UNIT-V

TRANSPORT LAYER
Transport layer the need for process-to-process delivery. The Internet model has three protocols at the transport layer: UDP, TCP,
and SCTP. First we discuss UDP, which is the simplest of the three. We see how we can use this very simple transport layer
protocol that lacks some of the features of the other two. We then discuss TCP, a complex transport layer protocol. We finally
discuss SCTP, the new transport layer protocol that is designed for multi homed , multi stream applications such as multimedia.
UDP

UDP provides connectionless, unreliable, datagram service. Connectionless service means that there is no logical connection
between the two ends exchanging messages. Each message is an independent entity encapsulated in a datagram.
UDP does not see any relation (connection) between consequent datagram coming from the same source and going to the same
destination.
UDP has an advantage: it is message-oriented. It gives boundaries to the messages exchanged. An application program may be
designed to use UDP if it is sending small messages and the simplicity and speed is more important for the application than
reliability.

User Datagram
UDP packets, called user datagram, have a fixed-size header of 8 bytes made of four fields, each of 2 bytes (16 bits).
. The 16 bits can define a total length of 0 to 65,535 bytes. However, the total length needs to be less because a UDP user datagram
is stored in an IP datagram with the total length of 65,535 bytes. The last field can carry the optional checksum

UDP Services
 Process-to-Process Communication.
 Connectionless Services
 Flow Control
UDP is a very simple protocol. There is no flow control’
 Error Control
There is no error control mechanism in UDP except for the checksum.
 Checksum

UDP checksum calculation includes three sections: a pseudo header, the UDP header, and the data coming from the application
layer. The pseudo header is the part of the header of the IP packet in which the user datagram is to be encapsulated with some fields
filled with 0s
Typical Applications
The following shows some typical applications that can benefit more from the services of UDP
1.UDP is suitable for a process that requires simple request-response communication with little concern for flow and error control
2.UDP is suitable for a process with internal flow- and error-control mechanisms. For example, the Trivial File Transfer Protocol
(TFIP)
3.UDP is a suitable transport protocol for multicasting. Multicasting capability is embedded in the UDP software
4.UDP is used for management processes such as SNMP
5.UDP is used for some route updating protocols such as Routing Information Protocol (RIP)
6.UDP is normally used for interactive real-time applications that cannot tolerate uneven delay between sections of a received
message

TRANSMISSION CONTROL PROTOCOL:

Transmission Control Protocol (TCP) is a connection-oriented, reliable protocol. TCP explicitly defines connection
establishment, data transfer, and connection teardown phases to provide a connection-oriented service.

TCP Services:
 Process-to-Process Communication
 Stream Delivery Service
 Segments
 Full-Duplex Communication
 Reliable Service
Format:
The segment consists of a header of 20 to 60 bytes, followed by data from the application program.The header is 20 bytes if there
are no options and up to 60 bytes if it contains options.

Source port address This is a 16-bit field that defines the port number of the application program in the host that is sending the
segment.
Destination port address This is a 16-bit field that defines the port number of the application program in the host that is receiving
the segment.
Sequence number This 32-bit field defines the number assigned to the first byte of data contained in this segment.
Acknowledgment number This 32-bit field defines the byte number that the receiver of the segment is expecting to receive from
the other party.
Header length This 4-bit field indicates the number of 4-byte words in the TCP header. The length of the header can be between
20 and 60 bytes.
Reserved. This is a 6-bit field reserved for future use.
Control. This field defines 6 different control bits or flags as shown in Figure .One or more of these bits can be set at a time.
URG: Urgent pointer is valid
ACK: Acknowledgment is valid
PSH: Request for push
RST: Reset the connection
SYN: Synchronize sequence numbers
FIN: Terminate the connection
Window size. The length of this field is 16 bits, which means that the maximum size of the window is 65,535 bytes.
Checksum. This 16-bit field contains the checksum. The calculation of the checksum for TCP follows the same procedure as the
one described for UDP.
Urgent pointer. This l6-bit field, which is valid only if the urgent flag is set, is used when the segment contains urgent data.
Options. There can be up to 40 bytes of optional information in the TCP header.

TCP Features:
Numbering System
• Byte Number
• Sequence Number
• Acknowledgment Number
.
Flow Control
TCP, unlike UDP, provides flow control.
Error Control
To provide reliable service, TCP implements an error control mechanism.
Congestion Control
TCP, unlike UDP, takes into account congestion in the network.

