Certainly, let's go through your explanation of how DNS uses UDP for transport layer and the comparison with TCP:

DNS and Use of UDP for Transport Layer:

• DNS is an example of an application layer service that utilizes UDP instead of TCP for transport.
• The key difference between TCP and UDP is that UDP is connectionless, reducing the need for setup and teardown of connections.
• DNS requests and responses usually fit within a single UDP datagram, making UDP suitable for this scenario.
• DNS can generate substantial traffic, particularly when full resolutions are required.

TCP-Based DNS Lookup:

• If DNS lookup were done via TCP, it involves a three-way handshake for connection setup and teardown.

• For a full DNS lookup:

  

  1. SYN packet sent (1 packet).
  2. SYN-ACK packet sent, ACK response needed (3 packets).
  3. Request sent, ACK response needed (5 packets).
  4. Multiple handshakes and responses with root, TLD, and authoritative name servers (11 packets each, totaling 27).
  5. Final request and response for IP address (11 packets, total 38).
  6. Response to initial request, ACK response (2 packets).
  7. Four-way handshake for connection teardown (4 packets).
  8. A total of 44 packets at minimum.

UDP-Based DNS Lookup:

• In UDP (at port 53), a simplified process involves fewer packets.
  1. Initial computer sends UDP packet to local name server (1 packet).
  2. Recursive server sends UDP packet to root server, and so on (7 packets).
  3. Local name server responds to DNS resolver (1 packet).
  4. A total of 8 packets.

Efficiency of UDP for DNS:

• UDP requires much less overhead compared to TCP for simple tasks like DNS.
• DNS usually involves lookups to obtain IP addresses to send additional data, not merely out of curiosity.
• UDP's simplicity is suitable for lightweight tasks like DNS, reducing unnecessary complexity.

Error Recovery in UDP:

• While UDP lacks the error recovery of TCP, DNS's application layer offers a simple solution.
• DNS resolver repeats requests if no response is received, similar to how TCP handles errors.

Complexity and DNS Over TCP: