Transport Layer

The network layer is responsible for host-to-host communication at the computer level, delivering messages only to the destination computer.
However, this delivery is incomplete, as the message still needs to be directed to the specific computer process. Therefore, the primary role of the transport layer is to provide communication services directly to the application processes running on different hosts.
The transport layer establishes a logical communication channel between application processes on different hosts. Despite not being physically connected, application processes utilize the logical communication facilitated by the transport layer to exchange messages.
For instance, when using WhatsApp on a mobile phone, the connection between two phones is established by the network layer, but the communication between my WhatsApp application and my friend's WhatsApp application is facilitated by the transport layer.
The transport layer protocol is specifically responsible for delivering messages to the appropriate process. It ensures end-to-end or process-to-process communication, addressing the intricacies (complexities) of connecting and communicating between specific applications running on different hosts.
Common transport layer protocols include Transmission Control Protocol (TCP) and User Datagram Protocol (UDP), each offering different features and trade-offs in terms of reliability and speed.
Transport layer protocols use port numbers to identify specific processes on a host, enabling the correct delivery of messages to the intended applications.
Through flow control and error detection mechanisms, the transport layer ensures reliable and ordered delivery of messages between applications.
Transport layer services contribute to the abstraction of the underlying network complexities, allowing application developers to focus on high-level communication without worrying about the complexities of network configuration.

Connection-Oriented and Connectionless Services in Transport Layer

Connection-Oriented Services:
- Connection-oriented services in the transport layer involve the establishment of a dedicated, reliable, and ordered communication channel before data exchange begins.
- These services ensure a virtual circuit or connection is established between the sender and receiver, providing a reliable and predictable data transfer mechanism.
- Transmission Control Protocol (TCP) is a prime example of a connection-oriented protocol, offering features such as acknowledgment, flow control, and error checking to ensure the integrity of data delivery.
- Connection-oriented services are suitable for applications where data accuracy, sequencing, and completeness are critical, such as file transfer and web browsing.
Connectionless Services:
- Connectionless services in the transport layer involve communication without the establishment of a dedicated connection beforehand.
- These services are faster and less resource-intensive but do not guarantee ordered or error-checked delivery of data.
- User Datagram Protocol (UDP) is a classic example of a connectionless protocol, providing a lightweight and quick data transfer mechanism without the overhead of connection setup and maintenance.
- Connectionless services are suitable for real-time applications, such as online gaming, live streaming, and voice over IP (VoIP), where speed is prioritized over guaranteed data delivery.

Transmission Control Protocol (TCP) vs. User Datagram Protocol (UDP)

Transmission Control Protocol (TCP):
- TCP is a connection-oriented protocol that provides reliable, ordered, and error-checked delivery of data between applications.
- It establishes a virtual circuit or connection between the sender and receiver before exchanging data, ensuring that data is delivered accurately and in the correct order.
- TCP includes features such as flow control, acknowledgment of received data, and retransmission of lost or corrupted packets to guarantee the integrity of the communication.
- Example: Web browsing, file transfer (e.g., FTP), and email (e.g., SMTP) commonly use TCP to ensure accurate and complete data delivery.
User Datagram Protocol (UDP):
- UDP is a connectionless protocol that provides faster, simpler, and less reliable communication compared to TCP.
- It does not establish a dedicated connection before data transfer, and it does not perform error checking or retransmission of lost packets.
- UDP is suitable for applications where speed and low overhead are prioritized over guaranteed data delivery, such as real-time applications like online gaming, live streaming, and voice over IP (VoIP).
- Example: Domain Name System (DNS) uses UDP for quick and lightweight communication to resolve domain names to IP addresses.
Comparison:
- TCP is ideal for applications that require reliable, ordered, and error-checked delivery, making it suitable for scenarios where data accuracy is crucial.
- UDP, on the other hand, is suitable for applications where speed and simplicity are prioritized, and occasional packet loss or disorder is acceptable.
- The choice between TCP and UDP depends on the specific requirements of the application and the trade-offs between reliability and speed.

Services provided by the Transport Layer

The services offered by the transport layer share similarities with those of the data link layer. While the data link layer operates within a single network, the transport layer extends its services across an internetwork, encompassing multiple interconnected networks. Unlike the data link layer, which governs the physical layer, the transport layer exercises control over all the lower layers of the OSI model.

