Lecture 13

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Transcript Lecture 13

Protocols and Protocol Suit
Review
Lecture 13
Overview
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Network Access Layer
Transport Layer
Protocols
Protocol Data Unit
Protocol Architecture
TCP/IP Stack
Layered Approach and its Advantages
Router
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Network Access Layer
Q:- What is the major function of the network access layer?
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OSI Model
Application
Layer 7
Presentation
Layer 6
Session
Layer 5
Transport
Layer 4
Network
Layer 3
Data link
Layer 2
Physical
Layer 1
7 layers OSI model
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Physical Layer Functions
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Establishment and termination of a connection to a
communication medium
Process for effective use of communication resources
(e.g., contention resolution and flow control)
Conversion between representation of digital data in the
end user’s equipment
The physical layer is responsible for movements of individual bits from one hop (node) to the next.
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Data Link Layer Functions
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Responds to service requests from the network layer and issues
requests to the physical layer.
Provides functional and procedural means to transfer data between
network entities and to detect and correct errors that may occur in
the physical layer.
Concerned with:
 Framing
 Physical addressing (MAC address)
 Flow Control
 Error Control
 Access Control
The data link layer is responsible for moving frames from one hop (node) to the next.
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Hop-to-hop Delivery
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Network Layer Functions
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Provides for transfer of variable length sequences from source to
destination via one or more networks
Responds to service requests from the transport layer and issues
requests to the data link layer
Concerned with:
 Data Packet
 Logical addressing (IP address)
 Routing
The network layer is responsible for the delivery of individual packets from the source host to the destination host.
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Source to Destination Delivery
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Transport Layer Functions
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Provides transparent data transfer between end users
Responds to service requests from the session layer and issues
requests to the network layer.
Concerned with:
 Service-point addressing
 Segmentation and reassembly
 Connection control and Flow Control (end-to-end)
 Error Control
The transport layer is responsible for the delivery of a message from one process to another.
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Reliable Process to Process Delivery
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Session Layer Functions
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Provides mechanism for managing a dialogue between end-user
application processes
Responds to service requests from the presentation layer and issues
requests to the transport layer
Supports duplex or half- duplex operations.
Concerned with:
 Dialogue control
 Synchronization (Check point)
The session layer is responsible for dialog control and synchronization.
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Presentation Layer Functions
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Relieves application layer from concern regarding syntactical
differences in data representation with end-user systems
Responds to service requests from the application layer and issues
requests to the session layer
Concerned with:
 Translation
 Encryption
 Compression
The presentation layer is responsible for translation, compression, and encryption.
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Application Layer Functions
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Interfaces directly to and performs common application services for
application processes
Issues service requests to the Presentation layer
Specific services provided:
 Network virtual terminal
 File transfer, access and management
 Mail services
 Directory services
 HTTP, FTP, DHCP…
The application layer is responsible for providing services to the user.
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OSI Layered Model
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TCP/IP Protocol
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The lower four layers correspond to the layer of the OSI model
The application layer of the TCP/IP model represents the three
topmost layers of the OSI model.
The layers in the TCP/IP protocol suite do not exactly match
those in the OSI model. The original TCP/IP protocol suite was
defined as having four layers: host-to-network, internet,
transport, and application. However, when TCP/IP is compared
to OSI, we can say that the TCP/IP protocol suite is made of
five layers: physical, data link, network, transport, and
application.
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TCP/IP Protocol stack
OSI layers
TCP/IP layers
Application
DNS
Presentation
Application
Session
Transport
Network
Data link
Physical
FTP,
Telnet,
SMTP
TCP
IP
OSPF
DHCP
UDP
ICMP
IGMP
Lower level vendor implementations
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Addressing
Four levels of addresses are used in an internet employing the
TCP/IP protocols: physical, logical, port, and specific.
Topics discussed in this section:
Physical Addresses
Logical Addresses
Port Addresses
Specific Addresses
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Addressing
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Addressing
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Example
In Figure below a node with physical address 10
sends a frame to a node with physical address 87.
