Mobile computing Mobile Transport Layer

UNIT-III
Mobile Network Layer
1. Mobile IP
i) Goals, Assumptions & Requirements
ii) Entities and Terminology
iii) IP Packet Delivery
iv) Agent Advertisement & Discovery
v) Registration
vi) Tunneling & Encapsulation
vii) Optimizations
2. DHCP
i) Basic Configuration
ii) Client Initialization
CONTENTS

Goal of Network Layer
• Goal of Routing Protocols
– decrease routing-related overhead
– find short routes
– find ―stable‖routes (despite mobility)
• Goal of Mobile IP
– Supporting end-system mobility while
maintaining scalability, efficiency and
compatibility in all
systems.

• Traditional routing
– based on IP address; network prefix
determines the subnet
– change of physical subnet implies
• change of IP address (conform to new subnet), or
• special routing table entries to forward packets to
new subnet
Mobile IP : Motivation

Quick Solution
• Changing of IP address
– Use DHCP to have a new IP address when
mobile device moves to a new subnet, but
then the new address may not be known
to anyone
– Take help of DNS to update the entry, but
DNS updates take long time
– TCP connections break
– security problems
• Changing entries in routing tables
– change routing table entries as the Mobile
Node moves from one network to another
does not scale with the number of mobile
hosts and frequent changes in the

Requirements
• Compatibility
– support of the same layer 2 protocols as IP
– no changes to current end-systems and routers required
– mobile end-systems can communicate with fixed systems
• Transparency
– mobile end-systems keep their IP address
– continuation of communication after interruption of link
possible
– point of connection to the fixed network can be changed
• Efficiency and scalability
– only little additional messages to the mobile system required
(connection typically via a low bandwidth radio link)
– world-wide support of a large number of mobile systems
• Security
– authentication of all registration messages

Terminology
• Mobile Node (MN) - is an end-system that can change the point of connection to the
network without changing its IP address.
• Home Network (HN) – is the subnet the MN belongs to with respect to its IP address.
• Foreign Network (FA) – is the current subnet the MN visits.
• Correspondent Node (CN) – is a fixed or Mobile Node act as partner for communication
with MN.
• Care-of Address (COA)
– address of the current tunnel end-point for the MN (at FA or MN)
– actual location of the MN from an IP point of view
– can be chosen, e.g., via DHCP
• Home Agent (HA)
– Is a system (or router) located in the home network of the MN,
– registers the location of the MN, then tunnels IP datagrams to the COA
• Foreign Agent (FA)
– system in the current foreign network of the MN, typically a router
– typically the default router for the MN

Mobility: Vocabulary
home network:
permanent
“home” of mobile
(e.g., 128.119.40/24)
home agent: entity that
will perform mobility
functions on behalf of
mobile, when mobile is
remote
wide area
network
Permanent address:
address in home
network, can always be
used to reach mobile
e.g., 128.119.40.186
correspondent:
wants to
communicate with
mobile
visited network:
network in which mobile
currently resides (e.g.,
79.129.13/24)
Permanent address:
remains constant
(e.g., 128.119.40.186)
Care-of-address:
address in visited
network.
(e.g., 79,129.13.2)
Foreign agent: entity in visited
network that performs
mobility functions on behalf of1
mobile.

Mobile computing Mobile Transport Layer

Three Phases
To communicate with a remote host, a mobile host goes through three
phases: agent discovery, registration, and data transfer.

Phase-III : Data Transfer or IP PacketDelivery
• Step 1: CN send the packet as usual to the IP address
of MN.
• Step 2: The HA intercepts the packet and the forwarded
into the subnet as usual, but encapsulated and tunneled
to the COA.
• Step 3: The FA now decapsulates the packet and
forwards the original packet with CN as source and MN
as destination to the MN
• Step 4: The MN sends the packet as usual with its own
fixed IP address as source and CN‘s address as the
destination.

