Skip to content

Stage 9: TCP vs UDP

Key takeaways

  • TCP = reliable, ordered, connection-based — costs overhead
  • UDP = fire-and-forget, connectionless — costs nothing but reliability
  • Choice of protocol is a direct trade: correctness vs speed

Both operate at the Transport layer (Layer 4). Both use ports to identify applications, as covered in DNS and Ports. That's where the similarity ends.

TCP (Transmission Control Protocol)

The three-way handshake

Before any data flows, TCP establishes a connection: 

Client → Server:  SYN
Server → Client:  SYN-ACK
Client → Server:  ACK
Only after this handshake does actual data start flowing. This is the overhead cost of TCP's reliability guarantees — every connection pays this latency upfront (one full round-trip) before useful work begins.

What TCP guarantees

  • Ordered delivery — segments are sequenced and reassembled in the correct order even if they arrive out of order
  • Reliability — lost segments are detected (via acknowledgments) and retransmitted
  • Flow control — the receiver can tell the sender to slow down if it's being overwhelmed
  • Congestion control — TCP backs off when it detects network congestion, to avoid making things worse for everyone sharing the link

When to use TCP

Anything where correctness matters more than raw speed: web traffic, file transfers, email, databases — losing or reordering a byte in a bank transaction or a downloaded file is unacceptable.

UDP (User Datagram Protocol)

  • Connectionless — no handshake, just send
  • No guarantees — no ordering, no retransmission, no flow control
  • Much lower overhead and latency as a direct result

When to use UDP

Anything where a late or dropped packet is worse than a slightly wrong one: video calls, live streaming, online gaming, and — notably — DNS queries, covered in DNS and Ports, which use UDP by default because a single small query just isn't worth handshake overhead; if it's lost, the client just retries the whole query rather than needing byte-level retransmission.

Side-by-side

TCP UDP
Connection Yes (handshake) No
Reliability Guaranteed (retransmits) None
Ordering Guaranteed None
Overhead Higher Lower
Speed Slower Faster
Typical use HTTP, FTP, SSH, databases DNS, video/voice calls, gaming

A common follow-up interview question

"If UDP has no reliability, how does video calling still mostly work?" — reliability is rebuilt at the application layer when needed. A video call would rather drop a frame and keep moving than pause the entire call to retransmit an old one — so the application itself decides how to handle loss, rather than relying on the transport layer to force retransmission the way TCP would.

Cloud connection

A proxy or load balancer may terminate one TCP connection and create a separate backend connection. In Kubernetes, a Service's port is the stable port clients use, while targetPort identifies the port on which selected Pods listen.

Practice in Packet Tracer

Next

DNS & Ports →