COMPUTER NETWORKS NOTES Unit 4

KARPAGAM INSTITUTE OF
TECHNOLOGY
Course Code with Name :CS8591/Computer
Networks
Prepared by
M.Sudha,AP/IT
1
Karpagam Institute of Technology
4/5/2022

UNIT – IV
TRANSPORT LAYER
4/5/2022 Karpagam Institute of Technology 2
Introduction – Transport Layer Protocols –
Services – Port Numbers – User Datagram
Protocol – Transmission Control Protocol –
SCTP.

INTRODUCTION
 The transport layer is the fourth layer of the
OSI model and is the core of the Internet
model.
 It responds to service requests from the
session layer and issues service requests
to the network Layer.
 The transport layer provides transparent
transfer of data between hosts.
 It provides end-to-end control and
information transfer with the quality of service
needed by the application program.
 It is the first true end-to-end layer,
implemented in all End Systems (ES).

TRANSPORT LAYER
FUNCTIONS / SERVICES
 The transport layer is located between
the network layer and the application
layer.
 The transport layer is responsible for
providing services to the application
layer; it receives services from the
network layer.

TRANSPORT LAYER
FUNCTIONS / SERVICES
 The services that can be provided by
the transport layer are
1. Process-to-Process Communication
2. Addressing : Port Numbers
3. Encapsulation and Decapsulation
4. Multiplexing and Demultiplexing
5. Flow Control
6. Error Control
7. Congestion Control

Process-to-Process
Communication
 The Transport Layer is responsible for
delivering data to the appropriate
application process on the host
computers.
 This involves multiplexing of data from
different application processes, i.e.
forming data packets, and adding source
and destination port numbers in the
header of each Transport Layer data
packet.
 Together with the source and destination
IP address, the port numbers constitutes
a network socket, i.e. an identification
address of the process-to-process

Addressing: Port Numbers
 Ports are the essential ways to address multiple
entities in the same location.
 Using port addressing it is possible to use more
than one network-based application at the same
time.
 Three types of Port numbers are used :
Well-known ports - These are permanent port
numbers. They range between 0 to 1023.These
port numbers are used by Server Process.
Registered ports - The ports ranging from 1024 to
49,151 are not assigned or controlled.
Ephemeral ports (Dynamic Ports) – These are
temporary port numbers. They range between
49152–65535.These port numbers are used by
Client Process.

Encapsulation and
Decapsulation
 To send a message from one process to
another, the transport-layer protocol
encapsulates and decapsulates messages.
 Encapsulation happens at the sender site.
The transport layer receives the data and
adds the transport-layer header.
 Decapsulation happens at the receiver site.
When the message arrives at the
destination transport layer, the header is
dropped and the transport layer delivers the
message to the process running at the
application layer. 4/5/2022 Karpagam Institute of Technology 10

Multiplexing and
Demultiplexing
 Whenever an entity accepts items from
more than one source, this is referred to as
multiplexing (many to one).
 Whenever an entity delivers items to more
than one source, this is referred to as
demultiplexing (one to many).
 The transport layer at the source performs
multiplexing
 The transport layer at the destination
performs demultiplexing

Flow Control
 Flow Control is the process of managing
the rate of data transmission between
two nodes to prevent a fast sender from
overwhelming a slow receiver.
 It provides a mechanism for the receiver to
control the transmission speed, so that
the receiving node is not overwhelmed with
data from transmitting node.

Flow Control

Error Control
 Error control at the transport layer is
responsible for
1. Detecting and discarding corrupted
packets.
2. Keeping track of lost and discarded
packets and resending them.
3. Recognizing duplicate packets and
discarding them.
4. Buffering out-of-order packets until the
missing packets arrive.
 Error Control involves Error Detection and
Error Correction 4/5/2022 Karpagam Institute of Technology 14

Error Control

Congestion Control
 Congestion in a network may occur if the load on the
network (the number of packets sent to the network)
is greater than the capacity of the network (the
number of packets a network can handle).
 Congestion control refers to the mechanisms and
techniques that control the congestion and keep the
load below the capacity.
 Congestion Control refers to techniques and
mechanisms that can either prevent congestion,
before it happens, or remove congestion, after it has
happened
 Congestion control mechanisms are divided into two
categories,
1. Open loop - prevent the congestion before it
happens. 4/5/2022 Karpagam Institute of Technology 16