A TCP Connection
 TCP is connection-oriented. a connection-oriented transport protocol establishes a logical path between the source and
destination.
.
 In TCP, connection-oriented transmission requires three phases: connection establishment, data transfer, and connection
termination.

Connection Establishment
TCP transmits data in full-duplex mode. When two TCPs in two machines are connected, they are able to send segments to each
other simultaneously.

Three- Way Handshaking

The connection establishment in TCP is called three-way handshaking. an application program, called the client, wants to make a
connection with another application program, called the server, using TCP as the transport-layer protocol The process starts with
the server.
.
A SYN segment cannot carry data, but it consumes one sequence number.
A SYN + ACK segment cannot carry data, but it does consume one sequence number.
An ACK segment, if carrying no data, consumes no sequence number
Data Transfer
After connection is established, bidirectional data transfer can take place. The client and server can both send data and
acknowledgments.

Connection Termination
Any of the two parties involved in exchanging data (client or server) can close the connection ,although it is usually initiated by the
client. Most implementations today allow two options for connection termination: three-way handshaking.

ATM
Asynchronous Transfer Mode (ATM) is an International Telecommunication Union- Telecommunications Standards Section (ITU-
T) standard for cell relay wherein information for multiple service types, such as voice, video, or data, is conveyed in small, fixed-
size cells. ATM networks are connection-oriented. ATM technology has been implemented in a very broad range of networking
devices.
Benefits of ATM
 Dynamic bandwidth for bursty traffic meeting application needs and delivering high utilization of networking
resources.
 Smaller header with respect to the data to make the efficient use of bandwidth
 Can handle Mixed network traffic very efficiently.
 Cell network: All data is loaded into identical cells that can be transmitted with complete predictability and uniformity.
 Class-of-service support for multimedia traffic allowing applications with varying throughput and latency requirements
to be met on a single network.
 Scalability in speed and network size supporting link speeds of T1/E1 to OC–12 (622 Mbps).
 Common LAN/WAN architecture allowing ATM to be used consistently from one desktop to another.
 International standards compliance in central-office and customer-premises environments allowing for multivendor
operation.

ATM NETWORKS :
Public ATM Network:
o Provided by public telecommunications carriers (e.g., AT&T, MCI WorldCom, and Sprint)
o Interconnects private ATM networks
o Interconnects remote non-ATM LANs
o Interconnects individual users
Private ATM Network:
o Owned by private organizations
o Interconnects low speed/shared medium LANs (e.g., Ethernet, Token Ring, FDDI) as a backbone network
o Interconnects individual users as the front-end LAN for high performance or multimedia applications

ATM Network Interfaces

An ATM network consists of a set of ATM switches interconnected by point-to-point ATM links or interfaces. ATM switches
support two primary types of interfaces: UNI and NNI as shown in Fig. The UNI (User-Network Interface) connects ATM end
systems (such as hosts and routers) to an ATM switch. The NNI (Network-Network Interface) connects two ATM switches. UNI
and NNI can be further subdivided into public and private UNIs and NNIs.

Figure 4.6.3 UNI and NNI interfaces of the ATM

ATM Cell Format
ATM transfers information in fixed-size units called cells. Each cell consists of 53 octets, or bytes as shown in Fig. The first 5
bytes contain cell-header information, and the remaining 48 contain the payload (user information).

Header Payload
5 bytes 48 bytes

Figure ATM cell Format

An ATM cell header can be one of two formats: UNI or NNI. The UNI header is used for communication between ATM endpoints
and ATM switches in private ATM networks. The NNI header is used for communication between ATM switches.
  Generic Flow Control (GFC)—Provides local functions, such as identifying multiple stations that share a single ATM
interface. This field is typically not used and is set to its default value of 0 (binary 0000).
 Virtual Path Identifier (VPI)—In conjunction with the VCI, identifies the next destination of a cell as it passes
through a series of ATM switches on the way to its destination.
 Virtual Channel Identifier (VCI)—In conjunction with the VPI, identifies the next destination of a cell as it passes
through a series of ATM switches on the way to its destination.
 Payload Type (PT)—Indicates in the first bit whether the cell contains user data or control data.
 Cell Loss Priority (CLP)—Indicates whether the cell should be discarded if it encounters extreme congestion as it
moves through the network. If the CLP bit equals 1, the cell should be discarded in preference to cells with the CLP bit
equal to 0.
 Header Error Control (HEC)—Calculates checksum only on the first 4 bytes of the header. HEC can correct a single
bit error in these bytes, thereby preserving the cell rather than discarding it.
ATM Virtual Connections
ATM standard defines two types of ATM connections: virtual path connections (VPCs), which contain virtual channel connections
(VCCs) as shown in Fig. A collection of virtual circuits can be bundled together into a virtual path connection. A virtual path
connection can be created from end-to-end across an ATM network.