The two nodes are connected by a link (bus
topology LAN). As the figure shows, the computer
with physical address 10 is the sender, and the
computer with physical address 87 is the receiver.
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Example
Most local-area networks use a 48-bit (6-byte)
physical address written as 12 hexadecimal digits;
every byte (2 hexadecimal digits) is separated by a
colon, as shown below:
07:01:02:01:2C:4B
A 6-byte (12 hexadecimal digits) physical address.
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Example
Figure shows a part of an internet with two routers connecting three LANs. Each device (computer or router) has a pair of
addresses (logical and physical) for each connection. In this case, each computer is connected to only one link and therefore
has only one pair of addresses. Each router, however, is connected to three networks (only two are shown in the figure). So
each router has three pairs of addresses, one for each connection.
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Example
Figure below shows two computers communicating via the Internet. The sending computer is running three processes at
this time with port addresses a, b, and c. The receiving computer is running two processes at this time with port addresses
j and k. Process a in the sending computer needs to communicate with process j in the receiving computer. Note that
although physical addresses change from hop to hop, logical and port addresses remain the same from the source to
destination.
The physical addresses will
change from hop to hop, but the
logical addresses usually remain
the same.
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TCP/IP Protocol stack
OSI layers
TCP/IP layers
Application
DNS
Presentation
Application
Session
Transport
Network
Data link
Physical
FTP,
Telnet,
SMTP
TCP
IP
OSPF
DHCP
UDP
ICMP
IGMP
Lower level vendor implementations
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Internet Protocol (IP)
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Provides connection-less, best-effort service for
delivery of packets through the inter-network
Best-effort: No error checking or tracking done
for the sequence of packets (datagrams) being
transmitted
Upper layer should take care of sequencing
Datagrams transmitted independently and may
take different routes to reach same destination
Fragmentation and reassembly supported to
handle data links with different maximum –
transmission unit (MTU) sizes
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Internet Control Message
Protocol (ICMP)
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Companion protocol to IP
Provides mechanisms for error reporting and
query to a host or a router
Query message used to probe the status of a
host or a router
Error reporting messages used by the host and
the routers to report errors
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Internet Group Management
Protocol (IGMP)
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Used to maintain multicast group membership
within a domain
Similar to ICMP, IGMP query and reply
messages are used by routers to maintain
multicast group membership
Periodic IGMP query messages are used to
find new multicast members within the domain
A member sends a IGMP join message to the
router, which takes care of joining the
multicast tree
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Dynamic Host Configuration
Protocol (DHCP)
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Used to assign IP addresses dynamically in a
domain
Extension to Bootstrap Protocol (BOOTP)
Node Requests an IP address from DHCP server
Helps in saving IP address space by using same
IP address to occasionally connecting hosts
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Internet Routing Protocols
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Routing Information Protocol (RIP)
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An intra-domain distance vector routing protocol
Uses the Bellman-Ford algorithm to calculate routing table
Distance information about all the nodes is conveyed to the
neighbors.
