1

MOBILE COMPUTINGUnit II

Dr Gnanasekaran Thangavel

2

UNIT II MOBILE INTERNET PROTOCOL AND

TRANSPORT LAYER Overview of Mobile IP – Features of Mobile IP – Key Mechanism in Mobile IP –

route Optimization. Overview of TCP/IP – Architecture of TCP/IP- Adaptation of

TCP Window – Improvement in TCP Performance.

3

4

Mobile IPMobile IP enables an IP node to retain the same IP address and maintain existing communications while traveling from one link to another.

5

Mobile IP

Tunneling

6

Mobile IP Components

Mobile node (MN)

Home agent (HA)

Foreign agent (FA)

Correspondent Node (CN)

7

Mobile node (MN)The Mobile Node is a device such as -a cell phone, -personal digital assistant, -or laptop

whose software enables network roaming capabilities.

8

Home agent (HA)Home Agent is a router on the home network serving as the anchor point for communication with the Mobile Node; it tunnels packets from a device on the Internet, called a Correspondent Node, to the roaming Mobile Node.

(A tunnel is established between the Home Agent and a reachable point for the Mobile Node in the foreign network.)

9

Foreign agent (FA)

The Foreign Agent is a router that may function as the point of attachment for the Mobile Node when it roams to a foreign network, delivering packets from the Home Agent to the Mobile Node.

10

Correspondent Node (CN)

End host to which MN is corresponding (eg. a web server)

11

How Mobile IP WorksThe Mobile IP process has three main phasesthe following sections. Agent Discovery

A Mobile Node discovers its Foreign and Home Agents during agent discovery. Registration

The Mobile Node registers its current location with the Foreign Agent and Home Agent during registration.

TunnelingA reciprocal tunnel is set up by the Home Agent to the care-of address (current location of the Mobile Node on the foreign network) to route packets to the Mobile Node as it roams.

12

Two IP addresses for mobile node

Home address: static

Care-of address: topologically significant address

The care-of address is the termination point of the tunnel toward the Mobile Node

when it is on a foreign network. The Home Agent maintains an association

between the home IP address of the Mobile Node and its care-of address, which is

the current location of the Mobile Node on the foreign or visited network

13

Features of Mobile IP Transparency : IP address should not be any effect of mobility on any ongoing

communication Compatibility: It should compatible with existing Internet protocols. Security: It should provide users with secure communication over internet. Efficiency : It should neither result in large number of message nor should it incur too

much computational overheads Scalability: It should also be scalable to support billions of moving hosts worldwide.

14

Key Mechanism in Mobile IPThree basic mechanisms

1. Discovering the care-of address2. Registering the care-of address3. Tunneling to the care-of address

15

1.Discovery the care-of-addressExtension of ICMP Router Advertisement• Home agents and foreign agents broadcast agent advertisements at

regular intervalsAllows for the detection of mobility agentsLists one or more available care-of addressesInforms the mobile node about special featuresMobile node selects its care-of addressMobile node checks whether the agent is a home agent or foreign

agent– Mobile node issues an ICMP router solicitation message

16

Mobile IP Agent Advertisement Message

17

2. RegistrationOnce a mobile node has a care-of address, its home agent must find out about it

18

19

3. Tunneling to the care-of address

20

Mobile IP Tunneling

Across Internet

21

Tunneling takes place to forward an IP datagram from home agent to a care-of-address.

When a home agent receives a packet addressed to a mobile host , it forwards the

packet to the care-of-address using IP-within-IP(encapsulation)

Using IP-within-IP, the home agent inserts a new IP header in front of the IP header of

any datagram.

Destination address is set to the home agent’s address.

Source address is set to the home agents address.

After stripping out the first header, IP processes the packet again.

22

Two Tunneling Methods

IP-within-IP Encapsulation Minimal Encapsulation

23

Route Optimization All the data packets to the mobile node go through the home agent. There will be a heavy traffic between HA and CN causing latency to increase. To overcome this problem the following route optimization needs• Enable direct notification of the corresponding host• Direct tunneling from the corresponding host to the mobile host.• Binding cache maintained at the corresponding hostThe association of the home address with care-of-address is called binding

24

Overview of TCP/IPTCP/IP protocol suite is a collection of a large number of protocols.The four layer of protocols are • Application layer• Transport layer • Internet layer• Network layer • TCP/IP allows any of the standard protocols to be used for network

interface

25

26

Architecture of TCP/IPTCP/IP protocol consists of four layers. 1. Application layer, 2.Transport layer, 3.Internet layer and 4. Network access layer

27

1. Application layer: Establish communication with other applications which may be running on separate hosts. Example http,ftp and telnet. 2. Transport layer: The purpose of Transport layer is to permit devices on the source and destination hosts to carry on a

conversation. Provides reliable end-to-end data transfer services. Also referred as host-to-host protocol. The main protocols included at Transport layer are

I. TCP (Transmission Control Protocol) and II. UDP (User Datagram Protocol).

3. Internet layer Internet layer pack data into data packets known as IP datagrams, which contain source and destination address (logical address or IP address) information that is used to

forward the datagrams between hosts and across networks.

