Before reliable data transmission occurs over Transmission Control Protocol (TCP), the communicating endpoints must establish a connection via an ordered exchange known as the **three-way handshake**:

1. **SYN**: The client sends a synchronize message to request initiating a connection.
2. **SYN-ACK**: The server responds with an acknowledgment and agreement.
3. **ACK**: The client sends a final acknowledgment, establishing the active connection.

Actual data transfer begins only after this three-way handshake completes. While this setup provides reliable, ordered data delivery by numbering and confirming packets, it makes TCP slightly slower to start than connectionless protocols like UDP, which send data immediately without a handshake.

The TCP Three-Way Handshake

User Datagram Protocol (UDP) is a lightweight transport layer protocol designed for fast, unguaranteed data transmission. Operating on top of IP, UDP transmits data immediately without establishing a connection or running a handshake beforehand.

Unlike TCP, UDP provides no packet numbering, no arrival confirmations, and no automatic retransmission for lost data. It follows a direct "send and forget" approach, sending packets with minimal overhead and no delay.

UDP is suited for scenarios where speed and timeliness matter more than perfection. In applications such as live video streaming, voice calls, and online games, a late-arriving packet is effectively useless because the moment it represents has passed. In such cases, skipping lost packets and continuing without interruption is preferable to stopping and waiting for retransmissions.

User Datagram Protocol (UDP)

Both TCP and UDP run on top of IP, but they make opposite architectural trade-offs to serve different networking needs. The rule of thumb in transport protocol selection is TCP for accuracy and UDP for timeliness.

When downloading a file or requesting a web page, accuracy is essential. TCP detects missing sequence numbers, holds transfer flow until retransmitting the exact lost packet, and adjusts its speed according to network conditions. While this ensures that no data is missing or out of sequence, the connection setup and retransmission logic make TCP slightly slower to start and potentially susceptible to delays.

In contrast, live media streaming and real-time communications prioritize low latency over complete reliability. Streaming systems using UDP trade away guaranteed delivery to avoid freezing playback over a dropped packet, often pairing with mechanisms like adaptive bitrate to lower video quality dynamically when bandwidth drops rather than stalling the stream.

Transport Trade-offs: Accuracy versus Timeliness

University of Michigan - Ann Arbor

Ch.3 Transmission and Routing Protocols - Demystifying Network Architecture: Protocols, Addressing, and Packet Flow @ University of Michigan - Ann Arbor

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