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network-programmingtcpembeddedethernet

What is the border of roles of network interfaces in MCUs?

I am an embedded software developer who has any experience with TCPIP on connected devices. Also, I am not a software protocol expert, so I am a bit confusing about TCPIP protocol stack + responsiblities of its various phy layers.

First of all, I have experiences with such protocols like UART, SPI, CAN, USB... As you know, the phy layer directly affects you while selecting the protocol you used at the software level. For example, if you use usb and you build a software protocol on it, you do not occasionally deal with some details like checking corrupted frame in your sofware protocol, because phy layer of it guarantees this operation. CAN also has some CAN Controller facilities like crc and bit stuffing so, it is really reliable. But the situation is not the same for simple peripherals like UART/USART. Let's say you are using a bluetooth module to upgrade your firmware, you need to be aware of almost everything that can occur while communicating like delays, corrupted frames, payload validating etc.

Briefly, i am trying to understand the exact role of newtork interfaces come included in MCUs, that are interfaced with RJ45 phy sockets directly. In another words, imagine that I wrote a server application on my pc. Also i configured and ran an application in my development board which has an RJ45 socket and it runs as a client. Also imagine they established a connection over TCP. So, what will be the situation at the client side, when i send a 32 bytes of data to the socket from the server side? What will I see at the lowest level of MCU that is an RxCompleteInterrupt()? Are the data I sent and some other stuffs appended to the TCP packet guaranteed to be delivered by the eth controller in the MCU and ethernet controller of my PC? OR am i responsible (or the stack i used) check all the things necessary to validate whether the frame is valid or not?

I tried to be as clear as it would be. Please if you have experience, then try to write clean comments. I am not a TCPIP expert, maybe I used some wrong terminology, please focus the main concept of the question.

Thanks folks.

Solution

  • If you don't have any prior experience with the TCP/IP protocol suite, I would strongly suggest you to have a look at this IBM Redbook, more specifically at chapters 2, 3 and 4.

    This being said:

    1. So, what will be the situation at the client side, when i send a 32 bytes of data to the socket from the server side? What will I see at the lowest level of MCU that is an RxCompleteInterrupt()?

      You should have received an Ethernet frame in your buffer. This Ethernet frame should contain an IP packet. This IP packet should contain a TCP packet, which payload should consist in your 32 bytes of data. But there will be several exchanges between the client and the server prior to your data to be received, because of TCP being a connection-oriented protocol, i.e. several Ethernet frames will be sent/received.
    2. Are the data I sent and some other stuffs appended to the TCP packet guaranteed to be delivered by the eth controller in the MCU and ethernet controller of my PC? OR am i responsible (or the stack i used) check all the things necessary to validate whether the frame is valid or not?

      The TCP packet will ultimately be delivered, but there there are not warranties that your Ethernet frames and IP packets will be delivered, and will arrive in the right order. This is precisely the job of TCP, as a connection-oriented protocol, than to do what is needed so that the data you are sending as a TCP payload will ultimately be delivered. Your MCU hardware should be the one responsible for validating the Ethernet frames, but the TCP/IP stack running on the MCU is responsible for validating IP and TCP packets and the proper delivery of the data being sent/received over TCP.

    You can experiment with TCP on a Linux PC using netcat, and capture the exchange using Wireshark or tcpdump.

    Create a 'response' file containing 32 bytes:

    echo 0123456789ABCDEFGHIJKL > response.txt

    Start Wireshark, and filter on lo interface using filter tcp port 1234
    Start a TCP server listening on TCP port 1234, which will send the content of response.txt upon receiving a connection from the client:

    netcat -l 1234 < response.txt

    In another console/shell, connect to the server listening on tcp/1234, and display what was received:

    netcat localhost 1234
0123456789ABCDEFGHIJKL

    On Wireshark, you should see the following Wireshark Network Capture, and be able to expand all frames/packets of the full exchange using the IBM Redbook as a reference.
    Your 32 bytes of data will be in the payload section of a TCP packet sent by the server.