Understanding Internet Protocols: Overheads, Limitations, and the OSI Model

Introduction

The internet was built on a diverse suite of protocols—each with unique roles, overheads, and limitations. In this post, we’ll explore standard protocols (TCP, UDP, HTTP/1.1 to HTTP/3, QUIC, SMTP, FTP variants, and more), examine their operational details, including timing and packet formation, and explain their place within the OSI model (layers 1–7).

1. OSI Model: A Framework for Network Communications

The Open Systems Interconnection (OSI) model divides network communications into seven layers:

• Layer 1 – Physical: The transmission of raw bits over a physical medium (e.g., cables, radio waves).
• Layer 2 – Data Link: Framing, error detection, and media access control (e.g., Ethernet, Wi-Fi).
• Layer 3 – Network: Logical addressing and routing (e.g., IP).
• Layer 4 – Transport: End-to-end communication and reliability (e.g., TCP, UDP).
• Layer 5 – Session: Establishing, maintaining, and terminating sessions between applications.
• Layer 6 – Presentation: Data translation, encryption, and compression (e.g., TLS/SSL encryption).
• Layer 7 – Application: Network services and user-facing protocols (e.g., HTTP, SMTP, FTP).

Each protocol we discuss fits within one or more of these layers, affecting how data is handled and transmitted across networks.

2. Transport Layer Protocols (OSI Layer 4)

Transmission Control Protocol (TCP)

• Purpose: Provides reliable, ordered, and error-checked delivery of data.
• Overheads & Limitations:
  • Connection Establishment: A three-way handshake is used to introduce initial latency.
  • Flow Control & Congestion Control: Adds overhead with continuous adjustments to network conditions.
  • Header Size: 20 bytes (or more with options) are typically added to every segment.
• OSI Mapping: Operates at Layer 4.
• Standard Ports: Used by many application protocols (e.g., HTTP on port 80/443, SMTP on port 25/587).

User Datagram Protocol (UDP)

• Purpose: Offers a connectionless-mode of transmission with minimal overhead.
• Overheads & Limitations:
  • Speed vs. Reliability: Faster than TCP due to no connection setup, but does not guarantee delivery or order.
  • Use Cases: Suitable for real-time applications (e.g., video conferencing, online gaming) where speed is prioritized over accuracy.
• OSI Mapping: Also a Layer 4 protocol.
• Standard Ports: These are used by protocols such as DNS (port 53) and streaming services.

3. Application Layer Protocols (OSI Layer 7)

Hypertext Transfer Protocol (HTTP) Evolution

HTTP has seen significant evolution, especially in response to performance bottlenecks and security requirements:

• HTTP/1.1:
  • Features: Persistent connections and pipelining (though limited by head-of-line blocking).
  • Limitations: Each request waits for the previous one to finish, causing delays.
  • Ports: Traditionally, port 80 is used for HTTP.
• HTTP/2:
  • Features: Multiplexing of requests, header compression, and server push capabilities.
  • Limitations: It is still built on TCP so packet loss can delay all multiplexed streams.
  • Ports: Often still uses port 443 as HTTPS is the dominant mode.
• HTTP/3:
  • Features: Built on QUIC (which runs over UDP), reducing latency through faster connection establishment and eliminating head-of-line blocking.
  • Impact: With the rise of HTTPS and HTTP/3, port 80 (HTTP) is becoming nearly redundant as secure communications over port 443 dominate.

QUIC Protocol

• Purpose: Designed to improve TCP by reducing latency and offering enhanced multiplexing.
• Overheads & Benefits:
  • Built on UDP: Lower connection establishment overhead.
  • Integrated Security: Encryption is built in by default.
  • Performance: Reduces latency, particularly in networks with packet loss.
• OSI Mapping: While operating at the transport level (Layer 4), its security integration brings aspects of Layer 6 into play.
• Everyday Use: Backbone of HTTP/3 implementations.