The services provided by transport layer protocols can be categorized into five main areas:

End-to-End Delivery: Ensuring the reliable delivery of messages between the source and destination, regardless of the number of intermediate networks.
Addressing: Managing the addressing scheme to identify the source and destination processes in the communication.
Reliable Delivery: Ensuring the accuracy and completeness of data delivery through:
- Error Control: Detecting and correcting errors in transmitted data.
- Sequence Control: Maintaining the correct order of data packets during transmission.
- Loss Control: Minimizing data loss during the transmission process.
- Duplication Control: Preventing the occurrence of duplicated data packets.
Flow Control: Managing the rate of data transmission to prevent overwhelming the receiving device or network.
Multiplexing: Efficiently handling multiple communication streams through:
- Upward Multiplexing: Combining data streams from multiple processes into a single communication channel.
- Downward Multiplexing: Distributing data streams from a single source process to multiple destination processes.

User Datagram Protocol (UDP)

User Datagram Protocol (UDP) operates at the Transport Layer and is an integral part of the Internet Protocol suite, commonly known as UDP/IP suite.
Unlike Transmission Control Protocol (TCP), UDP is characterized as an unreliable and connectionless protocol. Consequently, there is no need to establish a dedicated connection before initiating data transfer.
While TCP is the predominant transport layer protocol for most Internet services, providing guaranteed delivery, reliability, and additional features, these benefits come with added overhead and latency.
UDP, on the other hand, plays a crucial role in scenarios that demand real-time services, such as online gaming, voice or video communication, and live conferences. In these applications, where high performance is paramount, UDP allows packets to be dropped instead of processing delayed packets. Notably, UDP lacks error checking, resulting in bandwidth savings.
User Datagram Protocol (UDP) stands out for its efficiency in terms of both latency and bandwidth, making it a preferred choice for time-sensitive and performance-critical applications.

UDP Header

The UDP header is a fixed and simple 8-byte structure, in contrast to TCP, which can vary in size from 20 to 60 bytes.
The first 8 bytes of the UDP header contain all the necessary information, while the remaining part is dedicated to data transmission.
UDP port number fields are each 16 bits long, allowing for a port number range defined from 0 to 65535. Port number 0 is reserved for special purposes. These port numbers play a crucial role in distinguishing between different user requests or processes.

The UDP header consists of the following fields, each serving a specific purpose:
1. Source Port: Identifies the port of the sender, helping in the proper routing of the response.
2. Destination Port: Specifies the port of the intended recipient, guiding the delivery of the data.
3. Length: Indicates the total length of the UDP datagram, including both the header and the data.
4. Checksum: Facilitates error detection, ensuring the integrity of the transmitted data.

Applications of UDP:

Used for simple request-response communication when the data size is small, reducing concerns about flow and error control.
Suitable for multicasting due to its support for packet switching.
Utilized in routing update protocols like RIP (Routing Information Protocol).
Preferred for real-time applications that cannot tolerate uneven delays in message sections.
Implementations using UDP as a transport layer protocol include:
- NTP (Network Time Protocol)
- DNS (Domain Name Service)
- BOOTP, DHCP
- NNP (Network News Protocol)
- Quote of the day protocol
- TFTP, RTSP, RIP
Tasks performed by the application layer through UDP:
- Trace Route
- Record Route
- Timestamp
UDP takes a datagram from the Network Layer, attaches its header, and swiftly sends it to the user, contributing to its fast operation.
UDP is considered a null protocol if you remove the checksum field, reducing the demand on computer resources.
Effective when using Multicast or Broadcast for data transfer.
Commonly employed for the transmission of real-time packets, particularly in multimedia applications.

Application Layer

The Application Layer is the topmost layer of the OSI model, responsible for providing network services directly to end-users and applications.
- Here "network services" refer to the set of functionalities and capabilities that the Application Layer provides to end-users and applications. The Application Layer serves as the interface between the network and the software applications that users interact with.
- Network services at the Application Layer include a variety of communication protocols and services that facilitate communication and data exchange between software applications on different devices within a network. These services can encompass tasks such as data formatting, encryption, decryption, authentication, and other application-specific functionalities.
It serves as the interface between the software applications and the underlying network infrastructure.
Application layer protocols enable communication and data exchange between software applications on different devices.
Examples of Application Layer protocols include HTTP (Hypertext Transfer Protocol), SMTP (Simple Mail Transfer Protocol), and FTP (File Transfer Protocol).
These protocols define the rules and conventions for communication, ensuring seamless interaction between applications running on diverse devices.
One practical example illustrating the role of the Application Layer is the communication between users on WhatsApp. When you send a message on WhatsApp, the application layer protocols manage the process of composing, sending, and receiving messages, providing a user-friendly and efficient communication experience.
The Application Layer encapsulates the complexities of network communication, allowing diverse applications to interact without being concerned with the underlying network details.
It plays a pivotal role in supporting a wide range of network-based applications, including web browsing, email services, and instant messaging.

