#include "netlib.h"
/* netd - a minimal IP stack over SLIP. Milestone A: answer ICMP echo (ping).
   Our address is 10.0.0.2; the SLIP peer (gateway) is 10.0.0.1.

   Everything is done on raw byte arrays so endianness is explicit (network
   byte order = big-endian): field[hi], field[lo]. */

/* Internet checksum (RFC 1071): 16-bit one's-complement sum with carry fold. */
static unsigned int cksum(unsigned char *d, unsigned char len) {
    unsigned int sum = 0, w;
    unsigned char i = 0;
    while ((unsigned char)(i + 1) < len) {
        w = ((unsigned int)d[i] << 8) | d[i + 1];
        sum += w; if (sum < w) sum++;
        i += 2;
    }
    if (len & 1) { w = (unsigned int)d[len - 1] << 8; sum += w; if (sum < w) sum++; }
    return ~sum;
}

void main(void) {
    unsigned char pkt[128];
    unsigned char len, ihl, i, t;
    unsigned int c;
    puts("netd: IP/ICMP up (10.0.0.2)"); nl();
    for (;;) {
        len = slip_recv((char *)pkt, 127);
        if (len < 28) continue;                 /* IP(20) + ICMP(8) minimum */
        if ((pkt[0] >> 4) != 4) continue;       /* IPv4 only */
        ihl = (pkt[0] & 0x0f) << 2;
        if (pkt[9] != 1) continue;              /* protocol 1 = ICMP */
        if (pkt[ihl] != 8) continue;            /* ICMP type 8 = echo request */

        /* turn the request into a reply, in place */
        for (i = 0; i < 4; i++) { t = pkt[12 + i]; pkt[12 + i] = pkt[16 + i]; pkt[16 + i] = t; }
        pkt[8] = 64;                            /* reset TTL */
        pkt[ihl] = 0;                           /* ICMP type -> echo reply */
        pkt[ihl + 2] = 0; pkt[ihl + 3] = 0;     /* recompute ICMP checksum */
        c = cksum(pkt + ihl, (unsigned char)(len - ihl));
        pkt[ihl + 2] = (unsigned char)(c >> 8); pkt[ihl + 3] = (unsigned char)c;
        pkt[10] = 0; pkt[11] = 0;               /* recompute IP header checksum */
        c = cksum(pkt, ihl);
        pkt[10] = (unsigned char)(c >> 8); pkt[11] = (unsigned char)c;

        slip_send((char *)pkt, len);
    }
}