28

4. Network access layer The Network Access Layer of the TCP/IP model is associated with the Physical Layer (Layer 1) and the Data Link

layer (Layer 2) of the OSI model. The Network Access Layer's function is to move bits (0s and 1s) over the network medium such as coaxial cable,

optical fiber, or twisted pair copper wire. The OSI Physical layer is responsible for converting the frame into a stream of bits suitable for the transmission

medium and synchronizes signals for the actual transmission. On the destination device, the Physical layer reassembles these signals into a data frame. The OSI Data Link layer is again subdivided into the following two sub layers according to their function: Media Access Control (MAC) Sublayer :— MAC sublayer provides an interface with the network adapter. Logical Link Control (LLC) Sublayer :— LLC sublayer is responsible for error-checking functions for frames

delivered also responsible for managing links between communicating devices.

29

Comparison of TCP/IP and OSI network models

30

Comparison of TCP/IP and OSI network models

31

Adaptation of TCP Window TCP deploys a flow control technique to control congestion in a network. Traffic congestion occurs when the rate at which data is injected by a host into the

network exceeds the rate at which data can be delivered to the network.I. Router buffer overflow.II. Receiver buffer overflow A flow control technique by TCP helps adapt the rate at sending host end and to

prevent overrun at the slow receiver. The flow control mechanism by TCP called the sliding window protocol.

32

Improvements of TCP Performance Traditional networks Transport protocols typically designed for

Fixed end-systems Fixed, wired networks

TCP congestion control Packet loss in fixed networks typically due to (temporary) overload situations Routers discard packets as soon as the buffers are full TCP recognizes congestion only indirectly via missing acknowledgements Retransmissions unwise, they would only contribute to the congestion and make it even

worse Slow-start algorithm as reaction

33

Classification of Schemes

05/02/2023 34

TCP Slow Start Sender calculates a congestion window for a receiver

Start with a congestion window size equal to one segment

Exponential increase of the congestion window up to the congestion threshold, then

linear increase

Missing acknowledgement causes the reduction of the congestion threshold to one

half of the current congestion window

Congestion window starts again with one segment

05/02/2023 35

Congestion avoidance It starts where slow start stops. Once congestion window reaches the congestion threshold level, then after that if an

acknowledgement is received the window size is increased linearly ic the window size doubling is avoided.

The TCP increases its rate linearly by adding one additional picket to its window at each transmission time. If congestion is detected at any point the TCP is reduces its transmission rate to half the previous value. Makes to right transmission rate.

The is scheme is less aggressive than slow start phase.

05/02/2023 36

Fast Retransmit/ Fast Recovery TCP sends an acknowledgement only after receiving a packet. If a sender receives several acknowledgements for the same packet, this is due to a

gap in received packets at the receiver. However, the receiver got all packets up to the gap and is actually receiving packets Therefore, packet loss is not due to congestion, continue with current congestion

window (do not use slow-start)

05/02/2023 37

TCP in mobile networksTCP assumes congestion if packets are dropped typically wrong in wireless networks, here we often have packet loss due to transmission

errors furthermore, mobility itself can cause packet loss, if e.g. a mobile node roams from one

access point (e.g. foreign agent in Mobile IP) to another while there are still packets in transit to the wrong access point and forwarding is not possible

The performance of an unchanged TCP degrades severely however, TCP cannot be changed fundamentally due to the large base of installation in the

fixed network, TCP for mobility has to remain compatible the basic TCP mechanisms keep the whole Internet together

05/02/2023 38

Indirect TCP (I -TCP) Indirect TCP or I-TCP segments the connection

no changes to the TCP protocol for hosts connected to the wired Internet, millions of computers use (variants of) this protocol

optimized TCP protocol for mobile hosts splitting of the TCP connection at, e.g., the foreign agent into 2 TCP connections, no real end-to-end

connection any longer hosts in the fixed part of the net do not notice the characteristics of the wireless part

mobile hostaccess point (foreign agent) wired Internet

“wireless” TCP standard TCP

05/02/2023 39

I-TCP socket and state migration

mobile hostaccess point2

Internet

access point1

socket migrationand state transfer

05/02/2023 40

Indirect TCP II Advantages

no changes in the fixed network necessary, no changes for the hosts (TCP protocol) necessary, all current optimizations to TCP still work

transmission errors on the wireless link do not propagate into the fixed network simple to control, mobile TCP is used only for one hop between, e.g., a foreign agent and mobile

host therefore, a very fast retransmission of packets is possible, the short delay on the mobile hop is

known Disadvantages

loss of end-to-end semantics, an acknowledgement to a sender does not any longer mean that a receiver really got a packet, foreign agents might crash

higher latency possible due to buffering of data within the foreign agent and forwarding to a new foreign agent