Simple Mail Transfer Protocol (SMTP)

SMTP is an application layer protocol designed for efficiently and reliably sending emails over the internet.

Working:

The SMTP server is consistently in a listening mode, ready to handle incoming connections.
A client initiates a TCP connection with the SMTP server.
The SMTP server listens for a connection and establishes a connection on the designated port.
Once the connection is established, the client informs the SMTP server of its intention to send an email.
Upon confirmation, the client sends the email to its mail server through the SMTP server.
SMTP facilitates the transfer of the email from the sender's mail server to the receiver's mail server.

SMTP is involved in the email transmission process twice:
1. Between the sender and the sender's mail server.
2. Between the sender's mail server and the receiver's mail server.
Note: To receive or download the email,
- Another protocol is required between the receiver's mail server and the recipient.
- The two most commonly used protocols for this purpose are POP3 (Post Office Protocol version 3) and IMAP (Internet Message Access Protocol).

Characteristics of SMTP

Push Protocol: SMTP operates as a push protocol, meaning it initiates the transfer of data without waiting for a request. It actively pushes emails from the sender to the receiver.
Transport Layer: SMTP utilizes TCP (Transmission Control Protocol) at the transport layer, ensuring reliable and orderly data transmission.
Port Number: SMTP uses port number 25 to establish communication between the sender and the recipient's mail server.
Persistent TCP Connections: SMTP employs persistent TCP connections, allowing it to send multiple emails in a single connection, which enhances efficiency.
Connection Oriented: SMTP is a connection-oriented protocol, establishing a dedicated connection between the sender and the mail server for the duration of the email transfer.
In-Band Protocol: SMTP is an in-band protocol, meaning the control information and data share the same communication channel. This simplifies the communication process but may limit scalability.
Stateless Protocol: SMTP is a stateless protocol, implying that each email transaction is independent. The server does not retain information about the previous interactions, treating each email as a new transaction.

Important Points about SMTP

Text-Based Protocol: SMTP is a pure text-based protocol, meaning it can only handle messages containing 7-bit ASCII text. It is not designed to transfer other types of data like images, video, audio, executable files, binary objects, or text data in languages represented in 8-bit codes such as French, Japanese, Chinese, etc.
MIME Extension: Multipurpose Internet Mail Extensions (MIME) extends the limited capabilities of SMTP. It is an extension to the internet email protocol designed to enable users to send and receive graphics, audio files, video files, and other non-text data within email messages. MIME was specially designed for SMTP.
Stateless Protocol: SMTP is a stateless protocol, meaning it does not maintain the state of its clients. If an email is requested to be sent twice, the SMTP server resends it without recognizing that the email has already been sent.
Receiver's Side: SMTP is not used at the receiver's side. Instead, a pull protocol like POP3 or IMAP is needed. The receiver periodically checks if there are any new emails from the mail server.
Sender and Receiver Machines: SMTP is not run directly between sender and receiver machines. This is because machines cannot always be ON. The functionality has been divided between the client and the mail server. The mail server receives the mail on behalf of its client and manages the client's mailbox.
Client Authentication: SMTP is not suitable for client authentication, as it does not require authentication. It allows anyone on the Internet to send emails to anyone or even to a large group of people. SMTP Auth, short for SMTP Authentication, has been introduced separately to provide authentication features.

Post Office Protocol (POP)

POP is an application layer protocol designed for facilitating the retrieval and management of emails on a remote server.

Purpose

Message Access: POP serves as a message access protocol, allowing clients to retrieve or download emails from their remote mail server.
POP3 Version: The most widely used version of POP is POP version 3 (POP3).

Working

To Retrieve a Message:
- The client initiates a TCP connection using port 110.
- The client identifies itself to the server during the connection establishment.
- Client issues a series of POP3 commands to interact with the server.