Figure 4.6.6 Virtual channel connections of ATM

ATM Switching Operations

The basic operation of an ATM switch is straightforward: The cell is received across a link with a known VPI/VCI value. The
switch looks up the connection value in a local translation table to determine the outgoing port (or ports) of the connection and the
new VPI/VCI value of the connection on that link.

ATM Reference Model

The ATM architecture uses a logical model to describe the functionality that it supports. ATM functionality corresponds to the
physical layer and part of the data link layer of the OSI reference model.

The ATM reference model, as shown in Fig. 4.6.9, consists of the following planes, which span all layers:

 Control—This plane is responsible for generating and managing signaling requests.

 User—This plane is responsible for managing the transfer of data.
 Management—This plane contains two components:
 Layer management manages layer-specific functions, such as the detection of failures and protocol problems.
 Plane management manages and coordinates functions related to the complete system.

The ATM reference model consists of the following ATM layers:

  Physical layer—Analogous to the physical layer of the OSI reference model, the ATM physical layer manages the
medium-dependent transmission. Cells are converted into a bit stream

Figure 4.6.9 ATM reference model

 ATM layer—Combined with the ATM adaptation layer, the ATM layer is roughly analogous to the data link layer of the
OSI reference model. The ATM layer is responsible for the simultaneous sharing of virtual circuits over a physical link
and passing cells through the ATM network. To do this, it uses the VPI and VCI information in the header of each ATM
cell.
 ATM adaptation layer (AAL)—Combined with the ATM layer, the AAL is roughly analogous to the data link layer of
the OSI model. The AAL is responsible for isolating higher-layer protocols from the details of the ATM processes. The
adaptation layer prepares user data for conversion into cells and segments the data into 48-byte cell payloads.
 Higher Layer: Finally, the higher layers residing above the AAL accept user data, arrange it into packets, and hand it to
the AAL.
ATM Applications

 ATM is used in both LANs and WANs

 Multimedia virtual private networks and managed services
 Frame-relay backbones
 Internet backbones:
 Carrier infrastructures for the telephone and private-line networks
Cryptography
The word cryptography has come from a Greek word, which means secret writing. In the present day context it refers to the tools
and techniques used to make messages secure for communication between the participants and make messages immune to attacks
by hackers. The message to be sent through an unreliable medium is known as plaintext, which is encrypted before sending over
the medium. The encrypted message is known as ciphertext, which is received at the other end of the medium and decrypted to get
back the original plaintext message.

In this lesson we shall discuss various cryptography algorithms, which can be divided into two broad categorize - Symmetric key
cryptography and Public key cryptography.

Symmetric Key Cryptography

The cipher, an algorithm that is used for converting the plaintext to ciphertex, operates on a key, which is essentially a specially
generated number (value). To decrypt a secret message (ciphertext) to get back the original message (plaintext), a decrypt
algorithm uses a decrypt key. In symmetric key cryptography, same key is shared, i.e. the same key is used in both encryption and
decryption as shown in Fig. The algorithm used to decrypt is just the inverse of the algorithm used for encryption.

Symmetric key cryptography algorithms are simple requiring lesser execution time. As a consequence, these are commonly used
for long messages. However, these algorithms suffer from the following limitations:
 Requirement of large number of unique keys. For example for n users the number of keys required is n(n-1)/2.
 Distribution of keys among the users in a secured manner is difficult. Figure .

Traditional Symmetric Key ciphers : Two types:substitution, Transpositional Cipher .Substitution again divided into two types:
Monoalphabetic Substitution , Polyalphabetic Substitution
Monoalphabetic Substitution
As shown in Fig. in this approach a character in the ciphertext is substituted by another character shifted by three places, e.g. A is
substituted by D. Key feature of this approach is that it is very simple but the code can be attacked very easily.
Polyalphabetic Substitution
This is an improvement over the Caesar cipher. Here the relationship between a character in the plaintext and a character in the
ciphertext is always one-to-many. Key feature of this approach is that it is more complex and the code is harder to attack
successfully.

