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NetServicesMin - arp failure?

Topic last updated 19 Apr 2012, by Hugh S. 2 replies
Hugh S
19 Apr 2012

I'm attempting to build a UDP-based data logger using the StrippedDownNetServices library. (Sorry, I put the wrong library name in the posting title!) The mbed application will acquire data and then send UDP packets to a remote host. Unfortunately, I'm having trouble getting past the first step. In the attached test program I would expect to see a data packet arriving at 192.168.1.8 once per second but I see nothing. 

#include "mbed.h"
#include "EthernetNetIf.h"
#include "UDPSocket.h"
    
int main() {
    
    EthernetNetIf eth(
        IpAddr(192,168,1,2), //IP Address
        IpAddr(255,255,255,0), //Network Mask
        IpAddr(192,168,1,1), //Gateway
        IpAddr(192,168,1,1)  //DNS
    );
    
    eth.setup();
    Host destHost(IpAddr(192,168,1,8), 55555);
    Host localHost(IpAddr(), 10000);

    UDPSocket udpSocket;
    udpSocket.bind(localHost);
 
    char sendBuffer[8] = { '1', '2', '3', '4', '5', '6', '7', '8'};

    for (int i = 0; i < 1000; i++) {    
        udpSocket.sendto(sendBuffer, 8, &destHost);
        wait(1);
    }
    
}

I used tcpdump to try to figure out what the problem might be and to my surprise, the mbed network stack doesn't seem to be responding to arp replies. I see the following network traffic repeated over and over again once per second.

16:01:14.271964 arp who-has hugh-laptop.local tell 192.168.1.2
	0x0000:  ffff ffff ffff 0002 f7f0 3079 0806 0001  ..........0y....
	0x0010:  0800 0604 0001 0002 f7f0 3079 c0a8 0102  ..........0y....
	0x0020:  0000 0000 0000 c0a8 0108 0000 0000 0000  ................
	0x0030:  0000 0000 0000 0000 0000 0000            ............
16:01:14.271982 arp reply hugh-laptop.local is-at 00:01:4a:9d:d5:95 (oui Unknown)
	0x0000:  0002 f7f0 3079 0001 4a9d d595 0806 0001  ....0y..J.......
	0x0010:  0800 0604 0002 0001 4a9d d595 c0a8 0108  ........J.......
	0x0020:  0002 f7f0 3079 c0a8 0102                 ....0y....

What in the world am I doing wrong? BTW, I wrote another test program to see if Ethernet packets are making out of the mbed. It appears that they are. Here's the test program.

#include "mbed.h"

Ethernet eth;

int main() {
    char buf[16];
    buf[0] = 0;

    while(1) {
        eth.write(buf, 16);
        eth.send();
        buf[0] += 1;
        wait(1);
    }   
}

And here is some typical network traffic.

15:58:46.479969 arp-#0 for proto #0 (0) hardware #1 (0)
	0x0000:  d2ff ffff ffff 0001 4a9d d595 0806 0001  ........J.......
	0x0010:  0000 0000 0000 0000 0000 0000 0000 0000  ................
	0x0020:  0000 0000 0000 0000 0000 0000 0000 0000  ................
	0x0030:  0000 0000 0000 0000 0000 0000            ............
15:58:47.479979 arp-#0 for proto #0 (0) hardware #1 (0)
	0x0000:  d3ff ffff ffff 0001 4a9d d595 0806 0001  ........J.......
	0x0010:  0000 0000 0000 0000 0000 0000 0000 0000  ................
	0x0020:  0000 0000 0000 0000 0000 0000 0000 0000  ................
	0x0030:  0000 0000 0000 0000 0000 0000            ............

My hardware setup is an LPC1768 plugged into a coolcomponents development board. I'm using a Linksys Etherfast 10/100 switch to link my computer and the mbed.

Adam Green
19 Apr 2012

You probably need to call Net::poll() to allow the network stack to process packets (such as ARP) between your UDP sends. Your code is probably sitting in a wait() call when the ARP packets arrive.

I haven't actually tried this code snippet below but I just wanted to have something that showed calling Net::poll() as often as possible while still sending your UDP packet on a 1 second interval.

    Timer sendTimer;

    sendTimer.start();
    for (;;) 
    {
        Net::poll();
        if (sendTimer.read() > 1.0f)
        {    
            udpSocket.sendto(sendBuffer, 8, &destHost);
            sendTimer.reset();
        }
    }
Hugh S
19 Apr 2012

HA! That fixes it! I am humbled. :)