Characteristics of Post Office Protocol (POP)

Pull Protocol: POP is a pull protocol, implying that the client initiates communication and pulls emails from the server when needed.
Transport Layer: POP utilizes TCP at the transport layer for reliable communication.
Port Number: POP uses port number 110 for communication between the client and the server.
Persistent TCP Connections: POP employs persistent TCP connections, allowing the client to maintain a continuous connection for multiple interactions with the server.
Connection-Oriented: POP is a connection-oriented protocol, ensuring a reliable and ordered data exchange between the client and the server.
In-Band Protocol: POP is an in-band protocol, meaning that control information and data share the same communication channel.
Stateful and Stateless: POP is stateful during a session while the mail is being downloaded, ensuring that the server maintains awareness of the client's actions. However, it becomes stateless across sessions.

Internet Message Access Protocol (IMAP)

IMAP is an application layer protocol.
It enables clients to receive or download emails from their remote mail server.
IMAP has become more widely used than POP for managing emails.

Characteristics of IMAP

Pull Protocol: IMAP is a pull protocol, allowing clients to initiate communication and pull emails from the server when needed.
Transport Layer: IMAP uses TCP at the transport layer to ensure reliable communication between the client and the server.
Port Number: IMAP uses port number 143 for communication between the client and the server.
Persistent TCP Connections: IMAP employs persistent TCP connections, enabling clients to maintain continuous connections for multiple interactions with the server.
Connection-Oriented: IMAP is a connection-oriented protocol, ensuring a reliable and ordered data exchange between the client and the server.
In-Band Protocol: IMAP is an in-band protocol, meaning that control information and data share the same communication channel.
Stateful Protocol: IMAP is a stateful protocol, maintaining the state of the client's actions across the session for efficient interaction.
Distributed Mailboxes: IMAP distributes mailboxes across multiple servers, allowing for better organization and accessibility of emails.

POP vs. IMAP

Hyper Text Transfer Protocol (HTTP)

It is an application layer protocol that facilitates communication between clients and servers on the World Wide Web.
In simple words, HTTP, or Hypertext Transfer Protocol, is the foundation of data communication on the World Wide Web. It is a set of rules that allows web browsers and servers to communicate. When you type a web address (URL) into your browser and hit Enter, your browser uses HTTP to request the web page from the server. The server then sends back the requested web page, and your browser displays it for you to see. HTTP is like a language that enables the exchange of information between your computer and the web server, making it possible for you to access and view websites on the internet.

Purpose

HTTP is primarily used for the retrieval of data from websites throughout the internet.
It operates on top of the TCP/IP suite of protocols, providing a standardized way for web browsers to communicate with web servers.

Working

HTTP operates on a client-server model where:
- The web browser acts as the client.
- The client communicates with the web server hosting the website.

When a client requests information (such as clicking on a hyperlink) from the website server:
The browser sends a request message to the HTTP server for the requested objects. Then:
- HTTP establishes a connection between the client and server through TCP.
- HTTP sends a request to the server, which collects the requested data.
- HTTP sends the response with the objects back to the client.
- HTTP closes the connection.

HTTP Connections

Non-persistent: In non-persistent HTTP connections:

A new connection is established for each request and response exchange between the client and server.
After serving the requested content, the connection is immediately closed.
Subsequent requests require opening new connections, incurring additional overhead.

Persistent: In persistent HTTP connections:

A single connection is reused for multiple request-response exchanges between the client and server.
The connection remains open after serving a request, allowing for subsequent requests to use the same connection.
This reduces the overhead of opening and closing connections for each interaction.

HTTP Characteristics and Notes

HTTP uses TCP at the transport layer.

This choice is made because:

Unlike UDP, TCP ensures reliable data delivery through a Three-way handshake.
TCP guarantees retransmission of lost packets, providing a reliable communication channel.
HTTP lacks built-in reliability features, necessitating the use of TCP for reliable data transfer.
If HTTP used UDP, it would need to manage sessions independently, including packet retransmission.

It is important to note:

Any service not using TCP must have built-in reliability features.

HTTP uses port number 80.

HTTP clients use port 80 to send and receive requested web pages from an HTTP server.
HTTP servers respond to all requests at port 80.

HTTP 1.0 is non-persistent, and HTTP 1.1 is persistent.