05/02/2023 41

Snooping TCP (S-TCP) Transparent extension of TCP within the foreign agent buffering of packets sent to the mobile host lost packets on the wireless link (both directions!) will be retransmitted immediately by the mobile host or

foreign agent, respectively (so called “local” retransmission) the foreign agent therefore “snoops” the packet flow and recognizes acknowledgements in both directions, it

also filters ACKs changes of TCP only within the foreign agent (+min. MH change)

„wired“ Internet

buffering of data

end-to-end TCP connection

local retransmission correspondenthostforeign

agent

mobilehost

snooping of ACKs

05/02/2023 42

Snooping TCP II Data transfer to the mobile host

FA buffers data until it receives ACK of the MH, FA detects packet loss via duplicated ACKs or time-out fast retransmission possible, transparent for the fixed network

Data transfer from the mobile host FA detects packet loss on the wireless link via sequence numbers, FA answers directly with a NACK to

the MH MH can now retransmit data with only a very short delay

Advantages: Maintain end-to-end semantics No change to correspondent node No major state transfer during handover

Problems Snooping TCP does not isolate the wireless link well May need change to MH to handle NACKs Snooping might be useless depending on encryption schemes

05/02/2023 43

Mobile TCP(M-TCP) Special handling of lengthy and/or frequent disconnections M-TCP splits as I-TCP does

unmodified TCP fixed network to supervisory host (SH) optimized TCP SH to MH

Supervisory host no caching, no retransmission monitors all packets, if disconnection detected

set sender window size to 0 sender automatically goes into persistent mode

old or new SH reopen the window Advantages

maintains semantics, supports disconnection, no buffer forwarding Disadvantages

loss on wireless link propagated into fixed network adapted TCP on wireless link

44

Mobile TCP(M-TCP)

05/02/2023 45

Fast retransmit/fast recovery Change of foreign agent often results in packet loss

TCP reacts with slow-start although there is no congestion Forced fast retransmit

as soon as the mobile host has registered with a new foreign agent, the MH sends duplicated acknowledgements on purpose

this forces the fast retransmit mode at the communication partners additionally, the TCP on the MH is forced to continue sending with the actual window size and not to

go into slow-start after registration Advantage

simple changes result in significant higher performance Disadvantage

further mix of IP and TCP (to know when there is a new registration), no transparent approach

05/02/2023 46

Freeze-TCPMobile hosts can be disconnected for a longer time no packet exchange possible, e.g., in a tunnel, disconnection due to overloaded cells or mux.

with higher priority traffic TCP disconnects after time-out completely

TCP freezing MAC layer is often able to detect interruption in advance MAC can inform TCP layer of upcoming loss of connection TCP stops sending, but does not assume a congested link MAC layer signals again if reconnected

Advantage scheme is independent of data and TCP mechanisms (Ack,SN) => works even with IPsec

Disadvantage TCP on mobile host has to be changed, mechanism depends on MAC layer

05/02/2023 47

Selective retransmission TCP acknowledgements are often cumulative

ACK n acknowledges correct and in-sequence receipt of packets up to n if single packets are missing quite often a whole packet sequence beginning at the gap has to be

retransmitted (go-back-n), thus wasting bandwidth Selective retransmission as one solution

RFC2018 allows for acknowledgements of single packets, not only acknowledgements of in-sequence packet streams without gaps

sender can now retransmit only the missing packets Advantage: much higher efficiency Disadvantage

more complex software in a receiver, more buffer needed at the receiver

05/02/2023 48

Transaction oriented TCPTCP phases connection setup, data transmission, connection release using 3-way-handshake needs 3 packets for setup and release, respectively thus, even short messages need a minimum of 7 packets!

Transaction oriented TCP RFC1644, T-TCP, describes a TCP version to avoid this overhead connection setup, data transfer and connection release can be combined thus, only 2 or 3 packets are needed

Advantage efficiency

Disadvantage requires changed TCP mobility no longer transparent

49

50

ReferencesBook: Prasant Kumar Pattnaik, Rajib Mall, “Fundamentals of Mobile Computing”, PHI Learning

Pvt. Ltd, New Delhi – 2012. http://www.omnisecu.com/tcpip/tcpip-model.phphttp://www.computernetworkhomeworkhelp.com/comparison-models/http://www.tcpipguide.com/free/t_InternetProtocolMobilitySupportMobileIP.htmPPT: https://cs.uccs.edu/~chow/pub/master/ycai/mobile_computing_mtu.ppthttp://www.slideshare.net/ayyakathir/it6601-mobile-computing-54556306https:// cs.winona.edu/lin/cs313/CS313DataLink2.ppt

51

Thank You

Questions and Comments?