TCP vs UDP: When to Use What, and How TCP Relates to HTTP

Introduction: The Internet Runs on Rules

The internet is not magic. It’s a carefully negotiated contract between machines.

Every email you send, every video you stream, every API call your backend makes—none of it happens randomly. Data moves because rules exist that define how information is packetized, how it is delivered, and what happens when things go wrong.

At the heart of this contract sit two transport protocols: TCP and UDP.

They solve the same problem—moving data between systems—but with different philosophies. One prioritizes reliability and order, the other prioritizes speed and minimal overhead. Understanding when to use each is not academic trivia; it’s a core architectural decision that impacts performance, scalability, and user experience.

And layered on top of this is HTTP, often misunderstood, frequently misattributed, and routinely confused with TCP itself.

Let’s clear the fog in this blog!!

---

What Are TCP and UDP? (High-Level View)

At a very high level, TCP and UDP are transport-layer protocols. Their job is simple in theory:

Move data from one machine to another.

How they choose to do that is where everything changes.

UDP — The Fast Announcement System:

The User Datagram Protocol is also called connectionless, unreliable transport protocol.

There are no host-to-host communication, It relies on Process-to-Process communication (based on Ports). Unlike TCP, UDP does not establish a connection before sending data, nor does it guarantee delivery, order, or error correction.

Now the questions arise, "If there are no reliability, no error-handling, no ordering, then why to use it??"

Well with every disadvantages, there also comes some advantages. UDP is a simple protocol that use minimum overhead. If a process wants to send a message but doesn't care whether it's reached or not, it can use UDP. Sending a message by using UDP takes much less interaction between sender and receiver than using DCCP, SCTP, TCP making it ideal for time-sensitive applications where speed is prioritised over reliability..

UDP Header: UDP packets, also called as User Datagrams, have a fixed size header of 8 bytes.

Field	Size	Description
Source Port	16 bits	Sender’s port number.
Destination Port	16 bits	Receiver’s port number.
Length	16 bits	Total length of UDP header and data.
Checksum	16 bits	Used for error detection (optional in IPv4, mandatory in IPv6).

Unlike TCP, checksum in UDP is optional. If checksum is not calculated then the field will be filled with 1s.

Features:

• No Reliability
• No Error control & Flow control
• No Retransmission
• Fast
• No ordering
• No Congestion

Common Use Cases of UDP:

• Broadcasting
• Video Conferencing
• Voice over IP (Services like WhatsApp voice calls, Wi-Fi calling)
• Online Gamings
• Live Video Streaming
• Music Streaming
• DNS

TCP — The Reliable Courier

Transmission Control Protocol is a Connection-oriented, Reliable transport protocol.

It's a core protocol of the Internet Protocol suite (TCP/IP), responsible for reliable, ordered, and error-checked delivery of data between applications over a network. As it's a Process-to-Process protocol like UDP, therefore it uses the Port numbers. Developed in the 1970s by Vint Cerf and Bob Kahn, TCP operates at the Transport Layer (Layer 4) of the OSI model and ensures that data sent from one device reaches another correctly and in the same order it was sent.

Now a question should arise "How this Protocol can achieve the Reliability, Error Handling, Order Checking??"

A reliable connection is established between Client and Server via a Three-way Handshake before data transfer even begins. It ensures that both sides are synchronised and ready to communicate using Positive Acknowledgement with Retransmission (PAR):

• Each segment sent must be acknowledged.
• If a segment is lost or corrupted (detected using the checksum), the receiver discards it, and the sender retransmits.

But before knowing about 3-way handshake in detail, first we need to know about "What is Segment??"