Open Shortest Path First (OSPF)
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Based on shortest path algorithm, sometimes also known as
Dijkstra algorithm
Hosts are partitioned into autonomous systems (AS)
AS is further partitioned into OSPF areas that helps boarder
routers to identify every single node in the area
Link-state advertisements sent to all routers within the same
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hierarchical area
Internet Routing Protocols
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Border Gateway Protocol (BGP)
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Intra-autonomous systems communicate with each
other using path vector routing protocol
Each entry in the routing table contains the
destination network, the next router, and the path
to reach the destination
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Example
Interior Router
BGP Router
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TCP
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A routing table maintained at each
node, indicating the best known
distance and next hop to get there
Calculate w(u,v), is the cost
associated with edge uv
Calculate d(u), the distance of
node u from a root node
For each uv, find minimum d(u,v)
Repeat n-1 times for n-nodes
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Application Layer
 Top three layers (session,
presentation, and application)
merged into application layer
Routing using Bellman-Ford Algorithm
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Root
Abstract model of a wireless network in the form of a graph
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TCP (ctd)
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1
6
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3
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Abstract model of a wireless
network in the form of a graph
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1
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2
4
0
Pass 3
Pass 4
0
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*
*
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3
4
0
Pass 0
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Pass 2
To Node
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Pass 1
8 8
0
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8
8 8
Pass 1
3
8
0
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8 8
Pass 0
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8
0
Root
8 8
To Node
0
7
3
1
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Pass 2
*
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0
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0
0
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3
1
2
Pass 3
*
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0
4
0
0
4
3
1
2
Pass 4
*
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0
4
0
Successive calculation of distance D(u)
from node 0
0
Predecessor from node 0 to other
network nodes
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TCP over Wireless
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The wireless domain is not only plagued by the mobility
problem, but also by high error rates and low BW
Traditional TCP: provides a connected-oriented, reliable,
and byte stream service
TCP functions: flow-control (controlled by sliding
window), congestion-control (congestion window), data
segmentation, retransmission, and recovery
Slow Start: resets the congestion window (CW) size to
one and let threshold to half of the current CW size
 Double the CW on every successful transmission
until the CW reach threshold and after that increases
the CW by one for each successful transmission
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Solutions for Wireless Environment
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Networking layering provides good abstraction
in the network design
Wireless networks are interference limited, and
the information delivery capability is closely
dependent on current channel quality
Adoption in physical and link layer broadcast
could lead to efficient resource usage
Protocol changes need to be made in MSs and
mobile access points to ensure compatibility
with existing TCP applications
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End-to-End Solutions
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TCP-SACK
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WTCP Protocol
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Selective Acknowledgement and Selective Retransmission.
Sender can retransmit missing data due to random
errors/mobility
Separate flows for wired (Sender to AP) and wireless (AP
to MS) segments of TCP connections
Local retransmission for mobile link breakage
AP sends ACK to sender after timestamp modification to
avoid change in round trip estimates
Freeze-TCP Protocol
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Mobile detects impending handoff
Advertises Zero Window size, to force the sender into Zero
Window Probe mode
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End-to-End Solutions (Cont’d)
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Explicit Band State Notification (EBSN)
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Local Retransmission from BS (AP) to shield wireless
link errors
EBSN message from BS to Source during local
recovery
Source Resets its timeout value after EBSN
Fast Retransmission Approach
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Tries to reduce the effect of MS handoff
MS after handoff sends certain number of duplicate
ACKs
Avoids coarse time-outs at the sender, accelerates
retransmission
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Link Layer Protocols
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Snoop Protocol
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Transport layer aware Snoop Agent at BS
Agent monitors all TCP segments destined to MS,
caches it in buffer
Also monitors ACKs from MS
Loss detected by duplicate ACKs from MS or local
time-out
Local Retransmission of missing segment if cached
Suppresses the duplicate ACKs
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Split TCP Approach
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Indirect TCP: splits the TCP connection into two
distinct connections, one is MS and BS and
another is BS and corresponding node (CN)
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The AP acts as a proxy for MS
The AP acknowledges CN for the data sent to MS and
buffers this data until it is successfully transmitted to
MS
Handoff may take a longer time as all the data
