Core Internet Protocols: DNS, HTTP, TCP/IP, Email, TFTP, IP Addressing

Posted on May 25, 2025 in Contemporary Music Composition

DNS Fundamentals

DNS, or Domain Name System, is a fundamental component of the internet that facilitates human-friendly web addresses (like www.example.com) to be translated into IP addresses that computers use to identify each other on the network. In essence, DNS acts as the internet’s phone book, allowing users to access websites using easily recognizable domain names instead of having to remember complex numerical IP addresses.

How DNS Resolution Works

  1. User Enters a Domain Name: When a user types a domain name into their web browser, such as “www.example.com,” the computer needs to find the corresponding IP address to connect to the server hosting that website.
  2. Local DNS Resolver: The user’s computer checks its local DNS resolver (usually provided by the Internet Service Provider or configured by the user). If the resolver already has the IP address for the requested domain in its cache, it returns the address to the computer, and the process is complete.
  3. DNS Query: If the local resolver doesn’t have the IP address, it sends a DNS query to the root DNS servers. These servers are a critical part of the DNS infrastructure and manage the root level of the DNS hierarchy.
  4. Top-Level Domain (TLD) Servers: The root DNS servers direct the query to the appropriate Top-Level Domain (TLD) servers. TLDs are the last part of the domain name (e.g., .com, .org, .net).
  5. Authoritative DNS Servers: The TLD servers direct the query to the authoritative DNS servers for the specific domain (e.g., example.com). These authoritative servers have information about the IP addresses associated with the domain.
  6. IP Address Resolution: The authoritative DNS servers respond to the query with the IP address of the requested domain.
  7. Cache Update: The local resolver caches the IP address for future use, reducing the need to repeat the entire process for subsequent requests to the same domain.
  8. Connection Established: The computer can now use the obtained IP address to establish a connection with the web server hosting the requested website.

DNS Message Types and Resolution

DNS messages are like conversations between computers on the internet, helping them find each other by translating human-friendly domain names into computer-friendly IP addresses. Here’s a simple breakdown of a DNS query process:

  1. Question (Query): Imagine you want to visit a website (let’s say www.example.com). Your computer asks, “Hey, does anyone know the IP address for www.example.com?”
  2. Root Server: The computer then asks a big internet directory, called the Root Server, “Do you know where I can find information about .com domains?” The Root Server points your computer in the right direction.
  3. Top-Level Domain (TLD) Server: Next, your computer goes to a TLD Server specific to “.com” and asks, “Hey, who’s in charge of ‘example.com’?” The TLD Server says, “Check with example.com’s boss.”
  4. Authoritative Server: Your computer then asks the authoritative server of “example.com,” “What’s the IP address for www?” The authoritative server replies, “Here it is!”
  5. Answer: Your computer gets the IP address and says, “Great, thanks!” Now, armed with the IP address, it can connect to the server that hosts www.example.com.

Key Terms in DNS Resolution

i) Recursive Resolution

Definition: In a recursive DNS resolution, a DNS resolver (usually operated by your Internet Service Provider or a public DNS service) is responsible for fulfilling the entire resolution process on behalf of the requesting client.

Process: When a client requests the resolution of a domain name, the recursive resolver starts the resolution process by querying root DNS servers. It then follows the chain of authoritative DNS servers, obtaining the necessary information to ultimately provide the client with the IP address associated with the requested domain.

ii) Iterative Resolution

Definition: In an iterative DNS resolution, the DNS resolver does not fully resolve the query on behalf of the client. Instead, it iteratively queries other DNS servers, providing partial information to the client and allowing the client to continue the resolution process.

Process: The iterative resolver starts by querying the root DNS servers and, instead of providing the final result to the client, it returns a referral to authoritative servers responsible for the Top-Level Domain (TLD). The client then queries these TLD servers, and the process continues until the authoritative server for the specific domain provides the final IP address.

iii) Caching

Definition: Caching in DNS refers to the temporary storage of previously resolved DNS queries. Once a DNS resolver obtains the IP address associated with a domain name, it stores this information in its cache for a specified time (known as Time-to-Live or TTL).

Purpose: Caching is implemented to improve the efficiency of DNS resolution. If the same domain is requested again within the TTL period, the resolver can provide the cached result instead of repeating the entire resolution process. This reduces the time and resources required for subsequent requests and helps to alleviate the load on DNS servers.

Classful IP Addressing

Classful addressing refers to an older addressing scheme used in the early days of the Internet and networking. This addressing scheme divides IP addresses into three main classes: Class A, Class B, and Class C. Each class has a specific range of IP addresses and was designed to accommodate different network sizes. Classful addressing has largely been replaced by Classless Inter-Domain Routing (CIDR) in modern networking, but understanding the basics of classful addressing is still valuable for historical context.

