TCP vs UDP When to Use What, and How TCP Relates to HTTP
Source: Dev.to
What are TCP and UDP? (high level)
TCP (Transmission Control Protocol)
- Reliable, connection‑oriented network protocol.
- Establishes a connection using a three‑way handshake.
- Guarantees that data reaches the destination without loss or corruption.
- Maintains the correct order of packets and retransmits lost packets when needed.
- Because of these checks, TCP is slower than UDP but extremely reliable.
UDP (User Datagram Protocol)
- Fast, connectionless network protocol.
- Sends packets without establishing a connection or checking whether they arrive.
- Does not guarantee delivery, ordering, or retransmission of lost data.
- This makes UDP much faster, but less reliable.
Typical use cases
- TCP: chat applications, messaging apps, email, file downloads, web browsing (HTTP/HTTPS).
- UDP: video calls, live streaming, online gaming, voice calls.
Key differences between TCP and UDP
| Feature | TCP | UDP |
|---|---|---|
| Connection type | Connection‑oriented | Connectionless |
| Handshake | 3‑way handshake required | No handshake |
| Reliability | Guarantees delivery (retransmits) | No guarantee, no retransmission |
| Packet ordering | Maintains order | May arrive out of order |
| Speed | Slower due to overhead | Faster |
| Error checking | Yes (checksums, retransmission) | Minimal (checksum only) |
| Typical applications | Chats, emails, downloads, web browsing | Streaming, gaming, VoIP |
When to use TCP
Use TCP when data accuracy matters more than speed and losing data is unacceptable.
- Data must be delivered correctly and in order.
- Examples:
- Sending important emails.
- Downloading files.
- Chatting or messaging where message integrity is crucial.
A small delay is acceptable if it ensures that the data is not corrupted.
When to use UDP
Use UDP when speed is critical and occasional data loss is tolerable, especially for real‑time communication.
- Low latency is more important than perfect reliability.
- Examples:
- Video calls.
- Live streaming.
- Online gaming.
If a few packets are lost, it is better to continue than to wait for retransmission.
What is HTTP and where it fits
HTTP (HyperText Transfer Protocol) defines:
- What data is requested.
- What data is sent back.
It does not decide how the data is delivered safely; that responsibility belongs to the transport layer (TCP or UDP).
Example HTTP request/response
GET / HTTP/1.1
Host: www.facebook.comHTTP/1.1 200 OK
Content-Type: text/html
...HTML content...The browser sends an HTTP request, the server processes it, and then returns an HTTP response containing the requested resource.
Relationship between HTTP and TCP
- HTTP sits on top of TCP and uses TCP to deliver web data reliably.
- HTTP only defines the rules of communication (request/response format).
- Reliability, ordering, and retransmission are handled by TCP.
Therefore, traditional HTTP chooses TCP because web pages require correct, complete data. While UDP is faster, its lack of reliability makes it unsuitable for most HTTP traffic.
Summary
- TCP: Reliable, ordered, slower – ideal for data where correctness is essential.
- UDP: Fast, unordered, less reliable – ideal for real‑time media where speed outweighs occasional loss.
- HTTP: Needs reliability → uses TCP as its transport protocol.