An Introduction to TCP/IP and Network Protocol Structures in the DoD Model

1. TCP/IP History

The TCP/IP model originated in the 1970s and became the official ARPAnet standard in 1983. Developed under IETF guidance, it laid the foundation for today’s Internet.

2. DoD Model vs. OSI Model

Process/Application: Equivalent to OSI’s Application, Presentation, and Session layers
Host-to-Host: Corresponds to the OSI Transport layer
Internet: Maps to OSI’s Network layer
Network Access: Covers OSI’s Data Link and Physical layers

3. Application Layer Protocols

FTP (TCP 20,21), SFTP/SSH (TCP 22)
Telnet (TCP 23), SMTP (TCP 25), DNS (TCP/UDP 53)
DHCP (UDP 67/68), TFTP (UDP 69), HTTP/HTTPS (TCP 80/443)
POP3 (TCP 110), IMAP (TCP 143), SNMP (UDP 161/162)
LDAP (TCP 389), LDAPS (TCP 636)
MySQL (TCP 3306), RDP (TCP 3389)
SIP (TCP/UDP 5060/5061), RTP (UDP/TCP 5004/5005)

4. Host-to-Host Layer Protocols

Feature	TCP	UDP
Sequencing	Yes	No
Reliability	Yes	No
Connection-Oriented	Yes	No
Overhead	High	Low
Acknowledgment	Yes	No
Flow Control	Yes (Windowing)	No

5. Port Numbers

Protocol	Port	Transport Layer
FTP	20, 21	TCP
DNS	53	TCP/UDP
HTTP	80	TCP
HTTPS	443	TCP
SMTP	25	TCP
DHCP	67, 68	UDP
RDP	3389	TCP

6. Internet Layer Protocols

IP: Logical addressing and routing
ICMP: Error messaging and network diagnostics
ARP: Resolves IP to MAC address
RARP: Resolves MAC to IP address
GRE: Tunneling various protocols
IPSec: Secure communication via AH and ESP

7. Data Encapsulation Process


Application Layer → User Data  
Transport Layer → Segment (TCP/UDP Headers)  
Network Layer → Packet (IP Headers)  
Data Link Layer → Frame (MAC + CRC)  
Physical Layer → Bits transmitted

8. Conclusion

TCP/IP delivers scalable, secure, and reliable communication. Understanding its layered architecture enables network management, troubleshooting, and design of robust infrastructures across varied platforms.