3. OSI Model, TCP/IP Model, TCP vs UDP and the 3-Way Handshake

3.4 TCP vs UDP

By Ravindra Bagale Last updated: 15 October 2026

Aata transport layer che don hero – TCP aani UDP. Class madhe mala nehmi vicharla jata: "Sir, TCP reliable aahe tar sagla TCP var ka nahi?" Karan reliability chi kimmat aahe – handshake, acknowledgement, retransmission. Live video call madhe ek frame gela tar chalel, pan wait kelela chalnar nahi.

Feature	TCP	UDP
Connection	Connection-oriented (3-way handshake SYN, SYN-ACK, ACK)	Connectionless
Reliability	Guaranteed delivery, ordering, retransmission	Best effort; may lose packets
Speed / overhead	Slower, 20-byte header	Faster, 8-byte header
Uses	Web (HTTP/1.1, HTTP/2), SSH, e-mail, databases	DNS, DHCP, VoIP, streaming, gaming, HTTP/3 (QUIC)
The difference is visible in the headers themselves – TCP carries much more control information:

Header field	TCP	UDP
Source / destination port	Yes	Yes
Sequence and acknowledgement numbers	Yes	No
Flags (SYN, ACK, FIN, RST, PSH, URG)	Yes	No
Window size (flow control)	Yes	No
Checksum	Yes	Yes (optional in IPv4)

sudo ss -tn              # established TCP connections
sudo ss -un              # UDP sockets
dig example.com          # DNS query – normally over UDP 53

Why this matters for security

TCP's handshake is abused in SYN flood attacks (half-open connections exhaust the server). UDP has no handshake, so the source IP is easy to spoof – this enables amplification DDoS through open DNS, NTP or memcached servers. Scanning is different too: TCP ports answer clearly, UDP scanning (nmap -sU) is slow and ambiguous, so admins often forget UDP services that are quietly open.

Ravindra Bagale's Tip

Many students remember only that "DNS runs over UDP". But a large response or a zone transfer (AXFR) goes over TCP 53. Don't forget this when writing a firewall rule or reading Nmap results – DNS needs both UDP and TCP 53.

Practice task

Make a two-column list of ten services (HTTP, SSH, DNS, DHCP, SNMP, MySQL, RDP, NTP, SMTP, TFTP) and mark each as TCP, UDP or both. Then add a live video call and an online game, and write one reason for your choice for each.

Extended teaching notes

Why this lesson matters

Use this lesson to strengthen systems you own: understand the risk, practise safely in a lab, then write and apply the fix.

Apply the fix across stacks

Same class of bug, same idea in every language.

Java / Spring Boot

Parameterise queries, encode output, CSRF tokens, limited Actuator, dependency scanning.

ASP.NET Core

EF Core / parameterised SQL, Razor encoding, HTTPS, Data Protection.

Node.js / Express

helmet, parameterised SQL, npm audit, avoid eval and shell concat.

Python / Django / Flask / FastAPI

ORM, autoescape, pip-audit, Bandit; never shell=True with user input.

PHP / Laravel

PDO/Eloquent, Blade escaping, composer audit.

Go / Ruby

database/sql placeholders / ActiveRecord; html/template / ERB escaping; govulncheck / Brakeman.

nginx / Apache / Tomcat

Hide versions, TLS, security headers, least-open ports.

MySQL / PostgreSQL / MongoDB / Redis

Private networks, auth on, least-privilege users, no 0.0.0.0/0.

AWS / Azure / GCP

Roles not long-lived keys, private data stores, cloud audit logs on.

Mini case study

Suppose we are on a purple-team exercise for an Indian mid-size company (sample data). Red notes a technique from this lesson against a lab clone. Blue must show the control that stops it, the log that would have fired, and the ticket that hardens production. That is the standard this course trains you for.

Defender's worksheet (do this today)

Write the lesson title, date and the exact lab target you are allowed to use.
List three owned systems (VMs, apps, or cloud accounts) where this lesson applies.
Run one safe check from the lesson on a lab system only. Paste the command and output.
Write a two-sentence fix you would give a teammate.
Name one related public advisory (CERT-In, CISA, NVD, or a vendor bulletin) with its date.

Indian workplace (sample data)

Suppose we are a junior analyst at Infosys, TCS, Wipro or a fintech SOC in Bengaluru/Pune. Turn this lesson into one checklist item with an owner, a due date and a place to store evidence (ticket + screenshot).

Global practice

Compare your notes with OWASP, CIS Benchmarks, NIST CSF/IR guidance or a cloud Well-Architected security question. Write five lines on what overlaps.

Everyday analogy

Explain the risk and the fix to a family member without jargon. If they can repeat the warning, you understand it well enough to teach juniors.

Key terms

Control: a safeguard (patch, config, process, detection).
Finding: a weakness backed by evidence.
Remediation: the fix plus how you verified it.
Scope: systems you have written permission to touch.
Least privilege: only the access the task needs.
Defence in depth: several layers so one failure does not end the story.

Common mistakes

Skipping written permission "just for a quick scan".
Practising on hostel, office or public targets without authorisation.
Collecting tool output without writing the fix.
Copying attack steps onto production to "verify". Defenders verify with versions, configs and patches.
Publishing screenshots that contain secrets, cookies or internal hostnames.

Recap

Evidence, fix, owner, date.
Own lab only.
Fix > spectacle.