Phase – I : Agent Discovery
• HA and FA periodically send
Advertisement messages into air
• MN listens to these messages and
detects, if it is in the home or a foreign
network
– If MN does not wish to wait for the periodic
advertisement, it can send out Agent
Solicitation messages that will be
responded by HA or FS
• MN reads a COA from the FA
advertisement messages

❑ agent advertisement: foreign/home agents advertise service by
broadcasting ICMP messages (typefield = 9)
R B H F M G V
b it s
re s e rv e d
t y p e = 16
t y p e = 9 c o d e = 0 c h e c k s u m
ro u te r a d d re s s
s ta n d a rd
IC M P fie ld s
m o b ilit y a g e n t
a d v e rt is e m e n t
e x te n s io n
le n g th s e q u e n c e #
re g is tra tio n lif e tim e
0 o r m o re c a re -of -
a d d re s s e s
0 8 16 24
R bit: registration
required
H,F bits: home
and/or foreign agent
Mobile IP: agent discovery

Field Description
• Type – ICMP packet type ( 9 – Router
Advt)
• Code – 0 - agent routes traffic for
mobile/non- mobile node – 16 – only for
mobile traffic
• #address – number of router addresses
• Life time – length of time this advertise
valid
• Preference – choose the router, most
eager one to get a new node.

Phase-II: Registration
• MN signals COA to the HA via the FA, HA
acknowledges via FA to MN
– these actions have to be secured by
authentication

Registration
Two Ways : 1. Via FA
2. directly with HA

Registration Request
❑ Type is set to 1 for registration request
❑ Flags –S – simultaneous bindings, B – receive Broadcast pkts, D –
de-capsulation, M & G – Minimul & generic encapsulation, T –
reverse tunnel (replaced with V)
❑ Identification used for replay protection
❑ Uses UDP messages

Registration request flag field bits

❑ Type field set to 3
❑ Extensions must at least contain parameter for authentication
❑ Code field describes status information, e.g. why the
registration failed. These include
• authentication failed, ID mismatch , unknown HA
Registration Reply

Tunneling and Encapsulation
• It describes the mechanism used for forwarding packets
between the HA and COA.
• A tunnel establishes the virtual pipe for data packets
between a tunnel entry and tunnel end
• Encapsulation is the mechanism of
point.
taking a packet
consisting of packet header and data and put into a data
part of another packet.
• Outer Header – Header of New Packet
• Inner Header – identical to header of Original Packet or
can be computed during encapsulation.
• There are different ways of performing the encapsulation –
IP-in-IP (mandatory), minimal and generic routing.

IP-in-IP Encapsulation
• Tunneling
between HA
and COA

Field Description
• The outer IP header source & destination address identify the tunnel
endpoints (i.e. HA & FA).
• Ver – is ‗4‘for IP protocol.
• TOS – now it is DS in the context of Differentiated Services.
• The inner IP header source and destination address identify the
original sender & recipient
• Other headers for authentication might be added to outer header.
• Some outer IP header fields are copied from the inner IP fields (TOS),
most are re-computed (checksum, length) based on new datagram.
• The outer TTL must be high enough so that packet can reach the
tunnel end-point. The inner TTL decremented by one only, that is,
whole tunnel to be considered as single hop.

• We can save space by recognizing that much of the inner header can
be derived from the outer header
– Copy inner header
– Modify protocol field to be 55, for the minimal protocol
– Destination address replaced by tunnel exit
– If encapsulator isn‘t originator of message, replace source address
with address of encapsulator
– Increment total length by the size of the additional header (either
12 or 8 octets)
– Recompute checksum
• Sender sends all packets via HA to MN
• Higher latency and network load
Minimal Encapsulation

Generic Routing Encapsulation
It allows the encapsulation of packets of one protocol suite in to the
payload portion of a packet of another protocolsuite.
Fig: GRE

• The inefficient behavior of a non-
optimized mobile IP is called
Triangular Routing.
• In this all packets to through the
HA. This cause unnecessary
overhead between CN & HA and
HA & COA/FA .
• One way to optimize the route is
inform the CN of the current
location of MN.
• The optimized mobile IP needs 4
additional messages.
Optimizations

Four Messages
• Binding Request (BR) – Any node wants to know the current
location of MN can send BR to the HA.
• Binding Update (BU) – It is sent by the HA to CN reveals the current
location of MN.
• Binding ACK (BA) – If requested, a node returns this ACK after
receiving a BU.
• Binding Warning (BW) – If a node decapsulates a packet for an MN,
but it is not the current FA for this MN, this nods send this BW.

