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[lwip-users] Ethernet raw frames?

From: Michael Anburaj
Subject: [lwip-users] Ethernet raw frames?
Date: Fri, 7 Feb 2003 14:01:30 -0800 (PST) 
Hi,

I tried to run Adam’s example code (18.1 Using the API
from lwip.pdf - which I have also appended to this
email at the end) on my MIPS hardware running UCOS-II
RTOS. I came across this issue while doing the
following:

>From the PC, through Internet explorer (ie) browsed to
this URL “http://169.254.140.28/”. Where
169.254.140.28 is the IP address set on the MIPS
hardware.
The program on the MIPS side gets the HTTP GET packet
& executes the following lines of code.

/* Send the header. */
netconn_write(conn, http_html_hdr,
sizeof(http_html_hdr),
NETCONN_NOCOPY);

When this line gets executed, the following things
happen (I tried to follow the code as much as
possible, so let me know if I assumed things wrongly):

tcp_enqueue() gets called at some point <from
tcpip_thread>. It creates two pbufs, seg->p & p.
seg->p is of type PBUF_RAM.
p is of type PBUF_ROM
p is chained with seg->p. “pbuf_chain(seg->p, p);”
seg->p carries the ACK packet with the TCP header
p carries the pointer to the constant ‘http_html_hdr’.
And no header attached to it.
Later the IP layer calls the netif->output() function
with the pbuf (p chained to seg->p). Note: the 1st
Packet, ACK is complete with its header & stuff. But
the second packet has just the raw string contained in
‘http_html_hdr’.
When this is sent to the PC, the Internet explorer
receives the following 2 packets:
ACK packet to IPAddress 169.254.140.27 <PC’s IP
address> from 169.254.140.28 <MIPS hardware>
Ethernet II frame type (I guest its a raw ethernet
frame) to MAC address 43:6f:6e:74:65:6e from
74:2d:74:79:70:65. The string carried by
‘http_html_hdr’, “Content-type: text/html\r\n\r\n" is
getting interpreted for src & dst MAC addresses, since
this string is sent all by itself as raw frame.

This is the case with ‘indexdata’ also.

Please bare with me, Though I have plenty of Embedded
Real-time experience with ARM & MIPS like RISCs.. I am
a newbe when it comes to ethernet & TCP/IP protocols.
So please answer to my questions below with as much
details as possible:

Is it ok for us (lwIP) to send the ‘http_html_hdr’
info as raw ethernet frame following the ACK packet
that’s complete with TCP/IP headers?
If not why is this happening? Please help me resolve
this issue.

I can also send the Ethernet capture ran on the PC
using Ethereal. Actually the UCOS-II port running on
the MIPS Atlas board was done by me & I am trying to
get lwIP working on it. This is purely an experimental
project, which I am conducting at my Home. Just for
learning stuff about TCP/IP.

Thanks,
-Mike.






/* A simple HTTP/1.0 server using the minimal API. */

#include "lwip/sys.h"
#include "lwip/api.h"
#include "consol.h"


/* This is the data for the actual web page.
Most compilers would place this in ROM. */
const static char indexdata[] =
"<html> \
<head><title>A test page</title></head> \
<body> \
This is a small test page. \
</body> \
</html>";

const static char http_html_hdr[] =
"Content-type: text/html\r\n\r\n";


/* This function processes an incomming connection. */
static void process_connection(struct netconn *conn)
{
        struct netbuf *inbuf;
        char *rq;
        u16_t len;
        int i;

        /* Read data from the connection into the netbuf
inbuf.
        We assume that the full request is in the netbuf. */
        if((inbuf = netconn_recv(conn)) == NULL)
        {
                netconn_close(conn);
                return;
        }
                
        netbuf_data(inbuf, (void **)&rq, &len);

        /* Check if the request was an HTTP "GET /\r\n". */
        if(rq[0] == 'G' && rq[1] == 'E' &&
        rq[2] == 'T' && rq[3] == ' ' &&
        rq[4] == '/' && rq[5] == '\r' &&
        rq[6] == '\n')
        {
                /* Send the header. */
                if(netconn_write(conn, (void *)http_html_hdr,
sizeof(http_html_hdr), NETCONN_NOCOPY) != ERR_OK)
                        printf("***Write1Err***\n");

                /* Send the actual web page. */
                if(netconn_write(conn, (void *)indexdata,
sizeof(indexdata), NETCONN_NOCOPY) != ERR_OK)
                        printf("***Write2Err***\n");

                /* Close the connection. */
                netconn_close(conn);
        }
}

static void 
tcpecho_thread(void *arg)
{
        struct netconn *conn, *newconn;

        /* Create a new TCP connection handle. */
        conn = netconn_new(NETCONN_TCP);

        /* Bind the connection to port 80 on any local IP
address. */
        netconn_bind(conn, NULL, 80);

        /* Put the connection into LISTEN state. */
        netconn_listen(conn);

        /* Loop forever. */
        while(1)
        {
                /* Accept a new connection. */
                newconn = netconn_accept(conn);
                
                if(newconn != NULL)
                {
                        /* Process the incomming connection. */
                        process_connection(newconn);
                
                        /* Deallocate connection handle. */
                        netconn_delete(newconn);
                }
        }               
}
        
void tcpecho_init(void)
{
        sys_thread_new(tcpecho_thread, NULL);  
}

void LAPP_vInit()
{
        tcpecho_init();

        /* Block for ever. */
        sem = sys_sem_new(0);
sys_sem_wait(sem);
}


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