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Evgeniy Berezovsky
Asked: 2018-08-30 21:04:56 +0800 CST

All TCP packets have the PSH flag set - who / what would be responsible for that?

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On a server->client TCP stream, all packets from the server have the PSH flag set. Is good old Nagle responsible for this flag being set?

tcp

3 Answers

  1. Best Answer

    The push flag (PSH or P) is set by the sending host. If the sending application uses write or send of 100 bytes, each packet will have PSH set. If the sender sends 2000 bytes, it will be sent in two packets, and the second will have the PSH flag, assuming standard frame size of 1500.

    The push flag indicates to the receiving host that the received data should be passed to the receiving application immediately.

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  2. The setting of the Push Flag is usually not controlled by the sending application, but by the sending TCP layer. Most modern TCP/IP stacks set the PSH bit at the end of the buffer supplied to send() .

    Source: Improve latency for TCP by not waiting for Push flag

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  3. Setting the P bit all the time is a standard correct, but inefficient behavior that you might see coming from old TCP stacks.

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