Change of FA with an Optimized mobile IP

Mobile Transport Layer
❑ Traditional TCP
❑ Indirect TCP
❑ Snooping TCP
❑ Mobile TCP
❑ Fast retransmit/recovery
❑ Transmission freezing
❑ Selective retransmission
❑ Transaction oriented TCP
CONTENTS

❑Provide mobility support to the
applications at transport Layer.
❑Keep original concepts while doing
TCP improvements for mobile and
wireless networks.
❑Increase TCP‘s performance.

Introduction
❑ Supporting mobility up to the network layer is not enough to support
mobility support for applications.
❑ Most application rely on a transport layer, such as TCP in case of
internet.
❑ Two functions of the transport layer in the internet are
– Checksumming user data
– Mux/demux of data to/from application
❑ While network layer only addresses a host, ports in UDP/TCPallow
dedicated services to be addressed.
❑ The connectionless UDP does not offer much more than TCP, so, this
unit concentrates only on TCP.
❑ Mobility Support in IP (like Mobile IP) is already enough for UDP towork.
❑ TCP has built-in mechanism to behave in a ‗network friendly‘ manner.

Traditional TCP
Transport protocols typically designed for
– Fixed end-systems
– Fixed, wired networks
Research activities
– Performance
– Congestion control
– Efficient retransmissions
Congestion control
– packet loss in fixed networks typically due to (temporary) overload
situations
– router have to discard packets as soon as the buffers arefull
– TCP recognizes congestion only indirect via missing
acknowledgements, retransmissions unwise, they would only
contribute to the congestion and make it evenworse
– slow-start algorithm as reaction

Slow-start
– sender calculates a congestion window for areceiver
– start with a congestion window size equal to one segment
– exponential increase of the congestion window up to thecongestion
threshold, then linear increase
– missing acknowledgement causes the reduction of thecongestion
threshold to one half of the current congestion window
– congestion window starts again with one segment
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 thereceiver
– however, the receiver got all packets up to the gap and is actually
receiving packets
– therefore, packet loss is not due to congestion, continuewith
current congestion window (do not useslow-start)
Traditional TCP (2)

Implications on mobility
TCP 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 inMobile 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 thelarge
base of installation in the fixed network, TCP for mobility has to
remain compatible
– the basic TCP mechanisms keep the whole Internettogether

Classical TCP
Improvements
❑Indirect TCP (I-TCP)
❑Snooping TCP
❑Mobile TCP (M-TCP)
❑Fast retransmit/fast recovery
❑Transmission/time-out freezing
❑Selective retransmission
❑Transaction-oriented TCP
(T/TCP)

Indirect TCP (I-TCP)
Indirect TCP segments the connection
– no changes to the TCP protocol for hosts connected to thewired
Internet, millions of computers use (variants of) thisprotocol
– optimized TCP protocol for mobile hosts
– splitting of the TCP connection at, e.g., the foreign agent into 2TCP
connections, no real end-to-end connection anylonger
– hosts in the fixed part of the net do not notice the characteristics of
the wireless part
mobile host access point
(foreign agent) ―w
ired―Internet
“wireless” TCP standard TCP
Correspondent
Host

Indirect TCP (I-TCP) [2]
mobile host
access point2
Internet
access point1
socket migration
and state transfer
Figure: I-TCP socket and state migration
Correspondent
Host

Indirect TCP or I-TCP [3]
Advantages
– no changes in the fixed network necessary, no changes for thehosts
(TCP protocol) necessary, all current optimizations to TCP still work
– transmission errors on the wireless link do not propagate into thefixed
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, theshort
delay on the mobile hop is known
Disadvantages
– loss of end-to-end semantics, an acknowledgement to a senderdoes
now not any longer mean that a receiver really got a packet, foreign
agents might crash
– higher latency possible due to buffering of data within theforeign
agent and forwarding to a new foreign agent

Snooping TCP
―Transparent‖extension of TCP within t
h
e
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 filtersACKs
correspondent
host
– chanlogceasloreftTraCnPsmonislsyiow
nithifnortehiegnforeignagent
agent
―w
ired―Internet
mobile
host
snooping of ACKs buffering of data
end-to-end TCPconnection