Transpositional Cipher
The transpositional cipher, the characters remain unchanged but their positions are changed to create the ciphertext. Figure
illustrates how five lines of a text get modified using transpositional cipher.

Simple Modern Ciphers:

Modern cipher uses several simple ciphers to achieve its goal.
XOR Cipher
The first one discussed here is called the XOR cipher because it uses the exclusive-or operation as defined in computer science.
An XOR operation needs two data inputs plaintext, as the first and a key as the second. Note that in an XOR cipher, the size of the
key, the plaintext, and the ciphertext are all the same.
Rotation Cipher
Another common cipher is the rotation cipher, in which the input bits are rotated to the left or right.

Substitution Cipher: S-box

An S-box (substitution box) parallels the traditional substitution cipher for characters. The input to an S-box is a stream of bits with
length N; the result is another stream of bits with length M. And N and M are not necessarily the same.

Transposition Cipher: P-box

A P-box (permutation box) for bits parallels the traditional transposition cipher for characters.It performs a transposition at the bit
level; it transposes bits. It can be implemented in software or hardware, but hardware is faster. P-boxes, like S-boxes, are normally
keyless.
We can have three types of permutations in P-boxes: the straight permutation, expansion permutation, and compression
permutation .as shown in Figure A straight permutation cipher or a straight P-box has the same number of inputs as outputs. In
other words, if the number of inputs is N, the number of outputs is also N. In an expansion permutation cipher, the number of
output ports is greater than the number of input ports. In a compression permutation cipher, the number of output ports is less than
the number of input ports.

Modern Round Ciphers

The ciphers of today are called round ciphers because they involve multiple rounds,where each round is a complex cipher made
up of the simple ciphers that we previously described. The key used in each round is a subset or variation of the general key called
the round key. If the cipher has N rounds, a key generator produces N keys, K1 K2, ..., KN, where K1 is used in round 1, K2 in
round 2, and so on.
we introduce two modem symmetric-key ciphers: DES and AES.
One example of a complex block cipher is the Data Encryption Standard (DES). DES was designed by IBM and adopted by the
U.S. government as the standard encryption method for nonmilitary and nonclassified use. The algorithm encrypts a 64-bit
plaintext block using a 64-bit key, as shown in Figure.
DES has two transposition blocks (P-boxes) and 16 complex round ciphers (they are repeated). Although the 16 iteration round
ciphers are conceptually the same, each uses a different key derived from the original key.
The initial and final permutations are keyless straight permutations that are the inverse of each other. The permutation takes a 64-
bit input and permutes them according to predefined values.
Triple DES
Triple DES, popularly known as 3DES, is used to make DES more secure by effectively increasing the key length. Its operation is
explained below:
 Each block of plaintext is subjected to encryption by K1, decryption by K2 and again encryption by K1 in a sequence as
shown in Fig.
 CBC is used to turn the block encryption scheme into a stream encryption scheme

Figure Triple DES encryption technique

Advanced Encryption Standard (AES)

The Advanced Encryption Standard (AES) was designed because DES's key was too small. Although Triple DES increased the
key size, the process was too slow. AES is a very complex round cipher. AES is designed with three key sizes: 128, 192, or 256
bits. Table shows the relationship between the data block, number of rounds, and key size.The structure and operation of the other
configurations are similar. The difference lies in the key generation.

Initial XOR operation followed by 10 rounds .The last round is slightly different from preceding rounds ,it is missing one
operation. structure each round as shown in figure.
 Structure Each Round

Public key Cryptography(Asymmetric key cryptography)

In public key cryptography, there are two keys: a private key and a public key. The public key is announced to the public, where as
the private key is kept by the receiver. The sender uses the public key of the receiver for encryption and the receiver uses his
private key for decryption as shown in Fig. 8.1.16.