A TCP segment consists of data bytes to be sent and a header that is added to the data by TCP as shown:

The header of a TCP segment can range from 20-60 bytes. 40 bytes are for options. If there are no options, a header is 20 bytes else it can be of upmost 60 bytes. Header fields:

1. Source Port / Destination Port (16 bits each): Identify sending and receiving applications.
2. Sequence Number (32 bits): Position of the first byte in this segment, used for ordering.
3. Acknowledgement Number (32 bits): Next byte expected by the receiver (confirms data received).
4. Header Length (HLEN): Size of the header (5–15 words, i.e., 20–60 bytes).
5. Window Size: Receiver’s buffer size (for flow control).
6. Checksum: Error detection (mandatory).
7. Urgent Pointer: Position of urgent data (if URG flag is set).
8. Control Flags (1 bit each):

• URG: Urgent data
• ACK: Acknowledgement valid
• PSH: Push data immediately
• RST: Reset connection
• SYN: Synchronize sequence numbers
• FIN: Terminate connection

So what actually happens in 3-way handshake is stated below step-by-step:

• Step 1 (SYN): In the first step, the client wants to establish a connection with a server, so it sends a segment with SYN (Synchronize Sequence Number) which informs the server that the client is likely to start communication.
• Step 2 (SYN + ACK): Server responds to the client request with SYN-ACK signal bits set. Acknowledgement(ACK) signifies the response of the segment it received and SYN signifies with what sequence number it is likely to start the segments with.
• Step 3 (ACK): The client acknowledges the response of the server and they both establish a reliable connection with which they will start the actual data transfer.

Like the establishemt of the connection, Three-way Handshake is also used in connection termination. Though there's also another option available which is Four-way Handshake with half-close, but in most of the modern case implementation, 3-way handshake is used.

Features:

• Reliable
• Error control & Loss-detection
• Flow control
• Retransmission
• Slow
• Ordering
• Congestion Control

Common Use Cases of TCP:

• Web-Browsing (HTTP, HTTPS)
• Text-Communication
• Email (SMTP, IMAP)
• File Transfer (FTP, SFTP)

---

What Is HTTP—and Where It Fits

The Hypertext Transfer Protocol (HTTP) is an Application level protocol that is mainly used to access the data that are on the World Wide Web (www). Basically it enables the connection in between the server and the clients (Web Browsers).
It operates on a request-response model, where a client sends a request to a server, which then responds with the requested resource, such as an HTML page(texts, image, or video). HTTP is stateless, meaning each request is independent. It runs over TCP and has evolved through versions like HTTP/1.1, HTTP/2, and HTTP/3, with each improving performance, security, and efficiency.
HTTP functions as a combination of FTP and SMTP.
HTTP uses the services of TCP on the port 80.

Understanding HTTP Request and Response

HTTP Request

An HTTP request is how your browser asks the server for something. It includes:

• HTTP Version: The version of HTTP (like HTTP/1.1 or HTTP/2) that is being used.
• URL: The specific address of the resource (e.g., https://www.example.com/about).
• HTTP Method: The type of action being requested (e.g., GET to retrieve information, POST to send data, PUT to send document from server etc).
• HTTP Request Headers: Extra information (metadata) about the request, like what kind of browser you're using or what kind of content you’re expecting.
• HTTP Request Body: In some cases, the request will include a body that contains data (e.g., when you submit a form).

HTTP Response

An HTTP response is the server’s answer to your request. It includes:

• HTTP Status Code: A number that tells you if the request was successful or not (e.g., 200 OK means everything is fine, 404 Not Found means the requested page doesn’t exist, 500 Internal Server Error means server site related error might be crash).
• Response Headers: Information about the response (Metadata), like what kind of data is being sent (e.g., Content-Type: text/html means it’s an HTML page).
• Response Body: The content that the server sends back (e.g., HTML code that the browser will use to display the webpage).

More about HTTP Methods

An HTTP method, sometimes referred to as an HTTP verb, indicates the action that the HTTP request expects from the queried server. For example,

Method	Action
GET	Client requests from the Server
HEAD	Request information about document but the not the document itself
POST	Sends information from Client to Server
PUT	Sends document from Server to Client
DELETE	Request the Sever to delete the specified resource
TRACE	Echoes incoming request
CONNECT	Reserved
OPTION	Inquiry about available options
COPY	Requests the server to copy the specified page to another network address

More about HTTP Status Codes

HTTP status codes are standardized three-digit numbers returned by a server in response to a client's request. They are grouped into five classes based on the first digit:

• 1xx (Informational): The request was received, and the process is continuing.
• 2xx (Success): The request was successfully received, understood, and accepted.
• 3xx (Redirection): Further action is required to complete the request.
• 4xx (Client Error): The request contains bad syntax or cannot be fulfilled.
• 5xx (Server Error): The server failed to fulfill an apparently valid request.

