ESP-HOSTED FG packet classification problem

[dreamcmi]

I am studying the communication process of FG mode, and now I am wondering how the host distinguishes control packets or 802.3 packets. I noticed that there is a double tlv packet of endpoint+data, but I don't know how it works specifically. So my question is when the MCU acts as the host in FG mode, how does the MCU determine whether the received data packet is a network data packet or a control command packet? Thank you.

[mantriyogesh]

Hello @dreamcmi ,

The network packet is never serialised. Only control packet is serialised. Control packet uses ESP_SERIAL_IF interface in header. So basically after serialization, the receiver knows if it needs to de-serialise or not using this interface type.

Here is very much deftailed explanation of control path: https://github.com/espressif/esp-hosted/blob/master/esp_hosted_fg/docs/common/contrl_path.md

APIs: https://github.com/espressif/esp-hosted/blob/master/esp_hosted_fg/docs/common/ctrl_apis.md

[mantriyogesh]

Please feel free to ping if something is not clear or not covered in this documentation.

[dreamcmi]

Thanks for your reply. I understand a little bit now. Let me briefly describe it.

1. In the master-slave direct interaction data packets of esp-hosted FG, there is a fixed header esp_payload_header. Every time a data packet is received, the correctness of this header must be determined first.
2. When the header is correct, then determine the if_type type. If it is ESP_SERIAL_IF, it means that this is a control data packet. If it is ESP_STA_IF or ESP_AP_IF, it means that this is an 802.3 frame from sta or ap mode.
3. After determining the packet type, if it is a control packet, enter the deserialization function for parsing. If it is a network packet (802.3), enter the network protocol stack for processing.
4. When the host (such as stm32) wants to send a packet to the slave (such as esp32c3), if it wants to send a control packet, it will be serialized first, and then a header of theESP_SERIAL_IF type will be added. If it wants to send an 802.3 network packet, it will be added with a header of ESP_STA_IF or ESP_AP_IF, and then sent it via SPI or SDIO. The above is my understanding, and the following are the questions.
5. Is there anything wrong with my explanation above?
6. I think ESP_HCI_IF should indicate a BLE data packet? What type of packet does ESP_PRIV_IF represent?

[mantriyogesh]

Hello @dreamcmi ,

1. Your understanding is absolutely correct. ESP_HCI_IF is bluetooth packet.

ESP_PRIV_IF is packet that is private packet in between slave and host. However, if you wish to send something small, you can use TLV format in ESP_PRIV_IF.

As ESP_PRIV_IF is generally for some capabilities exchange, we use single byte denoting specific capability or configuration. If you need more than a byte, you might need to serialise (endian-ness)

ESP_PROV_IF is discouraged for large control packets, for the same reason.

2. In your block diagram, right side, there is typo, want to send a network packet

3. If you are interested in MCU based host, you can refer to https://github.com/espressif/esp-hosted/tree/feature/esp_as_mcu_host Where you can connect two ESP chipsets and evaluate the solution. And later can adapt to your host using port layer added.

[mantriyogesh]

Branch in (3) above is much advanced, with a lot of optimizations, bug fixes and lwip network stackand NimBLE bluetooth stack integrated.

Master branch is maintained and developed for Linux only.

Edit: correct spelling of nimble

[dreamcmi]

Thank you very much for your explanation, it helped me a lot.

[mantriyogesh]

Please find updated comment: https://github.com/espressif/esp-hosted/issues/454#issuecomment-2283900891