What does the PSH flag mean in TCP?

PSH = Push - it tells the receiver to deliver buffered data to the application immediately instead of waiting (reduces latency for interactive apps).

PSH = Push

The PSH flag tells the receiver to pass the data to the application immediately rather than buffering it.

Purpose:

• Reduces latency for interactive applications
• Forces immediate delivery to application layer
• Common in real-time applications

When used:

• Interactive sessions (SSH, Telnet)
• When application needs data immediately
• Often combined with ACK

Tip: "PSH = Push it up now!" - don't buffer, deliver to the application right away.

Go deeper:

• RFC 9293 §3.1 — Header Format — defines the PSH control bit and the push function for prompt delivery to the application.
• TCP segment structure — Flags — Wikipedia's control-bit table placing PSH in context.