网友张巍提供的tcpip协议栈,是一个成功稳定的以太网接口上的TCP/IP协议程序。里面包括有说明

				//********************************************************************************
				//* 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;
				}