Transport layer is a 4th layer
from the top. The role of the transport layer is to provide the communication
services directly to the application processes running on different hosts.
The
transport layer provides a logical communication between application processes
running on different hosts. Although the application processes on different
hosts are not physically connected, application processes use the logical
communication provided by the transport layer to send the messages to each
other.
A
computer network provides more than one protocol to the network applications.
For example, TCP and UDP are two transport layer protocols that provide a
different set of services to the network layer. The transport layer protocols
are implemented in the end systems but not in the network routers.
All
transport layer protocols provide multiplexing/demultiplexing service. It also
provides other services such as reliable data transfer, bandwidth guarantees,
and delay guarantees.
Each
of the applications in the application layer has the ability to send a message
by using TCP or UDP. The application communicates by using either of these two
protocols. Both TCP and UDP will then communicate with the internet protocol in
the internet layer. The applications can read and write to the transport layer.
Therefore, we can say that communication is a two-way process.
Services provided by the Transport Layer
1. End-to-end delivery: The transport layer
transmits the entire message to the destination. Therefore, it ensures the
end-to-end delivery of an entire message from a source to the destination.
2. Addressing : Data generated by an
application on one machine must be transmitted to the correct application on
another machine. In this case, addressing is provided by the transport layer.
The transport layer provides the user address which is specified
as a station or port. The port variable represents a particular TS user of a
specified station known as a Transport Service access point (TSAP). Each
station has only one transport entity.
The transport layer protocols need to know which upper-layer
protocols are communicating.
3. Reliable delivery:
The
transport layer provides reliability services by retransmitting the lost and
damaged packets.
The reliable delivery has four aspects:
- Error
control:
The primary role of reliability is Error Control. In reality,
no transmission will be 100 percent error-free delivery. Therefore,
transport layer protocols are designed to provide error-free transmission.
The data link layer also provides the error handling mechanism,
but it ensures only node-to-node error-free delivery. However, node-to-node
reliability does not ensure the end-to-end reliability.
The transport layer performs the checking for the errors
end-to-end to ensure that the packet has arrived correctly.
- Sequence
control:
The second aspect of the reliability is sequence control which is
implemented at the transport layer.
On the sending end,
the transport layer is responsible for ensuring that the packets received from
the upper layers can be used by the lower lay.
- Loss
control:
Loss Control is a third aspect of reliability. The transport layer ensures
that all the fragments of a transmission arrive at the destination, not
some of them. On the sending end, all the fragments of transmission are
given sequence numbers by a transport layer. These sequence numbers allow
the receiver’s transport layer to identify the missing segment.
- Duplication
control:
Duplication Control is the fourth aspect of reliability. The transport
layer guarantees that no duplicate data arrive at the destination.
Sequence numbers are used to identify the lost packets; similarly, it
allows the receiver to identify and discard duplicate segments.
4. Flow Control: Flow control is used
to prevent the sender from overwhelming the receiver. If the receiver is
overloaded with too much data, then the receiver discards the packets and
asking for the retransmission of packets. This increases network congestion and
thus, reducing the system performance.
The transport layer is responsible for flow control. It uses the
sliding window protocol that makes the data transmission more efficient as well
as it controls the flow of data so that the receiver does not become
overwhelmed. Sliding window protocol is byte oriented rather than frame
oriented.
5. Multiplexing: The transport layer uses the multiplexing to improve
transmission efficiency.
Multiplexing can occur in two ways:
Upward multiplexing: Upward multiplexing means multiple transport layer
connections use the same network connection. To make more cost-effective, the
transport layer sends several transmissions bound for the same destination
along the same path; this is achieved through upward multiplexing.
Downward multiplexing: Downward multiplexing means one transport layer connection
uses the multiple network connections. Downward multiplexing allows the
transport layer to split a connection among several paths to improve the
throughput. This type of multiplexing is used when networks have a low or slow
capacity.
Elements of Transport
Protocols
The fourth layer in the OSI model is known as the transport layer
from the top. This layer provides the communication services directly to the
running application processes on different hosts. Even though these processes
on various hosts are not linked physically, and use logical communication to transmit
the messages to each other.
The protocols of this layer are implemented within the end
systems, not in the routers of the network.
For instance, the two transport layer protocols like TCP &UDP
provides different services to the network layer.
All the protocols in this layer provide different services
like multiplexing, de-multiplexing, reliable data transfer, bandwidth
& delay guarantees.
Elements of Transport Layer
The elements of this layer include the following.
Service-point Addressing
Computers frequently run several programs at a time, because of
this reason, the delivery of source to destination in two computers is not
possible. So this layer is used for an exact kind of address to its header
which is known as a port address or service point address. Through this
address, every packet arrives at the correct computer & also transport
layer attains the total message to the exact method on that computer.
Segmentation & Reassembly
In a segmentation process, a message can be separated into communicable
segments where each segment includes a sequence number that enables the layer
to rebuild the message. Once the message arrives from the source system to the
destination system then the message can be reassembled properly, packets can be
identified and replaced because of the lost data in transmission.
Connection Control
These are two types namely connectionless and connection-based.
Connectionless based Transport Layer
In connectionless, the transport layer treats every packet like an
individual & transmits it to the destination computer. In this kind of
transmission, the receiver doesn’t transmit an acceptance to the sender regarding
a packet receipt.
Connection based Transport Layer
This layer makes a connection with the layer at the destination of
the computer before sending the packets toward the target. The connection can
be created by using the following steps: (Three way hand shaking mechanism)
- Establishment of Connection
- Transfer of Data
- Termination Connection
So connection-based service is used for the entire data
transmission, whereas connectionless service is less consistent
Multiplexing & De-multiplexing
Several packets are transmitted across a system from various
applications by using control systems and these are found within the transport
layer. This layer allows packets using different methods, and these packets are
distinguished through their port numbers & transmit them to the layer once
adding correct headers.
In the De-multiplexing process, the data can be obtained which is
transmitted from different processes. It gets the data segments from this layer
to deliver it to the suitable method running on the machine at the receiver
end.
Flow Control
The transport layer is also accountable for the flow control
between the contiguous layers of the TCP/IP model. It doesn’t execute
across a single link even it executes a continuous node. By impressive flow
control methods, loss of data can be stopped from the sender & slow
receiver.
For example, it uses the sliding window protocol technique. In
this technique, the receiver transmits a window back toward the sender to
update the received data size.
Error Control
This is also achieved back-to-back similar to the data link layer.
In this layer, the total message can appear at the receiving end of the layer
without any fault. The improvement of error can be attained through the packet
re-transmission. The integrity of data can be checked once the data has arrived
or not by using the services like the ACK & NACK to update the sender.
