Web 51 - ARP

ARP - Address Resolution Protocol

ARP - Address Resolution Protocol is described in RFC826 (An Ethernet Address Resolution Protocol). ARP is used in Ethernet networks to determine the MAC address from a known IP address. The method is very simple and truly distributed - each node responds only to requests for its own MAC address. Whenever a node needs to find a MAC address from an IP address, it sends a broadcast message to all nodes (address 0xFFFFFFFFFFFF) containing the IP address in question and a request for the respective MAC address. The other node which detects its own IP address in such a message replies with a packet containing its MAC address. For correct operation, Web51 needs to implement both parts - it must be able to receive and reply to ARP requests, as well as ask for another station's MAC address and process the reply. Algorithms implementing ARP are detailed below.

Processing of received ARP packets

void ProcessARP (void) {
if(rx_eth_pkt.pkt.arp.TIPA == flash_my_ip) {
  switch (rx_eth_pkt.pkt.arp.Op) {
    case ARPT_RQ:

ARP request received; send a reply.

      tx_eth_pkt.hdr.pktDest = EthRcvHdr.pktSrc;
      tx_eth_pkt.hdr.pktSrc = flash_my_ether;
      tx_eth_pkt.hdr.pktType = 0x0806;
      tx_eth_pkt.pkt.arp.HwType = 0x0001;
      tx_eth_pkt.pkt.arp.PrType = 0x0800;
      tx_eth_pkt.pkt.arp.HwLen = 6;
      tx_eth_pkt.pkt.arp.PrLen = 4;
      tx_eth_pkt.pkt.arp.Op = ARPT_REPLY;                 //ARP 0x0002=reply
      tx_eth_pkt.pkt.arp.SHwA = flash_my_ether;           //senders hardware address
      tx_eth_pkt.pkt.arp.SIPA = flash_my_ip;              //senders IP address
      tx_eth_pkt.pkt.arp.THwA = rx_eth_pkt.pkt.arp.SHwA;  //target hardware address
      tx_eth_pkt.pkt.arp.TIPA = rx_eth_pkt.pkt.arp.SIPA;  //target IP address
      xmit_frame(sizeof(rx_eth_pkt.hdr) + sizeof(rx_eth_pkt.pkt.arp));
      return;
    case ARPT_REPLY:

Reply to an ARP request was received, extract and store the MAC address.

      if (rx_eth_pkt.pkt.arp.SIPA == ip_gateway) {

Reply with gateway MAC address was received.

        mac_gateway = rx_eth_pkt.pkt.arp.SHwA;
        return;
      if (rx_eth_pkt.pkt.arp.SIPA == IPsrcAddr23) {

Reply with MAC address of "telnet" target was received.

        MacSrcAddr23 = rx_eth_pkt.pkt.arp.SHwA;
        return;
      }
    default:
      return;
  }
}
return;

Transmission of an ARP packet

Construct and send a request for gateway MAC address.

void MakeArpGwRq (void) {
  tx_eth_pkt.hdr.pktDest = eth_brdcast;       //Broadcast Address
  tx_eth_pkt.hdr.pktSrc = flash_my_ether;     //packet source address
  tx_eth_pkt.hdr.pktType = 0x0806;            //ARP
  tx_eth_pkt.pkt.arp.HwType = 0x0001;         //hardware type
  tx_eth_pkt.pkt.arp.PrType = 0x0800;         //protocol type
  tx_eth_pkt.pkt.arp.HwLen = 6;               //hardware address length
  tx_eth_pkt.pkt.arp.PrLen = 4;               //protocol address length
  tx_eth_pkt.pkt.arp.Op = ARPT_RQ;            //ARP operation (1=request, 2=reply)
  tx_eth_pkt.pkt.arp.SHwA = flash_my_ether;   //senders hardware address
  tx_eth_pkt.pkt.arp.SIPA = flash_my_ip;      //senders IP address
  tx_eth_pkt.pkt.arp.THwA = eth_zero;         //target hardware address
  tx_eth_pkt.pkt.arp.TIPA = flash_ip_gateway; //target IP address
  xmit_frame(sizeof(rx_eth_pkt.hdr) + sizeof(rx_eth_pkt.pkt.arp));
  return;
}