Snooping TCP [2]
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 fixednetwork
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
Integration of the MAC layer
– MAC layer often has similar mechanisms to those ofTCP
– thus, the MAC layer can already detect duplicated packets dueto
retransmissions and discard them
Problems
– snooping TCP does not isolate the wireless link as good as I-TCP
– snooping might be useless depending on encryptionschemes

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 bufferforwardin1g

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 foreignagent,
the MH sends duplicated acknowledgements on purpose
– this forces the fast retransmit mode at the communicationpartners
– 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 higherperformance
Disadvantage
– further mix of IP and TCP, no transparent approach

Transmission/time-out
freezing
Mobile hosts can be disconnected for a
longer time
– no packet exchange possible, e.g., in a tunnel, disconnectiondue
to overloaded cells or multiplexing 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 now not assume a congestedlink
– MAC layer signals again if reconnected
Advantage
– scheme is independent of data
Disadvantage
– TCP on mobile host has to be changed, mechanism dependson

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 packetsequence
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, notonly
acknowledgements of in-sequence packet streams withoutgaps
– sender can now retransmit only the missing packets
Advantage
– much higher efficiency
Disadvantage
– more complex software in a receiver, more buffer needed atthe

Transaction oriented TCP
(T/TCP)
TCP phases
– connection setup, data transmission, connectionrelease
– using 3-way-handshake needs 3 packets for setup and release,
respectively
– thus, even short messages (one byte) need a minimum of7
packets!
Transaction oriented TCP
– RFC1644, T-TCP, describes a TCP version to avoid this overhead
– connection setup, data transfer and connectionrelease can be
combined
– thus, only 2 or 3 packets are needed
Advantage
– efficiency
Disadvantage
– requires changed TCP
– mobility not longer transparent

Client Server
Connection
setup
Data
transmission
TCP SYN
TCP SYN/ACK
TCPACK
HTTP request
HTTP response
GPRS: 500ms!
>15 s
no data
Connection
release
Transaction oriented TCP [2]
Figure: Example TCP Connection setup overhead

Comparison of different
P
a
A pp
p r op
a c h
roa
Mc
e c h
h
a n ie
s m s forA d
a
v a n “
ta gm
e s obiD
lis
e
a d”
v a n tT
a g eC
s
In d ire c t T C P s p lits T C P co n n e c tio n
in to tw o c o n n e c tio n s
is o la tio n o f w ir e le s s
lin k , s im p le
lo ss o f T C P s e m a n tic s ,
h ig h e r la te n c y a t
h a n d o v e r
S n o o p in g T C P ―
s n o o p s ‖d a ta a n d
a c k n o w le d g e m e n ts , lo c a l
re tra n s m is s io n
tra n s p a re n t fo r e n d -to -
e n d c o n n e c tio n , M A C
in te g ra tio n p o s s ib le
p ro b le m a tic w ith
e n c ryp tio n , b a d is o la tio n
o f w ir e le s s lin k
M -T C P s p lits T C P co n n e c tio n ,
c h o k e s s e n d e r via
w in d o w s iz e
M a in ta in s e n d -to -e n d
s e m a n tic s , h a n d le s
lo n g te rm a n d fre q u e n t
d is c o n n e c tio n s
B a d is o la tio n o f w ir e le s s
lin k , p ro c e s s in g
o v e rh e a d d u e to
b a n d w id th m a n a g e m e n t
F a s t re tr a n s m it/
fa s t re c o ve ry
a vo id s s lo w -s ta rt a fte r
ro a m in g
s im p le a n d e ffic ie n t m ix e d la ye rs , n o t
tra n s p a re n t
T ra n s m is s io n /
tim e -o u t fre e z in g
fre e z e s T C P s ta te a t
d is c o n n e c t, re s u m e s
a fte r re c o n n e c tio n
in d e p e n d e n t o f c o n te n t
o r e n c ry p tio n , w o rk s fo r
lo n g e r in te rru p ts
c h a n g e s in T C P
re q u ire d , M A C
d e p e n d a n t
S e le c tiv e
re tra n s m is s io n
re tra n s m it o n ly lo s t d a ta ve ry e ffic ie n t s lig h tly m o re c o m p le x
re c e iv e r s o ftw a re , m o re
b u ffe r n e e d e d
T ra n s a c tio n
o rie n te d T C P
c o m b in e c o n n e c tio n
s e tu p /r e le a s e a n d d a ta
tra n s m is s io n
E ffic ie n t fo r c e rta in
a p p lic a tio n s
c h a n g e s in T C P
re q u ire d , n o t tra n s p a re n t

Database Hoarding Techniques
A mobile device cannot store a large database due
to memory constraints.
The large databases are available on the servers,
remote computing systems, or networks.
Retrieving the required data from a database
server during every computation- impractical due
to time constraints
A mobile device is not always connected to the
server or network; neither does the device retrieve
data from a server or a network for each
computation.