acknowledged by AP and not transmitted to MS must
be buffered at the new AP
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Indirect TCP
Wireless
link
MS
Wired
Domain
AP
CN
(Acts as proxy)
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Split TCP Approach (Cont’d)
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M-TCP Protocol
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Split the connection into wired component and
wireless component
BS relays ACKs for sender only after receiving ACKs
from MS
In case of frequent disconnections, receiver can
signal sender to enter in persist mode by advertising
Zero Window size
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Impact of Mobility
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Handoffs occur in wireless domains when an
MN moves into a new BS’s domain
The result of the packet loss during handoff is
slow start
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The solution involves artificially forcing the sender to go into
fast retransmission mode immediately, by sending DUP ACK
after the handoff, instead of go into slow start
Using multicast: the MN is required to define a
group of BSs that it is likely to visit in the near
future
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Reduce the handoff latency: Only one BS is in contact with the
MN and the others buffer the packets addressed to the
multicast address
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Internet Protocol Version 6
(IPv6)
 Designed to address the unforeseen growth of the internet
and the limited address space provided by IPv4
 Features of IPv6:
• Enhanced Address Space: 128 bits long, can solve the
problem created by limited IPv4 address space (32 bits)
• Resource Allocation: By using “Flow Label”, a sender can
request special packet handling
• Modified Address Format: Options and Base Header are
separated which speeds up the routing process
• Support for Security: Encryption and Authentication
options are supported in option header
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IPv4 Header Format
Version
(4 bits)
Header
length (4
bits)
Type of
service (8
bits)
Identification (16 bits)
Time to live
(8 bits)
Protocol
(8 bits)
Total length (16 bits)
Flags
(3 bits)
Fragment offset
(13 bits)
Header checksum (16 bits)
Source address (32 bits)
Destination address (32 bits)
Options and padding (if any)
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IPv6 Header Format
Address Space
Resource Allocation
Modified Header Format
Support for Security
Version
Traffic Class
Flow Label
Payload Length
Next
Hop
Header
Limit
Source Address
Destination Address
Data
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Format of IPv6
Name
Bits
Function
Version
4
IPv6 version number
Traffic Class
8
Internet traffic priority delivery value
Flow Label
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Used for specifying special router handling from source to
destination(s) for a sequence of packets
Payload Length
Next Header
Hop Limit
16,
unsigned
8
8,
unsigned
Specifies the length of the data in the packet. When set to zero,
the option is a hop-by-hop Jumbo payload
Specifies the next encapsulated protocol. The values are
compatible with those specified for the IPv4 protocol field
For each router that forwards the packet, the hop limit is
decremented by 1. When the hop limit field reaches zero, the
packet is discarded. This replaces the TTL field in the IPv4
header that was originally intended to be used as a time based
hop limit
Source Address
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The IPv6 address of the sending node
Destination Address
128
The IPv6 address of the destination node
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Differences between IPv4 and
IPv6
 Expanded Addressing Capabilities
 Simplified Header Format
 Improved Support for Options and
Extensions
 Flow Labeling Capabilities
 Support for Authentication and Encryption
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Network Transition from IPv4 to IPv6
• Dual IP-Stack:
 IPv4-hosts and IPv4-routers have an IPv6-stack, this
ensures full compatibility to not yet updated systems
• IPv6-in-IPv4 Encapsulation (Tunneling):
 Encapsulate IPv6 datagram in IPv4 datagram and
tunnel it to next router/host
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Network Access Layer
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The Internet Protocol Suite (commonly known as TCP/IP) is the set of
communications protocols used for the Internet and other similar networks.
Transmission Control Protocol (TCP) and the Internet Protocol (IP)
The Internet Protocol Suite may be viewed as a set of layers. Each layer solves a set
of problems involving the transmission of data, and provides a well-defined service to
the upper layer protocols based on using services from some lower layers.
The TCP/IP model consists of four layers. This layer architecture is often compared
with the seven-layer OSI Reference Model. From lowest to highest, these are
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the Network Access Layer,
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the Internet Layer,
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the Transport Layer,
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and the Application Layer
The TCP/IP Network Access Layer can
encompass the functions of two lower layers
of theOSI reference Model:
Data Link, and Physical.
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Network Access Layer
Q:- What is the major function of the network access layer?
Ans: The network access layer is concerned with the
exchange of data between a computer and the network to
which it is attached.
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Transport Layer Recap
Q:- What tasks are performed by the transport layer?
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Isolates messages from lower and upper layers
Breaks down message size
Monitors quality of communications channel
Selects most efficient communication service necessary for a
given transmission
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Transport Layer
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Concerned with reliable transfer of information between
applications
Independent of the nature of the application
Includes aspects like flow control and error checking
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Transport Layer Recap
Q:- What tasks are performed by the transport layer?
Ans:- The transport layer is concerned with data reliability
and correct sequencing.
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