Advantages:
The pair of keys can be used with any other entity
The number of keys required is small
Disadvantages:
It is not efficient for long messages
Association between an entity and its public key must be verified
RSA
The most popular public-key algorithm is the RSA (named after their inventors Rivest, Shamir and Adleman) as shown in Fig. Key
features of the RSA algorithm are given below:
 Public key algorithm that performs encryption as well as decryption based on number theory
 Variable key length; long for enhanced security and short for efficiency (typical 512 bytes)
 Variable block size, smaller than the key length
 The private key is a pair of numbers (d, n) and the public key is also a pair of numbers (e, n)
 Choose two large primes p and q (typically around 256 bits)
 Compute n = p x q and z = (p-1)x(q-1)
 Choose a number e
 Find d such that e x d =1 mod (p-1)x(q-1)
 e and n to the public; Z and d secret.
 For encryption: C = Pe (mod n) For decryption: P = Cd (mod n)

Figure The RSA public key encryption technique

Diffie-Hellman
RSA is a public-key cryptosystem that is often used to encrypt and decrypt symmetric keys. Diffie-Hellman, on the other hand, was
originally designed for key exchange. In the Diffie-Hellman cryptosystem, two parties create a symmetric session key to exchange
data without having to remember or store the key for future use.
Let us see how the protocol works when Alice and Bob need a symmetric key to communicate. Before establishing a symmetric
key, the two parties need to choose two numbers p and g. The first number, p, is a large prime number on the order of 300 decimal
digits (1024 bits). The second number is a random number. These two numbers need not be confidential. They can be sent through
the Internet; they can be public.
Procedure
Figure shows the procedure. The steps are as follows:

--------------------------------------
K=gXY mod p

Step 1: Alice chooses a large random number x and calculates R1=gx mod p.
Step 2: Bob chooses another large random number y and calculates R2 = gY mod p.
Step 3: Alice sends R1 to Bob. Note that Alice does not send the value of x; she sends only R1-
Step 4: Bob sends R2 to Alice. Again, note that Bob does not send the value of y,he sends only R2.
Step 5: Alice calculates K = (R2)X mod p.
Step 6: Bob also calculates K = (R1)Y mod p.
The symmetric key for the session is K.
(gx mod p)Y mod p =(gY mod p)X mod p =gxy mod p

Network security:
Network security can provide one of the five services Four of these services are related to the message exchanged using the
network:
• message confidentiality
• Integrity
• Authentication
• no repudiation.
• The fifth service provides entity authentication or identification

Message Confidentiality
Message confidentiality or privacy means that the sender and the receiver expect confidentiality. The transmitted message must
make sense to only the intended receiver. To all others, the message must be garbage. When a customer communicates with her
bank, she expects that the communication is totally confidential.
 Confidentiality with Symmetric-Key
 Confidentiality with Asymmetric-Key Cryptography

To provide confidentiality with symmetric-key cryptography, a sender and a receiver need to share a secret key. In the past
when data exchange was between two specific persons , it was possible to personally exchange the secret keys.

The problem we mentioned about key exchange in symmetric-key cryptography for privacy culminated in the creation of
asymmetric-key cryptography. Here, there is no key sharing; there is a public announcement.
Message Integrity
Message integrity means that the data must arrive at the receiver exactly as they were sent. There must be no changes during the
transmission, neither accidentally nor maliciously.
 There One way to preserve the integrity of a document was traditionally through the use of a fingerprint.
 The electronic equivalent of the document and fingerprint pair is the message and digest pair.
 To preserve the integrity of a message, the message is passed through an algorithm called a cryptographic hash
function.
 The function creates a compressed image of the message that can be used like a fingerprint. Figure 16.8 shows the
message, cryptographic hash function and message digest.

The message digest needs to be

safe from change.

• The message digest is created at the sender site and is sent with the message to the receiver. To check the integrity of a
message, or document, the receiver creates the hash function again and compares the new message digest with the one
received. If both are the same, the receiver is sure that the original message has not been changed. Of course, we are
assuming that the digest has been sent secretly.

Message authentication:
Figure 16.8 Message and
Message authentication is a service beyond message integrity. In message authentication the receiver needs to be sure of the
digest
sender's identity and that an imposter has not sent the message.
A message digest guarantees the integrity of a message—it guarantees that the message has not been changed. A message digest,
however, does not authenticate the sender of the message. When Alice sends a message to Bob, Bob needs to know that the
message is really from Alice. To provide message authentication, Alice needs to provide proof that it is she who is sending the
message and not an impostor. A message digest per se cannot provide such a proof. The digest created by a cryptographic hash
function is normally called a modification detection code (MDC). What we need for message authentication is a message
authentication code (MAC).

Message authentication code (MAC)

Digital signatures
We are all familiar with the concept of a signature. A person signs a document to show that it originated from him/her or was
approved by him/her. The signature is proof to the recipient that the document comes from the correct entity. In other words, a
signature on a document, when verified, isFigure 16.10
a sign of authentication—the document code
Message authentication is authentic. When Alice sends a message to
Bob, Bob needs to check the authenticity of the sender: he needs to be sure that the message comes from Alice and not Eve.
Bob can ask Alice to sign
16.34 the message electronically. In other words, an electronic signature can prove the authenticity of Alice

as the sender of the message. We refer to this type of signature as a digital signature.