Database Hoarding Techniques
Rather, the device caches some specific data, which may be required
for future computations, during the interval in which the device is
connected to the server or network.
Cache is a list or database consisting of saved items or records at the
device, which the device saves for faster access at a later time, rather
than reselecting or re-tuning or re-fetching the data when required.
The cached items are the ones which the device fetches by request ,
demand or subscription to the server and saves in the list or database.
Caching entails saving a copy of select data or a part of a database
from a connected system with a large database.
The cached data is hoarded in the mobile device database as and
when the device connects to the server or network
Hoarding of the cached data in the database ensure that even when the
device is not connected to a network or server(disconnected mode),
the data required from the database is available instantly for computing

Example-caching and hoarding:
Consider the train schedules in a railway timetable.
The schedule of specific trains to and from the
device user location is stored in the huge
databases of the servers and networked of the
company operating the trains.
The user device caches and hoards some specific
information from the database and the hoarded
device-database is used during computations from
retrieving the data for a specific set of train
schedules

Database Hoarding
Database hoarding may be done at the
application tier itself. The following figure shows a
simple architecture in which a mobile device API
directly retrieves the data from a database.
It also shows another simple architecture in which
a mobile device API directly retrieves the data from
a database through a program, for ex: IBM DB2
Everyplace (DB2e).

(a)API at mobile device sending queries and retrieving data from local
database (Tier 1) (b)API at mobile device retrieving data fromdatabase
using DB2e (Tier1)

Database Hoarding
Both the two architectures belong to the class of
one-tier database architecture.
Some examples are downloaded ringtones, music
etc. IBM DB2 Everyplace (DB2e) is a relational
database engine which has been designed to
reside at the device.
It supports J2ME and most mobile device
operating systems.

Hoarding multi-tier database
In two-tier or multi-tier databases the databases reside in the remote
servers and the copies of these databases are cached at the client
tiers.
This type of architecture is known as client-server computing
architecture.
Cache is a list or database consisting of saved items or records at the
device.
The device may select and save the item from a set of records
broadcasted or pushed by a server or the device may access (by
request ,demand, or subscription) the item at the server and and save
it to the list or database
Databases are hoarded at the application or enterprise tier, where the
database server uses business logic and connectivity for retrieving the
data and then transmitting it to the device.
The server provides and updates local copies of the database at each
mobile device connected to it

(a) Distributed data cachesin mobile devices (b) Similar architecturefor
a distributed cachememory in multiprocessorsystems

The computing API at the mobile device
(first tier) uses the cached local copy.
At first tier (tier 1), the API uses the cached
data records using the computing
architecture.
From tier 2 or tier 3, the server retrieves and
transmits the data records to tier 1 using
business logic and synchronizes the local
copies at the device.

Advantage of hoarding
•The advantage of hoarding is that there is no access
latency (delay in retrieving the queried record from
the server over wireless mobile networks).
• The client device API has instantaneous data access
to hoarded or cached data.
• After a device caches the data distributed by the
server, the data is hoarded at the device.
•The disadvantage of hoarding is that the consistency of
the cached data with the database at the server needs
to be maintained.

Data Caching
• Hoarded copies of the databases at the servers
are distributed or transmitted to the mobile
devices from the enterprise servers or
application databases.
•The copies cached at the devices are equivalent
to the cache memories at the processors in a
multiprocessor system with a shared main
memory and copies of the main memory data
stored at different locations.