Overview of Classful IP Address Classes

  • Class A Addresses:

    • Range: 1.0.0.0 to 126.0.0.0
    • Default Subnet Mask: 255.0.0.0
    • Networks: Supports a large number of networks, each with a large number of hosts.

  • Class B Addresses:

    • Range: 128.0.0.0 to 191.255.0.0
    • Default Subnet Mask: 255.255.0.0
    • Networks: Suitable for a moderate number of networks, each with a moderate number of hosts.

  • Class C Addresses:

    • Range: 192.0.0.0 to 223.255.255.0
    • Default Subnet Mask: 255.255.255.0
    • Networks: Supports a large number of networks, each with a small number of hosts.

Each class has a fixed boundary between the network and host portions of the IP address. For example, in a Class A address, the first octet represents the network, and the remaining three octets represent host addresses. Class B uses the first two octets for the network, and Class C uses the first three octets.

One of the limitations of classful addressing was its inefficiency in allocating IP addresses, as each class had a fixed size, and organizations were often assigned more addresses than they needed. This led to the development of CIDR, which allows for more flexible allocation of IP addresses by using variable-length subnet masks.

TFTP Message Types

TFTP (Trivial File Transfer Protocol) is a simple, lightweight protocol often used for transferring files between devices on a network. TFTP operates over UDP (User Datagram Protocol) and is commonly employed in scenarios where a more feature-rich protocol like FTP (File Transfer Protocol) is not necessary. TFTP messages are packet-based and serve different purposes during file transfer. Here are the main types of TFTP messages:

  • Read Request (RRQ) – Opcode 1:

    Sent by a TFTP client to request the transfer of a file from a TFTP server. The client specifies the filename it wants to read along with the transfer mode (e.g., “netascii” or “octet”).

  • Write Request (WRQ) – Opcode 2:

    Sent by a TFTP client to request the transfer of a file to a TFTP server. Similar to the RRQ, the client specifies the filename and transfer mode for writing the file.

  • Data – Opcode 3:

    Sent by the TFTP server to the client (or vice versa) to transmit a block of data during a file transfer. The data block contains a portion of the file being transferred.

  • Acknowledgment (ACK) – Opcode 4:

    Sent by the TFTP client or server to acknowledge the successful receipt of a data block. The acknowledgment packet contains the block number of the successfully received data block.

  • Error – Opcode 5:

    Sent by the TFTP server to the client (or vice versa) to indicate an error condition. The error packet includes an error code and an error message, providing information about the nature of the error.

  • Option Acknowledgment – Opcode 6:

    Used in TFTP options negotiation. Sent by the side that has received a request with options to acknowledge the agreed-upon options.

  • Option Negotiation Error – Opcode 7: 

    Sent by the side that has received a request with options to indicate an error in the option negotiation.

HTTP Transactions, Request-Response, and Methods

HTTP Transactions

An HTTP transaction refers to a sequence of communications between a client and a server over the Hypertext Transfer Protocol (HTTP). It consists of an HTTP request sent by the client to the server and the corresponding HTTP response sent by the server back to the client. This exchange allows the client to request resources from the server and receive responses.

HTTP Request and Response Messages

HTTP Request Message:

An HTTP request message is sent by a client to request a specific action from a server. It includes the following components:

  • Request Line: Specifies the HTTP method, the URI (Uniform Resource Identifier) of the requested resource, and the HTTP version.
  • Headers: Additional information about the request (e.g., Host, User-Agent, Content-Type).
  • Body (Optional): Data sent to the server, typically used in POST or PUT requests.
Example of an HTTP Request Message:
GET /index.html HTTP/1.1
Host: www.example.com
User-Agent: Mozilla/5.0

HTTP Response Message:

An HTTP response message is sent by a server in response to an HTTP request. It includes the following components:

  • Status Line: Specifies the HTTP version, a status code indicating the result of the request, and a reason phrase.
  • Headers: Additional information about the response (e.g., Content-Type, Content-Length).
  • Body (Optional): Data sent from the server to the client, such as the requested resource or an error message.
Example of an HTTP Response Message:
HTTP/1.1 200 OK
Content-Type: text/html
Content-Length: 123

<!DOCTYPE html>
<html>
<head>
  <title>Example Page</title>
</head>
<body>
  <h1>Hello, World</h1>
</body>
</html>

HTTP Methods

HTTP methods (or HTTP verbs) define the actions that can be performed on a resource. The common HTTP methods include:

  • GET:

    Retrieve data from the server. It should have no side effects.

