TCP/IP协议栈源代码！

wensean

适合对tcp/ip协议感兴趣的人看看，同时帮助初学者了解协议栈的知识。

//********************************************************************************
//* Resolve an IP address to a hardware address; if not found,initiate query
//* and return NULL.  If an address is returned, the interface driver may  
//* send the packet; if NULL is returned,res_arp() will have saved the packet
//* on its pending queue,so no further action (like freeing the packet) is
//* necessary.
uint8 * res_arp(
                 struct iface *iface,                /* Pointer to interface block */
                 enum   arp_hwtype hardware,        /* Hardware type */
                 int32  target,                        /* Target IP address */
                 struct mbuf **bpp)                /* IP datagram to be queued if unresolved */               
{
    register struct arp_tab *arp;
    struct ip ip;
   
    if((arp = arp_lookup(hardware,target)) != NULL && arp->state == ARP_VALID)
            return arp->hw_addr;
    if(arp != NULL){
            /* Earlier packets are already pending, kick this one back
             * as a source quench
             */
            ntohip(&ip,bpp);
            icmp_output(&ip,*bpp,ICMP_QUENCH,0,NULL);
            free_p(bpp);
    } else {
            /* Create an entry and put the datagram on the
             * queue pending an answer
             */
            arp = arp_add(target,hardware,NULL,0);
            enqueue(&arp->pending,bpp);
            arp_output(iface,hardware,target);
    }
    return NULL;
}
/* Look up the given IP address in the ARP table */
struct arp_tab *
arp_lookup(hardware,ipaddr)
enum arp_hwtype hardware;
int32 ipaddr;
{
        register struct arp_tab *ap;

        for(ap = Arp_tab[hash_ip(ipaddr)]; ap != NULL; ap = ap->next){
                if(ap->ip_addr == ipaddr && ap->hardware == hardware)
                        break;
        }
        return ap;
}