Transport Protocols: UDP and TCP

Overview of Transport Protocols

• Transport protocols operate at the transport layer of the internet's five-layer structure:
  • Application Layer
  • Transport Layer
  • Network Layer
  • Link Layer
  • Physical Layer
• Purpose of Transport Protocols:
  • Allow multiple applications to use one network connection simultaneously
  • Use of ports (about 65,000 per connection) for communication:
    • Example: Application using Port 12437 to send a message to Port 80.

UDP (User Datagram Protocol)

Characteristics of UDP

• Packet size:
  • Smaller than TCP (UDP headers: 8 bytes vs TCP headers: 20 bytes)
• Connectionless:
  • No need to establish a connection before sending data.
• Control over data transmission:
  • More flexibility in sending data at any time.

Limitations of UDP

• Error detection:
  • Primitive 16-bit checksum; unreliable, does not recover from errors.
• No guarantee of packet delivery:
  • Does not resend lost packets.
• No in-order delivery:
  • Packets may arrive out of order.
• No congestion control:
  • Will attempt to send all packets even in a busy network.

Conclusion on UDP

• Lightweight but not reliable.

TCP (Transmission Control Protocol)

Characteristics of TCP

• Connection-based:
  • Requires a connection to be established via a three-way handshake:
    • Initiator asks to connect.
    • Acceptor replies.
    • Acknowledgement confirms connection establishment.
• Reliability features:
  • Delivery acknowledgements: Sender gets confirmation that data was received.
  • Retransmission: Resends lost packets after timeout.
  • In-order delivery: Rearranges packets to ensure correct order.
  • Congestion control: Delays transmission during network congestion to minimize loss.

Limitations of TCP

• Larger header size than UDP due to additional features.
• Potential latency issues due to congestion control:
  • Example: Real-time applications (e.g., Skype) may experience delays.
• Higher overhead due to acknowledgements and retransmissions.

Conceptual Differences

• UDP: Message-oriented (data sent in distinct chunks).
• TCP: Stream-oriented (continuous flow of data, sender does not slice packets).

Application Scenarios

Text Communication

• TCP recommended:
  • Reliable, supports in-order delivery, and retransmission.
• UDP may lead to lost or out-of-order text.

File Downloads

• TCP preferred:
  • Retransmission of lost parts necessary for file integrity.

Remote Access Protocols (e.g., SSH)

• TCP used for delivery acknowledgements.

Multimedia Streaming

• Historically, UDP chosen:
  • Lower overhead, tolerates some packet loss.
• Increasingly, TCP is being used:
  • Benefits of reliability when bandwidth allows.
• Security issues:
  • Some firewalls block UDP, necessitating TCP use.

Small Transactions (e.g., DNS lookups)

• UDP is efficient:
  • Reduces overhead of connection management.

Summary

• Choose between UDP and TCP based on application requirements.
  • UDP for bandwidth-intensive apps tolerating packet loss.
  • TCP for reliability and ordered delivery.