Data Caching cotd..
Cache Access Protocols: A client device caches the
pushed (disseminated) data records from a server.
Caching of the pushed data leads to a reduced access
interval as compared to the pull (on-demand) mode of data
fetching.
Caching of data records can be-based on pushed 'hot
records' (the most needed database records at the client
device).
Also, caching can be based on the ratio of two
parameters—access probability (at the device) and pushing
rates (from the server) for each record.
This method is called cost-based data replacement or
caching

Data Caching cotd..
Pre-fetching: Pre-fetching is another alternative to caching
of disseminated data.
The process of pre-fetching entails requesting for and
pulling records that may be required later.
The client device can pre-fetch instead of caching from the
pushed records keeping future needs in view.
Pre-fetching reduces server load. Further, the cost of
cache-misses can thus be reduced.
The term 'cost of cache-misses' refers to the time taken in
accessing a record at the server in case that record is not
found in the device database when required by the device
API.

Caching Invalidation
Mechanisms
A cached record at the client device may be invalidated.
This may be due to expiry or modification of the record at
the database server.
Cache invalidation is a process by which a cached data
item or record becomes invalid and thus unusable because
of modification, expiry, or invalidation at another computing
system or server.
Cache invalidation mechanisms are used to synchronize
the data at other processors whenever the cache-data is
written (modified) by a processor in a multiprocessor
system.

Caching Invalidation Mechanisms
contd…
A cache consists of several records. Each record is called
a cache-line, copies of which can be stored at other
devices or servers.
The cache at the mobile devices or server databases at
any given time can be assigned one of four possible tags
indicating its state
—modified (after rewriting), exclusive, shared, and
invalidated (after expiry or when new data becomes
available) at any given instance.
These four states are indicated by the letters M, E, S, andI,
respectively (MESI).

The states indicated by the various tags
are as follows:
a)The E tag indicates the exclusive state which means that
the data record is for internal use and cannot be used by
any other device or server.
b) The S tag indicates the shared state which indicates that
the data record can be used by others.
c) The M tag indicates the modified state which means that
the device cache
d)The I tag indicates the invalidated state which means
that the server database no longer has a copy of the record
which was shared and used for computations earlier.

The following figure shows the four possible states of a data
record i at any instant in the server database and its copy at
the cache of the mobile devicej.
Fourpossiblestates (M,E,S,or/) of a data record /at any instance at the server
database and device jcache

Cache consistency
Another important factor for cache
maintenance in a mobile environment is
cache consistency (also called cache
coherence).
This requires a mechanism to ensure that a
database record is identical at the server as
well as at the device caches and that only
the valid cache records are used for
computations.

Data Cache Maintenance in Mobile
Environment
Device needs a data-record while running an application.
A request must be sent to the server for the data
record(this mechanism is called pulling)
The time taken for the application software to access a
particular record is known as access latency.
Caching and hoarding the record at the device reduces
access latency to zero.
Data cache maintenance is necessary in a mobile
environment to overcome access latency.
Data cache inconsistency means that data records cached
for applications are not invalidated at the device when
modified at the server but not modified at the device

Data cache consistency can be maintained by
the three methods:
Cache invalidation mechanism(server-initiated
case)
Polling mechanism(client-initiated case)
Time-to-live mechanism(client-initiated case)

Cache invalidation mechanisms
Cache invalidation mechanisms in mobile
devices are triggered or initiated by the
server.
There are four possible invalidation
mechanisms –
Stateless asynchronous,
stateless synchronous,
stateful asynchronous and
stateful synchronous.

Stateless Asynchronous:
Stateless Asynchronous: A stateless mechanism entails broadcasting
of the invalidation of the cache to all the clients of the server.
The server does not keep track of the records stored at the device
caches.
It just uniformly broadcasts invalidation reports to all clients irrespective
of whether the device cache holds that particular record or not.
The term 'asynchronous' indicates that the invalidation information for
an item is sent as soon as its value changes.
The server advertises the invalidation information only. The client can
either request for a modified copy of the record or cache the relevant
record when data is pushed from the server.
The server advertises as and when the corresponding data-record at
the server is invalidated and modified (deleted or replaced).

The advantage of the asynchronous approach is that there
are no frequent, unnecessary transfers of data reports, thus
making the mechanism more bandwidth efficient.
The disadvantages of this approach are—
(a) every client device gets an invalidation report, whether
that client requires that copy or not and
(b) client devices presume that as long as there is no
invalidation report, the copy is valid for use in
computations.
Therefore, even when there is link failure, the devices may
be using the invalidated data and the server is unaware of
state changes at the clients after it sends the invalidation
report.

