UNIT 4 -TRANSPORT LAYER
UNIT 4 -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.
Services provided by
the Transport Layer
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:
· The
Process to Process Delivery
o End-to-end delivery
o Addressing
o Reliable delivery
o Flow control
o Multiplexing
Process to Process Delivery
While Data Link Layer
requires the MAC address (48 bits address contained inside the Network
Interface Card of every host machine) of source-destination hosts to correctly
deliver a frame and the Network layer requires the IP address for appropriate
routing of packets, in a similar way Transport Layer requires a Port number to
correctly deliver the segments of data to the correct process amongst the
multiple processes running on a particular host. A port number is a 16-bit
address used to identify any client-server program uniquely.
Process to Process Delivery
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
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.
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.
o 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.
Addressing
o According to the layered model, the transport
layer interacts with the functions of the session layer. Many protocols combine
session, presentation, and application layer protocols into a single layer
known as the application layer. In these cases, delivery to the session layer
means the delivery to the application layer. 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.
o 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.
o The transport layer protocols need to know which
upper-layer protocols are communicating.
Transport Layer protocols
o The transport layer is
represented by two protocols: TCP and UDP.
o The IP protocol in the
network layer delivers a datagram from a source host to the destination host.
o Nowadays, the operating
system supports multiuser and multiprocessing environments, an executing
program is called a process. When a host sends a message to other host means
that source process is sending a process to a destination process. The
transport layer protocols define some connections to individual ports known as
protocol ports.
o An IP protocol is a
host-to-host protocol used to deliver a packet from source host to the
destination host while transport layer protocols are port-to-port protocols
that work on the top of the IP protocols to deliver the packet from the
originating port to the IP services, and from IP services to the destination
port.
o Each port is defined by
a positive integer address, and it is of 16 bits.
UDP
o UDP stands for User Datagram Protocol.
o UDP is a simple
protocol and it provides nonsequenced transport functionality.
o UDP is a connectionless
protocol.
o This type of protocol
is used when reliability and security are less important than speed and size.
o UDP is an end-to-end
transport level protocol that adds transport-level addresses, checksum error
control, and length information to the data from the upper layer.
o The packet produced by
the UDP protocol is known as a user datagram.
User Datagram Format
The user datagram has
a 16-byte header which is shown below:
Where,
o Source port address: It defines the
address of the application process that has delivered a message. The source
port address is of 16 bits address.
o Destination port
address: It defines the address of the application process that will
receive the message. The destination port address is of a 16-bit address.
o Total length: It defines the
total length of the user datagram in bytes. It is a 16-bit field.
o Checksum: The checksum is a
16-bit field which is used in error detection.
Disadvantages of UDP protocol
o UDP provides basic
functions needed for the end-to-end delivery of a transmission.
o It does not provide any
sequencing or reordering functions and does not specify the damaged packet when
reporting an error.
o UDP can discover that
an error has occurred, but it does not specify which packet has been lost as it
does not contain an ID or sequencing number of a particular data segment.
TCP
o TCP stands for
Transmission Control Protocol.
o It provides full
transport layer services to applications.
o It is a
connection-oriented protocol means the connection established between both the
ends of the transmission. For creating the connection, TCP generates a virtual
circuit between sender and receiver for the duration of a transmission.
Features Of TCP protocol
o Stream data transfer: TCP protocol
transfers the data in the form of contiguous stream of bytes. TCP group the
bytes in the form of TCP segments and then passed it to the IP layer for
transmission to the destination. TCP itself segments the data and forward to
the IP.
o Reliability: TCP assigns a
sequence number to each byte transmitted and expects a positive acknowledgement
from the receiving TCP. If ACK is not received within a timeout interval, then
the data is retransmitted to the destination.
