Understanding TCP vs UDP: Handshakes, Data Transfer, and Key Differences

Role of TCP and UDP

Both TCP and UDP operate at the transport layer to carry application data such as files, video streams, and images.

Key differences

TCP is connection‑oriented and provides reliable, ordered delivery. It is suited for scenarios that require high communication quality, such as file transfer, email, and web browsing.

UDP is connection‑less, prioritizes low latency over reliability, and tolerates occasional packet loss. Typical use cases include DNS queries, voice calls, live video streaming, and tunneling protocols like VXLAN.

TCP communication process

Three‑way handshake

The client initiates a connection with a SYN packet. If the server accepts, it replies with a SYN‑ACK packet. The client then sends an ACK packet, completing the handshake and establishing a reliable channel. This three‑step exchange prevents stale or duplicate connection attempts caused by delayed packets.

Data transmission

Large data blocks are split into multiple segments. TCP assigns a sequence number to each byte, starting at zero, and includes these numbers in segment headers. The receiver acknowledges received data with an ACK containing the next expected sequence number, enabling detection of lost or out‑of‑order packets and supporting retransmission.

Four‑way termination

Either side can initiate closure by sending a FIN packet. The peer acknowledges with an ACK, then sends its own FIN when ready to close. The original side replies with a final ACK. The waiting period after the last ACK ensures that any lost ACKs are retransmitted, guaranteeing a clean shutdown over an unreliable network.

UDP protocol overview

UDP sends datagrams without establishing a connection or tracking state, resulting in minimal processing overhead and low CPU/memory usage. Because UDP provides no guarantees for delivery, ordering, or integrity, it is less reliable than TCP and is best suited for applications tolerant of occasional loss.

Code example

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