What Are TCP and UDP?

What Are TCP and UDP?

TCP and UDP are two of the most fundamental protocols in networking. Yet, most explanations stop at “one is reliable, the other is fast.” After spending time truly understanding them — how they work, when to use them, how they behave under attack — I decided to share what I’ve learned in depth.

This post isn’t just theory. It includes hands-on practice plans, real-world examples, and security perspectives. If you’re building networked systems, apps, or just learning cybersecurity, this is for you.

What is TCP? (Transmission Control Protocol)

Definition

TCP is a connection-oriented, reliable, and ordered transport protocol. It ensures that data sent between two systems arrives completely, in order, and without duplication.

You’re using TCP every time you load a web page, send an email, or SSH into a server. It’s the backbone of most reliable internet communication.

Connection Process: 3-Way Handshake

Before any data transfer, TCP establishes a session between client and server:

1. SYN: Client requests a connection
2. SYN-ACK: Server acknowledges the request
3. ACK: Client confirms and connection is established

Client                  Server
   |------ SYN -------->|
   |<----- SYN+ACK -----|
   |------ ACK -------->|

Key Features of TCP

• Reliability: Lost packets are retransmitted
• Ordering: Packets arrive in the order they were sent
• Flow Control: Sender respects receiver’s capacity (sliding window)
• Congestion Control: Adapts to network conditions to avoid overload
• Error Checking: Built-in checksum to detect corrupted data

TCP Segment Structure (Header)

Field	Description
Source/Dest Port	Identifies endpoints
Sequence Number	Tracks the order of packets
ACK Number	Confirms received packets
Flags	SYN, ACK, FIN, RST
Window Size	Buffer capacity info
Checksum	Error detection

What is UDP? (User Datagram Protocol)

Definition

UDP is a connectionless, unreliable, but very fast transport protocol. It sends packets (datagrams) without establishing a connection and without guaranteeing delivery, order, or integrity.

It follows a fire-and-forget model. No acknowledgments, no retries — just raw speed.

Key Features of UDP

• No connection: Just send and go
• Minimal overhead: Very small headers (8 bytes)
• No retransmission or ordering
• Perfect for real-time data where speed matters more than perfection

Used in things like DNS, VoIP, video streaming, multiplayer gaming.

UDP Header Fields

• Source Port
• Destination Port
• Length
• Checksum (optional)

TCP vs UDP – Technical Comparison

Feature	TCP	UDP
Connection	Yes (3-way handshake)	No
Reliability	Yes	No
Packet Ordering	Yes	No
Error Checking	Robust	Basic or none
Flow/Congestion Ctrl	Yes	No
Speed	Slower	Faster
Typical Usage	Web, SSH, Email	DNS, VoIP, Online Games

When Should You Use TCP vs UDP?

Use TCP When:

• You need reliable communication (e.g., file transfer, web requests)
• Packet order and delivery must be guaranteed
• Applications like SSH, HTTPS, FTP

Use UDP When:

• Speed is critical (e.g., voice/video)
• Real-time communication with some loss is acceptable
• You need broadcast or multicast
• Applications like DNS, VoIP, online games, IPTV

TCP and UDP from a Security Perspective

TCP Security Risks

• SYN Flood: Overwhelms server with incomplete connections
• TCP Hijacking: Intercepts session and injects data
• RST Injection: Forces connections to close

UDP Security Risks

• UDP Flooding: Saturates bandwidth with fake requests
• Spoofing: Easily forged source IP addresses
• Amplification Attacks: Especially via DNS due to lack of session tracking

My Practice Plan to Master TCP & UDP

Now that I’ve learned the theory, I’m building a hands-on practice routine to solidify everything:

1. Capture TCP/UDP packets with Wireshark
   Analyze how TCP does sequencing, retransmission, and how UDP just pushes data out raw.
2. Simulate connections using Netcat
   Use nc to test TCP and UDP ports, send manual packets, and observe differences.
3. Write TCP/UDP servers in Python
4. TCP:
   
   import socket s = socket.socket(socket.AF_INET, socket.SOCK_STREAM) s.bind(('0.0.0.0', 1234)) s.listen(1) conn, addr = s.accept() data = conn.recv(1024) print("TCP received:", data.decode())
   
   
   UDP:
   
   import socket s = socket.socket(socket.AF_INET, socket.SOCK_DGRAM) s.bind(('0.0.0.0', 1234)) data, addr = s.recvfrom(1024) print("UDP received:", data.decode())
   
5. Test DNS traffic
   Using dig and nslookup, observe how UDP handles DNS and when it falls back to TCP.
6. Analyze simple DoS attacks in test environments
   Study how SYN flood affects TCP servers, and how UDP services can be abused in flood/amplification attacks.

In Summary: Reliability vs Speed

• TCP gives you reliability, ordering, and control — perfect for critical communication.
• UDP gives you raw speed and low latency — ideal for real-time apps where “close enough” is good enough.

Neither is better in every case. Use the right tool for the job.