ESP-NOW is yet another protocol developed by Espressif, which enables multiple devices to communicate with one another without using Wi-Fi. The protocol is similar to the low-power 2.4GHz wireless connectivity that is often deployed in wireless mouses. So, the pairing between devices is needed prior to their communication. After the pairing is done, the connection is secure and peer-to-peer, with no handshake being required. espressif 
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> ESP-NOW applies the IEEE802.11 Action Vendor frame technology, along with the IE function developed by Espressif, and CCMP encryption technology, realizing a secure, connectionless communication solution.
* ESP8266 Non-OS SDK, API Reference. pdf
ESP8266 Wi-Fi SoC offers a complete and self-contained Wi-Fi networking solution; it can be used to host applications or to offload Wi-Fi networking functions from another application processor. When the ESP8266 hosts application, it boots up directly from an external flash. It has an integrated cache to improve the performance of system’s running applications. Alternately, serving as a Wi-Fi adapter, wireless internet access can be added into any microcontroller-based design with simple connectivity through UART interface or the CPU AHB bridge interface. ESP8266EX is amongst the most integrated Wi-Fi chips in the industry; it integrates the antenna switches, RF balun, power amplifier, low noise receive amplifier, filters, and power management modules. Thus, it requires minimal external circuitry, and the entire solution, including front-end module, is designed to occupy minimal PCB area. ESP8266EX also integrates an enhanced version of Tensilica’s L106 Diamond series 32-bit processor, with on-chip SRAM, on top of its Wi-Fi functionalities. It is integrated with external sensors and other application specific devices through its GPIOs. Codes for such applications are provided as examples in the SDK. Sophisticated system-level features include fast sleep/wake switching for energy-efficient VoIP, adaptive radio biasing for low-power operations, advanced signal processing, spur cancellation and radio co-existence features for common cellular, Bluetooth, DDR, LVDS, LCD interference mitigation. The SDK based on ESP8266 IoT platform offers the users an easy, fast and efficient way to develop IoT devices. This programming guide provides overview of the SDK as well as detailed description of the APIs. It is written to help embedded software developers program on ESP8266 IoT platform.
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* George Stafanick, 802.11 – Action Frames. post
> The 802.11 standard section 8.5 comments on action frames. Action frames are interesting. Action frames can be triggered by access points or client stations. The action frame provides information and direction as in what to do. The 802.11 standard comments about action frames in 17 different sections of subsection 8.5. While many of these aren't used by vendors today some important ones are. Lets review some comments about action frames and then head to the frame captures.
* wikipedia :
> Action frame: extending management frame to control a certain action. Some of the action categories are Block Ack, Radio Measurement, Fast BSS Transition, etc. These frames are sent by a station when it needs to tell its peer for a certain action to be taken. For example, a station can tell another station to set up a block acknowledgement by sending an ADDBA Request action frame. The other station would then respond with an ADDBA Response action frame.
* Channel switch and quiet attack: New DoS attacks exploiting the 802.11 standard. ieee
> The amendment 802.11h, released in 2003, extends the capabilities for exchanging management information between stations. 802.11h defines so called action frames, which belong to a certain action category, e.g., spectrum management. Each action category defines its own information elements (IE) tailored to certain management tasks. These IEs can be part of dedicated action frames or be included in other management frames, e.g., beacons.