Detailed Explanation of the TCP PUSH Flag (PSH) Function and Mechanism

1. Basic Concept of the PSH Flag
The TCP segment header has six control flags (such as SYN, ACK, FIN, etc.), and PSH (PUSH) is one of them. When the sender sets PSH to 1, it indicates that the receiver should deliver the data to the upper-layer application as soon as possible, rather than waiting for the buffer to fill up. Its design purpose is to reduce data latency in the TCP buffer and avoid the impact of the "sticky packet" problem on scenarios with high real-time requirements.

2. The Working Mechanism of PSH

• Sender Behavior:
  When the application layer calls a send function (such as send()) and sets the PSH flag (or the kernel triggers it automatically), TCP will immediately send the data currently in the buffer and set the PSH bit to 1. For example, interactive applications (like entering commands in SSH) may trigger PSH for each input to ensure data is sent promptly.
• Receiver Behavior:
  Upon receiving a segment with PSH=1, the receiving TCP layer will immediately deliver the data to the application layer, instead of waiting for subsequent data to fill the buffer. For example, if the receive buffer already has some data but has not reached the read threshold, PSH will force the push of the existing data.

3. Interaction Between PSH and Buffers

• Default Behavior Without PSH:
  To optimize efficiency, TCP may combine multiple small data transmissions (e.g., Nagle's algorithm) or wait for the buffer to reach the MSS (Maximum Segment Size) before sending. Similarly, the receiver may accumulate data to a certain amount before submitting it to the application layer.
• Intervention Effect of PSH:
  PSH breaks the buffering delay, similar to "flushing the pipeline." However, note:
  1. PSH does not guarantee that data arrives separately (it may be merged with other segments);
  2. Modern operating systems often ignore PSH (handled automatically by the kernel), and the application layer typically does not control it directly.

4. Practical Applications and Limitations

• Typical Scenarios:
  Remote login (e.g., Telnet), real-time message transmission, and other scenarios requiring low-latency feedback.
• Control in Programming:
  It is possible to indirectly influence PSH behavior through socket options (e.g., TCP_NODELAY to disable Nagle's algorithm), but directly setting PSH requires low-level interfaces (e.g., send(..., MSG_PUSH)).
• Precautions:
  PSH is not a reliability mechanism (it does not affect retransmission or acknowledgment), and network devices (such as routers) do not process this flag. Overuse may reduce throughput (by increasing the proportion of small packets).

5. Example Illustration
Assume a client sends data twice:

1. Send "Hel" (PSH=0), data is temporarily stored in the send buffer;
2. Send "lo!" (PSH=1), triggering the transmission of both "Hel" and "lo!", and the receiver immediately delivers "Hello!" to the application layer.
   Without PSH, the receiver might wait for more data before delivery, causing display delay.

Summary
The PSH flag is an optimization in TCP for interactive traffic, improving responsiveness by reducing buffering delay. However, its actual effectiveness is influenced by the operating system and network environment. In modern development, reliance is often placed on higher-layer protocols (e.g., HTTP/2 frame control) or socket parameter tuning.