Espressif Systems ESP Faq page 150

Chapter 4. Software framework
4.10.32 [LWIP] What is the retransmission interval of TCP？
When ESP32 acts as the sender, the initial retransmission interval of the TCP protocol is usually set to 3 seconds.
If the receiver does not send an ACK message, the next retransmission interval will be determined based on the
Jacobson algorithm. The retransmission interval will be exponentially increased. Specially, it will be increased by
2, 4, 8, 16, 32 seconds gradually. This retransmission interval is not fixed and you can adjust it by changing some
parameters such as timeout time and the size of sliding window.
4.10.33 [LWIP] What is the maximum number of sockets that can be created?
32 for most, and the default number is 10.
4.10.34 [Sleep] What kinds of sleeping mode does ESP32 have? What are the differences?
• There are mainly three sleeping modes: Modem sleep, Light sleep and Deep sleep.
– Modem sleep: the station WMM sleeping mode specified in the Wi-Fi protocol (the station sends NULL
data frame to tell the AP to sleep or wake up). The Modem sleep mode is enabled automatically after
the station connected to AP. After entering this mode, the RF block is disabled and the station stays
connected with the AP. The Modem sleep mode will be disabled after the station disconnected from the
AP. The ESP32 can also be configured to decrease the CPU's clock frequency after entering Modem
sleep mode to further reduce its current.
– Light sleep: this is a station sleep mode based on Modem sleep mode. The difference between is that,
besides for the RF block being disabled, the CPU will also be suspended in this mode. After exiting from
Light sleep mode, the CPU continues to operate from where it stopped.
– Deep sleep: a sleeping mode un-specified in the Wi-Fi protocol. During Deep sleep mode, all the blocks
except for RTC is disabled, and the station cannot be connected to AP. After exiting from this mode, the
whole system will restart to operate (similar to system restart).
4.10.35 [Sleep] How can I enable the dynamic frequency scaling function for ESP32 in
Modem-sleep mode?
You can enable this function by selecting menuconfig > Component Config > Power Man-
agement > Support for power management > Enable dynamic frequency scal-
ing (DFS) at startup (NEW).
4.10.36 [Sleep] How low can the speedstep function go for ESP32 in modem sleep mode？
For now, the CPU clock can go down to as low as 40 MHz.
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