Stateless Synchronous
• Stateless Synchronous: This is also a stateless mode, i.e., the
server has no information regarding the present state of data
records at the device caches and broadcasts to all client devices.
•However, unlike the asynchronous mechanism, here the server
advertises invalidation information at periodic intervals as well
as whenever the corresponding data-record at server is
invalidated or modified.
• This method ensures synchronization because even if the in-
between period report is not detected by the device due to a link
failure, the device expects the period-end report of invalidation
and if that is not received at the end of the period, then the
device sends a request for the same (deleted or replaced).
• In case the client device does not get the periodic report due to
link
• failure, it requests the server to send the report.

Polling mechanism(client-initiated case)
Polling means checking the state of the data record, from
the server and determining whether the record is in the
valid, invalid, modified, or exclusive state.
Each cached record copy is polled whenever required by
the application software during computation
The device connects to the server and finds out whether
the cached data record copy at the device has become
invalid or has been modified at the server
If the record is found to be modified or invalidated, then the
device requests for the modified data and replaces the
earlier cached record copy.

Time-to-live mechanism(client-initiated case)
Each cached record is assigned a TTL value
The TTL assignment is adaptive for previous
update intervals of that record.
The client device requests the server to check
whether the cached data record is invalid or
modified at the end of the TTL time.
If it is modified , then the device requests the
server to replace the invalid cached record with the
modified data.
when TTL is set 0, the TTL mechanism is
equivalent to the polling mechanism

Client-server computing for mobile
computing and adaptation
The network architecture can be designed so that a node is either client
or server
A client node runs application software which depends on server nodes
resources(files, databases, web pages, processor power or other
devices or computers connected or networked to it)
The server node has larger resources and computing power than the
client nodes.
This architecture is known as client-server computing architecture
It is different from peer-to-peer architecture, where each node on the
network has similar resources and the various nodes depend on each
others resources
Client-server architecture is used for mobile computing
Mobile devices function as client nodes due to their resources
constraints.

Two Network Based Computing
Architectures
Client-Server:
Client nodes dépends on server resources
A client requests the server for data or
responses which the client then uses in
computations
The client can either access the data
records at the server or cache these records
through broadcasts or distribution from the
server

Client-server Computing
• An N-tier architecture (N = 1, 2, …)
•On the same computing system (not on a
network), then the number of tiers, N = 1
• When the client and the server are on
different computing systems on the network,
then N = 2

Server networks or connecting to
other computing systems
• if server connecting to other systems
provide additional resources to the server for
the client then N > 2
• N > 1 means that the client device at tier
1 connects to the server at tier 2 which,
in
turn, may connected to other

Application server in two-tier client–
server computing architecture
A client is a program(API) used to retrieve records
from databases.
A server is a program that connects to database
and sends the outputs(response) to the client
A server is defines as a computing system, which
responds to request from one or more clients
A client is defined as a computing system, which
request the server for a resources or for executing
a task.

Two-tier Client–Server Architecture

APIs and Synchronization API
• Various APIs synchronization with each
other
• Synchronization─ means that when copies
at the server-end modifies, the cached
copies accordingly modified
• The APIs designed independent of
hardware and software platforms as far as
possible as different devices may have
different platforms

Three-tier Client–Server Architecture
•The application interface, the functional
logic, and the database are maintained at
three different layers
• The database is associated with the
enterprise server tier (tier 3)
• Only local copies of the database exist at
mobile devices

Three-tier Client–Server Architecture
Data records at tier 3 are sent to tier 1 through
synchronization-cum-application server at tier 2.
The synchronization-cum application server has
synchronization server programs, which retrieves data
records from the enterprise tier (tier 3) using business logic,
The enterprise tier connects to the databases using a
connectivity protocol and sends the database records as
per the business logic query to tier 2
There is an in-between server, called synchronization
which sends and synchronizes the copies at the multiple
devices

Mobile computing Mobile Transport Layer

More Related Content

What's hot (20)

Similar to Mobile computing Mobile Transport Layer (20)

More from Jyothishmathi Institute of Technology and Science Karimnagar (20)

Recently uploaded (20)

Mobile computing Mobile Transport Layer