    GET /page.html HTTP/1.1

  • POST:

    Submit data to the server for processing.

    POST /data HTTP/1.1
Content-Type: application/x-www-form-urlencoded
Content-Length: 13

name=John+Doe

TCP/IP Protocol Suite: Layers and Communication Units

The TCP/IP protocol suite is commonly represented using the OSI (Open Systems Interconnection) model, which is a conceptual framework that standardizes the functions of a telecommunication or computing system into seven abstraction layers. The TCP/IP model, however, is a four-layer model that closely aligns with the OSI model. Here’s a simplified representation of the TCP/IP protocol suite and its responsibilities at each layer, along with the unit of communication at each layer:

  1. Application Layer:

    • Responsibility: Provides network services directly to end-users or applications. Handles high-level protocols, user interfaces, and application-level data.
    • Unit of Communication: Messages, data, or requests generated by applications.

  2. Transport Layer:

    • Responsibility: Ensures reliable end-to-end communication. Manages flow control, error correction, and retransmission of lost data. Multiplexes multiple connections onto a single network interface.
    • Unit of Communication: Segments or datagrams. For TCP (Transmission Control Protocol), it’s called segments, and for UDP (User Datagram Protocol), it’s called datagrams.

  3. Internet Layer:

    • Responsibility: Facilitates internetworking, including routing and addressing. Translates logical addresses (IP addresses) to physical network addresses (MAC addresses). Routes data packets between different networks.
    • Unit of Communication: Packets or datagrams. For example, IP packets.

  4. Link Layer:

    • Responsibility: Manages the physical connection to the network medium. Handles access to the physical network, including framing and addressing. Detects and corrects errors at the physical layer.
    • Unit of Communication: Frames or frames with physical layer addressing. For example, Ethernet frames.

Communication Flow in TCP/IP

  1. Application Layer to Application Layer: Communication begins with an application generating a message or data. The message is passed down through the layers.
  2. Transport Layer to Transport Layer: The transport layer segments the data into segments or datagrams.
  3. Internet Layer to Internet Layer: The internet layer encapsulates the transport layer’s segments into packets, adding IP headers.
  4. Link Layer to Link Layer: The link layer encapsulates the internet layer’s packets into frames, adding link layer headers and trailers.

Email Architecture: Four Scenarios Explained

  1. Simple Email Architecture:

    In a basic email architecture, there are two main components:

    • User’s Device (Client): This is where the user accesses their email. It could be a desktop email client (e.g., Microsoft Outlook) or a web-based email client (e.g., Gmail).
    • Email Server: This server stores and manages the user’s email messages. It handles the sending and receiving of emails. Common email server protocols include SMTP (Simple Mail Transfer Protocol) for sending emails and IMAP (Internet Message Access Protocol) or POP3 (Post Office Protocol) for retrieving emails.

    Communication Flow:

    • The user creates or reads emails using the email client.
    • When sending an email, the email client uses SMTP to communicate with the email server to send the message.
    • When receiving emails, the email client uses IMAP or POP3 to retrieve messages from the email server.

  2. Corporate Email Architecture:

    In a corporate setting, the email architecture may involve additional components:

    • Mail Transfer Agent (MTA): Handles the routing and transfer of emails between servers. It uses SMTP for communication.
    • Mail Delivery Agent (MDA): Responsible for delivering emails to the recipient’s mailbox.

    Communication Flow:

    • User interacts with the email client.
    • SMTP is used to send emails to the sender’s MTA.
    • MTA routes the email to the recipient’s MTA.
    • The recipient’s MTA delivers the email to the recipient’s mailbox using an MDA.
    • The recipient retrieves emails using IMAP or POP3.

  3. Webmail Architecture:

    Webmail services are accessed through web browsers and have their architecture:

    • Webmail Server: This server hosts the webmail application and stores user emails.

    Communication Flow:

    • Users access the webmail client through a web browser.
    • The webmail client communicates with the webmail server.
    • The webmail server stores and manages user emails.
    • Sending emails involves the webmail server using SMTP.
    • Retrieving emails involves the webmail server using IMAP or POP3.

  4. Mobile Email Architecture:

    For mobile email, the architecture is adapted for use on smartphones and tablets:

    • Mobile Email Client: Specialized email clients designed for mobile devices.

    Communication Flow:

    • Users access their email through a mobile email client.
    • The mobile email client communicates with the email server, similar to the desktop client.
    • The email server handles the storage and management of email.