net: proper socket API in the kernel (SYS_NET) - no more SLIP in userland

The network stack moves into the kernel. src/socket.asm owns SLIP framing,
IPv4, RFC1071 checksums, and ICMP; programs now speak a socket API through one
syscall (SYS_NET, DE=&netreq dispatched by op): net_socket/connect/send/recv/
close/poll (c/sock.h). No program touches SLIP, IP headers, or checksums.

- Socket table (4 sockets) + tx/rx buffers in WRAM0; our IP = 10.0.0.2.
- net_pump: drains the link, reassembles SLIP frames, demuxes IPv4. Inbound
  ICMP echo requests are auto-answered in-kernel, so the GB replies to pings
  whenever any process pumps RX.
- ICMP sockets: send() emits an echo request to the connected peer; recv()
  returns the matching reply (with a spin/yield timeout).

ping.c is now a ~15-line socket client; netd.c is just `for(;;){net_poll();
yield();}`. Verified over tunbridge:
    /# ping 1.1.1.1     -> replies from the real internet (kernel builds it all)
    host# ping 10.0.0.2 -> 4/4, 0% loss (kernel auto-answers)

Gotchas recorded: gbos.inc isn't a make dep (touch asm after editing); this
crt0 doesn't copy initializers (fill arrays at runtime); and the arg string at
0xA000 overlaps _DATA, so parse targets must sit past it (big buffer first).
UDP and TCP sockets build on this same core next.

1 files changed, 7 insertions, 73 deletions

diff --git a/c/netd.c b/c/netd.c
index bce6ea7..b07abd2 100644
--- a/c/netd.c
+++ b/c/netd.c
@@ -1,78 +1,12 @@
-#include "netlib.h"
-/* netd - a minimal IP stack over SLIP. Our address is 10.0.0.2, peer 10.0.0.1.
-   Milestones: A) ICMP echo, B) UDP echo. Raw byte arrays, network byte order
-   (big-endian) handled explicitly. */
-
-static unsigned char pkt[256];
-
-/* one's-complement sum (RFC 1071) with a running seed, carry-folded each add */
-static unsigned int sum16(unsigned char *d, unsigned int len, unsigned int sum) {
-    unsigned int w, i = 0;
-    while (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;
-}
-
-/* pseudo-header sum for UDP/TCP: src+dst IP, protocol, segment length */
-static unsigned int pseudo(unsigned char proto, unsigned int seglen) {
-    unsigned int sum = sum16(pkt + 12, 8, 0), w;
-    w = proto;  sum += w; if (sum < w) sum++;
-    w = seglen; sum += w; if (sum < w) sum++;
-    return sum;
-}
-
-static void swap4(unsigned char a, unsigned char b) {
-    unsigned char i, t;
-    for (i = 0; i < 4; i++) { t = pkt[a + i]; pkt[a + i] = pkt[b + i]; pkt[b + i] = t; }
-}
-static void swap2(unsigned char a, unsigned char b) {
-    unsigned char t;
-    t = pkt[a]; pkt[a] = pkt[b]; pkt[b] = t;
-    t = pkt[a + 1]; pkt[a + 1] = pkt[b + 1]; pkt[b + 1] = t;
-}
-
-/* recompute the IP header checksum over ihl bytes, then send 'len' total */
-static void ip_send(unsigned char ihl, unsigned int len) {
-    unsigned int c;
-    pkt[10] = 0; pkt[11] = 0;
-    c = ~sum16(pkt, ihl, 0);
-    pkt[10] = (unsigned char)(c >> 8); pkt[11] = (unsigned char)c;
-    slip_send((char *)pkt, (unsigned char)len);
-}
+#include "sock.h"
+/* netd - the network daemon. It just pumps the kernel RX path; the kernel's
+   socket layer owns SLIP/IP and auto-answers inbound ICMP echo requests, so
+   running netd makes the Game Boy respond to pings. No SLIP/IP here anymore. */
 
 void main(void) {
-    unsigned char len, ihl;
-    unsigned int c, seglen;
-    puts("netd: IP up (10.0.0.2)"); nl();
+    puts("netd: up (10.0.0.2)"); nl();
     for (;;) {
-        len = slip_recv((char *)pkt, 255);
-        if (len < 20 || (pkt[0] >> 4) != 4) continue;
-        ihl = (pkt[0] & 0x0f) << 2;
-        pkt[8] = 64;                                  /* TTL for any reply */
-
-        if (pkt[9] == 1) {                            /* ICMP */
-            if (len < (unsigned char)(ihl + 8) || pkt[ihl] != 8) continue;   /* echo request */
-            swap4(12, 16);
-            pkt[ihl] = 0;                             /* -> echo reply */
-            pkt[ihl + 2] = 0; pkt[ihl + 3] = 0;
-            c = ~sum16(pkt + ihl, (unsigned int)(len - ihl), 0);
-            pkt[ihl + 2] = (unsigned char)(c >> 8); pkt[ihl + 3] = (unsigned char)c;
-            ip_send(ihl, len);
-
-        } else if (pkt[9] == 17) {                    /* UDP echo */
-            if (len < (unsigned char)(ihl + 8)) continue;
-            seglen = (unsigned int)(len - ihl);
-            swap4(12, 16);                            /* swap IP addresses */
-            swap2(ihl, ihl + 2);                      /* swap UDP ports */
-            pkt[ihl + 6] = 0; pkt[ihl + 7] = 0;
-            c = ~sum16(pkt + ihl, seglen, pseudo(17, seglen));
-            if (c == 0) c = 0xffff;                   /* UDP: 0 means "none" */
-            pkt[ihl + 6] = (unsigned char)(c >> 8); pkt[ihl + 7] = (unsigned char)c;
-            ip_send(ihl, len);
-        }
+        net_poll();     /* drain RX, answer pings */
+        yield();
     }
 }