Code	Name	Class	Description
100	Continue	Informational	The client should continue with the request.
101	Switching Protocols	Informational	The server is switching protocols as requested.
102	Processing	Informational	The server has received and is processing the request.
103	Early Hints	Informational	Used to return metadata (e.g., link headers) before the final response.
200	OK	Success	The request succeeded and the response is returned.
201	Created	Success	The request succeeded and a new resource was created.
202	Accepted	Success	The request was accepted for processing but not completed.
203	Non-Authoritative Information	Success	The response includes modified content from a proxy.
204	No Content	Success	The server successfully processed the request, but returns no content.
205	Reset Content	Success	The client should reset the document view.
206	Partial Content	Success	The server is returning partial content (e.g., for range requests).
207	Multi-Status	Success	Multiple status codes are included in the response (used in WebDAV).
208	Already Reported	Success	The response is not a new one, but already reported in a previous response.
226	IM Used	Success	The server fulfilled the request using the IM (Instance Manipulation) method.
300	Multiple Choices	Redirection	The resource has multiple representations; client must choose one.
301	Moved Permanently	Redirection	The resource has been permanently moved to a new URI.
302	Found	Redirection	The resource is temporarily located at a different URI.
303	See Other	Redirection	The response is available at a different URI; use GET method.
304	Not Modified	Redirection	The resource has not been modified since the last request (used with caching).
305	Use Proxy	Redirection	The requested resource must be accessed through a proxy.
307	Temporary Redirect	Redirection	The resource is temporarily located at a different URI; method must not change.
308	Permanent Redirect	Redirection	The resource has been permanently moved; method must not change.
400	Bad Request	Client Error	The request has malformed syntax or invalid data.
401	Unauthorized	Client Error	Authentication is required and has failed or not been provided.
402	Payment Required	Client Error	Reserved for future use (e.g., digital payment systems).
403	Forbidden	Client Error	The server refuses to authorize the request.
404	Not Found	Client Error	The requested resource could not be found.
405	Method Not Allowed	Client Error	The method is not supported for the requested resource.
406	Not Acceptable	Client Error	The server cannot produce a response matching the client’s Accept headers.
407	Proxy Authentication Required	Client Error	The client must authenticate with the proxy.
408	Request Timeout	Client Error	The server timed out waiting for the request.
409	Conflict	Client Error	The request conflicts with the current state of the resource.
410	Gone	Client Error	The resource is no longer available and will not be available again.
411	Length Required	Client Error	The server requires a Content-Length header.
412	Precondition Failed	Client Error	One or more preconditions in the request headers failed.
413	Payload Too Large	Client Error	The request entity is too large.
414	URI Too Long	Client Error	The request URI is too long.
415	Unsupported Media Type	Client Error	The media type is not supported by the server.
416	Range Not Satisfiable	Client Error	The requested range is invalid or not satisfiable.
417	Expectation Failed	Client Error	The server cannot meet the expectation in the Expect header.
418	I'm a teapot	Client Error	A humorous response (RFC 2324); not implemented in real servers.
421	Misdirected Request	Client Error	The request was directed to a server that cannot handle it.
422	Unprocessable Entity	Client Error	The server understands the content type and syntax, but cannot process it.
423	Locked	Client Error	The resource is locked and cannot be modified.
424	Failed Dependency	Client Error	The request failed due to a failure in a previous request.
425	Too Early	Client Error	The server is unwilling to risk processing a request that might be replayed.
426	Upgrade Required	Client Error	The client must upgrade to a different protocol.
428	Precondition Required	Client Error	The request must be conditional (e.g., If-Match).
429	Too Many Requests	Client Error	The user has sent too many requests in a given time (rate limiting).
431	Request Header Fields Too Large	Client Error	The request headers are too large.
451	Unavailable For Legal Reasons	Client Error	The resource is unavailable due to legal reasons.
500	Internal Server Error	Server Error	The server encountered an unexpected condition.
501	Not Implemented	Server Error	The server does not support the requested functionality.
502	Bad Gateway	Server Error	The server received an invalid response from an upstream server.
503	Service Unavailable	Server Error	The server is temporarily unable to handle the request (e.g., maintenance).
504	Gateway Timeout	Server Error	The server did not receive a timely response from an upstream server.
505	HTTP Version Not Supported	Server Error	The server does not support the HTTP version used in the request.
506	Variant Also Negotiates	Server Error	A server-side negotiation error occurred.
507	Insufficient Storage	Server Error	The server cannot store the representation needed to complete the request.
508	Loop Detected	Server Error	A server detected a loop in the request processing.
510	Not Extended	Server Error	Further extension is required to fulfill the request.
511	Network Authentication Required	Server Error	The client must authenticate to access the network.

