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Transport Layer Introduction
The OSI Model's Transport Layer is the following layer (Layer-4)
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As with all other levels,
this layer communicates with the Transport layer on the remote
host
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The transport layer receives data from the top layer (the
application layer), breaks it into smaller size segments, numbers
each byte, and sends it to the bottom layer (the network layer)
for delivery
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It numbers every byte in the segment
and maintains their accounting
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• This layer provides end-to-end delivery of data
between hosts which may or may not belong to the
same subnet
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End-to-End Communication
A process on one host uses TSAPs, commonly referred to as port
numbers, to locate its peer host on a different network
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For example, when a DHCP client wants to communicate with
remote DHCP server, it always requests on port number 67
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The two main Transport layer protocols are:
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Transmission Control Protocol
It provides reliable communication between two hosts
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Transmission Control Protocol
One of the most crucial protocols in the family of Internet
Protocols is the transmission Control Protocol (TCP)
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Features
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TCP is reliable protocol
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TCP ensures that the data reaches intended
destination in the same order it was sent
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TCP requires that
connection between two remote points be established
before sending actual data
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TCP provides end-to-end communication
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TCP operates in Client/Server point-to-point mode
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e
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Header
The length of TCP header is minimum 20 bytes long and
maximum 60 bytes
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Destination Port (16-bits) - It identifies destination port
of the application process on the receiving device
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Acknowledgement Number (32-bits) - When ACK flag
is set, this number contains the next sequence number
of the data byte expected and works as
acknowledgement of the previous data received
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Reserved (3-bits) - Reserved for future use and all are
set zero by default
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o CWR - When a host receives packet with ECE
bit set, it sets Congestion Windows Reduced
to acknowledge that ECE received
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▪ If SYN bit is set to 1, ECE means that
the device is ECT capable
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ACK - It indicates that Acknowledgement
field has significance
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o PSH - When set, it is a request to the receiving
station to PUSH data (as soon as it comes) to
the receiving application without buffering it
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▪ It is used to reject a segment
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o SYN - This flag is used to set up a connection
between hosts
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Because packets with SYN and FIN flags have
sequence numbers, they are processed in
correct order
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e
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Checksum - This field contains the checksum of
Header, Data and Pseudo Headers
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Options - It facilitates additional options which are not
covered by the regular header
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If this field contains data less
than 32-bit, padding is used to cover the remaining bits
to reach 32-bit boundary
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Ports numbers can range from
0 – 65535 which are divided as:
System Ports (0 – 1023)
• User Ports ( 1024 – 49151)
• Private/Dynamic Ports (49152 – 65535)
Connection Management
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TCP communication works in Server/Client model
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Three-way handshaking is used for connection management
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The server responds with an
acknowledgement with its own Sequence number and an ACK of
the client's segment that is one higher than the client's Sequence
number
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Release
Either of server and client can send TCP segment with FIN flag
set to 1
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Bandwidth Management
TCP uses the concept of window size to accommodate the need
of Bandwidth management
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TCP uses slow start phase by using window
size 1 and increases the window size exponentially after each
successful communication
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When the acknowledgement of this segment received the
windows size is doubled to 4 and next sent the segment sent will
be 4 data bytes long
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If an acknowledgement is missed, i
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data lost in transit network
or it received NACK, then the window size is reduced to half and
slow start phase starts again
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Along with that, it uses
sequence numbers to synchronize itself with the remote host
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The Sender knows which last data segment was received by the
Receiver when it gets ACK
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If the sequence number of a segment recently received does not
match with the sequence number the receiver was expecting,
then it is discarded and NACK is sent back
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Multiplexing
The technique to combine two or more data streams in one
session is called Multiplexing
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The client itself
uses a randomly generated port number from private port
number pools
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For example,
a client requests a web page which in turn contains different
types of data (HTTP, SMTP, FTP etc
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This enables the client system to receive multiple connection
over single virtual connection
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Congestion Control
When large amount of data is fed to system which is not capable
of handling it, congestion occurs
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TCP sets a window size telling the
other end how much data segment to send
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• When keep-alive time expires, the host sends a probe
to check if the connection still exists
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• If the acknowledgement of sent data does not receive
within the Retransmission time, the data segment is
sent again
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• To resume the session a host needs to send Window
Size with some larger value
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• When the Persist timer expires, the host re-sends its
window size to let the other end know
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Timed-Wait:
• After releasing a connection, either of the hosts waits
for a Timed-Wait time to terminate the connection
completely
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• Timed-out can be a maximum of 240 seconds (4
minutes)
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It provides sequence number to
each of byte sent in segment
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e
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When a TCP Server crashes mid-way communication and restarts its process it sends TPDU broadcast to all its hosts
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User Datagram Protocol
The User Datagram Protocol (UDP) is simplest Transport Layer
communication protocol available of the TCP/IP protocol suite
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UDP is said to be an unreliable transport protocol but it uses IP
services which provides best effort delivery mechanism
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This shortcoming makes this
protocol unreliable as well as easier on processing
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For example, in case of
video streaming, thousands of packets are forwarded towards its
users
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The best delivery
mechanism of underlying IP protocol ensures best efforts to
deliver its packets, but even if some packets in video streaming
get lost, the impact is not calamitous and can be ignored easily
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Features
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UDP is used when acknowledgement of data does not
hold any significance
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UDP is simple and suitable for query based
communications
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UDP does not provide congestion control mechanism
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UDP is stateless
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UDP Header
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UDP header is as simple as its function
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• Destination Port - This 16 bits information, is used
identify application level service on destination
machine
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It is 16-bits field and
minimum value is 8-byte, i
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the size of UDP header
itself
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IPv4 has this
field as optional so when checksum field does not
contain any value it is made 0 and all its bits are set to
zero
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This layer exists in both layered Models because of its
significance, of interacting with user and user applications
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A user may or may not directly interacts with the applications
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Because this layer is on the top of the layer stack, it
does not serve any other layers
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When an application layer protocol wants to communicate with
its peer application layer protocol on remote host, it hands over
the data or information to the Transport layer
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There’is an ambiguity in understanding Application Layer and its
protocol
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except those applications which interact with the
communication system
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On the other hand, when we use a Web Browser, which is
actually using Hyper Text Transfer Protocol (HTTP) to interact
with the network
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Another example is File Transfer Protocol, which helps a user to
transfer text based or binary files across the network
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Hence, irrespective of which software you use, it is the protocol
which is considered at Application Layer used by that software
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Client Server Model
Two remote application processes can communicate mainly in
two different fashions:
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Peer-to-peer: Both remote processes are executing at
same level and they exchange data using some shared
resource
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In client-server model, any process can act as Server or Client
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A system can act as Server and Client simultaneously
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This
may also happen that both client and server processes reside on
the same machine
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The second process acting as a Client
also opens a socket but instead of waiting for an incoming
request, the client processes ‘requests first’
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It can either
be an information sharing or resource request
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One process (client) calls the
procedure lying on remote host
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Both processes are allocated stubs
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It passes all the
parameters pertaining to program local to it
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Kernel sends the data over the network and the other
end receives it
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The parameters are passed to the procedure and the
procedure is then executed
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