Persistent connections, as in HTTP 1.1, improve performance by 20% compared to non-persistent connections.

HTTP 1.0 is a connectionless protocol:

After serving a single HTTP request, the connection is closed and not reused.
HTTP 1.0 without connection keep-alive is considered connectionless.

HTTP is an in-band protocol:

HTTP passes control data (commands) and main data over the same connection.
Both control data and main data are processed without distinction or priority for control data.

HTTP is a stateless protocol:

HTTP servers do not maintain any state information.
After sending a response, the server forgets about the client.
Each new request is treated independently, and the connection is automatically closed after generating the response.

If HTTP were a stateful protocol:

It would allow browser windows to engage connections with the web server for extended periods.
This could lead to a situation of reaching the maximum connections of a web server, even with many idle connections.

File Transfer Protocol (FTP)

FTP is an application layer protocol.
In simple words, FTP, or File Transfer Protocol, is a way to move files from one computer to another over the internet. It's like a digital courier service. If you have a file on your computer that you want to share or send to someone else, you can use FTP to upload (send) that file to a server on the internet. The other person can then use FTP to download (receive) the file from the same server. It's a convenient method for sharing files between computers, especially when the computers are not physically close to each other.

Purpose:

FTP is designed for exchanging files over the internet.
It facilitates users in uploading and downloading files from the internet.

Working:

FTP establishes two TCP connections between the client and the server:

One connection is dedicated to transferring data.
The other connection is utilized for transferring control information.

Additional Information:

FTP operates on two modes: Active mode and Passive mode.
- In Active mode, the client initiates the connection, and the server responds.
- In Passive mode, the server initiates the data connection, and the client responds. Passive mode is useful when the client is behind a firewall.
FTP uses different commands for controlling the file transfer operations, such as RETR (retrieve a copy of the file) and STOR (store a copy of the file).
FTP can work with or without user authentication, depending on the server's configuration.
Security concerns with FTP have led to the development of more secure alternatives, such as SFTP (SSH File Transfer Protocol) and FTPS (FTP Secure).

Characteristics of FTP

FTP uses TCP at the transport layer: FTP relies on the Transmission Control Protocol (TCP) for reliable communication between the client and server.
FTP uses port number 21 for the control connection: Port 21 is designated for the control connection, where commands and responses between the client and server are exchanged.
FTP uses port number 20 for the data connection: Port 20 is reserved for the data connection, facilitating the actual transfer of files between the client and server.
FTP establishes persistent TCP connections for the control connection: The control connection remains open throughout the FTP session, handling commands, and responses.
FTP uses non-persistent connections for the data connection: The data connection is established as needed for file transfers and closed after the transfer is complete.
FTP is a connection-oriented protocol: FTP maintains a connection between the client and server, ensuring a reliable exchange of commands and data.
FTP is an out-of-band protocol: FTP uses separate connections for control information (out-of-band) and data, enhancing efficiency and flexibility.
FTP is not mentioned as SMTP; it is a stateful protocol: FTP maintains the state of the session, remembering user login information and the current working directory.

Important Notes

Emails can't be sent using FTP.
- FTP requires the establishment of a connection between the client and server before transferring files.
- Both the client and server need to be online simultaneously for FTP file transfers.
- Due to this requirement, emails are not sent using FTP.
FTP can transfer one file at a time.
- FTP is designed for transferring a single file at a time in either direction between the client and server.
FTP is a stateful protocol.
- The client establishes a control connection for the duration of an FTP session.
- FTP sessions typically span multiple data transfers, making it a stateful protocol.

Transport Layer & Application Layer

Transport Layer

Connection-Oriented and Connectionless Services in Transport Layer

Transmission Control Protocol (TCP) vs. User Datagram Protocol (UDP)

Services provided by the Transport Layer

User Datagram Protocol (UDP)

UDP Header

Applications of UDP:

Application Layer

Simple Mail Transfer Protocol (SMTP)

Characteristics of SMTP

Important Points about SMTP

Post Office Protocol (POP)

Working

Characteristics of Post Office Protocol (POP)

Internet Message Access Protocol (IMAP)

Characteristics of IMAP

POP vs. IMAP

Hyper Text Transfer Protocol (HTTP)

Working

HTTP Connections

HTTP Characteristics and Notes

File Transfer Protocol (FTP)

Purpose:

Working:

Characteristics of FTP

Important Notes