Construct and send a request for MAC of a "telnet" target.

void MakeArpPoiRq (void) {
  tx_eth_pkt.hdr.pktDest = eth_brdcast;       //Broadcast Address
  tx_eth_pkt.hdr.pktSrc = flash_my_ether;     //packet source address
  tx_eth_pkt.hdr.pktType = 0x0806;            //ARP
  tx_eth_pkt.pkt.arp.HwType = 0x0001;         //hardware type
  tx_eth_pkt.pkt.arp.PrType = 0x0800;         //protocol type
  tx_eth_pkt.pkt.arp.HwLen = 6;               //hardware address length
  tx_eth_pkt.pkt.arp.PrLen = 4;               //protocol address length
  tx_eth_pkt.pkt.arp.Op = ARPT_RQ;            //ARP operation (1=request, 2=reply)
  tx_eth_pkt.pkt.arp.SHwA = flash_my_ether;   //senders hardware address
  tx_eth_pkt.pkt.arp.SIPA = flash_my_ip;      //senders IP address
  tx_eth_pkt.pkt.arp.THwA = eth_zero;         //target hardware address
  tx_eth_pkt.pkt.arp.TIPA = flash_ip_point;   //target IP address
  xmit_frame(sizeof(rx_eth_pkt.hdr) + sizeof(rx_eth_pkt.pkt.arp));
  return;
}

ARP implementation in Web 51

Previous algorithms may look complicated at the first glance; however, in most cases they simply fill the transmitted packet in with constant, or "almost" constant values. To take advantage of this, the packet is pre-constructed in the processor's EEPROM. Constant portions are initialized at compile-time and then downloaded into the processor together with firmware; "almost" constant parts are initialized whenever the processor is reset or configuration is cloned -- required parameters (IP/MAC addresses) are copied into respective fields of pre-constructed packets. Following example details the transmission of an ARP request for gateway MAC address.

First, a peek into the processor's EEPROM.
arpgwreq:.word  0xFFFF,0xFFFF,0xFFFF  ;PKT_DEST (Broadcast Address)
my3eth:  .ascii "??????"              ;PKT_SRC = myether
         .word  0x0806  ;PKT_TYPE     ;0806 = ARP
         .word  0x0001  ;ARP_HWTYPE   ;hardware type  0001
         .word  0x0800  ;ARP_PRTYPE   ;protocol type  0800
         .byte  6       ;ARP_HWLEN    ;hardware address length  06
         .byte  4       ;ARP_PRLEN    ;protocol address length  04
         .word  ARPT_RQ               ;ARP operation (1=request, 2=reply)
my4eth:  .ascii "??????"              ;senders hardware address = myether
my2ip:   .ascii "????"                ;senders IP address = myip
         .word  0x0000,0x0000,0x0000  ;target hardware address
gw1ip:   .ascii "????"                ;target IP address
sizeofarpgwreq equ ($ - arpgwreq)
All '?'s are filled in during cloning with the real MAC address of Web 51 (my3eth, my4eth), IP address of Web 51 (my2ip), or gateway IP address (gwlip).

The actual implementation of gateway MAC address request is very simple - all that is needed is to transmit the pre-constructed packet.
ProcessArpGwRq:
         mov    r7,#0		; use service stack
         lcall  changeStack
         clr    flagArpGwRq
         LCALL  pcode
         .pcode pe2x PKT_DEST, arpgwreq, #BYTE sizeofarppointreq
         .pcode xmit_frame #SIZE_OF_ETH_PKT_HDR+SIZE_OF_ARP
         .byte  0
         RET
Consistent use of pre-constructed packets greatly improves processing speed and therefore shortens the response time of a Web 51 server.