UNIT-4
1 Transport Layer
o The transport layer is a 4th layer from the top.
o The main role of the transport layer is to provide the communication services directly to
the application processes running on different hosts.
o 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.
o The transport layer protocols are implemented in the end systems but not in the network
routers.
o 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.
o All transport layer protocols provide multiplexing/demultiplexing service. It also
provides other services such as reliable data transfer, bandwidth guarantees, and delay
guarantees.
o 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.
The services provided by the transport layer are similar to those of the data link layer. The data
link layer provides the services within a single network while the transport layer provides the
,services across an internetwork made up of many networks. The data link layer controls the
physical layer while the transport layer controls all the lower layers.
The services provided by the transport layer protocols can be divided into five categories:
o End-to-end delivery
o Addressing
o Reliable delivery
o Flow control
o Multiplexing
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.
Reliable delivery: The transport layer provides reliability services by retransmitting the lost and
damaged packets. The reliable delivery has four aspects:
o Error control
o Sequence control
o Loss control
o Duplication control
,Error Control
o 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.
o 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.
o The data link layer checks for the error between each network. If an error is introduced
inside one of the routers, then this error will not be caught by the data link layer. It only
detects those errors that have been introduced between the beginning and end of the link.
Therefore, the transport layer performs the checking for the errors end-to-end to ensure
that the packet has arrived correctly.
Sequence Control
o The second aspect of the reliability is sequence control which is implemented at the
transport layer.
o 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 layers. On the receiving end, it
ensures that the various segments of a transmission can be correctly reassembled.
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.
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.
Sliding window protocol (Refer from unit 2)
2 Multiplexing
The transport layer uses the multiplexing to improve transmission efficiency. Multiplexing can
occur in two ways:
o 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.
1 Transport Layer
o The transport layer is a 4th layer from the top.
o The main role of the transport layer is to provide the communication services directly to
the application processes running on different hosts.
o 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.
o The transport layer protocols are implemented in the end systems but not in the network
routers.
o 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.
o All transport layer protocols provide multiplexing/demultiplexing service. It also
provides other services such as reliable data transfer, bandwidth guarantees, and delay
guarantees.
o 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.
The services provided by the transport layer are similar to those of the data link layer. The data
link layer provides the services within a single network while the transport layer provides the
,services across an internetwork made up of many networks. The data link layer controls the
physical layer while the transport layer controls all the lower layers.
The services provided by the transport layer protocols can be divided into five categories:
o End-to-end delivery
o Addressing
o Reliable delivery
o Flow control
o Multiplexing
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.
Reliable delivery: The transport layer provides reliability services by retransmitting the lost and
damaged packets. The reliable delivery has four aspects:
o Error control
o Sequence control
o Loss control
o Duplication control
,Error Control
o 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.
o 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.
o The data link layer checks for the error between each network. If an error is introduced
inside one of the routers, then this error will not be caught by the data link layer. It only
detects those errors that have been introduced between the beginning and end of the link.
Therefore, the transport layer performs the checking for the errors end-to-end to ensure
that the packet has arrived correctly.
Sequence Control
o The second aspect of the reliability is sequence control which is implemented at the
transport layer.
o 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 layers. On the receiving end, it
ensures that the various segments of a transmission can be correctly reassembled.
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.
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.
Sliding window protocol (Refer from unit 2)
2 Multiplexing
The transport layer uses the multiplexing to improve transmission efficiency. Multiplexing can
occur in two ways:
o 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.