The receiving TCP uses the sequence number to reassemble the segments if they
arrive out of order or to eliminate the duplicate segments.
o Flow Control: When receiving
TCP sends an acknowledgement back to the sender indicating the number the bytes
it can receive without overflowing its internal buffer. The number of bytes is
sent in ACK in the form of the highest sequence number that it can receive
without any problem. This mechanism is also referred to as a window mechanism.
o Multiplexing: Multiplexing is a
process of accepting the data from different applications and forwarding to the
different applications on different computers. At the receiving end, the data
is forwarded to the correct application. This process is known as
demultiplexing. TCP transmits the packet to the correct application by using
the logical channels known as ports.
o Logical Connections: The combination
of sockets, sequence numbers, and window sizes, is called a logical connection.
Each connection is identified by the pair of sockets used by sending and
receiving processes.
o Full Duplex: TCP provides Full
Duplex service, i.e., the data flow in both the directions at the same time. To
achieve Full Duplex service, each TCP should have sending and receiving buffers
so that the segments can flow in both the directions. TCP is a
connection-oriented protocol. Suppose the process A wants to send and receive
the data from process B. The following steps occur:
o Establish a connection
between two TCPs.
o Data is exchanged in
both the directions.
o The Connection is
terminated.
TCP Segment Format
Where,
o Source port address: It is used to
define the address of the application program in a source computer. It is a
16-bit field.
o Destination port
address: It is used to define the address of the application program
in a destination computer. It is a 16-bit field.
o Sequence number: A stream of data
is divided into two or more TCP segments. The 32-bit sequence number field
represents the position of the data in an original data stream.
o Acknowledgement number: A 32-field
acknowledgement number acknowledge the data from other communicating devices.
If ACK field is set to 1, then it specifies the sequence number that the
receiver is expecting to receive.
o Header Length (HLEN): It specifies the
size of the TCP header in 32-bit words. The minimum size of the header is 5
words, and the maximum size of the header is 15 words. Therefore, the maximum
size of the TCP header is 60 bytes, and the minimum size of the TCP header is
20 bytes.
o Reserved: It is a six-bit
field which is reserved for future use.
o Control bits: Each bit of a control
field functions individually and independently. A control bit defines the use
of a segment or serves as a validity check for other fields.
There are total six types of flags in control
field:
o URG: The URG field
indicates that the data in a segment is urgent.
o ACK: When ACK field is
set, then it validates the acknowledgement number.
o PSH: The PSH field is
used to inform the sender that higher throughput is needed so if possible, data
must be pushed with higher throughput.
o RST: The reset bit is
used to reset the TCP connection when there is any confusion occurs in the
sequence numbers.
o SYN: The SYN field is
used to synchronize the sequence numbers in three types of segments: connection
request, connection confirmation ( with the ACK bit set ), and confirmation
acknowledgement.
o FIN: The FIN field is
used to inform the receiving TCP module that the sender has finished sending
data. It is used in connection termination in three types of segments:
termination request, termination confirmation, and acknowledgement of
termination confirmation.
o Window Size: The window is a
16-bit field that defines the size of the window.
o Checksum: The checksum is a
16-bit field used in error detection.
o Urgent pointer: If URG flag is
set to 1, then this 16-bit field is an offset from the sequence number
indicating that it is a last urgent data byte.
o Options and padding: It defines the
optional fields that convey the additional information to the receiver.
Differences
b/w TCP & UDP
|
Basis for Comparison |
TCP |
UDP |
|
|
Definition |
TCP establishes a virtual circuit before transmitting the
data. |
UDP transmits the data directly to the destination computer
without verifying whether the receiver is ready to receive or not. |
|
|
Connection Type |
It is a Connection-Oriented protocol |
It is a Connectionless protocol |
|
|
Speed |
slow |
high |
|
|
Reliability |
It is a reliable protocol. |
It is an unreliable protocol. |
|
|
Header size |
20 bytes |
8 bytes |
|
|
acknowledgement |
It waits for the acknowledgement of data and has the ability
to resend the lost packets. |
It neither takes the acknowledgement, nor it retransmits the damaged frame. |
|
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