PORT NUMBERS
 A transport-layer protocol usually has several
responsibilities.
 One is to create a process-to-process
communication.
 Processes are programs that run on hosts. It could
be either server or client.
 A process on the local host, called a client, needs
services from a process usually on the remote host,
called a server.
 Processes are assigned a unique 16-bit port number
on that host.
 Port numbers provide end-to-end addresses at the
transport layer
 They also provide multiplexing and demultiplexing at
this layer. 4/5/2022 Karpagam Institute of Technology 17

PORT NUMBERS

 ICANN (Internet Corporation for Assigned
Names and Numbers) has divided the port
numbers into three ranges:
◦ Well-known ports
◦ Registered
◦ Ephemeral ports (Dynamic Ports)

WELL-KNOWN PORTS
 These are permanent port numbers used by
the servers.
 They range between 0 to 1023.
 This port number cannot be chosen
randomly.
 These port numbers are universal port
numbers for servers.
 Every client process knows the well-known
port number of the corresponding server
process.
 For example, while the daytime client
process, a well-known client program, can
use an ephemeral (temporary) port number,
52,000, to identify itself, the daytime server
process must use the well-known

WELL-KNOWN PORTS

EPHEMERAL PORTS
(DYNAMIC PORTS)
 The client program defines itself with a port
number, called the ephemeral port number.
 The word ephemeral means “short-lived” and
is used because the life of a client is normally
short.
 An ephemeral port number is recommended
to be greater than 1023.
 These port number ranges from 49,152 to
65,535
 They are neither controlled nor registered.
They can be used as temporary or private
port numbers.

REGISTERED PORTS
 The ports ranging from 1024 to 49,151
are not assigned or controlled.

TRANSPORT LAYER
PROTOCOLS
 Three protocols are associated with the
Transport layer.
 They are
(1) UDP –User Datagram Protocol
(2) TCP – Transmission Control Protocol
(3) SCTP - Stream Control Transmission
Protocol
 Each protocol provides a different type of
service and should be used appropriately.

 UDP - UDP is an unreliable connectionless
transport-layer protocol used for its simplicity and
efficiency in applications where error control can be
provided by the application-layer process.
 TCP - TCP is a reliable connection-oriented
protocol that can be used in any application where
reliability is important.
 SCTP - SCTP is a new transport-layer protocol
designed to combine some features of UDP and
TCP in an effort to create a better protocol for
multimedia communication.

USER DATAGRAM PROTOCOL (UDP)
 User Datagram Protocol (UDP) is a connectionless,
unreliable transport protocol.
 UDP adds process-to-process communication to best-
effort service provided by IP.
 UDP is a very simple protocol using a minimum of
overhead.
 UDP is a simple demultiplexer, which allows multiple
processes on each host to communicate.
 UDP does not provide flow control , reliable or
ordered delivery.
 UDP can be used to send small message where
reliability is not expected.
 Sending a small message using UDP takes much less
interaction between the sender and receiver.
 UDP allow processes to indirectly identify each other
using an abstract locator called port or mailbox

UDP PORTS
 Processes (server/client) are identified by an abstract
locator known as port.
 Server accepts message at well known port.
 Some well-known UDP ports are
7–Echo, 53–DNS, 111–RPC, 161–SNMP, etc.
 < port, host > pair is used as key for demultiplexing.
 Ports are implemented as a message queue.
 When a message arrives, UDP appends it to end of the
queue.
 When queue is full, the message is discarded.
 When a message is read, it is removed from the queue.
 When an application process wants to receive a
message, one is removed from the front of the queue.
 If the queue is empty, the process blocks until a
message becomes available.

UDP DATAGRAM (PACKET) FORMAT
 UDP packets are known as user datagrams
.
 These user datagrams, have a fixed-size
header of 8 bytes made of four fields, each
of 2 bytes (16 bits).

Source Port Number
 Port number used by process on source host with 16
bits long.
 If the source host is client (sending request) then the
port number is an temporary one requested by the
process and chosen by UDP.
 If the source is server (sending response) then it is
well known port number.
Destination Port Number
 Port number used by process on Destination host with 16 bits
long.
 If the destination host is the server (a client sending request)
then the port number is a well known port number.
 If the destination host is client (a server sending response) then
port number is an temporary one copied by server from the
request packet.