Digital signature process.

Figure shows the digital signature process. The sender uses a signing algorithm to sign the message. The message and the
signature are sent to the recipient

The recipient receives the message and the signature and applies the verifying algorithm to the combination. If the result is
true, the message is accepted, otherwise it is rejected.

Message Nonrepudiation Figure 16.11 The digital signature process

Message nonrepudiation means that a sender must not be able to deny sending a message that he or she, in fact, did send. The
burden of proof falls on the receiver. For example, when16.36
a customer sends a message to transfer money from one account to
another, the bank must have proof that the customer actually requested this transaction.

A digital signature provides three out of our initial five security services: message,authentication, message integrity and non-
repudiation. We have seen the first two, the third can be done using the following figure.
Entity Authentication
In entity authentication (or user identification) the entity or user is verified prior to access to the system resources (files, for
example). For example, a student who needs to access her university resources needs to be authenticated during the logging
process. This is to protect the interests of the university and the student.Entity authentication is a technique designed to let one
Figure 16.13 Non-repudiation using digital signatures
party prove the identity of another party. An entity can be a person, a process, a client or a server. The entity whose identity
needs 16.39
to be proved is called the claimant: the party that tries to prove the identity of the claimant is called the verifier.
Verification categories
In entity authentication, the claimant must identify themselves to the verifier. This can be done with one of three kinds of
witnesses:
 Something known. This is a secret known only by the claimant that can be checked by the verifier. Examples are a
password, a PIN, a secret key and a private key.
 Something possessed. This is something that can prove the claimant’s identity. Examples are a passport, a driver’s license,
an identification card and a credit card
 Something inherent. This is an inherent characteristic of the claimant. Examples are conventional signatures, fingerprints,
voice, facial characteristics, retinal pattern and handwriting.

Quality of service (QoS):

Quality of service (QoS) refers to any technology that manages data traffic to reduce packet loss, latency and jitter on the network.
QoS parameters:
Organizations can measure QoS quantitatively by using several parameters, including the following:

 Packet loss happens when network links become congested and routers and switches start dropping packets. When packets are

dropped during real-time communication,

 Jitter is the result of network congestion, timing drift and route changes. Too much jitter can degrade the quality of voice and

video communication.

 Latency is the time it takes a packet to travel from its source to its destination. Latency should be as close to zero as possible. If

a voice over IP call has a high amount of latency, it can experience echo and overlapping audio.
 Bandwidth is the capacity of a network communications link to transmit the maximum amount of data from one point to

another in a given amount of time. QoS optimizes the network by managing bandwidth and setting priorities for applications

that require more resources than others.

 Mean opinion score (MOS) is a metric to rate voice quality that uses a five-point scale, with a five indicating the highest

quality.
Transport service primitives:
A service is specified by a set of primitives. A primitive means operation. To access the service a user process can access these
primitives. These primitives are different for connection oriented service and connectionless service.
There are five types of service primitives:

1. LISTEN : When a server is ready to accept an incoming connection it executes the LISTEN primitive. It blocks waiting
for an incoming connection.
2. CONNECT : It connects the server by establishing a connection. Response is awaited.
3. RECIEVE: Then the RECIEVE call blocks the server.
4. SEND : Then the client executes SEND primitive to transmit its request followed by the execution of RECIEVE to get the
reply. Send the message.
5. DISCONNECT : This primitive is used for terminating the connection. After this primitive one can’t send any message.
When the client sends DISCONNECT packet then the server also sends the DISCONNECT packet to acknowledge the
client. When the server package is received by client then the process is terminated.

Connection Oriented Service Primitives

 There are 4 types of primitives for Connection Oriented Service :

CONNECT This primitive makes a connection

DATA, DATA-ACKNOWLEDGE, Data and information is sent using thus

EXPEDITED-DATA primitive

CONNECT Primitive for closing the connection

RESET Primitive for reseting the connection

Connectionless Oriented Service Primitives

 There are 2 types of primitives for Connectionless Oriented Service:

UNIDATA This primitive sends a packet of data

FACILITY, Primitive for enquiring about the performance of the

REPORT network, like delivery statistics.