The FA can provide all
the services to the MN during its visit to
the foreign network. The FA has COA (Care
Of Address) act as a tunnel end point for
forwarding packets to the MN. It also
provide security to MN because it is a
Care of Address (COA): The COA defines the
current location of the MN from IP point of
view. All the IP packets to the MN are
delivered to the COA, not directly to the
MN. Here the packet delivery takes
through tunneling method, so COA is act
as a tunnel endpoint. There are two types
Foreign Agent COA: This COA is located at
FA. It mean the COA is an IP address of
FA. Now FA acts as a tunnel endpoint and
then forward the packets to MN.
Co-located COA: When MN in the foreign
network, suppose it obtain temporary
additional IP address, which act as COA is
known as co-located COA. If this address
is topologically correct, then the MN acts
as a tunnel endpoint.
Home Agent (HA): The HA provides several
services to MN and is located in the home
network. Actually the tunneling of packets
to MN starts at HA. The HA maintains a
location register has an information of
MN’s current location. There are three
alternatives for the implementation of HA.
• HA can be implemented on a router, which
is responsible for home network. Really it
is a best position, because without
optimizing the IP all the packets for MN go
through the router.
• Generally changing of router software is
not possible, so HA can implemented on
any arbitrary node in the subnet. One
disadvantage of this method is if the MN is
the foreign network, the packets are
double crossing the router.
• Home network is not necessary at all. The
HA could again on router, but only act as a
manager for MN belonging to a virtual home
network. With this solution all the MNs are
always in foreign network.
IP packet Delivery (Working of IP):
Let us consider an example network as
shown in the diagram.
Here correspondent node wants to send IP
packets to MN and it does not know
anything about MN’s current location and
sends the packets as usual to the IP
address of MN. It mean CN sends IP
packets containing MN’s IP address is des-
-tination address and CN’s IP address as
packets to the home agent which is in the
• The HA intercepts the packet, and
identifies that the MN is currently not in
• Now the packet is encapsulated by COA
and tunneled to foreign network. Here new
IP is put in front of the old IP with new
destination IP (COA).
• The foreign agent now decapsulate the
packet, it removes the additional header
and forward the packet with CN as source
address and MN as destination address to
• Now transmitting the data from MN to CN
is very simple, MN sends the packet with
its own IP address as source address and
CN’s address as destination address. Here
the router in foreign network (FA) act as a
default router and forward the packets.
Packet Delivery & Handover Management:
correspondent node (CN) and the mobile
node (MN) which are moving around
different networks and the CN may be the
fixed or mobile, how the services are
handed over between the different
CASE-1: CN is a fixed node and MN in home
• Let we consider CN is a fixed node in its
own network, (corresponding network) and
MN is also in home network, now CN
transmits the connection establishment
message using IP protocol, through CA
(corresponding agent) to the home agent
(HA) of MN.
• In this message source IP address is the IP
address of CN and destination IP address
is the IP address of MN.
• Now the HA receives and then delivers it to
the MN, the HA also receives the response
from MN and send back to CN.
CASE-2: CN is a mobile node in its home
network, and MN is also in its home
• In this case the CN transmit the packet for
connection establishment to HA of MN
• Here source IP address is the IP address of
CN, and destination IP address is the IP
address of MN.
• Now the HA receives and then delivers it to
the MN, the HA also receives the response
from MN and send back to CN.
CASE-3: CN is fixed in its home network and
MN is in the foreign network.
establishment message using IP protocol,
through CA (corresponding agent) to the
home agent (HA) of MN. In this message
source IP address is the IP address of CN
and destination IP address is the IP
address of MN.
• Now HA of MN receives the packet for MN
and identifies that the MN is not available
in home network. So HA encapsulate the
receiving packet with new IP header which
source address is IP address of CN and
destination address is the care of address
(COA) of MN in the foreign network.
• Now the foreign agent (FA) receives the
packet with new header and decapsulate
it, then deliver to the MN.
• To send back the information from MN to
CN, the MN is able to directly transfers the
data to CN through FA, because the IP
address of CN is available at MN after
decapsulating the packet – That is the
source address is turned as destination
address in reply.
CASE-4: CN is mobile node at foreign
network and MN is at the home network.
• In this case the CN sends the connection
establishment message to the HA of MN
through the FA where it is located.
• Once the connection established, the data
is exchanged between the HA and FA.
CASE-5: CN is mobile node at foreign
network and MN is at the foreign network.
• In this case, we carefully considered that
the CN is at the foreign network FA1 and
MN is at another foreign network FA2.
• Now the CN sends the connection
establishment packet through FA1 to the
HA of MN. But MN recognizes that MN is
not available in home network, so it
encapsulate the packet with new header
and then transmit to the FA2.
• Now the data transmissions are carried in
between the FA1 and FA2.
CASE-6: CN is mobile node in its home
network and MN is the at the foreign
• This case is similar to CASE-3 except the
CN is mobile node in its home netwrok.
• We know that the mobile node always
moving between the different network,
here one of the initial problem is, how to
find out a mobile node when it is moved
into other networks. The mobile IP
describes two procedures to discover the
MN in foreign networks; one is agent
Agent Advertisement: In this method, the
home and foreign agents advertise their
• This advertisement message can broadcast
into the entire subnet. To frame this
message we use Internet Control Message
Protocol (ICMP) with some mobility
• The routers in the fixed network can also
obey this protocol and advertise their
routing services periodically to the
attached links. The following diagram
shows the agent advertisement packet with
• Here the upper part represents the ICMP
packet and the lower part consists
extensions needed for mobility.
• Here TTL field of the IP packet is set to 1,
to avoid forwarding the packet to the next
node. The IP destination address is set
advertisement, it may be a multicast
address or broadcast address.
• The type field of ICMP is set to 9. The code
field is 0, if the agent route the packets
from non mobile nodes, or code is 16 it
means the agent route the traffic only from
• # address field represents, the number of
addresses advertised with the packet. The
Life time field indicates the length of the
time this advertisement is valid. Preference
levels for each address help a node to
choose the router that is most eager to
service to a new node.
• In the extension field contains, type is set
to 16, length is depending on the number
of COA (care of addresses) advertise with
the message, sequence number indicates,
the total number of advertisements sent
since initialization. The registration life
a node can request during registration.
• The bit R shows, the registration with the
agent is required even when MN using
colocated COA. B bit shows, the agent is
currently too busy to accept the new
registration. The bit H indicates, the agent
works as a home agent, F indicates the
agent works as foreign agent. The bits M
encapsulation, M represents minimal
encapsulation. The V bit specified use of
header compression ‘r’ bit set to 0, and
ignored. T bit indicates reverse tunneling.
• Now a mobile node in the subnet an
receive agent advertisement from either its
home agent or a foreign agent. This is one
way for MN to discover the location.
Agent Solicitation: If no agent advertisement
is present or the MN has not received a
COA, or the inter arrival time is too high,
then the mobile node sends the agent
• The solicitations are based on the RFC
1256 protocol and we must ensure that
the solicitation messages do not flood the
• In agent solicitation, the MN search for FA
messages. Generally a mobile node send
three solicitations per second.
• If the node does not receive an answer to
its solicitations it must decrease the rate of
solicitations exponentially to avoid flooding
the network unit it reaches a maximum
interval between solicitations.