Length
 This field denotes the total length of the UDP
Packet (Header plus data)
 The total length of any UDP datagram can be
from 0 to 65,535 bytes.
Checksum
 UDP computes its checksum over the UDP
header, the contents of the message body, and
something called the pseudoheader.
 The pseudoheader consists of three fields from
the IP header—protocol number, source IP
address, destination IP address plus the UDP
length field.
Data
 Data field defines tha actual payload to be
transmitted.
 Its size is variable.

UDP SERVICES
Process-to-Process Communication
 UDP provides process-to-process
communication using socket
addresses, a combination of IP
addresses and port numbers.

UDP SERVICES
Connectionless Services
 UDP provides a connectionless service.
 There is no connection establishment and no
connection termination .
 Each user datagram sent by UDP is an
independent datagram.
 There is no relationship between the different
user datagrams even if they are coming from
the same source process and going to the
same destination program.
 The user datagrams are not numbered.
 Each user datagram can travel on a different
path.

UDP SERVICES
Flow Control
 UDP is a very simple protocol.
 There is no flow control, and hence no
window mechanism.
 The receiver may overflow with incoming
messages.
 The lack of flow control means that the
process using UDP should provide for this
service, if needed.

UDP SERVICES
Error Control
 There is no error control mechanism in UDP
except for the checksum.
 This means that the sender does not know if a
message has been lost or duplicated.
 When the receiver detects an error through the
checksum, the user datagram is silently
discarded.
 The lack of error control means that the
process using UDP should provide for
this service, if needed.

UDP SERVICES
Checksum
 UDP checksum calculation includes three
sections: a pseudoheader, the UDP header,
and the data coming from the application
layer.
 The pseudoheader is the part of the header
in which the user datagram is to be
encapsulated with some fields filled with 0s.

UDP SERVICES
Optional Inclusion of Checksum
 The sender of a UDP packet can choose
not to calculate the checksum.
 In this case, the checksum field is filled with
all 0s before being sent.
 In the situation where the sender decides to
calculate the checksum, but it happens that
the result is all 0s, the checksum is
changed to all 1s before the packet is sent.
 In other words, the sender complements
the sum two times.

UDP SERVICES
Congestion Control
 Since UDP is a connectionless protocol, it
does not provide congestion control.
 UDP assumes that the packets sent are
small and sporadic(occasionally or at
irregular intervals) and cannot create
congestion in the network.
 This assumption may or may not be true,
when UDP is used for interactive real-time
transfer of audio and video.

UDP SERVICES
Encapsulation and Decapsulation
 To send a message from one process
to another, the UDP protocol
encapsulates and decapsulates
messages.

UDP SERVICES
Queuing
 In UDP, queues are associated with
ports.
 At the client site, when a process starts,
it requests a port number from the
operating system.
 Some implementations create both an
incoming and an outgoing queue
associated with each process.
 Other implementations create only an
incoming queue associated with each
process.

UDP SERVICES
Multiplexing and Demultiplexing
 In a host running a transport protocol
suite, there is only one UDP but
possibly several processes that may
want to use the services of UDP.
 To handle this situation, UDP
multiplexes and demultiplexes.

APPLICATIONS OF UDP
 UDP is used for management processes such as
SNMP.
 UDP is used for route updating protocols such as
RIP.
 UDP is a suitable transport protocol for
multicasting. Multicasting capability is embedded in
the UDP software
 UDP is suitable for a process with internal flow and
error control mechanisms such as Trivial File
Transfer Protocol (TFTP).
 UDP is suitable for a process that requires simple
request-response communication with little concern
for flow and error control.
 UDP is normally used for interactive real-time
applications that cannot tolerate uneven delay

The Transport Services
 The transport protocol should provide
to higher-level protocols.
 The transport entity that provides
services to transport service users,
which might be an application
process.

The Transport Services
 The following categories of service are
useful for describing the transport
service.
Type of service
Quality of service
Data transfer
User interface
Connection management
Expedited delivery
Status reporting
Security

COMPUTER NETWORKS NOTES Unit 4

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COMPUTER NETWORKS NOTES Unit 4