Simple Mail Transfer Protocol (SMTP)

• Purpose: Handles sending email messages.
• Overheads & Limitations:
  • Reliability: Runs over TCP for reliable delivery.
  • Security Issues: Originally lacked strong authentication and encryption; STARTTLS was introduced to secure transmissions.
• OSI Mapping: Application layer (Layer 7), though it relies on TCP (Layer 4).
• Standard Ports: Traditionally uses port 25, with secure variants on ports 465 or 587.

File Transfer Protocols

• FTP (File Transfer Protocol):
  • Purpose: Transferring files between systems.
  • Limitations: Lacks encryption, leaving credentials and data vulnerable.
  • Ports: Port 21 is typically used for control and a separate port for data.
  • OSI Mapping: Application layer (Layer 7) relying on TCP.
• FTPS (FTP Secure) & FTP over TLS:
  • Enhancements: Wraps FTP communications within TLS/SSL to provide encryption.
  • Overheads: Additional processing for encryption, but dramatically increases security.
  • Ports: Often the same as FTP, with negotiation for secure channels.
• SCP (Secure Copy Protocol):
  • Purpose: Securely copy files using the SSH protocol.
  • Limitations: Lacks advanced file transfer features such as resume and directory listings.
  • OSI Mapping: Operates at the application layer (Layer 7) with underlying secure transport (SSH typically on TCP port 22).

4. Other Commonly Used Protocols and Their Ports

Many other protocols play a crucial role in internet communication, each assigned specific ports:

• DNS (Domain Name System):
  • Port: 53 (UDP/TCP)
  • Role: Translates human-readable domain names to IP addresses (operates primarily at Layer 7 using UDP, with TCP fallback for large queries).
• IMAP/POP3 (Email Retrieval):
  • Ports: IMAP typically on 143 (or 993 for IMAPS), POP3 on 110 (or 995 for POP3S)
  • Role: Retrieve email messages from a server, working at the application layer (Layer 7) over TCP.
• Telnet/SSH:
  • Ports: Telnet on 23; SSH on 22
  • Role: Remote login and command execution. SSH provides secure access, whereas Telnet does not encrypt data.

5. Packet Formation and Timing

Packet Structure

Every packet transmitted over the internet contains:

• Headers:
  • Layer 3 (Network): IP headers include source/destination IP addresses, version, and fragmentation info.
  • Layer 4 (Transport): TCP or UDP headers include port numbers, sequence numbers (for TCP), and flags.
• Payload: The actual data being carried, which might be an HTTP request, an email message, or file data.

Timing Considerations

• Latency:
  • TCP’s handshake and retransmission mechanisms introduce delays. In high-latency environments, this can slow down data delivery.
• Throughput:
  • UDP’s minimal overhead leads to faster transmission but risks data loss, requiring higher-level applications to handle error correction if needed.
• Protocol Efficiency:
  • HTTP/3 and QUIC are designed to reduce latency further by allowing multiple streams to be sent concurrently and reducing connection setup times.

OSI Layers in Packet Transmission

• Layers 1 & 2:
  • Concerned with the physical and data link aspects, ensuring bits are reliably transmitted over cables or wireless channels.
• Layer 3:
  • The IP protocol governs the routing of packets.
• Layer 4:
  • TCP/UDP manage how packets are delivered reliably (or quickly, in UDP’s case).
• Layers 5-7:
  • Protocols like HTTP, FTP, and SMTP ensure the application’s data is correctly formatted, secured, and delivered to the appropriate software on the receiving end.

Conclusion

Understanding the interplay between various internet protocols—from the fundamental transport mechanisms of TCP and UDP to the evolving world of HTTP/3 and QUIC—reveals the complexity behind everyday network communications. By examining their overheads, limitations, and integration within the OSI model, we gain a deeper insight into how data is efficiently and securely transmitted globally. Whether you’re troubleshooting network issues or designing a new application, appreciating these protocols and their layered architecture is key to optimizing performance and security.