✅ Note: The most commonly encountered codes are 200 (OK), 404 (Not Found), 400 (Bad Request), 500 (Internal Server Error), and 301/302 (Redirects).

You don’t need to memorize these flags—but understanding why they exist explains how TCP guarantees order and reliability.

---

The Relationship Between TCP and HTTP (Deep Dive)

The Critical Clarification: HTTP Runs on Top of TCP

At a fundamental level, HTTP and TCP operate at different layers and solve different classes of problems.

• TCP operates at the transport layer
• HTTP operates at the application layer

This separation is intentional and foundational to how the internet scales.

When we say “HTTP runs on top of TCP”, we mean this literally in terms of layering:

HTTP does not send data directly over the network.
It relies on TCP to carry its data reliably from one endpoint to another.

HTTP is completely unconcerned with:

• Packet loss
• Retransmissions
• Ordering of bytes
• Congestion control

All of that responsibility is delegated to TCP.

Step-by-Step: What Actually Happens During an HTTP Request

When you type a URL into a browser and hit Enter, the following sequence occurs (simplified but accurate):

a. TCP Connection Establishment

• The client and server perform a TCP handshake.
• This creates a reliable, bidirectional communication channel.
• From this point on, both sides agree on sequence numbers, window sizes, and delivery guarantees.

b. HTTP Constructs the Request

• HTTP formats the request as structured text (method, headers, body).
• Example: GET /api/users HTTP/1.1
• HTTP does not worry about packet boundaries or fragmentation.

c. TCP Transmits the Data

• TCP breaks the HTTP message into segments.
• Each segment is numbered, tracked, and retransmitted if lost.
• TCP ensures all bytes arrive, in order, exactly once.

d. HTTP Interprets the Response

• Once TCP delivers the byte stream, HTTP parses it.
• Status codes, headers, and body are interpreted at the application level.

---

Why HTTP Does Not (and Cannot) Replace TCP

A common misconception is assuming that HTTP could somehow “handle everything” on its own. This is architecturally incorrect.

Different Responsibilities, Different Layers

HTTP defines semantics

• What does this request mean?
• Is this a GET or POST?
• What does this response mean?
• How should the response be interpreted?

TCP defines mechanics

• How do bytes move across unreliable networks?
• What happens when packets are lost?
• Are the packets sent in order?
• How is congestion managed?

HTTP is deliberately designed without transport logic. This is not a limitation—it is a strength.

---

Conclusion: Protocols Are Design Decisions, Not Just Theory

TCP, UDP, and HTTP are not competing technologies—they are collaborating layers, each solving a different class of problems.

• TCP exists to make unreliable networks usable
• UDP exists to make real-time systems possible
• HTTP exists to standardize communication on the web

Choosing between TCP and UDP is not about which protocol is “better”.
It’s about what your system values more—reliability or latency, correctness or responsiveness.
HTTP does not replace TCP.
It depends on it.
Once you understand this layering, networking stops feeling abstract. You stop memorizing protocols and start designing systems intentionally.
And that is the real goal—not passing an exam, but making informed architectural choices in real-world software.

---

Follow for more beginner-friendly breakdowns of core software engineering concepts.